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Forum Index : Windmills : rectification here or there
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old mate's mate Newbie  Joined: 11/08/2010 Location: AustraliaPosts: 15 |
team, salutations ..... the (sometimes dodgy) conventional wisdom around here is that AC current is less prone to transmission losses than DC. IF this is correct do you think it would be better to position the bridge rectifiers close to the battery bank and transmit AC from the HAWT? In the case of a 7 phase F&P rewire this would mean running 7 wires, it would be a little more expensive and a little more cumbersome, somewhat inelegant perhaps, BUT: Do you think it would improve losses due to transmission ? Thoughts and comments appreciated Keep the dream alive etc OMM |
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Downwind Guru  Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
Hi OMM, It would be to yours and our advantage to keep this as one thread rather than 2, as you had posted this twice in 2 sections. Most of us read all sections of the forum. You are right there has been many discussions on this subject of AC verses DC and i still dont have a black and white answer, and it always brings different arguments. My view is if the cable run is not over long than best to keep it AC if you can, this also allows for more applications and future changes if needed. I would rather all the hardware be in one place like in the battery shed. In a different situation like KarlJ mill it was @80+m run and was more practical to do a heavy DC cable run. I would also concider running a light data cable with the DC cable if it was a long run, as it is always handy to have a control line out to the mill for data transfer or remote switching if needed in the future. I see the argument of AC/DC more of what suits your setup best, as there is advantages and disadvantages for both systems. Dealing with losses is one thing dealing with what is practical is another. Take my advice and please yourself. Pete. Sometimes it just works |
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| old mate's mate Newbie Joined: 11/08/2010 Location: AustraliaPosts: 15 |
Thanks Pete, sorry about the double post, just to clarify, could you expand on why it was more practical to do a heavy DC cable run for the 80m + implementation? You write: "My view is if the cable run is not over long than best to keep it AC", does this imply that if it IS a long cable run then DC is the preferred option ? I am easily confused OMM |
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| RossW Guru Joined: 25/02/2006 Location: AustraliaPosts: 495 |
Both have advantages. Polyphase AC (3phase and up) have distinct benefits in flexability, like being able to short the mill as a brake, or monitor the raw AC for speed detection etc. I also like having the rectifier inside where I can work on it any time, any weather conditions, and be able to check its temperature etc. Against that, high-frequency (and the F&P is moderately high frequency) appears to have a demonstrably higher loss (probably due to stray capacitance and cable inductance as much as anything) than rectifying and sending as DC. This isn't likely to have any any measurable effect with short cable runs, but the longer the run, the worse the effects. |
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| Downwind Guru Joined: 09/09/2009 Location: AustraliaPosts: 2333 |
I agree with what Ross said. There appears to be a reported higher loss over a longer distance with high frequency AC. This is why some prefer to run DC. When running DC the cable size is bigger to handle the current without introducing too high of a resistance to the run. (cable loss) Where as with AC the number of conductors are higher and the current per conductor is lower but combined its effectively the same. I dont know what the magic distance is, that would be said to be better to run Dc compaired to AC. Im not even prepared to take a guess, as there is so many egos and opionions here that have shown in the past on this subject. As i and Ross agree we would much rather have AC run to the Battery shed and have all our equipment safe in one place where practically possiable. This might not be practical for you, and you have told us amost nothing of your setup. You did mension a seven phase setup, and this might make it not practical for a AC run if you have a good distance to cover. (7 conductors compared to 2 ) For me i would want to do things like measure the rpm of the mill and log the mill data etc. So access to the AC would be important for me, and would run the AC if it was practical, but my reasons might not be the same as your needs. Pete. Sometimes it just works |
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| rogeriko Newbie Joined: 10/08/2010 Location: GreecePosts: 4 |
Contrary to popular belief it is a lot better to send DC down a long cable run than AC. Almost all undersea cables are DC!! the longest being Holland to Norway a distance of 360 miles so just put the rectifiers up the pole and run two regular thickness wires to the batteries. A little resistance in the wire maybe 0.25 of an ohm will make the turbine spin a little faster and produce more energy anyway.But this is a whole other subject. |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
I think the believe AC was better for long distances came from the public battle between Edison and Tesla. Edison had built DC power stations and was feeding DC power to a few towns, it was a big money earner for him. Tesla jumped up and said AC was better for long distances because it was easy to use transformers to step up and down the voltage, and the motors were much simpler to build. In times past it was very easy to convert 240VAC into 11,000VAC and back again. The higher voltage means thiner cable can be used to carry the same power. It was relatively hard to convert DC to high voltage and back. In the end Tesla won, AC was the cheapest way to go by far. I would tend to use AC for a 3 phase setup, rectify at the batteries. 3 code AC power cable is cheap compared to 2 core power cable that can cary the equivalent power. A length of 3 core 15A cable is much cheaper than the same length of 2 core 20A cable. For a 7 phase set up, 7 core trailer wire would be used, and that would be more expensive than a 2 core cable. So I would retify at the windmill and run DC. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| old mate's mate Newbie Joined: 11/08/2010 Location: AustraliaPosts: 15 |
fellow enthusiasts, greetings ..... thank you very much for the rapid and informative replies. The knowledge base behind this forum is excellent ! My question is answered in full. I will feed your information into the algorithm, parse the results via a fuzzy confusion matrix and get back to you ..... thanks again OMM |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
fuzzy confusion matrix I got one of those. I leave it switched on for convenience.  The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Robb Senior Member  Joined: 01/08/2007 Location: AustraliaPosts: 221 |
This info about on basslink may be of interest. 400kv DC at 500 megawatt over 290 km under the sea. |
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| davef Guru Joined: 14/05/2006 Location: New ZealandPosts: 499 |
My 20 cents worth . . . I believe the issue with high frequency AC not looking as good as DC is that the AC waveform generated off a F&P is quite rich in harmonic energy. If you don't reliably transmit those harmonics, ie maintain the waveform you will lose some of the energy you would have captured if you rectified at the tower. However, DC losses in the cable would reduce that gain. How much, ?? High frequency pure sine wave AC there will be some transmission losses due to "skin effect", which at frequencies of 1-10kHz is still going to be quite low. As you get up to frequencies, ie 50-200kHz skin effect becomes a problem at high power levels, search <Litz wire>. http://en.wikipedia.org/wiki/High-voltage_direct_current 3% per 1000km for AC (50-60Hz?). My brain can't work out the change in loss due skin effect versus frequency, but I'd be surprised if it was 10 times higher at 500-600Hz (guess about 2 times higher). Even if it was 10% per 1000km, that is 0.01% per 1 km. Don't think skin loss is a factor for the average home power setup. The area that needs further clarification, in my mind, is with AC you enter the world of reactive components. The above article talks about cable capacitance *** Long undersea high voltage cables have a high electrical capacitance, since the conductors are surrounded by a relatively thin layer of insulation and a metal sheath. The geometry is that of a long co-axial capacitor. Where alternating current is used for cable transmission, this capacitance appears in parallel with load. Additional current must flow in the cable to charge (and discharge at the rate of the operating frequency) the cable capacitance, which generates additional losses in the conductors of the cable. Additionally, there is a dielectric loss component in the material of the cable insulation, which consumes power. *** however I think most people would not be using 3 pieces of coaxial cable to transmit their 3 phase AC. With ordinary 3 phase (parallel, maybe lightly twisted conductors) cable there is still going to be some capacitance. Don't see any mention of inductance or transmission line effects, ie mismatch losses. Must do those measurements and circuit simulations as I will need to install my system in about one year's time. Dave |
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