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Current Transformer (CT) - Construction and Working Principle

The large alternating currents which can not be sensed or passed through normal ammeter, and current coils of wattmeters, energy meters can easily be measured by use of current transformers along with normal low range instruments.

A transformer is a device which consists of two windings called primary and secondary. It transfers energy from one side to another with suitable change in the level of current or voltage.
Related: Working Principle of Transformer
A current transformer (CT) is a type of transformer that is used to measure AC current. It produces an alternating current (AC) in its secondary which is proportional to the AC current in its primary. Current transformers, along with voltage or potential transformers are Instrument transformer.

Current Transformer Symbol / Circuit Diagram 

A current transformer (CT) basically has a primary coil of one or more turns of heavy cross-sectional area. In some, the bar carrying high current may act as a primary. This is connected in series with the line carrying high current.
The secondary of the current transformer is made up of a large number of turns of fine wire having small cross-sectional area. This is usually rated for 5A. This is connected to the coil of normal range ammeter.


Related: Why Current Transformer (CT) Seconday Should not be Open ?

Working Principle of CT

These transformers are basically step up transformers i.e. stepping up a voltage from primary to secondary. Thus the current reduces from primary to secondary. 

So from current point of view, these are step down transformer, stepping down the currant value considerably from primary to secondary. 

Let,
N1 = Number of Primary Turns
N2 = Number of Secondary Turns
I1 = Primary Current
I2 = Secondary Current

For a transformer,
I1∕I = N2/N1

As N2 is very high compared to N1, the ratio I1 to I2 is also very high for current transformers. Such a current ratio is indicated for representing the range of current transformer. 

For example, consider a 500:5 range then it indicates that C.T. steps down the current from primary to secondary by a ratio 500 to 5. 

I1∕I = 500/5

Knowing this current ratio and the meter reading on the secondary, the actual high line current flowing through the primary can be obtained. 

Construction of Current Transformer

There are three types of constructions used for the current transformers which are,
  1. Wound Type CT
  2. Torroidal (Window) Type CT
  3. Bar Type CT

Wound Type Current Transformer – The transformers primary winding is physically connected in series with the conductor that carries the measured current flowing in the circuit. The magnitude of the secondary current is dependent on the turn’s ratio of the transformer. 

Torroidal (Window) Type Current Transformer – These do not contain a primary winding. Instead, the line that carries the current flowing in the network is threaded through a window or hole in the torroidal transformer. Some current transformers have a “split core” which allows it to be opened, installed, and closed, without disconnecting the circuit to which they are attached. 

Bar-type Current Transformer – This type of current transformer uses the actual cable or bus-bar of the main circuit as the primary winding, which is equivalent to a single turn. They are fully insulated from the high operating voltage of the system and are usually bolted to the current carrying device. 

1. Wound Type Current Transformer

In wound type construction the primary is wound for more than one full turn on the core.
The construction is shown below.
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 a 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 transfonncrs are shown in the figure below.

The core material for wound type is nickel-iron alloy or an oriented electrical steel. Before installing the secondary winding on 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 the Figure.

The insulation on the bar type primary is bakelized paper tube or a resin directly moulded on the bar. Such bar type primary is the integral part of the current transformer. The core and the secondary winding are 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 short circuit in the circuit in which the current transfomter 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.

Uses / Advantages of Current Transformer

Current transformers are used extensively for measuring current and monitoring the operation of the power grid.

Along with voltage leads, revenue-grade CTs drive the electrical utility's watt-hour meter on virtually every building with three-phase service and single-phase services greater than 200 amperes. 

High-voltage current transformers are mounted on porcelain or polymer insulators to isolate them from ground. 

Current transformers can be mounted on the low voltage or high voltage leads of a power transformer.

Often, multiple CTs are installed as a "stack" for various uses. For example, protection devices and revenue metering may use separate CTs to provide isolation between metering and protection circuits, and allows current transformers with different characteristics (accuracy, overload performance) to be used for the devices.