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:
EQUATION1867 V1 EN-US (Equation 203)
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
cc_i
that flows in
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
cc_i
, can be calculated as:
_
Im( )
cc i i i L
I I K I= - - ´
EQUATION1868 V1 EN-US (Equation 204)
where Im signifies the imaginary part of the expression in brackets and K
i
is a constant which
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
di
, with the following formula:
di i cc _ i i
V C I X= ´ ´
EQUATION1988-ANSI V1 EN-US (Equation 205)
where X
i
is the short-circuit reactance for transformer i and C
i
, is a setting parameter named
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
di
will have
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:
EQUATION1989-ANSI V1 EN-US (Equation 206)
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
i
would then be
Section 14 1MRK 502 066-UUS B
Control
874
Technical manual
To Next Page
Loading...
