ABB REG670 Applications Manual page 750

Section 24
Requirements
744
in power system protection, is the first official document that highlighted this
development. So far remanence factors of maximum 80% have been considered when
CT requirements have been decided for ABB IEDs. Even in the future this level of
remanent flux probably will be the maximum level that will be considered when
decided the CT requirements. If higher remanence levels should be considered, it
should often lead to unrealistic CT sizes.
Thus, now there is a need to limit the acceptable level of remanent flux. To be able to
guarantee the performance of protection IEDs, we need to introduce the following
classification of CTs.
There are many different standards and a lot of classes but fundamentally there are
four different types of CTs:
• Very High Remanence type CT
• High Remanence type CT
• Low Remanence type CT
• Non Remanence type CT
The Very High Remanence (VHR) type is a CT with closed iron core (for example.
protection classes TPX, P, PX according to IEC, class C, K according to ANSI/IEEE)
and with an iron core material (new material, typically new alloy based magnetic
materials) that gives a remanent flux higher than 80 % of the saturation flux.
The High Remanence (HR) type is a CT with closed iron core (for example,
protection classes TPX, P, PX according to IEC, class C, K according to ANSI/IEEE)
but with an iron core material (traditional material) that gives a remanent flux that is
limited to maximum 80 % of the saturation flux.
The Low Remanence (LR) type is a CT with small airgaps in the iron core (for
example, TPY, PR, PXR according to IEC) and the remanent flux limit is specified to
be maximum 10% of the saturation flux.
The Non Remanence (NR) type is a CT with big airgaps in the core (for example,
TPZ according to IEC) and the remanent flux can be neglected.
It is also possible that different CT classes of HR and LR type may be mixed.
CT type VHR (using new material) should not be used for protection CT cores. This
means that it is important to specify that the remanence factor must not exceed 80 %
when ordering for example, class P, PX or TPX CTs. If CT manufacturers are using
new core material and are not able to fulfill this requirement, the CTs shall be specified
with small airgaps and therefore will be CTs of LR type (for example, class PR, TPY
or PXR). Very high remanence level in a protection core CT can cause the following
problems for protection IEDs:
1. Unwanted operation of differential (i.e. unit) protections for external faults
2. Unacceptably delayed or even missing operation of all types of protections (for
example, distance, differential, overcurrent, etc.) which can result in loosing
protection selectivity in the network
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
1MRK 502 071-UEN -
Application manual