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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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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
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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 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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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 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