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DISCUSSION

1.

Application of differential relay

Microprocessor-based current differential relays offer superior protection for power transmission

lines. The key advantages over distance relays include better sensitivity for high resistance faults,

100% line protection, and better performance in the single-pole-tripping mode, particularly during

evolving and cross-country faults

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. In other words, it is not

so much the relay construction as the way the relay is connected in a circuit that makes it a

differential relay.

Most differential-relay applications are of the current-differential type

The following figure represents the system element that is protected by the differential relay

There are three general categories of differential relays used in bus applications:

Differentially connected overcurrent (instantaneous or inverse time)

Percentage-restrained differential

High-impedance differential

In deciding what type of protection system to apply for a specific application, protection engineers

consider cost, complexity, reliability, and performance. The performance attributes they evaluate

are selectivity, sensitivity, and speed.

Differential protection is often applied on bus protection for its high selectivity. All current into and

out of a zone of protection is measured. The zone of protection isprecisely determined by thelocation of the current transformers that define the differential zone. With high selectivity, a

differential relay does not need to have any intentional time delay to coordinate with relays in

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adjacent zones. Thus, differential protection can provide relatively high speed. As mentioned above,

there are a number of different types of differential relays. Each has different levels of performance

with regard to ultimate speed, selectivity, and sensitivity.

2. Biasing of Relay

In the differential relays theoretically the all the conditions and the characteristics are identical. But,in practical this is difference due to the inequality of the two CTs. This difference leads the relay to

flow of spill current even if there is no fault. If the spill current exceeds the set value then the relay

may result an undesirable operation. To prevent this, in practice the biasing of relay is required.

To make the differential relay more stable to external faults and improve relay quality, its

respectively to operation was increased by inserting restraining coils. Two restraining (Biasing) coils

and one operating are used as shown in figure. Restraining coils will opposite the operation of

operating coil. The relay will operate only when the operating force is higher than restraining force.

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OBSERVATION SHEET

NAME : I N Kodikara

INDEX NO : 110300F

GROUP : G 10

DATE OF PER : 04.06.2014

DATE OF SUB :

INSTRUCTED BY :

When Rs = 0& I= 5A

When I= 5A & IR = 25mA

when Rs= 0& IR = 25mA

Wire resistance

Rp ()

10 9 8 7 6 5 4

Spill current

IR(A)

59 54 48 43 36 27 17

Wire resistance

Rp ()

10 9 8 7 6 5

stabilizing

resistance

Rs ()

127.9 109.5 85.5 64.1 32.9 2.5

Wire resistance

Rp () 10 9 8 7 6 5

Primary

current I (A)

2.3 2.6 3.0 3.3 4.0 5.0

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when Rs= 0& IR = 25mA

Wire resistance

Rp ()

0 20 40 60 80 100

Spill current

IR(A)

0.21 0.215 0.24 0.26 0.27 0.29

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Rp () IR(A)

10 59

9 54

8 48

7 43

6 36

5 27

4 17

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Spill current (IR) Vs pilot wire resistance (Rp)

10

20

30

40

50

60

70

3.5 4.5 5.5 6.5 7.5 8.5 9.5 10.5

spill

current(A)

Pilot wire resistance ()

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Rp () Rs ()

10 127.9

9 109.5

8 85.5

7 64.1

6 32.9

5 2.5

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Stabilizing resistance (Rs) Vs pilot wire resistance(Rp )

0

20

40

60

80

100

120

140

4 5 6 7 8 9 10 11

Rs()

Rp()

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Rp () I(A)

10 2.3

9 2.6

8 3.0

7 3.3

6 4.0

5 5.0

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Pilot wire resistance (Rp) Vs primary current(I)

0

1

2

3

4

5

6

4 5 6 7 8 9 10 11

Rp (

)

I (A)

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Rp () I(A)

0 0.21

20 0.215

40 0.24

60 0.26

80 0.27

100 0.29

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0.2

0.21

0.22

0.23

0.24

0.25

0.26

0.27

0.28

0.29

0.3

0 20 40 60 80 100 120

I(A)

Rp ()

Primary current (I) Vs pilot wire resistance (Rp)