-ETHOD OF OPERATION3*
6
#
' #
5NBALANCED LOAD PROTECTION
4HE UNIT IS EQUIPPED WITH AN UNBALANCED LOAD PRO
TECTION WHICH IS ADVANTAGEOUS FOR PROTECTION OF MO
TORS WHICH ARE SWITCHED BY VACUUM CONTACTORS WITH
ASSOCIATED FUSES 7HEN RUNNING ON SINGLE PHASE THE
MOTORS DEVELOP SMALL AND PULSATING TORQUES SO THAT
WITH UNCHANGED TORQUE LOAD THE MOTOR WILL BE QUICK
LY THERMALLY OVERLOADED &URTHERMORE THERMAL OVER
LOADING OF THE MOTOR MAY ARISE BY UNSYMMETRICAL
SYSTEM VOLTAGE %VEN SMALL UNBALANCED SYSTEM VOLT
AGES MAY LEAD TO LARGE SLIP LOAD CURRENTS BECAUSE OF
THE SMALL NEGATIVE SEQUENCE REACTANCES
4HE UNBALANCED LOAD PROTECTION DETECTS ADDITIONAL
LY INTERRUPTIONS SHORT
CIRCUITS AND SWOPPED
PHASE CONNECTIONS OF THE CURRENT TRANSFORMER CIR
CUITS 3INGLE
PHASE AND TWO PHASE SHORT CIRCUITS
CAN BE DETECTED EVEN WHEN THE FAULT CURRENT IS TOO
SMALL TO BE DETECTED BY THE OVERCURRENT TIME PROTEC
TION
)N THE UNBALANCED LOAD PROTECTION OF THE 3* THE
FUNDAMENTAL WAVE OF THE PHASE CURRENTS IS FILTERED
OUT AND SEPARATED INTO SYMMETRICAL COMPONENTS
NEGATIVE SEQUENCE )
AND POSITIVE SEQUENCE )
4HE RATIO )
)
.
)
.
RATED RELAY CURRENT IS EVALUATED
FOR UNBALANCED LOAD DETECTION
4HE UNBALANCED LOAD PROTECTION HAS TWO
STAGE DE
SIGN )F THE FIRST ADJUSTABLE THRESHOLD )
IS REACHED
TIMER 4
)
IS STARTED THE SECOND ADJUSTABLE THRESH
OLD )
STARTS THE TIMER 4
)
SEE &IGURE
7HEN THE ASSOCIATED TIME HAS ELAPSED TRIP COM
MAND IS ISSUED
&ILTERING OF THE NEGATIVE SEQUENCE CURRENT IS POSSI
BLE AS LONG AS THE HIGHEST OF THE THREE PHASE CURRENTS
IS AT LEAST TIMES RATED CURRENT OF THE RELAY AND ALL
PHASE CURRENTS ARE SMALLER THAN TIMES RATED CUR
RENT
&IGURE SHOWS THE LOGIC DIAGRAM OF THE UNBAL
ANCED LOAD PROTECTION
T
4
)
4
)
)
)
)
TRIP STAGE )
TRIP STAGE )
4RIP AREA
&IGURE 4RIP TIME CHARACTERISTIC OF THE UNBALANCED LOAD PROTECTION
ONLY FOR MODEL 3*J JJJJ JJJJ
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