ABB REC670 Applications Manual page 399

1MRK 511 401-UEN A
Bay control REC670 2.2 IEC
Application manual
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 function
blocks (one TR8ATCC function for each transformer in the parallel group).
TR8ATCC 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 data, analog as well as binary, via GOOSE is made cyclically every 300
ms.
The busbar voltage U is measured individually for each transformer in the parallel
B
group by its associated TR8ATCC function. These measured values will then be
exchanged between the transformers, and in each TR8ATCC block, the mean value
of all U values will be calculated. The resulting value U
B
each IED instead of U
B
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 U , differs from U
B
parameter VTmismatch) and for more than a pre set time (setting parameter
tVTmismatch) an alarm signal VTALARM will be generated.
The calculated mean busbar voltage U
value BusVolt under Main menu/Test/Function status/Control/
TransformerVoltageControl(ATCC,90)/TR8ATCC:x.
Measured current values for the individual transformers must be communicated
between the participating TR8ATCC functions, in order to calculate the circulating
current.
for the voltage regulation, thus assuring that the same value
with more than a preset value (setting
Bmean
is shown on the local HMI as a service
Bmean
Section 14
Control
SEMOD159053-159 v5
will then be used in
Bmean
393