in other words, the transformer with the higher tap position will have the higher V
L
value and the transformer with the lower tap position will have the lower V
L
value.
Consequently, when the busbar voltage increases, T1 will be the one to tap down, and
when the busbar voltage decreases, T2 will be the one to tap up. The overall
performance will then be that the runaway tap situation will be avoided and that the
circulating current will be minimized.
Parallel control with the circulating current method
Two transformers with different turns ratio, connected to the same busbar on the HV-
side, will apparently show different LV-side voltage. If they are now connected to the
same LV busbar but remain unloaded, this difference in no-load voltage will cause a
circulating current to flow through the transformers. When load is put on the
transformers, the circulating current will remain the same, but now it will be
superimposed on the load current in each transformer. Voltage control of parallel
transformers with the circulating current method means minimizing of the circulating
current at a given voltage target value, thereby achieving:
1. that the busbar or load voltage is regulated to a preset target value
2. that the load is shared between parallel transformers in proportion to their ohmic
short circuit reactance
If the transformers have equal percentage impedance given in the respective
transformer MVA base, the load will be divided in direct proportion to the rated power
of the transformers when the circulating current is minimized.
This method requires extensive exchange of data between the TR8ATCC (90) function
blocks (one TR8ATCC (90) function for each transformer in the parallel group).
TR8ATCC (90) function block can either be located in the same IED, where they are
configured in PCM600 to co-operate, or in different IEDs. 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 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 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. This works such that, if a measured
voltage V
B
, differs from V
Bmean
with more than a preset value (setting parameter
VTmismatch) and for more than a pre set time (setting parameter tVTmismatch) an
alarm signal VTALARM will be generated.
1MRK504116-UUS C Section 3
IED application
665
Application manual
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