A heterogeneous multiplex modular multilevel converter topology and a control method thereof

Abstract

This invention discloses a heterogeneous multiplexed modular multilevel converter topology and its control method. The topology includes three shaping arms connected in series on the AC bus. The output port of any one shaping arm is connected to the DC side via two multiplexing circuits. Each multiplexing circuit consists of a three-phase switching bridge and multiplexing arms, dynamically connected to any one of the shaping arms to provide voltage support. By designing an energy balance angle α modulation strategy, the multiplexing arms are controlled to alternately multiplex within half the power frequency cycle of the three-phase shaping arms, and the α angle can be dynamically adjusted according to the modulation index m and phase φ to achieve half-cycle energy balance, significantly improving the power density and device utilization of the topology. By constructing multiplexing circuits composed of two three-phase switching bridges and multiplexing arms, this invention can save nearly 3 / 4 of the number of sub-modules, significantly reducing the number of required control devices. The series adjustable reverse voltage sub-module on the AC side improves the fault isolation level of the topology, thereby improving the overall economy and reliability of the topology.

Description

Technical Field

[0001] This invention belongs to the technical field of power generation, transformation or distribution, and specifically relates to a heterogeneous multiplexed modular multilevel converter topology and its control method. Background Technology

[0002] In recent years, modular multilevel converters (MMCs) have been widely used in flexible DC power transmission and distribution due to their advantages such as high waveform quality, low operating losses, and high integration. However, in practical applications, MMCs require a large number of semiconductor devices and capacitors, leading to high switching costs, large footprint, and low power density. Regarding other basic forms of MMC topologies, one is the hybrid cascaded MMC structure, which includes shaping arms and bridge switching circuits. The shaping arms cascade a large number of full-bridge sub-modules, increasing the number of power semiconductor devices. Another is the arm-selective MMC structure, which operates at a specific AC-side voltage to DC-side port voltage ratio, resulting in poor flexibility in voltage input / output range. One typical topology is the alternating arm multilevel converter (AAMC), where half of the arms are configured with full-bridge sub-modules, achieving a lightweight MMC topology and improving fault isolation capability. However, the modulation range of AAMCs is limited. Despite widening the overlap conduction angle of the upper and lower bridge arms to 60°, it still suffers from limited modulation range, a large number of components, and the need for DC-side support capacitors. Facing resource constraints such as weight and size limitations in offshore wind power converter platforms, optimizing the existing MMC topology, adjusting control methods, and ensuring a reduction in the number of submodules required for the same power conversion, thereby improving the lightweight and reliability of converter equipment, will significantly promote the large-scale transmission capacity of offshore wind power and other new energy sources. Summary of the Invention

[0003] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art. The present invention provides a heterogeneous multiplexed modular multilevel converter topology and its control method, which has a simple structure, flexible operation and control, and solves the problems of large number of MMC sub-modules, large footprint and low power density.

[0004] To solve the above-mentioned technical problems, this invention adopts the following technical solution: a heterogeneous multiplexed modular multilevel converter topology, comprising three-phase shaping arms and two multiplexing circuits; the three-phase shaping arms are each composed of cascaded sub-modules; the two multiplexing circuits are each composed of a three-phase switching bridge and multiplexing arms; T j1 Signal switching devices and S j1The signal switching devices and the upper multiplexing bridge arm constitute the j-phase upper multiplexing circuit; T j2 Signal switching devices and S j2 The switching devices of the signal and the lower multiplexing bridge arm form a j-phase upper multiplexing circuit, where j is A, B, or C. The switching devices in the same phase do not conduct simultaneously, i.e., T... j1 +T j2 +S j1 +S j2 ≤1.

[0005] Furthermore, the upper multiplexing circuit and the lower multiplexing circuit have the same structure, both including a three-phase switching bridge and a multiplexing bridge arm. Each phase of the shaping bridge arm consists of one set (fully controlled, half-controlled, or uncontrolled devices) and another set of fully controlled devices. In the upper multiplexing circuit, phase j S... j1 The lower ends of the signal switching devices are connected to the negative terminal of the upper multiplexing bridge arm, and their upper ends are connected to the negative terminal of the corresponding phase shaping bridge arm; in the upper multiplexing circuit, T j1 The upper ends of the switching devices for the signal are all connected to the positive terminal of the upper multiplexing bridge arm, and their lower ends are all connected to the negative terminal of the corresponding phase shaping bridge arm; in the lower multiplexing circuit, phase j S j2 The upper ends of the switching devices of the signal are connected to the positive terminal of the lower multiplexed bridge arm, and their lower ends are connected to the negative terminal of the corresponding phase shaping bridge arm; in the lower multiplexed bridge arm, phase T of phase j... j2 The upper ends of the switching devices of the signal are all connected to the negative terminal of the corresponding phase shaping bridge arm, and their lower ends are all connected to the negative terminal of the lower multiplexing bridge arm; the positive terminal of the upper multiplexing bridge arm is connected to the positive terminal of the DC side, and the negative terminal of the lower multiplexing bridge arm is connected to the negative terminal of the DC side.

[0006] Furthermore, the proposed basic control method for the topology can be described as follows: An energy balance angle α is introduced to ensure energy balance between the shaping bridge arm and the multiplexing circuit during half a power frequency cycle, with current i... j =I m sin(ωt+φ j () is defined as the reference phase:

[0007]

[0008] ω is the angular frequency, φ j These correspond to the phases of phase j. In upper bridge arm mode: current flows through T. j1 Or S j1 The same applies when reusing the lower arm mode.

[0009] Furthermore, the topological energy balance method can be described as follows: through the energy balance angle α, in the upper bridge arm mode... Time: j-phase alternating current i j Flow through the J-phase shaping bridge arm; S j1 In phase j alternating current i jThe α phase before and after the zero point is When the circuit is activated, the j-phase alternating current i j The current flows through the j-phase shaping bridge arm and the upper multiplexing bridge arm. The j-phase shaping bridge arm achieves energy balance during the two-stage charging and discharging, while the upper multiplexing bridge arm... Phase energy balance. Similarly, the working principle of the lower arm mode is the same as that of the upper arm mode.

[0010] Furthermore, when phase j is in the upper arm mode, T j1 or S j1 Conductivity, T j2 and S j2 The energy balance of the turn-off, shaping arm and upper multiplexed arm is achieved during half-cycle operation; when phase j is in lower arm mode, T... j1 and S j1 Shutdown, T j2 or S j2 The conduction, shaping bridge arm and lower multiplexed bridge arm half-cycle energy balance.

[0011] This invention provides a control method for a heterogeneous multiplexed modular multilevel converter topology. The method further includes: introducing an energy balance angle α to balance the energy of the shaping arm and the multiplexing circuit during half a power frequency cycle, so that the current i j =I m sin(ωt+φ j () is defined as the reference phase: ; Where ω is the angular frequency, φ j These correspond to the phases of phase j; in upper bridge arm mode: current flows through T j1 Or S j1 The same applies when reusing the lower arm mode.

[0012] According to claim 6, a control method for a heterogeneous multiplexed modular multilevel converter topology is characterized in that the capacitor voltage V of the upper multiplexed bridge arm is... u and the capacitor voltage V of the lower multiplexed bridge arm l For V dc / 2,V dc This refers to the DC bus voltage, and the voltage before and after the phase current crosses zero. Phased charge and discharge energy balance, signal S of all controlled devices in the upper multiplexing circuit. j1 The fully controlled device signal S in the multiplexing circuit j2 The expression is: ; When phase A is reused, the energy fluctuation balance of the reused bridge arm within half a cycle can be expressed as: ; Wherein, ΔWu It is the half-cycle energy fluctuation of the upper multiplexed bridge arm, ΔW l It is the half-cycle energy fluctuation of the lower multiplexed bridge arm; The AC-side shaping arm achieves energy balance during half the power frequency cycle, and the reference voltage V of the j-phase shaping arm... jref for: ; When the phase-shaping bridge arm is in upper bridge arm mode, it reuses the bridge arm energy fluctuation balance within half a cycle, which is expressed as: .

[0013] Compared with the prior art, the present invention has the following beneficial effects:

[0014] 1. The present invention includes two multiplexing circuits and a three-phase shaping bridge arm; the shaping bridge arm works in conjunction with the multiplexing circuit to shape the output voltage within the power frequency cycle, thereby achieving a wide range of voltage input and output.

[0015] 2. The multiplexing circuit composed of the constructed three-phase switching bridge and multiplexing bridge arm can significantly reduce the number of power devices required for the topology and improve economic efficiency;

[0016] 3. The shaping bridge arm is composed of cascaded adjustable reverse voltage sub-modules (full bridge sub-module, diode clamping sub-module and clamping dual capacitor sub-module, etc.), which can effectively realize AC and DC fault isolation and improve operational reliability.

[0017] 4. When the MMC topology of this invention is working, the bridge arm selection switch and the bridge arm guiding switch switch switch every half power frequency cycle. At the same time, the shaping bridge arm is periodically reused, and the reused bridge arm is three-phase multiplexed, which greatly reduces the number of MMC topology submodules used, thereby reducing the size of the MMC and increasing the power density of the MMC. Attached Figure Description

[0018] Figure 1 This is a topology diagram of a heterogeneous multiplexed modular multilevel converter provided in an embodiment of the present invention;

[0019] Figure 2 In a heterogeneous multiplexed modular multilevel converter provided in an embodiment of the present invention, T j1 and T j2 Topological graph;

[0020] Figure 3 This is a multiplexed bridge arm topology diagram provided in an embodiment of the present invention for a heterogeneous multiplexed modular multilevel converter;

[0021] Figure 4 This is a specific heterogeneous multiplexed modular multilevel converter topology provided in an embodiment of the present invention;

[0022] Figure 5 (a) and (b) Three-dimensional relationship with two different expressions under different power factor angles φ and modulation intensities m;

[0023] Figure 6 (a) and (b) are waveforms of the A-phase shaping bridge arm modulation voltage plus multiplexed bridge arm voltage, A-phase AC side current, A-phase AC side voltage, and A-phase shaping bridge arm modulation voltage under different energy balance angles α when the power factor angle φ is less than 0, provided in the embodiments of the present invention.

[0024] Figure 7 (a) and (b) are waveforms of the A-phase shaping bridge arm modulation voltage plus multiplexed bridge arm voltage, A-phase AC side current, A-phase AC side voltage, and A-phase shaping bridge arm modulation voltage under different energy balance angles α when the power factor angle φ is greater than 0, provided in the embodiments of the present invention.

[0025] Figure 8 This is a block diagram of an energy balance control strategy for a heterogeneous multiplexed modular multilevel converter provided in an embodiment of the present invention.

[0026] Figure 9 The number n of shaping bridge arms provided in this embodiment of the invention is... c The relationship between modulation index m and power factor angle φ.

[0027] Figure 10 When the modulation ratio is 0.9 in the embodiment of the invention, the modulation voltage V of the A-phase shaping bridge arm under this modulation method is... aref Phase A shaping bridge arm voltage V a AC three-phase current v j The capacitor voltage V of the two multiplexed bridge arms u With V l and the voltage V of each submodule of phase A aP The simulation results are shown in the figure. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only some, not all, of the embodiments of this invention.

[0029] This invention proposes a heterogeneous multiplexed modular multilevel converter topology, such as... Figure 1As shown, it includes: phase A shaping bridge arm, phase B shaping bridge arm, phase C shaping bridge arm, upper multiplexing circuit, and lower multiplexing circuit. In the topology proposed in this application, the three-phase AC side shaping bridge arm and the two DC side multiplexing circuits constitute the core architecture. The multiplexing circuit is composed of a heterogeneous combination of a three-phase switching bridge and multiplexing bridge arms, wherein the signal corresponding to each switch in phase j of the three-phase switching transistors is T. j1 and S j2 T j2 and S j2 T j1 and T j2 The corresponding switching device can be a fully controlled device, a partially controlled device, or an uncontrolled device, such as... Figure 2 As shown. In this embodiment, T j1 and T j2 Select uncontrolled diodes and choose large capacitors for the multiplexed bridge arms. Figure 3 The AC side shaping arm is selected as a full-bridge submodule. Based on the above selection, a topology principle analysis is carried out, and the principle analysis of other device-based topologies is similar. Through the energy balance angle α modulation strategy, the multiplexed arm is alternately multiplexed by the three-phase shaping arm within half a power frequency cycle. This heterogeneous combination and cyclic multiplexing collaborative working mechanism works together to provide stable voltage support and energy balance regulation for the system. Based on the heterogeneity of the device composition and the efficient multiplexing characteristics of the arm resources, this topology is named "A Heterogeneous Multiplexed Modular Multilevel Converter Topology". This name reflects both its physical structure of heterogeneous device combination and modular core attributes, and accurately describes the cross-phase cyclic and dynamic commissioning operation mechanism of the multiplexed arm.

[0030] Figure 4 In the topology shown, the upper multiplexing circuit and the lower multiplexing circuit have the same structure, both including a three-phase switching bridge and capacitors. Any one phase of the switching bridge in the multiplexing circuit consists of one set of switching devices (fully controlled, half-controlled, or uncontrolled devices) and another set of fully controlled devices. In the upper multiplexing circuit, phase j S... j1 The lower ends of the signal switching devices are connected to the negative terminal of the upper multiplexing bridge arm, and their upper ends are connected to the negative terminal of the corresponding phase shaping bridge arm; in the upper multiplexing circuit, T j1 The upper ends of the switching devices for the signal are all connected to the positive terminal of the upper multiplexing bridge arm, and their lower ends are all connected to the negative terminal of the corresponding phase shaping bridge arm; in the lower multiplexing circuit, phase j S j2 The upper ends of the switching devices of the signal are connected to the positive terminal of the lower multiplexed bridge arm, and their lower ends are connected to the negative terminal of the corresponding phase shaping bridge arm; in the lower multiplexed bridge arm, phase T of phase j... j2 The upper ends of the switching devices of the signal are all connected to the negative terminal of the corresponding phase shaping bridge arm, and their lower ends are all connected to the negative terminal of the lower multiplexing bridge arm; the positive terminal of the upper multiplexing bridge arm is connected to the positive terminal of the DC side, and the negative terminal of the lower multiplexing bridge arm is connected to the negative terminal of the DC side.

[0031] With phase A AC side current i a As a reference phase, the three-phase AC side voltage v j With current i j The expression is:

[0032]

[0033] Where I m Vm is the maximum value of the AC side current, Vm is the maximum value of the AC side voltage, φ is the power factor angle, and ω is the angular frequency. The expression for the modulation index m is defined as:

[0034]

[0035] The value of m ranges from 0 to 1.

[0036] The working principle of this topology is to introduce an energy balance angle α to achieve energy balance between the shaped arm and the multiplexed arm during the half-power frequency cycle, thereby enabling energy balance to be achieved in both arm modes during the half-power frequency cycle.

[0037]

[0038] φ j These correspond to the phases of phase j. In upper bridge arm mode: current flows through T. j1 Or S j1 When the j-phase current i j >0 and S j1 =0, current flows through T j1 Switch to S at this time j1 =1, then the capacitor voltage of the upper multiplexed bridge arm immediately clamps the j-phase diode T. j1 The voltage drop across the diode causes it to cut off momentarily, and the current switches to flow through S. j1 The same applies when reusing the lower arm mode. When α is greater than 0, the energy balance of this topology can be described as follows: through the energy balance angle α, in the upper arm mode... Time: j-phase alternating current i j Managing the energy of the J-phase shaping bridge arm; S j1 In phase j alternating current i j The α phase before and after the zero point is When the circuit is activated, the j-phase alternating current i j The energy of the J-phase shaping arm and the upper multiplexed arm is managed. The J-phase shaping arm achieves energy balance during the two-stage charging and discharging, and the upper multiplexed arm... Phase energy balance: The working principle of the current bridge arm mode is the same as that of the upper bridge arm mode; when α is less than 0, the energy balance method of this topology can be described as follows: Through the energy balance angle α, in the upper bridge arm mode... Time: j-phase alternating current i jManaging the energy of the J-phase shaping bridge arm; S j1 In phase j alternating current i j The α phase before and after the zero point is When the circuit is activated, the j-phase alternating current i j The energy of the J-phase shaping arm and the upper multiplexed arm is managed. The J-phase shaping arm achieves energy balance during the two-stage charging and discharging, and the upper multiplexed arm... During the phase of energy balance, the current bridge arm mode works in the same way as the upper bridge arm mode.

[0039] In summary, the switching states can be obtained as follows (for ease of describing the switching states of uncontrolled devices, the state of a device is considered to be 1 when current flows through it, and 0 otherwise. Analysis shows that for the four switching devices of phase j, exactly one of them is 1 at any given time):

[0040]

[0041] The capacitor voltage V of the upper multiplexed bridge arm u and the capacitor voltage V of the lower multiplexed bridge arm l All are V dc / 2. Using the above expressions for the switching devices and Kirchhoff's voltage law, the modulation voltage V of the j-phase shaping bridge arm can be obtained. jref The expression is:

[0042]

[0043] Depend on Figure 4 It can be known that the current of the j-phase shaping bridge arm is the AC side current i. j For multiplexed bridge arms, ensuring the conduction time of all controlled devices in phase j is symmetrical about the current zero-crossing point when the multiplexed bridge arm is reused in phase j can achieve energy balance of the multiplexed bridge arm. Instantaneous power is the product of voltage and current, and the bridge arm energy fluctuation is the integral of instantaneous power. Given the bridge arm modulation voltage and bridge arm current from the above analysis, the shaping bridge arm energy fluctuation W for phase j within half a cycle is... j It must be zero to obtain:

[0044]

[0045] Solving the above equation, we can derive the relationship between α, m, and φ as follows:

[0046]

[0047] Clearly, the angle α changes dynamically with different values ​​of m and φ. For the same m and φ, the expression for α depends on the relationship between α and 0, and can be divided into two cases: one where α is greater than 0, and the other where α is less than 0. The precise expression for α is:

[0048]

[0049] When the power balance angle α satisfies the above condition, the energy of the shaping bridge arm is balanced during half-power frequency cycle. The relationship between α and m, φ is as follows: Figure 5 As shown. When α is greater than 0, the maximum value of α is π / 2, as... Figure 5 As shown in (a), as φ increases in the range [-π / 2, 0], α decreases, and as φ increases in the range [0, π / 2], α increases; conversely, when α is less than 0, the minimum value of α is -π / 2, as... Figure 5 As shown in (b), α increases as φ increases in the range [-π / 2, 0], and decreases as φ increases in the range [0, π / 2]. From the above equation or Figure 5 It can be seen that α is symmetrically distributed about φ=0 in both cases. In summary, for the same m and φ, there are two different α angles that can achieve energy balance of the reused bridge arm within half a cycle.

[0050] Based on the above analysis, α has two different expressions for the same φ angle. The value of α directly affects the bridge arm voltage V. jref The magnitude of α affects the number of SMs and the configuration of switching devices. Therefore, this paper analyzes the impact of two different selections of α on device selection. Analyzing the expression for α, the absolute value of α is always greater than φ. In this embodiment, phase A is used as an example to analyze the relevant parameters of phase A. The other two phases are similar, and the results can be obtained. Figure 6 and Figure 7 The effect of different α values ​​on the modulation voltage of phase A bridge arm under different φ angles, where the phase A shaping bridge arm voltage + multiplexed bridge arm capacitor voltage refers to the voltage when in upper bridge arm mode and S a1 When = 1, the voltage of the A-phase shaping arm plus the capacitor voltage of the multiplexed arm is V. aref -V dc / 2, when in lower bridge arm mode and S a2 When = 1, the voltage of the A-phase shaping arm plus the capacitor voltage of the multiplexed arm is V. aref +V dc / 2, that is, the expression for the sum of the A-phase shaping arm voltage and the multiplexed arm voltage within one cycle is:

[0051]

[0052] Combination Figure 6 and Figure 7 From the formula for α, we know that when φ∈[-π / 2,π / 2], the voltage V of phase A bridge arm is... aref The two different α values ​​only affect the sum of the capacitor voltage of the multiplexed arm and the voltage of the shaping arm (at this time, the capacitor voltage of the multiplexed arm means when S...). j1When =1, the voltage of the j-phase multiplexed bridge arm is V. dc / 2; when S j2 When = 1, the capacitor voltage of the j-phase multiplexed bridge arm is -V. dc / 2; the capacitor voltage of the j-phase multiplexed bridge arm is 0 at other times), which only affects the selection of the multiplexing time (when α is greater than 0, the multiplexing time of the upper bridge arm mode is π-α≤ωt≤π+α and the multiplexing time of the lower bridge arm mode is 2π-α≤ωt≤2π+α; when α is less than 0, the multiplexing time of the upper bridge arm mode is α≤ωt≤-α and the multiplexing time of the lower bridge arm mode is π+α≤ωt≤π-α), and has no effect on the bridge arm voltage. Therefore, we can take... And define this expression as the energy equation.

[0053] The energy balance control strategy of this topology is as follows: Figure 8 As shown, the reference value α of the current energy balance angle is calculated using the modulation index m and the power factor angle φ. ref Then, the average submodule voltage V of the j-phase shaping bridge arm. smj_ave Reference value V of the voltage of phase j submodule smj_ref The fine-tuning amount Δα1 of the energy balance angle is obtained through closed-loop control. The effect of the fine-tuning amount Δα1 on the energy balance angle differs between rectifier and inverter operating conditions. The fine-tuning amount Δα1 is superimposed under inverter operating conditions, while the fine-tuning amount Δα1 superimposed under rectifier operating conditions is then compared with the reference value α of the energy balance angle. ref The superposition is used to determine the upper or lower bridge arm mode. The turn-off time of the switching transistor is finely adjusted via a micro-adjustment Δα1, thereby achieving closed-loop control of the submodule capacitors of the j-phase shaping bridge arm to maintain energy balance. The capacitor voltage V of the upper multiplexed bridge arm is used as the control factor. u The capacitor reference voltage V of the upper multiplexed bridge arm u_ref The fine-tuning amount Δα2 of the energy balance angle and the capacitor voltage V of the lower multiplexed bridge arm are obtained through closed-loop control. l The capacitor reference voltage V of the lower multiplexed bridge arm l_ref The fine adjustment Δα3 of the energy balance angle is obtained through closed-loop control, and then compared with the reference value α of the energy balance angle. ref This superposition affects the conduction timing of both the upper and lower fully controlled devices. By fine-tuning the discharge timing of the capacitors in the multiplexed bridge arms using adjustments Δα2 and Δα3, closed-loop control is achieved to maintain energy balance between the capacitors in the upper and lower multiplexed bridge arms. The actual switching state expression at this time is:

[0054]

[0055] The above formula is a logical timing judgment.

[0056] Figure 10 In this embodiment of the invention, when the modulation ratio is 0.9, the power P = 1MW, and V m 9kV was selected. dcA 20kV bridge was selected, with 6 sub-modules per phase shaping arm, and the capacitor voltage V of each multiplexed bridge arm was selected. dc / 2, the reference voltage V for each submodule can be obtained through calculation. sm =1666.7V, then the voltage of the multiplexed bridge arm capacitor V u= V l =10kV, the capacitance of the submodule capacitor and the capacitance of the multiplexed bridge arm are both 3mF, and the power factor angle cosφ=1. Under this modulation method, the A-phase shaping bridge arm modulation voltage V aref Phase A shaping bridge arm voltage V a AC side three-phase current i a i b i c The voltage V of the two multiplexed bridge arm capacitors u With V l The voltage V of the six sub-modules of the A-phase shaping bridge arm ap Simulation results are shown in the graph. The vertical axis represents the corresponding voltage / current values ​​in volts per ampere, and the horizontal axis represents the simulation time in seconds. Figure 10 This demonstrates that the feasibility of the topology has been verified.

[0057] Derivation of the selection of the number of shaping bridge arm sub-modules.

[0058] Taking phase A as an example. Since the energy of the phase A shaping arm is balanced over half a power frequency cycle, only half a power frequency cycle needs to be analyzed. The modulation voltage V of the phase A shaping arm... aref for:

[0059]

[0060] There is also phase A AC side voltage v a :

[0061]

[0062] Where V m =mV dc / 2, the number of sub-modules n in a single bridge arm of a traditional MMC can be expressed as:

[0063]

[0064] As previously analyzed, the two possible values ​​of α do not affect the modulation voltage and current of the shaping arm, but only the multiplexing time of the multiplexed arm. The relationship between the multiplexed arm capacitor current at the two times is as follows: Figure 6 and Figure 7 It can be seen that this is consistent, therefore this analysis only selects the case where α is a positive number.

[0065]

[0066] Therefore, the number of sub-modules required for the shaping bridge arm is n. c This can be used to determine:

[0067]

[0068] From the above equations, we can obtain n c The variation law of m and φ is as follows Figure 9 As shown in the figure. This 3D plot clearly illustrates n c The trend of m∈[0,1] and φ∈[-π / 2,π / 2]. c The image is symmetrical about φ=0. When m=0.55 and φ=±0.0321, the number of shaping bridge arm sub-modules reaches the minimum value of 0.255n; while when φ=±π / 2, the number of shaping bridge arm sub-modules reaches the maximum value of 0.5n.

[0069] The selection of the shaping bridge arm submodule and the capacitance value of the multiplexed bridge arm capacitor is derived.

[0070] Since the three phases are symmetrical, taking phase A as an example, the capacitance value of the submodule reflects the volume and power density of the submodule. According to Kcl's law, the shaping arm current of phase A can be obtained as follows:

[0071]

[0072] Since this design is a half-power frequency cycle energy balance, we only need to consider α≤ωt≤π+α. Furthermore, as previously analyzed, the two possible values ​​of α do not affect the modulation voltage and current of the shaping arm, but only the reuse time of the multiplexed arm. The relationship between the multiplexed arm capacitor current at the two times is as follows: Figure 6 and Figure 7 It can be seen that this is consistent, therefore this analysis only selects the case where α is a positive number.

[0073]

[0074] The energy accumulation in the bridge arm is the bridge arm voltage V. aref With bridge arm current i a The product is the bridge arm power P. A The integral is expressed as follows:

[0075]

[0076] As can be seen from the above formula, the integral quantity has four processes within half a power frequency cycle: discharge-charge-discharge-charge.

[0077] Determining a specific volatility expression is quite difficult. Therefore, this design derives a rough mathematical expression. When the specific operating conditions are determined, the response parameters can be selected by substituting the derived expression into the expression.

[0078] The energy fluctuation of phase A bridge arm is equal to the bridge arm energy W.a Maximum value minus bridge arm energy W a The minimum value is expressed as:

[0079]

[0080] For the capacitance analysis of the multiplexed bridge arms, since the upper and lower multiplexed bridge arms are symmetrical, this design only analyzes the capacitance of the upper multiplexed bridge arm.

[0081] According to Kirchhoff's current law, the current i of the upper multiplexed bridge arm capacitor is... u The expression for the power frequency period is:

[0082]

[0083] Based on the previous analysis:

[0084]

[0085] From the above equation, it can be seen that the capacitor of the multiplexed bridge arm is in energy balance during 1 / 3 of the power frequency cycle. Therefore, analysis only requires selecting π-α≤ωt≤π+α. Since the capacitor voltage V of the multiplexed bridge arm... u The current i in the upper multiplexed bridge arm is constant and greater than 0, therefore when π-α≤ωt≤π, the current i in the upper multiplexed bridge arm is constant. u =i a >0 discharge, when π≤ωt≤π+α, the capacitor current i of the upper multiplexed bridge arm u =i a <0 charging. Therefore, the energy fluctuation of the upper multiplexed arm is equal to the energy W of the upper multiplexed arm. u The maximum value minus the upper multiplexed bridge arm W u The minimum value is expressed as:

[0086]

[0087] Based on the relationship between the maximum volatility of the submodule and the energy volatility, the maximum energy storage volatility per phase arm is... and the maximum fluctuation of energy storage in the reusable bridge arm It can be represented as

[0088]

[0089] Where n c V represents the number of submodules in each phase arm. smref V is the reference voltage for the shaping bridge arm submodule. u To reuse the capacitor reference voltage of the bridge arm, C csm It is the submodule capacitance value of each phase arm, C. m Here, ε1 represents the capacitance value of the multiplexed bridge arm, and ε2 represents the voltage fluctuation rate of the capacitor in the multiplexed bridge arm. Therefore, the capacitance value C of the shaping bridge arm SM is...csm The capacitance value C of the multiplexed bridge arm m The selection criteria can be expressed as follows:

[0090]

[0091] Based on the fact that the capacitance fluctuation of the sub-modules and the capacitance fluctuation of the reused bridge arms are both within 5%, it can be seen that the shaping bridge arm capacitance of this topology is about 0.4mF and the reused bridge arm capacitance is about 69uF. Compared with the traditional MMC, the number of sub-modules and capacitance requirements are significantly reduced, the capacitor volume is significantly reduced, and the economic advantages are improved.

[0092] The embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A heterogeneous multiplexed modular multilevel converter topology, characterized in that, The topology includes a three-phase shaping bridge arm and two multiplexing circuits; the three-phase shaping bridge arms are each composed of cascaded adjustable reverse voltage submodules; the multiplexing circuits include an upper multiplexing circuit and a lower multiplexing circuit, both of which include a three-phase switching bridge and a multiplexing bridge arm; T j1 Signal switching devices and S j1 The signal switching devices and the upper multiplexing bridge arm constitute the j-phase upper multiplexing circuit; T j2 Signal switching devices and S j2 The signal switching devices and the lower multiplexing bridge arm constitute a j-phase lower multiplexing circuit, where j is a, b, or c; where T j1 Signal switching devices and T j2 The switching device for the signal can be any one of a fully controlled device, a semi-controlled device, or an uncontrolled device, S j1 Signal switching devices and S j2 The switching devices for the signal are fully controlled devices; the switching devices in the same phase do not conduct simultaneously, i.e., T j1 +T j2+ S j1 +S j2 ≤1; The multiplexed bridge arm is formed by cascading sub-modules or is a capacitor.

2. The heterogeneous multiplexed modular multilevel converter topology according to claim 1, characterized in that, The switching bridge of any phase in the multiplexing circuit is composed of a set of switching devices and another set of fully controlled devices; in the upper multiplexing circuit, phase j S j1 The lower ends of the signal switching devices are connected to the negative terminal of the upper multiplexing bridge arm, and the upper ends are connected to the negative terminal of the corresponding phase shaping bridge arm; in the upper multiplexing circuit, phase T... j1 The upper ends of the switching devices for the signal are all connected to the positive terminal of the upper multiplexing bridge arm, and the lower ends are all connected to the negative terminal of the corresponding phase shaping bridge arm; in the lower multiplexing circuit, phase j S j2 The upper ends of the switching devices for the signal are connected to the positive terminal of the lower multiplexing bridge arm, and the lower ends are connected to the negative terminal of the corresponding phase shaping bridge arm; in the lower multiplexing circuit, phase T... j2 The upper ends of the switching devices of the signal are all connected to the negative terminal of the corresponding phase shaping bridge arm, and the lower ends are all connected to the negative terminal of the lower multiplexing bridge arm; the positive terminal of the upper multiplexing bridge arm is connected to the positive terminal of the DC side, and the negative terminal of the lower multiplexing bridge arm is connected to the negative terminal of the DC side.

3. The heterogeneous multiplexed modular multilevel converter topology according to claim 1, characterized in that, The multiplexing circuit has any phase switching device T j1 and T j2 Composed of any one of the following types of devices connected in series: fully controlled devices, semi-controlled devices, and uncontrolled devices, the multiplexing circuit uses any one phase switching device S. j1 and S j2 It consists of a series of fully controlled components.

4. The heterogeneous multiplexed modular multilevel converter topology according to claim 1, characterized in that, The shaping bridge arm is composed of cascaded sub-modules with adjustable back pressure, and the sub-modules are full-bridge sub-modules, clamped double sub-modules, or cross-type sub-modules, etc.

5. The heterogeneous multiplexed modular multilevel converter topology according to claim 1, characterized in that, The reused bridge arm is a valve string or a large capacitor composed of cascaded sub-modules.

6. A control method for a heterogeneous multiplexed modular multilevel converter topology, used to control the topology as described in any one of claims 1 to 5, characterized in that, The control method is as follows: the voltage is modulated by the j-phase shaping bridge arm and the upper multiplexing circuit or the lower multiplexing circuit together, and the energy balance is satisfied within half of the power frequency cycle; the upper bridge arm mode is defined when the voltage is modulated by the upper j-phase shaping bridge arm and the upper multiplexing circuit together, and the lower bridge arm mode is defined when the voltage is modulated by the upper j-phase shaping bridge arm and the lower multiplexing circuit together.

7. The control method for a heterogeneous multiplexed modular multilevel converter topology according to claim 6, characterized in that, The method also includes: introducing an energy balance angle α to balance the energy of the shaping bridge arm and the multiplexing circuit during half-power frequency cycles, so that the current i j =I m sin(ωt+φ j () is defined as the reference phase: ； Where ω is the angular frequency, φ j These correspond to the phases of phase j respectively; in upper bridge arm mode: current flows through T j1 Or S j1 The same applies when reusing the lower arm mode.

8. The control method for a heterogeneous multiplexed modular multilevel converter topology according to claim 6, characterized in that, The capacitor voltage V of the upper multiplexed bridge arm u and the capacitor voltage V of the lower multiplexed bridge arm l For V dc / 2,V dc This refers to the DC bus voltage, and the voltage before and after the phase current crosses zero. Phased charge and discharge energy balance, signal S of all controlled devices in the upper multiplexing circuit. j1 The fully controlled device signal S in the multiplexing circuit j2 The expression is: ； When phase A is reused, the energy fluctuation balance of the reused bridge arm within half a cycle can be expressed as: ； Wherein, ΔW u It is the half-cycle energy fluctuation of the upper multiplexed bridge arm, ΔW l It is the half-cycle energy fluctuation of the lower multiplexed bridge arm; The AC-side shaping arm achieves energy balance during half the power frequency cycle, and the reference voltage V of the j-phase shaping arm... jref for: ; When the phase-shaping bridge arm is in upper bridge arm mode, it reuses the bridge arm energy fluctuation balance within half a cycle, which is expressed as: 。

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