Various types of protective relays are used to protect power system networks. In this article, we will discuss the classification of protective relays.
The protective relays are classified according to function, actuating quantity, construction, or component used.
The following is the classification of protective relays;
Electromagnetic (Electromechanical) Type Relay
The relay works on the principle of electromagnetic. According to this principle, the armature is attracted by the magnetic force produced by undesirable current or movement of the plunger in a solenoid.
This relay can be actuated by AC or DC quantities. There are various types of electromagnetic relays listed below.
In a solenoid relay, the plunger or iron core moves into a solenoid, and the operation of the relay depends on the movement of the plunger.
Attracted Armature Type Relay
In attracted armature type relay, the operate need to set the current setting. When the circuit current exceeds the limit, the armature gets attracted by the magnetic force produced by the undesirable current.
This type of relay operates in the current rating, so the current setting in the relay plays a vital role in this relay.
Balanced Beam Type
In a balanced beam type relay, the armature is fastened to a balanced beam. For normal current, the beam remains horizontal, but when the current exceeds, the armature gets attracted, and the beam gets tilted, causing the required operation.
Induction Type Relay
The relay works on the principle of electromagnetic induction is known as an induction type relay; this type of relay only be used with the AC quantities.
According to the construction, there are two types of induction relays;
Induction Disc Type
In this relay, a metal disc can rotate between two electromagnets. Alternating currents energize the electromagnets.
There are two types of construction; shaded pole type and watthour meter type.
Induction Cup Type
Electromagnets acted as a stator in induction cup type relay and were energized by relay coils. And the rotor is a metallic cylindrical cup shape.
Directional Type Relay
The directional type relay works on the direction of current and power flow in the circuit. The various types of these relays are;
Reverse Current Type
The relay is actuated when the current direction is reversed or the current phase becomes more than the predetermined value.
Reverse Power Type
The relay is actuated when the phase displacement between applied voltage and current attains a specified value.
Relay Based on Operation Time
In relays, the time between the instant of relay operation and the instant at which tripping of contacts take place is called operation time. Based on the operation time, the relays are classified as below.
In instantaneous type relay, there is no time between the relay operation and tripping of contact; however, there is no instantaneous time delay provided.
Definite Time Lag Type
In a definite time lag relay, intentionally, a definite time delay is provided between the operation of relay and tripping contacts.
Inverse Time Lag Type
In an inverse time lag relay, the operating time is approximately inversely proportional to the magnitude of the actuating quantity.
Distance Type Relay
The distance relay works on the principle of measurement of voltage to current ratio. There are two coils; one coil is energized by current while the other is energized by voltage.
The torque produced is proportional to the ratio of two quantities. The relay will operate when the ratio is reduced from a predefined value.
There are many types of distance relays used in the network.
In impedance relay, the ratio of voltage and current is nothing but an impedance proportional to the distance of the relay from the fault point.
The operating time is proportional to the reactance. And the reactance is proportional to the distance of the relay from the fault point.
Admittance relay is also known as mho relay. In the admittance relay, the operating time is proportional to admittance.
Differential Type Relay
The differential relay operates when the vector difference of two or more electrical quantity exceeds a predefined value. These relays are classified as;
Current Differential Relay
In a current differential relay, the relay compares the currents entering and leaving the section. Under a fault condition, the current difference is significant compared to the normal state.
Voltage Differential Relay
In voltage differential relay, two transformers are used so that the secondaries of both transformers are connected in series. In normal conditions, the induced EMFs are in the opposite position. And in an abnormal state, the primary carry different currents due to which induced EMFs no longer remains in the opposite, and it operates the relay.
Other Types of Relays
There are different types of relays that are used in power system networks.
Undervoltage, current, Power Relay
This relay will operate when the circuit’s voltage, current, or power falls below a predefined value.
Overvoltage, current, Power Relay
This relay will operate when the circuit’s voltage, current, or power rises above a predefined value.
The thermal relay actuates due to the heat produced by the current in the relay coil.
This type of relay uses sensors and electronics methods. This type of relay is also known as a digital relay, and it requires additional circuits.
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