TIME AND CURRENT GRADING FOR PROTECTION RELAY

TYPE MK1000 & MK2000

MOHAMAD NAIM B. MOHAMAD

This report is submitted in partial fulfillment of the requirements for the

Bachelor of Electrical Engineering (Industrial Power)

Faculty of Electrical Engineering

Universiti Teknikal Malaysia Melaka

ABSTRACT

The main purpose of this project is to investigate the characteristic time grading

and current grading using the inverse definite minimum time (IDMT) protection relay

type MK1000 and MK2000.Generally, time grading can be define as the minimum time

to carry the fault current before nearest protection relay disconnect the supply in circuit.

Current grading can be used to obtain the correct discrimination in the circuit where it

will be a large different ratio of fault current to rated current in the network. Meanwhile

the other objectives of this project are to develop a proper test procedure for the time

grading and current grading. Beside that computer simulation can be done by using the

ERACS software in order to verify the testing results. This project also develops the

current injection set with using the autotransformers and toroidal core as the main

element. The main purpose of this current injection to produce secondary current around

100 ampere. At the end this project, this research can help the student to understand the

basic operation time grading and current grading in the operation protection relay

especially the IDMT operation in the low voltages system.

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ABSTRAK

Tujuan utama projek ini dijalankan adalah untuk mengenal pasti ciri-ciri yang

terdapat pada “current grading” dan “time grading” yang digunakan dalam pemasangan

geganti perlindungan didalam sistem elektrik. Secara umunya “current grading” dapat

diterangkan dengan masa yang diambil paling singkat yang diambil untuk mengesan

arus kegagalan didalam sistem sebelum geganti perlindungan yang paling hampir

memutuskan bekalan kuasa didalam sistem. “Current grading” pula boleh digunakan

untuk mendapatkan diskriminasi betul dalam sistem di mana arus kegagalan dan arus

didalam sistem akan menjadi satu nisbah yang besar. Selain daripada itu, objektif lain

projek ini adalah menyiapkan kaedah pengujian yang melibatkan “current grading” dan

“time grading”. Projek ini juga akan melibatkan penggunaan perisian ERACS dalam

membuktikan praktikal dan teori adalah seiring. Selain daripada projek ini juga

merekabentuk alat suntikan arus kedua yang akan diguna pakai dalam menjalankan

eksperiment nanti. Dalam mereka bentuk alat suntikkan arus kedua, dua kompenen

utama akan digunakan iaitu “toroidal core” dan pengubah automatik. Diakhir projek ini,

diharap kajian yang dilakukan ini dapat membantu pelajar untuk lebih memahami

tentang “current grading” dan “time grading” dalam yang melibatkan sistem

perlindungan terutama yang melibatkan geganti jenis IDMT didalam sistem voltan

rendah.

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CHAPTER I

INTRODUCTION

1.1 Introduction

The main purpose of this project is to investigate the characteristic time grading

and current grading using the inverse definite minimum time (IDMT) protection relay

type MK1000 and MK2000.Generally, time grading can be define as the minimum time

to carry the fault current before nearest protection relay disconnect the supply in circuit.

Current grading can be used to obtain the correct discrimination in the circuit where it

will be a large different ratio of fault current to rate current in the network. Meanwhile,

the other objectives of this project are to develop a proper test procedure for the time

grading and current grading. Beside that computer simulation can be done by using the

ERACS software in order to verify the testing results. This project also develops the

current injection set with using the autotransformers and toroidal core as the main

element. The main target of this current injection set is to produce secondary current

around 100 amperes. At the end this project, this project can help the students to

understand the basic of operation time grading and current grading in the operation

protection relay especially the IDMT operation in the low voltages system.

1.2 Project Objective.

The main purposes of this project are:

Analysis characteristic time grading and current grading

Simulate the current grading and time grading characteristic for IDMT (inverse

definite time minimum) relay type MK1000 & MK2000

Produce 100 ampere current output in secondary current injection set. By using

toroidal core and autotransformers as main element.

1.3 Problem Statement.

Problem related with the project are:

Define the best discrimination by using time or current and both

Discriminate the healthy and faulty equipment or the circuit by current, time and

both current and time.

Define the minimum time interval between time setting of adjacent circuit

breaker to prevent the simultaneous operation

Define the time grading and current grading characteristics.

1.4 Scope

Scopes of this project research are:

This project only related 3 phase and single phase (415 and 240) protection

system

The type protection relay will be use MK1000 and MK2000

Secondary injection test on protection relay with using 100ADM MK 2

2

1.5 Expected Results

The expected results from the project are:

Can be define the characteristic time grading and current grading

Secondary current injection set can be operate smoothly

The simulation results are able to verify the hardware operation when the

hardware implemented soon.

3

CHAPTER II

LITERATURE REVIEW

2.1 Introduction

This project research will discuss the principle of coordination protection system

in the system. Coordination protection relay can be classified by three types namely

discrimination by time, discrimination by current and discrimination current and time or

both. The main common aims for the three methods are to give the correct coordinating

relay in system. Beside that, the others target of this project research to increase the

general knowledge about protection relay such as the circuit breaker, time grading,

current grading and utilizing protection relay among the student and fresh engineer

especially related with the protection system.

2.2 Relay as Protection Devices

Relay has been designed to measure or receive the signal in others word to make

it operate or send the signal to another device such as circuit breaker. Protection relay

will operate when the abnormal condition occurs in the system and protection relay will

provide signal at circuit breaker to trip the circuit. The circuit breaker will open and the

fault area will be isolate to minimize fault current. [3]. Nowadays, relay can be classified

by three type such as electromechanical, static and microprocessor relay.

Figure 2.1 Basic Relay

Electromechanical relay can be dividing into two important parts called as

“moving” and “static”. When the flow current equal to the setting value, torque will

occurs in the moving part and will touch the static part hence the connecter will close

and circuit breakers will trip. Electromechanical relay is easy to maintenance and the

cost installation is cheaper than others relay.

5

Figure 2.2 Electromechanical Relay

Another relay that based on electromechanically operation is “induction relay”,

“attracted-armature relay”, “moving–coil relay”, “thermal relay” and “timing relay”.

Induction relay have been apply to watt-metric relay, KVAR relay, over current relay

and over voltages or under voltages relay. Inductions relay construction based on

kilowatt meter. Attracted armature relay construction is based on electromagnet element

and it used to connect the armature. Induction relay normally use as “all-or-nothing

relay” and “measurement relay”. Thermal relay is operated based on the heat. Thermal

relay has been used as an overload relay because it has more accuracy and has a long

delay time. Timing relay is using in protection system and can be classified into three

groups such as short circuit relays, medium-value accurate-time delays and long time

relays.

Static relay is usually using in electronic equipment. This relay use electronic

circuit as a base circuit to operate. Static relay was used in power protection circuit

because it can protect the sensitive equipment from fault current. However this relay is

more expensive than electromechanical relay. Beside that, it is easy to maintenance and

has the long live time.

6

Figure 2.3 Block Diagram Static Relay [4]

Figure 2.4 Block Diagram Microprocessor Relay [5]

Relay based on microprocessor is a relay where using the software and operate

based on binary code. The different of this relay is programming has been used and the

basic concept from the conventional relay has been changed for comparison and the

binary code. Relay type microprocessor can store the fault of data that already happened

previously.

2.3 Protection System with using Single Processor for Low Voltages

7

Single Processor is use the network of communication system. This network

functions to analyze data from equipment by separate. All data will send to main

computer. In this network, main computer will send the signal to the devices and finally

the signal has been prescribed by protocol. System communication type Ethernet include

protocol collision-detection multiple-access (CSMA/DA) has been used in this system.

[4].This concept was operating by sending data from the device to main processor. The

sending data include the voltage, current and devices status. In the network,

microprocessors will used to analyze data and make the decision related to the system.

The advantage of this system is the processor can detect all information related to the

system. Protection system and controller was design depend on electrical signal like

current magnitude and phase angle. As a conclusion this system has a lot of advantages

on protection system where the extra protection to feeder device through the main device

on current executioner without affects the system.

2.4 Discrimination by Time

Discrimination by time is the basic idea to add time lag features to control the

relays of the number circuit breaker in the protection system. Discrimination by time can

be defined as the nearest circuit breaker to the fault will operate first. The minimum time

interval between time settings of adjacent circuit will be considering before setting the

time to prevent the simultaneous operation [1].The minimum time interval is required to

the breaker to clear the fault in the system. Besides, the minimum time interval setting

must be match with the relay and the current transformers (CT) [1]. Discrimination by

time has a big problem where the problem occurred from discrimination by time will

increase in time fault clearance. The time will be increase if the fault occurs closed at the

power source especially when MVA rating value is highest. [2]

8

Figure 2.5 Radial Systems with Time Discrimination

The figure shows the system been provided at the circuit breaker for each section

L, K, J, and H in the feed of the power system. Each protection system have been

comprises by inverse definite minimum time (IDMT) over-current relay in which the

operation of the current sensitive element simply initiates the time delay element.

Provide the current element is setting below the fault current where this element plays

the no part in the achievement of the discrimination. It is because the relay sometimes

described as an independent definite time delay relay since its operating time is for

practical purposes independent of the level of over-current. It is time delay element

therefore which provides the means discrimination. The relay at zone H is set at the

shortest time delay permissible to allow a fuse to blow for a fault on the secondary side

of transformer at zone G.

2.5 Discrimination by Current

Discrimination by current relies on the fact that the fault current varies with the

position of the fault. To discriminate by current the value of impedance must be

considered. Relay in the system was controlling the various circuit breaker where are set

to operate at suitable tapered values such as nearest relay at the fault to trip the breakers

[1].Discrimination by current is not practical this because the impedance between two

circuit breaker must be concern to get the correct coordination[2]

2.6 Discrimination by both Current and Time

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Discrimination by current and time is combination by current and time where the

time is inversely proportional to the faulted current level and the actual characteristic is a

function of both “current” and “time” setting [1].For example when the faulted current is

high level the circuit breaker will take the shortest time to clearance the fault current in

the system. It will be more efficient. In order to carry out a system analysis before relay

coordination all impedance must be calculate. Discrimination by both is the best way in

relay coordination relay in the system.

2.7 Summary

In this has been discussing basic operation of relay as protection device. Beside

that, these chapters also discuss the coordination the relay in systems. Before

coordination relay is applied to the system, we must know the basic discrimination for

the system. Discrimination can be classified by three ways, co-ordination by time, and

current, both.

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CHAPTER III

THEORY

3.1 Introduction

Power system has been used to supply the electrical energy to customer with

efficient and the energy called as a burden. It has two main components called as real

power and reactive power during the sending of electrical at home. This energy was

delivered by used the transmission line.

The power system was provided to the customer by using three phase system and

single system depends on their load. In Malaysia, “Suruhanjaya Tenaga” had decided

where the power factor is 0.85 to each customer. During the electrical installation the

contractor has considered the voltage drop, current, cost, protection system and the load

that will be used in the future.

Electrical installation in industry and the building that using a lower voltage 415

V need a matching design. Each design is refer to the safety aspect, practical, effective

cost, efficient and follow each rules has been decided by government.

11KV Underground Cable

415V Underground Cable

415V Underground Cable &

Cable in Building

415V/240 V PVC Or

PVC/PVC Cable

415V/240 V PVC Or

PVC/PVC Cable

Figure 3.1 Low Voltage Distributions Systems

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TNBSubstation

ElectricSubstation

Main Switch Board

Small Switch Board

DB Board

Load

3.2 Faults in Electric System

In the electric system, fault will obtained at the point in the power system or

equipment. The fault can be divided into several types and the fault can destroyed the

system, equipment and user.

• Three phase fault - short circuit among the three phase supply or fault among

three phases with the earth.

• Fault line to earth – short circuit between phases with the earth.

• Fault line to line – short circuit among two phases line.

• Fault two lines with the earth – short circuit among two phases with earth.

3.3 Protection Relay

Protection relay is devices that use to measure the level of damage that happen in

the system. Beside that, the protection relay also used to control the operation of the

circuit breaker. If the leakage of current occurred in the circuit for a short or temporary

time, the protection relay will send the signal to circuit breaker. If over current or the

earth leakage occurs during the same time and this situation can damage the system and

equipment, protection relay will send the signal to circuit breaker to operate.

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3.4 Relay Coordination

Coordination was indicated in protection system and relay coordination. Correct

current relay application requires knowledge of the fault current that flow in each

part on of the network. Since large scale impracticable, system analysis must be

used. The data requires for a relay setting study are [1]:

• A one line-diagram of the power system involved, showing the type and

rating of the protective devices and their associated current transformers.

• The impedances in ohm, percent or per unit of all power transformers,

rotating machines and feeder circuits

• The maximum and minimum values of the short circuits current that are

expected to flow through each protective device.

• The starting current requirement of motor and the starting and stalling time of

induction motors.

• Maximum peak load current through protective devices

• Decrement curves showing the rate of decay of the fault current supplied by

the generators

• Performances curves of the current transformers.

Relay coordination use in relay installation by using the discrimination concept. The

basic rules for correct relay coordination are:

• Use same relay operating characteristic in series for each others.

• Ensure the relay farthest from the source has current setting equal to or less

than relays behind, that the primary current required to operate the relay in

front is equal to or less than the primary current required to operate relay

behind it.

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Discrimination concept can be explained by the nearest relay where the fault zone will

operate first and separated the circuit to smallest fault zone. By using discrimination

concept, the fault occurs in shortest time hence the system and the user will be avoided

from the accident. Discrimination can be achieved by three ways:

• Discrimination by time

• Discrimination by current

• Discrimination by both (current and time)

The main purpose of the discrimination is to give the correct discriminating with

protection relay installation in the system.

3.4 Inverse Definite Minimum Time (IDMT) Protection Concept

Inverse time means when the current increase, time will be decrease. It can be

achieved by the current in the circuit is high the protection relay will take the shortest

time to operate. Definite Minimum Time can be explain when the fault current is high,

the protection relay will take shortest time to operate and the protection relay has a fixed

time depend on the time delay setting.

Fi

g ur

e

3.2 IDMT Construction

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3.5 Determine the size of bus bar comply with the current rate

However if the busbar sizes are not specified, then the busbar rating shall be

based on a current density of not more than 1.5 A/sq. mm. in nay case, the busbar rating

shall not be less than the rating of the incoming switchgear. All busbar either horizontal

or vertical shall be of the same size. The main busbar shall be run for the full length of

the switch board without reduction in size. . Neutral busbar shall be full size and full

length as the phase busbars. The main busbar shall be arranged in horizontal plane in the

order red-yellow-blue-neutral from back to front on each panel connections should be

red-yellow-blue-

neutral

TABLE 3.1 Bus

Bars Rating

Busbar Size

(mm x mm)

Current Rating

(A)

6.3 x 40 400

6.3 x 50 500

6.3 x 63 600

6.3 x 70 700

6.3 x 80 800

6.3 x 90 900

6.3 x 100 1000

2 x 6.3 x 63 1200

2 x 6.3 x 80 1600

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3.6 Fault Current

When the current is over the rating it can be define as over current. For the

conductor, the rating values depend on the capacity value to conduct the current. This

value can be obtained from the nominal value or setting value on current protection

devices. Over current can be divided into over current on the load and current damage.

3.7 Element Low- Set Normal Curve

Figure 3.3 Normal Inverse Curves

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Characteristic curve setting depends on normal inverse because this protection

relay can be used for the low voltage protection. This situation allowed the circuit

breaker in series each others in the system. The setting time at the others protection relay

on the system. There are no special coordination requirements compared with other

types of protective equipment farther out on the system. The fault level at the near and

far ends of the system does not vary significantly.

Time equation (t) = 1

14.0

−Iset

If

3.8 IDMT in Distribution System

This topics will presents the application of the overcurrent protection in

distribution systems with inverse definite minimum time (IDMT) setting in distribution

system. Where in this topic will discuss the operation inverse definite minimum (IDMT)

typical in ring main distribution system with IDMT time grading based protection

scheme. Overcurrent protection is earliest protection principle, has been playing a

dominant role in power distribution system protection since the beginning of last

century. The overcurrent principle based on the time-graded protection scheme has such

shortcomings where the relay located near to source has longest operating time setting,

where the fault level is the highest [1]. The maximum tripping time is might be longer to

prevent excessive disturbance of the power system. In addition, the tripping can only be

made at one end (the source end) of the protected line section for radial connected line or

open ring systems.

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Figure 3.4 Basic Ring Systems in Distribution

In the system the protection relay R1, R2, R3, R4, R5, R6, R7 and R8 has been

installed in multi-section ring main power cable system in the line C-1, C-2, C-3 and C-4

respectively. B1, B2, B3, B4, B5, B6, B7 and B8 in the system function as are circuit

breakers. The BUS B, BUS C and BUS D in system have been connected to the load in

the system on each cable section.

Figure 3.5 Clockwise grading

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of IDMT relay

Figure 3.6 Anticlockwise grading of IDM relay

From the graph, we can see the relays R1 and R8 have the longest operating time

to clear the fault. It because the relay installed near sources have longest operating times

in the section to protect the faults current in such sections will have the highest current

levels which can only be allowed to persist for short periods. Even though it is easy to

select the operating times needed to ensure the correct discrimination when relays with

definite time lags are used, but it have the weakness where it must be used restricted to

networks with relatively few series connected sections. From the graph it can be

concluded that the relay R2, R4, R7 and R5 is setting for high operating time and R1,

R3, R6 and R8 is setting for the lower operating time.

In the ring system at above, the protection relay has been divided by two groups

directional where the relay has been operated in clockwise operation and anticlockwise

operation. In the system, relay R1, R3, R5 and R7 will operate in clockwise directional

and R2, R4, R6 and R8 will operate in anticlockwise directional to protect the system

when the fault occurs. As a example, if the fault occurring on cable section C-1, the

directional relays R1, R2, R4, R6 and R8 will detect the fault since the fault is in the

forward direction to these relays and relay R2 will operated where it has been setting the

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fastest time where the R2 will operate first to open its connected circuit breaker B2. The

opening of the circuit breaker B2 means that the fault is on the protected cable section

C-1, and R1 will accelerated trip it associated circuit breaker B1 and isolate the faulted

cable section C-1.Circuit breaker B2 will isolate the fault from cable sections C-2, C-3

and C-4 to protect the equipment. However, if the circuit breaker B2 does not clear the

fault on the section C-1, the relay R1 will start to detect the system balanced condition

and the signals will send to determine the operation performed by the isolated circuit

breaker.

When a fault occurs on section C-2, the protection relays R1, R3, R4, R6 and R8

will detect the fault. The relay R4 which has the fastest time setting curve in the group

will operate first to trip its connected circuit breaker B4.The circuit breaker B4 will

operated, where the fault will be isolated from cable sections C-3 and C-4, and relays R6

and R8 will restrain from operation and if circuit breaker B4 does not clear the fault on

the cable sections C-1 and C-2. The relays R1 will change its operation mode from

accelerate to inverse definite minimum time (IDMT) time operation since it fails to

detect the operation of the remote circuit breaker R3 within the time interval of 0.3s.

This would be the time period from the tripping of the relay R3 to the opening of the

circuit breaker B3 if a fault is detected on C-1. The relay R3, however, detects the

operation of the circuit breaker B4 using the new technique after a fixed time delay of

0.3s and accelerated trips its associated circuit breaker B3 to isolate the faulted cable

section C-2. At the same time, the fault is also isolated from section C-1, which enables

the relay R1 to restrain. Again, the operating speed of relay R3 is significantly increased

in this case

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3.9 Transformers

This topic will discuss the criteria of power transformers construction where the

power transformer has been used to produce the current until 100 ampere. Today

transformers are important devices in electrical engineering. Actually, a transformer is a

device that changes ac electric power at one voltage level through the action of a

magnetic field. It consists of two or more coil of wire wrapped around a common

ferromagnetic core [Chapman]. A transformer is consisting of two coils that are

electrically isolated from each others but are wound on same magnetic core. The coil to

which the source supplies the power is called primary winding. The coil that delivers

power to the load is called the secondary winding. Either the winding may be connected

to source or the load. Basic formula for transformers can be defined as:

Vs

Vp =

Ip

Is =

Ns

Np

Where P is referring to primary side and S is refer to secondary side. Today transformers

came in all shapes and size. Transformers can be defined by various types in the industry

today.

• Generator Transformers

• Power Transformers

• Distribution Transformers

• Pole-Mounted Lighting Transformers

• Grounding Transformers

• Regulating Transformers

• Welding Transformers

• Converter Transformers

• Instrument Transformers (CT and PT)

22

Some of the transformers at above could be autotransformers. However, this

chapter is more focusing on power transformers where power transformer has been used

to produce the current until 100 ampere.

The purpose of a transformer is to convert ac power at one voltage level to ac

power of the same frequency at another voltage level. Transformer also used for variety

of the other purpose.

Figure 3.7 Core-form transformers construction

Power transformers are constructed on one of two types of core. One type of

construction consists of a simple rectangular laminated piece of steel with the

transformer winding wrapped around two side of the rectangle. This type of

constructions is known as core form. The other type consists of a three-legged laminated

core with the winding wrapped around the center leg. This type of construction is known

23

as shell form. The main purpose of thin lamination electrically isolated from each other

in order to minimize eddy current.

Figure 3.8 Shell-form transformers

Figure 3.9 The Hysteresis Curve of the Transformers [4]

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