SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 64 of 74
The effects of CVT transient errors on distance relays of the block average type are
well documented. The possible effects are:
a) Maloperation for faults in the reverse direction of the relay.
b) Transient overreach for forward faults beyond the protected zone.
c) Slow operation for faults within the protected zone.
Generally, the faster and more sensitive a distance relay is, the worse the possible
extent of these problems.
The amplitude and waveform of CVT error and hence its effect on a particular type
of relay, depend on the type of CVT, the burden on the CVT, type of ferro-
resonance damping and the point on wave of fault incidence. The system
impedance ratio (SIR) at the relaying point determines whether a given CVT error
can seriously affect the performance of the relay, since the SIR controls the
amplitude of the true relay input signals relative to the CVT error. Practically it has
been found that if the SIR is less than about 9, the effect of CVT transient errors on
most distance relays is negligible, even for CVT with a poor dynamic response.
The development program of Quadramho included a concerted effort to eradicate
the worst effects of CVT errors. The policy was:
a) To completely eliminate maloperation for reverse faults.
b) To reduce transient overreach to a negligible level.
c) To allow the relay comparators to be slowed only to the extent necessary to
prevent reverse maloperation and transient overreach.
The following techniques are used to meet these objectives:
a) Maintenance of true directional reference signal (or polarising signal) at all
times by the use of an adequate proportion of square wave cross-polarising or
synchronous polarising signals.
b) The use of a comparator which, although capable of fast operation, has built-in
safeguards against maloperation caused by non-power system frequency
components of the inputs. These safeguards inherently extend signal processing
time as the true inputs become more contaminated with low frequency CVT
errors.
c) Fitting a fast directional comparator for each phase to reinforce the action of
methods a and b for the very critical case of a close-up fault behind the relay at
high values of SIR. The directional comparator uses current and polarising
voltage as measuring quantities, both of which are independent of CVT errors.
If the directional comparator detects a reverse current flow it inhibits operation
of the quadrilateral or mho main comparator to completely eliminate the
possibility of wrong operation.
This method of using a directional unit to inhibit the main comparator avoids the
problems of slow operating times and signal races on resetting which often beset
other types of multiple comparators.
d) The use of a switched filter to pre-condition the voltage signal to the measuring
units and eliminate excessive delays to comparator operation. Such delays
would otherwise be caused by the presence of large slowly decaying low
frequency CVT errors for faults inside a comparator zone, but near to the
boundary of operation, at high values of SIR.
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