ABB RET670 - Page 698

0 0 0
1
( 37 ) 90 53 j = - - - = -
EQUATION1939 V1 EN (Equation 430)
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 V
L
being smaller or greater than V
B
if the ratio Rline/Xline is not
adjusted.
Figure
289 shows an example of this where the settings of Rline and Xline for j = 11°
from figure
288 has been applied with a different value of j (j = 30°).
V
B
jI
T
*Xline
V
L
Xline
Rline
Zline
I
T
I
T
*Rline
j=30
0
j
1
j
2
DV
j
1
=11
0
-90
0
=-79
0
en06000630_ansi.vsd
ANSI06000630 V1 EN
Figure 289: Transformer with reverse reactance regulation poorly adjusted to the
power factor
As can be seen in figure 290, the change of power factor has resulted in an increase of
j2 which in turn causes the magnitude of V
L
to be greater than VB. 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
430, 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
Section 3 1MRK504116-UUS C
IED application
692
Application manual