'Relay' and its Types in Electrical Circuits

Kathmandu : Urja Khabar presents an informative column, "Urja Gyan", every Friday. This week we focus on relays and their types. Last week, we discussed electrical tools and equipment used in the power sector.

1. Relay

In power lines, relays are used to automatically disconnect the line in case of faults, such as short circuits or excessive current (overcurrent), to protect the equipment. Consider the power transformer in a substation, for instance. The secondary side of the transformer is connected with a three-phase line, which afterwards connects to the transmission line. In case of any fault such as wire falling on the ground or line touching any other, excessive current flows, which is termed as fault. If the fault current is not stopped in time, it may burn the transformer and further damage the power system. The fault or overcurrent condition is detected by the relay, sensed, and the line is disconnected automatically in milliseconds to protect the equipment.

2. Types of Relays

Relays are designed for specific purposes and are named accordingly.

• Overcurrent Relay: It protects the line from faults or overcurrent conditions.
• Earth Fault Relay: Operates when the line comes in contact with the ground or when there is an unbalanced load current in the line.
• Differential Relay: Used primarily for the protection of power transformers or generator windings.
• Undervoltage Relay: Operates when the voltage falls below a predetermined level and thus initiates tripping of the line.
• Overvoltage Relay: Operates when the voltage rises above a predetermined value and thus disconnects the line.
• MHO Relay: It is also known as an admittance relay, a voltage-controlled directional relay, and is used for the protection of long transmission lines.
• Frequency Relay: It ensures that the frequency of voltage generated by a generator is constant. If not, it trips the generator.
• Synchro-check Relay: It is used for synchronization of a running generator with an incoming generator or busbar. If synchronization is not correct, it prevents the incoming generator from connecting to the busbar.
• Buchholz Relay: This relay detects gas formation inside a transformer due to internal faults and disconnects the incoming breaker of the transformer.
• Impedance/Distance Relay: Indicates the distance or area of an earth fault or phase-to-phase fault from the substation. Modern equipment often includes fault locators that show the precise distance in meters, speeding up repairs and restoration of power supply.
• Directional Relay: Detects fault current flowing in only one direction. In case of faults on parallel buses or lines, it prevents fault currents from flowing in both directions.
• Reverse Current Relay: It is an application for protection of generators. It acts when the generator starts taking the current from the busbar rather than supplying it.

Multiple Choice Questions:

1. The simple word for relay can be _________.
a. Inductor
b. Switch
c. Capacitor
d. Resistor

Answer: Switch

2. What is the following used for?

(a) Distribution transformer
(b) Current transformer
(c) Power transformer winding
(d) Potential transformer winding

Answer: Power transformer winding

3. Under what conditions does a Buchholz relay connected to a transformer operate?

(a) Gas formation due to oil
(b) Overcurrent
(c) Earth fault
(d) None of the above

Answer: Gas formation due to oil

4. Where is a synchro-check relay used?

(a) Motor protection
(b) Parallel operation of transformers
(c) Parallel operation of generators
(d) None of the above

Answer: Parallel operation of generators

5. What does taping on the current coil of a relay adjust?

(a) Current
(b) Voltage
(c) Number of turns
(d) Capacity power

Answer: Capacity power

6. What kind of fault is detected when current coming from the feeder is unbalanced?

(a) Unbalanced
(b) Overload
(c) Underload
(d) None of the above

Answer: Unbalanced

7. A reverse current relay is used, by and large, for the protection of which equipment?

(a) Transformer
(b) Motor
(c) Capacitor bank
(d) Generator

Answer: Generator