RU2269196C1 - Voltage converter built around combined circuit arrangement - Google Patents

Abstract

FIELD: regulation and reactive power correction in power systems; inverters for high-voltage frequency-controlled electric drives.

SUBSTANCE: proposed converter is built around combined circuit arrangement incorporating three-phase bridge circuit of voltage inverter (with series-interconnected semiconductor devices of IGCT, IGBT, and other types) with one or more series-interconnected single-phase bridge voltage converters (whose semiconductor devices are not interconnected in series)connected to each of its phase outputs. All change-over operations are made in three-phase bridge circuit whose arms form valves with series-interconnected semiconductor devices and which forms output voltage base of converter at low frequency (such as that equal to supply mains frequency). Bridge arms incorporating series-interconnected semiconductor devices having different on/off delay times are changed over by means of switching circuits specially inserted in circuit.

EFFECT: enhanced reliability reduced power loss in building converter combined circuit, reduced dynamic loss in semiconductor devices.

3 cl, 9 dwg

Description

The invention relates to a converter technique, namely to high-voltage converters requiring the use of serial connection of semiconductor controlled devices, and is intended for a device for regulating and compensating reactive power in power systems. Such a converter can also be used as an inverter in a high-voltage frequency-controlled electric drive.

Known application in devices for regulating and compensating reactive power in power systems of a three-level pulse-width converter [1]. This is the simplest configuration solution, but it requires high-frequency switching to implement it. To implement such a converter in the higher voltage class, it is necessary to use a series connection of controlled semiconductor devices. Even with a slight spread in turn-off and turn-on delays in series-connected devices, voltage equalization requires dividing circuits of significant power, the energy loss of which is many times higher than the dynamic energy loss in the device itself. Theoretically, it is also possible to select identical devices without dividing circuits, but in practice it is not feasible. Circuits with transformer addition, when the voltages of several inverters are added using special intermediate transformers, require additional costs.

Known converters for a controlled electric drive, made according to a combined circuit [2], consisting of a three-phase two-level (forming two voltage levels: positive and negative) or three-level (forming three voltage levels: positive, negative and zero) of the base voltage inverter, one or more in series connected single-phase bridge inverter circuits in each phase at the output of the base inverter and the control system of all inverters in a pulse-width manner. This embodiment is closest to the present invention. Both the two-level and three-level basic three-phase inverters are executed at a voltage level several times higher than the voltage in single-phase bridge circuits, and forms the voltage base, which is close in effective value, but unsatisfactory in shape. Moreover, the switching frequency of a three-phase inverter is much lower than the switching frequency in single-phase bridge circuits. Single-phase bridge inverters add to the base. As a result, a total signal of the desired shape is formed. This is a good technical solution for voltages of the order of 6–7 kV (as indicated in [2]). To implement such a converter in the higher voltage class, the use of serial connection of semiconductor devices is required, and in this case, as in [1], voltage equalization requires dividing circuits of significant power, the energy loss of which is many times higher than the dynamic energy loss in the device itself .

The invention is known [3], which reduces the dynamic losses of transistors in a transistor inverter made according to a bridge circuit by introducing into each arm an additional circuit that reduces the slew rate of the voltage across each of the transistors by reducing the slew rate of the capacitor discharge current. The use of a converter with such chains in a two-level three-phase circuit is possible, but assumes a low switching frequency and, therefore, the quality of the output voltage of the converter is unsatisfactory for the tasks. In the three-phase three-level circuit, the proposed chains do not work.

The purpose of the present invention is to increase reliability and reduce power losses when constructing a combined circuit of a high voltage converter using a serial connection of semiconductor controlled devices. The technical solution allows the serial connection of devices in which there is a spread in the delay time for turning off (turn off) and turning on (turn on).

Achieving this goal in a voltage converter made according to a combined circuit, which includes a basic three-phase bridge voltage inverter for gates formed by series-connected controlled semiconductor devices in each arm of the bridge, having a two-, three- or more multi-level structure, and single-phase bridge voltage inverters connected to each phase output of the base inverter, one or more connected in series by alternating current in each phase, is provided introducing into each arm of the base inverter switching circuits from series-connected capacitor and switching diodes, as well as switching reactors connected in series with the shoulder valve, respectively, from the plus or minus side of the base inverter and / or its phase output, and the switching circuit diodes connected directly to the valve shoulder according to him regarding switching reactors. The capacitors introduced into the basic inverter circuit are connected in parallel to the arm valves and are separated from them by switching diodes. Also, in each phase of the base inverter, regenerative units are introduced that return switching energy to DC voltage capacitors. The recuperating units, the outputs included between the DC bus of the base inverter, the inputs are connected respectively between the positive bus, negative bus or phase output and the connection points of the capacitors and diodes of the switching circuits. In the above construction of the circuit, a control algorithm is selected according to which the base circuit is switched at a low frequency comparable to the network frequency, so fast switching is not required. The formation of the monitored signal occurs on cascaded single-phase inverter bridges due to pulse-width modulation in each bridge circuit and a shift in the control of each of the single-phase bridges, with a higher switching frequency possible, since serial connection of semiconductor devices is not supposed.

To clarify the invention, Fig. 1a shows a block diagram of a converter for regulating and compensating reactive power in power supply systems of electric networks, the basis of the output voltage in which is formed using a three-level three-phase bridge inverter;

on figb shows a structural diagram of the phase of a three-level three-phase bridge inverter;

on figv shows a block diagram of energy recovery units used in the phases of a three-phase inverter;

on figa presents a structural diagram of the Converter, the basis of the output voltage in which is formed using a two-level three-phase bridge inverter;

on figb shows a structural diagram of the phase of a two-level three-phase bridge inverter;

on figv shows a structural diagram of the phase of a two-level three-phase bridge inverter with the inclusion of switching reactors in the midpoint of the phase;

figure 3 shows the structure of the control system of the Converter;

figure 4 shows the waveforms of voltages on three series-connected IGCT-thyristors (4500 V, 4000 A) with a delay off of two of them by 1 μs in the presence of switching circuits in the circuit;

figure 5 shows the oscillograms explaining the principle of operation of the proposed circuit.

The device and selection criteria for the type and number of necessary elements of the claimed technical solution in its static state are described according to the schemes of figure 1 and figure 2.

We give options for constructing a three-phase inverter according to a three- and two-level scheme.

1. The structure of the Converter, the basis of the output voltage in which is formed using a three-level three-phase bridge inverter, is presented in figa.

A three-level three-phase bridge inverter 1 consists of three phases (2, 3, 4) and a capacitor bank (5, 6) with a middle (neutral) point 7. To the phase outputs of a three-phase inverter (points 8, 9, 10) are connected serially connected single-phase bridge inverters (11 (1) ... 11 (n), 12 (1) ... 12 (n), 13 (1) ... 13 (n)). The contact buses of the combined converter (points A, B, C) are connected to the mains (without or with filtration, depending on the requirements of the application).

To ensure switching of the gates 14, 15, 16, 17 (fig.1b - phase 2) of the phase of the bridge inverter 2, 3, 4 with the valves formed by the serial connection of fully controlled semiconductor devices (with reverse diodes and protective circuits), in the phase circuit of the inverter capacitor banks are included (in FIG. 1b: 18 between points 20 and 21; 19 between points 22 and 23) to limit the rate of change of voltage across the valves (hereinafter, dU / dt). The value of the capacitance of capacitor banks is selected from the calculation of the dU / dt limit at an acceptable level based on the requirement not to exceed the maximum permissible operating voltage on semiconductor devices, taking into account the spread in their on / off time (when turned off, the fastest and when turned on the slowest in the absence of additional measures, they are fully energized). To ensure switching of valves without inrush currents through reverse diodes (14 (1) ... 14 (n2), 15 (1) ... 15 (n2). 16 (1) ... 16 (n2), 17 (1 ) ... 17 (n2)) and shunt (24, 25) diodes, to reduce the level of overvoltage on the valves due to the inductance of the installation of filtering capacitor banks of constant voltage (in figa: 5 and 6), as well as to limit the current switching faults during breakdowns in the phase circuit of the inverter include switching reactors (Fig.1b: 26 between points 28 and 29; 27 between points 30 and 8; 31 between points 8 and 32; 33 between points 34 and 35), limiting the rate of change of current ( di / dt) h through the valves. The inductance value of switching reactors is selected from the calculation of the di / dt limit at an acceptable level, based on the requirement not to exceed the maximum permissible repetitive current through semiconductor devices. To output reactor energy after switching the valve, switching diodes are included in the circuit (in Fig. 1b: 36 between points 20 and 29; 37 between points 21 and 27; 38 between points 22 and 33; 39 between points 23 and 34) and regenerative converters ( on figb: 40 between points 29 and 20, 41 between points 21 and 22 and 42 between points 23 and 35), returning switching energy back to DC capacitors 5 and 6 (with the exception of losses in the converters themselves).

2. The structure of the Converter, the basis of the output voltage in which is formed using a two-level three-phase bridge inverter, is presented in figa

A two-level three-phase bridge inverter 1 consists of three phases (2, 3, 4) and a capacitor bank 43. Three-phase inverters (points 8, 9, 10) are connected to the phase outputs of single-phase bridge inverters 12 (1) ... 12 (n ), 13 (1) ... 13 (n), 14 (1) ... 14 (n)). The contact bars of the combined converter (points A, B, C) are connected to the mains (without or with filtration, depending on the requirements of the application).

To ensure the switching of the valves 44, 45 (fig.2b) of the phase of the bridge inverter 2, 3, 4 with the valves formed by the serial connection of fully controlled semiconductor devices (with reverse diodes and protective circuits), capacitor banks are included in the phase circuit of the inverter (in Fig. 2a: 46 between points 48 and 8; 47 between points 8 and 49) to limit dU / dt on the valves. The value of the capacitance of capacitor banks is selected from the calculation of the dU / dt limitation at an acceptable level based on the requirement not to exceed the maximum permissible operating voltage on semiconductor devices, taking into account the spread in their on / off time.

To ensure switching of valves without inrush currents through reverse (44 (1) ... 44 (n3), 45 (1) ... 45 (n3),) diodes, to reduce the level of overvoltage on the valves due to the inductance of mounting a filtering capacitor DC batteries (in Fig. 2a: 43), as well as for limiting the short circuit current during breakdowns, switching reactors are included in the inverter phase circuit (in Fig. 2b: 50 between points 29 and 51; 53 between points 52 and 35), limiting rate of change of current (di / dt) through the valves. The inductance value of switching reactors is selected from the calculation of the di / dt limit at an acceptable level, based on the requirement not to exceed the maximum permissible repetitive current through semiconductor devices. To output reactor energy after switching the valve, switching diodes (in Fig.2b: 54 between points 48 and 51; 55 between points 52 and 49) and regenerative converters (in Fig.2b: 56 between points 29 and 48, 57 between points 49 and 35), which return the switching energy back to the DC capacitor bank 43 (with the exception of losses in the converters themselves).

By analogy with [3], switching reactors can be turned on at the midpoint of the phase (pigv).

We explain the operation principle of the proposed converter using an example of a version with a three-phase bridge three-level inverter as a base inverter.

The control system (figure 3) using the blocks of the PWM modulator generates in a given sequence the control pulses of the valves of all inverters. In this case, the three-level three-phase bridge inverter forms one of the three voltage levels (positive, negative and zero) between the midpoint 7 and the phase terminals of the three-phase inverter (points 8, 9, 10) (U3 in Fig. 5). Switching of valves is carried out by the control system in accordance with the accepted algorithm. The control system has several control loops, among which two global loops can be distinguished:

- the amplitude value of the current through the reactor output filter (signal i in figure 3);

- according to the amplitude value of the voltage from the output (voltage transformer on the busbars connecting the capacitors of the output filter) - (signal and figure 3).

In addition, to develop fast processes, local control loops for instantaneous current and voltage values from the output filter, as well as several local control loops for current and voltage from DC capacitors of the base inverter and single-phase bridges of the cascade inverter, are organized.

(REFO на фиг.5) отслеживаемого ею выходного сигнала. Базовый трехфазный мостовой инвертор формирует основу (U3 на фиг.5) выходного напряжения преобразователя с низкой частотой переключения (S1³...S12³ a, b, c) на фиг.3). Разница между заданием

и основой U3 формируется однофазными мостовыми инверторами посредством широтно-импульсной модуляции (ШИМ). Управление каждым из однофазных мостовых инверторов ((S1⁰...S4⁰ a, b, c), (S1¹...S4^l a, b, c), (S1²...S4² a, b, c) на фиг.3) осуществляется с фазовым сдвигом относительно друг друга, за счет чего увеличивается результирующая частота пульсаций выходного напряжения.In general, the control system is built on the principle of maintaining a given value

(REFO in FIG. 5) of the output it monitors. The basic three-phase bridge inverter forms the basis (U3 in Fig. 5) of the output voltage of the converter with a low switching frequency (S1 ³ ... S12 ³ a, b, c) in Fig. 3). Difference between task

and the base U3 is formed by single-phase bridge inverters by means of pulse width modulation (PWM). Control of each of the single-phase bridge inverters ((S1 ⁰ ... S4 ⁰ a, b, c), (S1 ¹ ... S4 ^l a, b, c), (S1 ² ... S4 ² a, b, c ) in figure 3) is carried out with a phase shift relative to each other, due to which the resulting ripple frequency of the output voltage increases.

The circuit includes recuperative converters that return switching energy to a DC capacitor bank 5, 6 (with the exception of losses in the converters themselves). The operation of one of the possible options for the regenerating Converter explains figv. When the valve control is turned off (for example 14), the energy of the switching reactor (for example 26) charges the capacitor 58 through the switching diodes (for example 36). On average, the constant voltage of the capacitor 58 is converted by the inverter 59 to an alternating voltage of increased frequency. The transformer 60 increases it to the voltage level on the capacitor bank 5, 6 of the DC link, and then after rectification by the rectifier 61, the voltage is supplied to the capacitor bank. Thus, the reactive switching energy from the reactor is returned to the storage capacitor bank.

In general, when regulating a three-phase three-level inverter (including the possibility of several switching of the same valve in a half-cycle), depending on the sign of the voltage generated at the output, the following types of switching are possible:

• U> 0, i> 0 (where U is the inverter phase voltage, i is the inverter phase current): from the transistors of the valve 14 to the diodes 24 and vice versa when the transistors 15 are on;

• U <0, i <0: from the transistors of the valve 17 to the diodes 25 and vice versa with the transistors 16 turned on (switching processes proceed similarly to the previous paragraph due to the symmetry of the circuit);

• U> 0, i <0: from the return diodes of the valves 14 and 15 to the transistors 16 and diodes 25 and vice versa.

• U <0, i> 0: from reverse diodes of valves 16 and 17 to transistors 15 and diodes 24 and vice versa (during switching, the processes proceed similarly to the previous paragraph due to the symmetry of the circuit);

Thus, of the eight types of switching (including reverse switching), four types can be distinguished in principle:

1) from the external (upper) controlled valve to the shunt diode of the same inverter arm;

2) on the contrary, from a shunt diode to a controlled valve;

3) from the return diodes of the valves of one arm to the internal controlled valve and the shunt diode of the other arm of the same phase;

4) on the contrary, from a controlled valve and a shunt diode to reverse diodes.

Consider the processes that occur with various types of switching in the presence of switching circuits in the circuit,

1) Switching of the first type, for example, when switching current from a controlled valve 14 to a shunt diode 24, is carried out in two stages.

In the pre-switching state, the current is carried out by a switching reactor 26, controlled valves 14, 15, reactor 27. The capacitor 18 is discharged, and the capacitor 19 is charged to a voltage level in the DC link Ud - voltage between DC buses 29 and 35 (accurate to the voltage in the blocks energy recovery).

Switching starts by locking the controlled valve 14. The switching diodes 36 and 37 are unlocked and the capacitors 18 and 19 are smoothly recharged, while the capacitor 18 is charged through a circuit 26-36-18-37-27- (another phase) - (DC link 6- 5) -26, and the capacitor 19 is recharged along the circuit 42-19-40-37-27- (other phase) - (DC link 5-6) -42. At the end of the first switching stage, the capacitors 18 and 19 are recharged to the same voltage level equal to half of Ud (accurate to voltage 40 ... 42) and the shunt diode 24 is unlocked.

At the second stage of switching, the current through the shunt diode gradually increases, and the current through the reactor 26 and diode 36 gradually decreases, causing further recharging of the capacitors 18 and 19 (by an amount corresponding to voltages of 40 ... 42). When the current through the reactor 26 and the diodes 36 reaches zero, the diodes 36 are locked, and the switching process ends.

2) Switching of the second type, for example, when switching current from the shunt diode 24 to the controlled valve 14 is carried out in three stages.

In the pre-switching state, current is carried out by shunt diodes 24, controlled valve 15, reactor 27. Capacitors 18 and 19 are charged to the same voltage level (accurate to voltage in energy recovery units 40 ... 42).

Switching begins by unlocking the controlled valve 14. The current through the shunt diodes 24 gradually decreases, and after 14 it increases at a speed limited by the inductance of the switching reactors 26 and 27. The first stage of switching ends when the current through the diodes 24 drops to zero.

At the second stage of switching, capacitor banks 18 and 19 are recharged (18 is discharged to zero along the circuit 18-40-26-14-15-27-31-38-41-18, and 19 is charged to full voltage in the DC link by circuit 19-42- (DC link 6-5) -26-14-15-27-31-38-19). The second stage ends when the voltage at 18 becomes equal to zero.

At the third stage, the switching diodes 36, 37 and the "excess" current, which occurs in 26, 27, 31 due to overcharge 18, 19, open to the nominal value through the recovery units 40 and 41, respectively, after which the switching diodes are locked and the switching ends .

3) Consider the processes of switching the third type, for example, from the return diodes of the valves 14, 15 to the controlled valve 16 and the shunt diode 25. Switching of the third type is carried out in two stages.

In the pre-switching state, the current is conducted by the reactor 27, the return diodes of the valves 14, 15, and the reactor 26. The capacitor bank 18 is discharged to zero and 19 is charged to the full voltage Ud (accurate to the voltage in the energy recovery units 40 ... 42).

Switching begins by unlocking valve 16: the current through it begins to increase smoothly, and through diodes 14, 15 gradually decrease. At the same time, there is a smooth recharge of the capacitor banks 18, 19: 18 is charged through the circuit 18-37-27- (other phase) - (DC link 6-5) -26-36-18, and 19 is discharged through the circuit 19-41-37 -27- (other phase) - (DC link 5-6) -42-19. At the end of the first switching stage, the capacitors 18 and 19 are recharged to the same voltage level equal to half Ud (accurate to the voltage in the energy recovery units 40 ... 42). The main current (load) flows through the circuit 31-16-25, to which the overcharge current 18, 19 is added.

At the second stage of switching, a switching diode 38 is opened and overcurrents through reactors 26, 27, 31, caused by overcharging 18, 19, fall through the recovery units 40, 41, respectively. At the end of the second stage, the currents through the reactors 26 and 27 become equal to zero, and after 31 - nominal, the switching diodes 36 ... 38 are locked and the switching ends.

4) Consider the processes of switching the fourth type, for example, from a controlled valve 16 and a shunt diode to the reverse diodes of the valves 14, 15. Switching of the fourth type is carried out in two stages.

In the pre-switching state, the current is conducted by the reactor 31, the controlled valve 16 and the shunt diodes 25. The capacitor banks 18, 19 are charged to the same voltage level equal to half of Ud (accurate to the voltage in the energy recovery units 40 ... 42).

Switching begins by locking the controlled valve 16. The flow of current through 16 and 25 stops; the switching diode 38 is unlocked and a smooth recharge of the capacitor banks 18 and 19: 18 begins; it is discharged along the circuit 18-40- (DC link 5-6) - (load) -31-38-41-18, and 19 is charged through the circuit 19- 42- (DC link 6-5) - (load) -31-38-19. The first stage ends when the voltage at 18 becomes equal to zero, and 19 is equal to half the voltage Ud.

At the second stage of switching, the reverse diodes 14, 15 are unlocked, through which the load current begins to flow. Simultaneously with the diodes 14, 15, switching diodes 36, 37 are opened; through 37, the current of reactor 31 smoothly drops (along the circuit 31-38-41-37-27-31), caused by overcharging 18, 19, and after 36 the differential current of valves 14, 15 flows and the current of reactor 26 smoothly increases. At the end of the second stage the current through 31 becomes equal to zero, after 26 it becomes nominal, the switching diodes 36 ... 38 are locked and the switching ends. The load current at the end of the switching flows through the circuit 27-15-14-26- (DC link 5-6) - (load) -27.

In all four types of switching, the rate of change of voltage on the capacitor banks 18, 19 is determined by the value of their capacitance and the value of the load current. The rate of change of voltage across the valves 14 ... 17 when they are locked is equal to the rate of change of voltage across the capacitors 18, 19. Thus, in a situation where one of the series-connected semiconductor devices of the valve closes before the others (due to the available scatter of delays in switching off the devices control channels), the voltage on it rises smoothly and does not exceed the maximum allowable value by the moment the other devices are unlocked. An additional effect is to reduce dynamic losses in semiconductor devices.

As an example of the operation of the inventive switching circuit, Fig. 4 shows the voltage waveforms of three series-connected IGCT thyristors (4500 V, 4000 A) with a delay of two of them being turned off by 1 μs. The voltage spread on the devices in the closed state was about 600 V, which is acceptable taking into account the voltage reserves recommended by the manufacturers of semiconductor devices.

The oscillograms shown in figure 5, explain the operation of the Converter as a whole. The basic three-phase bridge inverter forms the basis of the U3 output voltage of the converter with a low switching frequency (in the example, this is the network frequency - 50 Hz). The remaining voltage difference is formed by single-phase bridge inverters. Each of the single-phase bridge inverters is controlled with a phase shift relative to each other, due to which the resulting ripple frequency of the output voltage increases. In the example, the switching frequency of the power switches in each of the bridges is 267 Hz, and the resulting modulation frequency is 800 Hz.

In figure 5, the following notation:

REF0 - reference signal,

US is the phase voltage of a three-level three-phase bridge inverter,

V3 - reference for series-connected bridge phase inverters, equal to the difference between the phase reference and the voltage on the phase of the three-phase inverter,

U3 is the resulting voltage of the converter.

Information sources

1. STATCOM Based on Multimodules of Multilevel Converters Under Multiple Regulation Feedback Feedback Yiqiang Chen and Boon-Teck OoiDEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 14, No. 5, SEPTEMBER 1999.

2. US patent US 6621719 B2 (Sep.16, 2003) Converter with additional voltage addition or subtraction at the output STEIMER PETER (CH); VEENSTRA MARTIN (CH).

3. Mustafa G.M., Baregamyan G.V., Ruditsky R.Sh., Kurpina E.V .: Reducing the dynamic losses of inverter transistors by changing the slew rate of capacitors. Copyright certificate No. 989711 (01.15.83), UDC 621.314.57 (088.8).

Claims (3)

1. A voltage converter made according to a combined circuit that includes a basic bridge multilevel (at least three-level) three-phase voltage inverter for valves formed by the serial connection of fully controllable semiconductor devices in the bridge arms connected respectively between the plus or minus DC bus and the corresponding phase output, as well as one or more series-connected alternating current single-phase bridge inverters included in Each phase at the output of the base inverter, characterized in that a chain of series-connected capacitor, a first switching diode connected to the first capacitor plate and a second switching diode connected to the second capacitor plate, and two switching reactors are introduced into each arm of the basic inverter the indicated circuit is connected in parallel to the shoulder valve so that the first switching diode is connected to the valve from the DC bus side, one switching reactor is connected between the corresponding the corresponding DC bus and the connection point of the first switching diode and the shoulder valve, and the second switching reactor is between the connection point of the second switching diode with the shoulder valve and the corresponding phase output, while the switching diodes are connected to the shoulder valves with the same electrodes; three recovery units are introduced into each phase of the inverter, two of which are connected by inputs each between the corresponding DC bus and the connection point of the capacitor and the first switching diode, while the input of the third recovery unit is connected between the connection points of the capacitors of the adjacent phase arms with the second switching diodes, and the outputs all phase recovery units are connected between the DC buses of the base inverter. 2. A voltage converter made according to a combined circuit including a basic bridge two-level three-phase voltage inverter for valves formed by a serial connection of fully controllable semiconductor devices in the bridge arms connected respectively between the plus or minus DC bus and the corresponding phase output, as well as one or several series-connected alternating current single-phase bridge inverters connected to each phase at the output of the base and an inverter, characterized in that a chain of series-connected capacitor and a switching diode, a switching reactor and a recovery unit is introduced into each arm of the base inverter, said chain being connected in parallel to the shoulder valve so that the switching diode is connected to the valve with the same electrode on the DC bus side, and a switching reactor is connected between the connection point of the switching diode with the valve and this DC bus; in this case, the recovery unit is connected by an input between the corresponding DC bus and the connection point of the switching diode and the capacitor, and the output is connected to the DC buses of the base inverter. 3. A voltage converter made according to a combined circuit including a basic bridge two-level three-phase voltage inverter for valves formed by series connection of fully controllable semiconductor devices in the bridge arms connected respectively between the plus or minus DC bus and the corresponding phase output, as well as one or several serially connected alternating current single-phase bridge inverters included in each phase at the output of the base inverter ertor, characterized in that a chain of consecutively connected capacitor and switching diode, a switching inductor and a recovery unit is introduced into each arm of the base inverter, said circuit being connected in parallel to the shoulder valve chain so that the switching diode is connected to the valve by the same electrode from the side of the corresponding phase output , a switching inductor is connected between this phase output and the connection point of the switching diode and the valve, and a recuperating unit with an input connected between the annym phase terminal and the connection point of the capacitor and the switching diode, and the output is connected to the DC bus bars of the base of the inverter.

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