Alstom SHPM 101, Руководство по обслуживанию

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SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 44 of 74
5.10 Single pole tripping
The level detectors are also beneficial when single pole tripping of the circuit
breakers is required when using line VTs. Following a single phase to ground fault
and a single pole trip, the output of the ground fault comparator is blocked by the
resetting of the relevant phase current level detector and the comparator is forced
to count down by the relevant pole dead signal. Thus the relay resets correctly
even though the presence of residual current due to load and the presence of
sound phase cross polarising, may appear as an impedance within the relay
characteristic.
5.11 Phase selection
A problem with full scheme distance relays is that heavy close-up single phase
faults can sometimes intrude into the operating characteristics of the phase fault
comparators, causing a three pole trip where a single pole trip would be
appropriate. In Quadramho a special neutral current level detector is used to block
the phase fault comparators to prevent this type of incorrect trip.
This neutral current detector, known as the “high set” (LDHSN), has a setting level
which is biased by the maximum amplitude of phase difference current flowing at
any time. The three signals
I
A
–
I
B
.
I
B
–
I
C
and
I
C
–
I
A
are rectified by a precision three
phase full wave rectifier and a peak level formed by a fast charge slow discharge
smoothing circuit. A fixed proportion of this level is then used as the value V
REF
in a
level detector of the type previously described. See Figure 58.
In order to prevent chatter of the output and also to give the level detector a
minimum sensitivity under no load or very low load conditions, a fixed minimum
reference level is “OR”ed with the variable reference.
For practical reasons, an upper limit to the reference level is used to ensure that
under very heavy fault current conditions the detector will operate correctly and not
be limited by internal power supply rails.
The signal input to the neutral detectors is derived by summing together the
individual phase current replica signals, in order to maintain the relay sensitivity
independent of residual compensation setting.
A similar “low set” neutral current detector (LDLSN) is used to enable the ground
fault comparators, thereby preventing wrong operation of a ground fault
comparator under heavy close up phase-phase fault conditions. See Figure 58.
The low set neutral detector is also used by the Voltage Transformer Supervision
feature since it has a high degree of immunity from operational unbalanced load
conditions.
Biasing the neutral current detector has distinct advantages. The detector can be
set sensitive enough to operate for all single phase faults which could cause phase
fault comparator maloperation, without any risk of the detector picking up on
neutral spill current during phase-phase faults. Neutral spill current arises from
mismatched current transformers, CT saturation, etc. The biasing also ensures that
the phase fault comparators are general]y enabled during two phase to ground
faults, permitting the relay to give its fastest possible three pole trip. For two phase
to ground faults with high resistance in the neutral, only the phase fault
comparators are enabled, avoiding possible measuring errors which ground fault
comparators can exhibit under these conditions. For conditions where the fault
resistance places the fault impedance just outside the ground fault comparator
characteristic, but with sufficient neutral spill current to still block the phase fault