!PPENDIX
3*
6
# ' #
4HE
HIGH SET ) STAGE
REQUIRES THAT IT MUST NOT PICK UP ON A SHORT CIRCUIT ON THE LOWER VOLTAGE SIDE 4HIS IS
ENSURED WHEN IT IS SET HIGHER THAN
U
K
TIMES RATED CURRENT IE
|
! ! 7ITH A SAFETY MARGIN WE
SELECT ! /N THE SECONDARY SIDE ))
.
4HE
OVERCURRENT STAGE ))
P
REPRESENTS THE BACK UP STAGE FOR THE DOWNSTREAM PROTECTIVE RELAYS AND MUST BE
GRADED WITH THESE 4HE PICK UP VALUE SHOULD LIE APPROXIMATELY TO ABOVE THE MAXIMUM PRESUMED
LOAD CURRENT INCLUDING PERMISSIBLE OVERLOAD 7E SELECT AS SETTING VALUE FOR EXAMPLE ))
.
IE )
!
4HE
UNBALANCED LOAD PROTECTION
CAN ALSO BE USED ON TRANSFORMERS IN ORDER TO DETECT LOW CURRENT SINGLE
PHASE OR TWO PHASE FAULTS 4HIS ALLOWS FOR EXAMPLE TO DETECT SINGLE PHASE SHORT CIRCUIT ON THE LOWER VOLT
AGE SIDE OF THE TRANSFORMER EVEN WHEN THERE IS NO ZERO SEQUENCE CURRENT ON THE HIGHER VOLTAGE SIDE CONNEC
TION GROUP $Y
4HE POWER TRANSFORMER TRANSFORMS SYMMETRICAL CURRENT ACCORDING TO ITS TRANSFORMATION RATIO
L
P
.
TR
L
S
.
TR
TRANSFORMATION RATIO
4HIS IS ALSO VALID FOR THE NEGATIVE SEQUENCE CURRENTS )
L
P
.
TR
L
S
)N CASE OF SINGLE PHASE FAULT )
A
)
A
)
A
|
)
A
AND THUS
)
A
.
TR
)
A
)N CASE OF TWO PHASE FAULT )
B
)
A
)
A
)
A
)
A
o
|
)
A
AND THUS
)
A
.
TR
)
A
4HE ASSUMED DATA OF THIS EXAMPLE RESULT IN
.
TR
K6
KV
)F WE SET )
|
)
.
AT THE RELAY ON THE HIGHER VOLTAGE SIDE THE MINIMUM DETECTABLE FAULT CURRENT ON THE LOWER
VOLTAGE SIDE IS )
A
|
.
TR
|
)
|
|
|
! ! WITH SINGLE PHASE FAULT AND
o
|
.
TR
|
|
!
! WITH TWO PHASE FAULT 4HIS CORRESPONDS TO OR OF THE RATED TRANSFORMER CURRENT
4HE UNBALANCED LOAD PROTECTION CANNOT DISTINGUISH BETWEEN FAULT ON THE HIGHER VOLTAGE SIDE AND ON THE LOWER
VOLTAGE SIDE 4HEREFORE THE TIME DELAY 4)
MUST BE COORDINATED WITH THE TIME GRADING OF THE DOWNSTREAM
PROTECTION RELAYS
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