Protective Relays: Function, Features & Operation

A protective relay is basically an electrical device that detects a fault in a power system and initiates the operation of the circuit breaker to isolate the defective section or component from the rest of the system. In other words, the prime function of protective relays is the timely and discriminative clearance of system faults. In practice a particular relay is usually set to ensure that its response is such that its operation is coordinated with that of other relays on a system.

Protective relays detect the abnormal conditions in the electrical circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The electrical quantities that may change under fault conditions include: voltage, current, frequency and phase angle. Any changes in one or more of these electrical quantities, is relayed as a fault signal that denotes their presence, type and location to the protective relays. The instant the fault is detected, the protective relay operates to close the trip circuit of the circuit breaker. This results in the opening of the breaker and disconnection of the faulty circuit.

Essential Requirements of Protective Relays

The fundamental function of a protective relay is to cause the quick removal from service of any section or component of the power system when it begins to operate in an abnormal manner or to interfere with the effective operation of the rest of the system. For a protective relay system to perform this function adequately, it should have the following qualities:

• Be Selective
• Quick Speed
• Reliable
• Sensitive
• Simple
• Cost effective

Selective – A well designed and efficient relay system should be selective, i.e. it should be able to detect the point at which the fault occurs and cause the opening of the circuit breakers closest to the fault with minimum or no damage to the system.

Speed – The relay system should disconnect the faulty portion as fast as possible for the following reasons:

• Electrical equipment may be damaged if they are made to carry the fault currents for a long period.
• A failure on the system leads to a great reduction on the system voltage. If the faulty section is not disconnected promptly, then the low voltage created by the fault may shut down consumer’s motors and generators on the system may become unstable.
• The high speed relay system minimizes the possibility of development of one type of fault into another more severe type.

Sensitive – the relay system should be able to operate with low value of actuating quantity. Sensitivity of a relay is a function of the volt-amperes input to the coil of the relay needed to cause its operation. The smaller the volt-ampere input needed to cause the relay operation, the more sensitive is the relay.

Reliable – the relay system should be reliable i.e. should be able operate under the preset conditions without a problem.

Simple – a simple relay system makes it easier for it to be maintained.

Cost effective – cost is a key factor when in it comes to choosing a particular protection scheme however when the equipment to be protected is of highest significance, such as generators and main transmission lines, then this factor of cost becomes secondary.

Operation of a Protective Relay

Let’s consider the following relay circuit (only a single phase of 3-phase system shown here):

The relay circuit connections can be split into three sections:

• First part is the primary winding of a current transformer (CT) which is connected in series with the line to be protected.
• Second part consists of secondary winding of CT and the relay operating coil.
• Third part is the tripping circuit which may be either ac or dc. It comprises of a source of supply, the trip coil of the circuit breaker and the relay stationary contacts.

When a short circuit takes place at point S on the transmission line, the current flowing in the line increases tremendously. As a result, a heavy current flows through the relay coil, causing the relay to operate by closing its contacts. This in turn closes the trip circuit of the circuit breaker, making the circuit breaker to open and consequently isolates the faulty part from the rest of the system. In this manner, the relay ensures the safety of the circuit apparatus from damage and normal working of the healthy section of the system.

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