In a typical transformer, the primary and secondary windings are concentrically wound on a single iron core column, with the low-voltage winding inside and the high-voltage winding outside. (In contrast, the primary and secondary windings of a welding machine transformer are mounted on separate iron core columns.)
When a transformer is operating under load, as the secondary current increases, the primary current must also increase accordingly to maintain a constant main magnetic flux in the iron core and balance the secondary current.
The secondary active power of a transformer is generally calculated as: Transformer rated capacity (KVA) × 0.8 (transformer power factor) = KW.
Power transformers mainly include:
A. A desiccant (silica gel cartridge): This contains silica gel. The insulating oil in the oil conservator (oil tank) is connected to the atmosphere through the desiccant. The desiccant absorbs moisture and impurities from the air to maintain good insulation performance of the transformer's internal windings. Discoloration or deterioration of the silica gel can easily cause blockages.
B. Oil Level Gauge: Reflects the transformer oil level, generally around +20°C. If too high, drain oil; if too low, add oil. In winter, when temperatures are low and the load is light, the oil level change is minimal, or it may drop slightly. In summer, when the load is heavy, the oil temperature rises, and the oil level also rises slightly; both are normal.
C. Oil Tank: Regulates the oil level in the tank and prevents rapid oxidation of the transformer oil. It has a filler hole at the top.
D. Explosion-proof Pipe: Prevents sudden pressure surges in the tank that could cause an explosion.
E. Signal Thermometer: Monitors the transformer's operating temperature and sends a signal. It indicates the temperature of the upper layer of oil in the transformer. The transformer coil temperature should be 10°C higher than the upper layer oil temperature. National standards stipulate that the maximum operating temperature of the transformer windings is 105°C (i.e., at an ambient temperature of 40°C), and the upper layer temperature should not exceed 95°C. It is generally advisable to set the monitored temperature (upper layer oil temperature) at or below 85°C.
F. Tap Changer: Changes the voltage ratio by altering the high-voltage winding taps, increasing or decreasing the number of winding turns.
Since U1/U2 = W1/W2 and U1W2 = U2W1,
therefore, U2 = U1W2/W1.
Most transformers are no-load tap changers, requiring a power outage for operation. They are typically available in three ranges: +5%, 0%, and -5% (primary voltage 10.5KV, 10KV, 0.95KV; secondary voltage 380V, 400V, 420V). They are generally set to range II at the factory.
G. Gas Signal Relay: (Gas Relay) Provides protection against light and heavy gas signals. The upper contact is for a light gas signal, generally used for alarm purposes to indicate abnormal transformer operation; the lower contact is for a heavy gas signal, which, upon activation, sends a signal and simultaneously trips the circuit breaker, drops the trip card, and triggers an alarm. Generally, a gas relay filled with oil indicates no gas. If gas is present in the oil tank, it will enter the gas relay, and when it reaches a certain level, the gas will displace the stored oil, causing the contacts to activate.
Opening the gas relay cover reveals two adjusting rods on top; unscrewing one cap releases the gas from the relay. The other adjusting rod is the protection operation test button. When operating on energized surfaces, insulated gloves must be worn, and safety must be emphasized.