47764564 differential protection

7/29/2019 47764564 Differential Protection

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DIFFERENTIAL PROTECTION

PROTECTION OF AC MACHINESAGAINST INTERNAL FAULTS

Document BySANTOSH BHARADWAJ REDDYE mail: [email protected]

More Papers and Presentations available on above site


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INTERNAL & EXTERNAL FAULTS

Why a normal O/C relay is inadequate ?

TR 1


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DIFFERENTIAL RELAY

- SHOULD TRIP INSTANTANEOUSLYFOR AN INTERNAL FAULT

- SHOULD NOT TRIP FOR AN EXTERNAL FAULT

- NORMAL SETTINGS FOR DIFFERENTIAL PICK UP = 15%


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Differential Protection

ProtectedEquipment

50 Ie2 - Iel

Ip

Ip - Iel Ip - Ie2

IpTA TA


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Differential Protection

ProtectedEquipment

IL IR IL IR

T.A T.A


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PRINCIPLE OF OPERATION OF DIFFERENTIAL RELAYS

CIRCULATING CURRENT PRINCIPLE

G1


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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 ISHIGHER 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


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MOTTO OF A DIFFERENTIAL RELAY

SHOULD OPARATE FOR INTERNAL FAULT

SHOULD NOT OPERATEFOR EXTERNAL FAULT

SATABILITY OFDIFFERENTIAL RELAY


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TYPES OF DIFFERENTIAL RELAYS

CIRCULATING CURRENT(HIGH IMPEDANCE)

PERCENTAGE BIASED(LOW IMPEDANCE)


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CIRCULATING CURRENTHIGH IMPEDANCE DIFFERENTIAL RELAY

I1 - I2 RELAY OPERATINGCURRENT

TRIP IN TERMS OFSETTING CT PRIMARY

(20% OF In)

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


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Id

Is =20%

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

0.5 1.51.0 2.52.0

RELAY TRIP AREA


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DISADVANTAGES :

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

A) UNDER NORMAL OPERATRING CONDITIONSB) UNDER THROUGH FAULT CONDITIONS

C) WHENEVER THE TAPS ARE CHANGED INTRANSFORMERS

THIS IS MAINLY DUE TO :DC OFFSET EFFECT ANDASSYMMETRIC CT RESPONSE UNDER LARGETHROUGH FAULT CURRENTS.


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CIRCULATING CURRENT(HIGH IMPEDENCE) DIFFERANTIAL RELAY

ADD ASTABILISING RESISTOR TO IMPROVEPERFORMANCE


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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 REALINTERNAL FAULT.

FOR TOTAL IMPROVEMENT

USE PERCENTAGE BIASED RELAY


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BIASED DIFFERENTIAL

I1 - I2 RELAY OPERATINGCURRENT

(I1+I2)/2 RELAY RESTRAININGCURRENT

TRIP SETTING = (I1- I2)

(I1 +I2)/2(BIAS %)

NORMALLY BIAS % = 20%

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

VALUE EXCEEDS TRIP SETTING


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BIASED DIFFERENTIAL

Refer excel sheet


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PERCENTAGE BIASED DIFFERENTIAL RELAY

INTERNAL FAULTEXTERNAL FAULT

BIAS PERCENTAGE


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PERCENTAGE BASED DIFFERENTIAL RELAY BIASE CHARECTERSTICS

I1+I22

%Id

( I1-I2)

RELAY NOT TO OPERATE

RELAY OPERATES

SUM CURRENT

DIFFERENCE

CURRENT

MIN I1-I2SLOPE

DUAL SLOPE ?


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DIFFERENTIAL RELAYS

INTERPOSING CTs

IN TRANSFORMER DIFFERENTIAL RELAYS

FOLOWING COMPENSATIONS WILL BEREQUIRED FOR:

A) PROVIDING MAGNITUDE COMPENSATION

B) PHASE ANGLE COMPENSATION DUETO VECTOR GROUP SHIFT BETWEENPy & Sy


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DIFFERENTIAL RELAYS

INTERPOSING CTs

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

IN NUMERICAL RELAYS THE SOFTWARE INSIDEINSIDE THE RELAY TAKES CARE OF THESE

COMPENSATION


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TRANSFORMER VECTOR GROUPS

1. YY0

2. YY6

3. YD1

4. YD11

R

YB

R

YB

R

YB

R

YB

R

YB

YB


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PHASE ANGLE COMPENSATIONTHROUGH INTERPOSING CTS


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CONNECTION DIAGRAM MD32-T RELAY

WHY REF ?


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RESTRAINTS IN DIFFERENTIAL RELAYS

2ND HARMONIC RESTRAINT

WHEN A TRANSFORMER IS SWITCHED ON,

THE DIFFERENTIAL RELAY MAY SENSE ADIFFERENTIAL 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 OPERATIONAT THE TIME OF SWTICH ON BY THE2ND HARMONIC RESTRAINT FEATURE.


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RESTRAINTS IN DIFFERENTIAL RELAYS

5TH HARMONIC RESTRAINT

WHEN OVER FLUXING OCCURS IN A TRANSFORMER

THE DIFFERENTIAL RELAY MAY SENSE ADIFFERENTIAL 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 OPERATIONAT THE TIME OF OVER FLUXING BY THE5TH HARMONIC RESTRAINT FEATURE.


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SALIET FEATURES OF MD32 T

1. PERCENTAGE BIASED2. DUAL SLOPE3. DISPLAYS OF Id4. VECTOR/AMPLITUDE

COMPENSATION(W/O ICTs)

5. 2ND HARMONIC RESTRAINT

6. 5TH HARMONIC RESTRAINT7. WAVE FORM CAPTURE8. INBUILT REF

47764564 differential protection

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