GE P444 User Manual

P
44x/EN ST/Hb6
Setting
(ST) 4
-78 MiCOM P40 Agile
P442, P444
5 CURRENT TRANSFORMER REQUIREMENTS
Two calculations must be performed – one for the three phase fault current at the zone 1
reach, and one for earth (ground) faults. The higher of the two calculated Vk voltages must
be used:
5.1 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).
5.2 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.
5.3 Recommended CT classes (British and IEC)
Class X current transformers with a knee point voltage greater than or equal to the
calculated one can be used.
Class 5P protection CTs can be used, noting that the equivalent knee point voltage they offer
can be approximated from:
Vk = (VA x ALF) / In + (R
CT
x ALF x In)
Where:
VA = Volt-ampere load rating,
ALF = Accuracy Limit Factor,
In = CT nominal secondary current.
5.4 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 the IEC). The
equivalence formula is:
Vk = [ (C rating in volts) x 1.05 ] + [ 100 x R
CT
]

GE P444 Specifications

General

Brand	GE
Model	P444
Category	Protection Device
Language	English