ABB RELION RET670, Applications Manual

Page: 781 / 1016

for T2 and T1. This in turn would be misinterpreted as a circulating current, and would
upset a correct calculation of I
cc
. Thus, if the actual connection is as in the left figure
the capacitive current I
C
needs to be compensated for regardless of the operating
conditions and in ATCC this is made numerically. The reactive power of the capacitor
bank is given as a setting Q1, which makes it possible to calculate the reactive
capacitance:
2
C
V
Q1
X
=
EQUATION1981-ANSI V1 EN-US
(Equation 510)
Thereafter the current I
C
at the actual measured voltage V
B
can be calculated as:
B
C
C
V
X
I
3
×
=
EQUATION1982-ANSI V1 EN-US
(Equation 511)
In this way the measured LV currents can be adjusted so that the capacitor bank current
will not influence the calculation of the circulating current.
Three independent capacitor bank values Q1, Q2 and Q3 can be set for each
transformer in order to make possible switching of three steps in a capacitor bank in
one bay.
Power monitoring
SEMOD159053-251 v4
The level (with sign) of active and reactive power flow through the transformer, can be
monitored. This function can be utilized for different purposes for example, to block
the voltage control function when active power is flowing from the LV side to the HV
side or to initiate switching of reactive power compensation plant, and so on.
There are four setting parameters P> , P< , Q> and Q< with associated outputs in
TR8ATCC (90) and TR1ATCC (90) function blocks PGTFWD, PLTREV, QGTFWD
and QLTREV. When passing the pre-set value, the associated output will be activated
after the common time delay setting
tPower .
The definition of direction of the power is such that the active power P is forward when
power flows from the HV-side to the LV-side as shown in figure
371 . The reactive
power Q is forward when the total load on the LV side is inductive ( reactance) as
shown in figure
371 .
1MRK 504 163-UUS A
Section 15
Control
Transformer protection RET670 2.2 ANSI 775
Application manual