ABB RELION REX640 Technical Manual page 1556

Section 9
Control functions
9.10.5.6
1550
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 (Auto parallel)
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, OL5ATCC 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 when 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.
Automatic voltage regulation of parallel transformers (Auto parallel)
A circulating current may occur between transformers 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 OL5ATCC. For these configurations, the
Minimizing Circulating Current (MCC) or Negative Reactance Principle (NRP)
1MRS759142 C
chapter.
REX640
Technical Manual