A high-power-density three-phase multiplex submodule type MMC topology and modulation method

Abstract

The application discloses a high-power-density three-phase multiplex sub-module type MMC topology and a modulation method, and belongs to the technical field of power generation, power transformation or power distribution. The application designs a three-phase multiplex sub-module composed of a three-phase half-bridge circuit and a parallel capacitor, each phase comprising three series switches of upper, middle and lower switches, wherein the upper switch and the lower switch adopt the same driving signal, and the middle switch signal is complementary to the upper switch and the lower switch. The MMC topology structure comprises upper and lower conventional bridge arms and upper and lower multiplex bridge arms, wherein the multiplex bridge arms are composed of three-phase multiplex sub-modules in series. The working states of the same-phase half-bridge switches of all three-phase multiplex sub-modules in the multiplex bridge arm are consistent, that is, all three-phase multiplex sub-modules are simultaneously multiplexed and cut off. Based on the topology structure, a modulation method is provided, in which the multiplex bridge arm is frequency-cycled by the conventional bridge arm and the energy balance of the multiplex bridge arm is realized, the [0, 1] modulation of the proposed topology can be realized, compared with the traditional MMC, the number of capacitors can be reduced by nearly 1 / 3, and the light weight level of the device is greatly improved.

Description

TECHNICAL FIELD

[0001] The application relates to a few-capacitor MMC topology, and particularly discloses a high-power-density three-phase multiplexing sub-module (TMSM) type MMC topology and a modulation method, relates to the power system technology, and belongs to the technical field of power generation, power transformation or power distribution. BACKGROUND

[0002] In recent years, the modular multilevel converter (MMC) topology structure has shown the advantages of modularity, scalability and low harmonic content, and promoted its application in wind power, photovoltaic and other new energy gathering and sending occasions. At present, China has also built flexible HVDC transmission projects represented by Zhangbei and Chongqing-Hubei based on the MMC topology. However, in actual application, the semi-bridge MMC and full-bridge MMC, as the main representatives of the flexible HVDC converter station, adopt a large number of power devices and capacitors, and among them, the sub-modules occupy most of the volume of the MMC, causing problems such as large land occupation, large weight and large operation loss of the converter station, which particularly limits the construction and development of the offshore converter platform based on the MMC.

[0003] At present, the MMC topology has the following two basic forms: one is a three-phase series type topology structure, which uses two-level voltage type converters and sub-module valve strings to construct a three-phase parallel hybrid MMC, which can reduce the number of a large number of sub-modules, but it is difficult to ensure the voltage balance of a large number of devices in series; the other is a new type of topology structure constructed by combining the sub-module valve string and the traditional voltage source converter (VSC). This topology structure is constructed by combining the full-bridge sub-module (FBSM) valve string and the two-level VSC, the two-level VSC works at the fundamental frequency, and the FBSM valve string modulation can greatly reduce the number of sub-modules, but the two-level VSC bridge arm needs to be in series for voltage balance.

[0004] In order to reduce the number of sub-modules, using bridge arm multiplexing technology is an effective way to improve the lightness and economy of the existing MMC topology. The patent with publication number CN113595424A proposes a three-phase bridge arm multiplexing MMC topology, which uses sub-module valve strings to construct a multiplex bridge arm. By controlling the parallel three-phase switch bridge, the multiplex bridge arm is time-multiplexed, which improves the lightness level of the traditional MMC. However, the three-phase switch bridge needs a large number of power devices in series to withstand the modulation voltage of the multiplex bridge arm. The static and dynamic voltage sharing technology of existing power devices in series is not mature, which reduces the engineering application feasibility. Therefore, solving the technical problem of static and dynamic voltage sharing of power devices in series in the bridge arm multiplexing MMC topology and exploring a new MMC topology scheme with high lightness and high engineering feasibility will provide a solution for large-scale new energy efficient and low-cost transmission. SUMMARY

[0005] The invention aims to solve the problems of the prior art. The invention provides a three-phase multiplex sub-module MMC topology with high power density and a modulation method. The three-phase multiplex sub-module is connected in series to construct a multiplex bridge arm, reducing the number of capacitors. The energy balance of the multiplex bridge arm is achieved by modulating the three-phase multiplex sub-module to be fully inserted or fully removed. The power devices of the three-phase multiplex sub-module only withstand the capacitor voltage, solving the technical problem of power devices in series in the existing bridge arm multiplexing MMC topology, and achieving the purpose of constructing a few-capacitor MMC topology and controlling the stable operation of the MMC topology.

[0006] To achieve the above invention purpose, the invention adopts the following technical scheme:

[0007] A three-phase multiplex sub-module MMC topology with high power density, comprising: three-phase upper conventional bridge arms, three-phase lower conventional bridge arms, three-phase upper multiplex bridge arms, and three-phase lower multiplex bridge arms.

[0008] Each upper conventional bridge arm is composed of at least one sub-module in series, each lower conventional bridge arm is composed of at least one sub-module in series, the upper end of each lower conventional bridge arm is connected to the lower end of the corresponding upper conventional bridge arm to form a bridge arm midpoint, each upper multiplex bridge arm is composed of at least one three-phase multiplex sub-module in series, the upper end of each upper multiplex bridge arm is connected to the positive electrode of the DC port, the lower end of each upper multiplex bridge arm is connected to the upper end of the corresponding upper conventional bridge arm, each lower multiplex bridge arm is composed of at least one three-phase multiplex sub-module in series, the upper end of each lower multiplex bridge arm is connected to the lower end of the corresponding lower conventional bridge arm, and the lower end of each lower multiplex bridge arm is connected to the negative electrode of the DC port. When the current of any phase conventional bridge arm flows through the corresponding phase multiplex bridge arm, all three-phase multiplex sub-modules are inserted. When the current of any phase conventional bridge arm bypasses the corresponding phase multiplex bridge arm, all three-phase multiplex sub-modules are removed.

[0009] As a further optimization scheme of the high-power-density three-phase multiplex sub-module type MMC topology, the three-phase multiplex sub-module comprises a three-phase switching half-bridge circuit and a multiplex capacitor, any one-phase half-bridge circuit is composed of an upper switch, an intermediate switch and a lower switch connected in series, the current input ends of the upper switches in the half-bridge circuits of the phases are connected in parallel, the intermediate point formed by connecting the current output end of the upper switch in any one-phase half-bridge circuit with the current input end of the intermediate switch is the upper end of the corresponding one-phase half-bridge circuit, the intermediate point formed by connecting the current output end of the intermediate switch in any one-phase half-bridge circuit with the current input end of the lower switch is the lower end of the corresponding one-phase half-bridge circuit, the current output ends of the lower switches in the half-bridge circuits of the phases are connected in parallel, and the multiplex capacitor is connected in parallel with the half-bridge circuits of the phases; the driving signals of the same-phase half-bridge circuits of all the three-phase multiplex sub-modules in the same multiplex bridge arm are the same; in any one-phase half-bridge circuit, the driving signals of the upper switch and the lower switch are the same, and the driving signal of the intermediate switch is complementary to the driving signals of the upper switch and the lower switch.

[0010] The upper ends of the half-bridge circuits of the phases in the first three-phase multiplex sub-module constitute the upper end of the upper / lower multiplex bridge arm, the upper ends of the half-bridge circuits of the phases in the kth three-phase multiplex sub-module are connected with the lower ends of the half-bridge circuits of the phases in the (k-1)th three-phase multiplex sub-module, the lower ends of the half-bridge circuits of the phases in the kth three-phase multiplex sub-module are connected with the upper ends of the half-bridge circuits of the phases in the (k+1)th three-phase multiplex sub-module, and the lower ends of the half-bridge circuits of the phases in the mth three-phase multiplex sub-module constitute the lower end of the upper / lower multiplex bridge arm, wherein 1≤k≤m, and k and m are integers.

[0011] As a further optimization scheme of the high-power-density three-phase multiplex sub-module type MMC topology, the upper switch, the intermediate switch and the lower switch are one or more of a fully controlled device, a combined device comprising a fully controlled device and a semi-controlled device, other semiconductor devices or mechanical switches.

[0012] As a further optimization scheme of the high-power-density three-phase multiplex sub-module type MMC topology, the upper regular bridge arm of each phase further has a corresponding phase upper bridge arm inductor connected in series, the lower regular bridge arm of each phase further has a corresponding phase lower bridge arm inductor connected in series, and the phase bridge arm points are respectively connected to corresponding phase alternating currents through filter inductors.

[0013] As a further optimization scheme of the high-power-density three-phase multiplex sub-module type MMC topology, the number of sub-modules connected in series in the upper regular bridge arm of each phase is equal to the number of sub-modules connected in series in the lower regular bridge arm of the corresponding phase, and the number of three-phase multiplex sub-modules connected in series in the upper multiplex bridge arm of each phase is equal to the number of three-phase multiplex sub-modules connected in series in the lower multiplex bridge arm.

[0014] As a further optimization scheme of the high-power-density three-phase multiplex sub-module type MMC topology, the sub-module is a half-bridge type sub-module, a full-bridge type sub-module or other functional type sub-module.

[0015] A modulation method of a high-power-density three-phase multiplex sub-module type MMC topology, according to the intersection of two-phase alternating voltage, the bridge arm energy balance angle γ is set, and according to the bridge arm energy balance angle, the upper / lower conventional bridge arm of each phase is controlled to time-share the upper / lower multiplex bridge arm in the power frequency cycle.

[0016] As a further optimization scheme of the modulation method of the high-power-density three-phase multiplex sub-module type MMC topology, the upper / lower conventional bridge arm of each phase is controlled to time-share the upper / lower multiplex bridge arm in the power frequency cycle according to the bridge arm energy balance angle, specifically, the driving signals of the upper switch, the middle switch and the lower switch of any one phase half-bridge circuit in the three-phase multiplex sub-module are obtained through the following control strategy, and the control strategy is:

[0017] According to the rectification and inversion two working mode states, the closed-loop energy balance angle deviation Δγ of the upper / lower multiplex bridge arm energy regulation is obtained a , and the overall energy balance angle γ±Δγa is obtained.

[0018] The voltage sharing control is performed on each three-phase multiplex sub-module in the upper / lower multiplex bridge arm, and the closed-loop fine tuning is performed on the voltage sharing energy balance angle deviation Δγ d related to the upper / lower switch conduction duty cycle in the three-phase multiplex sub-module.

[0019] According to the overall energy balance angle γ±Δγa, the voltage sharing energy balance angle deviation Δγ d , the real-time phase of alternating voltage and the multiplex control logic, the driving signals of the upper switch, the middle switch and the lower switch of any one phase half-bridge circuit in the three-phase multiplex sub-module are obtained.

[0020] As a further optimization scheme of the modulation method of the high-power-density three-phase multiplex sub-module type MMC topology, the driving signals of the upper switch, the middle switch and the lower switch of any one phase half-bridge circuit in the three-phase multiplex sub-module obtained satisfy the following constraints: Wherein, S jlk1 , S jlk2 , S jlk3 are the states of the upper switch, the middle switch and the lower switch of the kth TMSM in the lower multiplex bridge arm, ω is the angular frequency, and φ j is the initial phase of the j-phase voltage.

[0021] As a further optimization scheme of the modulation method of the high-power-density three-phase multiplex sub-module type MMC topology, the bridge arm energy balance angle γ satisfies: 0≤γ≤2π / 3, then the multiplex bridge arm modulation voltage and the conventional bridge arm modulation voltage satisfy: The traditional MMC j-phase lower bridge arm modulation voltage V jl and the lower multiplex bridge arm modulation voltage V l can be expressed as: The multiplexing bridge arm is in charge-discharge energy balance in the power frequency period multiplexing stage, and then:

[0022] According to the operation of the multiplexing bridge arm energy balance angle γ in 0≤γ≤2π / 3, the multiplexing bridge arm realizes charge-discharge balance in the power frequency period.

[0023] Further, the voltage multiplexing relationship, the modulation voltage and the energy balance of the upper multiplexing bridge arm and the upper conventional bridge arm can be inferred from the multiplexing relationship, the modulation voltage and the energy balance mode of the lower multiplexing bridge arm and the lower conventional bridge arm.

[0024] The application adopts the above technical scheme and has the following beneficial effects:

[0025] (1) The application constructs a multiplexing bridge arm of the MMC topology by constructing a novel three-phase multiplexing sub-module, all of which are put into or cut off, each phase bridge arm of each three-phase multiplexing sub-module multiplexes a capacitor, which greatly reduces the number of capacitors; the capacitor voltage of the three-phase multiplexing sub-module and the capacitor voltage of the conventional bridge arm sub-module are configured consistently, when the TMSM is multiplexed by any phase conventional bridge arm, the upper and lower switches are turned on, and only the middle switch bears the capacitor voltage; when the TMSM is bypassed by any phase conventional bridge arm, the middle switch is turned on, and the upper and lower switches jointly bear the capacitor voltage, which is consistent with the pressure bearing of the conventional bridge arm sub-module device, and fundamentally solves the consistency problem of the power device in series connection; in addition, the MMC topology proposed in the application has more excellent characteristics in terms of modularization, reduction of the number, volume and efficiency of the traditional MMC sub-module, and improvement of the power density.

[0026] (2) The application provides a modulation strategy for the MMC topology proposed in the application, in which all three-phase multiplexing sub-modules in the multiplexing bridge arm are multiplexed and bypassed by any phase conventional bridge arm, and the energy balance of the multiplexing bridge arm is realized through the set energy balance angle and the voltage balance control, so that the MMC topology can work stably.

[0027] (3) The MMC topology proposed in the application maintains the operation characteristics of the traditional MMC, and provides an efficient, compact, lightweight and economical topology scheme with high application value for the current Shaguo new energy direct current collection and sending and offshore converter platform construction. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 A three-phase multiplexing sub-module type MMC topology structure is provided.

[0029] Figure 2Fig. 3(a) is a DC bus voltage waveform of a three-phase multiplexing sub-module type MMC topology multiplexing bridge arm in a power frequency cycle according to an embodiment; Fig. 3(b) is a circuit topology diagram of the multiplexing bridge arm in mode 1 in the power frequency cycle shown in Fig. 3(a); Fig. 3(c) is a circuit topology diagram of the multiplexing bridge arm in mode 2 in the power frequency cycle shown in Fig. 3(a); Fig. 3(d) is a circuit topology diagram of the multiplexing bridge arm in mode 3 in the power frequency cycle shown in Fig. 3(a); Fig. 3(e) is a circuit topology diagram of the multiplexing bridge arm in mode 4 in the power frequency cycle shown in Fig. 3(a); Fig. 3(f) is a circuit topology diagram of the multiplexing bridge arm in mode 5 in the power frequency cycle shown in Fig. 3(a); and Fig. 3(g) is a circuit topology diagram of the multiplexing bridge arm in mode 6 in the power frequency cycle shown in Fig. 3(a).

[0030] Fig. 3(a) is a DC bus voltage waveform of a three-phase multiplexing sub-module type MMC topology multiplexing bridge arm in a power frequency cycle according to an embodiment; Fig. 3(b) is a circuit topology diagram of the multiplexing bridge arm in mode 1 in the power frequency cycle shown in Fig. 3(a); Fig. 3(c) is a circuit topology diagram of the multiplexing bridge arm in mode 2 in the power frequency cycle shown in Fig. 3(a); Fig. 3(d) is a circuit topology diagram of the multiplexing bridge arm in mode 3 in the power frequency cycle shown in Fig. 3(a); Fig. 3(e) is a circuit topology diagram of the multiplexing bridge arm in mode 4 in the power frequency cycle shown in Fig. 3(a); Fig. 3(f) is a circuit topology diagram of the multiplexing bridge arm in mode 5 in the power frequency cycle shown in Fig. 3(a); and Fig. 3(g) is a circuit topology diagram of the multiplexing bridge arm in mode 6 in the power frequency cycle shown in Fig. 3(a).

[0031] Fig. 3(a) is a DC bus voltage waveform of a three-phase multiplexing sub-module type MMC topology multiplexing bridge arm in a power frequency cycle according to an embodiment; Fig. 3(b) is a circuit topology diagram of the multiplexing bridge arm in mode 1 in the power frequency cycle shown in Fig. 3(a); Fig. 3(c) is a circuit topology diagram of the multiplexing bridge arm in mode 2 in the power frequency cycle shown in Fig. 3(a); Fig. 3(d) is a circuit topology diagram of the multiplexing bridge arm in mode 3 in the power frequency cycle shown in Fig. 3(a); Fig. 3(e) is a circuit topology diagram of the multiplexing bridge arm in mode 4 in the power frequency cycle shown in Fig. 3(a); Fig. 3(f) is a circuit topology diagram of the multiplexing bridge arm in mode 5 in the power frequency cycle shown in Fig. 3(a); and Fig. 3(g) is a circuit topology diagram of the multiplexing bridge arm in mode 6 in the power frequency cycle shown in Fig. 3(a).

[0032] Figure 4 Fig. 4 is a block diagram of an energy balance control strategy of a three-phase multiplexing sub-module type MMC topology multiplexing bridge arm according to an embodiment.

[0033] Fig. 5(a) is a conventional MMC topology structure according to an embodiment; Fig. 5(b) is a three-phase bridge arm multiplexing type MMC topology structure according to an embodiment; and Fig. 5(c) is a three-phase multiplexing sub-module type MMC topology structure according to an embodiment.

[0034] Figure 6 Fig. 6 is a waveform diagram of a simulation of a three-phase multiplexing sub-module type MMC sub-module voltage and bridge arm modulation voltage acquisition according to an embodiment.

[0035] Fig. 6 is a waveform diagram of a simulation of a three-phase multiplexing sub-module type MMC sub-module voltage and bridge arm modulation voltage acquisition according to an embodiment. c SM1 to SMm are the first to mth three-phase multiplexing sub-modules. m SM1 to SMm are the first to mth three-phase multiplexing sub-modules. jlk1 SM1 to SMm are the first to mth three-phase multiplexing sub-modules. jlk2 SM1 to SMm are the first to mth three-phase multiplexing sub-modules. jlk3 SM1 to SMm are the first to mth three-phase multiplexing sub-modules. DETAILED DESCRIPTION

[0036] Embodiments of the present application will be described below with reference to the accompanying drawings.

[0037] The application provides a three-phase multiplexing sub-module MMC (Three phase Multiplexing Sub-Module Modular Multilevel Converter) topological structure as shown in Figure 1 Fig. 1, which comprises three-phase bridge arms, wherein an upper bridge arm of any phase is composed of a conventional upper bridge arm, a multiplexing upper bridge arm and an upper bridge arm inductor in series, a lower bridge arm of any phase is composed of a conventional lower bridge arm, a multiplexing lower bridge arm and a lower bridge arm inductor in series, the upper end of the conventional upper bridge arm of any phase is connected to the lower end of the corresponding multiplexing upper bridge arm, the upper ends of the multiplexing upper bridge arms are respectively connected to the positive poles of the DC ports, the lower end of the conventional lower bridge arm of any phase is connected to the upper end of the corresponding multiplexing lower bridge arm, the lower ends of the multiplexing lower bridge arms are respectively connected to the negative poles of the DC ports, the upper bridge arm inductor and the lower bridge arm inductor of the same phase are connected to form the midpoint of the bridge arm of the phase, and the midpoints of the bridge arms of the phases are respectively connected to the AC power of the corresponding phase through a filter inductor L. The conventional bridge arm is composed of a first half-bridge sub-module (HBSM) SM1 to a cth half-bridge sub-module SM c in series, and the multiplexing bridge arm is composed of a first three-phase multiplexing sub-module TMSM1 to an mth three-phase multiplexing sub-module TMSM m in series, and m is greater than or equal to 1.

[0038] The kth three-phase multiplexing sub-module TMSM k has a structure as shown in Figure 2 Fig. 1 (a), which comprises a three-phase three-switch half-bridge circuit and a multiplexing capacitor C mk . The three-phase three-switch half-bridge circuit of any phase comprises upper switches S jlk1 , middle switches S jlk2 and lower switches S jlk3 in series, the current output end of the upper switch S jlk1 is connected to the current input end of the middle switch S jlk2 to form a middle point as the upper end of the j-phase three-switch half-bridge circuit of the kth three-phase multiplexing sub-module, the current output end of the middle switch S jlk2 is connected to the current input end of the lower switch S jlk3 to form a middle point as the lower end of the j-phase three-switch half-bridge circuit of the kth three-phase multiplexing sub-module, the current input ends of the upper switches in the three-switch half-bridge circuits of the phases are connected in parallel, the current output ends of the lower switches in the three-switch half-bridge circuits of the phases are connected in parallel, and the multiplexing capacitor C mk is connected in parallel with the single circuits of the three-switch half-bridge circuits of the phases, j is a, b or c, k = 1, 2, …, m, and m is an integer greater than or equal to 1; the upper switch S jlk1 and the lower switch S jlk3 of the three-switch half-bridge circuit of the j phase adopt the same driving signal, and the middle switch S jlk2 adopts a driving signal different from Sjlk1 , S jlk3 Drive signals are complementary, multiplexing capacitors are in S jlk1 and lower switch S jlk3 is on and the middle switch S jlk2 is off, access j-phase three-switch half-bridge circuit, multiplexing capacitors are in S jlk1 and lower switch S jlk3 is off and the middle switch S jlk2 is on, bypassed by j-phase three-switch half-bridge circuit.

[0039] Further, the switches in the TMSM are one or more of fully controlled devices, combination devices including fully controlled devices and semi-controlled devices, other semiconductor devices or mechanical switches, and here a new three-phase multiplexing sub-module is constructed using IGBT devices, as shown in (b) of Figure 2 Since the TMSM-MMC topology is symmetrical up and down, and the abc three-phase working principle is consistent, the subsequent analysis takes the a-phase lower regular bridge arm and the a-phase lower multiplexing bridge arm as an example for analysis.

[0040] The TMSM connection mode is shown in (c) of Figure 2 , and the middle point formed by S k and S jlk1 of the kth TMSM jlk2 is connected to the middle point formed by S k-1 and S jl(k-1)2 of the k-1th TMSM jl(k-1)3 , the middle point formed by S jlk2 and S jlk3 is connected to the middle point formed by S k+1 and S jl(k+1)1 of the k+1th TMSM jl(k+1)2 , and the remaining TMSMs are connected in turn. The lower end of the a-phase lower regular bridge arm is connected to the middle point formed by S jl11 and S jl12 , the middle point formed by S jlm2 and S jlm3 is connected to the negative pole of the DC bus.

[0041] Regarding the basic multiplexing mode of TMSM-MMC, as shown in Figures 3(a) to 3(g) , when the lower multiplexing bridge arm is multiplexed by the j-phase lower regular bridge arm, S jlk1 and S jlk3 in the j-phase three-switch half-bridge circuit of all TMSMs in the lower multiplexing bridge arm are simultaneously turned on, S jlk2 is off, achieving simultaneous multiplexing of all TMSMs in the lower multiplexing bridge arm; when the lower multiplexing bridge arm is bypassed, S jlk2 in the j-phase three-switch half-bridge circuit of all TMSMs in the lower multiplexing bridge arm is turned on, S jlk1 and Sjlk3 At the same time, all TMSM of the lower multiplexed bridge arm are bypassed by the lower conventional bridge arm.

[0042] In order to realize the energy balance of the multiplexed bridge arm, a modulation strategy is proposed, in which the multiplexed bridge arm is periodically multiplexed by the conventional bridge arm at the power frequency and the energy balance is achieved. As shown in FIG. 3(a), by optimizing the operating range of the energy balance angle γ of the multiplexed bridge arm, the modulation range is widened to [0, 1], and the operating characteristics of the traditional MMC are further improved. According to the multiplexing principle of the multiplexed bridge arm, the bridge arm energy balance angle γ is set at the voltage crossing point of the two-phase alternating current side, and γ satisfies the operating principle: 0≤γ≤2π / 3 in the power frequency cycle. If the γ adjustment angle is greater than 2π / 3, the multiplexed bridge arm cannot balance the energy fluctuation in the power frequency cycle, the power transmitted by the conventional bridge arm and the power transmitted by the multiplexed bridge arm no longer match in the power frequency cycle, and the MMC topology cannot work stably.

[0043] According to the multiplexing principle shown in FIG. 3(a), the detailed working mode of the multiplexed bridge arm is as follows:

[0044] Mode 1: in ω2≤ωt≤ω7, the switching state of {S alk1 / S alk3 ,S alk2} is {1, 0}, at this time the a-phase lower conventional bridge arm current ial flows through the a-phase half-bridge circuit of all TMSM of the lower multiplexed bridge arm;

[0045] Mode 2: in 0≤ωt≤ω2, ω7≤ωt≤ω9, the switching state of {S alk1 / S alk3 ,S alk2} is {0, 1}, at this time the a-phase lower conventional bridge arm current ial bypasses all TMSM of the lower multiplexed bridge arm;

[0046] Mode 3: in 0≤ωt≤ω1 and ω5≤ωt≤ω9, the switching state of {S blk1 / S blk3 ,S blk2} is {1, 0}, at this time the b-phase lower conventional bridge arm current ibl flows through the b-phase half-bridge circuit of all TMSM of the lower multiplexed bridge arm;

[0047] Mode 4: in ω1≤ωt≤ω5, the switching state of {S blk1 / S blk3 ,S blk2} is {0, 1}, at this time the b-phase lower conventional bridge arm current ibl bypasses all TMSM of the lower multiplexed bridge arm;

[0048] Mode 5: in 0≤ωt≤ω4, ω8≤ωt≤ω9, the switching state of {S clk1 / S clk3 ,S clk2The switch state of the lower bridge arm of the c-phase is {1, 0}, and at this time, the c-phase lower conventional bridge arm current icl flows through the c-phase half-bridge circuit of all TMSMs of the lower multiplex bridge arm;

[0049] Mode 6: ω4≤ωt≤ω8, {S clk1 / S clk3 ,S clk2 The switch state of the lower bridge arm of the c-phase is {1, 0}, and at this time, the c-phase lower conventional bridge arm current icl flows through the c-phase half-bridge circuit of all TMSMs of the lower multiplex bridge arm;

[0050] Further, the switch state of the TMSM three-phase half-bridge circuit in the power frequency cycle is represented as:

[0051]

[0052] In the formula, S jlk1 , S jlk2 , S jlk3 are the states of the upper switch, the middle switch and the lower switch of the kth TMSM in the lower multiplex bridge arm respectively; ω is the angular frequency; φ j is the initial phase of the j-phase voltage;

[0053] The energy balance angle γ is in the range of 0≤γ≤2π / 3, and the multiplexing relationship between the conventional bridge arm and the multiplex bridge arm voltage is:

[0054]

[0055] In the formula, V al is the modulation voltage of the a-phase lower bridge arm of the traditional MMC topology, V cal is the modulation voltage of the lower conventional bridge arm, and V l is the modulation voltage of the lower multiplex bridge arm;

[0056] The modulation voltage of the conventional lower bridge arm of the traditional MMC and the modulation voltage of the multiplex bridge arm of the TMS-MMC can be represented as:

[0057]

[0058] In the formula, V jl is the modulation voltage of the j-phase lower conventional bridge arm; V dc is the DC bus voltage; and α is the proportion of the modulation voltage of the multiplex bridge arm in the DC bus voltage;

[0059] Then, according to the working mode and the basic voltage modulation of the multiplex bridge arm, it can be seen that the energy balance control of the multiplex bridge arm is the key to the stable operation of the TMS-MMC. Under the condition that the modulation degree m is certain, the relationship between the multiplex bridge arm capacitor current and the working mode of the multiplex bridge arm current can be represented as:

[0060]

[0061] where, i j is the j-phase phase current, I dc is the DC bus current, S jlm1 is the state of the upper switch of the mth TMSM in the lower multiplexing bridge arm, C TMSM is the TMSM capacitance value, V TMSM (t) is the TMSM capacitor voltage at time t;

[0062] The above formula shows that the multiplexing bridge arm capacitor energy fluctuation is reflected in the capacitor voltage fluctuation; the multiplexing bridge arm energy balance control is closed-loop controlled by the multiplexing bridge arm capacitor voltage, and according to the states of the rectification and inversion working modes, the closed-loop energy balance angle deviation Δγ a participating in the multiplexing bridge arm energy regulation is obtained, and further superimposes the voltage sharing control deviation component Δγ d between the multiplexing submodules to realize the multiplexing submodule energy balance. The multiplexing bridge arm energy balance control strategy is shown in Figure 4 . The method for obtaining Δγ a is that the product of the low-pass filtered value of the sum of the capacitor voltages ∑V cmm of all three-phase multiplexing submodules and mV smref is PI regulated, V smref is the reference value of the conventional bridge arm submodule capacitor voltage, and the multiplexing submodule voltage sharing control can refer to the conventional bridge arm submodule voltage sharing control idea. On the basis of the overall energy balance angle γ±Δγa, the voltage sharing energy balance angle deviation Δγ d of the multiplexing submodule upper / lower tube conduction duty cycle is fine-tuned through the submodule average value closed-loop voltage sharing to ensure the voltage sharing balance between the multiplexing submodules. Finally, according to the overall energy balance angle γ±Δγa, the voltage sharing energy balance angle deviation Δγ d , the real-time phase θ j of the alternating voltage, and the multiplexing control logic, the drive signals S jlm1 , S jlm2 , and S jlm3 of the upper switch, the middle switch, and the lower switch of any one-phase half-bridge circuit in the three-phase multiplexing submodule are obtained.

[0063] Regarding the size of the TMSM and SM capacitor value of the multiplexing bridge arm, the maximum bridge arm energy fluctuation is the fundamental basis for selecting the capacitor value. Further, the power transmitted by the multiplexing bridge arm is shown in the following formula:

[0064]

[0065] The maximum energy fluctuation ΔE larm of the lower multiplexing bridge arm can be represented as:

[0066]

[0067] Where t1 and t2 are the time when all TMSM are put into and cut off respectively when only the a-phase normal bridge arm current flows through the lower multiplex bridge arm; I m is the maximum amplitude of alternating current; φ is the phase voltage and phase current phase difference.

[0068] The normal bridge arm transmits power P carm (t) is represented as:

[0069]

[0070] The maximum energy fluctuation ΔE of the lower normal bridge arm is carm which can be represented as:

[0071]

[0072] Where t3 and t4 are the zero-crossing time of the a-phase normal bridge arm current respectively;

[0073] The relationship between the maximum energy fluctuation of the bridge arm and the sub-module capacity is:

[0074]

[0075] Where C csm , C TMSM are the capacitance capacity of the normal bridge arm SM and TMSM respectively; N csm , N TMSM are the number of normal bridge arm and multiplex bridge arm sub-modules respectively; ε is the capacitance voltage fluctuation rate; V sm is the capacitance voltage of the normal bridge arm sub-module.

[0076] Based on the engineering parameters: DC bus voltage ±200kV, transmission capacity 400MVA, AC voltage 311kV, the number of SM in a single bridge arm is 250, the capacitance voltage fluctuation rate ε is 5%, the sub-module voltage is 1.6kV, and the sub-module capacity is 12mF. Under the condition of unity power factor and modulation degree m=0.9, it is calculated that the capacitance of the normal bridge arm of the TMSM-MMC is about 9.8mF, and the capacitance voltage of the multiplex bridge arm is about 1.83mF; compared with the traditional MMC, the capacitance demand of the topology structure is greatly reduced, which greatly reduces the volume of the capacitor and improves the economy.

[0077] The existing MMC, three-phase bridge arm multiplexing MMC (TPMA-MMC), and three-phase multiplexing sub-module MMC topology are compared. As shown in FIGS. 5(a), 5(b), and 5(c), it can be found that, compared with the traditional MMC, the TPMA-MMC saves the number of SMs required by the MMC as a whole by constructing a multiplexing bridge arm composed of half-bridge sub-modules. Compared with the TMSM-MMC, the devices in the three-phase switching circuit in the TPMA-MMC have a large pressure and need to be connected in series in a large number, while the TMSM constructed by the TMSM-MMC solves the problem of series connection of the three-phase switching bridge devices in the TPMA-MMC on the basis of meeting the multiplexing function, and improves the engineering application value of the TMSM-MMC.

[0078] Further, based on the system simulation parameters: DC bus voltage ±10kV, system capacity S = 1MVA, multiplexing bridge arm sub-module 3, conventional bridge arm sub-module 9, sub-module voltage 1.67kV, modulation ratio 0.9, the simulation results are as shown in FIGS. 6(a), 6(b), and 6(c). Figure 6 a b c abc three-phase currents are respectively shown in FIGS. 6(a), 6(b), and 6(c). au al a-phase upper and lower conventional sub-module bridge arm currents are respectively shown in FIGS. 6(a), 6(b), and 6(c). u l upper and lower multiplexing sub-module bridge arm currents are respectively shown in FIGS. 6(a), 6(b), and 6(c). cau cal A-phase upper and lower conventional sub-module bridge arm voltages are respectively shown in FIGS. 6(a), 6(b), and 6(c). mu ml upper and lower multiplexing bridge arm voltages are respectively shown in FIGS. 6(a), 6(b), and 6(c). causm calsm A-phase upper and lower conventional bridge arm sub-module voltages are respectively shown in FIGS. 6(a), 6(b), and 6(c). musm mlsm upper and lower multiplexing bridge arm sub-module voltages are respectively shown in FIGS. 6(a), 6(b), and 6(c).From the simulation results, the modulation of the conventional bridge arm and the multiplexing bridge arm, and the capacitor voltage of the conventional SM and the TMSM are all stable, and all the control strategies remain the same as the traditional MMC. From the simulation results, the modulation of the conventional bridge arm and the multiplexing bridge arm, and the capacitor voltage of the conventional SM and the TMSM are all stable, and all the control strategies remain the same as the traditional MMC. The proposed topology solves the problem of series connection of the three-phase switching bridge devices by constructing a new multiplexing sub-module (TMSM) and using the TMSM to construct a multiplexing bridge arm, and improves the engineering application value of the MMC topology. Compared with the traditional MMC, taking the working condition m = 0.9 as an example, the number of capacitors of the proposed topology structure is reduced by 21.3%, which greatly reduces the land occupation area and weight of the MMC converter station, and greatly improves the lightness level of the MMC. ​​​​​​​​

[0079] The embodiments of the present application are described in detail above with reference to the accompanying drawings, but the present application is not limited to the above-described embodiments, and various changes can be made by those skilled in the art within the scope of knowledge acquired from the above description, without departing from the spirit of the present application.

Claims

1. A high power density three-phase multiplexed sub-module type MMC topology, characterized in that, The application relates to a three-phase full-bridge rectifier, comprising: a three-phase upper bridge arm, each phase upper bridge arm being composed of at least one sub-module in series, a three-phase lower bridge arm, each phase lower bridge arm being composed of at least one sub-module in series, the upper end of each phase lower bridge arm being connected to the lower end of the corresponding phase upper bridge arm to form a phase bridge arm midpoint, a three-phase upper multiplex bridge arm, each phase upper multiplex bridge arm being composed of at least one three-phase multiplex sub-module in series, the upper end of each phase upper multiplex bridge arm being connected to the positive pole of a direct current port, the lower end of each phase upper multiplex bridge arm being connected to the upper end of the corresponding phase upper bridge arm, and a three-phase lower multiplex bridge arm, each phase lower multiplex bridge arm being composed of at least one three-phase multiplex sub-module in series, the upper end of each phase lower multiplex bridge arm being connected to the lower end of the corresponding phase lower bridge arm, the lower end of each phase lower multiplex bridge arm being connected to the negative pole of the direct current port, the three-phase multiplex sub-module comprising a three-phase switch half-bridge circuit and a multiplex capacitor, any phase half-bridge circuit being composed of an upper switch, an intermediate switch and a lower switch in series, the current input end of the upper switch in each phase half-bridge circuit being connected in parallel, the intermediate point formed by the connection of the current output end of the upper switch and the current input end of the intermediate switch in any phase half-bridge circuit being regarded as the upper end of the corresponding phase half-bridge circuit, the intermediate point formed by the connection of the current output end of the intermediate switch and the current input end of the lower switch in any phase half-bridge circuit being regarded as the lower end of the corresponding phase half-bridge circuit, the current output end of the lower switch in each phase half-bridge circuit being connected in parallel, the multiplex capacitor being connected in parallel with each phase half-bridge circuit; the driving signals of the same phase half-bridge circuits of all three-phase multiplex sub-modules in the same multiplex bridge arm are the same; in any phase half-bridge circuit, the driving signals of the upper switch and the lower switch are the same, the driving signal of the intermediate switch is complementary to the driving signals of the upper switch and the lower switch, the upper ends of the phase half-bridge circuits in the first three-phase multiplex sub-module form the upper ends of the upper and lower multiplex bridge arms, the upper ends of the phase half-bridge circuits in the kth three-phase multiplex sub-module are connected to the lower ends of the phase half-bridge circuits in the (k-1)th three-phase multiplex sub-module, the lower ends of the phase half-bridge circuits in the kth three-phase multiplex sub-module are connected to the upper ends of the phase half-bridge circuits in the (k+1)th three-phase multiplex sub-module, the lower ends of the phase half-bridge circuits in the mth three-phase multiplex sub-module form the lower ends of the upper and lower multiplex bridge arms, 1<=k<=m, k and m are integers; when the current of any phase bridge arm flows through the corresponding phase multiplex bridge arm, all three-phase multiplex sub-modules are put into use; when the current of any phase bridge arm bypasses the corresponding phase multiplex bridge arm, all three-phase multiplex sub-modules are cut off.

2. The high power density three-phase multiplexed sub-module type MMC topology of claim 1, wherein, The upper switch, the intermediate switch and the lower switch are one or more of full-controlled devices, combined devices comprising full-controlled devices and half-controlled devices or mechanical switches.

3. The high power density three-phase multiplexed sub-module type MMC topology of claim 1, wherein, Each phase upper bridge arm further comprises a corresponding phase upper bridge arm inductor in series, each phase lower bridge arm further comprises a corresponding phase lower bridge arm inductor in series, and each phase bridge arm midpoint is connected to a corresponding phase alternating current through a filtering inductor.

4. The high power density three-phase multiplexed sub-module type MMC topology of claim 1, wherein, The number of sub-modules in series in each phase upper bridge arm is equal to the number of sub-modules in series in the corresponding phase lower bridge arm, the number of three-phase multiplex sub-modules in series in each phase upper multiplex bridge arm is equal to the number of three-phase multiplex sub-modules in series in the lower multiplex bridge arm.

5. The high power density three-phase multiplexed sub-module type MMC topology according to claim 1, characterized in that, The sub-module is a half-bridge type sub-module or a full-bridge type sub-module.

6. The modulation method of a high-power-density three-phase multiplexed sub-module type MMC topology according to claim 1, characterized in that, The multiplexing bridge arm energy balance angle γ is set according to the intersection of two-phase alternating voltages, and the upper and lower bridge arms of each phase are controlled to time-share and multiplex the upper and lower multiplexing bridge arms in a power frequency cycle according to the bridge arm energy balance angle.

7. The modulation method of a high-power-density three-phase multiplexed sub-module type MMC topology according to claim 6, characterized in that, The upper and lower bridge arms of each phase are controlled to time-share and multiplex the upper and lower multiplexing bridge arms in a power frequency cycle according to the bridge arm energy balance angle, and specifically, the driving signals of the upper switch, the middle switch and the lower switch of any one phase half-bridge circuit in the three-phase multiplexing sub-module are obtained through the following control strategy, and the control strategy is as follows: According to the two working mode states of rectification and inversion, the closed-loop energy balance angle deviation Δγ of the upper and lower multiplex bridge arms participating in energy regulation is obtained a , and the overall energy balance angle γ±Δγa is obtained. The voltage balancing control is performed on each three-phase multiplexing sub-module in the upper and lower multiplexing bridge arms, and a closed-loop fine adjustment is performed on the voltage balancing energy balance angle deviation Δγ related to the conduction duty cycles of the upper and lower switches in the three-phase multiplexing sub-modules d ; According to the overall energy balance angle γ±Δγa, the equalizing energy balance angle deviation Δγ d , the alternating voltage real-time phase and the multiplexing control logic, the driving signals of the switch, the middle switch and the lower switch on any phase half-bridge circuit in the three-phase multiplexing sub-module are obtained.

8. The modulation method of a high-power-density three-phase multiplexed sub-module type MMC topology according to claim 7, characterized in that, The driving signals of the switches, the middle switches and the lower switches on any one phase half-bridge circuit in the acquired three-phase multiplexing sub-module satisfy the following constraints: , wherein S jlk1 , S jlk2 , S jlk3 are the states of the upper, middle and lower switches of the kth three-phase multiplexing sub-module in the lower multiplexing bridge arm, ω is the angular frequency, and φ j is the initial phase of the j-phase voltage.

9. The modulation method of a high-power-density three-phase multiplexed sub-module type MMC topology according to claim 8, characterized in that, The multiplexing bridge arm energy balance angle γ satisfies: 0≤γ≤2π / 3.

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