What is restraining current in differential relay?

This is the differential current, which is the phasor or instantaneous sum of the currents flowing into the zone of protection. Restraint current. This is some measure of the current flowing through the zone of protection.

What is biased differential relay?

The biased-differential relay is a low-impedance relay and compares with the high-impedance relay as follows: Advantages of biased scheme: Can operate with CTs with a lower knee point and less accuracy, since the relay is of a low-impedance type. The bias feature compensates for any CT.

What is the function of differential protection relay?

One of the most important protection in power system is differential protection. It is used to protect the electrical equipment against internal fault such as internal winding short circuit or bus bar phase to phase short circuit, inter turn short, winding puncture etc.

Why is a biased differential relay preferred over a simple differential relay?

Biased differential relay is preferred because its operation is not affected by the trouble arising out of the difference in the CTs ratios for high values of external short circuit currents.

What is the purpose of restraining coil?

The restraining coil controls the sensitive characteristic of the relay. It restricts the unwanted tripping of the transformer due to the imbalance current. The restraining coil also restrains the harmonics in the inrush current.

What is the purpose of back up protection?

The backup protection provides the backup to the main protection during a short circuit whenever it fails in operation or is cut out for repairs. The backup protection is essential for the proper working of the electrical system.

What is the minimum current in the relay coil at which the relay starts to operate?

pick-up current
The minimum current in the relay coil at which the relays start to operate is known as pick-up current. So as long as the current in the relay is less than the pick-up value, the relay does not operate and the circuit breaker controlled by it remains in the closed position.

What is restraining coil and operating coil?

The operating coil places between the midpoint of the restraining coil. The restraining coil controls the sensitive characteristic of the relay. It restricts the unwanted tripping of the transformer due to the imbalance current. The restraining coil also restrains the harmonics in the inrush current.

What do you mean by differential protection?

Differential protection is a unit-type protection for a specified zone or piece of equipment. It is based on the fact that it is only in the case of faults internal to the zone that the differential current (difference between input and output currents) will be high.

Why Is percent differential protection superior to simple differential protection?

So generally the percentage differential relay scheme is more stable than a simple differential relay scheme. And stable means that the relay will operate only for real internal fault in the protected zone, and it will never operate for through faults or due to any other mismatch.

Why are differential relays more sensitive than over current relays?

Differential relays are used to protect transformers and generators from local faults. Overcurrent relays depend on excess current for their operation. Differential relays are very sensitive to the faults occurred within the zonee of protection . …

What is the function of restraining coil in differential relay?

What is the function of restraining coil in differential relay? as well as restraining current?.. restraining coil is given to avoid mal-functioning of differential relay.

What is the difference between high-impedance relay and biased-differential relay?

The biased-differential relay is a low-impedance relay and compares with the high-impedance relay as follows: Advantages of biased scheme: Can operate with CTs with a lower knee point and less accuracy, since the relay is of a low-impedance type. The bias feature compensates for any CT.

How does percentage differential relay protection work?

Percentage-differential relay protection can be applied to the system elements having more than two terminals, as in the three-terminal application shown in Fig. 3.62. Each of the three restraining coils has the same number of turns, and each coil develops a torque independently of others.

What happens to a CT relay when it fails?

During normal operating conditions the three secondary currents of CTs are balanced and no current flows through the relay coil. But during fault in the protected zone, the balance is disturbed and differential current flows through the relay operating coil and when the differential current exceeds the relay’s pick-up value, the relay operates.