Characteristics of Protection Relays, Classification, Universal Torque Equation, Capacitor Protection Relays

Characteristics of Protection Relays 

Any Protection Relay should fulfill the following functional Characteristics.

Selectivity and


Reliability means that the relay will act when it is required to act. This is ensured by making sure


Sensitivity refers to the characteristic of the relay to act when the actual fault conditions occur. Sensitivity is usually represented in terms of the minimum volt-amperes required for the relay operation.


The relay should act according to the present time delay. It should neither operate too fast or too slow. If it is too slow it can cause damage to the equipment, if it operates too fast it may unnecessarily trip equipment for transient faults.


Selectivity refers to the ability of the relay to discriminate between faults. This is critical as only the smallest possible section of the power system should be taken out of line in the event of a fault. The relay should be able to discriminate between a transient fault and a through fault. It should be able to differentiate between downstream faults and immediate faults.
that all the components of the protection from the voltage and current signals to the dc power supply for the trip circuit to the internal components of the relay are checked for for functionality and integrity. The failure of any one of these components can result in the failure of the relay to act affecting the reliability.

Classification of Relays

Relays can be classified on the basis of their function into five broad categories. They are Protective, Regulating, reclosing synchronism Check and Synchronizing, monitoring and Auxilliary.

Protection Relays
Protection relays are used in generators, transformers, feeders, transmission lines, etc. The primary function of these relays is to continually monitor a specific parameter such as current, voltage or power and to generate alarm/isolate the system or device in the situation of deviation from set limits for the parameter or a fault. For instance, an overcurrent relay may be programmed to operate when the current in a feeder exceeds a certain predetermined limit. These relays generally obtain their feedback from current or voltage transformers.

Regulating Relays
These relays are used to regulate a specific parameter such as the output voltage of a transformer. These relays operate a control equipment such as the tap changer of a transformer. These relays are not designed to respond to fault conditions.

Reclosing Relays,
These relays are used to put the system into operation. These relays are used to synchronize lines and feeders. These relays usually are used in connecting different components of an electrical distribution system such as generators, feeders, transformers, etc. They also come into play when restoring the system after a fault.

Monitoring relays
These relays are used to monitor conditions in a system such as the direction of power flow and generate alarms when there are deviations. Examples include the low forward power relay which generates an alarm when the power in a direction falls below the minimum set points. They are also used to monitor the continuity of systems such as pilot wires.

Auxilliary Relays
These relays are used generally for contact multiplication. The single contact available in a relay is used to trip a number of breakers. Besides, these relays are also to isolate the relay from other equipments such as breakers.

The Universal Torque Equation for Protection Relays

The Universal Torque Equation is a equation which governs the application of all types of relays. The equation has variables and constants which can be ignored for specific functions.

This equation can be used to describe the operation of any Electrical Relay by changing the signs of some of the terms or ignoring them entirely.

For example, to describe the overcurrent relay, K2 and K3 can be considered zero while K will be negative as it is used to describe the restraining torque.

The Equation will then become


In the case of a directional power relay, K1 and K2 can be considered to be zero while K can be considered to be negative.

Capacitor Protection Relays 

Capacitors Protection Relays are dedicated relays which are designed to provide a range of protection functions for capacitors.

Capacitors are widely used in power systems for VAr regulation and PF control.  They are also used for filtering harmonics.

Capacitor banks need to be protected against overload by harmonic currents. They also need to be protected against system overvoltages.

Capacitor Protection Relays consist of a number of different protection elements such as overcurrent, overvoltage, differential protection, etc.   They also have protective interlocks such as preventing an energised capacitor from being connected to the network.