3
Area of
application
The voltage regulator SPAU 341 C is intended
to be used for regulating the voltage of power
transformers with on-load tap-changers in dis-
tribution substations. The connections required
for a simple voltage regulating function is the
measured phase-to-phase voltage U
12
and the
raise and lower output contacts. If the line drop
compensation, the minimizing circulating cur-
rent or the overcurrent blocking feature is to be
used, one or more phase currents have to be
measured. If only one phase current is meas-
ured, it is always connected to the terminals of
the phase current I L1, and the current to be
measured is selected with the software switches
SGF2/6 and SGF2/7 of the automatic voltage
regulating module SPCU 1D50.
Description of
operation
The purpose of the regulator is to maintain a
stable secondary voltage of the power trans-
former. The basis for this operation is the refer-
ence voltage, which is set by the user. By adding
or decreasing various compensation factors, the
regulator calculates a control voltage from the
reference voltage. Hence, the control voltage is
the desired transformer secondary voltage to be
maintained by the regulator. Then the control
voltage is compared with the voltage measured
and the difference between these two forms the
regulating process error.
Since the tap-changer changes the voltage in
steps, a certain error has to be allowed. This er-
ror, called bandwidth, is also set by the user. If
the measured voltage fluctuates within the band-
width, the regulator is inactive. Should, on the
other hand, the measured voltage be outside
these bandwidth limits, an adjustable delay T1
starts. This delay T1 remains active as long as
the measured voltage is outside the hysteresis
limits of the bandwidth. The factory setting of
the hysteresis limits is 90%.
Should the measured voltage still be outside the
hysteresis, when the delay counter T1 reaches
its setting value, the raise or lower output relay
is activated and the motor drive of the tap-
changer is operated. If, on the other hand, the
measured voltage falls within the hysteresis lim-
its, the delay counter is reset.
Should one tap-changer operation not be
enough to regulate the transformer voltage
within the hysteresis limits, a second adjustable
delay T2, usually with a shorter time setting than
T1, starts.
The delays T1 and T2 can be selected either
with definite or inverse time characteristic. In-
verse time characteristic means, that the delay
is inversely proportional to the regulator error,
i.e the delay is inversely proportional to the dif-
ference between the control voltage and the
measured voltage.
The voltage regulating function will be described
more in detail in the document 1MRS 750111-
MUM EN.
Fig.1. Voltage regulating function
U
t
T1 start
T1 start
T1
Up, control voltage
hysteresis limit
bandwidth limit
Um, measured voltage
Lower
T1 reset
