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Participant

Join Date: Jan 2011
Posts: 2

Knee Point Voltage For Ct

01/29/2011 5:11 AM

i need to calculate knee point voltage for ct for classes X , METERING and 5p10. what is the accuracy formula for those classes in IEC standards. and i need exciting current calculation also. I referred lot of formulas but which formula is correct for that. for class 5p10 and metering, knee point voltage is not available in name plate details, how to test in site for those classes without given of knee point voltage.

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Power-User

Join Date: Jun 2010
Posts: 348
#1

Re: knee point voltage for ct

01/29/2011 6:10 AM

The manufacturer would have provided the magnetizing characteristic curves in the manuals if not ask for it from the manufacturer.

If you have to carry out the test at site; connect a variable AC supply at the secondary of CT, a voltmeter and an ammeter connected in the circuit would provide the readings so you can plot a graph.

In 5p10 the first element 5P indicates accuracy class, last number 10 gives you composite error.

Composite error: 5P is 5%, 10 P is 10% and 15P is 15%

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Associate

Join Date: Jun 2019
Posts: 50
#10

Re: knee point voltage for ct

07/04/2019 9:48 AM

The point at which when the voltage is increased 10% and correspondingly the current increase 50%. The point is called the knee point voltage.

The knee point voltage has importance when the CT is used for zone or differential protection.

Guru

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#11

Re: knee point voltage for ct

07/11/2019 3:01 PM

WARNING!!!

LINKS IN THE ABOVE POST GO TO A VIRUS INFECTED SITE.

DO NOT CLICK ON THEM!

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Guru

Join Date: Sep 2009
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#2

Re: knee point voltage for ct

01/29/2011 6:44 AM
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Associate

Join Date: Jun 2019
Posts: 50
#8

Re: knee point voltage for ct

06/29/2019 6:10 AM
Guru

Join Date: Feb 2009
Location: Houston, USA
Posts: 947
#3

Re: Knee Point Voltage For Ct

01/29/2011 9:26 AM

For the knee point calculation of class X (PS) CT, see this thread (especially the comment # 3 and # 6):

- MS

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Guru

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#4

Re: Knee Point Voltage For Ct

01/30/2011 9:02 AM

Knee Point Voltage needs to be calculated only for Class X (or) Class PS CTs.

Associate

Join Date: Jun 2019
Posts: 50
#6

Re: Knee Point Voltage For Ct

06/29/2019 6:08 AM

The knee point voltage of CT is very important parameter for the CT used for differential protection. The system fault current and the burden of the CT should be known for calculating the knee point voltage. The CT manufacturer asks for the following information before designing the PS class CT.

1. System fault current

2. RCT- Lead wire resistance and the relay burden

Guru

Join Date: Oct 2009
Posts: 2061
#9

Re: Knee Point Voltage For Ct

06/29/2019 7:27 AM

In what context you have made this comment in response to my above comment? What is the relevance?

Participant

Join Date: Nov 2009
Posts: 3
#5

Re: Knee Point Voltage For Ct

01/31/2011 4:31 AM

The knee point voltage is defined as that minimum sinusoidal e.m.f. (r.m.s.) at rated power frequency when applied to the secondary terminals of the transformer, all other terminals being open-circuited, which when increased by 10 % causes the r.m.s. exciting current to increase by no more than 50 %

So, if you don't have the magnetizing characteristic curves from the manufacturer, you will need to obtain with a variable AC supply (measuring the voltage and the current in the secondary circuit).

A class 5P10, is a different protection core. In this case it is suppose that you know the burden, and the internal resistance of the secondary, then:

emf = FLP * Isn * SQRT((Rct+Rb)2+Xb2)

where: emf -secondary limiting e.m.f. the product of the accuracy limit factor, the rated secondary current and the vectorial sum of the rated burden and the impedance of the secondary winding. THIS emf IS NOT THE KNEE POINT VOLTAGE

FLP - accuracy limit factor = 10

Isn - Rated secondary current

Rct - secondary winding d.c. resistance in ohms corrected to 75 °C

Rb - rated value of the secondary connected resistive burden in ohms.

Xb – rated value of he secondary connected inductance burden in ohms

(The burden usually is expressed in VA with a power factor 0.8)

And the knee point voltage we can be estimated as:

Vkp = (15/20) * emf

Vkp = Kx * Isn * (Rct + Rb) * Isn

(where Kx for the 5P10 class CT is about 7.5)

Associate

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Posts: 50
#7

Re: Knee Point Voltage For Ct

06/29/2019 6:09 AM
Participant

Join Date: Feb 2020
Posts: 3
#12

Re: Knee Point Voltage For Ct

02/20/2020 11:51 PM

When 10% increase in voltage gives you 50% increase in excitation current, then that voltage is known as knee point voltage. To measure the knee point voltage, At first demagnetize the CT and then apply voltage gradually from secondary keeping primary winding open-circuited. While doing this, the above phenomena will be observed then that voltage is recorded as the knee point voltage of the current transformer

Knee point calculation of CT

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