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Question
how to calculate knee point voltage of current transformer?
 Question Submitted By :: Electrical-Engineering
I also faced this Question!!     Answer Posted By  
 
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 ?    129 Yes 21 No
Rajendra Soni
 
Answer
# 2
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 ?    42 Yes 10 No
Balamurugan R
 
 
 
Answer
# 3
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 ?    66 Yes 37 No
Saleh Abdulsalam Trabelsi
 
Answer
# 4
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
 
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
 
Answer
# 6
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 ?    4 Yes 0 No
Shakir Mehmood
 
Answer
# 7
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
 
Answer
# 8
CT Knee Point Voltage Calculation Sheet



Primary Secondary Volt
Resistance Resistance Length
In MVA In AMP In kV In ohm In
ohm/km in Meter In %
1 CT ratio of the trasformer REF Protection -
1200 1 - - - - -
2 Transformer Rating, MVA 20 - - -
- - - -
3 % Impedance, %Z - - - -
- - - 13.9
4 Voltage of Line Side, KVLV - - -
11 - - - -
5 CT Secondary Resistance, RCT (ohm) - -
- - 6 - - -
6 Resistance of control cable of 2.5 Sq. mm.
- 8.5 -
7 Loop Length of Control Cable - - -
- - - 120


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 3 No
Arvind Negi
 
Answer
# 9
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
 
Answer
# 10
Knee point voltage, Vkn = (VA X ALF)/ In

VA= Burden =30 (Say)
ALF= Accu. Limit. Factor = 20 (Say)
In= Rated Secondar Current =5 amp (Say)
Then Vkn= 120 volts

If CT Ratio is 2000:5 then In=5
Class is 5P20 then ALF=20, P=Protection CT
 
Is This Answer Correct ?    3 Yes 1 No
Arup
 

 
 
 
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