Buchholz relay in transformers

Introduction

Transformers are crucial components in electrical systems, they are made up of many parts of which a Buchholz relay is part of. Buchholz relay in transformers is a very important component/device.

The Buchholz relay features two cylinders: a solid non-metallic cylinder that houses the mercury switch and a smaller solid metal counterbalance cylinder. Both cylinders are connected and rotate around the same axis, with their rotation controlled by mechanical stops.

• When the Relay is Empty of Oil: The weight of the non-metallic switch cylinder dominates, making the switch system rest against the bottom stop with the mercury switch in the closed-circuit position.

• When the Relay is Full of Oil: As the relay fills with oil, the different densities of the cylinders cause the switch cylinder to appear lighter. This shift allows the weight of the counterbalance cylinder to prevail, rotating the system until it reaches the top stop and placing the mercury switch in the open position.

Fault Detection

Minor Faults:

• • When a minor fault occurs in the transformer, fault currents produce heat that decomposes the transformer oil and generates gas bubbles. These gas bubbles rise and collect in the Buchholz relay.
  • The collected gas displaces the oil in the relay, causing the upper float to rise and close the upper mercury switch. This switch is part of the alarm circuit, which then activates to signal the presence of a minor fault.
  • The amount of gas collected helps gauge the fault’s severity. However, during minor faults, the gas typically does not affect the lower float, so the lower mercury switch remains unaffected.

Major Faults:

• • • For major faults, such as a phase-to-earth short circuit, the fault generates more heat and produces a large volume of gas. This gas moves with enough force to tilt the lower float in the Buchholz relay.
    • The movement of the lower float closes the lower mercury switch, which triggers the tripping mechanism. This action isolates the transformer from the power supply to prevent further damage.

Operations

Fault Detection and Response

Incipient Fault Detection 

When a minor or incipient fault occurs within a transformer, it generates gas that collects in the top of the Buchholz relay housing. As gas accumulates, the oil level in the relay housing decreases. This results in the alarm switch becoming increasingly exposed above the oil level. Over time, as the oil level continues to fall, the weight of the switch cylinder becomes dominant, causing the switch to rotate and eventually trigger the alarm.

Trip Operation

 In the event of a serious fault, gas is produced at a rapid rate, causing a surge of oil through the relay. This surge impacts a flap attached to the trip element, causing it to rotate around its axis. As a result, the mercury switch is closed, activating the tripping devices to isolate the transformer.

Oil Loss Response

 If there is a significant loss of oil from the transformer, both the alarm and trip elements of the relay will operate sequentially, similar to the process described for gas accumulation. The oil level in the double-element relay can be monitored using the graduated scales visible through the windows on either side of the relay.

Functions

• By detecting gas buildup early, The Buchholz relay helps in identifying faults before they lead to catastrophic failures.
• It provides valuable information for planning maintenance activities and avoiding unplanned outages.

Useful Tips for Transformer Maintenance

Regular Inspections

Regularly inspect the Buchholz relay and other transformer components to ensure they are functioning correctly.

Oil Testing

Periodically test the transformer oil for gas content and other indicators of degradation. This helps in early detection of potential issues. here a breakdown voltage test (BDV) is usually done to verify the dielectric strength of the oil of the transformer.

Monitor Temperature

Keep an eye on the transformer’s operating temperature. Excessive heat can accelerate oil decomposition and lead to more severe faults.

Maintenance Schedule

 Follow a proactive maintenance schedule to address any signs of wear or malfunction before they escalate into major issues.