4 DISTANCE PROTECTION REQUIREMENTS
Zone 1 Reach Point Accuracy (RPA)
V
k
³
K
RPA
´
I
fZ1
(1+ X/R)(R
CT
+ R
L
)
where:
● V
k
= Required CT knee point voltage (volts)
● K
RPA
= Fixed dimensioning factor = 0.6
●
I
fZ1
= Maximum secondary phase fault current at Zone 1 reach point (A)
● X/R = Primary system reactance/resistance ratio
● R
CT
= CT secondary winding resistance
● R
L
= Single lead resistance from CT to IED
Zone 1 Close-up Fault Operation
An additional calculation must be performed for all cables and lines where the source impedance ratio (SIR) may
be less than SIR = 2.
V
k
³
K
max
I
fmax
´
I(R
CT
+ R
L
)
where:
K
max
= Fixed dimensioning factor = 1.4
I
fmax
= Maximum secondary phase fault current
Then, the highest of the two calculated knee points must be used.
Note:
It is not necessary to repeat the calculation for earth faults, as the phase reach calculation is the worst-case for CT
dimensioning.
Chapter 16 - Current Transformer Requirements P543i/P545i
422 P54x1i-TM-EN-1
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