A transformer mainly consists of a core, windings, tank, conservator, bushings, tap changers, Buchholz relays, and other components. The functions of each part are as follows:
The core is the magnetic circuit part of the transformer. To reduce hysteresis and eddy current losses under alternating flux, the core is made of high-quality silicon steel sheets with a thickness of 0.35 mm or thinner. Currently, cold-rolled grain-oriented silicon steel sheets with high magnetic permeability are widely used to reduce the size and weight of the transformer, save wiring, and minimize heat loss caused by wire resistance.
The core consists of two parts: the core columns and the yokes. Windings are mounted on the core columns, while the yokes connect the core columns to form a closed magnetic circuit. Based on the arrangement of windings in the core, transformers can be classified into two types: core-type and shell-type (or simply "core" and "shell").
Single-phase double-core column: This type has two core columns connected by upper and lower yokes to form a closed magnetic circuit. Both high-voltage and low-voltage windings are mounted on the core columns. Typically, the low-voltage winding is placed closer to the core, while the high-voltage winding is on the outside to meet insulation requirements.
Three-phase three-core column and five-core column: The three-phase five-core column structure adds two side yokes (without windings) to the three-core column design. This reduces the cross-sectional area and height of the upper and lower yokes, thereby lowering the overall height of the transformer.
In large-capacity transformers, cooling oil channels are often installed in the core to dissipate heat generated by core losses effectively during oil circulation.
Windings are arranged on the core in two basic configurations: concentric and interleaved.
Core-type transformers generally use concentric windings. The low-voltage winding is placed closer to the core, while the high-voltage winding is on the outside. Insulation gaps and cooling oil channels are left between the windings and between the windings and the core. Concentric windings can be further classified into cylindrical, spiral, continuous, and interwoven types based on their structural characteristics.
The active part of an oil-immersed transformer (windings and core) is housed in a tank filled with transformer oil. The tank is made of welded steel plates. For medium and small transformers, the tank consists of a shell and a cover. Large transformers often adopt a "bell-type" tank structure for easier maintenance. Modern large-capacity transformers use fully sealed tanks to prevent oil leakage.
The conservator, also known as the oil reservoir, is a cylindrical container mounted horizontally on the transformer tank. It is connected to the tank via a curved pipe and has an oil level gauge to monitor oil volume changes. The conservator's volume is typically 8%–10% of the total oil volume in the tank. It stores and replenishes oil as the oil expands or contracts due to temperature changes, ensuring the tank remains full. It also reduces oil degradation by minimizing contact with air.
The Buchholz relay is a key protective device for transformers. It detects internal faults such as oil level drops, insulation breakdown, core or winding moisture, overheating, or discharge faults. Installed on the connecting pipe between the tank and conservator, it operates quickly and sensitively. When minor faults occur, gases accumulate in the relay, triggering an alarm. Severe faults cause strong oil flow, activating the heavy gas trip mechanism to cut off the transformer's power supply.
The tap changer adjusts the voltage ratio of the transformer. It can be either off-load or on-load. Off-load tap changers switch without power, while on-load tap changers allow switching under load using a transition circuit to prevent short circuits between taps.
The cooling system includes radiators, fans, and oil pumps. Radiators increase the surface area for heat dissipation. Cooling methods include natural air cooling, forced air cooling, and forced oil-water cooling.
Transformer insulation is divided into main insulation and longitudinal insulation. Main insulation refers to the insulation between windings and ground, between phases, and between windings of different voltage levels. Longitudinal insulation refers to insulation within the same winding, such as between layers, turns, and between the winding and electrostatic shield.
This comprehensive description highlights the key components and their functions in transformer design, emphasizing their roles in ensuring efficient and safe operation.
