An Oil Surge Relay, or OSR, is a type of mechanical relay. It operates in response to a sudden oil surge.
Need for Separate Diverter Tank
Every modern power transformer includes an On-Load Tap Changer (OLTC). During tap-changing operations, some arcing always occurs at the diverter switch contacts. It is desirable to change tap positions from one to another without any arcing. However, practically, it is impossible to make this change completely arc-free. Due to this arcing, the oil inside the diverter compartment becomes carbonized. If this process were to occur in the main transformer tank, the oil would rapidly deteriorate due to carbonization caused by the arcing during tap changes.
To prevent degradation of the main tank oil, we provide a separate oil chamber, known as the diverter tank, for the tap changer switch. The diverter tank is physically attached to the main tank. Although it does not have any piping connection with the main tank. In other words, the oil of the diverter tank does not mix with the oil of the main tank. As a result, any deterioration or carbonization in the diverter tank does not affect the quality of oil in the main transformer tank.
Needs of Oil Surge Relay (OSR)
Now, since the diverter tank also contains oil, it must have its own separate conservator tank, similar to the main transformer. Just as the main tank uses a Buchholz Relay, the diverter tank also requires an oil-actuated protective device. Therefore, we install an Oil Surge Relay (OSR) in the pipe connecting the diverter tank and its conservator tank. This relay is similar in principle to a Buchholz Relay, but there is one major difference. The Oil Surge Relay does not have any alarm contacts. It has only a trip contact, which operates during a fault condition.
If any major internal fault occurs inside the diverter tank, typically due to loose connections, contact misalignment, or mechanical failure, heavy arcing occurs within the diverter switch. This expansion creates a surge of oil flow through the connecting pipe toward the conservator tank. On the way, this high-speed oil surge actuates the Oil Surge Relay (OSR).
Construction of the Oil Surge Relay
The OSR casing is a strong iron enclosure that forms a small oil chamber. It has one inlet towards the diverter tank and one outlet towards the diverter conservator tank. Inside the OSR, there is a flap mechanism positioned in the path of oil flow.
Under normal breathing or slow oil movement, the oil velocity is too low to disturb the flap. Hence, during normal operation, the relay remains in its resting position. The flap mechanism has a permanent magnet attached to it. A reed switch is mounted at a certain distance. The reed switch has a normally open (NO) contact. Actually, the moving contact of the reed switch is either made of ferromagnetic material (iron) or a piece of iron is attached to it.
Working of the REED Switch
During normal operation, the magnet stays away from the reed switch, keeping the contacts open. When a sudden oil surge occurs, the flap deflects rapidly, bringing the magnet near the reed switch. The magnet then attracts the moving contact of the reed switch and hence makes it close. As a result, it sends a trip signal to isolate the transformer.
There is a reset lever and a test lever, which we have not shown in our diagram. After the clearance of the fault, we can only manually reset the OSR using the external reset lever. Pushing this lever returns the flap to its normal position.
On the relay, there is a small terminal box from where we get the two terminals of the NO contact of the reed switch.
Working Principle of the OSR
Normal Condition
During natural breathing of the transformer, oil flows gently through the OSR without disturbing the flap mechanism. Even during tap-changing operations, small or moderate arcing may occur. Gas forms at the diverter contacts, rises, and finally moves to the conservator tank through the OSR line. However, this slow movement of gas bubbles does not disturb the position of the flap mechanism inside the OSR.
Fault Condition
Since the diverter chamber is much smaller than the main transformer tank, it experiences a very sudden and intense pressure surge during such faults. A heavy fault or severe arcing occurred due to any loose connection or misalignment of the tap contact creates an oil surge. Actually, this arcing produces a sudden rise in temperature and pressure, which in turn causes a rapid expansion of oil inside the diverter tank. When the oil velocity exceeds its threshold value, the flap mechanism rotates about its hinge. As it rotates, the magnet attached to the flap moves closer to the reed switch.
The reed switch has an NO contact. Because of the magnetic attraction, the moving contact (made of ferromagnetic material, or fitted with a ferromagnetic insert) comes toward the magnet. As a result, it touches the fixed contact and closes the NO contact. Closing of this contact initiates the tripping signal to the circuit breaker associated with the transformer. This action immediately disconnects the transformer from the system, preventing damage or fire.
Reset
There is a reset lever and a test lever in an OSR. If the OSR operates, it indicates a major fault in the transformer diverter tank or the OLTC mechanism. Therefore, without physically inspecting the transformer, one should not reset the OSR. There is no provision to normalize an OSR remotely without going to the transformer physically. Because the reset mechanism is purely mechanical and provided within the relay itself.
Remember: Once the OSR operates, the transformer must not be re-energized until the OSR is physically reset at the site.
Why does OSR have no Alarm Contact?
The diverter chamber produces gas even during normal tap-changing, because tap-changing cannot be 100% arc-free. These slowly developing gases would accumulate and nuisance-operate an alarm if an alarm contact were provided, even when there is no actual fault. To avoid unnecessary alarms, the OSR is provided with only a trip contact, not an alarm. This is the key difference between a Buchholz relay and an OSR.
Buchholz vs OSR (key differences)
- Buchholz Relay: has both alarm and trip contacts; mounted between the main tank and main conservator; can auto-reset after gas is released and conditions normalize.
- OSR: has only a trip contact; mounted between the diverter tank and its conservator; requires manual/mechanical reset at the site.
Advantages of OSR
- Very quick fault detection in the OLTC diverter chamber.
- Simple mechanical design → robust and reliable.
- No external power supply required.
- Reliable tripping protection.
Limitations of OSR
- Cannot detect slow-developing faults (no alarm stage).
- Requires manual reset after operation. We can not reset it electrically from the control room.
- Applicable only to oil-filled OLTC systems.