Power quality improvement using upqc with soft computing method: Fuzzy logic

POWER QUALITY IMPROVEMENT POWER QUALITY IMPROVEMENT POWER QUALITY IMPROVEMENT POWER QUALITY IMPROVEMENT

USING UPQC WITH SOFT COMPUTING USING UPQC WITH SOFT COMPUTING USING UPQC WITH SOFT COMPUTING USING UPQC WITH SOFT COMPUTING

METHOD: FUZZY LOGICMETHOD: FUZZY LOGICMETHOD: FUZZY LOGICMETHOD: FUZZY LOGIC




By:

Sakti Prasanna Muduli

Along with

R C MuduliA K MohapatraP C JenaS K MahallikH Parida




Along with:

R C Muduli Mohapatra

P C Jena S K Mahallik H Parida

DedicatedDedicatedDedicatedDedicated

ToToToTo

All my FriendsAll my FriendsAll my FriendsAll my Friends

CONTENTS Chapter 1: Introduction…………………………………………………………………01 1.1 Overview…………………………………………………………………………..01 1.2 Motivations………………………………………………………………………..02 1.3 Objectives…………………………………………………………………………03 1.4 Organisation of thesis……………………………………………………………03

Chapter 2: Power Quality Problem……………………………………………………04 2.1 Introduction……………………………………………………………………….....04 2.2 Linear and Non-linear Loads……………………………………………………...04 2.2.1 Linear load……………………………………………………………………….04 2.2.2 Non Linear load………………………………………………………………....05 2.3 Major Power Quality problem…………………………………………………...…06 2.3.1 Short Duration Voltage variations…………………………………………..…06 2.3.2 Long Duration Voltage variations…………………………………………...…06 2.3.3 Transients………………………………………………………………………...07 2.3.4 Waveform Distortion……………………………………………………………..08 2.4 Frequency Variation…………………………………………………………………..09 2.5 Harmonics………………………………………………………………………………09 2.7 Power Quality…………………………………………………………………………..09 2.7.1 Voltage dip…………………………………………………………………………10 2.7.2 Voltage dip mitigation……………………………………………………………..10 2.9 Power quality Monitoring……………………………………………………………..13 2.9.1 Asset load monitoring…………………………………………………………….13 2.9.2 Phase Balancing………………………………………………………………….13 2.9.3 Load Balancing……………………………………………………………………14 2.9.4 Distributed Energy Resource management(DER)……………………………14 2.9.5 Direct monitoring and control of DER…………………………………………..14 2.9.6 Shutdown or Islanding verification for DER……………………………………14 2.10 Power Quality controller based on VSC…………………………………………..15 2.11 Voltage Source Converter…………………………………………………………..15 2.12 Distribution Static Compensator……………………………………………………16 2.13 Dynamic Voltage Restorer(DVR)…………………………………………………..17 2.14 UPFC………………………………………………………………………………….18

Chapter 3: Proposed Scheme for Power Quality Improvement………………………19 3.1 Measurement blocks……………………………………………………………….20 3.2 Fuzzy logic Controller………………………………………………………………20 3.2.1 Fuzzy logic Background……………………………………………………….21 3.2.2 Applying truth value……………………………………………………………21 3.2.3 Linguistic Variables…………………………………………………………….22 3.3 PWM Pulse Generator……………………………………………………………...24 3.4 UPQC…………………………………………………………………………………24

3.4.1 Series APF………………………………………………………………………26 3.4.2 Shunt APF……………………………………………………………………….26 3.4.3 DC Link Capacitor………………………………………………………………26 3.4.4 Design of UPQC………………………………………………………………...27 3.4.5 configuration of proposed UPQC……………………………………………...29

Chapter 4: System Model and Results…………………………………………………..32 4.1 Simulink Based System model: With UPQC……….…………………………… 32 4.2 Results: Comparison of 3 ph voltage wave forms- with & without UPQC…….34 4.3 Results: Comparison of DC volt Waveforms –with & without UPQC………….34 4.4 Results: Comparison of volt waveforms under transients………………………35

Chapter 5: Conclusion and Future Scope………………………………………………37 5.1 Conclusion……………………………………………………………………………37 5.2 Future Scope………………………………………………………………………...37

Reference………………………………………………………………………………….39

LIST OF FIGURES

Figure No. Title Page No.

2.1 Basic building blocks of D-STATCOM 17

2.2 Standard configuration of DVR 18

2.3 Basic configuration of UPFC 19

3.1 Whole System block diagram 20

3.2 Fuzzy logic 22

3.3 Fuzzy logic system 23

3.4 Simple UPQC scheme 25

3.5 UPQC with Energy storage 30

3.6 Equivalent Circuit diagram for UPQC 30

4.1 Simulink based system model without UPQC 33

4.2 Result without UPQC(Voltage sag) 34

4.3 Result with UPQC(no voltage sag) 34

4.4 DC voltage without UPQC 35

4.5 DC voltage with UPQC 35

4.6 Transients 36

4.7 Compensated system under transients 36

ABSTRACT

In a powers system network there are many problems related to power quality.When the term ‘Power Quality’ comes just after that ‘Voltage Quality’ comesalong with. We can say Power Quality is directly referred by voltage quality. So to improve power quality of a system we use different devices such as active power filters. Active power filters are classified into two types those are Shunt Active Power Filter and Series Active Power Filter and combination of both is known as “UPQC (Unified Power Quality Conditioner)”. UPQC is used to mitigate voltage sag and transients due to sudden load variation, fault, capacitor switching etc. Series APF reduces all voltage related problems and shunt APF reduces all current related problems, Harmonics in addition to reactive power support. The control scheme used is based on “Fuzzy logic” as it is a multi-valued logic and simple to understand define a rule.

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CHAPTER1

INTRODUCTION

1.1 Overview

In today’s world there is great importance of electrical energy as it is the most

famous from of energy and all are massively relying on it. Without supply of electricity

life cannot be imagined. At the same time the quality and continuousness of the electric

power supplied is also very important for the efficient functioning of the end user

equipment. Many of the commercial and industrial loads require high quality undisturbed

and constant power. Thus maintaining the qualitative power is topmost important in

today’s world. Electric power quality (PQ) has become the concern of utilities, end

users, manufacturers, and all other customers. Power quality is the set of parameters

defining the properties of power supply delivered to the users in normal operating

conditions in terms of continuity of supply and characteristics of voltage (magnitude,

frequency, symmetry, waveform etc.). Modern electronic equipments and devices, such

as microprocessors, microcontrollers, telecommunications equipment and sensitive

computerized equipments etc. are susceptible to PQ problems.

Poor PQ has become a more important concern of both power suppliers and

customers. The global bill for poor power quality is more than 500 billion Euros per year

which is 50% of the turnover of the global electricity sector. For many business uses,

the cost of poor PQ is higher than the electricity bill and the cost is rising.Due to power

electronics devices there is serious effect on quality and continuousness of electric

supply. Because of power electronics devices there is uninterrupted power supply,

flicker, harmonics, voltage fluctuations etc. There is also PQ problems such as voltage

rise/dip due to network faults, lightning, switching of capacitor banks. With the excessive

uses of non-linear load (computer, lasers, printers, rectifiers) there is reactive power

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disturbances and harmonics in power distribution system. It is very essential to

overcome this type of problems as its effect may increase in future and cause adverse

effect.

Traditionally passive filters were used for reactive power disturbances and

harmonics generation but there is many problems with them like they are large in size,

resonance problem, effect of source impedance on performance. Active Power Filters

are used for power quality enhancement. Active power filters can be classified

according to system configuration. Active power filters are of two types series and

shunt. Combining both series APF & shunt APF we get a device known as UPQC.

UPQC eliminates the voltage and current based distortions together. A Shunt

APF eliminates all kind of current problems like current harmonic compensation,

reactive power compensation, power factor enhancement. A Series APF compensates

voltage dip/rise so that voltage at load side is perfectly regulated. The Shunt APF is

connected in parallel with transmission line and series APF is connected in series with

transmission line. UPQC is formed by combining both series APF and shunt APF

connected back to back on DC side. In this section, UPQC and fuzzy logic controller is

used to mitigate power quality problem efficiently. UPQC is made by combining both

shunt APF and series APF. Fuzzy logic is very efficient controller used to eradicate

power qualities issues and problems.

In this thesis power quality of system was improved by using UPQC and fuzzy

logic. First simulation of shunt APF was done after that series APF was done. And after

that combining both device simulation of UPQC was done using fuzzy logic controller.

1.2 Motivation

At present scenario, one of the serious problems in electrical systems is the

increasing number of power electronics devices that are used by industries as well as

residences. These devices which need high-quality energy to work properly, at the

same time are the most responsible ones for injections of harmonics in the distribution

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system. The goal of this work consists in expand this control strategy of fuzzy logic to

deal with the power quality problem issues using UPQC. UPQC is an integration of

shunt and series compensation to limit the harmonic contamination. Simulation results

on MATLAB can be illustrated in order to verify the accuracy of the expanded control

strategy. UPQC is used to solve all problems related to voltage and current harmonics

and improve power quality.

1.3 Objective

� To explore the soft computing techniques fuzzy logic for generating gate pulse

for the UPQC.

� Study the UPQC model for power quality improvement.

1.4 Organization of Thesis

Chapters 1 – So far in the chapter1 Basic introduction to our thesis, Motivation,

Literature survey and Objective have been discussed.

Chapter2–This chapter discusses main theory related to the Power Quality(PQ),

mathematical modeling etc. and various method adopted for the improvement in the

process based on literature review.

Chapter 3- This chapter discusses modeling techniques used, Operations and design

principle. The chapter further demonstrates about the basic principles behind the

proposed scheme.

Chapter 4 –This chapter shows various performances on MATLAB/SIMULINK

environment for the adopted scheme.

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Chapter 5 –This chapter shows Concludes with performance analysis methods and

results of our adopted technique and further advancement for implementing new

innovation in this specific topic.

Finally, the thesis ended with references taken for the detailed case study on the topic.

CHAPTER2

POWER QUALITY PROBLEM

2.1 Introduction In the day-today life, and in many industries there is very huge use of power electronics devices, Programmable logic circuits (PLC), semiconductor devices, and adjustable speed drives due to this there is power quality problems. There is also many external and internal factors that affect the quantity and quality of power delivered. Many network faults, switching of capacitor banks, voltage sag/swell, lightning, and harmonics also cause power quality problems. Mainly loads work at 50 Hz and 60 Hz frequencies but there are many loads which work at integer multiple of 50 Hz or 60 Hz frequencies. Because of these loads there is harmonics in power system. 2.2 Linear and Non- linear loads 2.2.1 Linear load The loads which have current and voltage waveform sinusoidal are linear loads. The current at any time is proportional to voltage. Linear load only change relative timing (phases) between current and voltage there is no change in shape of current waveform. According to Ohm’s law

I(t)=�(�)

�

According to this law if waveform of voltage is sinusoidal then the waveform of current also will be sinusoidal. Linear loads are capacitor and inductor. If capacitor is in load side then the current will lead voltage. If inductor is in load side then voltage will lead. So waveforms in both cases will be out of phase. Power factor defined in case of linear load.

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Power factor = ��

�� = cos∅

Table 2.1 Linear load

2.2.2 Non-Linear loads In non-linear loads the shape of current waveform changes its shape from

original shape. Non-linear loads produce harmonics with original fundamental

component of AC current. Non-linear loads examples are power electronics devices like

BJT,MOSFET. Given in table 2.2.

Table 2.2

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2.3 Major power quality problems 2.3.1 Short duration voltage variation Due to faults there is voltage rise (swells), voltage dip (sag), or complete loss of

voltages (interruptions) which are temporary for certain interval of time depending upon

the type of fault occurred and location of fault. The duration is around 1 min for short

voltage variation. Also if duration of fault is for few millisecond then it is short duration

voltage variation.

(i)Voltage sag:- Voltage sag is also called voltage dip . The rms line voltage decreases

to 10 % to 90 % of nominal line voltage. The time interval for voltage dip is about 0.5

cycle to 1 min. The equipment which causes voltage dip are induction motor starting

etc. Voltage dip is shown in figure below.

(ii)Voltage swell:- Voltage swell is also called voltage rise. The rms line voltage

increases from 1.1 % to 1.8% of nominal line voltage. The duration for voltage rise is

around 0.5 cycles to 1 min. The voltage swell is caused due to energizing the large

capacitor bank and shutting down the large loads. Voltage swell is shown in fig 2.2

(iii) Interruption:- Interruption is degradation in current or line voltage up to 0.1 pu of

the nominal value. It is for the time period of 60 seconds and not going beyond it. The

causes of interruption are failures in equipment, faults in power systems, control

malfunctions.

2.3.2. Long- Duration voltage variation The long duration voltage variations are caused because of sustained interruptions,

under voltages and over voltages.

(i)Sustained interruptions:- When there is zero supply voltage for a interval of time

more than 60 sec, it is considered as sustained interruption in case of long duration

voltage variation.

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ii) Under voltages:- It is the reduction in rms ac voltage to lower than 90 % at power

frequency for a time interval 60 sec or may be greater than it. The switching off of

capacitor banks and switching on of loads cause under voltage as far as voltage

regulation device on the system bring back the voltage to the given tolerance limits. The

under voltage is also caused due circuits which are overloaded.

(iii) Over voltages:- It is the rise in rms ac voltage to more than 110 % at power

frequency for a time interval of more than 60 sec. Over voltages are caused due to the

wrong tap settings of transformers and switching of loads.

2.3.3 Voltage fluctuations

Fluctuations in voltage are irregular or repeated variations in magnitude of source

voltage due to sudden change in real and reactive power drawn by the load. The

characteristics of voltage fluctuation depend upon type of loads. The magnitude of

voltage fluctuation does not rise above 12 10% of nominal supply voltages. The Lamp

flicker is the effect of voltage fluctuations. Loads that cause fluctuations in voltages are

arc furnaces, arc welders, air conditioner units, rolling mills, cyclo-converters, and

equipment with excessive motor speed changes.

2.3.4 Voltage Unbalance

Voltage unbalances occur when there is difference in magnitude of phases or line

voltages and phase angle is different from balanced conditions. Voltage unbalance is

due to different loads in the phases causing drops in voltages at phase – line

impedance.

2.3.5 Transients

Transients are sudden & small change in current and voltage signals for a very less

period of time.

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(i) Impulsive transients:- Impulsive transient are variation in current, voltage or both on

power line in one direction (unidirectional). The causes of impulsive transients are

switching in power distribution systems, inductive loads switching, lightning. The

impulsive transients can be removed with help of zener diode which suppresses the

transient voltage.

(ii)Oscillatory transients:- Oscillatory transients are transients which have swing

(bidirectional) i.e. rapid change of polarity of current , voltage or both on power line.

Capacitors switching which help in power factor correction.

2.3.6 Waveform Distortion

A power system network tries to generate a sinusoidal voltage and current waveform

but due to certain problem it is not able to generate the sinusoidal nature waveform and

distortions occurs.

There are many causes of waveform distortion:-

(i)DC Offset:- A DC offset is a presence of DC voltage or current in a AC power

system. Due to DC offset the signal shifts from its actual reference position.

(ii) Noise:- Noise is unwanted electrical signals. It is caused due to interference in

communication network. The unwanted signals are superimposed in powers system

current or voltages which are in phase or in neutral conductors.

(iii) Notching:- Notching is voltage disturbances caused periodically due to transfer of

current from one phase to another when power electronics equipment are commutated.

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(iii)Harmonics:- The harmonics are sinusoidal currents and voltages which operate at

frequencies that are integer multiple of fundamental frequency. The 50 Hz and 60 Hz

are fundamental frequency. The harmonics are caused due to non- linear loads.

(iv) Inter harmonics:- Inter harmonics are harmonics which are not at the frequencies

that are integer multiple of fundamental frequency (50 Hz or 60 Hz). That are caused

due to induction furnaces, cyclo-converters, arc furnaces, static frequency converters.

2.4 Frequency variations

In a power system many equipment and devices are made to operate at fundamental

frequency. But there is variation in frequency due sudden disturbances in supply or

demand. Frequency variations are mainly caused due to failure of generators and

switching of loads.

2.5 Harmonics

Harmonics are sinusoidal voltage & current which operate at integer multiple of

fundamental frequency. In power Quality Corporation some index values has been

developed to determine the service quality and distortions caused due to harmonics.

2.7 Power Quality

The term power quality is not universally agreed upon but the concept has become a

very important aspect of power delivery. Other terminology in use is quality of power

supply and voltage quality.

Interest in power quality has recently increased mainly due to the following factors:

• Equipment has become more sensitive to voltage disturbances.

• Equipment causes voltage disturbances. The number of loads fed via power

electronic converters has recently increased. These present a challenge in

ensuring power quality.

• There is a growing need for standardization and performance criteria.

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• The power quality can be measured. Harmonic currents and voltage dips are no

longer difficult to measure.

The quality of electrical power supply is a set of parameters which describe the process

of electric power delivery to the user under normal operating conditions, determine the

continuity of supply (short and long supply interruptions) and characterize the supply

voltage (magnitude, asymmetry, frequency, and waveform shape). Power quality

phenomena can be divided into two types.

• A characteristic of voltage or current (e.g., frequency or power factor) is never

exactly equal to its nominal and desired value. The small deviations are called

voltage variations or current variations.

• Occasionally the voltage or current deviates significantly from its normal or ideal

wave shape. These sudden deviations are called events.

Power quality events are the phenomena which can lead to tripping of equipment, to

interruption of the production or of plant operation, or endanger power system

operation. This includes interruptions, under voltages, overvoltage, phase angle jumps

and three phase unbalance.

2.7.1 Voltage Dip:

A voltage dip is a short time (10 ms to 1 minute) event during which a reduction in

r.m.s voltage magnitude occurs. It is often set only by two parameters, depth/magnitude

and duration. The voltage dip magnitude is ranged from 10% to 90% of nominal voltage

(which corresponds to 90% to 10% remaining voltage) and with a duration from half a

cycle to 1 min. Voltage dip in a three phase system affects both phase to phase(line

voltage) and phase to ground(phase voltage).

2.7.2 Voltage Dip Mitigation:

Voltage dips in transmission and distribution systems can be mitigated in different

ways. At present, a wide range of very flexible controllers, which capitalize on newly

available power electronics components, are emerging for custom power applications .

These devices are used to control and stabilize voltage in the Power System. These

devices consist of static VAR generator or absorber and a suitable controlling power

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electronic device. These devices provide fast-acting reactive power compensation to

power system networks. These devices are connected on transmission systems to

improve voltage profile and system stability during both normal and contingency system

conditions. The use of these devices helps to increase transmission capacity and

stabilizes voltage in different buses over a wide range of loads. These devices also

compensate the reactive power demand of the widely varying loads. If the load in the

system is very high, the demand of reactive power is also very high, so there will be

high amount of reactive power flow in the system and it causes the voltage drop in the

line. Therefore, the voltage at the receiving end will decrease. Similarly, if the load in the

system is very low, voltage at the receiving end of the line increases due to charging

current (Ferranti effect). It means that if the generated reactive power is less than the

consumed reactive power in the system, the voltage drops and vice versa. Therefore,

the variation of voltage is because of imbalance in generation and consumption of

reactive power in the system.

As the power quality problems are originated from utility and customer side, the

solutions should come from both and are named as utility based solutions and customer

based solutions respectively. The best examples for those two types of solutions are

FACTS devices (Flexible AC Transmission Systems) and Custom power devices.

FACTS devices are those controlled by the utility, whereas the Custom power devices

are operated, maintained and controlled by the customer itself and installed at the

customer premises. Both the FACTS devices and Custom power devices are based on

solid state power electronic components. As the new technologies emerged, the

manufacturing cost and the reliability of those solid state devices are improved; hence

the protection devices which incorporate such solid state devices can be purchased at a

reasonable price with better performance than the other electrical or pneumatic devices

available in the market. Some of these Custom Power Devices are: Series-connected

compensator like DVR (Dynamic Voltage Restorer), shunt-connected compensator like

DSTATCOM (Distribution Static compensator), Series and shunt compensator like

UPQC (Unified Power Quality Conditioner) and SSTS (Solid State Transfer Switch).

Among these, the UPQC is an effective custom power solution which consists of two

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back to back connected IGBT based voltage sourced bidirectional converters with a

common DC bus.

Unified Power Quality Conditioner (UPQC) is an integration of shunt active

power filter and series active power filter. The series portion compensates for supply

voltage harmonics and voltage unbalances, acts as a harmonic blocking filter and

damps power system oscillations. The shunt portion compensates load current

harmonics, reactive power and load current unbalances. In addition, it regulates the DC

link capacitor voltage. The power supplied or absorbed by the shunt portion is the power

required by the series compensator and the power required to cover losses. A Unified

Power Quality Conditioner that operates in simultaneous voltage and current control

modes. UPQC combines the operations of a Distribution Static Compensator

(DSTATCOM) and Dynamic Voltage Restorer (DVR) together. In the voltage control

mode it can make bus voltage at load terminal sinusoidal against any unbalance,

harmonic or flicker in the source voltage or unbalance or harmonic in the load current. In

the current control mode, it draws a balanced sinusoidal current from the utility bus

irrespective of unbalance and harmonic in either source voltage or load current. General

configuration of UPQC In present case UPQC with right shunt topology is considered. In

right shunt topology, shunt active power filter will come right side of series active power

filter that is near to load so that the currents flowing through the series transformers will

be balanced sinusoidal. UPQC voltage references are calculated based on Fourier

series, extraction of fundamental sequence components using half cycle running

(moving) averaging, current references are calculated using the instantaneous

symmetrical component theory. At the PCC of shunt active power filter, the voltage is

load voltage which is balanced sinusoidal after compensating the source voltages by

series active power filter. So we can use the instantaneous symmetrical component

theory for calculating the source reference currents, which will be in phase with the load

voltage, so that source will supply only average active power, remaining part of

oscillating active power and total reactive power will be supplied by the UPQC. A

mathematical model for UPQC with right shunt topology considering non-linearity in load

is derived.

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2.9 Power Quality Monitoring

Today for some larger customers and at select locations on the grid we are able

to monitor harmonics, wave form, phase angles and other power quality indicators. The

need continues to grow as large screen televisions and other consumer electronics

devices are increasingly adding harmonics to the system. With the newest metering

technology some power quality monitoring is built into the meter and more is on the

way. While not every house needs to monitor power quality, a percentage of the meters

deployed should probably have this advanced capability.

2.9.1 Asset Load Monitoring

With Connectivity Verification and Geo-Location information it is possible to

group the devices in a tree structure that correctly shows connection points in the grid.

With the ability to read intervals from the meters it is then possible to build a picture of

the load that each asset (e.g. transformers, conductors, etc.) are subjected to. This

allows an operator to monitor heavily loaded assets and look for ways to off load some

of the demand from that asset. It also allows a maintenance planner to prioritize what

maintenance should be done to maximize the reliability of the grid, as part of a reliability

centered maintenance program.

2.9.2 Phase Balancing

One of the least talked about issues with losses in the distribution grid today is

single phase load and the imbalance it can cause between the phases. These losses

have seldom been measured in the grid and little study has been done of the amount of

phase imbalance on the grid today. In early studies the chronic phase imbalance in

several circuits that were monitored averaged over 10 percent. While correction is hard

when the circuit is run as single phase laterals, in many cases there is enough load on

the feeder portion of the circuit to allow rebalancing of the circuit to eliminate more than

half of the chronic phase imbalance.

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2.9.3 Load Balancing

Where there is an option to move a portion of the load from one circuit to

another, the instrumentation is not always available to make good choices or to be able

to forecast the load in a way that makes the movement pro-active instead of reactive.

Automated feeder switches, and segmentation devices are becoming more and more

common in the grid. The ability to use metering data to support the operation of these

devices will only increase their value to the grid operator. Today with information only at

the substation end of the circuit, it is tough to determine where on the circuit the load

really is and where to position segmentation and when to activate a segmentation

device when more than one is available. Operators today typically learn the right way by

trial and error on the system.

2.9.4 Distributed Energy Resource (DER) Management

In the future, more and more of the resources on the grid will be connected to

the distribution network and will complicate the operation of the grid for the future.

Failure to integrate these resources into the grid and understand their impact will only

degrade the operation of the grid and its reliability. It is no longer an option to deal with

distributed resources, the time for refusing to allow them has passed. The only choice is

to either embrace them and manage their impact or ignore them and suffer the

consequences.

2.9.5 Direct Monitoring and Control of DER

Some DER units at customer sites could be monitored in “near-real-time” and

possibly directly controlled by the utility or a third party (e.g. an aggregator) via the AMI

system, in an equivalent manner to load control.

2.9.6 Shut-Down or Islanding Verification for DER

Each time an outage occurs that affect the power grid with DER, the DER should

either shut down or island itself from the rest of the grid, only feeding the “microgrid” that

is directly attached to. In many cases the shut-down or islanding equipment in smaller

installations is poorly installed or poorly maintained. This leads to leakage of the power

into the rest of the grid and potential problems for the field crews.

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Each time an outage occurs, meters that are designed to monitor net power can

tell if the islanding occurred correctly, if they are installed at the right point in the system.

This reporting can minimize crew safety and allow the utility to let the customer know

that maintenance is required on their DER system. In most cases when the islanding

fails, other problems also exist that reduce the efficiency of the DER system, costing the

customer the power that they expected to get from the

2.10 Power Quality Controllers based on VSC

Power quality control will be one of the issues addressed in smart grids. Power

electronics is expected to be the main enabling technology in this area. The most widely

used active power quality controllers based on power electronics are FACTS devices

FACTS (Flexible AC Transmission System) are a power electronic based system and

other static equipment that provide control of one or more AC transmission system

parameters to enhance controllability and increase power transfer capability. The

FACTS devices like Unified Power Flow Controller , Unified Power Quality Conditioner,

Distribution Synchronous Compensator, Dynamic Voltage Restorer are expected to gain

widespread use in smart distribution networks for power quality control. Power quality

controllers which are based on voltage source converter are explained in the following

topics. These controllers are DSTATCOM, DVR and UPFC. The emphasis in this paper

is on the capability of these devices in mitigating voltage dips in a distribution system.

2.11 Voltage Source Converter:

A voltage-source converter is a power electronic device, which can generate a

sinusoidal voltage with any required magnitude, frequency and phase angle. The

converter is normally based on an energy storage device, which will supply the

converter with a DC voltage. The solid-state electronics in the converter is then

switched to get the desired output voltage. The controller generates the required

switching pattern. The VSC is a basic component of devices used for mitigation of

voltage dips and harmonic distortion. In VSC based HVDC transmission schemes

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described herein, the switching of IGBT valves follows a pulse width modulation pattern.

This switching control allows simultaneous adjustment of amplitude and phase angle of

the converter ac output voltage with constant dc voltage even with a two level converter.

With these two independent control variables, separate active and reactive power

control loops can be used for regulation.

2.12 Distribution Static Compensator

When the STATCOM is applied in distribution system is called DSTACOM

(Distribution-STACOM) and its configuration is the same, or with small modifications,

oriented to a possible future amplification of its possibilities in the distribution network at

low and medium voltage, implementing the function so that we can describe as flicker

damping, harmonic filtering and short interruption compensation. Distribution STATCOM

(DSTATCOM) exhibits high speed control of reactive power to provide voltage

stabilization, flicker suppression, and other types of system control. The dstatcom

utilizes a design consisting of a GTO- or IGBT-based voltage sourced converter

connected to the power system via a multi-stage converter transformer. The dstatcom

protects the utility transmission or distribution system from voltage sags and/or flicker

caused by rapidly varying reactive current demand. In utility applications, a dstatcom

provides leading or lagging reactive power to achieve system stability during transient

conditions. The dstatcom can also be applied to industrial facilities to compensate for

voltage sag and flicker caused by non-linear dynamic loads, enabling such problem

loads to co-exist on the same feeder as more sensitive loads. The dstatcom

instantaneously exchanges reactive power with the distribution system without the use

of bulky capacitors or reactors. In most applications, a dstatcom can use its significant

short-term transient overload capabilities to reduce the size of the compensation system

needed to handle transient events. The short-term overload capability is up to 325% for

periods of 1 to 3 seconds, which allows applications such as wind farms and utility

voltage stabilization to optimize the system’s cost and performance. The dstatcom

controls traditional mechanically switched capacitors to provide optimal compensation

on a both a transient and steady state basis.

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Figure 2.1 Basic Building Blocks of The DSTATCOM

2.13 Dynamic Voltage Restorer:

Dynamic Voltage Restorer (DVR) is a voltage controller having the same

building blocks as a DSTATCOM but its coupling transformer is connected in series with

the ac system .The resulting voltage at the load bus bar equals the sum of the grid

voltage and the injected voltage from the DVR. The converter generates the reactive

power needed while the active power is taken from the energy storage. The energy

storage can be different depending on the needs of compensation. If the injected

voltage V DVR is kept in quadrature with IL, no active power injection by the DVR is

required to correct the voltage. It requires only the injection of reactive power which is

generated by the DVR itself. DVR can be kept in quadrature with IL only up to a certain

value of voltage dip and beyond which the quadrature relationship cannot be maintained

to correct the voltage dip. For such a case, injection of active power into the system is

essential which must be provided by the energy storage system of the DVR.

18

Figure 2.2 Standard Configuration Of A DVR

2.14 Unified Power Flow Controller

The Unified Power Flow Controller (UPFC) is used to control the power flow in a

transmission system by controlling the impedance, voltage magnitude and phase angle.

The basic structure of the UPFC consists of two voltage source converters (VSCs);

where one converter is connected in parallel to the transmission line through a shunt

transformer while the other is in series with the transmission line through a series

transformer. Both VSCs are connected to each other by a common dc link including a

storage capacitor. The shunt inverter is used for voltage regulation at the point of

connection injecting reactive power into the line and to balance the real power flow

exchanged between the series inverter and the transmission line. The series inverter

can be used to control the real and reactive line power flow inserting voltage with

controllable magnitude and phase in series with the transmission line. Thereby, the

UPFC can fulfill functions of reactive shunt compensation, active and reactive series

compensation and phase shifting.

19

Figure 2.3 Basic Configuration Of UPFC

CHAPTER3

PROPOSED SCHEME

UPQC DESIGN FOR POWER QUALITY IMPROVEMENT

In this proposed scheme we have introduced UPQU as compensator with Fuzzy

logic controller as control element which provides gate pulses to SCRs used in the

active filters in the UPQC. The figure below describes the whole system for voltage

compensation and hence to improve the power quality.

20

The main subsystems of the above given scheme are described in detail below

3.1 Measurement Block:

This block consists of some Voltmeters and Ammeters connected to the

system through PTs and CTs. These meters collect the line parameters (volt and

current), compare with the predefined values and create the error signals. These error

signals are fed to the controller block for further processing.

3.2 Fuzzy Logic Controller:

The FACTS device used in the system requires proper gate pulse for operation.

PWM generator generates pulse for SCRs used in UPQC and it requires a reference

wave for generating the reference signal. This reference signal is obtained from Fuzzy

Logic Controller. Actually Fuzzy logic controller is a soft computing method used for

generating desired output signal using predefined rules.

Figure 3.1 Whole System Block Diagram

21

3.2.1 Fuzzy logic background

Fuzzy logic is a form of many-valued logic in which the truth values of variables

may be any real number between 0 and 1, considered to be "fuzzy". By contrast, in

Boolean logic, the truth values of variables may only be 0 or 1, often called "crisp"

values. Fuzzy logic has been extended to handle the concept of partial truth, where the

truth value may range between completely true and completely false. Furthermore,

when linguistic variables are used, these degrees may be managed by specific

(membership) functions.

The term fuzzy logic was introduced with the 1965 proposal of fuzzy set

theory by Lotfi Zadeh. Fuzzy logic has been applied to many fields, from control theory

to artificial intelligence. Fuzzy logic had however been studied since the 1920s,

as infinite-valued logic—notably by Łukasiewicz and Tarski.

Humans and animals often operate using fuzzy evaluations in many everyday

situations. In the case where someone is tossing an object into a container from a

distance, the person does not compute exact values for the object weight, density,

distance, direction, container height and width, and air resistance to determine the force

and angle to toss the object. Instead the person instinctively applies quick “fuzzy”

estimates, based upon previous experience, to determine what output values of force,

direction and vertical angle to use to make the toss.

3.2.2 Applying truth values:

A basic application might characterize various sub-ranges of a continuous

variable. For instance, a temperature measurement for anti-lock brakes might have

several separate membership functions defining particular temperature ranges needed

to control the brakes properly. Each function maps the same temperature value to a

truth value in the 0 to 1 range. These truth values can then be used to determine how

the brakes should be controlled.

Figure

In this image, the meanings of the expressions

represented by functions mapping a temperature scale. A point on that scale has three

"truth values" — one for each of the three functions. T

represents a particular temperature that the three arrows (truth values) gauge. Since the

red arrow points to zero, this temperature may be interpreted as "not hot". The orange

arrow (pointing at 0.2) may describe it as "sligh

0.8) "fairly cold".

3.2.3 Linguistic variables:

While variables in mathematics usually take numerical values, in fuzzy logic

applications, the non-numeric are often used to facilitate the expression of

facts. A linguistic variable is

variables is that they can be modified via linguistic hedges applied to primary terms.

These linguistic hedges can be associated with certain functions. Fuzzification

operations can map mathematical input values into fuzzy membership functions. And

the opposite de-fuzzifying operations can be used to map a fuzzy output

functions into a “crisp” output value that can be then used for decision or control

purposes. Fuzzy logic is a logic having many values. Unlike the binary logic system,

here the reasoning is not crisp ,rather it is approximate and having a vague

The variables in fuzzy logic system may have any value in between 0 and 1 and hence

22

Figure 3.2 Fuzzy logic temperature control

In this image, the meanings of the expressions cold,


one for each of the three functions. The vertical line in the image



arrow (pointing at 0.2) may describe it as "slightly warm" and the blue arrow (pointing at


numeric are often used to facilitate the expression of

facts. A linguistic variable is young or its antonym old. However, the value of linguistic


can be associated with certain functions. Fuzzification


fuzzifying operations can be used to map a fuzzy output


Fuzzy logic is a logic having many values. Unlike the binary logic system,

here the reasoning is not crisp ,rather it is approximate and having a vague


cold, warm, and hot are


he vertical line in the image



tly warm" and the blue arrow (pointing at


numeric are often used to facilitate the expression of rules and

old. However, the value of linguistic


can be associated with certain functions. Fuzzification


fuzzifying operations can be used to map a fuzzy output membership


Fuzzy logic is a logic having many values. Unlike the binary logic system,

here the reasoning is not crisp ,rather it is approximate and having a vague boundary.


23

this type of logic system is able to address the values of the variables those lie between

completely truth and completely false.

The variables are called linguistic variables and each linguistic variable is

described by a membership function which has a certain degree of membership at a

particular instance.

System based on fuzzy logic carries out the process of decision making by

incorporation of human knowledge into the system. Fuzzy inference system is the major

unit of a fuzzy logic system. The decision making is an important part of the entire

system. The fuzzy inference system formulates suitable rules and based on these rules

the decisions are made. This whole process of decision making is mainly the

combination of concepts of fuzzy set thoery, fuzzy IF-THEN rules and fuzzy reasoning.

The fuzzy inference system makes use of the IF-THEN statements and with the help of

connectors present (such as OR and AND), necessary decision rules are constructed.

The basic Fuzzy inference system may take fuzzy inputs or crisp inputs depending upon

the process and its outputs, in most of the cases, are fuzzy sets. Fuzzy sets in X Fuzzy

sets in Y.

Figure. 3.3 A pure fuzzy system

24

The fuzzy inference system can be called as a pure fuzzy system due to the fact

that it takes fuzzy sets as input and produces output that are fuzzy sets. The fuzzy rule

base is the partresponsible for storing all the rules of the system and hence it can also

be called as the knowledge base of the fuzzy system. Fuzzy inference system is

responsible for necessary decision making for producing a required output. In most of

the practical applications where the system is used as a controller, it is desired to have

crisp values of the output rather than fuzzy set values. Therefore a method of

defuzzification is required in such cases which converts the fuzzy values into

corresponding crisp values. In general there are three main types of fuzzy infernece

systems such as :-

Mamdani model, Sugeno model and Tsukamoto model. Out of these three, Mamdani

model is the most popular.

3.3 PWM Pulse generator:

Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is

a modulation technique used to encode a message into a pulsing signal. Many digital

circuits can generate PWM signals (e.g., many microcontrollers have PWM outputs).

They normally use a counter that increments periodically (it is connected directly or

indirectly to the clock of the circuit) and is reset at the end of every period of the PWM.

When the counter value is more than the reference value, the PWM output changes

state from high to low (or low to high). This technique is referred to as time

proportioning, particularly as time-proportioning control– which proportion of a

fixed cycle time is spent in the high state.

3.4 UPQC(Unified Power Quality Conditionor):

Basically UPQC (Unified Power Quality conditioner) is a equipment which is used

for compensate for voltage distortion and voltage unbalance in a power system so that

the voltage at load side is completely balance and sinusoidal & perfectly regulated and

also it is used to compensate for load current harmonics so that the current at the

source side is perfectly sinusoidal and free from distortions and harmonics. UPQC is a

25

combination of a Shunt Active power filter and Series Active power filter. Here Shunt

Active power filter (APF) is used to compensate for load current harmonics and make

the source current completely sinusoidal and free from harmonics and distortions. Shunt

APF is connected parallel to transmission line. Here Series APF is used to mitigate for

voltage distortions and unbalance which is present in supply side and make the voltage

at load side perfectly balanced, regulated and sinusoidal. Series APF is connected in

series with transmission line. UPQC consists of two voltage source inverters connected

back to back through a DC link capacitor in a single phase, three phase-three wire,

three phase-four wire configuration. The inverter in shunt APF is controlled as a variable

current source inverter and in series APF is controlled as a variable voltage source

inverter. Earlier passive filters where also used for compensation of harmonics and

voltage distortion but due to their many disadvantages they are not used nowadays.

Figure 3.4 Simple UPQC Scheme

26

3.4.1 Series APF: In a transmission line series APF is generally connected in series. It is connected to

the transmission line with the transformer. Series APF is a voltage source inverter

connected in series with transmission line. It is used to compensate or mitigate the

problems which come due to voltage distortions and voltage unbalances. The series

APF injects a compensating voltage so that load voltage will be perfectly balanced and

regulated. Controlling of series inverter is done by PWM (pulse width modulation)

techniques. Here we used Hysteresis band PWM techniques as it implementation is

easy. Also its response is fast. Its details are explained in subsequent sections.

3.4.2 Shunt APF: In a transmission line shunt APF is generally connected in parallel. Shunt APF is

used to compensate for distortions & harmonics which are produced due to current. Due

to non- linear load there is harmonics in load current, so to keep source current

completely sinusoidal and distortion free we uses Shunt APF. Shunt APF injects

compensating current so that the source current is completely sinusoidal and free from

distortions. Controlling of Shunt APF is done by hysteresis band PWM techniques. In

hysteresis band PWM techniques output current follows the reference and current and

is within the fixed hysteresis band.

3.4.3 DC link capacitor: The two voltage source inverters are connected back to back through a DC

capacitor. DC capacitor is provides a DC voltage for working of both the inverter. The

DC capacitor also provides a real power difference between source and load during the

transient period and also acts as a energy storage elements. During steady state real

power supplied by source should be equal to the sum real power demand of load & a

small amount of power which compensates for active filter. DC capacitor voltage should

be equal to reference value but due to disturbance in real power balance between

source and load due to change in load conditions the DC capacitor value is changed

from reference value.

27

3.4.4 Design Of Upqc Controller:

The control algorithm for series APF is based on unit vector template generation

scheme where as the control strategy for shunt APF is discussed in this section. Based

on the load on the 3P4W system, the current drawn from the utility can be unbalanced.

In this paper, the concept of single phase P-Q theory. According to this theory, a single

phase system can be defined as a pseudo two-phase system by giving π/2 lead or π /2

lag that is each phase voltage and current of the original three phase systems. These

resultant two phase systems can be represented in α-β coordinates, and thus P-Q

theory applied for balanced three phase system can also be used for each phase of

unbalanced system independently. The actual load voltages and load currents are

considered as α-axis quantities, whereas the π/2 lead load or π/2 lag voltages and π/2

lead or π/2 lag load currents are considered as β-axis quantities. In this paper, π/2 lead

is considered to achieve a two-phase system for each phase. The major disadvantage

of p–q theory is that it gives poor results under distorted and/or unbalanced input/utility

voltages. In order to eliminate these limitations, the reference load voltage signals

extracted for series APF are used instead of actual load voltage unbalanced input/utility

voltages. In order to eliminate these limitations, the reference load voltage signals

extracted for series APF are used instead of actual load voltage

The electric power industry in the 21st century will see dramatic changes in both

its control and communication infrastructure. These changes are the result of mainly

three factors: 1) the push toward a deregulated industry, 2) the development of more

efficient and/or less polluting energy resources that are cost competi-tive with traditional

power generation sources, and 3) the continued electrification and integration of infor-

mation technology into most facets of our everyday lives has resulted in a need for a

better reliability and improved power quality than the existing power grid can supply. In

this context and with the progress of electronics component technology some efficient

solu-tions as the unified power quality conditioner (UPQC) are used. It consists of

combined series and shunt active filters for simultaneous compensation of voltage and

current.

28

The UPQC can compensate not only harmonic currents and unbalances of a

non-linear load, but also voltage harmonics and unbalances of the power sup-ply, which

improves the power quality offered for other harmonic sensitive loads. There are also

different custom power devices such as dynamic voltage restorer (DVR), which

improves the quality of power supply, distribution static compensator (DSTATCOM),

which compensates current unbalance and harmonics of non-linear loads, and

combined SVC with DSTATCOM, which generates reactive power and compensates

load current simultaneously.

The main objective of this work is the improvement of the performances of UPQC

with respect to the variation of the frequency. In order to achieve this goal, we present in

this paper, a new method based on GA which allows the determination of the optimized

parameters and consequently an optimal operating point of the system, a wide pass

band with a unity gain of both the ratio of the compensating voltage to the voltage

generated by the series PWM converter and the ratio of the compensating current to the

current generated by the shunt PWM converter, and a no load normal total impedance,

in other words without resonance. The results given by the GA method is com-pared

with the results given by the classical method, which uses iterative calculations under

certain conditions to find the optimal parameters that satisfy all constraints. The GA

method seems to be more efficient and gives better results. Therefore, the following

presented results are interesting and confirm the efficiency of the proposed new

method.

The equivalent circuit of UPQC that has been considered in this study is presented

in Figure 1. In this figure the primary side is the network side and the secondary side is

the PWM converter side.

29

3.4.5 Configuration of proposed UPQC:

The UPQC is utilized for simultaneous compensation of the load current and the

voltage disturbance at the source side. Normally the UPQC has two voltage-source

inverters of three-phase four-wire or three-phase three-wire configuration.

One inverter, called the series inverter is connected through transformers

between the source and the common connection point. The other inverter, called the

shunt inverter is connected in parallel through the transformers. The series inverter

operates as a voltage source, while the shunt inverter operates as a currentsource. The

UPQC has compensation capabilities for the harmonic current, the reactive power

compensation, the voltage disturbances, and the power flow control. However, it has

nocompensation capability for voltage interruption because no energy is stored. This

paper proposes a new configuration for the UPQC that has the super-capacitors for

energy storage connected to the dc link through the DC/DC converter. The proposed

UPQC can compensate the voltage interruption in thefundamental component of the A-

B transformed current, iα′ = sin(ω1t) and iβ′ = cos(ω1t) . The calculated active power S

p′ and reactive power S q′ includes the positive sequence fundamental component of

the source voltage vs .

30

Figure 3.5 Upqc System Inter Connected With Energy Storage

3.4.5 Circuit Description

Figure 3.6 Euivalent Circuit Diagram For Upqc

Taking the load voltage, VL as a reference phasor and suppose the lagging power

factor of the load is CosφL we can write

31

Where factor k represents the fluctuation of source voltage, defined as,

The UPQC is assumed to be lossless and therefore, the active power demanded by the

load is equal to the active power input at PCC. The UPQC provides a nearly unity power

factor source current, therefore, for a given load condition the input active power at PCC

can be expressed by the following equations,

The above figure shows the whole system which we developed using MATLAB Simulink

as tool.

32

Table 4.1 Whole system Parameter values

Serial No. Parameter Measurement Value

1 System Voltage 800V

2 Coupling Transformer 100:1

3 DC link Capacitor 1000 micro F

4 Volt measurement Components As Required

5 Current measurement components As Required

6 System Frequency 50Hz

CHAPTER4

SYSTEM MODEL & RESULTS

4.1 Simulink based system model: with UPQC

We have designed a simple model showing three phase transmission line with

line voltage measuring instruments and wave form viewer. A fault block is introduced in

order to examine the effect of fault on the system voltage. Here is the Simulink model

showing all the required parts of the power system. At end of the transmission line ac

voltage is converted to dc form only for better observation.

33

Figure 4.1 Simulink based syetm model: with upqc

34

4.2 Results: Comparison of Three phase voltage waveforms-with& without

UPQC

These are the result wave forms from an uncompensated line and compensated

line. Voltage sag is introduced in line by the fault box, effect of which is shown in fig 4.2

and in fig 4.3 the compensated voltage waveform is shown which is fully

compensated(i.e.,100% compensated) in three phase waveform presentation.

Figure 4.2 Result without UPQC(Voltage sag) Figure 4.3 Result with UPQC(No voltage sag)

4.3 Results: Comparison of dc voltage waveforms-with& without UPQC

Fig 4.4 and in Fig 4.5 the compensated and uncompensated voltage waveform in

reference to DC analysis respectively.

Voltage Voltage

Time Time

35

Figure 4.4 DC voltage without UPQC Figure 4.5 DC voltage with UPQC

4.4 Results: Comparison of Voltage waveforms under transients

With significant change in fault box it produces transient, and this transient is

introduced in the system line. Then in fig 4.6 and 4.7 the three phase voltage

waveforms under transients compensated and uncompensated is compared

respectively.

Time(ms)

DC volt

DC volt

Time(ms)

36

Figure 4.6 Transients without UPQC controller

Figure 4.7 Transients with UPQC controller

37

Individual analyzing and comparison of different results under different conditions like

voltage sags and transients, reveals that system is fully compensated (or 100%

compensated). Hence we can say that the system as a whole performed satisfactorily

(as expected).

CHAPTER-5

CONCLUSION & FUTURE SCOPE

In this chapter the general conclusions of this thesis and suggestions for future work

have been listed. The following conclusions have been drawn from this thesis.

5.1 Conclusion

Unified quality conditioner was studied and investigated in this thesis for power

quality improvement. UPQC is a type of advance hybrid filter which uses series APF for

removal of voltage related problems like voltage dip/rise, fluctutaion, imbalance and

shunt APF for removal of harmonics in current harmonics. What type of problems are

there in power quality was studied and discussed. UPQC system is developed and

discussed in detail. The simulink models of Shunt APF, Series APF, UPQC are

developed.

� UPQC model was developed by joining Shunt APF and series APF back to back

using DC capacitor. The controlling techniques used here are Fuzzy logic

controller.

� The simulation is done and voltage sag, transients are eliminated.

38

5.2 Future Scope

This model can be enhanced and enriched to terminate the power quality problems in a

power system. The various ways for doing that –

� The prototype of this UPQC model can be established in laboratory.

� Other soft computing techniques like Neural Network(NN), Genetic

Algorithm(GA), etc can also be used.

� We can connect wind turbines, solar energy system that is renewable source of

energy to UPQC to get improved power in consumer ends during serious

conditions

39

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