Mater Trip Relay 86 Working Function and Significance of Master Trip Relay?
Another important auxiliary relay in power system protection is Mater trip relay. Mater trip relays are the final command of the breaker tripping circuits. Mater trip relays is nothing but a collection of all tripping circuits. if the fault detected by any of the protection relay means at the same time the protection relay trips the associated circuit breaker through the mater tripping relay circuits. The master trip relay’s output is wired to the circuit breaker trip Coil.
Note: Each Zones of protection have separate master trips section. Example, generator protection have individual master trip relays and Grid section have separate etc.
Master trip relays are classified in to three categories according to the level of fault detection:
- Low level trip— ANSI Code => 86 L
- Medium level trip —- ANSI Code => 8M
- Heavy trip—– ANSI code => 86 H
Low Level Trip 86L:
To trip the auxiliary circuit breaker these relays are used. Which means if the fault level is low the relay does not require to trip the entire power system. These relays are used for annunciations systems.
Medium level master trip relays 86M:
The power system suffers due to the tolerable fault such as over current faults, Earth Faults, Instantaneous over current, Power plant auxiliary power failure etc, in such a condition Medium mater trip relay operates. It does not trip the turbine of any other mechanical device*. If the fault come from grid means, first grid master trip acts then the generator master trip operate if the fault is high.
*all mechanical equipment failure tripping will be done by master trip relay 86M only. i.e oil pressure low, ACW (Auxiliary cooling Water) flow low, MCW (Main Cooling Water) Pressure low etc but some of the mechanical equipments like turbine activity equipment such as MOP or AOP, low Steam inlet pressure or temperature… are wired with 86H relay
- Generator Breaker trips.
- Annunciation alarm for 86M operation.
Heavy Trip 86H:
If the fault is heavy, then the relay operates. These relays are called as Lock out relays. These relays only operate when the fault is heavy or might have chance to damage the electrical or mechanical equipment. 86H output is wired with turbine tripping, Grid breaker tripping and Generator breaker tripping.
Tripping equipments: Generator Breaker, Turbine trip, Annunciation alarms
The relay operates when..
- Instantons Earth Fault
- Restricted Earth Fault
- Standby Earth fault
- Differential relay
- 86 H- Turbine Side
Resetting Mater Trip Relays:
If the fault is none, then these relay resets, otherwise we cannot reset the relay.
Significance of Master Trips Relays:
- All the protection relays cannot be connected in directly to each circuit breaker’s trip coil. if we do so, the breaker trip coil may get damage due to continues tripping command (breaker tripping circuit does not have anti pumping relays). To avoid these, the master trip relays are implemented. Mater trip relay takes the continues command from multiple relays are gives single command to the breaker trip coils.
- Decreases the circuit Complexity
- Reduces the cabling Cost. Circuit breakers are field equipment but relays are in control room, therefore we do not want to lay the cable for each relay to the circuit breaker. Instead of that, By installing, Mater trip relays we can convert in to single contact.
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