CT voltage 7

[deltawhy]

Hello all,

Can anyone explain mathematically how the voltage in the secondary of a current transformer is kept to a minimum? I understand that open circuiting a CT will cause some havoc, but I can't seem to derive why mathematically.

Thanks,
Daniel

 

[stevenal]

scottf,

When you look at voltage drop on the primary being a function only of the conductor impedance, you are completely neglecting the mutual inductance caused by the presence of the secondary coil. If you insist on using a more complete model than what I've suggested, you might want to avoid going to the opposite extreme.

"The bottom line point is that I think you're using an ideal transformer alone to explain something incorrectly, which leads to the reader drawing a false and dangerous conclusion."

I really thought I had dealt with the incompleteness of the model in the last paragraph of the FAQ. Perhaps I should move it to the front for those who can't make it through a few short paragraphs? If you have suggestions, please use the comment button at the FAQ. Please understand that I have no desire to go into much more detail. I'll leave that to another FAQ writer.

By the way. I'm curious what happens when your open circuit voltage of 32kV is applied to 600V rated insulation?

 

[scottf]

Stevenal-

Seems you're taking this a little personal. That's not my intent.

As for the mutual inductance, it can be considered, and you'll find that it's negligible when discussing voltage drop over a CT's primary. I've worked for an instrument transformer manufacturer for almost 20 yrs and I've done/been witness to plenty of testing and calculating on this topic.

On your FAQ, it's not that I "can't make it through a few short paragraphs", the point is the FAQ leads the reader to the wrong conclusion. I hope no one gets hurts by drawing the wrong conclusions from your FAQ.

As for "my" open circuit voltage of 32 kV applied to 600V insulation....there is normally a lot of damage and melting of secondary terminal blocks. Unfortunately, I know of more than one specific case of a worker being killed and/or seriously injured due to open-circuited CTs.

 

[waross]

Hi fellows. I have an old text book that describes and shows a picture of a "Wireless reactor". This was several cores similar to window transformer cores that were stacked beside the transformer. One of the secondary conductors passed through the cores which added enough inductive reactance to aid in paralleling transformers with dissimilar impedance voltages. To put it in perspective, the voltage drop across the cores would be a few percent of the terminal voltage. A CT may be only 10% 0r 5% of that value, but still above the micro-volt level you may see with a burden on the CT secondary.
Lets keep it simple and save discussion of fault conditions to a later time.

Bill
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"Why not the best?"
Jimmy Carter