A modular multi-level converter with energy alternating reset

Abstract

The present invention provides a modular multi-level converter EAR‑MMC with energy alternate reset, which is composed of at least two-phase circuits, and the phase bridge arm of each phase circuit is composed of two sub-module strings and two parallel semiconductor switch strings connecting the sub-module strings, wherein: the outer ends of the two sub-module strings are respectively connected to the positive and negative ends of the DC bus; the midpoint of the first semiconductor switch string is connected to the AC end; the midpoint of the second semiconductor switch string is connected to the midpoint of the second semiconductor switch string of the other phase bridge arm through an AC inductor. The phase bridge arm of each phase circuit of the EAR‑MMC switches the switch devices on both sides of the semiconductor switch string of the phase bridge arm, and controls when the phase bridge arm outputs an AC voltage, the two sub-module strings of the phase bridge arm alternately perform AC end voltage shaping and energy reset in the positive and negative half cycles of the AC voltage. Under the same conditions, the number of sub-modules and the number of fully controlled devices required by the EAR‑MMC are only 50% of those of the MMC, which is conducive to the miniaturization and lightweight design of systems such as flexible direct current transmission (VSC‑HVDC).

Description

Technical Field

[0001] The invention relates to the field of multi-level converters, and in particular to a modular multi-level converter with energy alternately reset. Background Art

[0002] Since the invention of MMC (Modular Multilevel Converter) in 2003, the problem of dynamic voltage balancing, which requires the simultaneous opening and closing of switching devices required for two-level high-voltage converters, has been eliminated because the switching devices can withstand small voltage changes and can be turned on and off at different times. At the same time, MMC has a modular structure and is easy to expand to higher levels, so MMC has been widely used in the high-voltage field.

[0003] Since the phase bridge arm of MMC is composed entirely of cascaded submodules, during operation, 50% of the submodule capacitors must be dynamically connected to the DC path to act as DC support capacitors, and the other 50% of the submodule capacitors must be removed and placed in bypass mode to obtain a "shaped" sinusoidal output voltage at the AC end. Therefore, the number of cascaded submodules required to form the upper and lower bridge arm submodule strings in the MMC phase bridge arm is large, and the sum of the DC side voltages of these submodules is twice the DC side voltage. It is the 50% dynamically redundant number of submodules in the phase bridge arm that leads to the shortcomings of the MMC device, such as large size, heavy weight, and high cost.

[0004] With the rapid development of the new energy industry, large-scale new energy DC grid-connected applications have shown explosive growth, especially the demand for offshore wind power to be connected to the grid through transmission, which has put forward higher requirements on multi-level converters in terms of reducing size, weight and cost. Summary of the invention

[0005] The present invention provides a modular multi-level converter with energy alternate reset, which is used to solve the defects of large volume, heavy weight and high cost of MMC device caused by a large number of cascaded sub-modules required to form upper and lower bridge arm sub-module strings in the MMC phase bridge arm in the prior art, reduce the volume and weight of the multi-level converter, and reduce the cost.

[0006] The present invention provides a modular multi-level converter with energy alternate reset, the multi-level converter is composed of at least two-phase circuits, the phase bridge arm of each phase circuit is composed of two sub-module strings and two parallel semiconductor switch strings connecting the sub-module strings, wherein one end of the sub-module string connected in series with the upper bridge arm of the phase bridge arm is connected to the positive terminal of the DC bus; one end of the sub-module string connected in series with the lower bridge arm of the phase bridge arm is connected to the negative terminal of the DC bus; the semiconductor switch string includes a first semiconductor switch string and a second semiconductor switch string, the midpoint of the first semiconductor switch string is connected to the AC end; the midpoint of the second semiconductor switch string is connected to the midpoint of the second semiconductor switch string of the other phase bridge arm through an AC inductor; the first semiconductor switch string is used as a load current of the AC end between the upper and lower bridges. The switching work of the upper bridge arm submodule string of the phase bridge arm is carried out, and the second semiconductor switch string is used as the reset current switching work of the energy storage element of the submodule string energy resetting; the phase bridge arm of each phase circuit of the multi-level converter controls the output of the AC voltage by switching the switching devices on both sides of the semiconductor switch string of the phase bridge arm, and the two submodule strings of the phase bridge arm are alternately ac terminal voltage shaping and energy resetting in the positive and negative half cycles of the AC voltage: in the positive half cycle of the AC voltage, the upper bridge arm submodule string of the phase bridge arm shapes the output half-wave voltage, and the lower bridge arm submodule string of the phase bridge arm performs energy reset; in the negative half cycle of the AC voltage, the lower bridge arm submodule string of the phase bridge arm shapes the output half-wave voltage, and the upper bridge arm submodule string of the phase bridge arm performs energy reset.

[0007] According to the modular multi-level converter with alternate energy reset provided by the present invention, the submodule string is formed by cascading half-bridge submodules and / or full-bridge submodules having energy storage units.

[0008] According to the modular multi-level converter with alternate energy reset provided by the present invention, when the load connected to the modular multi-level converter is resistive and capacitive, the semiconductor switch string is composed of fully-controlled devices with static voltage-equalizing resistors connected in series, half-controlled thyristor devices connected in series, or a mixed series of fully-controlled devices and half-controlled thyristor devices; when the load connected to the modular multi-level converter is inductive, the semiconductor switch string is composed of fully-controlled devices with static voltage-equalizing resistors connected in series, or a mixed series of fully-controlled devices and half-controlled thyristor devices.

[0009] According to the modular multi-level converter with energy alternating reset provided by the present invention, when the switch devices on both sides of the semiconductor switch string are controlled to be turned on and off, the current flowing through the semiconductor switch string flows into or out of the midpoint on one of the two sides of the midpoint of the switch string; when the current needs to be switched from one side of the midpoint of the switch string to the other side, the switch devices on both sides of the midpoint of the switch string perform switching in a zero voltage state.

[0010] According to the modular multi-level converter with alternate energy reset provided by the present invention, when the phase bridge arm outputs an AC voltage, the two sub-module strings of the phase bridge arm respectively perform AC end voltage shaping and energy reset alternately in the positive and negative half cycles; the sub-module string of the phase bridge arm resets the energy of its own energy storage element via the second semiconductor switch string and the AC inductor in the half cycle not participating in the AC end voltage shaping.

[0011] According to the modular multi-level converter with alternate energy resetting provided by the present invention, when the sub-module string of the phase bridge arm performs energy resetting, the opening of the upper and lower bridge arm sub-module strings is controlled to ensure that the energy storage elements of the upper bridge arm sub-module string of at least one phase bridge arm and the energy storage elements of the lower bridge arm sub-module string of at least one phase bridge arm form a DC voltage path to play the role of a DC side support capacitor.

[0012] According to the modular multi-level converter with energy alternating reset provided by the present invention, when the multi-level converter is a three-phase converter, the AC inductor is connected to the midpoint of each of the second semiconductor switch strings of each of the phase bridge arms through a Y-type connection or a △-type connection.

[0013] According to the modular multi-level converter with alternate energy reset provided by the present invention, by controlling each of the sub-module strings and each of the semiconductor switch strings, the reset current for energy reset flowing through the second semiconductor switch string and the AC inductor is less than one third of the AC end load current flowing through the first semiconductor switch string.

[0014] According to the modular multi-level converter with alternate energy reset provided by the present invention, fully-controlled devices are used at the positions adjacent to both sides of the midpoint of the semiconductor switch string, and absorption capacitors are connected in parallel at both ends of the two series-connected fully-controlled devices to form a half-bridge circuit. When the current needs to be switched from one side of the midpoint of each semiconductor switch string to the other, the absorption capacitor is used to absorb the transient overvoltage generated on the fully-controlled device when the current is switched.

[0015] The modular multi-level converter with energy alternate reset provided by the present invention is composed of at least two-phase circuits, and the phase bridge arm of each phase circuit is composed of two sub-module strings and two parallel semiconductor switch strings connecting the sub-module strings, wherein one end of the sub-module string connected in series with the upper bridge arm of the phase bridge arm is connected to the positive terminal of the DC bus; one end of the sub-module string connected in series with the lower bridge arm of the phase bridge arm is connected to the negative terminal of the DC bus; the semiconductor switch string includes a first semiconductor switch string and a second semiconductor switch string, the midpoint of the first semiconductor switch string is connected to the AC end; the midpoint of the second semiconductor switch string is connected to the midpoint of the second semiconductor switch string of the other phase bridge arm through an AC inductor; the first semiconductor switch string is used as an AC The switching work of the load current flowing through the upper and lower bridge arm submodule strings, and the second semiconductor switch string is used as the reset current switching work of the energy resetting of the submodule string energy storage element; the multilevel converter is used to control the two submodule strings of the phase bridge arm when the phase bridge arm outputs the AC voltage by switching the switch devices on both sides of the semiconductor switch string, and the AC end voltage shaping and energy resetting are performed alternately in the positive and negative half cycles: in the positive half fundamental wave cycle, the upper bridge arm submodule string of the phase bridge arm shapes the output half-wave voltage, and the lower bridge arm submodule string of the phase bridge arm performs energy resetting; in the negative half fundamental wave cycle, the lower bridge arm submodule string of the phase bridge arm shapes the output half-wave voltage, and the upper bridge arm submodule string of the phase bridge arm performs energy resetting. The present invention can greatly save the number of submodules and devices in the modular multilevel converter and the DC transmission system, which is conducive to the miniaturization and lightweight design of systems such as flexible DC transmission, thereby reducing the design cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0017] Figure 1 Schematic diagram of the structure of a modular multi-level converter provided by an embodiment of the present invention.

[0018] Figure 2 This is one of the structural schematic diagrams of the semiconductor switch string provided in the embodiment of the present invention.

[0019] Figure 3 This is the second structural schematic diagram of the semiconductor switch string provided in an embodiment of the present invention.

[0020] Figure 4 This is the third structural schematic diagram of the semiconductor switch string provided in an embodiment of the present invention.

[0021] Figure 5This is the fourth structural schematic diagram of the semiconductor switch string provided in an embodiment of the present invention.

[0022] Figure 6 It is one of the structural schematic diagrams of the submodule string provided in the embodiment of the present invention.

[0023] Figure 7 This is the second structural diagram of the submodule string provided in an embodiment of the present invention.

[0024] Figure 8 This is the third structural diagram of the submodule string provided in an embodiment of the present invention.

[0025] Figure 9 It is one of the working modes of the modular multi-level converter in the first embodiment.

[0026] Figure 10 This is the second working mode of the modular multi-level converter in the first embodiment.

[0027] Figure 11 This is the third working mode of the modular multi-level converter in the first embodiment.

[0028] Figure 12 This is the fourth working mode of the modular multi-level converter in the first embodiment.

[0029] Figure 13 This is the fifth working mode of the modular multilevel converter in the first embodiment.

[0030] Figure 14 This is the sixth working mode of the modular multi-level converter in the first embodiment.

[0031] Figure 15 This is the seventh working mode of the modular multi-level converter in the first embodiment.

[0032] Figure 16 This is the eighth working mode of the modular multi-level converter in the first embodiment.

[0033] Figure 17 This is the ninth working mode of the modular multi-level converter in the first embodiment.

[0034] Figure 18 This is the tenth working mode of the modular multi-level converter in the first embodiment.

[0035] Figure 19 This is the eleventh working mode of the modular multi-level converter in the first embodiment.

[0036] Figure 20 This is the twelfth working mode of the modular multi-level converter in the first embodiment.

[0037] Figure 21This is a working principle diagram of the submodule string in the first embodiment when performing energy resetting work and serving as a DC side support capacitor.

[0038] Figure 22 It is the reset current generated in the positive and negative half-cycle AC inductance in the first embodiment.

[0039] Figure 23 It is one of the working modes of the modular multilevel converter in the second embodiment.

[0040] Figure 24 This is the second working mode of the modular multilevel converter in the second embodiment.

[0041] Figure 25 This is the third working mode of the modular multilevel converter in the second embodiment.

[0042] Figure 26 This is the fourth working mode of the modular multi-level converter in the second embodiment.

[0043] Figure 27 This is the fifth working mode of the modular multilevel converter in the second embodiment.

[0044] Figure 28 This is the sixth working mode of the modular multi-level converter in the second embodiment.

[0045] Figure 29 This is a working principle diagram of the chain submodule unit in the second embodiment when performing energy resetting work and serving as a DC side support capacitor.

[0046] Figure 30 It is the reset current generated in the positive and negative half-cycle AC inductance in the second embodiment. DETAILED DESCRIPTION

[0047] In order to make the purpose, technical solution and advantages of the present invention clearer, the technical solution of the present invention will be clearly and completely described below in conjunction with the drawings of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0048] In order to solve the problems of large size, heavy weight and high cost of MMC devices caused by the number of 50% dynamically redundant sub-modules in the MMC phase bridge arm, the present invention provides an energy alternate resetting modular multilevel converter (EAR-MMC for short). Under the same conditions, the number of sub-modules and the number of fully controlled devices required by EAR-MMC are only 50% of those of MMC. Therefore, it can be used for the miniaturization and lightweight design of systems such as flexible direct current transmission (VSC-HVDC).

[0049] Combine the following Figures 1 - 30 A modular multilevel converter of the present invention is described.

[0050] In some possible embodiments of the present invention, a multi-level converter EAR-MMC is composed of at least two-phase circuits, and the phase bridge arm of each phase circuit is composed of two sub-module strings and two parallel semiconductor switch strings connecting the sub-module strings, wherein one end of the sub-module string connected in series with the upper bridge arm of the phase bridge arm is connected to the positive terminal of the DC bus; one end of the sub-module string connected in series with the lower bridge arm of the phase bridge arm is connected to the negative terminal of the DC bus; the semiconductor switch string includes a first semiconductor switch string and a second semiconductor switch string, the midpoint of the first semiconductor switch string is connected to the AC end; the midpoint of the second semiconductor switch string is connected to the midpoint of the second semiconductor switch string of the other phase bridge arm through an AC inductor; the first semiconductor switch string is used as a load current of the AC end between the upper and lower bridge arms. The switching work of the module string, the second semiconductor switch string is used as the reset current switching work of the energy resetting of the sub-module string energy storage element; the phase bridge arm of each phase circuit of the multi-level converter controls the output of the AC voltage by switching the switching devices on both sides of the semiconductor switch string of the phase bridge arm, and the two sub-module strings of the phase bridge arm are alternately ac terminal voltage shaping and energy resetting in the positive and negative half cycles of the AC voltage: in the positive half cycle of the AC voltage, the upper bridge arm sub-module string of the phase bridge arm shapes the output half-wave voltage, and the lower bridge arm sub-module string of the phase bridge arm performs energy reset; in the negative half cycle of the AC voltage, the lower bridge arm sub-module string of the phase bridge arm shapes the output half-wave voltage, and the upper bridge arm sub-module string of the phase bridge arm performs energy reset.

[0051] In some possible implementations, such as Figure 1 As shown, Figure 1 is one of the structural diagrams of the modular multi-level converter provided in the embodiment of the present invention. In this embodiment, the EAR-MMC is composed of j Mutually( j=A, B, C) circuit, each phase bridge arm of each phase circuit includes two submodule links (Submodule Link, SML), two parallel semiconductor switch links (Semiconductor Switch Link, SSL) and an AC inductor; the two submodule links are upper bridge arm submodule links SML jP and lower bridge arm submodule string SML jN The two parallel semiconductor switch strings are the first semiconductor switch string and the second semiconductor switch string. The first semiconductor switch string is divided into the upper bridge arm first semiconductor switch string SSL jP1 and the first semiconductor switch string of the lower bridge arm SSL jN1 , the second semiconductor switch string is divided into the upper bridge arm second semiconductor switch string SSL jP2 and the second semiconductor switch string of the lower bridge arm SSL jN2 For simplicity, this figure only shows the typical semiconductor switch string structure of fully controlled devices and half-controlled thyristor devices in series; the AC inductor for energy reset is L j .

[0052] In a possible embodiment, the upper bridge arm submodule string SML jP The outer end 12 j Connect to the positive terminal 11P of the DC bus; the lower bridge arm submodule string SML jN The outer end 13 j Connect to the negative terminal 11N of the DC bus.

[0053] In a possible embodiment, the upper bridge arm submodule string SML jP The other end is connected to the first connection point 21 j ; Lower bridge arm submodule string SML jN The other end is connected to the second connection point 22 j .

[0054] In a possible embodiment, the first semiconductor switch string SSL jP1 and the second semiconductor switch string SSL jP2 The intersection point is connected to the first connection point 21 j ; The first semiconductor switch string SSL jN1 and the second semiconductor switch string SSL jN2 Connect to the second connection point 22j .

[0055] In a possible embodiment, the first semiconductor switch string SSL jP1 and the first semiconductor switch string SSL jN1 The intersection point is the third connection point 23 j , the third connection point 23 j As an AC end, the AC end is connected to an AC power source or an AC load.

[0056] In a possible embodiment, the second semiconductor switch string SSL jP2 and the second semiconductor switch string SSL jN2 The intersection point is the fourth connection point 24 j .

[0057] In some possible implementations, when the EAR-MMC is a three-phase converter, the AC inductor is connected to the midpoint of each second semiconductor switch string of each phase bridge arm through a Y-type connection or a △-type connection.

[0058] Specifically, see Figure 1 , EAR-MMC is a three-phase converter, the AC inductance L j One end of the fourth connection point 24 j connected; three AC inductors L j The other ends of the inductors are connected together to form a Y connection, or three AC inductors L j The other end is connected to the 24 k ( k =A, B, C, but k ≠ j ) form a △ type connection.

[0059] On the basis of the above embodiments, the semiconductor switch string is composed of fully controlled devices or half-controlled thyristor devices or a mixture of fully controlled devices and half-controlled thyristor devices connected in series with static voltage-sharing resistors. Figures 2 - 5 As shown. The static voltage-equalizing resistor is connected in parallel at both ends of the semiconductor switch to ensure that the voltages at both ends of each semiconductor switch device are substantially the same when the semiconductor switch string is in the off state. The first semiconductor switch string is used for switching the AC load current flowing in the upper and lower bridge arm submodule strings, and the second semiconductor switch string is used for resetting the energy of the submodule string energy storage element. The current switching of the two semiconductor switch strings occurs when the voltage at both ends of the semiconductor switch string is close to zero.

[0060] That is to say, when controlling the switching devices on both sides of the semiconductor switch string to turn on and off, the current flowing through the semiconductor switch string flows into or out of the midpoint on one of the two sides of the midpoint of the switch string; when the current needs to be switched from one side of the midpoint of the switch string to the other, the switching devices on both sides of the midpoint of the switch string perform switching under zero voltage state.

[0061] In a possible embodiment, when the load is resistive and capacitive, the semiconductor switch string is composed of fully controlled devices with static voltage-equalizing resistors connected in series, half-controlled thyristor devices connected in series, or a mixed series of fully controlled devices and half-controlled thyristor devices; when the load is inductive, the semiconductor switch string is composed of fully controlled devices with static voltage-equalizing resistors connected in series, or a mixed series of fully controlled devices and half-controlled thyristor devices.

[0062] Specifically, fully controlled devices can control their on and off, while half-controlled thyristor devices can only control their on but not their off. When the semiconductor switch string is composed entirely of half-controlled thyristor devices, it can only be automatically turned off when the current in the switch string is less than the holding current of the half-controlled thyristor device. Therefore, the switch string composed entirely of half-controlled thyristor devices can only be used in resistive and capacitive load situations, and cannot be used in inductive load situations.

[0063] In a possible embodiment, fully-controlled devices are used adjacent to both sides of the midpoint of the semiconductor switch string, and absorption capacitors are connected in parallel at both ends of the two series-connected fully-controlled devices to form a half-bridge circuit, which is suitable for when the current needs to be switched from one side of the midpoint of each semiconductor switch string to the other. The absorption capacitor is used to absorb the transient overvoltage generated on the fully-controlled device when the current is switched.

[0064] Specifically, in order to adapt to various load conditions, the third connection point 23 in the first semiconductor switch string j and a fourth connection point 24 in the second semiconductor switch string j At least one fully controlled device is usually used on both sides, and the remaining semiconductor switches in the semiconductor switch string are semi-controlled thyristor devices. Figure 1 In the illustrated embodiment, the first semiconductor switch string and the second semiconductor switch string each use a fully-controlled device on both sides of the midpoint.

[0065] When the first semiconductor switch series SSL jP1 and SSL jN1 When the current switches back and forth, the second semiconductor switch series SSL jP2 and SSL jN2 When the current switches, the fully controlled device is quickly turned off, causing the current to drop rapidly and there is a large stray inductance in the loop. Therefore, at the third connection point 23 j and the fourth connection point 24 jThe full-control devices on both sides are connected in parallel with the absorption capacitors C jS A half-bridge circuit is formed to absorb the electromagnetic energy of the stray inductance in the switch string loop, thereby effectively suppressing the overvoltage at both ends of the fully controlled device.

[0066] The absorption capacitance is usually determined by the following formula:

[0067] (1)

[0068] In the formula, C S is the absorption capacitor, L s is the stray inductance, I c is the cut-off current of the fully controlled device, U p The overvoltage peak value for fully controlled device shutdown. U c It is the rated voltage that the fully controlled device withstands during the off-state.

[0069] On the basis of the above embodiments, the submodule string is formed by a half-bridge submodule or a full-bridge submodule or a mixed cascade of a half-bridge submodule and a full-bridge submodule, and is used for shaping the AC terminal voltage or for energy reset in conjunction with an AC inductor. Figures 6 - 8 A half-bridge submodule is a two-quadrant submodule formed by connecting a submodule energy storage unit in parallel with two fully-controlled devices arranged in a half-bridge form; a full-bridge submodule is a four-quadrant submodule formed by connecting a submodule energy storage unit in parallel with four fully-controlled devices arranged in a full-bridge form.

[0070] In a possible embodiment, the two sub-module strings of each phase bridge arm of the EAR-MMC are used for shaping the positive half-cycle and negative half-cycle of the AC terminal voltage respectively; during the half-cycle when the sub-module string of each phase bridge arm does not participate in the AC terminal voltage shaping work, the energy of its own energy storage element is reset via the second semiconductor switch string and the AC inductor.

[0071] Based on the above embodiments, the working principle and characteristics of EAR-MMC are explained.

[0072] Based on the topology of the EAR-MMC converter, the AC side voltage and current are:

[0073] (2)

[0074] in, U m and I m They represent the fundamental frequency amplitude of AC voltage and AC current respectively, ωtrepresents the angular frequency of the power frequency, φ represents the power factor angle, φ j Indicates the initial phase of the three-phase voltage. In order to describe the relationship between AC voltage and DC voltage, the voltage modulation index is defined m for:

[0075] (3)

[0076] According to Kirchhoff's voltage law, the expression of the voltage across the upper bridge arm submodule string and the lower bridge arm submodule string of EAR-MMC is obtained:

[0077] (4)

[0078] When the output voltage on the AC side is in the positive half cycle, the upper bridge arm submodule string in the EAR-MMC phase bridge arm is a "shaping" submodule string, which is used for AC end voltage shaping, and outputs the voltage and current required by the AC side through the first semiconductor switch string path; the lower bridge arm submodule string in the phase bridge arm is a "reset" submodule string, which does not participate in the AC end voltage shaping, and forms a path through the second semiconductor switch string and the AC inductor, and performs energy reset work of submodule capacitor voltage balance in conjunction with the "reset" submodule string of other phase bridge arms. When the output voltage on the AC side is in the negative half cycle, the lower bridge arm submodule string in the EAR-MMC phase bridge arm is a "shaping" submodule string, which is used for AC end voltage shaping, and outputs the voltage and current required by the AC side through the first semiconductor switch string path; the upper bridge arm submodule string in the phase bridge arm is a "reset" submodule string, which does not participate in the AC end voltage shaping, and forms a path through the second semiconductor switch string and the AC inductor, and performs energy reset work of submodule capacitor voltage balance in conjunction with the submodule string in other phase bridge arms that does not participate in the AC end voltage shaping.

[0079] When the submodule strings of each phase bridge arm are performing energy resetting work, the energy storage units of the upper bridge arm submodule string of at least one phase bridge arm and the energy storage units of the lower bridge arm submodule string of at least one phase bridge arm form a DC voltage path, and the voltages at both ends of the upper and lower submodule strings are equal to the DC side voltage, playing the role of a DC side support capacitor. In a possible embodiment, when the submodule strings of each phase bridge arm are performing energy resetting work, the opening of the upper and lower bridge arm submodule strings is controlled to ensure that the energy storage elements of the upper bridge arm submodule string of at least one phase bridge arm and the energy storage elements of the lower bridge arm submodule string of at least one phase bridge arm form a DC voltage path, so as to play the role of a DC side support capacitor.

[0080] Specifically, by controlling the on and off of the switch strings on both sides of the first semiconductor switch string, the AC load current can flow through one of the two sides of the first semiconductor switch string, and the current flows into or out of the midpoint of the first semiconductor switch string; by controlling the on and off of the switch strings on both sides of the second semiconductor switch string, the sub-module strings involved in the energy reset work can be connected to the AC inductor respectively to generate a reset current for energy reset, and the reset current flows into or out of the midpoint of the second semiconductor switch string and flows through one of the two sides of the second semiconductor switch string to balance the upper bridge arm sub-module capacitor voltage or the lower bridge arm sub-module capacitor voltage respectively.

[0081] The upper bridge arm submodule string and the lower bridge arm submodule string of each phase of EAR-MMC are used for half-cycle AC end voltage shaping and half-cycle voltage balance energy reset work respectively, and switch the switch string states on both sides of the first semiconductor switch string and the second semiconductor switch string at the AC end voltage zero crossing; therefore, when the AC end load current switches from one side of the midpoint of the first semiconductor switch string to the other side, the switch devices on both sides of the midpoint of the first semiconductor switch string are switched under zero voltage state; when the reset current switches from one side of the midpoint of the second semiconductor switch string to the other side, the switch devices on both sides of the midpoint of the second semiconductor switch string are switched under zero voltage state. In summary, since the switch devices on both sides of the first semiconductor switch string and the second semiconductor switch string are switched under zero voltage state, the series devices of the semiconductor switch string do not need dynamic voltage balancing measures, but only static voltage balancing measures.

[0082] One of the prerequisites for the stable operation of the energy alternating reset modular multi-level converter proposed in the present invention is the voltage balance of the sub-module capacitors. Therefore, it is necessary to ensure that the energy added to the sub-module capacitors in a fundamental wave cycle is equal to zero. The energy added by the upper bridge arm "shaping" sub-module string during the positive half-cycle of the AC terminal voltage of each phase of the EAR-MMC for the AC terminal voltage waveform shaping is:

[0083] (5)

[0084] in, T is the fundamental period.

[0085] In order to make the EAR-MMC converter run stably, the upper bridge arm "reset" submodule string performs voltage balance energy reset work through the second semiconductor switch string and the AC inductor during the negative half cycle of the AC terminal voltage; if the working range of the upper bridge arm "reset" submodule string and the AC inductor to perform voltage balance energy reset during the negative half cycle of the AC terminal voltage is [ t 1 , t 2], the interval is not greater than the entire interval of the positive half-cycle of the AC terminal voltage and not less than two-thirds of the positive half-cycle interval of the AC terminal voltage (i.e. T / 3≤ t 2 - t 1 ≤ T / 2), that is, the interval for energy reset is between 120° and 180°. In this interval, the energy reduced by the upper bridge arm "reset" submodule string is:

[0086] (6)

[0087] According to the energy balance principle, formula (5) and formula (6) should be equal, so that the energy of the EAR-MMC sub-module capacitor can increase to zero within one fundamental cycle, thereby achieving sub-module capacitor voltage balance.

[0088] In addition, by comparing equation (5) and equation (6), ( U dc / 2) is greater than ( U dc / 2- u j ) and the interval of energy reset in the negative half-cycle of the AC terminal voltage is not greater than the entire interval of the positive half-cycle of the AC terminal voltage and is not less than two-thirds of the interval of the positive half-cycle of the AC terminal voltage. At the same time, formula (5) and formula (6) are equal, so i jb Less than i j , that is, the reset current flowing through the second semiconductor switch string and the AC inductor is smaller than the AC end load current flowing through the first semiconductor switch string.

[0089] The following will be divided into two specific embodiments to illustrate the working principle and control strategy of the present invention.

[0090] Embodiment 1:

[0091] When the voltage at the A phase AC terminal is in the positive half cycle, the A phase upper bridge arm submodule series SML AP It is a "shaping" submodule string - used for AC voltage shaping, A phase lower bridge arm submodule string SML AN It does not participate in the AC voltage shaping work, and is the "reset" submodule string - through the second semiconductor switch string of phase A SSL AN2 and AC inductance L A Combined voltage balance energy reset work; when the A phase AC end voltage is negative half cycle, the A phase lower bridge arm submodule series SMLAN It is a "shaping" submodule string - used for AC voltage shaping, A phase upper bridge arm submodule string SML AP It does not participate in the AC voltage shaping work, and is the "reset" submodule string - through the second semiconductor switch string of phase A SSL AP2 and AC inductance L A The energy resetting work of voltage balancing is carried out in combination. In this embodiment, the AC inductor adopts a Y-type connection.

[0092] Obviously, each of the A-phase submodule string, the B-phase submodule string, and the C-phase submodule string has a "shaping" submodule string for AC voltage shaping, and each has a "reset" submodule string for resetting voltage balance. The two SMLs of different phase bridge arms performing "reset" work form a DC voltage path through their respective second SSLs and AC inductors, acting as DC support capacitors, thereby eliminating the need for a dedicated DC side series capacitor branch.

[0093] Table 1 is the control strategy of EAR-MMC embodiment 1. The upper bridge arm sub-module string and the lower bridge arm sub-module string of each phase in EAR-MMC embodiment 1 of the present invention are implemented according to the control strategy of Table 1.

[0094] Table 1

[0095]

[0096] The working modes of the converter embodiment 1 of the present invention are described in stages below.

[0097] Mode II [0,π / 6]: such as Figure 9 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively greater than zero, less than zero and greater than zero, and the first semiconductor switch string of phase A is SSL AP1 , B phase first semiconductor switch string SSL BN1 and the first semiconductor switch string of phase C SSL CP1 Conducting, A phase upper bridge arm submodule string SML AP 、B phase lower bridge arm submodule string SML BN And C phase upper bridge arm submodule string SML CP The second semiconductor switch string of phase B is connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL BP2 and the second semiconductor switch string of phase C SSL CN2Conducting, B phase upper bridge arm submodule string SML BP And C phase lower bridge arm submodule string SML CN The "reset" submodule string is connected to the AC inductor to form a DC voltage path. All submodules are put into use to play the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset. The second semiconductor switch string of phase A is turned off, and the lower bridge arm submodule string of phase A is turned off. SML AN resection.

[0098] Mode I-II [π / 6,π / 3]: Figure 10 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively greater than zero, less than zero and greater than zero, and the first semiconductor switch string of phase A is SSL AP1 , B phase first semiconductor switch string SSL BN1 and the first semiconductor switch string of phase C SSL CP1 Conducting, A phase upper bridge arm submodule string SML AP 、B phase lower bridge arm submodule string SML BN And C phase upper bridge arm submodule string SML CP It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AN2 and the second semiconductor switch string of phase B SSL BP2 Conducting, A phase lower bridge arm submodule string SML AN And the B phase upper bridge arm submodule string SML BP The "reset" submodule string is connected to the AC inductor to form a DC voltage path. All submodules are put into use, playing the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset. The second semiconductor switch string of phase C is turned off, and the lower bridge arm submodule string of phase C is turned off. SML CN resection.

[0099] Mode I-III [π / 3,π / 2]: such as Figure 11 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively greater than zero, less than zero and less than zero, and the first semiconductor switch string of phase A is SSL AP1 , B phase first semiconductor switch string SSL BN1and the first semiconductor switch string of phase C SSL CN1 Conducting, A phase upper bridge arm submodule string SML AP 、B phase lower bridge arm submodule string SML BN And C phase lower bridge arm submodule string SML CN It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AN2 and the second semiconductor switch string of phase B SSL BP2 Conducting, A phase lower bridge arm submodule string SML AN And the B phase upper bridge arm submodule string SML BP The "reset" submodule string is connected to the AC inductor to form a DC voltage path. All submodules are put into use, playing the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset. The second semiconductor switch string of phase C is turned off, and the upper bridge arm submodule string of phase C is turned off. SML CP resection.

[0100] Mode I-IV [π / 2,2π / 3]: Figure 12 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively greater than zero, less than zero and less than zero, and the first semiconductor switch string of phase A is SSL AP1 , B phase first semiconductor switch string SSL BN1 and the first semiconductor switch string of phase C SSL CN1 Conducting, A phase upper bridge arm submodule string SML AP 、B phase lower bridge arm submodule string SML BN And C phase lower bridge arm submodule string SML CN It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AN2 and the second semiconductor switch string of phase C SSL CP2 Conducting, A phase lower bridge arm submodule string SML AN And C phase upper bridge arm submodule string SML CPThe "reset" submodule string is connected to the AC inductor to form a DC voltage path. All submodules are put into use, playing the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset. The second semiconductor switch string of phase B is turned off, and the upper bridge arm submodule string of phase B is turned off. SML BP resection.

[0101] Mode IV [2π / 3,5π / 6]: such as Figure 13 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively greater than zero, greater than zero and less than zero, and the first semiconductor switch string of phase A is SSL AP1 , B phase first semiconductor switch string SSL BP1 and the first semiconductor switch string of phase C SSL CN1 Conducting, A phase upper bridge arm submodule string SML AP 、B phase upper bridge arm submodule string SML BP And C phase lower bridge arm submodule string SML CN It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AN2 and the second semiconductor switch string of phase C SSL CP2 Conducting, A phase lower bridge arm submodule string S ML AN And C phase upper bridge arm submodule string SML CP The "reset" submodule string is connected to the AC inductor to form a DC voltage path. All submodules are put into use to play the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset. The second semiconductor switch string of phase B is turned off, and the lower bridge arm submodule string of phase B is turned off. SML BN resection.

[0102] Mode I-VI [5π / 6,π]: such as Figure 14 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively greater than zero, greater than zero and less than zero, and the first semiconductor switch string of phase A is SSL AP1 , B phase first semiconductor switch string SSL BP1 and the first semiconductor switch string of phase C SSL CN1 Conducting, A phase upper bridge arm submodule string SML AP、B phase upper bridge arm submodule string SML BP And C phase lower bridge arm submodule string SML CN The second semiconductor switch string of phase B is connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL BN2 and the second semiconductor switch string of phase C SSL CP2 Conducting, B phase lower bridge arm submodule string SML BN And C phase upper bridge arm submodule string SML CP The "reset" submodule string is connected to the AC inductor to form a DC voltage path. All submodules are put into use to play the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset. The second semiconductor switch string of phase A is turned off, and the lower bridge arm submodule string of phase A is turned off. SML AN resection.

[0103] Mode I-VII [π,7π / 6]: such as Figure 15 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively less than zero, greater than zero and less than zero, and the first semiconductor switch string of phase A is SSL AN1 , B phase first semiconductor switch string SSL BP1 and the first semiconductor switch string of phase C SSL CN1 Conducting, A phase lower bridge arm submodule string SML AN 、B phase upper bridge arm submodule string SML BP And C phase lower bridge arm submodule string SML CN The second semiconductor switch string of phase B is connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL BN2 and the second semiconductor switch string of phase C SSL CP2 Conducting, B phase lower bridge arm submodule string SML BN And C phase upper bridge arm submodule string SML CP The "reset" submodule string is connected to the AC inductor to form a DC voltage path. All submodules are put into use, playing the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset work. The second semiconductor switch string of phase A is turned off, and the upper bridge arm submodule string of phase A is turned off.SML AP resection.

[0104] Modes I-VIII [7π / 6,4π / 3]: Figure 16 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively less than zero, greater than zero and less than zero, and the first semiconductor switch string of phase A is SSL AN1 , B phase first semiconductor switch string SSL BP1 and the first semiconductor switch string of phase C SSL CN1 Conducting, A phase lower bridge arm submodule string SML AN 、B phase upper bridge arm submodule string SML BP And C phase lower bridge arm submodule string SML CN It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AP2 and the second semiconductor switch string of phase B SSL BN2 Conducting, A phase upper bridge arm submodule string SML AP And the B phase lower bridge arm submodule string SML BN The "reset" submodule string is connected to the AC inductor to form a DC voltage path. All submodules are put into use, playing the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset. The second semiconductor switch string of phase C is turned off, and the upper bridge arm submodule string of phase C is turned off. SML CP resection.

[0105] Mode I-IX [4π / 3,3π / 2]: Figure 17 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively less than zero, greater than zero and greater than zero, and the first semiconductor switch string of phase A is SSL AN1 , B phase first semiconductor switch string SSL BP1 and the first semiconductor switch string of phase C SSL CP1 Conducting, A phase lower bridge arm submodule string SML AN 、B phase upper bridge arm submodule string SML BP And C phase upper bridge arm submodule string SML CPIt is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AP2 and the second semiconductor switch string of phase B SSL BN2 Conducting, A phase upper bridge arm submodule string SML AP And the B phase lower bridge arm submodule string SML BN The "reset" submodule string is connected to the AC inductor to form a DC voltage path. All submodules are put into use, playing the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset. The second semiconductor switch string of phase C is turned off, and the lower bridge arm submodule string of phase C is turned off. SML CN resection.

[0106] Mode IX [3π / 2,5π / 3]: such as Figure 18 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively less than zero, greater than zero and greater than zero, and the first semiconductor switch string of phase A is SSL AN1 , B phase first semiconductor switch string SSL BP1 and the first semiconductor switch string of phase C SSL CP1 Conducting, A phase lower bridge arm submodule string SML AN 、B phase upper bridge arm submodule string SML BP And C phase upper bridge arm submodule string SML CP It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AP2 and the second semiconductor switch string of phase C SSL CN2 Conducting, A phase upper bridge arm submodule string SML AP And C phase lower bridge arm submodule string SML CN The "reset" submodule string is connected to the AC inductor to form a DC voltage path. All submodules are put into use to play the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset. The second semiconductor switch string of phase B is turned off, and the lower bridge arm submodule string of phase B is turned off. SML BN resection.

[0107] Mode I-XI [5π / 3,11π / 6]:Figure 19 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively less than zero, less than zero and greater than zero, and the first semiconductor switch string of phase A is SSL AN1 , B phase first semiconductor switch string SSL BN1 and the first semiconductor switch string of phase C SSL CP1 Conducting, A phase lower bridge arm submodule string SML AN 、B phase lower bridge arm submodule string SML BN And C phase upper bridge arm submodule string SML CP It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AP2 and the second semiconductor switch string of phase C SSL CN2 Conducting, A phase upper bridge arm submodule string SML AP And C phase lower bridge arm submodule string SML CN The "reset" submodule string is connected to the AC inductor to form a DC voltage path. All submodules are put into use, playing the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset. The second semiconductor switch string of phase B is turned off, and the upper bridge arm submodule string of phase B is turned off. SML BP resection.

[0108] Mode I-XII [11π / 6,2π]: Figure 20 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively less than zero, less than zero and greater than zero, and the first semiconductor switch string of phase A is SSL AN1 , B phase first semiconductor switch string SSL BN1 and the first semiconductor switch string of phase C SSL CP1 Conducting, A phase lower bridge arm submodule string SML AN 、B phase lower bridge arm submodule string SML BN And C phase upper bridge arm submodule string SML CP The second semiconductor switch string of phase B is connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL BP2 and the second semiconductor switch string of phase C SSLCN2 Conducting, B phase upper bridge arm submodule string SML BP And C phase lower bridge arm submodule string SML CN The "reset" submodule string is connected to the AC inductor to form a DC voltage path. All submodules are put into use, playing the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset work. The second semiconductor switch string of phase A is turned off, and the upper bridge arm submodule string of phase A is turned off. SML AP resection.

[0109] Based on the control strategies of the upper bridge arm submodule strings and the lower bridge arm submodule strings of each phase in the first embodiment, the control strategies of the first semiconductor switch string and the second semiconductor switch string of each phase in the first embodiment are obtained, as shown in Table 2.

[0110] Table 2

[0111]

[0112] According to the control strategy and mode of the submodule strings and semiconductor switch strings of each phase in the first embodiment, the semiconductor switch strings on both sides of the third connection points of the A phase, the B phase and the C phase are alternately turned on and off for 180° in one fundamental wave cycle, and the AC output voltage of each phase passes through zero when the on state and the off state are switched; the semiconductor switch strings on both sides of the fourth connection points of the A phase, the B phase and the C phase are turned on for 120° in one fundamental wave cycle, and the voltage across the semiconductor switch strings can be set to zero when the on state and the off state are switched;

[0113] Therefore, the current flowing through the semiconductor switch string flows into or out of the midpoint on one side of the midpoint of the switch string; when the current needs to be switched from one side of the midpoint of the switch string to the other side, the switching devices on both sides of the midpoint of the switch string are switched under zero voltage state; the series devices of the semiconductor switch string do not require dynamic voltage equalization measures.

[0114] In the first embodiment, each of the upper bridge arm and the lower bridge arm has only one "reset" submodule string to form a DC side path, and the DC side support capacitance range of each "reset" submodule string is 120°. Figure 21 shown.

[0115] In order to enable the converter proposed in the present invention to operate normally and stably, the voltage of the submodule capacitor needs to be balanced, so the energy added by the submodule capacitor in one fundamental wave cycle should be equal to zero.

[0116] According to Mode II to Mode I-VI and Kirchhoff's law, the voltage output on the AC side of phase A in the positive half cycle of the fundamental wave is u A for:

[0117] (7)

[0118] The output AC load current is:

[0119] (8)

[0120] The modulation voltage of the upper bridge arm "shaping" submodule string in the positive half cycle is:

[0121] (9)

[0122] Therefore, the energy added by the upper bridge arm "shaping" submodule string in the positive half cycle is:

[0123] (10)

[0124] In order to make the converter run stably, according to mode I-VII to mode I-XII, during the negative half cycle of phase A voltage, the "shaping" submodule string of the lower bridge arm of the converter is used to shape the negative half cycle of phase A voltage, and the "reset" submodule string of the upper bridge arm generates a reset current during the (2 / 3)p period of mode I-VIII to mode I-XI to offset the energy added in the positive half cycle, that is, to reset the voltage balance. At the same time, all submodules are put into use and combined with the second semiconductor switch string of phase B or phase C and the AC inductor to form a DC path, which plays a role in supporting the DC side capacitor. If the reset current generated is i Ab , that is, the current flowing through the inductor of phase A i LA , then during the negative half cycle of phase A voltage, the reset current i Ab The energy of the upper bridge arm submodule string is reduced to:

[0125] (11)

[0126] According to the principle of energy conservation, formula (10) should be equal to formula (11), and it is calculated that the upper bridge arm "reset" submodule string needs to generate a current with an average value of negative. At this time, the energy increase of the upper bridge arm submodule string in one fundamental wave cycle is zero, and the submodule capacitor voltage is balanced. The principle of the lower bridge arm of phase A is the same as that of the upper bridge arm.

[0127] Taking the direction of the A-phase inductor current flowing into the midpoint of the second semiconductor switch string as the reference direction, according to modal analysis and energy reset principle, the bridge arm "resets" the submodule string in the positive half cycle of the A-phase. SML AN The combined AC inductor generates a reset current to reduce the energy added in the first half cycle. The inductor current has the same direction as the lower bridge arm current, so the A-phase inductor generates a current with a negative average value in the positive half cycle.

[0128] A phase negative half cycle upper bridge arm "reset" submodule string SML AP A reset current will flow to balance the energy added in the first half cycle. The reset current is opposite to the direction of the upper bridge arm current. Taking the direction of the inductor current as the reference direction, the A-phase inductor in the negative half cycle will generate a reset current with an average value of positive, and the amplitude should be equal to the amplitude of the negative reset current in the positive half cycle.

[0129] From the above analysis and the mode of Example 1, it can be seen that when the "reset" submodule string of the upper bridge arm or lower bridge arm of phase A performs the energy reset work of voltage balance and plays the role of DC side support capacitor, the upper bridge arm or lower bridge arm of phase A conducts 120° in half a cycle and the reset current average amplitude generated by the positive and negative half-cycle AC inductance is the same. If the reset current, that is, the inductor current, appears in the form of a square wave, such as Figure 22 As shown, the reset current of phase A is:

[0130] (12)

[0131] According to the energy balance principle, equation (6) can be rewritten as:

[0132] (13)

[0133] According to the principle of energy conservation, formula (10) should be equal to formula (13), and the following is obtained by calculation:

[0134] (14)

[0135] By passing the reset current generated by the AC inductor through the "reset" sub-module string of the upper bridge arm or the lower bridge arm and satisfying equation (14), the energy added to the EAR-MMC sub-module capacitance within a fundamental cycle is achieved to be zero, thereby achieving sub-module capacitance voltage balance.

[0136] In applications such as flexible DC transmission, the m Generally, it will be higher than 0.85. m =0.85Substitute into formula (14), and we get:

[0137] (15)

[0138] Therefore, the current flowing through the second semiconductor switch series I b is always smaller than the current flowing through the first semiconductor switch series I m , and the former is generally less than one-third of the latter.

[0139] Embodiment 2:

[0140] When the voltage at the A phase AC terminal is in the positive half cycle, the A phase upper bridge arm submodule series SML AP It is a "shaping" submodule string - used for AC voltage shaping, A phase lower bridge arm submodule string SML AN It does not participate in the AC voltage shaping work, and is the "reset" submodule string - through the second semiconductor switch string of phase A SSL AN2 and AC inductance L A Combined voltage balance energy reset work; when the A phase AC end voltage is negative half cycle, the A phase lower bridge arm submodule series SML AN It is a "shaping" submodule string - used for AC voltage shaping, A phase upper bridge arm submodule string SML AP It does not participate in the AC voltage shaping work, and is the "reset" submodule string - through the second semiconductor switch string of phase A SSL AP2 and AC inductance L A The energy resetting work of voltage balancing is carried out in combination. In this embodiment, the AC inductor adopts a Y-type connection.

[0141] Obviously, each of the A-phase submodule string, the B-phase submodule string, and the C-phase submodule string has a "shaping" submodule string for AC voltage shaping, and each has a "reset" submodule string for voltage balance reset. The three SMLs performing voltage balance energy reset work form a DC voltage path through their respective second SSLs and AC inductors, acting as DC support capacitors, thereby eliminating the need for a dedicated DC side series capacitor branch.

[0142] As shown in Table 3, Table 3 is the control strategy of the upper bridge arm sub-module string and the lower bridge arm sub-module string of each phase in the second embodiment of the EAR-MMC of the present invention.

[0143] Table 3

[0144]

[0145] The working modes of the converter embodiment 2 of the present invention are described in stages below.

[0146] Mode II-I [0,π / 3]: Figure 23 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively greater than zero, less than zero and greater than zero, and the first semiconductor switch string of phase A is SSL AP1 , B phase first semiconductor switch string SSL BN1 and the first semiconductor switch string of phase C SSLCP1 Conducting, A phase upper bridge arm submodule string SML AP 、B phase lower bridge arm submodule string SML BN And C phase upper bridge arm submodule string SML CP It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AN2 , B phase second semiconductor switch string SSL BP2 and the second semiconductor switch string of phase C SSL CN2 Conducting, A phase lower bridge arm submodule string SML AN 、B phase upper bridge arm submodule string SML BP And C phase lower bridge arm submodule string SML CN It is the "reset" submodule string, which is connected to the AC inductor of each phase and forms a DC voltage path. All submodules are put into use to play the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset work for voltage balance.

[0147] Mode II-II [π / 3,2π / 3]: Figure 24 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively greater than zero, less than zero and less than zero, and the first semiconductor switch string of phase A is SSL AP1 , B phase first semiconductor switch string SSL BN1 and the first semiconductor switch string of phase C SSL CN1 Conducting, A phase upper bridge arm submodule string SML AP 、B phase lower bridge arm submodule string SML BN And C phase lower bridge arm submodule string SML CN It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AN2 , B phase second semiconductor switch string SSL BP2 and the second semiconductor switch string of phase C SSL CP2 Conducting, A phase lower bridge arm submodule string SML AN 、B phase upper bridge arm submodule string SMLBP And C phase upper bridge arm submodule string SML CP It is the "reset" submodule string, which is connected to the AC inductor of each phase and forms a DC voltage path. All submodules are put into use to play the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset work for voltage balance.

[0148] Mode II-III [2π / 3,π]: Figure 25 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively greater than zero, greater than zero and less than zero, and the first semiconductor switch string of phase A is SSL AP1 , B phase first semiconductor switch string SSL BP1 and the first semiconductor switch string of phase C SSL CN1 Conducting, A phase upper bridge arm submodule string SML AP 、B phase upper bridge arm submodule string SML BP And C phase lower bridge arm submodule string SML CN It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AN2 , B phase second semiconductor switch string SSL BN2 and the second semiconductor switch string of phase C SSL CP2 Conducting, A phase lower bridge arm submodule string SML AN 、B phase lower bridge arm submodule string SML BN And C phase upper bridge arm submodule string SML CP It is the "reset" submodule string, which is connected to the AC inductor of each phase and forms a DC voltage path. All submodules are put into use to play the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset work for voltage balance.

[0149] Mode II-IV [π,4π / 3]: such as Figure 26 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively less than zero, greater than zero and less than zero, and the first semiconductor switch string of phase A is SSL AN1 , B phase first semiconductor switch string SSL BP1 and the first semiconductor switch string of phase C SSL CN1Conducting, A phase lower bridge arm submodule string SML AN 、B phase upper bridge arm submodule string SML BP And C phase lower bridge arm submodule string SML CN It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AP2 , B phase second semiconductor switch string SSL BN2 and the second semiconductor switch string of phase C SSL CP2 Conducting, A phase upper bridge arm submodule string SML AP 、B phase lower bridge arm submodule string SML BN And C phase upper bridge arm submodule string SML CP It is the "reset" submodule string, which is connected to the AC inductor of each phase and forms a DC voltage path. All submodules are put into use to play the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset work for voltage balance.

[0150] Mode II-V [4π / 3,5π / 3]: Figure 27 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively less than zero, greater than zero and greater than zero, and the first semiconductor switch string of phase A is SSL AN1 , B phase first semiconductor switch string SSL BP1 and the first semiconductor switch string of phase C SSL CP1 Conducting, A phase lower bridge arm submodule string SML AN 、B phase upper bridge arm submodule string SML BP And C phase upper bridge arm submodule string SML CP It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AP2 , B phase second semiconductor switch string SSL BN2 and the second semiconductor switch string of phase C SSL CN2 Conducting, A phase upper bridge arm submodule string SML AP 、B phase lower bridge arm submodule string SML BNAnd C phase lower bridge arm submodule string SML CN It is the "reset" submodule string, which is connected to the AC inductor of each phase and forms a DC voltage path. All submodules are put into use to play the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset work for voltage balance.

[0151] Mode II-VI [5π / 3,2π]: Figure 28 As shown, in this stage, the modulated output voltages of phase A, phase B and phase C are respectively less than zero, less than zero and greater than zero, and the first semiconductor switch string S of phase A SL AN1 , B phase first semiconductor switch string SSL BN1 and the first semiconductor switch string of phase C SSL CP1 Conducting, A phase lower bridge arm submodule string SML AN 、B phase lower bridge arm submodule string SML BN And C phase upper bridge arm submodule string SML CP It is a "shaping" submodule string, connected to the AC side to establish the required voltage and current output from the AC end to the AC power supply or AC load. SSL AP2 , B phase second semiconductor switch string SSL BP2 and the second semiconductor switch string of phase C SSL CN2 Conducting, A phase upper bridge arm submodule string SML AP 、B phase upper bridge arm submodule string SML BP And C phase lower bridge arm submodule string SML CN It is the "reset" submodule string, which is connected to the AC inductor of each phase and forms a DC voltage path. All submodules are put into use to play the role of DC side support capacitor. At the same time, its energy storage element and AC inductor jointly generate reset current to perform energy reset work for voltage balance.

[0152] Based on the control strategy of the upper bridge arm sub-module string and the lower bridge arm sub-module string of each phase of Example 2, the control strategy of the first semiconductor switch string and the second semiconductor switch string of each phase of Example 2 is obtained, as shown in Table 4. Table 4 is the control strategy of the first semiconductor switch string and the second semiconductor switch string of each phase of Example 2.

[0153] Table 4

[0154]

[0155] From the control strategies and modes of the sub-module strings and semiconductor switch strings of each phase in the second embodiment, it is known that the semiconductor switch strings on both sides of the third connection points of phase A, phase B and phase C and the semiconductor switch strings on both sides of the fourth connection points are alternately turned on and off 180° within one fundamental wave cycle, and the AC output voltage of each phase passes through zero when the on state and the off state are switched.

[0156] Therefore, the current flowing through the semiconductor switch string flows into or out of the midpoint on one side of the midpoint of the switch string; when the current needs to be switched from one side of the midpoint of the switch string to the other side, the switching devices on both sides of the midpoint of the switch string are switched under zero voltage state; the series devices of the semiconductor switch string do not require dynamic voltage equalization measures.

[0157] Similar to the first embodiment, in order to enable the converter of the second embodiment of the present invention to operate normally and stably, the voltage of the submodule capacitor needs to be balanced, so the energy added by the submodule capacitor in one fundamental wave cycle should be equal to zero. The energy added by the upper bridge arm submodule string in the positive half cycle is still expressed by formula (10). The difference between the second embodiment and the first embodiment lies in the energy expression generated by the reset current.

[0158] It is known from the modality of the second embodiment that when the sub-module string of the upper bridge arm or the lower bridge arm of each phase performs the energy reset work of voltage balance and plays the role of DC side support capacitor, the upper bridge arm or the lower bridge arm of each phase is turned on 180° in half a cycle and the average amplitude of the reset current generated by the positive and negative half-cycle AC inductance is the same, that is, the "reset" sub-module string of each phase is fully put into use, and the three "reset" sub-module strings jointly serve as DC side support capacitors. Each "reset" sub-module string is turned on 180° in each fundamental wave cycle and plays the role of DC support capacitor, such as Figure 29 shown.

[0159] If the reset current appears in the form of a sine wave, that is, the current generated by the AC inductor is a sine wave, such as Figure 30 As shown, the inductor current is:

[0160] (16)

[0161] According to the energy balance principle, equation (11) can be rewritten as:

[0162] (17)

[0163] According to the principle of energy conservation, formula (10) should be equal to formula (17), and the following is obtained by calculation:

[0164] (18)

[0165] Embodiment 2 By passing the reset current generated by the AC inductor through the "reset" submodule string of the upper bridge arm or the lower bridge arm and satisfying equation (18), the energy added to the submodule capacitor of the converter of the present invention in one fundamental wave cycle is achieved to be zero, thereby achieving submodule capacitor voltage balance.

[0166] In applications such as flexible DC transmission, the m Generally, it will be higher than 0.85. m =0.85Substituting into formula (18), we get

[0167] (19)

[0168] Therefore, the current flowing through the second semiconductor switch series I b is always smaller than the current flowing through the first semiconductor switch series I m , and the former is generally less than one-third of the latter.

[0169] The modular multilevel converter and DC power transmission system provided by the present invention form a modular multilevel converter through a mixed connection of a submodule string and a semiconductor switch string, realize the alternating reset of the energy of the upper and lower bridge arms, and can save 50% of the number of submodules compared to MMC; and in the above two embodiments, it is shown that by controlling each submodule string and each semiconductor switch string, the reset current for energy reset flowing through the second semiconductor switch string and the AC inductor can be less than one-third of the AC end load current flowing through the first semiconductor switch string. At the same time, in order to avoid the problem of dynamic voltage balancing of devices in the semiconductor switch string, the semiconductor switch string in the embodiment of the present invention operates in a zero voltage switching state; in order to make the voltage balance control of the submodule capacitor simpler, the submodule string in the embodiment of the present invention does not have the circulation influence between the bridge arms of each phase.

[0170] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A modular multi-level converter with energy alternating reset, characterized in that: The multi-level converter is composed of at least two-phase circuits, and the phase bridge arm of each phase circuit is composed of two sub-module strings and two parallel semiconductor switch strings connecting the sub-module strings, wherein one end of the sub-module string connected in series with the upper bridge arm of the phase bridge arm is connected to the positive terminal of the DC bus; one end of the sub-module string connected in series with the lower bridge arm of the phase bridge arm is connected to the negative terminal of the DC bus; the semiconductor switch string includes a first semiconductor switch string and a second semiconductor switch string, the midpoint of the first semiconductor switch string is connected to the AC end; the midpoint of the second semiconductor switch string is connected to the midpoint of the second semiconductor switch string of the other phase bridge arm through an AC inductor; the first semiconductor switch string is used for switching work of the AC end load current flowing in the upper and lower bridge arm sub-module strings, and the second semiconductor switch string is used for resetting current switching work of resetting the energy of the sub-module string energy storage element; The phase bridge arm of each phase circuit of the multilevel converter switches the switch devices on both sides of the semiconductor switch string of the phase bridge arm, and controls when the phase bridge arm outputs an AC voltage, the two sub-module strings of the phase bridge arm alternately perform AC terminal voltage shaping and energy reset in the positive and negative half cycles of the AC voltage: in the positive half cycle of the AC voltage, the upper bridge arm sub-module string of the phase bridge arm shapes the output half-wave voltage, and the lower bridge arm sub-module string of the phase bridge arm resets the energy; in the negative half cycle of the AC voltage, the lower bridge arm sub-module string of the phase bridge arm shapes the output half-wave voltage, and the upper bridge arm sub-module string of the phase bridge arm resets the energy.

2. The modular multi-level converter with energy alternating reset according to claim 1, characterized in that: The submodule string is formed by cascading half-bridge submodules and / or full-bridge submodules having energy storage units.

3. The modular multi-level converter with energy alternating reset according to claim 1, characterized in that: When the load connected to the modular multi-level converter is resistive and capacitive, the semiconductor switch string is composed of fully-controlled devices with static voltage-equalizing resistors connected in series, half-controlled thyristor devices connected in series, or a mixed series of fully-controlled devices and half-controlled thyristor devices; when the load connected to the modular multi-level converter is inductive, the semiconductor switch string is composed of fully-controlled devices with static voltage-equalizing resistors connected in series, or a mixed series of fully-controlled devices and half-controlled thyristor devices.

4. The modular multi-level converter with energy alternating reset according to claim 1, characterized in that: When the switching devices on both sides of the semiconductor switch string are controlled to turn on and off, the current flowing through the semiconductor switch string flows into or out of the midpoint on one of the two sides of the midpoint of the switch string; when the current needs to be switched from one side of the midpoint of the switch string to the other, the switching devices on both sides of the midpoint of the switch string perform switching under zero voltage state.

5. The modular multi-level converter with energy alternating reset according to claim 2, characterized in that: When the phase bridge arm outputs an AC voltage, the two sub-module strings of the phase bridge arm perform AC end voltage shaping and energy resetting alternately in the positive and negative half cycles of the AC voltage respectively; the sub-module string of the phase bridge arm resets the energy of its own energy storage element via the second semiconductor switch string and the AC inductor within the half cycle not participating in the AC end voltage shaping work.

6. The modular multi-level converter with energy alternating reset according to claim 1 or 5, characterized in that: When the submodule string of the phase bridge arm performs energy resetting, the opening of the upper and lower bridge arm submodule strings is controlled to ensure that the energy storage elements of the upper bridge arm submodule string of at least one phase bridge arm and the energy storage elements of the lower bridge arm submodule string of at least one phase bridge arm form a DC voltage path to play the role of a DC side support capacitor.

7. The modular multi-level converter with energy alternating reset according to claim 1 or 5, characterized in that: When the multi-level converter is a three-phase converter, the AC inductor is connected to the midpoint of each of the second semiconductor switch strings of each of the phase bridge arms by a Y-type connection or a △-type connection.

8. The modular multi-level converter with energy alternating reset according to claim 1 or 3, characterized in that: By controlling each of the submodule strings and each of the semiconductor switch strings, a reset current for energy reset flowing through the second semiconductor switch string and the AC inductor is less than one third of an AC end load current flowing through the first semiconductor switch string.

9. The modular multi-level converter with energy alternating reset according to claim 1 or 4, characterized in that: Fully-controlled devices are used in the positions adjacent to the midpoint of the semiconductor switch string, and absorption capacitors are connected in parallel at both ends of the two series-connected fully-controlled devices to form a half-bridge circuit. This circuit is suitable for when the current needs to be switched from one side of the midpoint of each semiconductor switch string to the other. The absorption capacitor is used to absorb the transient overvoltage generated on the fully-controlled device when the current is switched.

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