Cascade h-bridge photovoltaic grid-connected inverter power balance optimization control method and system

Abstract

The application discloses a cascaded H-bridge photovoltaic grid-connected inverter power balance optimization control method and system, and belongs to the technical field of power balance optimization. The method comprises the following steps: acquiring the circuit parameters of the cascaded H-bridge photovoltaic grid-connected inverter, and calculating the modulation voltage of each H-bridge module; calculating the modulation degree of all H-bridge modules based on the modulation voltage of the H-bridge module, and determining the maximum modulation degree among them; comparing the maximum modulation degree with a preset linear modulation reference value to obtain a modulation degree error signal; generating an adaptive zero-sequence voltage coefficient based on the modulation degree error signal; calculating a complete zero-sequence voltage according to the system power imbalance degree, and multiplying the complete zero-sequence voltage by the adaptive zero-sequence voltage coefficient to generate an adaptive zero-sequence voltage; and injecting the adaptive zero-sequence voltage into the modulation wave of the three-phase H-bridge module. The application avoids over-modulation of the H-bridge module, cooperates with the two-level inverter to participate in power regulation, and simultaneously suppresses the DC side voltage fluctuation.

Description

Technical Field

[0001] This invention belongs to the field of power balance optimization technology, specifically relating to a power balance optimization control method and system for a cascaded H-bridge photovoltaic grid-connected inverter. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] With the continued growth of global energy demand, solar energy has become one of the most promising clean energy sources due to its abundant resources and wide distribution. As a key device connecting photovoltaic power generation systems to the power grid, the performance of photovoltaic inverters directly affects the power generation efficiency and grid connection quality of the photovoltaic system.

[0004] In large-scale photovoltaic (PV) power generation systems, cascaded H-bridge (CHB) multilevel PV inverters have attracted widespread attention due to their modular structure, multilevel output, ease of expansion, and ability to be directly connected to medium- and high-voltage power grids. In this topology, each H-bridge module is powered by an independent PV array, facilitating multi-path maximum power point tracking. However, due to factors such as sunlight, temperature, and shading, the output power of each PV array often differs, leading to phase-to-phase power imbalance. This imbalance causes asymmetrical three-phase grid-connected currents, making it difficult to meet grid connection specifications and, in severe cases, even causing system instability.

[0005] To address the phase-to-phase power imbalance problem in CHB photovoltaic inverters, existing technologies mainly focus on two directions: One method is to achieve power redistribution by injecting zero-sequence voltage. This type of method redistributes active power in the three-phase H-bridge modules by superimposing a fundamental frequency zero-sequence voltage onto the three-phase modulation voltage, thereby achieving inter-phase power balance. However, when the power imbalance is significant, the required zero-sequence voltage amplitude increases significantly, which can easily cause the H-bridge modules to enter an overmodulation state, leading to control failure.

[0006] Secondly, the power self-balancing capability can be enhanced by improving the topology. For example, a three-phase two-level inverter can be connected in series with a three-phase CHB to create an energy interconnection loop between the three phases. This method improves the power balancing capability to a certain extent, but when the imbalance is large, the two-level inverter needs to bear a large amount of regulation power, resulting in significant second-harmonic power fluctuations on its DC side, which affects system stability.

[0007] In summary, existing control strategies still have limitations in expanding the power balancing capability of hybrid DC bus cascaded H-bridge photovoltaic grid-connected inverters. These limitations include the tendency of zero-sequence voltage compensation methods to cause overmodulation, and the limitation of topology self-balancing methods on the capacity of two-level inverters and DC-side fluctuations. Summary of the Invention

[0008] The purpose of this invention is to overcome the shortcomings of the prior art and provide a power balance optimization control method and system for cascaded H-bridge photovoltaic grid-connected inverters. When the output power of the photovoltaic array has a large difference, it avoids over-modulation of the H-bridge module, coordinates the two-level inverters to participate in power regulation, and suppresses DC side voltage fluctuations.

[0009] To achieve the above objectives, the present invention is implemented through the following technical solution: In a first aspect, the technical solution of the present invention provides a power balance optimization control method for a cascaded H-bridge photovoltaic grid-connected inverter, comprising: Obtain the circuit parameters of the cascaded H-bridge photovoltaic grid-connected inverter; Each phase of the cascaded H-bridge photovoltaic grid-connected inverter consists of multiple independent DC bus H-bridge modules and a three-phase two-level inverter connected in series. The DC side of each H-bridge module and the three-phase two-level inverter is connected to a photovoltaic array. The modulation voltage of each H-bridge module is calculated based on the obtained circuit parameters. The modulation degree of all H-bridge modules is calculated based on the modulation voltage of the H-bridge modules, and the maximum modulation degree is determined. The maximum modulation index is compared with a preset linear modulation reference value to obtain a modulation index error signal; an adaptive zero-sequence voltage coefficient is generated based on the modulation index error signal; wherein the adaptive zero-sequence voltage coefficient is between 0 and 1; The complete zero-sequence voltage is calculated based on the degree of power imbalance in the system, and the complete zero-sequence voltage is multiplied by the adaptive zero-sequence voltage coefficient to generate the adaptive zero-sequence voltage. An adaptive zero-sequence voltage is injected into the modulation wave of the three-phase H-bridge module to suppress overmodulation of the H-bridge module and achieve three-phase power balance.

[0010] In at least one embodiment, the three-phase two-level inverter is specifically a three-phase three-bridge-arm two-level inverter.

[0011] In at least one embodiment, the modulation voltage of each H-bridge module is calculated based on the acquired circuit parameters, specifically including: A notch filter is used to filter the sum of the DC bus voltage of the three-phase two-level inverter and the DC side voltages of all H-bridge modules, and the output of the filter is used as a positive-sequence active current reference value. Based on this positive-sequence active current reference value, the positive-sequence active current is controlled to the positive-sequence active current reference value and the positive-sequence reactive current is controlled to the positive-sequence reactive current reference value by the PI regulator. The final output of the PI regulator is added to the grid voltage feedforward value, and then the total modulation voltage of the inverter is calculated by coordinate transformation. A 100Hz notch filter is used to filter the DC bus output voltage of each H-bridge module, and a PI regulator is used to adjust and control the output modulation voltage coefficient increment. The number of converters in each phase is used as the modulation voltage coefficient feedforward of the H-bridge module to obtain the modulation voltage coefficient of all H-bridges in each phase. This coefficient is then multiplied by the total modulation voltage of the inverter to calculate the modulation voltage of each HB.

[0012] In at least one embodiment, the modulation index of all H-bridge modules is calculated based on the modulation voltage of the H-bridge modules, and the maximum modulation index is determined. Specifically, this includes: dividing the modulation voltage amplitude by the DC side voltage amplitude to obtain the modulation index of all H-bridge modules, and then using the max module to determine the maximum modulation index in the region.

[0013] In at least one embodiment, the maximum modulation degree is compared with a preset linear modulation reference value to obtain a modulation degree error signal, specifically expressed as follows:

[0014] In the formula, To adjust for system errors; For maximum adjustment mechanism.

[0015] In at least one embodiment, generating an adaptive zero-sequence voltage coefficient based on the modulation error signal specifically includes: inputting the modulation error into a PI regulator, controlling it internally, then passing the output of the PI regulator through a 0-1 limiting circuit to ensure that the output of the PI regulator is controlled between 0 and 1, and finally subtracting the limiting result from the preset linear modulation reference value to obtain the adaptive zero-sequence voltage coefficient.

[0016] In at least one embodiment, the required zero-sequence voltage for compensation is calculated based on the degree of system power imbalance, specifically expressed as follows:

[0017]

[0018] In the formula, The magnitude of the zero-sequence voltage to be compensated; The phase of the zero-sequence voltage to be compensated; This refers to the voltage amplitude of the power grid. The initial phase angle of the grid current; Phase A power imbalance; Phase B power imbalance; For C-phase power imbalance; For intermediate parameters; , and For segmented judgment criteria; This is the ratio of the rated power of the three-phase CHB to the rated power of the inverter; This represents the relative output power coefficient of TTLI.

[0019] In at least one embodiment, the segmentation criterion is specifically expressed as follows:

[0020] In the formula, Phase A power imbalance; For phase B power imbalance; This refers to the power imbalance of phase C.

[0021] Secondly, the technical solution of the present invention also provides an adaptive zero-sequence voltage compensation method, including: Obtain the circuit parameters of the cascaded H-bridge photovoltaic grid-connected inverter, determine whether the three-phase power is balanced, if so, no compensation is needed; otherwise, perform zero-sequence voltage compensation based on the topology self-balancing control strategy. After zero-sequence voltage compensation, it is determined whether the DC-side second harmonic fluctuation meets the requirements and whether the three-phase power can be restored to balance. If the DC-side second harmonic fluctuation meets the requirements and the three-phase power can be restored to balance, no further additional control strategy is required. Otherwise, full zero-sequence voltage compensation is performed based on the full zero-sequence voltage compensation strategy applicable to hybrid CHB inverters. After complete zero-sequence voltage compensation, it is determined whether the three-phase HB is over-modulated. If so, the power balance optimization control method of the cascaded H-bridge photovoltaic grid-connected inverter as described in the first aspect is used for power balance control.

[0022] Thirdly, the technical solution of the present invention also provides a power balance optimization control system for a cascaded H-bridge photovoltaic grid-connected inverter, comprising: The parameter acquisition module is configured to acquire the circuit parameters of the cascaded H-bridge photovoltaic grid-connected inverter. Each phase of the cascaded H-bridge photovoltaic grid-connected inverter consists of multiple independent DC bus H-bridge modules and a three-phase two-level inverter connected in series. The DC side of each H-bridge module and the three-phase two-level inverter is connected to a photovoltaic array. The modulation calculation module is configured to: calculate the modulation voltage of each H-bridge module based on the acquired circuit parameters, calculate the modulation degree of all H-bridge modules based on the modulation voltage of the H-bridge modules, and determine the maximum modulation degree among them. The adaptive zero-sequence voltage coefficient generation module is configured to: compare the maximum modulation degree with a preset linear modulation reference value to obtain a modulation error signal; and generate an adaptive zero-sequence voltage coefficient based on the modulation error signal; wherein the adaptive zero-sequence voltage coefficient is between 0 and 1. The adaptive zero-sequence voltage generation module is configured to: calculate the complete zero-sequence voltage based on the degree of power imbalance in the system, and multiply the complete zero-sequence voltage by the adaptive zero-sequence voltage coefficient to generate the adaptive zero-sequence voltage; The compensation module is configured to inject an adaptive zero-sequence voltage into the modulation wave of the three-phase H-bridge module, thereby suppressing over-modulation of the H-bridge module and achieving three-phase power balance.

[0023] The beneficial effects of the above-described technical solution of the present invention are as follows: This invention introduces an adaptive zero-sequence voltage compensation mechanism, which dynamically adjusts the zero-sequence voltage injection amplitude according to the real-time modulation of the H-bridge modules. This ensures that all H-bridge modules always operate within the linear modulation region, effectively avoiding the overmodulation problem that traditional zero-sequence compensation methods are prone to when the power imbalance is large. At the same time, by coordinating the adaptive zero-sequence voltage compensation with the topology self-balancing control strategy, the three-phase two-level inverter and the cascaded H-bridge modules jointly participate in power regulation. This not only significantly increases the inter-phase power balance factor of the system from 57.71% in the existing complete zero-sequence compensation strategy to 62.42%, but also effectively reduces the DC-side double-frequency power fluctuations caused by the two-level inverters alone undertaking power regulation, thereby comprehensively improving the system's operational stability and grid-connected power quality. Attached Figure Description

[0024] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0025] Figure 1 This is a schematic diagram of the power balance optimization control method for cascaded H-bridge photovoltaic grid-connected inverters disclosed in Embodiment 1 of the present invention; Figure 2 This is a topology diagram of the cascaded H-bridge photovoltaic grid-connected inverter system disclosed in Embodiment 1 of the present invention; Figure 3 This is a system distributed control block diagram disclosed in Embodiment 1 of the present invention; Figure 4 This is a waveform diagram of the three-phase HB modulation voltage using TSBS and FZSVCS as disclosed in Embodiment 1 of the present invention; Figure 5 This is a control block diagram of the power balance optimization control strategy for a cascaded H-bridge photovoltaic grid-connected inverter disclosed in Embodiment 1 of the present invention; Figure 6 This is disclosed in Embodiment 1 of the present invention. Follow A schematic diagram illustrating the changing pattern; Figure 7 These are experimental results of PBS and PBF corresponding to different control strategies disclosed in Embodiment 1 of the present invention; Figure 8 This is a flowchart of the adaptive zero-sequence voltage compensation disclosed in Embodiment 2 of the present invention. Detailed Implementation

[0026] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0027] Example 1 As introduced in the background section, in large-scale photovoltaic (PV) power generation systems, compared to low-voltage, low-power systems, large-capacity PV systems have lower unit power generation costs, thus offering greater advantages in practical applications. However, traditional centralized inverters and string inverters have certain limitations in further increasing system power levels. Cascaded H-Bridge (CHB) multilevel PV inverters have received widespread attention in recent years due to their modular structure, multilevel output, and ease of expansion. This structure can achieve high-voltage output by increasing the number of H-bridge (HB) modules, allowing direct connection to medium- and high-voltage power grids. Each HB module is powered by an independent PV array, facilitating multi-path maximum power point tracking and improving overall system power generation efficiency. However, because the output power of the PV arrays connected to each HB module is often affected by environmental factors such as sunlight, temperature, and shading, it can easily lead to phase-to-phase and intra-phase power imbalances in the system. Among these issues, inter-phase power imbalance leads to asymmetrical three-phase grid-connected current, making it difficult to meet grid connection specifications; while intra-phase power imbalance may cause some HB modules to output power exceeding their transmission capacity, resulting in grid-connected current distortion or even overcurrent. Therefore, how to effectively solve the power imbalance problem in CHB photovoltaic inverters has become an important technical issue that urgently needs to be studied.

[0028] Currently, many papers have presented innovative work on the phase-to-phase power balance optimization control of H-bridge converters. The paper "Y. Yu, G. Konstantinou, B. Hredzak and VG Agelidis, 'On extending the energy balancing limit of multilevel cascaded H-bridge converters for large-scale photovoltaic farms,' 2013 Australasian Universities Power Engineering Conference (AUPEC), Hobart, TAS, Australia, 2013" derives a fundamental frequency zero-sequence voltage injection method using instantaneous power theory and phasor theory. To address the overmodulation problem of three-phase modulation voltage after zero-sequence voltage compensation, based on fundamental frequency zero-sequence voltage compensation, the literature "Y. Yu, G. Konstantinou, B. Hredzak and VG Agelidis, "Power Balance Optimization of Cascaded H-Bridge Multilevel Converters for Large-Scale Photovoltaic Integration," in IEEE Transactions on Power Electronics, vol. 31, no. 2, pp. 1108-1120, Feb. 2016" proposes an optimal zero-sequence injection and a simplified optimal zero-sequence voltage injection strategy based on Fourier analysis. However, compared to the fundamental frequency zero-sequence voltage compensation strategy, the optimal zero-sequence voltage injection strategy only improves the power balance capability by 16% over the entire operating range, and its improvement is still limited.

[0029] Clearly, when the output power difference of the photovoltaic array leads to interphase power imbalance in the three phases, the existing control strategy still has limitations in improving the power balance capability of the three-phase CHB, and the power balance capability of CHB needs to be improved according to the topology. The paper “Y.-C. Su, P.-H. Wu and P.-T. Cheng, 'Control and Modulation of the Hybrid Cascaded Converter Based on Distributed Architecture,' in IEEE Transactions on Industry Applications, vol. 56, no. 2, pp. 1553-1562, March-April 2020” studies a hybrid DC bus topology. A conventional three-phase three-bridge-arm two-level inverter (TTLI) is connected in series with a three-phase CHB photovoltaic inverter to create an energy interconnection loop between the three phases. A corresponding Topology Self-Balancing Strategy (TSBS) is proposed. However, when the three-phase power imbalance is significant, the two-level inverter needs to handle a large amount of regulation power, which may lead to a significant power fluctuation of twice the power frequency on the DC side and increase the DC bus voltage ripple, affecting system stability. The paper “S. Farzamkia, M. Zhang, H. Zou and AQ Huang, “Combining Common DC-Link Utilization and Zero-Sequence Injection for Power Balancing in Hybrid Cascaded H-Bridge PV Inverters,” in IEEE Transactions on Industrial Electronics, vol. 72, no. 5, pp. 4943-4955, May 2025,” proposes a Full Zero-Sequence Voltage Compensation Strategy (FZSVCS) for hybrid CHB inverters. This strategy achieves power balance by injecting zero-sequence voltage into the three-phase CHB. However, when the imbalance is significant, the required zero-sequence voltage amplitude increases substantially, potentially forcing the CHB modules into an overmodulation state. Furthermore, this method fails to fully leverage the energy regulation potential of TTLI, and the overall system power regulation capability still needs optimization.

[0030] In summary, existing technologies still have limitations in expanding the power balance capability of three-phase hybrid DC bus cascaded H-bridge photovoltaic grid-connected inverters, and it is necessary to solve the problems of over-modulation and large DC-side voltage fluctuations caused by inter-phase power imbalance. To overcome the shortcomings of the existing technologies, in a typical embodiment of the present invention, as shown in Figures to... Figure 8 As shown in the figure, this embodiment discloses a power balance optimization control method for a cascaded H-bridge photovoltaic grid-connected inverter, which specifically includes the following steps: S1. Obtain the circuit parameters of the cascaded H-bridge photovoltaic grid-connected inverter; Each phase of the cascaded H-bridge photovoltaic grid-connected inverter consists of multiple independent DC bus H-bridge modules and a three-phase two-level inverter connected in series. Each H-bridge module and the DC side of the three-phase two-level inverter are connected to a cluster of photovoltaic arrays. S2. Calculate the modulation voltage of each H-bridge module based on the obtained circuit parameters, calculate the modulation degree of all H-bridge modules based on the modulation voltage of the H-bridge modules, and determine the maximum modulation degree among them; S3. Compare the maximum modulation index with the preset linear modulation reference value to obtain the modulation index error signal; generate an adaptive zero-sequence voltage coefficient based on the modulation index error signal; wherein the adaptive zero-sequence voltage coefficient is between 0 and 1; S4. Calculate the complete zero-sequence voltage based on the degree of power imbalance in the system, and multiply the complete zero-sequence voltage by the adaptive zero-sequence voltage coefficient to generate the adaptive zero-sequence voltage; S5. Inject the adaptive zero-sequence voltage into the modulation wave of the three-phase H-bridge module to suppress over-modulation of the H-bridge module and achieve three-phase power balance.

[0031] The following section provides a detailed explanation of the specific implementation process and theoretical mechanism of the power balance optimization control method for the cascaded H-bridge photovoltaic grid-connected inverter.

[0032] S1. Obtain the circuit parameters of the cascaded H-bridge photovoltaic grid-connected inverter.

[0033] In this step, the cascaded H-bridge photovoltaic grid-connected inverter is specifically a three-phase hybrid DC bus cascaded H-bridge (HybridCHB, H-CHB) photovoltaic grid-connected inverter, whose topology is as follows: Figure 2 As shown. Each phase of this H-CHB photovoltaic grid-connected inverter consists of... The system consists of an H-bridge (HB) module with an independent DC bus and a three-phase two-level inverter connected in series. Each HB module and the DC side of the three-phase two-level inverter are connected to a photovoltaic array. As a further implementation, the three-phase two-level inverter is specifically a three-phase three-bridge-arm two-level inverter (TTLI).

[0034] like Figure 2 As shown, and These represent the output voltage and output current of the photovoltaic array in CHB, respectively. ; This indicates the DC bus voltage of a three-phase two-level inverter; This indicates the output current of the photovoltaic array in a three-phase two-level inverter. express Xiangdi The fundamental component of the AC output voltage of HB is ; and They represent The fundamental components of the total output voltage of the phase HB converter and the output voltage of the three-phase two-level inverter are respectively and . , and This represents the voltage at the point of common coupling (PCC) of the three phases, where... This is the effective value of the grid voltage; , and This represents the three-phase current injected into the PCC by the three-phase two-level inverter; , and This represents the active power transferred from the three-phase two-level inverter to the PCC. It is worth noting that the ratio of the DC-side voltage of each HB to the DC-side voltage of the three-phase two-level inverter is set to... ,Right now K can be adjusted according to system requirements.

[0035] In this step, the DC bus of each H-bridge module of the three-phase hybrid DC bus cascaded H-bridge photovoltaic grid-connected inverter is sampled to obtain the DC bus output voltage of each H-bridge module. and output current ,in, The DC bus voltage of the three-phase two-level inverter is obtained by sampling the DC bus voltage. The voltage at the three-phase common coupling point is obtained by sampling the three-phase common coupling point. , and And the three-phase current injected into the PCC by the three-phase two-level inverter. , and .

[0036] S2. Calculate the modulation voltage of each H-bridge module based on the obtained circuit parameters, calculate the modulation degree of all H-bridge modules based on the modulation voltage of the H-bridge modules, and determine the maximum modulation degree among them.

[0037] In this step, because compensation is needed for the unbalanced three-phase power transmission of the CHB inverter, the three-phase power transmission of the TTLI is usually unbalanced. Therefore, the DC-side voltage of the TTLI exhibits twice the power frequency ripple, requiring a 100Hz notch filter to compensate. and all HB DC side voltages The sum is filtered and controlled by a PI regulator to... The controller's output is the positive-sequence active power reference value of the current loop. Based on this positive sequence active current reference value Then, the positive sequence active current is controlled by a PI regulator. Control as Positive sequence reactive current Controlled to positive sequence reactive power reference value ,like Figure 3 The TTLI controller section is shown. The final output of the PI regulator is added to the grid voltage feedforward value, and then the total modulation voltage of the inverter can be calculated through coordinate transformation (i.e., dq / abc transformation). Through the analysis of Mutually Summing the modulation voltage coefficients of each HB and subtracting them from 1, we can obtain the TTLI in... Phase modulation voltage coefficient This leads to the modulation voltage of TTLI.

[0038] Considering the presence of double the power frequency ripple in the DC-side voltage of the HB bridge, a 100Hz notch filter is first used to measure the DC bus output voltage of each H-bridge module. Filter the signal and control it using a PI controller. The PI controller outputs the modulation voltage coefficient increment. Since each phase contains... One bridge arm of HB and TTLI, totaling A converter, therefore As the modulation voltage coefficient of HB fed forward, we obtain H-bridge modulation voltage coefficient Combine it with the modulation voltage Multiply to calculate the modulation voltage for each HB. ,like Figure 3 The HB controller section is shown.

[0039] The modulation index of all H-bridge modules is calculated based on the modulation voltage of the H-bridge modules, and the maximum modulation index is determined. Specifically, the modulation voltage amplitude is divided by the DC-side voltage amplitude to obtain the modulation index of all H-bridge modules, and the maximum modulation index in the region is determined by the max module.

[0040] S3. Compare the maximum modulation index with the preset linear modulation reference value to obtain the modulation index error signal; generate an adaptive zero-sequence voltage coefficient based on the modulation index error signal; wherein the adaptive zero-sequence voltage coefficient is between 0 and 1.

[0041] In this step, to avoid algebraic loops and suppress the influence of transient spikes on regulation, the maximum regulation is set... The modulation error signal is obtained by comparing it with the preset linear modulation reference value 1, which is specifically expressed as follows: (1) In the formula, To adjust for system errors.

[0042] Adjustment error After input to the PI controller, the output is then passed through a 0-1 limiting circuit to ensure it remains between 0 and 1. Finally, the adaptive zero-sequence voltage coefficient is obtained by subtracting the limiting value from the preset linear modulation reference value of 1. The adaptive zero-sequence voltage coefficient Between 0 and 1.

[0043] S4. Calculate the complete zero-sequence voltage based on the degree of power imbalance in the system, and multiply the complete zero-sequence voltage by the adaptive zero-sequence voltage coefficient to generate the adaptive zero-sequence voltage.

[0044] In this step, to quantitatively characterize the degree of power imbalance in the system, a phase-to-phase power imbalance parameter is defined to address the interphase power imbalance problem. Specifically, it is expressed as follows: (2) In the formula, Representation system The actual output active power of all HB modules; This is the ratio of the rated power of the three-phase CHB to the rated power of the inverter; This is the value of the system's rated output power.

[0045] Considering that the photovoltaic array connected to the TTLI may also deviate from its rated operating condition due to environmental fluctuations, the relative output power factor of the TTLI is defined. Specifically, it is expressed as follows: (3) In the formula, This refers to the total output power of the TTLI three-phase system. The TTLI relative output power coefficient is used to characterize the degree of change of the actual output power of the TTLI relative to the rated value. It can reflect the power fluctuation caused by changes in environmental conditions of the photovoltaic array on the TTLI side, as well as the degree of deviation of the system operating state from the rated operating condition, and provide a quantitative basis for subsequent power balance analysis and control strategy design.

[0046] Based on the TSBS-FZSVC concept, the required zero-sequence voltage for compensation is calculated according to the degree of power imbalance in the system. Its amplitude and phase They are represented as follows: (4) (5) In the formula, This refers to the voltage amplitude of the power grid. The initial phase angle of the grid current; Phase A power imbalance; Phase B power imbalance; For C-phase power imbalance; For intermediate parameters, ; , and The segmented criterion is specifically expressed as follows: (6) The calculated complete zero-sequence voltage With the adaptive zero-sequence voltage coefficient generated in step S3 Multiplication achieves scaling of the completely zero-sequence voltage, generating an adaptive zero-sequence voltage. .

[0047] S5. Inject the adaptive zero-sequence voltage into the modulation wave of the three-phase H-bridge module to suppress over-modulation of the H-bridge module and achieve three-phase power balance.

[0048] In this step, an adaptive zero-sequence voltage will be generated. The final modulation voltage of each HB module is formed by injecting a three-phase modulation wave. , 、 ,like Figure 5 As shown.

[0049] When injecting adaptive zero-sequence voltage If the three-phase power imbalance cannot be completely eliminated afterward, the remaining unbalanced power is automatically handled by the three-phase two-level inverter. The modulation voltage coefficient of each phase of the three-phase two-level inverter is... Depend on Determined, and adapts to the injected adaptive zero-sequence voltage It adapts to changes and adjusts itself, thereby expanding the operating range of the system that can achieve power balance and improving its stability and robustness under complex operating conditions.

[0050] In this step, the modulation voltage of each module is divided by its own DC bus voltage to obtain the normalized modulation wave. The modulation wave of the H-bridge module uses a carrier phase-shifted sinusoidal pulse width modulation strategy to generate the switching drive signal, while the modulation wave of the three-phase two-level inverter uses a standard sinusoidal pulse width modulation strategy to generate the switching drive signal. By injecting an adaptive zero-sequence voltage, when the output power difference of the photovoltaic array causes phase-to-phase power imbalance, the modulation degree of all H-bridge modules is always limited to less than or equal to 1, thus suppressing overmodulation of the H-bridge modules and achieving three-phase power balance.

[0051] If the three-phase HB maximum modulation The zero-sequence voltage is less than 1, calculated according to equations (4) and (5). This itself will not cause overmodulation problems in HB, so the output of the PI controller is 0, and the calculated value is... That is, 1. At this point, the adaptive zero-sequence voltage compensation strategy degenerates into the original completely zero-sequence voltage. By adaptively constraining the amplitude of the zero-sequence voltage, the amplitude of the HB modulation voltage can be effectively limited to avoid over-modulation, while maximizing the power balance potential of the zero-sequence voltage within the constraint boundary.

[0052] To demonstrate the feasibility of the method proposed in this embodiment, the theoretical mechanism of the method proposed in this embodiment will be explained below.

[0053] Due to various complex factors, the power generation of the H-CHB photovoltaic grid-connected inverter varies. This difference in power generation inevitably leads to phase-to-phase power imbalance, which in turn leads to three-phase phase-to-phase power imbalance. This imbalance in phase-to-phase power imbalance will result in asymmetrical three-phase grid-connected current, and may even cause other equipment connected to the photovoltaic system on the bus to malfunction.

[0054] The existing H-CHB FZSVC scheme utilizes the topology's power self-balancing capability and fundamental frequency zero-sequence voltage compensation characteristics to distribute power regulation. It reconstructs the three-phase voltage by injecting a completely zero-sequence voltage, thereby achieving inter-phase power redistribution and maintaining three-phase output balance. Based on the TSBS-FZSVC concept and combined with relevant conditions, the required completely zero-sequence voltage for compensation can be obtained. As shown in equations (4) to (6).

[0055] After complete zero-sequence voltage compensation, the three-phase HB equivalent modulation coefficients satisfy... This indicates that the completely zero-sequence voltage compensation strategy can theoretically achieve complete balance of three-phase active power. Furthermore, the TTLI three-phase modulation coefficient... , , The equivalent modulation coefficients of 1 and the three-phase HB respectively satisfy the following conditions: By subtraction, since the three-phase HB modulation coefficients are equal, we can obtain... This means that in FZSVC, the voltage distribution ratio of TTLI in the three phases is equal and independent of the degree of power imbalance. In other words, the regulation of three-phase power imbalance is mainly undertaken by the zero-sequence voltage injection on the HB side, while TTLI does not further participate in inter-phase energy regulation. Consequently, the capacity of TTLI is not fully utilized for power balancing. When the power imbalance is significant, the amplitude of the modulation voltage on the HB side will be forced to increase to meet compensation requirements, thus significantly increasing the risk of the system entering the over-modulation region.

[0056] by =0.46、 =1、 =1、 Taking a value of 1 as an example, after compensating for a completely zero-sequence voltage, the amplitudes of the modulation voltages in both phases A and B show a significant increase. For example... Figure 4 As shown, The three-phase HB modulation system is adopted when TSBS is used; The figures show the modulation indices of the three-phase HB after zero-sequence voltage compensation. The comparison shows that zero-sequence voltage compensation significantly improves the modulation indices of phases B and C. When the required modulation voltage amplitude further increases, exceeding the linear modulation range allowed by the DC side voltage, the FZSVC will enter the overmodulation region and lose its proper power redistribution capability, thus limiting the usability of the strategy under deep imbalance conditions.

[0057] In summary, the TSBS strategy's balancing capability is limited by TTLI capacity and DC voltage constraints, and it introduces significant second-harmonic power fluctuations under phase-to-phase power imbalance conditions. While the FZSVC strategy can achieve redistribution under large power differences, it is prone to inducing HB overmodulation and fails to fully utilize the phase-to-phase power matching potential of TTLI. Therefore, this embodiment proposes an Adaptive Zero-Sequence Voltage Compensation (AZSVC) strategy for the power balance optimization control method of cascaded H-bridge photovoltaic grid-connected inverters. Combined with the existing TSBS strategy, this strategy can maintain the zero-sequence voltage phase while adjusting the adaptive zero-sequence voltage coefficient. ( Adjusting the zero-sequence voltage amplitude allows for the redistribution of three-phase power while minimizing the unbalanced power required by the TTLI and suppressing the risk of the HB side entering the overmodulation region.

[0058] Inject adaptive zero-sequence voltage Afterwards, the equivalent active power borne by the three-phase HB side satisfies: (7) In the formula, It is an adaptive zero-sequence voltage coefficient; The power generated by the X-phase photovoltaic array; This represents the amplitude of the grid current.

[0059] To facilitate the analysis of constraint boundaries, the following will use... or For example, under this operating condition, phase C (HB) bears the largest proportion of power and is most prone to overmodulation. When phase C (HB) is in a critical state, the required modulation voltage amplitude on its AC side should meet the DC side voltage constraint, specifically expressed as follows: (8) Meanwhile, from equation (7), the power of phase C can be obtained as follows: (9) Then the equivalent modulation coefficient of the C-phase HB side can be obtained. Specifically, it is expressed as: (10) Similarly, voltage amplitude constraints and power relationships between phase A and phase B can be established, specifically expressed as follows: (11) Furthermore, the equivalent modulation coefficients of phase A and phase B on the HB side can be obtained. and Specifically, it is expressed as: (12) To simplify the expression, the intermediate variable D is defined as: (13) And further define C1 and C2 as: (14) Compared to FZSVC, the TSBS-AZSVC proposed in this embodiment, which combines the adaptive zero-sequence voltage compensation strategy with the TSBS strategy, only adjusts the amplitude of the completely zero-sequence voltage without changing its phase. Therefore, equation (8) can be rewritten as: (15) Simplifying equation (15) yields the adaptive zero-sequence voltage coefficient. The quadratic equation in one variable is expressed as follows: (16) In the formula, The coefficient is specifically represented as; (17) Solving equation (16) yields the adaptive zero-sequence voltage coefficient. Considering In the solution process, integer roots that satisfy the physical constraints are selected, specifically expressed as: (18) When the working conditions are met or Phase A is more prone to overmodulation; similarly, critical constraints for phase A can be established and corresponding results obtained. The expression is: (19) When the working conditions are met or At this time, phase B is more prone to overmodulation, and the critical constraints for phase B can be established similarly to obtain the corresponding values. The expression is: (20) In summary, the adaptive zero-sequence voltage coefficient It can be uniformly represented as: (twenty one) The variables and segmentation criteria in the formula are as follows: (twenty two) Figure 6 A more intuitive display Follow The changing pattern, among which Indicates the system imbalance. =1, =1, =1, =0.75, =1. Figure 6 This indicates that, in Within the range, Follow The increase of shows a monotonically decreasing trend, and in It converges to zero at that point. This means that when When the time degenerates into a completely zero-sequence voltage compensation strategy, the amplitudes of the three-phase modulated voltages in TTLI are completely consistent, thus minimizing the second harmonic ripple. On the other hand, when When the deviation from 1, the imbalance increases rapidly, and the greater the deviation, the more significant the increase, indicating that the system imbalance has a significant impact on the system's stability. It exhibits high sensitivity. Furthermore, different... The steepness of the curve varies significantly under different conditions. When The smaller the value, the more severe the power reduction in phase A (HB), the greater the imbalance of the three-phase modulation voltage amplitude in TTLI. This will increase significantly, leading to a further exacerbation of DC-side second harmonic fluctuations.

[0060] Due to the aforementioned adaptive zero-sequence voltage coefficient The analytical solution requires segmented discussion based on the degree of imbalance and involves numerous algebraic operations and criterion judgments, thus increasing the online computational burden on the controller. To reduce implementation complexity, this embodiment proposes a simplified acquisition method based on closed-loop regulation for the power balance optimization control of cascaded H-bridge photovoltaic grid-connected inverters. This method can generate the power balance online without explicitly solving the segmented analytical expression. This closed-loop strategy is centered on the modulation margin constraint on the HB side. It monitors the modulation index of each HB unit in real time and uses its maximum value. As a constraint, the maximum modulation index is expressed as:

[0061] By adjusting the zero-sequence voltage injection amplitude, Maintaining at the critical boundary without entering overmodulation (i.e. And try to be as close as possible to 1). Under this constraint, while ensuring that HB always works in the linear modulation region, the ability of zero-sequence voltage to redistribute three-phase power can be fully utilized; when the adaptive zero-sequence compensation has reached the modulation constraint and still cannot achieve complete power balance, the remaining unbalanced power will be coordinated and distributed by the TTLI of the common DC bus, thereby expanding the operating range of the system that can achieve power balance and improving its stability and robustness under complex operating conditions.

[0062] Based on the above mechanism analysis, it is proven that the power balance optimization control method for cascaded H-bridge photovoltaic grid-connected inverters proposed in this embodiment can keep the modulation voltage of the cascaded H-bridge module within the linear modulation range by introducing an adaptive zero-sequence voltage compensation mechanism. This can effectively avoid the over-modulation problem that is easy to occur in traditional zero-sequence compensation methods. It is proven that it is entirely feasible to achieve three-phase power balance while suppressing the over-modulation of the H-bridge module by introducing an adaptive zero-sequence voltage.

[0063] To verify the effectiveness of the power balance optimization control method for cascaded H-bridge photovoltaic grid-connected inverters proposed in this embodiment, this embodiment also conducts an experimental comparison between the proposed method and existing power balance optimization strategies. , , , Under the given conditions, the power matching space (PBS) and power balancing factor (PBF) corresponding to different control strategies are as follows: Figure 7 As shown, phase-to-phase power matching can be achieved at operating points within the PBS range; outside this range, three-phase power balance cannot be guaranteed and overmodulation risk will be present. Figure 7 It can be seen that when using the TSBS strategy, the PBF is 18.43%, indicating that theoretically only 18.43% of the phase-to-phase power mismatch conditions can be effectively allocated by this method. In contrast, the FZSVC strategy introduces complete zero-sequence voltage compensation on the basis of TSBS, increasing the PBF to 57.71%. Furthermore, the method proposed in this embodiment achieves synergistic utilization of the HB cell and TTLI capacity while adaptively compensating for zero-sequence voltage, thereby obtaining a larger PBS, with its PBF increasing to 62.42%, a further improvement of 8.16% compared to TSBS-FZSVC, demonstrating stronger phase-to-phase power matching capability.

[0064] Example 2 In a typical embodiment of the present invention, this embodiment discloses an adaptive zero-sequence voltage compensation method based on the cascaded H-bridge photovoltaic grid-connected inverter power balance optimization control method disclosed in Embodiment 1, such as... Figure 8 As shown, the specific steps include: T1. Obtain the circuit parameters of the cascaded H-bridge photovoltaic grid-connected inverter and determine whether the three-phase power is balanced. If so, no compensation is needed; otherwise, zero-sequence voltage compensation is performed based on the topology self-balancing control strategy (TSBS). T2. After zero-sequence voltage compensation, determine whether the DC side second harmonic fluctuation meets the requirements and whether the three-phase power can be restored to balance. If the DC side second harmonic fluctuation meets the requirements and the three-phase power can be restored to balance, no additional control strategy is required. Otherwise, perform full zero-sequence voltage compensation based on the full zero-sequence voltage compensation strategy (FZSVCS) applicable to hybrid CHB inverters. T3. After complete zero-sequence voltage compensation, determine whether the three-phase HB is over-modulated. If so, use the cascaded H-bridge photovoltaic grid-connected inverter power balance optimization control method disclosed in Example 1 to perform power balance control.

[0065] Example 3 In a typical embodiment of the present invention, this embodiment provides a power balance optimization control system for a cascaded H-bridge photovoltaic grid-connected inverter, comprising: The parameter acquisition module is configured to acquire the circuit parameters of the cascaded H-bridge photovoltaic grid-connected inverter. Each phase of the cascaded H-bridge photovoltaic grid-connected inverter consists of multiple independent DC bus H-bridge modules and a three-phase two-level inverter connected in series. The DC side of each H-bridge module and the three-phase two-level inverter is connected to a photovoltaic array. The modulation calculation module is configured to: calculate the modulation voltage of each H-bridge module based on the acquired circuit parameters, calculate the modulation degree of all H-bridge modules based on the modulation voltage of the H-bridge modules, and determine the maximum modulation degree among them. The adaptive zero-sequence voltage coefficient generation module is configured to: compare the maximum modulation degree with a preset linear modulation reference value to obtain a modulation error signal; and generate an adaptive zero-sequence voltage coefficient based on the modulation error signal; wherein the adaptive zero-sequence voltage coefficient is between 0 and 1. The adaptive zero-sequence voltage generation module is configured to: calculate the complete zero-sequence voltage based on the degree of power imbalance in the system, and multiply the complete zero-sequence voltage by the adaptive zero-sequence voltage coefficient to generate the adaptive zero-sequence voltage; The compensation module is configured to inject an adaptive zero-sequence voltage into the modulation wave of the three-phase H-bridge module, thereby suppressing over-modulation of the H-bridge module and achieving three-phase power balance.

[0066] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A power balance optimization control method for cascaded H-bridge photovoltaic grid-connected inverters, characterized in that, include: Obtain the circuit parameters of the cascaded H-bridge photovoltaic grid-connected inverter; Each phase of the cascaded H-bridge photovoltaic grid-connected inverter consists of multiple independent DC bus H-bridge modules and a three-phase two-level inverter connected in series. The DC side of each H-bridge module and the three-phase two-level inverter is connected to a photovoltaic array. The modulation voltage of each H-bridge module is calculated based on the obtained circuit parameters. The modulation degree of all H-bridge modules is calculated based on the modulation voltage of the H-bridge modules, and the maximum modulation degree is determined. The maximum modulation index is compared with a preset linear modulation reference value to obtain a modulation index error signal; an adaptive zero-sequence voltage coefficient is generated based on the modulation index error signal; wherein the adaptive zero-sequence voltage coefficient is between 0 and 1; The complete zero-sequence voltage is calculated based on the degree of power imbalance in the system, and the complete zero-sequence voltage is multiplied by the adaptive zero-sequence voltage coefficient to generate the adaptive zero-sequence voltage. An adaptive zero-sequence voltage is injected into the modulation wave of the three-phase H-bridge module to suppress overmodulation of the H-bridge module and achieve three-phase power balance.

2. The power balance optimization control method for cascaded H-bridge photovoltaic grid-connected inverters as described in claim 1, characterized in that, The three-phase two-level inverter is specifically a three-phase three-bridge-arm two-level inverter.

3. The power balance optimization control method for cascaded H-bridge photovoltaic grid-connected inverters as described in claim 1, characterized in that, The modulation voltage of each H-bridge module is calculated based on the obtained circuit parameters, specifically including: A notch filter is used to filter the sum of the DC bus voltage of the three-phase two-level inverter and the DC side voltages of all H-bridge modules, and the output of the filter is used as a positive-sequence active current reference value. Based on this positive-sequence active current reference value, the positive-sequence active current is controlled to the positive-sequence active current reference value and the positive-sequence reactive current is controlled to the positive-sequence reactive current reference value by the PI regulator. The final output of the PI regulator is added to the grid voltage feedforward value, and then the total modulation voltage of the inverter is calculated by coordinate transformation. A 100Hz notch filter is used to filter the DC bus output voltage of each H-bridge module, and a PI regulator is used to adjust and control the output modulation voltage coefficient increment. The number of converters in each phase is used as the modulation voltage coefficient feedforward of the H-bridge module to obtain the modulation voltage coefficient of all H-bridges in each phase. This coefficient is then multiplied by the total modulation voltage of the inverter to calculate the modulation voltage of each HB.

4. The power balance optimization control method for cascaded H-bridge photovoltaic grid-connected inverters as described in claim 1, characterized in that, The modulation index of all H-bridge modules is calculated based on the modulation voltage of the H-bridge module, and the maximum modulation index is determined. Specifically, the modulation voltage amplitude is divided by the DC side voltage amplitude to obtain the modulation index of all H-bridge modules, and the maximum modulation index in the region is determined by the max module.

5. The power balance optimization control method for cascaded H-bridge photovoltaic grid-connected inverters as described in claim 1, characterized in that, The maximum modulation depth is compared with a preset linear modulation reference value to obtain the modulation depth error signal, which is specifically expressed as: In the formula, To adjust for system errors; For maximum adjustment mechanism.

6. The power balance optimization control method for cascaded H-bridge photovoltaic grid-connected inverters as described in claim 1, characterized in that, The adaptive zero-sequence voltage coefficient is generated based on the modulation error signal. Specifically, the modulation error is input into the PI regulator, and after being controlled internally by the PI regulator, the output of the PI regulator is passed through a 0-1 limiting circuit to ensure that the output of the PI regulator is controlled between 0 and 1. Finally, the adaptive zero-sequence voltage coefficient is obtained by subtracting the limiting result from the preset linear modulation reference value.

7. The power balance optimization control method for cascaded H-bridge photovoltaic grid-connected inverters as described in claim 1, characterized in that, The required zero-sequence voltage for compensation is calculated based on the degree of power imbalance in the system, specifically expressed as follows: In the formula, The magnitude of the zero-sequence voltage to be compensated; The phase of the zero-sequence voltage to be compensated; This refers to the voltage amplitude of the power grid. The initial phase angle of the grid current; Phase A power imbalance; For phase B power imbalance; For C-phase power imbalance; For intermediate parameters; , and For segmented judgment criteria; This is the ratio of the rated power of the three-phase CHB to the rated power of the inverter; This represents the relative output power coefficient of TTLI.

8. The power balance optimization control method for cascaded H-bridge photovoltaic grid-connected inverters as described in claim 7, characterized in that, The segmented criterion is specifically expressed as follows: In the formula, Phase A power imbalance; For phase B power imbalance; This refers to the power imbalance of phase C.

9. An adaptive zero-sequence voltage compensation method, characterized in that, include: Obtain the circuit parameters of the cascaded H-bridge photovoltaic grid-connected inverter, determine whether the three-phase power is balanced, if so, no compensation is needed; otherwise, perform zero-sequence voltage compensation based on the topology self-balancing control strategy. After zero-sequence voltage compensation, it is determined whether the DC-side second harmonic fluctuation meets the requirements and whether the three-phase power can be restored to balance. If the DC-side second harmonic fluctuation meets the requirements and the three-phase power can be restored to balance, no further additional control strategy is required. Otherwise, full zero-sequence voltage compensation is performed based on the full zero-sequence voltage compensation strategy applicable to hybrid CHB inverters. After complete zero-sequence voltage compensation, it is determined whether the three-phase HB is over-modulated. If so, power balance control is performed using the cascaded H-bridge photovoltaic grid-connected inverter power balance optimization control method as described in any one of claims 1-8.

10. A power balance optimization control system for a cascaded H-bridge photovoltaic grid-connected inverter, characterized in that, include: The parameter acquisition module is configured to acquire the circuit parameters of the cascaded H-bridge photovoltaic grid-connected inverter. Each phase of the cascaded H-bridge photovoltaic grid-connected inverter consists of multiple independent DC bus H-bridge modules and a three-phase two-level inverter connected in series. The DC side of each H-bridge module and the three-phase two-level inverter is connected to a photovoltaic array. The modulation calculation module is configured to: calculate the modulation voltage of each H-bridge module based on the acquired circuit parameters, calculate the modulation degree of all H-bridge modules based on the modulation voltage of the H-bridge modules, and determine the maximum modulation degree among them. The adaptive zero-sequence voltage coefficient generation module is configured to compare the maximum modulation degree with a preset linear modulation reference value to obtain a modulation error signal; An adaptive zero-sequence voltage coefficient is generated based on the modulation error signal; wherein, the adaptive zero-sequence voltage coefficient is between 0 and 1; The adaptive zero-sequence voltage generation module is configured to: calculate the complete zero-sequence voltage based on the degree of power imbalance in the system, and multiply the complete zero-sequence voltage by the adaptive zero-sequence voltage coefficient to generate the adaptive zero-sequence voltage; The compensation module is configured to inject an adaptive zero-sequence voltage into the modulation wave of the three-phase H-bridge module, thereby suppressing over-modulation of the H-bridge module and achieving three-phase power balance.

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