Best wave to go from 120vac to 28vdc

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Twoskinsoneman

Twoskinsoneman

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Facility Senior Electrician
Got a customer who wants to have the full use of a 13.0 kw genset with either 120vac or 240vac output in the form of 28vdc.
ALL the loads are 28vdc. What method do you recommend?
Thanks


edited for capacity typo
 
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Twoskinsoneman said:
Got a customer who wants to have the full use of a 13000kw genset with either 120vac or 240vac output in the form of 28vdc.
ALL the loads are 28vdc. What method do you recommend?
Thanks
Click to expand...

13,000 kw at 28vdc is about 465,000 amps.
 
How clean does the DC need to be? My first step would be to get to about 24-32VAC using a Sola/Heviduty S19H10A or S20H10A. Those transformers are good for 416 and 312 amps respectively, so two would be needed.

The rest of the circuit would depend on the ripple that's allowable on the DC - decisions like whether to use a half- or full-wave rectification and how much smoothing capacitance is necessary.

Link to serious rectifiers:D

Of course these guys have COTS 1500W 28VDC supplies like the HSP 28-53 that output 53A @ 28VDC with 150mV noise and 60 mV ripple. A bank of those would keep things humming!
 
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Thanks for the great information.
A little more info I've received is that is needs to be very clean dc. Some of the equipment is sensitive. What kind of power loss can I expect to have. I'm thinking around 20% loss. Is that right. If they have a 12500watt load I'm thinking I'm going to be around 15000watt genset.
 
Twoskinsoneman said:
What kind of power loss can I expect to have. I'm thinking around 20% loss. ... If they have a 12500watt load I'm thinking I'm going to be around 15000watt genset.
Click to expand...

If you go the power supply route then you're dead-on with regard to loss, theoretically. The unit I mentioned is spec'ed at 78% efficiency @ full load. I'd never spec a genset at 100% output, however. You'll invariably run into temperature and/or fuel derate issues and cause lots of wear and tear running it flat out. If your DC load requirement is 12.5kW, then I'd shoot for 12.5kW / .75 (efficiency rounded down) / 0.96 (worst case PF of power supply) * 1.25 (capacity fudge factor) = 21.7 kVA (note that the worst case PF on the supply is 0.96). The overall efficiency is 57.6%, but that also gives a little wiggle room for expansion and gives the genset some output compliance headroom for changes in genset loading.

This doesn't take cabling losses into account. 400+ amps will create a bit of I2R heating in the DC cables!
 
Even more manly PS

Even more manly PS

These guys have a Javelin III PS that's rated 5400W out @ 28Vdc. That's 192+ amps per supply! These might be more cost-effective than the other ones, but I think the word "Military" might mean $$$.
 
My questions would be:

1) Do they already have a generator that they want to use, or are they purchasing the generator.

2) How clean must the 28V be. "Very clean" is _not_ a specification. You need to know the allowable voltage range, and also the noise frequency requirements. (For example, one of those switching power supplies will produce an output that is very close to the nominal 28V, but may have lots of high frequency hash...whereas a battery bank will have its voltage vary all over the place depending upon state of charge, but will have very very low noise.)

3) What is the actual power requirement? Average? Peak? It may be better to have a smaller generator with a battery bank to provide for peak power, as well as a certain amount of filtering.

4) What sort of voltage regulation is required? Do you need 'kelvin' voltage sensing? (With high current power supplies, voltage drop in the supply conductors is a serious issue, so a standard trick is to measure the voltage at the load, and use that as part of the feedback loop which regulates the output voltage of the supply.)

IMHO it would be a waste to have a 240V genset and then use a transformer and rectifier to get to the 28V...but if a switching power supply is appropriate, then starting with a higher voltage is totally reasonable; the switching power supply has to have transformers (or inductors) anyway, and because they operate at high frequency they will be very small.

Depending upon the load, you may be far better off with a large number of small switching power supplies, each handling a specific portion of the load.

If you simply want to buy a high current 28V genset, consider using a welder :)

-Jon
 
Sounds like he needs a big rectifier like they use in plating operations. Xantrex/Sorenson is a popular brand, and they make some that go up to 500 amps easy, and you just type in the VDC on the frontpanel that you want from it. There are other companies of less recognizable names that make rectifiers that go into the many thousands of amps.


http://www.elgar.com/products/SG/SG_Overview.htm

sga6u.jpg
 
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petersonra said:
I vote for multiple smaller supplies rather than one big one. The large well regulated supplies tend to have reliability issues.
Click to expand...

I'm thinking this is good thinking.
Thanks for the great suggestions. Marc we have trusted the Xantrex name for a long time but I've never used that SG series dc supply. I just took a quick glance at it but I thought it said 3 phase only. Anyway I'm still waiting for answers but I did find out it is going in one of these.
871209828_b86ca986f6.jpg
 
The fact that they are moving may change some things on installation. Be careful of vibrations and jarring as electronics especially large bridges can be sensitive to such things. Also have you thought of a DC Gen set?

Edit: For spelling
 
MJJBEE said:
The fact that they are moving may change some things on installation. Be careful of vibrations and jarring as electronics especially large bridges can be sensitive to such things. Also have you thought of a DC Gen set?

Edit: For spelling
Click to expand...

Thought about it. Don't know if I could find a 28vdc genset to make 12.5kw.
Also the only reason I'm involved in the project is because I am a Kohler dealer and the outfit wants Kohler.
 
I work with DC plants all the time, I have done about a hundred of 10,000- amps and more at 48 and 24.

To get the best effeciency you will want Switch Mode Rectifiers that run high 90% made by folks like Marconni, Peco, and Tyco.
 
Aircraft Power Unit

Aircraft Power Unit

28VDC sounds a little like aircraft power. Airports are equipped with large GPU (ground power units) that convert AC to DC and 400Hz for various aircraft. There might be something out there.

For high quality power (low ripple), you need high-frequency switch-mode power supplies. There are many large 5VDC power supply companies that have units that can be placed in series or parallel. Several 120VAC-5VDC units, in combined series-parallel will give you the voltage and current needed using off-the-shelf units with redundancy.

You would want a power supply with a high power factor, low harmonics input stage so that the crest factor of a rectifier front end with its high harmonics do not adversely impact the generator.
 
Twoskinsoneman said:
Thanks for the great information.
A little more info I've received is that is needs to be very clean dc. Some of the equipment is sensitive. What kind of power loss can I expect to have. I'm thinking around 20% loss. Is that right. If they have a 12500watt load I'm thinking I'm going to be around 15000watt genset.
Click to expand...

You have to take into consideration that most rectifiers will put a heavy harmonic burden on the generator so you need to consult with the genset manufacturer. This is an application that screams for engineering.

If you have various type loads as far as how clean DC you need for them you may want to separate the "dirty" from the "clean". Higher quality power suplpies such as Acopian, Kepco are less efficent than plain battery charger type inverters and higher cost per VA.

Voltage loss is just need to be balanced.

You can consider dedicated AC/DC power supply for each equipment or a mixture of centralized and distributed.

Of course a DC genset is always an option.
 
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