A power coordination method for preventing and treating over-modulation of grid-connected operation of a photovoltaic storage integrated quasi-z-source inverter facing photovoltaic power fluctuation

Abstract

This invention discloses a power coordination method for preventing overmodulation in grid-connected quasi-Z-source inverters with integrated photovoltaic and energy storage systems, addressing photovoltaic power fluctuations. The method involves collecting photovoltaic voltage, photovoltaic current, and battery voltage to calculate the available photovoltaic power and constructing a feasible active power boundary based on the DC-side energy balance relationship of the quasi-Z-source impedance network. When the original virtual synchronous generator active power reference value exceeds this feasible boundary, the method calculates the power derating online and corrects the active power reference value, keeping the inverter operating point within the linear modulation region. This invention eliminates the need for additional battery current closed-loop or quasi-Z-source network voltage-current closed-loop; overmodulation prevention under photovoltaic power fluctuations is achieved solely through supervisory layer power reference correction. It is suitable for applications involving the retrofitting of existing photovoltaic power plants into grid-connected photovoltaic-energy storage systems.

Description

Technical Field

[0001] This application belongs to the field of new energy grid-connected converter control technology, specifically involving a power coordination method for preventing overmodulation in the grid operation of a photovoltaic-storage integrated quasi-Z source inverter oriented towards photovoltaic power fluctuations. In particular, it involves a control method that, under conditions of low irradiance, rapid changes in photovoltaic irradiance, or partial shading of the photovoltaic array, corrects the active power reference value of the virtual synchronous generator to maintain linear modulation operation of the photovoltaic-storage integrated quasi-Z source inverter and avoid overmodulation. Background Technology

[0002] In recent years, with the increasing proportion of new energy power generation such as photovoltaics in the power system, the proportion of traditional synchronous generators has decreased, leading to a reduction in the system's equivalent inertia and voltage support capability. To improve the active support capability of new energy grid-connected systems, grid-connected converter control has gradually attracted attention. Grid-connected converters can actively establish voltage amplitude, frequency, and phase angle, and simulate the inertia and damping characteristics of synchronous generators through droop control, virtual synchronous generator control, and other methods, thereby improving the frequency and voltage support capability of new energy grid-connected systems.

[0003] The literature “Liu Hui, Yu Siqi, Sun Dawei, et al. Review of Control Technology and Principle of Grid-based Converters [J]. Proceedings of the CSEE, 2025, 45(1): 277-297” reviews the control technology of grid-based converters, pointing out that grid-based control can enable the converter to exhibit voltage source characteristics and has the ability to autonomously construct voltage and frequency. This type of method provides a control basis for the transformation of photovoltaic power plants from grid-connected operation to grid-based operation. However, for photovoltaic systems, their active power support capacity is still limited by the available power on the photovoltaic side. When irradiance decreases or local shading occurs, if grid-based control still maintains a high active power demand, a mismatch between the available power on the source side and the power demand on the grid side is likely to occur.

[0004] To improve the operating capability of photovoltaic systems within a wide DC voltage range, Z-source / quasi-Z-source inverters are applied to photovoltaic grid-connected systems. Quasi-Z-source inverters possess continuous input current and boost capability, can boost the DC-side voltage through a shoot-through state, and utilize impedance networks to complete energy storage and transfer. The literature "Li Yuan, Peng Fangzheng. Constant voltage control strategy for capacitors in photovoltaic grid-connected systems using Z-source / quasi-Z-source inverters [J]. Journal of Electrical Engineering, 2011, 26(5):62-69" studies the capacitor voltage control problem of Z-source / quasi-Z-source inverters in photovoltaic grid-connected systems, pointing out that photovoltaic cells are greatly affected by ambient temperature and light intensity, and their output voltage varies over a wide range. Z-source / quasi-Z-source inverters can improve the system's adaptability to changes in photovoltaic-side voltage.

[0005] In the grid-based retrofitting of existing photovoltaic power plants, if only virtual synchronous generator control is introduced from the inverter control layer, the system's active power support capacity is still limited by the instantaneous photovoltaic power. If additional battery energy storage units are added, traditional solutions typically require configuring battery-side DC / DC converters, increasing hardware costs and control complexity. The integrated photovoltaic-energy storage quasi-Z-source inverter can directly connect battery energy storage units to the DC side of the quasi-Z-source impedance network, providing energy buffering without adding additional battery-side DC / DC converters. Therefore, it is suitable for the grid-based retrofitting of existing photovoltaic power plants.

[0006] However, under the control of a virtual synchronous generator, the integrated photovoltaic-storage quasi-Z-source inverter still faces new operational challenges. When photovoltaic irradiance decreases, or local shading or changes in the maximum power point lead to a reduction in available photovoltaic power, if the active power reference value of the virtual synchronous generator is not adjusted in time, the active power demand on the AC side may exceed the available energy on the DC side. In this situation, the diode current decreases or even ceases in the non-shoo-through state of the quasi-Z-source impedance network, the effective charging time of the capacitor is shortened, and the DC link voltage support capability decreases, ultimately causing the inverter to enter the overmodulation region, resulting in output voltage distortion and reduced system operational stability.

[0007] In summary, the existing technology still has the following shortcomings:

[0008] 1) Existing photovoltaic grid control methods do not adequately consider the matching relationship between available power fluctuations on the photovoltaic side and active power demand on the AC side;

[0009] 2) Existing research on grid-connected Z-source / quasi-Z-source photovoltaics mainly focuses on boost control, capacitor voltage control and grid-connected control, with insufficient research on overmodulation prevention under grid-connected operation;

[0010] 3) Existing methods lack an active power reference value correction strategy based on the DC-side energy boundary. Therefore, it is necessary to propose a power coordination method for preventing overmodulation during grid operation of photovoltaic power fluctuation integrated quasi-Z-source inverters. Summary of the Invention

[0011] To address the aforementioned shortcomings, this application proposes a power coordination method for preventing overmodulation during grid-connected operation of photovoltaic (PV) power fluctuation-integrated quasi-Z-source inverters. This method uses the PV-storage quasi-Z-source inverter as the power conversion structure for the grid-connected retrofit of existing PV power plants. The battery energy storage unit is directly connected to the DC side of the quasi-Z-source impedance network, providing an energy buffer for the grid-connected operation of the virtual synchronous generator (VSR). Simultaneously, addressing the issue that when PV irradiance decreases, local shading occurs, or the maximum power point changes, leading to a reduction in available PV power, the active power demand of the VSR may exceed the available DC energy, causing overmodulation, this application establishes a feasible active power boundary based on the DC-side energy transfer relationship of the quasi-Z-source impedance network. Based on this boundary, the active power reference value of the VSR is adjusted online, thereby enabling the system to maintain linear modulation operation under PV power fluctuation conditions.

[0012] The objective of this application can be achieved through the following technical solutions:

[0013] A power coordination method for preventing overmodulation during grid operation of photovoltaic power fluctuation integrated quasi-Z-source inverters includes the following steps:

[0014] Step S1: Establish a grid-connected system model for a photovoltaic-storage integrated quasi-Z source inverter.

[0015] A main circuit topology for a photovoltaic-storage integrated quasi-Z-source inverter is constructed. This topology includes a photovoltaic array, a quasi-Z-source impedance network, a battery energy storage unit, a three-phase inverter bridge, an AC filter, a point of common coupling, and the power grid. The photovoltaic array is connected to the input side of the quasi-Z-source impedance network, and the battery energy storage unit is directly connected to the DC side of the quasi-Z-source impedance network. The quasi-Z-source impedance network is connected to the point of common coupling via the three-phase inverter bridge and the AC filter. The quasi-Z-source impedance network includes a first inductor L1, a second inductor L2, a first capacitor C1, a second capacitor C2, and diodes, used to achieve continuous input current on the photovoltaic side, DC-side voltage boost, and battery energy buffering. A virtual synchronous generator control strategy is adopted to achieve grid-connected operation. The inverter output voltage phase angle and frequency commands are generated through an active power control loop, and the inverter output voltage amplitude commands are generated through a reactive power control loop.

[0016] The active power control equation for the virtual synchronous generator is expressed as:

[0017] ;

[0018] Where J and D represent the moment of inertia and damping coefficient of the virtual synchronous generator, respectively. T IN and T OUT These represent the input torque and the output torque, respectively. T REFThe reference torque is represented by ω, and f represents the output angular frequency and frequency of the virtual synchronous generator, respectively. g and f g Let k represent the grid synchronization angular frequency and the grid frequency, respectively. f Indicates the frequency droop factor;

[0019] The reactive voltage control equation is expressed as:

[0020] ;

[0021] Where E is the virtual internal potential amplitude command, E0 is the rated internal potential amplitude, and Q... REF and Q OUT These are the reactive power reference value and the output reactive power, U. REF U and k represent the voltage reference value and output voltage, respectively. Q and k u These are the reactive power regulation coefficient and the voltage compensation coefficient, respectively.

[0022] Step S2: Analyze the DC-side energy transfer state of the photovoltaic-storage integrated quasi-Z source inverter under photovoltaic power fluctuation conditions, and determine the diode current constraint conditions that the quasi-Z source impedance network needs to meet for normal operation.

[0023] The integrated photovoltaic-storage quasi-Z-source inverter operates in both shoot-through and non-shoot-through states. In shoot-through state, the three-phase inverter bridge arms are short-circuited, the diodes in the quasi-Z-source impedance network are off, the impedance network inductors store energy, and the capacitors release energy. In non-shoot-through state, the three-phase inverter bridge is in an active or zero-vector state, the diodes in the quasi-Z-source impedance network are on, and the photovoltaic array, battery storage units, and impedance network jointly transfer energy to the three-phase inverter bridge, while the impedance network capacitors receive supplementary charging. When photovoltaic irradiance decreases, the photovoltaic array is partially shaded, or the photovoltaic output power drops rapidly, if the virtual synchronous generator active power reference value remains high, a mismatch occurs between the available photovoltaic power and the AC-side active power demand. At this time, the diode conduction current in the quasi-Z-source impedance network decreases in the non-shoot-through state, and may even be interrupted in some non-shoot-through sections, resulting in a shortened effective charging time for the impedance network capacitors. As the capacitor voltage decreases, the DC link voltage support capability weakens, and the inverter may eventually enter the overmodulation region.

[0024] Therefore, to ensure that the quasi-Z source impedance network is in a normal energy transfer state, the average current of the diode in the non-straight-through state should remain positive, and the constraint condition is expressed as:

[0025] ;

[0026] Among them, I PV I is the average current on the photovoltaic side. BThe average output current on the battery side; when When this time, it indicates that the battery energy storage unit is in a discharging state.

[0027] Step S2.1, collect DC side operating parameters:

[0028] Photovoltaic side voltage V PV Average current I on the photovoltaic side PV Battery voltage V B and the system equivalent power loss P loss ;

[0029] Step S2.2, calculate the available photovoltaic power:

[0030] ;

[0031] Among them, P PV The available power of photovoltaics;

[0032] Step S2.3: Establish the relationship between photovoltaic power and battery power:

[0033] ;

[0034] Among them, P B This refers to the battery-side power.

[0035] Step S2.4: Based on the energy conservation relationship between the DC and AC sides, establish the power balance equation:

[0036] ;

[0037] Among them, P INV P is the active power output of the inverter's AC side. loss This represents the system's equivalent power loss.

[0038] Step S2.5: Under the control of the virtual synchronous generator, the active power P output from the AC side of the inverter is... INV Equivalent to the virtual synchronous generator active power reference value Pref, combined with the constraint that the average diode current remains positive, the feasible boundary of active power for grid-connected operation of the photovoltaic-storage integrated quasi-Z-source inverter is obtained:

[0039] ;

[0040] Where Pref is the reference value of the active power of the virtual synchronous generator;

[0041] Step S2.6: The feasible boundary of active power is written in discrete sampling form. In the k-th sampling period, the upper limit of the allowed active power is expressed as:

[0042] ;

[0043] Among them, P lim [k] represents the feasible active power boundary in the k-th sampling period, P PV [k] represents the available photovoltaic power in the k-th sampling period, V B [k] represents the battery voltage in the kth sampling period, P loss [k] represents the system's equivalent power loss during the kth sampling period.

[0044] Step S3: Based on the photovoltaic side voltage, photovoltaic side average current, battery voltage, and system equivalent power loss, establish the active power feasible boundary for the grid-connected operation of the photovoltaic-storage integrated quasi-Z source inverter.

[0045] Step S3.1: Obtain the original active power reference value P of the virtual synchronous generator. ref,0 [k];

[0046] Step S3.2, the original active power reference value P ref,0 [k] and the feasible boundary P of active power lim [k] compares;

[0047] When the following conditions are met: At that time, it was determined that the current active power demand of the virtual synchronous generator did not exceed the feasible operating boundary of the DC side of the quasi-Z source inverter, and the system did not have the risk of entering the over-modulation region;

[0048] Step S3.3: Calculate the active power derating based on the comparison results:

[0049] ;

[0050] Wherein, P0[k] is the active power derating in the kth sampling period;

[0051] Step S3.4: Generate the corrected virtual synchronous generator active power reference value based on the active power derating.

[0052] ;

[0053] in, This is the corrected reference value of the active power of the virtual synchronous generator in the kth sampling period;

[0054] when At that time, there were:

[0055] ;

[0056] when At that time, there were:

[0057] ;

[0058] This ensures that the reference value of the active power of the virtual synchronous generator does not exceed the feasible operating boundary of the quasi-Z source inverter under the current photovoltaic power conditions.

[0059] Step S4: Compare the original active power reference value of the virtual synchronous generator with the active power feasible boundary, calculate the active power derating, and generate the corrected active power reference value.

[0060] Step S5: Input the corrected active power reference value into the virtual synchronous generator control loop to enable the photovoltaic-storage integrated quasi-Z source inverter to maintain linear modulation operation under photovoltaic power fluctuation conditions.

[0061] The corrected active power reference value The input virtual synchronous generator active power control loop generates inverter output voltage phase angle and frequency commands. The reactive voltage control loop generates inverter output voltage amplitude commands based on reactive power reference values, voltage reference values, output reactive power, and output voltage. The output voltage phase angle, frequency commands, and voltage amplitude commands together generate a three-phase voltage reference signal, which is then used by the quasi-Z source inverter modulation strategy to generate three-phase inverter bridge switching signals.

[0062] The photovoltaic (PV) side maximum power point tracking (MPPT) control operates in parallel with the power coordination method. MPPT control adjusts the PV side operating point and generates the direct-on duty cycle of the quasi-Z-source inverter. The power coordination method corrects the virtual synchronous generator (VSG) active power reference value based on the available PV power. This power coordination method does not directly adjust the battery current, nor does it additionally set up a voltage or current closed loop in the quasi-Z-source impedance network. Instead, it indirectly coordinates the power relationship between the PV side, battery side, and AC side by correcting the VSG active power reference value.

[0063] Compared with the prior art, this application has the following advantages:

[0064] 1. This application uses the photovoltaic-storage integrated quasi-Z source inverter as the power conversion structure for the grid-based transformation of existing photovoltaic power plants. It directly connects the battery energy storage unit to the DC side of the quasi-Z source impedance network to provide energy buffer for the grid-based operation of virtual synchronous power generation.

[0065] 2. Based on the DC-side energy transfer relationship of the quasi-Z source impedance network, this application establishes a feasible boundary for active power and corrects the reference value of the active power of the virtual synchronous generator online according to the boundary, so that the system can maintain linear modulation operation under photovoltaic power fluctuation conditions;

[0066] 3. This application does not require the addition of a battery current closed loop or a quasi-Z source network voltage and current closed loop. It can achieve overmodulation prevention under photovoltaic power fluctuations simply by using the supervisory layer power reference correction. It is suitable for application scenarios where existing photovoltaic power plants are being upgraded to grid-type photovoltaic-storage systems. Attached Figure Description

[0067] Figure 1 This is a structural diagram of a grid-connected system for a photovoltaic-storage integrated quasi-Z source inverter according to an embodiment of this application;

[0068] Figure 2 This is a schematic diagram of the feasible boundary of active power under photovoltaic power fluctuation conditions in the embodiments of this application;

[0069] Figure 3 This is a flowchart of the supervisory power coordination control according to an embodiment of this application;

[0070] Figure 4 This is a system operation waveform diagram when the power coordination method is not used in the embodiments of this application;

[0071] Figure 5 This is a waveform diagram of the system operation after adopting the power coordination method in the embodiments of this application. Detailed Implementation

[0072] To enhance understanding of this application, the following detailed description of the embodiments is provided in conjunction with the accompanying drawings.

[0073] Example 1;

[0074] This embodiment provides a power coordination method for preventing overmodulation during grid operation of photovoltaic power fluctuation-integrated quasi-Z-source inverters with photovoltaic power storage, specifically including the following steps:

[0075] Step S1: Establish a grid-connected system model for a photovoltaic-storage integrated quasi-Z source inverter.

[0076] Figure 1 This is a structural diagram of a grid-connected system for a photovoltaic-storage integrated quasi-Z-source inverter, as described in an embodiment of this application. Figure 1 As shown, the system includes a photovoltaic array, a quasi-Z-source impedance network, a battery energy storage unit, a three-phase inverter bridge, an AC filter, a point of common coupling, and the power grid. The photovoltaic array is connected to the input side of the quasi-Z-source impedance network, the battery energy storage unit is directly connected to the DC side of the quasi-Z-source impedance network, and the quasi-Z-source impedance network is connected to the point of common coupling through the three-phase inverter bridge and the AC filter. The quasi-Z-source impedance network includes a first inductor L1, a second inductor L2, a first capacitor C1, a second capacitor C2, and diodes, used to realize continuous input current on the photovoltaic side, DC-side voltage boost, and battery energy buffering. A virtual synchronous generator control strategy is used to achieve grid-connected operation. The active power control loop generates inverter output voltage phase angle and frequency commands, and the reactive power control loop generates inverter output voltage amplitude commands.

[0077] The active power control equation for the virtual synchronous generator is expressed as:

[0078] ;

[0079] Where J and D represent the moment of inertia and damping coefficient of the virtual synchronous generator, respectively. T IN and T OUT These represent the input torque and the output torque, respectively. T REF The reference torque is represented by ω, and f represents the output angular frequency and frequency of the virtual synchronous generator, respectively. g and f g Let k represent the grid synchronization angular frequency and the grid frequency, respectively. f Indicates the frequency droop factor;

[0080] The reactive voltage control equation is expressed as:

[0081] ;

[0082] Where E is the virtual internal potential amplitude command, E0 is the rated internal potential amplitude, and Q... REF and Q OUT These are the reactive power reference value and the output reactive power, U. REF U and k represent the voltage reference value and output voltage, respectively. Q and k u These are the reactive power regulation coefficient and the voltage compensation coefficient, respectively.

[0083] Step S2: Analyze the DC-side power constraint relationship of the photovoltaic-storage integrated quasi-Z source inverter under photovoltaic power fluctuation conditions.

[0084] The integrated photovoltaic-storage quasi-Z-source inverter operates in both shoot-through and non-shoot-through states. In shoot-through state, the three-phase inverter bridge arms are short-circuited, the diodes in the quasi-Z-source impedance network are off, the impedance network inductors store energy, and the capacitors release energy. In non-shoot-through state, the three-phase inverter bridge is in an active or zero-vector state, the diodes in the quasi-Z-source impedance network are on, and the photovoltaic array, battery storage units, and impedance network jointly transfer energy to the three-phase inverter bridge, while the impedance network capacitors receive supplementary charging. When photovoltaic irradiance decreases, the photovoltaic array is partially shaded, or the photovoltaic output power drops rapidly, if the virtual synchronous generator active power reference value remains high, a mismatch occurs between the available photovoltaic power and the AC-side active power demand. At this time, the diode conduction current in the quasi-Z-source impedance network decreases in the non-shoot-through state, and may even be interrupted in some non-shoot-through sections, resulting in a shortened effective charging time for the impedance network capacitors. As the capacitor voltage decreases, the DC link voltage support capability weakens, and the inverter may eventually enter the overmodulation region.

[0085] Therefore, to ensure that the quasi-Z source impedance network is in a normal energy transfer state, the average current of the diode in the non-straight-through state should remain positive, and the constraint condition is expressed as:

[0086] ;

[0087] Among them, I PV I is the average current on the photovoltaic side. B The average output current on the battery side; when When this time, it indicates that the battery energy storage unit is in a discharging state.

[0088] Step S3: Establish the active power feasible boundary for the grid-connected operation of the photovoltaic-storage integrated quasi-Z source inverter.

[0089] Step S3.1: Collect DC side operating parameters:

[0090] Photovoltaic side voltage V PV Average current I on the photovoltaic side PV Battery voltage V B and the system equivalent power loss P loss ;

[0091] Step S3.2, calculate the available photovoltaic power:

[0092] ;

[0093] Among them, P PV The available power of photovoltaics;

[0094] Step S3.3: Establish the relationship between photovoltaic power and battery power:

[0095] ;

[0096] Among them, P B This refers to the battery-side power.

[0097] Step S3.4: Based on the energy conservation relationship between the DC and AC sides, establish the power balance equation:

[0098] ;

[0099] Among them, P INV P is the active power output of the inverter's AC side. loss This represents the system's equivalent power loss.

[0100] Step S3.5: Under the control of the virtual synchronous generator, the active power P output from the AC side of the inverter is... INV Equivalent to the virtual synchronous generator active power reference value Pref, combined with the constraint that the average diode current remains positive, the feasible boundary of active power for grid-connected operation of the photovoltaic-storage integrated quasi-Z-source inverter is obtained:

[0101] ;

[0102] Where Pref is the reference value of the active power of the virtual synchronous generator;

[0103] Step S3.6, Figure 2 This is a schematic diagram of the feasible boundary of active power under photovoltaic power fluctuation conditions in an embodiment of this application. Figure 2 As shown, the active power feasible boundary divides the system operating region into a linear modulation operating region and an overmodulation risk region. When the available photovoltaic power decreases, if the active power reference value of the virtual synchronous generator remains unchanged, the system operating point may cross the active power feasible boundary and enter the overmodulation risk region; if the active power reference value is reduced according to the active power feasible boundary, the system operating point can remain within the linear modulation operating region. In discrete control, the upper limit of the allowed active power in the k-th sampling period is expressed as:

[0104] ;

[0105] Among them, P lim [k] represents the feasible active power boundary in the k-th sampling period, P PV [k] represents the available photovoltaic power in the k-th sampling period, V B [k] represents the battery voltage in the kth sampling period, V. PV [k] represents the photovoltaic side voltage during the kth sampling period, P loss [k] represents the system's equivalent power loss during the kth sampling period.

[0106] Step S4: Compare the original active power reference value of the virtual synchronous generator with the active power feasible boundary, calculate the active power derating, and generate the corrected active power reference value.

[0107] Step S4.1, Figure 3 This is a flowchart illustrating the supervised power coordination control process according to an embodiment of this application. Figure 3 As shown, the controller first obtains the original active power reference value P of the virtual synchronous generator. ref,0 [k];

[0108] Step S4.2, the original active power reference value P ref,0 [k] and the feasible boundary P of active power lim [k] compares;

[0109] When the following conditions are met: At that time, it was determined that the current active power demand of the virtual synchronous generator did not exceed the feasible operating boundary of the DC side of the quasi-Z source inverter, and the system did not have the risk of entering the over-modulation region;

[0110] Step S4.3: Calculate the active power derating based on the comparison results.

[0111] ;

[0112] Wherein, P0[k] is the active power derating in the kth sampling period;

[0113] Step S4.4: Generate the corrected virtual synchronous generator active power reference value based on the active power derating.

[0114] ;

[0115] in, This is the corrected reference value of the active power of the virtual synchronous generator in the kth sampling period;

[0116] when At that time, there were:

[0117] ;

[0118] when At that time, there were:

[0119] ;

[0120] This ensures that the reference value of the active power of the virtual synchronous generator does not exceed the feasible operating boundary of the quasi-Z source inverter under the current photovoltaic power conditions.

[0121] The corrected active power reference value is input into the virtual synchronous generator active power control loop, which generates the inverter output voltage phase angle and frequency commands. The reactive power and voltage control loop generates the inverter output voltage amplitude command based on the reactive power reference value, voltage reference value, output reactive power, and output voltage. The output voltage phase angle, frequency command, and voltage amplitude command together generate a three-phase voltage reference signal, which is then used by the quasi-Z source inverter modulation strategy to generate three-phase inverter bridge switching signals.

[0122] In this embodiment, the photovoltaic (PV) side maximum power point tracking (MPPT) control and the power coordination method operate in parallel. MPPT control is used to adjust the PV side operating point and generate the direct-on duty cycle of the quasi-Z-source inverter. The power coordination method is used to correct the virtual synchronous generator active power reference value based on the available PV power. Since this application does not directly adjust the battery current, nor does it add additional voltage or current closed loops to the quasi-Z-source impedance network, it can achieve overmodulation prevention under PV power fluctuations while maintaining a simple control structure.

[0123] Example 2;

[0124] This embodiment revolves around Figure 4 and Figure 5 This demonstrates the operational performance of this application under experimental conditions of linearly decreasing irradiance.

[0125] This embodiment adopts the same grid-connected system structure of the integrated photovoltaic-storage quasi-Z-source inverter as Embodiment 1, and is verified on a hardware-in-the-loop experimental platform. The experimental platform consists of a real-time simulator, a control board, a host computer, and an oscilloscope. The real-time simulator is used to run the main circuit of the integrated photovoltaic-storage quasi-Z-source inverter and the grid model. The control board is used to run virtual synchronous generator control, maximum power point tracking control, and the supervised power coordination method described in this application. The oscilloscope is used to record the dynamic responses such as AC side active power P, reactive power Q, quasi-Z-source impedance network first capacitor voltage VC1, three-phase voltage Vabc at the point of common coupling, and three-phase output current Iabc.

[0126] To verify the experimental operating characteristics of the system under conditions of continuous decrease in photovoltaic power, this embodiment sets the photovoltaic irradiance to decrease linearly from 600 W / m² to 400 W / m² within 5 seconds. During this process, the available photovoltaic power gradually decreases as the irradiance decreases, and the virtual synchronous generator grid control still needs to maintain the active power output on the AC side. Therefore, this operating condition can reflect the matching problem between the decrease in available photovoltaic power and the active power demand of the grid.

[0127] Figure 4The figures show experimental waveforms of active power P, reactive power Q, first capacitor voltage VC1 of the quasi-Z source impedance network, three-phase voltage Vabc at the point of common coupling, and three-phase output current Iabc without the power coordination method. Figure 4 In the experimental conditions shown, the active power reference value of the virtual synchronous generator remains unchanged from the original setpoint. As the irradiance decreases, the available photovoltaic power gradually decreases, and the energy supplied by the DC side of the quasi-Z source inverter gradually becomes insufficient. When the available photovoltaic power cannot meet the current active power demand, the diode current in the non-straight-through state of the quasi-Z source impedance network decreases, the effective charging capacity of the capacitor decreases, causing the voltage VC1 of the first capacitor in the quasi-Z source impedance network to gradually decrease, and the voltage support capability of the DC link weakens accordingly.

[0128] Depend on Figure 4 It can be seen that the system enters an overmodulation state at approximately t=2.5 s. At this time, the voltage VC1 of the first capacitor of the quasi-Z source impedance network drops significantly, the three-phase voltage Vabc at the point of common coupling shows obvious distortion, and the three-phase output current Iabc increases abnormally. This indicates that without the power coordination method, the linear decrease in irradiance will lead to a mismatch between the available photovoltaic power and the active power demand of the virtual synchronous generator, and further cause DC-side energy transfer imbalance and inverter overmodulation, reducing the stability of the system grid operation.

[0129] Figure 5 The experimental waveforms of active power P, reactive power Q, first capacitor voltage VC1 of the quasi-Z source impedance network, three-phase voltage Vabc at the point of common coupling, and three-phase output current Iabc after adopting the power coordination method of this application are shown. Figure 5 In the experimental conditions shown, the irradiance decrease condition is similar to... Figure 4 To maintain consistency, the controller collects real-time data on the photovoltaic side voltage VPV, photovoltaic side average current IPV, and battery voltage VB, calculates the available photovoltaic power PPV, and calculates the current allowable active power upper limit Plim based on the active power feasible boundary. When the original active power reference value Pref,0 of the virtual synchronous generator is detected to exceed the current active power feasible boundary, the supervisory power coordination loop calculates the active power derating P0 and generates a corrected active power reference value Pref*.

[0130] After the corrected active power reference value Pref* is input into the virtual synchronous generator control loop, the AC-side active power P is coordinated and adjusted according to the current available photovoltaic power, enabling the quasi-Z source impedance network to maintain effective energy transfer and preventing continuous over-discharge of the capacitor. Figure 5 It can be seen that after adopting the method of this application, the first capacitor voltage VC1 of the quasi-Z source impedance network can be maintained within the range that supports linear modulation operation. The three-phase voltage Vabc and the three-phase output current Iabc at the common connection point do not show serious distortion caused by overmodulation. The system can maintain stable network operation under the experimental condition of linearly decreasing irradiance.

[0131] Depend on Figure 4 and Figure 5 The comparison shows that without the method of this application, the linear decrease in irradiance will cause the available photovoltaic power to gradually fall below the active power requirement of the virtual synchronous generator, resulting in an imbalance in the DC-side energy transfer, a decrease in VC1, and overmodulation of the inverter, which will further cause Vabc distortion and abnormal increase in Iabc. After adopting the method of this application, the reference value of the active power of the virtual synchronous generator can be corrected in real time according to the feasible boundary of the active power of the DC side, so that the active power P of the AC side is coordinated with the available power of the photovoltaic side, maintaining the voltage support capability of VC1, and maintaining the normal dynamic response of Vabc and Iabc, thereby avoiding overmodulation and improving the grid operation stability of the photovoltaic-storage integrated quasi-Z source inverter under the condition of continuous decrease in photovoltaic power.

[0132] The preferred embodiments of this application have been described in detail above. It should be understood that those skilled in the art can make numerous modifications and variations based on the concept of this application without inventive effort. Therefore, any technical solutions that can be obtained by those skilled in the art based on the concept of this application through logical analysis, reasoning, or limited experimentation on the basis of existing technology should be within the scope of protection defined by the claims.

Claims

1. A method for overmodulation prevention and power coordination in grid-connected operation of photovoltaic power fluctuation integrated quasi-Z-source inverters, characterized in that, Includes the following steps: Step S1: Obtain the DC-side operating parameters of the photovoltaic-storage integrated quasi-Z source inverter and the original active power reference value of the virtual synchronous generator under grid-connected operation conditions; Step S2: Calculate the available photovoltaic power based on the DC side operating parameters, and determine the feasible boundary of active power based on the DC side energy balance relationship of the quasi-Z source impedance network; Step S3: Compare the original active power reference value of the virtual synchronous generator with the active power feasible boundary to determine whether there is an over-modulation risk at the current operating point; Step S4: When the original active power reference value does not exceed the feasible active power boundary, keep the original active power reference value unchanged; When the original active power reference value exceeds the active power feasible boundary, the active power derating is calculated and a corrected active power reference value is generated. Step S5: Input the corrected active power reference value into the virtual synchronous generator control loop to enable the photovoltaic-storage integrated quasi-Z source inverter to maintain linear modulation operation under photovoltaic power fluctuation conditions.

2. The method for overmodulation prevention and power coordination in grid operation of a photovoltaic-storage integrated quasi-Z-source inverter oriented towards photovoltaic power fluctuations, as described in claim 1, is characterized in that... The specific method for step S1 is as follows: A grid-connected system model for a photovoltaic-storage integrated quasi-Z-source inverter is established, as detailed below: A main circuit topology for a photovoltaic-storage integrated quasi-Z-source inverter is constructed. This topology includes a photovoltaic array, a quasi-Z-source impedance network, a battery energy storage unit, a three-phase inverter bridge, an AC filter, a point of common coupling, and the power grid. The photovoltaic array is connected to the input side of the quasi-Z-source impedance network, and the battery energy storage unit is directly connected to the DC side of the quasi-Z-source impedance network. The quasi-Z-source impedance network is connected to the point of common coupling through the three-phase inverter bridge and the AC filter. The quasi-Z-source impedance network includes a first inductor L1, a second inductor L2, a first capacitor C1, a second capacitor C2, and diodes, used to achieve continuous input current on the photovoltaic side, DC-side voltage boost, and battery energy buffering. The photovoltaic-storage integrated quasi-Z source inverter adopts a virtual synchronous generator control strategy to achieve grid-connected operation. It generates inverter output voltage phase angle and frequency commands through the active power control loop, and generates inverter output voltage amplitude commands through the reactive voltage control loop. The active power control equation for the virtual synchronous generator is expressed as: ； Where J and D represent the moment of inertia and damping coefficient of the virtual synchronous generator, respectively. T IN and T OUT These represent the input torque and the output torque, respectively. T REF The reference torque is represented by ω, and f represents the output angular frequency and frequency of the virtual synchronous generator, respectively. g and f g Let k represent the grid synchronization angular frequency and the grid frequency, respectively. f Indicates the frequency droop factor; The reactive voltage control equation is expressed as: ； Where E is the virtual internal potential amplitude command, E0 is the rated internal potential amplitude, and Q... REF and Q OUT These are the reactive power reference value and the output reactive power, U. REF U and k represent the voltage reference value and output voltage, respectively. Q and k u These are the reactive power regulation coefficient and the voltage compensation coefficient, respectively.

3. The method for overmodulation prevention and power coordination in grid operation of a photovoltaic-storage integrated quasi-Z-source inverter oriented towards photovoltaic power fluctuations, as described in claim 1, is characterized in that... Step S2 further includes: The DC-side energy transfer state of the photovoltaic-storage integrated quasi-Z source inverter under photovoltaic power fluctuation conditions is analyzed, and the diode current constraint conditions that the quasi-Z source impedance network must meet for normal operation are determined as follows: The photovoltaic-storage integrated quasi-Z source inverter includes a shoot-through state and a non-shoot-through state during operation. In the shoot-through state, the three-phase inverter bridge arms are short-circuited, the quasi-Z source impedance network diodes are turned off, the impedance network inductor stores energy, and the capacitor releases energy to the inductor and subsequent circuits; in the non-shoot-through state, the three-phase inverter bridge is in an active state or a zero-vector state, the quasi-Z source impedance network diodes are turned on, and the photovoltaic array, battery energy storage unit and impedance network jointly transfer energy to the inverter bridge, while the impedance network capacitor is recharged. When photovoltaic irradiance decreases, photovoltaic array is partially shaded, or photovoltaic output power drops rapidly, if the reference value of the active power of the virtual synchronous generator exceeds the upper limit of the allowable active power determined under the current operating conditions, a mismatch will occur between the available photovoltaic power and the active power demand on the AC side, causing the diode conduction current of the quasi-Z source impedance network to decrease in the non-straight-through state. When the diode current is interrupted in part of the non-straight-through region, the effective charging time of the impedance network capacitor is shortened, the capacitor voltage gradually decreases, the DC link voltage support capability is weakened, and the inverter eventually enters the overmodulation region. Therefore, to ensure that the quasi-Z source impedance network is in a normal energy transfer state, the average current of the diode in the non-straight-through state should remain positive, and the constraint condition is expressed as: ； Among them, I PV I is the average current on the photovoltaic side. B The average output current on the battery side; when When this time, it indicates that the battery energy storage unit is in a discharging state.

4. The method for overmodulation prevention and power coordination in grid operation of a photovoltaic-storage integrated quasi-Z-source inverter oriented towards photovoltaic power fluctuations, as described in claim 3, is characterized in that... In step S2, the method for determining the feasible boundary of active power is as follows: Step S2.1, collect DC side operating parameters: Photovoltaic side voltage V PV Average current I on the photovoltaic side PV Battery voltage V B and the system equivalent power loss P loss ; Step S2.2, calculate the available photovoltaic power: ； Among them, P PV The available power of photovoltaics; Step S2.3: Establish the relationship between photovoltaic power and battery power: ； Among them, P B This refers to the battery-side power. Step S2.4: Based on the energy conservation relationship between the DC and AC sides, establish the power balance equation: ； Among them, P INV P is the active power output of the inverter's AC side. loss This represents the system's equivalent power loss. Step S2.5: Under the control of the virtual synchronous generator, the active power P output from the AC side of the inverter is... INV Equivalent to the virtual synchronous generator active power reference value Pref, combined with the constraint that the average diode current remains positive, the feasible boundary of active power for grid-connected operation of the photovoltaic-storage integrated quasi-Z-source inverter is obtained: ； Where Pref is the reference value of the active power of the virtual synchronous generator; Step S2.6: The feasible boundary of active power is written in discrete sampling form. In the k-th sampling period, the upper limit of the allowed active power is expressed as: ； Among them, P lim [k] represents the feasible active power boundary in the k-th sampling period, P PV [k] represents the available photovoltaic power in the k-th sampling period, V B [k] represents the battery voltage in the kth sampling period, P loss [k] represents the system's equivalent power loss during the kth sampling period.

5. A power coordination method for preventing overmodulation during grid operation of a photovoltaic-storage integrated quasi-Z-source inverter oriented towards photovoltaic power fluctuations, as described in claim 4, is characterized in that... Step S3 specifically includes the following methods: Step S3.1: Obtain the original active power reference value P of the virtual synchronous generator. ref,0 [k]; Step S3.2, the original active power reference value P ref,0 [k] and the feasible boundary P of active power lim [k] compares; When the following conditions are met: At that time, it was determined that the current active power demand of the virtual synchronous generator did not exceed the feasible operating boundary of the DC side of the quasi-Z source inverter, and the system did not have the risk of entering the over-modulation region; Step S3.3: Calculate the active power derating based on the comparison results: ； Wherein, P0[k] is the active power derating in the kth sampling period; Step S3.4: Generate the corrected virtual synchronous generator active power reference value based on the active power derating. ； in, This is the corrected reference value of the active power of the virtual synchronous generator in the kth sampling period; when At that time, there were: ； when At that time, there were: ； This ensures that the reference value of the active power of the virtual synchronous generator does not exceed the feasible operating boundary of the quasi-Z source inverter under the current photovoltaic power conditions.

6. The method for overmodulation prevention and power coordination in grid operation of a photovoltaic-storage integrated quasi-Z-source inverter oriented towards photovoltaic power fluctuations, as described in claim 5, is characterized in that... Step S5 specifically includes the following methods: The corrected active power reference value The input virtual synchronous generator active power control loop generates inverter output voltage phase angle and frequency commands; the reactive voltage control loop generates inverter output voltage amplitude commands based on reactive power reference values, voltage reference values, output reactive power, and output voltage; the inverter output voltage phase angle, frequency commands, and inverter output voltage amplitude commands are combined to generate a three-phase voltage reference signal, which is then used by a quasi-Z source inverter modulation strategy to generate a three-phase inverter bridge switching signal; Simultaneously, the photovoltaic side maximum power point tracking control and the power coordination method operate in parallel. The maximum power point tracking control is used to adjust the photovoltaic side operating point and generate the direct-on duty cycle of the quasi-Z source inverter. The power coordination method is used to correct the active power reference value of the virtual synchronous generator based on the available photovoltaic power.

7. A power coordination method for preventing overmodulation during grid operation of a photovoltaic-storage integrated quasi-Z-source inverter oriented towards photovoltaic power fluctuations, as described in claim 4, is characterized in that... The equivalent power loss P of the system loss The system's equivalent power loss P includes at least one of the following: three-phase inverter bridge switching losses, three-phase inverter bridge conduction losses, quasi-Z source impedance network inductance losses, quasi-Z source impedance network capacitance losses, diode losses, and AC filter losses. loss It can be obtained through any one of the following methods: fixed ratio estimation, offline experimental calibration, efficiency curve lookup, or real-time power difference calculation.

8. The method for overmodulation prevention and power coordination in grid operation of a photovoltaic-storage integrated quasi-Z-source inverter oriented towards photovoltaic power fluctuations, as described in claim 1, is characterized in that... The photovoltaic power fluctuation includes at least one of the following: a step decrease in photovoltaic irradiance, a continuous decrease in photovoltaic irradiance, partial shading of the photovoltaic array, a change in the maximum power point caused by temperature changes, and rapid fluctuations in photovoltaic output power.

9. A method for overmodulation prevention and power coordination in grid operation of a photovoltaic-storage integrated quasi-Z-source inverter oriented towards photovoltaic power fluctuations, as described in claim 1, is characterized in that... The method is applicable to the grid-based retrofit of existing photovoltaic power plants. The battery energy storage unit is directly connected to the DC side of the quasi-Z source impedance network, without the need for an additional battery-side DC / DC converter.

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