ABB REG670 Applications Manual page 567

1MRK 502 071-UEN -
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
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
IED instead of U for the voltage regulation, thus assuring that the same value is used
B
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 with more than a preset value (setting parameter VTmismatch)
Bmean
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.
The calculated circulating current I
service value ICIRCUL under Main menu/Test/Function status/Control/
TransformerVoltageControl(ATCC,90)/TR8ATCC:x.
Bmean
is shown on the local HMI as a service
Bmean
for transformer "i" is shown on the HMI as a
cc_i
Section 15
Control
will then be used in each
B
561
,