3 Mounting and Commissioning
272
7UM61 Manual
C53000-G1176-C127-3
3.3.8 Checking the Current Circuits
General The checks of the current circuits are performed with the generator to ensure correct
CT circuit connections with regard to cabling, polarity, phase sequence, CT ratio etc.,
not in order to verify individual protection functions in the device.
Preparation Switch unbalanced load protection (address 1701) and overload protection (address
1601) to Block. Relay. With the primary system voltage-free and earthed, install a
three-pole short-circuit bridge which is capable of carrying rated current (e.g. earthing
isolator) to the generator line-side terminals.
DANGER!
Energized equipment of the power system ! Capacitive coupled voltages at discon-
nected equipment of the power system !
Non-observance of the following measure will result in death, severe personal injury
or substantial property damage.
Primary measurements must only be carried out on disconnected and grounded
equipment of the power system !
After the preparatory measures all current transformer circuits (protection, measuring,
metering etc.) can be checked with the remanent excitation.
Test Instruction Then the checks of the current transformer circuits are carried out with max. 20 % of
the rated transformer current. Tests with generator currents of more than 20 % are not
normally required for digital protection. Operation of the generator at rated current
during commissioning may only be necessary when the short-circuit characteristic is
measured for the first time.
Amplitude Values The currents can be read out from the device front panel or from the PC via the oper-
ator interface under operational measured values and compared with the actual mea-
sured values. If significant deviations are found, the CT connections are not correct.
Checking Phase
Rotation
The phase rotation must conform with the configured phase sequence (address 271
under Power System Data 1); otherwise an indication “Fail Ph. Seq.” will be
output. The allocation of measured values to phases must be checked and corrected,
if necessary. The negative sequence component I2 of the currents can be read out
under the operational measured values. It must be approximately zero. If this is not
the case, check for crossed current transformer leads:
If the unbalanced load amounts to about 1/3 of the phase currents then current is
flowing in only one or in only two of the phases.
If the unbalanced load amounts to about 2/3 of the phase currents, then one current
transformer has wrong polarity.
If the unbalanced load is about the same as the phase currents, then two phases have
been crossed.
After correcting the wrong connection, the test must be repeated.
Remove short circuit bridges.
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