The Buchholz relay has two oil-filled chambers with floats and relays arranged vertically one over the other. If high eddy currents, local overheating, or partial discharges occur within the tank, bubbles of resultant gas rise to the top of the tank. These rise through the pipe between the tank and the conservator.
As gas bubbles migrate along the pipe, they enter the Buchholz relay and rise into the top chamber. As gas builds up inside the chamber, it displaces the oil, decreasing the level. The top float descends with oil level until it passes a magnetic switch which activates an alarm.
Typically, inspection windows are provided so that the amount of gas and relay operation may be viewed during testing. If the oil level falls low enough (conservator empty), switch contacts in the bottom chamber are activated by the bottom float. These contacts are typically connected to cause the transformer to trip.
Probable cause and action taken analysis while Buchholz trip
Probable Cause for Buchholz alarm/trip | Actions to be taken |
Collection of Gas | Make the transformer off. Do not test charge the transformer without checking and testing of the transformer Check odor of gas/air Check for flame test Check color Check smell Check color of flame Test the oil for DGA |
Collection of trapped air | If above test do not confirm for gas, then it is air This may be due to improper release of the air before charging of the transformer Release the air from main tank and relay |
Entry of moisture in terminal block | Clean the terminal box and replace the gasket |
Blocking of breather pipe | Verify breather pipe line for free flow of air |
Turbulence in oil due to simultaneous operation of pump | Verify time delay in operation of pumps |
In correct buchholz relay pipeline | Verify angle of inclination for Buchholz relay and rectify the same |
Blockage of Buchholz relay pipe | Verify the same and attend |
Mal operation of switches in Buchholz relay due to oxidation or puncture of sleeve on wires provided on switch | Verify operation of Buchholz relay by draining of oil or injecting air or replace the relay |
Key action taken while transformer trip in 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.
Probable cause and action taken analysis while PRV and OLTC PRV/OSR trip
Probable Cause for PRV and OLTC PRV/OSR trip | Action to be taken |
Excessive oil filling in conservator | Verify the oil level and take corrective action |
Entry of moisture in terminal block | Replace the gasket |
Blockages in relay pipe | Verify the valves |
OLTC trouble | Do not test charge the transformer |
Opening of transition resistor | Verify by continuity test during tap change Replace the transition resistor |
Mal operation due to entry of moisture in terminal box | Replace the gasket of terminal box |
Drop in oil level in diverter switch chamber | replace by fresh oil |
Key action taken while transformer trip in OTI/WTI
OTI(Oil Temperature Indicator)– It is dial type thermometer, works on the vapour pressure principle.While transformer trip in OTI, check oil temperature setting.
WTI(Winding Temperature Indicator)-It is also similar to OTI but has some changes. It consists of a probe fitted with 2 capillaries. Capillaries are connected with two separate bellows(operating/compensating). These bellows are connected with temperature indicator.
Probable cause and action taken analysis while OTI /WTI –high alarm/trip
Probable cause for OTI /WTI –high alarm/trip | Action to be taken |
Blockage in cooler circuit | Make the transformer off Check valves in cooler circuit |
Transformer Over load | Arrange for load shedding or load transfer |
Inadequate cooling | Verify proper working of fan & pump |
Wrong setting of OTI/WTI for alarm/trip | Verify settings as per name plate data of transformer |
Problem with sensing element of OTI/WTI | Verify oil in the pockets of OTI/WTI sensors Verify the calibration of OTI/WTI |
Wrong connection of CT secondary for WTI CT | Verify connections by ratio test of CT |
