Alstom SHPM 101, Service Manual

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SERVICE MANUAL R5888C
QUADRAMHO Chapter 2
Page 48 of 74
iii) the indication LED operates and latches.
iv) the alarm contact 97X–1 closes.
v) the comparators are held reset and blocked.
vi on the permissive overreach scheme the weak infeed echo feature is blocked.
vii) the switch on to fault is blocked.
5.16.2 Principle
The VTS feature operates by detecting the zero phase sequence voltage (V
0
)
arising from failure of the voltage supply. The VTS is stopped from operating on
ground faults on the transmission line (which also gives rise to V
0
) by a circuit
which detects zero phase sequence current. Zero phase sequence current is only
caused by line faults.
5.16.3 Outputs
The VTS provides visual indication by the V~FAIL LED on the scheme logic module
and remote alarm via the auxiliary relay contact 97X–1. If the VTS is required to
block the tripping action of the relay, switch SW3 on the scheme logic module
must be set to the right hand position. Since the distance relay cannot perform
correctly if the voltage supply is faulty, the green “Relay Available” LED is
extinguished and the “Relay Inoperative” alarm contact 97Y–1 closes if VTS is set
to block the relay. The following descriptions assume that SW3 is set to the right
hand position, unless otherwise stated.
5.16.4 Level detector settings
The zero phase sequence voltage level detector settings must be above the
maximum level of V
0
possible in a healthy three phase voltage supply (10% V
n
)
and below the mimimum amount of V
0
produced when there is an open circuit or
short circuit in the voltage supply (33% V
n
). A level of V
0
= 15% V
n
has been
chosen as the setting.
The zero phase sequence current level detector is the low set neutral current unit.
This level detector LDLSN has a self compensating setting which adjusts to the
amount of phase-phase current flowing in the line and therefore cannot be picked
up by unbalance in load current (see Section 5.8). This is the same level detector
which is used for checking ground fault comparator operation, so there is no
degradation in distance relay sensitivity when the feature is used for blocking
comparator operation.
5.16.5 Speed of operation
The circuit is required to respond to loss of voltage faster than the relay
comparators, in order that maltrips can be blocked. The V
0
detector therefore has
to be reasonably fast in operation. However, for ground faults of the transmission
line, the
I
0
detector has to store the comparators from producing an output (which
would delay tripping) and so the
I
0
detector (LDLSN) must be quicker than the V
0
detector.
5.16.6 Implementation
The feature is contained in the scheme logic module, with the exception of the
output relay circuits and the current level detectors. The level detectors are
implemented as hardware circuits. The rest of the logic, described in the following
sections, is effected by software in the microprocessor scheme logic.
The block diagram, Figure 63, is drawn as a logic equivalent circuit.