Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Patent
1991-11-19
1993-02-16
Stephan, Steven L.
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
363 36, 363 37, H02J 336
Patent
active
051876515
DESCRIPTION:
BRIEF SUMMARY
It is become progressively more usual to utilize high voltage direct current for the purpose of transmitting electrical power over long distances, particularly because of the lower line costs achieved thereby. A large cost entailed by such systems lies in the current converters and in the transformers used. Consequently, it is not realistic to place several complete receiving stations of relatively low power along such a high voltage line.
For the purpose of discharging, i.e. tapping-off, lower powers, typically powers of less than 10% of the total power transmitted, it has been suggested that power discharge stations are connected in series in the d.c. circuit, the power discharged or tapped-off corresponding to a line voltage drop. The simplest of these solutions involves connecting a three-phase current converter bridge in series with the d.c. circuit. The converter bridge is connected directly to the three-phase network via a transformer. The bridge is line-commutated, which means that commutation of the valves is achieved with the aid of the a.c. voltage in the three-phase network.
A first drawback with systems of this kind is that line-controlled commutation requires the provision of synchronous machines, which becomes expensive in the case of small or average-size networks. A second drawback is that the transformer is expensive in relation to the power, while a third drawback is that disturbances and interference in the a.c. network can result in disturbances in the commutating process in the converter and therewith in the main network.
One object of the present invention is to provide a series energy discharge system capable of eliminating these drawbacks. Another object is to provide an energy discharge system which will enable small powers to be delivered to smaller networks in an economically viable manner. This is highly significant in the case of a public service company which can expect little understanding of its reluctance to deliver electric current to a sparsely populated area through which the cost-encumbered power line is drawn.
These and other objects are achieved in accordance with the invention by means of a system having the characteristic features set forth in claim 1.
The invention enables a small amount of energy to be taken from a large HVDC-network with the aid of relatively inexpensive components, so that the cost of said delivery is found defensible. All that need be connected to the high line voltage is a converter bridge with associated commutating equipment controlled via optical fibers from earth and the primary winding of a transformer, and within which winding all that need be accommodated is a voltage drop which corresponds to the power discharged or tapped off.
In order to form the concepts, there can be imagined a case which involves an HVDC-network of known kind with a voltage of .+-.500 kV and a maximum current of 1600 A. Connected to a conductor in this network is a first converter bridge and the primary winding of a transformer. Both may be single-phase. The primary winding must be capable of withstanding the whole of the maximum current of 1600 A and shall be insulated for 500 kV against the transformer core. On the other hand, it is not necessary to place over the winding more than one voltage corresponding to the power discharged, e.g. a maximum of 50 kV at a maximum power of 80 MW. The cost of the transformer can also be lowered, if the alternating current taken out is given a higher frequency than the normal network frequency, e.g. a frequency of 120-150 Hz. There can then be taken from the secondary side of the transformer, e.g. 40 kV, 2 kA, which is rectified to a local d.c. voltage of 40 kV in a second converter, which is assumed to be force commutated. Preferably, a three phase voltage is produced from this d.c. voltage by conversion in a third converter, which converter may be force commutated to a network frequency of 50 or 60 Hz, according to the case in question, at a voltage suitable for local transmission.
The local d.c. network is preferably made
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Asea Brown Bourrc AB
Stephan Steven L.
Sterrett Jeffrey
LandOfFree
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