Siemens SIPROTEC 7SJ531 - Page 77

-ETHOD OF OPERATION3*
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2EVERSE &ORWARDS &ORWARDS 2EVERSE
)
%
LAGGING)
%
LEADING
)
%A
ACTIVE COMPONENT
&IGURE  $IRECTIONAL EARTH FAULT MEASUREMENT CHARACTERISTIC
EXAMPLE ) | COS É
)N NETWORKS WITH ISOLATED STARPOINT THE EARTH FAULT
CURRENT FLOWS AS A CAPACITIVE CURRENT FROM THE HEALTHY
LINES VIA THE MEASURING POINT TO THE POINT OF FAULT 4HIS
CAPACITIVE CURRENT DETERMINES THE DIRECTION
)N NETWORKS WITH ARC SUPPRESSION COILS THE 0ETERSEN
COIL SUPERIMPOSES A CORRESPONDING INDUCTIVE CUR
RENT ON THE CAPACITIVE EARTH FAULT CURRENT WHEN AN
EARTH FAULT OCCURS SO THAT THE CAPACITIVE CURRENT AT
THE POINT OF FAULT IS COMPENSATED $EPENDENT UPON
THE POINT OF MEASUREMENT IN THE NETWORK THE RESUL
TANT MEASURED CURRENT CAN HOWEVER BE INDUCTIVE OR
CAPACITIVE AND THE REACTIVE CURRENT IS THEREFORE NOT
SUITABLE FOR THE DETERMINATION OF DIRECTION )N THIS
CASE ONLY THE OHMIC RESIDUAL CURRENT WHICH RESULTS
FROM THE LOSSES OF THE 0ETERSEN COIL CAN BE USED FOR
DIRECTIONAL DETERMINATION 4HIS EARTH FAULT OHMIC CUR
RENT IS ONLY A FEW PERCENT OF THE CAPACITIVE EARTH FAULT
CURRENT
)N THE LATTER CASE IT MUST BE NOTED THAT DEPENDENT
UPON THE LOCATION OF THE PROTECTIVE RELAY A CONSIDER
ABLE REACTIVE COMPONENT MAY BE SUPERIMPOSED
WHICH IN THE MOST UNFAVOURABLE CASES CAN ATTAIN 
TIMES THE ACTIVE COMPONENT %VEN THE EXTREMELY
HIGH ACCURACY OF THE CALCULATION ALGORITHM IS THEN IN
ADEQUATE IF THE CURRENT TRANSFORMERS DO NOT EXACTLY
CONVERT THE PRIMARY VALUES
4HE MEASUREMENT INPUT CIRCUIT OF THE RELAY FOR HIGHLY
SENSITIVE EARTH FAULT DETECTION IS PARTICULARLY DE
SIGNED FOR THIS PURPOSE AND PERMITS AN EXTREMELY
HIGH SENSITIVITY FOR THE DIRECTIONAL DETERMINATION OF
THE WATTMETRIC RESIDUAL CURRENT )N ORDER TO UTILIZE THIS
SENSITIVITY IT IS RECOMMENDED THAT WINDOW
TYPE CUR
RENT TRANSFORMERS BE USED FOR EARTH FAULT DETECTION IN
COMPENSATED NETWORKS !S EVEN THE CORE BALANCE
TRANSFORMERS HAVE AN ERROR OF ANGLE THE PROTECTION
SYSTEM ALLOWS THE SETTING OF FACTORS WHICH DEPEN
DENT UPON THE REACTIVE CURRENT WILL CORRECT THE ERROR
ANGLE
&URTHER EXPLANATION CONCERNING THE CHARACTERISTIC
AND SYMMETRY AXIS ARE GIVEN IN THE SETTING HINTS IN
3ECTION 
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