Page 334 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

6 Distance relay

7 Impedance relay
8 Admittance relay
9 Reactance relay
Relay is one of the main device used for switch gear protection networks to protect the transmission lines,
transmission equipments and sub station equipments. The equipments used for transmission and in substation for
distribution such as transformers, lightening arrestors,earth switches, isolators, CTs & PTs etc; are very costly and
needs continuous protection from damage. Replacement or repairs are not easy and to provide an uninterrupted
supply to consumers. So, protection of these devices/equipments are very essential

Reasons for over current, Over voltage and under voltage fault:
Many reasons constituted for over current, over and under voltage or earth faults; type of fault and the cause
effect is listed in Table 1.
Table 1

Sl No Type of Fault Cause Effect
1 Phase to neutral short - Insulation failure - High current flow in line.
- Components failure - Fire
- Human error`
2 Phase to phase short in - Tree branches falls on line - Very high current flows
transmission lines - Snakes crossing on tower lines - Fire
and - Extensive damage of
- Birds falls equipments
- Strong winds
- Natural calamities
- Riots, and human made faults
3 Phase to ground fault - Insulation failure - High current flow in line
- Component failure - Fire
- Low voltage
4 Lightening storm etc; - Natural calamities - Very high current flows
- Fire
- High voltage spikes
5 Sudden removal of heavy - Fuse failure - High voltage
load
6 Increasing Load beyond - Human Error - Low voltage in line
the rated level - Over loading the line

Sensors used for Relays
The relay cannot accept the total line voltage or load current. A small part of the electrical quantity is supplied to
the relay through sensors. A current transformer popularly known as CT and a potential transformer PT, is serves
the purpose of sensors in current relay and voltage relay. Various input and output ratios are in practice to supply
the sensing quantity to the relays according to the load conditions.
Working principle of current relay

The electro magnetic relay widely using in the substation and transmission lines are serves the protection from the
disaster conditions. The latest version of modern static or digital relays are now a days out dated the conventional
electro magnetic relays, because of their many of advancements compare to electro magnetic relay. (Fig 1)
Fig 2 shows the front panel setting of a electric magnetic relay.
1 Time multiplier setting (TMS)
2 Trip flag

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CITS : Power - Electrician & Wireman - Lesson 106-116 CITS : Power - Electrician & Wireman - Lesson 106-116