Current Transformer (CT) – Construction and Working Principle

1. Wound Type Current Transformer
In wound type construction the primary is wound for more than one full turn on the core.

Wound Type Current Transformer
The construction of Wound Type Current Transformer is shown above.
In a low voltage wound type current transformer, the secondary winding is wound on a bakelite former. The heavy primary winding is directly wound on the top of the secondary winding with suitable insulation in between the two.
Otherwise, the primary is wound completely separately and then taped with suitable insulating material and assembled with the secondary on the core.
The current transformers can be ring type or window type. Some commonly used shapes for the stampings of window type current transformers are shown in the figure below.
The core material for wound type is nickel-iron alloy or oriented electrical steel. Before installing the secondary winding on the core it is insulated with the help of end collars and circumferential wraps of pressboards. Such pressboards provide additional insulation and protection to the winding from damage due to the sharp corners.
2. Bar Type Current Transformer
In this type of current transformer, the primary winding is nothing but a bar of suitable size. The construction is shown in Figure.

Bar Type Current Transformer
The insulation on the bar type primary is bakelized paper tube or resin directly molded on the bar. Such bar type primary is an integral part of the current transformer. The core and the secondary winding are the same in bar type transformer.
The stampings used for the laminations in current transformers must have high cross-sectional area than the ordinary transformers. Due to this, the reluctance of the interleaved comers remains as low as possible. Hence the corresponding magnetizing current is also small.
The windings are placed very close to each other so as to reduce the leakage reactance. To avoid the corona effect, in bar type transformer, the external diameter of the tube is kept large.
The windings are so designed that without damage, they can withstand short circuit forces which may be caused due to a short circuit in the circuit in which the current transformer is inserted.
For small line voltages, the tape and varnish are used for insulation. For line voltages above 7 kV, the oil-immersed or compound filled current transformers are used.
Construction of High Voltage Outdoor Current Transformer