If the functions are located in different IEDs they must communicate via GOOSE interbay
communication on the IEC 61850 communication protocol. Complete exchange of TR8ATCC (90)
data, analog as well as binary, via GOOSE is made cyclically every 300 ms.
The main objectives of the circulating current method for parallel voltage control are:
1. Regulate the busbar or load voltage to the preset target value.
2. Minimize the circulating current in order to achieve optimal sharing of the reactive load
between parallel transformers.
The busbar voltage V
B
is measured individually for each transformer in the parallel group by its
associated TR8ATCC (90) function. These measured values will then be exchanged between the
transformers, and in each TR8ATCC (90) block, the mean value of all V
B
values will be calculated.
The resulting value V
Bmean
will then be used in each IED instead of V
B
for the voltage regulation,
thus assuring that the same value is used by all TR8ATCC (90) functions, and thereby avoiding that
one erroneous measurement in one transformer could upset the voltage regulation. At the same
time, supervision of the VT mismatch is also performed.
Figure
462 shows an example with two transformers connected in parallel. If transformer T1 has
higher no load voltage it will drive a circulating current which adds to the load current in T1 and
subtracts from the load current in T2.
Load
T1
IL
T2
VB
VL
Load
VL
IL
+
+
ANSI06000484-2-en.vsd
VT2
ZT2
IT2
ZT1
VT1
IT1
ICC...T2
ICC...T1
ICC...T1
ICC...T2
IT2
IT1
ANSI06000484 V2 EN-US
Figure 462: Circulating current in a parallel group of two transformers
It can be shown that the magnitude of the circulating current in this case can be approximately
calculated with the formula:
T1 T 2
cc _ T1 cc _ T 2
T1 T 2
V V
I I
Z Z
-
= =
+
EQUATION1987-ANSI V1 EN-US
(Equation 202)
Section 14 1MRK 502 066-UUS B
Control
872
Technical manual
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