Areva MiCOM P441, Техническое руководство

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Application Notes P44x/EN AP/E33
MiCOM P441/P442 & P444 Page 189/220
7. CURRENT TRANSFORMER REQUIREMENTS
Two calculations must be performed – once for the three phase fault current at the zone 1
reach, and once for earth (ground) faults. The highest of the two calculated Vk voltages
must be used:
7.1 CT Knee Point Voltage for Phase Fault Distance Protection
Vk
≥
K
RPA
x I
F Z1
x (1+ X/R) . (R
CT
+ R
L
)
Where:
Vk = Required CT knee point voltage (volts),
K
RPA
= Fixed dimensioning factor = always 0.6
I
F Z1
= Max. secondary phase fault current at Zone 1 reach point (A),
X/R = Primary system reactance / resistance ratio,
R
CT
= CT secondary winding resistance (
Ω
),
R
L
= Single lead resistance from CT to relay (
Ω
).
7.2 CT Knee Point Voltage for Earth Fault Distance Protection
Vk
≥
K
RPA
x I
Fe Z1
x (1+ Xe/Re) . (R
CT
+ 2R
L
)
Where:
K
RPA
= Fixed dimensioning factor = always 0.6
I
Fe Z1
= Max. secondary earth fault current at Zone 1 reach point (A),
Xe/Re = Primary system reactance / resistance ratio for earth loop.
7.3 Recommended CT classes (British and IEC)
Class X current transformers with a knee point voltage greater or equal than that calculated
can be used.
Class 5P protection CTs can be used, noting that the knee point voltage equivalent these
offer can be approximated from:
Vk = (VA x ALF) / In + (R
CT
x ALF x In)
Where:
VA = Voltampere burden rating,
ALF = Accuracy Limit Factor,
In = CT nominal secondary current.
7.4 Determining Vk for an IEEE “C" class CT
Where American/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 R
CT
]
8. DDB DESCRIPTION FOR ALL TYPES P441/P442 & P444 MODELS