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Description of
operation
4
When the residual voltage of the monitored
network exceeds the set start level of the low-set
residual overvoltage stage U
ule SPCU 1C6 starts. If the fault persists the
U
> stage trips after the set operate time t> has
0
elapsed. Correspondingly, the high-set residual
overvoltage stage U
>> starts when its set start
0
level is exceeded and trips when its set operate
time t>> has elapsed.
When one of the phase-to-phase voltages of the
monitored network falls below the set start level
of the undervoltage stage U<, the relay module
SPCU 3C15 starts. If the undervoltage situa-
tion persists the U< stage trips after the set (at
definite time mode) or calculated (at inverse
time mode) operate time t< has elapsed. Corre-
spondingly, the under voltage stage 3U<< starts
when all three phase-to-phase voltages simulta-
neously fall below the set start level and trips
when its set operate time t<< has elapsed.
The operation of the undervoltage stage U< can
be given inverse time characteristic or definite
time characteristic. At inverse time characteris-
Fig. 1. Operation diagram for the undervoltage relay module SPCU 3C15 when selector switch
SG1/5 = 1, i.e. starting of the undervoltage stage is blocked at voltage levels under 0.2 x U
U</U
Set start level of the undervoltage stage U<
n
SS1
Start signal of the undervoltage stage U<
TS1
Trip signal of the undervoltage stage U<
tic the operate time is a function of the meas-
ured voltage and the set time multplier k and
> the relay mod-
the operate time is shorter the lower the meas-
0
ured voltage. The undervoltage stage 3U<< fea-
tures definite time characteristic only.
By turning switch SG1/5 on the front panel of
the undervoltage relay module SPCU 3C15 in
position 1, the operation of the undervoltage
stage U< can be blocked, when at least one of
the measured phase-to-phase voltages falls be-
low 0.2 x U
dervoltage stage U< can be blocked by the start
signal of the undervoltage stage 3U<<, switch
SG1/8 in position 1.
Tripping of both operation stages of the under-
voltage relay module and the residual voltage
relay module can be blocked by means of an
external control signal applied to the control
input 10-11 of the relay. The operation stages
to be blocked are selected with switches 4 and 5
of switch group SGB in the relay modules re-
spectively, see section "Signal flow diagram".
. Further, the operation of the un-
n
.
n
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