ABB ret650 - Page 235

To achieve a more correct regulation, an adjustment to a value of j2 slightly less
than -90° (2 – 4° less) can be made.
The effect of changing power factor of the load will be that j2 will no longer be
close to -90° resulting in U
L
being smaller or greater than U
B
if the ratio Rline/
Xline is not adjusted.
Figure 83 shows an example of this where the settings of Rline and Xline for j =
11° from figure 82 has been applied with a different value of j (j = 30°).
U
B
jI
T
*Xline
U
L
Xline
Rline
Zline
I
T
I
T
*Rline
j=30
0
j
1
j
2
DU
j
1
=11
0
-90
0
=-79
0
en06000630.vsd
IEC06000630 V1 EN-US
Figure 83: Transformer with reverse reactance regulation poorly adjusted to
the power factor
As can be seen in figure 83, the change of power factor has resulted in an increase
of j2 which in turn causes the magnitude of U
L
to be greater than U
B
. It can also
be noted 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 56, 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.
1MRK 504 169-UEN A Section 11
Control
Transformer protection RET650 2.2 IEC 229
Application manual