Stability of a Differential Relay

Stability of a differential relay

Instructed By: K. L .J .Jayarathna

Name

: MHMS. Kariapper

Index No

: 100239P

Group

: G5

Date of Performance

: 14/08/2013

Date of Submission

: 28/08/2013

OBSERVATION SHEET Name

: Kariapper MHMS

Index Number

: 100239P

Experiment

: Stability of a differential relay

Group

: 05

Date of Performance

: 14-08-2013

Instructed By

:

1. Variation of spill current for different values of pilot wire resistance Stabilizing resistance Rs=0 Primary current I=5A Pilot resistance ( Ω ) 3 4 5 6 7 8 9 10

Spill current ( mA ) 10 24 42 54 70 78 88 102

Secondary current through CT2 ( A ) 1 1 1 1 0.95 0.95 0.9 0.9

2. Variation of stabilizing resistance for different values of pilot wire resistance

Spill current Primary current Pilot resistance ( Ω ) 7 8 9 10

IR = 50 mA I = 5A Stabilizing resistance ( Ω ) 3.4 30.6 45.8 58.3

3. Variation of primary current with different values of pilot wire resistance

Stabilizing resistance Rs=0 Spill current IR = 50 mA

Pilot resistance ( Ω ) 6 7 8 9 10

Primary current ( A ) 5.1 4.1 3.52 3.45 3

Variation of primary fault current with different values of pilot resistance when the secondary terminals of one current transformer is reversed Applied voltage V=0 Stabilizing resistance Rs=0 Spill current IR = 100 mA Pilot resistance ( Ω ) 102 74.3 55.3 35.7 13.4

Primary current (m A ) 350 320 295 275 270

Graphs :

1) Spill current Vs pilot wire resistance ( when Rs=0, Primary current 5A)

Pilot wire resistance ( Ω )

Spill current ( mA )

0

0

3

10

4

24

5

42

6

54

7

70

8

78

9

88

10

102

The graph of Spill current Vs Pilot wire resistance 120

110

Spill current ( mA )

100

90

80

70

60

50

40

30

20

10

0 0

2

4

6

8

10

Pilot wire resistance (Ω)

12

2) Stabilizing resistance Vs Pilot wire resistance (when IR=0, Primary current 5A )

Pilot resistance ( Ω )

Stabilizing resistance ( Ω )

7

3.4

8

30.6

9

45.8

10

58.3

The graph of Stabilizing resistance Vs Pilot wire resistance

70

60

Stabilizing resistance (Ω)

50

40

30

20

10

0 5

6

7

8

9

10

Pilot wire resistance (Ω)

11

3) Primary current Vs Pilot wire resistance (Rs=0, IR=0 ) Pilot resistance ( Ω )

Primary current ( A )

6

5.1

7

4.1

8

3.52

9

3.45

10

3

The graph of Primary current Vs Pilot wire resistance

6

5

Primary current (A)

4

3

2

1

0 5

6

7

8

9

Pilot wire resistance (Ω)

10

11

4) Primary fault current Vs Pilot wire resistance (V = 0, Rs=0, IR = 100mA)

Pilot resistance ( Ω ) 102

Primary current (m A ) 350

74.3

320

55.3

295

35.7

275

13.4

270

The graph of Primary fault current Vs Pilot wire resistance

360

350

340

Primary fault current (mA)

330

320

310

300

290

280

Pilot wire resistance (Ω) 270 20

30

40

50

60

70

80

90

100

110

Discussion: (1) Applications of differential relay

Differential relays take a variety of forms, depending on the equipment they protect. The definition of such a relay is “one that operates when the vector difference of two or more similar electrical quantities exceeds a predetermined amount. It will be seen later that almost any type of relay, when connected in a certain way, can be made to operate as a differential relay.. Most differential-relay applications are of the “current-differential” type. The simplest example of such an arrangement is shown. The dashed portion of the circuit of represents the system element that is protected by the differential relay. This system element might be a length of circuit, a winding of a generator, a portion of a bus, etc. A current transformer (CT) is shown in each connection to the system element. The secondaries of the CTÕs are interconnected, and the coil of an overcurrent relay is connected across the CT secondary circuit. This relay could be any of the a-c types that we have considered.

(2) Biasing of relay

Large external fault may cause false operation of simple differential relay. To make the differential relay more stable to external faults and to improve relay quality, restraining coils can be inserted. Two restraining (biasing) coils and one operating coil are used. Restraining coils will oppose the operation of operating coil. The relay will operate only when the operating force is higher than restraining force.