ABB Relion 670 Series Applications Manual page 606

Section 25
Requirements
• 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
No information is available about how frequent the use of the new iron core material is for
protection CT cores, but it is known that some CT manufacturers are using the new material
while other manufacturers continue to use the old traditional core material for protection CT
cores. In a case where VHR type CTs have been already installed, the calculated values of E
HR type CTs, for which the formulas are given in this document, must be multiplied by factor
two-and-a-half in order for VHR type CTs (i.e. with new material) to be used together with ABB
protection IEDs. However, this may result in unacceptably big CT cores, which can be difficult
to manufacture and fit in available space.
Different standards and classes specify the saturation e.m.f. in different ways but it is possible
to approximately compare values from different classes. The rated equivalent limiting
secondary e.m.f. E
requirements for the IED. The requirements are also specified according to other standards.
25.1.2 Conditions
The requirements are a result of investigations performed in our network simulator. The
current transformer models are representative for current transformers of high remanence
and low remanence type. The results may not always be valid for non remanence type CTs
(TPZ).
The performances of the protection functions have been checked in the range from
symmetrical to fully asymmetrical fault currents. Primary time constants of at least 120 ms
600
according to the IEC 61869–2 standard is used to specify the CT
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