Three-Phase Transformer Winding Connections

Consider three 1-\(\Phi\) identical transformer each rating of \(10 KVA, 200V/ 100V, 50 Hz\)
Current Rating in HV and LV winding \[\begin{aligned} I_{HV} = & 10000/200 = 50 A\\ I_{LV} = & 10000/100 = 100A \end{aligned}\]
Total KVA that can be supplied to the load: \(\sqrt{3}V_LI_L = \sqrt{3} (\sqrt{3}100) = 30 KVA\)

Generally used for small, high-voltage transformers.
Because of star connection, number of required turns/phase is reduced (as \(V_p=1/\sqrt{3}V_L\)).
Thus, the amount of insulation required is also reduced.
This connection can be used only if the connected load is balanced

Problems Associated With Star-Star Connection The Y-Y connection has two very serious problems:

Not satisfactory for the unbalance load in the absence of a neutral connection. If the neutral is not provided, then the phase voltages become severely unbalance when the load is unbalanced.
The Y-Y connection contains a third harmonics, and in balanced conditions, these harmonics are equal in magnitude and phase with the magnetising current. Their sum at the neutral of star connection is not zero, and hence it will distort the flux wave which will produce a voltage having a harmonics in each of the transformers

The unbalanced and third harmonics problems of Y-Y connection can be solved by using the solid ground of neutral and by providing tertiary windings.

Total KVA that can be supplied to the load: \(\sqrt{3}V_LI_L = \sqrt{3}100(\sqrt{3}100) = 30KVA\)

This connection is mainly used in step down transformer at the substation end of the transmission line

The main use of this connection is at the substation end of the transmission line where the voltage is to be stepped down
This can be used to provide 3-pase 4-wire service (Y-with neutral grounded)
The ratio between secondary to primary line voltage is \(1/\sqrt{3}\) times the transformation ratio of each transformer
There is a \(30^\circ\) shift between the primary and secondary line voltage which means that a \(Y-\Delta\) transformer bank cannot be paralleled with either a \(Y-Y\) or \(\Delta-\Delta\) bank
Also third harmonic currents flows in the \(\Delta\) to provide a sinusoidal flux

Used to step-up the voltage as for example, at the beginning of high tension transmission system
3-phase 4 wire service (Y-neutral grounded)
Connection gained popularity because it can serve both the 3-phase power equipment and single-phase lightning circuits
Because of \(30^\circ\) shift between primary and secondary line voltages and line currents, it is impossible to parallel such a bank with a \(\Delta-\Delta\) or \(Y-Y\) bank
The ratio of primary voltage is \(\sqrt{3}\) times the transformation ratio of each transformer

This connection is generally used for large, low-voltage transformers.
Number of required phase/turns is relatively greater than that for Y-Y connection
This connection can be used even for unbalanced loading
Another advantage of this type of connection is that even if one transformer is disabled, system can continue to operated in open delta connection but with reduced available capacity

Used when one of the TFs in \(\Delta-\Delta\) bank is disabled and the service is to be continued until the faulty TF is repaired or replaced.
Also used for small three phase loads where installation of full three TF bank is unnecessary
The total load carrying capacity of open-delta connection is 57.7% than that would be for delta-delta connection