ABB Relion 670 series, Техническое руководство

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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 pickup 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
M13039-328 v4
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 pickup 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
M13039-331 v4
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" for more 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 used for these calculations. The only
difference is that the matrix algorithm is fed by instantaneous values of currents, that is, samples.
Harmonic and waveform block criteria
M13039-334 v4
The two block criteria are the harmonic restrain and the waveform restrain. These two criteria
have the power to block a trip command by the traditional differential protection, which produces
pickup signals by applying the differential currents, and the bias current, to the operate - restrain
characteristic.
Harmonic restrain
M13039-337 v6
The harmonic restrain is the classical restrain method traditionally used with power transformer
differential protections. The goal is to prevent an unwanted trip command due to magnetizing
inrush currents at switching operations, or due to magnetizing currents at over-voltages.
The magnetizing currents of a power transformer flow only on one side of the power transformer
and are therefore always the cause of false differential currents. The harmonic analysis (the 2
nd
and the 5
th
harmonic) is applied to the instantaneous differential currents. Typical instantaneous
differential currents during power transformer energizing are shown in figure 40 . The harmonic
analysis is only applied in those phases, where pickup signals have been set. For example, if the
content of the 2
nd
harmonic in the instantaneous differential current of phase A is above the
setting
I2/I1Ratio, then a block signal is set for that phase, which can be read as BLK2H_A output
of the differential protection.
1MRK 502 066-UUS B Section 6
Differential protection
135
Technical manual