Contents
Ratio of Potential Transformers
It is a common practice to indicate ratio of Potential transformers as Rated primary voltage/Rated secondary voltage e.g. for a 132 kV P.T., suitable for connection between phase and g round, and having only one secondary winding rated at 110  volts, 

the ratio shall be indicated as

132 kV / 110              

  Ö3           Ö3             

If the two secondary windings are required, each rated at  110    

                                                                                                    Ö3     volts, the ratrio shall be indicated 

as

132 kV / 110   - 110  volts.

 Ö3         Ö3       Ö3  
The rated primary voltage is usually the system voltage which shall appear across the primary winding when the transformer is connected into the system. For nominal system voltages of 33 kV (H.S.V. 36 kV) and above, it is the usual practice to connect three single phase P.Ts. in Star formation so that the phase to ground voltage (the system neutral in majority of such cases is effectively earthed) appears across the primary winding. As this voltage will be the nominal system voltage divided by 3, the ratio is indicated as above.

Potential Transformers for use with system voltages from 440 V to 22 kV it is also a practice to connect single phase P.Ts. in V/V formation so that the line to line volytage appears across the primary winding. In such cases the ratio shall be indicated as11 kV/110 volts for P.Ts suitable for use on 11 kV system (HSV 12 kV).

Indoor three phase transformers are manufactured and used on systems upto 22 kV voltage class. If it is proposed to use a 3-phase transformer as 6.6 kV system, the ratio shall be indicated as 6.6 kV/110 volts. In these cases it shall also be necessary to specify the requirement of individual winding configuration such Star/Star, V/V, Star/Star-open delta etc. along with the desired construction required for H.T. and L.T. neutral i.e. H.T. neutral formed but not brought out or H.T. neutral formed and brought out (for star connected H.T. windings). For star connected L.T. neutral is usually formed and brought out.

If open-delta winding is required to be provided on a three phase P.T., it shall be necessary to adopt a five-limb construction to provide the return path for the flux. In such a case the P.T. is likely to be much more expensive, and hence it may be desirable to consider use of three single phase P.Ts connected Star/Star-open delta.