Construction and principle of operation of current transformer

Current transformer (C.T.):

The current transformers are of family two types. They are

  1. Wound type
  2. Bar type

Wound type C.T.

(Wound type C.T.)
(Stampings for window type CTs.)

A current transformer in which the primary winding consists of more than one full turn wounded on core is called wound type. The simplest form of current transformer is the ring type or window type which is shown in the figure (b), which shows three commonly used shapes. such as stadium, circular and rectangular orifices. The core is either a nickel-iron or an oriented electrical steel which is continuously wound type. But current transformers contain hot rolled steel will consist of stack of ring stampings. Before putting secondary winding on the core, the current transformer using hot rolled steel is insulated by means of end collars and circumferential wraps of elephantine or presspahn. These pressboards act as insulation medium and also protects the secondary winding conductor from mechanical damage due to sharp corners. The secondary winding conductor is put on the core by a toroidal winding machine But if the number of secondary winding turns is small, then hand winding can be used to put the winding on core. Once the secondary winding is placed on the core, the ring type transformer exterior end collars and circumferential insulating wraps.

Bar type C.T.

In a bar type current transformer, the core and secondary windings are same as in ring type transformer, but the fully insulated bar conductor constituting the single turn primary winding is now an integral part of current transformer. The insulation provided on the primary winding conductor is either bakelized paper tube or a resin directly molded on the bar.

When using bar type primary, the external diameter of the tube should be large to keep the voltage gradient, in the dielectric at its surface, to an acceptable value in order to avoid corona effect.

The windings are designed to withstand the large short circuit forces that are caused when a short circuit takes place on the system in which current transformer is connected. These windings are separately wound and are insulated by tape and varnish for small line voltages. Above 7kv voltages the transformers are oil immersed or compound filled.

Working Principle:

Current transformer connection:

The current transformer is used to measure current. The primary of a current transformer consists of a few turns of thick cross-section connected in series with the high current. Most of the time, secondary winding is formed by just one turn of the primary by taking the line conductor. The secondary winding consists of a large number of turns of fine wire designed for either 5A or 1A rating. Thus, a current transformer is step up transformer. The current transformer has the secondary winding short circuited through low impedance of the ammeter. The core material for C.T. is usually low -loss silicon steel, mu metal. The core has high permeability and low loss. To keep losses low flux density is kept low. The core lamination is rectangular or ring shaped. But ring type core is joint less but difficult to wind. Windings must be rigid to withstand extremely large mechanical forces produced by short-circuit currents. For low and medium voltages, tape and varnish insulations are used while for high voltage working current transformer may be compound filled or oil immersed.

 The current transformer ratio is not equal to the ratio of secondary to primary turns, mainly because of the effect of the magnetizing current. The primary current can be assume as the sum of two currents. The first to balance secondary current so that primary and secondary m.m.f may balance and second being the no load current I0. The component l0 sides being responsible for a slight error in the current ratio, is also responsible for a p angle error. The transformer must be carefully designed to minimize the ratio and phase angle errors.

The current transformer is never operated in open circuit due to two reasons.

  • There will be no secondary m.m.f. and since the primary current is fixed, the core flux will increase enormously. This will cause large eddy current and hysteresis losses and the resulting high temperature may dana the insulation or even the core.
  •  A very high voltage will be induced in the multiturn secondary and this high voltage may be dangerous both to life and to insulation.

Also Read- Construction of single-phase transformers

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