What types of current transformer?

  1. RING TYPE CTs : A CT which has an opening in the center to accommodate a primary conductor (busbar or cable) through it.
  2. BAR -PRIMARY CTs 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 CTs : A CT having a primary winding of more than ane full turn wound on the core.

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

Current transformers are also classified in accordance with their nature of application, i.e.
MEASURING CURRENT TRANSFORMER & PROTECTIVE CURRENT TRANSFORMER. CTs can be manufactured to suit above individual application or combined dual purpose application.

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

2) SUMMATION CTs : 'When current flowing, in more than one feeder is required to be meteredi summation current transformer are used. These CTs 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 CTs These CTs are Ring type construction & can be mounted on Busduct or Bushing turrert of power transformer.

4) CORE BALANCE CTs : These are ring type CTs & 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 : 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 : These CTs 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.