What is the use 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 use of restraining coil in differential relay?
In this type of relay, there are restraining coils in addition to the operating coil of the relay. The restraining coils produce torque opposite to the operating torque. Under normal and through fault conditions, restraining torque is greater than operating torque. Thereby relay remains inactive.
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 are relay coils?
A relay is an electrically operated switch. They commonly use an electromagnet (coil) to operate their internal mechanical switching mechanism (contacts). When a relay contact is open, this will switch power ON for a circuit when the coil is activated.
What is Idmt relay?
IDMT relay is inverse definite minimum time relay. It is one in which Time of operation is inversely proportional to magnitude of fault current near pickup value and becomes substantially constant slightly above the pickup value of the Relay.
What are the demerits of static relay?
Disadvantages of static relays: The components used by the static relay are very sensitive to the electrostatic discharges. The relay is easily affected by the high voltage surges. Thus, precaution should be taken for avoiding damages through voltage spikes. The relay has less overloading capacity.
What is the function of a restraining coil?
A restraining coil is one of the active coils inside what is called a differential (protection) relay. Differential relays are used to protect power-plant generators, large transmission transformers and short sections of very high current parrallel paths in transmission lines.
What is the differential current in the operating and restraining coil?
The differential current in the operating coil is proportional to (I 1 -I 2 ), and the restraining coil is proportional to (I 1 -I 2 )/2 as the operating current is connected to the midpoint of the restraining coil. For external faults both I 1 and I 2 increase and thereby the restraining torque increase, which prevents the maloperation.
What is the ratio of the operating current and restraining current?
The above graph shows that the ratio of their operating current and restraining current is fixed percentage. This relay is also called the biased differential relay because the restraining coil is also called a bias coil as it provides additional flux.