Full name of PRV is Pressure relief valve
Full name of OSR is Oil surge Relay
Definition of PRV
When the pressure in the tank rises above pre- determined safe limit, this valve operates & performs the following functions: –
- Allows the pressure to drop by instantaneously opening the port.
- Gives visual indication of valve operation by raising a flag.
- Operates a micro switch, which gives trip command to breaker.
Overview of PRV
It is mounted on the top plate of transformer main tank.It is provided with a pressure die cast aluminium flange with nitrile gasket for mounting on transformer.
The stainless steel diaphragm is loaded with two reverse wound spring and seals 75 mm and 150 mm port.
If there is major fault inside the transformer tank, and due to that, the pressure inside the tank rises above a pre-set limit, the diaphragm gets lifted instantaneously.
It gives way throwing gas and oil out side and hence excessive pressure drops. The diaphragm then restores to its original position.The lift of the diaphragm is utilized to operate a flag indicator and micro switch.
If PRV does not operate properly and relieve pressure within few milliseconds, the tank may burst.
In large transformers, PRV is mounted on all 3 phase OLTC diverter switch compartment.In 66 KV class transformers, PRV is mounted on OLTC compartment top plate.
The NO(Normally Open) contact of micro switch is used for tripping of transformer.Operating pressure set on the PRV is 0.42 kg/sqcm, 0.49 kg/ sqcm, 0.56 kg/ sqcm, 0.70 kg/sqcm.
How to Check PRV operation manually
The operation of PRV can be done by lifting the plunger (Plunger operates switch). By shorting contact externally by link, trip circuit can also be checked.
Definition of OSR
It is an oil operated relay having one contact to trip the transformer on occurrence of violent oil surge arising due to mal operation in OLTC.It is connected between OLTC tank & conservator tank of OLTC.
Overview of OSR
It is connected in a pipe between OLTC and OLTC conservator.In large transformer, 3 nos. are provided for 3 phases and in small transformers, one no. is provided.
OSR are two types:
OSR with self reset float:
It is like Buchholz relay but with only one mercury switch. It is operated when there is sudden surge in the OLTC during operation of tap or low oil level.It is self reset type.It is provided with Test/Service knob.
OSR without float:
It is operated when ther is sudden oil surge in the OLTC during operation of tap.Once operated, it is required to reset from top side reset switch on the relay.It is not operated due to low oil level.It is provided with Test/Reset switch on top cover of relay.
How to Check OSR operation manually
This relay can be checked by pressing test switch provided on top side. Here only one contact is provided which gives trip signal on operation of float. By shorting contact externally by link, trip circuit can also be checked.
