Because the transformer impedance is dominantly inductive, it is possible to use just the
transformer reactances in the above formula. At the same time this means that T1 circulating
current lags the busbar voltage by almost 90°, while T2 circulating current leads the busbar
voltage by almost 90°.
See figure
463.
I
T2
I
L
I
T1
I
CC_T1
I
CC_T2
V
T2
V
B
V
T1
C
T1
*I
CC_T1
*Z
T1
C
T2
*I
CC_T2
*Z
T2
2*Vdeadband
T2 Receives Cir_Curr T1 Produces Cir_Curr
I
L
= I
T1
+ I
T2
Icc_T1 = Imag {IT1- (ZT2/(ZT1+ZT2)) * IL}
Icc_T2 = Imag {IT2- (ZT1/(ZT1+ZT2)) * IL}
en06000525_ansi.vsd
ANSI06000525 V1 EN-US
Figure 463: Vector diagram for two power transformers working in parallel
Thus, by minimizing the circulating current flow through transformers, the total reactive power
flow is optimized as well. In the same time, at this optimum state the apparent power flow is
distributed among the transformers in the group in proportion to their rated power.
In order to calculate the circulating current, measured current values for the individual
transformers must be communicated between the participating TR8ATCC (90) functions. It should
be noted that the Fourier filters in different IEDs run asynchronously, which means that current
and voltage phasors cannot be exchanged and used for calculation directly between the IEDs. In
order to “synchronize” measurements within all IEDs in the parallel group, a common reference
must be chosen. The most suitable reference quantity for all transformers, belonging to the same
parallel group, is the busbar voltage. This means that the measured busbar voltage is used as a
reference phasor in all IEDs, and the position of the current phasors in a complex plane is
calculated in respect to this reference. This is a simple and effective solution, which eliminates any
additional need for synchronization between the IEDs regarding TR8ATCC (90) function.
At each transformer bay, the real and imaginary parts of the current on the secondary side of the
transformer are calculated from measured values, and distributed to the TR8ATCC (90) functions
belonging to the same parallel group.
As mentioned before, only the imaginary part (reactive current component) of the individual
transformer current is needed for the circulating current calculations. The real part of the current
1MRK 502 066-UUS B Section 14
Control
873
Technical manual
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