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Line differential protection (DIFL)
Line differential protection (DIFL)
When testing the differential protection it is important to be aware of that actions taken
locally may cause operation of the terminal in the remote end.
At commissioning and after changes in the current circuits, the trip signals at both ter-
minals must be blocked permanently before the dc supply is connected. This precaution
is taken to avoid unwanted local and remote trip in case of error in the current circuits.
The blocking can not be done by the COMBITEST test switch. It has to be made on the
station side of the test switch because the test switch disconnects the current inputs to
the terminal and thus causes current unbalance in the differential circuit. This will cause
the remote end terminal to trip if not blocked. The blocking of the trip signal must be
maintained, until the test has been performed at both terminals.
Any work in any of the terminals, such as injecting current or to short-circuit the current
during load transfer on the line, will result in a local and REMOTE end trip.
When the current balance is disturbed by any action, the protections at both line ends
must be blocked.
Before performing any work in the current circuit to the line differential protection, the
TestMode must be activated and saved. The protection will not enter into test mode
without saving the command.
When the protection is equipped with a COMBITEST test switch, inserting the test han-
dle will automatically force the protection into test mode. This function is also achieved
by activating the Test digital input. The test mode command must be saved to be acti-
vated.
It is sufficient if one of the terminals is set in test mode. The trip function is blocked
automatically at both line ends, when one of the protection terminals is in test mode.
Thus the protection can be tested without any manual actions in the remote station.
Testing can be performed only when the terminals communicate with each other and if
one and one only of the terminals is in test mode.
To activate the local trip relays during the test for operating time measurement, the
blocking of the trip is overridden in the terminal in test mode by the command "Local
trip".
For a complete test of the protection, the tests must be repeated at both terminals.
Chapter 11
Verifying settings by secondary
injection
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