A Buchholz relay is a protective device used in oil-immersed transformers to detect and respond to faults such as internal electrical faults or mechanical failures that may occur within the transformer's oil-filled tank. It operates based on the principle of gas accumulation in the transformer oil due to these faults. The Buchholz relay is primarily used to protect larger transformers, typically those with ratings above a few MVA (Mega Volt-Amperes).
Here's how a Buchholz relay works:
Installation and Positioning: The Buchholz relay is usually installed in the piping connecting the transformer's main tank and its conservator tank, which is a container connected to the main tank to compensate for oil volume changes due to temperature variations. The relay is positioned vertically between the main tank and the conservator tank.
Construction: The Buchholz relay consists of two main components: a float chamber and a mercury switch. The float chamber is located within the piping, and it's partially filled with oil. It has a hinged float suspended within it. The mercury switch is situated above the float chamber and is designed to tilt and make or break electrical contacts based on the position of the float.
Normal Operation: During normal operation, the transformer functions properly, and the oil circulates smoothly between the main tank and the conservator. The float within the float chamber remains in a lower position, and the mercury switch remains in its normal state.
Fault Detection: If a fault occurs within the transformer, such as a short-circuit between windings or a localized heating due to a partial disconnection, the fault generates heat that causes the insulation materials to degrade. This degradation produces gases such as hydrogen and methane. These gases, being lighter than oil, tend to rise and accumulate at the top of the main tank.
Gas Accumulation: As gas accumulates within the main tank due to the fault, it travels towards the conservator tank through the connecting piping. When the gas accumulates in the float chamber of the Buchholz relay, it displaces some of the oil in the chamber, causing the float to rise.
Mercury Switch Activation: As the float rises, it tilts the mercury switch. The mercury switch is connected to the transformer's protection circuitry. When it tilts, it either closes or opens an electrical contact, depending on its design. This change in the mercury switch's state triggers an alarm or initiates a protective action, such as tripping the transformer, to prevent further damage.
Alarm and Tripping: Depending on the severity of the fault and the design of the protection scheme, the Buchholz relay can either send a warning signal or directly trip the transformer. The response depends on the number of gas bubbles detected in the float chamber, which relates to the magnitude of the fault.
Maintenance and Resetting: After a fault event, the Buchholz relay needs to be reset once the transformer has been inspected and repaired. This involves draining any accumulated gas and oil from the relay, ensuring proper functioning for future operations.
In summary, a Buchholz relay is a crucial protection device for oil-immersed transformers that detects internal faults by monitoring gas accumulation caused by degradation of insulation materials. It responds by triggering an alarm or initiating a protective action to prevent further damage to the transformer.