CURRENT TRANSFORMER

Current transformer- Definition, Principle, Equivalent circuit and types.

Definition:-

A current transformer is an “Instrument transformer”, used along with measuring or protective devices, in which the secondary current is proportional to the primary current (under normal conditions of operation) and differs from it by an angle which is approximately zero.

Current transformers perform the following functions:

·        Current transformers supply the protective relays with currents of magnitude proportional to those of power circuit but sufficiently reduced in magnitude.

·        The measuring devices cannot be directly connected to the high magnitude supplies. Hence current transformers are used to supply those devices with currents of magnitude proportional to those of power.

·        A current transformer also isolates the measuring instruments from high voltage circuits..

Principle:-

The basic principle of current transformer is same as that of the power transformer. Like the power transformer current transformer also contains a primary and a secondary winding. Whenever an alternating current flows through the primary winding alternating magnetic flux is produced, which then induces alternating current in the secondary winding.

In case of current transformers the load impedance or “burden” is very small. Therefore the current transformer operates under short circuit conditions. Also the current in the secondary winding does not depend load impedance but depends on the current flowing in the primary winding. The current transformer basically consists of an iron core on which primary winding and secondary winding are wound.

The primary winding of the transformer is connected in series with the load and carries the actual current flowing to the load while the secondary winding is connected to a measuring device or a relay. The number of secondary turns is proportional to the current flowing through the primary. (i.e.) larger the magnitude of current flowing through the primary, more the number of secondary turns.

The ratio of primary current to the secondary current is known as the current transformation ratio of the CT. Usually the current transformation ratio of the CT is high. Normally the secondary ratings are of the order 5 A, 1 A, 0.1 A whereas the primary rating vary from 10 A to 3000 A or more. The CT handles very less power.

Rated burden can be defines as the product of current and voltage at the secondary side of the CT. It is measured in volt ampere (VA). The secondary of a current transformer should not be disconnected from its Rated burden while current is flowing in the primary. As the primary current is independent of the secondary current, the entire primary current acts as a magnetizing current when secondary is opened. This results in deep saturation of the core which cannot return to normal state and so the CT is no longer usable.

Types of Current transformers:-

Based on the function performed by the current transformer it can be classified in to as follows:

1. Measuring current transformers:-

·        These current transformers are used along with the measuring devices for the measurement of current, energy and power.

2. Protective current transformers:-

·        These current transformers are used along with the protection equipments such as trip coils, relays etc.

Based on the function construction it can be classified in to as follows:

1) Bar Type: This type of current transformer consists of a bar of suitable size and Material forming an integral part of the transformer.

2) Wound Type: This type of CTs have a primary winding of more than one full turn Wound over the core.

3) Window Type: CTs of this type have no primary winding. The secondary wind of the CT is placed around the current flowing conductor. The magnetic electric field created by current flowing through the conductor induces current in the secondary winding which is used for measurement.

Secondary current rating:-

The value of rated secondary current is 5A. the secondary current rating of 2A and 1A may also be used in some cases if the number of secondary turns is low the ratio cannot be adjusted within the required limits by addition or removal of one turn, if the length of secondary connecting lead is such that the burden due to them at higher secondary current would be excessive.

The disadvantage of making transformer with lower secondary current rating are that they produce much higher voltage if they are ever accidentally left open circuited. For this reason it is better to adopt 5 A rating at the secondary.

Terminology of current transformer:-

Rated transformation ratio:

·        Rated transformation ratio is defines as the ratio of the rated primary current to the rated secondary current.

Current error (ratio error):

·        The percentage error in the magnitude of the secondary current is defined by the following formula

·        Ratio error = (KnIs - Ip)/ Ip x 100%

·        Ip, Is = primary and secondary winding currents respectively,

·        Kn = turns ratio

Accuracy class:

·        It is a classification assigned to a transformer. In the case of metering CT’s, accuracy class shall be 2, 0.5, 1.3 and 5.

Phase displacement:

·        The difference in phase between the primary and the secondary current phasors, the direction of the phasors being chosen such that the angle is zero for a perfect transformer.

Rated secondary current:

·        The value of rated secondary current shall be 5 A. The secondary currents rating of 2 and 1 A may also be used in some case.

Rated burden:

·        The product of current and voltage at the secondary side of the CT is called as rated burden. It is measured in volt ampere (VA).

Temperature rise:

·        The temperature rise of current transformer winding when carrying a rated primary current, at rated frequency and with rated burden should not exceed the approximate values given in table.