Alstom SHPM 101, Руководство по обслуживанию

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SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 37 of 74
peak value of the square wave V
KA
produces the same voltage at the input of IC2
as 16% of the peak value of the sinewave VA under healthy supply conditions.
The squared output V
QA
from IC2 is phase shifted 90
°
by a shift register (which is
not shown in Figure 39) and the resultant signal V
POL
is then supplied to the
comparator. The phase of V
QA
is dominated by input V
A
throughout all types of
fault except ground fault conditions involving the A phase and three phase faults
which cause the supply to collapse. For these conditions of low V
A
voltage the V
KA
input predominates. Indeed if V
A
collapses to less than 16% of rated voltage, V
A
ceases to have any influence at all on the polarising signal and the comparator
becomes effectively fully cross polarised. The resistive expansion of the
characteristic for low voltage ground faults is, therefore, much stronger than for a
conventional partially cross polarised mho relay. (See Figure 41). The synchronous
polarising also causes similar large resistive expansion for low voltage three phase
faults.
The B–C polarising mixing circuit is explained with the aid of Figure 40.
Under B–C and three phase fault conditions, V
KA
has the same phase as the
memory signal V
MA
and as explained the square wave voltage V
KA
is mixed with
the sinewave V
A
on resistors R5 and R6. The values of these resistors are selected
so that, under a state of healthy supply voltage, the peak value of V
KA
has 16% of
the effect of the peak value of V
A
at the input of the amplifier 1C2. The resultant
squarewave at the output of IC3 is phase retarded by 90
°
using a shift register to
produce a squarewave V
NA
. A set of resistors R9, R10, R11, R12 are used to mix
V
NA
with V
B
and V
C
in such proportions that the peak value of V
NA
corresponds to
16% of the peak value of V
B
–V
C
at this input of squaring amplifier IC4. The output
V
QBC
of IC4 is phase shifted through a lagging angle of 90
°
by a shift register (not
shown) and is then supplied to the comparator.
The phase of V
QBC
is determined largely by the zero crossings of V
B
–V
C
under all
types of fault conditions except B–C, B–C–G and three phase faults which cause
the B–C voltage to collapse. For these conditions of low B–C voltage the V
NA
input
dominates the phase of the polarising signal. V
NA
in turn is controlled by V
A
if this
fault involves the B–C or B–C–G phases, or by V
MA
if the fault involves all three
phases. Therefore, if V
B
–V
C
collapses below 16%, the B–C unit is effectively fully
cross polarised and consequently the resistive expansion of the impedance
characteristic is greater than for a conventional partially cross polarised relay.
Also the resistive expansion also applies to three phase faults.
The combination of the sinewave faulty phase voltage with the square wave cross
polarising (or synchronous polarising) voltage results in a phase displacement of
the resultant polarising signal from its prefault position, which is different from that
of a conventional partially cross polarised mho. Figure 41 shows the relationship
of phase displacements of the faulty phase and polarising signals for Quadramho
and conventionally polarised comparators, drawn for a typical fault voltage
amplitude. In Quadramho the displacement of the polarising signal is zero until the
faulted phase is displaced by more than a critical angle Ø
°
, the capture angle.
Once the critical angle is exceeded, the polarising voltage phase displacement
rises linearly with the faulted phase voltage displacement.
The explanation for this behaviour is shown in Figures 42 and 43. Figure 42,
shows an example of the composition of the polarising signal in Quadramho.
The faulted phased sinewave is drawn here for a fault voltage of 25%, displaced
by 30
°
lagging, relative to the prefault values. This signal is summed with the
square wave cross polarising signal the magnitude of which is 16% of the prefault