Degree project in

Value of Fast Switching Devices toElectric Distribution Networks

An Approach to Reduce Voltage Sags and Interruptions inDistribution Networks

SINDURI KASALA

Stockholm, Sweden 2014

XR-EE-ETK 2014:002

Electromagnetic EngineeringMaster of Science

ABSTRACTPower Quality (PQ) has gained a lot of importance in the last decade. Several solutions

to power quality problems have been proposed and developed. With the advent of solid statetechnology and power electronics in the power system protection devices, faster switching isachievable. In order to control and minimize the power quality problems which occur in ex-tremely short times of less than 100ms, the need arises for a selection of devices that can switchfaster than today. Also, the economic losses that occur in the network due to the power qualityproblems increase the incentive to transform the existing devices into faster and e�cient de-vices. This transformation can be seen as valuable from both a technical and economical pointof view to the distribution networks today where a large number of customers are connected.However, in order to interpret the value these fast switching devices render to the distributionnetwork a prior study is required.

This thesis presents a picture of the devices that can be suitable for fast switching in today’sdistribution network, and how to determine their value to the distribution network. Further itsummarizes the research work related to this field. The description of the devices and technicalaspects is presented first. A literature review of proposed devices is given. The technical aspectsof power quality and its problems is described. An approach to estimate the value of the fastswitching devices is detailed from di↵erent literature. The study shows that fast switchingdevices can be worthy to invest in when seen from a distribution network’s perspective providedthat di↵erent technical aspects are taken into account.

Keywords: Power Quality, Fast Switching Devices, Voltage Sags and Interruptions

ACKNOWLEDGEMENTSI would like to start by thanking ABB for letting me do this thesis at “ABB AB Corporate

Research Center”, Vasteras between July and December 2013. I would like to thank LarsLiljestrand, my adviser at ABB for his guidance and valuable suggestions throughout the work.I appreciate his patience, motivation and immense knowledge. I would also like to thank mymanager Magnus Backman for giving the opportunity to work and providing me with all theresources and making me comfortable during my work.I would like to express my kind and sincere gratitude to my examiner Prof. Goran Engdhal forhis continuous support to my master thesis research and study. I would also like to thank myadviser Dr. Nathaniel Taylor for his insightful comments on the weekly reports. His continuousguidance helped me sail throughout the journey of study and research. This thesis wouldn’thave been possible without him.Last but not the least, I take this opportunity to express my profound gratitude to my parents,sister and wonderful friends for their love, motivation and continuous support throughout theresearch work.

Contents

1 Introduction 11.1 Purpose . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Devices suitable for fast switching 42.1 On-Load Tap Changers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.2 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.1.3 Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

2.2 Circuit Breakers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42.2.1 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52.2.2 Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.3 Electrical Fuses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3.1 Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.3.2 Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.4 Fault Current Limiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.4.1 Super-conducting fault current limiters . . . . . . . . . . . . . . . . . . . . 72.4.2 Solid-state fault current limiters . . . . . . . . . . . . . . . . . . . . . . . 7

2.5 Transfer Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82.5.1 Mechanical Transfer Switch . . . . . . . . . . . . . . . . . . . . . . . . . . 82.5.2 Static Transfer Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

3 Power Quality 103.1 Power Quality Problems- Voltage Sags . . . . . . . . . . . . . . . . . . . . . . . . 10

3.1.1 What is Voltage Sag? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.2 Magnitude Calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103.1.3 Sag Duration Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1.4 Sags in 3-phase systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.1.5 Propagation of sags to lower voltage levels due to the presence of trans-

formers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113.2 Equipment behaviour during sags . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

3.2.1 Personal Computers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123.2.2 Contactors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.2.3 Process Control Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . 13

3.3 Adjustable speed AC drives connected to motors . . . . . . . . . . . . . . . . . . 133.3.1 Household Appliances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133.3.2 Lamps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

4 Literature review of existing and published fast switching devices 154.1 Methodology of Review . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.2 Overview of Research Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 154.3 Operation Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174.4 Components Employed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.5 Industrial Scenario . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.5.1 Circuit Breakers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184.5.2 On-Load Tap Changers . . . . . . . . . . . . . . . . . . . . . . . . . . . . 194.5.3 Automatic Transfer Switches . . . . . . . . . . . . . . . . . . . . . . . . . 194.5.4 Fast Switching Devices Research and Development . . . . . . . . . . . . . 204.5.5 I

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Current Limiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

4.5.6 Ultra Fast Earthing Switch . . . . . . . . . . . . . . . . . . . . . . . . . . 224.5.7 CapThor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

5 Comparison of the Canadian and NewZealand Power Quality Surveys 245.1 Technical Aspects of the Canadian Survey . . . . . . . . . . . . . . . . . . . . . . 245.2 Technical Aspects of the New Zealand Survey . . . . . . . . . . . . . . . . . . . . 245.3 Analysis of Voltage Sag charts obtained from the Surveys . . . . . . . . . . . . . 245.4 Analysis of the frequency of voltage sags occurring in di↵erent phases . . . . . . 275.5 Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

6 Voltage Control 316.1 Voltage Analysis for Distribution Systems . . . . . . . . . . . . . . . . . . . . . . 316.2 Distribution System with Integrated Generation . . . . . . . . . . . . . . . . . . 326.3 Role of Fast Switching in Voltage Control . . . . . . . . . . . . . . . . . . . . . . 336.4 Role of Fast Switching in the Integrated Network . . . . . . . . . . . . . . . . . . 33

7 Estimation of Value 347.1 Frequency of voltage sags and estimation . . . . . . . . . . . . . . . . . . . . . . 347.2 Calculation of voltages during three phase unbalance . . . . . . . . . . . . . . . . 34

7.2.1 Single Phase Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357.3 Phase-Phase Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367.4 Two Phase to Ground Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367.5 Area of Vulnerability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377.6 Area of Severity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377.7 Estimated Sag Frequency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 377.8 Types of Customers and the impacts of voltage sags on the di↵erent customers . 38

7.8.1 Residential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387.8.2 Industrial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 387.8.3 Commercial . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

7.9 Costs associated with Voltage Sags . . . . . . . . . . . . . . . . . . . . . . . . . . 397.9.1 Direct Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397.9.2 Restart Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 397.9.3 Hidden Costs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

7.10 Method to Estimate the Cost of Sags . . . . . . . . . . . . . . . . . . . . . . . . . 397.11 Who has to be paid for the loss . . . . . . . . . . . . . . . . . . . . . . . . . . . . 407.12 Estimating the costs of Fast Switching Devices . . . . . . . . . . . . . . . . . . . 407.13 Cases illustrating fast switching devices in a Distribution Network . . . . . . . . 417.14 Case1: Network installed with backup generation . . . . . . . . . . . . . . . . . . 41

7.14.1 Case2: Radial Distribution Network . . . . . . . . . . . . . . . . . . . . . 417.14.2 Case3: Ring Main Unit Distribution Network . . . . . . . . . . . . . . . . 437.14.3 Case4: Choice of Devices and Feasibility . . . . . . . . . . . . . . . . . . . 44

7.15 Weighing the loss costs against the cost of the devices . . . . . . . . . . . . . . . 447.16 Value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 467.17 Economic Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 477.18 Possible future distribution network with Fast Switching Devices . . . . . . . . . 49

8 Fast Switching Devices for future Electricity Networks 508.1 Today’s Electricity Grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 508.2 Future Electricity Grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

8.2.1 Distributed Generation Impacts . . . . . . . . . . . . . . . . . . . . . . . . 528.2.2 Smart Grid Technology Impacts . . . . . . . . . . . . . . . . . . . . . . . 53

8.3 Role of Fast Switching Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

9 CONCLUSIONS & FUTURE WORK 54

10 55

List of Figures

1 Di↵erent aspects of Power Quality [1] . . . . . . . . . . . . . . . . . . . . . . . . 22 Mechanism of a tap changer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 V-I characteristics of a fault current limiter [2] . . . . . . . . . . . . . . . . . . . 74 Solid-State fault current limiter [3] . . . . . . . . . . . . . . . . . . . . . . . . . . 85 Single line diagram of a medium voltage static transfer switch system [4] . . . . . 96 Voltage divider model for Sag magnitude calculation . . . . . . . . . . . . . . . . 107 Phasor diagrams for di↵erent types of sags. Obtained from [5] . . . . . . . . . . . 128 Standard Voltage Tolerance Curves (Data obtained from [5]) . . . . . . . . . . . 139 Principle of I

s

Current Limiter . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2110 Internal View of CapThor with plasma switch on the left and mechanical switch

on the right. [6] . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2311 Voltage Sag Charts for Industrial Customer Group at Primary and Secondary

Voltage Levels according to the Canadian Survey . . . . . . . . . . . . . . . . . . 2512 Voltage Sag Charts for Commercial Customer Group at Primary and Secondary

Voltage Levels according to the Canadian Survey . . . . . . . . . . . . . . . . . . 2613 Annual Voltage Sag Chart for Bairds 11kV Zone Substation in New Zealand

(Data obtained from [7]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2714 Voltage Sag Charts for Commercial Customer Group at Primary and Secondary

Voltage Levels according to the Canadian Survey . . . . . . . . . . . . . . . . . . 2815 Average number of Voltage Sags per phase for Bairds 11Kv zone substation in

New Zealand (Data obtained from [7]) . . . . . . . . . . . . . . . . . . . . . . . . 2916 Distribution Feeder (from [8]) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3117 Voltage profile for a Distribution Feeder (from [9]) . . . . . . . . . . . . . . . . . 3218 Distribution Feeder integrated with Distributed Generation (from [8]) . . . . . . 3319 An example of area of vulnerability ([10]) . . . . . . . . . . . . . . . . . . . . . . 3720 A simple network with Backup Generation installed . . . . . . . . . . . . . . . . 4121 A radial distribution network . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4222 A Ring Main distribution network . . . . . . . . . . . . . . . . . . . . . . . . . . 4323 A simple network with parallel supply . . . . . . . . . . . . . . . . . . . . . . . . 4424 PQ+Solution Costs for di↵erent solutions . . . . . . . . . . . . . . . . . . . . . . 4725 Possible future distribution network with fast switching devices installed . . . . . 4926 A schematic of Today’s Electricity Grid . . . . . . . . . . . . . . . . . . . . . . . 5027 A schematic of Future Electricity Grid . . . . . . . . . . . . . . . . . . . . . . . . 52

List of Tables

1 Sags experienced in 3 phase systems . . . . . . . . . . . . . . . . . . . . . . . . . 112 Propagation of Sags through Transformers. Obtained from [5] . . . . . . . . . . . 123 Components employed to improve the performance of the protection and control

equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 184 Circuit Breakers manufacturers ([],[11],[12] . . . . . . . . . . . . . . . . . . . . . . 185 On-load tap changers manufacturers ([13],[14],[15]) . . . . . . . . . . . . . . . . . 196 Automatic Transfer Switches ([16],[17]) . . . . . . . . . . . . . . . . . . . . . . . . 197 Threshold voltage levels during monitoring period . . . . . . . . . . . . . . . . . 248 Example value of weighing factors, Estimated Sag Frequency and Number of

Equivalent Interruptions for a year. Data obtained from ([18],[19]) . . . . . . . . 409 Investment costs of Fast Switching Devices obtained from([18]) . . . . . . . . . . 4010 Example value of weighing factors, Estimated Sag Frequency and Number of

Equivalent Interruptions for a year. Data obtained from ([18],[19]) . . . . . . . . 4511 Weighing the Investment costs of Fast Switching Devices against the Cost of sags

or interruptions per year. Data obtained from([18], [19]) . . . . . . . . . . . . . . 4612 NPV method applied for di↵erent solutions. Data obtained from ([20]) . . . . . . 47

List of Abbreviations

OLTC On-Load Tap Changer

CBEMA Computer Business Equipment Manufacturers Association

ITIC Information Technology Industrial Council

OLTC On-Load Tap Changer

PSPICE Personal Simulation Program with Integrated Circuit Emphasis

FCL Fault Current Limiter

ATS Automatic Transfer Switch

SSTS Solid-State Transfer Switch

UFES Ultra Fast Earthing Switch

AOS Area of Severity

NPV Net Present Value

UPS Uninterruptible Power Supply

DG Distributed Generation

1 Introduction

Distribution Systems play a significant role as a part of the modern Power Systems. Anykind of fault in the distribution system will a↵ect the customers connected to the network.The term Power Quality (PQ) has gained a lot of interest in the last decade. According tothe Institute of Electrical and Electronics Engineers (IEEE) dictionary, the definition of powerquality is, “Power quality is the concept of powering and grounding sensitive equipment in amatter that is suitable to the operation of that equipment” [21]. Voltage sags and interruptionscome under the di↵erent classification of the power quality problems. A detailed study ondistribution system power quality is of great importance in order to have an idea how di↵erentkind of power quality problems have an impact on the customers who are connected to thesystem. Fast switching devices are a current trend in order to switch the circuit during faultswith a time lapse of much less than a cycle. Currently two fast switching devices namelyfast-acting mechanical switch and Hybrid commutation device are being developed by twoPh.D. students in the department of Electromagnetic Engineering, KTH. ABB, the leadingmanufacturer for power system components has developed devices that can switch in a timeof 4ms. On-load tap changers (OLTCs) at the distribution level that could change the voltageratio of the transformers still maintaining the voltage sags under the limits and resulting in noflicker in the equipment is also a key aspect to be studied.

The aim of the project is to evaluate how the fast switching devices that are discussed abovecould be useful for a greater distributed generation and how their application in distributednetwork could lead to a potential increase in the value for customers connected to the network.The potential advantages of the fast switching devices that can limit the fault current withimmediate reset and result in shorter or shallow sags have to be studied. The economic side ofthe project is to investigate how the application of these devices can be an advantage to themanufacturers in terms of the net profit of the proposed devices.

1.1 Purpose

The purpose of this report is to give an insight of what devices in the distribution networkcan be used for faster switching and how will faster switching impact the distribution system andits problems. The devices when installed in the system need to be studied from both technicaland economical point of view in order to estimate their value and draw further conclusions.

1.2 Scope

This thesis aims to open a path to the development and application of fast switching devicesby estimating their value. The research on power quality is ongoing and the area is vast withseveral aspects to be dealt with. Therefore, there is a need to delimit the scope of this thesisin order to show what aspects of power quality have been taken into consideration and whatother aspects need to be studied further.

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Figure 1: Di↵erent aspects of Power Quality [1]

From the figure 1, it is observed that electric power quality can be divided into severalcategories [1].

• Modeling and analysis is an important section of power quality which consists of modelingpower system components and analyzing the waveforms through proposed time domainmethods, transformed domain methods and simulation of the existing circuit.

• Stochastic analysis has been researched to a huge extent with several papers using standardmethods such as monte carlo methods. Measurement and instrumentation techniques havebeen proposed and several test standards for measurements such as American NationalStandards Institute (ANSI), International Electro-technical Commission (IEC) and IEEEare being practised.

• Sources of power quality problems have been studied and grounding systems have beenidentified as one of the sources other than loads, equipment and components.

• E↵ects of power quality on the loads and equipment is a wide area of power quality andhas been studied through several experiments. It is discussed briefly in section in thisreport.

• Mitigation of power quality problems is another important area where the research hasbeen ongoing. The mitigation devices that have been proposed and developed in this volt-age regulation equipment such as adaptive var compensators, power electronic solutionsand power conditioning equipment.

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• Fundamental concepts of power quality have been studied and detailed in various litera-ture.

Although each aspect has been considered for further research and several papers have beenpublished, there are still areas which need to be explored. This thesis focuses on finding solutionsto power quality problems, particularly; voltage sags and interruptions. Fast switching devicesas a solution to power quality problems is investigated. The solution that has been presentedcomes under the category of mitigation of power quality problems.

1.3 Structure

This report has the following structure.Chapter 1 describes the purpose and scope of the project.Chapter 2 describes the devices that are suitable for fast switching.Chapter 3 introduces power quality problems and describes their causes and impacts on the

distribution network.Chapter 4 presents a summary of related literature and also the state of the art of the

technology.Chapter 5 presents a comparison of two di↵erent surveys conducted to predict the behavior

and frequency of sags. Also some interpretations and conclusions are discussed.Chapter 6 describes the voltage control in a distribution system and the role of fast switching

devices in the voltage control.Chapter 7 is a compilation of both the technical and economical aspects related to power

quality problems. The main idea of this chapter is to arrive at an economical value that thefast switching devices can provide to the network.

Chapter 8 steps into the future grid and gives a glimpse of how can fast switching de-vices be incorporated into the future grids which employ smart grid technology and distributedgeneration.

Chapter 9 draws conclusions from the work and presents the future prospects for the project.

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2 Devices suitable for fast switching

2.1 On-Load Tap Changers

2.1.1 Introduction

A tap changer is a switching device that is on a transformer, and switches between the tapspresent on the transformer winding thus varying the transformer ratio. The number of tapsvaries from design to design and the desired range of voltage for example 10 taps with a voltagedi↵erence of 0.8% between each tap. On-load tap changers (OLTCs) play a vital role in voltageregulation in the power system distribution networks. There are two main switching techniquesused in the tap changers, namely resistor type switching and reactance type switching. Both thetechniques use tap changers which are in oil filled transformers either in the same compartmentas that of the transformer or in a di↵erent compartment with a bridging contact in order toconnect the taps. During the recent decade OLTCs using vacuum switching techniques withvacuum interrupters in oil, in which the switching takes place in vacuum have been proven tobe very e↵ective.

2.1.2 Operation

A conventional tap changer uses the principle of make before break in order to vary thetransformer ratio while maintaining the supply to the load. There are two important conditionsfor an OLTC operation, first being the regulating step must never be short circuited and thesecond being the load current must never be interrupted. The principle make before breakimplies that the load is connected to two adjacent taps, the load current is switched to thedesired one and the other tap is then disconnected so that it ensures the continuity of supplyto the load. The two main switching techniques namely resistance switching and reactanceswitching employ resistors or impedances during the transition of the load current from onetap to the other. The idea is to limit the circulating current when the two taps are bridgedand to transfer the load current to the other tap without any interruption of supply. There aretwo di↵erent types of switches employed in OLTCs namely the selector switch and the diverterswitch. The selector switch first selects the desired tap to switch. Later the diverter switchperforms the transfer of load current from one tap to the other. Figure 1 shows the mechanismof the tap changer using a tap selector and a diverter switch.

2.1.3 Performance

Switching time: Conventional tap changers require a few minutes for switching whereas therecently developed tap changers require 3 to 10 sec for their operation [22].

Life Time: Conventional tap changers have a lifetime of 100,000 operations whereas therecent technology of vacuum switching tap changers have a life time of 500,000 to 600,000operations [23].

2.2 Circuit Breakers

Circuit breakers are the vital components of the power system switch-gear which are capableof interrupting short circuit currents that appear during abnormal condition in the transmissionand distribution networks. During the normal conditions, circuit breakers are used to make orbreak circuits both for operation and maintenance. In the protection system of the transmissionand distribution networks circuit breakers operate in coordination with relays which sense a faultin the circuit and notify the circuit breakers. On receiving a trip signal from the protective relaysthe circuit breaker operates in order to interrupt the fault current.

Circuit breakers are divided into many types based on di↵erent criterion namely voltage,location, external design, interrupting media. Voltage criterion refers to the circuit breakers

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Figure 2: Mechanism of a tap changer

that are designed based on their voltage application such as low voltage, medium voltage, highvoltage and extra high voltage circuit breakers. Location criterion refers to the circuit breakersthat are designed based on the location where they are installed such as indoor and outdoorcircuit breakers. External design criterion refers to the circuit breakers which have di↵erentouter physical structure such as dead tank circuit breaker which contains a vessel at groundpotential with interrupters at high potential and live tank circuit breaker which contains a vesselat high potential with interrupters at a high potential above the ground. Interrupting mediacriterion refers to the circuit breakers which employ di↵erent medium of interruption such as airblast, oil, SF6 and vacuum circuit breakers. Until the 1960s when the SF6 and vacuum circuitwere introduced all the circuit breakers used air and oil as the medium of interruption. SF6 andvacuum circuit breakers are the dominant circuit breaker technologies in today’s market.SF6 isdominating for high voltage above 72 kV and vacuum for medium voltage below 36 kV. Vacuumtechnology at medium voltage levels is preferred in China, Japan and USA whereas SF6 circuitbreaker are preferred in Europe and Middle East countries. Oil circuit breakers are still in usein China, India, Eastern Europe and Latin America [24].

2.2.1 Operation

Circuit breakers are designed to carry very high currents which they are rated for and areintended to possess the capability of breaking the circuit when a fault occurs. A circuit breakerconsists of a pair of contacts which are closed and carry the current in the circuit during thenormal operating conditions. When a fault occurs in the system a very high fault currentflows through the contacts which has to be interrupted in order to protect the system fromdamage.The contacts are opened at any random phase angle of the current. The opening ofthe contacts will cause an arc, which could be interrupted at the current zero crossing whenthe power (P = U

arc

I) in the arc is zero. The main task of the circuit breaker is to interruptshort circuit currents by using various techniques such blowing air, using insulating medium tocool the arc, gas blast, using oil which vaporizes and produces gases which quench the arc. Arecovery voltage occurs across the circuit breaker after the current interruption, and the circuit

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breaker has to be designed to withstand this voltage.

2.2.2 Performance

Switching time: Typical switching time of circuit breakers is tens of milliseconds. Magneti-cally actuated medium-voltage circuit breakers can operate with 40ms [24].

2.3 Electrical Fuses

A fuse is an electrical device which consists of a piece of conducting material that carriesthe current during normal circuit conditions and melts during over current in order to preventdamage of the healthy parts of the circuit. The piece of conducting material generally referredto as a fuse element or a fuselink has a low melting point so that it does not achieve excess heatwhen normal current flows. During an over-current, the fuselink attains a very high temperature,thus melting itself. Fuses usually contain either one element or many elements depending ontheir breaking capacity. Silver and copper are the most preferred materials that are used inmost of the modern fuselinks[25]. Fuses are generally classified into three types namely highvoltage, low voltage and miniature. High voltage fuses are for voltage applications above 1000Vand low voltage fuses are for voltage applications below 1000V. Miniature fuses are based ontheir physical dimensions.

2.3.1 Operation

A fuse consists of a fuse base and a fuse carrier which is fitted in a fuse holder. Fuse bodies aremade of ceramic and glass which provides insulation and resistance to thermal shock. The fusecarrier is the component that contains the fuse element. During the normal circuit conditionsthe element which is in the circuit conducts the current. In a faulted condition when a veryhigh current flows through the element it attains a high temperature and melts, thus breakingthe contacts of the circuit and preventing the circuit from damage. In high voltage fuses withvery high breaking capacities the breaking of the currents leads to creation of an arc which hasto be interrupted. The current limiting fused is designed to create several series connected arcs,which will cause a high total arc voltage. This will enable a current limitation and interruptionof the current, when the arc voltage is larger than the system voltage. This phenomenon is noteasily achieved since the heat distribution in the element during the pre-arcing period is notuniform and there are variations in thermal conditions and dimensions in the element.

2.3.2 Performance

Time-Current Characteristics: The switching time for a fuse is dependent on the time-current characteristics of the fuse. The lower the current the more the delay in the switchingtime of the fuse and for higher currents the switching time has almost no delay. Fast-blow fusemay require twice the rated current to blow in 0.1s but for higher currents the time will beshorter and a slow-blow fuse may require twice its rated current to blow in tens of seconds.

2.4 Fault Current Limiter

If the fault current during distribution system faults can be limited at the feeding end thenthe equipment that are connected both upstream and downstream do not experience the wholefault current and hence damage can be avoided. The device that limits the current to a non-zerovalue during a fault yet protecting the other parts of the system from being a↵ected is called afault current limiter. Limiting a fault current is done by changing the impedance in the circuit.The ideal V-I characteristics of a fault current limiter are shown in figure 3. The impedance of

6

the device when the current is less than the set point is zero. During a fault, when the currentreaches the set point the device impedance changes to limit the value of the fault current.

Figure 3: V-I characteristics of a fault current limiter [2]

The fault current limiters can be classified into di↵erent types based on the technology usedto limit the fault current and also whether the fault current is interrupted or not.

2.4.1 Super-conducting fault current limiters

As the name suggests, these current limiters use super conducting materials with the abilityto change their resistance by varying temperature. In the steady state the super conductingmaterial has no impedance and hence very low steady-state losses. When a fault occurs thefault current rises the super-conducting material quenches and in turn increasing its impedancethus limiting the fault current. Four di↵erent principles namely series resistance, shielded induc-tance, saturated inductance and air-gap current limiting devices employed by super-conductingmaterial for fault current limiting are described in [26]. Fault currents can be limited withinthe first half cycle.

2.4.2 Solid-state fault current limiters

One example of solid-state fault current limiters consist of a switch which is made by con-necting two semiconductor GTO thyristors in inverse parallel. A current limiting impedance isconnected in parallel to the switch. During normal operation, the switch conducts the current.When a fault occurs the switch is turned o↵ and the fault current is diverted to the currentlimiting impedance which limits the fault current. The switch is connected in parallel with anover voltage limiting surge arrester and a snubber circuit to limit the rate of rise of transientrecovery voltages across the thyristors. A schematic of a solid-state fault current limiter andthe current characteristics are shown in figure 4a and figure 4b. It can be observed from thefigure that fault current is limited to a value which is non-zero but under the limits, by using asolid-state fault current limiters.

7

(a) A schematic of the solid-state fault current limiter

(b) Fault Current characteris-tics with and without a solid-state fault current limiter

Figure 4: Solid-State fault current limiter [3]

2.5 Transfer Switches

2.5.1 Mechanical Transfer Switch

Mechanical transfer switches use circuit breakers or switches to perform the transfer betweentwo sources. The mechanical devices include motor operated air draw-out breakers, solenoidoperated vacuum or gas breakers, motor driven switches. The isolation of the sources is per-formed through open contacts. The transfer switch closes the first breaker of one power sourcebefore the second breaker of the other power source is opened. Either the the used infeedercan be first disconnected before connecting the alternative infeeder or the alternative infeederis first connected and then the used infeeder is dis-connected. This means that at least one ofthe two sources is always connected to the load. The transfer time is at least 11

2

cycles.

2.5.2 Static Transfer Switch

A static transfer switch is a switch that can transfer power between two di↵erent sources incase of a fault in the feeding power source. An example of a static transfer switch consists oftwo parallel thyristors, During normal conditions, thyristor 1 is continuously fired and conductsthe load current. When a disturbance occurs, the thyristor 1 is disabled from firing and thethyristor 2 is fired. The current commutates to the other source or supply in a very less timeusually less than half a cycle after the disturbance is detected. [4] proposes a medium voltagestatic transfer switch with a transfer switch less than 4ms.

In order the detect the disturbance very fast, the static transfer switch is employed with acontrol logic that initiates the transfer. A single line diagram of a static transfer switch systemis shown in figure 5.

8

Figure 5: Single line diagram of a medium voltage static transfer switch system [4]

During normal operating conditions, the bypass switches are open and the disconnectswitches are closed. The controller transfers the load to the alternate source from the pri-mary source in case of a fault or interruption by opening the static switch of the primary sourceand closing the static switch corresponding to the alternate source.

9

3 Power Quality

3.1 Power Quality Problems- Voltage Sags

Power Quality problems refer to the obstacles that are faced in order to improve the powerquality of power systems. Voltage sags are a part of the power quality problems and are usuallyseen in appreciable numbers during faults that occur in distribution systems. This section dealswith voltage sags, their calculations and their e↵ects on the distribution networks.

3.1.1 What is Voltage Sag?

Voltage sag is a decrease in the level of voltage in the feeder or on a load of a distributionnetwork, which persists until the fault in the system is cleared. It is usually called as voltagesag in North America and voltage dip in Europe. Voltage sag can be defined as a decreasein voltage level below 90% and above 10% of the pre-sag voltage. Voltage sags are caused byabnormal conditions such as short circuits or faults on parallel feeders, motor starting. Thereare contradictions on which voltage (pre-sag voltage or nominal voltage) has to be taken intoconsideration when calculating the magnitude of the sag. Based on the severity in the magnitudethe sags can be classified as shallow and deep sags. Shallow sags have large magnitude of voltagethus being less severe while deep sags have low magnitude of voltage thus being severe.

3.1.2 Magnitude Calculations

While calculating the magnitude of the sag di↵erent techniques are applied, which use theRMS value, peak value or the fundamental value of the nominal voltage to obtain the magnitudeof the sag. For a simple network under consideration, the sag magnitude is estimated as thevoltage at the point of common coupling. The point of common coupling is a common point onthe feeder from where both the fault and the load are fed.

Figure 6: Voltage divider model for Sag magnitude calculation

Vsag

is given by

Vsag

=Zf

E

Zs

+ Zf

(1)

Where, E is the source voltage, Zf

is the fault impedance and Zs

is the source impedance.Radial systems are common among the low voltage and medium voltage networks. The abovecalculation of sag voltage is applicable to a radial network.

Similarly the sag magnitude calculations for non-radial networks, sub transmission loops,branches with loops and Meshed systems can be obtained using the voltage divider model andextending it to the di↵erent configurations of the network which have several impedances. Innon-radial networks the concept of the point of common coupling no longer remains the same

10

as there local generators present in the network which keeps up the local voltage by feedinginto the fault. This helps in mitigating sags. For sub-transmission loops, a fault in one branchcan cause a sag in the other branch, which should be taken into account while calculating thesag magnitude. In meshed systems, matrix calculations are applied and solved using computersoftware since the system gets complicated with many voltage, current and impedance valuescoming into picture.

3.1.3 Sag Duration Calculation

Setting a threshold value for the voltage magnitude and comparing it with the measuredrms voltage can calculate duration of sag. The period during which the measured voltage isbelow the threshold is the sag duration. Usually the sag magnitude is determined by the faultclearing time in the network.

3.1.4 Sags in 3-phase systems

In 3-phase systems, sags that occur can cause a voltage drop in one phase, two phase orall three phases depending on the type of fault that occurs in the system for example single-phase fault, phase-phase fault etc. Table 1 shows the sags experienced due to single-phase andphase-phase faults.

Table 1: Sags experienced in 3 phase systems

3.1.5 Propagation of sags to lower voltage levels due to the presence of transform-ers

In distribution networks, when a fault occurs at a higher voltage level due to which voltagesdrop, the voltage sags propagate through the network to the lower voltage levels due to the pres-ence of transformers and hence are transformed to di↵erent kinds of sags. The transformationof the sags depends on the connection of the transformers.

Generally, the primary and secondary of the three-phase transformers are connected as wye-wye, delta-delta, wye-delta and delta-wye. wye-wye transformers with neutrals grounded do notchange the voltage where as the remaining types of transformers change the voltage levels. Zerosequence voltages play and important role in propagation of voltage sags through transformers asthese components cannot pass through all types of connections except wye-wye connection withneutral points grounded. This means that zero sequence voltages are eliminated in other typeof connections resulting in di↵erent kinds of sags. Also the delta-wye and wye-delta connectedtransformers can swap the phase and line voltages and thus result in voltages on the secondaryside which are a di↵erence in the voltages on the primary side.

Summarizing the influence of the transformer connections and load connections on the dis-tribution levels, Figure 7 and Table 2 can be obtained. Figure 7 shows the phasor diagrams ofthe di↵erent types of sags.

Table 2 shows the transformation of sags due to propagation to lower voltage levels in thepresence of transformers.

11

Figure 7: Phasor diagrams for di↵erent types of sags. Obtained from [5]

Table 2: Propagation of Sags through Transformers. Obtained from [5]

3.2 Equipment behaviour during sags

In our day-to-day life we always wonder why a quick restart of the desktop computer ora lamp flicker happens. Sometimes the CD player resets by its own without our knowledge.All these events are associated to the behaviour of equipment during voltage sags. Equipmentbehaviour is generally explained in terms of voltage tolerance of the equipment i.e. how muchpercentage of the voltage can drop, and how long can the equipment tolerate the drop withoutmalfunctioning. The voltage tolerance curve of equipment shows the equipment behaviour todi↵erent voltage drops for specific duration of time. Since voltage sags are characterized by avoltage drop for a particular duration of time, the voltage tolerance curve specifies the behaviourof equipment to di↵erent types of sag.

The first proposed voltage tolerance curve for PCs was the Computer Business EquipmentManufacturers Association (CBEMA) Curve which was later revised in 1995 and named as In-formation Technology Industrial Council (ITIC) curve and is specific to equipment that operateat a rated voltage of 120V and frequency 60Hz. In the year 2000 another voltage-tolerance curvefor Semiconductor Processing Equipment was proposed which is known as SEMI F47 Curve.Figure 8 indicates the three di↵erent voltage-tolerance curves.

As can be seen from Figure 8, the voltage tolerance curves clearly specify the magnitudebelow which and the duration above which the equipment turns o↵ or starts to malfunction.

3.2.1 Personal Computers

Personal Computers (PCs) are a predominant part of domestic, commercial and industrialequipment. PCs when subjected to voltage sags with shorter duration of time will restart afterthe completion of the sag whereas if the duration is longer they might turn o↵ completely. Thebehaviour of the PCs also depends on the voltage levels during the sag. Apart from the hardwaremalfunctions like restarting or shut down, sometimes the PCs may lockup the operations thatare being executed such as read, write or copy from a CD ROM [27].

12

Figure 8: Standard Voltage Tolerance Curves (Data obtained from [5])

3.2.2 Contactors

Contactors are electrical switching devices used to control motors or for heating, lightingetc. During a large interruption contactors drop out, thus disconnect the motor from the circuit.Since voltage sags are small interruptions, contactors withstand sags until a certain after whichthey drop out.

3.2.3 Process Control Equipment

Process control equipment are extremely sensitive to voltage sags. Some process controllersmay send incorrect control signals which could lead to process malfunctions in the industries[5]. The consequences might be of serious concern to process industries since a huge loss canoccur due to process failure.

3.3 Adjustable speed AC drives connected to motors

Adjustable speed AC drives which are connected to motors are also sensitive to voltagesags. The drive controller will drive during sags or interruptions to protect the power electronicequipment. There will be torque variations and drop in speed of the motor controlled by theadjustable speed drives, which will a↵ect the process being controlled. The motor speed in somecases can drop to zero and the motor may or may not re-accelerate which can lead to damageof equipment.[5].

3.3.1 Household Appliances

House appliances like microwave, electric rice cooker, CD player generally tolerate voltagesags up to certain levels. If the sags are deeper and persist for a longer duration of time, theappliances switch o↵ and have to be switched on again.

13

3.3.2 Lamps

Most of the lamps flicker due to voltage sags, which is not of serious concern but maybe disturbing to the eyes of the people near the lamp. Sometimes the lamps may switch o↵completely and fail to re-start if the sag is very deep.

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4 Literature review of existing and published fast switching de-vices

The need for protection and control in the power systems has grown in the past decadeowing to the power outages and faults that pose a barrier to the system which cause powerquality problems. Also the penetration of distributed generation in the system leads to voltagerise at di↵erent buses. Consequently, the design of the equipment needed for protection andcontrol has to be modified in order to maintain the reliability of supply in the power system.Fast switching technology in the protection equipment such as on-load tap changers, circuitbreakers, current limiters etc.is grown significantly in the field of power system protection andcontrol.

The aim of this review is to examine the overall trend in the research on fast switching tech-nology and to provide an insight of how the technology has been applied to various equipmentfor protection. The review will summarize the technical aspects in general from the researchpapers and give a picture of the contribution of the devices to the system.

4.1 Methodology of Review

The research papers studied in the review focus on the fast switching technology and devicesthat use fast switching techniques mainly in distribution system protection. The databases ando�cial websites that were searched to find the papers and articles include: Google Scholar, IEEExplore, KTH B Primo (O�cial Library data base for the KTH-Royal Institute of Technology),ABB website. While selecting the papers and articles, the intention was to limit the period ofresearch to the past decade. Hence, the research papers are mainly the ones published duringthe years 2000-2013.

The keywords used during the search were: fast switching, fast switching devices, fast circuitbreakers, fast on-load tap changers, fast switching medium voltage equipment. The searchprovided several papers out of which the ones during the above specified period were chosen forreview.

4.2 Overview of Research Studies

With the advent of solid state technology in the field of engineering, it has been applied tomany devices for enhancing their performance.

• C. Alvarez, J. Alamar, B. R. Gimenez, and A. Montenegro. Solid state devices for pro-tection in distribution systems: A new proposal for solid state transfer switch (SSTS). InProceedings of 9th International Conference on Harmonics and Quality of Power, Orlando,Florida, USA, 1-4 Oct, pages 456–461. Institute of Electrical and Electronic Engineers,2000This paper discusses the performance of the solid state transfer switch (SSTS) that canbe installed in the distribution network with di↵erent topologies. The device proposedis for medium voltage applications i.e. the range is 13.8kV to 15kV. The paper gives agood picture on how the solid state transfer switch can perform several functions whenconnected in di↵erent topologies. The switch was implemented on a prototype that wastested. The results obtained from the tests show that the SSTS can operate in an timeless than 4ms while the actual switching time is 500µs.

• J. Faiz and H. Javidnia. Fast response solid-state on-load transformer tap changer.In Proceedings of 8th International Conference on Power Electronics and VariableSpeed drives, London, 18-19 Sep, pages 355–359. Institute of Electrical and ElectronicEngineers, 2000This paper proposes a new method for on-load tap changer switching using solid state and

15

vacuum switches. The simulation of the proposed method is performed and the resultsobtained show that the speed of the tap changer is less than 100ms when compared tothe mechanical tap changing technique with a speed of 5s for changing from one tap tothe other.

• S. M. Bashi. Microcontroller based fast on-load tap changer for small power transformer.Journal of Applied Sciences, pages 999–1003, 2005This paper tests a prototype of a semiconductor tap changer triggered by a Micro-controller when there is an increase or decrease in the voltage levels above or below thespecified limits. The Micro-controller detects the variation and sends a signal to the triacin the tap changer which then changes the taps to higher or lower voltage depending onan increase or decrease. The overall operation time was 0.4s which was much less thanthe mechanical tap changer.

• W. Holaus and K. Frolich. Ultra-fast switches - A new element for the medium voltagefault current limiting switchgear. In Proceedings of Power Engineering Society meeting,NewYork, USA, 23-31 Jan, volume 1, pages 299–304. Institute of Electrical and ElectronicEngineers, 2000This paper presents an economic approach to the fast fault current limiting circuit breakerreferred as FCLCB in the paper by using simple ultra fast switches. The approachis to interrupt the fault current using the commutation principle. The hybrid circuitbreaker technique is used without involving power semiconductors in the design. Thepower semiconductor switches are replaced by a series of fast breakers. The choice of theswitches was made through a prior study of the commutation technique and hence thenumber of series breakers that are required in each switch is estimated. The approach wastested and the results show that the FCLCB proposed can be used for ratings of 24kV,3/40kA. The feasibility of the approach has been put forward for further investigation.

• C. Meyer, S. Schroder, and R.W. De Doncker. Design of solid-state circuit breakersfor medium-voltage systems. In Proceedings of Transmission and Distribution Con-ference,Dallas, USA, 7-12 Sep, volume 2, pages 798–803. Institute of Electrical andElectronic Engineers, 2003This paper proposes di↵erent topologies for Medium-Voltage Circuit Breakers usingdi↵erent semiconductors namely GTO, IGBT and Gate Commutated Thyristor (GCT).The di↵erent proposed topologies are compared on basis of their costs and on state lossesof the semi conductors. An optimization of the costs of the di↵erent topologies wasperformed. From the comparison, the economic aspect was found to be a crucial factorfor the proposed topologies whereas the technical aspects had minor di↵erences.

• G. K. George and G. T. Heydt. Novel concept for medium voltage circuit breakers usingmicroswitches. IEEE Transcations on Power Delivery, Volume 21, No. 1, 2006This paper proposes a new concept for switching in medium voltage circuit breakersusing micro-electro-mechanical switches (MEMS) in series or parallel in order to performthe current interruption. A topology with series and parallel connected MEMS withdiodes connected against each switch is explained in detail. The closing and openingphenomena of the topology has been discussed with the aid of graphical figures. Thetopology was tested for current interruption using Personal Simulation Program withIntegrated Circuit Emphasis (PSPICE) software and the results obtained were shown.The results obtained prove the feasibility of the topology when included in the circuit

16

breakers but the actual design of the circuit breakers with the topology is challengingand hence needs further study.

• Fan Xing-Ming, Zhang Xin, Huang Zhi-chao, Yang Jia-zhi, Hua dong Liu, and Ji yanZou. A fast making vacuum circuit breaker research and its applications. In 24th Inter-national symposium on discharges and electrical insulation in vacuum, Braunschweig,Germany, Aug 30-Sep 3, pages 146–149. Institute of Electrical and Electronic Engineers,2010This paper presents a novel concept of fault current limiting circuit breaker which isbased on a hybrid arrangement of semi-conductors, temperature dependent resistors anda fast opening mechanical switch which employs electrodynamic repulsion drive. Theprinciple of the interrupting circuit when placed in a power system network is describedin detail and the performance is evaluated. The fast mechanical switch and its mechanismis illustrated. Laboratory test results for the circuit breaker are presented and lastly theeconomic aspects of the proposed device are discussed.

• Po Tai Cheng and Yu Hsing Chen. Design and implementation of solid-state transferswitches for power quality enhancement. In Proceedings of 35th Annual IEEE PowerElectronics Specialists Conference, Auchen, Germany, pages 1108–1114. Institute of Elec-trical and Electronic Engineers, 2004This paper proposes a design and implementation of static transfer switches for power qual-ity enhancement. The static switches are controlled by a control method which coverts the3-phase voltages in the system to Synchronous reference frame in order to identify voltage.The voltage magnitude obtained from the voltages in the d-q frame are compared with athreshold voltage in order to detect a voltage sag in the system. Laboratory test resultsare shown and explained further. Also the proposed control method is compared withother methods presented in relevant literature in order to prove its e↵ectiveness.

4.3 Operation Time

The operating times of any switching device is the vital parameter that decides whether itis a fast switching device. The devices that are discussed in various papers and articles have awide range of operating times.

• [28] tests di↵erent topologies of hybrid switches and the results obtained show operatingtime of 1/4th of a cycle.

• The new scheme for on-load tap changer proposed in [29] is simulated and the results givea tap changing time of 100ms.

• [30] tests a prototype of a semiconductor on-load tap changer triggered by a micro-controller and the results obtained show a reacting time of 0.4s or 400ms.

• [32] proposes a new topology for medium voltage circuit breakers with solid state switchingdevices whose simulation results give an operating time as fast as 100µs.

• [31] tests a new approach by using ultra fast switching elements consisting of semicon-ductor switches for fault current limiting circuit breakers and the results give an arcextinguishing time of several 100µs.

• [33] proposes a topology for circuit breakers using MEMS which can aim an arc lessinterruption of the current by the end of the first half cycle.

17

• [34] presents laboratory test results which show that the fault making vacuum circuitbreaker with a response time of 8ms.

• [35] proposes a control method that detects voltage sags within 5ms time.

4.4 Components Employed

In order to enhance the performance of the protection and control equipment like OLTCs,circuit breakers or transfer switches, di↵erent techniques using the technologies like power elec-tronics, solid state, MEMS or semiconductor are applied. These techniques employ di↵erentcomponents like diodes, thyristors, IGBTs, GCTs, GTOs depending on the rated voltage andcurrent ratings for the equipment. Table 3 shows di↵erent components that have been employedin the various research studies.

Table 3: Components employed to improve the performance of the protection and control equipment

Literature Componets[28] GTO, SCR[29] GTO, Vacuum Switches[30] NWT-0020 FOSHCll Microcontroller, GTO[32] IGBT, GCT, GTO[31] IGCT, Series Breaks[33] MEMS, Diodes[34] GTO, PTC-Resistors,Diodes[35] Thyristors,DSP,PLL

4.5 Industrial Scenario

This section gives an overview of the devices developed by di↵erent manufacturers of powersystem protection and control devices. The idea is to look at the devices and their characteristicsthat are sold in the market today by di↵erent manufacturers. Also the ongoing research anddevelopment by di↵erent companies and institutions in the area of fast switching is summarizedin a table showing di↵erent companies and institutions. The existing fast switching devicesdeveloped by ABB are discussed.

4.5.1 Circuit Breakers

Table 4 lists di↵erent manufacturers of circuit breakers including ABB and provides detailsabout the technologies employed and operating times.

Table 4: Circuit Breakers manufacturers ([],[11],[12]

Company Technologies employed Quickest Operating TimesABB Vacuum Interruption, Mag-

netic Actuation55ms-60ms

Siemens Magnetic actuation, VacuumInterruption

Opening Time <75ms; ClosingTime<50ms

Schneider Electric Magnetic Actuation, VacuumInterruption

Operating Sequence:O-0.3s-CO-15s-C

Eaton Vacuum Interruption 83ms

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4.5.2 On-Load Tap Changers

Table 5 lists di↵erent manufacturers of on-load tap changers including ABB and providesdetails about the technologies employed and switching time between two taps.

Table 5: On-load tap changers manufacturers ([13],[14],[15])

Company Technologies employed Fast Operating TimesABB Oil Interruption,Vacuum Interrup-

tion2s

Maschinenfabrik Rein-hausen

Oil Interruption, Vacuum Interrup-tion

3-10s

Hyundai Oil Interruption, Vacuum Interrup-tion

4.4s

Huaming Oil Interruption, Vacuum Interrup-tion

Not specified

4.5.3 Automatic Transfer Switches

Table 6 lists di↵erent manufacturers of automatic transfer switches and provides the transfertimes employed.

Table 6: Automatic Transfer Switches ([16],[17])

Company Transfer TimesABB 2s-4s (low voltage)GE Industrial Solutions 30ms-80ms

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4.5.4 Fast Switching Devices Research and Development

Company/Institution Area DescriptionGE Global Research The Ultimate Circuit

BreakerEmploying Micro Electro Mechani-cal Systems(MEMS) technology toprotect the industrial equipmentfrom disruptive power surges. Po-tential advantages include faster re-sponse time to surges, reducingpower losess and ensuring a saferand reliable control. [36]

Silicon Power Solid State Fault Cur-rent Limiter

Monitoring the main bus anddynamically inserting additionalimpedance in the line in case of afault. Applications are faster faultcurrent limiting, fault ride throughand virtual susbstations.[37]

Siemens MV Circuit Breaker Vacuum interrupter with a mag-netic actuator. Interrupts the cur-rent faster than the melting time ofthe fuse. Also known as fusesaversince it saves the fuses from blow-ing during transient faults. It hasof on-board microprocessor control,wireless connectivity, event historyand can be integrated with SCADAfor remote control.[38]

University of Arkansas Solid State Fault Cur-rent Limiter

Uses the silicon carbide semi-conductor technology in a conven-tional fault current limiter and lim-its the fault current during surges.[39]

U.S Army ResearchLaboratory

Bi-directional scal-able solid state circuitbreaker

The invention is for fault protec-tion for the hybrid electric vehiclesystems using the solid state tech-nology. A gate driver designed toenable inherent over-current protec-tion feature for the control of highspeed bi-directional solid state cir-cuit breakers. Interruption speedsare in the range of tens to hundredsof micro seconds when compared totens of milliseconds for a conven-tional circuit breaker.[40]

Eaton Electronic VoltageRegulator (EVR)microprocessor-controlled tap changer

The tap is automatically activatedthrough a silicon-controlled rectifier(SCR) and the tap changing is initi-ated within one cycle at zero currentcrossing. [41]

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4.5.5 Is

Current Limiter

Is

current limiter is a device that limits the short circuit current when a fault occurs in anetwork. It is used when the short circuit current exceeds the ratings for the switch gear andthe circuit breakers. The I

s

current limiter limits this current by transferring the current to afuse connected in parallel which limits the fault current in less than 1ms. The fuse is a currentlimiting fuse with a high arc voltage, which will force a current zero crossing. It does not waituntil the natural current zero crossing. This can bee seen in Figure 9, where I

2

reaches zerobefore I

1

. It has to be zero before the peak is reached in I1

. The condition is that the peakvalue of I

1

+ I2

is not higher than I1

.

(a) Single line diagram of a bus tie with a short circuit currentof 31.5kA and an Is current limiter installed[42]

(b) Waveform of the fault current with and without the IsCurrent Limiter[42]

Figure 9: Principle of Is Current Limiter

The instantaneous current and the rate of rise of the current is constantly measured bymeans of an electronic device. When a fault occurs in the network the short circuit currentrises and when it exceeds the set points of the rate of rise of current the electronic device the I

s

limiter trips the circuit thus limiting the short circuit current before it reaches its peak value.The principle of the Is current limiter is shown in Figure 9.

ABB’s recent production of the Is

current limiters have current limiting capacity in therange of 140kA

rms

-210kArms

. Current limiters have wide range of applications in improvingpower quality, maintaining network impedance and also by the use of I

s

current limiter existing

21

bus bar or cables do not have to be changed since the short circuit current is kept below theratings of existing equipment.

4.5.6 Ultra Fast Earthing Switch

As the name indicates, Ultra Fast Earthing Switch (UFES) is the product of ABB whichcan extinguish an arc during a fault in a duration of 4ms. It extinguish arc the by creatinga parallel current path to ground for the short circuit current. The short circuit current willcommutate to the parallel switch if the resistance of the switch is low enough that voltage acrossthe switch is lower than the arc voltage at the fault.

The UFES system consists of optical sensors which sense the arc during a fault and intimatethe detection Unit. When the criteria / conditions for tripping are fulfilled, the tripping unittrips the circuit by sending a signal to the primary switching element which extinguishes thearc in 4ms after the detection [43].

The advantages of the UFES are its speed in suppressing the arc, the safety in investingand also for the personnel and the saving in regard with switchgear repair after arc extinction.Applications of UFES include protection of medium voltage equipment and dry transformers.

4.5.7 CapThor

In order to protect high voltage equipment, ABB has developed a fast protection schemewith a new component called CapThor. CapThor is part of fast protective device scheme forseries capacitors. CapThor is a hermetically sealed very fast power switch. It can operate anumber of times to bypass the series capacitor so that it can be reinstated without problems.CapThor is connected in parallel with the series capacitor. The switch is normally open andthe line current flows through the series capacitor. The voltage across the series capacitor isgiven by the current through the capacitor. The voltage across the capacitors will rise if a shortcircuit occurs downstream the line causing a high current through the series capacitor. TheCapThor will close and by-pass capacitor and make the short circuit current to flow throughthe CapThor instead of the series capacitor

An internal view of CapThor is shown in figure 10. It consists of a high power plasma switchwhich is connected in parallel to a fast mechanical switch. Both the switches are housed on highpressure insulator chambers that are filled with gas. The plasma switch consists of an electrodearrangement to which conducting electric arc is injected by an external triggering circuit whichgives a “close” signal. The injected arc is directed into the main electrode gap by means ofmagnetic forces that are created by a current loop in the triggering circuit. The plasma switchhas a high current making capability and the time needed for the switch to conduct on receivingthe close signal is in the range of 0.3-1ms.

22

Figure 10: Internal View of CapThor with plasma switch on the left and mechanical switch on the right.[6]

The fast mechanical switch consists of a moving contact that can alternate between open andclose positions. The switch is actuated by thomson magnetic mirror e↵ect of repulsive forces.The injection to the mechanical switch is also performed by an external source consisting ofcharged capacitors. The switch has a very high current making capability with closing andopening times for the switch are less than 5ms.

23

5 Comparison of the Canadian and NewZealand Power QualitySurveys

The Canadian Electricity Network (CEA) and a New Zealand utility, Vector have conducteda 3 year and 5 year survey respectively in order to analyze the power quality of the electricity intheir regions. In this chapter a detailed analysis of both the surveys and a comparison of howthe Power Quality in terms of Voltage Sags varies according to the voltage levels and di↵erentconnections in the networks will be discussed. All the data for analysis and comparison isobtained from [44] and [7].

5.1 Technical Aspects of the Canadian Survey

22 utilities and 550 sites from all over Canada participated in the survey. The customersincluded commercial, industrial and residential customers. All the sites were monitored overa 25 day period. During the survey, monitoring threshold levels were set to the voltage levelswith a time duration of 80ms-10s as shown in Table 7. Table 7 illustrates for each voltage level,what is the threshold voltage level below which any voltage anomaly can be called a sag.

Table 7: Threshold voltage levels during monitoring period

Voltage Level Voltage Sags Duration(80ms-10s)120 Volts 110Vrms120/208 Volts 110Vrms347/600Volts 318Vrms

5.2 Technical Aspects of the New Zealand Survey

The New Zealand Survey was conducted on the entire network with a total number of 12Zone Substations. The monitored data is given for the Substation Bairds which has bus barvoltage down to 11kV from the higher levels.

5.3 Analysis of Voltage Sag charts obtained from the Surveys

The number of voltage sags per phase per month per site versus the percentage of sites thatwere monitored in the Canadian survey. As mentioned in the section above, the survey wasconducted on di↵erent customer groups and at di↵erent voltage levels i.e. primary and secondary.Monitoring at primary and secondary voltage levels was done separately. The number of voltagesags per phase per month per site versus the percentage of sites that were monitored in theCanadian survey are plotted for industrial customer group as shown in Figure 11.

24

(a) Voltage Sag Chart for Industrial Customer Group at Sec-ondary Voltage Levels

(b) Voltage Sag Chart for Industrial Customer Group at Pri-mary Voltage Levels

Figure 11: Voltage Sag Charts for Industrial Customer Group at Primary and Secondary Voltage Levelsaccording to the Canadian Survey

As can be seen from both the Figures 11a and 11b, a considerable percentage of sites withindustrial customers from the Canadian survey experience no sags at all, at both primary andsecondary voltage levels. It can also be inferred from the graphs that the maximum percentageof sites experience only 1-2 sags per phase per month at secondary voltage levels and 0-1 sags perphase per month at the utility primary voltage level, which means the quality of power suppliedto the industrial customer group is quite high and also the sags occurring at the secondaryvoltage levels are are slightly higher than that of the utility primary voltage levels.

The number of voltage sags per phase per month per site versus the percentage of sites that

25

were monitored in the Canadian survey are plotted for commercial customer group as shown inFigure 12.

(a) Voltage Sag Chart for Commercial CustomerGroup at Secondary Voltage Levels

(b) Voltage Sag Chart for Commercial CustomerGroup at Primary Voltage Levels

Figure 12: Voltage Sag Charts for Commercial Customer Group at Primary and Secondary VoltageLevels according to the Canadian Survey

As can be seen from both the Figures 12a and 12b, similar to the industrial customer groupa considerable percentage of sites of the commercial customer group from the Canadian surveyexperience no sags at all, at both primary and secondary voltage levels. It can also be inferredfrom the graphs that the maximum percentage of sites experience only 1-2 sags per phase permonth at secondary voltage levels (120/208 V) and 0-1 sags per phase per month at the utilityprimary voltage levels, which means the sags occurring at the secondary voltage levels are areslightly higher than that of the utility primary voltage levels.

The percentage of remaining voltage versus the expected number of voltage sags per annumat a zone substation-Bairds in New Zealand, is shown in Figure 13. The New Zealand surveywas conducted for a period of 5 years and hence the data obtained is very reliable and gives awider picture of the substation.

26

Figure 13: Annual Voltage Sag Chart for Bairds 11kV Zone Substation in New Zealand (Data obtainedfrom [7])

It can be observed from the Figure 13, that the highest number of sags that occurred have aremaining voltage of 80-90% in a time duration of 0-0.2s. This means that most of the voltagesags that occur do not usually lead to an outage of a substation or a power system. The highestnumber of sags that occurred is 8 and the least is no sags.

5.4 Analysis of the frequency of voltage sags occurring in di↵erent phases

A voltage sag incident is defined according to [44] as an event where a voltage sag occursin one or more phases within a short interval of time i.e. less than 1s. At any given instance oftime, in a 3-ph system, voltage sags can occur in one phase or more phases and sometimes theremaining voltage can be low thus leading to an outage. In this case the number of voltage sagincidents is equal to one whereas the number of voltage sags can be one or more. The number ofsites that participated in the Canadian survey versus the total number of voltage sags occurredhas been plotted for both industrial and commercial customer group as shown in Figure 14aand 14b.

It can be seen from Figure 14a and 14b that most of the voltage sags occur in 1 phase or 2phases. Sags in all three phases are low in number when compared to one phase or two phases.Also the total number of voltage sags per phase is highest for the sites 3, 6 and 9 with 8 sagsper phase for industrial customer group and the total number of sags per phase is highest forsite 6 with 9 sags per phase for commercial customer group.

27

(a) Number of Voltage Sags per phase for IndustrialCustomer Group

(b) Number of Voltage Sags per phase for Commer-cial Customer Group

Figure 14: Voltage Sag Charts for Commercial Customer Group at Primary and Secondary VoltageLevels according to the Canadian Survey

The voltage sag magnitude versus the number of sags occuring per annum plotted for thezone substation-Bairds in New Zealand is as shown in Figure 15.

28

Figure 15: Average number of Voltage Sags per phase for Bairds 11Kv zone substation in New Zealand(Data obtained from [7])

From Figure 15, it can observed that the average number of voltage sags per annum occursis highest for the voltage level 80-90% with 5.03 one phase sags, 2.32 two phase sags and 3.29three phase sags respectively. It can also be seen that the 3 phase sags are common in all thevoltage levels.

5.5 Summary and Conclusions

In the above section a real time scenario of occurrence of sags is given from two di↵erentparts of the world i.e. Canada and New Zealand. The Canadian and New Zealand survey arevery di↵erent from each other in terms of technical aspects and also the Canadian survey focuseson many sites whereas the New Zealand survey provides information about one zone substation.Even though there are di↵erences in the surveys their main aim was to look at the power qualityof the participating sites or substation. No description about the causes of the power qualityproblems is provided. The data from the surveys serves as a good base for the interpretationof voltage sags and their occurrence. Some interesting points that can be concluded from thesurveys are

• The Canadian survey gives a picture of the number of sags occurring in a huge numberof participating sites whereas the New Zealand survey focuses on a single substation indetail.

• The data obtained from the New Zealand survey is for a 5 year period and hence morereliable when compared to the Canadian survey which was conducted for a one monthperiod.

• It can be seen from the Figures 11a and 11b, that the maximum percentage of sitesexperience 0-1 or 1-2 voltage sags per phase per month per site for industrial customergroup; whereas from Figures 12a and 12b, it can be said that the maximum percentage

29

of sites experience 1-2 voltage sags per phase per month per site for commercial customergroup. This number is totally di↵erent when considering the annual number of sags thatoccurred in Bairds substation as seen in the Figure 13, where 8 sags is the highest numberof sags that occurred.Combining all the annual sags occurring at the Bairds substation,the total annual sags can fit better with the results of the Canadian survey.

• When considering the frequency of voltage sags occurring in di↵erent phases Figure 14aand 14b show that the maximum number of sites experience one phase sags and theoccurrence of sags in di↵erent phases is random whereas from Figure 15 it can be seenthat for a single substation under consideration the frequency of 3-phase voltage sags ishigher and they occur at almost every voltage range.

30

6 Voltage Control

In today’s world, in order to cater the energy demands while realizing the ultimate goal ofa sustainable future, renewable sources of energy play an important role in power generation.However, integrating renewable energy with the existing power system while maintaining thevoltage within the specified limits is a complex task which requires a prior study. The conceptof generating electricity by means of small energy sources at lower voltage levels in the powersystem is termed as distributed generation. Distributed generation is termed as generationfrom photo voltaic, wind energy, combined heat power (CHP) or any other sources of energy insmall amounts. In this section the focus is on how the impact of distributed generation on thenetwork be mitigated using the concept of fast switching. The question is where in the networkcan the fast switching be applied in order to limit the voltage levels when distributed generationis included.

6.1 Voltage Analysis for Distribution Systems

In a distributed system where the feeders are connected to many customers it is crucialto maintain the voltage at the customers’ connection points under permissible limits. Alsothe speed of the variation in the voltage and the phase angle of the voltage must be underspecified limits. In order to analyze the voltage profile on a distribution feeder, consider asimple distribution feeder as shown in Figure 16.

Figure 16: Distribution Feeder (from [8])

VS

represents the sending end voltage, R+ jX is the impedance of the feeder, P and Q arethe active and reactive power flows through the feeder, P

L

and QL

are the active and reactivepowers consumed by the load, V

R

is the receiving end voltage. The power flow in the feeder isfrom the sending end to the receiving end. The power supplied to the feeder is given by theequation

P + jQ = VS

I⇤ (2)

I =P � jQ

VS

(3)

where, I is the current flowing through the feeder. The voltage at sending end can be writtenas

VS

= VR

+ I(R+ jX) (4)

Substituting the current value from equation 3 in equation 4, the equation for VS

modifies as

VS

= VR

+P � jQ

VS

(R+ jX) (5)

The voltage drop between the sending and receiving end can be obtained as shown in equation

�V = VS

� VR

=RP +XQ

VS

+ jXP �RQ

VS

(6)

31

From eq(6), if the second term can be neglected, due to the voltage drop being small comparedto the sending or receiving voltages, then the voltage drop is given by

�V =RP +XQ

VS

(7)

This voltage drop is of serious concern as the decrease in voltage on the distribution feederwill have a direct impact on the customers connected to the feeder. In order to maintain thevoltage levels, boosting transformers equipped with tap changers are located at di↵erent partsof the feeder. These transformers have di↵erent turn ratios in such a way that the farthesttransformer has the lowest turns ratio thus resulting in the secondary voltage to be higherenough to compensate for the drop at the remote end. By using the transformers with tapchangers, the voltage can be boosted to 5% [9]. Figure 17 shows the voltage across a distributionfeeder during maximum as well as minimum load.

Figure 17: Voltage profile for a Distribution Feeder (from [9])

From the figure, it is evident how the voltage profile changes with the change in the loadand how it has to be maintained within the upper and lower limits. When the voltage is belowthe under voltage limit or above the over voltage limit, the utility is not fulfilling its obligationswhich could lead to abnormal situations. Therefore, the necessity for voltage control in thedistribution system arises.

6.2 Distribution System with Integrated Generation

When distributed generation is integrated to the distribution system, there is a drasticchange in the voltage profile owing to the injected active power into the network and also thechange in the direction of the power flow. Consider the same distribution feeder shown inFigure 16 with a distributed generation and also a shunt compensation installed in the feederas shown in Figure 18.

In the Figure 18, VGEN

is the voltage at the point where the distributed generation isinstalled, P

L

and QL

are the active and reactive powers consumed by the load, PG

and QG

are the active and reactive powers injected by the distributed generator, QC

is the reactivepower absorbed or exported by the shunt compensator installed. With distributed generationthe voltage profile and the power flows in the feeder are a↵ected. The voltage drop that is givenin eq (7) is changed due to the distributed generation and is given by the equation

�V = VGEN

� VS

=R(P

G

� PL

) +X(±QC

�QL

±QG

)

VGEN

(8)

32

Figure 18: Distribution Feeder integrated with Distributed Generation (from [8])

where, ±QC

and ±QG

depend on whether reactive power is being absorbed by or exportedfrom the synchronous compensator and the distributed generation. Depending on the activeand reactive power flows values in the eq (8), the voltage rise in the system varies. Withdistributed generation in a network, the first step is to know what capacity the generator hasand how much of the incoming power is acceptable to the distribution feeder. This is relatedto the hosting capacity which is defined as the maximum amount of distributed generationthat can be connected to the distribution feeder without resulting in any abnormal situation orinterruption of the customers [9]. The hosting capacity is given by the equation

Pmax

=U2

R�max

(9)

where Pmax

is the maximum hosting capacity of the feeder, U is the nominal voltage level onthe feeder, R is the resistance of the feeder and �

max

is the relative voltage margin. Dependingon the capacity of the distributed generator connected, the power flow in the feeder can varyand also reverse its direction.

6.3 Role of Fast Switching in Voltage Control

Voltage Control using on-load tap changers has gained a lot of importance for research inthe last decade. Research papers consider voltage control strategies using Automatic VoltageRelays as a pronounced technique to enhance the ability of the OLTC’s [45][46]. By usingautomatic voltage controller that can notify the on-load tap changers when there is a voltagerise or drop in the network, the voltage levels can be maintained within the specified limits.

With the advent of vacuum technology in the on-load tap changer design, the tap changersare much more e�cient. Apart from installing transformers in the system that can boost thevoltage, it is also important to switch the voltage from one level to the other to be able to changethis boost level during operation, in order to maintain a well-regulated voltage to the customers.This can be achieved by load tap changers with fast switching techniques thus maintaining thevoltage within the limits as quick as possible. The on-load tap changers can switch to di↵erentvoltage levels within a few seconds of time. If they are made faster than today i.e. up to onecycle switching time, the voltage profile of the distribution feeder will look flat, which meansthat the voltage drop across the length of the feeder is reduced.

6.4 Role of Fast Switching in the Integrated Network

Fast switching on-load tap changers can play a vital role in maintaining the voltage at thecustomer connection points by switching the voltage levels as soon as the additional generatoris added to the distribution feeder. With distributed generation, there is a rise in the voltagelevels which could result in unexpected changes. Also the distributed generation at di↵erentpoints in the feeder can result in a further voltage rise which should be controlled as fast aspossible. This can also be achieved to a great extent by fast switching.

33

7 Estimation of Value

Power quality problems are of serious concern for the distribution networks and the technicaland economical losses that occur to the customers due to the power quality problems mainlyvoltage sags, interruptions, harmonics or flicker cannot be neglected. Technical loss is to a greatextent related to economic loss, for example, if a device in a network fails it has to be repairedor replaced which requires an additional investment, which in turn is an economic loss. Thisinvestment may vary when it comes to a network owner and a customer. Depending on thetype of customer, the loss caused due to sags may or may not be a great impact technicallyor economically. For example, a residential customer might not experience any economical lossdue to sags whereas an Industrial customer might need to face a huge loss due to the voltagesags since some processes might have to shut down. A major question then arises-“Who isresponsible for the loss and who has to pay for it?”. The answer to this question is vagueand cannot be decided without a proper study and analysis of the impacts of power qualityproblems in a distribution network. Then also comes the question-“Is there a way to mitigateor prevent the power quality problems?”. The answer to the latter question has to considerthe benefits of having mitigation techniques both technically and economically. Fast switchingdevices discussed in the previous section can play an important role in preventing the powerquality problems but as described above there is a need to see how they benefit the networkand customers and hence to look at their value to the distribution network.

In this chapter an overview of di↵erent methods to estimate the frequency of voltage sags andinterruptions is given in the first part. The next section classifies di↵erent kinds of customersthat are connected to the distribution networks and the impacts on di↵erent customers due tovoltage sags. Further, a detailed description about who has to pay the loss due to sags is given.In the last sections, the costs of fast switching devices are estimated and weighed against theloss due to voltage sags and the value of the devices is estimated.

7.1 Frequency of voltage sags and estimation

Frequency of voltage sags or interruptions is an important factor in the distribution systemssince it provides a base for estimating any kind of loss that can occur due to voltage sags orinterruptions. Various methods for the stochastic estimation of voltage sags have been discussedin [5]. The magnitude of sag at a point in a feeder due to a fault occurring in the feeder canbe calculated using the voltage divider model as described in section (3.1). The voltage dividermodel can be extended to a network during unbalanced faults by considering a sensitive load busand a fault on another line as described in [47]. Sag is characterized by means of a magnitudeand duration. These are the main characteristics which determine whether the sag is deep orlong enough that it can cause a serious damage to the equipment. The estimated sag frequencyis defined as the average number of sags that occur during a year.

7.2 Calculation of voltages during three phase unbalance

The sag voltages during short circuit faults that are unbalanced can be calculated by us-ing the symmetrical components theory. The voltage divider model can be extended to theunbalanced fault by splitting the voltage in to three sequence components namely positive,negative and zero. The voltages during di↵erent kind of faults can be obtained by connectingthe equivalent networks of positive, negative and zero sequence according to type of fault thatoccurs [5]. For the voltage calculations V

1

, V2

and V0

represent the positive, negative and zerosequence voltage respectively. Z

s1 ,Zs2 ,Zs0 are the source impedance values and Z

f1 ,Zf2 ,Zf0 arethe feeder impedance values at the point of common coupling, for the sequence components.E is the source voltage for the positive sequence network. The voltage equations obtained fordi↵erent faults are as shown in the following subsections.

34

7.2.1 Single Phase Faults

For single phase faults, the three sequence networks are connected in series and the sym-metrical component voltages obtained are

V1

=Zf1 + Z

s2 + Zf2 + Z

s0 + Zf0

(Zf1 + Z

f2 + Zf0) + (Z

s1 + Zs2 + Z

s0)(10)

V2

=Zs2

(Zf1 + Z

f2 + Zf0) + (Z

s1 + Zs2 + Z

s0)(11)

V0

=Zs0

(Zf1 + Z

f2 + Zf0) + (Z

s1 + Zs2 + Z

s0)(12)

The phase voltages calculate from the symmetrical component voltages for the faulted and thenon-faulted phases are

Va

= 1� Zs1 + Z

s2 + Zs0

(Zf1 + Z

f2 + Zf0) + (Z

s1 + Zs2 + Z

s0)(13)

Vb

= a2 � a2Zs1 + aZ

s2 + Zs0

(Zf1 + Z

f2 + Zf0) + (Z

s1 + Zs2 + Z

s0)(14)

Vc

= a� aZs1 + a2Z

s2 + Zs0

(Zf1 + Z

f2 + Zf0) + (Z

s1 + Zs2 + Z

s0)(15)

where, a is given by e-j120.If the system is solidly grounded, the source impedances are equal and therefore from the

equations 13, 14 and 15, the voltage equations obtained for the solidly grounded system in thefaulted and non-faulted phases are

Va

= � Zs1

1

3

(Zf1 + Z

f2 + Zf0) + Z

s1

(16)

Vb

= a2 (17)

Vc

= a (18)

If the system considered is impedance grounded i.e. the positive and negative sequenceimpedances are assumed to be equal and then the voltage equations for the faulted and non-faulted phases are given by

Va

= 1� 3Zs1

(2Zf1 + Z

f0) + (2Zs1 + Z

s0)(19)

Vb

= a2 (20)

Vc

= a (21)

35

7.3 Phase-Phase Faults

For phase-phase faults, the positive and negative sequence networks are connected in parallel.The zero sequence currents and voltages are zero. Therefore, the sequence voltages at the pointof common coupling are given by

V1

= E � EZs1

(Zf1 + Z

f2) + (Zs1 + Z

s2)(22)

V2

=Zs2

(Zf1 + Z

f2) + (Zs1 + Z

s2)(23)

V0

= 0 (24)

The phase voltages obtained from the sequence voltages with E = 1 are

Va

= a2 � Zs1 � Z

s2

(Zf1 + Z

f2) + (Zs1 + Z

s2)(25)

Vb

= a2 � a2Zs1 � aZ

s2

(Zf1 + Z

f2) + (Zs1 + Z

s2)(26)

Vc

= a� aZs1 � a2Z

s2

(Zf1 + Z

f2) + (Zs1 + Z

s2)(27)

7.4 Two Phase to Ground Faults

For the two phase to ground faults, all the sequence networks are connected in parallel andthe sequence voltages obtained at the point of common coupling are given by

V1

= 1�(Z

s1)(Zs0 + Zf0 + Z

s2 + Zf2)

D(28)

where,

D = (Zs0 + Z

f0)(Zs1 + Zf1 + Z

s2 + Zf2) + (Z

s1 + Zf1)(Zs2 + Z

f2) (29)

V2

=(Z

s2)(Zs0 + Zf0)

D(30)

V0

=(Z

s0)(Zs2 + Zf2)

D(31)

The phase to ground voltages obtained from the sequence voltages for the faulted and non-faulted phases are given by

Va

= 1 +(Z

s2 � Zs1)(Zs0 + Z

f0)

D+

(Zs0 � Z

s1)(Zs2 + Zf2)

D(32)

Vb

= a2 +(aZ

s2 � a2Zs1)(Zs0 + Z

f0)

D+

(Zs0 � a2Z

s1)(Zs2 + Zf2)

D(33)

Vc

= a+(a2Z

s2 � aZs1)(Zs0 + Z

f0)

D+

(Zs0 � aZ

s1)(Zs2 + Zf2)

D(34)

36

7.5 Area of Vulnerability

A fault that occurs on one bus can cause a voltage sag at a sensitive load that is connectedto another bus. The area around the sensitive load in which any fault occurring can cause avoltage sag in the load is known as Area of Vulnerability, as shown in figure 19.

Figure 19: An example of area of vulnerability ([10])

The area of vulnerability can be a↵ected by di↵erent factors like equipment sensitivity, typeof fault and nature of faults [48]. The area of vulnerability can be obtained by calculating thephase voltages during a fault and comparing these voltages with the voltage threshold for eachline [47]. This comparison can be performed in a computer software thus simulating the wholenetwork to determine the area of vulnerability for di↵erent lines to which sensitive equipment isconnected. The purpose of finding the area of vulnerability is illustrated in [10], which describesin detail application of the area of vulnerability concept to determine the frequency of voltagesags.

7.6 Area of Severity

The area of severity is defined as the area in which any faults occurring can lead to voltagesags at di↵erent sensitive loads. The area of severity (AOS) can be obtained by the intersectionof all the area of vulnerabilities for each sensitive load. If the number of sensitive load pointsin a network is N , then the AOS will have N di↵erent severity levels and a fault occurring inAOS

N

will cause voltage sags at all the N sensitive load points [10].

7.7 Estimated Sag Frequency

The Estimated Sag frequency can be calculated by first determining the area of Vulnerabilityand then multiplying the fault rates of the lines in the area of vulnerability with the length of

37

the lines. For unbalanced faults, the area of vulnerability is calculate separately for each phaseusing the residual voltage equations described in (section) and then the ESF is calculated. Fora single phase load the estimated sag frequency is given by [47]

ESFUF

=3X

i=1

3X

j=1

"mX

B=1

BFR+nX

L=1

IL

⇥ LFR

#(35)

The estimated sag frequency for a balanced fault is given by

ESFBF

=mX

B=1

BFR+nX

L=1

IL

⇥ LFR (36)

By assuming that all the three phases have equal probability of fault occurrence, the ESF fora single phase load can be obtained by adding the ESF for balanced and unbalanced faults onone phase which is given by

ESFSPL

=ESF

UF

3+ ESF

BF

(37)

where, i is the type of unbalanced fault (SLGF, LLF, DLGF); j is the phase; m is the totalnumber of buses under the area of vulnerability; n is the total number of lines under the areaof vulnerability; BFR is the bus failure rate for each fault type; LFR is the line failure rate foreach fault type; I

L

is the length of the line that is present inside the area of vulnerability.

7.8 Types of Customers and the impacts of voltage sags on the di↵erentcustomers

In a distribution network, the utilities may classify the customers based on di↵erent criteriasuch as load profile, voltage levels at which the electricity is delivered, end-user applications etc.According to the service they receive, the customers in a distribution network may be classifiedas residential, industrial and commercial.

7.8.1 Residential

The residential customers include private houses or apartments which receive low voltagelevels of electricity. The energy is consumed by this group is for refrigerating, air conditioning,cooking, drying and operating electric or electronic appliances.

7.8.2 Industrial

The industrial customers comprise all the manufacturing, mining, construction, agricultural,forestry, fishing industries or companies which receive electricity at medium voltage levels.The energy is consumed by this group mainly for the manufacturing processes running in theindustries and partly for lighting, heating or air conditioning purposes.

7.8.3 Commercial

The commercial customers consist of any kind of business establishments such as hotels,restaurants, hospitals, street lighting, educational or government institutions, retail stores,wholesale business establishments, social and religious institutions which receive electricity atlow voltage levels. The energy is consumed for lighting, heating, air conditioning and by severalelectronic appliances like loud speakers, computers, television sets etc. .

38

7.9 Costs associated with Voltage Sags

The financial loss associated with voltage sags can vary for di↵erent utilities and customergroups depending on the type of sag, its duration and its impact. The impact of voltage sagcould be a stoppage of the process running in an industry, shut down of PCs or flicker of lamps.The financial loss or the costs of voltage dips to the customers can be categorized into threetypes as described below [49].

7.9.1 Direct Costs

These are the costs that contribute to a huge loss. They include loss of production, equip-ment damage, loss of raw material due to stoppage of a process, energy loss, salary costs dueto the non-productive hours, lost hours, extra costs for maintenance, penalties etc.

7.9.2 Restart Costs

These are the costs associated with repair of equipment and restarting of a process. Theyinclude equipment or product repair or replacement costs, restart cost for a process, additionallabor costs to recover for the lost production etc.

7.9.3 Hidden Costs

These are the costs which are not palpable but intellectual which cannot be neglected.They include cost of decreased competitiveness, decreased reputation, customer dissatisfaction,employee annoyance etc.

7.10 Method to Estimate the Cost of Sags

According to [18], the costs of sags can be estimated by considering their impact on thedistribution system and the customer. In order to estimate the cost of sags, the first step is tocalculate all the above mentioned costs during an interruption. The next step step is to sumup all the above costs in order to get the total cost for an interruption.

TCi

= DCi

+RCi

+HCi

(38)

where, TCi

is the total cost for one interruption, DCi

are the direct costs for one interruption,RC

i

are the restart costs for one interruption, HCi

are the hidden costs for one interruption.When the total cost of interruption is determined, the next step is to calculate the weighingfactors for each type of sag i.e. sag down to 50% or sag down to 70% etc. If the weighing factorfor an interruption is 1, then the weighing factor for sag down to 70% that causes an impactof 10% that an interruption causes is 0.1. The weighing factors can be multiplied with theestimated frequency of sags during a year to give the number of equivalent interruptions.

NEI = W.F ⇥ ESF (39)

where, NEI is the number of equivalent interruptions, W.F is the weighing factor for each typeof sag and ESF is the estimated sag frequency obtained from Equation 37. The final step is tomultiply the total cost for one interruption with the number of equivalent interruptions.

COS = TCi

⇥NEI (40)

where, COS is the estimated cost of sags. Table 8 shows examples of weighing factors, estimatedfrequency of sags or interruptions and the number of equivalent interruptions caused.

39

Table 8: Example value of weighing factors, Estimated Sag Frequency and Number of Equivalent Inter-ruptions for a year. Data obtained from ([18],[19])

Type of event Weighing Factor Estimated Frequency(Sags or interruptions)

Number of EquivalentInterruptions

Interruption 1 5 5Sag down to 50% 0.8 3 2.4Sag down torange 50% and70%

0.4 15 6

Sag down torange 70% and90%

0.1 35 3.5

Total 16.9

7.11 Who has to be paid for the loss

The impact of voltage sags and interruptions are not limited to a particular group. Theindustrial customers are the ones who su↵er a huge loss of more than $10,000 per trip due tothe process trips. The residential customers experience a shutdown of PCs or other electronicappliances. The commercial customers su↵er from lamp flickers or turn o↵ some electronicappliances etc. The utilities su↵er from customer dissatisfaction apart the repair costs forequipment and labor.

In general when a sag or interruption occurs it is a loss on both customer and utility’sperspective. The customers may claim their loss from the utilities for the poor power qualityand the utilities may blame the customers for having sensitive equipment installed. Both theperspectives of the customers and the utilities seem to be justified since the financial loss is hugein most of the cases. When the issue is seen from the utilities point of view it is reasonable forthe customers to have equipment which are not very sensitive so that they can operate in case ofshallow sags but the utility has to make sure that deep sags or interruptions do not occur in thenetwork. When seen from the customer’s perspective the utilities should have proper protectionagainst the sags or interruptions. There needs to be a trade o↵ when it comes to paying for theloss i.e. when the customers notice sags even after installing proper equipment and mitigationdevices, they need to be ready to pay to the utility for installing protection devices and if theutility fails to provide better power quality even after having better protection devices then theutility needs to su↵er for the loss. Indeed, the customers need to cooperate with the utilities forinvesting in better solutions which thereafter will benefit both the customers and the utilities.

7.12 Estimating the costs of Fast Switching Devices

The fast switching devices today use advanced technology for their operation and hence thecost of these devices are deemed to be high. In general, it is hard to estimate how much canthe devices cost until the manufacturer provides details about the devices. Also the costs mayvary depending on the rating and application of the devices. Table 9 summarizes the costs ofsome fast switching devices and solutions. It can be seen how much should a utility invest forinstalling the devices in the network.

Table 9: Investment costs of Fast Switching Devices obtained from([18])

Device Total cost in $

Static Switch(10 MVA) 600,000Fast Transfer Switch(10 MVA) 150,000

40

7.13 Cases illustrating fast switching devices in a Distribution Network

In this section di↵erent cases which give a picture of the advantages of having fast switchingdevices in a distribution network are described. The aim of the illustration is to know whathappens during severe situations in a network and how can the fast switching devices resolvethe issues without neglecting the economic constraints.

7.14 Case1: Network installed with backup generation

Consider a simple distribution network with the incoming feeders that are supplying power.The network is supplied with the backup incoming feeder in case of a fault or interruption.Manual switching is often employed to switch the supply to the back up generation. This maycause interruption of power to the customers until the switching is done manually. In case ofindustrial customers, the losses due to the interruption can be huge. If a fast transfer switch

Figure 20: A simple network with Backup Generation installed

is installed in the system as shown in Figure 20, the supply can be transferred to the backup generation in a duration of few milliseconds. A static transfer switch which uses thyristorsfor switching is described in [5]. This can avoid any kind of interruption to the customers.This is an e�cient way to prevent interruptions and also provide protection to the equipment.In order to ensure fast switching, the detection of a sag or an interruption must also be fast.Hence the transfer switch is generally combined with a detection scheme. A fast transfer schemecan reduce a voltage sag duration to half cycles which otherwise would have lasted for severalcycles. Also the fault current limiter can limit the fault current before it a↵ects other parts ofthe network in case a fault occurs near to the transformer. Considering the sensitive processequipment in the process industries, the high sag rate, installing fast switching devices can saveon the huge interruption costs every time by protecting sensitive equipment and maintainingsupply to the customers. The interruption costs when weighed against cost of the devices willbe larger and hence the benefit is clear to the customer.

7.14.1 Case2: Radial Distribution Network

In order to assess the impact of voltage sags a typical radial distribution network is con-sidered as shown in Figure 21. The network consists of high voltage incoming feeders and atransformer equipped with an on load tap changer. The main bus bars i.e. bus bar 1 suppliespower to the two other bus bars i.e. bus bar 2 and bus bar 3 respectively which are then con-nected to lines which comprise the feeding network. The voltage is transformed to lower levelat the feeding networks and the feeders supply the customer loads. During a fault anywhere in

41

Figure 21: A radial distribution network

the system, a fault current occurs which may a↵ect the other parts of the system. If a fault Foccurs in the system shown in Figure 21, the fault current can rise rapidly and a↵ect the busbars and all components upstream the fault through which the fault current flows. The faultmight also cause severe sags to the customers connected to the local bus bar 10 and also causeshallow sags to the customers connected to the local bus bar 3 and local bus bar 4. The sagscan penetrate, a↵ecting customers connected to local bus bar 1, local bus bar 2 and local busbar 9. If a fault current limiter is installed in the line where the fault occurs, it limits the faultcurrent to such an extent that no other customers connected to that line are e↵ected except theones that are connected to the local bus bar 10 until the fault is cleared. In addition the faultcurrent limiter can prevent the thermal stress, mechanical stress and damages at the locationof the fault.

Installing local generators at each level of the network for example a local generator supplyingload to the customers connected to local bus bar 1 and local bus bar 2 will not a↵ect the rateof sags since it takes time to start up a generator and voltage sags occur at the instant of thefault. Therefore, a better choice would be to install a fault current limiter at each feeder in thenetwork which would limit the fault current in a very short duration of time.

Fault at local bus bar 1 or local bus bar 2 can cause deep sags to the customers connectedto the respective bus bars. Shallow sags can also be experienced by the customers connected tothe local bus bars 9, 3, 4 and 10. The sags can be reduced due to the e↵ects of the transformers

42

connected at each bus bar. A solution to this problem is to redesign the circuit breaker so thatthey trip in less than a cycle thus restraining the sags that last for more than a cycle frompenetrating to the other buses.

7.14.2 Case3: Ring Main Unit Distribution Network

Consider a Ring Main distribution network as shown in Figure 22. The network comprisesprimary feeders that supply electrical energy to the network, load break switches which isolateany part of the network in case of an emergency or fault and the feeders connected to thecustomers. This configuration is e�cient as the faulty feeder can be isolated from the networkthus maintaining continuity of supply through the other feeders.

Figure 22: A Ring Main distribution network

When a fault occurs on the line supplying all the customer feeders, the fault current risesrapidly; this can cause damage to not only the nearest feeder but also the other feeders sincethey are connected to the same line. The circuit breakers detect the fault and trip thus isolatingthe nearest feeder. This restricts the fault to a particular region so that the fault current doesn’tpenetrate in the other parts of the network. If the relay connected to the circuit breaker fails todetect the fault current or if there is a delay in the operation, it is likely that the whole networkcan be a↵ected. A solution to this is to install a fault current limiter at each of the ring mainunits in the line as shown in Figure 22. This limits the fault current almost instantaneouslythus protecting the other part of the network from damage. For the same case, an Ultra-FastEarthing Switch which can extinguish an arc by earthing in 4ms can be installed in the line to

43

prevent any damages at the fault location.

7.14.3 Case4: Choice of Devices and Feasibility

Consider a simple network as shown in Figure 23 with parallel incoming feeders. The shortcircuit current rating of the switchgear is exceeded when using two incoming feeders or addinga local generator in the system therefore, fault current limiters can be installed in the systemas shown in the Figure 23.

In economic point of view considering the system shown in Figure 23, the circuit breakersare the ones installed in today’s distribution network. Now if all of them have to be replacedwith fast switching circuit breakers, the costs would be very high. The ratings for the FCL isgiven by the prospective short circuit current of the system. The FCL should ensure that shortcircuit ratings of the Circuit Breaker and switch gear is not exceeded. This could limit the faultcurrent before it rises rapidly and hence the circuit breakers switch with their normal operatingtimes. The cost of a fault current limiter will be less when compared to several fast switchingcircuit breakers.This might not solve the problem of voltage dips.

Voltage dips caused by faults in the outgoing feeders in the distribution system could eitherbe solved by installing a FCL on each of the outgoing feeders or a faster Circuit Breaker oneach of the outgoing feeders.

Figure 23: A simple network with parallel supply

This system is a good example to look at the network in di↵erent perspectives in order toattain a trade-o↵ between the technical and economic aspects of installing fast switching devicesin a system.

7.15 Weighing the loss costs against the cost of the devices

The cost for each sag or interruption is dependent on how deep and severe it is. This costmay vary for every event and also according to the number of equipment tripped. as discussedin section 6. The cost for each sag has been assumed in research papers ([20], [18]). The typicalcost of a sag for a sag according to ([20], [18]) is $ 25,000 and $ 40,000. Based on this value andaccording to the method described in section 6, the cost of sags during a year can be calculatedand weighed against the cost of investment for a fast switching device.

44

Table 10: Example value of weighing factors, Estimated Sag Frequency and Number of EquivalentInterruptions for a year. Data obtained from ([18],[19])

Type of so-lution

TypicalCost($)

Operatingand Main-tenanceCosts(%ofinitialcosts peryear)

Interruption(%)

Sag downto 50%

Sag downto range50% and70%

Sag downto range70% and90%

CVT (con-trols)

1000/kVA 10 0 20 70 100

DynamicSag Cor-rector

250/kVA 5 0 20 90 100

FlywheelridethroughTechnolo-gies

500/kVA 5 70 100 100 100

UPS 500/kVA 15 100 100 100 100StaticSwitch

600,000(10MVA)

5 100 80 70 40

FastTransferSwitch

150,000(10MVA)

5 80 70 60 40

In order to see how e↵ective can each solution to mitigate sags and interruptions, acomparison for each device is shown Table 10. A brief description of the devices in the table isgiven below.

Constant Voltage Transformer: Constant-voltage transformer is a saturating transformerused as a voltage regulator. It consists of a tank circuit composed of a high-voltage resonantwinding and a capacitor that can produce constant average voltage output for varying inputcurrent or load. The primary is on one side of a magnet shunt and a tuned circuit coil andsecondary on the other side. The voltage regulation is achieved by the magnetic saturation inthe section present around the secondary of the transformer.

Dynamic Sag Corrector: It can boost the incoming A.C line voltage by more than 100%.It can also provide ride-through protection against outages.The ride-through function isdependent on the stored energy whereas the voltage boost function is not limited by the storedenergy in the system.

Uninterruptible Power Supply (UPS): It is an electrical device, that provides power tothe load in case of emergency when the input source fails. UPS provides near-instantaneousprotection from input power interruptions, by supplying energy stored in batteries or a flywheel.The runtime of most uninterruptible power sources is relatively short (only a few minutes).

Flywheel ride through technologies: Flywheel electricity storage systems use electric energyinput which is stored in the form of kinetic energy. Kinetic energy can be achieved by meansof a rotor that called a spins in a nearly friction less enclosure. During emergency conditions

45

when backup power is required, the inertia of the rotor allows it to continue spinning and thekinetic energy is converted into electricity.It can be observed from the Table 10 that the number of sags or interruptions avoided byinstalling each mitigation device is di↵erent. It can also be seen that installation of UPS atevery equipment terminal can reduce almost all the sags or interruptions that can occur butthe drawback for this solution is the UPS failure rate which cannot be neglected. Looking atthe investment costs of the two fast switching solutions i.e. Static Switch and the Fast TransferSwitch from the Table 10 it can be said that the solutions are expensive when compared to theother solutions. Now if they are weighed against the costs of sags obtained from ([20], [18]), theactual value that they can contribute to network can be estimated. From Table 11, the costs ofthe fast switching devices are compared against the cost of sags and interruptions per year. Itcan be said that the cost of the devices are less than the cost of the sags but the e↵ectivenessof the device can only be estimated by comparing the amount of mitigation it provides to thesags or interruptions. It should be noted that the values are obtained from ([20], [18]) and arenot concerning to any particular company or product.

Table 11: Weighing the Investment costs of Fast Switching Devices against the Cost of sags or interrup-tions per year. Data obtained from([18], [19])

Device Total cost in $ Cost of Sags per year

Static Switch 600,000 676,000Fast Transfer Switch(10 MVA) 150,000 676,000

7.16 Value

For a network involving customers and Utility, the value of a solution that can mitigate mostnumber of sags or interruptions is technically and economically decisive. Therefore, there shouldbe a trade-o↵ between both the aspects in order to estimate the value that can be beneficial toboth the customers and the utility. Cost of fast switching devices, cost of sags and the amountof mitigation have been considered in the previous sections in order to estimate the value of thesolutions. A simple method is to calculate the di↵erence of the cost of the fast switching deviceand the product of the number of sags that have been mitigated with the cost of each sag orinterruption as shown in equation 41. This will give an exact value or benefit that the solutionprovides.

V O = COS

� Tci

⇥NOM (41)

where, COS

is the cost of the solution Tci

is the cost per sag or interruption NOM is theNumber of Mitigated Sags or Interruptions

The Net Present Value (NPV) method provides a good analysis for a project in order tocalculate the profit of the investments. The annual net income is discounted to the project’sstarting value with a discount rate and provides a best solution by comparing the obtainedNPV. A positive value of income indicates the project is valid for the invested income. In aproject with di↵erent solutions or investments, the one with the largest NPV is accepted. Themethod is described in the equations below.

NPV =nX

i=0

(CI� CO)i

(1 + i0

)�i (42)

where, CI is the cash inflows, CO is the outflows of the ith year, n is the project life cycle andi0

is the discount rate.

46

By applying the NPV method for estimating the value of the solutions to mitigate sags orinterruptions, Equation 43 can be obatined.

NPV = �C0

+nTX

n=0

fn

Csag

� Coperating

(1 + r)n(43)

where, Csag

is the cost of one sag or interruption,fn

is the number of sags after improvementby the solution, C

operating

is the operating cost of the equipment, nT

is the life cycle of theequipment.

By assuming cost per sag as $25,000, the NPV method is applied to four di↵erent solutionsin [20]. The equipment life is assumed to be 10years, the interest rate is 5%. The resultsobtained are shown in Table 12. It can be seen that the Fast transfer switch is the best solutionaccording to the method. If the cost per sag or interruption varies, the NPV value for eachsolution may also vary accordingly.

Table 12: NPV method applied for di↵erent solutions. Data obtained from ([20])

Type of Solution Total Invest-ment in $

No of sags re-duced

Losses pertain-ing to the re-duced sags in $

NPV in $

UPS 2,158,300 0 266,750 -9.8457DynamicVolatge Re-storer

831,660 5.0121 141,447 26.0598

Static Switch 831,660 2.4850 204,625 74.8454Fast TransferSwitch

207,915 3.6679 175,053 114.3840

7.17 Economic Analysis

According to [18], the objective is to minimize the annual costs i.e power quality (PQ costs+ Solution Costs) in order to arrive at conclusions. For a $40,000 cost per sag, 15year life time;interest rate of 10

Figure 24: PQ+Solution Costs for di↵erent solutions

47

It can seen from the figure that the base cost is the annual PQ cost without any solutionemployed. The annual costs with di↵erent solution employed show that fast transfer switch andstatic switch reduce the base cost to a large extent and also the annual cost is less compared tothe base cost.

48

7.18 Possible future distribution network with Fast Switching Devices

A possible design of a distributed network installed with fast switching devices is as shownin figure 25. All the circuit breakers and on-load tap changers for transformers can be madefaster than today. Additionally, fault current limiters, fuse savers, fast transfer switch, ultra fastearthing switch can be installed at various parts in the network while considering the technicalfeasibility and also the investment in economical perspective. The idea is to mainly reduce thepower quality problems namely voltage sags and interruptions to deliver high quality power inthe network.

Figure 25: Possible future distribution network with fast switching devices installed

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8 Fast Switching Devices for future Electricity Networks

While envisaging for a sustainable future where the technological advances revolutionize theworld, the need for transforming today’s electricity networks is increasing. Electricity gridswith distributed generation and Smart grid technologies well integrated are the next step tothis transformation. This transformation is spreading worldwide. This section details today’selectricity grid and visualizes the future grid where Smart grid technologies and distributedgeneration is incorporated. The drivers, challenges and advantages of tomorrow’s grid arediscussed. The role of fast switching devices and where can they be incorporated in tomorrow’sgrid is described.

8.1 Today’s Electricity Grid

A schematic of how today’s electricity grid looks is shown in Figure 26. The electricitygenerated from the hydro power, coal and nuclear power plants is transformed to higher voltagelevel which exceed 300 kV and is transmitted through the Extra High Voltage Transmissionsystem over very long distances. This electricity is converted to a slightly lower voltage levelsi.e. 110 kV and transmitted by means of the High Voltage Transmission System.

Figure 26: A schematic of Today’s Electricity Grid

The electricity produced by industrial power plants is incorporated into the high voltagesystem by converting it into higher levels of voltage. Also, the electricity is tapped by Factorieswhich require high voltage levels for their processes. Further, the electricity is transmitted tothe urban and rural networks by converting to lower voltage levels upto 50 kV. The industrial

50

customers or factories are provided with electricity by converting to voltage levels up to 11 kV.The electricity is transmitted to the Urban networks which consists of households, buildings,hospitals etc where the voltage levels are brought down to 440V or 230V. The same low voltagelines transmit electricity to the rural networks which consists of houses, farms etc. The voltage isconverted by using transformers at di↵erent points in the network. The transmission is throughoverhead lines or underground cables depending on climatic factors.

8.2 Future Electricity Grid

With the advent of Smart grid technology, control of remote power plant or substations isno more cumbersome. Also the incorporation of the distributed generation in the existing gridhas gained a lot of importance in the past decade. A future electricity grid with smart gridtechnology and distributed generation incorporated in the grid is shown in Figure 27. A smartgrid control center which connects to several points in the grid and controls di↵erent processesand devices through communication protocols. Smart meters are installed at di↵erent pointsin the network which can record the consumption of energy and communicate to the controlcenter. Any kind of fault occurring in the grid can be easily located and cleared by using theadvanced technology.

51

Figure 27: A schematic of Future Electricity Grid

8.2.1 Distributed Generation Impacts

The impacts of distributed generation on a grid is an ongoing research topic. The amountof distributed generation that can be incorporated into a grid without e↵ecting the stabilityof the grid is determined by hosting capacity approach that is discussed in section(6.2). Thedistributed generation in the grid has various impacts on the power quality aspects, protectionand losses of both distribution and the transmission network [50]. The drivers for the dis-tributed generation in the network make it a reliable solution for environment protection andsustainability for the future grid [51]. Distributed generation can also improve the reliability ofsupply in the grid in various ways [50].

52

8.2.2 Smart Grid Technology Impacts

Smart grid technology in a future electricity grid will serve as the most e�cient solution bycatering the growing demands of energy whilst maintaining the power quality of the grid. Thetechnology provides data integration, monitoring and control of the grid, e↵ective protection andrestoration during emergency conditioned like faults. The idea is to use Wireless communicationnetworks to communicate with various locations and act accordingly in case of any kind of faultor power quality disturbances. Integrating smart grid technology in the grid will enhance itsperformance in various aspects and therefore is an ongoing discussion as the next step to theelectricity grids.

8.3 Role of Fast Switching Devices

As discussed in the previous section when transforming the existing grid into a future gridby integrating di↵erent technologies, there is a need to recognize the scope of the fast switchingdevices in the future grid. Since the scope of the thesis is limited to distribution networks, theother parts of the grid are not taken into consideration for further discussion.

In the future grid with smart grid technology, all the devices for protection and controlwill be controllable by the control center which constantly monitors all the data in the systemincluding the distribution network. In case of a fault, the operator issues signals to the respectiveprotection and control devices to operate and clear the fault. Since the wireless communicationnetworks are extremely fast, the signals sent to the protection and control devices must operatethe devices in less than a cycle which means that the devices installed must also be extremelyfast. For instance, if a circuit breaker operates in a time period of 55ms-75ms after it receivesthe command signal from the control center, the other signals for restoring the power have tobe delayed which might cause interruptions to the customers. Hence, if the circuit breaker isfaster than today the future electricity grid will be much more e�cient.

Distributed generation (DG) integration in the system can serve as a good solution in thefuture distribution network but it has some impacts like voltage variations, harmonic distortionsetc. When operated as a backup generation during peak load, the DG can be accompanied witha fast transfer switch which can connect the DG into the grid and disconnect when the peak loadperiod ends. During the voltage variations that occur due to the integration of the distributedgeneration in the distribution network, the on-load tap changers can switch the voltage to avoidany kind of equipment failure or interruptions in the network.

53

9 CONCLUSIONS & FUTURE WORK

While there is an enormous amount of research and work going on in this field, this report isan attempt to show that fast switching devices can have a good value in the future distributionsystems. There are several solutions which use power electronics to make the devices faster thantoday while the others try to improve their mechanical designs by using thomson coil actuatorsor other high speed mechanisms.

The devices need to be investigated further in detail by comparing which among mechanicaland solid state technology is beneficial.

While estimating the economical value of the fast switching devices in the distributionnetwork there are several factors that have to be taken into consideration; optimization usingseveral constraints is needed. Therefore, when looking at larger distribution networks, thecomputation can get complex which needs numerical tools to arrive at appropriate results.

With the research so far and the developments, it can be concluded that investing in devicesthat can be as fast as less than a cycle to operate is beneficial. Both utilities and the customerscan be benefited when seen from a distribution network’s point of view. Also since there canbe a reduction in the power quality problems to a large extent, this solution can be next stepfor an e�cient power system in the future.

This report focuses on specific areas of power quality and considers the possibility of in-stalling the fast switching devices in distribution networks. The speed of the devices consideredmay be independent of the other devices that are inter-connected to them in some cases. Forexample, operating times of fast switching circuit breakers need to be considered in interconnec-tion with the relay time settings. This is an aspect to be studied further. Also, when consideringthe impact of the fast switching speeds on the distribution network, the study is specific to mit-igation e↵ects of power quality is emphasized. But the impact of this solution on other powerquality problems such as harmonics etc need to investigated further. The stochastic analysislacks real time information which might be an important aspect that needs to be studied.

============================

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10

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