ABB RELION 670 Series Technical Manual page 880

Section 14
Control
will, however, be used to calculate the total through load current and will be used for the line
voltage drop compensation.
The total load current is defined as the sum of all individual transformer currents:
k
å
=
I
L
=
i 1
EQUATION1867 V1 EN-US
where the subscript i signifies the transformer bay number and k the number of parallel
transformers in the group (k≤ 8). Next step is to extract the circulating current I
bay i. It is possible to identify a term in the bay current which represents the circulating current.
The magnitude of the circulating current in bay i, I
= -
I
cc i _
EQUATION1868 V1 EN-US
where Im signifies the imaginary part of the expression in brackets and K
depends on the number of transformers in the parallel group and their short-circuit reactances.
The TR8ATCC (90) function automatically calculates this constant based on the transformer
reactances which are setting parameters, and shall be given in primary ohms calculated from each
transformer rating plate. The minus sign is added in the above equation in order to get a positive
value of the circulating current for the transformer that generates it.
In this way each TR8ATCC (90) function calculates the circulating current of its own bay.
A plus sign means that the transformer produces circulating current while, a minus sign means
that the transformer receives circulating current.
As a next step, it is necessary to estimate the value of the no-load voltage in each transformer. To
do that the magnitude of the circulating current in each bay is first converted to a voltage
deviation, V
=
V C
di
EQUATION1988-ANSI V1 EN-US
where X
Comp which serves the purpose of alternatively increasing or decreasing the impact of the
circulating current in the TR8ATCC (90) control calculations. It should be noted that V
positive values for transformers that produce circulating current and negative values for
transformers that receive circulating current.
Now the magnitude of the no-load voltage for each transformer can be approximated with:
=
V V
i Bmean
EQUATION1989-ANSI V1 EN-US
Generally speaking, this value for the no-load voltage can then be put into the voltage control
function in a similar way as for the single transformer described previously. V
874
I
i
- ´
Im( I K I )
i i L
, with the following formula:
di
´ ´
I X
i cc _ i i
is the short-circuit reactance for transformer i and C
i
+
V
di
, can be calculated as:
cc_i
i
, is a setting parameter named
i
1MRK 502 066-UUS B
(Equation 203)
that flows in
cc_i
(Equation 204)
is a constant which
(Equation 205)
will have
di
(Equation 206)
would then be
i
Technical manual