Protection Functions
PRS-7367 149
triggered by phase overcurrent protection operation phase judged by 94Trip protection; For earth
protection, the fault distance can be calculated only when both of the Phase N and FPS are valid.
3.28.2.2 Fault distance calculation
As soon as a fault condition is recognized, the fault distance calculation is started using one of the
seven impedance measuring elements. 21FL employs independent algorithms for each fault type
in order to achieve optimal performance.
Single-phase earth faults
When the individual earth faults are located at different feeders, the voltage and current of fault
phase will be changed, also, the zero CT will affect the result of normal phases, this is up to the
ratio of zero impendence to positive impendence in fault position. So in the single-phase earth
faults, the zero impendence will be considered. The calculation formula is as following:
Up/(Ip+K*3I0),K=(Z0-Z1)/(3*Z1) (p=A,B,C)
The detail is shown in table 3.27.1
Two-phase earth faults
It is used for the phase-to-phase short circuit faults and also for the phase-to-phase-to-earth faults
if the individual earth-faults are located in the same feeder. The calculation formula is as following:
Upp/(Ipp) (pp=AB, BC, CA)
The detail is shown in table 3.27.1
Three-phase earth faults
It is used only for the three-phase short circuit the impedances, the three phases are symmetrical,
user can use only two phases to calculate the impendence. In 21FL, the phase BC is chosen. The
calculation formula is as following:
UBC/(IBC)
The detail is shown in table 3.27.1
Calculation formula
The fault distance is calculated by a calculation formula, as shown in following:
L = LineLe*Im(Zx)/(PosZ*sin(PosAng))
Where, the Im(Zx) is the imaginary part of Zx,
is the angle of voltage U
and current I.
3.28.3 21FL Application Scope
The main objective of line protection and monitoring terminals is fast, selective and reliable
operation for faults on a protected line section. Besides this, information on the distance to fault is
very important for those involved in operation and maintenance. Reliable information on the fault
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