ABB REF615 F - Automatic Voltage Regulation of Parallel Transformers

it is difficult to predict the actual voltage levels in the feeder lines in such a case,
and lowering the voltage at the substation can have harmful effects in the far end of
the network. However, the Rv Pwr flow allowed setting allows also negative LDC
terms to be taken into equation.
The topology changes in the network can cause changes to the equivalent
impedance value of the network. If the change is substantial, the setting groups can
be used to switch between different setting values for Line drop V React and Line
drop V Ris. In practice this means that the boolean-type information from the
topology change is connected to the active setting group change.
The use of the LDC equation in the case of parallel transformers is described in the
Automatic voltage regulation of parallel transformers chapter.
Reduce Set Voltage (RSV) input
The system frequency decreases when the active power production in the network
is smaller than its consumption. Either the power supply has to be increased or
some loads have to be shed to restore the power balance.
The simplest way to decrease the load is to reduce the voltage level by giving a
lower band center voltage value to the regulators. For this purpose, OLATCC has
the setting group parameter Band reduction. The RSV input activation results in
reduction. If this input is set to TRUE, a set target voltage value is decreased by
Band reduction. If more than one RSV reduction steps are desired, the setting group
change has to be used where different Band reduction values are supported. The
decreased value is kept as a target value as long as the RSV input is TRUE.
Because the decrease of frequency indicates a need to reduce the load, it is
practical to connect the start signal of an underfrequency function block to the RSV
digital input.
It depends on the load characteristics how much the load is reduced as the voltage
drops. For instance, purely resistive loads are proportional to the square of the
voltage, whereas motor drives based on frequency controllers may draw constant
power despite small voltage changes.
The status of the RSV input can be read from the RSV input data.
9.5.10 Automatic voltage regulation of parallel transformers
It is likely that a circulating current between transformers occurs if two or more
transformers with slightly different ratios are energized in parallel. This is due to
the unbalanced short circuit impedances of the parallel transformers. To avoid such
currents, the tap changers of the transformers should be adjusted to achieve
equilibrium. If the transformers are assumed identical, the tap (voltage) steps and
tap positions should also match. In this case, the Master/Follower principle can be
used. However, unequally rated transformers with different tap steps can be
connected in parallel and these configurations can also be managed by the tap
changer control function. For these configurations, the Minimizing Circulating
Section 9 1YHT530004D05 D
Control functions
736 615 series
Technical Manual