Large scale generation of electric power is usually 3-phase at generated voltages of 13.2 kV or somewhat higher. Transmission is generally accomplished at higher voltages of 110, 132, 275, 400 and 750 kV for which purpose 3-phase transformers are necessary to step up the generated voltage to that of the transmission line. Next, at load centres, the transmission voltages are reduced to distribution voltages of 6,600, 4,600 and 2,300 volts. Further, at most of the consumers, the distribution voltages are still reduced to utilization voltages of 440, 220 or 110 volts. Years ago, it was a common
practice to use suitably interconnected three single-phase transformers instead of a single 3-phase transformer. But these days, the latter is gaining popularity because of improvement in design and manufacture but principally because of better acquaintance of operating men with the three-phase type. As compared to a bank of single-phase transformers, the main advantages of a 3 phase transformer are that it occupies less floor space for equal rating, weighs 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 Fig. 33.1 in which only primary windings have been shown interconnected in star and put across 3- phase supply. The three cores are 120° apart and their empty legs are shown in contact with each other. The centre 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 also zero.
leg is removed. In that case any two legs will act as the return for the third just as in a 3-phase system any two conductors act as the return for the current in the third conductor. This improved design is shown in Fig. 33.2 (a) where dotted rectangles indicate the three windings and numbers in the cores and yokes represent the directions and magnitudes of fluxes at a particular instant. 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 as shown in Fig. 33.2(b). But some saving in iron can be achieved in
or Vee but this is not always feasible). However, in the case of a 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 the faulty transformer can be readily replaced by a single spare.
Three-phase Transformer Connections
There are various methods available for transforming 3-phase voltages to higher or lower 3-phase voltages i.e. for handling a considerable amount of power. The most common connections are
Scott connection or T – T connection.