1 - HT panel protection relay:

HT panel is used for distribution of 11 KV / 33 KV power supply.The HT power supply is received from GO switch and distributed to the transformer.
HT panel has two types supply section one is receiving or incomer section and 2nd is distribution or feeder section. so we can categories it two types incomer safety and outgoing safety.

Incomer safeties:

1. Over current + earth fault relay.
2. Trip circuit supervision relay.
3. Master trip Relay.
4. Annunciator Panel.
5. Hooter

Outgoing safeties:

1. Over current + earth fault relay.
2. Trip circuit supervision relay.
3. Master trip Relay.
4. Restrict earth fault relay.
5. Auxiliary alarm relay of three way (OTI, WTI, Buchholz).
6. Auxiliary trip relay of three way (OTI, WTI, Buchholz).
7. Annunciator Panel.
8. Hooter

2 - Protection relay wiring diagram:

3 - Types of HT panel protection relay:

HT panel protection relay are used according different types faults as over current, earth fault etc.

A - Over current and earth fault (IDMT) relay:

Over current and earth fault relay is used for HT panel protection from over current and earth fault. It is used in HT panel incomer and outgoing feeder both for protection. It is known as IDMT (inverse definite minimum time) relay because it works as IDMT function.

Working principle

If the fault current is high than trip time will be minimum. If the fault current is low than trip time is high or adjustable as per requirement. Nowadays it is available in digital type numeric relay with three phase and over current relay.
It has two no’s functions available. One is three phase over current relay and earth fault relay in HV system. It is best for using in overhead feeder and underground feeders.
It is microprocessors based with wide range field.it can be controlled with different characteristic of trip time. It has also time delayed selected program for over current and earth fault relay.

B - Trip circuit supervision relay:

The trip circuit supervision relay is a monitoring relay and it is known as numerical name 96 S. it is used for continuous monitoring of breaker trip circuit healthy or not healthy in HT panel in any premised or substation. If the trip circuit will not ok then it will provide an alarm for avoiding any fault.
In an electrical panel trip circuit of the circuit breaker is very important and critical. During fault condition If the trip circuit did not find any trip sensing than breaker will not be disconnected and resultant by this problem may be very loss.
So, keeping the trip circuit healthy the TCS relay is used so that system always remains healthy.

working principal

1. Monitoring the complete trip circuit of the circuit breaker. working principal
2. Identify the faults in the supervised circuits, like the not available voltage, the circuit to be open, contact flashing in connections, and resistance abnormal in wires, contacts, and trip coils.
3. Low-level measuring current enables application in sensitive or high resistance circuits.
4. False signals are avoided for circuit breaker smooth operations.
5. Operation indication by a single dual-color LED Complete range of rated voltages either AC or DC.
6. Galvanic isolation is possible between auxiliary source and supervised circuit Very low burden on the auxiliary source.

C - Master trip relay:

Master relay is not a monitoring relay. It is used as an isolation in protection relay and breaker trip coil. it is used in HT Panel for safety of monitoring relay. It is received multiple commands from protection relay and then a single command is received by breaker trip coil from master trip coil.

Working principal

Master trip relay (MTR) is used for keeping safe conditions to the all protection relay.it is received tripping commands from auxiliary alarm and tripping relay, over current, and earth fault relay then it provides an own a single tripping command to the breaker trip circuit.
If by chance a fault occurs in the breaker trip circuit resultant a major fault current will pass towards the master trip relay from the trip circuit in this situation MTR relay will be affected but the protection relay will be safe.

D - Restrict earth fault relay.

Restricted earth fault relay is used for protection of an earth fault from Restricted zone. It is used transformer HT side. restricted earth fault relay is used for transformer grounded faut in HT side.
It is a very highly sensitive relay as 2-8% and other differential relay works as sensing 20-40%. It works as fast due to the simple measuring process with the help of one winding. It protects a zone between two measuring points.

Working principle

Restricted earth fault relay is installed restricted zone like as transformer primary winding to primary CT in HT panel. It is operated minimum leakage current as 5% but other earth fault relay is operated on more than 20% leakage current. if the earth’s fault current is less than 20% then it will be very difficult to protect the system.
So, 100% earth fault protection is not possible by other normal earth fault relays. So, remove for uncertainty situation and keep in mind for 100% internal earth fault protection, we used very sensitively restricted earth fault relay.

E - Auxiliary alarm relay of three way (OTI, WTI, Buchholz).

Auxiliary protection relay is used for transformer protection like as oil temp, winding temp and Buchholz relay. it is used in HT panel as two types as one is alarm and 2nd is trip protection relay.
The auxiliary alarm relay is used for receiving fault alarm signal from transformer protection device (OTI, WTI, Buchholz) after crossing set value for alarm and after that it provides alarm signal to the master trip relay for activating annunciator panel and hooter. resultant annunciator panel and hooter is activated for alarm fault.

F - Auxiliary trip relay of three way (OTI, WTI, Buchholz).

Auxiliary protection relay is used for transformer protection like as oil temp, winding temp and Buchholz relay. it is used in HT panel as two types as one is alarm and 2nd is trip protection relay.
The auxiliary trip relay is used for receiving fault trip signal from transformer protection device (OTI, WTI, Buchholz) after crossing set value for trip and after that it provides trip signal to the master trip relay for activating annunciator panel and circut breaker shunt relay. resultant annunciator panel and shunt relay is activated and breaker will be tripped..

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