Vk is knee point of CT it mean when 10% increase in voltage
varies the current 50% at that point we determine the knee
point voltage. In othe words excitation test of CTs,
connect a single phase variable power supply leads to the
CT seccondary leads k & l or s1, s2 along with ampere meter
& voltage meter & than apply voltage start from zero &
raise incremental simultaneously draw a graph on each
increments by plotting current on X-axis & voltage on Y-
axis.
Magnetizing current is those which is used in core & tank
of transformer due to leakage flux, which is furthur
devided into hysteris losses & eddy current losses.
We can find magnetizing current by open circuit test.
5P10 means: 5 for composite errors in percentage, P for
protection & 10 mean overlaod factor by multiply 10 times
in others words CT will not saturate by passing 10 times
higher current.
knee point means at which the core tends towards
saturation .magnetising current means to produce flux in
the core it will take some current it is called magnetising
current.before knee point the graph between Vk and Imag
will be linear,at which point linear curve getting
disturbed .
if it is 5P20 we can calculate from following formula
1.2*VA*ALF/rated secondary current.
magnetizing current it will be mA it may be .5 or .7 of current
Dear friend,
Knee Point voltage is nothing but a point at which the CT
saturates. Thatsy its always better to design the CTs which
can operate without saturation even at a voltage appears in
the CT during fault condition. Pls find below the following
formula for calculating the Knee point voltage.
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
ugrasen.vishawakrma@gmail.com
the resstance of a thermistor is 5000 ohm at 25degreeC
and coefficient is .04/degCel. A measurement with lead
resitance of 10 ohm will cause an error of
If there is unit power factor in a power house and a capacitor bank used in a factory at resistive load . In this case, will there leading power factor in power house across that factory.
WHICH IS IS BEST OPTION FOR 33 KV LINE INCOMER.I AM
DESINGING THE PANEL FOR NEW PROJECTS AND I WOULD LIKE TO GET
YOUR OPINION ON THIS
VCB OR SF6.
PLEASE HELP ME WITH YOUR REPLIES.
THANKS
D P SINGH
Site Map
