Static relays

Brief

What is a static relay

There are plenty of relays in the market but what separates the static relay is that this relay does not have any moving parts in it and it purely contains only electronic elements (such as diodes, BJT, etc.) therefore, it is also known as a solid-state relay.

Block diagram of static relay

The static relay consists of a rectifier, amplifier, output unit, and relay measuring circuit. The relay measuring circuit consists of the level detectors, logic gate, and the comparators (amplitude and phase comparators).

The Block diagram of the static relay is shown below

block diagram of static relay

The transmission line is connected to the CT or PT i.e., that transmission line feeds the input to the current transformer or the potential transformer.

The output of the CT is supplied as an input to the rectifier. Here, the rectifier is responsible for rectifying the input alternating current signal into the direct current (linear) signal. And that direct current signal is applied to the relay measuring unit.

The relay measuring unit performs the most important action required in the static relay system. It detects the level of the input signal through the level detectors, compares the magnitude and the phase of the signal through the comparators, and performs the logic gate operations.

Two types of comparators are present in this relay unit amplitude comparator and a phase comparator.

The amplitude comparator compares the magnitude of the input signal and the phase comparator compares the phase difference of the input quantity.

The output of the relay measuring unit is applied to the amplifier. The amplifier amplifies the magnitude of the signal and fed it to the output device.

The output device energizes the trip coil which trips the circuit breaker.

The amplifier, relay measuring unit, and the output device need an additional dc supply for their operations. This is a major disadvantage that static relay has.

Static relay working principle

To check whether there is any fault in the system or not, the solid-state relay proceeds in the following manner.

First, the CT or PT takes the input quantity signal (it can be voltage or current) from the transmission line and fed the output to the rectifier. The rectifier then converts the alternating current signal into the direct current signals and passes its output to the relay measuring unit.

Now the relay measuring unit detects the level of the input signal. Then comparators present in the unit compare the magnitude and the phase of the signal. The comparators compare the input signal to check whether the signal is faulty or not.

Then the amplifier amplifies the magnitude of the signal and sends it to the output device which energizes the trip coil which trips the circuit breaker.

Advantages

  • It shows high accuracy
  • Less maintenance required
  • Response time is very fast
  • Static relays provide resistance to vibrations and shock
  • Resetting time is very quick
  • Consume less power and they draw power from auxiliary dc supply
  • Compact size
  • Operates for a very long period

Disadvantages

  • It is sensitive to temperature and voltage. A spike in temperature or voltage can cause component damage or complete failure.
  • Expensive than the electromagnetic relay
  • It needs a secondary supply
  • It has less overloading capacity

Applications

  • Overcurrent schemes
  • Earth fault protection schemes

Leave a Comment

Your email address will not be published. Required fields are marked *