ABB RET670 - Conditions

The low remanence type has a specified limit for the remanent flux. This CT is made
with a small air gap to reduce the remanence to a level that does not exceed 10% of the
saturation flux. The small air gap has only very limited influences on the other
properties of the CT. Class PR, TPY according to IEC are low remanence type CTs.
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 60044 – 6 standard is used to specify
the CT requirements for the IED. The requirements are also specified according to
other standards.
2.1.2 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.
Section 2 1MRK504116-UUS C
Requirements
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Application manual