5/28/2018 The Formula for Knee Point Voltage Is
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The formula for Knee Point Voltage is:
Vkp = K * If/CTR * (RCT + RL + RR)
in which,
K = Constant, conventionally taken as 2.0
Vkp = The minimum Knee Point Voltage
If = Maximum Fault Current at the location, in Amperes
CTR = CT Ratio
RCT = CT Secondary Winding Resistnace, in Ohms
RL = 2-way Lead Resistance, in Ohms
RR = Relay Burden, in Ohms
KNEE POINT VOLTAGE TEST....Q.What is the knee point voltage?A.It
is the point at where 50% of secondary current increases in order
to increase 10% of voltage insecondary winding.
Q. Why it is important in only PS class? what about metering
class and over current andearth fault
protection (5P class) ?A. for metering and 5P class, secondary
circuit is small so the voltage drop across burden is
small,itimplies that voltage across secondary winding is small,then
it will not saturate in normal case.But inPS(the class used only
for differential protection purpose) class secondary burden is very
high sothe voltage across secondary winding is also high which may
be divert our CT to saturate position innormal condition also,so
generally by designing specially(secondary current only 1amps, as
to minimizeohimic loss in secondary circuit). So generally
manufacture provide knee point voltage maximumcompared to other
classes).
Q. What is burden?A.The load connected to secondary winding (sum
of relay impedance or meter impedance and controlcable resistance
and CT secondary winding resistance).
Q. why we are doing this test?
A.We are just checking the manufacture specification is correct
or not.
