Siemens 7SD502, Instruction Manual

Page: 32 / 178

-ETHOD OF OPERATION 3$
6
# ' #
&OR THE CONNECTION SHOWN IN &IGURE THE
WEIGHTING FACTORS 7 OF THE MIXED CURRENTS FOR THE
VARIOUS FAULT CONDITIONS AND THE RATIOS TO THAT GIVEN
BY THE PHASE SYMMETRICAL FAULTS ARE SHOWN IN
4ABLE /N THE RIGHT HAND SIDE IS THE
COMPLEMENTARY MULTIPLE OF RATED CURRENT WHICH
7 o WOULD HAVE TO BE IN ORDER TO GIVE THE MIXED
CURRENT I
-
M! IN THE SECONDARY CIRCUIT )F THE
CURRENT SETTING VALUES ARE MULTIPLIED WITH THIS FACTOR
THE ACTUAL PICKUP VALUES RESULT
4HE TABLE SHOWS THAT 3$ IS MORE SENSITIVE TO
EARTH FAULTS THAN TO THOSE WITHOUT EARTH PATH COMPO
NENT 4HIS INCREASED SENSITIVITY IS DUE TO THE FACT THAT
THE SUMMATION TRANSFORMER WINDING IN THE CT STAR
POINT CONNECTION )
%
RESIDUAL CURRENT REFER TO &IGURE
HAS THE LARGEST NUMBER OF TURNS AND THUS THE
WEIGHTING FACTOR 7
o
, , , SYM
&AULT TYPE 7
7 o
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) FOR I
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M!
4ABLE 4YPES OF FAULT AND WEIGHTING FACTOR WITH STANDARD CT CONNECTION , , %
)N EARTHFREE NETWORKS OR THOSE WITH 0ETERSON COIL
EARTHING IT IS NOT DESIRABLE TO TRIP THE LINE WHEN A
SINGLE EARTH FAULT OCCURS )F A DOUBLE EARTH FAULT OC
CURS CLEARANCE OF ONLY ONE OF THE TWO FAULT CONDI
TIONS IS OFTEN DESIRED
4HE DELIBERATELY DIFFERENT CONTRIBUTIONS OF THE THREE
PHASE CURRENTS DUE TO THE SUMMATION TRANSFORMER
PRIMARY RATIOS EFFECT A PHASE PREFERENTIAL CLEARANCE
SEQUENCE WHICH IS DETERMINED BY THE WAY IN WHICH
THE CTS ARE CONNECTED 4HE LINE SECTION WHICH HAS
THE FAULT ON THE THUS DETERMINED PREFERENTIAL PHASE
WILL BE CLEARED 4HE PERFORMANCE OF THE DIFFERENTIAL
RELAY WHICH PROTECTS A LINE ON WHICH ONE OF THE TWO
EARTH FAULTS LIES DEPENDS ON THE RATIO OF THE CURRENT
CONTRIBUTIONS AT EACH END 4O ACHIEVE PROPERLY SE
LECTIVE DOUBLE EARTH FAULT PERFORMANCE OF THE PROTEC
TIVE SYSTEM IN A NETWORK BOTH THE DIFFERENTIAL AND DIS
TANCE RELAYS SHOULD BE OPERATED WITH THE SAME
PHASE PREFERENCE SEQUENCE
)N DIFFERENTIAL SCHEMES WHICH USE SUMMATION CUR
RENT TRANSFORMERS ACYCLIC PREFERENCE IS POSSIBLE
&IGURE SHOWS THE CONNECTIONS FOR THE MOST USUAL
CASES OF ACYCLIC PREFERENCE 4HE OTHER POSSIBILITIES
CAN BE ACHIEVED BY USING ALTERNATIVE CT CONNEC
TIONS
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3$
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3$
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, BEFORE , BEFORE , , BEFORE , BEFORE ,
&IGURE $OUBLE EARTH FAULT PREFERENCE BY PHASE CHANGE