Line Losses

[drysider]

One of the issues that comes up regularly is that of line loss between the generator and the battery system. Is there a reason to rectify the generated AC at the source? If you have a 48 volt wind generator, it should be possible to put a 1:10 transformer on it (one at each end) and increase the voltage while decreasing the amperage by a factor of ten. The frequency would not matter....in fact a higher frequency would make the transformer a bit more efficient. The resulting AC delivered to the second transformer (thru a much smaller wire) would be down converted and rectified to 48 vdc for battery charging.

The main reason Westinghouse won the AC-DC battle was the ease with which you can alter the AC voltages to avoid excessive line losses. DC (of equivalent voltage) actually has less line loss since there is no reactive component. However, the technology for handling high voltage DC is relatively new, and low voltage (AC or DC) will always mean higher currents and higher losses.

Pat

[kitno455]

frequency most definately does matter. off-the shelf EI core transformers have ratings for that. i have purchased two different models of the same basic transformer before, and the manufacturer specifically made one of them 50hz and up, the other 60 hz and up. according to them, using the 60hz model at 50hz lowered the current carrying spec somewhat, but going up quite a ways in freq was ok on both.

with a wind genny, unless it has alot of poles, you may have to worry about transformer heating at low speeds. of course, output is lower them, so that may not matter much.

i bet others will disagree

allan

[chux0r]

If you have transformers in your system, you can use half the number (3 or 1 depending on phase). Just wire your stator for 480V, then just step down to 48 for the batteries.

I was thinking about this myself... because of the abundance of 12V components, I wanted the battery/inverter end to be 12V, but I don't want to carry 12V from the tower or elsewhere.  That way, my massive cables can be 3 feet long (batteries to bus, inverter to bus), and the rest can be thinner.

[Flux]

If you choose to use the transformer approach it makes sense to wind the stator for high voltage and use transformers only to step down, step up and down is not such a good idea from the point of cost or losses.

I certainly wouldn't go wild with the voltage, 480 would be very demanding on the stator insulation and the problems of making it completely water proof would be considerable. I wouldn't want to guarantee that my cast in resin stators would stand up in rainstorms at 480v.

Also the frequency at cut in is likely to be rather low and it would be better to use more smaller magnets to keep the frequency as high as possible. Even so you will probably have to use standard transformers below their rated voltage to cope with the low frequency, so that using star connected transformers rated for 480v you may only be able to work the stator at 240v to keep magnetising current reasonable at cut in. If you over flux the transformers you will have trouble with start up .

For very long runs the transformer idea may work out cheaper than thicker cables .

Flux

[tannoyman]

Hi Drysider

I don't know how AWG compares to the cable size over here in the UK (CSA ) cross sectonal area in millimetres squared,

Do you run single cables,on the ground or buried under ground?, or 2 core in one sheath?

If the cost of large cable is high , it may be cost effective to buy smaller multi core cables and parralell the cores

Erik

[PHinker]

This is along the lines of what I'm starting to experiment with since the best spot on my site is probably 500'+ from the house.  I've made a magnet rotor with 30 poles made from one inch by half inch by half inch neo magnets.  According to my back-of-the-envelope calculations, a 240v cutin should happen around 300 RPM and I'll need 30 coils with 275 turns each.  I have it set up on my lathe and need to wind some test coils to see how close my numbers are.  With this rotor, I should see 75 hz at 300 RPM.

The picture is a little dark but I think you can see it.  Instead of pouring resin around the magnets, I took a piece of copolymer polypropolene and slotted it for the magnets.  The magnets are glued in but I put some tape around the edge just to make sure I didn't have magnetic projectiles during the RPM testing.

[Trivo]

Fixed your pic for you

Trivo

[scottsAI]

60Hz frequency matters on very long cable runs, Miles. Nothing you need worry about. Three phase has a slight advantage, the third wire allows 59% more power. This is 9% better than using DC, with the same amount of copper. Three phase does need the third wire, but the copper is the big cost, insulation is not costly.

Better to wind the machine for the high voltage eliminating the transformer. A good transformer should be 97-99% efficient, but that is still 1-3% loss. A number most are trying to keep the loss in the power cable down to. A 1kw transformer will cost $100 if lucky or more. The higher voltages can save a huge amount on the wire. 250 feet of single strand  #4/0 cost $192 (ebay). Three conductor #14, 250 feet cost $22!!

Since most of you are building your own generators, no reason not to wind for the higher voltages. Most wire is rated for 600v, I do not know if that is RMS or peek voltage, if peek then anything above 400v may be a problem.

Have fun.

[commanda]

Biggest problem with transferring DC is corrosion caused by electrolysis between dissimilar metals. I did hear of an underground irrigation system installed here in Australia, where they had to rip it out & convert it all to run on AC for this very reason. Just something to be aware of, especially wherever you join or terminate cables.

Also, if you look in my diary, I have done some testing with the F&P and transformers.

Amanda

[srnoth]

Hi there,

Just a thought. It seems to me that most high wattage transformers are very expensive, so I was wondering why couldn't one make one's own transformer, and design in specifically for the task?

Cheers,

Stephen.

[richhagen]

Although I didn't write it for this, the little wire resistance calculator I wrote here:

http://www.fieldlines.com/story/2004/11/27/12326/032

can be used to convert if you calculate resistance in a fixed length of one type of wire size units (square mm, AWG, ect) and then after you calculate the results, blank out the wire size field, and set the wire size units to the type you wish to convert to.  Rich

[scoraigwind]

Three phase has a slight advantage, the third wire allows 59% more power. This is 9% better than using DC, with the same amount of copper. Three phase does need the third wire, but the copper is the big cost, insulation is not costly.

I disagree.  If you are using 3-phase to feed a rectifier then the transmission is less efficient than DC after the rectifier.  

see http://www.scoraigwind.com/CABLE/index.htm

[Flux]

I have seen this 59% figure somewhere before, perhaps it applies when comparing standard sinusoidal ac where current always flows in the 3 phases, with dc.

It certainly is not the case for a 3 phase rectifier especially with alternators with low inductance.

Flux

[Vernon]

AC does create more voltage drop at a given current than DC due to the inductive reactance of the circuit. This would have greatest effect at high rotational speeds (frequency) and widely separated conductors.

One solution that helps with voltage drop and allows the recovery of energy at low wind speeds is the step up regulator. The idea would be to use a 12V nominal DC machine and a 48 Volt battery. Lets assume that there is a light wind, the generator is turning but only putting out 8 volts, there is no charging because the battery is of course at 12V in the normal application. We can however make the battery 48V and add a step up inductor at the generator output. A transistor, controlled by a computer PWM output grounds the output end of the inductor, even at 8V current starts to ramp up. When the current reaches an output determined by the wind speed the transistor opens and the energy stored in the field is dumped, through a diode, into the output capacitor which charges until 48 + V is reached and the battery circuit draws current. The circuit transmitts DC at 55 volts even though you have a 12V generator, the swiching converter is controlled to transmit the power available until output approaches a limiting voltage ... say 60V; it then leaves the transitor on for excess periods to load and brake the turbine. This solution allows getting some energy at any wind speed that results in tubine rotation, maximum power transfer to the load, less transmission loss due to 55V nominal transmission and the ability to tailor the PWM to avoid stalling ... the turbine can be operated at its maximum power point.

[Vernon]

The power transfered by a 3 phase circuit is equal to the line amps X the line to line volts X 1.732 (the square root of 3). Since a single phase circuit does not have the mulitplier and is just the line voltage x the line current its power transfer capacity at the same current and voltage is 57.73% of the three phase version.

These figures, of course, imply unity power factor.

[scottsAI]

Facts! I love them. Thanks for the link. unfortunately I do not believe it's correct. The math is confusing the conversion of Line current to RMS with calculations with three phase power. Rather complex. I will take a stab at a solution.

Three phase Vs DC, what is the True story?

Lets agree 100Vrms = 100VDC, same with current (I), and Power (P).

Why do I want this?

If DC power is power RMS then the peak or RMS currents are not important, plus they get very confusing.

The load whether a resistor or a 3 phase bridge will follow the equations below.

I'm willing to go into the peak currents and such to prove it. Will take a while. Might just be easier to do an experiment. What do you suggest?

Calculation for DC is easy, 400VDC driving a 1200w load.

Current is 3amps.

Now the same thing with 3 phase. Not so easy.

Each phase will have one third of the load or 1200 / 3 = 400w

The phase current will be 400w/400Vrms = 1amp RMS.

The line current will be the sum of the two adjoining phases, the math is

I line = square root 3   *  I phase = 1 * 1.73 = 1.73 amps RMS.

Assumption: Wire cost is based on the amount of copper in it.

To compare power transmission with 3 or 2 wires, each solution will have the same copper content.

Given: 3 phase wire impedance is 3 ohms. (We can use resistance to)

For the DC wire we take the third wire split it in half, parallel one each to the two wires. 6 ohms comes from splitting the third wire in half.

So, the DC wires resistance is R = (3 * 6) / (3 + 6) = 2 ohms.

Power loss in the wires. R * I * I = P

DC is 2 ohms * 3 amps ^2 = 18w each wire or 36 watts total.

3 phase is 3 ohms * 1.73 ^2 = 9 w each wire or 27 watts total.

Wire power loss in 3 phase is 75% of the DC (100%).

For very long runs 300 miles, the 60 Hz inductance and capacitance become a bigger problem and DC wins the day. A huge down side to DC is the necessary equipment on the receiving end to convert it back to AC. Not the case here.

How was that? Did I mess up any of the math? Checked it against my book.

Have fun.

[Flux]

This is correct for a normal distribution system with a resistive load.

The discussion here is about a rectifier feeding a battery. Power can only be defined as battery voltage x dc current. In this case the reference that Hugh gave is correct and cable losses for a given weight of copper will be less on the dc side.

Flux

[scottsAI]

OK, Flux, I may have to bow to your wisdom, but not just yet:-)

I see the Vrms and VDC will not be the same, the battery is dominating the voltage, so Vrms and VDC are not equal as I assumed. This will change the equations.

I worked on them, are very complex.

I attempted to simulate (easier) this to get some pertly wave forms. I did not get the same wave form with the two solutions. I have an old simulator, no diodes that can handle 30 amps. So, will hunt down a newer one. http://www.beigebag.com/ has an interesting package. Anybody use it?

Could use some suggestions on generator impedance, line, battery etc. to design a system and simulate it?? I have seen a 800w generator with 700 foot line, this a good start? Thanks.

Have fun.