Transformers are devices able to transform the voltage of an electrical supply from one level to another. This is necessary because voltage is normally distributed at a level higher than that used by machinery in industry: higher voltages used in the distribution system reduces energy losses in the distribution lines.
Transformers are static machines made up of a core comprising a number of ferromagnetic plates, with the primary and secondary coils wound around the opposite sides of the core. The transformation rate of the voltages is given by the ratio V2/V1 (see Figure 3.23).
If P1 is the electrical power entering the transformer, P2 the power exiting and PL the losses, then the power balance is:
and the transformer efficiency can be written as:
The losses are of two main types: losses in the iron components and losses in copper components. Losses in iron are caused by hysteresis and eddy currents inside ferromagnetic core plates; such losses are proportional to V2 and are from about 0.2 to 0.5 % of nominal power Pn (= P2). Losses in copper are caused by the Joule effect in copper coil; such losses are proportional to I2, and are estimated roughly from 1 to 3 % of nominal power Pn (at 100 % of the load).
Since a transformer works on average with a load factor x lower than 100 %, (Peffective = x Pn), it can be demonstrated that the relationship between the transforming efficiency and the load factor follows the curve in Figure 3.24 (for a 250 kVA transformer). In this case, the transformer has a maximum point at a value of about 40 % of the load factor.
Whatever the power of the transformer is, the relationship between efficiency and load factor always shows a maximum, set normally on average at around 45 % of the nominal load.
Due to this distinctive behaviour, it is possible to evaluate the following options in an electrical power (transformer) substation:
●if the global electric load is lower than 40 - 50 % Pn, it is energy saving to disconnect one or more transformers to load the others closer to the optimal factor
●in the opposite situation (global electric load higher than 75 % Pn), only the installation of additional capacity can be considered
●when repowering or updating the transformer substation, installing low loss transformers, that show a reduction of losses from 20 to 60 % is preferred.
P1 = P2 + PL
其效率公式 η=P2 / P1 = (P1-PL) / P1 = 1 –PL / P1
The optimisation criteria are applicable to all transformer rooms. Optimising the loading is estimated to be applicable in 25 % of cases.
The number of new transformer power installed/repowered every year in industry is estimated to be 5 % and low loss transformers can be considered in these new/repowered cases.
In the case of the installation of low loss transformers with respect to ‘ normal series’ transformers, or in substitution of low efficiency transformers operating at present, payback times are normally short, considering that transformers operate for a high number of hours/year.
Energy Efficiency (2009) 3.5.4