Electric power conversion systems – Current conversion – Using semiconductor-type converter
Reexamination Certificate
2001-12-03
2003-02-11
Sterrett, Jeffrey (Department: 2838)
Electric power conversion systems
Current conversion
Using semiconductor-type converter
C363S137000
Reexamination Certificate
active
06519169
ABSTRACT:
FIELD OF THE INVENTION AND PRIOR ART
The present invention relates to a VSC-converter for converting direct voltage into alternating voltage and conversely and which has at least two phase legs having each at least two current valves connected in series, said valves consisting of at least a semiconductor element of turn-off type and a rectifying member connected in anti-parallel therewith, a mid point of the phase leg between said valves being adapted to form a phase output and to be connected to a phase of an alternating voltage network
Such a VSC-converter for connection between a direct voltage network and an alternating voltage network is already known through for example the thesis “PWM and control of two and three level High Power Voltage Source Converters” by Anders Lindberg, Royal Institute of Technology, Stockholm, 1995 in which publication a plant for transmitting electric power through a direct voltage network for high voltage direct current (HVDC) while utilizing such a converter is described. Before the creation of this thesis plants for transmitting electric power between a direct voltage network and an alternating voltage network have been based upon the use of network commutated CSC(Current Source Converter)-converters in stations for power transmission. However, in this thesis a totally new concept is described, which is based on instead using VSC(Voltage Source Converter)-converters for forced commutation for transmitting electric power between a direct voltage network being voltage stiff therethrough, in the case in question for high voltage direct current, and alternating voltage networks connected thereto, which offers several considerable advantages with respect to the use of network commutated CSC-converters in HVDC, among which it may be mentioned that the consumption of active and reactive power may be controlled independently of each other and there is no risk for commutation faults in the converter and thereby no risk for transmitting commutation faults between different HVDC-links, which may take place for network commutated CSC-s. Furthermore, there is a possibility to feed a weak alternating voltage network or a network without any generation of its own (a dead alternating voltage network). There are also further advantages.
The invention is not restricted to this application, but the converter may just as well be intended for conversion in an SVC in which the direct voltage network is then replaced by a DC-intermediate link. “Network” is also to be given a very broad sense, and it has not to be a question about any such networks in the normal meaning of this word. The voltages of the direct voltage side of the converter are advantageously high, 10-400 kV, preferably 50-400 kV.
When transmitting direct voltage on a direct voltage network connected to the converter it is desired to have a voltage being as high as possible, since the transmission losses are reduced when the voltage increases. However, an increase of this voltage means that the number of semiconductor elements of turnoff type connected in series in the current valves of the converter has to be higher so as to together be able to take the voltage required, which is distributed among them. This means that the cost for the high number of semiconductor elements of turn-off type and rectifying members (rectifying diodes) will be high. This cost is particularly unfavourable when transmitting low powers on the direct voltage network, since the fact that the power is proportional to the voltage will mean a much lower voltage across the converter than the voltage it is dimensioned for, so that this and also the conductor of the direct voltage network, which is designed for higher voltages, are poorly utilized. Thus, the cost per volt becomes in such a case very high.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a VSC-converter of the type defined in the introduction, which is more cost efficient than the converters already known with respect to the ability to generate certain voltage levels on the direct voltage side of the converter.
This object is according to the invention obtained by connecting the phase legs of such a converter in series, and the opposite ends of the series connection formed by an outer end of the respective outer phase leg in the series connection are intended to be connected to a pole conductor each of a direct voltage network.
By connecting the phase legs of the converter in series in this way between the two pole conductors of the direct voltage network each current valve will at a determined voltage of the direct voltage side have a lower voltage to hold in the blocking state thereof than in VSC-converters already known, where the different phase legs are connected in parallel between the two pole conductors of the direct voltage side, so that a lower number of semiconductor elements of turn-off type and rectifying members connected in series or such ones dimensioned for lower voltages and thereby being less expensive may be used in each current valve for obtaining the voltage in question. Another way to express this is to say that for a given set of semiconductor elements of turn-off type and rectifying members of the converter and thereby a given cost thereof a higher voltage may be obtained on the direct voltage side than for a VSC-converter already known through the design of the converter according to the invention, so that the cost per volt may be reduced. More exactly, a cost so defined per volt will be at the most ½ (in the case of a converter having only two-phase legs) of the cost of a converter of this type already known. However, in the practice three phases are mostly used which then means a reduction of the cost per volt with respect to semiconductor elements of turn-off type and rectifying members to a third, and it is also especially advantageous to connect exactly three phase legs in series in this way, since the mutual phase displacement of 120° of the voltage therebetween results in a uniform power flow through the converter at the series connection of the phase legs.
According to a preferred embodiment of the invention a transformer is connected to each phase output between this and the alternating voltage network phase belonging thereto. A transformer normally already there for adaption of the voltage between the alternating voltage network and the direct voltage network may in this way be utilized for realizing said series connection. One winding of the respective transformer is then preferably connected through a first end to the phase output of the phase leg and through a second end in that way to the current valve that a closed loop through the current valve is formed.
According to another preferred embodiment of the invention one winding of the respective transformer is connected through a first end to the phase output of the phase leg and through a second end to a midpoint between two capacitors connected in series in parallel with the current valves of the phase leg. The invention may in this way easily be realized, in which each phase leg then preferably has two current valves connected in series, as in a conventional 2-pulse bridge. The return current to the alternating voltage network will in this case go through the two capacitors.
According to another preferred embodiment of the invention each phase leg has a NPC-connection, i.e. four current valves connected in series, in which one point of the phase leg between the two inner valves of the series connection forms said phase output, and a series connection of two so called clamping rectifying members directed in the same direction with respect to said series connection as said rectifying members is connected between on one hand a point between one outer valve in the series connection and the inner valve next thereto and on the other a point between the other outer valve of the series connection and the inner valve next thereto with a midpoint between the two clamping rectifying members connected to a zero potential defined by capacitors connected
Asplund Gunnar
Bijlenga Bo
ABB AB
Dykema Gossett PLLC
Sterrett Jeffrey
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