Starlit Electricals Current Transformer

There are following types and subtypes of Current Transformers.

1) L.T Tape wound/Cast Resin type CT.

        i.            Ring/window type and

        ii.            Wound primary type

2) L.T/ H.T Cast Resin CT.

        i.          Ring/window type and

        ii.         Wound primary upto 33 KV with Indoor/ Outdoor application.

3) H.T Indoor / Outdoor Oil Immersed type C.T/ P.T upto 66KV.

Function of Current Transformer

The main function of the Current Transformer is to reproduce the Primary current on its secondary side. The Current Transformer isolates the devices connected to their secondary circuit from the network system. Current Transformers are specially designed for measuring and protection applications.

Standard Terminology used for CT:

Rated primary current:

The Current to be measured and transformed to lower value

Rated secondary current:

The current on the secondary side of the CT Typical values of secondary current are 1A or 5A. For WTI Current Transformers 3 A secondary current is also specified. For special differential protection, secondary currents of 1/ Rt 3 A and 5/ Rt 3 A are also specified.

Accuracy class:

For metering CT s, accuracy class is typically 0.2, 0.5,1 or 3. This means that the errors have to be within the limits specified in the standards for that particular accuracy class. The metering CT has to be accurate from 5% to 120% of the rated primary current at the specified power factor.

Class PS/ PXCT:

In balance systems of protection, CT s with a high degree of similarity in their characteristics is required. These requirements are met by Class PS /PX CT s. Their performance is defined in terms of a knee-point voltage (KPV), the magnetizing current (Imag) at the knee point voltage or 1/2 or 1/4 the knee-point voltage, and the resistance of the CT secondary winding corrected to 75C. Accuracy is defined in terms of the turn’s ratio.

Knee point voltage:

That point on the magnetizing curve where an increase of 10% in the flux density (voltage) causes an increase of 50% in the magnetizing force (current).

Exciting Current:

The current measured in mA@ Knee point Voltage

Secondary Resistance:

The secondary resistance of C.Tr generally measured @ 75 deg C.

A General table indicating the Application of Tests

A number of routine and type tests have to be conducted on CT s before they can meet the standards specified above. The tests can be classified as:

1. Accuracy tests to determine whether the errors of the CT are within specified limits.
2. Composite Error Test: To verify whether the Trs comply with the accuracy limit factor indicated on the nameplate
3. Knee Point Voltage Test: To verify that the knee point is equal to or greater than the knee point indicated on the nameplate
4. Inter turn insulation Test: To check the insulation between winding
5. Dielectric insulation tests such as power frequency withstand voltage test on primary and secondary windings for one minute,
6. Impulse tests with 1.2u/50 wave,
7. Temperature rise tests.
8. Short time current tests
9. Verification of terminal markings and polarity. The routine tests are carried out in the test lab located in our factory premises itself. The type tests are conducted at NABL accredited labs such as ERDA or CPRI located in India.