CT Remanence Measurement 1

[bigmantony]

Dear Engineers,
Does any know of, or have, any equipment that measures the Remanence of CT's?

 

[electricpete]

Do an excitation test and abnormal remnant magnetism (such as from disconnecting secondary terminals under load) will cause abnormally high excitation current.

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[amps21]

Hi Bigmantony,

As u may know, opening CTs under load should not be done, if u need to do it, extreme care should be exercised. I dont know the reason why u need to do this test (CT magnetic retentivity) but i think V-I curve from secondary excitation test should depict CT core condition. The VI curve can be corelated with a BH curve (if CT design parameters are known) an d may be a BH curve can be plotted by repetition of excitation cycles. Although i have not done this but i am sure it can be done (if need be)

Thanks

 

[waross]

In support of Bigmantony's statement.

"As u may know, opening CTs under load should not be done, if u need to do it, extreme care should be exercised."

I still remember one of my instructors talking about "The Big Orange Basket-Ball" which was the last thing a technician saw for several hours after opening a ct secondary under load.
Personally, I lost a multi-meter. I was checking voltages on a terminal strip in the control panel of a 600 KW diesel set. Someone had left an unused and open circuited ct connected to a terminal. My meter was on the 1000 Volt setting. It made a buzzing sound inside and never worked again. I considered myself extremely lucky that the only loss was a multi meter.

 

[amps21]

hi Waross,

The statement was mine. But anyways thanks for supporting. The big orange basket ball - Short and straight forward description.

 

[randyman]

Bigmantony, et al...

Was wondering why you want to measure the remanence of the CT anyway?? Depending on the ratio, type and physical size you may be able to view the B-H curve direct with a scope and simple integrator, provided you also have the means to excite the core. If you are still interested, give me some speific details on the CT in question and I will see if I can help you.

BTW, I do not think that electricpete was saying to open circuit the CT, but just to compare the exciting current at some known or typical curve value to see if remanence is present. In some cases you can determine that and once the CT is driven into saturation and slowly reduced back to zero, the remanence will be removed.

And I agree that you should never open circuit the secondary while the primary circuit is energized, for several reasons. First and foremost, the open circuit voltage developed can be hazardous to personnel. And could damage the CT winding in the process, or at least magnetize the core.

 

[bigmantony]

It touching that so many of you are concerned for my well-being and staying alive. The evils of open cct CT's has drifted slightly from the thread.

Remanence is a test our customer wants us provide for certain CT's. Gapped cores (oh joy!)

I have carried out the test in the past using a digital scope and integration. Bit fiddly but not an ideal solution for a manufacturing environment.

I was looking for something dedicated and easy for a test operator to use. Found a CT Analyzer by Omicron that may do, but it does loads of other thing as well. Looks like a handy peice of kit. Omicron also do stand alone remanence measuring equipment. I am meeting a rep tomorrow for a demo.
It may be of interest to "randyman".

http://www.omicron.at/products/primary/ctanalyzer/

Please keep your ideas coming when they blossom.