Conditions - ABB REG670 Applications Manual

1MRK 502 071-UEN -
24.1.2
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
Application manual
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 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
the CT requirements for the IED. The requirements are also specified according to
other standards.

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, 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, 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
for HR type CTs, for which the formulas are
al
according to the IEC 61869–2 standard is used to specify
al
Section 24
Requirements
745