8.4.4.4 DIFFERENTIAL CURRENT TEST (“DIFF CUR”)
This test is for two-terminal and three-terminal systems. In a three terminal system Startup Mode is working with
true currents and is not performing pre-fault load subtraction. The values that are used are true CT currents as
is the case in two terminal systems.
NOTE
IT IS ASSUMED THAT WE HAVE A QUIESCENT SYSTEM, THERE ARE NO ALARMS OR ACTIVE
FAULTS AND THERE ARE NO TAPPED LOADS IN THE SYSTEM. CURRENT FLOWING INTO
ANY TAPS ON THE LINE WILL AFFECT THE ACCURACY OF THE “DIFF CURR” TEST.
For this test to be accurate all stations must have successfully passed the Phase Sequence Test.
The local, remote 1 and remote 2 relays should be in Startup Mode and the Phase Sequence Test should show
each of the stations passing the test before proceeding.
If the user has not already done so, the shunt should be removed from the PHASE A current.
After “SEQ TEST” message is displayed, begin the Differential Current test by pressing the display keypad but-
ton [ENT] once. Alternatively, pressing keys [.] ,[4] and [ENT] consecutively at any time while in STARTUP
MODE will display the Differential Current test screen.
The display will indicate “DIFF CUR” on line one and “ NO DATA” on line two of the display, for up to ten sec-
onds, or until valid data has been computed and received by the relay. The message “ NO DATA” indicates that
the test has begun but no valid data is yet available to display.
Once valid data has been received from all units, the display will show the largest value computed in the sys-
tem. This value is the arithmetic sum or differential current between PHASE A currents at all units. Note that in
a three terminal system that is properly phased, only one station will compute an arithmetic sum or differential
current. This value will be displayed at all terminals. In a 2 terminal system, both stations will compute a differ-
ential current and both should be the same.
The display should show no current if all units are wired correctly. The DIFF CURR at each station will ap-
proach 0.00 amps. If the phases are connected incorrectly the display will show a value above 1 amp. Typi-
cally, this will indicate that the CT connections at the local or remote stations are phased improperly with re-
spect to each other.
NOTE
IT IS POSSIBLE FOR A NON-ZERO VALUE OF DIFFERENTIAL CURRENT TO BE VALID IF
THERE IS A TAPPED LOAD IN THE SYSTEM AND THE DIFFERENTIAL CURRENT IS EQUAL TO
THE CURRENT IN THE TAP.
RFL 9300 RFL Electronics Inc.
October 20, 2004 8 - 29 (973) 334-3100
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