P54x/EN AP/La4 Application Notes
(AP) 6-84
MiCOM P543, P544, P545 & P546
AP
Or
K 65
This is valid for (If x X/R) 600
For higher (If x X/R) up to 1600:
K = 256
6.3 Zone 1 reach point accuracy (RPA)
Vk K
RPA
x IF Z1 x (1+ X/R). (RCT + RL)
Where:
Vk = Required CT knee point voltage (volts)
K
RPA
= Fixed dimensioning factor = always 0.6
IF
Z1
= Max. secondary phase fault current at Zone 1 reach point (A)
X/R = Primary system reactance/resistance ratio
RCT = CT secondary winding resistance ()
RL = Single lead resistance from CT to relay ()
6.4 Zone 1 close-up fault operation
An additional
calculation must be performed for all cables, and any lines where the source
impedance ratio might be less than SIR = 2.
Vk Kmax x IF max x (RCT + RL)
Where:
Kmax = Fixed dimensioning factor = always 1.4
IF max = Max. secondary phase fault current (A).
Then, the highest of the two calculated knee points must be used. Note that it is not
necessary to repeat the calculation for earth faults, as the phase reach calculation (3) is the
worst-case for CT dimensioning.
6.5 Time delayed distance zones
Whe
n a time delayed distance zone is being used, there is no need to calculate the required
Vk separately. This is due to the employed time delay (usually more than 3 times the primary
time constant for a fault at the remote bus of the protected feeder), which overrides the
transient conditions. When it is insisted to do some calculations for the time delayed distance
zone, then we should use the following equation
Vk > If (RCT + RL)
Where If is the current for a fault at the remote bus of the protected feeder (in other words,
the through fault current for the current differential function)
6.6 Determining Vk for an IEEE “C" class CT
Where Ameri
can/IEEE standards are used to specify CTs, the C class voltage rating can be
checked to determine the equivalent Vk (knee point voltage according to IEC). The
equivalence formula is:
Vk = [(C rating in volts) x 1.05] + [100 x RCT]
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