Voltage Factor

Attention is invited to Table 2 of I.S. 3156/1992 clause 6.6.2 reproduced below which can be used for determining the voltage factor applicable to various systems.

                                                  Table 2
     Highest System Voltages upto and including 72.5 kV.

Method of connecting the primary winding and system earthing condition

Rated Voltage Factor

Rated Time

Between lines in any network between transformer star point and earth in any network



Between line in and earth in any effective earthed neutral system (see 3.33.1)

1.2 1.5

Continuous  30 seconds

Between line and earth in a non-effectively earthed neutral system (see 3.33.2) with automatic earth fault tripping.
1.2 1.9

Continuous  30 seconds

Between line and earth in an isolated neutral system without automatic earth-fault tripping (see 3.30) or in a resonant earthed system (see 3.31) without automatic earth-fault tripping.
1.2 1.9
Continuous   8 Hours

Note - Reduced rated times are permissible by agreement between the manufacturer and the user.
It will be seen that the voltage factor is dependant upon the system earthing condition.
It may be appreciated that specification of too high, a voltage factor can complicate the design of P.Ts., particularly when the specified burden is high. Against this it may please be noted that specification of too low, a voltage factor may result into failure of P.Ts. in the event of a fault on the system. In the case of isolated neutral systems the voltage appearing across the open-delta winding is 3 times the rated secondary voltage of the individual secondary winding.