Capacitor voltage control method for hybrid modular multilevel converter

Abstract

The present invention proposes a capacitor voltage control method suitable for a hybrid modular multi-level converter. The present invention expands the optimal point limit of AAMC energy balance and expands the voltage regulation range through current commutation modulation and capacitor voltage compensation control. In addition, the present invention can solve the influence of the commutation interval caused by the bridge arm inductance on the energy balance. First, the switch module in the bridge arm is adjusted by the current commutation modulation method so that it switches at the zero point of the bridge arm current. The current commutation process is analyzed and derived in detail through analytical expressions. The mechanism of capacitor voltage drop caused by the rectification process is further explained, which provides a theoretical basis for the design of the controller. On this basis, voltage compensation control of capacitor voltage drop is proposed to achieve normal operation of the capacitor in four quadrants. Finally, the effectiveness of the proposed modulation and control strategies under different conditions is verified by simulation and experimental results.

Description

Technical Field

[0001] This invention belongs to the field of flexible DC power transmission and distribution technology, specifically relating to a capacitor voltage control method suitable for hybrid modular multilevel converters. Background Technology

[0002] Modular multilevel converters have recently attracted widespread attention in high-voltage direct current transmission systems due to their advantages of high modularity, high robustness, and high controllability. However, traditional half-bridge modular multilevel converters (MMCs) require a large number of sub-modules and lack DC fault ride-through capability.

[0003] Based on this, the academic community has proposed various hybrid modular multilevel converters (MMCs), where the bridge arms are composed of a mixture of submodules and power devices. This topology possesses the structural characteristics and advantages of MMCs, and has DC fault interruption capability. It employs an alternating conduction mode for the bridge arms, eliminating commutation between arms, and requires fewer power devices, resulting in lower cost. However, due to the commutation mode, the energy balance of the bridge arms needs to be considered. When considering the arm inductance, overlapping conduction intervals occur during commutation, leading to energy exchange between upper and lower arms. This causes the DC component of the submodule capacitor to experience amplitude drops due to the commutation overlap angle, a phenomenon more pronounced when the arm inductance is large. To address this issue, it is necessary to research submodule capacitor voltage compensation control strategies for modular multilevel converters. Summary of the Invention

[0004] In view of the above, the present invention provides a capacitor voltage control method suitable for hybrid modular multilevel converters. This method takes into account the influence of bridge arm inductance and designs current commutation modulation, which solves the problem of capacitor voltage amplitude drop and ensures stable output of the converter.

[0005] This invention provides a capacitor voltage control method suitable for hybrid modular multilevel converters, comprising the following steps:

[0006] (1) Analyze the expressions for bridge arm voltage and bridge arm current that take into account the submodule capacitor and bridge arm inductor, and thus obtain the relationship between the commutation overlap angle and the system parameters during the commutation process of the upper and lower bridge arms. The system parameters are voltage modulation ratio and power factor angle.

[0007] (2) By using the bridge arm voltage modulation signal and the bridge arm current expression, the DC component of the submodule capacitor voltage is obtained, and a mathematical model of the submodule capacitor voltage is established.

[0008] (3) Design a feedback controller to compensate for the voltage amplitude of the submodule capacitor. The output of the feedback controller is superimposed with the DC component of the bridge arm voltage reference signal to form a control closed loop, thereby realizing the compensation control of the capacitor voltage of the hybrid modular multilevel converter.

[0009] The bridge arm of the hybrid modular multilevel converter described in this invention is composed of a valve string section with sub-modules connected in series and a switching module with power devices connected in series.

[0010] As a preferred embodiment of the present invention, the specific implementation process of step (1) is as follows:

[0011] (1.1) Taking into account the influence of bridge arm inductance, overlap will occur when the upper and lower bridge arms commutate. Assuming that the voltage at the midpoint of the bridge arm is a square wave, let the bridge arm switch at the zero crossing point of the AC current and design a current commutation modulation method.

[0012] (1.2) The bridge arm voltage is the sum of the valve string voltage, the switching module voltage, and the inductor voltage. The valve string voltage expression and the switching module modulation signal are obtained according to the current commutation modulation method. During the commutation period, the sum of the valve string voltage and the inductor voltage is equal to the DC bus voltage. Based on the relationship between the valve string voltage and the inductor voltage during the commutation period, and taking into account the sub-module capacitance and the bridge arm inductance, the inductor voltage expression and the bridge arm current expression are obtained.

[0013] (1.3) From the expression of the arm current obtained in step (1.2), the DC component of the arm current is obtained; according to the energy balance, the relationship between the AC side current and the DC bus current is obtained; according to the topological symmetry, the relationship between the DC component of the arm current and the DC bus current is obtained; by combining the above relationships, the relationship between the commutation overlap angle of the upper and lower arms and the influence of system parameters can be obtained.

[0014] The relationship between the AC side current and the DC bus current in step (1.3) is as follows:

[0015]

[0016] Among them, i dc IC is the DC bus current, and I1 is the AC side current amplitude. θ is the power factor angle, and m is the voltage modulation ratio.

[0017] As a preferred embodiment of the present invention, the specific implementation process of step (2) is as follows:

[0018] (2.1) The number of sub-modules can be obtained from the expression of the bridge arm valve string voltage. The upper bridge arm valve string voltage is calculated using the capacitor voltage fluctuation rate, the DC component of the capacitor voltage and the number of upper bridge arm sub-modules. The lower bridge arm valve string voltage is calculated using the DC component of the capacitor voltage, the bridge arm current, the sub-module capacitance and the number of lower bridge arm sub-modules. Based on the relationship between the valve string voltage, the switch module voltage and the inductor voltage, the expression of the switch module voltage is obtained, and the expression of the DC component of the switch module voltage is further obtained.

[0019] (2.2) Since the expression obtained in step (2.1) is related to the power factor angle, commutation overlap angle and voltage modulation ratio, and combined with the relationship between the DC bus voltage, valve string voltage and the DC component of the switching module voltage, the expression of the sub-module capacitor voltage under the influence of system parameters is obtained, that is, the mathematical model of the sub-module capacitor voltage. Through this expression, it is found that the module capacitor voltage has an amplitude drop under the influence of parameters such as the power factor angle, commutation overlap angle and voltage modulation ratio.

[0020] Furthermore, the mathematical model for the submodule capacitor voltage in step (2.2) is as follows:

[0021]

[0022] Among them, u dc The DC bus voltage, u C0 β is the DC component of the submodule capacitor voltage, β is the commutation overlap angle, L is the bridge arm inductance value, C is the bridge arm capacitance value, N is the number of individual bridge arm submodules, ω is the angular frequency, and ε is the capacitor voltage fluctuation rate.

[0023] As a preferred embodiment of the present invention, the specific implementation process of step (3) is as follows:

[0024] (3.1) Generate bridge arm voltage reference signal based on bridge arm voltage; take the average value of submodule capacitor voltage as the controlled variable, select the cutoff frequency and phase margin according to the system performance requirements, and design a feedback controller by combining the relationship between the DC component of valve string voltage, the DC component of switch module voltage and DC bus voltage.

[0025] (3.2) The controller output is superimposed with the DC component of the bridge arm voltage reference signal to form a control closed loop, and the capacitor voltage is controlled by adjusting the number of sub-modules switched.

[0026] Furthermore, step (3) involves designing a feedback controller using the following formula:

[0027] u SM_dc =1 / 2u dc -u DS_dc

[0028] Among them, u SM_dcFor the DC component of the valve series voltage, u DS_dc This represents the DC voltage component of the switching module.

[0029] Compared with existing technologies, this invention designs current commutation modulation based on the influence of bridge arm inductance and analyzes the energy exchange under this modulation. This invention analyzes the mechanism of capacitor voltage sag during rectification and proposes voltage compensation control to ensure the normal operation of the topology in all four quadrants. The effectiveness of the proposed modulation and control strategies under different conditions is verified through simulation and experiments. This invention considers the influence of inductance under actual operating conditions and designs current commutation modulation and voltage compensation strategies to ensure the effectiveness of the topology, thereby fully leveraging the advantages of hybrid modular multilevel converters (MMCs) in terms of low cost and high efficiency. Furthermore, this invention is tested on an experimental platform, demonstrating that the topology can operate stably under a wide voltage regulation range under the proposed modulation and control strategies, effectively verifying the practicality of this invention through actual data. Attached Figure Description

[0030] Figure 1 This is a topology diagram of the T-shaped bridge arm alternating conduction modular circulating circuit in an embodiment of the present invention.

[0031] Figure 2 This is a waveform diagram of current commutation modulation in an embodiment of the present invention.

[0032] Figure 3 This is a diagram showing the relationship between voltage modulation ratio, commutation overlap angle, and power factor in an embodiment of the present invention.

[0033] Figure 4 This is a diagram showing the relationship between the DC component of the capacitor voltage, the commutation overlap angle, and the power factor in an embodiment of the present invention.

[0034] Figure 5 This is a block diagram of voltage compensation control in the control method of the present invention.

[0035] Figure 6 This is a Bode plot of the voltage compensation feedback control in an embodiment of the present invention.

[0036] Figure 7 This is a system control block diagram of the control method of the present invention.

[0037] Figure 8 The simulation results of AC side voltage and current are shown in the embodiment of the present invention when the power factor angle is 0 and the voltage modulation ratio is 0.7.

[0038] Figure 9 In this embodiment of the invention, the power factor angle is... Simulation results of AC side voltage and current when the voltage modulation ratio is 0.7.

[0039] Figure 10 The simulation results of AC side voltage and current are shown in the embodiment of the present invention when the power factor angle is 0 and the voltage modulation ratio is 0.9.

[0040] Figure 11 The simulation results show the capacitor voltage of phase A submodule in the case of no voltage compensation in this embodiment of the invention.

[0041] Figure 12 The simulation results show the capacitor voltage of phase A submodule under voltage compensation in this embodiment of the invention. Detailed Implementation

[0042] To describe the present invention in more detail, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0043] The bridge arm of the hybrid modular multilevel converter of the present invention is composed of a valve string section with sub-modules connected in series and a switching module with power devices connected in series, which are connected in series. The capacitor voltage control method for the hybrid modular multilevel converter proposed in this invention includes the following steps:

[0044] (1) Analyze the expressions for bridge arm voltage and bridge arm current that take into account the submodule capacitor and bridge arm inductor, and thus obtain the relationship between the commutation overlap angle and the system parameters during the commutation process of the upper and lower bridge arms. The system parameters are voltage modulation ratio and power factor angle.

[0045] (2) By using the bridge arm voltage modulation signal and the bridge arm current expression, the DC component of the submodule capacitor voltage is obtained, and a mathematical model of the submodule capacitor voltage is established.

[0046] (3) Design a feedback controller to compensate for the voltage amplitude of the submodule capacitor. The output of the feedback controller is superimposed with the DC component of the bridge arm voltage reference signal to form a control closed loop, thereby realizing the compensation control of the capacitor voltage of the hybrid modular multilevel converter.

[0047] The implementation process of step (1) in a specific embodiment is as follows:

[0048] like Figure 1 As shown, taking the T-type Alternate Arm Multilevel Converter (T-AAMC) as an example, each phase of the hybrid modular multilevel converter consists of two DC arms and one AC arm. The DC arms are composed of half-bridge sub-modules and power devices connected in series. The DC arms of the T-AAMC consist of arm inductors L, half-bridge sub-modules (HBSMs), and director switches (ds). The voltage at the midpoint of the DC arm is marked as u.mj From this, we can derive the expressions for the voltages of the relevant modules.

[0049] The voltages of the upper and lower DC bridge arms of phase j are respectively expressed as u jpDC and u jnDC :

[0050]

[0051] Where u dc The DC bus voltage, u mj This is the voltage at the midpoint between the upper and lower bridge arms.

[0052] The corresponding sub-module's input quantity n jp n jn :

[0053]

[0054] Where N is the number of individual bridge arm submodules, m is the voltage modulation ratio, and φ j Let ω be the phase angle of phase j, and ω be the angular frequency.

[0055] The voltage and current on the AC side of phase j are respectively expressed as u j and i j :

[0056]

[0057] Where U1 is the AC side voltage amplitude, I1 is the AC side current amplitude, and φ is the power factor angle.

[0058] The voltage value at the midpoint u mj for:

[0059]

[0060] Where m dc This is the DC voltage modulation ratio.

[0061] Depend on Figure 1 As shown in the structural diagram, during the overlapping conduction period, the sum of the voltages of the upper and lower DC bridge arm valve strings and the sum of the inductor voltages equal the DC bus voltage.

[0062] u jpsm +u jnsm +u jpL +u jnL =u dc

[0063] Where u jpsm and u jnsm These represent the voltages of the upper and lower bridge arm valve series, u and u, respectively. jpL and u jnL For the upper and lower bridge arm inductors.

[0064] according to Figure 2 The schematic diagram shows the upper and lower bridge arm currents i in four stages. jp i jn Meanwhile, assuming the upper arm current in the overlapping conduction interval is f(t), then we have:

[0065]

[0066] Then we can obtain the expression for the inductor voltage:

[0067]

[0068] Where L is the inductance of the bridge arm, and during commutation, both the upper and lower bridge arms are conducting simultaneously, then:

[0069]

[0070] Where C is the capacitance value of the submodule. The relationship between the valve string voltage and the bridge arm current can also be used to obtain the valve string voltage relationship. Combining the above formulas, we get f(t) as follows:

[0071]

[0072] Where L pu and C pu Here, represents the per-unit value of the bridge arm inductance and submodule capacitance, and β is the commutation overlap angle. Based on this, we obtain the following schematic diagrams: AC side voltage and current waveforms of the T-AAMC, valve string voltage and the modulation signal of the switching module, and bridge arm current waveforms. Figure 2 According to the current expression, the DC component of the bridge arm current can be obtained. Due to the symmetry of the three-phase structure, this DC component is one-third of the DC current. Based on this relationship, we can obtain:

[0073]

[0074] Where T is the period, the power factor angle is obtained from this. The relationship between the commutation overlap angle β and the voltage modulation ratio m is as follows: Figure 3 As shown.

[0075] The implementation process of step (2) in a specific embodiment is as follows:

[0076] Taking the A-phase DC upper bridge arm as an example, in the fourth stage, the capacitor voltage of the bridge arm submodule is the lowest, so the capacitor voltage fluctuation rate ε can be used to represent the upper bridge arm valve string voltage in this stage:

[0077] u apsm =n ap ·u cmin =(1-ε)u co

[0078] Where u apsm For the voltage of the upper bridge arm valve series of phase A, n ap The number of submodules deployed on the upper arm of phase A, u min This represents the minimum capacitor voltage, while the lower bridge arm valve series voltage can be expressed as the DC component of the capacitor voltage plus the voltage fluctuation:

[0079]

[0080] Where u Cave The average value of the capacitor voltage, u Crip This represents the capacitor voltage ripple. Combining this with the inductor voltage, the DC component of the upper bridge arm switching module voltage can be obtained:

[0081]

[0082] Thus, the expression for the DC component of the capacitor voltage is obtained:

[0083]

[0084] Where ε is the capacitor voltage fluctuation rate, according to β and The relationship between m and the DC component of the capacitor voltage can be obtained. The relationship of m is as follows: Figure 4 Therefore, the range of capacitance values ​​can be obtained.

[0085] The implementation process of step (3) in a specific embodiment is as follows:

[0086] Based on the above expression, the energy exchange of the AC bridge arm per cycle is calculated. Setting this value to 0, the AC / DC voltage modulation ratio relationship under normal operating conditions can be obtained as follows:

[0087] m dc =π / 4·m

[0088] Based on this and the above formula, a dual closed-loop control is constructed to control the system power, where the inner loop uses current loop decoupling control and the outer loop is for power control.

[0089] Based on the above analysis, the sum of the reference value of the DC bridge arm valve string voltage and the reference value of the DC component voltage of the switching module is 12u. dc

[0090] To address the capacitor voltage sag, capacitor voltage compensation control is implemented using a PI control design, with the following relationship:

[0091]

[0092] U dcsm_ref Δ is the reference value for the DC bridge arm valve string voltage, and Δ is the average deviation of the capacitor voltage.

[0093] Based on this, draw the voltage compensation control block diagram, such as... Figure 5 As shown, this feedback control uses the capacitor voltage of the DC valve string submodule as the controlled variable. The output Δ is adjusted via a PI controller and the difference between this output and the power loop output is used to obtain the reference voltage for the DC valve string. The number of submodules connected is adjusted based on the capacitor voltage, and the voltage is controlled by the clamping effect of the DC bus voltage in the overlapping conduction interval.

[0094] according to Figure 5 It can be known that the open-loop transfer function G of the control loop is... oC It can be represented as:

[0095]

[0096] Choosing a cutoff frequency of 10Hz and a phase margin of 47 degrees, the corresponding PI parameters are obtained:

[0097]

[0098] The amplitude-frequency and phase-frequency characteristic curves of the voltage compensation control are thus obtained as follows: Figure 6 .

[0099] In addition, by Figure 5 It can be seen that this control loop and the power control are in parallel and will not affect the power loop. Combining the two controls, the system control block diagram can be obtained, as follows: Figure 7 As shown.

[0100] Building in MATLAB Figure 1 The HBSM-TAAMC shown here consists of 12 half-bridge submodules per bridge arm. The simulation system, rated at ±200V, is powered by both AC and DC power supplies. System parameters are shown in Table 1, including rated power and voltage, passive parameters, and controller parameters. Since the DC voltage is 160V, the submodule capacitor voltage should be 16.67V. The submodule capacitor and bridge arm inductance of the simulation system are 5mF and 30mH, respectively. The voltage-compensated PI controller parameter k in this embodiment is shown. p and k i They are 9.6 and 72 respectively.

[0101] Table 1. T-AAMC Simulation Parameters

[0102]

[0103] Simulation results are as follows Figure 8-10 As shown, the wide-range voltage regulation capability of TAAMC power transmission under the capacitor voltage control method proposed in this invention is verified. Figure 8 and Figure 9 The AC voltage modulation index m is 0.7, and the power factor angle is... 0 and Figure 8 and Figure 10 of All are 0, and m is 0.7 and 0.9 respectively. By comparing the output waveforms of the three, the operating capability of the control method in the full range can be verified. Figure 11 and Figure 12 The waveforms of the 12 capacitors in phase A are shown under the same operating conditions, with and without voltage compensation. Comparing the two, it can be seen that without voltage compensation, the capacitor voltage amplitude drops by 12%, while with voltage compensation control, the capacitor voltage is basically stable at 16.67V.

[0104] Under different modulation and power factor conditions, T-AAMC can operate normally and the capacitor voltage can be maintained at around 16.67V with a fluctuation rate of less than 3%. Therefore, the proposed control method can effectively maintain the capacitor voltage of the submodule.

[0105] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the appended claims.

Claims

1. A capacitor voltage control method applicable to a hybrid modular multi-level converter, characterized in that: The steps include: (1) Analyze the bridge arm voltage expression and bridge arm current expression taking into account the submodule capacitance and bridge arm inductance, so as to obtain the relationship between the commutation overlap angle and the influence of system parameters during the commutation of the upper and lower bridge arms, wherein the system parameters are the voltage modulation ratio and the power factor angle; (2) Obtain the DC component of the submodule capacitor voltage through the bridge arm voltage modulation signal and the bridge arm current expression, and establish a mathematical model of the submodule capacitor voltage; The specific implementation process of step (2) is as follows: (2.1) According to the bridge arm valve string voltage expression, the number of submodules put into operation can be obtained. The upper bridge arm valve string voltage is calculated using the capacitor voltage fluctuation rate, the DC component of the capacitor voltage, and the number of upper bridge arm submodules put into operation. The lower bridge arm valve string voltage is calculated using the DC component of the capacitor voltage, the bridge arm current, the submodule capacitance, and the number of lower bridge arm submodules put into operation. According to the relationship between the valve string voltage, the switch module voltage, and the inductor voltage, the switch module voltage expression is obtained, and further the expression of the DC component of the switch module voltage is obtained; (2.2) Since the expression obtained in step (2.1) is related to the power factor angle, the commutation overlap angle and the voltage modulation ratio, the expression of the submodule capacitor voltage under the influence of the system parameters is obtained by combining the relationship between the DC bus voltage, the valve string voltage and the DC component of the switch module voltage, that is, the mathematical model of the submodule capacitor voltage. Through this expression, it is found that the module capacitor voltage has an amplitude drop under the influence of the power factor angle, the commutation overlap angle and the voltage modulation ratio; The mathematical model of the submodule capacitor voltage in step (2.2) is: Among them, u dc is the DC bus voltage, u C0 is the DC component of the submodule capacitor voltage, β is the commutation overlap angle, L is the bridge arm inductance, C is the bridge arm capacitance, N is the number of single bridge arm submodules, ω is the angular frequency, ε is the capacitor voltage fluctuation rate, and m is the voltage modulation ratio; (3) Design a feedback controller to compensate for the submodule capacitor voltage amplitude. The feedback controller output is superimposed on the DC component of the bridge arm voltage reference signal to form a control closed loop to achieve compensation control of the capacitor voltage of the hybrid modular multilevel converter. The specific implementation process of step (3) is as follows: (3.1) Generate a bridge arm voltage reference signal based on the bridge arm voltage; take the average value of the submodule capacitor voltage as the controlled variable, select the cutoff frequency and phase margin according to the system performance requirements, and design a feedback controller based on the relationship between the DC component of the valve string voltage, the DC component of the switch module voltage and the DC bus voltage; (3.2) The controller output is superimposed on the DC component of the bridge arm voltage reference signal to form a control closed loop, and the capacitor voltage is controlled by adjusting the number of submodules switched on and off; The step (3) designs a feedback controller in combination with the following formula: in SM_dc =1 / 2u dc -in DS_dc Among them, u SM_dc is the DC component of the valve series voltage, u DS_dc is the DC voltage component of the switching module.

2. The capacitor voltage control method applicable to a hybrid modular multilevel converter according to claim 1, characterized in that: The bridge arm of the hybrid modular multi-level converter is composed of a valve string part in which submodules are connected in series and a switch module in which power devices are connected in series.

3. The capacitor voltage control method applicable to a hybrid modular multi-level converter according to claim 1, characterized in that: The specific implementation process of step (1) is as follows: (1.1) Considering the influence of the bridge arm inductance, there will be overlap when the upper and lower bridge arms are commutating. Assuming that the voltage at the middle point of the bridge arm is a square wave, the bridge arm is switched at the zero-crossing point of the AC current, and the current commutation modulation method is designed; (1.2) The bridge arm voltage is the sum of the valve string voltage, the switch module voltage and the inductor voltage. The valve string voltage expression and the switch module modulation signal are obtained according to the current commutation modulation method. The sum of the valve string voltage and the inductor voltage during the commutation period is equal to the DC bus voltage. According to the relationship between the valve string voltage and the inductor voltage during the commutation period, the submodule capacitance and the bridge arm inductance are taken into account to obtain the inductor voltage expression and the bridge arm current expression. (1.3) The DC component of the bridge arm current is obtained from the bridge arm current expression obtained in step (1.2); based on the energy balance, the relationship between the AC side current and the DC bus current is obtained; based on the symmetry of the topological structure, the relationship between the DC component of the bridge arm current and the DC bus current is obtained; combining the above relationship, the relationship between the commutation overlap angle and the influence of system parameters during the commutation of the upper and lower bridge arms can be obtained.

4. The capacitor voltage control method applicable to a hybrid modular multilevel converter according to claim 3, characterized in that: The relationship between the AC side current and the DC bus current in step (1.3) is: Among them, i dc is the DC bus current, I1 is the AC side current amplitude, is the power factor angle, and m is the voltage modulation ratio.

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