Redundant processor unit PM866AK02 3BSE081637R1

Redundant processor unit PM866AK02 3BSE081637R1

Redundant processor unit PM866AK02 3BSE081637R1

The current transformer requirements are based on the maximum fault current for faults in different positions. Maximum fault current will occur for three-phase faults or single phase-to-earth faults. The current for a single phase-to-earth fault will exceed the current for a three-phase fault when the zero sequence impedance in the total fault loop is less than the positive sequence impedance. When calculating the current transformer requirements, maximum fault current for the relevant fault position should be used and therefore both fault types have to be considered.

There are several different ways to specify CTs. Conventional magnetic core CTs are usually specified and manufactured according to some international or national standards, which specify different protection classes as well.

There are many different standards and a lot of classes but fundamentally there are three different types of CTs:

• High remanence type CT

• Low remanence type CT

• Non remanence type CT The high remanence type has no limit for the remanent flux.

This CT has a magnetic core without any airgaps and a remanent flux might remain almost infinite time. In this type of transformers the remanence can be up to around 80% of the saturation flux.

Typical examples of high remanence type CT are class P, PX, TPX according to IEC, class P, X according to BS (old British Standard) and non gapped class C, K according to ANSI/IEEE.