The transformer gas relay is a protective device installed on the top of oil-filled transformers. It performs two functions. It detects the slow accumulation of gases, providing an alarm after a given amount of gas has been collected.
Also, it responds to a sudden pressure change that accompanies a high rate of gas production (from a major internal fault), promptly initiating disconnection of the transformer.
An incipient fault or developing fault, usually causes slow formation of gas.
Examples of incipient faults are:
- Current flow through defective supporting and insulating structures;
- Defective joints at winding terminals causing heating;
- Minor tap changer troubles; and
- Core faults.
- Shorts between turns and windings; and
- Open circuits, which result in severe arcing.
Generation of Gas Due to Faults
Internal transformer electrical faults result in the production of ionized gases. A significant volume of gas is frequently generated in the early stages of a fault by rapid oil breakdown.
The generated gases rise through the oil to the top of the equipment and collect in the gas relay.
In the event of a gas alarm, it is necessary to sample and analyze the gas being generated. This analysis, together with knowledge of the rate at which gas is accumulating, will determine the proper course of action. If a fault is thought to be developing, the device must be removed from service.
Ignoring this early warning sign can lead to severe equipment damage as the fault progresses.
Operation of a Transformer Gas Relay
A typical transformer gas relay consists of two chambers, each performing a distinctive function. A simplified cross-section of a gas relay is shown in Figure 1.
The relay assembly consists of a gas accumulation chamber mounted directly over a pressure chamber. The accumulation chamber collects slowly produced gases. A float located in this partially oil-filled chamber moves as the gas volume increases. It operates an alarm switch when the amount of gas collected reaches a specified level.
An indicator coupled to the float also provides a means to monitor the rate at which gas is being generated.
The second chamber, a pressure chamber, connects directly to the transformer oil circuit. It connects vertically to the accumulation chamber, providing a path for the rising gas.
An air-filled bellows within the pressure chamber acts as the pressure change detector. A sudden pressure surge in the oil compresses the bellows and forces the
air within to move a diaphragm. The moving diaphragm actuates a switch that initiates tripping of the transformer.
Sudden pressures, such as oil circulating pump surges, are normal operating events and the relay must be set to ride through them. In practice, it is necessary to make sure the relay is set to operate at about 7 KPa (1 psi) above the maximum oil circulating pump surge pressure.
This is basically a diaphragm sealed pipe with its open end directed away from the transformer.
A significant increase in pressure bursts the diaphragm and discharges gases and hot oil with a possibility of resulting fire.
Buchholz Relay (VIDEO)
Resource: Science and Reactor Fundamentals – Electrical; CNSC Technical Training Group