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Question
how to calculate knee point voltage of current transformer?
 Question Submitted By :: Dilip
I also faced this Question!!     Rank Answer Posted By  
 
  Re: how to calculate knee point voltage of current transformer?
Answer
# 1
10 % increase in voltage gives you 50 % increase in
excitation current is called knee point voltage.
To measure this first demagnetise the CT and apply
voltage gradually from secondary keeping primary
winding open circuited. while doing this above
phenomeneo will be obsesrved.
 
Is This Answer Correct ?    126 Yes 21 No
Rajendra Soni
 
  Re: how to calculate knee point voltage of current transformer?
Answer
# 2
to calculate knee point voltage of current transformer by
formal
Vknee point = Isecandary(Rct+ Rcable+ Rburden)
where
Rct= resistenc of current transformer
Rcable =resistenc of cable =2Rcable
Rburden =burden of relay
IF Vknee point< manufacture voltage of current transformer
current transformer is good
IF Vknee point> manufacture voltage of current transformer
current transformer must be change
 
Is This Answer Correct ?    64 Yes 37 No
Saleh Abdulsalam Trabelsi
 
 
 
  Re: how to calculate knee point voltage of current transformer?
Answer
# 3
10 % increase in voltage gives you nearly 50 % increase in
excitation current is called knee point voltage or
breakdown voltage.
To measure this first demagnetise the CT and apply
voltage gradually(like 10% or 20% step) from secondary
keeping primary should be open circuited.
 
Is This Answer Correct ?    40 Yes 10 No
Balamurugan R
 
  Re: how to calculate knee point voltage of current transformer?
Answer
# 4
For calculating knee point voltage there are two methods,
they are
1.Low method
2.High method


1.low impedance method
Vk=24In(Rct+2Rl)

2.High impedance method
Vk=4IsRs

Then, Rs=If/Is(Rct+Rl)
where
Is,In-Rated Sec. Current of CT
Rct-CT resistance
Rl-Max. one way lead reistance from CT to Relay
 
Is This Answer Correct ?    9 Yes 21 No
S.arunkumar
 
  Re: how to calculate knee point voltage of current transformer?
Answer
# 5
The knee-point voltage will depend on the size and class of
the CT, and it can be very high on some CTs (several
hundreds of volts) Be very cautious, you will be working on
live equipment during the test.

Now you can just plot the current and voltage values on a
graph. This is the mag-curve of the CT. IEC classify the
knee point voltage as the point where a 10% increase in
voltage results in a 50% increase in current.
 
Is This Answer Correct ?    11 Yes 2 No
Pankaj
 
  Re: how to calculate knee point voltage of current transformer?
Answer
# 6
I am not familiar with the term saturation test but it seems
from the above posts like an exciting current test to
obtain the knee-point of the CT. With this test you can also
obtain the exciting curve (mag-curve) of the CT.

You'll need a variable voltage, (easiest obtain from a
variac - must be able to obtain the suitable voltage) a
voltmeter, an ammeter, a pencil and paper.You'll have to
measure the voltage and the current while you do the test.
Apply the voltage to the secondary winding with the primary
and other windings being open-circuited. Increase the
voltage until you reach a point where a small increase in
voltage results in a big increase in current.
Now slowly decrease the voltage to a few measuring points,
while you measure the current. (write it down) Something like:

205V - 1.1A
200V - 0.6A
190V - 0.238A
180V - 0.149A
170V - 0.114A
150V - 0.08A
125V - 0.061A
100V - 0.049A
50V - 0.028V
25V - 0.017A
0V - 0A

You have to decrease your voltage slowly to zero volt to
demagnetize the CT-core.

BEWARE The knee-point voltage will depend on the size and
class of the CT, and it can be very high on some CTs
(several hundreds of volts) Be very cautious, you will be
working on live equipment during the test.

Now you can just plot the current and voltage values on a
graph. This is the mag-curve of the CT. IEC classify the
knee point voltage as the point where a 10% increase in
voltage results in a 50% increase in current.
 
Is This Answer Correct ?    30 Yes 9 No
Pankaj Joshi
 
  Re: how to calculate knee point voltage of current transformer?
Answer
# 7
The knee-point voltage will depend on the size and class of
the CT, and it can be very high on some CTs Be very
cautious, you will be working on
live equipment during the test.

Now you can just plot the current and voltage values on a
graph. This is the mag-curve of the CT. IEC classify the
knee point voltage as the point where a 10% increase in
voltage results in a 50% increase in current.
 
Is This Answer Correct ?    5 Yes 5 No
Narendra Solanki
 
  Re: how to calculate knee point voltage of current transformer?
Answer
# 8
Is the answer given below is correct?
For calculating knee point voltage there are two methods,
they are
1.Low method
2.High method


1.low impedance method
Vk=24In(Rct+2Rl)

2.High impedance method
Vk=4IsRs

Then, Rs=If/Is(Rct+Rl)
where
Is,In-Rated Sec. Current of CT
Rct-CT resistance
Rl-Max. one way lead reistance from CT to Relay
 
Is This Answer Correct ?    7 Yes 4 No
Sunny
 
  Re: how to calculate knee point voltage of current transformer?
Answer
# 9
knee point Voltage = Burden x aLf
-------------
Class

E.g:10VA 5P20 CT.

To Get the Knee Point Voltage

=> 10VA x 20
--------- = 10 x 20
5P ------- = 40V
5
 
Is This Answer Correct ?    21 Yes 44 No
Gusu
 
  Re: how to calculate knee point voltage of current transformer?
Answer
# 10
Calculation:
1) Fault Current at Primary Side of CT (kA) = MVA/*100(%
Z*1.732*KLV) 7.55
2) Fault Current at Secondary Side of CT, If (A)=
6.29
3) Cable Lead Resistance, RL (Ohm)= 1.02


4) Knee Point Voltage Vk (Volt) = 2If(RCT+2RL)
101.2


REMARKS : -
Knee Point Voltage of CT provided by CGL for REF CT = 115V
and for Differental CT = 200V are in order.
 
Is This Answer Correct ?    6 Yes 4 No
Arvind Negi
 

 
 
 
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