Instrument Transformers

INSTRUMENT TRANSFORMERS

For measuring a large current in a d.c. circuit, we use low-range ammeter with a suitable shunt. The measurement of high d.c. voltage is made using a low-range voltmeter with a multiplier.

However, this method is not used for the measurement of high alternating currents and voltages for many good reasons. In order to measure high alternating currents and voltages, we employ specially designed transformers, called instrument transformers. These transformers facilitate the a.c. measurements with low-range a.c. instruments. There are two types of instrument transformers viz. (i) Current transformers (ii) Potential transformers.

Current Transformer (C.T.)

A current transformer (C.T.) is used to measure high alternating current in a power system. The primary of this transformer has a few turns of thick wire whereas the secondary has many turns of very fine wire as shown in Fig. 4.15. It is clear from the figure that a current transformer is simply a well designed step-up transformer. Since voltage is stepped up, the current is stepped down which can be measured with a low-range a.c. ammeter.

The primary of the current transformer is connected in series with the line whose current is to be measured as shown in Fig. 4.15. The secondary of the transformer is connected across a low-range (0-5A) a.c. ammeter. The line current (I_P) and a.c. ammeter current (I_S) are related as:

N_P I_P = N_S I_S

I_P / I_S = N_S / N_P

The primary to secondary current ratio (i.e., I_P / I_S) is called C.T. ratio (current transformation ratio)

I = C.T. ratio

Or I_P = I_S × C.T. ratio

i.e., Line current (I_P) = A.C. ammeter reading × C.T. ratio

Fig. 4.16. It consists of a ring-shaped laminated core which carries the secondary winding. The current carrying conductor itself acts as a one-turn primary that simply passes through the centre of the ring. The position of the primary is unimportant as long as it is more or less centred. This current transformer has the arrangement to open and close the ring shaped core so that current can be measured without opening the line.

The clamp-on current transformers are simple and inexpensive and are widely used in low-voltage (LV) and medium voltage (MV) lines in the power system.

Potential Transformer (P.T.)

A potential transformer (P.T.) is used to measure high alternating potential difference (voltage) in a power system. The primary of this transformer has many turns while the secondary has few turns as shown in Fig. 4.17.

It is clear from the figure that a potential transformer is simply a well-designed step- down transformer. The stepped down voltage is measured with a low-range a.c. voltmeter. The magnetic core of a potential transformer usually has a shell-type construction for better accuracy. In order to provide adequate protection to the operator, one end of the secondary winding is usually grounded.

The primary of the potential transformer is connected across the high-voltage line whose voltage is to be measured. A low-range (0-110 V) a.c. voltmeter is connected across the secondary. The line voltage (V) and a.c. voltmeter voltage (V) are related as:

V_P / V_S = N_P / N_S

The primary to secondary voltage ratio (i.e., V_P / V_S) is called P.T. ratio (potential transformation ratio).

V_P / V_S = P.T. ratio

or V_P = V_S P.T. ratio

i.e. Line voltage (VP)=A.C. voltmeter reading × P.T. ratio

Advantages of Instrument Transformers

In order to measure high alternating currents and voltages in a power system, we prefer instrument transformers to shunts and the following reasons:

(i) The errors due to stray inductance and capacitance in shunts, multipliers and their leads are eliminated.

(ii) The measuring circuit is isolated from the mains by the transformer.

(iii) We can use low-range and accurate a.c. instruments.

(iv) The length of the connecting leads from the transformer to the instrument is of lesser importance and leads may be of small cross-sectional area.

(v) By using a clip-on type of transformer core, the current in a heavy-current conductor can be measured without breaking the circuit.

Difference between C.T. and PT.

Few differences between C.T. and P.T. are listed below.