Electric power conversion systems – Current conversion – With voltage multiplication means
Reexamination Certificate
2001-12-07
2004-03-09
Berhane, Adolf D. (Department: 2838)
Electric power conversion systems
Current conversion
With voltage multiplication means
Reexamination Certificate
active
06704213
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the field of power electronics, and is based on a converter system for increasing a DC voltage, and on a wind power system, as claimed in the precharacterizing clause of the independent claims.
BACKGROUND OF THE INVENTION
A wide range of circuit systems, in particular conventional converter systems, are known for increasing a DC voltage. One suitable converter system, for example, is disclosed in U.S. Pat. No. 4,462,070. The converter system described there is used in particular for coupling two electrical networks. At the input, the converter system according to U.S. Pat. No. 4,462,070 has a DC circuit for this purpose, which is formed by two series-connected DC capacitances, which are interconnected at a center point connection. Furthermore, the DC circuit is connected to a voltage inverter which, as a single-phase bridge circuit, is designed for switching two voltage levels of the DC voltage in the DC circuit, and is used for producing an AC voltage by appropriate control of the switching elements in the voltage inverter. Furthermore, the output of the voltage inverter and the center point connection are each connected to one connection of the primary of a transformer. The secondary of the transformer is connected to a converter, which is used for setting a variable load-dependent direct current for an output circuit which is connected to the output of the converter and is formed by an inductance. The output circuit is coupled directly to the DC circuit, via a common connection of the voltage inverter and of the DC circuit. The converter provides an additional coupling between the output circuit and the DC circuit, and is connected to a further common connection of the voltage inverter and of the DC circuit.
The converter system according to U.S. Pat. No. 4,462,070 allows the already mentioned setting of a variable, load-dependent direct current. However, the cabling complexity is high, and causes additional material costs, due to the various couplings of the DC circuit to the output circuit. Furthermore, although the converter allows a load-dependent direct current to be set, it does not allow a variable, load-independent DC voltage to be set for the output circuit. In addition, the inductance of the output circuit is used as an energy store but, since it is physically large and complex, it causes additional material costs and requires a correspondingly large amount of space. Furthermore, the voltage inverter allows the production of an AC voltage but, since only two voltage levels of the DC voltage in the DC circuit can be switched, it causes considerable harmonics in the AC voltage that is produced, and these are undesirable. Furthermore, the withstand voltage of the switching elements in the voltage inverter restricts the amount of power which can be transmitted from one electrical network to the other, since the switching elements in the voltage inverter must be designed to withstand at least the value of the DC voltage in the DC circuit. Although appropriate switching elements which have a sufficiently high withstand voltage are generally known, such switching elements are expensive, and require complex additional circuitry.
In wind power systems, which are nowadays being increasingly used as alternative energy sources as energy resources disappear, transmission systems, in particular DC transmission systems such as long DC cables, are required for transmitting electrical power. One such wind power system is disclosed in DE 196 20 906 C2. The wind power system in this document has at least one wind power installation, with each wind power installation having a rotor-driven generator, which is connected via a rectifier unit to a DC transmission system. The DC transmission system is connected to a converter network coupling station, which is coupled via a network transformer to a conventional electrical supply network.
In a wind power system according to DE 196 20 906 C2, the DC transmission system is lossy, particularly if high power levels need to be transmitted, resulting from the use of a number of wind power installations. These losses increase drastically if, for example, the DC transmission system has a long DC line, such as those which are normally used for off-shore systems, owing to the long distance between the wind power installations at sea and the coast. At the moment, no suitable wind power system is known for solving these problems.
SUMMARY OF THE INVENTION
One object of the invention is therefore to specify a converter system for increasing a DC voltage, whose design is particularly simple and cost-effective, and which allows a high power level to be transmitted. A further aim is to develop a wind power system such that electrical energy is transmitted at high power levels from wind power installations in the wind power system via a DC transmission system to an electrical supply network with loss levels that are as low as possible and with little circuit complexity. These objects are achieved by the features of Claims 1 and 8. Advantageous developments of the invention are specified in the dependent claims.
The converter system according to the invention has a first voltage inverter, whose first and second input connections are connected to a DC circuit which is formed by at least two series-connected DC capacitances, which are interconnected at a center point connection. According to the invention, the center point connection is connected via a third input connection of the first voltage inverter to a center point voltage stage of the first voltage inverter, which is in the form of a stepping circuit for producing three switching voltage levels. The advantage of the stepping circuit and its connection to the center point connection is that more power can be transmitted than with a single-phase bridge circuit having two possible switching voltage levels, since twice the DC voltage can be used and switched in the DC circuit, with switching elements having the same withstand voltage. Furthermore, an advantageous reduction in harmonics is achieved, since three switching voltage levels can be switched, and it is thus possible to generate a more exact sinusoidal profile for the AC voltage that is to be produced.
In a wind power system having at least one wind power installation, each of which has a rectifier unit, and having a DC transmission system, the rectifier unit according to the invention is connected to the DC transmission system via the converter system for increasing the DC voltage according to the invention. This extremely advantageously means that the DC voltage, in particular the transmission DC voltage in the DC transmission system for transmitting a high power level, can be increased particularly easily and with very little circuit complexity, without having to redesign and use switching elements, particularly in the rectifier unit, for a higher withstand voltage. Furthermore, the increase in the transmission DC voltage advantageously results in the capability to transmit electrical energy from the wind power installation in the wind power system at a high power level, with minimal losses in the DC transmission system, to an electrical supply network.
REFERENCES:
patent: 4462070 (1984-07-01), Iida
patent: 5514944 (1996-05-01), Miyazaki
patent: 19620906 (1998-01-01), None
patent: 19620906 (1998-01-01), None
patent: 0630099 (1994-12-01), None
“Modular DC-DC converter for high-output voltage applications”, S.N. Manias, et al., IEEE Proceedings-B, vol. 140, No. 2, Mar. 1993, pp. 97-102.
“A Switched-Mode Three-Phase Three-Level Telecommunications Rectifier”, Roger Gules, et al., Dept. of Electrical Engineering, Power Electronics Institute, Federal University of Santa Catarina, Florianopolis, Brazil, Jun. 6, 1999, pp. 1-7.
“A Novel Three-Level ZVS PWM Inverter Topology for High-Voltage DC/DC conversion Systems with Balanced Voltage Sharing and Wider Load Range”, In-Ho Song, et al., Dept. of Electrical Engineering, Hanyang University, Oct. 6, 1996, pp. 973-979.
ABB (Schweiz) AG
Berhane Adolf D.
Burns Doane Swecker & Mathis L.L.P.
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