Path: utzoo!attcan!uunet!amara!khai From: khai@amara.uucp (Sao Khai Mong) Newsgroups: sci.electronics Subject: Re: Free power from 'whispering wires' ?? Message-ID:Date: 12 Jul 88 12:58:37 GMT References: <3677@tekgvs.GVS.TEK.COM> Sender: khai@amara.UUCP Organization: Applied Dynamics International, Inc. Lines: 77 In-reply-to: jans@tekgvs.GVS.TEK.COM's message of 11 Jul 88 17:25:25 GMT In article <3677@tekgvs.GVS.TEK.COM> jans@tekgvs.GVS.TEK.COM (Jan Steinman) writes: > > <<4. What percentage of the high-voltage lines that one sees in the countryside > are AC and what percentage are DC?>> > > > I don't think this is correct. I've seen pictures of huge (I mean 40-60 feet > tall) thyristors, which are supposedly used for line synching high voltage DC > transmission lines to a local grid. I believe the article mentioned that DC > transmission was efficient enough to more than overcome the conversion losses. > No, almost all lines are indeed AC; there are only about ten or so DC links in the USA. Seeing pictures does not imply their widespread use. Look at HT (high tension) lines across the country. You are sure to see only three or six wires (ignore the differently-sized protective ground wire) and that is almost a dead giveaway that it is an AC line. (Three phase xmission) HVDC transmission is quite a new idea compared to AC and there has not been enough time for it to be in widespread use. Perhaps because you live in Oregon, you may have had biased opinion: There is indeed at least two HVDC lines from Oregon or Washington to Southern California. It's called the Pacific intertie. Cons against DC 1. Those thyristors and associated control circuitry are very expensive and in most cases not worth the extra money. 2. Conversion to DC produce a lot of power line harmonics which require still more expensive filters to remove them. 3. HVDC switches are still infant technology. It is very hard to stop hundreds of amps flowing at xxx000's voltage. AC switches are easier because they use the zero crossing to extinguish the arc. As a result there are no HVDC networks; they are exclusively used for point to point transmission and reconversion back to AC. 4. HVDC transformation to higher and lower DC voltages is not feasible at the moment. Another factor against networks. 5. For unknown reasons, environmentalists oppose DC lines more than AC lines; hence it is more difficult to get right-of-way's to build DC lines. 6. Residential power distribution will always be AC for my lifetime. Some Pros for DC. 1. Good for connecting networks of different frequencies. Sometimes they even have DC transformers back-to-back without any lines. 2. You don't have to worry about power system stability and power transmission limits. In fact, power system stability can be enhanced by the use of DC with appropriate controls. Power system stability is the reason for those whole area blackouts, like the one in New York in 1965, etc. 3. More efficient when you talk only about transmission line losses. 4. Capability to operate using only one wire in emergencies. (Frowned upon by environmentalists, sailors who have to navigate over these cables., etc) 5. Underwater transmission (E.g. under the English channel) because DC at steady state does not care about the high capacitance of underwater cables. 6. Great thesis topic for budding PhDs. This is by no means complete. Remember, Thomas Edison lost out to Westinghouse, Tesla, et al, and would still lose today. -- --------------------+------------------------------------------------------- ...uunet!amara!khai | Applied Dynamics International, 3800 Stone School Road khai@amara.uucp | Ann Arbor, Michigan 48105, U.S.A (313) 973-1300 --------------------+-------------------------------------------------------