Half-wave shaping multi-level alternating current converter topology of industrial and low frequency weak coupling

Abstract

The invention discloses an industrial and low frequency weak coupling half-wave shaping multi-level AC converter topology, which belongs to the technical field of AC-AC conversion, and comprises an I-type converter and an II-type converter which are respectively arranged on a low frequency side and a power frequency side, the I-type converter and the II-type converter adopt heterogeneous half-wave shaping multi-level converters, and the I-type converter and the II-type converter are connected with the I-type converter and the II-type converter. The I-type current converter and the II-type current converter are respectively composed of three phases of independent phase units, and each phase of the I-type current converter is composed of a multi-level shaping unit I and a half-wave reversing unit I which are connected in parallel; and each phase of the II-type converter is formed by connecting a multi-level shaping unit II and a half-wave commutation unit II in an embedded manner. The half-wave shaping multi-level AC converter topology with low frequency weak coupling is adopted, the core lies in weakening the strong constraint of a traditional AC-DC-AC complete decoupling topology on a DC link, the DC link is allowed to have coupling of harmonic components on the two sides, and the DC bus voltage has flexible adjustment capability.

Description

Technical Field

[0001] The present invention relates to the technical field of AC-AC conversion, and in particular to a low-frequency weakly coupled half-wave shaping multi-level AC converter topology. Background Art

[0002] As a beneficial complement to industrial frequency and DC transmission, low-frequency transmission technology is becoming an important option for supporting renewable energy development in areas with weak grid infrastructure, such as deserts and deep seas. As the "heart" of low-frequency transmission technology, the power density of high-voltage, high-power AC converters is one of the key indicators determining their scalability.

[0003] The modular multilevel matrix converter (M3C), currently widely used in engineering, uses a nine-arm structure to achieve AC-AC conversion, offering advantages such as a small number of submodules and high power density. However, the power components at both the power frequency and low frequency sides are directly coupled within the arms, and multiple power loops exist within the converter. This complicates the control of decoupling the power and low frequency electrical quantities in the arms, resulting in poor adaptability to complex grid disturbances. The back-to-back modular multilevel converter (BTB-MMC) uses a 12-arm structure to achieve AC-DC-AC conversion, completely decoupling the power conversion between the power frequency and low frequency sides through the DC link. This offers advantages such as simple control and strong grid adaptability. However, compared to the M3C, the number of submodules is significantly increased, resulting in lower power density. Existing AC converter topologies that combine direct power frequency coupling with AC-DC-AC decoupling struggle to achieve the balance between high power density and low decoupling control complexity.

[0004] Half-Wave Shaping-based MMC (HWS-MMC) multi-level bridge arms only require modulation to generate a half-sine wave, which is then flipped to a full sine wave by a reversing switch. This significantly reduces the number of submodules and improves power density. Back-to-back HWS-MMCs with the same structure can form an AC-DC converter topology, facilitating decoupled control of power conversion on the industrial and low-frequency sides. However, this approach faces limitations in improving power density. First, the DC port characteristics of identical HWS-MMC structures are either voltage or current sources, which are mismatched, requiring DC voltage support and filtering during connection. Second, the inherent energy balance control issues of the half-wave shaping topology increase voltage stress in the bridge arms, requiring an increase in the number of submodules. These factors limit the actual power density improvement achieved by the converter.

[0005] How to inherit the advantages of simple decoupling control of AC-DC-AC conversion topology and further improve power density is a key issue that needs to be faced in the large-scale application of low-frequency transmission technology. Summary of the Invention

[0006] The object of the present invention is to provide a low-frequency weakly coupled half-wave shaping multi-level AC converter topology to solve the problems mentioned in the background technology.

[0007] To achieve the above objectives, the present invention provides a power-low frequency weakly coupled half-wave shaping multilevel AC converter topology, comprising a type I converter and a type II converter, respectively arranged on the low-frequency side and the power-frequency side. The type I converter and the type II converter employ heterogeneous half-wave shaping multilevel converters. Both the type I converter and the type II converter are composed of three independent phase units. Each phase of the type I converter comprises a multilevel shaping unit I and a half-wave commutation unit I connected in parallel; each phase of the type II converter comprises a multilevel shaping unit II and a half-wave commutation unit II connected in an embedded manner. The three phase unit circuits of the type I converter are connected in series on the DC side, with both ends forming the positive and negative ends of the DC bus; the three phase unit circuits of the type II converter are connected in parallel on the DC side, and are simultaneously connected in parallel to the positive and negative ends of the DC bus; The I-type converter is connected to each phase of the low-frequency side AC power supply through three single-phase transformers; the secondary sides of the three single-phase transformers are respectively connected to the three half-wave commutation units I of the I-type converter, and the primary sides of the three single-phase transformers are connected in a star shape.

[0008] Preferably, the multi-level shaping unit I is composed of a cascade of full-bridge sub-modules; the half-wave commutation unit I adopts a full-bridge circuit structure, the DC end of the half-wave commutation unit I is connected in parallel with the two ends of the multi-level shaping unit I, and the AC end of the half-wave commutation unit I is connected to a single-phase transformer and connected to each phase of the low-frequency side AC system; the half-wave commutation unit I includes four commutation switches, each of which is composed of multiple IGBTs or IGCTs connected in series.

[0009] Preferably, the multi-level shaping unit II of each phase in the type II converter is embedded in the half-wave commutation unit II, and the half-wave commutation unit II of each phase is composed of two DC side commutation switches and two AC side commutation switches. The two AC side commutation switches are composed of multiple IGBTs connected in forward and reverse series, and the two DC side commutation switches are composed of multiple IGBTs connected in unidirectional series; the two AC side commutation switches are AC side commutation switch 1 and AC side commutation switch 2 respectively; the two DC side commutation switches are DC side commutation switch 1 and DC side commutation switch 2 respectively; the multi-level shaping unit II is composed of a mixed cascade of half-bridge and full-bridge sub-modules, and both ends of the multi-level shaping unit II are connected to the positive and negative ends of the DC bus through the DC side commutation switch, and both ends of the multi-level shaping unit II are connected to each phase of the power frequency side AC system through the AC side commutation switch.

[0010] Preferably, the multi-level shaping unit I and the multi-level shaping unit II respectively generate multi-level sinusoidal half-waves of corresponding system frequency and shape according to the sinusoidal reference waves of the phase voltages on both sides, and the half-wave commutation unit I and the half-wave commutation unit II respectively flip or longitudinally shift the generated multi-level sinusoidal half-waves according to the sinusoidal reference waves of the phase voltages on both sides, so as to obtain a complete multi-level sinusoidal full wave on the AC side.

[0011] Preferably, the multi-level shaping unit 1 generates a multi-level sinusoidal half-wave 1; the reversing switch includes a reversing switch 1, a reversing switch 2, a reversing switch 3 and a reversing switch 4, the reversing switch 1 and the reversing switch 4 are located on a diagonal line of the half-wave reversing unit 1, constituting a reversing switch group 1, the reversing switch 2 and the reversing switch 3 are located on another diagonal line of the half-wave reversing unit 1, constituting a reversing switch group 2; the two reversing switch groups are alternately turned on and off according to the fundamental frequency period of the low-frequency side, and when the phase voltage When the sinusoidal reference wave is positive, the first and fourth reversing switches are turned on, the second and third reversing switches are turned off, and the AC side outputs a positive half-wave of the multi-level sinusoidal voltage. When the phase voltage sinusoidal reference wave is negative, the second and third reversing switches are turned on, the first and fourth reversing switches are turned off, and the AC side outputs a negative half-wave of the multi-level sinusoidal voltage. Within one low-frequency cycle, a complete multi-level sinusoidal full wave I is obtained based on the positive and negative half-waves of the multi-level sinusoidal voltage.

[0012] Preferably, the multi-level sinusoidal half-wave I generated by the multi-level shaping unit I is a two-pulsating voltage waveform with a peak facing upward; the voltage phases of the three multi-level sinusoidal half-waves I generated by the three-phase multi-level shaping unit I are sequentially 120° apart, and the DC bus voltage obtained at the positive and negative ends of the DC side after series superposition is based on the low-frequency side frequency as the reference fundamental wave. u dc It is a six-pulse voltage, and each pulse cycle is The DC side of the I-type converter exhibits controlled voltage source characteristics, and the DC voltage of the I-type converter is a low-frequency six-pulse DC bus voltage. u dc .

[0013] Preferably, the multi-level shaping unit II generates a multi-level sinusoidal half-wave II; the AC side reversing switch 1 and the DC side reversing switch 2 in the half-wave reversing unit II constitute a switch group 1, and the AC side reversing switch 2 and the DC side reversing switch 1 constitute a switch group 2, and the two switch groups are alternately turned on and off according to the fundamental frequency cycle of the power frequency side; when the phase voltage sinusoidal reference wave is positive, the AC side reversing switch 2 and the DC side reversing switch 1 are turned on, and the AC side reversing switch 1 and the DC side reversing switch 2 are turned off, and the AC current flows to the positive end of the DC bus, and the multi-level sinusoidal half-wave generated by the multi-level shaping unit II is translated downward , is the steady-state value of the DC bus voltage, and is flipped upward to obtain the positive half-wave of the AC phase voltage; when the phase voltage sinusoidal reference wave is negative, the AC side reversing switch 1 and the DC side reversing switch 2 are turned on, the AC side reversing switch 2 and the DC side reversing switch 1 are turned off, the AC current flows to the negative end of the DC bus, and the multi-level sinusoidal half-wave generated by the multi-level shaping unit II is shifted downward , the negative half-wave of the AC phase voltage is obtained. Within one power frequency cycle, a complete multi-level sinusoidal full wave II is obtained based on the positive half-wave and negative half-wave of the multi-level sinusoidal voltage.

[0014] Preferably, the output current of each phase of the DC side of the II type converter is a single pulse sinusoidal half-wave current within one power frequency cycle, and the phases of the three-phase DC output currents are 120 degrees apart in sequence. The power frequency side frequency is used as the reference fundamental wave, and the total current output to the DC bus after the three phases are connected in parallel is is a six-pulse DC current, and the period of each pulse is The DC side of the type II converter exhibits a controlled current source characteristic, and the DC current of the type II converter is a six-pulse DC current at an industrial frequency. .

[0015] Preferably, the DC side of the type I converter exhibits controlled voltage source characteristics, and the DC side of the type II converter exhibits controlled current source characteristics. The DC side characteristics of the type I converter and the type II converter match, and are directly connected through a DC bus. The DC links of the type I converter and the type II converter are coupled with a low-frequency six-pulse DC voltage and an industrial frequency six-pulse DC current.

[0016] Preferably, the type I converter side adopts a third harmonic injection method to achieve energy balance in a wide voltage range, and the type II converter side adopts a commutation unit phase shift modulation method to achieve energy balance in a wide range.

[0017] Therefore, the present invention adopts the above-mentioned low-frequency weakly coupled half-wave shaping multi-level AC converter topology, which is suitable for AC-AC conversion scenarios such as low-frequency power transmission and has the following beneficial effects: (1) Two heterogeneous half-wave shaping multilevel converters with matching DC-side controlled source characteristics are introduced, and two AC systems with different frequencies are interconnected through a DC link. The DC link does not require voltage support and filtering components, which can reduce the number of sub-modules while reducing the amount of capacitors and inductors, thereby maximizing power density. The DC link of this topology can achieve no coupling of fundamental power components between the industrial and low-frequency sides, and only weak coupling of harmonic components. Therefore, the power conversion and regulation between the industrial and low-frequency sides can be decoupled.

[0018] (2) The core of this topology is to weaken the strong constraints of the traditional AC / DC fully decoupled topology on the DC link, allowing the coupling of harmonic components on both sides of the DC link, and the DC bus voltage has flexible adjustment capabilities; this topology realizes the interconnection of AC systems of different frequencies by selecting a half-wave shaping MMC structure (one side is a controlled voltage source and the other side is a controlled current source) that matches the DC side characteristics, without the need for additional DC voltage support and filtering devices, breaking through the type limitations of the existing industrial and low-frequency direct coupling and fully decoupled AC / AC conversion topologies, and giving full play to the high power density advantages of the half-wave shaping unit; at the same time, there is only weak coupling of the 6th harmonic component between the converters on both sides, and there is no coupling of the fundamental and second harmonic components, making the industrial and low-frequency power decoupling control simple.

[0019] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a schematic structural diagram of a low-frequency weakly coupled half-wave shaping multi-level AC converter topology according to an embodiment of the present invention; Figure 2 This is a diagram showing the topological structure and working principle of the phase unit of the low-frequency side I-type converter of the present invention; Figure 3 This is a diagram showing the topological structure and working principle of the phase unit of the power frequency side II type converter of the present invention; Figure 4 The topological structure diagram of the half-bridge submodule HBSM and the full-bridge submodule FBSM of the present invention; Figure 5 The DC output current of the half-wave commutation unit I of the low-frequency side I-type converter according to the embodiment of the present invention is and the multi-level shaping unit I current The simulation waveform of Figure 6 The multi-level sinusoidal half-wave I voltage, the multi-level sinusoidal full-wave I voltage and the DC bus voltage on the I-type converter side of the embodiment of the present invention are u dc The simulation waveform of Figure 7The DC positive terminal current of the a-phase unit of the power frequency side II type converter according to the embodiment of the present invention is and DC bus current The simulation waveform of Figure 8 : The simulated waveforms of the multi-level sinusoidal half-wave II voltage and the multi-level sinusoidal full-wave II voltage on the Type II converter side according to an embodiment of the present invention; Figure 9 1 is a simulation waveform of the AC current on the power frequency side and the AC current on the low frequency side according to an embodiment of the present invention. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. The components of the embodiments of the present invention generally described and shown in the drawings herein can be arranged and designed in various different configurations. Therefore, the following detailed description of the embodiments of the present invention provided in the drawings is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0022] It should be noted that similar reference numerals and letters denote similar items in the following drawings, and therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0023] Example like Figure 1 As shown, the present invention provides a low-frequency and low-frequency weakly coupled half-wave shaping multilevel AC converter topology, including a type I converter and a type II converter respectively arranged on the low-frequency side and the power frequency side. The type I converter and the type II converter adopt heterogeneous half-wave shaping multilevel converter (HWS-MMC), as shown in FIG. Figure 2-Figure 3 As shown, the DC sides of the type I converter and the type II converter exhibit controlled voltage source characteristics and controlled current source characteristics, respectively, and the two characteristics match and are directly connected through a DC bus.

[0024] Both Type I and Type II converters consist of three independent phase units. Each phase unit of the Type I converter includes a multilevel shaping unit I and a half-wave commutation unit I, while each phase unit of the Type II converter includes a multilevel shaping unit II and a half-wave commutation unit II. Each phase of the Type I converter consists of a multilevel shaping unit I and a half-wave commutation unit I connected in parallel; each phase of the Type II converter consists of a multilevel shaping unit II and a half-wave commutation unit II connected in an embedded manner.

[0025] like Figure 1 As shown, the three phase unit circuits of the type I converter are connected in series on the DC side, with both ends forming the positive and negative ends of the DC bus; the three phase unit circuits of the type II converter are connected in parallel on the DC side, and are also connected in parallel to the positive and negative ends of the DC bus.

[0026] The multilevel shaping unit I of each phase of the I-type converter is composed of a cascade of full-bridge sub-modules (FBSMs). The half-wave commutation unit I of each phase adopts a full-bridge (H-bridge) circuit structure. Its DC end is connected in parallel with the two ends of the multilevel shaping unit I, and its AC end is connected to a single-phase transformer, which is then connected to each phase of the low-frequency AC system. The half-wave commutation unit I of each phase contains four commutation switches, each of which is composed of multiple insulated gate bipolar transistors (IGBTs) (or integrated gate-commutated thyristors (IGCTs)) connected in series.

[0027] The multi-level shaping unit II of each phase of the type II converter is embedded in the half-wave commutation unit II. The half-wave commutation unit II of each phase consists of two DC side commutation switches and two AC side commutation switches, of which the two AC side commutation switches (AC side commutation switch S 11 and AC side reversing switch 2S 12 ) is composed of multiple IGBTs connected in series in forward and reverse directions, and two DC side reversing switches (DC side reversing switch - S 21 and DC side reversing switch 2S 22 ) is composed of multiple IGBTs connected in series in one direction; the multi-level shaping unit II can be composed of a cascade of half-bridge submodules HBSM (or a mixture of half-bridge submodules and full-bridge submodules), whose two ends are connected to the positive and negative terminals of the DC bus through the DC side reversing switch, and whose two ends are connected to each phase of the AC system on the power frequency side through the AC side reversing switch. The structure of the full-bridge submodule FBSM and the half-bridge submodule HBSM is as follows Figure 4 shown.

[0028] In this embodiment, the I-type converter is connected to each phase of the low-frequency side AC power supply through three single-phase transformers; the secondary side of the single-phase transformer is respectively connected to the three half-wave commutation units I of the I-type converter to provide electrical isolation between the three-phase circuits; the primary side of the single-phase transformer adopts a star connection method.

[0029] In this embodiment, the multilevel shaping units I and II of the type I converter and the type II converter respectively generate multilevel sinusoidal half-waves corresponding to the system frequency and shape based on the sinusoidal reference waves of the phase voltages on both sides. The half-wave commutation units I and II of the type I converter and the type II converter respectively flip or longitudinally shift the generated multilevel sinusoidal half-waves based on the sinusoidal reference waves of the phase voltages on both sides, thereby obtaining a complete multilevel sinusoidal full-wave on the AC side. Specifically, For Type I converter: Figure 2 As shown, the multi-level shaping unit I generates a multi-level sinusoidal half-wave I; the reversing switches include reversing switch 1 Q1, reversing switch 2 Q2, reversing switch 3 Q3, and reversing switch 4 Q4. Reversing switch 1 Q1 and reversing switch 4 Q4 are located on one diagonal line of the half-wave reversing unit 1, forming reversing switch group 1, and reversing switch 2 Q2 and reversing switch 3 Q3 are located on the other diagonal line of the half-wave reversing unit 1, forming reversing switch group 2. The two reversing switch groups are alternately turned on and off according to the fundamental frequency cycle of the low-frequency side. When the phase voltage sinusoidal reference wave is positive, reversing switch 1 Q1 and reversing switch 4 Q4 are turned on, and reversing switch 2 Q2 and reversing switch 3 Q3 are turned off. The AC side outputs the positive half-wave of the multi-level sinusoidal voltage. When the phase voltage sinusoidal reference wave is negative, reversing switch 2 Q2 and reversing switch 3 Q3 are turned on, and reversing switch 1 Q1 and reversing switch 4 Q4 are turned off. The AC side outputs the negative half-wave of the multi-level sinusoidal voltage. In one low-frequency cycle, a complete multi-level sinusoidal full wave I is obtained based on the positive half-wave and negative half-wave of the multi-level sinusoidal voltage.

[0030] The multi-level sinusoidal half-wave I generated by the multi-level shaping unit I is a two-pulsating voltage waveform with an upward peak. The voltage phases of the three multi-level sinusoidal half-waves I generated by the three-phase multi-level shaping unit I are 120° apart. The DC bus voltage obtained at the positive and negative ends of the DC side after being superimposed in series is u dc It is a six-pulse voltage (with the low-frequency side frequency as the reference fundamental wave), and each pulse cycle is Therefore, the DC side of the I-type converter presents a controlled voltage source characteristic, and its voltage is the low-frequency six-pulse DC bus voltage u dc .

[0031] For Type II converter: Figure 3 As shown, the multi-level shaping unit II generates a multi-level sinusoidal half-wave II; the AC side reversing switch S in the half-wave reversing unit II 11 and DC side reversing switch 2S 22 Consists of switch group 1, AC side reversing switch 2 S 12 and DC side reversing switch S 21 The two switch groups are switched on and off alternately according to the fundamental frequency cycle of the power frequency side. When the phase voltage sinusoidal reference wave is positive, the AC side reversing switch S2 in the switch group 2 is 12 and DC side reversing switch S 21 Open, the AC side reversing switch S in switch group 1 11 and DC side reversing switch 2S 22 Turn off, AC current flows to the positive end of the DC bus, and the multi-level sine half-wave generated by the multi-level shaping unit II is shifted downward , is the steady-state value of the DC bus voltage, and is flipped upward to obtain the positive half-wave of the AC phase voltage; when the phase voltage sinusoidal reference wave is negative, the AC side reversing switch S in the switch group 1 is 11 and DC side reversing switch 2S 22 Open, the AC side reversing switch 2 S in switch group 2 12 and DC side reversing switch S 21 Turn off, AC current flows to the negative end of the DC bus, and the multi-level sine half-wave generated by the multi-level shaping unit II is shifted downward , and the negative half-wave of the AC phase voltage is obtained. Within one power frequency cycle, a complete multi-level sinusoidal full wave II is obtained based on the positive half-wave and negative half-wave of the multi-level sinusoidal voltage.

[0032] The output current of each phase of the DC side of the II type converter is a single pulse sinusoidal half-wave current within one power frequency cycle. The phases of the three-phase DC output currents are 120 degrees apart. The total current output to the DC bus after the three phases are connected in parallel is It is a six-pulse DC current (with the power frequency side frequency as the reference fundamental wave), and the period of each pulse is Therefore, the DC side of the Type II converter at the power frequency side presents a controlled current source characteristic, and its current is the power frequency six-pulse DC current. .

[0033] In this embodiment, the DC side of the Type I converter exhibits controlled voltage source characteristics, and the DC side of the Type II converter exhibits controlled current source characteristics. The two characteristics match and can be directly connected through the DC bus. No voltage support and filtering components are required in the DC link. Therefore, there is coupling between the low-frequency six-pulse DC voltage and the industrial frequency six-pulse DC current in the DC link of the AC converter, that is, there is only weak coupling of the system frequency harmonic components on both sides, and there is no coupling of the fundamental power components on both sides.

[0034] In this embodiment, since half-wave commutation introduces a limitation on the modulation ratio due to the energy balance of the bridge arm capacitors, both the Type I and Type II converters need to adopt appropriate methods to perform wide-range energy balance regulation.

[0035] The Type I converter uses a third-harmonic injection method to achieve energy balance over a wide voltage range. This involves superimposing a third-harmonic component on the three-phase AC modulated wave, which allows the zero-crossing point of the composite voltage of the multi-level units to coincide with the fundamental wave, ensuring soft switching of the commutation units at zero. The third harmonic is a zero-sequence component, and the components in the line voltage cancel each other out, so injecting the third harmonic does not affect the quality of the line voltage waveform. Furthermore, by adjusting the amplitude of the injected third-harmonic voltage, the amplitude ratio of the AC side fundamental voltage to the DC voltage can be varied within a certain range, enabling wide-range regulation of the AC side voltage.

[0036] Type II converters use a commutation unit phase shift modulation method to achieve wide range energy balance. For Type II converters, the phase shift modulation strategy is to make the switching point of the half-wave commutation unit II lag behind the zero crossing point of the port voltage fundamental (or reference voltage). θ Angle, assuming the initial phase of the reference voltage is 0, the switching function of the half-wave commutation unit II (Take phase a as an example): ; in, represents the phase shift angle, Indicates angular frequency, Indicates time.

[0037] Phase shift angle θ The calculation formula is: ; in, is the modulation ratio, and , is the steady-state value of the DC bus voltage, is the amplitude of the reference phase voltage, It is the angle at which the AC current on the power frequency side lags behind the AC voltage.

[0038] In this embodiment, the capacitor voltages of the three-phase multi-level shaping unit I and the three-phase multi-level shaping unit II modules of the type I converter and the type II converter need to be balanced between phases and within phases.

[0039] Based on Matlab / Simulink according to Figure 1 The topology shown is used to build a system simulation model.

[0040] Reference Figure 5 It can be seen that the current of the multi-level shaping unit I branch on the low-frequency side is the DC output current of the half-wave commutation unit I minus the six-pulse current of the DC bus.

[0041] Depend on Figure 6 It can be seen that the multi-level sinusoidal half-wave I generated by the low-frequency side multi-level unit I is a two-pulse voltage waveform with an upward peak; the three multi-level sinusoidal half-waves generated by the three-phase multi-level unit are 120° apart in phase, and the six-pulse DC bus voltage is obtained after being superimposed in series. u dc (taking the low-frequency side frequency as the reference fundamental wave), which is consistent with the above analysis.

[0042] Reference Figure 7 Because the positive terminal switch group of the half-wave commutation unit II of the type II converter on the power frequency side is only on for half a cycle, the positive terminal current of each phase of DC is a single pulse current. The phases of the positive terminal currents of the three-phase DC are 120° apart, and after parallel superposition, a six-pulse current of the DC bus is formed. .

[0043] Reference Figure 8 It can be seen that the multi-level sinusoidal half-wave voltage II of the type II converter is translated and flipped by the commutation unit to obtain the multi-level sinusoidal full-wave voltage II.

[0044] Reference Figure 9 , the current frequency on the power frequency side is 50Hz, and the current frequency on the low frequency side is 20Hz.

[0045] In summary, the relevant simulation results prove the effectiveness of the present invention. The present invention provides a low-frequency weakly coupled half-wave shaping multi-level AC converter topology, which is suitable for AC-AC frequency conversion scenarios such as low-frequency power transmission. For example, an offshore wind farm outputs low-frequency electric energy, which is connected to a transformer via a 20Hz submarine cable and then transmitted to the low-frequency weakly coupled AC converter provided in the embodiment of the present application, and then transmitted to a 50HZ industrial frequency AC power grid.

[0046] Therefore, the present invention adopts the above-mentioned half-wave shaping multi-level AC converter topology with weak coupling between the industrial and low frequencies. The DC link of this topology can achieve no coupling of fundamental power components between the industrial and low frequency sides, and only weak coupling of harmonic components. Therefore, the power conversion and regulation between the industrial and low frequency sides can achieve decoupling control.

[0047] 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 the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A low-frequency, weakly coupled, half-wave shaping multi-level AC converter topology, characterized by: The invention comprises a type I converter and a type II converter respectively arranged on the low-frequency side and the power frequency side. The type I converter and the type II converter adopt heterogeneous half-wave shaping multilevel converters. The type I converter and the type II converter are both composed of three independent phase units. Each phase of the type I converter is composed of a multilevel shaping unit I and a half-wave commutation unit I in parallel; each phase of the type II converter is composed of a multilevel shaping unit II and a half-wave commutation unit II in an embedded connection. The three phase unit circuits of the type I converter are connected in series on the DC side, with both ends forming the positive and negative ends of the DC bus; the three phase unit circuits of the type II converter are connected in parallel on the DC side, and are simultaneously connected in parallel to the positive and negative ends of the DC bus; The I-type converter is connected to each phase of the low-frequency side AC power supply through three single-phase transformers; the secondary sides of the three single-phase transformers are respectively connected to the three half-wave commutation units I of the I-type converter, and the primary sides of the three single-phase transformers are connected in a star shape.

2. The industrial low-frequency weakly coupled half-wave shaping multi-level AC converter topology according to claim 1, characterized in that: The multi-level shaping unit I is composed of a cascade of full-bridge sub-modules; the half-wave commutation unit I adopts a full-bridge circuit structure, the DC end of the half-wave commutation unit I is connected in parallel with the two ends of the multi-level shaping unit I, and the AC end of the half-wave commutation unit I is connected to a single-phase transformer and connected to each phase of the low-frequency side AC system; the half-wave commutation unit I includes four commutation switches, each of which is composed of multiple IGBTs or IGCTs connected in series.

3. The industrial low-frequency weakly coupled half-wave shaping multi-level AC converter topology according to claim 1, characterized in that: The multi-level shaping unit II of each phase in the type II converter is embedded in the half-wave commutation unit II. The half-wave commutation unit II of each phase is composed of two DC-side commutation switches and two AC-side commutation switches. The two AC-side commutation switches are composed of multiple IGBTs connected in forward and reverse series, and the two DC-side commutation switches are composed of multiple IGBTs connected in unidirectional series; the two AC-side commutation switches are AC-side commutation switch 1 and AC-side commutation switch 2 respectively; the two DC-side commutation switches are DC-side commutation switch 1 and DC-side commutation switch 2 respectively; the multi-level shaping unit II is composed of a mixed cascade of half-bridge and full-bridge sub-modules, and both ends of the multi-level shaping unit II are connected to the positive and negative ends of the DC bus through the DC-side commutation switch, and both ends of the multi-level shaping unit II are connected to each phase of the power frequency side AC system through the AC-side commutation switch.

4. The industrial low-frequency weakly coupled half-wave shaping multi-level AC converter topology according to claim 1, characterized in that: The multi-level shaping unit I and the multi-level shaping unit II respectively generate multi-level sinusoidal half-waves of corresponding system frequency and shape according to the sinusoidal reference waves of the phase voltages on both sides. The half-wave commutation unit I and the half-wave commutation unit II respectively flip or longitudinally shift the generated multi-level sinusoidal half-waves according to the sinusoidal reference waves of the phase voltages on both sides to obtain a complete multi-level sinusoidal full wave on the AC side.

5. The industrial low-frequency weakly coupled half-wave shaping multi-level AC converter topology according to claim 2, characterized in that: The multi-level shaping unit 1 generates a multi-level sinusoidal half-wave 1; the reversing switch includes a reversing switch 1, a reversing switch 2, a reversing switch 3 and a reversing switch 4, wherein the reversing switch 1 and the reversing switch 4 are located on a diagonal line of the half-wave reversing unit 1, constituting a reversing switch group 1, and the reversing switch 2 and the reversing switch 3 are located on another diagonal line of the half-wave reversing unit 1, constituting a reversing switch group 2; the two reversing switch groups are alternately switched on and off according to the fundamental frequency period of the low-frequency side. When the phase voltage sinusoidal When the reference wave is positive, the reversing switch 1 and the reversing switch 4 are turned on, the reversing switch 2 and the reversing switch 3 are turned off, and the AC side outputs the positive half-wave of the multi-level sinusoidal voltage. When the phase voltage sinusoidal reference wave is negative, the reversing switch 2 and the reversing switch 3 are turned on, the reversing switch 1 and the reversing switch 4 are turned off, and the AC side outputs the negative half-wave of the multi-level sinusoidal voltage. Within one low-frequency cycle, a complete multi-level sinusoidal full wave I is obtained based on the positive half-wave and negative half-wave of the multi-level sinusoidal voltage.

6. The industrial low-frequency weakly coupled half-wave shaping multi-level AC converter topology according to claim 5, characterized in that: The multi-level sinusoidal half-wave I generated by the multi-level shaping unit I is a two-pulsating voltage waveform with a peak facing upward; the voltage phases of the three multi-level sinusoidal half-waves I generated by the three-phase multi-level shaping unit I are 120° apart in sequence, and the DC bus voltage obtained by superimposing them in series at the positive and negative ends of the DC side is u dc It is a six-pulse voltage, and each pulse cycle is The DC side of the I-type converter exhibits controlled voltage source characteristics, and the DC voltage of the I-type converter is a low-frequency six-pulse DC bus voltage. u dc .

7. The industrial low-frequency weakly coupled half-wave shaping multi-level AC converter topology according to claim 4, characterized in that: The multi-level shaping unit II generates a multi-level sinusoidal half-wave II; the AC side reversing switch 1 and the DC side reversing switch 2 in the half-wave reversing unit II constitute a switch group 1, and the AC side reversing switch 2 and the DC side reversing switch 1 constitute a switch group 2, and the two switch groups are alternately turned on and off according to the fundamental frequency cycle of the power frequency side; when the phase voltage sinusoidal reference wave is positive, the AC side reversing switch 2 and the DC side reversing switch 1 are turned on, the AC side reversing switch 1 and the DC side reversing switch 2 are turned off, the AC current flows to the positive end of the DC bus, and the multi-level sinusoidal half-wave generated by the multi-level shaping unit II is translated downward , is the steady-state value of the DC bus voltage, and is flipped upward to obtain the positive half-wave of the AC phase voltage; when the phase voltage sinusoidal reference wave is negative, the AC side reversing switch 1 and the DC side reversing switch 2 are turned on, the AC side reversing switch 2 and the DC side reversing switch 1 are turned off, the AC current flows to the negative end of the DC bus, and the multi-level sinusoidal half-wave generated by the multi-level shaping unit II is shifted downward , the negative half-wave of the AC phase voltage is obtained. Within one power frequency cycle, a complete multi-level sinusoidal full wave II is obtained based on the positive half-wave and negative half-wave of the multi-level sinusoidal voltage.

8. The industrial low-frequency weakly coupled half-wave shaping multi-level AC converter topology according to claim 7, characterized in that: The output current of each phase of the DC side of the II type converter is a single pulse sinusoidal half-wave current within one power frequency cycle. The phases of the three-phase DC output currents are 120 degrees apart. The power frequency side frequency is used as the reference fundamental wave. The total current output to the DC bus after the three phases are connected in parallel is is a six-pulse DC current, and the period of each pulse is The DC side of the type II converter exhibits a controlled current source characteristic, and the DC current of the type II converter is a six-pulse DC current at an industrial frequency. .

9. The industrial low-frequency weakly coupled half-wave shaping multi-level AC converter topology according to claim 1, characterized in that: The DC side of the type I converter exhibits controlled voltage source characteristics, and the DC side of the type II converter exhibits controlled current source characteristics. The DC side characteristics of the type I converter and the type II converter match, and are directly connected through a DC bus. The DC links of the type I converter and the type II converter are coupled with a low-frequency six-pulse DC voltage and an industrial frequency six-pulse DC current.

10. The industrial low-frequency weakly coupled half-wave shaping multi-level AC converter topology according to claim 4, characterized in that: The type I converter side adopts a third harmonic injection method to achieve energy balance in a wide voltage range, and the type II converter side adopts a commutation unit phase shift modulation method to achieve energy balance in a wide range.

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