ABB ret650 - Page 205

2
L
a ci
I
Usetadjust Uset S S
I Base
= + × +
IECEQUATION1978 V1 EN-US (Equation 49)
U
set, adjust
Adjusted set voltage in per unit
USet Original set voltage: Base quality is U
n2
S
a
Automatic load voltage adjustment factor, setting VRAuto
I
L
Load current
I2Base Rated current, LV winding
S
ci
Constant load voltage adjust. factor for active input i (corresponding to
LVAConst1, LVAConst2, LVAConst3 and LVAConst4 )
It shall be noted that the adjustment factor is negative in order to decrease the load
voltage and positive in order to increase the load voltage. After this calculation
U
set, adjust
will be used by TR1ATCC or TR8ATCC for voltage regulation instead of
the original value USet. The calculated set point voltage U
Set, adjust
is shown on the
local HMI as a service value under Main menu/Test/Function status/Control/
TransformerVoltageControl(ATCC,90).
Automatic control of parallel transformers
SEMOD159053-135 v5
Control of parallel transformers means control of two or more power transformers
connected to the same busbar on the LV side and in most cases also on the HV
side. Special measures must be taken in order to avoid a runaway situation where
the tap changers on the parallel transformers gradually diverge and end up in
opposite end positions.
Three alternative methods can be used for parallel control with the Automatic
voltage control for tap changer, single/parallel control TR8ATCC:
• master-follower method
• reverse reactance method
• circulating current method
In order to realize the need for special measures to be taken when controlling
transformers in parallel, consider first two parallel transformers which are
supposed to be equal with similar tap changers. If they would each be in automatic
voltage control for single transformer that is, each of them regulating the voltage
on the LV busbar individually without any further measures taken, then the
following could happen. Assuming for instance that they start out on the same tap
position and that the LV busbar voltage U
B
is within USet ± DU, then a gradual
increase or decrease in the load would at some stage make U
B
fall outside USet ±
DU and a raise or lower command would be initiated. However, the rate of change
of voltage would normally be slow, which would make one tap changer act before
the other. This is unavoidable and is due to small inequalities in measurement and
so on. The one tap changer that responds first on a low voltage condition with a
1MRK 504 169-UEN A Section 11
Control
Transformer protection RET650 2.2 IEC 199
Application manual