Static Relay - its Definition, Block Diagram, Advantages & Limitations

It can be seen that there is a rapid increase in the expansion of transmission and distribution systems and power stations day by day. This results in the increase of duty imposed on the switchgear equipment. Since the relays now have to perform much more complicated functions which have become difficult in the case of electromagnetic relays due to their increased mechanical complexity, cost, testing, and maintenance.


But due to the advancements in semiconductor technology, the static relays supersede all the conventional electromagnetic relays that do not use any moving parts. A static relay or solid-state relay is a relay that employs electronic components such as semiconductor diode, transistors, thyristors, logic gates, etc for the protection of the power system.


The static relay works on the comparison or measurement of electrical quantities. It uses comparators, level detectors, zero-crossing detector, etc that uses static electronic circuits for the comparison, and gives an output signal for tripping of the circuit breaker. The static relay doesn't use any moving parts such as armature in an electromagnetic relay. But the tripping circuit used to disconnect breaker contacts can be either electronic or electromagnetic. Let us see the basic components of static relays.


Basic Elements of Static Relay :

The below shows the block diagram of the static relay with essential elements.

Static Relay - its Definition, Block Diagram, Advantages & Limitations

Input Element :

Generally, the input to the relay will be either current or voltage or both the quantities obtained from the CT and PT connected in the circuit to be protected. But in the case of static relays, it cannot be directly connected to the CT and PT. Thus an electronic circuit such as a rectifier is used to convert the input quantity obtained from the CT and PT into a convenient form that can be measurable by the measuring element. Some mixing circuits such as op-amp and adder are also used in the input element circuit and sometimes the input signal obtained can be from a transducer or combination of various signals.


Measuring Element :

It is the major element in the whole relay. Basically, it comprises a comparator that compares the output signal from the input element with a preset value. It generates an output signal based on the comparison i.e., when there is a fault in the circuit it protects, and the signal is applied to the output element which in turn drives the tripping circuit.


Output Element :

The output element is basically an amplifier. The output obtained from the measuring element needs to be amplified before applying it to the trip circuit. Thus the output element strengthens the output signal obtained from the measuring element with the help of an amplifier which is used to drive the trip circuit. Other than amplifiers it also contains multipliers that combines the signal with other signals to delay them.


Feed Element :

The feed element provides the required dc supply to all the electronics circuits along with the tripping circuit present in the relay.

Advantages of Static Relays :

  • In static relays moving parts and contacts are greatly reduced as the designed response is being developed by incorporating solid-state components like transistors and diodes, without mechanical motion. Hence, friction or contact losses are absent as a result of minimum. maintenance is required.
  • Static relays have a high speed of operation.
  • They have greater sensitivity as amplification of signals can be obtained very easily by the use of semiconductor devices.
  • Resetting time and overshoots can be reduced.
  • The use of printed circuits avoids wiring errors and facilitates the rationalization of batch production.
  • Several functions can be accommodated in a static relay.
  • Static relays have superior characteristics and accuracy.
  • Static relays consume less power compared to conventional electromechanical relays and provide less burden on the CTs and PTs.
  • By making use of the linear couplers in place of CTs reduces the cost and solves the difficulty of the DC component of the fault current.
  • Simplified testing and servicing are possible.
  • They have excellent mechanical stability which is not possible with the electromechanical relays.
  • They are not affected by the vibrations as they are compact in size.
  • They have a long life.

Limitations of Static Relays :

  • The characteristics of semiconductors devices (such as transistors, diodes, etc.) used in static relays vary with temperature and aging.
  • They are sensitive to voltage transients.
  • They are not robust in construction.
  • The reliability of static relays depends upon a large number of small components and their electrical connections.
  • Easily affected by surrounding interference.
  • Static relays have low short-time overload capacity compared with electromechanical relays.
  • These relays could not meet practical requirements and hence are never used for commercial purposes.

Difference Between Static and Electromagnetic Relays :

Static RelaysElectromagnetic Relays
Static relays have no moving parts. All the components are static.Electromagnetic relays possess moving parts which lead to problems such as contact bounce, arcing, contact erosion, spring restraint, etc.
Static relays possess a reduced burden on protective current transformers and potential transformers due to lower VA requirements of the static circuits.In electromagnetic relays, the burden on the CTs and PTs is high due to higher VA.
Static relays have a very small operating time, hence it gives a faster response.The response is comparatively slow.
Relaying characteristics could be controlled more precisely. But these characteristics vary with temperature and aging.Relaying characteristics are lesser precise due to the moving core in the electromagnetic relays.
In this type of relays, the sensitivity attained is more due to the amplification.Less sensitive due to higher VA.
Compact size due to small-sized ICs.Bulky in size due to the presence of armature disc etc.
Comparatively less robust.Comparatively robust.
This type of relay requires lesser maintenance.Maintenance required is more due to the bearing friction, contact trouble, etc.
Static relays are reliable.Electromechanical relays have high reliability due to the smaller number of components and experience gained in manufacturing practices.
Overload capacity is comparatively less.Good overload capacity.
Comparatively lesser manufacturing difficulties.Manufacturing difficulties are more.

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