Current Transformer Technical Specifications

Current transformers are used to convert alternating current of higher value to lower value depending upon the requirement. Current Transformers has many applications in electrical systems. 

Current Transformers usually have two types of designs:-

1.      Window type

2.      Ring Type

There are generally two type of applications where Current Transformers are used:-

1.      Measurement Type Current Transformers

2.      Protection Type current Transformers

Measurement type CT’s design requires the inclusion of core and winding which will which will when connected to its rated burden; perform within the limits of error as indicated by the standard’s specification. It is an advantage for a measurement type transformer to saturate above this range, which provides a protection against damage to instruments by limiting the secondary current when surge currents or faults appear in the primary circuit.

There are following Specification of current Transformers:-

Rated Transformation Ratio:-

It is the ratio of rated primary current to rated secondary current

Rated primary current: 

It is primary current which is to be Transformed to secondary current as per requirement. Usually rated primary current is the current flowing in system whose value to be converted to as per requirement. CT should be selected according to this current.

Rated secondary current: 

This is the secondary current of Transformer. This current is obtained after Transformation by CT. Typical values of secondary current are 5 A and 1A.

Rated burden: 

This is expressed in VA and is an apparent power of the secondary circuit of CT. It is specified on CT’s as per rated secondary current of CT’s at 0.8 Power factor.

Accuracy class: 

Accuracy class will depend upon type of application where CT is used. In case of metering accuracy class is 0.2, 0.5, 0.1, 1, 3 or 5.  It means that the errors have to be within the limits specified in the standards for that particular accuracy class. The accuracy class of measuring CT is 0.2 it means the error limit will be of 0.2%. This means that if we try to measure 100 A with a 0.2 class CT, the measured value may be either 100.2 or 99.8 A.

For protection type CT’s  the CT s should pass both the ratio and phase errors at the specified accuracy class, usually 5P or 10P where 5 stands for 5% and 10 stands for 10% and P stands for Protection.

 Current error (ratio error): 

This error is introduced into measurement by CT generally due to Transformation error. This error is difference between primary current to be measured and Secondary current multiplied by Transformation ratio.

This is expressed by formula as below

Current error, percent =[ (K X.Is)- (Ip)] x 100 / Ip

Where, K= rated transformation ratio

Ip= actual primary current

Is= actual secondary current

Accuracy limit factor: 

This is error value of primary current upto which the CT complies with composite error requirements.

Phase displacement:

Phase displacement as clear from its name is difference in phase between the primary and secondary current vectors.

 The phase displacement is said to be positive when the secondary current vector leads the primary current vector. It is usually express in minutes.

Composite error:-

This error is the subsequent limitation of the harmonic content of the secondary current which is necessary for the correct operation of certain types of relays.

Diameter of CT:-

Bore diameter of the CT shall not be less than 40 mm. Ring type CTs shall have suitable clamp to fix the CT to panel Board.

Highest Withstand Voltage: -

This is highest voltage which is needed to withstand by CT without getting faulty. This doesn’t include the voltage variations in case of fault conditions.

General Technical Specifications of Current Transformers

Standards applicability: -

IS/IEC 60044-1 (2003): Instrument Transformers, Part 1: Current Transformers

BS 3938:1973 :Specification for current transformers

Insulation class: -

E (120°C max)

Test voltage: -

For Ring (Window) type CT : 4KV 50 Hz for 1 min

For Wound type CT : 3KV 50 Hz for 1 min

Operating frequency: -

50Hz

Rated primary rating: -

1A to 7500A

Rated secondary output: -

5A standard (1A on request)

Rated burden: -

1, 1.25, 1.5, 2.5, 3.75, 5, 7.5, 10, 12.5, 15, 20, 30, 45, 60, 100 VA

Class of accuracy: -

0.2, 0.2S for laboratory and power measurement

0.5, 0.5S for accurate measuring, kWh

1 for general measurement

3 for indicating instruments

Ambient temperature:

- -20°C…+45°C

Storage temperature: -

-50°C…+80°C

Thermal short circuit current (Ith):-

40xIn for Wound type CT and

60xIn for Bus Bar type CT.

Dynamic short circuit current (Idyn):-

2.5xIth

Instrument security factor (FS): -

2.5, 5, 10

Limits of current error and phase displacements for measuring current transformers

(Classes from 0.1 to 1)

Limits of current error and phase displacements for measuring current transformers

(For special applications)

Limits of current error and phase displacements for measuring current transformers

(Classes 3 and 5)