DIFFERENTIAL PROTECTION

PROTECTION OF AC MACHINESAGAINST INTERNAL FAULTS

INTERNAL & EXTERNAL FAULTS

Why a normal O/C relay is inadequate ?

TR 1

DIFFERENTIAL RELAY

- SHOULD TRIP INSTANTANEOUSLYFOR AN INTERNAL FAULT

- SHOULD NOT TRIP FOR AN EXTERNAL FAULT

- NORMAL SETTINGS FORDIFFERENTIAL PICK UP = 15%

Differential Protection

P ro tec ted E q u ip m en t

5 0 Ie2 - Ie l

Ip

Ip - Ie l Ip - Ie2

IpTA TA

Differential Protection

P ro tec ted E q u ip m en t

IL IR

IL IR

T.A T.A

PRINCIPLE OF OPERATION OF DIFFERENTIAL RELAYS

CIRCULATING CURRENT PRINCIPLE

G1

INTERNAL / EXTERNAL FAULTS

I1 I2 Id

100 100 0 NORMAL LOAD

500 500 0 THROUGH FAULT (500%)

500 300 200 INTERNAL FAULT (200%)

100 80 20 INTERNAL FAULT (20%)

NOTE :1. THROUGH FAULT CONDITION HAS TO BE DETECTED BY

NORMAL O/C RELAY (SINCE FAULT CURRENT IS HIGHER THAN 100%)

2. INTERNAL FAULT HAS TO BE CLEARED BY DIFFERENTIALRELAY - SINCE FAULT CURRENT IS LESS THAN FULL LOAD,NORMAL O/C RELAY WILL NOT PICK UP

MOTTO OF A DIFFERENTIAL RELAY

SHOULD OPARATE FORINTERNAL FAULT

SHOULD NOT OPERATEFOR EXTERNAL FAULT

SATABILITY OF DIFFERENTIAL RELAY

TYPES OF DIFFERENTIAL RELAYS

CIRCULATING CURRENT(HIGH IMPEDANCE)

PERCENTAGE BIASED(LOW IMPEDANCE)

CIRCULATING CURRENTHIGH IMPEDANCE DIFFERENTIAL RELAY

I1 - I2 RELAY OPERATINGCURRENT

TRIP IN TERMS OF SETTING CT PRIMARY

(20% OF In)

RELAY WILL TRIP IF I1-I2 IS GREATER THAN 20% 0F In(In = RATING OF CT PRIMARY)

CIRCULATING CURRENTHIGH IMPEDANCE DIFFERENTIAL RELAY

Id

Is =20%

Average current Ir /In =((I1 +I2)/2)/In

0.5 1.51.0 2.52.0

RELAY TRIP AREA

CIRCULATING CURRENTHIGH IMPEDANCE DIFFERENTIAL RELAY

DISADVANTAGES :

DUE TO CT CHARACTERISTICS MISMATCH,RELAY CAN DETECT A CONDITION WHEREI1-I2 EXCEEDS SET VALUE

A) UNDER NORMAL OPERATRING CONDITIONS B) UNDER THROUGH FAULT CONDITIONSC) WHENEVER THE TAPS ARE CHANGED IN

TRANSFORMERS

THIS IS MAINLY DUE TO :DC OFFSET EFFECT AND ASSYMMETRIC CT RESPONSE UNDER LARGE THROUGH FAULT CURRENTS.

CIRCULATING CURRENT(HIGH IMPEDENCE) DIFFERANTIAL RELAY

ADD A STABILISING RESISTORTO IMPROVEPERFORMANCE

CIRCULATING CURRENTHIGH IMPEDANCE DIFFERENTIAL RELAY

STABILISING RESISTOR

WILL IMPROVE PERFORMANCE OF RELAYFOR CT CHARACTERISTICS MISMATCH - TO CERTAIN LIMITS

BUT RELAY HAS BECOME INSENSITIVE DUE TO ADDITION OF RESISTOR.RELAY MAY NOT TRIP FOR A REAL INTERNAL FAULT.

FOR TOTAL IMPROVEMENT USE PERCENTAGE BIASED RELAY

BIASED DIFFERENTIAL

I1 - I2 RELAY OPERATINGCURRENT

(I1+I2)/2 RELAY RESTRAINING CURRENT

TRIP SETTING = (I1- I2)

(I1 +I2)/2(BIAS %)

NORMALLY BIAS % = 20%

RELAY WILL TRIP WHEN (2*(I1 - I2)/(I1+I2))*100VALUE EXCEEDS TRIP SETTING

PERCENTAGE BIASED DIFFERENTIAL RELAY

INTERNAL FAULTEXTERNAL FAULT

BIAS PERCENTAGE

PERCENTAGE BASED DIFFERENTIAL RELAY BIASE CHARECTERSTICS

I1+I2 2

%Id

( I1-I2)

RELAY NOT TO OPERATE

RELAY OPERATES

SUM CURRENT

DIFFERENCE CURRENT

MIN I1-I2SLOPE

DUAL SLOPE ?

DIFFERENTIAL RELAYS

INTERPOSING CTs

IN TRANSFORMER DIFFERENTIAL RELAYSFOLOWING COMPENSATIONS WILL BE REQUIRED FOR:

A) PROVIDING MAGNITUDE COMPENSATION

B) PHASE ANGLE COMPENSATION DUETO VECTOR GROUP SHIFT BETWEEN Py & Sy

DIFFERENTIAL RELAYS

INTERPOSING CTs

INELECTROMECHANICAL / SOLIDSTATE RELAYSINTERPOSING CTs ARE USED FOR PROVIDING THE MAGNITUDE AND VECTOR COMPENSATION

IN NUMERICAL RELAYS THE SOFTWARE INSIDE INSIDE THE RELAY TAKES CARE OF THESECOMPENSATION

TRANSFORMER VECTOR GROUPS

1. YY0

2. YY6

3. YD1

4. YD11

R

YB

R

YB

R

YB

R

YB

R

YB

R

Y

B

PHASE ANGLE COMPENSATION THROUGH INTERPOSING CT’S

CONNECTION DIAGRAM MD32-T RELAY

WHY REF ?

RESTRAINTS IN DIFFERENTIAL RELAYS

2ND HARMONIC RESTRAINT

WHEN A TRANSFORMER IS SWITCHED ON,THE DIFFERENTIAL RELAY MAY SENSE A DIFFERENTIAL CURRENT AND INITIATE TRIP.

THIS IS DUE TO THE MAGNETISING CURRENTINRUSH ON ONE SIDE OF THE TRANSFORMER.THIS WILL RESULT IN A LARGE VALUE OFI1-I2 FOR ABOUT 4 OR 5 CYCLES.

RELAY IS RESTRAINED FROM OPERATION AT THE TIME OF SWTICH ON BY THE 2ND HARMONIC RESTRAINT FEATURE.

RESTRAINTS IN DIFFERENTIAL RELAYS

5TH HARMONIC RESTRAINT

WHEN OVER FLUXING OCCURS IN A TRANSFORMERTHE DIFFERENTIAL RELAY MAY SENSE A DIFFERENTIAL CURRENT AND INITIATE TRIP.

THIS IS DUE TO THE HIGH LEVEL OF FLUXPRESENT IN THE CORE CAUSING AN ARTIFICIALI1-I2 FOR A SHORT TIME.

RELAY IS RESTRAINED FROM OPERATION AT THE TIME OF OVER FLUXING BY THE 5TH HARMONIC RESTRAINT FEATURE.

SALIET FEATURES OF MD32 T

1. PERCENTAGE BIASED2. DUAL SLOPE 3. DISPLAYS OF Id4. VECTOR/AMPLITUDE

COMPENSATION (W/O ICTs)

5. 2ND HARMONIC RESTRAINT6. 5TH HARMONIC RESTRAINT7. WAVE FORM CAPTURE8. INBUILT REF