For measuring a large current is a d.c. circuit, we use low-range ammeter with suitable shunt. The measurement of high d.c. voltage is made using a low-range voltmeter with a multiplier.
However, shunt and multiplier is not used for the measurement of high alternating currents and voltages respectively for many good reasons.
In order to measure high alternating currents and voltages, we employ specially constructed accurate ratio transformers, called instrument transformers.
There are two types of instrument transformers;
- Current Transformers (CT)
- Potential Transformers (PT)
Current transformers are employed in the ac circuits where the measuring current exceeds the safe current of measuring instruments.
Potential transformers are employed in the ac circuits where the measurement voltage exceeds 750 volt.
- Current Transformers (CT)
Current transformers (CT) are used to measure high alternating current in a power system. The current transformers are basically step-up transformers. The primary winding having one or a few turns of thick wire is connected in series with the line whose current is to be measured. The secondary consists of a large number of turns of fine wire and connected across the ammeter.As regards voltage, the transformer is of step-up variety but it is obvious that current will be stepped down. Thus, if the current transformer has primary to secondary current ratio of 100:5, then it steps up the voltage 20 times whereas it steps down the current to 1/20th of its actual value.
The working of current transformer is slightly different to that of an ordinary power transformer. In case of current transformer, the ‘Burden’ on the secondary is very small, therefore, it is considered to be short circuited. Hence, current transformer works under short circuit conditions. Moreover, the current in secondary windings is not governed by its load impedance rather it depends upon the current flowing through the primary winding.
From construction point of view, there are two types of current transformers which are commonly used in laboratories and panels. There are two types of current transformers (i) Clamp-on or clip-on type and (ii) Bar type.
- Clamp-on or clip-on type current transformer: One of the most commonly used current transformers is the one known as clamp-on or clip-on type. It has laminated core which is so arranged that it can be opened out at hinged section by merely pressing a trigger like projection when is core is thus opened, it permits the admission of very heavy current carrying bus bars or feeders whereupon the trigger is released and the core tightly closed by a spring. An ammeter is connected across the secondary winding of the transformer which measures the current flowing through the conductor directly. It is a portable instrument and generally used in the laboratories for testing purposes.
- Bar type current transformer: It has a circular ring type core over which secondary is wound. An ammeter is connected across the secondary. When a bar conductor or bus bar is inserted through it, the ammeter measures the current flowing through bar conductor directly. These are generally used with the instruments placed on panels or used with protective relays.
- Potential Transformers
The potential transformers are basically step down transformers. The connections of a voltmeter when used in conjunction with a potential transformer for measurement of high ac voltages. The voltage to be measured is applied across the primary winding which has a large number of turns.
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. Upto voltage of 5,000V, potential transformers are usually of the dry type, between 5,000 and 11,000 volts, they may be either dry type or oil immersed type, and although for voltages above 11,000V or 11KV they are always oil immersed type.