Siemens 7SD502, Instruction Manual

Page: 38 / 178

-ETHOD OF OPERATION 3$
6
# ' #
-EASURED VALUE FORMATION FOR
DIFFERENTIAL PROTECTION USING TWO
PILOT WIRES
&IGURE ILLUSTRATES A DIFFERENTIAL CURRENT COMPARI
SON SCHEME WITH TWO PILOT WIRES !T EACH END THREE
CURRENTS ARE MEASURED
) IS THE LOCAL CURRENT INTO THE LINE
)
B
IS THE CURRENT FLOWING THROUGH THE LOCAL SHUNT RE
SISTOR 2
B
)
A
IS THE CURRENT FLOWING THROUGH THE PILOT WIRE
LOOP
4WO OF THESE CURRENT ARE SUFFICIENT TO CALCULATE THE
THIRD BUT IN 3$ ALL THE THREE CURRENTS ARE MEA
SURED IN ORDER TO GAIN A PLAUSIBILITY CHECK WITH THESE
QUANTITIES
$IFFERENTIAL AND STABILIZING QUANTITIES CAN BE CALCU
LATED BY MEANS OF THE MEASURED CURRENTS THE INTER
NAL RESISTANCES 2
A
AND 2
B
AND THE PILOT WIRE RESIS
TANCE 2
X
WHICH MUST BE ENTERED TO THE DEVICE DURING
SETTING USING THE FOLLOWING FORMULAE
4HE LOOP EQUATION OF THE LOOP WHICH IS ILLUSTRATED BY
THE DOTTED LINE GIVES US
) )
A
| 2
B
)
A
| 2
X
2
A
) )
A
| 2
B
)
A
| 2
X
2
A
4HE FOLLOWING SIMPLIFICATIONS SHOULD BE AGREED
2
B
2 REFERENCE RESISTANCE
2
X
2
A
X | 2 LONGITUDINAL RESISTANCE
4HUS THE LOOP EQUATION CAN BE WRITTEN
) )
A
| 2 | )
A
| X | 2 ) )
A
| 2
!FTER REDUCTION OF THE 2 VALUES AND SOME REAR
RANGEMENT WE OBTAIN
) ) | )
A
| X
OR
) ) | X | )
A
4HIS IS DIRECTLY THE DIFFERENTIAL CURRENT 4HE CURRENT OF
THE REMOTE LINE END IS REQUIRED IN ORDER TO OBTAIN THE
STABILIZING CURRENT 4HE FOLLOWING APPLIES
) ) ) ) | X | )
A
)
&INALLY ALL QUANTITIES WHICH ARE NEEDED FOR THE DIFFER
ENTIAL PROTECTION PROCESSING ARE AVAILABLE
)
DIFF
\ ) ) \ \ | X | )
A
\
)
STAB
\ ) \ \ ) \ \ ) \\ | X | )
A
) \
&OR LINES WITH THREE TERMINAL TEED LINES EACH SUB
STATION HAS TWO DIFFERENTIAL RELAYS EACH CONNECTED
THROUGH A PILOT WIRE PAIR TO ONE OF THE OTHER ENDS )N
ORDER TO OBTAIN THE DIFFERENTIAL AND STABILIZING CURRENT
FOR ALL THE THREE LINE ENDS EACH OF THE TWO LOCAL RELAYS
MUST BE FED BY THE PILOT WIRE CURRENT OF THE OTHER
RELAY 4HE DIFFERENTIAL AND STABILIZING CURRENT CAN BE
CALCULATED IN EACH OF THE RELAYS FROM THE PILOT WIRE
CURRENT )
A
AND THE LOCAL CURRENT ) USING THE SAME
METHOD AS DESCRIBED ABOVE FOR TWOTERMINAL LINES
CF 3ECTION
#4 #4
3TATION 3TATION
) )
) )
A
)
B
2
B
)
A
)
)
B
2
B
2
A
2
A
2
X
2
X
I I
&IGURE $IFFERENTIAL PROTECTION USING TWO PILOT WIRES FUNCTION SCHEME