High voltage and current cannot be measured by ordinary volt-meter and ampere meter. Instrument transformer are used for this purpose. It has two types.
- Current transformer – used to measure high alternating current
- Potential transformer – used to measure high alternating voltage
This type of transformer is used to measure large current in high voltage alternating current circuits. The primary of this type transformer has one or more turns of thick wire. The primary is connected in series with the line whose current is to be measured as shown in Figure 1. The secondary of the transformer consist of large number of turns of fine wire. A low range ammeter is connected across the secondary winding. It is a step-up transformer i.e. it raises the voltage but stepping down the current at the secondary. Thus, knowing the reading of the ammeter and the current ratio of the transformer, the line current can be calculated e.g.
K = 20
I2 = 4
I1 / I2 = 20
I1 = 20 X 4 = 80 A
One of the most commonly used current transformer is the one known as clamp-on or clip on type. It has a laminated core which is so arranged that it can be opened and very heavy current feeder can be passed through it. Then a spring arrangement is closed. The feeder acts as a single turn primary where secondary is connected across the standard ammeter. Thus, current is noted from the calibrated ammeter directly. It should be noted that if for any reason ammeter is taken out from the secondary, then the secondary must be short circuited. Otherwise, the unopposed primary m.m.f. will setup high flux and hence high loss in the core, which will produce heat and high voltage at the secondary.
Figure 1: Current Transformer(P for Primary and S for Secondary)
It is an accurate ratio step down transformer. A standard low range voltmeter is connected on its secondary. The reading of the voltmeter when multiplied by the transformation ratio gives the primary i.e. input voltage which is to be measured. These transformers are of shell type and of small rating. (40-100) W. the secondary is completely insulated from high voltage primary. It is also grounded for safty purposes. See figure 2.
Figure 2: Potential Transformer