INSTRUMENT & CURRENT TRANSFORMERS

In D.C. circuits when large currents are to be measured, it is usual to use low-range ammeters with suitable shunts. For measuring high voltages, low-range voltmeters are used with a high resistance connected in series with the voltmeter. But it is not convenient to use this method with alternating current and voltage instruments. For this purpose, specially constricted accurate-ratio instrument transformers are employed in conjunction with standard low-range A.C. instruments. These instrument transformers are of two kinds: (1) current transformers for measuring large alternating currents and (2) potential transformers for measuring high alternating voltages.

These transformers are used with low range ammeters to measure currents in high voltage alternating current circuits. In addition to insulating the instrument from the high voltage line, they step-down the current in a known ratio. The current or series transformer has a primary coil of one or more turns of thick wire connected in series with the line whose current is to be measured. The secondary consists of large number of turns of line wire and connected across the ammeter terminals.
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 secondary to primary turn ratio of 100:5, then it steps up the voltage in the 20:1 ratio whereas it steps down the current in the ratio 1:20. Hence if we know current ratio of the transformer and the reading of the A.C. ammeter, the line current can be calculated. One of the most commonly used current transformers is one known as clamp-on or clip-on type. It has a laminated core, which is so arranged that it can be opened out at a hinged section by merely pressing a trigger like projection. When the 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 is tightly closed by a spring. The current carrying conductor or feeder acts as a single turn primary whereas the secondary is connected across the standard ammeter conveniently mounted in the handle.

It should be noted that, since the ammeter resistance is very low, the current transformer normally works short-circuited. If for any reason the ammeter is taken out of the secondary winding, then this winding must be short-circuited. If this is not done, then due to the absence of counter amp-turns of the secondary, the unopposed primary m.m.f. will set up an abnormally high flux in the core, which will produce excessive core loss with subsequent heating and a high voltage across the secondary terminals. This is not the case with ordinary constant potential transformers, because their primary currents is determined by the load on their secondary whereas in a current transformer, the primary current is determined entirely by the load on the system and not by the load on its own secondary. Hence the secondary of a current transformer should never be left open under any circumstances.