It is not possible to use line current directly for measurement and relaying purpose due to its high value and insulation problem. Current Transformers which is widely known as 'CT' is a piece of electrical equipment which converts line current (primary current) in to small standard current values which are suitable for the devices to which it is connected e.g. measuring instruments, meters, protection relays.

BASIC FUNCTIONS OF Current Transformers ( CT ) ARE

• To reduce line current to a value which is suitable for standard measuring instruments, relays etc.
• To isolate the measuring instruments, meters, relays etc. from line voltage of an installation.
• To protect measuring instruments against short circuit currents.
• To sense abnormalities in current and give current signals to protective relays to isolate the defective system.

CURRENT TRANSFORMERS ARE BROADLY CLASSIFIED AS

1. Ring Type Current Transformers ( CT ) : A CT which has an opening in the center to accommodate a primary conductor (busbar or cable) through it.
2. Bar-Primary Current Transformers ( CT ) A CT in which the primary winding consists of bar of suitable size and, material forming an integral part of the CT.
3. Wound Type Current Transformers : A CT having a primary winding of more than ane full turn wound on the core.

Ring Type ( Rectangular Type Current Transformers ) CT are normally preferred over other types of CT. because they are simple in construction, mechanically stronger and cheaper. In a Ring Type / Bar Primary Type Current Transformers the working ampere-turns are determined by the primary current and therefore necessarily, the accuracy that can be offered with these CT becomes progressively inferior as the rated primary current decreases.'-If higher accuracy and burdens are required for CT of low primary current wound types CT are used.

Current Transformers are also classified in accordance with their nature of application, i.e.

Measuring Current Transformer & Protective Current Transformer. CT can be manufactured to suit above individual application or combined dual purpose application.

FOLLOWING ARE FEW GENERAL TYPES OF Current Transformers ( CT ).

1) Interposing Current Transformers ( CT ) : These CT are used in conjuction with main CT to alter the ratio of main CT or to provide isolation to meters or relays from main CT secondary circuit. Primary current of these CT are generally lower than 1 0 amp. Due to which they are always wound primary types.

2) Summation Current Transformers ( CT ) : 'When current flowing, in more than one feeder is required to be meteredi summation Current Transformers are used. These CT are provided with more than one primary winding and one common secondary. The standard primary & secondary currents are 5 or 1 amp. Summation Current Transformers are generally manufacture-confirming to ISS 6949.

3) Bushing Type Or Busduct Current Transformers ( CT ) : These are Ring type construction & can be mounted on Busduct or Bushing turrert of power transformer.

4) Core Balance Current Transformers ( CT ) : These are Ring Type Current Transformers ( CT ) & suitable for the measurement of the sum of three phase currents in a 3-ph cable. Under normal operating conditions this sum is zero. In the event of an earth-fawt the sum of the current is equal to the zero sequence current. These CTs are generally used with English Electric make CTUM 15 static relay. It is necessary to specify leakage current to be detected along with minimum setting of the relay and size of cable at the time of ordering CT.

5) Furnace Transformers ( CT ) : These transformers are generally split core type which can be easily mounted on the bus of furnace transformer. Primary current of these transformers are o value 10,000 Amp & above.? Special precaution is necessary to reduce the heat de@eloped in the transformers due to eddy current during design and manufacture.

6) Precision Grade Current Transformers ( CT ) : These Current Transformers ( CT ) are of accuracy of 0.1, 0.2 or 0.5 and used as a standard current transformer to check

accuracies of other transformer. These Current Transformers are either wound primary or ring type and manufactured -in. teakwood cases.