Transformer Components and Their Functions
Windings
Primary and secondary windings are electrical conductors neatly coiled around an iron core. The primary winding connects to a power source, while the secondary winding connects to the load, receiving energy induced from the primary winding.
The transformation ratio depends solely on the number of turns in the primary coil (N1) and the secondary coil (N2).
Core
The core plays a crucial role by channeling the magnetic flux from the primary winding to the secondary winding.
It consists of rolled ferrosilicon sheets (to reduce eddy currents), insulated and layered to form a solid iron block. Both the coils and the core must be insulated from each other. Paper, cardboard, and paint are used for insulation, and wood is used for support. These materials are tightly pressed to minimize noise and vibration.
Insulating Oil
High-voltage transformers are typically immersed in insulating oil. This oil provides insulation between energized parts and transfers heat from the core to the tank’s exterior.
Common insulating liquids include silicone and mineral insulating oil derived from petroleum. Ascarel, previously used, is now banned due to its environmental impact.
Main Tank
The tank houses the transformer’s active parts and dissipates the heat transferred from the core and windings via the insulating oil. Tanks are constructed from reinforced iron sheets for structural support.
Radiators
Radiators, attached to the tank’s exterior, enhance cooling by increasing air circulation and heat transfer from the oil. They are made of sheet metal with open palettes at the ends, allowing the hotter oil to rise and the cooler oil to return to the bottom.
Expansion Tank (Conservator)
The expansion tank compensates for oil volume fluctuations caused by temperature changes due to varying loads. Installed externally at the transformer’s highest point, it accommodates the expanding oil. Typically, the oil volume in the conservator should be between 25% and 50% of its capacity.
Oil Level Indicator
This indicator shows the oil volume inside the tank. It can be located on the conservator or directly on the tank (in transformers without a conservator). Transformers with conservators often include a contact (like a micro-switch) to trigger an alarm if the oil level reaches a critical point.
Air Dryer (Silica Gel Breather)
Air entering and exiting the conservator, due to oil volume changes, passes through the air dryer to remove moisture. The air dryer contains silica gel crystals, which absorb moisture. The silica gel is blue under normal conditions and turns pink when saturated. It can be regenerated by heating.
Thermometer
Oil temperature is monitored as it reflects the internal temperature of the transformer. The thermometer uses mercury, which expands with heat and moves a pointer along a scale. Often, the oil thermometer includes a contact to activate cooling fans (if the transformer has forced cooling).
Thermal Imaging (Winding Temperature Indicator)
The winding is the hottest and most rapidly heating part of the transformer, and its temperature is related to the load. High temperatures degrade the insulation. A current transformer (CT) connected in series with the primary winding measures the load. The CT’s secondary terminals are connected in series with a resistance placed in an oil-filled container. As the transformer load increases, the current in the winding and the CT also increases, heating the resistance and the oil. This expansion triggers a pointer on a thermometer. At high temperatures, a contact triggers alarms, and if the temperature continues to rise, another contact activates the protection system, tripping the breaker and isolating the transformer.
Pressure Relief Valve
The pressure relief valve protects the transformer from excessive internal pressure caused by arcing or insulation failure. It consists of a curved tube mounted on the tank’s top lid. Under high pressure, a membrane ruptures, releasing the pressure. Modern high-voltage transformers use relief valves instead of burst pipes.
Gas Relay (Buchholz Relay)
This device protects oil-immersed and conservator-equipped transformers from internal faults. Faults generate gas bubbles (sometimes flammable). Located between the tank and the conservator, the relay has two floats: one detects slow gas accumulation (triggering an alarm), and the other detects a rapid, continuous gas flow, tripping the circuit breaker to isolate and protect the transformer.
Bushings and Insulators
Bushings and insulators provide electrical isolation between energized conductors and the transformer’s frame. Common materials include porcelain and glass. They can be rigid or suspension type, and their designs include pin, pedestal, support, and pass-through types.
Cooling Systems
Several cooling methods prevent dangerous temperature rises in transformer insulation:
- Natural Cooling: Natural oil circulation
- Forced Air Cooling: Fans mounted on radiators increase airflow and heat transfer.
- Forced Oil Circulation: Oil is circulated through external cooling systems.
- Water Cooling: Water is used to cool the oil.