ABB RET670 Commissioning Manual page 286

Section 12
Primary injection testing
280
3. Connect all transformers in the parallel group to the same busbar on the
secondary side.
4. Open the test display for Transformer Voltage Control on the LHMI in Main
menu/Test/Function status/Control/TransformerVoltageControl(ATCC,
90).
5. Manually execute Raise commands to step up the tap changer for transformer
T1 to two steps above the setting for the other transformers in the parallel
group.
6. Check that the value of IPRIM is below the overcurrent blocking level
(Iblock)
7. Check that the value of ICIRCUL is below the circulating current limit
(CircCurLimit).
8. Set the control mode to Automatic for all transformers.
9. For transformer T1, adjust the parameter Comp on the local HMI in Main
menu/Settings/Setting group N/Control/
TransformerVoltageControl(ATCC,90)/TR8ATCC:x/ParCtrl/Compso
that the LOWER output is activated due to circulating current.
Comp is a setting for circulating current Compensating Factor, and it is
effectively a multiplier value to change the sensitivity of the voltage
regulation function to measured values of circulating current. A nominal
value of 200 for Comp should be appropriate to achieve sensitive voltage
regulation. Smaller values may lead to a normal state where tap changers for
parallel transformers are well out of step, and values significantly higher will
cause over-sensitivity in the voltage control function and tap changer hunting
behavior. It is an important outcome of the testing process that the
compensating factor is checked for each transformer to ensure sensitive but
stable operation for circulating currents.
For example, if there are three transformers connected in parallel, and the tap
changer of transformer T1 is two steps over the tap changer of T2 and T3, the
circulating current detected by the VCTR for T1 will be the sum (with
opposite sign) of the current measured at T2 and T3. The currents measured
at T2 and T3 will ideally be about the same values. If the voltage is close to
the upper limit of the UDeadband, the tap changer of T1 will try to decrease
1MRK 504 165-UEN A
IEC13000252-1-en.vsd
Transformer protection RET670 2.2 IEC
Commissioning manual