Line-to-Line Sensing: Analog Vs Digital?

Jerry Avins <jya@ieee.org> wrote:
(snip)
>> Apparently there is something wrong in your assumptions here. One case
>> that introduces signficant voltage between G and N is when the system
>> is corner-grounded. We'd like to be able to operate in this
>> configuration.
 
> Apparently there is: my apologies. It may not violate code in your
> case -- I never understood why it should, but I can guess -- but it is
> surely unusual.

As I understand it, it has to be grounded somewhere.  If it floats,
it could float to any high voltage (leakage from the transformer).

The obvious choices for the ground are the wye-center, center of
one side of the delta, and one corner of the delta (or leg of wye).
In the case of an actual delta, it takes a funny transformer
to ground the center of the wye.

The last choice does sound unusual, and I don't follow the code
close enough to know, but it does seem possible.  If you don't
have a center tap on the transformer, it seems the choice that
is left.

-- glen

On May 16, 3:13&#4294967295;pm, glen herrmannsfeldt <g...@ugcs.caltech.edu> wrote:
> Jerry Avins <j...@ieee.org> wrote:
>
> (snip)
>
> >> Apparently there is something wrong in your assumptions here. One case
> >> that introduces signficant voltage between G and N is when the system
> >> is corner-grounded. We'd like to be able to operate in this
> >> configuration.
> > Apparently there is: my apologies. It may not violate code in your
> > case -- I never understood why it should, but I can guess -- but it is
> > surely unusual.
>
> As I understand it, it has to be grounded somewhere. &#4294967295;If it floats,
> it could float to any high voltage (leakage from the transformer).
>
> The obvious choices for the ground are the wye-center, center of
> one side of the delta, and one corner of the delta (or leg of wye).
> In the case of an actual delta, it takes a funny transformer
> to ground the center of the wye.
>
> The last choice does sound unusual, and I don't follow the code
> close enough to know, but it does seem possible. &#4294967295;If you don't
> have a center tap on the transformer, it seems the choice that
> is left.

For instrumentation purposes, a small Scott T (or even three
resistors) does the job, but that won't do for distribution because
the neutral has to be able to withstand full fault current for several
cycles.

Jerry
--
Engineering is the art of making what you want from things you can get.