Transformer Components: Bushings, Expansion Tank, Probes, Buchholz Relay

Transformer Bushings (Medium and Low Voltage)

Bushings (medium and low voltage): Bushings are insulators installed in the cover (lid) of the transformer that allow the inner conductors to be connected to the external terminals and windings. They are typically porcelain and glazed brown on the outside. Medium-voltage (MV) bushings are larger than low-voltage (LV) bushings.

Expansion Tank and Oil Level

Expansion Tank: The expansion tank is a reservoir located above the transformer tank (cuba) and connected to it by a pipe. It is partially filled to a controlled level so that, when the oil heats and expands, the excess oil flows into the expansion tank. The expansion tank is usually located on the right side of the main tank (when viewed from the standard servicing position). Its function is to accommodate changes in oil level caused by temperature variations and to maintain proper oil filling of the transformer.

Temperature Probe and Thermometers

The probe: The probe indicates the transformer coolant (oil) temperature, which is an indicator of the transformer load. Glass thermometers using alcohol were historically common. Modern transformers often use dial (bimetal) thermometers. When the transformer is fully enclosed or encapsulated, thermistor probes (PTC type) are typically installed instead.

Secondary Voltage Regulation

Secondary voltage regulation: The secondary (low-voltage) output depends on the load level and may fall below the permitted value under heavy load. To maintain voltage under load without interrupting service, many transformers use tap-changing devices. On-load tap changers (OLTC) or tap links allow modification of the secondary voltage in steps, commonly 2.5% to 5% per step (and rarely up to 10%).

Many companies require multi-primary linking options. A multi-primary configuration allows reconfiguration of the primary connections so that, for example, a transformer with a nominal primary of 24 kV can be reconnected to operate with a different supply voltage (for example 15 kV) without changing the secondary voltage.

Buchholz Relay and Fault Detection

The Buchholz relay: The Buchholz relay is a gas-actuated protection device used on oil-filled transformers with a conservator or expansion device. If an internal fault causes overheating or arcing inside the tank (for example: internal short circuits, earth leakage, or other failures), gases generated by decomposition of the oil are collected and detected by the Buchholz relay, which provides an alarm and/or trip signal.

What it detects:

1. Internal arcing: Cut or break of an internal connection or an electric arc inside the transformer; the oil vaporizes and gas is produced.
2. Leakage to tank: Leakage of current from the winding to the transformer tank (earth leakage).
3. Short between windings: Short circuit between windings or sudden and severe electrical stress.
4. Oil aging: Degradation or aging of the insulating liquid that reduces its insulating properties and promotes internal discharges.
5. Core defects: Defects in the magnetic core, which produce abnormal heating by eddy currents.
6. Oil leakage: Coolant (oil) leak through joints or cracks, causing a decline in oil level.

DGPT Detector Protection (Top-Mounted)

The DGPT protection detector: The DGPT device is also applied to fully filled transformers and is normally mounted at the top of the tank as a probe or assembly. It performs the following functions:

1. Gas detection: Detects the emission of gases from the transformer oil.
2. Oil level detection: Detects an accidental or abnormal drop in oil level in the main tank.
3. Pressure detection: Detects excessive increases in pressure exerted by the oil on the tank.
4. Temperature contacts: Contains two temperature contacts for two temperature limits. The first contact acts on an alarm, and the second causes disconnection (trip).
5. Oil level display: Displays the oil level using a float and a transparent cylinder (level gauge).

Notes: All protective and monitoring devices described above are part of the transformer’s safety and operational systems. Proper maintenance, correct installation, and periodic testing ensure reliable detection of internal faults, temperature excursions, oil level changes, and gas emissions.