Construction method and modulation method of three-port multi-frequency MMC topology based on bridge arm multiplexing

Abstract

The invention discloses a construction method and a modulation method of a three-port multi-frequency MMC topology based on bridge arm multiplexing, and relates to the technical field of power electronics. The construction method comprises the steps that sub-modules of a traditional double-bridge-arm MMC are integrated and optimized into a three-bridge-arm structure through a current conversion branch, independent energy interaction of three ports is achieved through an auxiliary bridge arm, multi-frequency operation modes of power frequency, frequency division, direct current and the like are supported at the same time, and therefore the utilization rate of the bridge arm sub-modules is increased, and the energy utilization rate of the MMC is increased. And the overall size and the manufacturing cost of the converter are reduced. Based on three-port multi-frequency coordination control and power balance requirements, a modulation method of the topological structure is provided, a multi-frequency coordination modulation strategy is adopted, frequency offset is performed through an auxiliary bridge arm to reduce cross interference of port frequency, and output waveform quality is improved through multi-objective optimized capacitor voltage balance control. And meanwhile, direct-current side faults are effectively eliminated by means of the auxiliary bridge arm.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a method for constructing and modulating a three-port multi-frequency MMC topology based on bridge arm multiplexing. Background Technology

[0002] The offshore wind power industry is currently experiencing large-scale development and is gradually expanding into deep-sea areas. Against this backdrop, Frequency Division Transmission (FFTS), as a power transmission technology suitable for transmitting deep-sea wind power to onshore areas, has demonstrated significant advantages: compared to conventional power frequency transmission, its long-distance transmission capacity can be increased by three times; compared to DC transmission schemes, it has lower conversion costs and outstanding economic benefits, thus attracting widespread attention from the academic and engineering communities and has been successfully applied in practical projects in Taizhou and Hangzhou. However, in the eastern coastal areas where offshore wind power resources are abundant and electricity loads are highly concentrated, to improve the overall efficiency of wind energy utilization and transmission, deep-sea wind farms based on FFTS not only need to be connected to high-voltage DC transmission systems for long-distance transmission but also need to achieve local consumption within the local power frequency grid. This requires flexible interconnection and efficient energy transmission between multiple frequency systems, and a composite multi-port frequency converter capable of interconnecting DC, power frequency AC, and FFTS ports is the core key equipment for achieving this goal.

[0003] To integrate multiple ports with different frequencies, multiple two-port converters typically share an AC / DC bus. Among inverter topologies capable of AC-DC conversion, the Multi-Module Converter (MMC) is a commonly used topology in flexible DC transmission projects. It uses submodule cascading to increase voltage levels, but in high-voltage applications, it suffers from a large number of submodules, high conversion costs, and large size. Furthermore, only half of the submodules in an MMC are active at any given time during normal operation, with the other half idle in bypass mode, resulting in low submodule utilization. Moreover, since commonly used half-bridge MMCs lack fault-blocking capability, half-bridge submodules need to be replaced with full-bridge submodules to achieve fault blocking in the event of a DC-side fault, further increasing the number of components and cost. Current research often employs hybrid MMCs, combining half-bridge and full-bridge submodules, to reduce the number of power devices while retaining DC fault-blocking capability. However, in industrial applications, the proportion of full-bridge submodules in hybrid MMCs is reduced to at most half, still resulting in a significant number of power devices, and the low submodule utilization remains unresolved. Furthermore, the reduction in the proportion of full-bridge submodules has not led to a decrease in the overall number of submodules, nor has it reduced the number of capacitors. In MMCs, capacitors account for 50% to 70% of the total volume and 80% of the total weight, making them the main factors contributing to the large size and weight of MMCs. Therefore, the capacitor utilization rate and the number of power devices in current MMCs are both low.

[0004] Common frequency converter topologies capable of AC-AC conversion include matrix converters, hexagonal MMCs, nonagonal MMCs, and Y-MMCs. Matrix converters offer high power density, require no intermediate DC link, and can achieve AC-AC conversion, but suffer from high coupling on both sides, leading to complex control strategies. Hexagonal MMCs achieve AC-AC conversion using only six bridge arms, but their topology dictates only one internal circulating current channel. Without additional compensation equipment, strong constraints on reactive power and circulating current must be satisfied on both sides, making control strategies difficult to implement. Nonagonal MMCs, as an extension of hexagonal MMCs, can achieve AC-AC-AC conversion, but their control is even more complex. Y-MMCs have a simple structure, no circulating current, and can achieve AC-AC conversion. Due to the inherent limitations of these structures, developing three-port multi-frequency MMC topologies with bridge arm multiplexing characteristics and their corresponding modulation strategies is crucial for overcoming existing technological bottlenecks and promoting the development of multi-energy complementary systems and AC / DC hybrid power grids. Summary of the Invention

[0005] The purpose of this invention is to provide a method for constructing and modulating a three-port multi-frequency MMC topology based on bridge arm multiplexing, aiming to solve or improve at least one of the above-mentioned technical problems.

[0006] To achieve the above objectives, the present invention provides the following solution:

[0007] A method for constructing a three-port multi-frequency MMC topology based on bridge arm multiplexing includes:

[0008] A novel commutation branch is formed by connecting a diode and a thyristor in reverse parallel and connecting them in series with an IGCT. The novel commutation branch is then connected to a traditional MMC topology, which transforms the traditional dual-arm structure into a three-arm structure and reuses the middle arm, resulting in a bridge arm reuse type MMC topology.

[0009] Based on the bridge arm multiplexing type MMC topology, the converter branch is connected to the auxiliary bridge arm to form a three-port multi-frequency MMC topology based on bridge arm multiplexing to realize multi-frequency energy transmission; wherein, each phase auxiliary bridge arm is set to two, and the two auxiliary bridge arms are connected to different frequency ports.

[0010] Optionally, the construction process of the novel converter branch is as follows: connecting two IGCTs connected in reverse series with N-1 diodes and thyristors connected in reverse parallel.

[0011] Optionally, the three-bridge arm structure includes an upper bridge arm, a middle bridge arm, and a lower bridge arm; each bridge arm includes N cascaded half-bridge sub-modules.

[0012] Optionally, the auxiliary bridge arm is constructed by connecting N / 2 full-bridge sub-modules end to end in sequence, with one end connected to the converter branch and the other end connected to the AC port.

[0013] The present invention also provides a modulation method for a three-port multi-frequency MMC topology based on arm multiplexing, which is applied to the MMC topology constructed by the method described above, and includes:

[0014] Based on the three-port multi-frequency MMC topology, a multi-frequency cooperative modulation strategy is adopted for processing. Frequency cancellation is performed through the auxiliary arm to reduce the cross interference of port frequencies, and the output waveform quality is improved through the multi-objective optimized capacitor voltage balance control. At the same time, the DC fault current is cleared by means of the auxiliary arm; wherein, the multi-objective optimized capacitor voltage balance control includes: the nearest level approximation strategy for the arm switching moment, the carrier phase-shifted modulation strategy for other moments, the carrier stacking modulation strategy for the auxiliary arm, the port current control strategy, the DC side voltage control strategy, the power balance control strategy, and the voltage equalization control strategy between units in the arm.

[0015] Optionally, the specific process of the nearest level approximation strategy for the arm switching moment includes:

[0016] First, the belonging of the middle arm is judged according to the quantity relationship between the number of upper-arm sub-modules Np and the number of lower-arm sub-modules Na in each phase of the three-port multi-frequency MMC topology, so as to determine the opening and closing of the two commutation branches; the switching process of the belonging of the middle arm is as follows:

[0017] Based on the nearest level approximation strategy, when Np > Na, commutation branch 1 is turned off and commutation branch 2 is turned on; when Np < Na, commutation branch 1 is turned on and commutation branch 2 is turned off; when Np = Na, commutation branch 1 is turned on and commutation branch 2 is turned on.

[0018] Optionally, the specific process of the port current control strategy includes:

[0019] The AC side port currents are respectively subjected to closed-loop control through two current inner loops, and the two port currents respectively obtain voltage control commands through PI controllers.

[0020] Optionally, the specific process of the DC side voltage control strategy includes:

[0021] The DC side voltage control is placed on the 50 / 3HZ frequency side as the outer loop, and a current control signal is generated through a PI controller.

[0022] Optionally, the specific process of the voltage equalization control strategy between units in the arm is:

[0023] In the bridge arm reuse type MMC topology, the capacitor voltages in the corresponding bridge arms are sorted and charged and discharged according to the direction of the bridge arm current. At the moment of switching on and off of the converter branch, a sub-module of the reused bridge arm is turned on to prevent overvoltage of the converter branch. In the auxiliary bridge arms, the capacitor voltages are sorted and charged and discharged according to the direction of the bridge arm current.

[0024] Optionally, the process of clearing the DC fault current includes:

[0025] In the event of a fault, all IGBTs on the auxiliary bridge arm are locked out, which cuts off the DC-AC energy transmission path and clears the fault current.

[0026] According to specific embodiments provided by the present invention, the present invention discloses the following technical effects:

[0027] This invention discloses a construction method and modulation method for a three-port multi-frequency MMC topology based on bridge arm multiplexing. The construction method includes: integrating the sub-modules of a traditional dual-bridge-arm MMC through the converter branch, optimizing it into a three-bridge-arm structure, and using auxiliary bridge arms to achieve independent energy interaction of the three ports. It simultaneously supports multiple frequency operating modes such as power frequency, frequency division, and DC, thereby improving the utilization rate of the bridge arm sub-modules and reducing the overall size and manufacturing cost of the converter. Based on the requirements of three-port multi-frequency coordinated control and power balance, the proposed modulation method for this topology adopts a multi-frequency cooperative modulation strategy. Frequency cancellation is performed through auxiliary bridge arms to reduce cross-interference at port frequencies, and multi-objective optimized capacitor voltage balance control improves the output waveform quality. Simultaneously, the auxiliary bridge arms are used to effectively clear DC-side faults. Attached Figure Description

[0028] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This is a schematic diagram of the three-port multi-frequency MMC topology based on bridge arm multiplexing in this embodiment;

[0030] Figure 2 This is a diagram of the three-port multi-frequency MMC topology control strategy based on bridge arm multiplexing in this embodiment;

[0031] Figure 3 This is a diagram of the AC side voltage signal of the three-port multi-frequency MMC topology based on bridge arm multiplexing in this embodiment;

[0032] Figure 4This is a diagram of the AC side current signal of the three-port multi-frequency MMC topology based on bridge arm multiplexing in this embodiment;

[0033] Figure 5 This is a diagram of the AC side voltage signal of the three-port multi-frequency MMC topology based on bridge arm multiplexing in this embodiment;

[0034] Figure 6 This is a diagram of the AC side current signal of the three-port multi-frequency MMC topology based on bridge arm multiplexing in this embodiment;

[0035] Figure 7 This is a DC-side voltage diagram of a three-port multi-frequency MMC topology based on bridge arm multiplexing in this embodiment;

[0036] Figure 8 This is a capacitor voltage diagram of a three-port multi-frequency MMC topology based on bridge arm multiplexing in this embodiment;

[0037] Figure 9 This is a power diagram of the AC side 1 of the three-port multi-frequency MMC topology based on bridge arm multiplexing in this embodiment;

[0038] Figure 10 This is a power diagram of the AC side 2 of the three-port multi-frequency MMC topology based on bridge arm multiplexing in this embodiment;

[0039] Figure 11 This is a fault AC side current diagram of the three-port multi-frequency MMC topology based on bridge arm multiplexing in this embodiment.

[0040] Figure 12 This is a fault AC side current diagram of the three-port multi-frequency MMC topology based on bridge arm reuse in this embodiment. Detailed Implementation

[0041] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0042] The purpose of this invention is to provide a method for constructing and modulating a three-port multi-frequency MMC topology based on bridge arm multiplexing, aiming to solve or improve at least one of the above-mentioned technical problems.

[0043] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0044] like Figures 1-12As shown in the figure, the present invention provides a construction method and corresponding modulation method for a three-port multi-frequency MMC topology based on arm multiplexing, which is applicable to the energy interaction scenarios of multi-energy complementary systems and AC / DC hybrid power grids. The specific steps are as follows:

[0045] Step 1: A new commutation branch is formed by connecting a diode and a thyristor in reverse parallel and connecting them in series with an IGCT. The specific structure of the new commutation branch is that two IGCTs connected in reverse series and (N - 1) diodes and thyristors connected in reverse parallel are connected in series. The arm switching switch in the off state needs to bear the output voltage of the multiplexed arm. Therefore, the on-off link is composed of two IGCTs connected in reverse series to control the synchronous on-off of the circuit; the voltage withstand link is composed of (N - 1) thyristors and diodes connected in reverse parallel.

[0046] Step 2: A selection switch composed of two new commutation branches is connected to the traditional MMC topology. Each phase in the MMC is connected to two new commutation branches. One end of the two commutation branches is connected to each other, and the other end is connected to the MMC.

[0047] Step 3: The MMC topology is changed from a two-arm structure to a three-arm structure by two commutation branches and the middle arm is multiplexed. Each phase of the MMC is changed from a hybrid sub-module arm composed of two cascaded 2N sub-modules to a three-arm structure composed of three half-bridge arms with N sub-modules, and is connected to the commutation branch.

[0048] Step 4: An auxiliary arm composed of N / 2 full-bridge modules is connected to the commutation branch to form a three-port multi-frequency MMC topology based on arm multiplexing to achieve multi-frequency energy transmission. Among them, there are two auxiliary arms in each phase and they are connected to the connection point with the commutation branch, and the two auxiliary arms are connected to different frequency ports.

[0049] Step 5: Build a modulation strategy suitable for the topology of the three-port multi-frequency MMC based on arm multiplexing obtained in Step 4;

[0050] Judge the ownership of the middle arm by the quantity relationship between the number of sub-modules Np on the upper arm and the number of sub-modules Na on the lower arm of each phase of the MMC, so as to determine the opening and closing of the two commutation branches. For example, when Np > Na, commutation branch 1 is turned off and commutation branch 2 is turned on; when Np < Na, commutation branch 1 is turned on and commutation branch 2 is turned off; when Np = Na, commutation branch 1 is turned on and commutation branch 2 is turned on. Control the opening and closing of the three-arm sub-modules through the nearest level approximation strategy (NLM).

[0051] Step 6: Match the modulation strategy of the three-port multi-frequency MMC topology based on arm multiplexing obtained in Step 5 with appropriate port current control, DC side voltage control, power balance control, and voltage equalization control between units in the arm.

[0052] The port current control is as follows: the AC side port current is controlled in a closed loop through two inner current loops, and the voltage control command for each port current is obtained through a PI controller.

[0053] The DC-side voltage control is as follows: the DC-side voltage control is placed on the 50 / 3HZ frequency side as the outer loop, and a current signal is generated by the PI controller.

[0054] Power balance control is achieved by controlling the current command at one end and simultaneously adding front-end direct voltage control to feed energy to the grid, thereby achieving power balance.

[0055] The voltage equalization control between bridge arms is as follows: For the main bridge arms, the withstand voltage of the converter branch needs to be considered. Therefore, according to the assigned intermediate bridge arms, the capacitor voltages within each bridge arm are sorted, and charging and discharging are performed according to the direction of the bridge arm current. At the moment the converter branch is turned on and off, a sub-module is reused to prevent overvoltage in the converter branch. For the auxiliary bridge arms, the capacitor voltages are sorted, and charging and discharging are performed according to the direction of the bridge arm current.

[0056] Step 7: Adapt a suitable fault clearing scheme based on the three-port multi-frequency MMC topology modulation strategy obtained in Step 6, which is based on bridge arm multiplexing. During a fault, all IGBTs on the auxiliary bridge arm are locked, the DC-AC side energy transmission path is cut off, the fault current is cleared, and the system can quickly return to steady state after the fault is recovered.

[0057] To illustrate the solution of this invention, the proposed method was verified on the MATLAB experimental platform. The system parameter settings used in the simulation are shown in Table 1.

[0058] Table 1 Simulation System Parameter Settings

[0059]

[0060]

[0061] like Figure 1 As shown, in a three-port multi-frequency MMC topology based on bridge arm multiplexing, each phase has three bridge arms (upper, middle, and lower) and two auxiliary bridge arms. The upper, middle, and lower bridge arms have N half-bridge submodules and an inductor L, while the auxiliary bridge arms have N / 2 full-bridge submodules. The DC-side bus voltage is U. dcThe half-bridge submodule contains two IGBTs and two diodes connected in anti-parallel, along with a parallel energy storage capacitor. Controlling the IGBTs' on / off states controls the circuit connection or disconnection of each submodule. The submodule can output two voltage levels: 0 and +Uc. The full-bridge submodule contains four IGBTs and four diodes connected in anti-parallel, along with a parallel energy storage capacitor. Controlling the IGBTs' on / off states controls the circuit connection or disconnection of each submodule. The submodule can output three voltage levels: 0, +Uc, and -Uc. Multiplexing the intermediate bridge arms maintains a constant number of connected submodules (2N), ensuring stable capacitor voltage and DC voltage. The auxiliary bridge arm on this side cancels out frequency components from the other side to obtain the voltage and current on this side.

[0062] like Figure 2 As shown, it is a three-port multi-frequency MMC control diagram based on bridge arm multiplexing. The three-port multi-frequency MMC based on bridge arm multiplexing is decoupled and modulated by control methods such as power outer loop, current inner loop, direct voltage loop, and capacitor voltage equalization.

[0063] like Figure 3 As shown, when AC side 1 operates at 50 / 3Hz, without additional filtering, the AC voltage exhibits good sinusoidal characteristics with an amplitude of 35kV.

[0064] like Figure 4 As shown, when AC side 1 operates at 50 / 3Hz, without additional filtering devices, the AC current is a sine wave with relatively good waveform quality and an amplitude of 715A.

[0065] like Figure 5 As shown, when AC side 2 operates at 50Hz, without additional filtering devices, the AC voltage exhibits good sinusoidal characteristics with an amplitude of 35kV.

[0066] like Figure 6 As shown, when AC side 2 operates at 50Hz without additional filtering, the AC current is a sine wave with good waveform quality and an amplitude of 2800A.

[0067] like Figure 7 As shown, the DC side voltage is 140kV, and the waveform fluctuation range is small, approximating a straight line.

[0068] like Figure 8 The figure shows the average voltage of the bridge arm capacitors. It can be seen that the capacitor voltage fluctuates around 11.45kV with a small fluctuation range, achieving a good voltage equalization effect.

[0069] like Figure 9The figure shows the power waveform on the AC side 1. It can be seen that the active power fluctuates around 30MW with a small fluctuation range, and the reactive power fluctuates around 0MW with a small fluctuation range.

[0070] like Figure 10 The figure shows the AC side power waveform 2. It can be seen that the active power fluctuates around 100MW with a small fluctuation range, and the reactive power fluctuates around 0MW with a small fluctuation range.

[0071] like Figure 11 The figure shows the current waveform of AC side 1 when there is a fault. It can be seen that the current drops to 0 after the fault occurs in 0.5s, and the current quickly returns to steady state after the fault is restored in 0.7s.

[0072] like Figure 12 The figure shows the current waveform of AC side 1 when AC side 2 is faulty. It can be seen that the current drops to 0 after the fault occurs in 0.5s, and the current quickly returns to steady state after the fault is restored in 0.7s.

[0073] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0074] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A construction method of a three-port multi-frequency MMC topology based on bridge arm multiplexing, characterized in that, The application relates to a novel converter branch, a bridge-arm-reused MMC topology structure, a three-port multi-frequency MMC topology structure, and a multi-frequency collaborative modulation strategy. The novel converter branch is formed by connecting a diode and a thyristor in reverse parallel connection and connecting the diode and the thyristor in series with an IGCT. The three-bridge-arm structure comprises an upper bridge arm, an intermediate bridge arm and a lower bridge arm; each bridge arm comprises N cascaded half-bridge submodules.

2. The construction method of three-port multi-frequency MMC topology based on bridge arm multiplexing according to claim 1, characterized in that, The auxiliary bridge arm is formed by connecting N / 2 full-bridge submodules in sequence in a head-to-tail mode, with one end connected to the converter branch and the other end connected to an alternating-current port.

3. The method of claim 1, wherein, The application relates to a novel converter branch, a bridge-arm-reused MMC topology structure, a three-port multi-frequency MMC topology structure, and a multi-frequency collaborative modulation strategy.

4. The construction method of three-port multi-frequency MMC topology based on bridge arm multiplexing according to claim 1, characterized in that, The multi-objective optimization capacitor voltage balance control comprises a nearest level approximation strategy for bridge-arm switching time, a carrier phase-shift modulation strategy for other time, a carrier layering modulation strategy for the auxiliary bridge arm, a port current control strategy, a direct-current side voltage control strategy, a power balance control strategy and a voltage balance control strategy between units in the bridge arm.

5. A modulation method of a three-port multi-frequency MMC topology based on bridge arm multiplexing, applied to an MMC topology constructed by the method in any one of claims 1-4, characterized in that, The nearest level approximation strategy for bridge-arm switching time comprises the following steps: The number relationship between the number Np of the upper bridge arm submodules and the number Na of the lower bridge arm submodules in each phase of the three-port multi-frequency MMC topology structure is used to determine the attribution of the intermediate bridge arm, so as to determine the opening and closing of the two converter branches.

6. The modulation method of three-port multi-frequency MMC topology based on bridge arm multiplexing according to claim 5, characterized in that, When Np>Na, converter branch 1 is closed and converter branch 2 is opened; when Np<Na, converter branch 1 is opened and converter branch 2 is closed; and when Np=Na, converter branch 1 is opened and converter branch 2 is opened. The port current control strategy comprises the following steps: Two current inner loops are used to respectively close-loop control the alternating-current side port currents, and the two port currents respectively pass through PI controllers to obtain voltage control instructions.

7. The modulation method of three-port multi-frequency MMC topology based on bridge arm multiplexing according to claim 5, characterized in that, The direct-current side voltage control strategy comprises the following steps: The direct-current side voltage control is placed on the 50 / 3Hz frequency side as an outer loop, and a current control signal is generated through a PI controller.

8. The modulation method of three-port multi-frequency MMC topology based on bridge arm multiplexing according to claim 5, characterized in that, The voltage balance control strategy between units in the bridge arm comprises the following steps: ​ 9. The modulation method of three-port multi-frequency MMC topology based on bridge arm multiplexing according to claim 5, characterized in that, ​ In the bridge arm multiplexing MMC topology, the capacitor voltage in the corresponding bridge arm is sorted, and charging and discharging are performed according to the bridge arm current direction, and a sub-module of the multiplexing bridge arm is turned on at the turn-on and turn-off moment of the commutation branch, so as to prevent overvoltage of the commutation branch; in the auxiliary bridge arm, the capacitor voltage is sorted, and charging and discharging are performed according to the bridge arm current direction.

10. The modulation method of three-port multi-frequency MMC topology based on bridge arm multiplexing according to claim 5, characterized in that, The clearing process of the DC fault current comprises: When a fault occurs, all IGBTs on the auxiliary bridge arm are locked, at this time the energy transmission path of the DC-AC side is cut off, and the fault current is cleared.

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