5/23/13 CR4 - Thread: How to Calculate CT Ratio and PT Ratio

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AnonymousPoster

How to Calculate CT Ratio and PT Ratio10/16/2009 1:08 AM

Please tell me how to calculate the CT ratio and PT ratio for 11kV?

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TornadoGuru

Join Date: May 2009Location: Ketchikan, AK,USAPosts: 12931Good Answers: 463

Re: How to Calculate CT Ratio and PT Ratio10/16/2009 11:47 PM

There may be variations, but usually the desired output from a CT is 0-5 amps. Determine the amps of the current passing through the centerof the CT, and select the ratio accordongly. For PT, determine thevoltage applied to the primary coil, and the output voltage you need.Select a ratio to match.

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jmuellerGuru

Join Date: Jul 2006Posts: 916Good Answers: 74

Re: How to Calculate CT Ratio and PT Ratio10/18/2009 12:14 AM

Friend,

When you are connecting to a meter, you also would need the meter'sfaceplate to be printed with the proper scaled values. On ourswitchgear, the incoming voltage is nominally 12.5kV and the PT ratiois 12,000:120 or 100:1. This gives a secondary voltage that is well-within the normal insulation range of building wires. Before applyingany PT's, it would be well to determine the actual type of connectionfor the incoming power--Wye (Star) or Delta, because this will alsodictate the ratio and connections for the PT's.

Similarly to this, for CT's the same scaling of the meter faceplate is

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5/23/13 CR4 - Thread: How to Calculate CT Ratio and PT Ratio

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needed. It may be ordered with a 0-5A movement, but scaled so the5A corresponds to the full-scale primary current of the PT.

--JMM

brichGuru

Join Date: Oct 2009Location: sometimesWales,UK.. sometimesLibya!Posts: 1071Good Answers: 76

Re: How to Calculate CT Ratio and PT Ratio10/17/2009 5:16 AM

CT output is 0 to 5 amps, if you are connecting to a map chart recordersuch as a Bristol. The idea is to keep the output from CT to chartrecorder at 2.5 amps, so with that in mind read thefollowing...........................Current Transformers Electronic andmechanical measuring devices monitor the amperage in theswitchboard. To monitor the motor current, it must be reduced to alevel that is easier to measure. If the amperage drawn by the motor isnot reduced, measuring devices have to be large and cumbersome inorder to house components capable of withstanding high currents. Mostsubmersible pump system measuring devices (motor controllers) aredesigned to operate from 0 A to 5 A. To reduce or scale-down theactual current in the switchboard to an acceptable level, currenttransformers are used. Current Transformer Ratios The currenttransformers should be sized so that under normal operating conditions,secondary control amperage levels are between 2.5 Aand 3.5 A. This isdone to obtain the best motor controller performance and to have thepen track in the center of the recording ammeter char t. The currenttransformer size calculation is — current transformer size = expectedamperage through the switchboard x 1.6................nextpart............Example Motor and Switchboard Information 39 A, 990 Vrequired motor surface voltage; MDFH size 3, 1,500 V switchboard.Using current transformer size = expected amperage through theswitchboard x 1.6, current transformer size = 39 A x 1.6 = 62 A Choosea current transformer with a ratio as close to 62:5 as possible. (In thiscase, a 75:5 ratio.) (39/75) x 5 = 2.6 A hope this helps

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AnonymousPoster

Re: How to Calculate CT Ratio and PT Ratio11/06/2009 7:55 AM

Hi, Could you please tell me where the 1.6 factor (which is applied tothe load current in your calculation) comes from? is this just applied toallow for flexibility in full load current? or have i missed something?

brichGuru

Join Date: Oct 2009Location: sometimesWales,UK.. sometimesLibya!Posts: 1071Good Answers: 76

Re: How to Calculate CT Ratio and PT Ratio11/06/2009 8:25 AM

it is the co-efficient used to calculate the correct answer. I askedthat question years ago, that was the answer, so I used it. hopethis helps

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Anonymous Re: How to Calculate CT Ratio and PT

5/23/13 CR4 - Thread: How to Calculate CT Ratio and PT Ratio

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Poster Ratio11/06/2009 9:21 AM

HAHA, thanks, about as much help as everyone else who i haveasked. (None).

brichGuru

Join Date: Oct 2009Location: sometimesWales,UK.. sometimesLibya!Posts: 1071Good Answers: 76

Re: How to Calculate CT Ratio and PTRatio11/06/2009 9:30 AM

My friend, if you don't like the answer, there's not much I can doabout it, your question "what is the 1.6 for?" was answered. It ispart of the formula I have used for many years without fail. Youhave asked this forum to which many people reply free of chargeto help other people, technical and non-technical to find answersto their technical problems. If you are not happy with the answeror ridicule the person suppling the answer, then don't use thisforum, and remain ignorant. All you need to know is that withoutthe "1.6" in the formula you will never get the correct answer yourequire. So unless you can prove to me you know more than I doon this matter, I have nothing more to say. I am always open tolearn, but I suspect you are not, hence your comments.

__________________The square root of nothing is what you make it!

AnonymousPoster

Re: How to Calculate CT Ratio and PTRatio11/06/2009 9:45 AM

Dont get me wrong, i appreciate the reply, but as anengineering student, submitting technical information toprofessional institutes to demonstrate an understanding of basicengineering theory, i am trying to gain an understanding of WHYa factor of 1.6 is applied to a current, not simply "that it mustbe done". Where this information is sourced is not important, butunderstanding it is key.

brichGuru

Join Date: Oct 2009Location: sometimesWales,UK.. sometimesLibya!Posts: 1071Good Answers: 76

Re: How to Calculate CT Ratio and PTRatio11/06/2009 9:51 AM

that my friend is a FAR FAR better reply, rather than theprevious answer. I thought you might be in college/university,so give me time and will TRY and get a FULLER answer for you.However I would ask this.... why do you apply 3.14 to everyequation when calculating the circumference of a circle?Because it's "Pie" and we know it works, same theory, but I willlook for answer for you OK?

__________________The square root of nothing is what you make it!

jmuellerGuru

Join Date: Jul 2006Posts: 916Good Answers: 74

Re: How to Calculate CT Ratio and PTRatio11/08/2009 2:02 AM

Guest,

5/23/13 CR4 - Thread: How to Calculate CT Ratio and PT Ratio

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In reading the exchange you are part of, I believe the answerof the 1.6 factor is embedded in the original answer. IF youare wanting to get the current reading marginally above themidpoint of the meter range (as suggested for keeping a chartrecording pen near the middle of its full scale), then the 1.6 issimply a scaling factor for shortening the range of the full-scale reading. That may be a valid approach, depending on thenature of your data needs. Problems with this approach includean increased % reading error and decreased ability to readvery low loads.

All-in-all, your approach needs to be determined problem beingsolved. If accuracy is important, scaling the reading near full-scale is better. If ability to read overloads is important, scalingfor the normal load at (say) 60% of full-scale is morereasonable. Whatever you do, make sure that any assumptionsand limits to the validity of the readings are included in yourdocumentation. That also is good basic engineering practice.

--JMM

brichGuru

Join Date: Oct 2009Location: sometimesWales,UK.. sometimesLibya!Posts: 1071Good Answers: 76

Re: How to Calculate CT Ratio and PT Ratio11/06/2009 10:31 AM

would you like to give me your email address? mine iscontact@submersiblepumpstandards.com

__________________The square root of nothing is what you make it!

AnonymousPoster

Re: How to Calculate CT Ratio and PT Ratio11/06/2009 11:46 AM

my email address is rgold@acdp.co.uk

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