Mounting and Commissioning
3.3 Commissioning
SIPROTEC, 7UM62, Manual
C53000-G1176-C149-7, Release date 03.2010
419
3.3.12 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 func-
tions 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!
Primary measurements may only be carried out with the generator at stand–still 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 cur-
rent. 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 charac-
teristic 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 operator interface under oper-
ational measured values and compared with the actual measured values. If significant deviations are found,
the CT connections are not correct.
Phase Rotation
The phase rotation must conform to 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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