Siemens 7SD502, Instruction Manual

Page: 34 / 178

-ETHOD OF OPERATION 3$
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#ONNECTION FOR INCREASED EARTH
CURRENT SENSITIVITY
)N CERTAIN SYSTEMS EARTH FAULT CURRENTS OF EVEN LOWER
VALUES MUST BE DETECTED AND CLEARED AS IT IS POSSI
BLE WITH THE STANDARD CONNECTION EG BECAUSE OF AN
EARTHING RESISTANCE RESISTANCE EARTHED SYSTEMS
4O A CERTAIN EXTENT THIS CAN BE ACHIEVED BY USING AN
INTERMEDIATE TRANSFORMER )4 AS SHOWN IN &IGURE
$EPENDENT ON THE BURDEN ON THE CURRENT TRANSFORM
ERS THE CURRENT IN THE STAR POINT CONDUCTOR CAN BE IN
CREASED TO APPROXIMATELY DOUBLE THE NORMAL VALUE
0HASE FAULT SENSITIVITY IS THE SAME AS WITH THE STAN
DARD CONNECTION
%ARTH FAULT SENSITIVITY WILL THEN BE INCREASED ACCORD
ING TO THE TRANSFORMATION RATIO OF THE INTERMEDIATE
TRANSFORMER )4 4HE INTERMEDIATE TRANSFORMER CAN BE
CONNECTED AS AN AUTOTRANSFORMER SINCE NO GALVANIC
SEPARATION IS NECESSARY
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&IGURE #ONNECTION , , % WITH IN
CREASED EARTH CURRENT SENSITIVITY
5NEQUAL CT RATED CURRENTS
4HE MEASURING CIRCUITS OF THE RELAY ARE DESIGNED FOR
EQUAL SECONDARY OUTPUTS FROM THE TWO SUMMATION
TRANSFORMERS UNDER HEALTHY LINE CONDITIONS 4HIS IS
AUTOMATIC IF THE CTS HAVE EQUAL RATIO AT THE TWO LINE
ENDS IE EQUAL PRIMARY AND EQUAL SECONDARY RATING
%VEN IF THE CTS AT THE LINE ENDS HAVE THE SAME PRIMA
RY RATING BUT THE SECONDARY AT ONE END IS ! AND AT
THE OTHER ! THE USE OF THE MODEL 3$ FOR THE
! CT AT ONE END AND MODEL 3$ FOR ! AT THE
OTHER END WILL ENSURE CORRECT OPERATION
)F THE CTS HAVE DIFFERENT PRIMARY RATINGS MATCHING
TRANSFORMERS MUST BE USED TO CORRECT THE RELATION
SHIP #ONVENTIONALLY MATCHING TRANSFORMERS ARE IN
STALLED AT THE CTS WITH THE LOWER PRIMARY RATING 3IE
MENS MATCHING TRANSFORMERS !- !! ARE
SUITABLE CONNECTED AS SHOWN IN &IGURE
)F IT IS NECESSARY TO USE MATCHING TRANSFORMERS TO COR
RECT THE CT RATIO AT THE SAME TIME AS IT IS REQUIRED TO
INCREASE THE SENSITIVITY TO EARTH FAULTS ACCORDING TO
3ECTION THE CIRCUIT SHOWN IN &IGURE IS OF
COURSE ADEQUATE 4HE TRANSFORMATION RATIO OF THE IN
TERMEDIATE TRANSFORMER IN THE EARTH CURRENT PATH IS
THEN ACCORDINGLY CALCULATED )T IS NOT NECESSARY TO
USE A FURTHER INTERMEDIATE TRANSFORMER
%XAMPLES AND HINTS FOR CALCULATION OF INTERMEDIATE
CTS ARE GIVEN IN 3ECTION
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&IGURE -ATCHING OF UNEQUAL PRIMARY CUR
RENTS