Nominal Power Of Transformers
The main parameters of the transformer are the rated power, nominal primary and secondary voltages, transformation factor, short circuit voltage, idle loss and short circuit loss.
Nominal power The transformer is defined by its thermal mode. This is the power which the transformer can give long at nominal ambient temperature conditions which are regulated by the GOST: the maximum ambient temperature is 35 °C and the average annual temperature of 5 °C.
The temperature of the transformer corresponding to the long-moulded nominal power shall be allowed to exceed the ambient temperature by not more than 70°. If the thermal mode is met within the permitted and ambient conditions, the rated temperature conditions are within 20 years.
The length of service is determined by the degree of isolation, i.e. by the gradual loss of its elasticity, mechanical and electrical strength. Insulation ageing is most likely to result in short circuits in transformer shafts.
Load capacity The transformer depends on ambient temperature. If the average annual temperature differs from the GOST temperature of 5°, the rated power of the transformer (kV-A) under the conditions may be recalculated according to the formula
Short-term reloading of transformers is permitted in emergency cases. The overload value and allowable time of this1 overload regardless of ambient temperature (but not above 35 °C) are given in Table 11.1.
The oil transducers allow for a lengthy overload of each of the currents by 5 per cent above the rated response rate if the response voltage does not exceed the rated.
If the maximum of the typical (average) load schedule is less than the nominal transformer powerin winter months, an additional 1 per cent transshipment of the transformer is allowed per cent of the unloading in the summer but not more than 15 per cent (the dry load shall not exceed 150 per cent of the nominal load).
