Parallel Operation of Single-Phase Transformers - GATE EE Electrical Machines

For same pu leakage impedance, parallel operation of transformers become feasible.

Total kVA = 1050+180 = 1230 kVA.

out of all, polarity must be correct for the connected transformers.

3m-1 phases are displaced 120° in opposite and (3m+1) in the same direction as the fundamental. Here ‘m’ is even integer. So 5th harmonic is in negative sequence and 7th harmonic component in positive sequence.

As the leakage reactances are not same for both the transformers, then Ia < I2 and Ib < I/2.
So the kVA will also be less than sum of individual A and B.

As xa > xb, leakage impedance angle of A will be more than that of B, so lesser power factor.

Leakage impedance in ‘ohms’ are inverse ratio of their respective kVA ratings.

All are advantages of having parallel connection of transformers.

Voltage regulation is the change in secondary terminal voltage from no load to full load at a specific power factor of load and the change is expressed in percentage.

E₂ = no-load secondary voltage

V₂ = full load secondary voltage

Voltage regulation for the transformer is given by the ratio of change in secondary terminal voltage from no load to full load to no load secondary voltage.

+ sign is used for lagging loads and

- ve sign is used for leading loads

Hence voltage regulation can be negative only for capacitive loads

In transformer minimum voltage regulation occurs when the power factor of the load is leading.

The voltage regulation of the transformer is zero at a leading power factor load such as a capacitive load.

For zero voltage regulation, E₂ = V₂

> IR cos φ = IX sin φ (negative sign represents leading power factor loads)

This is the leading power factor at which voltage regulation becomes zero while supplying the load.

2 kVA, 100/200 V transformer

A2_winding = 200/100 = 2

((kVA)_auto )_max = (A2_winding + 1) × (kVA)_2winding

= (2 + 1) × 2 = 6