CLASSIFICATION OF PROTECTION CURRENT TRANSFORMERS IN IEC/BSEN61869-2

IEC61869-2/BSEN61869-2 commonly define protection current transformers in terms of composite error at an accuracy limit factor. In simple terms this means how accurate the current transformer will remain when the primary current flowing is many times higher than in normal conditions i.e. in a fault situation.

General Protection Class Current Transformers
The classification of protection current transformers follows the following simple formula:

10 P 10
Number before letter indicates composite error achieved in percentage terms Number after letter indicates factor of primary current up to which composite error will be achieved

Manufacturers of protection devices will normally specify the classification for the protection current transformer intended to operate the particular protection device concerned.

In addition the classification of protection current transformers indicates accuracy class:

5P - current transformer will have a ratio error of +/- 1% and phase error not exceeding 60 minutes

10P - current transformer will have a ratio error of +/- 3% (no level of phase error specified).

Specialist Protection Class PX Current Transformers
In some balanced protection systems where sensitivity and stability levels required are high, classes 5P10 or 10P10 may not be adequate. In these situations a Class PX current transformer will usually be specified. Class PX as defined in IEC61869-2/BSEN61869-2 allows the manufacturer of a protection device to detail more specific requirements of the current transformer so as to achieve a more sensitive protection scheme. Typically this will involve specifying the following information for the current transformer:

1) Turns ratio
2) Minimum kneepoint voltage
3) Maximum secondary resistance
4) Maximum excitation current at the rated minimum kneepoint voltage

To enquire about Class PX current transformers contact us with the above information together with any physical limiting dimensions and the aperture size required. If not all the above information is available, details of relay type, lead run from current transformer to relay and the fault current expected to flow through the zone protected by the current transformers will be required in order to provide a quotation.