CT- CURRENT TRANSFORMER

A current transformer (CT) is used for measurement of alternating electric currents. Current transformers, together with voltage (or potential) transformers (VT or PT), are known as instrument transformers. When current in a circuit is too high to apply directly to measuring instruments, a current transformer produces a reduced current accurately proportional to the current in the circuit, which can be conveniently connected to measuring and recording instruments. A current transformer isolates the measuring instruments from what may be very high voltage in the monitored circuit. Current transformers are commonly used in metering and protective relays in the electrical power industry.

Current Transformer Burden

Burden is the load imposed on the secondary of the CT at rated current and is measured in VA (product of volts and amps). The accuracy class applies only to loads at rated VA and below, down to one quarter VA .

The burden on the secondary of a CT includes the effect of pilot leads, connections etc , as well as the instrument burden itself.

In situations where the meter is remote from the current transformer, the resistance of the pilot wires may exceed the meter impedance many times in these cases it is often economical to use 1 amp meters and CTs. 

Instrument Type	Approximate Burden
Ammeter	0.5 - 5VA
Voltmeter	2 - 5VA
Frequesncy Meter	1 - 5VA
Protection Relay	0.2 - 30VA

The diagram below shows the burden imposed on the CT due to a run of pilot wire. It will be seen that a pilot loop of 2.5mm2 wire, 60 metres long (30 metres distance) has a load of 12.5 VA on a 5 amp CT but only 0.5VA on a 1 amp CT.

CT Ratio

Current transformers where the secondary current under normal operation is practically proportional to the primary current. e.g 3000/1A would be 3000A primary (input) with 1A secondary (output) which in effect would be 3000 turns of copper around the steel with winding wire or 3000:1 proportional ratio.

In the case of 3000/5A again the primary would be 3000A with a 5A secondary, however as the ratio is proportional  primary to secondary there would be only 600 turns of winding wire or 600:1 proportional ratio.

CT Accuracy Class or Current Transformer Class

A CT is similar to a electrical power transformer to some extent, but there are some difference in construction and operation principle. For metering and indication purpose, accuracy of ratio, between primary and secondary currents are essential within normal working range. Normally accuracy of current transformer required up to 125% of rated current; as because allowable system current must be below 125% of rated current. Rather it is desirable the CT core to be saturated after this limit since the unnecessary electrical stresses due to system over current can be prevented from the metering instrument connected to the secondary of the CT as secondary current does not go above a desired limit even primary current of the CT rises to a very high value than its ratings. So accuracy within working range is main criteria of a CT used for metering purpose. The degree of accuracy of a metering CT is expressed by CT accuracy class or simply current transformer class or CT class.