On Sun, 12 Mar 2006 22:33:41 -0600, Neal McLain wrote:
> John McHarry <jmcharry@comcast.net> wrote
>> Some places the power company, at least in the past, got cheap and used
>> two pole pigs to deliver an "open delta" where the third phase is
>> imputed. On a warm spring day with the sun shining and the birds
>> singing, this works fine. When the loads get out of balance, all sorts
>> of evil ensues.
> Why would ambient air temp affect load balance? I would think that load
> balance would be determined by the customer's instantaneous demand.
I was being a bit flippant, like saying something is so loud it can be
heard five miles upwind on a foggy day.
>> There is, however, a reason for both wye and delta connections. Non
>> linear loads, and ever more are with the prevalence of switching power
>> supplies, generating harmonics. Multiples of the third harmonic, called
>> triplen (from triple n) currents add in phase on the neutral. A
>> wye-delta transformation traps them and keeps them out of the upstream
>> system, where large currents on the neutral can wreak havoc.
> Are wye-delta and delta-wye transformations ever used in secondary
> distribution circuits? The ones I've seen were always in primary
> distribution or sub-transmission.
I don't know, but I think you know more about this subject than I
do. They may do it within substations, or perhaps they have other
means of getting triplens off the neutral.
I do know switching power supplies cause some horrid current
waveforms, mostly symmetric, so rich in odd harmonics. I used to
volunteer engineering help at a public radio station. We were having
trouble getting the UPSs and emergency generator to play together, so,
after seeing the top clipped off the generator voltage waveform, I
hooked a clamp on current probe into the scope. It looked like a pig's
breakfast! Pretty much all the loads are switching supplies or
fluorescent lights. The input to the UPSs looks a bit better, but is
still extremely non linear. The wonder is that it worked at all. I
believe they are now trying to raise the money for a much larger
generator in the hopes it can cope with the insult.
John McHarry (jmcharry@comcast.net)