Alstom SHPM 101, Service Manual

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SERVICE MANUAL R5888B
QUADRAMHO Chapter 1
Page 6 of 27
In the majority of applications, the SOTF enable timer switch will be set to the
200ms position, so that the feature will be available as soon as possible after the
line becomes dead. With this short time setting, the SOTF feature will be enabled
during autoreclose dead times, so that upon reclosure a SOTF trip is possible.
This is usually advantageous for most distance schemes, since a persistent fault in
the remote end of a protected line section can be cleared instantaneously after
reclosure of the local breaker, rather than after the Zone 2 time delay.
Where it is desired that a SOTF indication is not given after autoreclosure, or dual
shot autoreclosing is to be employed, then the 110s SOTF enable timer setting
should be used. This will ensure that the SOTF feature could not be enabled during
autoreclosure dead times. If a SOTF trip was allowed to occur on autoreclose, the
distance relay would also give a block autoreclose signal to the autoreclose relay
and any second autoreclose shot would be prevented.
SOTF tripping is only possible for the initial 240ms after line circuit breaker
closure. During this time with an option switch set to one position, the operation of
any distance comparator will give an instantaneous trip. With the option switch set
to the other position, the pick-up of any current level detector without its
corresponding voltage level detector picking-up within 20ms, will give an
instantaneous trip.
With the relay set to give a SOTF trip for any distance comparator operation, then
any fault existing on the protected line, including a close-up, 3 phase bolted fault
would be cleared. For the latter fault, where line VTs are used, there would be no
memory voltage to allow Zone 1 or Zone 2 distance comparator operation, but
Zone 3 will operate as it has a fixed offset. Faults at the remote end of the line will
also be cleared instantaneously by a SOTF trip when the local circuit breaker is
closed.
In some situations, it may be possible for the magnetising inrush current of banked
transformers at the end of a line, or particularly of teed-off transformers, to cause
transient operation of the Zone 3 comparators on line energisation, resulting in an
incorrect SOTF trip. In such a situation, the SOTF trip option switch should be set,
so the SOTF tripping will only occur when a current level detector picks up without
the corresponding voltage level detector picking up.
On versions of Quadramho, for use with miniature circuit breakers, the switch-on-
to-fault feature is blocked via the opto coupler when the MCB is open.
14.1 Performance of relay for earth faults
On resistance-earthed systems, the quadrilateral characteristic is beneficial for
ground fault protection in that, unlike the cross-polarised mho characteristic, its
faults resistance coverage is not reduced with increased earthing resistance behind
the relaying point.
Quadramho requires its low-set neutral current detector to operate before its
ground fault impedance elements are enabled. In an application where a system is
insulated behind the relaying point, the neutral current produced in the relay for
forward ground faults would be below the sensitivity of the low-set neutral current
detector, which would prevent operation of the ground fault impedance elements.
For reasons given in Chapter 2, the low-set neutral current detector has a sensitivity
which is biased according to the magnitude of the greatest phase difference
current, (vectoral difference).