Electrical transmission or interconnection systems – Anti-induction or coupling to other systems – Magnetic or electrostatic field control
Patent
1996-12-20
1998-07-07
Sircus, Brian
Electrical transmission or interconnection systems
Anti-induction or coupling to other systems
Magnetic or electrostatic field control
307 90, 307104, 333 12, 174 32, 174 35R, H02G 720
Patent
active
057774013
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to circuitry designed to reduce the magnetic field in the vicinity of a multiphase power line for transferring electrical current, wherein at least one of the phases is split into two branches for the transfer of a respective partial phase current and forming along a line portion a closed loop, in which a circulating current is induced so as to reduce the magnetic field, at least one compensation device being coupled to said closed loop for adjusting the self-reactance of the closed loop in such a way that the magnetic field is minimized in the vicinity of the power line.
2. Description of the Related Art
Such a circuitry is disclosed in WO 95/20835 (Vattenfall AB), wherein capacitive elements, for example capacitors, are directly connected in the closed loop as a compensation device.
In comparison with conventional three-phase arrangements, such a circuitry will reduce the magnetic field to approximately one sixth, wherein the phase split provides a reduction of one third and the compensation device provides a further fifty percent reduction.
However, such compensation devices being directly connected to the closed loop must be dimensioned in such a way that they can carry the primary partial phase current as well as the induced circulating current. Therefore, the capacitors must be dimensioned for transferring 70-100% of the phase current in a three-phase arrangement.
Furthermore, the availability of the power line is lowered because of the risk of failure of any of the capacitors.
SUMMARY OF THE INVENTION
With this background, the object of the present invention is to provide, in a circuitry of the kind stated in the first paragraph, an improved compensation device which may be dimensioned for substantially smaller currents and which does not affect the availability of the power line.
According to the invention, this object is achieved in that the compensation device is included in a separate circuit which is coupled to the closed loop via a transformer designed to transfer said circulating current, but not said partial phase currents, to the compensation device. By such an indirect coupling of the compensation device, the latter may be made much smaller and lighter, e.g. in the form of a filter, in particular comprising a capacitor. A possible short-cut or a cut-off in the separate circuit does not affect the power transfer as such, but only the reduction of the magnetic field. Thus, the availability of the power line will not be affected.
These advantages will very well balance the complication involved in adding a transformer.
Preferably, the two branches of the closed loop will include mutually opposite windings on a transformer core of the transformer, the core being coupled to the separate circuit by means of a third winding. This third winding may comprise a substantially smaller cable than the two phase branch conductors, in particular in case the opposite windings are dimensioned so that the excitation in the transformer core, originating from the primary partial phase currents, will be negligible. Thus, the transformer will only transfer the circulating current, induced in the closed loop, to the separate circuit and the associated compensation device. The various components included therein may therefore be dimensioned for smaller currents.
Preferably, the dimensions are such that the induced circulating current in the closed loop will be 15% to 25% of the respective phase current in a three-phase arrangement, whereby a total reduction of the magnetic field to about one sixth may be obtained, like in the previously known circuitry, as compared to a conventional power line with three-phase conductors.
At power lines supported in the air, the circuitry, i.e. the transformer and the associated separate circuit and the compensation device, may likewise be suspended above the ground.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be explained more fully below in conjunction with a preferred embodiment with ref
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Ganjian Peter
Sircus Brian
Vattenfall AB (Publ.)
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