Instrument Transformer

INSTRUMENT TRANSFORMER

Transformers are used in ac systems for the measurement of current, voltage, power and energy. Instrument transformers are classified according to their uses as Current Transformer (CT) and Potential Transformer (PT).

1. Current Transformer (CT)

It has a primary winding with a small number of turns, connected in series with a line carrying the current to be measured. The secondary winding has a larger number of turns and is connected to a current meter or a relay coil as shown in fig. 5.20

where N = turns ratio = no. of turns of secondary winding / no. of turns of primary winding.

I0 = exciting current

I = secondary current

δ = angle between the secondary induced emf and secondary current and

α = angle between I0 and working flux

Phase angle θ is the angle by which I, when reversed, differs in phase from the primary current.

where

kn = nominal ratio = rated primary current / rated secondary current

2. Potential "Transformer (PT).

Potential transformer is used to operate voltmeters, potential coils of wattmeters and relays from high voltage lines. The primary winding of the transformer is connected across the lines carrying the voltage to be measured and the voltage circuit is connected across the secondary winding (Fig. 5.21)

The transformer ratio R is

where

N = turns ratio

Is = secondary winding current

Ic = exciting current

Im = magnetizing component of no-load (exciting) current

β = phase angle of secondary load circuit

Vs = secondary winding terminal voltage

r1, x1 = resistance and reactance of primary windings

R2, X2 = equivalent resistance and reactance of a transformer referred X2 to secondary side.

The phase angle θ is

Range Extension

Instrument transformers extend the range of the ac measuring instruments.

Advantages

1. Moderate size instrument transformers are used for metering.

2. Cheaper single range instrument can be used to cover a large current or voltage range. 

3. The metering circuit consumers lower power and is isolated from high power circuits. 

4. Several instruments can be operated from a single instrument transformer.