because one or more of the fundamental frequency differential currents entered the
operate region on the operate - restrain characteristic. So, this protection is not
independent of the traditional restrained differential protection - it is activated after
the first start signal has been placed.
If the same fault has been positively recognized as internal, then the unrestrained
negative sequence differential protection places its own trip request.
Any block signals by the harmonic and/or waveform criteria, which can block the
traditional differential protection are overridden, and the differential protection
operates quickly without any further delay.
This logic guarantees a fast disconnection of a faulty power transformer for any
internal fault.
If the same fault has been classified as external, then generally, but not
unconditionally, a trip command is prevented. If a fault is classified as external, the
further analysis of the fault conditions is initiated. If all the instantaneous differential
currents in phases where start signals have been issued are free of harmonic pollution,
then a (minor) internal fault, simultaneous with a predominant external fault can be
suspected. This conclusion can be drawn because at external faults, major false
differential currents can only exist when one or more current transformers saturate. In
this case, the false instantaneous differential currents are polluted by higher harmonic
components, the 2
nd
, the 5
th
etc.
Sensitive negative sequence based turn-to-turn fault protection
The sensitive, negative sequence current based turn-to-turn fault protection detects
the low level faults, which are not detected by the traditional differential protection
until they develop into more severe faults, including power transformer iron core. The
sensitive protection is independent from the traditional differential protection and is a
very good complement to it. The essential part of this sensitive protection is the
internal/external fault discriminator. In order to be activated, the sensitive protection
requires no start signal from the traditional power transformer biased differential
protection. If magnitudes of HV and LV negative sequence current contributions are
above the set limit for
IminNegSeq
, then their relative positions are determined. If the
disturbance is characterized as an internal fault, then a separate trip request will be
placed. Any decision on the way to the final trip request must be confirmed several
times in succession in order to cope with eventual CT transients. This causes a short
additional operating time delay due to this security count. For very low level turn-to-
turn faults the overall response time of this protection is about 30ms.
Instantaneous differential currents
The instantaneous differential currents are calculated from the instantaneous values of
the input currents in order to perform the harmonic analysis and waveform analysis
upon each one of them (see section
"Harmonic and waveform block criteria"
information).
The instantaneous differential currents are calculated using the same matrix
expression as shown in equation
and equation
. The same matrices A, B and C are
1MRK502052-UEN B
Section 6
Differential protection
131
Technical manual
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