GB2175757A

GB2175757A - Power controlled 3-phase rectifier DC supply

Info

Publication number: GB2175757A
Application number: GB08530390A
Authority: GB
Inventors: Isao Takahashi; Tadanori Tsukamoto; Hirohide Hirayama; Minoru Yamane; Yasuhiko Hosokawa
Original assignee: Mitsubishi Electric Corp
Current assignee: Mitsubishi Electric Corp
Priority date: 1985-03-27
Filing date: 1985-12-10
Publication date: 1986-12-03
Also published as: DE3603120A1; GB8530390D0; GB2175757B; JPS61224865A

Abstract

Each phase of a particular bridge rectifier 51,52,53 is controlled for a different phase angle, alpha a, alpha b, alpha c. By applying the inputs to three bridges with respective different phase sequences, an asymmetrical controlled bridge rectifier system provides a variable dc source, permitting reactive power control of low voltages at large current with a quick response. The supply is particularly for a Leonard system and the magnetic field coil for nuclear fusion. <IMAGE>

Description

SPECIFICATION Power conversion apparatus BACKGROUND OF THE INVENTION Field of the Invention The present invention provides, in a power conversion apparatus using switching elements such as thyristors, a power conversion apparatus to be employed for variable direct current power source, having reactive power control of low voltage and large current. The apparatus of the present invention is considered specifically effective as power sources for such equipment as the Leonard system and the magnetic field coil for nuclear fusion.

Description of the Prior Art Fig. 1 is a wiring diagram showing basic structure of a three-phase thyristor bridge, in which reference numeral 1 denotes a threephase a.c. power source, 2, 3, and 4 denote phase R, phase S, and phase T of the threephase a.c. power source 1, respectively, 5 denotes a three-phase thyristor bridge, 6, 7, and 8 denote bridge input lines phase U, phase V, and phase W of the three-phase thyristor bridge 5, respectively, and P and N denote output lines of the bridge. Incidentally, UP, VP, and WP denote thyristors connected between the phases U, V, and W and the output line P, respectively, and UN, VN, and WN are thyristors connected between phases U, V, and W and the output line N, respectively.

Fig. 2 is a waveform chart indicating changes in the output voltage and of conductive thyristors with time in the case where variable d.c. output voltage Ed, is provided by a three-phase thyristor bridge 5 as shown in Fig. 1 through a conventional trigger control system. As apparent from the chart, all the thyristors UP to WN are symmetrically controlled with the same phase angle a for a constant output d.c. voltage. In this case, d.c.

output current Id must be changed in order to control reactive power, and so, there was a demerit in the prior art that a quick response characteristic was not obtained in the case of reactive load.

Therefore, an asymmetric control system has been employed in which three-phase thyristor bridges 5-1, 5-2, and 5-3 are put in a multi-stage cascade-connection as shown in Fig. 3 (shown in Fig. 3 is an example of three-stage cascade-connection) and these three-phase thyristor bridges 5-1 to 5-3 are controlled with control angles, aa to ac, which are different from each other (the thyristors within each bridge are controlled with the same control angle).

The conventional multi-stage cascade-connection asymmetric control system has been suitable only for a high-voltage small-current power source, and it has not been suitable for a low-voltage large-current power source application since the number of thyristor bridges used must be increased unreasonably. And, if it was dealt with a single thyristor bridge, reactive power was unable to control unless the output current is controlled, and thus, it had a problem that a quick response characteristics was not obtained in the case of reactive load.

SUMMARY OF THE INVENTION The present invention has been made to solve such problems as mentioned above.

Namely, while the conventional thyristor conversion apparatus of multi-stage cascade-connection system was such that the control angles given were the same within the same thyristor bridge, although they were different between different thyristor bridges, the present invention has been directed to the provision of an apparatus which, in a power conversion apparatus of parallel multiple connection, enables that the switching elements of different phases in one bridge unit are asymmetrically controlled by being provided with control angles which are different from each other and both improvement in the power factor and control of the reactive power on the a.c. side are thereby attained.

Another object of the invention is to provide a rectifying system, or a trigger control system, which brings about improved power factor and small reactive power in a low output range.

The power conversion apparatus of the present invention has made it possible to provide, in a conversion apparatus of a parallel multiple connection system, the provision of three different control angles within the same bridge unit by changing the phase sequences when the input lines of the bridge units in parallel connection are connected to the a.c.

power source, and thereby to make possible asymmetrical control of a bridge unit. And the system used in the present invention is suited for conversion apparatus of a parallel multiple connection type for the low-voltage large-current power supply application. While this apparatus is by no means inferior to the conventional conversion apparatus of a multiple cascade-connection type with regard to the controllable range, it has an advantage that it eliminates the unnecessary cascade connection for the control of the reactive power and therefore achieves cost reduction.

BRIEF DESCRIPTION OF THE DRA WINGS Fig. 1 is a wiring diagram showing basic structure of a three-phase thyristor bridge; Fig. 2 is a waveform chart showing fundamental operation of the three-phase thyristor bridge of Fig. 1; Fig. 3 is a wiring diagram showing a threestage threephase thyristor bridge as an example of a prior art multistage cascade-connection asymmetric control system; Fig. 4 is a wiring diagram showing a thyristor conversion apparatus of parallel triple-connection according to an embodiment of the invention; Figs. 5(a) and 5(b) are diagrams showing output voltage waveforms, and equivalent circuits, respectively, Fig. 5(c) is a thyristor trigger controlling schedule for the thyristor conversion apparatus of Fig. 4;; Fig. 6 is a diagram showing reactive power controlling area in the parallel triple-connection asymmetric control system, Fig. 7(a) is an explanatory diagram for operation in a single-phase mode in the Ist area and Fig. 7(b) is that in a three-phase mode in the 2nd area; Fig. 8 is a graph showing characteristics between control angle a and output voltage according to a rectifying apparatus of the invention; Fig. 9 is a power circle diagram showing relationship between output voltage and reactive power according to the same; Fig. 10 is a graph showing characteristics between fundamental wave power factor and output voltage according to the same; Fig. 11 is a wiring diagram of another embodiment of the invention; Figs. 12(a)-12(d) are waveform diagrams for explaining operations of the same, respectively;; Fig. 13 is a firing sequence showing the manners for trigger controlling of thyristors in each mode; Fig. 14 is a graph showing characteristics between control angle a and output according to a rectifying apparatus of the invention; Fig. 15 is a power circle diagram showing relationship between output voltage and reactive power according to the same; Fig. 16 is a graph showing characteristics between fundamental wave power factor and output voltage according to the same; Figs. 17(a)-17(c) are diagrams for showing equivalent circuits, output voltage waveforms, and Fig. 17(d) is descriptions of methods for trigger control of the structure as shown in Fig. 4 operated as an inverter in each mode; Fig. 18 is a graph showing characteristics between control angle a and output voltage according to the apparatus of the same embodiment;; Fig. 19 is a power circle diagram relationship between output voltage and reactive power according to the same; Fig. 20 is a graph showing characteristics between fundamental wave power factor and output voltage according to the same; Figs. 21 to 26 are diagrams showing output voltage waveforms, triggered thyristors, and equivalent circuits for explaining methods for trigger controlling of the structure as shown in Fig. 11 operated as an inverter in each mode, respectively; Fig. 27 is a triggering sequence showing the manners for trigger controlling the thyristors in the rectifying apparatus of the structure as shown in Fig. 11 in each mode; Fig. 28 is a graph showing characteristics between control angle a and output voltage in the rectifying apparatus of the invention; Fig. 29 is a power circle diagram showing relationship between output and reactive power in the same; and Fig. 30 is a graph showing characteristic between fundamental wave power factor and output voltage in the same.

DESCRIPTION OF THE PREFERRED EMBODI MENT Fig. 4 shows a wiring diagram of a thyristor conversion apparatus of a parallel triple asymmetrical control system embodying the power conversion apparatus of the present invention.

Referring to Fig. 4, the three-phase thyristor bridges 51 to 53 are the same in their circuits as those shown in Fig. 1, but are different in phase sequences in the connections of the input lines of the respective threephase thyristor bridges with the three-phase power source 1, that is, R-U, S-V, and T-W connections are made in the three-phase thyristor bridge 51, S-U, T-V, and R-W connections are made in the three-phase thyristor bridge 52, and T-U, R-V, and S-W connections are made in the three-phase thyristor bridge 53, and also in that their output lines P are connected to a common line P through interphase reactors Lla to L3a, and their output lines N are connected to a common line N through the interphase reactors L1 b to L3b, respectively.The thyristors UP-UN, VP-VN, and WP-WN for the three phases in each of the thyristor bridges in the thyristor conversion apparatus of the present invention as described above are asymmetrically controlled at different control angles aa, aD, and ac.

Fig. 5 indicates an example of trigger controlling of the respective arms in the thyristor conversion apparatus of Fig. 4 and the output voltage waveform of the same, in which Figs.

5(a) and (b) show the output voltage waveforms and equivalent circuits in the singlephase mode (mode I) and three-phase mode (mode 11), respectively, and Fig. 5(c) indicates trigger controlled states of the thyristors in the respective modes. In the control system of the invention, sudden changes in the control angles can not be practiced, but the control is realized in the region where continuous control is possible as shown in Fig. 6. In the control region, the control can be carried out only by three different control angles a,, ab, and a, present in the same bridge (Fig. 4).

These control angles are defined in further divided four regions. That is, the Ist region is the single-phase mode, wherein the thyristors in the phases U and W, i.e., UP, UN, WP, and WN only are triggered as shown in Fig.

7(a). In this case, the control angles a, and a, can be decided univocally according to an active current p and a reactive current q. Fig.

7(b) shows the action in the three-phase mode in the 2nd region where only reactive power is added to the case of Fig. 7(a). The state of a, = ab 2a/3 is the single-phase mode as shown in Fig. 7(a), which can be continuously shifted to the three-phase mode as shown in Fig. 7(b). While both the 3rd and 4th regions are similarly of the three-phase mode, the control is made with the fixed a, = 0 in the 3rd region and with the fixed ab = a in the 4th region.

In the apparatus of the present invention, each of the three-phase full-wave rectifying units is driven, according to desired output d.c. voltage level, switched between the rectifying modes I and II, and yet, it is arranged that the composite output of the three-phase fullwave rectifying units 51 to 53 is obtained.

That is, assuming that the thyristors are ideal rectifying elements, and expressing the maximum value of the line voltage of the threephase a.c. power source 1 by Em, the d.c.

output voltage of the three-phase full-wave rectifying unit by Ed,, and the maximum value of the Ed, by Edo, when Ed, is given by Ed, = 0 - (3/71) Em and Edo by Edo = (3/71)Em, the first to third three-phase full-wave rectifying units 51 to 53 are operated in the mode I when Ed, = 0 to (2/3)Edo, and in the mode II when Ed, = (2/3)Edo to End0, and, further, the thyristors 11 to 16 forming each of these three-phase full-wave rectifying units are operated as indicated in Fig. 5(c).That is, in the mode I, UP and UN are phase controlled according to the desired output d.c. voltage level and WP and WN are set fixedly conductive (equivalent to a diode in its characteristic) and WP and WN are set fixedly nonconductive, and in the mode II, WP and WN are phase controlled and UP, UN, VP, and VN are set fixedly conductive, thus to realize asymmetric trigger control. As earlier mentioned, the outputs of these three-phase full-wave rectifying units 51 to 53 are connected in parallel through the interphase reactors L1a to L3a and Llb to L3b which compose three phase mutually coupled reactors respectively. So the momentary unbalance voltages among the outputs of these units are absorbed by the interphase reactors and circulating currents among the rectifying units are suppressed.Therefore these units can be regarded as independent ones and can be asymmetrically fire controlled not requiring isolation transformer at the input lines from the three-phase power source 1.

Furthermore,as the input lines from the threephase power source to the respective first to third three-phase full-wave rectifying units 51 to 53 are twisted in the connections from each other, the input currents from power supply never becomes asymmetrical, although the thyristors in the three-phase full-wave rectifier units are controlled in an asymmetrical manner.

Fig. 8 is a characteristic curve showing the relationship of the d.c. output voltage Ed, for the control angle a when the asymmetric control switched between the mode I and the mode II was carried out according to the levels of the output d.c. voltage Ed, as described above, and Fig. 9 is a power circle diagram showing the relationship between the normalized output voltage Ed/Edo represented in abscissas and the normalized reactive power O/(Ed,ldr, where Q is fundamental wave reactive power and Id is d.c. output current, represented in ordinates, and for the sake of comparison, that in-the case of the symmetrical control of three-phase full-wave rectification mentioned, b, is indicated besides that of the asymmetrical control of the embodiment of the present invention, a.As seen from Fig. 9, the radius of the arc in the power circle diagram in the case of the apparatus of the invention is about 1/3 of that of the conventional three-phase full-wave system and the fact that the generation of reactive power has been largely reduced is apparent.

Fig. 10 indicates the relationship of the fundamental wave power factor cosf 1 against the output voltage, in which a is for the case of asymmetric control by the embodiment of the invention and b is for the case of the conventional three-phase full-wave system. As apparent from the graph, the power factor in the low and medium output regions is greatly improved.

Although the most fundamental case where the three-phase full-wave rectifying unit is formed of six thyristors was mentioned in the above for the sake of simplicity of explanation, the similar effects to those obtained from the above mentioned embodiment are obtained even if the thyristors are connected in parallel and series depending upon voltage and current values, the three-phase full-wave rectifying units themselves are connected in parallel and series, or they are put in an inverseparallel connection for providing the output current in the reverse direction. And, although thyristors were employed as the elements forming the three-phase full-wave rectifying unit in the above explanation, other elements which can be conductive-phase controlled, such as transistors and gate turn-off thyristors, may provide the same effects.

While the above described trigger system is that which is aimed to maximize the power factor according to the d.c. output voltage values to be provided through three sets of three-phase full-wave rectifying units connected in parallel, the structure of the present invention basically enables asymmetrical control providing the phases within the same three-phase full-wave rectifying system with three different control angles without accompanying asymmetry in the input currents and suppressing flows of circular currents between units, and the control angle of the thyristors in each the rectifying unit is not to be limited to that as described in the foregoing with reference to the embodiment of the invention.

According to the embodiment as described above, by connecting a plurality of rectifying units in parallel, by switching the mode of the operation of the apparatus to the single-phase mode or poly-phase mode according to required output d.c. voltage levels, and by controlling, in an asymmetrical trigger manner, the switching elements constituting each the rectifying unit, the effect is obtained that a rectifying system achieving high power factor and producing small reactive power even in the low output voltage range can be provided.

Since, further, the phases of the input power lines are twisted between rectifying units, such an effect is obtained that the unbalance on the power source side due to the asymmetrical control can be prevented. And, when such a rectifying apparatus producing small reactive power is used as a load, if a flywheel generator or the like is used as the threephase a.c. power source, there is provided not only such an effect that the required apparent power becomes smaller but also the effect that variation of the output voltage in the generator can be reduced. Thus, the rectifying apparatus having these characteristics is considered to be effective when applied to such as the Leonard system requiring low-voltage.large-current supply and the power source for the field coil for nuclear fusion system generating large reactive power.

Now, another embodiment of the present invention will be described below with reference to accompanying drawings. Referring to Fig.

11, reference numerals 51, 52, 53, 54, 55, and 56 denote the first, second, third, fourth, fifth, and the sixth three-phase full-wave rectifying units, each including thyristors UP, UN, VP, VN, WP, and WN. L1a, L1b, .., L6a, L6b denote reactors. Although the internal structure of the first to sixth three-phase full-wave rectifying units 51 to 56 is the same as that of the unit 5 of Fig. 1, connections with the input lines from the three-phase a.c. power source 1 are made like this: R-U, S-V, and T W connections are made for the first and second three-phase full-wave rectifying units 51, 52, S-U, T-V, and R-W connections are made for the third and fourth three-phase full-wave rectifying units 53, 54, and T-U, R-V, and S W connections are made for the fifth and sixth three-phase full-wave rectifying unit 55, 56.And, the output lines P and N, connected in series with interphase reactors L1a, L1b, L2a, L2b ..., L6a, and L6b, respectively, are connected in parallel with each other.

The present embodiment of the invention is such that operates the first to sixth threephase full-wave rectifying units 51 to 56 in four modes switchable thereto according to desired output voltage levels. Assuming that the thyristors are ideal rectifying elements, and expressing the maximum value of the line voltage of the three-phase a.c. power source 1 by Em, the d.c. output voltage of the threephase full-wave rectifying unit by Ed,, and the maximum value of the Ed, by Edo, when Ed, becomes Ed, = O to (3/7r)Em and Edo becomes Edo = (3/Em, operation in a mode I is made when Ed, = 0 to (1/3)end,, operation in a mode Il is made when Ed = (1/3)Edo to 2/3end,, operation in a mode Ill is made when Ed, = (2/3)Edo to (5/6)end,, and operation in a mode IV is made when Ed, = (5/6)Edo to Edo. Descriptions about the modes I to IV will be given later.

Fig. 12 shows equivalent circuits of the first to sixth three-phase full-wave rectifying units 51 to 56 and output waveforms corresponding to each operation mode.

Fig. 13 is a sequence indicating states of trigger control of all the thyristors constituting the first to sixth three-phase full-wave rectifying units 51 to 56 in each mode during the time period of the phase cot of the a.c. power source being cot = O to 4n (two-cycle period).

As apparent from Figs. 12 and 13, during the time period ot = O to 27r in the mode I, all the three-phase full-wave rectifying units 51 to 56 are operated in a single-phase mode. That is, the thyristors UP of the first, third and the fifth three-phase full-wave rectifying units 51, 53, 55 are phase controlled according to desired output voltage levels, the thyristors VP and VN are set fixedly conductive (the same as an diode in its characteristic), and the thyristors WP, UN, and WN are set fixedly nonconductive, and the output voltage as shown in Fig. 12(a) is thereby provided.And, the thyristors UN in the second, fourth, and the sixth three-phase full-wave rectifying units 52, 54, and 56 are phase controlled, the thyristors VP and VN are set fixedly conductive, and the thyristors UP, WP, and WN are set fixedly nonconductive, whereby the same voltage as that in Fig. 12(a) is output. In the mode 11, operation in a single-phase is performed, that is, the thyristors UN of the first, third, and the fifth three-phase full-wave rectifying units 51, 53, 55 are phase controlled, the thyristors UP, VP, and VN are kept conductive, and the thyristors WP and WN are kept nonconductive, whereby the output voltage as shown in Fig. 12(b) is provided. And, the thyristors UP of the second, fourth, and sixth threephase full-wave rectifying units 52, 54, 56 are phase controlled, the thyristors VP, UN, and VN are kept conductive, and the thyristors WP and WN are kept nonconductive, whereby the same voltage as that in Fig.

12(b) is output. In the mode Ill, operation in a three-phase mode is performed, namely, the thyristors WP of the first, third, and the fifth three-phase full-wave rectifying units 51, 53, 55 are phase controlled, the thyristors UP, VP, UN, and VN are kept conductive, and the thyristors WN are kept nonconductive, whereby the output voltage as shown in Fig.

12(c) is provided. And, the thyristors WN of the second, fourth, and the sixth three-phase full-wave rectifying units 52, 54, 56 are phase controlled, the thyristors UP, VP, UN, and VN are kept conductive, and the thyristors WP are kept nonconductive, whereby the same voltage as that of Fig. 12(c) is output. In the mode IV, operation in a three-phase mode is performed, that is, the thyristors WN of the first, third, and the fifth three-phase full-wave rectifying units 51, 53, 55 are phase controlled, the thyristors UP, VP, WP, UN, and VN are kept conductive and the output voltage as shown in Fig. 12(d) is obtained. And, the thyristors WP of the second, fourth, and sixth three-phase full-wave rectifying units 52, 54, 56 are phase controlled, and the thyristors UP, VP, UN, VN, and WN are kept conductive, whereby the same voltage as that in Fig. 12(d) is output.

And, further, to bring to naught the difference in the average output voltages upward and downward from the neutral potential on an average, the ways of control of two sets of three-phase full-wave rectifying units of those constituting three pairs of rectifying units are exchanged with each other, that is, gate controlling manners for the thyristors UP and UN are exchanged with each other in the next time period of ot = 21t to 4n, and the output as shown in Fig. 12 is thereby obtained.

In each of the modes I to IV, the thyristors of the first and second, the third and fourth, and the fifth and sixth three-phase full-wave rectifying units are selectively controlled so as to produce 1800 of phase difference between their output voltages. And to absorb the momentary unbalance voltages among each rectifying units 51 to 56, the output sides of these units are connected in series with the interphase reactors Lla to L6a and Llb to L6b respectively before they are connected in parallel.In the structured apparatus of the invention, the output voltages of the first, third, and the fifth three-phase full-wave rectifying units 51, 53, 55 have 120 of phase difference between each other since their input terminals are connected with the output terminals of the three-phase power source in different phase sequence, and the output voltages of the second, fourth, and the sixth three-phase full-wave rectifying units have also 120 of phase difference between each other for the same reason.On the other hand, as stated above, there are phase differences of 180 between the output voltages of the first and second, the third and fourth, and the fifth and sixth threephase full-wave rectifying units, and thus, all the threephase full-wave rectifying units 51 to 56 come to have in their output voltages 60 of phase difference between each other. Therefore, the composite output current from the units connected in parallel via the reactors Lla to L6b becomes very smooth in each mode.

Fig. 14 shows characteristics of the output voltage Ed, for the control angle a when the asymmetric control was carried out being switched among four modes according to the levels of the output current and voltage as described above, and Fig. 15, a power circle diagram, shows the relationship between the output voltage Ed,/Edo represented in abscissas and the reactive power Q/(Ed,ld), where o is fundamental wave reactive power and Id is d.c. output current, represented in ordinates, and for the sake of comparison, that in the case of the symmetric control of threephase full-wave rectification of the prior art, b, is indicated besides that of the asymmetric control of the embodiment of the present invention, a.As seen from the diagram, the radius of the arc in the power circle diagram in the case of the apparatus of the invention is about 1/6 of that of the conventional threephase full-wave system and the fact that the generation of reactive power has been largely reduced is apparent.

Fig. 16 indicates the relationship of the fundamental wave power factor cosf 1 against the output voltage, Ed,/Edo, in which a is for the case of asymmetric control by the embodiment of the invention and b is for the case of the conventional three-phase full-wave system. As apparent from the graph, the power factor in the low and medium output regions is greatly improved.

Although the most fundamental case where the three-phase full-wave rectifying unit is formed of six thyristors was mentioned in the above for the sake of simplicity of explanation, the similar effects to those obtainable from the above mentioned embodiment are obtained even if the thyristors are connected in parallel and series depending upon required voltage and current values, the three-phase full-wave rectifying units themselves are connected in parallel and series, or they are put in an inverse-parallel connection for providing the output current in the reverse direction. And, although thyristors were used as the elements forming the three-phase full-wave rectifying unit in the above explanation, other elements which can be conductive-phase controlled, such as transistors and gate turn-off thyristors, may provide the same effects.

According to the embodiment as described above, by connecting a plurality of pairs of rectifying units in parallel, by switching among the modes according to required output current and voltage levels, and by controlling, in an asymmetrical trigger manner, the switching elements constituting the rectifying unit, the effect is obtained that a rectifying system attaining high power factor and generating small reactive power even in the low output voltage range is achieved. Since, further, the phase sequences of the input power lines are made different between pairs of rectifying units, such an effect is obtained that the unbalance on the power source side due to the asymmetrical control is prevented. And, when such a rectifying apparatus producing small reactive power is used as a load, if a flywheel generator or the like is used as the three-phase a.c.

power source, there is provided such an effect not only the required apparent power becomes smaller but also the output voltage variation of the generator can be reduced. Thus, the rectifying apparatus having these characteristics is considered to be effective when applied to such as a Leonard system which requires low-voltage large-current supply and the power source for the field coil for nuclear fusion system generating larger reactive power.

Now, another embodiment of the invention, that is, the power conversion apparatus as shown in Fig. 4 adapted to be operated as an inverter, or adapted to provide a negative d.c.

output from the d.c. output terminal, will be described in the following.

Fig. 17(a)-(d) show the trigger control system and output voltage waveform of the three-phase full-wave rectifying units 51-53 of Fig. 4, in which Fig. 17(a) shows the output voltage waveforms, conductive thyristors, and equivalent circuits of the three-phase full-wave rectifying units in a single-phase mode (hereafter to be called the mode I') and Figs. 17(b) and (c) show the same data in three-phase modes (hereafter to be called the mode II', and the same is further divided into the mode II'A for the case (B) and the mode ll'B for the case of Fig. 17(c)), and Fig. 17(d) shows a thyristor trigger control sequence in each the mode.In the present embodiment, each of the three-phase full-wave rectifying unit is switched so as to be operated in the mode 1', II'A, or II'B according to the desired output d.c. voltage level in the inverter region, and it is aimed that composite output of these threephase full-wave rectifying units 51 to 53.That is, assuming that the thyristor is an ideal rectifying element and expressing the value of the minimum angle of advance of phase control (the y limiter), by y, the maximum value of the line voltage of the three-phase a.c. power source 1 by Em, the d.c. output voltage of the threephase full-wave rectifying unit by Ed,, and the maximum value of Ed, by Edo, when, in the inverter region, Ed, is given by Ed, = (Em/1r) (1 - cosy) to -(Em/;r)cosy and Edo is given by Edo = (3/:::)Em, the first to third three-phase full-wave rectifying units 51 to 53 are operated in the mode I' when Ed, = (Edo/3) (1 - cosy) to --(2/3)Ed,cosy, in the mode IlA when Ed, = --(2/3)Ed,cosy to -(Ed0/3)(2.Scosy - (V3/2)siny, and in the mode II'B when Ed, = --(Ed,/3)(2.5cosy - (A/3/2) siny) to -Ed0cosy. And further, the thyristors 11 to 16 constituting each of the first to third three-phase full-wave rectifying units are subjected to asymmetrical trigger control in the respective modes as shown in Fig. 17.That is, in the mode I': VP and VN are controlled at control angle a (O < a ( it - y) according to the desired output d.c. voltage level, UP and UN are triggered at 7ry, and WP and WN are kept nonconductive; in the mode II'A, VP and VN are phase controlled at the control angle a (it/3 + y ( a ( - y) and triggered also at it - y, UP and UN are triggered at 2/3it - y, and WP and WN are triggered at it - y; and in the mode ll'B, UP and UN are phase controlled at a ((2/3)it - y < a < y), and VP, VN, WP, and WN are triggered at it - y. As earlier mentioned, the outputs of these three-phase full-wave rectifying units 51 to 53 are connected in parallel through the interphase reactors L1a to L3a and L1b to L3b which compose three-phase mutually coupled reactors respectively.So the momentary unbalance voltages among the outputs of these units are absorbed by the interphase reactors and circulating currents among the rectifying units are suppressed. Therefore these units can be regarded as independent ones and can be asymmetrically fire controlled not requiring isolation transformer at the input lines from the three-phase power source 1. Furthermore, as the input lines from the three-phase power source to the respective first to third threephase full-wave rectifying units 51 to 53 are twisted in the connections from each other, the input currents from power supply never becomes asymmetrical, although the thyristors in the three-phase full-wave rectifier units are controlled in an asymmetrical manner.

Fig. 18 shows characteristics of the output voltage Ed, against the control angle a when the asymmetrical control switched among the mode 1', mode II'A, and the mode II'B was carried out according to the levels of the output d.c. voltage Ed, as described above, and Fig. 19 is a power circle diagram showing the relationship between the output voltage Ed,/Edo represented in abscissas and the reactive power O/(Ed0.ld), where 0 is fundamental wave reactive power and Id is d.c. output current, represented in ordinates, and for the sake of comparison, that in the case of the symmetrical control of three-phase full-wave rectification of the prior art, b, is indicated besides that of the asymmetrical control of the embodiment of the present invention, a.

As seen from Fig. 19, the radius of the arc in the power circle diagram in the case of the apparatus of the invention is about 1/3 of that of the conventional three-phase full-wave system and the fact that the generation of reactive power has been largely reduced is apparent.

Fig. 20 indicates the relationship of the fundamental wave power factor cossf 1 against the output voltage, in which a is for the case of asymmetrical control by the embodiment of the invention and 6 is for the case of the conventional three-phase full-wave system. As apparent from the graph, the power factor in the low and medium output regions is greatly improved.

Although the most fundamental case where the three-phase full-wave rectifying unit is formed of six thyristors was mentioned in the above for the sake of simplicity of explanation, the similar effects to those obtained from the above mentioned embodiment are obtainable even if the thyristors are connected in parallel and series depending upon voltage and current values, the three-phase full-wave rectifying units themselves are connected in parallel and series, or they are put in an inverseparallel connection for providing the output current in the reverse direction. And, although thyristors were used as the elements forming the three-phase full-wave rectifying unit in the above explanation, other elements which can be conductive-phase controlled, such as transistors and gate turn-off thyristors, may provide the same effects.And, the reason why the case where the thyristors are phase controlled in the manner as described with reference to Fig. 17 was cited in the above embodiment was because it was aimed to make the variation of the rectified waveforms when switched from one mode to another as small as possible and thereby to attain a smooth change, but, even if the phase to be phase controlled is changed (for example, in the mode 1', UP and UN are phase controlled and VP and VN are triggered at it - 7), the ef- fects of the invention will not be changed if the units 51 to 53 are changed all alike.

While the above described trigger system was such that was aimed to maximize the power factor according to the negative d.c.

output voltage values to be produced by the three sets of three-phase full-wave rectifying units connected in parallel, the structure of the present invention basically enables asymmetrical control, within the same three-phase fullwave rectifying system, to provide its phases with three different control angles not accompanying asymmetry in the input currents and suppressing flows of circular currents between units, and so, the control angle of the thyristors in each the rectifying unit is not to be limited to that as described above with reference to the embodiment of the invention.

According to the embodiment as described above, in carrying out the inverter operation through connection of a plurality of rectifying units in parallel, switching of the operating mode of the apparatus to the single-phase mode or three-phase mode according to required output current and voltage levels, and controlling, in an asymmetrical trigger manner, of the switching elements constituting each of the rectifying units, the effect is obtained that a rectifying system attaining high power factor and producing small reactive power even in the low output voltage range can be provided.

Since, further, the phase sequence of the input power lines are changed between rectifying units, such an effect is obtained that the unbalance on the power source side due to the asymmetrical control can be prevented.

Now, another embodiment of the invention, that is, the power conversion apparatus as shown in Fig. 11 adapted to be operated as an inverter, or adapted to provide a negative d.c. output from the d.c. output terminal, will be described in the following.

The present embodiment is such that, in operating the six sets of the three-phase fullwave rectifying units connected in parallel as shown in Fig. 11 as an inverter apparatus, only one of the six thyristors constituting each three-phase full-wave rectifying unit is phase controlled, while others are held OFF or triggered at it - y or (2/3) it y, and operated in any of six modes I', Ill', III'A, III'B, IV'A, and IV'B (to be described later) by being switched thereto. Now, assuming that the thyristor is an ideal rectifying element and expressing the value of the minimum angle of advance of phase control (the y limiter) by y, the maximum value of the line voltage of the three .phase a.c. power source 1 by Em, the d.c.

output voltage of the three-phase full-wave rectifying unit by Ed,, and the maximum value of Ed, by Edo, when, in the inverter region, Ed, is given by Ed, = (mem/271) (1 - cosy) to -(3/it)Em cosy and Edo is given by Edo = (3/7c)Em (in the converter region), the first to sixth three-phase full-wave rectifying units 51 to 56 are operated: in the mode I' when Ed, = (Edo/6) (1 - cosy) to -(Ed0/3)cosy; in the mode 11' when Ed, = (Edo/3)cosy to --(2/3)Ed, cosy; in the mode III'A when Ed, = -(2/3)Ed,, cosy to End,/6 ((V3/2)siny - 4.5 cosy); in the mode III'B when Ed, = (Edo/6) ((V3/2 siny - 0.5 cosy) to -(5/6) Edo cosy; in the mode IV'A when Ed, = -(5/6)Ed, cosy to (Edo/6) ((V3/2 siny - 5.5 cosy); and in the mode IV'B when Ed, = (Edo/6) ((V3/2 siny - 5.5 cosy) to -Ed, cosy.

Figs. 21 to 26 are for explaining trigger controlling manners in each mode. In each figures, (a) is a diagram for indicating the out put voltage waveforms of the three-phase fullwave rectifying unit and the triggered phase of each thyristor, (b) is a diagram indicating the triggered thyristors, and (c) and (d) are for showing equivalent circuits of the three-phase full-wave rectifying units. For the sake of convenience of explanation hereafter, the three phase full-wave rectifying units 51, 53, and 55 will be called the group No. 1, 52, 54, and 56 will be called the group No. 2, the first one cycle of a consecutive two-cycle per iod of the three-phase a.c. power source 1 will be called region A and the next cycle will be called region B.

In the mode 1', as shown in Fig. 21, the three-phase full-wave rectifying units of the 1st group in the region A and the three-phase full-wave rectifying units of the 2nd group in the region B are controlled according to desired output d.c. voltage levels such that VP is phase-controlled by the control angle a (O to it - 7), UP is triggered at (2/3)n - y, VN is kept conductive, and UN, WP, and WN are kept nonconductive.And the three-phase fullwave rectifying units of the Ist group in the region B and those of the 2nd group in the region A are controlled such that the thyristors on the P side and those on the N side are symmetrically exchanged in their controlled manners, that is, VN is phase-controlled by the control angle a, UN is triggered at (2/3)it -- y, VP is kept conductive, and UP, WP, and WN are kept nonconductive.The reason why the trigger control manners for the thyristors constituting the three-phase full-wave rectifying units are adapted to be symmetrically exchanged between the 1st group and the 2nd group and the control manners for the 1st group and the 2nd group are exchanged for each cycle of the three-phase a.c. power supply 1 is because it is aimed to balance the phase currents in the three-phase power source and also to bring the difference in the average output voltages upward and downward from the neutral potential to naught on an average, and the same treatment is made for the following modes II' to IV'B in Figs. 22 to 26.

And the reason why the thyristors to be phase-controlled were chosen from those in the phase V is just because continuity from the waveform of the phase U being phasecontrolled in the rectifying operation was taken into consideration so that the variation of the voltage waveform at the time of the switching from the rectifying operation to the inverting operation may be kept as small as possible, and there is no other significance.

In the mode 11', as shown in Fig. 22, the three-phase full-wave rectifying units of the Ist group in the region A and those of the 2nd group in the region B are controlled so that VN is phase-controlled at the phase angle a (y to it - 7), UP and UW are triggered at (2/3)it - 7, VP is triggered at it - 7, and WP and WN are kept nonconductive, and the threephase full-wave rectifying units of the Ist group in the region B and those of the 2nd group in the region A are controlled so that VP is phase-controlled, UP and UN are triggered at (2/3)K - y, VN is triggered at it y, and WP and WN are kept nonconductive.

In the mode III'A as shown in Fig. 23, the three-phase full-wave rectifying units of the Ist group in the region A and those of the 2nd group in the region B are controlled so that VN is phase-controlled at the control angle a (-(1/3)it + y to -7) and also triggered at it - 7, UP and UN are triggered at (2/3)it - 7, VP and WN are triggered at it - 7, and WP is kept nonconductive, and the three-phase full-wave rectifying units of the 1st group in the region B and those of the 2nd group in the region A are controlled so that VP is phase-controlled at the control angle a and also triggered at it - 7, UP and UN are triggered at (2/3)it - 7, VM and WP are triggered at it - 7, and WN is kept nonconductive.

In the mode III'B as shown in Fig. 24, the three-phase full-wave rectifying units of the 1st group in the region A and those of the 2nd group in the region B are controlled so that UN is phase-controlled by the control angle a ((2/3) n - y to n - y), UP is triggered at (2/3)it - 7, VP, VN, and WN are triggered at it - 7, and WP is kept nonconductive, and the three-phase full-wave rectifying units of the 1st group in the region B and those of the 2nd group in the region A are controlled so that UP is phase-controlled, UN is triggered at (2/3)it - 7, VP, VN, and WP are triggered at it , y, and WN is kept nonconductive.

In the mode IV'A as shown in Fig. 25, the three-phase full-wave rectifying units of the Ist group in the region A and those of the 2nd .group in the region B are controlled so that VP is phase-controlled by the control angle a (-(1/3)n + 7 to -7) and also triggered at it - 7, UP is triggered at (2/3)n -- y, and UN, VN, WP, and WN are triggered at it , 7, and the three-phase full-wave rectifying units of the 1st group in the region B and those of the 2nd group in the region A are controlled so that VN is phase-controlled by the control angle a and also triggered at it - 7, UN is triggered at (2/3)it - 7, and UP, VP, WP, and WN are triggered at it - 7.

In the mode IV'B as shown in Fig. 26, the three-phase full-wave rectifying units of the 1st group in the region A and those of the 2nd group in the region B are controlled so that UP is phase-controlled by the control angle a ((2/3)it - 7 to it - 7), and UN, VP, VN, WP, and WN are triggered at it - 7, and the three-phase full-wave rectifying units of the 1st group in the region B and those of the 2nd group in the region A. are controlled so that UN is phase-controlled and UP, VP, VN, WP, and WN are triggered at n.-- 7.

Fig. 27 is a sequence showing the trigger control manners of the three-phase full-wave rectifying units in all the modes. As stated in the foot note of the sequence, it is showing the controlling manners in the region A. For the reason as stated earlier, the controlling manners in the region B are such that those for the Ist group and those for the 2nd group in the region A are there exchanged with each other and such exchanging is repeated on for each cycle of the three-phase a.c. power source 1.

In each of the modes I' to IV'B, the thyris tors of the first and second, the third and fourth, and the fifth and sixth three-phase fullwave rectifying units are selectively controlled so that there will be produced 180 of phase difference between their output voltages. And to absorb the momentary unbalance voltages among each rectifying units 51 to 56, the output sides of these units are connected in series with the interphase reactors Lla to L6b before they are connected in parallel.In the thus structured apparatus of the invention, the output voltages of the first, third, and the fifth three-phase full-wave rectifying units 51, 53, 55 ought to have 120 of phase difference between each other since their input terminals are connected with the output terminals of the three-phase power source in different phase sequence, and the output voltages of the second, fourth, and the sixth three-phase fullwave rectifying units 52, 54, 56 have also 1200 of phase difference between each other for the same reason. Thus all the three-phase full-wave rectifying units 51 to 56 come to have in their output voltages 60 of phase difference between each other.Therefore, the composite output current from the units connected in parallel via interphase reactors Lla to L6b becomes very smooth in every the mode.

Fig. 28 shows characteristics of the output voltage Ed, against the control angle a when the asymmetrical control, switched among six modes, was carried out according to the levels of the output current and voltage in the inverting operation as described above, and Fig. 29 is a power circle diagram showing the relationship between the output voltage Ed,/Edo represented in abscissas and the reactive power Q/(Edo ld), where Q is fundamental wave reactive power and Id is d.c. output current, represented in ordinates, and for the sake of comparison, that in the case of the symmetric control of three-phase full-wave rectification of the prior art, b, is indicated besides that of the asymmetric control of the embodiment of the present invention, a.As seen from the diagram, the radius of the arc in the power cycle diagram in the case of the apparatus of the invention is about 1/6 of that of the conventional three-phase full-wave system and the fact that the generation of reactive power has been largely reduced is apparent.

Fig. 30 indicates the relationship of the fundamental wave power factor cosf 1 against the output voltage, Ed,/Edo, in which a is for the case of asymmetric control by the embodiment of the invention and b is for the case of the conventional three-phase full-wave system. As apparent from the graph, the power factor in the low and medium output voltage regions is greatly improved.

Although the most fundamental case where the three-phase full-wave rectifying unit is formed of six thyristors was mentioned in the above for the sake of simplicity of explanation, the similar effects to those obtained from the above mentioned embodiment are obtained even if the thyristors are connected in parallel and series depending upon required voltage and current values, the three-phase full-wave rectifying units themselves are connected in parallel and series, or they are put in an inverse-parallel connection for providing the output current in the reverse direction. And, although thyristors were used as the elements forming the three-phase full-wave rectifying unit in the above explanation, other elements which can be conductive-phase controlled, such as transistors and gate turn-off thyristors, may provide the same effects.The reason why the thyristors which are phase-controlled are selected as described above with reference to Figs. 21 to 27 is because it is considered to minimize the variations in the output voltage waveforms at the time of switching from one mode to another and thereby to obtain a smooth change of the mode. Even if the phase-controlled phase is changed, the effects of the invention will not change if all the three-phase full-wave rectifying units 51 to 56 are changed accordingly.

According to the embodiment as described above, in carrying out the inverter operation through connection of a plurality of pairs of rectifying units in parallel, switching among the modes according to required output current and voltage levels, and controlling, in an asymmetrical trigger manner, of the switching elements constituting the rectifying unit, the effect is obtained that a rectifying system attaining high power factor, producing small reactive power even in the low output voltage range, and eliminating current flows between the rectifying units can be provided. Since, further, the phase sequences of the input power lines are twisted between pairs of rectifying units, such an effect is obtained that the unbalance on the power source side due to the asymmetrical control can be prevented.

As described so far, the apparatus of the invention, in power conversion apparatuses formed of parallel multiple bridge units for low-voltage large-current applications, exerts asymmetrical control of the switching elements on its arms and thereby provides the effects of improvement in the power factor on the a.c. side as well as suppression of the reactive power.

Claims

1. In a power conversion apparatus including a rectifying unit formed of a plurality of switching elements, which are capable of conductive-phase controlling, and adapted, with said rectifying unit connected with an a.c.

power source and said switching elements conduction-controlled, to output d.c. power from the output side of said rectifying unit, said power conversion apparatus having a plu rality of said rectifying units, which are connected in parallel to the a.c. power source with mutually different phase sequences given at their input terminals and joined together at their output terminals, said switching elements of the different phases within said rectifying units being adapted to be conduction-controlled asymmetrically at different control angles.

2. A power conversion apparatus including a rectifying unit formed of a plurality of switching elements, which are capable of conductive-phase controlling, and adapted, with said rectifying unit connected with an a.c.

power source and said switching elements conduction-controlled, to output d.c. power from the output side of said rectifying unit, wherein there are provided a plurality of rectifying units, which are connected in parallel to the a.c. power source with mutually different phase sequences given at their input terminals and joined together at their output terminals via serially connected reactors and adapted to be operated being switched between a singlephase operation mode and a poly-phase operation mode according to the d.c. output voltage value to be output thereby.

3. A power conversion apparatus according to claim 2, wherein said rectifying units consist of a first, second, and third three-phase full-wave rectifying units, said three-phase fullwave rectifying units being adapted to be operated, with its providable maximum d.c. output voltage value expressed by Edo, in a single-phase mode when the desired output voltage values are 0 to (2/3)Edo and in a threephase mode when the desired output voltage values are (2/3)Edo to Wed(,.

4. A power conversion apparatus according to claim 3, wherein, with the phases of the output terminals of the three-phase a.c. power source expressed by phases R, S, and T, and the phases of the input terminals of the first, second, and third three-phase full-wave rectifying units expressed by phases U, V, and W, the output terminals of said three-phase a.c. power source and the input terminals of said first, second, and third three-phase fullwave rectifying units are connected with different phase sequences given thereto, i.e., such that the connections (R-U, S-V, T-W), (S U, T-V, R-W), and (T-U, R-V, S-W) are made, the positive output terminals P and the negative output terminals N of each of the threephase full-wave rectifying units are respectively connected in parallel via serially connected interphase reactors, and, with the switching element constituting each of the three-phase full-wave rectifying units disposed in the phase U on the P side denoted by UP, that disposed in the phase U on the N side denoted by UN, that disposed in the phase V on the P side denoted by VP, that disposed in the phase V on the N side denoted by VN, that disposed in the phase W on the P side denoted by WP, and that disposed in the phase W on the N side denoted by WN, each the three-phase full-wave rectifying unit is controlled when operated in the single-phase operation mode such that said switching elements UP and UN are phase-controlled, VP and VN are kept conductive, and WP and WN are kept nonconductive, and each the threephase full-wave rectifying unit is controlled when operated in the three-phase operation mode such that said switching elements WP and WN are phase-controlled, and UP, UN, VP, and VN are kept conductive.

5. A power conversion apparatus including the rectifying unit formed of a plurality of switching elements, which are capable of conductive-phase controlling, and adapted, with said rectifying unit connected with an a.c.

power source and said switching elements conduction-controlled, to output d.c. power from the output side of said rectifying unit, wherein there are provided a plurality of pairs of said rectifying units, which are connected in parallel to the a.c. power source with different phase sequences given between each pair at their input terminals and joined together at their output terminals and adapted to be operated being switched between a single-phase operation mode and a poly-phase operation mode according to the d.c. output voltage values to be output thereby.

6. A power conversion apparatus according to claim 5, wherein there are provided three pairs of said rectifying units, each the pair being formed of two sets of three-phase fullwave rectifying units, and each said three phase full-wave rectifying unit, with the maxi mum value of the d.c. output voltage obtaina ble therefrom expressed by Edo, is operated: when the desired voltage values are 0 to (1/3)Edo, in a single-phase mode I; when the desired output voltage values are (1/3)Edo to (2/3)end,, in a single mode II different from the mode 1, when the desired output voltage values are (2/3)Edoto (5/6)end,, in a three phase mode Ill, and when the desired output voltage values are (5/6)Edo to Edo, in a three phase mode IV different from the mode Ill.

7. A power conversion apparatus according to claim 6, wherein, with the phases of the output terminals of the three-phase power source expressed by phases R, S, and T, and the phases of each the three-phase full-wave rectifying unit of each pair expressed by phases U, V, and W, the output terminals of said three-phase a.c. power source and the input terminals of the three-phase full-wave rectifying units of each pair are connected with different phase sequences provided between each pair, i.e., such that the connec tions (R-U, S-V, T-W), (S-U, T-V, R-W), and (T-U, R-V, S-W) are made, the positive output terminals P and the negative output terminals N of the three-phase full-wave rectifying units are respectively connected in parallel via seri ally connected interphase reactors, and, with the switching element constituting the threephase full-wave rectifying units disposed in the phase U on the P side denoted by UP, that disposed in the phase U on the N side denoted by UN, that disposed in the phase V on the P side denoted by VP, that disposed in the phase V on the N side denoted by VN, that disposed in the phase W on the P side denoted by WP, and that disposed in the phase W on the N side denoted by WN: in the single-phase mode I, one side of two three-phase full-wave rectifying units constituting each pair is controlled such that the switching element UP is phase-controlled, VP and VN are kept conductive, and WP, UN, and WN are kept nonconductive, and the other side of two three-phase full-wave rectifying units constituting each pair is controlled such that the switching element UN is phasecontrolled, VP and VN are kept conductive, and UP, WP, and WN are kept nonconductive; in the single-phase mode II, one side of two three-phase full-wave rectifying units constituting each pair is controlled such that the switching element UN is phase-controlled, UP, VP, and VN are kept forward conductive, and WP and WN are kept nonconductive, and the other side of two three-phase full-wave rectifying units constituting each pair is controlled such that the switching element UP is phasecontrolled, VP, UN, and VN are kept conductive, and WP and WN are kept nonconductive; in the three-phase mode Ill, one side of two three-phase full-wave rectifying units constituting each pair is controlled such that the switching element WP is phase-controlled, UP, VP, UN, and VN are kept conductive, and WN is kept nonconductive, and the other side of two three-phase full-wave rectifying units constituting each pair is controlled such that the switching element WN is phase-controlled, UP, VP, UN, and VN are kept conductive, and WP is kept nonconductive; and in the three-phase mode IV, one side of two three-phase fullwave rectifying units constituting each pair is controlled such that the switching element WN is phase-controlled and all the other switching elements are kept conductive and the other side of two three-phase full-wave rectifying units constituting each pair is controlled such that the switching element WP is phase-controlled and all the other switching elements are kept conductive, and further, the two three-phase full-wave rectifying units constituting the three pairs of rectifying units are adapted to be exchanged with each other in their trigger controlled manners at intervals of the period of one cycle of the a.c. power source.

8. A power conversion apparatus including the rectifying unit formed of a plurality of switching elements, which are capable of conductive-phase controlling, and adapted, with said rectifying unit connected with an a.c.

power source and said switching elements conduction-controlled to perform inverting operation thereby to provide negative d.c. output voltage from the output side of said rectifying unit, wherein there are provided a plurality of rectifying units, which are connected in parallel to the a.c. power source with different phase sequences provided at their input terminals and joined together at their output terminals via serially connected interphase reactors and adapted to be operated being switched between a single-phase operation mode and a poly-phase operation mode according to the d.c. output voltage level to be output thereby.

9. A power conversion apparatus according to claim 8, wherein said rectifying unit consists of a first, second, and third three-phase full-wave rectifying units, said three-phase fullwave rectifying units, with its providable maximum d.c. output voltage value expressed by Edo and the value of the minimum angle of advance of phase control (y limiter) expressed by y, are operated in a mode I' of the singlephase mode when the desired output voltage values are (Edo/3) (1 - cosy) to --(2/3)Ed, cosy, in the mode II'A of the three-phase mode when the desired output voltage values are -(2/3)Ed, cosy to -(Ed,/3) (2.5 cosy (R/3/2 siny)siny, and in the mode II'B of the three-phase mode when the desired output voltage values are -(Ed,/3) (2.5 cosy (5/3/2)siny) to -Ed, cosy.

10. A power conversion apparatus according to claim 9, wherein, with the phases of the output terminals of the three-phase a.c.

power source expressed by phases R, S, and T, and the phases of the input terminals of the first, second, and third three-phase fullwave rectifying units expressed by phases U, V, and W, the output terminals of said threephase a.c. power source and the input terminals of said first, second, and third threephase full-wave rectifying units are connected with different phase sequences provided therefor, i.e., such that connections (R-U, S-V, T W), (S-U, T-V, R-W), and (T-U, R-V, S-W) are made, the positive output terminals P and the negative output terminals N of each the threephase full-wave rectifying unit are respectively connected in parallel via serially connected interphase reactors, and, with the switching element constituting each the three-phase fullwave rectifying unit disposed in the phase U on the P side denoted by UP, that disposed in the phase U on the N side denoted by UN, that disposed in the phase V on the P side denoted by VP, that, disposed in the phase V on the N side denoted by VN, that disposed in the phase W on the P side denoted by WP, and that disposed in the phase W on the N side denoted by WN: each the three-phase full-wave rectifying unit is controlled in the mode I' such that said switching elements VP and VN are phase-controlled, UP and UN are triggered at (2/3)it - 7, and WP and WN are kept nonconductive; each the three-phase fullwave rectifying unit is controlled in the mode II'A such that said switching elements VP and VN are phase-controlled and triggered at it r, UP and UN are triggered at (2/3)it - 7, and WP and WN are triggered at it - 7; and each the three-phase full-wave rectifying unit is controlled in the mode II'B such that said switching elements UP and UN are phase-controlled and VP, VN, WP, and WN are triggered at it - 7.

11. A power conversion apparatus including the rectifying unit formed of a plurality of switching elements, which are capable of conductive-phase controlling, and adapted, with said rectifying unit connected with an a.c.

power source and said switching elements conduction-controlled to perform inverter operation thereby to output negative output voltage from the output side of said rectifying unit, wherein there are provided a plurality of pairs of said rectifying units, which are connected in parallel to the a.c. power source with different phase sequences provided between each pair at their input terminals and joined together at their output terminals and adapted to be operated being switched between a single-phase operation mode and a poly-phase operation mode according to the d.c. output voltage value to be output thereby.

12. A power conversion apparatus according to claim 11, wherein said rectifying units are provided in three pairs, each of the pairs is formed of two sets of three-phase full-wave rectifying units, and said three-phase full-wave rectifying units, with the maximum value of the d.c. output voltage obtainable therefrom expressed by Edo and the value of the minimum angle of advance of phase control (7 limiter) expressed by 7, are operated: when the desired output voltages are (1/6)Edo (1 cos;) -(Ed0/3)cosy, in a single-phase mode I'; when the desired output voltages are -(Ed- 0/3)cos7 to --(2/3)Ed, cosy. in a single-phase mode 11' different from the above mode I'; when the desired output voltages are -(2/3)Ed, cosy to End,/6 ((V3/2)siny - 4.5 cosy), in a three-phase mode III'A; when the desired output voltages are (Edo/6) ((V 3/2 siny - 4.5 cosy) to --(5/6)Ed, cosy, in a three-phase mode III'B different from the above mode III'A, when the desired output voltages are --(5/6)Ed, cosy to (Edo/6) ((V3/2 siny - 5.5 cosy), in a three-phase mode IV'A different from the above modes III'A and III'B; and when the desired output voltages are (Edo/6) ((V3/2 siny - 5.5 cosy) to --Ed, cosy, in a three-phase mode IV'B different from the above modes III'A, III'B, and IV'A.

13. A power conversion apparatus according to claim 12, wherein, with the phases of the output terminals of the three-phase power source expressed by phases R, S, and T, and the phases of each the three-phase full-wave rectifying unit of each pair expressed by phases U, V, and W, the output terminals of said three-phase a.c. power source and the input terminals of the three-phase full-wave rectifying units of each pair are connected with different phase sequences provided between each pair, i.e., such that the connections (R-U, S-V, T-W), (S-U, T-V, R-W), and (T-U, R-V, S-W) are made, the positive output terminals P and the negative output terminals N of the three-phase full-wave rectifying units are respectively connected in parallel via serially connected interphase reactors, and, with the switching element constituting the threephase full-wave rectifying units disposed in the phase U on the P side denoted by UP, that disposed in the phase U on the N side denoted by UN, that disposed in the phase V on the P side denoted by VP, that disposed in the phase V on the N side denoted by VN, that disposed in the phase W on the P side denoted by WP, and that disposed in the phase W on the N side denoted by WN: in the single-phase mode I' operation, one side of two three-phase full-wave rectifying units constituting each pair are controlled such that said switching element VP is phase-controlled, UP is triggered at (2/3) (it - 7), VN is kept conductive, and UN, WP, and WN are kept nonconductive, and the other side of two three-phase full-wave rectifying units constituting each pair are controlled such that said switching element VN is phase-controlled, UN is triggered at (2/3)(it - 7), and VP is kept forward conductive, and UP, WP, and WN are kept nonconductive; in the single-phase mode II' operation, one side of two three-phase fullwave rectifying units constituting each pair are controlled such that said switching element VN is phase-controlled, UP and UN are trig gered at (2/3) (it - 7), VP is triggered at it - 7, and WP and WN are kept nonconductive, and the other side of two three-phase fullwave rectifying units constituting each pair are controlled such that said switching element VP is phase-controlled, UP and UN are triggered at (2/3) (K - 7), VN is triggered at it - 7, and WP and WN are kept nonconductive; in the three-phase mode III'A operation, one side of two three-phase full-wave rectifying units constituting each pair are controlled such that said switching element VN is phase-controlled and triggered at it - 7, UP and UN are trig gered at (2/3) (K - 7), VP and WN are triggered at it - 7, and WP is kept nonconduc- tive, and the other side of two three-phase full-wave rectifying units constituting each pair are controlled such that said switching element VP is phase-controlled and triggered at it - 7, UP and UN are triggered at (2/3) (a - 7), VN and WP are triggered at it - 7, and WN is kept nonconductive; in the three-phase mode III'B operation, one side of two threephase full-wave rectifying units constituting each pair are controlled such that said switch ing element UN is phase-controlled, UP is triggered at (2/3) (it - 7), VP, VN and WN are triggered at it - 7, and WP is kept nonconductive, and the other side of two three-phase full-wave rectifying units constituting each pair are controlled such that said switching element UP is phase-controlled, UN is triggered at (2/3) (it - 7), VP, VN, and WP are triggered at it - 7, and WN is kept nonconductive; in the three-phase mode IV'A operation, one side of two three-phase full-wave rectifying units constituting each pair are controlled such that said switching element VP is phasecontrolled and triggered at it - 7, UP is triggered at (2/3) (it - 7), and UN, VN, WP, and WN are triggered at it - 7, and the other side of two three-phase full-wave rectifying units constituting each pair are controlled such that said switching element VN is phase-controlled and triggered at it - 7, UN is triggered at (2/3) (it - 7), and UP, VP, WP, and WN are triggered at it - 7;; in the three-phase mode IV'B operation, one side of two three-phase full-wave rectifying units constituting each pair are controlled such that said switching element UP is phase-controlled and UN, VP, VN, WP, and WN are triggered at it - 7, and the other side of two three-phase full-wave rectifying units constituting each pair are controlled such that said switching element UN is phase-controlled and UP, VP, VN, WP, and WN are triggered at it - 7, and further, the two three-phase full-wave rectifying units constituting the three pairs of rectifying units are adapted to be exchanged with each other in their trigger controlled manners at intervals of the period of one cycle of the a.c. power source.

14. A power conversion apparatus substantially as hereinbefore described with reference to Figure 4 or 11 of the accompanying drawings.