Current Transformer Requirements; Ct Knee Point Voltage For Phase Fault Distance Protection; Ct Knee Point Voltage For Earth Fault Distance Protection; Recommended Ct Classes (British And Iec) - GE MiCOM P40 Agile Technical Manual

P44x/EN ST/Hb6
(ST) 4-78
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
Where:
Vk =
K =
RPA
I =
F Z1
X/R =
R =
CT
R =
L
5.2

CT Knee Point Voltage for Earth Fault Distance Protection

≥
Vk
Where:
K =
RPA
I =
Fe Z1
Xe/Re =
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 =
Where:
VA =
ALF =
In =
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 =
K x I x (1+ X/R) . (R
RPA F Z1
Required CT knee point voltage (volts),
Fixed dimensioning factor
Max. secondary phase fault current at Zone 1 reach point (A),
Primary system reactance / resistance ratio,
CT secondary winding resistance (Ω),
Single lead resistance from CT to relay (Ω).
K x I x (1+ Xe/Re) . (R
RPA Fe Z1
Fixed dimensioning factor
Max. secondary earth fault current at Zone 1 reach point (A),
Primary system reactance / resistance ratio for earth loop.
(VA x ALF) / In + (R x ALF x In)
CT
Volt-ampere load rating,
Accuracy Limit Factor,
CT nominal secondary current.
[ (C rating in volts) x 1.05 ] + [ 100 x R
+ R )
CT L
= always 0.6
+ 2R )
CT L
= always 0.6
]
CT
Setting
MiCOM P40 Agile P442, P444