Theory of Potential Transformer

Theory of Potential Transformer Points : theory of potential transformer The general theory of potential transformer is similar to that for power transformers, except that, as the current in the burden is very small, the total primary current is not much greater than the “no-load” current. The voltage drops due to the resistance and leakage reactance of the windings are of great importance, as an accurate value ratio is required. The phasor diagram of PT.
The vectors AB, BC, CD & DE represent small voltage drops due to resistances and reactance’s of the transformer windings (they have been exaggerated for the sake of clarity). Since the drops as will as the phase angle “y” are small, the top portion. (a) can be drawn with negligible loss of accuracy. (b) where V₂ vector has been drawn parallel to the vector for V₁.
in these diagrams, V,’ is the secondary terminal voltage as referred to primary assuming transformation without voltage drops. All actual voltage drops have been referred to the primary. Vector AB represents total resistive drop as referred to primary i.e. lRO1. Similarly, BC represents total reactive drop as referred to primary i.e. I₂ X01.
In a voltage transformer, the relatively large no-load current produces appreciable resistive drops which have been represented by vectors CD and DE respectively.
Errors: There are following errors present in the PT.
(i) Ratio Error
(ii) Phase Angle Error