Conditions - Hitachi Relion 670 Series Applications Manual

Section 23
Requirements
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
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
requirements are also specified according to other standards.
23.1.2

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-ground, phase-to-phase and three-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.
332
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for HR type CTs, for which the formulas are
al
Phasor measurement unit RES670
1MRK511407-UUS Rev. N
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M11610-3 v1
M11610-4 v5
Application manual