HARDWARE IMPLEMENTATION OF ADAPTIVE

RELAYING SCHEME FOR MICROGRID

PROTECTION

Supervisor

Engr. Jamil Ahmed

Developed By

Khawaja Ahmad Hassan FA11-EPE-039

Umer Mukhtar FA11-EPE-022

Abdul Majid FA11-EPE-007

DEPARTMENT OF ELECTRICAL ENGINEERING

COMSATS INSTITUTE OF INFORMATION AND TECHNOLOGY

ABBOTTABAD June 2015

ii

DISSERTATION

Presented to the Faculty of

COMSATS Institute of Information and technology Abbottabad

in Partial Fulfillment

of the Requirements

for degree of

Bachelors of Electrical Power Engineering

iii

IN THE NAME OF ALLAH THE MOST

GRACIOUS,

THE MOST MERCIFUL”

iv

APPROVAL

This is to certify that we have read the thesis submitted by Khawaja Ahmad Hassan, Umer

Mukhtar and Abdul Majid having Registration No. FA11-EPE-039, FA11-EPE-022, FA11-EPE-007 in the session FA11 2015. It is our judgment that this thesis is of sufficient standard

to warrant it acceptances by the COMSATS Institute of Information Technology, Abbottabad

for the Bachelor Degree in Electrical Power Engineering.

1. External Examiner __________________________________

Signature ____________

2. Internal Examiner ___________________________________

Signature ____________

Department of Electrical Engineering

COMSATS Institute of Information Technology,

Abbottabad.

3. Supervisor Engr. Jamil Ahmed_

Signature ____________

4. Head of Department Dr. Abdul Rasheed

Signature ____________

Department of Electrical Engineering

COMSATS Institute of Information Technology,

Abbottabad

v

DEDICATION

I dedicate this to my parents, teachers, friends and fellow members without whom it was

almost impossible for me to complete my thesis work.

vi

UNDERTAKING

It is certified that project work titled “Hardware Implementation of Adaptive Relaying

Scheme for Micro grid Protection” is our own work. No portion of the work presented in this

project has been submitted in support of another award or qualification either at this

institution or elsewhere. Material from other sources are referenced where used in this report.

Khawaja Ahmad Hassan

FA11-EPE-039

Umer Mukhtar

FA11-EPE-022

Abdul Majid

FA11-EPE-007

vii

ACKNOWLEDGMENTS

First and foremost, we would like to thank Almighty Allah for blessing us with knowledge, courage, wisdom, and enthusiasm to help us achieve our goals and to complete our final

year project successfully.

We are immensely thankful to our parents for their invaluable love and financial support.

The deepest and most gratifying words of thanks to our supervisor Engr. Jamil Ahmed for

his priceless guidance and stimulation throughout the course of the project, and having believe in us.

Last, but not the least we would like to express our earnest gratitude to our friends Nabil Shahid, M. Shoaib, M. Hamza Chaudhry and M. Arsalan for their support to make this

project possible.

Thank you all!

viii

ABSTRACT

Currently, there has been a significant interest in deployment of renewable and low carbon

energy sources in the form of microgrids. Microgrids operates in two modes – stand-alone and grid-connected mode. This poses a problem with the protection of microgrid because

the fault current magnitude reduces drastically with the change of mode from grid connected

to stand-alone mode. Adaptive relaying scheme is used for this problem. By this scheme relay can judge the mode of operation of the electric power system and accordingly Change

its setting to suit the expected fault current magnitude. This report demonstrates how hardware implementation of Adaptive Relaying scheme for microgrid protection are made.

Test cases are also conducted to validate the operation of Adaptive Relaying scheme. Tests shows that relays adjust its settings by sensing the mode of operating with a slight time

delay.

ix

Table of Contents

UNDERTAKING ................................................................................................................................................. vi

ACKNOWLEDGMENTS ................................................................................................................................... vii

ABSTRACT ....................................................................................................................................................... viii

Chapter 1 ....................................................................................................................................................... xiii

Introduction .................................................................................................................................................... xiii

1 BACKGROUND............................................................................................................................................. xiii

1.1 Dependence of Electrical Power Supply to Modern Society ........................................................................ xiii

1.2 Microgrid ..................................................................................................................................................... xiii

1.2.1 Why do we need Microgrids ..................................................................................................................... xiv

1.2.2 Benefits of Microgrids .............................................................................................................................. xiv

1.2.3 Microgrids and Renewable Energy ........................................................................................................... xiv

1.2.4 Microgrids for Community Heating and Cooling .................................................................................. xv

1.3 Modes of Microgrids ..................................................................................................................................... xv

1.3 Objectives of Microgrid Protection ............................................................................................................... xv

1.4 Functional Requirements of Protection Relay ............................................................................................... xv

1.4.1 Reliability ................................................................................................................................................... xv

1.4.2 Selectivity .................................................................................................................................................. xvi

1.4.3 Speed ......................................................................................................................................................... xvi

1.4.4 Sensitivity .................................................................................................................................................. xvi

1.5 Possible reasons of Faults ............................................................................................................................ xvi

1.6 Faults and abnormal operating conditions ................................................................................................... xvii

1.6.1 Shunt Faults (Short Circuits) .................................................................................................................... xvii

1.6.3 Symmetrical faults................................................................................................................................... xviii

1.6.4 Unsymmetrical faults .............................................................................................................................. xviii

1.6.5 Series fault ................................................................................................................................................. xix

1.7 Parameters Change during faults .................................................................................................................. xix

1.8 Interconnection between Microgrid and Utility Grid .................................................................................... xx

Chapter 2 ....................................................................................................................................................... xxi

Literature Review ......................................................................................................................................... xxi

2 Protective Relay .............................................................................................................................................. xxi

2.2 Over Current Relay ...................................................................................................................................... xxi

2.2.1 Instantaneous Over Current Relay ........................................................................................................... xxii

2.2.2 Definite time Over Current Relay ........................................................................................................... xxiii

2.2.3 Inverse Time over Current Relay ............................................................................................................ xxiv

2.2.3.1 Very Inverse Time OCR ...................................................................................................................... xxiv

x

2.2.3.1 Extremely Inverse Time OCR .......................................................................................................... xxv

2.3 Distance Relay ............................................................................................................................................ xxv

2.3.1 Impedance Relay ...................................................................................................................................... xxv

2.3.2 Admittance Relay ..................................................................................................................................... xxv

2.3.3 Reactance Relay ....................................................................................................................................... xxv

2.4 Circuit Breaker ........................................................................................................................................... xxvi

2.4.1 Operating Mechanism: ............................................................................................................................ xxvi

2.4.2 Factors responsible for Electric Arc .................................................................................................... xxvi

2.4.4 Arc extinction methods ........................................................................................................................... xxvi

2.4.5 Types of Circuit Breakers....................................................................................................................... xxvii

2.4.6 Comparison of Insulating Methods for CBs [13]: .................................................................................. xxvii

2.5 Sulphur Hexafluoride Circuit Breaker: .................................................................................................... xxviii

2.6 Isolators .................................................................................................................................................... xxviii

Chapter 3 ..................................................................................................................................................... xxix

3 Adaptive Relaying ............................................................................................................ xxix

3.1 Challenges faced in adaptive relaying ........................................................................................................ xxix

3.2 IDMT relay characteristics ......................................................................................................................... xxix

3.3 MICROGRID Test System and Utility Model ............................................................................................ xxx

3.4 Adaptive Relaying Scheme Relay Model................................................................................................... xxxi

Relay Settings .................................................................................................................................................. xxxi

Pick up current ................................................................................................................................................. xxxi

Plug Setting Multiplier .................................................................................................................................... xxxii

Time Setting Multiplier ................................................................................................................................... xxxii

Chapter 4 ................................................................................................................................................... xxxvi

4 Components used in the Project ....................................................................................................... xxxvi

4.1 ATMEGA 328 .......................................................................................................................................... xxxvi

4.2 DPDT Relay ............................................................................................................................................. xxxvi

4.2.1 Coil Data ............................................................................................................................................... xxxvi

4.2.2 Contact Data ......................................................................................................................................... xxxvii

4.2.4 LCD 20x4 ............................................................................................................................................. xxxvii

4.3 Full wave rectifier circuitry .................................................................................................................... xxxviii

4.4 LM 7805(VOLTAGE REGULATOR) .................................................................................................. xxxviii

4.4.1 PIN DESCRIPTION ............................................................................................................................. xxxix

4.5 DC Battery ............................................................................................................................................... xxxix

Chapter 5 ......................................................................................................................................................... xl

5 Results ............................................................................................................................................................ xl

5.1 Fault on Microgrid (DG1) side while in stand-alone mode ............................................................................ xl

5.2 Fault on Microgrid DG1 side while in grid-connected mode ........................................................................ xli

xi

5.3 Fault on Utility side while in stand-alone mode ........................................................................................... xlii

5.4 Fault on Utility side while in grid-connected mode ..................................................................................... xlii

5.5 Fault on Microgrid (DG2) side in stand-alone mode .................................................................................. xliii

5.6 Fault on Microgrid (DG2 side) while in grid-connected mode ................................................................... xliv

5.7 Conclusion ................................................................................................................................................... xlv

5.8 Reference ..................................................................................................................................................... xlv

xii

Table of Figures

Figure 1: L-L-L-G Fault…………………………………………………………….……..xviii

Figure 2: L-L-L Fault……………………………………………………………………...xviii

Figure 3: L-G Fault…………………………………………………………………….……xix

Figure 4: L-L Fault…………………………………………………………………….……xix

Figure 5: L-L-G Fault………………………………………………………………….……xix

Figure 6: Interconnection between Microgrid and Utility Grid……………………….…….xx

Figure 7: Time/Current Characteristics of Instantaneous OCR……………………………xxii

Figure 8: Time/Current Characteristics of Definite Time OCR……………………….......xxiii

Figure 9: Time/Current Characteristics of Inverse Time OCR……………………............xxiv

Figure 10: Circuit Breaker Operating mechanism………………………………………….xxvi

Figure 11: IDMT characteristic curve……………………………………………………....xxx

Figure 12: Microgrid Test system and Utility Model……………………………………….xxx

Figure 13: Adaptive relay Microcontroller model………………………………………….xxxi

Figure 14: Time/Plug Setting curve of IDMT Relay……………………………………..xxxiii

Figure 15: ATMEGA 328 Microcontroller……………………………………………….xxxvi

Figure 16: LM 7805 Voltage Regulator…………………………………………………xxxviii

Figure 17: 12 Volt DC battery………………………………………………………...…..xxxix

Figure 18: Fault at DG 1 while in stand-alone mode…………………………………………xl

Figure 19: Load of DG1 shifted to DG2 in stand-alone mode………………………………..xli

Figure 20: Fault on DG1 side while in grid-connected mode………………………………..xli

Figure 21: Fault on the utility side while in stand-alone mode……………………………….xlii

Figure 22: Fault on the utility side while in grid-connected mode………………………….xliii

Figure 23: Fault on DG2 while in stand-alone mode………………………………………..xliv

Figure 24: Fault on DG2 side in grid-connected mode, Load2 shifted to utility generator…xlv

xiii

Chapter 1

Introduction

1 BACKGROUND

The conventional way one would think of use electricity is to tap into the distribution system

connected to national or regional grid system. The power plants or power sources feed into

this power plant at same point in the grid. Let’s consider the example of automobile it has its

own independent power source (alternator) and the storage device (battery). These devices and

all the other control systems for these gives the power requirements of the automobile.

Now consider a house which has its own power source, a system for power storage and a

control system for automatically controlling and meeting the power requirements of the house.

That kind of system does not depend on the regional or national grid. Such type of distributed

system having its own power sources and their control devices are called microgrids.

1.1 Dependence of Electrical Power Supply to Modern Society

The modern society has come to depend heavily on continuous and reliable availability of

electricity and good quality of electricity too. Banking networks, railway networks, computer

and telecommunication networks and industrial sector are few functions that can’t work without reliable and continuous source of electric power. Moreover the mind boggling number

of domestic users will be thrown into darkness without electricity. This is a very serious matter.

So the importance of continuous and dependable supply of electric power to modern society

can’t be exaggerated.

No power system can be designed in such a way that it will never fail or a fault can’t occur on that system. However the consequences of fault can be minimize by isolating the faulty area

as quickly as possible and in time to prevent the whole system from going down. There should be some appropriate and reliable protection schemes for the system. Protection is an art, skill

and science of applying and setting relays in such a manner that any fault in a system can be

handled properly by the input values of current and voltage taken by the CT’s and PT’s respectively.

1.2 Microgrid

Microgrids are electricity distribution systems containing load and distributed energy resources such as (distributed generators, storage devices and controllable loads) that can

to be operated in controlled, coordinated way either while connected to main power network

[1].Microgrids usually employ global renewable sources and contribute considerably towards reduction of greenhouse gas emission and global warming. Microgrids can be used

to share load during peak hours in power system.

xiv

1.2.1 Why do we need Microgrids

The centralized transmission system is obviously very beneficial but it has its demerits as well.

The energy loss is almost 8 -10 % [8].

Investment costs are very high on the transmission system and transformers and

distribution systems, right of way and other legal issues.

Management of grid is a constant juggling act which requires balancing between the

demand and the generation of the electric power.

Investment cost also increases by managing the maximum demand of the consumer which

requires additional capacity of generation.

If there is any abnormality or fault all the users will feel frequency, voltage fluctuations

blackouts and may be brownouts. And this will badly effect the life of the electrical

equipment’s as well.

1.2.2 Benefits of Microgrids

The Microgrid, can’t be considered as a replacement for the national grid, but can be really

beneficial for the arears which have abundant renewable sources.

Financial commitments of microgrids are much smaller compared to main grids.

Because of the renewable resources microgrids are more environmentally friendly with

carbon footprints comparatively lower.

Another benefit of microgrid is that it’s operating is fully automatic and require fewer

technical skills.

Microgrids are isolated from any grid disturbance or outage. If running in grid-connected

mode can be isolated by proper protection scheme (Adaptive Relaying).

Microgrids place the consumer out of the grip of large corporations that run the generation

networks.

1.2.3 Microgrids and Renewable Energy

Microgrids are cost effective only if you can tap into locally available renewable energy

resources. Solar energy is available everywhere but with limitations. Wind, mini

hydro, geothermal and bio mass are regionally available and can augment Solar energy. This

combined with a storage device, battery or super capacitors and backup diesel generator makes

Microgrids highly reliable and cheap.

Storage devices in large grid systems are not economically or technical proven. The advent of

latest technologies in nano batteries and nano super capacitors makes electricity storage a

reality in the smaller capacity range [7]. This is an advantage for Microgrids.

Advances in computerized control technology make it possible to have simple and efficient

controls with less human interference and is the key ingredient that makes Microgrids feasible.

xv

1.2.4 Microgrids for Community Heating and Cooling

Microgrids are all the more efficient in communities and regions that require heating or cooling

apart from electricity. Heat from large thermal power plants go to waste because it cannot be

economically transferred large distances. In Microgrid communities, because of the limited

geographical distances it is possible to use the waste heat from the power generation sources

for effective heating or cooling using chillers.

1.3 Modes of Microgrids

Microgrids operates in two modes – grid-connected mode and stand-alone mode. In stand-

alone mode the microgrid is giving power to its specified load only, it is not connected to any other power generation plant. In grid-connected mode the microgrid is connected to the

utility grid by a point of common coupling (PCC) through some isolating device. [2][3]

1.3 Objectives of Microgrid Protection

The main objective of the microgrid protection scheme is to detect any fault within the

microgrid in both the modes of operation (stand-alone mode and grid-connected mode)

secondly to detect any fault in the grid and isolate the microgrid from the main grid through a point of common coupling PCC breaker or switch, and thirdly to synchronize the

microgrid with the utility grid when the fault is removed. [2][3]

1.3.1 Primary objective of Protection

A primary objective of all power systems is to maintain a very high level of continuity

of service, and when intolerable conditions occur, to minimize the outage times. Loss

of power, voltage dips, and over voltages will occur, however, because it is impossible, as well

as impractical, to avoid the consequences of natural events, physical accidents, equipment

failure, or mal operation owing to human error.

1.4 Functional Requirements of Protection Relay

1.4.1 Reliability

The most important feature of protective relay is its reliability. The relay is always sensing the parameters of the line to which it is connected, if a system remains in secure state it will not

operate but as soon as there is fault the protective relay must be reliable enough to sense that

fault and give the signal to circuit breaker to open that isolated area. Reliability means the relay must operate so that power system always remain in controlled and secure mode.

xvi

1.4.2 Selectivity

That primary protective relay should operates which is in the faulty zone. In power system we have many relays and there are protection zones. If a fault is occurred not all the relays should

operate, our protection system must be intelligent enough to make a decision that where is the fault occurred and which relay is to be operated. However we have a backup protection scheme

with a slight delay in its operating time; if primary relay fails to give signal to the CT then backup relay will operate to isolate the fault area from the whole system so that the

consequences of the fault may be minimized. This further improves the protection scheme and make it more reliable.

1.4.3 Speed

The protective relay’s operation must be speedy. If a fault occur and the operating time of the

relay is not fast this fault will be passed to the other system as well and its consequences will be very harmful for the whole system If the relay associated with the fault location can’t

operate in proper time then the backup relay for that location should operate but with a slight time delay because without the time delay there is a chance that both these relays will operate

at the same time.. Hence it should neither be too slow which may result in damage to the

equipment nor should it be too fast which may result in undesired operation.

1.4.4 Sensitivity

The relay should be sensitive enough to sense the fault in both modes of operation of

microgrid. Modes of operation of microgrid affects the sensitivity and performance of

overcurrent protection, because the fault current magnitude in the grid-connected mode is more than in standalone mode (fault current will be dedicated by the available generation).

To overcome this problem we are using Adaptive relaying scheme.

1.5 Possible reasons of Faults

Natural events that can cause short circuits (faults) are lightning (induced voltage or direct strikes), wind, ice, earthquake, fire, explosions, falling trees, flying objects, physical

contact by animals, and contamination [4].

Accidents include faults resulting from vehicles hitting poles or contacting live equipment,

unfortunate people contacting live equipment, digging into underground cables, human

errors, and so on.

Considerable effort is made to minimize damage possibilities, but the elimination of all such

problems is not yet achievable [5].

Most faults in an electrical utility system with a network of overhead lines are one phase-to-ground faults resulting primarily from lightning- induced transient high voltage and

from falling trees and tree limbs.

In the overhead systems momentarily tree contacts by the wind can be very dangerous.

xvii

Ice, freezing snow, and wind during severe storms can cause many faults and much damage.

Fault occurrence can be quite variable, depending on the type of power system (e.g.,

overhead versus underground lines) and the local natural or weather conditions

In many instances, the flashover caused by temporary events does not result in

permanent damage if the circuit is interrupted quickly.

A common practice is to open the faulted circuit, permit the arc to extinguish naturally, and

then reclose the circuit. Usually, this enhances the continuity of service by causing only a

momentary outage and voltage dip.

Typical outage times are in the order of 1/2 to 1 or 2 min, rather than many minutes and hours.

1.6 Faults and abnormal operating conditions

1.6.1 Shunt Faults (Short Circuits)

When load current path is cut short mainly due to insulation failure we say shunt or short circuit fault has occurred. The insulation may fail because of its weakening or by over voltage.

Weakening can be caused by following factors:

Temperature

Ageing

Snow

Rain

Chemical pollution

Other factors

Over voltage may be internal (by switching) or external (by lightening).

Normally faults that are caused by the breakdown of insulation are temporary like if a fault is occurred on the transmission line due to insulators, can be wet by the rain; an arc is produced

by the fault if this arc is allowed to deionize by disconnecting the supply for a short time then the arc will disappear after a short time. This process of interruption followed by unintentional

tripping is called reclosure. In low voltage supply systems three reclosure attempts are allowed but in extra high voltage system only one is allowed because reliability of the system is on

stake. In this project as we have medium voltage level so we are considering only two reclosure

attempts.

Short circuit can be at times entire or total that fault is called dead short circuit. If the fault by

passes the entire load current through itself then we say metallic fault has occurred. The arc has a resistance that is entirely nonlinear in nature [9]. One such widely used model is due to

Warrington, which gives the arc resistance as:

R arc =8750(𝑆 + 3𝑢𝑡)

(𝐼1.4)

xviii

S distance in feet

u is the velocity of air in mph

t time in seconds

I fault current in Amperes

1.6.3 Symmetrical faults

These are very severe faults and occur infrequently in the power systems. These are also called

as balanced faults and are of two types namely line to line to line to ground (L-L-L-G) and line

to line to line (L-L-L).

Only 2-5 percent of system faults are symmetrical faults. If these faults occur, system remains balanced but results in severe damage to the electrical power system equipment’s.

a b c

Zf ZfZf ZfZf

b

Zf

(a)LLLG Fault(b)LLL Fault

a c

Figure: 1 L-L-L-G Fault Figure: 2 L-L-L Fault

1.6.4 Unsymmetrical faults

These are very common and less severe than symmetrical faults. There are mainly three types

namely line to ground (L-G), line to line (L-L) and double line to ground (LL-G) faults.

Line to ground fault (L-G) is most common fault and 65-70 percent of faults are of this type.

It causes the conductor to make contact with earth or ground. 15 to 20 percent of faults are double line to ground and causes the two conductors to make contact with ground. Line to line

xix

faults occur when two conductors make contact with each other mainly while swinging of lines

due to winds and 5- 10 percent of the faults are of this type. These are also called unbalanced faults since their occurrence causes unbalance in the system. Unbalance of the system means

that that impedance values are different in each phase causing unbalance current to flow in the phases. These are more difficult to analyze and are carried by per phase basis similar to three

phase balanced faults [10].

a b c

Zf Zf

b

Zf

(a)LG Fault (b)LL Fault

a c

Zf

b

Zf

(c)LLG Fault

a c

Figure: 3 L-G Fault Figure: 4 L-L Fault Figure: 5 L-L-G Fault

1.6.5 Series fault

Series fault are nothing but a break in the path of the current. Such faults occurs when a broken conductor touches another conductor or grounded part. Open circuit faults are often very

dangerous like the secondary circuit of a current transformer and the field circuit of a dc

machine if open can leads to dangerous consequences.

1.7 Parameters Change during faults

System faults usually, but not always, provide significant changes in the system quantities, which can be used to distinguish between tolerable and intolerable system conditions.

These changing quantities include

Over current

Over or under Voltage

Power factor and current direction

Contamination of the insulating quantities

Temperature

xx

The most common fault indicator is a sudden and generally significant increase in the current;

consequently, overcurrent protection is widely used

1.8 Interconnection between Microgrid and Utility Grid

The PCC between the Microgrid and Utility Grid is very important as it links the two grids. The PCC can be a circuit breaker or a static switch. To link the two grids we must take into account the voltage and frequency. Both Microgrid and Utility Grid should have the same voltage and frequency otherwise there is a possibility that one grid can become load and other as source, and the grids can collapse [4].

Utility Grid Microgrid

Figure: 6 Interconnection between Microgrid and Utility Grid

In this project we have two distributed generators named DG1 and DG2 with their specified

loads and protective devices are installed on each line from DG to the bus bar to protect the microgrid. Utility Grid is also supplying the load which is referenced 3 and protective

devices are also installed on the line which is feed from Utility Grid see Fig 3.

Central Generation Distributed Generation

Static Switch

xxi

Chapter 2

Literature Review 2 Protective Relay

The IEEE defines a protective relay as “a relay whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate

appropriate control circuit action”.

2.1 Relay Operation Protection is the science, skill, and art of applying and setting relays or fuses, or both, to

provide maximum sensitivity to faults and undesirable conditions, but to avoid their operation

under all permissible or tolerable conditions [9]. The basic approach throughout this course is to define the tolerable and intolerable conditions that may exist and to look for definite

differences that the relays or fuses can sense.

A protective relay receives signal in the form of current and voltage from the CT and PT respectively. When the fault occur relay pickups the current and initiate its operation. Both

failure to operate and incorrect operation can result in major system upsets involving increased equipment damage, increased personnel hazards, and possible long interruption of

service.

2.2 Over Current Relay A relay that operates or picks up when its current exceeds a predetermined value (setting value) is called Overcurrent Relay [9]. The relay has two settings. These are the time setting and the

Plug setting. The time setting decides the operating time of the relay while. The plug setting decides the current required for the relay to pick up. The plug-setting multiplier PSM is defined as follows:

xxii

Psm =𝐼𝑟𝑒𝑙𝑎𝑦

PS

Where I relay the current through the relay operating coil & PS is the plug-setting of the relay.

The value of PSM tells us about the severity of the current as seen by the relay. At PSM less than 1 means that normal load current is flowing. At PSM > 1, the relay is supposed to pick

up. Higher values of PSM indicate how serious the fault is.[9].

2.2.1 Instantaneous Over Current Relay

As the name suggested this type of relay will operate as soon as the current exceeds the pick

up current. We can’t adjust its operating time. Its operating time is fixed. But practically some time is taken by the moving contacts to change their position, so it will take some time to

operate. Practically the operating time of the instantaneous over current relay is of the order of few milliseconds.

.

Figure: 7 Time/Current Characteristics of Instantaneous OCR [14]

Operates when current its pick up current value.

Criteria of it operation is only magnitude of the current not it’s direction.

Its operating time is constant.

No intentional time delay.

Coordination of definite-current relays is based on the fact that the fault current varies

with the position of the fault because of the difference in the impedance between the fault and the source

The relay which is at the farther most distance will operate at low current value because

the impedance is high at hence lower current there.

The operating currents are progressively increased for the other relays when moving

towards the source.

Instantaneous relay operates in 0.1s or less.

Application: Instantaneous over current relays are used in the outgoing feeders..

xxiii

2.2.2 Definite time Over Current Relay

This is a type of over current relay has time dial setting and pick up setting. Its operating time can be adjusted but up to a definite limit that’s why it is given the name Definite Time OCR.

As soon as the current exceeds the pick up value the relay after the given time delay will operate.

Figure: 8 Time/Current Characteristics of Definite Time OCR [14]

Operating time of Definite Time Over Current Relay is merely constant,

Above the pick up value of the current Definite Time OCR operation is not dependent

of current magnitude.

Two settings on it Time dial setting and pick up current setting.

Coordination is easy.

Operating time is independent of fault location and in feed variation.

Drawback of Definite Time OCR Relay

The continuity in the supply can’t be maintained at the load end in the event of fault

[11].

Time delay is provided which is highly undesirable as in short circuits.

When additional load is added it’s very difficult to coordinate them.

It is not suitable for long distance transmission lines where rapid fault clearance is

necessary for stability [11].

Application

Can be used as a backup protection for the distance relays in transmission line protection with time delay.

Back up protection of power transformer in the differential protection scheme with time

delay.

xxiv

Main protection to outgoing feeders and bus couplers with adjustable time delay

setting.

2.2.3 Inverse Time over Current Relay It operation is inversely proportional to the fault current. For more severe faults it operates in

less time and for less severe faults it operates in more time.

Figure: 9 Time/Current Characteristics of Inverse Time OCR [14]

T

Operates when current exceeds the pick up current.

Operating time depends on the magnitude of fault current.

For lower values of fault current it gives inverse time characteristic and for higher values of the fault current it shows definite time characteristics.

If the Plug setting is below 10 it shows inverse time characteristics and if between 10

to 20 then definite time characteristics.

Application

IDMT relays are used for protection of distributed lines.

For the protection of microgrids with adaptive relaying capability.

2.2.3.1 Very Inverse Time OCR

Has more inverse characteristics than IDMT relay.

When the distance from the source increases the fault current there is comparatively low very inverse time OCR is used.

Because of the steepness in the characteristics can be used for ground faults [11].

xxv

Very inverse overcurrent relays are particularly suitable if the short-circuit current

drops rapidly with the distance from the substation.

The grading margin can be reduced from 0.3sec to 0.4 sec when very inverse time OCR are used.

When fault current is dependent on fault location Very Inverse time OCR is used.

2.2.3.1 Extremely Inverse Time OCR Has more inverse characteristics from IDMT relay and very inverse time OCR.

Very handy for the protection of machines against overheating.

Its operating time is approximately inversely proportional to the square of the current.

The use of extremely inverse overcurrent relays makes it possible to use a short time delay in spite of high switching-in currents [11].

Application Suitable for protection of distribution feeders with peak currents on switching in

(refrigerators, pumps, water heaters and so on) [11].

Suitable for the protection of generators, transformer and very expensive cables.

2.3 Distance Relay Distance relays respond to the voltage and current, i.e., the impedance, at the relay location. The impedance per mile is fairly constant so these relays respond to the distance between the

relay location and the fault location [11]. As the power systems become more complex and the fault current varies with changes in generation and system configuration, directional

overcurrent relays become difficult to apply and to set for all contingencies, whereas the distance relay setting is constant for a wide variety of changes external to the protected line.

There are three general distance relay types [10]. Each is distinguished by its application and

its operating characteristic.

2.3.1 Impedance Relay The impedance relay has a circular characteristic centered at the origin of the R-X diagram [11]. It is non-directional and is used primarily as a fault detector.

2.3.2 Admittance Relay The admittance relay is the most commonly used distance relay. It is the tripping relay in pilot schemes and as the backup relay in step distance schemes. Its characteristic passes through the

origin of the R-X diagram and is therefore directional [10]. In the electromechanical design it is circular, and in the solid state design, it can be shaped to correspond to the transmission line

impedance.

2.3.3 Reactance Relay The reactance relay is a straight-line characteristic that responds only to the reactance of the

protected line. It is non-directional and is used to supplement the admittance relay as a tripping

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relay to make the overall protection independent of resistance [10]. It is particularly useful on

short lines where the fault arc resistance is the same order of magnitude as the line length

2.4 Circuit Breaker

A circuit breaker is a mechanical device which is capable of making breaking and carrying currents under normal conditions

2.4.1 Operating Mechanism:

Circuit breaker has two contacts called fixed contact and moving contact. Moving contact is

used to make & break the circuit using stored energies in the form of spring or compressed air. FC contains a spring which holds the MC after closing.

Circuit breaker consists of two coils:

Closing coil which is used for closing the circuit.

Trip coil used for tripping the circuit.

When a fault occurs in a circuit the trip coils of the circuit breaker get energized and the moving

contacts are pulled apart by some mechanism, thus opening the circuit.

Figure: 10 Circuit Breaker Operating mechanism [14]

2.4.2 Factors responsible for Electric Arc

Potential difference between the contacts.

Ionized particles between the contacts.

2.4.4 Arc extinction methods

Cooling the arc.

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Inserting medium of high dielectric strength.

2.4.5 Types of Circuit Breakers

Oil Circuit Breakers

Vacuum Circuit Breakers

Air Blast Circuit Breakers

SF6 Circuit Breakers

2.4.6 Comparison of Insulating Methods for CBs [13]:

Property Air Oil SF6 Vacuum

No of Operations Medium Low Medium High

Soft Break Ability Good Good Good Fair

Monitoring of Medium N/A Manual Test Automatic Not Possible

Fire hazard Risk None High None None

Health hazard Risk None Low Low None

Economical Voltage Range Up to 1kv 3.3-22kv 3.3-800kv 3.3-36kv

The following table highlights the features for different types of circuit breakers:

Factor Oil Breakers Air Breakers Vacuum/ SF6

Safety Risk of explosion & Fire due

to increase in pressure during multiple operations

Emission of Hot Air &

ionized Gas to the surroundings

No Risk of

Explosion

Size Quite Large Medium Smaller

Maintenance Regular Oil replacement Replacement of Arcing

Contacts

Minimum

lubrication for Control

Device

Environmental

Factors

Humidity & Dust can change the internal properties &

effect the dielectric

Since sealed

, No effect due to

environment

Endurance Below Average Average Excellent

Type Arc Quenching Medium Voltage Range & Breaking

Capacity

Air Circuit Breakers Air at Atmosphere pressure 400 – 11Kv 5 – 750MVA

Oil Circuit Breakers Transformer Oil 3.3Kv – 200Kv 150 – 25000MVA

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Vacuum Circuit

Breaker

Vacuum Oil 3.3Kv – 33Kv 200 – 2000MVA

SF6 Circuit Breaker SF6 at 5kg/cm2 pressure 3.3Kv – 765Kv 1000 – 50000MVA

Rated Voltage Choice of Circuit Breaker Remarks

Below 1KV Air Breaker CB -

3.3KV – 33KV Vacuum CB, SF6, Oil CB Vacuum Preferred

132KV – 220KV Air Blast CB, SF6, Oil CB SF6 Preferred

400KV – 760KV Air Blast CB, SF6, Oil CB SF6 Preferred

2.5 Sulphur Hexafluoride Circuit Breaker:

It contains an arc interruption chamber containing SF6 gas. In closed position the contacts

remain surrounded by SF6 gas at a pressure of 2.8 kg/cm2 .During opening high pressure SF6 gas at 14 kg/cm2 from its reservoir flows towards the chamber by valve

mechanism.SF6 rapidly absorbs the free electrons in the arc path to form immobile negative ions to build up high dielectric strength. It also cools the arc and extinguishes it. After operation

the valve is closed by the action of a set of springs. Absorbent materials are used to absorb the

byproducts and moisture.

Advantages of SF6 CB

Very short arcing period due to superior arc quenching property of SF6 .

Can interrupt much larger currents as compared to other breakers.

No risk of fire.

Less maintenance.

No over voltage problem.

There are no carbon deposits.

2.6 Isolators

Isolator (disconnecting switch) operates under no load condition. It does not have any current

breaking capacity or current making capacity. Isolator is not even used for breaking load currents. Isolators are used in addition to circuit breakers and are provided on each side of

every circuit breaker to provide isolation and enable maintenance.

2.6.1 Operation sequence

While opening –Open circuit breaker first and then isolators

While closing –Close isolators first and then close circuit breakers.

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Chapter 3

3 Adaptive Relaying

Adaptive relaying is a scheme that have the capability to adopt the system parameters if

they are changed. Adaptive relaying refers to the fact that setting of relays will be changed or adopted on-line to suit the status of power system at different times.

In this type of relaying technique developed to suit the fact that the power system conditions

are continuously changing leading to change in fault current levels and distribution in the network [5].

3.1 Challenges faced in adaptive relaying

The main challenge that engineers face while implementing the adaptive relaying is changing the relay setting to suit the load, generation level and topology changes.

Overcoming this challenge is what makes this type of relaying more effective.

There are usually three different methods to reach coordination of relays. These methods are

• Trial and error method

• Topological analysis method

• Optimization method

In optimization method the parameters of relay are set in a manner to optimize the protection

of grid.

One possible approach to achieve minimum shock to the system due to faults would be to minimize a sum of the operating times of all primary relays [6].

3.2 IDMT relay characteristics

Inverse definite minimum time relay is used for relay modeling in this project. This function also enables the user to approach the coordination problem as linear programming problem

[7].

𝑡𝑜𝑝 =0.14(𝑇𝑀𝑆)

(𝑃𝑆𝑀0.02 − 1)

t= trip time, TMS= Time Multiplier Setting, PS= Plug Setting. Adaptive overcurrent relaying

scheme, it is considered that the protection elements required are the same as the ones used in

a conventional grid, transducer, circuit breaker and relays. The only specificity is that the relays have to be microprocessor based digital relays with adaptive capabilities. This method uses the

overall impedance of the system to calculate the current through it, if impedance is too low

xxx

Op

era

tin

g T

ime

(Se

con

ds)

Plug setting multiplier(Relay current as a multiple of plug setting)

IDMTVery Inverse

Extremely Inverse

Figure: 11 IDMT characteristic curve [10]

and current too high, in other words fault is present, it will trigger the circuit breaker to open

and isolate the fault.

3.3 MICROGRID Test System and Utility Model

Figure 12: Microgrid Test system and Utility Model

Central Generation

10 MVA, 132KV, 50 Hz

Utility Transformer

132KV/11KV, 50 Hz

DG 1

20KV, 11 KV, 50 Hz DG 2

20 KV, 11 KV, 50 Hz

Load 1

Load 2

Load 3

C

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3.4 Adaptive Relaying Scheme Relay Model

Figure 13: Adaptive relay Microcontroller model

Relay Settings

Pick up current

Current at which relay operates is called pick up current of the relay. As relay moving parts are not allowed to move freely because there is some controlling force acting on the contacts

which may be gravitational, spring or magnetic force. This force is called controlling force. There is another force called deflecting force which acts in the opposite direction to the

controlling force. When current in the relay coil increases, it further increases the magnetic field intensity and in result deflecting force increase from controlling force. So relay operates

as contacts move from one position to another position. The current at which the relay initiates

its operation is called pick up current of the relay.

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The current setting of the relay is expressed in percentage ratio of pick up current of the relay

to the current transformer rated secondary current. Sometimes it is also called plug setting.

Current setting = Pick up current * 100 %

CT secondary current

Suppose you want your over current relay to operate at 150% of the rated current. If the relay

is rated with 1 A rated current and is equal to secondary rated current of the transformer. Then

the relay would operate when the pick up current becomes 1.5 A.

As per definition Current setting = 1.5 * 100 % = 150 %

1

Plug Setting Multiplier

It is the ratio of current in the relay coil to pick up current of the relay.

Plug setting multiplier = Fault current in the relay coil

Pick up current

Suppose the current transformer is rated 500/1. Then the CT rated secondary current would be 1 A. If the current setting is 150% then the pick up current would be 1.5 A. Now if fault occurs

and magnitude of the fault current in the current transformer primary be 2000 A then the relay coil current changes to 4 A. Then Plug setting multiplier for the relay would be 4 = 2.66 A

1.5

Time Setting Multiplier

The operating time of microcontroller based relay can be adjusted. If we only adjust time

setting multiplier of the relay we cannot set its actual operating time as it also depends upon

the fault current, so we can say that actual time also depends upon plug setting multiplier. For

getting clear idea, let us have a practical example. Say a relay has time setting 0.1 and you

have to calculate actual time of operation for PSM 10. From time / PSM graph of the relay as

shown in the figure 14. The operating time of an electrical relay mainly depends upon two

factors:

1. How long distance to be traveled by the moving parts of the relay for closing relay

contacts [10].

2. How fast the moving parts of the relay cover this distance [10].

So far adjusting relay operating time, both of the factors to be adjusted. The adjustment of

travelling distance of an electromechanical relay is commonly known as time setting. This

adjustment is commonly known as time setting multiplier of relay [10]. The time setting dial

is calibrated from 0 to 1 in steps 0.05 sec.

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Figure: 14 Time/Plug Setting curve of IDMT Relay [11]

we can see the total operating time of the relay is 3 seconds. That means, the moving parts of

the relay take total 3 seconds to travel 100% travelling distance. As the time setting multiplier

is 0.1 here, actually the moving parts of the relay have to travel only 0.1 × 100% or 10% of

the total travel distance, to close the relay contacts. Hence the actual operating time of the relay

is 3 × 0.1 = 0.3 sec. i.e. 10% of 3 sec.

It is assumed at medium voltage level [7], it takes at least 2 cycles to pick up the fault and

another 3 to 5 cycles taken by the circuit breaker to open. Collectively it takes 5 to 7 cycles

(0.1 to 0.14sec in a 50 Hz frequency system) from the time when fault occurs and the time

when it is cleared.

Time to clearance = t + relay delay = circuit breaker delay. With the below equation in mind

the trip time of the above relay is assumed to be 0.3 secs. The TMS is derived from the

following equation.

TMS =(𝑃𝑆𝑀0.02−1)

0.14

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The CT ratio is selected on the base of the nominal current flowing through the buses on which

relays are connected. The ratio of current transformer differ in both modes (stand-alone mode

and grid-connected mode). Current transformer ratio is smaller in stand-alone mode as less

current is flowing in the buses in stand-alone mode. In microcontroller based relays its ratio

can be changed in both the modes.

The plug setting is chosen using a thumb of rule as mentioned by Bedekar et al. in [8].

PS > 1.25* maximum load current

PS < 2/3 * minimum fault current

For stand-alone mode of operation of Relay 1 and Relay 2

Maximum load current= 30A

Minimum fault current= 1950A

Therefore,

PS > 1.25* (30/200)

PS < 2/3 * (1950/200)

0.19 < PS < 6.5

For stand-alone mode of operation of Relay 3

Maximum load current= 230 A

Minimum fault current= 4000A

Therefore,

PS > 1.25* (280/300)

PS < 2/3 * (4000/300)

0.96 < PS < 8.88

For grid-connected mode of operation of Relay 1 and Relay 2

Maximum load current= 280 A

Minimum fault current= 6000A

Therefore,

PS > 1.25* (280/400)

PS < 2/3 * (6000/400)

0.875 < PS < 10

For grid-connected mode of operation of Relay 3

Maximum load current= 450 A

Minimum fault current= 4000A

Therefore,

PS > 1.25* (450/600)

PS < 2/3 * (4000/600)

0.94 < PS < 4.44

From the above calculations we assumed the Plug setting of 1 because it satisfy all modes of operation of all the three relays.

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Relay 1 Parameters

Stand-Alone

Mode

Grid-Connected

Mode

CT ratio 400:1 200:1

TM 0.1 0.1

PS 1 1

Relay 2 Parameters

Stand-Alone

Mode

Grid-Connected

Mode

CT ratio 400:1 200:1

TM 0.11 0.12

PS 1 1

Relay 3 Parameters

Stand-Alone

Mode

Grid-Connected

Mode

CT ratio 600:1 300:1

TM 0.1 0.11

PS 1 1

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Chapter 4

4 Components used in the Project

4.1 ATMEGA 328

ATMEGA 328 microcontroller is used to model the parameters of the system and to make the

system highly automatic.

High Performance, Low Power Atmel®AVR® 8-Bit Microcontroller Family.

Operating Voltage: ̶ 1.8 - 5.5V

Temperature Range: ̶ -40 C to 85 C

28-pin PDIP

Arduino ATMEGA 328 IC is used. In this IC the code logic is burned.

Figure 15: ATMEGA 328 Microcontroller [9]

Crystal oscillator of 16 MHz is used to provide oscillation to the controller and these oscillations are smoothen by two capacitors of 22 microfarad.

4.2 DPDT Relay

Double Pole Double through relay is used for shifting the load of the faulty grid to other grid having no fault assuming the later grid has the capacity to meet its load. DPDT has eight

terminals. When the relay coil gets energized with 75mA current minimum then the normally open (NO) and common (COM) are connected. When relay receives no power then normally

closed (NC) and common (COM) gets connected.

4.2.1 Coil Data

Coil Power: 1100 mW

Nominal Voltage: 12Vdc

Pick-Up Voltage: 9.6Vdc

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Drop-Out Voltage: 1.2Vdc

Maximum Voltage: 13.2Vdc

Coil Resistance: 160ohm

4.2.2 Contact Data

Contact Rating: 10A 250Vac / 30Vdc

Maximum Switching Voltage: 250Vac / 30Vdc

Maximum Switching Current: 10 A [9]

Maximum Switching Power: 2500 VA / 300 W

Mechanical Endurance: 10,000,000 Operations

Electrical Endurance: 100,000 Operations

Maximum Turn-On Time: 25 ms

Maximum Turn-Off Time: 25 ms

4.2.3 General

Dielectric Strength (Between Contacts): 1500 VAC 1 min

Insulation Resistance: 500 Mohm@500 VDC

Ambient Operating Temperature Range: -40°C to +70°C

Weight: 37g

4.2.4 LCD 20x4

LCD is used for displaying the results of the test cases and the parameters of the lines.

LCD Pin Function of LCD Pin Arduino Pin

1 Gnd Gnd

2 +5V 5V

3 Contrast NOT CONNECTED

4 RS (Register Select Pin) 12

5 R/W (Read/Write Pin) Gnd

6 E (Clock Enable Pin) 11

7 D0 NOT CONNECTED

8 D1 NOT CONNECTED

9 D2 NOT CONNECTED

10 D3 NOT CONNECTED

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11 D4 5

12 D5 4

13 D6 3

14 D7 2

15 Anode (for backlight) 5V

16 Cathode (for backlight) Gnd

4.3 Full wave rectifier circuitry

We have made a full wave rectifier circuit using 4 diodes (N14007) and a ripple or filter

capacitor is also used. The output from the current transformer is analog in nature it is then rectified through full wave rectifier and fed to the microcontroller and this signal is then

converted to digitized form through built in, Analog To Digital Convertor channel in library of the ATMEGA 328.

4.4 LM 7805(VOLTAGE REGULATOR)

Figure 16: LM 7805 Voltage Regulator [3]

7805 is a voltage regulator integrated circuit.

It is a member of 78xx series of fixed linear voltage regulator ICs.

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The voltage regulator IC maintains the output voltage at a constant value.

4.4.1 PIN DESCRIPTION

Pin

No

Function Name

1 Input voltage (5V-18V) Input

2 Ground (0V) Ground

3 Regulated output; 5V (4.8V-5.2V) Output

We have used LM 7805 for the voltage regulation. The input of 12 volts is given to the

regulator, on output we are getting 5 volts which are given to the microcontroller. Capacitor as a filter is used with the voltage regulator to remove any noise of distortion (100

microfarad). A diode is also used for blocking the voltage back to the battery.

4.5 DC Battery

12 volts DC battery is used to provide supply to the control devices like microcontroller. It will

give back up to the control network in case where main supply is disconnected by any fault or abnormal condition.

Figure 17: 12 volt DC battery [10]

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Chapter 5

5 Results Six test cases from test 5.1 to 5.6 have been carried out to check and validate the performance

of adaptive relaying scheme as described below.

5.1 Fault on Microgrid (DG1) side while in stand-alone mode

Consider figure 12 the static switch is open so the system is running in the stand-alone mode.

Now if fault 3 phase fault occurs at DG 1 then then the relay R1 and R2 sees the fault current

feeds through DG2 as DG2 is also connected through the bus 3.The fault current magnitude and it’s the operating time of the relay is also measured. The coordination between the relay R1 and

R2 is such so they have some difference in their operating time. The fault current at point A is measured to be 2400 Amperes for which the operating time of R1 and R2 is 0.3 and 0.35 seconds

respectively.

A Mimic is shown below which has one central generating system and two microgrids named Distributed generator 1 and Distributed generator 2 and loads which are shown by three green

panel lights respectively. The equipment’s used in this Mimic are of very low ratings so we graded the results according to the simulation results. After the occurrence of the fault as shown in figure

17 our system sense the load of the DG2 and if the supply of DG2 can handle the load of DG1 then this load is shifted to DG2 as shown in the figure 18. Relay R1 operates and gives signal to

the circuit breaker CB1 and hence the load of DG1 now shifted to DG2.

Figure 18: Fault at DG 1 while in stand-alone mode

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Figure 19: Load of DG1 shifted to DG2 in stand-alone mode

5.2 Fault on Microgrid DG1 side while in grid-connected mode

The static switch is closed and the system is running in interconnected mode. The loads are shared by all the three generation units. Now if a three phase fault occur at point A as shown

in figure 12, then Relay R1 sees the fault current feed through DG2 and utility generator and Relay R2 sees the fault current feed through only DG2. The fault current magnitude is 7500A.

The operating time for relay R1 is 0.1sec and for relay R2 is 0.15 sec respectively. In grid-

Figure 20: Load of DG1 Shifted to Main grid in grid -connected mode

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Connected mode the load is now shifted to main grid as the capacity of utility is generator is

more than DG2.

5.3 Fault on Utility side while in stand-alone mode

A three phase fault is occurred at point B as shown in the figure 12. The static switch is open,

so both the DG’s are not affected by this fault. Fault is only fed through the utility generator which is seen by the Relay R3. The fault current magnitude is 7000A and for this the trip time

for relay R3 is 0.18 sec. The relay R3 gives the signal to the circuit breaker C3 to open and thus disconnects the line. It is to be noted that load 3 is now off because the system is running

in stand-alone mode.

Figure 21: Fault on the utility side while in stand-alone mode

5.4 Fault on Utility side while in grid-connected mode Now the static switch is closed and the system is running in grid-connected mode. A three phase fault occurred at point B as shown in the figure 12. The relay R3 sees fault fed through

Utility generator, DG1 and DG2. The magnitude of the fault current is 10000A which is very high as the fault is fed through all the three generators. The operating time for R3 will be 1.1

seconds. The fault is very dangerous as the system is running in grid-connected mode and fault current magnitude is very high too. Relay R3 has IDMT characteristics and operates very

quickly to isolate or disconnect the line. Power needed for load 3 is very high and it can’t be meet by DG1 and DG2, so the load is off in this case too. It is shown in figure 21.

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Figure 22: Fault on the utility side while in grid -connected mode

5.5 Fault on Microgrid (DG2) side in stand-alone mode

Figure 23: Fault on DG2 while in stand-alone mode

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A three phase fault is occurred at point C as shown in the figure 12. Relay R2 sees the fault

current fed through the DG1 as the static switch is open not utility generator is not connected. It is to be noted that the rating of DG1 and DG2 is same so if a fault has occurred at point C

then the load 2 can’t be shifted on DG1. The magnitude of the fault current is 3000A, and for this the operating time of R2 is 0.3 sec. It is shown in the figure 22.

5.6 Fault on Microgrid (DG2 side) while in grid-connected mode

The static switch is closed and the system is running in grid-connected mode. A three phase fault occurred at point C as shown in the figure 12. The fault current is fed through DG1 and

main generator. The magnitude of the fault current is 6000A and for this operating time of relay R2 is 1.8sec. Now it is considered that main generator has the additional capacity to meet

the DG2 load also. So as the system was inn grid-connected mode, the load 2 is shifted to main

or utility generator. The DG2 is isolated. As shown in the figure below when fault occurred at point C the relay R2 after a time of 1.8 seconds gives signal to the circuit breaker C2 to open

the line and DG2 is disconnected from the rest of the system. Adaptive relaying scheme senses the mode of operation and gives a result that utility generator power is enough to meet load2.

So it is shifted to Utility generator. Another advantage of this scheme is as soon as the fault is cleared the load 2 will again be shifted to DG2.

Figure 24: Fault on DG2 side in grid-connected mode, Load2 shifted to utility generator

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5.7 Conclusion

As can be seen from tests 1 to 6, the fault currents in microgrids differs with the mode in which they are operating .In grid connected mode the fault current is generally higher than

that in stand-alone mode of operation. This is because more sources contribute to the fault current in the grid-connected mode. This influences the protection system used in

microgrids and in utility grids connected to micro-grids. The relay model developed using Arduino is based on the functionality of digital relays. Unlike electromagnetic relays, digital

relays have options such as mode of operation. So by Adaptive relaying scheme we can make protective system more intelligent; can sense the mode of operation of system and

adopt the respective changes.

5.8 Reference

[1] building-microgrid.lbl.gov/about-microgrids.

[2] J. Blackman and T. Domin, Protective Relaying Principles and Applications, 3rd Edition USA: CRC Press, 2007

[3] S. Chowdury, S. P. Chowdury and P.Crossley, Microgrids and active distribution networks, UK; the IET (UK), 2009

[4] D. Dondi, D. Dayoumi, C. Haederli, D. Julian and M. Super, “Network Integration of Distributed Power Generation”, Journal of Power Sources, Vol 106, Issues 1-2, pp.129, 2002

[5] J.D. Codling, S.A. House, J.H. Joice, K.M. Labhart, J.R.Richards, J.E. Tenbush,

“Adaptive Relaying” – “A new direction in Power System Protection” IEEE

potentials, Vol.15, Issue 1, pp.28-33, Feb/March 1996

[6] D. Naulhong, R, Wissanuwong and B. Pongsiri, “Optimal coordination of directional over current relays for back up distance relays using a hybrid genetic algorithm”, in

Proc. 2009 AESIEAP CEO Conference.

[7] S.Chowdhury, S.P. Chowdhury and P. Crossley, Microgrids and Active Distribution Networks, UK: The IET (UK), 2009

[8] http://www.instructables.com/id/78xx-Regulators-Ics/ ’ic picture lm 7805

[9] http://electrical-engineering-portal.com/distance-relays

[10] http://www.electrical4u.com/

[11] www.examgeneral.com/public/images/data/11316_8723.xm