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What is Vk and Imag i.e Knee point voltage & Magnetising 
How to calculate above values?
How to define accuracy class 5P10?
 Question Submitted By :: Electrical-Engineering
I also faced this Question!!     Answer Posted By  
# 1
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-
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.
Is This Answer Correct ?    77 Yes 9 No
Zahid Ali
# 2
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 .
Is This Answer Correct ?    29 Yes 9 No
V .selvakumar
# 3
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
Is This Answer Correct ?    13 Yes 2 No
# 4
Formula for vk is as follows


where if is fault current
rct is ct secondary resistance rl is lead resistance
Is This Answer Correct ?    10 Yes 3 No
# 5
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
Is This Answer Correct ?    6 Yes 0 No

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