Hitachi Relion 670 Series Applications Manual page 416

Section 22
Requirements
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
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 Hitachi Energy 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 according to the IEC 61869–2 standard is used to specify the CT requirements for the IED. The
al
requirements are also specified according to other standards.
22.1.2 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 have been considered at
the tests. The current requirements below are thus applicable both for symmetrical and asymmetrical
fault currents.
Depending on the protection function phase-to-earth, and phase-to-phase faults have been tested for
different relevant fault positions for example, close in forward and reverse faults, zone 1 reach faults,
internal and external faults. The dependability and security of the protection was verified by checking
for example, time delays, unwanted operations, directionality, overreach and stability.
The remanence in the current transformer core can cause unwanted operations or minor additional
time delays for some protection functions. As unwanted operations are not acceptable at all
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for HR type CTs, for
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