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 DIFFERENTIAL RELAY - SINCE FAULT CURRENT IS LESS THAN FULL LOAD, NORMAL O/C RELAY WILL NOT PICK UP
MOTTO OF A DIFFERENTIAL RELAY
SHOULD OPARATE FOR INTERNAL FAULT SHOULD NOT OPERATE FOR EXTERNAL FAULT
SATABILITY OF DIFFERENTIAL RELAY
TYPES OF DIFFERENTIAL RELAYS
CIRCULATING CURRENT (HIGH IMPEDANCE) PERCENTAGE BIASED (LOW IMPEDANCE)
CIRCULATING CURRENT HIGH IMPEDANCE DIFFERENTIAL RELAY
I1 - I2
RELAY OPERATING CURRENT
TRIP SETTING
IN TERMS OF CT PRIMARY (20% OF In)
RELAY WILL TRIP IF I1-I2 IS GREATER THAN 20% 0F In (In = RATING OF CT PRIMARY)
CIRCULATING CURRENT HIGH IMPEDANCE DIFFERENTIAL RELAY
Id
RELAY TRIP AREA
Is =20%
0.5
1.0
1.5
2.0
2.5
Average current Ir /In =((I1 +I2)/2)/In
CIRCULATING CURRENT HIGH IMPEDANCE DIFFERENTIAL RELAY
DISADVANTAGES : DUE TO CT CHARACTERISTICS MISMATCH, RELAY CAN DETECT A CONDITION WHERE I1-I2 EXCEEDS SET VALUE A) B) C)
UNDER NORMAL OPERATRING CONDITIONS UNDER THROUGH FAULT CONDITIONS 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 RESISTOR TO IMPROVE PERFORMANCE
CIRCULATING CURRENT HIGH IMPEDANCE DIFFERENTIAL RELAY
STABILISING RESISTOR
WILL IMPROVE PERFORMANCE OF RELAY FOR 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 OPERATING CURRENT
(I1+I2)/2
RELAY RESTRAINING CURRENT
TRIP SETTING = (BIAS %)
(I1- I2) (I1 +I2)/2
NORMALLY BIAS % = 20% RELAY WILL TRIP WHEN (2*(I1 - I2)/(I1+I2))*100 VALUE EXCEEDS TRIP SETTING
PERCENTAGE BIASED DIFFERENTIAL RELAY
INTERNAL FAULT EXTERNAL FAULT BIAS PERCENTAGE
PERCENTAGE BASED DIFFERENTIAL RELAY BIASE CHARECTERSTICS
( I1-I2)
DIFFERENCE CURRENT
RELAY OPERATES
RELAY NOT TO OPERATE
%Id
MIN I1-I2 SLOPE
I1+I2 2
SUM CURRENT
DUAL SLOPE ?
DIFFERENTIAL RELAYS
INTERPOSING CTs IN TRANSFORMER DIFFERENTIAL RELAYS FOLOWING COMPENSATIONS WILL BE REQUIRED FOR: A)
PROVIDING MAGNITUDE COMPENSATION
B)
PHASE ANGLE COMPENSATION DUE TO VECTOR GROUP SHIFT BETWEEN Py & Sy
DIFFERENTIAL RELAYS
INTERPOSING CTs INELECTROMECHANICAL / SOLIDSTATE RELAYS INTERPOSING CTs ARE USED FOR PROVIDING THE MAGNITUDE AND VECTOR COMPENSATION IN NUMERICAL RELAYS THE SOFTWARE INSIDE INSIDE THE RELAY TAKES CARE OF THESE COMPENSATION
TRANSFORMER VECTOR GROUPS
R
R
1. YY0 B
Y
B
R Y
2. YY6 B
Y R
B
3. YD1
Y
R
4. YD11
B
Y
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 CURRENT INRUSH ON ONE SIDE OF THE TRANSFORMER. THIS WILL RESULT IN A LARGE VALUE OF I1-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 TRANSFORMER THE DIFFERENTIAL RELAY MAY SENSE A DIFFERENTIAL CURRENT AND INITIATE TRIP. THIS IS DUE TO THE HIGH LEVEL OF FLUX PRESENT IN THE CORE CAUSING AN ARTIFICIAL I1-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. 2. 3. 4.
5. 6. 7. 8.
PERCENTAGE BIASED DUAL SLOPE DISPLAYS OF Id VECTOR/AMPLITUDE COMPENSATION (W/O ICTs) 2ND HARMONIC RESTRAINT 5TH HARMONIC RESTRAINT WAVE FORM CAPTURE INBUILT REF
