Power Sysyem Protection

1. Power System Protection
Sahdev Chandra Swarnakar
Lecturer
Dept. : EEE
Department of Electrical and Electronic Engineering
Green University of Bangladesh
Dhaka, Bangladesh

2. Content:
 CT
 Relay
 Fuse
 Directional Relay

3. Instrument transformers
In a modern power system, the circuits operate at very high voltages and carry current of thousands of amperes.
The measuring instruments and protective devices cannot work satisfactorily if mounted directly on the power
lines. This difficulty is overcome by installing instrument transformers on the power lines. The function of these
instrument transformers is to transform voltages or currents in the power lines to values which are convenient
for the operation of measuring instruments and relays. There are two types of instrument transformers viz.
(a) Current transformer (C.T.)
(b) Potential transformer (P.T.)
• The primary of current transformer is connected in the power line. The secondary winding provides for the
instruments and relays a current which is a constant fraction of the current in the line.
• Similarly, a potential transformer is connected with its primary in the power line. The secondary provides for
the instruments and relays a voltage which is a known fraction of the line voltage.
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4. (i) Current transformer (C.T.).
A current transformer in essentially a step-up transformer which steps down the current to a known ratio. The
primary of this transformer consists of one or more turns of thick wire connected in series with the line. The
secondary consists of a large number of turns of fine wire and provides for the measuring instruments and relays a
current which is a constant fraction of the current in the line. Suppose a current transformer rated at 100/5 A is
connected in the line to measure current. If the current in the line is 100 A, then current in the secondary will be
5A. Similarly, if current in the line is 50A, then secondary of C.T. will have a current of 2·5 A.Thus the C.T. under
consideration will step down the line current by a factor of 20.
(ii) Potential transformer (P.T.).
It is essentially a step down transformer and steps down the voltage to a known ratio. The primary of this
transformer consists of a large number of turns of fine wire connected across the line. The secondary winding
consists of a few turns and provides for measuring instruments and relays a voltage which is a known fraction of
the line voltage. Suppose a potential transformer rated at 66kV/110V is connected to a power line. If line voltage
is 66kV, then voltage across the secondary will be 110 V.
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5. Current transformer Potential transformer
Similar symbols of different apparatus can be found from V.K. Mehta 25th Chapter
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6. CT circuit connection. PT circuit connection.
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7. Fig. 1 The use of instrument transformers on a power line.
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8. Example of Rating of CT and PT & their advantages
Example of Rating:
• The potential transformer rated 66,000/110V provides a voltage supply for the potential coils of voltmeter and
wattmeter.
• The current transformer rated 1000/5 A supplies current to the current coils of wattmeter and ammeter.
Advantages:
The use of instrument transformers permits the following advantages :
(a) They isolate the measuring instruments and relays from high-voltage power circuits.
(b) The leads in the secondary circuits carry relatively small voltages and currents. This permits to use wires of
smaller size with minimum insulation.
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9. What is the problem if CT secondary IS OPENED ?
In the CT secondary, current is very low that’s why
voltage is very high. So, if the CT secondary is opened
then there is a high risk of electric shock.

10. Circulating current differential protection:

11. Problem of circulating current differential protection:
The protective scheme discussed in Section 2.11.1 gives a perfect discrimination if both the CTs have exactly
an identical saturation characteristic. However, in practice it is found that the characteristics of two CTs
never coincide exactly even if they are produced by the same manufacturer. This may lead to fl owing of a
spill current even though the primary currents are equal (no internal fault). If this spill current exceeds the
setting of the relay, an undesirable operation may occur. This spill current is especially large for a heavy
external (through) fault. One of the methods to avoid the uncalled operation of the relay is to provide a biased
differential scheme of protection.

12. Problem overcome by biased differential protection :

13. Problem of circulating current differential protection:

14. Problem of circulating current differential protection:

15. Fuse:

16. Induction type directional overcurrent relay:

18. Induction type directional overcurrent relay:

19. Best of luck