Relays: Top 10 Different Types, Their Construction, Operation & Applications

Introduction

relay is an electrical device that senses the abnormal condition arising in the electrical circuit. Current transformer and trip coil are usually used as auxiliary equipment of the relay. Their combination is such that the relay gets energized when a short-circuit or overload fault occurs and the relay automatically operates and separates the faulty part from the electrical system through circuit breaker and protects the equipment from the faulty current.

defined terminology

Main protection-

The protection system which operates first when a fault occurs is called main protection. Its operating time is very short.

Secondary protection (Back-up protection)-

The protection system which operates in case of failure of the main protection system is called Secondary protection or Back-up protection. Its operating time setting is kept higher than the Main protection system.

Circuit breaker back-up –

Circuit breaker back-up – Provides isolation from the supply source after the operation of the circuit breaker fails. The circuit breaker is called back-up or we can say that it provides another switching circuit in case of failure of the circuit breake

Selective protection –

Selective protection – This protection device detects which area the fault occurs in and isolates only that area from the circuit.

Residual current and voltage

The current and voltage which is equal to the algebraic sum of the currents and voltages of the entire circuit is called residual current and voltage.

Knee point voltage

This is the voltage of the transformer C.T. which is the sinusoidal voltage of fixed frequency applied at the secondary ends of the coil when all other windings are in open circuit condition.

Power consumption –

The electrical power consumed by the relay circuit at the prescribed voltage and current is called the power consumption of that relay. It is expressed in VA for relays operating in A.C. and in watts (W) for relays operating in Dc

Plug Setting Bridge:

A device which provides calibration and multiple connections for varying the relay operating current is called plug setting bridge.

Energizing quantity-

The electrical quantity which induces the relay to operate, i.e. the quantity whose change in its direction makes the relay operate, such quantity is called energizing quantity. For example- voltage, current etc. This quantity can be alone or can be with other quantity also.

Characteristics quantity:

The electrical variable quantity on whose value the designing of the relay depends and on whose value the relay is responsive.

Pick-up value

The value of the characteristic quantity of the relay at which the relay goes from off position to on position is called pick-up value.

Drop out or reset-

The value of characteristic night at which the relay goes from ON position to OFF position It is called drop out.

Operating time:

The time taken by the relay to sense the faulty state and to get activated in that faulty state and to isolate the switch is called operating time or the time from the time of occurrence of the fault till the extinguishing of the faulty state is called operating time.

Primary relay-

This relay which is connected indirectly in the protected circuit is called primary relay. It is connected directly in the line.

Secondary relay –

This relay is connected in the protected circuit with the help of C.T. or P.T. i.e. it is connected by the secondary coil of C.T. and P.T., it is called secondary relay.

Auxiliary relay –

The relay which operates by the operation of the primary and secondary relays is called the auxiliary relay.

Trip coil

Trip coil is the coil which is connected between the relay and the circuit breaker. In a way, it is the junction of the relay and the CB. The trip coil gets energized and energizes the tripping circuit which works to open the contact of the CB.

Trip circuit-

The trip circuit is connected to the battery but this circuit is not continuous in between. When the trip coil gets energized and pulls the plunger then this circuit becomes continuous (contact) and current starts flowing due to the battery supply. As soon as current flows in the trip circuit it pulls the plunger of the C.B. due to which the C.B. becomes open.

Applications of Relays in Circuits

Selectivity –

To disconnect a faulty line from the supply and protect it from faulty current. That is, disconnect the faulty line from the supply. Due to good selectivity, the reliability of the relay is high.

Sensitivity-

The relay should operate immediately at the minimum value of fault current. The quality of the relay operating at the minimum current is called sensitivity.

Reliability:

The quality of the protective mechanism to operate at its prescribed conditions or at its prescribed capacity is called reliability. The reliability of the system should be maximum.

Quickness –

Quickness is a form of sensitivity. Quickness means the short operating time of a relay. The shorter the operating time of a relay, the greater will be its quickness.

Stability-

Stability of the protection system refers to its operation and return to normal state. The relay operating in faulty state and getting activated again after coming to normal state determines the stability of the relay.

Types of Electrical Relays and working principle

Attracted armature or moving iron type relay

It is a relay in which the plunger or the whole part is attracted by the magnetic effect of the current. When the current flows more than a certain amount, then the value of the magnetic property also increases due to which the attraction force increases and the plunger or the whole part moves away from its place and the relay gets operated.

Solenoid type relay

This relay also works on the principle of magnetic attraction but in this the attraction force is generated by solenoid.

Moving coil type relay

The relay in which the current carrying coil moves in the magnetic field produced by the supply magnet i.e. current carrying coil is present in the revolving part also and deflection is produced in them by motoring action.

Induction type relay

This relay has a metal disc or cup placed in an alternating current magnetic field and eddy currents generate a rotating force in the metal disc or cup. The metal disc or cup should not be made of magnetic material. Such as aluminum AI disc.

Thermal type relay:

A relay whose operation is based on the heat produced by the current flowing through that part. The heat is produced according to Joule’s law.

H=I2Rt: Joule

These relays are of the following types-

• Bimetallic strip relay

• Uni-metallic strip relay

• Heat sink relay

Differential relay

A relay which operates on the difference between two or more electrical quantities in any two electrical circuits, such as voltage, current, power, phase difference etc. Merge price protection system.

Gas relay:

A relay which is operated by gas pressure, such as the one fitted on an oil transformer.(Buchholz Relay)

Static relay –

A relay which does not have any complete parts. Its efficiency is reduced due to the absence of complete parts .Like electronic relay etc.

Mechanical relay

The relay which works on the principle of mechanical pressure.

Hydraulic relay:

The relay which works on the principle of fluid pressure is called fluid relay.

Working Principle of a Relay

Relay is a type of sensing protective device. Electromagnetic relay is mainly used as relay but relays are of different types according to their operation and structure. In this chapter, different types of relays and their operating principles have been explained.

The internal structure of an electromagnetic relay is usually made of an iron core and a coil of copper wire. Similarly, the internal structure of an electro-thermal relay is usually made of a metallic leaf or heating element. The classification of relays is as follows-

• EMR- Electromagnetic relay

• ETR = Electrothermal relay

• SR = Static relay (static relay or electronic relay)

• MBR = Microprocessor based relay

• ATR = Attracted type relay

• ITR = Induction type relay

• FSR = Fully static relay

• SSR = Semi-static relay

• MITR = Moving iron type relay

• MCTR = Moving coil type relay

• RR = Reed relays (Road type relays)

Classification of relays according to technology

Relays can be classified into the following parts according to their operating capacity and structure:

1. Electromagnetic relay
2. Thermal relay
3. Static relay
4. Microprocessor based relay

The above classification has been done according to the operating principle of the relays. Relays can also be classified according to their structure but here the classification has been done according to the operating principle.

Classification of relays according to applications

1. Current relay
2. Overcurrent Relay

3. Under-current relay
4. Directional current relay
5. Non-directional current relay
6. Voltage relay
7. Over-voltage relay
8. Under-voltage relay
9. Frequency relay
10. Over-frequency relay
11. Under-frequency relay
12. Electric power relay
13. Over and under power relay
14. Directional power relay
15. Reverse directional power relay
    
    

Classification of relays according to faults

1. Over-load relay
2. Inter term short circuit fault relay
3. Earth faults relay
4. Open phase fault relay
5. Surge voltage relay
6. Automatic load limitation protection relay

According to timing characteristics

1. Instantaneous relay
2. Definite time lag relay
3. Inverse time lag relay
4. Inverse definite time lag relay

Difference between main protection and back-up protection

At the time of fault in the line, the main protection and primary protection systems operate first and the electrical power system Provides protection, while back-up protection operates in case of failure of main protection. That is why the time setting of back-up protection is kept higher than the main setting. The most common use of fuse as back-up protection is that which operates after the relay fails to operate.