When electric power is supplied to a locality, city or an area, a single transformer, capable of handing the required power demand, is installed. In some cases, it may be preferable to install two or more transformers in parallel, instead of one large unit. Though two or more transformers may be expensive than one large unit, yet this scheme possesses certain advantages described below:
(1) With two or more transformers, the power system becomes more reliable. For instance if one transformer develops fault, it can be removed and the other transformers can maintain the flow of power, though at a reduced level.
(2) Transformers can be switched off or on, depending upon the power demand. In this manner, the transformer losses decrease and the system becomes more economical and efficient in operation.
(3) The cost of a standby (or spare) unit is much less when two or more transformers are installed.
In any case, with the passage of time, electric power demand may become more than the rated kVA capacity of the already existing transformer or transformers. Under such circumstances, the need for extra transformer arises. Since the supply voltage has to remain constant, the extra unit must be connected in parallel.
Note that the parallel operation of transformers requires that their primary windings as well as their secondary windings, are connected in parallel. In this article, only the parallel operation of single-phase transformers is considered.
The various conditions which must be fulilled for the satisfactory parallel operation of two or more single-phase transformers, are as follows :
(A) The transformers must have the same voltage ratios, i.e. with the primaries connected to the same voltage source, the secondary voltages of all transformers should equal in magnitude.
(B) The equivalent leakage impedances in ohms should be inversely proportional to their respective kVA ratings. In other words, the per unit leakage impedances of the transformers based on their own kVA ratings must be equal.
(C) The ratio of equivalent leakage reactance to equivalent resistance, i.e. x/r should be same for all the transformers.
(D) The transformers must be connected properly, so far as their polarities are concerned. Out of the conditions listed above, condition
(D) must be strictly fulfilled. If the secondary terminals are connected with wrong polarities, large circulating currents will flow and the transformers may get damaged. Condition (A) should be satisfied as accurately as possible; since different secondary voltages would give rise toi undesired circulating currents. For conditions (B) and (C), some deviation is permissible. Thus the fulfilment of condition (D) is essential, whereas the fulfilment of other conditions is desirable.
Fig. 1.47 shows two single-phase transformers in parallel, connected to the same voltage source on the primary side. Terminals with proper polarity markings have been connected both on the H.V. and L.V. sides. A further check on the polarities can be applied by connecting a voltmeter V in series with the two secondaries. Zero voltmeter reading indicates proper polarities. If voltmeter reads the sum of the two secondary voltages, the polarities are improper and can be corrected by reversing the secondary terminals of any one transformer.
Electricalmode 