P54x/EN OP/La4 Operation
(OP) 5-42
MiCOM P543, P544, P545 & P546
OP
1.16 Conventional voltage transformer and capacitor VT applications
(Distance option only)
The MiCOM P54x achieves fast trip times due an optimized counting strategy. For faults on
angle and up to 80% of the set reach of the zone, a counter increments quickly to reach the
level at which a trip is issued. Near the characteristic boundary, the count increments slower
to avoid transient overreach, and to ensure boundary accuracy. This strategy is entirely
sufficient where conventional wound voltage transformers are used. Therefore, where
capacitor-coupled voltage transformers (CVT) are not
employed, the setting CVT Filters can
be set to Disabled.
Where capacitor-coupled voltage transformers are employed, then for a close-up fault the
transient component can be very large in relation to the fundamental component of fault
voltage. The relay has setting options available to allow additional filtering to be switched-in
when required, and the filter options to use depend on the likely severity of the CVT
transient. The two filtering methods are explained below.
1.16.1 CVTs with passive suppression of ferroresonance (Distance option only)
Passive
suppression employs an anti-resonance design, and the resulting transient/distortion
is fairly small. Sometimes such suppression is classed as a type 2 CVT. In passive CVT
applications, the affect on characteristic accuracy is generally negligible for source to line
impedance ratios of less than 30 (SIR < 30). However, at high SIRs it is advisable to use the
slower count strategy. This is achieved by setting CVT Filters to Passive.
It is important to note that by enabling this filter, the relay will not be slowed unless the SIR is
above that set. If the line terminal has an SIR below the setting, the relay can still trip
subcycle. It is only if the SIR is estimated higher than the setting that the instantaneous
operating time will be increased by about a quarter of a power frequency cycle. The relay
estimates the SIR as the ratio of nominal rated voltage Vn to the size of the comparator
vector IZ (in volts):
SIR = Vn/IZ
Where:
Vn = Nominal phase to neutral voltage
I = Fault current
Z = Reach setting for the zone concerned
Therefore for slower counting I would need to be low, as restricted by a relatively weak
infeed, and Z would need to be small as per a short line.
1.16.2 CVTs with active suppression of ferro
resonance (Distance option only)
Active suppression employs a tuned L-C circuit within the CVT. The damping of transients is
not as efficient as for the passive designs, and such suppression is often termed as being a
type 1 CVT. In active CVT applications, to ensure reach point accuracy the setting
CVT Filters is set to Active. The relay then varies the count strategy according to the
calculated SIR (= Vn / IZ). Subcycle tripping is maintained for lower SIRs, up to a ratio of 2,
with the instantaneous operating time increasing by about a quarter of a power frequency
cycle at higher SIRs.
Transients caused by voltage dips, however severe, will not have an impact on the relay’s
directional measurement as the MiCOM P54x uses voltage memory.
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