THREE-PHASE TRANSFORMERS

Large-scale generation of electric power is usually 3-phase at generated voltages of 13.2KV or somewhat higher. Transmission is generally accomplished at higher voltages of 66, 110, 132, 220, 275, 400, 765, 800KV for which purpose 3-phase transformers are necessary to step up the generated voltage to that of the transmission line. Next, at load centers, the transmission voltages are reduced to distribution voltages of 6600, 4600 and 2300 volts. Further, at most of the consumers, distribution voltages are still reduced to utilization voltages of 440, 220, or 110volts. Years ago, it was a common practice to use suitably interconnected three single 3-phase transformers, but these days, the latter is gaining popularity.

As compared with a bank of single phase transformers, the mains advantages of 3-phase transformer are that it occupies less floor space for equal rating, weight less, costs about 15% less and further, that only one unit is to be handled and connected. Like single-phase transformers, the three phase transformers are also of the core type or shell type. The basic principle of a 3-phase transformer is illustrated in which only primary windings interconnected in star and put across 3 -phase supply. The three cores are 1200 apart and their empty legs are in contact with each other. The center leg, formed by these three, carries the flux produced by the three phase currents IR, IY and IB. As at any instant IR + IY + IB = 0, hence the sum of three fluxes is always zero. Hence, it will make no difference if the common leg is removed. In that case, any two legs will act as the return for the third just as in a 3-phase current system any two conductors act as the return for the current in the third conductor.

It will be seen that at any instant, the amount of ‘up’ flux in any leg is equal to the sum of ‘down’ fluxes in the other two legs. The core type transformers are usually wound with circular cylindrical coils. In a similar way, three single-phase shell type transformers can be combined together to form a 3-phase shell type unit. But saving in iron can be achieved in constructing a single 3-phase transformer. It does not differ from three single-phase transformers put side by side. Saving in iron is due to the joint use of the magnetic paths between the coils. The three phase in this case are more independent than they are in the core type transformer, because each phase has a magnetic circuit independent of the other. One main drawback in a 3-phase transformer is that if any one phase becomes disabled, then the whole transformer has to be ordinarily removed from service for repairs. However in the case of 3-phase bank of single-phase transformers, if one transformer goes out of order, the system can still be run open -D at reduced capacity or a single spare can readily replace the faulty transformer.

All the conditions, which apply to the parallel operation of single-phase transformers also, apply to the parallel running of 3-phase transformers but with the following additions :(1) The voltage ratio must refer to the terminal voltage of primary and secondary. It is obvious that this ratio may not be equal to the ratio of the number of turns per phase. For example, if V1, V2 are the primary and secondary terminal voltages, then for Y/D connection, the turn ratio is V2 / (V1 / Ö3) =(Ö3 * V2 *) / V1.(2) The phase displacement between primary and secondary voltages must be the same for all transformers, which are to be connected for parallel operation.