ABB RELION RET670 Applications Manual page 775

1MRK 504 163-UUS A
Transformer protection RET670 2.2 ANSI
Application manual
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 is measured individually for each transformer in the parallel
B
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 values will be calculated. The resulting value V
B
in each IED instead of V
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 , differs from V
B
VTmismatch) and for more than a pre set time (setting parameter tVTmismatch) an
alarm signal VTALARM will be generated.
The calculated mean busbar voltage V
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 (90) 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.
When the circulating current is known, it is possible to calculate a no-load voltage for
each transformer in the parallel group. To do that the magnitude of the circulating
current in each bay, is first converted to a voltage deviation, V
for the voltage regulation, thus assuring that the same value
B
with more than a preset value (setting parameter
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
Bmean
, with equation 508:
di
769