DC Power Circuits

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jimmac49

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Can a DC 750 volt, 1500 amp, feeder be run in 3 separate steel conduits and connected in parallel. I seem to recall reading something about the cons of using steel conduits with DC power but I can seem to find it. NEC doesn't seem to have much information on these types of installations. Thanks
Jim
 
Re: DC Power Circuits

If the DC has ripple content (not filtered) it could still induce voltages into a metallic conduit or cable sheath.
It would be advisable to have a positive and negative wire in each conduit.

Ed
 
Re: DC Power Circuits

Jim,
I have a number of 1500 hp DC motors where we have paralled four 4" rigid steel conduits for the armature power. We have four 535kcmil DLO/MTW conductors in each raceway. Two + and two -. We have no problems with the steel raceways.
 
Re: DC Power Circuits

How does one obtain such DC voltages and currents? Motor generators or what? Do they make rectifiers that hefty?
 
Re: DC Power Circuits

Originally posted by rattus:
How does one obtain such DC voltages and currents? Motor generators or what? Do they make rectifiers that hefty?
Click to expand...
Rectifiers in most cases.
 
Re: DC Power Circuits

Many of the public transportation systems, including for example the elevated train in Chicago, operate at 1,500 volts DC. You?re not going to get that amount of DC power, without first generating it as AC, then using rectifiers to convert it to DC.

Several years ago, when I was in my MS program, I read about a significant research effort on the topic of transmitting large amounts of DC power across the country. Much of the research had to do with the conversion and inversion processes. The key advantage is that there would be a vast reduction in power losses within the transmission lines, as opposed to transmitting the same amount of power in AC.
 
Re: DC Power Circuits

We feed a traction power transformer with 480v three phase and then it feeds a 2000 kw 12 pulse rectifier, which then feeds the equipment.
 
Re: DC Power Circuits

I thought large amount of DC power over transmission lines was simply not practacle.

Were they still going to use high voltages? What's the equivelent of a DC transformer? DC to DC converters.
 
Re: DC Power Circuits

The Pacific HVDC Intertie is 500kVDC. China has some 800kVDC transmission systems.

There are definitely economic breakpoints, but its been years since I worked on them. Well regulated DC has little "skin-effect" so most of the entire conductor is used. Conversely inverting it back to "useable" distribution/utilization voltages is expensive.
 
Re: DC Power Circuits

Originally posted by physis:What's the equivelent of a DC transformer? DC to DC converters.
Click to expand...
Generate at high voltage AC. Convert to DC. Transmit over long distances. Invert to AC.
 
Re: DC Power Circuits

Not at all. With the high voltage, there is a small current, and therefore a small voltage drop. That is true for AC transmission as well. But with DC, you do not have any inductive or capacitive reactance. Therefore, the resistance is lower than the same line carrying AC current would have. That reduces the voltage drop even further.
 
Re: DC Power Circuits

There's no reactance , true. But resistance adds up over miles and miles.

I guess I'm just thinking that you can't push the voltage back up along the way. It's gonna be a lot smaller when it gets there still.
 
Re: DC Power Circuits

Sam, Although there may be a lot of volts lost over the miles, by comparison to 120 volt, by comparison to 500 Kilovolt it's nothing.

Think of the #12 solid copper conductor with 1.93 Ohms per thousand feet. At 120 volts a 2.4 KW load will draw 20 amps. A 5% voltage drop occurs in 75 feet (both ways).

Put that same 2.4 KW load on the end of a 500 KV line strung with #12 solid copper (I know its not practical, but pretend) and the 5% voltage drop occurs at a little over a quarter million miles.
 
Re: DC Power Circuits

Reminds me of an old guy who strung a long extension cord to power a light outside his service station. He powered it with a 6V battery, and the light was rather dim. He concluded that DC was no good for transmitting power! Didn't have a clue!

Sure there will be a voltage drop, but that drop should be less than it would for the same AC current.

[ December 28, 2004, 08:30 PM: Message edited by: rattus ]
 
Re: DC Power Circuits

Naw, Sam, it was the old man's brain that was worn out! He was a nice old gent though, but like many, he didn't understand electricity.
 
Re: DC Power Circuits

Physis, no I never worked on analog stuff at TI. Did some work on CODECS and CCD filters though at MOSTEK.
 
Re: DC Power Circuits

I guess we didn't work with a lot of the same kind of stuff.

[ December 30, 2004, 01:20 PM: Message edited by: physis ]
 
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