Oil Surge Relay [OSR] of Transformer:

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Oil Surge Relay-OSR is one of the auxiliary equipment of oil immersed type transformer.

It is mainly used to limit the damage to the on load tap changer in case of failure. OSR is placed in between OLTC(On Load Tap Changer) tank and OLTC conservator.

Oil surge relay responds only by rate of rise pressure resulting from internal arcing. When rate of rise pressure oil cross certain value then alarm and trip contacts operate.

A separation should be arranged such that the oil in OLTC chamber and transformer chamber should not be mixed together as shown in below fig.

Oil Surge Relay-OSR of Transformer
OSR Relay Mounting

In case of heavy faults due to surge in oil OSR relay will operate and causes tripping of both HT and LT breakers of the transformer.

Checklist for OSR Tripping:

The oil in OLTC tank may get contaminated, so the properties of transformer oil  may be changed after tripping of OSR relay. To ensure this the oil has to be tested and if any change in parameters observe, it has to be either replaced or oil filtration is needed.

The following testing have to do for the transformer oil

  • BDV Test:

Break Down Voltage test is conducted to know breakdown voltage or dielectric strength of the transformer oil by using BDV kit. The dielectric strength of the oil is the most important property of the oil. If the dielectric strength of the oil is reduced by water and impurities etc, a breakdown of the insulation can occur. Testing of the dielectric strength of the oil is normally conducted on site to get a quick check of the purity of the oil.

  • PPM Test:

Parts per Million test is to do for knowing the percentage of moisture content present in the transformer oil. Moisture content results dielectric strength of oil is reduced and flash over occurs.

  • DGA Test:

Dissolved Gas Analysis test is to identify various gas levels present in the transformer oil. Due to thermal and electrical stresses in the transformer the gases are dissolved in the transformer oil. The dissolved gases are mainly hydrocarbon gases like Methane, Ethane, Ethylene, Acetylene, Propane, Hydrogen. Carbon monoxide and Carbon dioxide. With the help of DGA the concentration of these gases in PPM can be known and can be cross checked with the IS standard.

  • Oil Level Monitoring:

The oil level must be monitored, a breakdown of the insulation occurs if the oil level gets too low.

  • Silica gel Dehydrating Breather:

The silica type breather is used for drying the air drawn into the oil conservator when the drop in load and temperature causes the oil to contract. The silica gel in the breather is able to absorb moisture to the extent of 20% of its own weight. When the silica gel gets saturated with moisture, the color changes from blue to pale pink and it has to be replaced.

Difference between Buchholz Relay and  Oil Surge Realy:

Generally there is confusion between operation of Buchholz relay and Oil surge relay.

So, Let me interpret the major differences between them.

Buchholz Relay
Oil Surge Realy
Buchholz relay is used for protection of oil filled transformers/reactors fitted with conservators against low oil level and internal faults.
OSR is used to report any malfunction developed inside the On Load Tap Changer Chamber.
Buchholz relay is Gas actuated Relay.
OSR is Oil actuated Relay.
Buchholz relay should be mounted in the connecting pipe between the transformer and conservator tank.
OSR is installed in the pipe between OLTC head and the oil conservator.
Buchholz relay is actuated by the gases flowing through and accumulated in relay.
OSR relay is actuated when the flow rate of oil exceeds the preset value.
Buchholz relay detects the fall in oil level due to formation of gases after any internal fault inside the transformer.
OSR detects excessive oil surge after any fault occurs inside OLTC Chamber.

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