High-power-density three-phase multiplexing sub-module type MMC topology and modulation method

Abstract

The invention discloses a high-power-density three-phase multiplexing sub-module type MMC topology and a modulation method, and belongs to the technical field of power generation, power transformation or power distribution. A three-phase multiplexing sub-module formed by connecting a three-phase half-bridge circuit and a capacitor in parallel is designed, each phase comprises an upper switch, a middle switch and a lower switch which are connected in series, the upper switch and the lower switch adopt the same driving signal, and the middle switch signal is complementary with the upper switch and the lower switch. The MMC topological structure comprises an upper conventional bridge arm, a lower conventional bridge arm, an upper multiplexing bridge arm and a lower multiplexing bridge arm, wherein the multiplexing bridge arm is formed by connecting three-phase multiplexing sub-modules in series. The working states of in-phase half-bridge switches of all the three-phase multiplexing sub-modules in the multiplexing bridge arm are consistent, namely all the three-phase multiplexing sub-modules are multiplexed and cut off at the same time. Based on a topological structure, a modulation method for multiplexing a multiplexing bridge arm by a conventional bridge arm power frequency period and realizing multiplexing bridge arm energy balance is provided, topological [0, 1] modulation can be realized, and compared with a 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 present invention relates to a MMC topology with few capacitors, and specifically discloses a high-power-density three-phase multiplexing sub-module (TMSM, Three phase Multiplexing Sub-Module) type MMC topology and a modulation method, which relates to power system technology and belongs to the technical field of power generation, power transformation or power distribution. Background Art

[0002] In recent years, the modular multilevel converter (MMC) topology has shown advantages such as modularity, scalability and low harmonic content, which has promoted its application in the collection and transmission of new energy sources such as wind power and photovoltaics. At present, my country has also built flexible direct current transmission projects represented by Zhangbei, Chongqing and Hubei based on the MMC topology. However, in actual applications, the flexible direct current converter station represented by half-bridge MMC and full-bridge MMC uses a large number of power devices and capacitors, and the sub-modules occupy most of the volume of the MMC, resulting in problems such as large area, heavy weight and large operating losses of the converter station, which especially limits the construction and development of offshore converter platforms based on MMC.

[0003] At present, there are two basic forms of MMC topology: one is a three-phase series topology, which uses a two-level voltage converter and a sub-module valve string to construct a three-phase parallel hybrid MMC, which can reduce the 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 topology that is constructed by integrating the sub-module valve string with the traditional voltage source converter (VSC). This topology is constructed by combining a full-bridge sub-module (FBSM) valve string with a two-level VSC. The two-level VSC works at the base frequency and cooperates with the FBSM valve string modulation, which can greatly reduce the number of sub-modules, but the two-level VSC bridge arm requires devices in series for voltage balance.

[0004] In order to reduce the number of submodules, the use of bridge arm reuse technology is an effective way to improve the lightweight and economic efficiency of the existing MMC topology. The patent with publication number CN113595424A proposes a three-phase bridge arm reused MMC topology, which uses submodule valve strings to construct reused bridge arms, and controls the three-phase switch bridge in parallel to achieve time-sharing reuse of reused bridge arms, which improves the lightweight level of traditional MMC. However, the three-phase switch bridge requires a large number of power devices in series to withstand the modulation voltage of the reused bridge arm, and the static and dynamic voltage balancing technology of the existing power devices in series is not yet mature, which reduces its feasibility in engineering applications. Therefore, solving the technical problems of static and dynamic voltage balancing of power devices in series in the bridge arm reused MMC topology and exploring new MMC topology solutions that are lightweight and highly feasible in engineering will provide solutions for the efficient and low-cost delivery of large-scale new energy. Summary of the invention

[0005] The purpose of the present invention is to address the deficiencies of the above-mentioned background technology and provide a high-power-density three-phase multiplexing sub-module type MMC topology and modulation method, by connecting the three-phase multiplexing sub-modules in series to construct a multiplexing bridge arm, thereby reducing the number of capacitors; by modulating the three-phase multiplexing sub-modules to fully engage or fully remove them, the energy balance of the multiplexing bridge arm is achieved, and the power devices of the three-phase multiplexing sub-modules only bear the capacitor voltage, thereby solving the technical difficulties of connecting power devices in series in the existing bridge arm multiplexing type MMC topology, and achieving the purpose of the invention of constructing a low-capacitor MMC topology and controlling the stable operation of the MMC topology.

[0006] The present invention adopts the following technical solutions to achieve the above-mentioned invention object:

[0007] A high power density three-phase multiplexing submodule type MMC topology, comprising: a three-phase upper conventional bridge arm, a three-phase lower conventional bridge arm, a three-phase upper multiplexing bridge arm and a three-phase lower multiplexing bridge arm;

[0008] Each phase upper conventional bridge arm is composed of at least one submodule connected in series, each phase lower conventional bridge arm is composed of at least one submodule connected in series, the upper end of each phase lower conventional bridge arm is connected to the lower end of the corresponding phase upper conventional bridge arm to form the midpoint of each phase bridge arm, each phase upper multiplexed bridge arm is composed of at least one three-phase multiplexed submodule connected in series, the upper end of each phase upper multiplexed bridge arm is respectively connected to the positive pole of the DC port, the lower end of each phase upper multiplexed bridge arm is connected to the upper end of the corresponding phase upper conventional bridge arm, each phase lower multiplexed bridge arm is composed of at least one three-phase multiplexed submodule connected in series, the upper end of each phase lower multiplexed bridge arm is connected to the lower end of the corresponding phase lower conventional bridge arm, and the lower end of each phase lower multiplexed bridge arm is respectively connected to the negative pole of the DC port; when the current of any phase conventional bridge arm flows through the corresponding phase multiplexed bridge arm, all three-phase multiplexed submodules are put into operation; when the current of any phase conventional bridge arm bypasses the corresponding phase multiplexed bridge arm, all three-phase multiplexed submodules are removed.

[0009] As a further optimization scheme of a three-phase multiplexing submodule MMC topology with high power density, the three-phase multiplexing submodule includes a three-phase switch half-bridge circuit and a multiplexing capacitor. Any phase half-bridge circuit is composed of an upper switch, a middle switch and a lower switch connected in series. The current input end of the upper switch in each phase half-bridge circuit is connected in parallel. The middle point formed by connecting the current output end of the upper switch and the current input end of the middle switch in any phase half-bridge circuit serves as the upper end of the corresponding phase half-bridge circuit. The middle point formed by connecting the current output end of the middle switch and the current input end of the lower switch in any phase half-bridge circuit serves 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 is connected in parallel, and the multiplexing capacitor is connected in parallel with each phase half-bridge circuit; the driving signals of the same phase half-bridge circuits of all three-phase multiplexing submodules in the same multiplexing 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, and the driving signal of the middle switch is complementary to the driving signals of the upper switch and the lower switch;

[0010] The upper end of each phase half-bridge circuit in the first three-phase multiplexing submodule constitutes the upper end of the upper / lower multiplexing bridge arm, the upper end of each phase half-bridge circuit in the kth three-phase multiplexing submodule is connected to the lower end of each phase half-bridge circuit in the k-1th three-phase multiplexing submodule, the lower end of each phase half-bridge circuit in the kth three-phase multiplexing submodule is connected to the upper end of each phase half-bridge circuit in the k+1th three-phase multiplexing submodule, and the lower end of each phase half-bridge circuit in the mth three-phase multiplexing submodule constitutes the lower end of the upper / lower multiplexing bridge arm, 1≤k≤m, k and m are both integers.

[0011] As a further optimization scheme for a high-power-density three-phase multiplexed sub-module MMC topology, the upper switch, the middle switch, and the lower switch are one or more of fully-controlled devices, combination devices including fully-controlled devices and half-controlled devices, other semiconductor devices, or mechanical switches.

[0012] As a further optimization scheme for a high-power-density three-phase multiplexed sub-module MMC topology, each phase upper conventional bridge arm is also connected in series with the corresponding phase upper bridge arm inductor, and each phase lower conventional bridge arm is also connected in series with the corresponding phase lower bridge arm inductor, and the midpoint of each phase bridge arm is connected to the corresponding phase AC power via a filter inductor.

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

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

[0015] A modulation method for a high power density three-phase multiplexing submodule MMC topology is disclosed. The bridge arm energy balance angle γ is set according to the intersection of the two-phase AC voltage, and the upper / lower conventional bridge arms of each phase are controlled to time-share multiplex the upper / lower multiplexed bridge arms within the power frequency cycle according to the bridge arm energy balance angle.

[0016] As a further optimization scheme of the modulation method of a high-power-density three-phase multiplexing submodule MMC topology, the upper / lower conventional bridge arms of each phase are controlled to multiplex the upper / lower multiplexing bridge arms in time within the power frequency cycle according to the bridge arm energy balance angle. Specifically, the drive signals of the upper switch, the middle switch and the lower switch of any phase half-bridge circuit in the three-phase multiplexing submodule are obtained through the following control strategy. The control strategy is:

[0017] According to the two working modes of rectification and inversion, the closed-loop energy balance angle deviation Δγ participating in the energy regulation of the up / down multiplexing bridge arm is obtained a , obtain the overall energy balance angle γ±Δγa;

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

[0019] Based on the overall energy balance angle γ±Δγa and the pressure-equalizing energy balance angle deviation Δγ d , AC voltage real-time phase and multiplexing control logic, and obtain the driving signals of the upper switch, middle switch and lower switch of any phase half-bridge circuit in the three-phase multiplexing sub-module.

[0020] As a further optimization scheme for the modulation method of a high power density three-phase multiplexing submodule type MMC topology, the drive signals of the upper switch, the middle switch and the lower switch of any phase half-bridge circuit in the three-phase multiplexing submodule are obtained to meet the following constraints: Among them, S jlk1 , S jlk2 , S jlk3 are the states of the upper switch, middle switch and lower switch of the kth TMSM in the lower multiplexing bridge arm, ω is the angular frequency, φ j is the initial phase of phase j voltage.

[0021] As a further optimization scheme for the modulation method of a high power density three-phase multiplexed sub-module MMC topology, the bridge arm energy balance angle γ satisfies: 0≤γ≤2π / 3, then the multiplexed bridge arm modulation voltage and the conventional bridge arm modulation voltage satisfy: Traditional MMC j-phase lower bridge arm modulation voltage V jl And the lower multiplexing bridge arm modulation voltage V l It can be expressed as: The charging and discharging energy of the reused bridge arm is balanced during the reuse phase of the power frequency cycle, then:

[0022] According to the multiplexing bridge arm energy balance angle γ running at 0≤γ≤2π / 3, the multiplexing bridge arm realizes charge and discharge balance within the power frequency cycle.

[0023] Furthermore, the voltage reuse relationship, modulation voltage and energy balance between the upper multiplexed bridge arm and the upper conventional bridge arm can be inferred by referring to the voltage reuse relationship, modulation voltage and energy balance between the lower multiplexed bridge arm and the lower conventional bridge arm.

[0024] The present invention adopts the above technical solution and has the following beneficial effects:

[0025] (1) The present invention constructs a multiplexed bridge arm of the MMC topology by constructing a new type of three-phase multiplexed submodule. The three-phase multiplexed submodules are all put into use or all removed. Each phase bridge arm of each three-phase multiplexed submodule reuses a capacitor, which greatly reduces the number of capacitors. The capacitor voltage of the three-phase multiplexed submodule is consistent with the capacitor voltage configuration of the conventional bridge arm submodule. When the TMSM is reused by any phase conventional bridge arm, the upper and lower switch tubes 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 tube is turned on, and the upper and lower switch tubes bear the capacitor voltage together, which is consistent with the pressure-bearing of the conventional bridge arm submodule device, and fundamentally solves the conduction consistency problem faced by the series connection of power devices. In addition, the proposed MMC topology shows more excellent characteristics in modularization, reducing the number, volume, efficiency and improving power density of traditional MMC submodules.

[0026] (2) A modulation strategy is provided for the MMC topology proposed in the present invention, in which all three-phase multiplexing sub-modules in the multiplexing bridge arm are multiplexed and bypassed by any phase conventional bridge arm. The energy balance of the multiplexing bridge arm is achieved by setting the energy balance angle and voltage balance control, thereby achieving stable operation of the MMC topology.

[0027] (3) The MMC topology proposed in the present invention maintains the operating characteristics of the traditional MMC modularization, and provides a highly efficient, compact, lightweight and economical topology solution with great application value for the current construction of Shagohuang new energy DC collection and transmission and offshore converter platforms. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 A three-phase multiplexing sub-module MMC topology structure of an embodiment.

[0029] Figure 2 Schematic diagram of optimizing the three-phase multiplexing submodule topology in the embodiment, wherein (a) is the three-phase multiplexing submodule topology structure in the embodiment, (b) is the novel three-phase multiplexing submodule topology structure mentioned in the embodiment, and (c) is a schematic diagram of the connection method of the novel three-phase multiplexing submodule mentioned in the embodiment.

[0030] FIG3(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 of an embodiment; FIG3(b) is a circuit topology diagram of the multiplexing bridge arm in mode 1 in the power frequency cycle shown in FIG3(a); and FIG3(c) is a circuit topology diagram of the multiplexing bridge arm in mode 2 in the power frequency cycle shown in FIG3(a);

[0031] Figure 3(d) is a circuit topology diagram of the multiplexed bridge arm in mode 3 during the power frequency cycle shown in Figure 3(a); Figure 3(e) is a circuit topology diagram of the multiplexed bridge arm in mode 4 during the power frequency cycle shown in Figure 3(a); Figure 3(f) is a circuit topology diagram of the multiplexed bridge arm in mode 5 during the power frequency cycle shown in Figure 3(a); Figure 3(g) is a circuit topology diagram of the multiplexed bridge arm in mode 6 during the power frequency cycle shown in Figure 3(a).

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

[0033] FIG. 5( a ) is a conventional MMC topology structure of an embodiment; FIG. 5( b ) is a three-phase bridge arm multiplexing MMC topology structure of an embodiment; and FIG. 5( c ) is a three-phase multiplexing submodule MMC topology structure of an embodiment.

[0034] Figure 6 The waveform diagram is obtained by simulating a three-phase multiplexing sub-module type MMC sub-module voltage and a bridge arm modulation voltage in the embodiment.

[0035] Description of the numbers in the figure: SM1~SM c The first to cth half-bridge submodules, TMSM1 to TMSM m are the 1st to mth three-phase multiplexing submodules, L is the filter inductor, S jlk1 , S jlk2 , S jlk3 are the upper switch, the middle switch, and the lower switch of the j-phase three-switch half-bridge circuit in the k-th three-phase multiplexing submodule. DETAILED DESCRIPTION

[0036] The embodiments of the present invention are described below in conjunction with the accompanying drawings.

[0037] The present invention proposes a three-phase multiplexing sub-module MMC (Three phase Multiplexing Sub-Module Modular Multilevel Converter, TMSM-MMC) topology structure as shown in FIG. Figure 1As shown, the topological structure includes three-phase bridge arms, the upper bridge arm of any phase is composed of the conventional bridge arm of the phase, the multiplexed bridge arm of the phase and the inductor of the upper bridge arm of the phase in series, the lower bridge arm of any phase is composed of the conventional bridge arm of the phase, the multiplexed bridge arm of the phase and the inductor of the lower bridge arm of the phase in series, the upper end of the conventional bridge arm of any phase is connected to the lower end of the multiplexed bridge arm of the corresponding phase, the upper end of the multiplexed bridge arm of each phase is respectively connected to the positive pole of the DC port, the lower end of the conventional bridge arm of any phase is connected to the upper end of the multiplexed bridge arm of the corresponding phase, the lower end of the multiplexed bridge arm of each phase is respectively connected to the negative pole of the DC port, the inductor of the upper bridge arm of the same phase is connected to the inductor of the lower bridge arm to form the midpoint of the bridge arm of the phase, and the midpoint of the bridge arm of each phase is respectively connected to the corresponding phase AC through the filter inductor L. The conventional bridge arm consists of the first half-bridge sub-module (HBSM, Half Bridge Sub-Module) SM1 to the cth half-bridge sub-module SM c The multiplexing bridge arm is composed of the first three-phase multiplexing submodule TMSM1 to the mth three-phase multiplexing submodule TMSM m Series composition, m≥1.

[0038] The kth three-phase multiplexing submodule TMSM k Structure Figure 2 As shown in (a), it includes a three-phase three-switch half-bridge circuit and a reuse capacitor C mk Any phase three-switch half-bridge circuit includes an upper switch S connected in series jlk1 , Middle switch S jlk2 and the lower switch S jlk3 , switch S jlk1 Current output terminal and intermediate switch S jlk2 The middle point formed by connecting the current input terminals is used as the upper end of the j-phase three-switch half-bridge circuit of the k-th three-phase multiplexing submodule, and the middle switch S jlk2 The current output terminal and the lower switch S jlk3 The middle point formed by the current input terminal connection is used as the lower end of the j-phase three-switch half-bridge circuit of the k-th three-phase multiplexing submodule. The current input terminals of the upper switches in each phase three-switch half-bridge circuit are connected in parallel, and the current output terminals of the lower switches in each phase three-switch half-bridge circuit are connected in parallel. The multiplexing capacitor C mk In parallel with each phase three-switch half-bridge single circuit, j is a or b or c, k = 1, 2, ..., m, m is an integer greater than or equal to 1; in the j-phase three-switch half-bridge circuit, the upper switch S jlk1 and the lower switch S jlk3 Using the same drive signal, the middle switch S jlk2 The driving signal and S jlk1 , S jlk3 The driving signals are complementary and the multiplexing capacitors are in S jlk1 and the lower switch S jlk3 The middle switch S is turned on jlk2 When the switch is turned off, the j-phase three-switch half-bridge circuit is connected, and the reuse capacitor is Sjlk1 and the lower switch S jlk3 Turn off and the intermediate switch S jlk2 When turned on, it is bypassed by the j-phase three-switch half-bridge circuit.

[0039] Furthermore, the switch in the TMSM is one or more of a fully controlled device, a combination of a fully controlled device and a half-controlled device, other semiconductor devices or a mechanical switch. Here, a new three-phase multiplexing submodule constructed using IGBT devices is used, such as Figure 2 As shown in (b) in the figure. Since the proposed TMSM-MMC topology is symmetrical in the upper and lower parts, and the working principles of the three phases abc are consistent, the subsequent analysis takes the conventional bridge arm under phase a and the multiplexed bridge arm under phase a as examples.

[0040] TMSM connection method is as follows Figure 2 As shown in (c), the kth TMSM k For example, S jlk1 and S jlk2 The intermediate points formed by the connection connect the k-1th TMSM k-1 Medium S jl(k-1)2 and S jl(k-1)3 The middle point formed by connecting jlk2 and S jlk3 The intermediate point formed by the connection connects the k+1th TMSM k+1 Medium S jl(k+1)1 and S jl(k+1)2 The middle point formed by connecting the other TMSMs is connected in sequence. The lower end of the conventional bridge arm of phase a is connected to S jl11 and S jl12 The middle point formed by connecting jlm2 and S jlm3 The middle point formed by the connection is connected to the negative pole of the DC bus.

[0041] The basic multiplexing method of TMSM-MMC is as follows: Figure 3(a) to Figure 3(g) As shown, when the lower reused bridge arm is reused by the j-phase lower conventional bridge arm, the S in the j-phase three-switch half-bridge circuit of all TMSMs in the lower reused bridge arm is jlk1 and S jlk3 At the same time, S jlk2 Shut down, so that all TMSMs in the lower multiplexing bridge arm are reused at the same time; when the lower multiplexing bridge arm is bypassed, that is, S in the j-phase three-switch half-bridge circuit of all TMSMs in the lower multiplexing bridge arm jlk2 On, S jlk1 and S jlk3 At the same time, all TMSMs in the lower multiplexing bridge arm are shut down to be bypassed by the lower normal bridge arm of phase j.

[0042] In order to achieve energy balance of the reused bridge arm, a modulation strategy is proposed in which the reused bridge arm is reused by the conventional bridge arm in the power frequency cycle and the energy is balanced. As shown in Figure 3(a), by optimizing the operating range of the energy balance angle γ of the reused bridge arm, the modulation range is widened to [0,1], further improving the operating characteristics of the traditional MMC; according to the basic principle of reuse of the reused bridge arm, the bridge arm energy balance angle γ is set at the voltage intersection of the two-phase AC side, and γ satisfies the operating principle that must be met in the power frequency cycle: 0≤γ≤2π / 3. If the γ adjustment angle is greater than 2π / 3, the reused bridge arm will not be able to balance the energy fluctuations in the power frequency cycle, the power transmitted by the conventional bridge arm and the power transmitted by the reused bridge arm will no longer match in the power frequency cycle, and the MMC topology will not be able to work stably.

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

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

[0045] Mode 2: In 0≤ωt≤ω2,ω7≤ωt≤ω9, {S alk1 / S alk3 ,S alk2}The switch state is {0,1}, at this time, the normal bridge arm current ial under phase a bypasses all TMSMs in the multiplexed bridge arm;

[0046] Mode 3: When 0≤ωt≤ω1 and ω5≤ωt≤ω9, {S blk1 / S blk3 ,S blk2} is in the switching state of {1,0}, at which time the normal bridge arm current ibl of phase b flows through the b-phase half-bridge circuits of all TMSMs in the lower multiplexing bridge arm;

[0047] Mode 4: When ω1≤ωt≤ω5, {S blk1 / S blk3 ,S blk2}The switch state is {0,1}, at this time, the normal bridge arm current ibl under phase b bypasses all TMSMs in the multiplexed bridge arm;

[0048] Mode 5: In 0≤ωt≤ω4,ω8≤ωt≤ω9, {S clk1 / S clk3 ,S clk2} is in the switching state of {1,0}, at which time the c-phase lower conventional bridge arm current icl flows through the c-phase half-bridge circuits of all TMSMs in the lower multiplexing bridge arm;

[0049] Mode 6: When ω4≤ωt≤ω8, {S clk1 / S clk3 ,S clk2}The switch state is {0,1}. At this time, the conventional bridge arm current icl in phase c bypasses all TMSMs in the multiplexed bridge arm.

[0050] Furthermore, the switching state of the TMSM three-phase half-bridge circuit in the power frequency cycle is expressed as:

[0051]

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

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

[0054]

[0055] Where V al is the modulation voltage of the lower bridge arm of phase a in the traditional MMC topology, V cal is the lower conventional bridge arm modulation voltage, V l Modulate the voltage for the lower multiplex bridge arm;

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

[0057]

[0058] Among them, V jl is the conventional bridge arm modulation voltage in phase j; V dc is the DC bus voltage; α is the proportion of the multiplexed bridge arm modulation voltage in the DC bus voltage;

[0059] Then, from the above-mentioned multiplexed bridge arm working mode and voltage basic modulation, it can be seen that the energy balance control of the multiplexed bridge arm is the key to the stable operation of TMS-MMC. When the modulation index m is constant in the topology structure, the relationship between the multiplexed bridge arm capacitor current and the multiplexed bridge arm current working mode can be expressed as:

[0060]

[0061] Among them, i j is the j-phase current, I dc is the DC bus current, S jlm1is the state of the upper switch of the mth TMSM in the lower multiplexing bridge arm, C TMSM is the capacitance value of TMSM, V TMSM (t) is the TMSM capacitor voltage at time t;

[0062] From the above formula, it can be seen that the energy fluctuation of the multiplexed bridge arm capacitor is reflected in the fluctuation of the capacitor voltage; the energy balance control of the multiplexed bridge arm is controlled by the closed-loop control of the multiplexed bridge arm capacitor voltage, and according to the two working modes of rectification and inversion, the closed-loop energy balance angle deviation Δγ involved in the energy regulation of the multiplexed bridge arm is obtained a , further superimpose the voltage control deviation component Δγ between multiplexed sub-modules d , to achieve energy balance of multiplexed submodules. Regarding the energy balance control strategy of multiplexed bridge arms, Figure 4 As shown. Get Δγ a The method is: the sum of the capacitor voltages ∑V in all three-phase multiplexing submodules cmm The low-pass filter value and mV smref The product of PI regulation, V smref is the reference value of the capacitor voltage of the conventional bridge arm submodule. The voltage balancing control of the multiplexed submodule can refer to the voltage balancing control idea of ​​the conventional bridge arm submodule. On the basis of the overall energy balance angle γ±Δγa, the voltage balancing energy balance angle deviation Δγ of the upper / lower tube conduction duty cycle of the multiplexed submodule is fine-tuned through the closed-loop voltage balancing of the submodule average value. d , ensuring the voltage balance between multiplexed submodules. Finally, based on the overall energy balance angle γ±Δγa and the voltage balance energy balance angle deviation Δγ d 、AC voltage real-time phase θ j and multiplexing control logic to obtain the driving signal S of the upper switch, middle switch and lower switch of any phase half-bridge circuit in the three-phase multiplexing submodule jlm1 , S jlm2 , S jlm3 .

[0063] Regarding the capacitance value of the multiplexed bridge arm TMSM and the conventional bridge arm SM, the maximum bridge arm energy fluctuation is the fundamental basis for selecting the capacitance value. Furthermore, the power transmitted by the multiplexed bridge arm is shown in the following formula:

[0064]

[0065] Then the maximum energy fluctuation ΔE of the lower multiplexing bridge arm is larm It can be expressed as:

[0066]

[0067] Where t1 and t2 are the moments when all TMSMs are switched on and off when only the current of the conventional bridge arm of phase a flows through the lower multiplexing bridge arm; I m is the maximum amplitude of the AC current; φ is the phase difference between the phase voltage and the phase current.

[0068] Conventional bridge arm transmission power P carm (t) is expressed as:

[0069]

[0070] Then the maximum energy fluctuation ΔE of the conventional bridge arm is carm It can be expressed as:

[0071]

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

[0073] The relationship between the maximum energy fluctuation of the bridge arm and the capacitance of the submodule is:

[0074]

[0075] In the formula, C csm , C TMSM are the capacitance values ​​of the conventional bridge arm SM and TMSM respectively; N csm 、N TMSM are the number of conventional bridge arm and multiplexed bridge arm submodules respectively; ε is the capacitor voltage fluctuation rate; V sm is the capacitor voltage of the conventional bridge arm submodule.

[0076] Based on the engineering parameters: DC bus voltage ±200kV, transmission capacity 400MVA, AC voltage 311kV, single bridge arm SM number 250, capacitor voltage fluctuation rate ε 5%, submodule voltage 1.6kV, submodule capacitance 12mF. Under unity power factor and modulation index m = 0.9, the conventional bridge arm capacitance of TMSM-MMC is calculated to be about 9.8mF, and the reused bridge arm capacitance voltage is about 1.83mF; the topology structure has greatly reduced capacitance requirements compared to traditional MMC, greatly reducing the capacitor volume and improving economic efficiency.

[0077] The existing MMC, three-phase bridge arm multiplexing MMC (Three Phase Multiplexing Arm MMC, TPMA-MMC), and three-phase multiplexing sub-module MMC topology are compared, as shown in Figure 5(a), Figure 5(b), and Figure 5(c). It can be found that compared with the traditional MMC, TPMA-MMC saves the number of SMs required for MMC as a whole by constructing a multiplexing bridge arm constructed by half-bridge sub-modules. Compared with TPMA-MMC, the devices in the three-phase switch circuit of TPMA-MMC are under greater pressure and require a large number of devices to be connected in series. The TMSM constructed by the proposed TMSM-MMC solves the problem of three-phase switch bridge devices in series in TPMA-MMC on the basis of meeting the multiplexing function, thereby improving the engineering application value of TMSM-MMC.

[0078] Furthermore, based on the system simulation parameters: DC bus voltage ±10kV, system capacity S=1MVA, 3 multiplexed bridge arm submodules, 9 conventional bridge arm submodules, submodule voltage 1.67kV, modulation ratio 0.9, the simulation results are as follows: Figure 6 As shown, a 、i b 、i c They are abc three-phase currents respectively; i au 、i al are the upper and lower conventional submodule bridge arm currents of phase a respectively; i u 、i l are the bridge arm currents of the upper and lower multiplexing submodules respectively; V cau 、V cal They are the upper and lower conventional submodule bridge arm voltages of phase A respectively; V mu 、V ml are the upper and lower multiplexing bridge arm voltages respectively; V causm 、V calsm They are the upper and lower conventional bridge arm submodule voltages of phase A respectively; V musm 、V mlsm They are the voltages of the upper and lower multiplexed bridge arm submodules, respectively. From the simulation results, the topological conventional bridge arm and multiplexed bridge arm modulation, conventional SM and TMSM capacitor voltages all operate stably, and all control strategies are consistent with traditional MMC. From the simulation results, the topological conventional bridge arm and multiplexed bridge arm modulation, conventional SM and TMSM capacitor voltages all operate stably, and all control strategies are consistent with traditional MMC. The proposed topology solves the problem of series connection of three-phase switch bridge arm devices by constructing a new type of multiplexed submodule (TMSM) and using TMSM to construct a multiplexed bridge arm, thereby improving the application value of MMC topology engineering. Compared with traditional MMC, taking the working condition m=0.9 as an example, the number of capacitors in the topological structure proposed in this application is reduced by 21.3%, which greatly reduces the footprint and weight of the MMC converter station, and greatly improves the lightweight level of MMC.

[0079] The embodiments of the present invention are described in detail above with reference to the accompanying drawings, but the present invention is not limited to the above embodiments, and various changes can be made within the knowledge scope of ordinary technicians in this field without departing from the purpose of the present invention.

Claims

1. A high power density three-phase multiplexing submodule type MMC topology, characterized in that: include: The three-phase conventional bridge arm, each phase conventional bridge arm is composed of at least one sub-module in series. The three-phase lower conventional bridge arm, each phase lower conventional bridge arm is composed of at least one sub-module connected in series, and the upper end of each phase lower conventional bridge arm is connected to the lower end of the corresponding phase upper conventional bridge arm to form the midpoint of each phase bridge arm. The three-phase multiplexing bridge arm, each phase multiplexing bridge arm is composed of at least one three-phase multiplexing sub-module connected in series, the upper end of each phase multiplexing bridge arm is respectively connected to the positive electrode of the DC port, and the lower end of each phase multiplexing bridge arm is connected to the upper end of the corresponding phase conventional bridge arm, and, The three-phase lower multiplexing bridge arm, each phase lower multiplexing bridge arm is composed of at least one three-phase multiplexing submodule connected in series, the upper end of each phase lower multiplexing bridge arm is connected to the lower end of the corresponding phase lower conventional bridge arm, and the lower end of each phase lower multiplexing bridge arm is respectively connected to the negative electrode of the DC port; When the current of any phase conventional bridge arm flows through the corresponding phase multiplexing bridge arm, all three-phase multiplexing submodules are put into operation; When the current of the conventional bridge arm of any phase bypasses the corresponding phase multiplexing bridge arm, all three-phase multiplexing sub-modules are removed.

2. A high power density three-phase multiplexing submodule MMC topology according to claim 1, characterized in that: The three-phase multiplexing submodule includes a three-phase switch half-bridge circuit and a multiplexing capacitor. The half-bridge circuit of any phase is composed of an upper switch, a middle switch and a lower switch connected in series. The current input terminals of the upper switches in each phase half-bridge circuit are connected in parallel. The middle point formed by connecting the current output terminal of the upper switch and the current input terminal of the middle switch in any phase half-bridge circuit serves as the upper end of the corresponding phase half-bridge circuit. The middle point formed by connecting the current output terminal of the middle switch and the current input terminal of the lower switch in any phase half-bridge circuit serves as the lower end of the corresponding phase half-bridge circuit. The current output terminals of the lower switches in each phase half-bridge circuit are connected in parallel, and the multiplexing capacitor is connected in parallel with each phase half-bridge circuit. The driving signals of the same phase half-bridge circuits of all three-phase multiplexing submodules in the same multiplexing 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, and the driving signal of the middle switch is complementary to the driving signals of the upper switch and the lower switch. The upper end of each phase half-bridge circuit in the first three-phase multiplexing submodule constitutes the upper end of the upper / lower multiplexing bridge arm, the upper end of each phase half-bridge circuit in the kth three-phase multiplexing submodule is connected to the lower end of each phase half-bridge circuit in the k-1th three-phase multiplexing submodule, the lower end of each phase half-bridge circuit in the kth three-phase multiplexing submodule is connected to the upper end of each phase half-bridge circuit in the k+1th three-phase multiplexing submodule, and the lower end of each phase half-bridge circuit in the mth three-phase multiplexing submodule constitutes the lower end of the upper / lower multiplexing bridge arm, 1≤k≤m, k and m are both integers.

3. According to the high power density three-phase multiplexing sub-module type MMC topology of claim 1, it is characterized in that: The upper switch, the middle switch and the lower switch are one or more of fully controlled devices, combined devices including fully controlled devices and half-controlled devices, other semiconductor devices or mechanical switches.

4. According to claim 1, a high power density three-phase multiplexing sub-module type MMC topology, characterized in that: The upper conventional bridge arm of each phase is also connected in series with the upper bridge arm inductor of the corresponding phase, and the lower conventional bridge arm of each phase is also connected in series with the lower bridge arm inductor of the corresponding phase. The midpoint of each phase bridge arm is connected to the corresponding phase AC through the filter inductor.

5. According to claim 1, a high power density three-phase multiplexing sub-module type MMC topology, characterized in that: The number of submodules connected in series in the conventional bridge arm on each phase is equal to the number of submodules connected in series in the conventional bridge arm below the corresponding phase, and the number of three-phase multiplexed submodules connected in series in the upper multiplexed bridge arm on each phase is equal to the number of three-phase multiplexed submodules connected in series in the lower multiplexed bridge arm.

6. A high power density three-phase multiplexing submodule MMC topology according to claim 1, characterized in that: The submodule is a half-bridge submodule, a full-bridge submodule or a submodule of another functional type.

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

8. The modulation method of a high power density three-phase multiplexing sub-module type MMC topology according to claim 7, characterized in that: According to the bridge arm energy balance angle, the upper / lower conventional bridge arms of each phase are controlled to time-share multiplex the upper / lower multiplexed bridge arms within the power frequency cycle. Specifically, the drive signals 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 through the following control strategy, and the control strategy is: According to the two working modes of rectification and inversion, the closed-loop energy balance angle deviation Δγ participating in the energy regulation of the up / down multiplexing bridge arm is obtained a , obtain the overall energy balance angle γ±Δγa; The voltage balancing control is performed on each three-phase multiplexing submodule in the upper / lower multiplexing bridge arm, and the closed-loop fine-tuning of the voltage balancing energy balance angle deviation Δγ related to the upper / lower switch conduction duty cycle in the three-phase multiplexing submodule is performed. d ; Based on the overall energy balance angle γ±Δγa and the pressure-equalizing energy balance angle deviation Δγ d , AC voltage real-time phase and multiplexing control logic, and obtain the driving signals of the upper switch, middle switch and lower switch of any phase half-bridge circuit in the three-phase multiplexing sub-module.

9. The modulation method of a high power density three-phase multiplexing sub-module type MMC topology according to claim 8, characterized in that: The obtained drive signals of the upper switch, the middle switch and the lower switch of any phase half-bridge circuit in the three-phase multiplexing submodule meet the following constraints: Among them, S jlk1 , S jlk2 , S jlk3 are the states of the upper switch, middle switch and lower switch of the kth TMSM in the lower multiplexing bridge arm, ω is the angular frequency, is the initial phase of phase j voltage.

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

Images