Siemens SIPROTEC 7SJ531 - Page 61

-ETHOD OF OPERATION3*
6

#
' #
 6OLTAGE PROTECTION
 -EASURED VALUE FORMATION
4HE VOLTAGE PROTECTION CAN EITHER BE FED BY THE THREE
PHASE
TO EARTH VOLTAGES 5
, %
5
, %
5
, %
OR
BY THE PHASE
TO PHASE VOLTAGES 5
, ,
5
, ,

5
, ,
 4HE LATTER CAN BE USED FOR PROCESSING BY THE
VOLTAGE PROTECTION EVEN IF FROM THE SWITCHGEAR ONLY
THE PHASE
TO EARTH VOLTAGES ARE AVAILABLE BE
CAUSE THE RELAY IS ABLE TO CALCULATE THE PHASE
TO
PHASE VOLTAGES FROM THE PHASE TO EARTH VOLTAGES
4HIS OPTION BETWEEN PHASE
TO EARTH AND PHASE
TO PHASE VOLTAGES ALLOWS EITHER TO EVALUATE PHA
SE
TO EARTH UNBALANCED VOLTAGES EG CAUSED BY
AN EARTH FAULT OR DISREGARD THEM PHASE
TO
PHASE
)T IS ALSO POSSIBLE TO CONNECT TWO PHASE
TO EARTH
VOLTAGES AND THE DISPLACEMENT VOLTAGE 5
%
 THE RELAY
THEN CALCULATES THE MISSING VOLTAGE FROM THOSE
WHICH ARE CONNECTED
4HE FUNDAMENTAL WAVES OF THE THREE FED
IN VOLTAGES
ARE FILTERED OUT BY A &OURIER ANALYSIS AND THEN PRO
CESSED IN THE FOLLOWING WAY AFTER FORMING THE ABSO
LUTE VALUES THE SMALLEST OF THE THREE VOLTAGES IS AS
SIGNED TO THE UNDERVOLTAGE PROTECTION AND THE HIGH
EST VOLTAGE TO THE OVERVOLTAGE PROTECTION
4HE FREQUENCY ADAPTATION IN THE RELAY IS DESIGNED FOR
THE SYSTEM FREQUENCY q (Z "EYOND THESE LIMITS
THE MEASURING ERROR INCREASES BEARING THE RISK OF
OVERFUNCTION OF THE UNDERVOLTAGE PROTECTION &OR THIS
REASON PICK
UP OF THE PROTECTION IS INHIBITED OUTSIDE
THE FREQUENCY ADAPTATION RANGE !N ALREADY EXISTING
PICK
UP HOWEVER WILL NOT BE CANCELLED WHEN LEAV
ING THIS RANGE
$EPENDING OF THE SWITCHGEAR DESIGN THE VOLTAGE
TRANSFORMERS CAN BE CONNECTED AT THE INCOMING
BUS
BAR OR THE OUTGOING FEEDER SIDE 4HESE DIF
FERENT CONFIGURATIONS RESULT IN A DIFFERENT REACTION BY
THE PROTECTION RELAY TO A FAULT 7HEREAS THE VOLTAGE
PERSISTS ON THE INCOMING SIDE AFTER A TRIP COMMAND
AND OPENING OF THE CIRCUIT BREAKER THE VOLTAGE ON THE
OUTGOING SIDE IS SWITCHED OFF 7HEN THE VOLTAGE IS
SWITCHED OFF PICK
UP OF THE UNDERVOLTAGE PROTEC
TION IS MAINTAINED )F RESET OF THE UNDERVOLTAGE PICK
UP IS REQUIRED THEN THE CURRENT CAN BE USED AS AN
ADDITIONAL CRITERION CURRENT CRITERION ## 4HUS THE
UNDERVOLTAGE PICK
UP IS MAINTAINED ONLY WHEN TO
GETHER WITH THE UNDERVOLTAGE CONDITION A MINIMUM
CURRENT ADDRESS  "2+ #,/3%$ IS EXCEEDED
4HIS CURRENT VALUE CAN BE PARAMETERIZED )F THE CUR
RENT DROPS BELOW THIS THRESHOLD AFTER THE OPENING OF
THE BREAKER THE PICK
UP RESETS
 5NDERVOLTAGE PROTECTION
4HE UNDERVOLTAGE PROTECTION RECOGNIZES VOLTAGE
DROPS ON TRANSMISSION LINES AND IN ELECTRICAL MA
CHINES AND AVOIDS IMPERMISSIBLE OPERATION CONDI
TIONS AND A POSSIBLE LOSS OF STABILITY
4HE UNDERVOLTAGE PROTECTION HAS TWO STAGES 5
AND 5 SO THAT A TIME
GRADED TRIPPING DEPEND
ING OF THE DEGREE OF THE VOLTAGE DROP CAN BE
ACHIEVED 6OLTAGE LIMITS AND TIME DELAYS CAN BE SE
LECTED INDIVIDUALLY FOR BOTH STAGES
4HE 5 STAGE FURTHERMORE OFFERS THE OPTION TO PARA
METERIZE THE RESET RATIO R AND TO SWITCH OVER FROM THE
NORMAL PICK
UP THRESHOLD TO AN INDIVIDUAL START UP
THRESHOLD DEPENDING ON A START
UP CRITERION 3# 2E
SET IS EFFECTED TOGETHER WITH RESET OF THE PICK
UP
THRESHOLD 4HE PICK
UP VALUE OF THE START UP
THRESHOLD IS



TIMES THE RESET VALUE 3WITCHING
OVER TO THE START
UP THRESHOLD MAY BECOME NECES
SARY WHEN THE LOAD IS MAINLY COMPOSED OF MOTORS
7HILE UNDER NORMAL OPERATING CONDITIONS THE MO
TORS WITH SYSTEM FREQUENCY CAN COPE WITH AN UNDER
VOLTAGE THE SAME UNDERVOLTAGE IS INSUFFICIENT FOR A
MOTOR START
UP FROM STANDSTILL ROTATING SPEED  
4HIS IS TAKEN INTO CONSIDERATION BY THE START
UP CRI
TERION 4HE START
UP CRITERION IS TRIGGERED BY A CORRE
SPONDING BINARY INPUT 5 3# OR BY THE CURRENT
MAGNITUDE &IGURE  SHOWS A TYPICAL FAULT CURVE
FOR VOLTAGE TRANSFORMERS CONNECTED TO THE INCOMING
BUS
BAR SIDE AND WITH ACTIVATED START UP CRITERI
ON 3# 4HE CURRENT CRITERION ## DESCRIBED UNDER
 IS NOT REQUIRED IN THIS CASE BECAUSE THE VOLT
AGE PERSISTS AFTER OPENING OF THE CIRCUIT BREAKER
7ITH A VOLTAGE DECREASE BELOW THE PICK
UP THRESH
OLD TRIPPING IS EFFECTED AFTER THE TIME DELAY 4
5
RECLOSING IS LOCKED AS LONG AS THE VOLTAGE REMAINS
BELOW THE RESET THRESHOLD /NLY AFTER FAULT CLEARANCE
IE WHEN THE VOLTAGE BECOMES HIGHER THAN THE RESET
THRESHOLD THE PICK
UP RESETS AND CLOSING IS RE
LEASED
4HE ACTIVATED START
UP CRITERION 3# ADDITIONALLY
MONITORS THE CURRENT 7HEN ALL THREE PHASE CURRENTS
ARE BELOW THE SET LIMIT 3# ) A START
UP CONDITION
IS ASSUMED AND SWITCH
OVER FROM THE PICK UP
THRESHOLD 5 TO THE START
UP THRESHOLD 5 3# IS EF
FECTED 7HEN THE START
UP CONDITION HAS ENDED THE
START
UP THRESHOLD IS MAINTAINED FOR A TIME 4 3#
WHICH CAN BE PARAMETERIZED !FTER THIS TIME HAS
ELAPSED IT IS ASSUMED THAT THE MOTORS HAVE RUN UP
AND THAT THE REGULAR PICK
UP THRESHOLD IS APPLICA
BLE
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