Electric power conversion systems – Current conversion – Including an a.c.-d.c.-a.c. converter
Patent
1999-09-20
2000-11-07
Nguyen, Matthew
Electric power conversion systems
Current conversion
Including an a.c.-d.c.-a.c. converter
363 34, H02M 547, H02J 300
Patent
active
061445673
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a plant for transmitting electric power comprising at least one VSC-converter adapted to convert direct voltage into alternating voltage and conversely and has an alternating voltage side to which at least one phase of an alternating voltage network is connected and a direct voltage side having two poles with a direct voltage thereacross.
The expression alternating voltage network is here to be given a very broad meaning comprising not only the case of a conventional alternating voltage network, but all types of electric connections on which an alternating voltage is present, such as for example output terminals of an AC-generator. With respect to the direct voltage side this may advantageously be connected to direct voltage networks for transmitting power, but it could have any other conceivable connections.
In order to illustrate but not in any way restrict the invention, the particular application of such a plant for transmitting electric power through a direct voltage network for High Voltage Direct Current (HVDC) may be mentioned. Such a plant has recently become known through the thesis "PWM and control of two and three level high power voltage source converters" by Anders Lindberg, Kungliga Tekniska Hogskolan, Stockholm, 1995, in which publication a plant for transmitting electric power through a direct voltage network for High Voltage Direct Current (HVDC) is described. Before the issuance of the thesis plants for transmitting electric power through a direct voltage network for High Voltage Direct Current have been based upon the use of line-commutated CSC(Current Source Converter)-converters in stations for power transmission. By development of IGBTs (Insulated Gate Bipolar Transistor=bipolar transistor having an insulated gate) for high voltage applications and the suitability to connect them in series in valves of converters, since they may easily be turned on and off simultaneously, VSC(Voltage Source Converter)-converters for forced commutation have now instead become an alternative. This type of transmission of electric power between the direct voltage network for High Voltage Direct Current being voltage-stiff therethrough and alternating voltage networks connected thereto offers several important advantages with respect to the use of line-commutated CSCs in HVDC. For example the consumption of active and reactive power may be controlled independently of each other and there is no risk of commutation failures in the converter and thus no risk of transmission of commutation failures between different HVDC-links, which may take place in line-commutation. Furthermore, it is possible to feed a weak alternating voltage network or a network without any generation of its own (a dead alternating voltage network). Further advantages are also there.
In a plant of such design, but also in plants of the type of other designs defined in the introduction, it is desired to manage without expensive transformers and still have the possibility to obtain exactly the voltage asked for on the direct voltage side. It is then desirable to obtain this while generating as low losses as possible on achieving this voltage and at the same time to obtain as low demands as possible upon the current handling capability of the components utilized for creating the voltage from the alternating voltage delivered to the alternating voltage side.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a device of the type defined in the introduction, through which the objects mentioned above are satisfied.
This is according to the invention obtained by providing the plant with at least one DC/DC-converter having two current valves connected in series and an inductance connected to a midpoint therebetween, and connecting the DC/DC-converter through the inductance to a first of the poles and through a first output terminal thereof connecting to one of the current valves to the second of the poles so as to produce an unbalanced step-up-transformation of
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Asplund Gunnar
Eriksson Kjell
Asea Brown Boveri AB
Nguyen Matthew
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