[Milsurplus] DC vs AC for power distribution

[Bruce Gentry]

One serious problem today is the third harmonic current in the neutral 
of a 3 phase 4 wire  distribution system feeding switching power 
supplies. Because current flows only at or near the peak of the wave to 
charge the condensors, the effects on the line are serious if there are 
large numbers of these power supplies. Office buildings and other places 
where there are  many 120 volt single phase computers can cause 
dangerous overloading of the neutral. There are various methods used to 
offset the effects or capture the wasted harmonic power, but having one 
master DC supply in the building might be a better way to deal with this 
problem. A six phase or zig zag transformer connection could give very 
good DC and yet keep the phases balanced and the harmonics to a far 
lower level, also would allow a convenient battery backup. I have not 
yet tried powering up a 120 volt AC switching power supply on 170 volts 
DC, but as long as all the power for the device is derived from the 
output of the supply, it should work fine.  The two major considerations 
I can see would be cooling fans, relays, or  transformer loads in the 
device that operate directly from the incoming line, and the rating of  
switches and fuses between the power source and the rectifier in the 
switching supply.

     Bruce Gentry

J. Forster wrote:
> IMO, the major benefit of DC HV transmission is the issue of synchronizing
> the grid. With DC, it is not an issue. With AC it is a nightmare.
>
> -John
>
> =============
>
>
>
>   
>> The problem being encountered with ultra high voltage transmission lines
>> these days is insulation (and air) breakdown at voltage peaks.  Since DC
>> doesn't have them, it is preferred over AC for maximum capacity lines.
>>
>> 73,
>> George
>> W5VPQ
>>
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>>     
>
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