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What is Static Relay ？

Edwiin

04/15/2025

What is Static Relay ？

Definition: A relay that has no moving parts is referred to as a static relay. In this type of relay, the output is generated by static components such as magnetic and electronic circuits. Even if a relay combines static elements with an electromagnetic relay, it is still called a static relay. This is because the static units are responsible for sensing the input and generating a response, while the electromagnetic relay is merely utilized for the switching operation.

The components of a static relay are depicted in the figure below. The input of the current transformer is connected to the transmission line, and its output is fed to the rectifier. The rectifier rectifies the input signal and forwards it to the relaying measuring unit.

The rectifying measuring unit consists of comparators, a level detector, and a logic circuit. The output signal from the relaying unit is obtained only when the input signal reaches the threshold value. The output of the relaying measuring unit serves as the input to the amplifier.

The amplifier magnifies the signal and provides the output to the output devices. The output device activates the trip coil only when the relay is in operation. The output is derived from the output devices only when the measurand has a well - defined value. Once activated, the output device issues a tripping command to the trip circuit.

Static relays respond solely to electrical signals. Other physical quantities such as heat, temperature, etc., need to be first converted into analogue or digital electrical signals before being used as inputs for the relay.

Advantages of Static Relay

The following are the advantages of static relays:

Static relays consume very little power. As a result, the burden on measuring instruments is reduced, and their accuracy is enhanced.
They offer a quick response, long lifespan, high reliability, high accuracy, and are shockproof.
The reset time of the relay is extremely short.
There are no thermal storage issues associated with them.
The relay amplifies the input signal, which increases its sensitivity.
The likelihood of unwanted tripping is low.
Static relays can operate easily in earthquake - prone areas due to their high shock resistance.

Limitations of Static Relay

The components used in static relays are highly sensitive to electrostatic discharges. Electrostatic discharges refer to the sudden flow of electrons between charged objects. Hence, special maintenance measures must be implemented for the components to prevent them from being affected by electrostatic discharges.
The relay is vulnerable to high - voltage surges. Therefore, precautions should be taken to avoid damage from voltage spikes.
The operation of the relay depends on electrical components.
It has a relatively low overloading capacity.
Static relays are more expensive compared to electromagnetic relays.
The construction of the relay is easily influenced by surrounding interference.

For integrated protection and monitoring systems, programmable microprocessor - controlled static relays are preferred.

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Power Systems Switchgear Circuit breaker

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