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Power Systems Lab GRIET/EEE

1

POWER SYSTEMSLAB

___ YEAR ___ SEM

EEE

By

Dr. J. Sridevi

Gokaraju Rangaraju Institute of Engineering &

Technology

Bachupally


2

GOKARAJU RANGARAJU INSTITUTE OF

ENGINEERING AND TECHNOLOGY

(Autonomous)

Bachupally , Hyderabad-500 072

CERTIFICATE

This is to certify that it is a record of practical work done in

the Power Systems Laboratory in ___ sem of __ year during

the year ________________________

Name:

Roll No:

Branch:EEE

Signature of staff member


3

INDEX

S.No Date Topic Page no. Signature of

the Faculty

1. Tripping Characteristics of Fuse & MCB 9

2. Characteristics of Over current relay for

Phase fault

16

3. Characteristics of Over current relay for

Earth fault

24

4. Characteristics of Induction disc type relay 31

5. Characteristics of over load relay 37

6. Testing of Differential relay 42

7. Model of a Transmission Line with

Lumped parameters

47

8. Characteristics of Over voltage Relay 52

9. Characteristics of Under voltage Relay 58

10. Zones Protection 64

11. Short circuit analysis 70

12. Tripping sequence of protective devices 75

13. Testing of Negative sequence Relay 80


4

ETAP SOFTWARE INTRODUCTION

ETAP stands for Electrical Transient Analysis Program, This amazing software suite covers a

wide range of electrical engineering domains; from software for network analysis to power

distribution. The wide range of software modules from ETAP are one of the most beneficial and

effective for electrical engineers. The Arc flash analysis software from ETAP allows engineers to

use simulation models to identify and mitigate arc flash hazards in the electrical power system

and other arc flash related issues.

Here we are using this software to analyse basic operation of power system during transients,

normal operation and fault conditions without actually interfacing with practical power system.

The main parts a power system we come across are as follows

1. Power Grid

2. Bus bar

3. Power Cable

4. Transformer

5. Circuit breaker

6. Relays

7. Fuse

8. Load(motor)

The detailed meaning of them is given below

1. Power Grid: An electrical grid is an interconnected network for delivering electricity to

consumers. It consists of generating stations that produce electrical power, high voltage

transmission lines that carry electrical power from distance sources to demand centres


5

and distribution lines that connect individual consumers. It looks as shown below in

ETAP.

2. Bus bar: A bus as regard to the power system is an electrical junction (node) .It is a strip

or bar of copper, brass or aluminium that conducts electricity within a

switchboard, distribution board, substation, battery bank, or other electrical apparatus. Its

main purpose is to conduct a substantial current of electricity.

3. Power Cable: A power cable is an assembly of one or more electrical conductors,

usually held together with an overall sheath. The assembly is used for transmission

of electrical power. Power cables may be installed as permanent wiring within buildings,

buried in the ground, run overhead, or exposed. In ETAP it looks as:

4. Transformer: Electrical power transformer is a static device which transforms

electrical energy from one circuit to another without any direct electrical connection and

with the help of mutual induction between two windings. It transforms power from one

circuit to another without changing its frequency but may be in different voltage level. It

steps up or steps down the voltage by changing the number of windings in primary and

secondary.

5. Circuit breaker: A circuit breaker is an automatically

operated electrical switch designed to protect an electrical circuit from damage caused by

http://en.wikipedia.org/wiki/Copper

http://en.wikipedia.org/wiki/Brass

http://en.wikipedia.org/wiki/Aluminium

http://en.wikipedia.org/wiki/Electricity

http://en.wikipedia.org/wiki/Electric_switchboard

http://en.wikipedia.org/wiki/Distribution_board

http://en.wikipedia.org/wiki/Electrical_substation

http://en.wikipedia.org/wiki/Battery_(electrical)

http://en.wikipedia.org/wiki/Electrical

http://en.wikipedia.org/wiki/Conductor_(material)

http://en.wikipedia.org/w/index.php?title=Cable_sheath&action=edit&redlink=1

http://en.wikipedia.org/wiki/Electric_power

http://en.wikipedia.org/wiki/Cable

http://en.wikipedia.org/wiki/Electrical_wiring

http://www.electrical4u.com/electrical-power-transformer-definition-and-types-of-transformer/

http://www.electrical4u.com/what-is-inductor-and-inductance-theory-of-inductor/#Mutual-Inductance

http://www.electrical4u.com/voltage-or-electric-potential-difference/

http://en.wikipedia.org/wiki/Electricity

http://en.wikipedia.org/wiki/Switch

http://en.wikipedia.org/wiki/Electrical_network


6

overload or short circuit. Its basic function is to detect a fault condition and interrupt

current flow. Unlike a fuse, which operates once and then must be replaced, a circuit

breaker can be reset (either manually or automatically) to resume normal operation.

Mainly here we come across two types of CB’s. They are

High Voltage Circuit Breaker (HVCB): High-voltage breakers are nearly

always solenoid-operated, with current sensing protective relays operated

through current transformers of about 72.5KV or higher.

Low Voltage Circuit Breaker (LVCB): Low-voltage (less than 1,000 VAC) types

are common in domestic, commercial and industrial application.

6. Relays: A relay is an electrically operated switch. Many relays use an electromagnet to

mechanically operate a switch. Relays are used where it is necessary to control a circuit

by a low-power signal (with complete electrical isolation between control and controlled

circuits), or where several circuits must be controlled by one signal. There are mainly

relays we use in ETAP. They are Over Current Relay, In-line Overload Protection Relay,

Voltage Relay, Differential Relay, Frequency Relay.

i. In-line Overload Relay:

A relay that opens a circuit when the load in the circuit exceeds a preset value, in

order to provide overload protection; usually respondsto excessive current, but ma

y respond to excessive values of power, temperature, or other quantitiesAlso kno

http://en.wikipedia.org/wiki/Overcurrent

http://en.wikipedia.org/wiki/Short_circuit

http://en.wikipedia.org/wiki/Fuse_(electrical)

http://en.wikipedia.org/wiki/Solenoid

http://en.wikipedia.org/wiki/Protective_relay

http://en.wikipedia.org/wiki/Current_transformer

http://en.wikipedia.org/wiki/Electric

http://en.wikipedia.org/wiki/Switch

http://en.wikipedia.org/wiki/Electromagnet


7

wn as overload release. In ETAP it is denoted by 49 which is ANSI code for In-

line overload relay.

ii. Over Current Relay:

A digital over current relay is a type of protective relay which operates when the

load current exceeds a pickup value. The ANSI device number is 50 for an

instantaneous over current (IOC) and 51 for a time over current (TOC). In a

typical application the over current relay is connected to a current transformer and

calibrated to operate at or above a specific current level. When the relay operates,

one or more contacts will operate and energize to trip (open) a circuit breaker.

iii. Voltage Relay: A

relay which operates when the system voltage when the system falls below or

above a certain preset value. For an under voltage relay the ANSI code is 27,

which is used as a standard in ETAP.

iv. Differential Relay:

A relay which responds to a difference in two voltages or currents. For example,

such a relay may have two coils and only respond when the respective currents of

said coils vary beyond a specified amount. It’s denoted by the ANSI code no.87 in

ETAP.

v. Frequency Relay:

Relay which functions at a predetermined value of frequency; may be an over-

frequency relay, an under-frequency relay, or a combinationof both. It is denoted

by the ANSI code no. 81 in ETAP to determine it’s specific function.

7. Fuse: A fuse is a type of low resistance resistor that acts as a sacrificial device to

provide overcurrent protection, of either the load or source circuit. Its essential

component is a metal wire or strip that melts when too much current flows through it,

interrupting the circuit that it connects. Short circuits, overloading, mismatched loads, or

http://www.dictionaryofengineering.com/definition/relay.html

http://www.dictionaryofengineering.com/definition/difference.html

http://en.wikipedia.org/wiki/Resistor

http://en.wikipedia.org/wiki/Sacrificial_device

http://en.wikipedia.org/wiki/Overcurrent

http://en.wikipedia.org/wiki/Electrical_network

http://en.wikipedia.org/wiki/Short_circuit


8

device failure are the prime reasons for excessive current. Fuses are an alternative

to circuit breakers. It can be seen in ETAP as:

8. Load (Motor):

The electric power delivered by a power source to a power user. If variations in voltage a

re small, load can becharacterized by magnitude of current. The term “load” is also often

applied to the device consuming the electric power—that is, to a piece of

equipment, such as a motor or a lighting device. In ETAP we come across mainly

Induction Motor, Synchronous Motor and Lumped load

http://en.wikipedia.org/wiki/Circuit_breaker


9

Date: Experiment-1

TRIPPING CHARACTERISTICS OF FUSE & MCB

AIM:To study the Time current characteristics of FUSE and MCB for given network.

SOFTWARE USED: ETAP Software

THEORY:

Time Current Characteristics of protective devices:

Time is plotted on the vertical axis and current is plotted on the horizontal axis of all time-

current characteristic curves. Log-log type graph paper is used to cover a wide range of times

and currents. Characteristic curves are arranged so that the area below and to the left of the

curves indicate points of "no operation,” and the area above and to the right of the curves

indicate points of "operation." The procedure involved in applying characteristic curves to a

coordination study is to select or set the various protective devices so that the characteristic

curves of series devices from the load to the source are located on a composite time-current

graph from left to right with no overlapping of curves. The result is a set of coordinated curves

on one composite time current graph.

The MCB has some advantages compared to fuse

1. It automatically switches off the electrical circuit during abnormal condition of the network

means in over load condition as well as faulty condition. The fuse does not sense but miniature

circuit breaker does it in more reliable way. MCB is much more sensitive to over current than

fuse.

2. Another advantage is, as the switch operating knob comes at its off position during tripping,

the faulty zone of the electrical circuit can easily be identified. But in case of fuse, fuse wire

should be checked by opening fuse grip or cutout from fuse base, for confirming the blow of fuse

wire.

3. Quick restoration of supply can not be possible in case of fuse as because fuses have to be

rewirable or replaced for restoring the supply. But in the case of MCB, quick restoration is

possible by just switching on operation.

http://www.electrical4u.com/electrical-fuse-hrc-fuse-high-rupturing-capacity/

http://www.electrical4u.com/electric-current-and-theory-of-electricity/


10

4. Handling MCB is more electrically safe than fuse.

Because of to many advantages of MCB over fuse units, in modern low voltage electrical

network, miniature circuit breaker is mostly used instead of backdated fuse unit.

Only one disadvantage of MCB over fuse is that this system is more costlier than fuse unit

system.

Working Principle Miniature Circuit Breaker

There are two arrangement of operation of miniature circuit breaker. One due to thermal

effect of over current and other due to electromagnetic effect of over current. The thermal

operation of miniature circuit breaker is achieved with a bimetallic strip whenever continuous

over current flows through MCB, the bimetallic strip is heated and deflects by bending. This

deflection of bimetallic strip releases mechanical latch. As this mechanical latch is attached with

operating mechanism, it causes to open the miniature circuit breaker contacts. But during short

circuit condition, sudden rising of current, causes electromechanical displacement of plunger

associated with tripping coil or solenoid of MCB. The plunger strikes the trip lever causing

immediate release of latch mechanism consequently open the circuit breaker contacts.


http://www.electrical4u.com/electrical-circuit-breaker-operation-and-types-of-circuit-breaker/






11

PARAMETERS:

SI.NO

COMPONENT

MANUFACTURER

RATING

OTHER

PARAMETERS TO

BE SPECIFIED

1.

2.

3.

4.

5.

6.

7.

8.

9.

Power Grid

Buses

Bus1

Bus2

Bus3

CT1

CT2

OCR

Cable

Fuse

Transformer

CB1

CB2

Synchronous

motor

---

----

----

----

ABB

GE Multilin

735/737

(Lib-220)

ICEA

Seimens

A500-2.54KV

Lib 1HP

0.46KV(MTR)

200MVASc

12.47kV

12.47KV

12.47KV

300:5

300:5

1000KVA

Pri-12.47KV

Sec-0.48KV

POWER-

1000KVA

Siemen Allis

LA-1600A

Siemens

Static-Trip III

2HP

0.46KV

X/R=5

1.Add circuit breaker.

2.choose any type of

relay

TCC KA

Phase-12.47

1.length-100

Tolerance-0

Impedance –typical

Z&X/R ratio


12

ETAP NETWORK DIAGRAM:

PROCEDURE:

Open the ETAP software in the pc.

On the right hand side of the software equipments are present.

Construct the circuit as shown in circuit diagram in “Edit mode” tab.

Click on the star protective devices icon present at the top of the edit mode.

Then select a part of the circuit and click on the star view at right.

The graphs are obtained and studied


13

ETAP Simulation diagram:


14

TIME CURRENT CHARACTERISTIC CURVES:


15

Result:

Signature of the faculty


16

Date: Experiment-2

CHARACTERISTICS OF OVER CURRENT RELAY FOR PHASE FAULT

AIM: To study the characteristics of over current realy for phase fault

APPARATUS: Over current relay

Auxiliary supply kit (Step-down Transformer-230/24V, Bridge rectifier, filter)

Auto transformer (0-230v, 2A)

Ammeter (0-2A)

Rheostat (110Ω, 1.8A)

THEORY:

The function of a relay is to detect abnormal conditions in the system and to initiate through

appropriate circuit breakers the disconnection of faulty circuits so that interference with the

general supply is minimized. Relays are of many types. Some depend on the operation of an

armature by some form of electromagnet. A very large number of relays operate on the induction

principle. When a relay operates it closes contacts in the trip circuit .The passage of current in

the coil of the trip circuit actuates the plunger, which causes operation of the circuit breaker,

disconnecting the faulty system.

OVER CURRENT PROTECTION:

The protective relaying which responds to a rise in current flowing through the protected element

over a pre-determined value is called 'overcurrent protection' and the relays used for this purpose

are known as overcurrent relays. Earth fault protection can be provided with normal overcurrent

relays, if the minimum earth fault current is sufficient in magnitude. The design of a

comprehensive protection scheme in a power system requires the detailed study of time-current

characteristics of the various relays used in the scheme. Thus it is necessary to obtain the time

current characteristics of these relays. The overcurrent relay works on the induction principle.

The moving system consists of an aluminum disc fixed on a vertical shaft and rotating on two

jeweled bearings between the poles of an electromagnet and a damping magnet. The winding of

the electromagnet is provided with seven taps (generally0, which are brought on the front panel,

and the required tap is selected by a push-in -type plug. The pick-up current setting can thus be


17

varied by the use of such plug multiplier setting. The operating time of all overcurrent relays

tends to become asymptotic to a definite minimum value with increase in the value of current.

This is an inherent property of the electromagnetic relays due to saturation of the magnetic

circuit. By varying the point of saturation, different characteristics can be obtained and these are

1. Definite time

2. Inverse Definite Minimum Time (IDMT)

3. Very Inverse

4. Extremely Inverse

Principle:

Overcurrent protection is practical application of magnitude relays since it picks up when the

magnitude of current exceeds some value (setting value). Overcurrent relays can be used to

protect practically any power system elements, i.e. transmission lines, transformers, generators,

or motors. As an example, a radial transmission line can be used. For a fault within the zone of

protection, the fault current is smallest at the end of the line and greatest at the relay end. If the

minimum fault current possible within the zone of protection is greater than the maximum

possible load current, it would be possible to define the operating principle as follows:

Where I is the current in the relay and is Ip the pickup setting of the relay.

Instantaneous overcurrent relays :

Its operation criterion is only current magnitude (without time delay). This type is applied to the

outgoing feeders.

Characteristics of instantaneous over current Relay


18

Definite Time Overcurrent Relays :

In this type, two conditions must be satisfied for operation (tripping), current must exceed the

setting value and the fault must be continuous at least a time equal to time setting of the relay.

Modern relays may contain more than one stage of protection each stage includes each own

current and time setting.

Characteristics of definite time over current Relay

Definite time overcurrent relay is the most applied type of over current. It is used as:

1. Back up protection of distance relay of transmission line with time delay.

2. Back up protection to differential relay of power transformer with time delay.

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

Inverse Time Overcurrent Relays

In this type of relays, operating time is inversely changed with current. So, high current will

operate overcurrent relay faster than lower ones. There are standard inverse, very inverse and

extremely inverse types

Characteristics of inverse time over current Relay


19

CIRCUIT DIAGRAM:

Experimental procedure:

1. Study the construction of the relay and identify the various parts.

2. Set the pick-up value of the current marked 1 A(100 % f. l current)

3. Set the Time Multiplier Setting (TMS) initially at 1.0.

4. Adjust the load current to about 1.8 times the f.l current. Record the time taken for the

overload condition.

5. Vary the value of the load current in steps and record the time taken for the operation of

the relay in each case with the help of the timer.

6. Repeat steps 5 and 6 for TMS values of 0.2, 0.4,0.6 and 0.8.

7. Repeat the above experiment with different pick up current values using the plug setting

bridge.


20

Tabular form-Range of Experimental values:

Pick-up current = 1 Amps

IP - Current setting: 10% = 0.1A

S.No Current(A) Current(A)

times the

plug setting

multiplier

Tp- Operating time in sec. for TMS of

1.0 0.8 0.6 0.4 0.2

1 0.12 1.2 38 30.4 22.8 15.2 7.6

2 0.13 1.3 28 22.4 16.8 11.2 5.6

3 0.14 1.4 22 17.6 13.2 8.8 4.4

4 0.16 1.6 17 13.6 10.2 6.8 3.4

5 0.18 1.8 14 11.2 8.4 5.6 2.8

Tabular form-practical values:


IP - Current setting :10%


times the

plug setting

multiplier


1.0 0.8 0.6 0.4 0.2

1

2

3

4

5


21

Model graph-Characteristics of over current relay

0

5

10

15

20

25

30

35

40

0 0.5 1 1.5 2

TIME(Sec)

PSM

TMS-1 TMS-0.8 TMS-0.6


22


23

RESULT:



24

Date: Experiment-3

CHARACTERISTICS OF OVER CURRENT RELAY FOR EARTH FAULT

Aim: To study the characteristics of over current relay for earth fault

Apparatus: Earth Fault relay



Ammeter (0-2A)


Theory:



general supply is minimized. Relays are of many types. Some depend on the operation of an





Overcurrent Protection

The protective relaying which responds to a rise in current flowing through the protected element

over a pre-determined value is called 'overcurrent protection' and the relays used for this purpose

are known as overcurrent relays. Earth fault protection can be provided with normal overcurrent

relays, if the minimum earth fault current is sufficient in magnitude. The design of a

comprehensive protection scheme in a power system requires the detailed study of time-current

characteristics of the various relays used in the scheme. Thus it is necessary to obtain the time

current characteristics of these relays. The overcurrent relay works on the induction principle.

The moving system consists of an aluminum disc fixed on a vertical shaft and rotating on two

jeweled bearings between the poles of an electromagnet and a damping magnet. The winding of

the electromagnet is provided with seven taps (generally0, which are brought on the front panel,

and the required tap is selected by a push-in -type plug. The pick-up current setting can thus be


25

varied by the use of such plug multiplier setting. The operating time of all overcurrent relays

tends to become asymptotic to a definite minimum value with increase in the value of current.

This is an inherent property of the electromagnetic relays due to saturation of the magnetic

circuit. By varying the point of saturation, different characteristics can be obtained and these are

1. Definite time

2. Inverse Definite Minimum Time (IDMT)

3. Very Inverse

4. Extremely Inverse

Principle:

Overcurrent protection is practical application of magnitude relays since it picks up when

the magnitude of current exceeds some value (setting value). Overcurrent relays can be used to

protect practically any power system elements, i.e. transmission lines, transformers, generators,

or motors. As an example, a radial transmission line can be used. For a fault within the zone of

protection, the fault current is smallest at the end of the line and greatest at the relay end. If the

minimum fault current possible within the zone of protection is greater than the maximum

possible load current, it would be possible to define the operating principle as follows:

Where I is the current in the relay and is Ip the pickup setting of the relay.

Protection against earth faults can be obtained by using a relay that responds only to the

residual current of the system, since a residual component exists only when fault current flows to

earth. The earth-fault relay is therefore completely unaffected by load currents, whether balanced

or not, and can be given a setting which is limited only by the design of the equipment and the

presence of unbalanced leakage or capacitance currents to earth. This is an important

consideration if settings of only a few percent of system rating are considered, since leakage

currents may produce a residual quantity of this order.


26

Circuit Diagram:



2. Set the pick-up value of the current marked 1 A(100 % f. l current)


4. Adjust the load current to about 1.8 times the f.l current. Record the time taken for the

overload condition.



6. Repeat steps 5 and 6 for TMS values of 0.2, 0.4,0.6 and 0.8.

7. Repeat the above experiment with different pick up current values using the plug setting

bridge.


27



IE - Current setting: 10% = 0.1A


times the

plug setting

multiplier

Tp- Operating time in sec. for

TMS of

1.0 0.8 0.6 0.4

1 0.12 1.2 35 26 20 12

2 0.13 1.3 20 16 10 7

3 0.14 1.4 14 11 8 5

4 0.16 1.6 12 9 7 4

5 0.18 1.8 10 8 6 3

Tabular form-practical values:


IE - Current setting :


times the

plug setting

multiplier

Tp- Operating time in sec. for

TMS of

1.0 0.8 0.6 0.4

1

2

3

4

5


28

Model graph-Characteristics of Earth fault relay

0

5

10

15

20

25

30

35

40

0 0.5 1 1.5 2

TIME(Sec)

PSM



29


30

RESULT:



31

Date: Experiment-4

CHARACTERISTICS OF INDUCTION DISC TYPE RELAY

Aim: To study the characteristics of induction disc type relay

Apparatus: Induction disc type relay



Ammeter (0-20A)


Theory:

The function of a relay is to detect abnormal conditions in the system and to initiate

through appropriate circuit breakers the disconnection of faulty circuits so that interference with

the general supply is minimized. Relays are of many types. Some depend on the operation of an





Induction disc type relay:

The electromagnetic induction disc relay is frequently used where the time of relay operation

should depend upon the amount of an overcurrent. The relay is essentially a small induction

motor. This is probably the most widely used protective relay in the industry. It starts to turn

when the current exceeds a (previously selected) threshold current, and rotates faster as the

current increases. This relay has one set of stationary contacts and one set which moves as the

disc turns. The distance which the disc must travel to close the contacts is adjusted by setting the

position of the time dial control. The magnitude of current which initiates disc movement is set

by the choice of the tap on the current coil. The results is that relay contact operation is


32

dependent upon the tap and the time dial settings. The relay timing can be varied from a few

cycles to as long as 30 seconds.

An induction relay works only with alternating current. It consists of an electromagnetic system

which operates on a moving conductor, generally in the form of a disc or cup, and functions

through the interaction of electromagnetic fluxes with the parasitic Fault currents which are

induced in the rotor by these fluxes. These two fluxes, which are mutually displaced both in

angle and in position, produce a torque.

Induction disc type Relay

Basic torque/current equation

T= Κ1.Φ1.Φ2 .sin θ

Where Φ1 and Φ2 are the interacting fluxes and θ is the phase angle between Φ1 and Φ2. It

should be noted that the torque is a maximum when the fluxes are out of phase by 90º, and zero

when they are in phase.

The relay's primary winding is supplied from the power systems current transformer via a plug

bridge, which is called the plug setting multiplier (psm). Usually seven equally spaced tappings

or operating bands determine the relays sensitivity. The primary winding is located on the upper

electromagnet. The secondary winding has connections on the upper electromagnet that are

energised from the primary winding and connected to the lower electromagnet. Once the upper

and lower electromagnets are energised they produce eddy currents that are induced onto the

metal disc and flow through the flux paths. This relationship of eddy currents and fluxes creates

torque proportional to the input current of the primary winding, due to the two flux paths been


33

out of phase by 90°.A restraining spring forces the disk to rotate in the direction that opens the

trip contacts while current creates operating torque to close the contacts. The net positive torque

closes the contacts. The IPU relay setting fixes the value of the pickup current. When the current

applied to the relay equals the pickup current, the contact closing torque just equals the

restraining torque and the disk will not move regardless of its position. If the applied current

increases above the pickup current, the disk will begin to rotate so that the trip contacts come

closer together

Wiring Diagram:


34



2. Set the pick-up value of the current as 2.5A

3. Adjust the load current to about 1.3 times the full load current. Record the time taken for

the overload condition.



5. Repeat the above experiment with different pick up current values.


Pick-up current = 2.5 Amps


times the

plug setting

multiplier

Operating

Time in

seconds

1

2

3

4

5


35

Model graph :Characteristics of induction disc type relay

0

5

10

15

20

25

30

35

40

0 0.5 1 1.5 2

TIME(Sec)

PSM



36

RESULT:



37

Date: Experiment-5

CHARACTERISTICS OF OVER LOAD RELAY

AIM:To study the characteristics of over load relay.


Theory:

Overload relays protect a motor by sensing the current going to the motor. Many of these use

small heaters, often bi-metallic elements that bend when warmed by current to the motor.When

current is too high for too long, heaters open the relay contacts carrying current to the coil of the

contactor. When the contacts open, the contactor coil de-energizes, which results in an

interruption of the main power to the motor. These contacts do not affect control power.

Overload relays and their heaters belong to one of three classes, depending on the time it takes

for them to respond to an overload in the motor. The overload relay itself will have markings to

indicate which class it belongs to. These include Class 10, 20, and 30. The class number

indicates the response time (in seconds). An unmarked overload relay is always Class 20.

Typical NEMA-rated overload relays are Class 20, but you can adjust many of them about 15%

above or below their normal trip current. IEC relays are usually Class 10, and you can usually

adjust them to 50% above their normal trip current.


38

PARAMETERS:

SI.NO

COMPONENT

MANUFACTURER

RATING

OTHER

PARAMETERS TO

BE SPECIFIED

1.

2.

3.

4.

5.

6.

7.

8.

9.

10.

Power Grid

Buses

Bus1

Bus2

Bus3

CT1

CT2

OCR

Cable

Fuse

Transformer

CB1

CB2

Induction motor

Thermal Over

load Heater

---

----

----

----

ABB

GE Multilin

735/737

(Lib-220)

ICEA

Seimens

A500-2.54KV

MTR Lib 1HP 0.46KV

General Electric

200MVASc

12.47kV

12.47KV

12.47KV

300:5

300:5

1000KVA

Pri-12.47KV

Sec-0.48KV

POWER-

1000KVA

Siemen Allis

LA-1600A

Siemens

Static-Trip III

2HP

0.46KV

0.48 kv

X/R=5



relay

TCC KA

Phase-12.47

1.length-100

Tolerance-0


Z&X/R ratio


39

ETAP Network Diagram:

PROCEDURE:




Click on the star protective devices icon present at the top of the edit mode.

Then select a part of the circuit and click on the star view at right.

The graphs are obtained and studied.


40

TIME CURRENT CHARACTERISTIC CURVES:


41

RESULT:



42

Date: Experiment-6

TESTING OF DIFFERENTIAL RELAY

Aim: To study the operation of Differential Relay

Apparatus: Differential Relay


Auto transformer (0-230v, 2A)-2 No.s

Ammeter (0-2A)-2 No.s

Rheostat (110Ω, 1.8A) -2 No.s

Theory:

The relay which is used to checks the difference between the output and input currents

for power system current in known as differential relay. The difference amongst the currents may

also be in phase angle or in magnitude or in eachthe angle and magnitude variations must be

zero. In case there's a difference which difference go beyond some value, the relay can work and

interconnected electrical fuse can disconnect.

Principle Operation of differential relay:

Consider a power transformer with transformation magnitude (ratio) relation 1:1 and (Y/Y)

connection and therefore the CT1 and CT2 ensure a similar transformation magnitude relation as

http://www.mytech-info.com/2013/11/principle-and-operation-of-differential.html


43

shown. The current flows within the primary side and secondary side of power transformer are

equal, presumptuous ideal power transformer. The secondary current I1 and I2 are same in

magnitude and reverse in direction. Therefore, the net current within the differential coil is nil at

load situation (without any fault), and therefore the relay won't operate.

External Fault Condition in Differential Relay:

Assigning the previous one the power transformer with an external fault F is shown in figure.

During this case the 2 currents I1, and I2 can increase to terribly high magnitudes values however

there's no modification in phase angle. Hence, net current within the differential coil continues to

be zero and therefore the relay won't operate.

Internal Fault Condition in Differential Relay:

http://www.mytech-info.com/2013/09/basic-working-principle-of-transformer.html

http://www.mytech-info.com/2013/09/basic-working-principle-of-transformer.html




44

An internal fault F is shown in this figure. Now, there are 2 anticipated conditions:

There’s other supply to feed the fault thus I2P includes a nonzero value Idiff = I1S + I2S which can

be terribly high and sufficient to function the differential relay.

Radial system, I2P = 0. So, Idiff = I1S and additionally the relay can work and disconnect the

breaker.

Circuit Diagram

Procedure:


2. Set the Bias current and current setting.

3. With zero bias current (ammeter A2=0), inject operate current in to phase A. When the

relay operates, shown bythe LED "Trip" illuminating, record the value of the current

indicated on ammeter A1.

4. Repeat the test with increasing bias currents up to 2times the relay rating.

5. Record the results in Table.


45

Model Tabular Form:

Initial

settings

Bias

settings

Bias current Ammeter A2 multiples of rated current in

0 1 1.5 2 2.5

Operate current, ammeter A1 Amps

10% 10% 0.1 0.11 0.16 0.21 2.26

20% 20% 0.2 0.21 0.33 0.44 0.56

30% 30% 0.3 0.35 0.53 0.71 0.88

40% 40% 0.4 0.5 0.75 1 1.25

50% 50% 0.5 0.67 1 1.33 1.67

- 70% 0.5 0.86 1.29 1.71 2.14

- 80% 0.5 1.08 1.62 2.15 2.69

Tabular form :practical readings

Intial

settings

Ammeter A2 multiples of rated current in

Operate current, ammeter A1 Amps

10%

20%

30%

40%

50%

-


46

RESULT:



47

Date: Experiment-7

MODEL OF A TRANSMISSION LINE WITH LUMPED PARAMETERS

AIM:To run the load flow for a given transmission line network


Theory:

The power-flow study, or load-flow study, is a numerical analysis of the flow of electric power

in an interconnected system. A power-flow study usually uses simplified notation such as a one-

line diagram and per-unit system, and focuses on various aspects of AC power parameters, such

as voltages, voltage angles, real power and reactive power. It analyzes the power systems in

normal steady-state operation.

Power-flow or load-flow studies are important for planning future expansion of power systems

as well as in determining the best operation of existing systems. The principal information

obtained from the power-flow study is the magnitude and phase angle of the voltage at each bus,

and the real and reactive power flowing in each line.

The goal of a power-flow study is to obtain complete voltage angle and magnitude information

for each bus in a power system for specified load and generator real power and voltage

conditions.[2] Once this information is known, real and reactive power flow on each branch as

well as generator reactive power output can be analytically determined. Due to the nonlinear

nature of this problem, numerical methods are employed to obtain a solution that is within an

acceptable tolerance.

The solution to the power-flow problem begins with identifying the known and unknown

variables in the system. The known and unknown variables are dependent on the type of bus. A

bus without any generators connected to it is called a Load Bus. With one exception, a bus with

at least one generator connected to it is called a Generator Bus. The exception is one arbitrarily-

selected bus that has a generator. This bus is referred to as the slack bus.

https://en.wikipedia.org/wiki/Numerical_analysis

https://en.wikipedia.org/wiki/One-line_diagram

https://en.wikipedia.org/wiki/One-line_diagram

https://en.wikipedia.org/wiki/Per-unit_system

https://en.wikipedia.org/wiki/AC_power

https://en.wikipedia.org/wiki/Busbar

https://en.wikipedia.org/wiki/Power-flow_study#cite_note-2

https://en.wikipedia.org/wiki/Slack_bus


48

PARAMETERS:

SI.NO

COMPONENT

MANUFACTURER

RATING

OTHER

PARAMETERS TO

BE SPECIFIED

1.

2.

3.

4.

5.

6.

7.

Buses

Bus1

Bus2

Bus3

Impedances

Z12

Z12

Z12

Sych.Generator 1

Sych.Generator 2

Lumped Load

----

----

----

-----

-----

-----

-----

-----

-----

100KV

100KV

100KV

Percentage base

KV-100kv

Percentage base

KV-100kv

Percentage base

KV-100kv

100KV,

250 MW

100KV,

250 MW

400MW,471MVA

R=2,X=4,Y=0

R=1,X=3,Y=0

R=1.25,X=2.5,Y=0

Reference or Slack

bus

Generator bus

Var limits:200&-100

Conventional model,

Rated 100kv


49

ETAP Network Diagram: Transmission line model

PROCEDURE:




Run the load flow analysis for a given network.

Get the power flow values at each bus.

And get the results through load flow result analyser


50

SIMULATION RESULTS:


51

RESULT:



52

Date: Experiment-8

CHARACTERISTICS OF OVER VOLTAGE RELAY

AIM: To study the characteristics of over voltage relay

APPARATUS: Over voltage relay



Voltmeter (0-200v)


THEORY:

OVERVOLTAGE PROTECTION



general supply is minimized. There are always a chance of suffering an electrical power system

from abnormal over voltages. These abnormal over voltages may be caused due to various

reason such as, sudden interruption of heavy load, lightening impulses, switching impulses etc.

These over voltage stresses may damage insulation of various equipments and insulators of the

power system. Although, all the over voltage stresses are not strong enough to damage insulation

of system, but still these over voltages also to be avoided to ensure the smooth operation of

electrical power system. These all types of destructive and non destructive abnormal over

voltages are eliminated from the system by means of overvoltage protection. For generator

protection an overvoltage relay is used to detect failure in voltage regulation. For transformers

and transmission lines, overvoltage protection is sometimes used to detect excessive voltages

Principle:

An overvoltage relay is one that operates when input voltage exceeds a predetermined(pick up)

value.over voltage relays must be instantaneous or time-delayed devices.In order to set a time

overvoltage relay,pickup voltage and time dial need to be specified and VT ration needs to be

documented.Time overvoltage relays start to time out every time input voltage exceeds the




53

setpoint.overvoltage relays complete their function and close the output contact when the

duration of the overvoltage exceeds the time delay described by the time voltage curve.

Inverse time characteristics of overvoltage relay

The inverse characteristic for overvoltage V>, is defined by the

following equation:

where:

t = operating time in seconds

TMS = time multiplier setting

V = applied input voltage

Vs = relay setting voltage

NOTE: this equation is valid for V> Vs


54

Circuit Diagram:



2. Set the pick-up value of the voltage .


4. Adjust the voltage from 1.2 to 1.8 times the pick up voltage step wise. Record the time

taken for the overvoltage condition.

5. Vary the value of the over voltage in steps and record the time taken for the operation of


6. Repeat steps 4 and 5 for TMS values of 0.8,0.6,0.5,0.2

7. Repeat the above experiment with different pick up voltage values using the plug setting

bridge.


55

Tabular form

Pick-up voltage = ------ volts

S.No Voltage(v) Voltage(v)

times the

plug setting

multiplier


1.0 0.8 0.6 0.5 0.2

1 1.2

2 1.3

3 1.4

4 1.6

5 1.8

Model graph-Characteristics of over voltage relay :


56


57

RESULT:



58

Date: Experiment-9

CHARACTERISTICS OF UNDER VOLTAGE RELAY

AIM: To study the characteristics of under voltage relay

APPARATUS: Under voltage relay



Voltmeter (0-200v)


THEORY:

UNDER VOLTAGE PROTECTION



general supply is minimized. There are always a chance of suffering an electrical power system

from abnormal under voltages. An undervoltage protection is used to disconnect motors at low

system voltage to prevent problems with inrush at system voltage recovery. Single-phase

versions connected phase-phase are used for asynchronous motors, whereas measuring of

positive sequence voltage is used for synchronous motors.

Principle:

An undervoltage relay is one that operates when input voltage drops below a predetermined

value(dropout value).Undervoltage relays are usually instantaneous devices.If time delays are

needed,timers,initiated on undervoltage relay,are utilized.Undervoltage relays should complete

their function every time input voltage drops below the setpoint.The dropout voltage needs to be

specified and VT ratio needs to be documented.A typical time voltage curve for undervoltage

relay is shown below.


59

Characteristics of under voltage relay

The inverse characteristic for undervoltage V<, is defined by the

following equation:

where:

t = operating time in seconds

TMS = time multiplier setting

V = applied input voltage

Vs = relay setting voltage

NOTE: this equation is valid for Vs>V


60

Circuit Diagram:



2. Set the drop out value of the voltage .


4. Adjust the voltage from 0.5 to 0.9 times the dropout voltage step wise. Record the time

taken for the under voltage condition.

5. Vary the value of the under voltage in steps and record the time taken for the operation of


6. Repeat steps 4 and 5 for TMS values of 0.8,0.6,0.5,0.2

7. Repeat the above experiment with different dropout voltage values using the plug setting

bridge.


61

Tabular form

Drop-out voltage = ----- volts

S.No Voltage(v) Voltage(v)

times the

plug setting

multiplier


1.0 0.8 0.6 0.5 0.2

1 0.9

2 0.8

3 0.7

4 0.6

5 0.5

Model graph- Characteristics of under voltage relay :


62


63

RESULT:



64

Date: Experiment-10

ZONES PROTECTION

AIM:To study the zones protection characteristics of Transformer and Motor zone of a given

network.


THEORY:

The protected zone is that part of a power system guarded by a certain protection and usually

contains one or at the most two elements of the power system. For a non-unit scheme, the zone

lies between the current transformers and the point or points on the protected circuit beyond

which the system is unable to detect the presence of a fault which is shown in figure. For a unit

scheme, the zone lies between the two or several sets of current transformers and the point or

points which together with the relays constitute the protective system

A power system is composed of number of sections (equipments) such as transformer , motor ,

generator, bus bar and transmission line. These sections are protected by protective relaying

systems comprising of Instrument Transformers, protective relays , circuit breakers (CB’s) and

communication equipment.These are called zones of protection.The protective system are

planned in such a way that the entire power system is collectively provided by them and thus no

part of the system is left unprotected In case of fault occurring on a section, its associated

protective relays should detect the fault and issue trip signals to open their associated CB’s to

isolate the faulted section from the rest of the power system in order to avoid further damage to

the power system.When a fault occurs, the protection scheme is required to trip only those circuit

breakers whose operation is required to isolate the fault. This property of selective tripping is

also called 'discrimination' and is achieved by two general methods:

1. Time Grading

Protection systems in successive zones are arranged to operate in times that are graded

through the sequence of equipments so that upon the occurrence of a fault, although a

number of protection equipments respond, only those relevant to the faulty zone complete

the tripping function. The others make incomplete operations and then reset. The speed of


65

response will often depend on the severity of the fault, and will generally be slower than for

a unit system.

2. Unit Systems

It is possible to design protection systems that respond only to fault conditions

occurring within a clearly defined zone. This type of protection system is known as

'unit protection'. Certain types of unit protection are known by specific names,

e.g. restricted earth fault and differential protection. Unit protection can be applied

throughout a power system and, since it does not involve time grading, is relatively fast

in operation. The speed of response is substantially independent of fault severity. Unit

protection usually involves comparison of quantities at the boundaries of the protected

zone as defined by the locations of the current transformers

Transformer Protection: The primary objective of the Transformer Protection is to detect

internal faults in the transformer with a high degree of sensitivity and cause subsequent de-

energisation and, at the same time be immune to faults external to the transformer i.e. through

faults. Sensitive detection and deenergisation enables the fault damage and hence necessary

repairs to be limited.

Motor Protection: The abnormalities in motor or motor faults may appear due to mainly two

reasons – 1.Conditions imposed by the external power supply network,

2.Internal faults, either in the motor or in the driven plant.

Unbalanced supply voltages, under-voltage, reversed phase sequence and loss of synchronism (in

the case of synchronous motor) come under former category. The later category includes bearing

failures, stator winding faults, motor earth faults and overload etc. The motor characteristics

must be very carefully considered in selecting the right motor protection scheme.

http://www.openelectrical.org/wiki/index.php?title=Earth_Fault_Protection

http://www.openelectrical.org/wiki/index.php?title=Relays#Differential_.28unit.29_protection


66

PARAMETERS:

SI.NO

COMPONENT

MANUFACTURER

RATING

OTHER

PARAMETERS

TO BE

SPECIFIED

1.

Power Grid

---

500MVASc

X/R =5

2.

Buses

Bus 1

Bus 2

Bus 4

Bus 5

---

13.5kV

0.48kV

0.48kV

0.48kV

---

3.

HVCB

ABB 15.5kV

In TCC kV=13.5

4.

Current

Transformer

-----

300/5

-----

5.

Fuses

Fuse 1

Fuse 2

General Electric

General Electric

12kV

6.25Kv

--------

6.

LVCB

CB1

CB2

CB3

CB4

Synchronous

Motor

ABB

ABB

ABB

ABB

-----

5kV

3.3kV

2kV

1.01kV

350HP

In TCC kV=13.5

In TCC kV=0.48

In TCC kV=0.48

In TCC kV=0.48

7. Lumped Load ------- 135kVA -----

8. Over Current

Relay

ABB ---- 1)Choose any type

of relay in OCR

2) Add CB in Output

9. Transformer ------ 1000kVA Impedance: Typical

Z and X/R

10. In-Line Relay Allen Bradley ---- -----

11. Cable ICEA (220 in library) 100ft Impedance:

R=5.75 X=5.75


67

ETAP Network Diagram:

Procedure:

Construct the above network diagram in ETAP software

Specify the ratings of each device as given in the parameter table

In star protective devices select the zone of protection and get the time current

characteristics for individual Transformer, Motor, Bus Bar zones


68

Characteristics of Transformer, Motor and Bus Bar zones:


69

RESULT:



70

Date: Experiment-11

SHORT CIRCUIT ANALYSIS

AIM: To perform short circuit analysis to a given network or transmission line.

SOFTWARE REQUIRED: ETAP

Theory: A short circuit analysis helps us to ensure that equipment are protected by establishing

proper interrupting rating of protective devices on power systems and is required to determine

the switch gear ratings and relay ratings.The short circuit calculations must be maintained and

periodically updated to protect the equipment.The short circuit in the system cannot always be

prevented; its effect can only be reduced by considering its consequences on the system at the

time of planning and design stage . The system components, transformers, cables, switchgears,

protection equipments etc must be designed and selected to havefault withstand capability to

match system fault current rating . The objectives of performing sort circuit study are:

To prepare basis for the selection of the interrupting equipment and also to verify adequacy of

existing interrupting equipment;

To determine the system protective device settings;

To coordinate protective devices

To determine the effects of the fault currents on various system components during the time the

fault persists;

Conceptualization, design and refinement of system layout, neutral grounding, and substation

grounding;

To ascertain the minimum short-circuit current.

Short Circuit analysis is required to ensure that existing and new equipment ratings are adequate

to withstand the available short circuit energy available at each point in the electrical system. A

Short Circuit Analysis will help to ensure that personnel and equipment are protected by

establishing proper interrupting ratings of protective devices (circuit breaker and fuses). If an

electrical fault exceeds the interrupting rating of the protective device, the consequences can be

devastating. It can be a serious threat to human life and is capable of causing injury, extensive

equipment damage, and costly downtime. On large systems, short circuit analysis is required to

determine both the switchgear ratings and the relay settings. No substation equipment can be


71

installed without knowledge of the complete short circuit values for the entire power distribution

system. The short circuit calculations must be maintained and periodically updated to protect the

equipment and the lives. It is not safe to assume that new equipment is properly rated.

Benefits of a Short Circuit Analysis

Performing a Short Circuit Study provides the following benefits:

Reduces the risk a facility could face and help avoid catastrophic losses.

Increases the safety and reliability of the power system and related equipment.

Evaluates the application of protective devices and equipment.

Identifies problem areas in the system.

Identifies recommended solutions to existing problems.

PARAMETERS:

SI.NO

COMPONENT

MANUFRACTURER

RATING

OTHER

PARAMETERS TO

BE SPECIFIED

1.

Power Grid

---

200MVASc

X/R=5

2.

Buses

Bus 1

Bus 2

Bus 3

---

12.47kV

12.47kV

0.48kV

-------

3.

HVCB

ABB

12kV

In TCC kV =12.47

4.

Transformer

-----

100MVA

Choose typical Z &

X/R

5.

Fuse

General Electric

6.25kV

-----

6.

LVCB

ABB

1.01kV

In TCC kV=0.48

7.

Synchronous

Motor

----- 1HP -----

8. Cable ICEA(220 in library) 100ft Impedance:

R=5.75 X=5.75


72

Circuit Diagram:

Procedure:

Construct the above circuit diagram in ETAP software.

Specify the ratings of the parameters as given in the table.

Perform the short circuit analysis for a given network by inserting fault at desired

location.

Get the fault current values after simulation


73

Simulation Results-


74

RESULT:



75

Date: Experiment-12

TRIPPING SEQUENCE OF PROTECTIVE DEVICES

AIM: To perform Tripping sequence of protective device for given network.

SOFTWARE USED:ETAP Software

THEORY:

Star Sequence-of-Operation evaluates, verifies, and confirms the operation and selectivity of the

protective devices for various types of faults for any location directly from the one-line diagram

and via normalized Time Current Characteristic Curve views.

Sequence-of-Operation provides a system-wide solution for an accurate and realistic

operating time and state of protective devices, such as relay, fuse, circuit breaker, trip devices,

contactor, etc. The operation time is calculated for each protective device based on its settings,

time current characteristic, and interlocks for a specified fault location and type.

Sequence of Operation Key Features

User-definable fault insertion location

View device operation sequence graphically

Device failure & backup operation

Detailed relay actions (27, 49, 50, 51, 51V, 59, 67, 79, 87)

Sequence of event viewer

Current summation

Normalized (shifted) Time Current Characteristic Curves

Phase & Ground faults (symmetrical & asymmetrical)

Flashing protective device via the one-line diagram

Coordination via One-Line Diagram

Graphically place a fault anywhere on the one-line diagram

Automatically calculate & display the fault current contributions on the one-line diagram

Evaluate the operating time & state of devices based on the actual fault current

contribution flowing through each individual device


76

Graphical animation of protective device operation

Globally view post fault actions & associated operating time via a tabulated event viewer

Examine the operation of protective devices via the one-line diagram.

PARAMETERS:

SI.NO

COMPONENT

MANUFACTURER

RATING

OTHER

PARAMETERS TO

BE SPECIFIED

1.

2.

3.

4.

5.

6.

7.

8.

9.

Power Grid

Buses

Bus1

Bus2

Bus3

CT1

CT2

OCR

Cable

Fuse

Transformer

CB1

CB2

Synchronous

motor

---

----

----

----

ABB

GE Multilin

735/737

(Lib-220)

ICEA

Seimens

A500-2.54KV

Lib 1HP

0.46KV(MTR)

200MVASc

12.47kV

12.47KV

12.47KV

300:5

300:5

1000KVA

Pri-12.47KV

Sec-0.48KV

POWER-

1000KVA

Siemen Allis

LA-1600A

Siemens

Static-Trip III

2HP

0.46KV

X/R=5



relay

TCC KA

Phase-12.47

1.length-100

Tolerance-0


Z&X/R ratio


77

ETAP NETWORK DIAGRAM:

PROCEDURE:

Open ETAP software in the computer.

Give the file name and save it before .

Open the edit mode tab and construct the circuit. .

Give the specified ratings to the equipments given in tabular column below.

Check for errors and now click on the “ star protective devices ” icon on the top of edit mode

tab.

On the right hand side there is a toolbox click on the red fault icon and apply fault at desired

location and get the sequence of protective devices.


78

Simulation Results: Sequence of Tripping observed for a given network


79

RESULT:



80

Date: Experiment-13

TESTING OF NEGATIVE SEQUENCE RELAY

Aim: To study the operation of Negative sequence relay.

Apparatus: Negative sequence relay



Ammeter (0-2A)


Theory:

Need for Negative Sequence Protection

Primary cause of motor failure is excessive heating, which if sustained over long time periods

will result in motor burn out. Overheating also reduces the life of motor. If a motor is

continuously over heated by just 10 degrees, its life can get reduced by almost 50%.

Overheating normally occurs due to over current, which in turn may be due to over loads or

locked rotor condition or low voltage or phase failure or repeat starts or phase unbalance.

Bimetallic relays are most economical solution for heating due to over loads. However they

suffer from inherent deficiencies like poor accuracy, rigid inverse time characteristics, poor

repeatability etc. They are totally insensitive to current unbalance, which is one of the major

contributors to overheating in motors.

Though the three-phase motor is supposed to be a balanced load, current unbalance occurs

frequently in motor feeders due to following:

voltage unbalance in the feeder supply

phase reversal

single phasing

Current unbalance in a motor is best represented by the presence of excessive negative sequence

component in the motor current. Consequently it is necessary to protect motors against negative

sequence.


81

When the power supply to the motor is unbalanced, the unbalanced voltage and the resulting

unbalanced currents in the three phases can be resolved into three balanced components as

follows :

Positive Sequence component: This component is in the same phase sequence as that of the

motor current. All its three phases are perfectly balanced - they are equal in magnitude and are

displaced by 120 degrees. The positive sequence component represents the amount of balance in

the power supply and consequently is instrumental in delivering useful power.

Negative Sequence component: This component has a phase sequence opposite to that of the

motor current hence the name negative sequence. It represents the amount of unbalance in the

feeder. All its three phases are perfectly balanced - they are equal in magnitude and are

displaced by 120 degrees. This component does not produce useful power - however by being

present it contributes to the losses and causes temperature rise.

Zero Sequence component: This, if present, represents extent of earth fault in the feeder. All

its three phases are in the same direction.

Negative sequence relay:

The negative relays are also called phase unbalance relays because these relays

provide protection against negative sequence component of unbalanced currents existing due to

unbalanced loads or phase-phase faults. The unbalanced currents are dangerous from generators

and motors point of view as these currents can cause overheating. Negative sequence relays are

generally used to give protection to generators and motors against unbalanced currents.

A negative sequence relay has a filter circuit which is operative only for negative

sequence components. Low order of over current also can cause dangerous situations hence a

negative sequence relay has low current settings. The earth relay provides protection for phase to

earth fault but not for phase to phase fault. A negative sequence relay provides protection against

phase to phase faults which are responsible to produce negative sequence components.

Induction Type Negative Sequence Relay:

It is commonly used negative sequence relay, The schematic diagram of this type of relay

is shown in the Fig

The central limb of upper magnet carries the primary which has a Centre tap. Due to this,

the primary winding has three terminal 1, 2 and 3. The section 1-2 is energized from the


82

secondary of an auxiliary transformer to R-phase. The section 2-3 is directly energized from the

Y-phase current.

The auxiliary transformer is a special device having an air gap in its magnetic circuit.

With the help of this, the phase angle between its primary and secondary can be easily adjusted.

Induction type Negative sequence relay

In practice it is adjusted such that output current lags by 120orather than usual 180ofrom

the input.

So, IR = Input current of auxiliary transformer

IR1 = Output current of auxiliary transformer

and IR1 lags IR by 120o

Hence the relay primary carries the current which is phase difference of IR1 and IR .

Positive Sequence Currents: The C.T secondary currents are shown in the Fig.(a). The Fig.(b)

shows the position of vector IR1 lagging IR by120o. The Fig..(c) shows the vector sum of IR1 and -

IY. The phase difference of IR1 and IY is the vector sum of IR1 and - IY. It can seen from the

Fig..(c) that the resultant is zero. Thus the relay primary current is zero and relay is inoperative

for positive sequence currents.


83

Positive sequence currents

Negative Sequence Currents: The C.T. secondary currents are shown in the Fig..(a). The

Fig..(b) Shows the position of IR1 lagging IR by 120o. The Fig..(c) Shows the vector difference of

IR1 and IY which is the relay current.

Under negative sequence currents, the vector difference of IR1 and IY results into a current I

as shown in the Fig. This current flows through the primary coil of the relay. Under the influence

of current I, the relay operates. The disc rotates to close the trip contacts and opens the circuit

breaker.

This relay is inoperative for zero sequence currents. But the relay can be made operative for the

flow of zero sequence currents also by providing an additional winding on the central limb of the

upper magnet of the relay. This winding is connected in the residual circuit of three line C.T.

This relay is called induction type negative and zero sequence relay. The schematic arrangement

of induction type negative and zero sequence relay is shown in the Fig.


84

Negative sequence currents

Induction type negative and zero sequence relay


85

Block Diagram:

CTNM 12 wiring diagram


86



2. Connect the circuit for phase B for unbalanced condition.

3. Increase the current and note the value of current & time at instant of tripping.

4. Repeat the above procedure by connecting circuit for phase C and tabulate the readings.

Tabular form:

S. No

Current (A)

Tripping Time (sec)

Phase B

Phase C

RESULT:



87

power systems lab griet/eee ps lab... · 2019-12-28 · power systems lab griet/eee 5 and...

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