ABB REL650 series, Applications Manual

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The non remanence type CT has practically negligible level of remanent flux. This type
of CT has relatively big air gaps in order to reduce the remanence to practically zero level.
In the same time, these air gaps reduce the influence of the DC-component from the
primary fault current. The air gaps will also decrease the measuring accuracy in the non-
saturated region of operation. Class TPZ according to IEC is a non remanence type CT.
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
al
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.
18.1.2 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.
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 maximum remanence has been considered for fault cases critical for the
security, for example, faults in reverse direction and external faults. Because of the almost
negligible risk of additional time delays and the non-existent risk of failure to operate the
remanence have not been considered for the dependability cases. The requirements below
are therefore fully valid for all normal applications.
It is difficult to give general recommendations for additional margins for remanence to
avoid the minor risk of an additional time delay. They depend on the performance and
economy requirements. When current transformers of low remanence type (for example,
TPY, PR) are used, normally no additional margin is needed. For current transformers of
high remanence type (for example, P, PX, TPS, TPX) the small probability of fully
asymmetrical faults, together with high remanence in the same direction as the flux
generated by the fault, has to be kept in mind at the decision of an additional margin. Fully
Section 18
1MRK 506 334-UUS A
Requirements
490
Application manual