## Errors in Potential Transformer :

#### The potential transformer also introduces the ratio error and phase angle error just like the current transformer.

### Ratio Error :

#### Due to a change in load conditions in the distribution system. In a potential transformer, the actual transformation ratio (R) (i.e., the ratio of primary to the secondary terminal voltage V_{p}/V_{s}) differs from its turns ratio (n) and is given by,

#### From the above equation, it is clear that the voltage across the secondary winding is not a constant fraction of the voltage across the primary winding and hence, an error called 'Ratio Error' is introduced in the measurement of voltage. It is also known as voltage error and it is defined as the deviation of actual transformation ratio (R) from the nominal ratio (K_{n}).

#### Where,- K
_{n} = Nominal ratio (i.e., rated primary voltage/rated secondary voltage).

_{n}= Nominal ratio (i.e., rated primary voltage/rated secondary voltage).### Phase Angle Error :

#### When the potential transformer is used in the measurement of power or energy, in order to obtain an accurate reading, the phase of secondary voltage should be exactly 180° out of phase with the primary voltage. But, in P.T.'s these voltages will not be displaced exactly by 180° in phase and differs by an angle called as phase angle.

This phase angle represents the phase angle error. The phase angle error in potential transformer is given as,

#### Where,- R
_{p}' = Equivalent resistance of the transformer referred to primary side. - X
_{p}' = Equivalent reactance of the transformer referred to primary side. - I
_{w} = Iron loss component. - I
_{m} = Magnetizing component. - I
_{s} = Secondary winding current. - V
_{s} = Secondary terminal voltage. - n = Turns ratio.
- δ = Secondary burden phase angle.

From the above two errors, it can be seen that the ratio and phase angle errors depend upon the no-load current (i.e., magnetizing and loss component of exciting or no-load current), impedance (i.e., resistance and reactance) of the windings, and secondary burden power factor.

_{p}' = Equivalent resistance of the transformer referred to primary side._{p}' = Equivalent reactance of the transformer referred to primary side._{w}= Iron loss component._{m}= Magnetizing component._{s}= Secondary winding current._{s}= Secondary terminal voltage.