Alstom SHPM 101, Service Manual

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SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 42 of 74
and has different polarisation (
I
Ph
R +
I
N
R). The “guard” zone is generated entirely
from inhibit comparators and so its operating speed is only a fraction of a cycle.
Due to the different polarisation employed for the reactance lines, under two phase
to ground fault conditions the reactance lines of the Zone 1 and corresponding
“guard” zone tilt with respect to each other (See Figure 52).
This action is used to advantage with the logic of Figure 53 to prevent operation of
the ground fault comparator. The ground fault comparators are allowed to operate
only if the corresponding “guard” zone, operates. For example, for the external
A–B–G fault condition depicted in Figure 52 which is 30% beyond Zone 1 reach
setting, it can be seen that the B phase Zone 1 reach line tilts such that the
measured B–G impedance appears within the B phase Zone 1 ground fault
characteristic.
The measured B–G impedance is also within the B phase “guard” zone
characteristic. This would cause the B phase Zone 1 ground fault element to
operate if it was not for the fact that the measured A–G impedance appears within
the A phase guard zone characteristic. The operation of the A phase guard zone
in conjunction with the B phase guard zone ensures that all ground fault Zone 1
comparators are inhibited due to logic action as depicted in Figure 53.
The “guard” zone system allows correct Zone 1 operation on single phase faults,
since the phase selection properties of the guard zone comparators ensure that
only the faulty phase guard Zone comparators operate. The 2:1 ratio of the
“guard” zone reach/Zone 1 reach ensures that the resistive coverage of Zone 1 is
not seriously affected by any angular “droop” of the “guard” zone reactance line
under load exporting conditions, caused by its non-optimum polarising quantity for
single phase faults.
Overreach of Zones 2 and 3 under two phase to ground fault conditions is less
serious than overreach of Zone 1 and can be tolerated provided that grading
problems between time delayed Zone 2 and Zone 3 back up trips do not occur.
To avoid having to provide guard zones for Zones 2 and 3, the polarising signal
(
I
Ph
R +
I
N
R) for these two zones provides a compromise between single phase and
two phase to ground fault requirements. Any consequent errors are bounded by
the accuracy claims for Zones 2 and 3.
5.7 The offset quadrilateral
The offset quadrilateral characteristic of Zone 3 is produced in a similar way to
that of Zones 1 and 2, except that no directional line is involved and a further
main comparator is used for the reverse reach, having vectors:
A5 =
I
Ph
R +
I
N
R
B5 = V+
I
Z’
This is shown in Figure 54.
The outputs of the Zone 3 forward main comparator and Zone 3 reverse main
comparator are “ANDed” to obtain the complete Zone 3 characteristic shape.
5.8 Level detectors
On de-energising a transmission line, line VTs can supply one input terminal of the
comparators with low frequency voltage waveforms, particularly if electro-magnetic
transformers are connected to the isolated line. To avoid the risk of false operation
of the relay comparators caused by the continuing presence of synchronous
polarising of the other input of the comparators, phase current level detectors are