The Buchholz Relay is a gas-actuated protection relay. It is a mechanical relay. All oil-immersed transformers that have a conservator tank should be equipped with a Buchholz relay.
A Buchholz relay can detect both slow-developing faults and major transient faults occurring within the transformer. It provides an early warning (alarm) for slow-developing faults and initiates a trip for severe internal faults.
Location and Mounting
We fit the relay in the pipe connecting the main transformer tank to the conservator. We have to fit it in a slightly inclined position (about 5°–10° upward towards the conservator). This inclination ensures that gases generated in the transformer tank rise naturally in the relay chamber.
Working and Construction Details
A typical Buchholz relay contains two main elements in an iron-cast casing. These are an upper float with a mercury switch and a lower float with a mercury switch.
Upper Float
The upper float arm can rotate against a hinge. The float arm carries a mercury switch as shown. The mercury switch closes an NO alarm contact. Due to any slowly developing internal fault, the generated gas accumulates in the upper part of the relay chamber. Obviously, this accumulation of gas causes the oil level to fall. As a result, the float comes down. Therefore, the float arm rotates and causes tilting of the mercury switch. Due to the tilting of the switch, the mercury inside the container of the switch spreads and touches both the terminals. Therefore, NO contact of the alarm circuit becomes closed.
Lower Float
There is a plate in front of the opening that lets oil pass through the Buchholz relay. This plate, along with a mercury switch, remains balanced on a hinge. The position of that plate is such that it always faces the inlet flow of oil through the relay.
When there is a gentle flow of oil due to the natural breathing of the transformer, the lower float arrangement will not tilt. Even during slow-developing faults in the transformer, the bubbles of gas come out from the same opening. Also, these bubbles will not disturb the alignment of the lower float.
However, when there is a major fault inside the transformer, there will be an oil surge due to sudden temperature rise and mechanical force developed in the winding. Due to this oil surge, the oil will rapidly come out from the inlet opening of the relay and go toward the conservator tank. This sudden oil movement disturbs the balance of the lower float arrangement. Hence, due to the oil pressure, it tilts. As a result, the alignment of the mercury switch attached to it becomes almost horizontal.
The mercury inside the switch spreads and hence touches both contacts of the switch. Therefore, it completes the associated trip circuit. Therefore, the closing of this NO contact of the mercury switch initiates the tripping of the circuit breaker. Finally, it isolates the transformer from the system.
Cock and Gas Release Valve
There is a gas release valve or cock provided at the top of the Buchholz relay. When the Buchholz relay operates the alarm, we need to investigate the accumulated gas inside the relay chamber to understand the nature of the fault inside the transformer.
If an alarm or fault signal appears, we must collect the gas accumulated in the upper chamber of the Buchholz relay by opening the gas release cock. By analyzing this gas, we can easily identify whether the internal fault is due to arcing or overheating, or any oil leakage.
The Buchholz relay often has a round-shaped or, in some cases, a square glass window. Through this inspection window, we can observe the oil level and detect the presence of gases.
In the Buchholz relay, there is a small terminal box with a cover. Inside this terminal box, there are usually three terminals — one common, one for the alarm, and one for the trip connection.
Advantages of Buchholz Relay
The advantages of the Buchholz relay are quite significant.
- It can detect internal faults well before they cause major damage by providing an early alarm. This gives engineers enough time to inspect and rectify internal issues before they develop into severe faults.
- It can also trip the circuit breakers on both sides to isolate the transformer from the system when a sudden internal fault occurs.
- The Buchholz relay is purely mechanical in operation, so it does not require any complicated electrical or electronic circuits. This makes it simple, robust, and almost maintenance-free for a long operating life.
- Another major advantage of the Buchholz relay is that it allows gas collection. It stores gases generated during internal faults and provides a proper outlet for collecting gas samples for analysis. After analyzing the gas collected from the Buchholz relay, one can easily determine the type of fault or defect present inside the transformer.
Limitations
However, there are some limitations.
- We can use a Buchholz relay only in oil-immersed transformers equipped with a conservator tank. We cannot use it in dry-type or hermetically sealed transformers.
- Another disadvantage is that when there is any oil leakage in the transformer, the leakage may create air bubbles inside the tank. These air bubbles can accumulate in the Buchholz relay chamber and cause an alarm. In such cases, the Buchholz relay gives an alarm even when there is no actual fault inside the transformer. Therefore, it may mislead the engineers.
Setting of Buchholz Relay
The typical operating settings are as follows:
- The weight of the lower float is so balanced that the trip operates only when the oil flow velocity is more than 1 to 1.5 meters per second. If the oil velocity is below this limit, it cannot disturb the balance of the lower float.
- The upper float gives an alarm only when the gas accumulation volume exceeds about 150 to 250 cubic centimeters.
These values may vary slightly depending on the relay manufacturer and transformer design.