ABB RELION RET670 Applications Manual page 802

Section 15
Control
796
j =11 0 -90 0 =-79 0
1
Rline
j
1
Xline Zline
j=30 0
I
ANSI06000630 V1 EN-US
Figure 375: Transformer with reverse reactance regulation poorly adjusted to the
power factor
As can be seen in figure 375, the change of power factor has resulted in an increase of
j2 which in turn causes the magnitude of V
that an increase 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 514, 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 Xline gives the sensitivity of the parallel regulation. If Xline is set too
low, the 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 Xline
can cause a hunting situation as the transformers will then be prone to over react on
deviations from the target value.
There is no rule for the setting of Xline such that an optimal balance between control
response 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. Xline equal
to half of the transformer 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 Xline value, as was discussed above, and
the disadvantage is then a high susceptibility to changing power factor.
T
to be greater than V
L
1MRK 504 163-UUS A
I
V
L
DV
jI *Xline
T
V
B
j
2
en06000630_ansi.vsd
. It can also be noted
B
Transformer protection RET670 2.2 ANSI
Application manual
*Rline
T