Setting : Residential area in rural Asia.
Public power supply: 11kV on 3 phases (no neutral) down to 220 V by 250 kVA transformer. Max power drawn is 200 kVA. Main load is fed by four 200 meter (650 ft) un-insulated, aluminium wires running to boundary of residential area. Wires carry 3 phases and neutral (A, B, C, N). Max load is 250 Amps on each phase, neutral 0-50 A. Wires are spaced about 12 inches on poles.
4 core insulated, twisted and shielded aluminium cables are available, but 8-10 times more expensive than the overhead wires and more complicated in terms of maintenance in this part of the world.
To prevent an unacceptable voltage drop under max. load the present overhead wires are being upgraded to higher capacity (thicker wires).
Based on the understanding that the main difference in overall impedance between the tightly spaced and twisted 4 core aluminium cable and the separated aluminium wires (given the same resistance in ohm) is the reactance (inductance and capacitance) caused by the spacing of the overhead wires, the question is:
Will installing two sets of wires in some kind of reverse configuration help reduce the reactance?
References:
A) In Karl Riley's book: "Tracing EMF in Building Wiring and Grounding", reverse phasing is defined in the glossary as: A simple method of reducing magnetic fields from double circuit transmission lines. One circuit has A, B, C, phase sequence, the other C, B, A. Simple, but highly effective. When created out of a single circuit this is called 'split phasing'.
B) In Fred Hartwell's article (March, 1993, EC&M): "Magnetic Fields from Water Pipes", the math is given for how, in a 2 conductor circuit (phase and neutral), the EMF drops off with the square of the distance; and if these 2 conductors are split into 4 in 'reverse phasing' the EMF drops off with the cube of the distance. The proportions are supposed to hold true for 3 phase circuits as well, even though the math gets more complex.
Less magnetic field (less EMF) should mean less overall reactance, or am I wrong?
Jim Dungar and Charlie B asked for more specs, such as wire spacing and length of wires. These are given above. Grateful for an answer, if you have one, including the math of how to calculate the result for different spacing and different lengths of wire, if available.
If two sets of wires is indeed a better solution, then what is the best configuration of the two sets?
Is it A, B, C, N on the upper set, and then C, B, A, N on the lower. Or is N, C, B, A better, or something else?
If split phasing does not offer any advantage, then a single, thicker set of wires will be installed.
A backup generator, also 250 kVA, used frequently, is placed further away from the generator (an additional 130 meters or 425 ft) to reduce noise. The overhead wires from the generator are connected with switchgear to the overhead wires mentioned above at the transformer site, giving a total run of 1100 ft., when using the generator. It is assumed that answers to above mentioned questions apply here as well. If not, please comment.
Please excuse the wordiness of these questions. No direct Internet connection here. This is being relayed by a friend in the U.S.
Thank you for your attention and help.
Public power supply: 11kV on 3 phases (no neutral) down to 220 V by 250 kVA transformer. Max power drawn is 200 kVA. Main load is fed by four 200 meter (650 ft) un-insulated, aluminium wires running to boundary of residential area. Wires carry 3 phases and neutral (A, B, C, N). Max load is 250 Amps on each phase, neutral 0-50 A. Wires are spaced about 12 inches on poles.
4 core insulated, twisted and shielded aluminium cables are available, but 8-10 times more expensive than the overhead wires and more complicated in terms of maintenance in this part of the world.
To prevent an unacceptable voltage drop under max. load the present overhead wires are being upgraded to higher capacity (thicker wires).
Based on the understanding that the main difference in overall impedance between the tightly spaced and twisted 4 core aluminium cable and the separated aluminium wires (given the same resistance in ohm) is the reactance (inductance and capacitance) caused by the spacing of the overhead wires, the question is:
Will installing two sets of wires in some kind of reverse configuration help reduce the reactance?
References:
A) In Karl Riley's book: "Tracing EMF in Building Wiring and Grounding", reverse phasing is defined in the glossary as: A simple method of reducing magnetic fields from double circuit transmission lines. One circuit has A, B, C, phase sequence, the other C, B, A. Simple, but highly effective. When created out of a single circuit this is called 'split phasing'.
B) In Fred Hartwell's article (March, 1993, EC&M): "Magnetic Fields from Water Pipes", the math is given for how, in a 2 conductor circuit (phase and neutral), the EMF drops off with the square of the distance; and if these 2 conductors are split into 4 in 'reverse phasing' the EMF drops off with the cube of the distance. The proportions are supposed to hold true for 3 phase circuits as well, even though the math gets more complex.
Less magnetic field (less EMF) should mean less overall reactance, or am I wrong?
Jim Dungar and Charlie B asked for more specs, such as wire spacing and length of wires. These are given above. Grateful for an answer, if you have one, including the math of how to calculate the result for different spacing and different lengths of wire, if available.
If two sets of wires is indeed a better solution, then what is the best configuration of the two sets?
Is it A, B, C, N on the upper set, and then C, B, A, N on the lower. Or is N, C, B, A better, or something else?
If split phasing does not offer any advantage, then a single, thicker set of wires will be installed.
A backup generator, also 250 kVA, used frequently, is placed further away from the generator (an additional 130 meters or 425 ft) to reduce noise. The overhead wires from the generator are connected with switchgear to the overhead wires mentioned above at the transformer site, giving a total run of 1100 ft., when using the generator. It is assumed that answers to above mentioned questions apply here as well. If not, please comment.
Please excuse the wordiness of these questions. No direct Internet connection here. This is being relayed by a friend in the U.S.
Thank you for your attention and help.