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Post by GTCGreg on Apr 15, 2018 15:38:31 GMT
as I recall, DC line losses are actualy slightly less than AC line losses. but it is much easier to step AC voltage up and down; which allows easier high voltage distribution. Edison's DC model was based on local generation plants, as I recall. True. The real advantage of AC was the ease at which the voltage could be stepped up for long distance transmission and then stepped back down for local distribution. With solid state voltage conversion, that now has changed. In fact, many of the long distance transmission lines are now DC. It's 33% more efficient just in the fact that you only need two wires instead of the 3 required for 3 phase AC. And as you stated, DC line loses are slightly less than AC because you aren't constantly charging and discharging the line capacitance 60 times per second. |
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Post by c64 on Apr 15, 2018 18:52:04 GMT
Actually, it is not "less wires" why DC is used. At very high voltages, the current flows on the surface only. You still need many wires since they are better than a few thicker wires. The bigger a circle, the more material per surface you need.
HVDC has less losses due to capacitance of the wires. AC causes a much bigger apparent current than the real current conveying the power. This is why transformers are rated in "VA" instead of "W". While WATT = VOLT x CURRENT, when WATT is used, COS(φ) must be 1. The greater the phase shift, the less energy can be processed by the transformer.
The problem with DC is galvanic corrosion (remember the prison break episode using salsa?). HVDC lines must swap the polarity every few hours in order to prevent "rotting" the wires and contacts.
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Post by GTCGreg on Apr 15, 2018 20:23:37 GMT
HVDC lines must swap the polarity every few hours in order to prevent "rotting" the wires and contacts. This is the first I've heard of them switching polarity. Wouldn't that make them really very low frequency AC lines? |
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Post by the light works on Apr 15, 2018 23:42:38 GMT
Actually, it is not "less wires" why DC is used. At very high voltages, the current flows on the surface only. You still need many wires since they are better than a few thicker wires. The bigger a circle, the more material per surface you need. HVDC has less losses due to capacitance of the wires. AC causes a much bigger apparent current than the real current conveying the power. This is why transformers are rated in "VA" instead of "W". While WATT = VOLT x CURRENT, when WATT is used, COS(φ) must be 1. The greater the phase shift, the less energy can be processed by the transformer. The problem with DC is galvanic corrosion (remember the prison break episode using salsa?). HVDC lines must swap the polarity every few hours in order to prevent "rotting" the wires and contacts. transmitting three phase AC requires three lines, phase A, phase B, and phase C. DC only requires two: positive and negative. stranded vs solid conductors for skin effect is a different matter entirely. and that is high frequency that makes skin effect more significant. |
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Post by GTCGreg on Apr 16, 2018 0:05:23 GMT
Another advantage of DC is that it's at its peak value all the time. AC is only at its peak for a very small percentage of each cycle. In fact, the AC equivalent (RMS) of DC is 70.7% of its peak. So right there, DC is almost 30% more efficient.
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Post by silverdragon on Apr 16, 2018 7:55:34 GMT
Actually, it is not "less wires" why DC is used. At very high voltages, the current flows on the surface only. You still need many wires since they are better than a few thicker wires. The bigger a circle, the more material per surface you need. HVDC has less losses due to capacitance of the wires. AC causes a much bigger apparent current than the real current conveying the power. This is why transformers are rated in "VA" instead of "W". While WATT = VOLT x CURRENT, when WATT is used, COS(φ) must be 1. The greater the phase shift, the less energy can be processed by the transformer. The problem with DC is galvanic corrosion (remember the prison break episode using salsa?). HVDC lines must swap the polarity every few hours in order to prevent "rotting" the wires and contacts. transmitting three phase AC requires three lines, phase A, phase B, and phase C. DC only requires two: positive and negative. stranded vs solid conductors for skin effect is a different matter entirely. and that is high frequency that makes skin effect more significant. Three Phase, four wires?. phase 1/2/3 needs "Neutral" return |
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Post by silverdragon on Apr 16, 2018 8:07:33 GMT
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Post by GTCGreg on Apr 16, 2018 8:21:24 GMT
You're looking at local distribution. Three phase long-distance transmission uses no neutral.
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Post by silverdragon on Apr 16, 2018 9:47:01 GMT
You're looking at local distribution. Three phase long-distance transmission uses no neutral. Duh...  oh yeah, of course. I never got that far, I only got certified for local distribution domestic and light engineering. |
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Post by the light works on Apr 16, 2018 14:23:32 GMT
transmitting three phase AC requires three lines, phase A, phase B, and phase C. DC only requires two: positive and negative. stranded vs solid conductors for skin effect is a different matter entirely. and that is high frequency that makes skin effect more significant. Three Phase, four wires?. phase 1/2/3 needs "Neutral" return not necessarily, you can use delta-wye transformers to set up a neutral for utilization voltage, and I believe that is done when they step down from transmission to distribution, as well, so single phase customers have a lower primary voltage. |
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Post by c64 on Apr 17, 2018 18:05:16 GMT
stranded vs solid conductors for skin effect is a different matter entirely. and that is high frequency that makes skin effect more significant. Yes, the skin effect is a matter of frequencies. But very high voltages play in the league of "static electricity" with their own effects. This also affects classic AC transmission lines. The center of the wire doesn't conduct any current. Very high tension lines used to have a steel cable in the center to support the weight with aluminium wires wrapped around for the actual conduction since aluminium is very light for its conductivity. Modern lines are different. Since the center doesn't conduct, plastic can be used for support which makes the line even lighter with the same conductivity. The plastic core is filled with fiber optic wires. This allows to make the distance between support points longer, saving metals while also conveying tremendous amounts of data over large distances.  |
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Post by c64 on Apr 17, 2018 18:43:22 GMT
You're looking at local distribution. Three phase long-distance transmission uses no neutral. Duh... oh yeah, of course. I never got that far, I only got certified for local distribution domestic and light engineering. Even local distribution doesn't really need the forth wire! It just depends on the load on the 3 phases. When symmetric, the neutral wire remains neutral and doesn't do anything. Usually, the neutral wire is still installed but not connected to the machine. Heavy machine tools usually are wired to the neutral but right behind the mains switch, the neural is discarded. Here is a picture of a frequency converter in the higher kW range:  The 3 phases are wired to the 3 screws of the big, black switch. This is the input of the converter. The big terminals on the bottom left are - Earth - U (1th phase), V (2th phase), W (3th phase) for the motor - 2x resistor for braking This converter is used e.g. for a 45kW (or more) spindle motor in a very big Okuma lathe. No neutral there. The electronics is connected between two phases. This load imbalance is very minor and won't cause more than a few millivolts in difference between phases. So instead of a 230V transformer, the electronics is equipped with a 400V transformer (in this case a switching PSU), that's all. Back to distribution: Normally, only a single phase is used for each load. Depending which loads are on and how big they are, you get a very high asymmetric load on the 3 phases. But the other homes are wired differently so each house will have a different asymmetric load. When there are many houses, they are very close to symmetric, the neutral carries currents mostly between the houses to balance the section of the grid. The neutral to the transformer feeding the section of the grid carries a very low current. E.g. the neutral of a transformer delivering 3x 50kA usually carries not more than a hundred amperes or two. In the US, most homes are connected to a single phase only. Here you need to think in streets rather than houses. Since the balance of each main transformer of the grid is very close to symmetric, a neutral wire is not required for long distance transmissions. Unless something goes wrong, e.g. a phase is missing in a section due to a blown fuse. Then the section is very unbalanced and a neutral on the long distance transmission lines would be required. In the past, what would be connected to such a neutral wire is just connected to a big grounder. So if a phase was lost, some worms died. Nowadays this is not allowed any more. Each input and output of a transformer station is equipped with a current sensing clamp which are connected to a computer by fiber optic wires. When the computer detects an imbalance somewhere and calculates that the current into the grounder is more than legal, it tries to calculate which phases on which sections must be shut down to preserve at least some service without polluting the ground with electricity. Until the early 1990s it had happened occasionally that roughly 1/3 of your rooms went dark in your house for half an hour or so (In the EU, not US). Nowadays it's almost always all or nothing. Another exception where there is no neutral wire was what we like to call "Farmer's Current". There are only 3 wires running to a barn or small sawmill used for a big electric motor. For lights, either there were two bulbs connected in series between two phases which gives you a dim light or 3 connected in a star configuration. A lot of barns had to be rewired with the ban of light bulbs, CFLs blow up when confronted with Farmer's current. |
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Post by c64 on Apr 17, 2018 18:49:55 GMT
Three Phase, four wires?. phase 1/2/3 needs "Neutral" return not necessarily, you can use delta-wye transformers to set up a neutral for utilization voltage, and I believe that is done when they step down from transmission to distribution, as well, so single phase customers have a lower primary voltage. Correct, but only as long as the total load is reasonably symmetric. If a single phase is lost, it causes a serious imbalance in the long distance lines causing damage if not dealt with. In the past, a simple big grounder was used to take care of that. Nowadays the section must fully shut down. (See my post above). |
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Post by c64 on Apr 17, 2018 18:58:07 GMT
Another advantage of DC is that it's at its peak value all the time. AC is only at its peak for a very small percentage of each cycle. In fact, the AC equivalent (RMS) of DC is 70.7% of its peak. So right there, DC is almost 30% more efficient. Not quite. AC voltage is rated as "effective". E.g. 100V AC causes the same work in ohmic loads as 100V DC. Light bulbs will glow as bright, heater will give the same heat and wires will cause the same losses. To do that, the peak voltage of AC has to be √2 times higher than DC. So U pp=U*√2 for AC while U pp=U for DC. Sowhat matters is the insulation and/or distance between wires which must be able to handle 1.44 times more peak voltage. For fuses, it doesn't matter at all. While there is 1.44 times more current in the maximum of the sine wave, there are neat relaxing breaks in between to cool down again. The average heat development inside the fuse is the same for AC and DC. But it does matter to anything sensitive to current densities like semiconductors! |
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Post by GTCGreg on Apr 17, 2018 19:12:06 GMT
To do that, the peak voltage of AC has to be √2 times higher than DC. So U pp=U*√2 for AC while U pp=U for DC. Sowhat matters is the insulation and/or distance between wires which must be able to handle 1.44 times more peak voltage. Or the RMS value of the AC is .707 that of DC, which is what I said. As for AC vs DC fuses, yes, they handle the same current, but the problem is when they try to interrupt that current. Because there is no interruption of DC current, like there is for AC, DC fuses must be much larger or incorporate other means to suppress the arcing when they open. |
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Post by c64 on Apr 17, 2018 19:35:24 GMT
To do that, the peak voltage of AC has to be √2 times higher than DC. So U pp=U*√2 for AC while U pp=U for DC. Sowhat matters is the insulation and/or distance between wires which must be able to handle 1.44 times more peak voltage. Or the RMS value of the AC is .707 that of DC, which is what I said. As for AC vs DC fuses, yes, they handle the same current, but the problem is when they try to interrupt that current. Because there is no interruption of DC current, like there is for AC, DC fuses must be much larger or incorporate other means to suppress the arcing when they open. RMS is the effective voltage (or current). U is always the effective (RMS) voltage and A always the effective (RMS) current. So AC is not smaller than DC. You confuse U pp with U RMS. U pp is not U, the correct formula symbol is Û (U with a roof on top). Since for AC using a sine wave: Û=U(RMS)*√2, you are not correct but also not quite wrong. The trouble is that U DC=Û DC and you treat the Û different for AC and DC which it is not! Û is the base reference for all signals, no matter how they look. And Û is most easy to identify on a scope. U (without the roof) which is RMS (or effective) on the other hand depends on the wave form: Rectangular (DC is just a special form of rectangular AC, an infinitive long period or low frequency) has a factor of 1, U = ÛTriangle or sawtooth is 2, Û = 2 * U or U = ½ * ÛAnd finally the sine wave has a factor of 1/√2: Û = √2 * U or U = Û * .707 (√2 ≈ 1.44) If you have a "normal" multimeter which expects a sine wave, you can convert knowing the true wave form and the factor of it. 20 years ago, a true RMS meter was very expensive. You could save the money knowing all form-factors of AC. Nowadays the difference between a common meter and RMS meter is less than $50. |
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Post by c64 on Apr 17, 2018 19:47:41 GMT
I know it is hard, "electricity 101" doesn't teach the true definition of formula symbols. 99.9% of all electricians never heard about any fancy symbols other than Ω.
per default, U is always the effective voltage, URMS unless otherwise marked, e.g. Upp.
Same for Watt. Watt is always the physical power which can directly translated to any form of power, e.g. heating power. Reactive power or "Wattless power" can't be expressed in Watt. What would a "wattless watt" be? That's why transformers are always rated in VA! VA can be watt for ohmic loads where current and voltage have no shift in phase. Apparent power must be written as VA since W is dedicated to true power.
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Post by GTCGreg on Apr 17, 2018 21:56:30 GMT
I know it is hard, "electricity 101" doesn't teach the true definition of formula symbols. 99.9% of all electricians never heard about any fancy symbols other than Ω. per default, U is always the effective voltage, U RMS unless otherwise marked, e.g. U pp. Same for Watt. Watt is always the physical power which can directly translated to any form of power, e.g. heating power. Reactive power or "Wattless power" can't be expressed in Watt. What would a "wattless watt" be? That's why transformers are always rated in VA! VA can be watt for ohmic loads where current and voltage have no shift in phase. Apparent power must be written as VA since W is dedicated to true power. I have a degree in electrical engineering. I know all about real power, reactive power, power factor and the RMS values of sine waves, triangle waves, square waves and any other shape wave you can shake a calculus book at. I don't need large print formulas. |
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Post by the light works on Apr 18, 2018 0:00:19 GMT
not necessarily, you can use delta-wye transformers to set up a neutral for utilization voltage, and I believe that is done when they step down from transmission to distribution, as well, so single phase customers have a lower primary voltage. Correct, but only as long as the total load is reasonably symmetric. If a single phase is lost, it causes a serious imbalance in the long distance lines causing damage if not dealt with. In the past, a simple big grounder was used to take care of that. Nowadays the section must fully shut down. (See my post above). considering that a dropped phase is caused BY damage, it still needs to be fixed, promptly. |
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Post by the light works on Apr 18, 2018 0:03:02 GMT
I know it is hard, "electricity 101" doesn't teach the true definition of formula symbols. 99.9% of all electricians never heard about any fancy symbols other than Ω. per default, U is always the effective voltage, U RMS unless otherwise marked, e.g. U pp. Same for Watt. Watt is always the physical power which can directly translated to any form of power, e.g. heating power. Reactive power or "Wattless power" can't be expressed in Watt. What would a "wattless watt" be? That's why transformers are always rated in VA! VA can be watt for ohmic loads where current and voltage have no shift in phase. Apparent power must be written as VA since W is dedicated to true power. I have a degree in electrical engineering. I know all about real power, reactive power, power factor and the RMS values of sine waves, triangle waves, square waves and any other shape wave you can shake a calculus book at. I don't need large print formulas. sneaker waves? |
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oh yeah, of course.