Potential Transformers

Potential transformers are extremely accurate-ratio stepdown transformers and are used in conjunction with standard low-range voltmeters (100-120 V) whose deflection when divided by transformation ratio, gives the true voltage on the primary or high voltage side. In general, they are of the shell type and do not differ much from the ordinary two-winding transformers except that their power rating is extremely small. Sine their secondary windings are required to operate instruments or relays or pilot lights, their ratings are usually of 40 to 100W. For safety, the secondary is completely insulated from the high voltage primary and is, in addition, grounded for affording protection to the operator. Figure (A) shows the connection of such a transformer.

Ratio and Phase-angle Errors Potential Transformers:

In the case of a potential transformers, we are interested in the ratio of the primary to the secondary terminal voltage and in the phase angle γ between the primary and reversed secondary terminal voltage V₂^′ .

The general theory of voltage transformer is the same as for the power transformers except that, as the current in the secondary burden is very small, the total primary current I₁ is not much greater than I₀.

In the phasor diagram of Figure (B) & (C), vectors AB, BC, CD and DE represent small voltage drops due to resistances and reactance of the transformer winding (they have been exaggerated for the sake of clarity). Since the drops as well as the phase angle γ are small, the top portion of diagram (Figure B) can be drawn with negligible loss of accuracy as in Figure (C) where V₂^′ vector has been drawn parallel to the vector for V₁.

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In these diagrams, V₂^′ 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. I₂^′ R₀₁. 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. Their values are I₀R₁ and I₀X₁ respectively.

Ratio Error of Potential Transformers

In the following theory, n would be taken to represent the ratio of primary turns to secondary turns (Article Instrument Transformer). Further, it would be assumed, as before, that n equals the nominal transformation ratio i.e. n = k_n. In other words, it would be assumed that V₁/V₂ = n, although, actually, V₁/V₂^′ = n.