Comment Re:Clarification on "twisted lines." (Score 4, Informative) 293
As an electrical Engineer with my primary background being power systems, ie generation, protection, transmission and conversion, I think I should correct you a bit.
For an electric power transmission line, this "loop" is the wires on the left and right sides of the power-line crossbar (OK, not all lines look like that, but the principle is the same). You can trace an imaginary line down one side of the power line and back on the other, enclosing a loop 12 feet wide and many miles long, with enormous area. This is one reason power lines are a bad idea for carrying RF signals; they make a GREAT antenna.
Not really true, most power transmission is done in 3 phases, with all 3 phases summing to a return path on the Neutral wire (which you don't need if everything is balanced, which transmission lines are close to so they omit it, using the ground for a neutral). Which you could really look at it as three return paths in the ground and three primary all at once, I suppose, but not technically correct. Now residential distribution might be single phase, but this is nothing compared to the amount of 3 phase out their right now.
Interesting note: Cross-country power lines ARE in fact twisted pairs, to prevent another interference type. At every Nth tower, you'll see the lines cross over so the left-hand line goes to the right. This results in loops of a half-mile length or so; useless for shielding from RF, but VERY important for protecting the grid from geomagnetic storms, where the Earth's magnetic field is pushed around by solar wind. Making the net loop area zero prevents the transmission line from acting as a giant DC generator and blowing out the switchgear, causing major blackouts (this happened in Canada in the 1970s, IIRC).
What you a describing is called transposition, and it has nothing to do with interference from magnetic storms. A single power line can be seen as a long resistor and inductor in series with a shunt capacitor to ground. Three lines can be seen as the same thing, however with a very small magnetic coupling between lines, often model as a transformer, and a capacitor between lines. Now there are a number of ways to calculate these values, and they are all based on the physical geometry of the line. So if A phase is next to B phase is next to C phase for 300 miles, then your get an unbalance because more A phase is couple into B than into C. When all of these calculations were made by hand, this made for some seriously heinous matrices, which are critical for stability calculations. To solve this problem you twist the wires, sort of. There are a number of different techniques to do this, IE just twisting 2 wires, and leaving one alone, doing all three. These towers are called crossover towers, and their use has been decreasing, due to the fact that at these locations there a higher percentage of transient faults occur (lightning strikes, squirrels getting zapped), which is a pretty big deal to people who make their money 'wheeling' power (transporting power through their systems). As well computers are used pretty extensively for modeling power lines (EMTDC or ATP) and they can deal with 1000x1000 matrix reduction way better than I can.
BTW solar storms did affect the Canadian outage, this is referred to as Geomagnetically Induced Currents (GIC). But it's not DC, it can't be it has to be AC to be seen by the relays that this effects. Basically it causes large ground currents to flow in and out of the system at unpredictable locations and magnitudes. When this happens, a lot of protective devices see a large ground current and assume they have a single line to ground fault and open up the breaker. This is really no big problem, open a breaker at full load is nothing compared to opening with a bolted 3 phase to ground fault right at the terminals of the breaker. If you go here he comments on "When power is restored, all thermostatically controlled electric loads come back on simultaneously. This stress, added to the higher demands of many devices such as motors and transformers, can draw up to 600% of normal load during restoration procedures." Which really has no bearing on this type of outage, because every long-term outage causes this affect, and it is well understood in the industry. Then he concludes "Such a blackout is also likely to cause transient voltage stresses and permanent damage to network equipment such as high-voltage breakers, transformers, and generation plants, which makes them unavailable for restoring power." Which is specious at best given the fact that the effect is seen in the grounding system and lighting strike arrestors are placed on lines to prevent large voltage stress from causing problems.
For an electric power transmission line, this "loop" is the wires on the left and right sides of the power-line crossbar (OK, not all lines look like that, but the principle is the same). You can trace an imaginary line down one side of the power line and back on the other, enclosing a loop 12 feet wide and many miles long, with enormous area. This is one reason power lines are a bad idea for carrying RF signals; they make a GREAT antenna.
Not really true, most power transmission is done in 3 phases, with all 3 phases summing to a return path on the Neutral wire (which you don't need if everything is balanced, which transmission lines are close to so they omit it, using the ground for a neutral). Which you could really look at it as three return paths in the ground and three primary all at once, I suppose, but not technically correct. Now residential distribution might be single phase, but this is nothing compared to the amount of 3 phase out their right now.
Interesting note: Cross-country power lines ARE in fact twisted pairs, to prevent another interference type. At every Nth tower, you'll see the lines cross over so the left-hand line goes to the right. This results in loops of a half-mile length or so; useless for shielding from RF, but VERY important for protecting the grid from geomagnetic storms, where the Earth's magnetic field is pushed around by solar wind. Making the net loop area zero prevents the transmission line from acting as a giant DC generator and blowing out the switchgear, causing major blackouts (this happened in Canada in the 1970s, IIRC).
What you a describing is called transposition, and it has nothing to do with interference from magnetic storms. A single power line can be seen as a long resistor and inductor in series with a shunt capacitor to ground. Three lines can be seen as the same thing, however with a very small magnetic coupling between lines, often model as a transformer, and a capacitor between lines. Now there are a number of ways to calculate these values, and they are all based on the physical geometry of the line. So if A phase is next to B phase is next to C phase for 300 miles, then your get an unbalance because more A phase is couple into B than into C. When all of these calculations were made by hand, this made for some seriously heinous matrices, which are critical for stability calculations. To solve this problem you twist the wires, sort of. There are a number of different techniques to do this, IE just twisting 2 wires, and leaving one alone, doing all three. These towers are called crossover towers, and their use has been decreasing, due to the fact that at these locations there a higher percentage of transient faults occur (lightning strikes, squirrels getting zapped), which is a pretty big deal to people who make their money 'wheeling' power (transporting power through their systems). As well computers are used pretty extensively for modeling power lines (EMTDC or ATP) and they can deal with 1000x1000 matrix reduction way better than I can.
BTW solar storms did affect the Canadian outage, this is referred to as Geomagnetically Induced Currents (GIC). But it's not DC, it can't be it has to be AC to be seen by the relays that this effects. Basically it causes large ground currents to flow in and out of the system at unpredictable locations and magnitudes. When this happens, a lot of protective devices see a large ground current and assume they have a single line to ground fault and open up the breaker. This is really no big problem, open a breaker at full load is nothing compared to opening with a bolted 3 phase to ground fault right at the terminals of the breaker. If you go here he comments on "When power is restored, all thermostatically controlled electric loads come back on simultaneously. This stress, added to the higher demands of many devices such as motors and transformers, can draw up to 600% of normal load during restoration procedures." Which really has no bearing on this type of outage, because every long-term outage causes this affect, and it is well understood in the industry. Then he concludes "Such a blackout is also likely to cause transient voltage stresses and permanent damage to network equipment such as high-voltage breakers, transformers, and generation plants, which makes them unavailable for restoring power." Which is specious at best given the fact that the effect is seen in the grounding system and lighting strike arrestors are placed on lines to prevent large voltage stress from causing problems.
