U
x
Setting
Line drop V React
φ Phase angle between U_A and I_A (ANGL_UA_IA)
By default, LDC is not active, but it can be activated by setting
LDC enable
to "True".
To keep the LDC term within acceptable limits in all situations, OL5ATCC has a
setting parameter
LDC limit
, which has a default value of 0.10 xU
n
. This gives the
maximum value for U
z
in
Equation 373
.
If more than one line is connected to the LV busbar, the equivalent impedance
is calculated and given as a parameter setting as shown in
Figure 917
for the
equivalent electrical circuit for calculating LDC. For example, if there are N number
of identical lines with identical loads in the substation, the R- and X-values needed
for the settings
Line drop V React
and
Line drop V Ris
are obtained by dividing the
resistance and the reactance of one line by N. Because the voltage drop is different
in lines with different impedances and load currents, it is necessary to make a
compromise when setting the
Line drop V React
and
Line drop V Ris
settings.
Raising the voltage in the point of lowest voltage must not lead to overvoltage
elsewhere.
By default, LDC is effective only on the normal active power flow direction. If the
active power flow in the transformer turns opposite, that is, from the regulated
side towards the system in the upper level, the LDC term is ignored, that is, set
to zero. In such a case, it is assumed that the feeding units at the regulated side
of the transformers maintain proper voltage levels. This can cause a conflict if the
transformer tries to reduce the voltage at the substation. Additionally, 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
. 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
Chapter 9.10.5.6 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.
1MRS759142 F
Control functions
REX640
Technical Manual
1705
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