SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 36 of 74
of large transient voltage errors from the CVTs, so that correct directional response
can be ensured.
2) To enable a fast operating time to be obtained for close up faults of all types in
the forward direction of the relay.
3) To provide expansion of the resistive coverage of the mho for faults with low
infeed currents, where arc resistance may be large.
Both the healthy phase and synchronous components are square wave signals of
amplitude 16% of the peak prefault voltage supply. Under unbalanced fault
conditions, the proportion of healthy phase polarising is enough to overcome the
effects of normal CVT transients. Under 3 phase fault conditions, the synchronous
polarising works in a similar way. Figure 36 shows that by adding a 16% square
wave to the CVT error, the correct zero crossings of the polarising voltage are
restored. The polarising signal is squared up and phase retarded by 90° to
become input B of the comparator
The unique shapes of the partially cross polarised mho practical polar
characteristics, shown in Figure 37, have been achieved by suitable choice of the
wave shape of the signals involved in the polarising mixing circuits.
In conventional polarising mixing circuits, all the signals are sinewaves, but in the
Quadramho the synchronous polarising and sound phase cross polarising
components are square waves. The advantages of these unique polar
characteristics are obtained with only one two input comparator, enabling
optimum operating time to be obtained. Due to the partial synchronous polarising
component, the resistive expansion is maintained for 3 phase faults. The top line of
the expanded characteristic is part of a fully cross polarised circle and moves with
prefault power flow so as to avoid overreach or underreach. Operating time
contours over the area of the characteristic are shown in Figure 38.
5.2.1 Partially cross-polarised MHO
The polarising quantity, V
POL
, is formed by squaring and summing circuits supplied
with the phase-ground voltages and the synchronous polarising signal. The circuits
are so arranged that the effect of synchronous polarising only becomes significant
when the phase-ground voltages are reduced under low voltage three phase fault
conditions. This is explained by reference to Figures 39 and 40, which show
circuit details for two typical phases of the polarising mixing circuits.
Taking first the A–G phase, Figure 39, the sinewave quantities V
B
and V
C
are
summed using two equal value resistors R1 and R2 and supplied to the inverting
input of a high gain amplifier IC1. The square wave synchronous polarising
voltage V
MA
is potentially divided with R3 and R4 and supplied to the non-
inverting input.
The resistor values are chosen so that the peak value of the waveform on the non-
inverting input is 16% of the peak value of the waveform on the other input under
healthy supply conditions. The square wave output V
KA
from IC1 retains the phase
information of the zero voltage crossings of the sum of the input waveforms.
The phase of V
KA
is dominantly controlled by the cross polarising signal V
B
+ V
C
for most types of fault, the exceptions being low voltage three phase and low
voltage B–C–G faults, where the synchronous polarising signal V
MA
becomes the
controlling quantity.
A proportion of V
KA
is then added to the self voltage V
A
using R5 and R6 and a
second high gain amplifier IC2. The values of R5 and R6 are arranged so that the
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