The rated power of each winding in a transformer refers to a continuous loading and it is a reference value for guarantees and tests concerning load losses and temperature rise. Where a transformer winding has different values of apparent power under different cooling conditions, the highest value is defined as the rated power. A two-winding transformer is given only one value of rated power, which is identical for both windings, but a multi-winding transformer may have reduced levels of rated power on auxiliary windings.
When rated voltage applied to the primary winding and rated current flows through the secondary winding, the transformer is carrying rated power for that set of windings. This definition of rated power, used by IEC, implies that the rated power is the value of power input to the transformer, including its own absorption of active and reactive power. The secondary voltage under full load differs from the rated voltage by the volt- age drop in the transformer.
In North America (ANSI/IEEE standard C57.1200) rated power is based on the output that can be delivered at rated secondary voltage, and allowance must be made so that the necessary primary voltage can be applied to the transformer. The difference between basing rated power on loaded voltages (IEC) or open-circuit voltages (ANSI/IEEE) is significant.
Although transformers are rated for continuous operation, it is possible to supply over- loads for limited periods. The analysis of overloading profiles is based on the deterioration of cellulose. At temperatures above the rated temperature, cellulose degrades at a faster rate, and the load cycle should therefore include a period of operation at lower temperature to balance effects of these faster ageing rates at higher temperature. In general, the ageing rate doubles for every 8°C rise in temperature.
When the ambient temperature is below the rated ambient temperature it is also permissible to overload the transformer, the limit of the overloading being the operating temperature of the winding. Under severe operational pressures it is sometimes necessary to overload a transformer well beyond its nameplate rating, but such an operation will usually result in shorter life. IEC 354 provides a guide to overloading of oil-filled transformers based on these principles.
A maximum winding temperature of 140°C should not be exceeded even under emergency conditions, because free gas can be produced by cellulose at temperatures above this, and gas bubbles may cause dielectric failure of the windings.
Parallel operation of transformers
For operational reasons it may be necessary to operate transformers connected directly in parallel. For successful parallel operation, the transformers must have:
• the same voltage ratings and ratio (with some tolerance)
• the same phase–angle relationship (clock-hour number)
• the same percentage impedance (with some tolerance)
It is not advisable to parallel-operate transformers with widely different power ratings as the natural impedance for optimal design varies with the rating of the trans- former. The power divides between parallel-connected transformers in a relationship which is inversely proportional to their impedances; a low-impedance transformer operated in parallel with a higher-impedance unit will pass the greater part of the power and may be overloaded. A mismatch in loading of up to 10 per cent is normally acceptable.