ABB RELION RET670, Commissioning Manual, Page: 293 / 328

Section 12 Primary injection testing
Whenever it becomes necessary to work on primary equipment, it is
essential that all the necessary switching, locking, grounding and safety
procedures are observed and obeyed in a rigid and formalized manner.
Operating and testing procedures should be strictly followed in order to
avoid exposure to live equipment.
A test with primary current through the protected zone is usually a final check that the
current circuits are correctly connected to the IED protection scheme. It is important to
have an appropriate source, which is able to inject sufficient current in the primary circuit
in order to distinguish between noise and real injected current. Therefore it is
recommended that the injection current should be at least 10% of rated CT primary
current.
12.1 Transformer Voltage Control ATCC
12.1.1 Load drop compensation function, LDC
The load drop compensation function can be tested directly with operational currents, in
other words, with the power transformer in service and loaded.
When the system is carrying load there is a difference between the busbar voltage (VB)
and the voltage at the load point (VL). This difference is load dependent and can be
compensated by the ATCC function.
The load current is fed into the ATCC function where parameters corresponding to the line
data for resistance and inductance are set. The voltage drop calculated by the LDC is
proportional to the voltage drop in the system up to the load point.
In the IED, this voltage is subtracted from the measured busbar voltage (VB) and the
result, corresponding to the voltage at the load point (UL) is presented to the ATCC
function for the purposes of voltage regulation. This voltage will be lower (if resistive or
inductive load current is applied) than the Vset voltage and ATCC increases the voltage
in order to achieve the correct system voltage at the load point.
1. Confirm the PST settings for Rline and Xline .
1MRK 504 165-UUS -
Section 12
Primary injection testing
Transformer protection RET670 2.2 ANSI 287
Commissioning manual