One common fundamental frequency bias current is used. The bias current is the
magnitude of the highest measured current in the protected circuit. The bias current is
not allowed to drop instantaneously, instead, it decays exponentially with a predefined
time constant. These principles make the differential IED more secure, with less risk
to operate for external faults. The “maximum” principle brings as well more meaning
to the breakpoint settings of the operate-restrain characteristic.
max( 1 , 2 , 3 , 1 , 2 , 3 )Ibias IL n IL n IL n IL t IL t IL t=
EQUATION1666 V1 EN (Equation 31)
IL1n IL1t
IL1n
IL1t
Idiff
IEC07000018_3_en.vsd
IEC07000018 V3 EN
Figure 51: Internal fault
IL1n IL1t
External fault: IL1n = - IL1t
IL1nIL1t
Idiff = 0
en07000019-2.vsd
IEC07000019 V2 EN
Figure 52: External fault
1MRK 502 048-UEN A Section 6
Differential protection
127
Technical manual
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