ABB Relion 670 Series Applications Manual page 429

1MRK 511 401-UEN Rev. K
The effect of changing power factor of the load will be that j2 will no longer be close to -90°
resulting in U
Figure 216
figure 215
IEC06000630 V1 EN-US
Figure 216: Transformer with reverse reactance regulation poorly adjusted to the power
As can be seen in figure 216, the change of power factor has resulted in an increase of j2 which
in turn causes the magnitude of U
in the load current aggravates the situation, as does also an increase in the setting of Zline
Rline and Xline ).
(
Apparently the ratio Rline/Xline according to equation 194, that is the value of j1 must be set
with respect to the power factor, also meaning that the reverse reactance method should not
be applied to systems with varying power factor.
The setting of
transformers will not pull together and a run away tap situation will occur. On the other hand, a
high setting will keep the transformers strongly together with no, or only a small difference in
tap position, but the voltage regulation as such will be more sensitive to a deviation from the
anticipated power factor. A too high setting of
transformers will then be prone to over react on deviations from the target value.
There is no rule for the setting of
and susceptibility to changing power factor is achieved. One way of determining the setting is
by trial and error. This can be done by setting e.g.
reactance, and then observe how the parallel control behaves during a couple of days, and then
tune it as required. It shall be emphasized that a quick response of the regulation that quickly
pulls the transformer tap changers into equal positions, not necessarily corresponds to the
optimal setting. This kind of response is easily achieved by setting a high
discussed above, and the disadvantage is then a high susceptibility to changing power factor.
A combination of line voltage drop compensation and parallel control with the negative
reactance method is possible to do simply by adding the required
Xline values separately to get the combined impedance. However, the line drop impedance has
a tendency to drive the tap changers apart, which means that the reverse reactance
impedance normally needs to be increased.
Bay control REC670
Application manual
being smaller or greater than U
L
shows an example of this where the settings of
has been applied with a different value of j (j = 30°).
j =11 0 -90 0 =-79 0
1
Rline
j
1
Xline Zline
j=30 0
I
T
factor
Xline gives the sensitivity of the parallel regulation. If Xline is set too low, the
Xline such that an optimal balance between control response
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if the ratio Rline/Xline is not adjusted.
B
to be greater than U . It can also be noted that an increase
L B
Xline can cause a hunting situation as the
Xline equal to half of the transformer
Rline and Xline for j = 11° from
I *Rline
U
T
L
DU
jI *Xline
T
U
B
j
2
en06000630.vsd
Xline value, as was
Rline values and the required
Section 15
Control
423