Modulation method for modular multilevel converter

Abstract

The application discloses a modular multilevel converter modulation method and belongs to the field of power systems. The modular multilevel converter topology comprises an upper bridge arm, a middle bridge arm, a lower bridge arm, a bridge arm switching switch group and a bridge arm inductor. The upper bridge arm, the middle bridge arm, the lower bridge arm and the bridge arm inductor of any phase are connected in series; the bridge arm switching switch group is connected at both ends of the middle bridge arm; an alternating current port is connected to the middle of the bridge arm switching switch group; the modular multilevel converter modulation method is characterized in that the voltage of the middle bridge arm is twice the frequency modulation, the on-off of the bridge arm switching switch group is controlled, the middle bridge arm is half-cycle multiplexed by the upper bridge arm and the lower bridge arm, meanwhile, the middle bridge arm can balance energy in a half-cycle of the frequency. In addition, the modulation method effectively improves the operation requirement of the topology under different operation conditions.

Description

Technical Field

[0001] This invention relates to the field of power systems, and more specifically to a modular multilevel converter modulation method. Background Technology

[0002] Modular multilevel converters (MMCs) currently demonstrate great potential in medium-voltage DC transmission and power exchange due to their high modularity, low harmonics, and high reliability. However, in actual DC transmission projects, MMCs have revealed problems such as large footprint, low power density, and high cost. The large number of MMC submodules is one of the reasons for these issues. Chinese patent CN111656670A (Modular Multilevel Converter) discloses an MMC topology where the intermediate conversion arm is multiplexed by the upper and lower conversion arms. By controlling the control unit, the intermediate conversion arm is multiplexed by the upper and lower conversion arms for half a power frequency cycle, thereby reducing the number of MMC submodules. However, this patent emphasizes that the upper / lower connection element is activated when the voltage of the lower / upper conversion arm is higher than that of the upper / lower conversion arm, thus requiring a specific modulation method to maintain the energy balance of the intermediate conversion arm. In one optimal submodule configuration, the number of intermediate conversion arm submodules is equal to the number of upper and lower conversion arm submodules. In this case, to realize the energy of the intermediate conversion arm, there needs to be a difference in AC side voltage and current, which reduces the transmission efficiency. At the same time, the reactive power transmitted in the topology will increase the fluctuation of the submodule capacitor voltage, thereby increasing the size of the submodule capacitor and increasing the cost. Summary of the Invention

[0003] To address the shortcomings of existing technologies, this invention proposes a modular multilevel converter modulation method.

[0004] The objective of this invention can be achieved through the following technical solutions:

[0005] A modulation method for a modular multilevel converter, wherein the topology of the modular multilevel converter includes: an upper bridge arm, a middle bridge arm, a lower bridge arm, a bridge arm switching group, and a bridge arm inductor; the upper bridge arm, middle bridge arm, lower bridge arm, and bridge arm inductor of any phase are connected in series; the bridge arm switching group is respectively connected to both ends of the middle bridge arm;

[0006] The modulation method includes the following steps:

[0007] The intermediate bridge arm voltage is modulated by twice the power frequency. When the lower switch of the bridge arm switching switch group is turned on and the upper switch is turned off, and the intermediate bridge arm is reused by the upper bridge arm for half the power frequency cycle, the upper bridge arm and the intermediate bridge arm jointly modulate the required bridge arm voltage, and the energy of the intermediate bridge arm is balanced; the lower bridge arm is modulated by normal sinusoidal voltage.

[0008] When the upper switch of the bridge arm switching switch group is turned on and the lower switch is turned off, the middle bridge arm is reused by the lower bridge arm at half the power frequency cycle. The lower bridge arm and the middle bridge arm jointly modulate the required bridge arm voltage, and the energy of the middle bridge arm is balanced; the upper bridge arm is modulated by normal sinusoidal voltage.

[0009] Optionally, the upper and lower tubes of any phase bridge arm switching switch group are complementaryly connected, and the operating state is as follows:

[0010]

[0011] m = 2v j / v dc

[0012] Where γ is the energy balance control angle of the multiplexed submodule bridge arm. Let j be the initial phase of the voltage. The phase difference between AC voltage and current, ω is the power frequency angular frequency, m is the modulation ratio, and v j V is the phase voltage on the AC side of phase j. dc S is the DC port voltage. j1 and S j2 These represent the switching states of the upper and lower tubes of the j-phase bridge arm switching switch group, respectively.

[0013] Optionally, the reference voltage signal of the intermediate bridge arm is a piecewise function, expressed as follows:

[0014]

[0015] Among them, v jm α1, α2, α3, and α4 are the modulation voltages of the middle bridge arm.

[0016] Optionally, the intermediate bridge arm achieves half-cycle energy balance. Energy balance can be achieved when the intermediate multiplexed bridge arm is half-cycle multiplexed by the upper bridge arm; energy balance can also be achieved when the intermediate multiplexed bridge arm is half-cycle multiplexed by the lower bridge arm, both based on the same balance principle. The energy accumulation when the intermediate bridge arm is half-cycle multiplexed by the lower bridge arm is expressed as:

[0017]

[0018] By calculating the above formula, we can obtain the energy balance control angle γ and modulation index m, and power factor of the lower and middle bridge arms with fixed α1, α2, α3, and α4. Relational expressions.

[0019] Optionally, both the upper and lower bridge arms can achieve energy balance during the power frequency cycle, and they share the same balance principle. The energy accumulation of the lower bridge arm during the power frequency cycle is expressed as follows:

[0020]

[0021] Optionally, the modulation voltage of any one phase upper and lower bridge arm is:

[0022]

[0023] Among them, v ju v jl These are the modulation voltages of the upper and lower bridge arms of phase j, respectively.

[0024] Optionally, the multiplexed submodule bridge arms and conventional submodule bridge arms of the topology of the modular multilevel converter are configured as half-bridge submodules, full-bridge submodules, other structure submodules, or hybrid submodules.

[0025] Optionally, the three-phase full-bridge switch of the modular multilevel converter topology is configured as a series fully controlled power device or a reverse parallel semi-controlled device.

[0026] The beneficial effects of this invention are:

[0027] By rationally designing the modulation voltage and energy balance control angle of the intermediate bridge arm, the half-cycle energy balance problem of the intermediate bridge arm under different operating conditions was solved. Simultaneously, the capacitance requirements of the submodules caused by the reactive power of the topology transmission were further reduced, thereby decreasing the converter size and cost. Attached Figure Description

[0028] The invention will now be further described with reference to the accompanying drawings.

[0029] Figure 1 This is a topology diagram of the modular multilevel converter in an embodiment of the present invention;

[0030] Figure 2 This is a modular multilevel converter modulation method in an embodiment of the present invention;

[0031] Figure 3 This is the bridge arm switching switch group drive signal in the embodiment of the present invention;

[0032] Figure 4 This is a graph showing the relationship between the energy balance control angle γ of the multiplexed submodule bridge arm and the modulation ratio m.

[0033] Figure 5 The above are the simulation results of the bridge arm voltage, current and submodule voltage under the modulation method when the modulation ratio is 0.9 in the embodiment of the present invention. Detailed Implementation

[0034] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] In some embodiments of the present invention, such as Figure 1 As shown, the modular multilevel converter topology includes an upper bridge arm, a middle bridge arm, a lower bridge arm, a bridge arm switching group, and a bridge arm inductor. The upper bridge arm, middle bridge arm, lower bridge arm, and bridge arm inductor of any phase are connected in series; the bridge arm switching group is connected to both ends of the middle bridge arm.

[0036] In some embodiments of the present invention, the multiplexed submodule bridge arm and the conventional submodule bridge arm can be selected as half-bridge submodules, full-bridge submodules, other structural submodules or hybrid submodules; the three-phase full-bridge switch can be composed of series-connected fully controlled power devices or anti-parallel semi-controlled devices;

[0037] In this embodiment, the upper, middle, and lower bridge arms all employ half-submodules (HBSMs). The following describes in detail a modular multilevel converter modulation method of the present invention, such as... Figure 2 As shown. The intermediate bridge arm voltage is modulated at twice the power frequency; when the intermediate bridge arm is multiplexed by the upper bridge arm for half a power frequency cycle, the upper and intermediate bridge arms jointly modulate the required bridge arm voltage, and the intermediate bridge arm energy is balanced; the lower bridge arm is modulated by a normal sinusoidal voltage. When the intermediate bridge arm is multiplexed by the lower bridge arm for half a power frequency cycle, the lower and intermediate bridge arms jointly modulate the required bridge arm voltage, and the intermediate bridge arm energy is balanced; the upper bridge arm is modulated by a normal sinusoidal voltage.

[0038] The drive signals for the upper and lower three-phase bridge switches are as follows: Figure 3 As shown, the upper and lower tubes of any phase bridge arm switching switch group are complementaryly connected, and the working state is as follows:

[0039]

[0040] m = 2v j / v dc (2)

[0041] Where γ is the energy balance control angle of the multiplexed submodule bridge arm. Let j be the initial phase of the voltage. The phase difference between AC voltage and current, ω is the power frequency angular frequency, m is the modulation ratio, and v j V is the phase voltage on the AC side of phase j. dc S is the DC port voltage. j1 and S j2These represent the switching states of the upper and lower tubes of the j-phase bridge arm switching group, respectively. When (S j1 ,S j2 When )=(1,0), the middle bridge arm is multiplexed by the lower bridge arm of phase j for half the power frequency cycle; (S j1 ,S j2 When )=(0,1), the middle bridge arm is multiplexed by the upper bridge arm of phase j at half the power frequency cycle.

[0042] The intermediate bridge arm modulation voltage is twice the power frequency modulation, and α1, α2, α3, and α4 are defined as follows:

[0043]

[0044] The reference voltage signal for the intermediate bridge arm is a piecewise function, as shown below:

[0045]

[0046] Among them, v jmmax The maximum modulation voltage of the j-phase intermediate multiplexed bridge arm can be expressed as:

[0047]

[0048] Furthermore, the modulation voltages of the upper and lower bridge arms can be expressed as follows:

[0049]

[0050] The energy balance principle of the middle bridge arm will be analyzed in detail below.

[0051] Because the energy balance is consistent when the middle bridge arm is reused by the upper and lower bridge arms, the analysis will be conducted using the middle and lower bridge arms of phase A as an example.

[0052] The energy accumulation of the intermediate bridge arm during the power frequency half-cycle is expressed as:

[0053]

[0054] By calculating (7), the energy balance control angle γ of the multiplexed submodule bridge arm, the modulation index m, and the power factor can be obtained. The relational expression is (8).

[0055]

[0056] when When the power factor is 1, i.e., when the converter is operating at unity power factor, equation (8) can be expressed as follows:

[0057]

[0058] Similarly, the upper and lower bridge arms can also achieve energy balance during the power frequency cycle. Taking the lower bridge arm as an example, its energy accumulation is (10).

[0059]

[0060] Within the range of 0 ≤ γ ≤ 2π, the calculation result is 0, indicating that the upper and lower bridge arms can also achieve energy balance under the proposed modulation method.

[0061] The following analysis examines the configuration of each bridge arm submodule.

[0062] Plotting the implicit function of (9) in the case of 0≤γ≤π / 2, 0≤m≤1, we can obtain the relationship between γ and modulation index m when the energy of the middle bridge arm is balanced, as follows: Figure 4 As shown. From Figure 4 It can be seen that when m=1, γ=0.1158, and at this time v ammax =0.2v dc The maximum modulation voltage of the upper and lower bridge arms is v. aumax =v almax =0.8v dc Compared to the traditional MMC, which has a total of 6n sub-modules (n for a single bridge arm, n = v), dc / v sm v sm (This refers to the submodule voltage), in which case 10% of the submodule can be saved. In an extreme case, when m = 0, γ = 1.231, at this time, v ammax =0.5v dc .

[0063] In addition, the capacitance requirements of the intermediate bridge arm and upper / lower bridge arm sub-modules are analyzed and calculated. Taking the transmission parameters of a converter station in Zhoushan as a reference, v sm The DC bus voltage is 1.6kV. dc The voltage is 400kV, with 250 submodules per bridge arm, ε = 5%, m = 0.9, and a capacity of 400MVA. Under this modulation method, the intermediate bridge arm HBSM is n. m =50, upper / lower arm HBSM adopts n c =200. Taking the middle bridge arm of phase A as an example, its maximum energy fluctuation in arcsin(m / 2) + γ ≤ ωt ≤ π - arcsin(m / 2) is

[0064]

[0065] The energy fluctuation of the intermediate bridge arm can also be expressed as

[0066] ΔE am =2n m C msm v sm 2ε (12)

[0067] ΔE was calculated am Given 0.13MW, combining equations (11) and (12), we obtain C. msm =10.15mF.

[0068] Similarly, the energy fluctuation of the lower bridge arm in phase A is

[0069]

[0070] The energy fluctuation of the lower bridge arm in phase A can also be expressed as...

[0071] ΔE al =2n al C csm v sm 2 ε (14)

[0072] ΔE was calculated al Given 0.542MW, combining equations (13) and (14), we obtain C. csm =10.58mF. Meanwhile, the capacitance of a submodule at a converter station in Zhoushan is 12mF. The capacitance requirement for the middle bridge arm submodule can be reduced by 15.4%, and the capacitance requirement for the lower bridge arm submodule can be reduced by 11.83%. Therefore, in summary, the proposed modular multilevel converter, using the proposed modulation method, not only reduces the number of submodules and lowers costs, but also significantly reduces the size and weight of the device.

[0073] Furthermore, the proposed modulation method for the modular multilevel converter was simulated and verified at ±10kV, S=1MVA. There were two intermediate bridge arm sub-modules, and ten upper and ten lower bridge arm sub-modules each. The sub-module voltage was 1.67kV, and the modulation ratio was 0.9. The simulation results are as follows: Figure 5 As shown, i au i al These are the upper and lower bridge arm currents of phase A, i am For the middle bridge arm current, v au v al These are the upper and lower bridge arm voltages of phase A, respectively, v am The voltage of the middle bridge arm, v ausm v alsm These are the voltages of the upper and lower bridge arm submodules of phase A, respectively, v amsm This refers to the voltage of the intermediate bridge arm submodule.

[0074] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0075] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A modular multilevel converter modulation method, characterized in that, The topology of the modular multilevel converter includes: an upper bridge arm, a middle bridge arm, a lower bridge arm, a bridge arm switching group, and a bridge arm inductor; the upper bridge arm, middle bridge arm, lower bridge arm, and bridge arm inductor of any phase are connected in series; the bridge arm switching group is connected to both ends of the middle bridge arm respectively; The modulation method includes the following steps: The intermediate bridge arm voltage is modulated by twice the power frequency. When the lower switch of the bridge arm switching switch group is turned on and the upper switch is turned off, and the intermediate bridge arm is reused by the upper bridge arm for half the power frequency cycle, the upper bridge arm and the intermediate bridge arm jointly modulate the required bridge arm voltage, and the energy of the intermediate bridge arm is balanced; the lower bridge arm is modulated by normal sinusoidal voltage. When the upper switch of the bridge arm switching group is turned on and the lower switch is turned off, the middle bridge arm is reused by the lower bridge arm for half a power frequency cycle. The lower bridge arm and the middle bridge arm jointly modulate the required bridge arm voltage, and the energy of the middle bridge arm is balanced; the upper bridge arm is modulated by normal sinusoidal voltage. The upper and lower switches of any phase bridge arm switching group are complementaryly connected, and their operating states are as follows: in, γ For the energy balance control angle of the middle bridge arm, φ j for j Phase voltage initial phase, φ The phase difference between AC side voltage and current. ω It is the power frequency angular frequency. m The modulation ratio, v j for j Phase voltage on the AC side, v dc This is the DC port voltage. S j1 and S j2 They are respectively j The switching status of the upper and lower switches of the phase bridge arm switching switch group; The modulation voltage signal of the intermediate bridge arm is a piecewise function, expressed as follows: in, v jm The modulation voltage for the middle bridge arm. α 1 , α 2 , α 3 and α 4 This is the proportionality coefficient; in, v jmmax yes j Maximum modulation voltage of the intermediate bridge arm; The intermediate bridge arm achieves energy balance during half-cycle power frequency operation. Energy balance can be achieved when the intermediate bridge arm is reused by the upper bridge arm during half-cycle power frequency operation; energy balance can also be achieved when the intermediate bridge arm is reused by the lower bridge arm during half-cycle power frequency operation, and both share the same balancing principle. The energy accumulation when the intermediate bridge arm is reused by the lower bridge arm during half-cycle power frequency operation is expressed as follows: By calculating the above formula, we obtain a fixed value. α 1 、α 2 、α 3 and α 4 At that time, the energy balance control angle between the lower bridge arm and the middle bridge arm γ With modulation ratio m Power factor cosφ Relational expressions.

2. The modular multilevel converter modulation method according to claim 1, characterized in that, Both the upper and lower bridge arms can achieve energy balance during the power frequency cycle, and they share the same balance principle. The energy accumulation of the lower bridge arm during the power frequency cycle is expressed as follows: v jl The modulation voltage of the lower bridge arm of phase j.

3. The modular multilevel converter modulation method according to claim 1, characterized in that, The modulation voltage of any one phase upper and lower bridge arm is: in, v ju , v jl These are the modulation voltages of the upper and lower bridge arms of phase j, respectively.

4. The modular multilevel converter modulation method according to claim 1, characterized in that, The multiplexed submodule bridge arms and conventional submodule bridge arms of the topology of the modular multilevel converter are configured as half-bridge submodules, full-bridge submodules, or hybrid submodules.

5. The modular multilevel converter modulation method according to claim 1, characterized in that, The topology of the modular multilevel converter is configured with bridge arm switching groups consisting of series-connected fully controlled power devices or anti-parallel semi-controlled devices.

6. A modular multilevel converter modulated by any one of the modulation methods described in claims 1 to 5.

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