Understanding Transformer Protection Systems

Transformer protection is crucial for ensuring the reliable operation and longevity of electrical power systems. Transformers are expensive and vital components, so detecting and isolating faults quickly is essential to prevent damage to the transformer itself and minimize disruption to the power supply. Various protective devices are employed to safeguard transformers against different types of internal and external faults.

Analyzing Common Transformer Protection Devices

Let's examine the devices mentioned in the options and their roles in electrical protection, focusing on transformer protection:

• Buchholz Relay: This is a gas-actuated relay commonly used for protection of oil-filled transformers against internal faults such as incipient faults, inter-turn faults, core faults, and minor insulation failures. It is installed in the pipe connecting the transformer tank to the conservator tank. Faults inside the transformer produce gas (due to decomposition of oil or insulation) or cause a sudden surge of oil. The Buchholz relay detects these phenomena.
• Pressure Switches: Pressure switches can be used in transformer protection, particularly to detect sudden pressure increases caused by severe internal faults like arc faults. A rapid rise in pressure indicates a major fault within the tank. While useful, they often supplement other protection schemes rather than being the primary protection for all internal faults.
• MHO Relay: A MHO relay is a type of distance relay. Distance relays are primarily used for transmission line protection, not typically for internal transformer protection. They measure impedance to determine the distance to a fault on a line.
• Merz Price Protection (Differential Protection): This is a very effective method for protecting transformers against internal faults. It works by comparing the current entering one side of the transformer with the current leaving the other side. Under normal conditions (and external faults), these currents, when normalized for the turns ratio, should be balanced. An internal fault causes an imbalance, triggering the protection. While crucial for transformers, the question asks which of the *following* is used, and Buchholz relay is a distinct device specifically associated with transformer tank protection.

Focusing on the Buchholz Relay

The Buchholz relay is uniquely designed for oil-immersed transformers with a conservator tank. Its operation depends on the effects of internal faults:

1. Incipient Faults (Minor Faults): Small internal faults like partial discharges or overheating cause slow decomposition of oil and insulation, producing gases. These gases accumulate in the upper part of the Buchholz relay chamber. As enough gas collects, the oil level inside the relay drops, tilting a float switch. This float switch typically operates an alarm circuit.
2. Severe Faults (Major Faults): Arcing faults or severe short circuits inside the transformer cause rapid oil decomposition and a sudden surge of oil upwards towards the conservator. This surge operates a second, lower float or a vane switch in the relay, which is designed to trip the circuit breaker and isolate the transformer quickly.

Therefore, the Buchholz relay provides both alarm (for minor faults) and tripping (for major faults) signals, offering valuable protection against various internal issues that might not be immediately detected by other protection schemes.

Considering the options provided, the Buchholz Relay is a distinct and widely used device specifically designed for and primarily associated with transformer protection systems, detecting a range of internal faults based on gas accumulation and oil surges.

Conclusion

Based on the analysis of the devices listed, the Buchholz Relay is a specific and common protective device used within the transformer protection system to detect internal faults.

Transformer Protection Revision Table

Additional Information on Transformer Protection

Transformer protection schemes often involve multiple layers of protection to cover various types of faults and ensure redundancy. While Buchholz relay is excellent for internal tank faults, other protection methods are also vital:

• Differential Protection (Merz Price): Provides fast and sensitive protection against internal winding faults and core faults.
• Overcurrent Protection: Protects against external phase-to-phase and phase-to-ground faults, as well as severe internal faults.
• Restricted Earth Fault (REF) Protection: A sensitive form of differential protection specifically for earth faults within the transformer windings.
• Overfluxing Protection: Protects against over-excitation which can cause core saturation and overheating.
• Thermal Protection: Monitors winding or oil temperature to prevent damage from overheating due to overload or cooling system failure.

The specific combination of protection used depends on the size, type, and importance of the transformer.