DIFFERENTIAL-PROTECTION.ppt

DIFFERENTIAL PROTECTION

PROTECTION OF AC MACHINES AGAINST INTERNAL FAULTS

INTERNAL & EXTERNAL FAULTS

TR 1

Why a normal O/C relay is inadequate ?

DIFFERENTIAL RELAY

-

SHOULD TRIP INSTANTANEOUSLY FOR AN INTERNAL FAULT

-

SHOULD NOT TRIP FOR AN EXTERNAL FAULT

-

NORMAL SETTINGS FOR DIFFERENTIAL PICK UP = 15%

Differential Protection

Ip

Ip TA

TA Protected Equipment

50

Ip - Iel

Ie2 - Iel Ip - Ie2

Differential Protection

T.A

T.A Protected Equipment

IL

IR

IL

IR

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