Protective Relays and Their Functional Characteristics

A protective relay is an electrical switch which can automatically operate when a fault or any other abnormal conditions occur in the electrical system. It sends a signal to turn on the alarm or indicator or trip a circuit breaker to separate the faulty part from the healthy section.

The primary function of a protective relay in an electrical circuit or system is to sense the fault and initiate the operation of a circuit breaker to which it is connected.

A protective relay is one of the most important components of an electrical protection system, as it is entirely responsible for detecting the faults in the system. For selecting a right protective relay for our electrical system, it is very important for us to understand the functional characteristics of a protective relay.

In this article, we will highlight all the major functional characteristics of a protective relay.

What is a Protective Relay?

A protective relay is nothing but an automatically operated electrical switch that can detect any fault or abnormal condition in the circuit and initiate the operation of circuit breaker to trip the power supply.

The primary function of a protective relay is to detect the fault and separate the faulty section from the healthy section of the circuit to minimize the losses or damages. A typical protection relay is shown in the following figure.

This relay measures and monitors the circuit parameters in real time and initiate the operation of a circuit breaker when any abnormal condition occurs in the circuit.

Characteristics of Protective Relay

To provide effective and reliable protection to the power system, a protective relay must have the following essential functional characteristics: Selective, Fast, Stable, Reliability, Sensitivity, Simple Construction and Installation Mechanism, and Cost-effective.

These are some essentially required characteristics that a protective relay must have. Let us discuss each of these characteristics in detail.

Selectivity of a Protective Relay

The ability of a protective relay to differentiate between the faulty section and the healthy section of the electrical system is termed as selectivity. It is also known as discrimination.

Selectivity is defined as the ability of a protective relay to distinguish whether a fault lies within its zone of protection or outside it, so that it can take the appropriate action.

Therefore, an efficient and reliable protective relay should be selective, so that it can detect the point correct point where the fault occurs and does not disturb the rest part of the circuit.

Depending on different parameters, the selectivity of a protective relay can be of several different types as explained below.

Total Selectivity - The total selectivity of a protective relay is defined as the ability to detect any possible overcurrent in the electrical system. If there is a pair of circuit breakers, then the total selectivity is said to exist if the protection system can handle any value of overcurrent. This ensures that the protection provided by the smaller circuit breaker in the pair is at least equal to the larger circuit breaker.

Partial Selectivity - When the protection system can handle overcurrent up to a certain value, which is called partial selectivity value (IS), then the selectivity is termed as partial selectivity. If the circuit current becomes greater than the value IS, then the protection selectivity between the two circuit breakers will not work as expected. If we consider a pair of circuit breakers, then the partial selectivity comes into picture when the protection system can handle overcurrent up to the value IS which is less than the rated current $\mathrm{I_{rated}}$ of both of the circuit breakers.

Current Selectivity - This type of selectivity is one in which the protection is provided by recognizing that the fault point is closer to the power source of the system and have a higher short-circuit current. In this type of selectivity, we set the instantaneous protections to different current values to determine the point of fault in the system.

Time-Current Selectivity - In this type of selectivity, if a fault or overcurrent occurs in the system, then the protection system on the load-side will trip the circuit breaker faster than that the protection system do on the source side.

Energy Selectivity - Energy selectivity is a particular type of selectivity in which the current limiting characteristics of certain circuit breakers is used to quickly stop the flow of current when a short-circuit occurs. This prevents the short-circuit current from reaching its maximum value and helps protecting the system.

Protective Relays Should Act Fast

It is desirable that the protective relay should isolate the faulty section from the healthy part of the system as fast as possible. So that it can results in the following benefits -

• Prevent equipment damage,
• Not create a low voltage condition in the system, and
• Reduce the risk of occurrence of more severe faults.

Therefore, the protective relay must have high speed operation.

Protective Relays Must Be Stable

A protective relay must be stable in terms of its operation, so that it can maintain its operational state until the fault in the system is not cleared.

High Reliability

In power systems, a fault or any abnormal condition can result in severe damages to the equipment or accidents to operating personals. Therefore, it is desirable that the protective relays are designed to have a reliable operation.

To ensure a high reliability of a protective relay, it must be tested for different operating conditions.

A Protective Relay Must Have Enough Sensitivity

Another important functional characteristic of a protective relay is its sensitivity. It is defined as the ability of a protective relay to sense and respond to a fault in the electrical system.

It is desirable that a protective relay must have enough sensitivity so that it can accurately detect the any abnormality or fault in the system.

Simple Construction and Installation Mechanism

It is also important that the protective relays must be designed have simple construction and installation mechanism. This also improves the reliability of the relay.

A Protective Relay Must Be Cost Effective

It is another important characteristic of a protective relay. A protective relay must be cost effective so that it can be adopted in a protection system without much affecting the overall cost of the system.

Conclusion

An efficient protective relay must have the above-mentioned characteristics so that it can be used in a protection system to assure high safety. All these characteristics ensure high performance and effectiveness of the relay in terms of protection.