Instrument transformer vs power transformer

An instrument transformer is a special transformer used to step down high voltages or currents to safe, measurable values for meters and relays. It isolates the measuring circuit from the high-voltage line. There are two types: the current transformer (CT) and the potential transformer (PT).

Types and working

Current transformer. The primary has few turns (often a single bar) and is connected in series with the line. The secondary has many turns and feeds an ammeter or relay coil (low impedance burden). Its secondary is nearly short-circuited. Secondary current is 5 A or 1 A.

$$I_1N_1 = I_2N_2 \Rightarrow \frac{I_1}{I_2} = \frac{N_2}{N_1} = \text{CT ratio}$$

Potential transformer. The primary is connected across the line, and the secondary feeds a voltmeter or pressure coil (high impedance burden). It works almost at no load, with secondary voltage 110 V.

$$\frac{V_1}{V_2} = \frac{N_1}{N_2}$$

  Line ----+----- Load       Line ------+------
           |                            |
          [CT]--(A)                    [PT]--(V)

Difference from the power transformer

Errors

For a CT, the exciting current $I_0$ is drawn from the primary but does not appear in the secondary, so the actual ratio $I_1/I_2$ differs from the turns ratio $N_2/N_1$ (ratio error), and $I_2$ is not exactly 180° from $I_1$ (phase-angle error). Both errors are kept small by a low-loss, high-permeability core operated at low flux density and by a low burden.