Title of Invention	"A POWER CONVERSION SYSTEM"
Abstract	A powar conversion system including a plurality of converter transformers. AC side windings of the converter transformers are connected in series for connecting to an AC power system. The power conversion system further includes a plurality of series connected voltage source type self-commutated converters for converting AC power into DC power or DC power into AC power. Each of the voltaga source type self-commutated converters is connected to one of DC aide windings of the converter transformers, respectively. The power conversion system further includes a plurality of DC voltage sources. Each of DC output sides of the voltage source type self-commutated converters is connected to one of the DC voltage sources, respectively. The power conversion system further includes a control system for controlling the voltage source type self-commutated converters such that each of DC voltages of the voltage source type self-commutated converters follows to one of DC voltage reference values for the voltage source type self-commutated converters, respectively.

Title of Invention

"A POWER CONVERSION SYSTEM"

Abstract

A powar conversion system including a plurality of converter transformers. AC side windings of the converter transformers are connected in series for connecting to an AC power system. The power conversion system further includes a plurality of series connected voltage source type self-commutated converters for converting AC power into DC power or DC power into AC power. Each of the voltaga source type self-commutated converters is connected to one of DC aide windings of the converter transformers, respectively. The power conversion system further includes a plurality of DC voltage sources. Each of DC output sides of the voltage source type self-commutated converters is connected to one of the DC voltage sources, respectively. The power conversion system further includes a control system for controlling the voltage source type self-commutated converters such that each of DC voltages of the voltage source type self-commutated converters follows to one of DC voltage reference values for the voltage source type self-commutated converters, respectively.

Full Text	The present invention relates to a power conversion system. BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a power conversion system, and more particularly to a power conversion system composed of voltage source type self-commutated converters in a multiplexed configuration and a control system of these voltage source type self-commutated converters when these voltage source type self-commutated converters are applied to such systems as DC transmission systems, fuel cells, battery energy storage systems, and reactive power compensation systems. DESCRIPTION OF THE RELATED ART A power conversion system composed of conventional voltage source type self-commutated converters in a multiplexed configuration and a control system of these voltage source type self-commutated converters will be described hereinafter with reference to FIGURE 14 and FIGURE 15. In FIGURE 14, 1 is an AC system, 2 is a potential transformer to measure an Ac system voltage of Ac system 1, 3 is a current transformer, and 4A, 4B are converter transformers to connect voltage source type self-commutated converters 5A, 5B to AC system 1. 6 is a DC voltage source composed of DC capacitor/ cell, etc., 1 is a DC voltage detector, and 8 is art active/reactive power detector to detect active and reactive powers that are output from converters 5A, SB by inputting detected values of potential trans-former 2 end current Lransformer 3. 9 is a DC voltage controller to input a difference between a DC voltage reference 51 and a DC voltage detected value 52 detected by uc voltage detector 7 and to control the DC voltage so as to make the difference zero. 10 is a reactive power controller to input a difference between a reactive power reference 53 and a reactive power detected value 54 detected by active/reactive power detactor B and to control the reactive power so as to make the deference zero. 11 is an AC current controller to control the AC current to a reference value by inputting on output of DC voltage controller 9, an output of reactive power controller 10, an AC current detected value 55 detected by current transformer 3 and an AC voltage detected value 56 detected by potential transformer 2, 12 is a pulse width modulation circuit to decide pulse patterns of self-turn-off devices composing each of self-commutated converters 5A, 5B according to the output of AC current controller 11, gate turn-off thyristors (hereinafter referred tc as Gio) and 14A - 14L are diodes, Furthermore, 1A, IB and 1C designate A-phase, B-phase and C-phase of AC system 1, respectively. Tha principle of control of active/reactive power of voltag* source type self-commutated converters 5A, 5B connected to AC system 1 is disclosed in a publication titled "Semiconductor Power Conversion Circuit (published from The Institute of Electrical Engineers of Japan), P.215 - 220, ate. and the detailed deacripfcion wiil be therefore omitted he:re. In addition, the principle and realizing a method of a constant currant control circuit are disclosed in Japanese Patent Publication (Kofcai) No. Hei 1-77110, and therefor, the detailed description will be omitted here. In FIGURE 15, the AC system side windings of converter transformers 4A, 4B are connected in series, the EC side windings thereof are connoted to each of converters 5A, 5B individually, and the DC outputs of voltage source type self commutated converters 5A, 5B are connected in parallel with • ach other. In. this configuration, as for the AC iiystem output voltage, the outputs of converters 5A and 53 are added through the AC windings of converter transformers 4A, 4B and hiyher harmonics thereof are negated. In addition, as the AC windings of converter transformers 4A, 4B are connected in series, the current values of converters 5A, 5B become the same. Further, ss the DC sides of converters 5A- 5R are converters SA, 5B become the same. In the configuration shown in FIGURE 14, higher harmonics can be decreased in the AC system output voltage by converter* 5A, B, and in addition/ no unbalanced current and voltage will be generated between converters 5A, SB. The multiplexed configuration of conventional voltage source type self-commutated converters shown in F1GURE 14 has the following problem when considering the application of it to, such as, a DC transmission system tc transmit DC power over a long distance. That is, as DC outputs of tie converters are connected in parallel, if the number of converters is increased for attaining the large capacity, DC current will increase accordingly. In case of the DC transmission system, a DC line is long and the resistance of a DC transmission line is large. If DC current increases, the power loss by the resistance of the DC transmition line also increases in proportion to a square of DC currant, and the efficiency of the system will drop. Therefore, when applying the conventional power conversion system to, such as, the DC transmission system, in case of increasing the systen capacity, it is preferred to increase the DC voltaghe rather than increasing the DC current from the viewpoint of reducing the loss. It can be also considered, as one method to increase the DC voltage, to increase rated DC voltage of the converters self-commutated converters with a large capacity, as it is not possible to increase switching frequency of the converter from the view point of reducing the switching loss, the multiplexing of converters becomes indispensable for reducing higher harmonics. As described above, instead of the conventional multiplexed configuration of converters to connect the DC sides of converters in parallel, a power conversion system with the multiplexed configuration of converters and its control system which does not increase DC current is demanded. SUMMARY OF THE INVENTION Accordingly, one object of this invention is to provide a power conversion system composed of voltage source type self-commutated converters in a multiplexed configuration and a control system of these voltage source type self-commutated converters which does not increase DC currents of these voltage source type self-commutated converters. Accordingly, there is provided a power conversion system, comprising a plurality of converter transformers; AC side windings of said converter transformers being connected in series for connecting to an AC power system; a plurality of series connected voltage source type self-commutated converters for converting AC power into DC power or DC power into AC power; each of said voltage source type self-commutated converters being connected to one of DC side windings of said converter transformers, respectively; a plurality of DC voltage sources; each of DC output sides of said voltage source type self-commutated converters being connected to one of said DC voltage sources, respectively; and control means for controlling said voltage source type self-commutated converters such that each of DC voltages of said voltage source type self-commutated converters follows to one of DC voltage reference values for said voltage source type self-commutated converters, respectively. According to one aspect of this invention, there is provided a power conversion system including a plurality of converter transformers. AC side windings of the converter transformers are connected in series for connecting to an AC power system. The power conversion system further includes a plurality of series connected voltage source type self-commutated converters for converting AC power into DC power or DC power into AC power. Each of the voltage source type self-commutated converters is connected to one of DC side windings of the converter transformers, respectively. The power converter further includes a plurality of DC voltage_ sources. Each of DC output sides of the voltags source type selt-coraautated converters is connected to one of the DC voltage sources, respectively. Th power converter further includes a DC voltage source connected in parallel with the series connected voltage source type self-com»uta:ed converters, and a control system for controlling l.he voltage source type self-commutated converters such that each of DC voltages of the voltage source type self-commutated i converters except one of the voltage source type self-coiwautated converters follows to one of DC voltage reference values for the voltage source type selff-commutated converters, respectively. According to another aspect of this Invention, there is provided a power conversion system including a plurality of converter transformers. AC side windings of the converter transformers are connected in series for connecting to an AC power system. The power conversion system further includes a plurality of scries connected voltage source type self-commutated converters for converting AC power into DC power or DC power into AC power. Each of the voltage source type self-commutated converters is connected to one of EC side windings of the converter transformers, respectively. The power conversion system further includes a plurality of DC voltage sources. Each of DC output sides of the voltage source tvrsa self -communated the DC voltage sources, respectively. The power conversion system further includes a DC voltage detector for detecting a DC voltage of the power conversion system, and a control system for controlling the voltage source type stlf-commutated converters such that each of DC voltaces of the voltage source type self-commuteted converters except one of the voltage source type self-commutated converters follows to one of DC voltage reference values for the voltace source type sell-commutated converters, respectively, and that the DC voltage of the power conversion system follows to a DC voltage reference value for the power conversion system. BRIEFE DESCRIPTION ON OF THE DRAWINGS A more complete appreciation of the invention and Many Of the attendant advantages thereof will ba readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: FIGURE 1 is a diagram showing the construction of a power conversion system according to a first embodiment of tnis invention; FIGURE 2 is a diagram showing the construction of a power conversion system according to a modification of the first embodiment of this invention; FIGURE 3 is a dianram showing the power conversion system according to & second embodiment of this invention; FIGURE 4 is a diagram showing the construction of a power conversion system according to a third embcdiment of this invention; FIGURE 5 is a diagram showing the construction of a power conversion system according to a fourth embodiment of this invention; FIGURE 6 is A diagram showing chc construction of a power conversion system according to a fifth embodiment of this invention; FIGURE 7 is a diagram showing the construction of a power' conversion system according to a modification of the fifth embodiment of this invention; FIGURE 8 is a diagram showing the construction of a power conversion system according to a sixth embodiment of this invention; FIGURE 9 is a diagram showing the construction of a power conversion system according to a modification of the sixth embodiment of this invention; FIGURE 10 is a diagram showing the construction of a part of a power conversion system according to a seventh embodiment of this invention} FIGURE 11 is a diagram showing the construction of a part of » power conversion system according to an eighth embodiment of this invention FIGURE 12 is a diagram showing the construction of a parL of a power conversion system according to a ninth embodiment of this invention; FIGURE 13 is a diagram showing the construction of a part of a power conversion system according to a tenth embodiment of this invention; FIGURE 14 is a diagram showing the construction of one example of a conventional power conversion ays ten.; and FIGURE 15 is a diagram showing 'the construction of one example of a part of the conventional power conversion system shown in FIGURE 14. DETRAILED DESCRIPTION OF THE PREFERREDEMBODIMENTS Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, the embodiments of this invention will be described below. FIGURE 1 is a diagram showing a power conversion system according to a first embodiment of the present invention, in FIGURE 1, the same components are assigned with the same reference numerals as in FIGURE 14 and FIGURE 15 and their explanations are omitted. in FIGURE 1, voltage source type salf-commutated as, DC capacitors 6A, 66, the negative side of the DC output of self-commutated converter 5A is connected to the positive side of the DC output of self-conunutated converter SB, and thereby self-commutated converters 5A and 53 are connected in series. In such the configuration as described above, it becomes possible to provide a power conversion system which makes DC voltage 2 times and DC current 1/2 when compared with a conventional multiplexed configuration of the unit converters (converters 5A, SB) with the equal capacity with the DC sides connected in parallel with each other. In the multiplexe configuration as shown in FIGURE 1, as for the AC system output voltage, the outputs of converters 5A and 5B are added through the AC windings of converter transformers 4A and 4B, and higher harmonics thereof are thereby negated. Further, as the AC winding* of converter transformers 4A, 4B are connected in series, the current values of converters 5A, 5B become equal. However, at the DC side* of converters 5A, SB are connectei in series. DC voltages of converters 5A, 53 may possibly be unbalanced by such factors as the delay in 3ignal transmission, the variations in switching, the variations in main circuit constants such as the capacitances of DC capacitors 6A, 6B, etc. So, in FIGURE 1, by inputting a difference between a DC and a DC voltage reference SlA into a DC voltage controller 9A, DC voltage ot converter 5A is controlled so That it becomes in accord with DC voltage reference 5lA of converter 5A. Similarly, by inputting a difference between a DC voltage of converter 5B detected by a DC voltage detector 78 and a DC voltage reference SIR into a DC voltage controller 98, DC voltage of converter 5B is controlled so that it becomes in accord with DC voltage reference 51B cf converter 58. As the reactive power does not directly relate to the DC voltage/ the circuit with respect to the reactive power can be the same as that shown in FIGURE 14, which is the conventional multiplexed configuration. An AC current controller is provided individually to each converter. In FIGURE 1, the current at the DC winging Side of converter transformer 4A is detected by a current transformer 3A as an AC current detected value 5SA, and is controlled by an AC current controller 11A by inputting an AC voltage detected value 56 from potential tranaforner 2 so that AC current detected value 55A becomes m accord with command values from DC voltage controller 9A and reactive power controller 10, Similarly, the current at the DC winding aide of converter transformer 4B is dete current transformer 3B as an AC current detected value 55B, ana is controlled by an AC currant controller IIB by detected value 55B becomes in accord with commanc values from DC voltage controller 9B and reactive power controller 10. Pulse width modulation circuits 12A, 12B decide pulse patterns of self-turn-off devices composing each of self-commutated converters 5A, SB according to the outputs of AC current controllers 11A, lis, respectively. In the multiplexed configuration as shown in FIGURE 1, if the DC voltage of converter 5A detected by DC voltage detector 7A is higher than DC voltage reference MA, DC voltage controller 9A outputs a current command to lower the DC voltage of converter 5A, and AC current controller 11A controls an output voltage command value for converter 5A so as to follow the current command value. As a result, the DC voltage of convtrter SA becomes in accord with DC voltage reference 51A. similarly the DC voltage of converter 5B is brouaht to agr«a with DC voltag reference 513. Accordingly, an unbalanced voltage between two converters 5A, SB is dissolved. FIGURE 2 ia a diagram showing a modification of the first embodiment shown in FIGURE 1. The same components as those shown in FIGURE 1, 14 and 15 are assigned with the same reference numerals and their explanations are omitted. 15 is an active power controller which, by inputting a difference between an active power reference 57 and an active power detected value 58 detected by active/reactive powor detector zero. In FIGURE 2, AC currant controller 11 Is provided instead of AC currant controllers 11 A, 11B in FIGURE 1. in FIGURE 2, AC current ia controlled by AC current controller 11 by controlling AC system current 55 detected by current transformer 3. AC current controller 11 controls AC system current 55 by inputting AC voltage detected value 56 so as to bring it to agree with an active current command value 59 1 which ia the output of active power controller 15 and a reactive current command value 60 which is the output of reactive power controller 10. In this time/ an output 61A of DC voltage controller 9A of converter 5A and an output 61B of DC voltage controller 9B of converter 5B are also input hr> AC current controller 11, respectively, to correct tie output of AC current controller 11 to pulse width modulation circuits 12A, 128 and thus, unbalanced voltage between two converters SA/ 5B is dissolved. In FIGURE 2, as DC voltages of two converters SA, 5B are adjusted by controlling thu AC system current by ona AC current controller 11 and correcting output voltage command values that are output to pulse width modulation circuits 12A, 12B, there is no interference between the AC currant control system and the DC voltage control system and thereby the power conversion system can, be stably controlled. FIGURE 3 shows a power conversion system aeco.rdxng to a same components already shown in FIGURES 1/ 2, U, 15 are assigned with the sane reference numerals and thuir explanations are omitted. 16 is a large capacity DC voltage source such as fuel cells, secondary cells, voltjge source type converters, etc. In FIGURE 3, the entire DC voltage value including DC voltages of salf-comunated converters 5A, 58 is decided by-this large capacity DC voltage source 16. Even when each converter 5A ,5B has a DC voltage reference and tries to control ita DC voltage, if a sum of respective DC voltage references does not agree with the output voltage of large capacity DC voltage source 16, each converter 5A, 5B is not able to control its DC voltage/ and the DC voltage controller is then saturated. So, as shown in FIGURE 3, one self-commutated converter 5B does not control its DC voltage and the remaining self-commutated converter 5A controls its DC voltage SO that the'DC voltage Of converter 5A detected by DC voltage detector 7A becomes in accord with DC voltage reference S1A. When DC voltage is controlled as shown in FIGURE 3, it is not necessary that converter 5B controls its DC voltage, because its DC voltage is primarily decided by a value of voltage of large capacity DC voltage source 16 minus DC voltage of converter 5A. As a result, DC voltage controller 9A is not saturated and it becomes possible to control desired active power. Mtnough the power conversion system is described using of more than two converters connected in series, when, except one converter, each of the remaining converters controls its DC voltage so as to agree with DC voltage reference of each converter, the power conversion system can be stably controlled without expanding unbalanced DC voltages between converters and saturating the DC voltage controller. FIGURE 4 shows a power conversion system acccrding to a third embodiment of the present invention. In FIGURE 4, the same components As those shown in already explained figures are assigned with the same reference numerals and their explanations are omitted. 7 is a DC voltage detector to detect DC voltage of large capacity DC voltage source 16, and 17A, 17B are operational amplifiers to multiply tne DC voltage detected by DC voltage detector 7 by fixed factors KA, KB, respectively. The output DC voltage of large capacity DC voltage source 16 such as fuel cells, secondary cells, etc. generally fluctuates by about ±{20 - 30)% according to the discharge state of cells and the DC current value. In the configuration shown in FIGURE 4, DC voltage references 51A, 51B of converters 5A, 5B are not fixed, but values obtained by multiplying the DC voltage by fixed factors KA, KB are used as DC voltage reference 5:,A, 51B of converters 5A, 5B, respectively, and converters 5A, SB control their output voltages to follow DC voltage: references 51A, 51B, respectively. As a result, no unbalance is the DC voltage of large capacity DC voltage source 16 fluctuates. Further, fixed factors KA, KB for DC voltage references 51A, 51B are so decided that the sum of factors KA, KB for all converters 5A, SB becomes 1. Although, the power conversion system is described using two converters in FIGURE 4, in a multiplexed configuration of more than two converters connected in series, if the DC voltage reference of each i converter is decided by multiplying the detected DC voltage value by a fixed factor and the DC voltage of each converter is controlled so as to follow the DC voltage reference, the power conversion system also can be operated stably without generating unbalanced DC voltage between converters even when voltage of the large capacity DC voltage source, such as fuel calls, secondary cells, etc. fluctuates. FIGURE 5 shows a power conversion system according to & fourth embodiment of the present invention. In FIGURE 5, the same components as those shown in the already explained figures are assigned with the same reference numerals and their explanations are omitted. In FIGURE 5, DC voltage is controlled for converter 5A and not for converter 5B, the same as the embodiment shown in FIGURE 3. In FIGURE 5, the entire DC voltage value including DC voltages of self-commutated converter 5A and 5B is decided by large capacity DC voltage source 16. As DC voltago reference voltage detector i by fixed factor KA, a sum of DC voltages of all converters oA, 5B does not differ from tho output voltage of large capacity DC voltage source 16. However, if there are errors, etc. of DC vol:age detectors of converters, these errors are accuroul.ated and the DC controller nay be saturated. Therefore, in th« embodiment shown in. FIGURE 5, one self-coimutated converter SB performs only the active power control without executing the DC voltage control. Remaining 8elf-coBBtiutat»d converter 5A controls its DC voltage so that the DC voltage detected by DC voltage detector ?A comes to agree with DC voltage reference 51A that is obtained by multiplying the entire DC voltage detected value by fixed factor KA. When the power conversion system, is controlled as shown in FIGURE 5, DC voltage of converter SB is primarily decided by the voltage of larcre capacity DC voltage source 16 minue DC voltage of converter 5A. Accordingly, even when there are errors of the detectors, the controller is not saturated and the power conversion system can be stably controlled. Further, although two converters are used for explanation in FIGURE 5, in the multiplexed configuration of more than, two converters connected in~saries, If DC voltage is so controlled that DC voltage of each converter except one Converter comes to agree with the DC voltage reference of each converter which is decided by multiplying the entire DC system car. be stably controlled without expanding unbalanced DC voltage between converters, and generating th» saturation of the DC voltage controller even when the DC vol.tage of the large capacity DC voltage source fluctuates. FIGURE 6 shows a power conversion system according to a fifth embodiment of the present invention. In FI3UBX 6, the same components shown in the already explained figures are assigned with the same reference numerals and the explanation is omitted. It, FIGURE 6, a DC voltage of added outputs of converters SA, 5B, that is, a DC output voltage of the power conversion system is detected by DC voltage detector 7. DC voltage of the power conversion system is controlled by DC voltage controller 9 so that the DC voltage detected by nc voltage detector 7 becomes in accord with DC voltage reterence 51 of the power conversion system. On the other hand, DC voltages of converter© >A, SD are controlled by DC voltage controllers 9A, 9B so th«.t they become in accord with DC voltage references 51A, SIB, respectively. In such the configuration, DC voltaje controller 9 is provided to control the entire DC voltage and the entire DC voltage is easily controllable when another power conversion system is connected to this power conversion system via a DC bus and it is required to maintain the entire DC voltage at a constant level. FIGURE / is a diagram showing a modification of the components as Chose shown in the already explained figures are assigned witn the same reference numerals and the explanation is omitted. In FIGURE 7, values obtained by multiplying the entire DC voltage detected by DC voltage detector 7 by fixed factors KA, KB are used as DC voltage references S1A. S13 of converters 5A, SB, respectively. In the configuration as shown in FIGURE "7, even when the entire DC voltage fluctuates transiently due to system failure, disturbance, etc., the DC voltage control system of converters 5A, SB control their DC voltages so as to take the balance against the entire DC voltage, respectively. Therefor*, overvoltage can be prevented from being applied to either one of the converters 5A, 5B. The action of this embodiment will further be described using FIGURE 6. In FIGURE 6, there are provided three DC voltage controllers: DC voltage controller 9 -o ccntrol the entire DC voltage of the power conversion system; DC voltage controller 9A to control DC voltage of converter 5A; and DC voltage controller 9B to control DC voltage of converter S3. On the other hand, as for the DC voltages, DC voltage of converter SA and that of converter SB are independent, respectively, while this entire DC voltage oT the power conversion system is primarily decided by adding DC voltage of converter 5A and that of converter SB. Therefore, there are three controllers for two independent variable and if cannot be controlled stably. So. the entire DC voltage is controlled at a relatively low speed in DC voltage controller 9. As for DC voltages of converters SA, 5B, so as to balance DC voltages between converters bA, 5B, they are controlled in DC voltage controllers 9A, 9B at high speeds, respectively. Thus, DC voltages can be stably controlled by changing responses of three DC controllers 9, 9A, 98, respectively. Although FIGURE 6 and FIGURE 7 are explained using a configuration of two converters, in case of a multiplexed configure of more than two converters, the same effect is obtained when an entire DC voltage controller anc. DC voltage controllers for respective converters are provided. FIGURE 8 is a diagram showing a power conversion system according to a sixth embodiment of the present invention. In FIGURE 1, the same components as those shown in the already explained figures are assigned with the same reference numerals and the explanation is omitted. In FIGURE 8, an entire DC voltage with the outputs of converters 5A, 5B added, that is, a DC output voJtage of the power conversion system, is detected by DC voltac-e detector 7. The DC voltage of the power conversion system detected by DC voltage detector 7 is controlled by DC voltage controller 9 -so that it becomes in accord with DC voltage reference 51 of the power conversion system. On the other hand, the DC voltage of converter 5 is controll by the voltage controller 9A so that it becomes in Accord with DC voltage reference 51A. In such the configuration, DC voltage controller 9 is provided to control the entire DC voltage and the entire DC voltage is easily controlled when another power conversion system is connected to this power conversion system via a DC bus and it is required to maintain the entire DC voltage at a constant level. Further, as the DC voltage of converter 5B is not controlled, the response of DC voltage controller 9 for the entire DC voltage can be decided independently from the response of DC voltage of DC voltage controller 9A of converter 5A. Therefore, this configuration is suited especially when it is necessary to make the entires DC voltage control fast when demanded by the system. FIGURE 9 is a diagram showing a modification of the sixth embodiment shown in FIGURE 8. In FIGURE 9, the same components as those shown in the already explained figures are assigned with the same reference numerals and the explanation is omitted, in FIGURE 9, a value obtained by multiplying the entire DC voltage detected by DC vjltage detector 7 by a fixed factor KA is made as DC voltage reference S1A of converter 5A. In the configuration as shown in FIGUBE 9. ever whan the entire DC voltage fluctuates transiently due to system failure, disturbance, etc., the DC voltage control system of balance against the entire DC voltage. Therefore, overvoltage can be prevented from being applied to either one of the converters SA, 5B. Although FIGURE 6 and FIGURE 9 are explained using a configuration of two converters, in case of a multiplexed configuration of more than two converters, the sane errect is obtained when an entire DC voltage controller and DC voltage controllers for respective converters are provided except one converter. FIGURE 10 is a diagram showing a part of a power conversion system according to a seventh embcdimert of the present invention. FIGURE 10 shows an example of the construction of AC current controller 11 shown in FIGURE 9. In FIGURE 10, 19 is a phase detector to detect a phase of the AC system voltage from AC system voltage detected value 56. 19A is a coordinate converter to convert the AC system voltage into orthogonal biaxial components from AC system voltage phase detected value 62 detected by phase detector 18 and AC systexu voltage detected value 56, and 19B is a coordinate converter to convert AC current detected value 55 into an active current component 63 and a reactive current component 64 using AC systea voltage phase detected value 62. 19C is a coordinate converter to compute the output voltage, command value to be given to pulse width modulation, circuit 12A of converter SA, and 19D is a coordinate converter to width modulation circuit 12B of converter SB. 11-1 13 an AC current controller to control As current by inputting a difference between active current command value 59 that is the output of DC voltage controller 9 shown in FIGURE 9 and active current component 63 that ia obtained through the coordinate conversion of AC current detected value 55 and make the difference zero. 11-2 is an AC current controller to control AC current by inputting a difference between traactive current command value 60 that is the output of reactive power controller 10 ahown in FIGURE 9 and reactive current component 64 that is obtained through the coordinate conversion of AC current detected value 55 and make the difference zero. Out of the actions of AC current controller li shown in FIGURE 10, a method to convert three-phaae AC voltage and current into DC quantities through the coordinate ronvsrsion and control them is an already generally known method as an AC current control method of voltage source type self-commutatad converter, and therefor, its explanation is omitted here. An active current correction value 61A shown in FIGURE 10 ia the output of DC voltage controller 3A that acts so as to control a DC voltage detected value 52A of converter 5A in - accord with the value that ia obtained by multiplying total DC voltage detected value 52 by^fixed factor KA in between converters SA and SB. The output voltage command value tor converter SA before the coordinate conversion that is input to coordinate converter 19C is corrected by active current correction value 61A. In the configuration shown in FIGURE 1C. as t:ie output voltage command value for converter 5A is corrected before the coordinate conversion by the DC voltage control system after controlling active current component and reactive current component of the AC system current independently, AC current controller 11 and DC voltage controller 9A. shown in FIGURE 9 can be controlled with less interference between them. FIGURE 11 is a diagram showing a part of a power conversion system according to an eighth embodimert of the present invention. In FIGURE 11 an example of the construction of current controller 11 already explained in FIGURE 7 is shown. In FIGURE 11, the same componeits as those already explained in FIGURE 10 are assigned with the same reference numerals and the explanation is omitted. In FIGURE 11/ AC current is controlled by AC current controller 11-1 so that active current command value 59 that is the output of DC voltage contiullm 9 of the power conversion system shown in FIGURE 7 becomes in accord with active current component 63. Further, AC current is controlled by AC current controller 11-2 so that reactive current command shown in FIGUR2 1 becomes in accord with reactive current component 64. Output voltage commend values for respective converters 5A, SB are corrected before the coordinate conversion by active current correction value 61A that is the output of voltage controller 9A to control DC voltage of converter 3A and active current correction value 61B that is the output of DC voltage controller 98 to control X voltage of converter SB, respectively. In the configuration shown in FIGURE 11, as th: output voltage command values for converters SA, 53 are corrected before the coordinate conversion by the DC voltage system after controlling active current component and reactive current component of the AC system current independently, AC current controller 11 and DC voltage controllers 9A, 9B shown in FIGURE 7 can be controlled with leas interference between them. Although FIGURE 10 and FIGURE 11 are explained using a configuration of two converters, in case of a multiplexed configuration of more than two converters, the iame effect ia obtained by correcting an output voltage command value of each converter by the output of the DC voltage controller of each converter. Further, the correction methods shown in FIGURE 10 and FIGURE 11 are explained with respect to the power conversion systems shown in FIGURE 9 and FIGURE 7, respectively. A conversion systems shown in FIGURES 2, 3, 4, 5, 6 and 8. FIGURE 12 is a diagram showing a part of a power conversion system according to a ninth embodiment of the present invention. FIGURE 12 shows an example of the construction of current controller 11 already explained in FIGURE 9. In FIGURE 12, the Same components as those shown in the already explained figures are assigned with the same reference numerals and the explanation is omitted. In FIGURE 12, 20 is a polarity judging device to judge the polarity or active current component 63, 21 is a switch that is changed over by the output of polarity judging device 20, and 22 is an inverter to change over the polarity of active current correction value 61A. In FIGURE 9, as AC side windings of converter transformers 4.A, 4B are connected in series, the waveforms of the output currents of covverters 5A, 5B become the same unless there are DC magnetizations, etc. in converter transformers 4A, 43. Accordingly, when DC voltages of DC capacitors 6A, 6B of converters 5A, 55 are unbalanced and it is desirable to adjust the unbalanced DC voltages by changing active powers of converter 5A, 5B, it is required t:o change voltage waveforms of converters 5A, SB, respectively. Generally, if the current waveforms are the sane, active powers can be changed by changing the amplitudes of output voltages, respectively. Here, the relation among the voltage of DC capacitor is considered. Now, it is assumed that the direction of current from the AC system to the converter is the forward direction. When operating as the rectifier, converter output current and converter output voltage are in the waveforms of the same polarity. When the amplitude of the output voitags le made larger, the active power in the direction of rectifier increases, and when the amplitude of the output voltage is made smaller, the active power in the direction of rectifier decreases. When considering the DC voltage of the DC capacitor, a converter provided with a DC capacitor of lower DC voltage increases the active power in the direction of rectifier by making the amplitude of its output voltage larger and thereby to charge the DC capacitor more, A converter provided with a DC capacitor of Mgher DC voltage reduces the aetivo power in the direction of rectifier by making the amplitude cf the output voltage smaller and thereby to suppress the charging of the DC capacitor. Next, inverter operation is considered. In inverter operation, converter output current and converter output voltage are in the waveforms of the opposite polarity. When the amplitude of the output voltage is made larger, the active power in the direction of inverter increases, and when the amplitude of the output voltage is made smaller, the When considering the DC voltage of the DC capacitor, a converter provided witn a DC capacitor of iower DC voltage decreases the active power in the direction of inverter by making the amplitude of its output voltage smaller and thereby to decraase the discharge of the DC capacitor. A converter provided with a DC capacitor of higher or. voltage increases the active power in the direction of inverter by making the amplitude of its DC voltage larger thereby to increase the discharge of the Dc capacitor. The relation between the magnitude of the DC voltage of the DC capacitor and the amplitude of the output voltage in the rectifier and inverter operation are summarizel as shown by Table. [Table] In Rectifier Operation In Invertat (Table Removed ) Generally, in tfte rectifier operation, an active current flows from the AC system to the converter, and. in the inverter operation, an active current flows from t'ie converter to the AC system. As can be see from Table, it is necessary to change the polarity of the correction for the amplitude of th output voltage based on the direction of the active current. 63 is judged by pclarity judging device 20. Switch 21 is changed over by the output of polarity judging device 20. As a result/ the correcting direction is changed by changing over an inverted active current correction value 61AA that is invented by inverter 22 and active current correction value 61A that is not inverted. In the configuration as shown in FIGURE 12, it is possible to continuously operate the power conversion system while taking the balance of DC capacitor voltages by properly correcting the,output voltage command values before the coordinate conversion even when the direction of the active current is changed. FIGURE 13 i.s a diagram showing a part of a power conversion system according to a tenth embodiment of the present invention. FIGURE 13 shows one example of the construction of current controller 11 already explained in FIGURE 1. In FIGURE 13, the same components as those shown in the already explained figures are assigned with the same reference numerals and the explanation is omitted. In FIGURE 13/ 20 is polarity judging device to judge the pol.arity of active current component 63. 21A, 21B are switches: that are changed over by the output of polarity judging device 20, and 22A, 22B are inverters to change over the polarities of active current correction values 61A, 61B, respectively. In . the configuration as shown in FIGURE 13, similarly to the embodiment shown in FIGURE 12, it is possible to continuously of DC capacitor voltages by properly correcting the output voltage command values before the coordinate conversion even when the direction of the active current is changed. Although FIGURE 12 and FIGURE 13 are explainec using a configuration of two converters, in case of a multiplexed configuration of more than two convertare, the aamo effect ia obtained when the polarity of the correction value by the DC voltage controller of each converter is changed ov»r according to the polarity of the active current component. Further, the correction methods shown in FIGURE 12 and FlGURE is are explained with respect to the power conversion systems shown in FIGURE 9 and FIGURE 7, respectively. A similar correction method is also applicable to the power conversion systems shown in FIGURES 2, 3, 4, 5, 6 and 8. In the embodiments described above, the DC voltage control is performed by using DC capacitors as DC voltage sources. If, for instance, fuel cells, batteries, eLc. are assumed to be used as DC voltage sources, this invention can be applied to such power conversion system by replacing DC voltage controllers by active power controllers which control the active power detected by active/reactive power detector 6. Further, although reactive power controller 10 is used in the embodiments described above, reactive power controller the AC voltage of AC system 1. Further, this invention is further applied to a power conversion system in which the voltage source types self-commutated converter described in the preceding emodiments can be composed of three single-phase bridge units composed of self-turn-off devices and diodes instead of three phase voltage source type self-commutated converter shown in FIGURE 15. As described above, according to the invention as it is possible to increase DC voltage and decrease DC current even if the capacity of the power conversion system is the same when compared with the multiplexed configuration with converters connacted in parallel, an economical pcwer conversion system that is capable of reducing power loss can be provided even for a system of which resistance of DC line becomes large in DC transmission for a long distance, etc. Obviously, numerous modifications and variaticns of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein. WE CLAIM: 1. A power conversion system (1), comprising: a plurality of converter transformers (4A, 4B); AC side windings of said converter transformers (4A, 4B) being connected in series for connecting to an AC power system; a plurality of series connected voltage source type self-commutated converters (5A, 5B) for converting AC power into DC power or DC power into AC power; each of said voltage source type self-commutated converters (5A, 5B) being connected to one of DC side windings of said converter transformers (4A, 4B), respectively; a plurality of DC voltage sources (6); each of DC output sides of said voltage source type self-commutated converters (5A, 5B) being connected to one of said DC voltage sources (6), respectively; and control means (9) for controlling said voltage source type self-commutated converters (5A, 5B) such that each of DC voltages of said voltage source type self-commutated converters (5A, 5B) follows to one of DC voltage reference values for said voltage source type self-commutated converters (5A, 5B), respectively. 2. A power conversion system (1), comprising: a plurality of converter transformers (4A, 4B); AC side windings of said converter transformers (4A, 4B) being connected in series for connecting to an AC power system; a plurality of series connected voltage source type self-commutated converters (5A, 5B) for converting AC power into DC power or DC power into AC power; each of said voltage source type self-commutated converters (5A, 5B) being connected to one of DC side windings of said converter transformers (4A, 4B), respectively; a DC voltage source (6) connected in parallel with said series connected voltage source type self-commutated converters (5A, 5B); and control means (9) for controlling said voltage source type self-commutated converters (5A, 5B) such that each of DC voltages of said voltage source type self-commutated converters (5A, 5B) except one of said voltage source type self-commutated converters (5A, 5B) follows to one of DC voltage reference values for said voltage source type self-commutated converters (5A, 5B), respectively. 3. The power conversion system (1) as claimed in claim 1, wherein a DC voltage source (6) connected in parallel with said series connected voltage source type self-commutated converters (5A, 5B); and a DC voltage detector (5A, 5B) for detecting a DC voltage of said power conversion system (1); wherein said control means (9) further includes means for determining said DC voltage reference values for said voltage source type self-commutated converters (5A, 5B) based on said DC voltage of said power conversion system (1) such that each of said DC voltage reference values for said voltage source type self-commutated converters (5A, 5B) shares said DC voltage of said power conversion system by one of predetermined ratios, respectively. 4. The power conversion system as claimed in claim 2, wherein a DC voltage detector (7) for detecting a DC voltage of said power conversion system (1); wherein said control means (9) further includes means for determining said DC voltage reference values for said voltage source type self-commutated converters (5A, 5B) except one of said voltage source type self-commutated converters (5A, 5B) based on said DC voltage of said power conversion system such that each of said DC voltage reference values for said voltage source type self-commutated converters except one of said voltage source type self-commutated converters shares said DC voltage of said power conversion system by one of predetermined ratios, respectively. 5. The power conversion system as claimed in claim 1, wherein a DC voltage detector (7) for detecting a DC voltage of said power conversion system (1); wherein said control means (9) further includes means for controlling said voltage source type self-commutated converters (5A, 5B) such that said DC voltage of said power conversion system follows to a DC voltage reference value for said power conversion system. 6. The power conversion system as claimed in claim 5, wherein said control means has means (9) for determining said DC voltage reference values for said voltage source type self-commutated converters based on said DC voltage of said power conversion system such that each of said DC voltage reference values for said voltage source type self-commutated converters shares said DC voltage of said power conversion system by one of predetermined ratios, respectively. 7. The power conversion system (1) as claimed in claim 6, wherein: in said control means (9), responses of DC voltage control of said voltage source type self-commutated converters (5A, 5B) are determined to be faster than a response of DC voltage control of said power conversion system. 8. A power conversion system (1), comprising: a plurality of converter transformers (4A, 4B); AC side windings of said converter transformers (4A, 4B) being connected in series for connecting to an AC power system; a plurality of series connected voltage source type self-commutated converters (5A, 5B) for converting AC power into DC power or DC power into AC power; each of said voltage source type self-commutated converters (5A, 5B) being connected to one of DC side windings of said converter transformers, respectively; a DC voltage detector (7) for detecting a DC voltage of said power conversion system (1); and control means (9) for controlling said voltage source type self-commutated converters (5A, 5B) such that each of DC voltages of said voltage source type self-commutated converters (5A, 5B) except one of said voltage source type self-commutated converters (5A, 5B) follows to one of DC voltage reference values for said voltage source type self-commutated converters (5A, 5B), respectively, and that said DC voltage of said power conversion system follows to a DC voltage reference value for said power conversion system (1). 9. The power conversion system (1) as claimed in claim 8, wherein said control means further includes means (9) for determining said DC voltage reference values for said voltage source type self-commutated converters (5A, 5B) except one of said voltage source type self-commutated converters (5A, 5B) based on said DC voltage of said power conversion system (1) such that each of said DC voltage reference values for said voltage source type self-commutated converters (5A, 5B) shares said DC voltage of said power conversion system (1) by one of predetermined ratios, respectively. 10. The power conversion system as claimed in claim 9, wherein: in said control means (9), responses of DC voltage control of said voltage source type self-commutated voltage source type self-commutated converters (5A, 5B) are determined to be faster than a response of DC voltage control of said power conversion system (1). 11. The power conversion system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

Full Text

The present invention relates to a power conversion system.
BACKGROUND OF THE INVENTION
Field of the Invention
This invention relates to a power conversion system, and more particularly to a power conversion system composed of voltage source type self-commutated converters in a multiplexed configuration and a control system of these voltage source type self-commutated converters when these voltage source type self-commutated converters are applied to such systems as DC transmission systems, fuel cells, battery energy storage systems, and reactive power compensation systems.
DESCRIPTION OF THE RELATED ART
A power conversion system composed of conventional voltage source type self-commutated converters in a multiplexed configuration and a control system of these voltage source type self-commutated converters will be described hereinafter with reference to FIGURE 14 and FIGURE 15. In FIGURE 14, 1 is an AC system, 2 is a potential
transformer to measure an Ac system voltage of Ac system 1, 3 is a current transformer, and 4A, 4B are converter transformers to connect voltage source type self-commutated converters 5A, 5B to AC system 1. 6 is a DC voltage source composed of DC capacitor/ cell, etc., 1 is a DC voltage detector, and 8 is art active/reactive power detector to detect active and reactive powers that are output from converters 5A, SB by inputting detected values of potential trans-former 2 end current Lransformer 3. 9 is a DC voltage controller to input a difference between a DC voltage reference 51 and a DC voltage detected value 52 detected by uc voltage detector 7 and to control the DC voltage so as to make the difference zero. 10 is a reactive power controller to input a difference between a reactive power reference 53 and a reactive power detected value 54 detected by active/reactive power detactor B and to control the reactive power so as to make the deference zero. 11 is an AC current controller to control the AC current to a reference value by inputting on output of DC voltage controller 9, an output of reactive power controller 10, an AC current detected value 55 detected by current transformer 3 and an AC voltage detected value 56 detected by potential transformer 2, 12 is a pulse width modulation circuit to decide pulse patterns of self-turn-off devices composing each of self-commutated converters 5A, 5B according to the output of AC current controller 11,
gate turn-off thyristors (hereinafter referred tc as Gio) and 14A - 14L are diodes, Furthermore, 1A, IB and 1C designate A-phase, B-phase and C-phase of AC system 1, respectively. Tha principle of control of active/reactive power of voltag* source type self-commutated converters 5A, 5B connected to AC system 1 is disclosed in a publication titled "Semiconductor Power Conversion Circuit (published from The Institute of Electrical Engineers of Japan), P.215 - 220, ate. and the detailed deacripfcion wiil be therefore omitted he:re. In addition, the principle and realizing a method of a constant currant control circuit are disclosed in Japanese Patent Publication (Kofcai) No. Hei 1-77110, and therefor*, the detailed description will be omitted here.
In FIGURE 15, the AC system side windings of converter transformers 4A, 4B are connected in series, the EC side windings thereof are connoted to each of converters 5A, 5B individually, and the DC outputs of voltage source type self* commutated converters 5A, 5B are connected in parallel with • ach other. In. this configuration, as for the AC iiystem output voltage, the outputs of converters 5A and 53 are added through the AC windings of converter transformers 4A, 4B and hiyher harmonics thereof are negated. In addition, as the AC windings of converter transformers 4A, 4B are connected in series, the current values of converters 5A, 5B become the same. Further, ss the DC sides of converters 5A- 5R are
converters SA, 5B become the same. In the configuration shown in FIGURE 14, higher harmonics can be decreased in the AC system output voltage by converter* 5A, *B, and in addition/ no unbalanced current and voltage will be generated between converters 5A, SB.
The multiplexed configuration of conventional voltage source type self-commutated converters shown in F1GURE 14 has the following problem when considering the application of it to, such as, a DC transmission system tc transmit DC power over a long distance. That is, as DC outputs of tie converters are connected in parallel, if the number of converters is increased for attaining the large capacity, DC current will increase accordingly. In case of the DC transmission system, a DC line is long and the resistance of a DC transmission line is large. If DC current increases, the power loss by the resistance of the DC transmition line also increases in proportion to a square of DC currant, and the efficiency of the system will drop. Therefore, when applying the conventional power conversion system to, such as, the DC transmission system, in case of increasing the systen capacity, it is preferred to increase the DC voltaghe rather than increasing the DC current from the viewpoint of reducing the loss.
It can be also considered, as one method to increase the DC voltage, to increase rated DC voltage of the converters
self-commutated converters with a large capacity, as it is not possible to increase switching frequency of the converter from the view point of reducing the switching loss, the multiplexing of converters becomes indispensable for reducing higher harmonics.
As described above, instead of the conventional multiplexed configuration of converters to connect the DC sides of converters in parallel, a power conversion system with the multiplexed configuration of converters and its control system which does not increase DC current is demanded.
SUMMARY OF THE INVENTION
Accordingly, one object of this invention is to provide a power conversion system composed of voltage source type self-commutated converters in a multiplexed configuration and a control system of these voltage source type self-commutated converters which does not increase DC currents of these voltage source type self-commutated converters.
Accordingly, there is provided a power conversion system, comprising a plurality of converter transformers; AC side windings of said converter transformers being connected in series for connecting to an AC power system; a plurality of series connected voltage source type self-commutated converters for converting AC power into DC power or DC power into AC power; each of said voltage source type self-commutated converters being connected to one of DC side windings of said converter transformers, respectively; a plurality of DC voltage sources; each of DC output sides of said voltage source type self-commutated converters being connected to one of said DC voltage sources, respectively; and control means for controlling said voltage source type self-commutated
converters such that each of DC voltages of said voltage source type self-commutated converters follows to one of DC voltage reference values for said voltage source type self-commutated converters, respectively.
According to one aspect of this invention, there is provided a power conversion system including a plurality of converter transformers. AC side windings of the converter transformers are connected in series for connecting to an AC power system. The power conversion system further includes a plurality of series connected voltage source type self-commutated converters for converting AC power into DC power or DC power into AC power. Each of the voltage source type self-commutated converters is connected to one of DC side windings of the converter transformers, respectively. The power converter further includes a plurality of DC voltage_
sources. Each of DC output sides of the voltags source type selt-coraautated converters is connected to one of the DC voltage sources, respectively. Th* power converter further includes a DC voltage source connected in parallel with the series connected voltage source type self-com»uta:ed converters, and a control system for controlling l.he voltage source type self-commutated converters such that each of DC voltages of the voltage source type self-commutated
i
converters except one of the voltage source type self-coiwautated converters follows to one of DC voltage reference values for the voltage source type selff-commutated converters, respectively.
According to another aspect of this Invention, there is provided a power conversion system including a plurality of converter transformers. AC side windings of the converter transformers are connected in series for connecting to an AC power system. The power conversion system further includes a plurality of scries connected voltage source type self-commutated converters for converting AC power into DC power or DC power into AC power. Each of the voltage source type self-commutated converters is connected to one of EC side windings of the converter transformers, respectively. The power conversion system further includes a plurality of DC voltage sources. Each of DC output sides of the voltage
source tvrsa self -communated
the DC voltage sources, respectively. The power conversion system further includes a DC voltage detector for detecting a DC voltage of the power conversion system, and a control system for controlling the voltage source type stlf-commutated converters such that each of DC voltaces of the voltage source type self-commuteted converters except one of the voltage source type self-commutated converters follows to one of DC voltage reference values for the voltace source type sell-commutated converters, respectively, and that the DC voltage of the power conversion system follows to a DC voltage reference value for the power conversion system.
BRIEFE DESCRIPTION ON OF THE DRAWINGS
A more complete appreciation of the invention and Many Of the attendant advantages thereof will ba readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
FIGURE 1 is a diagram showing the construction of a power conversion system according to a first embodiment of tnis invention;
FIGURE 2 is a diagram showing the construction of a power conversion system according to a modification of the first embodiment of this invention;
FIGURE 3 is a dianram showing the
power conversion system according to & second embodiment of this invention;
FIGURE 4 is a diagram showing the construction of a power conversion system according to a third embcdiment of this invention;
FIGURE 5 is a diagram showing the construction of a power conversion system according to a fourth embodiment of this invention;
FIGURE 6 is A diagram showing chc construction of a power conversion system according to a fifth embodiment of this invention;
FIGURE 7 is a diagram showing the construction of a power' conversion system according to a modification of the fifth embodiment of this invention;
FIGURE 8 is a diagram showing the construction of a power conversion system according to a sixth embodiment of this invention;
FIGURE 9 is a diagram showing the construction of a power conversion system according to a modification of the sixth embodiment of this invention;
FIGURE 10 is a diagram showing the construction of a part of a power conversion system according to a seventh embodiment of this invention}
FIGURE 11 is a diagram showing the construction of a part of » power conversion system according to an eighth embodiment of this invention
FIGURE 12 is a diagram showing the construction of a parL of a power conversion system according to a ninth embodiment of this invention;
FIGURE 13 is a diagram showing the construction of a part of a power conversion system according to a tenth embodiment of this invention;
FIGURE 14 is a diagram showing the construction of one example of a conventional power conversion ays ten.; and
FIGURE 15 is a diagram showing 'the construction of one example of a part of the conventional power conversion system shown in FIGURE 14.
DETRAILED DESCRIPTION OF THE PREFERREDEMBODIMENTS
Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, the embodiments of this invention will be described below.
FIGURE 1 is a diagram showing a power conversion system according to a first embodiment of the present invention, in FIGURE 1, the same components are assigned with the same reference numerals as in FIGURE 14 and FIGURE 15 and their explanations are omitted.
in FIGURE 1, voltage source type salf-commutated
as, DC capacitors 6A, 66, the negative side of the DC output of self-commutated converter 5A is connected to the positive side of the DC output of self-conunutated converter SB, and thereby self-commutated converters 5A and 53 are connected in
series.
In such the configuration as described above, it becomes possible to provide a power conversion system which makes DC voltage 2 times and DC current 1/2 when compared with a conventional multiplexed configuration of the unit converters (converters 5A, SB) with the equal capacity with the DC sides connected in parallel with each other.
In the multiplexe configuration as shown in FIGURE 1, as for the AC system output voltage, the outputs of converters 5A and 5B are added through the AC windings of converter transformers 4A and 4B, and higher harmonics thereof are thereby negated. Further, as the AC winding* of converter transformers 4A, 4B are connected in series, the current values of converters 5A, 5B become equal. However, at the DC side* of converters 5A, SB are connectei in series. DC voltages of converters 5A, 53 may possibly be unbalanced by such factors as the delay in 3ignal transmission, the variations in switching, the variations in main circuit constants such as the capacitances of DC capacitors 6A, 6B, etc.
So, in FIGURE 1, by inputting a difference between a DC
and a DC voltage reference SlA into a DC voltage controller 9A, DC voltage ot converter 5A is controlled so That it becomes in accord with DC voltage reference 5lA of converter 5A. Similarly, by inputting a difference between a DC voltage of converter 5B detected by a DC voltage detector 78 and a DC voltage reference SIR into a DC voltage controller 98, DC voltage of converter 5B is controlled so that it becomes in accord with DC voltage reference 51B cf converter 58.
As the reactive power does not directly relate to the DC voltage/ the circuit with respect to the reactive power can be the same as that shown in FIGURE 14, which is the conventional multiplexed configuration.
An AC current controller is provided individually to each converter. In FIGURE 1, the current at the DC winging Side of converter transformer 4A is detected by a current transformer 3A as an AC current detected value 5SA, and is controlled by an AC current controller 11A by inputting an AC voltage detected value 56 from potential tranaforner 2 so that AC current detected value 55A becomes m accord with command values from DC voltage controller 9A and reactive power controller 10, Similarly, the current at the DC winding aide of converter transformer 4B is dete current transformer 3B as an AC current detected value 55B, ana is controlled by an AC currant controller IIB by
detected value 55B becomes in accord with commanc values from DC voltage controller 9B and reactive power controller 10. Pulse width modulation circuits 12A, 12B decide pulse patterns of self-turn-off devices composing each of self-commutated converters 5A, SB according to the outputs of AC current controllers 11A, lis, respectively.
In the multiplexed configuration as shown in FIGURE 1, if the DC voltage of converter 5A detected by DC voltage detector 7A is higher than DC voltage reference MA, DC voltage controller 9A outputs a current command to lower the DC voltage of converter 5A, and AC current controller 11A controls an output voltage command value for converter 5A so as to follow the current command value. As a result, the DC voltage of convtrter SA becomes in accord with DC voltage reference 51A. similarly the DC voltage of converter 5B is brouaht to agr«a with DC voltag reference 513. Accordingly, an unbalanced voltage between two converters 5A, SB is dissolved.
FIGURE 2 ia a diagram showing a modification of the first embodiment shown in FIGURE 1. The same components as those shown in FIGURE 1, 14 and 15 are assigned with the same reference numerals and their explanations are omitted. 15 is an active power controller which, by inputting a difference between an active power reference 57 and an active power detected value 58 detected by active/reactive powor detector
zero.
In FIGURE 2, AC currant controller 11 Is provided instead of AC currant controllers 11 A, 11B in FIGURE 1. in FIGURE 2, AC current ia controlled by AC current controller 11 by controlling AC system current 55 detected by current transformer 3. AC current controller 11 controls AC system current 55 by inputting AC voltage detected value 56 so as to bring it to agree with an active current command value 59
1
which ia the output of active power controller 15 and a reactive current command value 60 which is the output of reactive power controller 10. In this time/ an output 61A of DC voltage controller 9A of converter 5A and an output 61B of DC voltage controller 9B of converter 5B are also input hr> AC current controller 11, respectively, to correct tie output of AC current controller 11 to pulse width modulation circuits 12A, 128 and thus, unbalanced voltage between two converters SA/ 5B is dissolved.
In FIGURE 2, as DC voltages of two converters SA, 5B are adjusted by controlling thu AC system current by ona AC current controller 11 and correcting output voltage command values that are output to pulse width modulation circuits 12A, 12B, there is no interference between the AC currant control system and the DC voltage control system and thereby the power conversion system can, be stably controlled.
FIGURE 3 shows a power conversion system aeco.rdxng to a
same components already shown in FIGURES 1/ 2, U, 15 are assigned with the sane reference numerals and thuir explanations are omitted. 16 is a large capacity DC voltage source such as fuel cells, secondary cells, voltjge source type converters, etc. In FIGURE 3, the entire DC voltage value including DC voltages of salf-comunated converters 5A, 58 is decided by-this large capacity DC voltage source 16. Even when each converter 5A ,5B has a DC voltage reference and tries to control ita DC voltage, if a sum of respective DC voltage references does not agree with the output voltage of large capacity DC voltage source 16, each converter 5A, 5B is not able to control its DC voltage/ and the DC voltage controller is then saturated. So, as shown in FIGURE 3, one self-commutated converter 5B does not control its DC voltage and the remaining self-commutated converter 5A controls its DC voltage SO that the'DC voltage Of converter 5A detected by DC voltage detector 7A becomes in accord with DC voltage reference S1A. When DC voltage is controlled as shown in FIGURE 3, it is not necessary that converter 5B controls its DC voltage, because its DC voltage is primarily decided by a value of voltage of large capacity DC voltage source 16 minus DC voltage of converter 5A. As a result, DC voltage controller 9A is not saturated and it becomes possible to control desired active power.
Mtnough the power conversion system is described using
of more than two converters connected in series, when, except one converter, each of the remaining converters controls its DC voltage so as to agree with DC voltage reference of each converter, the power conversion system can be stably controlled without expanding unbalanced DC voltages between converters and saturating the DC voltage controller.
FIGURE 4 shows a power conversion system acccrding to a third embodiment of the present invention. In FIGURE 4, the same components As those shown in already explained figures are assigned with the same reference numerals and their explanations are omitted. 7 is a DC voltage detector to detect DC voltage of large capacity DC voltage source 16, and 17A, 17B are operational amplifiers to multiply tne DC voltage detected by DC voltage detector 7 by fixed factors KA, KB, respectively. The output DC voltage of large capacity DC voltage source 16 such as fuel cells, secondary cells, etc.
generally fluctuates by about ±{20 - 30)% according to the
discharge state of cells and the DC current value.
In the configuration shown in FIGURE 4, DC voltage
references 51A, 51B of converters 5A, 5B are not fixed, but
values obtained by multiplying the DC voltage by fixed
factors KA, KB are used as DC voltage reference 5:,A, 51B of
converters 5A, 5B, respectively, and converters 5A, SB
control their output voltages to follow DC voltage: references
51A, 51B, respectively. As a result, no unbalance is
the DC voltage of large capacity DC voltage source 16 fluctuates.
Further, fixed factors KA, KB for DC voltage references 51A, 51B are so decided that the sum of factors KA, KB for all converters 5A, SB becomes 1. Although, the power conversion system is described using two converters in FIGURE 4, in a multiplexed configuration of more than two converters connected in series, if the DC voltage reference of each
i
converter is decided by multiplying the detected DC voltage value by a fixed factor and the DC voltage of each converter is controlled so as to follow the DC voltage reference, the power conversion system also can be operated stably without generating unbalanced DC voltage between converters even when voltage of the large capacity DC voltage source, such as fuel calls, secondary cells, etc. fluctuates.
FIGURE 5 shows a power conversion system according to & fourth embodiment of the present invention. In FIGURE 5, the same components as those shown in the already explained figures are assigned with the same reference numerals and their explanations are omitted. In FIGURE 5, DC voltage is controlled for converter 5A and not for converter 5B, the same as the embodiment shown in FIGURE 3.
In FIGURE 5, the entire DC voltage value including DC voltages of self-commutated converter 5A and 5B is decided by large capacity DC voltage source 16. As DC voltago reference
voltage detector i by fixed factor KA, a sum of DC voltages of all converters oA, 5B does not differ from tho output voltage of large capacity DC voltage source 16.
However, if there are errors, etc. of DC vol:age detectors of converters, these errors are accuroul.ated and the DC controller nay be saturated. Therefore, in th« embodiment shown in. FIGURE 5, one self-coimutated converter SB performs only the active power control without executing the DC voltage control. Remaining 8elf-coBBtiutat»d converter 5A controls its DC voltage so that the DC voltage detected by DC voltage detector ?A comes to agree with DC voltage reference 51A that is obtained by multiplying the entire DC voltage detected value by fixed factor KA.
When the power conversion system, is controlled as shown in FIGURE 5, DC voltage of converter SB is primarily decided by the voltage of larcre capacity DC voltage source 16 minue DC voltage of converter 5A. Accordingly, even when there are errors of the detectors, the controller is not saturated and the power conversion system can be stably controlled.
Further, although two converters are used for explanation in FIGURE 5, in the multiplexed configuration of more than, two converters connected in~saries, If DC voltage is so controlled that DC voltage of each converter except one Converter comes to agree with the DC voltage reference of each converter which is decided by multiplying the entire DC
system car. be stably controlled without expanding unbalanced DC voltage between converters, and generating th» saturation of the DC voltage controller even when the DC vol.tage of the large capacity DC voltage source fluctuates.
FIGURE 6 shows a power conversion system according to a fifth embodiment of the present invention. In FI3UBX 6, the same components shown in the already explained figures are assigned with the same reference numerals and the explanation is omitted. It, FIGURE 6, a DC voltage of added outputs of converters SA, 5B, that is, a DC output voltage of the power conversion system is detected by DC voltage detector 7. DC voltage of the power conversion system is controlled by DC voltage controller 9 so that the DC voltage detected by nc voltage detector 7 becomes in accord with DC voltage reterence 51 of the power conversion system.
On the other hand, DC voltages of converter© >A, SD are controlled by DC voltage controllers 9A, 9B so th«.t they become in accord with DC voltage references 51A, SIB, respectively. In such the configuration, DC voltaje controller 9 is provided to control the entire DC voltage and the entire DC voltage is easily controllable when another power conversion system is connected to this power conversion system via a DC bus and it is required to maintain the entire DC voltage at a constant level.
FIGURE / is a diagram showing a modification of the
components as Chose shown in the already explained figures are assigned witn the same reference numerals and the explanation is omitted. In FIGURE 7, values obtained by multiplying the entire DC voltage detected by DC voltage detector 7 by fixed factors KA, KB are used as DC voltage references S1A. S13 of converters 5A, SB, respectively. In the configuration as shown in FIGURE "7, even when the entire DC voltage fluctuates transiently due to system failure, disturbance, etc., the DC voltage control system of converters 5A, SB control their DC voltages so as to take the balance against the entire DC voltage, respectively. Therefor**, overvoltage can be prevented from being applied to either one of the converters 5A, 5B.
The action of this embodiment will further be described using FIGURE 6. In FIGURE 6, there are provided three DC voltage controllers: DC voltage controller 9 -o ccntrol the entire DC voltage of the power conversion system; DC voltage controller 9A to control DC voltage of converter 5A; and DC voltage controller 9B to control DC voltage of converter S3.
On the other hand, as for the DC voltages, DC voltage of converter SA and that of converter SB are independent, respectively, while this entire DC voltage oT the power conversion system is primarily decided by adding DC voltage of converter 5A and that of converter SB. Therefore, there are three controllers for two independent variable* and if
cannot be controlled stably.
So. the entire DC voltage is controlled at a relatively low speed in DC voltage controller 9. As for DC voltages of converters SA, 5B, so as to balance DC voltages between converters bA, 5B, they are controlled in DC voltage controllers 9A, 9B at high speeds, respectively. Thus, DC voltages can be stably controlled by changing responses of three DC controllers 9, 9A, 98, respectively.
Although FIGURE 6 and FIGURE 7 are explained using a configuration of two converters, in case of a multiplexed configure of more than two converters, the same effect is obtained when an entire DC voltage controller anc. DC voltage controllers for respective converters are provided.
FIGURE 8 is a diagram showing a power conversion system according to a sixth embodiment of the present invention. In FIGURE 1, the same components as those shown in the already explained figures are assigned with the same reference numerals and the explanation is omitted.
In FIGURE 8, an entire DC voltage with the outputs of converters 5A, 5B added, that is, a DC output voJtage of the power conversion system, is detected by DC voltac-e detector 7. The DC voltage of the power conversion system detected by DC voltage detector 7 is controlled by DC voltage controller 9 -so that it becomes in accord with DC voltage reference 51 of the power conversion system. On the other hand, the DC voltage of converter 5 is controll by the voltage
controller 9A so that it becomes in Accord with DC voltage reference 51A.
In such the configuration, DC voltage controller 9 is provided to control the entire DC voltage and the entire DC voltage is easily controlled when another power conversion system is connected to this power conversion system via a DC bus and it is required to maintain the entire DC voltage at a constant level. Further, as the DC voltage of converter 5B is not controlled, the response of DC voltage controller 9 for the entire DC voltage can be decided independently from the response of DC voltage of DC voltage controller 9A of converter 5A. Therefore, this configuration is suited especially when it is necessary to make the entires DC voltage control fast when demanded by the system.
FIGURE 9 is a diagram showing a modification of the sixth embodiment shown in FIGURE 8. In FIGURE 9, the same components as those shown in the already explained figures are assigned with the same reference numerals and the explanation is omitted, in FIGURE 9, a value obtained by multiplying the entire DC voltage detected by DC vjltage detector 7 by a fixed factor KA is made as DC voltage reference S1A of converter 5A.
In the configuration as shown in FIGUBE 9. ever whan the entire DC voltage fluctuates transiently due to system failure, disturbance, etc., the DC voltage control system of
balance against the entire DC voltage. Therefore, overvoltage can be prevented from being applied to either one of the converters SA, 5B.
Although FIGURE 6 and FIGURE 9 are explained using a configuration of two converters, in case of a multiplexed configuration of more than two converters, the sane errect is obtained when an entire DC voltage controller and DC voltage controllers for respective converters are provided except one converter.
FIGURE 10 is a diagram showing a part of a power conversion system according to a seventh embcdimert of the present invention. FIGURE 10 shows an example of the construction of AC current controller 11 shown in FIGURE 9. In FIGURE 10, 19 is a phase detector to detect a phase of the AC system voltage from AC system voltage detected value 56. 19A is a coordinate converter to convert the AC system voltage into orthogonal biaxial components from AC system voltage phase detected value 62 detected by phase detector 18 and AC systexu voltage detected value 56, and 19B is a coordinate converter to convert AC current detected value 55 into an active current component 63 and a reactive current component 64 using AC systea voltage phase detected value 62. 19C is a coordinate converter to compute the output voltage, command value to be given to pulse width modulation, circuit 12A of converter SA, and 19D is a coordinate converter to
width modulation circuit 12B of converter SB.
11-1 13 an AC current controller to control As current by inputting a difference between active current command value 59 that is the output of DC voltage controller 9 shown in FIGURE 9 and active current component 63 that ia obtained through the coordinate conversion of AC current detected value 55 and make the difference zero. 11-2 is an AC current controller to control AC current by inputting a difference between traactive current command value 60 that is the output of reactive power controller 10 ahown in FIGURE 9 and reactive current component 64 that is obtained through the coordinate conversion of AC current detected value 55 and make the difference zero.
Out of the actions of AC current controller li shown in FIGURE 10, a method to convert three-phaae AC voltage and current into DC quantities through the coordinate ronvsrsion and control them is an already generally known method as an AC current control method of voltage source type self-commutatad converter, and therefor*, its explanation is omitted here.
An active current correction value 61A shown in FIGURE 10 ia the output of DC voltage controller 3A that acts so as to control a DC voltage detected value 52A of converter 5A in - accord with the value that ia obtained by multiplying total DC voltage detected value 52 by^fixed factor KA in
between converters SA and SB. The output voltage command value tor converter SA before the coordinate conversion that is input to coordinate converter 19C is corrected by active current correction value 61A.
In the configuration shown in FIGURE 1C. as t:ie output voltage command value for converter 5A is corrected before the coordinate conversion by the DC voltage control system after controlling active current component and reactive current component of the AC system current independently, AC current controller 11 and DC voltage controller 9A. shown in FIGURE 9 can be controlled with less interference between them.
FIGURE 11 is a diagram showing a part of a power conversion system according to an eighth embodimert of the present invention. In FIGURE 11 an example of the construction of current controller 11 already explained in FIGURE 7 is shown. In FIGURE 11, the same componeits as those already explained in FIGURE 10 are assigned with the same reference numerals and the explanation is omitted. In FIGURE 11/ AC current is controlled by AC current controller 11-1 so that active current command value 59 that is the output of DC voltage contiullm 9 of the power conversion system shown in FIGURE 7 becomes in accord with active current component 63. Further, AC current is controlled by AC current controller 11-2 so that reactive current command
shown in FIGUR2 1 becomes in accord with reactive current component 64. Output voltage commend values for respective converters 5A, SB are corrected before the coordinate conversion by active current correction value 61A that is the output of voltage controller 9A to control DC voltage of converter 3A and active current correction value 61B that is the output of DC voltage controller 98 to control X voltage of converter SB, respectively.
In the configuration shown in FIGURE 11, as th: output voltage command values for converters SA, 53 are corrected before the coordinate conversion by the DC voltage system after controlling active current component and reactive current component of the AC system current independently, AC current controller 11 and DC voltage controllers 9A, 9B shown in FIGURE 7 can be controlled with leas interference between them.
Although FIGURE 10 and FIGURE 11 are explained using a configuration of two converters, in case of a multiplexed configuration of more than two converters, the iame effect ia obtained by correcting an output voltage command value of each converter by the output of the DC voltage controller of each converter.
Further, the correction methods shown in FIGURE 10 and FIGURE 11 are explained with respect to the power conversion systems shown in FIGURE 9 and FIGURE 7, respectively. A
conversion systems shown in FIGURES 2, 3, 4, 5, 6 and 8.
FIGURE 12 is a diagram showing a part of a power conversion system according to a ninth embodiment of the present invention. FIGURE 12 shows an example of the construction of current controller 11 already explained in FIGURE 9. In FIGURE 12, the Same components as those shown in the already explained figures are assigned with the same reference numerals and the explanation is omitted. In FIGURE 12, 20 is a polarity judging device to judge the polarity or active current component 63, 21 is a switch that is changed over by the output of polarity judging device 20, and 22 is an inverter to change over the polarity of active current correction value 61A.
In FIGURE 9, as AC side windings of converter transformers 4.A, 4B are connected in series, the waveforms of the output currents of covverters 5A, 5B become the same unless there are DC magnetizations, etc. in converter transformers 4A, 43. Accordingly, when DC voltages of DC capacitors 6A, 6B of converters 5A, 55 are unbalanced and it is desirable to adjust the unbalanced DC voltages by changing active powers of converter 5A, 5B, it is required t:o change voltage waveforms of converters 5A, SB, respectively.
Generally, if the current waveforms are the sane, active powers can be changed by changing the amplitudes of output voltages, respectively. Here, the relation among the
voltage of DC capacitor is considered.
Now, it is assumed that the direction of current from the AC system to the converter is the forward direction. When operating as the rectifier, converter output current and converter output voltage are in the waveforms of the same polarity. When the amplitude of the output voitags le made larger, the active power in the direction of rectifier increases, and when the amplitude of the output voltage is made smaller, the active power in the direction of rectifier decreases.
When considering the DC voltage of the DC capacitor, a converter provided with a DC capacitor of lower DC voltage increases the active power in the direction of rectifier by making the amplitude of its output voltage larger and thereby to charge the DC capacitor more, A converter provided with a DC capacitor of Mgher DC voltage reduces the aetivo power in the direction of rectifier by making the amplitude cf the output voltage smaller and thereby to suppress the charging of the DC capacitor.
Next, inverter operation is considered. In inverter operation, converter output current and converter output voltage are in the waveforms of the opposite polarity. When the amplitude of the output voltage is made larger, the active power in the direction of inverter increases, and when the amplitude of the output voltage is made smaller, the
When considering the DC voltage of the DC capacitor, a converter provided witn a DC capacitor of iower DC voltage decreases the active power in the direction of inverter by making the amplitude of its output voltage smaller and thereby to decraase the discharge of the DC capacitor. A converter provided with a DC capacitor of higher or. voltage increases the active power in the direction of inverter by making the amplitude of its DC voltage larger thereby to increase the discharge of the Dc capacitor.
The relation between the magnitude of the DC voltage of
the DC capacitor and the amplitude of the output voltage in
the rectifier and inverter operation are summarizel as shown
by Table.
[Table]
In Rectifier Operation In Invertat (Table Removed )
Generally, in tfte rectifier operation, an active current flows from the AC system to the converter, and. in the inverter operation, an active current flows from t'ie converter to the AC system.
As can be see from Table, it is necessary to change the polarity of the correction for the amplitude of th* output voltage based on the direction of the active current.
63 is judged by pclarity judging device 20. Switch 21 is changed over by the output of polarity judging device 20. As
a result/ the correcting direction is changed by changing over an inverted active current correction value 61AA that is invented by inverter 22 and active current correction value 61A that is not inverted. In the configuration as shown in FIGURE 12, it is possible to continuously operate the power conversion system while taking the balance of DC capacitor voltages by properly correcting the,output voltage command values before the coordinate conversion even when the direction of the active current is changed.
FIGURE 13 i.s a diagram showing a part of a power conversion system according to a tenth embodiment of the present invention. FIGURE 13 shows one example of the construction of current controller 11 already explained in FIGURE 1. In FIGURE 13, the same components as those shown in the already explained figures are assigned with the same reference numerals and the explanation is omitted. In FIGURE 13/ 20 is polarity judging device to judge the pol.arity of active current component 63. 21A, 21B are switches: that are changed over by the output of polarity judging device 20, and 22A, 22B are inverters to change over the polarities of active current correction values 61A, 61B, respectively. In . the configuration as shown in FIGURE 13, similarly to the embodiment shown in FIGURE 12, it is possible to continuously
of DC capacitor voltages by properly correcting the output voltage command values before the coordinate conversion even when the direction of the active current is changed.
Although FIGURE 12 and FIGURE 13 are explainec using a configuration of two converters, in case of a multiplexed configuration of more than two convertare, the aamo effect ia obtained when the polarity of the correction value by the DC voltage controller of each converter is changed ov»r according to the polarity of the active current component.
Further, the correction methods shown in FIGURE 12 and FlGURE is are explained with respect to the power conversion systems shown in FIGURE 9 and FIGURE 7, respectively. A similar correction method is also applicable to the power conversion systems shown in FIGURES 2, 3, 4, 5, 6 and 8.
In the embodiments described above, the DC voltage control is performed by using DC capacitors as DC voltage sources. If, for instance, fuel cells, batteries, eLc. are assumed to be used as DC voltage sources, this invention can be applied to such power conversion system by replacing DC voltage controllers by active power controllers which control the active power detected by active/reactive power detector 6.
Further, although reactive power controller 10 is used in the embodiments described above, reactive power controller
the AC voltage of AC system 1.
Further, this invention is further applied to a power conversion system in which the voltage source types self-commutated converter described in the preceding emodiments can be composed of three single-phase bridge units composed of self-turn-off devices and diodes instead of three phase voltage source type self-commutated converter shown in FIGURE 15.
As described above, according to the invention as it is possible to increase DC voltage and decrease DC current even if the capacity of the power conversion system is the same when compared with the multiplexed configuration with converters connacted in parallel, an economical pcwer conversion system that is capable of reducing power loss can be provided even for a system of which resistance of DC line becomes large in DC transmission for a long distance, etc.
Obviously, numerous modifications and variaticns of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.

WE CLAIM:
1. A power conversion system (1), comprising:
a plurality of converter transformers (4A, 4B);
AC side windings of said converter transformers (4A, 4B) being connected in series for connecting to an AC power system;
a plurality of series connected voltage source type self-commutated converters (5A, 5B) for converting AC power into DC power or DC power into AC power;
each of said voltage source type self-commutated converters (5A, 5B) being connected to one of DC side windings of said converter transformers (4A, 4B), respectively;
a plurality of DC voltage sources (6); each of DC output sides of said voltage source type self-commutated converters (5A, 5B) being connected to one of said DC voltage sources (6), respectively;
and control means (9) for controlling said voltage source type self-commutated converters (5A, 5B) such that each of DC voltages of said voltage source type self-commutated converters (5A, 5B) follows to one of DC voltage reference values for said voltage source type self-commutated converters (5A, 5B), respectively.
2. A power conversion system (1), comprising:
a plurality of converter transformers (4A, 4B);
AC side windings of said converter transformers (4A, 4B) being connected in series for connecting to an AC power system;
a plurality of series connected voltage source type self-commutated converters (5A, 5B) for converting AC power into DC power or DC power into AC power;
each of said voltage source type self-commutated converters (5A, 5B) being connected to one of DC side windings of said converter transformers (4A, 4B), respectively;
a DC voltage source (6) connected in parallel with said series connected voltage source type self-commutated converters (5A, 5B); and
control means (9) for controlling said voltage source type self-commutated converters (5A, 5B) such that each of DC voltages of said voltage source type self-commutated converters (5A, 5B) except one of said voltage source type self-commutated converters (5A, 5B) follows to one of DC voltage reference values for said voltage source type self-commutated converters (5A, 5B), respectively.
3. The power conversion system (1) as claimed in claim 1, wherein
a DC voltage source (6) connected in parallel with said series connected voltage source type self-commutated converters (5A, 5B); and
a DC voltage detector (5A, 5B) for detecting a DC voltage of said power conversion system (1);
wherein said control means (9) further includes means for determining said DC voltage reference values for said voltage source type self-commutated converters (5A, 5B) based on said DC voltage of said power
conversion system (1) such that each of said DC voltage reference values
for said voltage source type self-commutated converters (5A, 5B) shares said DC voltage of said power conversion system by one of predetermined ratios, respectively.
4. The power conversion system as claimed in claim 2, wherein
a DC voltage detector (7) for detecting a DC voltage of said power conversion system (1);
wherein said control means (9) further includes means for determining said DC voltage reference values for said voltage source type self-commutated converters (5A, 5B) except one of said voltage source type self-commutated converters (5A, 5B) based on said DC voltage of said power conversion system such that each of said DC voltage reference values for said voltage source type self-commutated converters except one of said voltage source type self-commutated converters shares said DC voltage of said power conversion system by one of predetermined ratios, respectively.
5. The power conversion system as claimed in claim 1, wherein
a DC voltage detector (7) for detecting a DC voltage of said power conversion system (1);
wherein said control means (9) further includes means for controlling said voltage source type self-commutated converters (5A, 5B) such that said DC voltage of said power conversion system follows to a DC voltage reference value for said power conversion system.
6. The power conversion system as claimed in claim 5, wherein said
control means has means (9) for determining said DC voltage reference
values for said voltage source type self-commutated converters based on
said DC voltage of said power conversion system such that each of said
DC voltage reference values for said voltage source type self-commutated converters shares said DC voltage of said power conversion system by one of predetermined ratios, respectively.
7. The power conversion system (1) as claimed in claim 6, wherein:
in said control means (9), responses of DC voltage control of said voltage source type self-commutated converters (5A, 5B) are determined to be faster than a response of DC voltage control of said power conversion system.
8. A power conversion system (1), comprising:
a plurality of converter transformers (4A, 4B);
AC side windings of said converter transformers (4A, 4B) being connected in series for connecting to an AC power system;
a plurality of series connected voltage source type self-commutated converters (5A, 5B) for converting AC power into DC power or DC power into AC power;
each of said voltage source type self-commutated converters (5A, 5B) being connected to one of DC side windings of said converter transformers, respectively;
a DC voltage detector (7) for detecting a DC voltage of said power conversion system (1); and
control means (9) for controlling said voltage source type self-commutated converters (5A, 5B) such that each of DC voltages of said
voltage source type self-commutated converters (5A, 5B) except one of
said voltage source type self-commutated converters (5A, 5B) follows to one of DC voltage reference values for said voltage source type self-commutated converters (5A, 5B), respectively, and that said DC voltage of said power conversion system follows to a DC voltage reference value for said power conversion system (1).
9. The power conversion system (1) as claimed in claim 8, wherein said control means further includes means (9) for determining said DC voltage reference values for said voltage source type self-commutated converters (5A, 5B) except one of said voltage source type self-commutated converters (5A, 5B) based on said DC voltage of said power conversion system (1) such that each of said DC voltage reference values for said voltage source type self-commutated converters (5A, 5B) shares said DC voltage of said power conversion system (1) by one of predetermined ratios, respectively.
10. The power conversion system as claimed in claim 9, wherein: in said control means (9), responses of DC voltage control of said voltage source type self-commutated voltage source type self-commutated converters (5A, 5B) are determined to be faster than a response of DC voltage control of said power conversion system (1).
11. The power conversion system substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

Documents:

2997-del-1997-abstract.pdf

2997-del-1997-claims.pdf

2997-del-1997-correspondence-others.pdf

2997-del-1997-correspondence-po.pdf

2997-del-1997-description (complete).pdf

2997-del-1997-drawings.pdf

2997-del-1997-form-1.pdf

2997-del-1997-form-13.pdf

2997-del-1997-form-19.pdf

2997-del-1997-form-2.pdf

2997-del-1997-form-3.pdf

2997-del-1997-form-4.pdf

2997-del-1997-form-5.pdf

2997-del-1997-gpa.pdf

2997-del-1997-petition-137.pdf

2997-del-1997-petition-138.pdf

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Patent Number

222397

Indian Patent Application Number

2997/DEL/1997

PG Journal Number

34/2008

Publication Date

22-Aug-2008

Grant Date

06-Aug-2008

Date of Filing

20-Oct-1997

Name of Patentee

KABUSHIKI KAISHA TOSHIBA

Applicant Address

72, HORIKAWA-CHO, SAIWAI-KU, KAWASAKI-SHI, KANAGAWA-KEN, JAPAN.

Inventors:

#	Inventor's Name	Inventor's Address
1	SHOICHI IROKAWA	C/O INTELLECTUAL PROPERTY DIVISION, TOSHIBA CORPORATION, 1-1-1, SHIBAURA, MINATO-KU, TOKYO, JAPAN.
2	HIROKAZU SUZUKI	C/O TOKYO ELECTRIC POWER COMPANY INCORPORATED, 1-1-3 UCHISAIWAI-CHO, CHIYODA-KU, TOKYO, JAPAN
3	KENICHI SUZUKI	C/O TOKYO ELECTRIC POWER COMPANY INCORPORATED, 1-1-3 UCHISAIWAI-CHO, CHIYODA-KU, TOKYO, JAPAN
4	NORIKO KAWAKAMI	C/O FUCHU WORKS, KABUSHIKI KAISHA TOSHIBA, 1 TOSHIBA-CHO, FURCHU-SHI, TOKYO, JAPAN.

PCT International Classification Number

H02M 7/48

PCT International Application Number

N/A

PCT International Filing date

PCT Conventions:

#	PCT Application Number	Date of Convention	Priority Country
1	P08-276371	1996-10-18	Japan