CT Reference Voltage

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BlueHornet
Posts: 56
Joined: Fri Dec 16, 2011 10:42 pm
Location: Victoria, Australia

CT Reference Voltage

Post by BlueHornet » Fri Jul 08, 2016 12:52 am

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Jameson
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Re: CT Reference Voltage

Post by Jameson » Fri Jul 08, 2016 9:22 am

Hello BlueHornet,

I would like to help you out but Im afraid I do not understand the nature of your question. Our Current Transformers output between 0 and 26.6mA on the secondary wire. This is in a ratio to the input on the primary wire (from 0 to 200 amps for example). The voltage output on the secondary is not at all consistent (open circuit is very different than when the CT is connected to a meter). As far as I know there is no voltage reference, I can ask around if you need more information.

Burden Resistance: 5 ohms
200 Amp CT: 7500 windings. Less windings on larger amperage CTs.

Somehow I have the feeling this does not answer your question.
Jameson
EKM METERING
http://www.ekmmetering.com
831.425.7371

BlueHornet
Posts: 56
Joined: Fri Dec 16, 2011 10:42 pm
Location: Victoria, Australia

Re: CT Reference Voltage

Post by BlueHornet » Fri Jul 08, 2016 8:09 pm

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BlueHornet
Posts: 56
Joined: Fri Dec 16, 2011 10:42 pm
Location: Victoria, Australia

Re: CT Reference Voltage

Post by BlueHornet » Tue Jul 12, 2016 2:21 am

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Jameson
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Re: CT Reference Voltage

Post by Jameson » Wed Jul 13, 2016 10:07 am

Hello BlueHornet,

I am still not sure how to answer your question. But I am guessing we are on different pages. For one thing it sounds like you think our Current Transformers have an internal resistor. They do not. So I may be using the Burden Resistor term wrong. Maybe it would be better to say that when our CT is connected to an Omnimeter there is a 5 Ohm burden resistance. Again I am not sure of the correct terminology here.

You are probably correct that IF you put a 5 Ohm resistor across the leads, the output voltage would be 0 to 133mV.

As far as I know 5amp CTs are built the same way and do not have an internal resistor built in.

Hope this helps!
Jameson
EKM METERING
http://www.ekmmetering.com
831.425.7371

BlueHornet
Posts: 56
Joined: Fri Dec 16, 2011 10:42 pm
Location: Victoria, Australia

Re: CT Reference Voltage

Post by BlueHornet » Wed Jul 13, 2016 2:25 pm

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Jameson
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Re: CT Reference Voltage

Post by Jameson » Wed Jul 13, 2016 2:55 pm

Hello BlueHornet,

As I mentioned before, I may be using the Burden Resistor term wrong. Maybe it would be better to say that when our CT is connected to an Omnimeter there is a 5 Ohm burden resistance. Again I am not sure of the correct terminology here.

Meaning I am not sure of the correct terminology, but the burden resistance of our meter and CT combination is 5 ohms. I do not know what the proper terminology of what our CT burden resistance is as a standalone product when not connected to a meter. If you have a way of testing a CT to better answer your question I would be happy to help and furnish you with the information that I find in testing. I am sorry if this has caused some sort of an issue.
Jameson
EKM METERING
http://www.ekmmetering.com
831.425.7371

BlueHornet
Posts: 56
Joined: Fri Dec 16, 2011 10:42 pm
Location: Victoria, Australia

Re: CT Reference Voltage

Post by BlueHornet » Wed Jul 13, 2016 11:58 pm

No Issue at all just trying to get some information so that I can better determine where and why I have some discrepancies with Omnimeter V4 readings especially the Current readings (Amps), which have absolutely nothing to do with sampling rate, cycle or sequencing as I can calculate them out correctly manually using a variety of formula and they can vary greatly from that which the EKM meter spits out more often than not. Even when the Power Factor is at an undisputed Unity - where PF equals 1.000000 and VAR equals zero.

Anyway, I'm happy for you to delete the whole thread.

BlueHornet
Posts: 56
Joined: Fri Dec 16, 2011 10:42 pm
Location: Victoria, Australia

Re: CT Reference Voltage

Post by BlueHornet » Tue Aug 02, 2016 1:09 am

Can't open a "New Topic" or reply to an existing thread using "Post Reply". This post has been submitted using the "Quick Reply" window and the submit button. Newly created posts cannot be edited either.
Last edited by BlueHornet on Tue Aug 02, 2016 1:49 pm, edited 1 time in total.

BlueHornet
Posts: 56
Joined: Fri Dec 16, 2011 10:42 pm
Location: Victoria, Australia

Alternative CT Checking Method

Post by BlueHornet » Tue Aug 02, 2016 4:40 am

So I've partly arrived at a conclusion for the incorrect values that are being reported by the EKM V4, not totally solved ATPIT.

I have several CT's that appear to be outside a reasonable value tolerance. When they are attached to a common load rail the instantaneous values are considerably different as is their resistance. All are 200A:26.6mA CT's.

Not attached to the meter and with no primary passing through the core for resistance measurements one of the CT's measures at 382.5 Ohms and returns an instantaneous value of 4846Watts when a 5000W resistive load is applied to the primary conductor each of the 3 CT's are measured on L1, L2 & L3 of a V4 Omnimeter at the same instance.
Another 2 measure 595.5 & 594.5 Ohms and the 594.5 Ohm CT returns a value of 5102 Watts and the last I tested measures 580.0 Ohms and returns an power value of 5086 Watts.It appears that each of them is out out of tolerance.

Whilst it is not necessarily the ideal method to check a CT's resistance using an ohm meter (Fluke 179) , it would appear that a more appropriate resistance value for a 200A:26.6mA CT (7018 windings) to return a more accurate power value which will also impact on the energy (kWh) registers, would more likely be in the range of 530 to 550 ohms or thereabouts? Maybe you could confirm what a typical resistance value range should be in order for the CT to read correctly.

These erroneous power values will obviously be reflected in the kWh energy values of the meter also.

3 of the 4 CT's have only been in operation for a few weeks since I have been attempting to resolve the erroneous Current (Amps) Value issues. The resistance of each disconnected CT was measured at an ambient temperature of approximately 18 C, 5 hours after they were tested to ensure that they all had reached a consistent ambient temperature.

Your feedback would be appreciated.

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