Dual-mode parallel competitive autonomous self-synchronizing voltage source string type photovoltaic power generation system

Abstract

The invention provides a dual-mode parallel competitive autonomous self-synchronizing voltage source string type photovoltaic power generation system, which comprises a photovoltaic array, a machine side converter and a grid side converter, the machine side converter comprises a plurality of paths of parallel DC / DC converters, and the network side converter comprises a plurality of DC / AC converters; the photovoltaic array is connected with the multiple DC / DC converters, the multiple DC / DC converters are connected to the same direct current bus, and the direct current bus is connected in a grid-connected mode through a DC / AC converter. In an MPPT operation interval, machine side MPPT control-network side inertia synchronous control is adopted; in a constant power interval, adopting machine side constant voltage control-network side virtual synchronous control; an MPPT control loop and a constant voltage control loop of the machine side converter operate at the same time, an inertia synchronous control loop and a virtual synchronous control loop of the network side converter operate at the same time, and in different operation intervals, autonomous smooth dynamic switching is completed through a dual-mode parallel competition autonomous strategy.

Description

Technical Field

[0001] This invention relates to the field of new energy power generation equipment technology, specifically to a dual-mode parallel competitive autonomous self-synchronizing voltage source string photovoltaic power generation system. Background Technology

[0002] The desert region possesses abundant photovoltaic resources, but its remote location far from load centers hinders its development. String photovoltaic inverters, as a fundamental power generation unit in this region, currently employ grid-connected control, exhibiting "current source" characteristics. This necessitates significant derating of power transmission channels, hindering power delivery and requiring numerous synchronous condensers, severely restricting the development of new energy sources. Against this backdrop, modifying the photovoltaic inverter control strategy to simulate the operating characteristics of a synchronous generator, enabling active support for grid transient voltages, inertial response, and primary frequency regulation, thus exhibiting "voltage source" characteristics, has become a hot research topic in string photovoltaic control within the desert environment.

[0003] Current research on voltage source control in string photovoltaic (PV) power generation systems mainly focuses on parallel energy storage to maintain DC bus voltage stability, with virtual synchronous control used in the grid-side converter and maximum power point tracking (MPPT) in the generator-side converter. Research on string PV systems without energy storage or with choppers is limited. Therefore, a coordinated grid-generator control structure is crucial for achieving voltage source characteristics in string PV systems across multiple operating ranges. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the purpose of this invention is to provide a dual-mode parallel competitive autonomous self-synchronizing voltage source string photovoltaic power generation system.

[0005] The dual-mode parallel competitive autonomous self-synchronizing voltage source string photovoltaic power generation system provided by the present invention includes:

[0006] Photovoltaic arrays, generator-side converters, and grid-side converters;

[0007] The machine-side converter includes a multi-channel parallel DC / DC converter, and the grid-side converter includes various DC / AC converters;

[0008] The photovoltaic array is connected to multiple DC / DC converters, which are connected to the same DC bus. This DC bus is connected to the grid via a DC / AC converter.

[0009] During the MPPT operating range, a machine-side MPPT control-grid-side inertial synchronization control is adopted; during the constant power range, a machine-side constant voltage control-grid-side virtual synchronization control is adopted.

[0010] The machine-side converter's MPPT control loop and constant voltage control loop operate simultaneously, while the grid-side converter's inertial synchronization control loop and virtual synchronization control loop operate simultaneously. In different operating ranges, autonomous smooth dynamic switching is achieved through a dual-mode parallel competition autonomous strategy.

[0011] Preferably, the self-synchronizing voltage source control strategy for the MPPT operating range includes:

[0012] The multiple Boost circuits in the machine-side converter operate at their respective MPPT points, and switch S M In M1 mode, the maximum power output under the current operating conditions is achieved; the grid-side inverter operates under inertial synchronous control, and the grid-side synchronous loop switching switch S... ω In M1 mode, disturbances to the DC bus voltage are introduced into the grid-connection frequency of the grid-side converter. This maintains the stability of the DC bus voltage while simultaneously synchronizing the inverter with the grid. The phase angle of the inverter output modulation voltage is:

[0013]

[0014] In the formula: ω0 represents the reference value of the power grid frequency, k pdc V represents the inertial synchronization proportional coefficient. dc This represents the actual value of the DC bus voltage, V. dcrefG This represents the given value of the grid-side inertial synchronous DC bus voltage; s represents the Laplace expression method for differential calculation.

[0015] Since inertial synchronization lacks damping, stabilizing damping is added during the generation of the modulated wave voltage amplitude. Its expression is as follows:

[0016]

[0017] In the formula: V t0 Q represents the given value of the peak potential within the inverter; gref Q represents the given value of reactive power in the power grid. g This represents the actual value of the current reactive power of the power grid; k pt and k it D represents the proportional and integral coefficients of the reactive power control loop, respectively; pss V represents the voltage damping stabilization compensation amount; t Indicates the magnitude of the modulated wave voltage; voltage stabilization switch S D In M1 mode; D pss The disturbance ΔV of the DC bus voltage dc get:

[0018]

[0019] In the formula: kpss T represents the damping stability proportional coefficient; c This represents the time constant of the high-pass filter.

[0020] Preferably, when operating within the MPPT range, the photovoltaic power generation system reduces active power output as grid frequency increases, causing the operating point of the generator-side converter's PV curve to shift to the right from the MPPT operating point, and the active power reference P... ref The instruction is:

[0021] P ref =V PV ·I PV +ΔP t (4)

[0022] In the formula, V PV Represented as the photovoltaic panel port voltage, I PV ΔP represents the output current of the photovoltaic panel. t This refers to the active power command for the photovoltaic power generation system participating in the grid frequency response offset;

[0023] Under inertial synchronous control, the DC bus voltage maps to the grid frequency, ΔP t Written as:

[0024] ΔP t =K c ·s·ΔV dc +K t ·ΔV dc (5)

[0025] Where: K c K represents the inertial response coefficient. t This represents the primary frequency modulation factor; -0.2P GN <ΔP t <0, P GN This refers to the rated power of the grid-side inverter.

[0026] Useful reference P ref The instruction obtains the reference value V of the photovoltaic panel port voltage using the MPPT algorithm. PVref The error between the voltage at the photovoltaic panel port and the voltage at the photovoltaic panel port is modulated by a PI controller to obtain the modulation signal PI. Vpv for:

[0027]

[0028] In the formula: k ppv k represents the proportional coefficient of the PI controller in MPPT control. ipv This represents the integral coefficient of the PI controller in MPPT control.

[0029] Preferably, the self-synchronizing voltage source control strategy for the constant power operating range and the low-voltage operating range of the power grid includes:

[0030] The multi-channel Boost circuit in the machine-side converter controls the DC bus voltage. Each channel controls the same DC bus voltage setpoint. Switch S... M In M1 mode;

[0031] The grid-side inverter employs virtual synchronous control, introducing the error signal between the power reference command and the actual value of the grid output active power into the grid-connected frequency of the grid-side converter through a virtual synchronous transfer function. The phase angle of its output modulation voltage is:

[0032]

[0033] In the formula: ω p Indicates the virtual synchronous output grid angular frequency error value; P gref Indicates the reference command for active power of the power grid; P g J represents the actual active power output of the grid-side converter; P D represents the virtual inertia coefficient of the virtual synchronization link. P This represents the virtual damping coefficient of the virtual synchronization link.

[0034] Preferably, when virtual synchronous control is used, damped stabilization already exists in the photovoltaic power generation system, and the output of the stabilization element is:

[0035] D pss =0 (8)

[0036] Grid-side synchronous switching switch S ω In M2 mode, voltage stabilization switch S D In M2 mode, the active power output of the photovoltaic inverter is adjusted while grid frequency synchronization is achieved. Under conditions of enhanced solar illumination and no grid faults, the grid-side converter participates in the grid frequency response. Its virtual synchronization control active power reference command is:

[0037] P gref =P GN +ΔP g (9)

[0038] =P GN +(-K tc ·ω p -K gc ·s·ω p )

[0039] Where: K tc K represents the primary frequency regulation coefficient of the grid-side converter. gc This represents the inertia response coefficient of the grid-side converter; -0.2P GN <ΔPg <0.1P GN ;

[0040] Machine-side switching switch S M In M2 mode, the reference value V of the DC bus voltage on the machine side is given. dcrefM The actual value of DC bus voltage V dc The error is converted into a modulation signal PI by the PI controller. Vdc for:

[0041]

[0042] In the formula: k pvdc k represents the proportional coefficient of the PI controller in constant voltage control. ivdc This represents the integral coefficient of the PI controller in constant pressure control.

[0043] Preferably, during steady-state operation of the power grid, in the MPPT operating range, the generator-side switch S M Operating in M1 mode, the network-side synchronization loop switching switch S ω In M1 mode, with enhanced illumination and increased generator-side output power, the grid-side active power output lags due to the smaller DC capacitor, leading to an increase in DC bus voltage and affecting the generator-side converter PI. Vdc As the voltage decreases, the photovoltaic (PV) operating point jumps to a new PV curve, and its output V... PVref Slightly increase the MPPT control output modulation signal PI Vpv Increase, dual-mode parallel competitive autonomous strategy selection final machine-side output is PI Vdc Switch S M When switched to M2 mode, the grid-side converter outputs active power P. g Get bigger, P gref ω remains unchanged p The output of the network side is reduced to ω, representing the dual-mode parallel competitive autonomous strategy selection. p Switch S D When operating in M2 mode, the system automatically switches to the constant power operating range.

[0044] Preferably, in the constant power operating range, the machine-side switch S M Operating in M2 mode, the grid-side synchronization loop switching switch S ω In M2 mode, as illumination decreases, the output power on the machine side decreases, the DC bus voltage gradually decreases, and PI... Vdc As the voltage increases, the operating point of the photovoltaic (PV) system shifts to the lower left, and its output V... PVref Slightly reduced, PI Vpv Reduce, dual-mode parallel competitive autonomous strategy selection machine-side output is PI Vpv Switch S MSwitching to M1 mode; the active power output of the grid-side converter is greater than the active power input of the grid side, the DC bus voltage decreases, ω dc As the grid-side output active power decreases, it gradually decreases until it equals the grid-side input active power, ω. p Increase, dual-mode parallel competitive autonomous strategy selection network-side output is ω dc Switch S D When operating in M1 mode, the system automatically switches to the MPPT operating range.

[0045] Preferably, during a low-voltage grid fault, the active power output of the grid-side converter is blocked, the DC bus voltage rises, the photovoltaic operating point shifts to the right on the original PV curve, and the output V... PVref Increase, PI Vpv Increase, PI Vdc Reduce, dual-mode parallel competitive autonomous strategy selection machine-side output is PI Vdc Switch S M It will switch to M2 mode, network side ω dc Increase, P gref It will decrease as the grid voltage decreases, and is related to P. g Keep it consistent, ω p The dual-mode parallel competitive autonomous strategy selection network-side output remains unchanged at ω. p Switch S D It operates in M2 mode.

[0046] Preferably, when the fault is recovered, the output power of the grid-side converter is greater than the input power of the generator-side converter, the DC bus voltage decreases, and PI... Vdc Increase, the photovoltaic returns to its original maximum power point, outputting V PVref Increase, PI Vpv Reduce, dual-mode parallel competitive autonomous strategy selection machine-side output is PI Vpv Switch S M It will automatically switch to M1 mode, network side ω dc Decrease, P gref With P g To maintain consistency, the dual-mode parallel competitive autonomous strategy selects the network-side output as ω. dc Switch S D It operates in M2 mode and automatically switches when a fault occurs or ends.

[0047] Preferably, the inverter power reference command design during a low-voltage grid fault includes:

[0048] During low-voltage ride-through, the photovoltaic inverter injects a dynamic reactive current into the power system that varies with the grid connection point voltage. The command for this dynamic reactive current is expressed as:

[0049] I Tq =K i(0.9-V orms )I N 0.2≤V orms ≤0.9 (11)

[0050] In the formula: I Tq The command K represents dynamic reactive current. i I represents the dynamic reactive current proportional gain of a photovoltaic inverter. N K represents the rated current of the photovoltaic inverter; under conditions of high reactive current output, K... i The inverter's reactive current varies with the terminal voltage, ensuring it outputs three times the normal current regardless of any percentage drop in grid voltage. orms Indicates the effective value of the grid voltage at the grid connection point;

[0051] Due to inverter capacity limitations, the active current reference command is as follows:

[0052]

[0053] In the formula: I Td Indicates the variable of active current that the inverter needs to output; I lim This indicates the maximum current limit that the inverter can output;

[0054] Under current source control, because the phase-locked loop locks the inverter phase angle with the grid phase angle, its active and reactive power are decoupled. Active power has a linear relationship with the d-axis current, and reactive power has a linear relationship with the q-axis current. Under voltage source control, there is a phase difference between the inverter and the grid, making decoupling impossible. In this case, the relationships between active and reactive power and the d-axis and q-axis currents are as follows:

[0055]

[0056] From equation (13), the active and reactive power reference commands during low-voltage ride-through are obtained:

[0057]

[0058] The voltage support function is achieved by modifying the power command as described above.

[0059] Among them, V od Indicates the D-axis component of the generated modulated wave voltage; I fd V represents the D-axis component of the generated output current; oq Indicates the Q-axis component of the generated modulated wave voltage; I fq This represents the Q-axis component of the generated output current.

[0060] Compared with the prior art, the present invention has the following beneficial effects:

[0061] (1) This invention proposes a dual-mode parallel competitive autonomous self-synchronizing voltage source string photovoltaic power generation system. In the MPPT operating range, the generator-side converter adopts MPPT control, and the grid-side converter uses inertial synchronization control to maintain DC bus voltage balance while achieving the maximum output power of the inverter. In the constant power operating range, the generator-side converter adopts constant voltage control, and the grid-side converter uses virtual synchronization control to achieve the power balance between the generator and grid-side converters by controlling the output power of the grid-side converter, and at the same time realizes autonomous grid frequency response.

[0062] (2) When switching between different operating ranges, two working modes operate simultaneously. The final loop is determined by the dual-mode parallel competition autonomous strategy, so as to achieve smooth switching between different operating ranges of the equipment and improve the dynamic performance during the switching process. Attached Figure Description

[0063] Other features, objects, and advantages of the present invention will become more apparent from the following detailed description of non-limiting embodiments with reference to the accompanying drawings:

[0064] Figure 1 This is a schematic diagram of a string photovoltaic inverter topology;

[0065] Figure 2 This is a diagram showing the operating status of a photovoltaic power generation system under full sunlight.

[0066] Figure 3 This is a block diagram of the self-synchronizing voltage source control for a string photovoltaic power generation system.

[0067] Figure 4a and Figure 4b The strategy is a dual-mode parallel competition switching strategy;

[0068] Figure 5 Figures (a) to (d) in the figure show the simulation results of frequency modulation in the MPPT operating range;

[0069] Figure 6 Figures (a) to (d) in the figure show the simulation results of frequency modulation in the constant power operation range;

[0070] Figure 7 Figures (a) to (b) show the simulation results of the dual-mode parallel autonomous switching strategy under low-pressure ride-through conditions. Detailed Implementation

[0071] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make several changes and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0072] Example

[0073] This invention proposes a dual-mode parallel competitive autonomous self-synchronizing voltage source string photovoltaic power generation system. Based on the requirements of different operating ranges, corresponding string photovoltaic voltage source strategies are proposed. During switching between different ranges, a dual-mode parallel autonomous switching method is used, with both modes operating simultaneously. Through the dual-mode parallel competitive autonomous strategy, a smooth switching effect is achieved, improving the safety and stability of the equipment. The invention also includes a multi-operating-range self-synchronizing voltage source control method and a dual-mode parallel voltage source control smooth switching method.

[0074] The string photovoltaic power generation system includes a photovoltaic array, a generator-side converter, and a grid-side converter; the generator-side converter includes multiple parallel DC / DC converters, and the grid-side converter includes various DC / AC converters. A schematic diagram of the string photovoltaic inverter topology is shown below. Figure 1 ;

[0075] The voltage source control of the photovoltaic power generation system adopts different control strategies according to different operating ranges of the photovoltaic system: in the MPPT operating range, it employs generator-side MPPT control and grid-side inertial synchronization control; in the constant power range, it employs generator-side constant voltage control and grid-side virtual synchronization control. The different operating ranges of the string photovoltaic power generation system under different light intensities are as follows: Figure 2 The self-synchronizing voltage source control block diagram of a string photovoltaic power generation system is as follows: Figure 3 ;

[0076] The aforementioned dual-mode parallel autonomous switching strategy for photovoltaic power generation systems refers to the simultaneous operation of the MPPT control loop and constant voltage control loop of the generator-side converter, and the simultaneous operation of the inertial synchronization control loop and virtual synchronization control loop of the grid-side converter, regardless of whether the MPPT operation range, constant power operation range, or low-voltage grid operation range is in use. Autonomous smooth dynamic switching is achieved through a dual-mode parallel competitive autonomous strategy in different operating ranges.

[0077] The self-synchronizing voltage source control strategy for the MPPT operating range can be designed as a multi-channel Boost circuit on the machine side operating at its respective MPPT point, with switch S... M In M1 mode, the maximum power output under the current operating conditions is achieved. The grid-side inverter operates under inertial synchronous control, and the grid-side synchronous loop switching switch S... ω In M1 mode, disturbances to the DC bus voltage are introduced into the grid-connection frequency of the grid-side converter. This maintains the stability of the DC bus voltage while simultaneously synchronizing the inverter with the grid. The phase angle of the inverter output modulation voltage is:

[0078]

[0079] In the formula: ω0 represents the reference value of the power grid frequency, k pdcV represents the inertial synchronization proportional coefficient. dc This represents the actual value of the DC bus voltage, V. dcrefG This represents the given value of the grid-side inertial synchronous DC bus voltage; s represents the Laplace expression method for differential calculation.

[0080] Since inertial synchronization lacks damping, stabilizing damping can be added during the generation of the modulated wave voltage amplitude. The expression for this is:

[0081]

[0082] In the formula: V t0 Q represents the given value of the peak potential within the inverter; gref Q represents the given value of reactive power in the power grid. g This represents the actual value of the current reactive power of the power grid; k pt and k it D represents the proportional and integral coefficients of the reactive power control loop, respectively; pss V represents the voltage damping stabilization compensation amount; t Indicates the magnitude of the modulated wave voltage; voltage stabilization switch S D In M1 mode.

[0083] D pss The disturbance ΔV of the DC bus voltage can be used as a measure. dc get:

[0084]

[0085] In the formula: k pss T represents the damping stability proportional coefficient; c This represents the time constant of the high-pass filter.

[0086] When operating within the MPPT (Maximum Active Power Response Time) range, the photovoltaic (PV) power generation system, having already reached its maximum active power output, cannot provide further active power to the grid. Therefore, when the grid frequency fluctuates, the system can only reduce active power output as the grid frequency increases, causing the operating point of the generator-side converter's PV curve to shift to the right from the MPPT operating point. The active power reference P... ref The instruction is:

[0087] P ref =V PV ·I PV +ΔP t (18)

[0088] In the formula, V PV Represented as the photovoltaic panel port voltage, I PV ΔP represents the output current of the photovoltaic panel. tThis represents the active power command for the photovoltaic power generation system participating in the grid frequency response offset. Under inertial synchronous control, the DC bus voltage maps to the grid frequency, ΔP. t It can be written as:

[0089] ΔP t =K c ·s·ΔV dc +K t ·ΔV dc (19)

[0090] Where: K c K represents the inertial response coefficient. t This represents the primary frequency modulation coefficient. Since ΔP t It cannot be greater than 0, nor can it be too small to reduce the accuracy of the MPPT algorithm; therefore, it is set to -0.2P. GN <ΔP t <0, P GN This refers to the rated power of the grid-side inverter.

[0091] Useful reference P ref The instruction obtains the reference value V of the photovoltaic panel port voltage using the MPPT algorithm. PVref The error between the voltage at the photovoltaic panel port and the voltage at the photovoltaic panel port is modulated by a PI controller to obtain the modulation signal PI. Vpv for:

[0092]

[0093] In the formula: k ppv k represents the proportional coefficient of the PI controller in MPPT control. ipv This represents the integral coefficient of the PI controller in MPPT control.

[0094] The self-synchronizing voltage source control strategy for the constant power operation range and the low-voltage grid operation range can be designed as a multi-path Boost circuit on the generator side controlling the DC bus voltage, with each path controlling the same DC bus voltage setpoint, and switch S... M In M1 mode, the grid-side inverter employs virtual synchronous control. The error signal between the power reference command and the actual value of the grid output active power is introduced into the grid-connected frequency of the grid-side converter through a virtual synchronous transfer function. The phase angle of its output modulation voltage is:

[0095]

[0096] In the formula: ω p Indicates the virtual synchronous output grid angular frequency error value; P gref Indicates the reference command for active power of the power grid; P g J represents the actual active power output of the grid-side converter; P D represents the virtual inertia coefficient of the virtual synchronization link.P This represents the virtual damping coefficient of the virtual synchronization link.

[0097] When using virtual synchronous control, damped stabilization already exists in the system. The output of the stabilization element is then:

[0098] D pss =0 (22)

[0099] Grid-side synchronous switching switch S ω In M2 mode, voltage stabilization switch S D In M2 mode, the active power output of the photovoltaic inverter is adjusted while simultaneously achieving grid frequency synchronization. Under conditions of enhanced solar illumination and no grid faults, the grid-side converter needs to participate in the grid frequency response, and its virtual synchronization control active power reference command is as follows:

[0100] P gref =P GN +ΔP g (twenty three)

[0101] =P GN +(-K tc ·ω p -K gc ·s·ω p )

[0102] Where: K tc K represents the primary frequency regulation coefficient of the grid-side converter. gc This represents the inertia response coefficient of the grid-side converter. Since the photovoltaic power generation system operates in a constant power region, the photovoltaic array has an active power margin, thus allowing for increased output active power when the grid frequency decreases, achieving bidirectional frequency regulation. Currently, the inverter can output 1.1P of active power under unity power factor operation. GN It can be set to -0.2P. GN <ΔP g <0.1P GN .

[0103] Machine-side switching switch S M In M2 mode, the DC bus voltage on the machine side is set to the reference value V. dcrefM The actual value of DC bus voltage V dc The error is converted into a modulation signal PI by the PI controller. Vdc for:

[0104]

[0105] In the formula: k pvdc k represents the proportional coefficient of the PI controller in constant voltage control. ivdc This represents the integral coefficient of the PI controller in constant pressure control.

[0106] The principle of the dual-mode parallel autonomous switching strategy:

[0107] (1) During steady-state operation of the power grid, in the MPPT operating range, the generator-side switch S M Operating in M1 mode, the network-side synchronization loop switching switch S ω In M1 mode, with enhanced illumination and increased generator-side output power, the grid-side active power output lags due to the smaller DC capacitor, leading to an increase in DC bus voltage and affecting the generator-side converter PI. Vdc As the voltage decreases, the photovoltaic (PV) operating point jumps to a new PV curve, and its output V... PVref Slightly increase the MPPT control output modulation signal PI Vpv Increase, dual-mode parallel competitive autonomous strategy selection final machine-side output is PI Vdc Switch S M When switched to M2 mode, the grid-side converter outputs active power P. g Get bigger, P gref ω remains unchanged p The output of the network side is reduced to ω, representing the dual-mode parallel competitive autonomous strategy selection. p Switch S D When operating in M2 mode, the system will automatically switch to the constant power operating range.

[0108] (2) During the constant power operation range, the machine-side switch S M Operating in M2 mode, the grid-side synchronization loop switching switch S ω In M2 mode, as illumination decreases, the output power on the machine side decreases, the DC bus voltage gradually decreases, and PI... Vdc As the voltage increases, the operating point of the photovoltaic (PV) system shifts to the lower left, and its output V... PVref Slightly reduced, PI Vpv Reduce, dual-mode parallel competitive autonomous strategy selection machine-side output is PI Vpv Switch S M Switching to M1 mode. The grid-side converter output active power exceeds the grid-side input active power, causing the DC bus voltage to decrease, ω dc As the grid-side output active power decreases, it gradually decreases until it equals the grid-side input active power, ω. p Increase, dual-mode parallel competitive autonomous strategy selection network-side output is ω dc Switch S D When operating in M1 mode, the system will automatically switch to the MPPT operating range.

[0109] (3) During a low-voltage grid fault, the active power output of the grid-side converter is blocked, the DC bus voltage rises, the photovoltaic operating point shifts to the right on the original PV curve, and the output V PVref Increase, PI Vpv Increase, PI VdcReduce, dual-mode parallel competitive autonomous strategy selection machine-side output is PI Vdc Switch S M It will switch to M2 mode, network side ω dc Increase, P gref It will decrease as the grid voltage decreases, and is related to P. g Keep it consistent, ω p The dual-mode parallel competitive autonomous strategy selection network-side output remains unchanged at ω. p Switch S D It operates in M2 mode.

[0110] (4) When the fault is recovered, the output power of the grid-side converter is greater than the input power of the generator-side converter, the DC bus voltage decreases, and PI Vdc Increase, the photovoltaic returns to its original maximum power point, outputting V PVref Increase, PI Vpv Reduce, dual-mode parallel competitive autonomous strategy selection machine-side output is PI Vpv Switch S M It will automatically switch to M1 mode, network side ω dc Decrease, P gref With P g To maintain consistency, the dual-mode parallel competitive autonomous strategy selects the network-side output as ω. dc Switch S D Operating in M2 mode, it can autonomously switch over when a fault occurs and ends. The dual-mode parallel contention switching strategy is shown in Figure 4.

[0111] The inverter power reference command design during low-voltage grid faults is as follows: During low-voltage ride-through, the photovoltaic inverter needs to inject a dynamic reactive current into the power system that varies with the grid connection point voltage. The command for this dynamic reactive current is expressed as follows:

[0112] I Tq =K i (0.9-V orms )I N 0.2≤V orms ≤0.9 (25)

[0113] In the formula: I Tq The command K represents dynamic reactive current. i I represents the dynamic reactive current proportional gain of a photovoltaic inverter. N K represents the rated current of the photovoltaic inverter; under conditions of high reactive current output, K... i It can vary with the terminal voltage, allowing the inverter to output 3 times the reactive current even when the grid voltage drops by any proportion; V orms Indicates the effective value of the grid voltage at the grid connection point;

[0114] Due to inverter capacity limitations, the active current reference command is as follows:

[0115]

[0116] In the formula: I Td Indicates the variable of active current that the inverter needs to output; I lim This indicates the maximum current limit that the inverter can output;

[0117] Under current source control, because the phase-locked loop locks the inverter phase angle to the grid phase angle, its active and reactive power can be decoupled. Active power has a linear relationship with the d-axis current, and reactive power has a linear relationship with the q-axis current. However, under voltage source control, there is a phase difference between the inverter and the grid phase angle, so decoupling is not possible. In this case, the relationship between active and reactive power and the d-axis and q-axis currents is as follows:

[0118]

[0119] From equation (27), the active and reactive power reference commands during low-voltage ride-through can be obtained:

[0120]

[0121] The voltage support function is achieved by modifying the power command as described above. V od Indicates the D-axis component of the generated modulated wave voltage; I fd V represents the D-axis component of the generated output current; oq Indicates the Q-axis component of the generated modulated wave voltage; I fq This represents the Q-axis component of the generated output current.

[0122] This invention provides a self-synchronizing voltage source control strategy for a string photovoltaic power generation system. Without altering the topology of the string photovoltaic power generation system or the control mode of the generator-side converter, voltage source control is achieved by changing the control strategy of the grid-side converter. The strategy includes the following steps:

[0123] Step 1: Design the MPPT operating range control structure based on the string photovoltaic power generation system according to equations (1) to (6), including grid-side converter inertial synchronization control, design of inertial synchronization proportional coefficient, power loop PI controller parameters and stabilization link control parameters based on grid-side converter system parameters, machine-side converter control structure and frequency modulation command, etc.

[0124] Step 2: Design the constant power operation range control structure based on the string photovoltaic power generation system according to equations (7) to (10), including the virtual synchronization control of the grid-side converter, the design of the virtual synchronization inertia coefficient and damping coefficient based on the grid-side converter system parameters, the power loop PI controller parameters, the machine-side converter control structure, and the frequency modulation command, etc.

[0125] Step 3: Design a switching strategy for different operating ranges of the string photovoltaic power generation system based on the principle of dual-mode parallel competitive switching strategy;

[0126] Step 4: Design the power command of the grid-side inverter based on the string photovoltaic power generation system during low-voltage grid faults according to equations (11) to (14).

[0127] To demonstrate the proposed self-synchronizing voltage source control strategy based on a string photovoltaic inverter, PSCAD / EMTDC is used in conjunction with a specific simulation example to further illustrate and verify the above method. A 320kW / 800V string photovoltaic inverter grid-connected system is established. The main circuit parameters of the system are shown in Table 1. Based on the steps described, voltage source control strategies based on inertial synchronization control and virtual synchronization control of the string photovoltaic inverter are designed. A frequency modulation strategy based on the string photovoltaic inverter voltage source control strategy and a smooth switching control strategy between different voltage source controls of the string photovoltaic inverter are also designed. The simulation results, including the DC bus voltage waveform, output current waveform, and output power waveform, are attached. Figure 5 Appendix Figure 6 and attached Figure 7 As shown.

[0128] Table 1 System Main Circuit Parameters

[0129]

[0130]

[0131] Appendix Figure 5 Simulation results show that during the MPPT operating range, the grid-side converter employs an inertial synchronous control strategy. Its DC bus voltage changes map to the grid frequency, adjusting the inverter's active power output accordingly. From 6s to 7s, the grid frequency increases by 0.01 pu, which translates to a 0.1 pu increase in DC voltage, and the grid-side converter's angular frequency increases by 0.01 pu. During grid frequency changes, the grid-side output active power is affected by both inertial response and primary frequency regulation commands, resulting in a maximum reduction of more than 0.02 pu. After the grid frequency stabilizes at 1.01 pu, only the primary frequency regulation command maintains a reduction of 0.02 pu. The grid frequency begins to recover at 10s and reaches 1 pu at 11s, at which point the inverter returns to its original operating state. The simulation results demonstrate that during the MPPT operating range, inertial synchronous control can achieve autonomous response to the grid frequency in string photovoltaic systems.

[0132] Appendix Figure 6Simulation results show that during the constant power operation range, the grid-side converter operates under virtual synchronous control, and the DC bus voltage is stabilized at 1300V by the machine-side converter. When the grid frequency changes, the inverter reflects this through changes in active power. From 6s to 7s, the grid frequency increases by 0.01 pu, and the grid-side converter's angular frequency increases by 0.01 pu. During the grid frequency change, the grid-side output active power is affected by both inertia response and primary frequency regulation commands, resulting in a maximum reduction in active power greater than 0.12 pu. After the grid frequency stabilizes at 1.01 pu, only the primary frequency regulation command has an effect. The grid frequency begins to recover at 10s and reaches 1 pu at 11s, at which point the inverter returns to its original operating state. From 14s to 15s, the grid frequency decreases by 0.01 pu, and the grid-side converter's angular frequency decreases by 0.01 pu. During grid frequency changes, the grid-side output active power is affected by both inertial response and primary frequency regulation commands, with a maximum increase in active power greater than 0.12 pu. After the grid frequency stabilizes at 1.01 pu, only the primary frequency regulation command takes effect. The grid frequency begins to recover at 18 seconds and reaches 1 pu at 19 seconds, at which point the inverter returns to its original operating state. Simulation results show that string photovoltaic systems operating in the constant power range can achieve bidirectional autonomous regulation of the grid frequency using virtual synchronization.

[0133] Appendix Figure 7 Simulation results show that (a) fault signal control switching and (b) loop contention control switching have similar effects when the grid experiences low voltage. However, after the low-voltage fault is recovered and both methods exit the fault state, the fault signal control switching method results in overshoot and fluctuations in the DC bus voltage, with a maximum overshoot of 2200V, far exceeding the hardware protection capability and causing the inverter to shut down. In contrast, the loop contention control switching method results in a smooth DC bus voltage transition without overshoot or fluctuations, ensuring stable inverter operation.

[0134] Those skilled in the art will understand that, in addition to implementing the system, apparatus, and their modules provided by this invention in purely computer-readable program code, the same program can be implemented in the form of logic gates, switches, application-specific integrated circuits, programmable logic controllers, and embedded microcontrollers by logically programming the method steps. Therefore, the system, apparatus, and their modules provided by this invention can be considered a hardware component, and the modules included therein for implementing various programs can also be considered structures within the hardware component; alternatively, modules for implementing various functions can be considered both software programs implementing the method and structures within the hardware component.

[0135] Specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various changes or modifications within the scope of the claims, which do not affect the essence of the present invention. Unless otherwise specified, the embodiments and features described in this application can be arbitrarily combined with each other.

Claims

1. A dual-mode parallel competitive autonomous self-synchronized voltage source string type photovoltaic power generation system, characterized in that, include: Photovoltaic arrays, generator-side converters, and grid-side converters; The machine-side converter includes a multi-channel parallel DC / DC converter, and the grid-side converter includes various DC / AC converters; The photovoltaic array is connected to multiple DC / DC converters, which are connected to the same DC bus. This DC bus is connected to the grid via a DC / AC converter. During the MPPT operating range, the machine-side MPPT control and grid-side inertial synchronization control are adopted; during the constant power range, the machine-side constant voltage control and grid-side virtual synchronization control are adopted. The machine-side converter's MPPT control loop and constant voltage control loop operate simultaneously, while the grid-side converter's inertial synchronization control loop and virtual synchronization control loop operate simultaneously. In different operating ranges, autonomous smooth dynamic switching is achieved through a dual-mode parallel competition autonomous strategy.

2. The dual-mode parallel competitive autonomous self-synchronizing voltage source string photovoltaic power generation system according to claim 1, characterized in that, The self-synchronization voltage source control strategy for the MPPT operating range includes: The multiple Boost circuits in the machine-side converter operate at their respective MPPT points, and switch S M In M1 mode, the maximum power output under the current operating conditions is achieved; the grid-side inverter operates under inertial synchronous control, and the grid-side synchronous loop switching switch S... ω In M1 mode, disturbances to the DC bus voltage are introduced into the grid-connection frequency of the grid-side converter. This maintains the stability of the DC bus voltage while simultaneously synchronizing the inverter with the grid. The phase angle of the inverter output modulation voltage is: wherein: ω0represents a reference value of the grid frequency, k pdc represents an inertia synchronous proportional coefficient, V dc represents an actual value of the DC bus voltage, V dcrefG represents a given value of the grid-side inertia synchronous DC bus voltage; s represents a differential calculation Laplace representation Since inertial synchronization lacks damping, stabilizing damping is added during the generation of the modulated wave voltage amplitude. Its expression is as follows: In the formula: V t0 Q represents the given value of the peak potential within the inverter; gref Q represents the given value of reactive power in the power grid. g This represents the actual value of the current reactive power of the power grid; k pt and k it D represents the proportional and integral coefficients of the reactive power control loop, respectively; pss V represents the voltage damping stabilization compensation amount; t Indicates the magnitude of the modulated wave voltage; voltage stabilization switch S D In M1 mode; D pss By the amount of disturbance ΔV of the DC bus voltage dc It is obtained: In the formula: k pss T represents the damping stability proportional coefficient; c This represents the time constant of the high-pass filter.

3. The dual-mode parallel competitive autonomous self-synchronized voltage source string-type photovoltaic power generation system according to claim 2, characterized in that, When operating within the MPPT range, the photovoltaic power generation system reduces active power output as grid frequency increases, causing the operating point of the generator-side converter's PV curve to shift to the right from the MPPT operating point. This affects the active power reference P... ref The instruction is: P ref = V PV · I PV + ΔP t (32) where V PV represents the photovoltaic panel port voltage, I PV represents the photovoltaic panel output current, ΔP t represents the photovoltaic generation system active power instruction for participating in the grid frequency response deviation; Under the inertia synchronous control, the DC bus voltage maps the grid frequency, ΔP t is written as: ΔP t = K c · s · ΔV dc + K t · ΔV dc (33) In the formula: K c K represents the inertial response coefficient. t This represents the primary frequency modulation factor; -0.2P GN <ΔP t <0, P GN This refers to the rated power of the grid-side inverter. Useful reference P ref The instruction obtains the reference value V of the photovoltaic panel port voltage using the MPPT algorithm. PVref The error between the voltage at the photovoltaic panel port and the voltage at the photovoltaic panel port is modulated by a PI controller to obtain the modulation signal PI. Vpv for: In the formula: k ppv k represents the proportional coefficient of the PI controller in MPPT control. ipv This represents the integral coefficient of the PI controller in MPPT control.

4. The dual-mode parallel competitive autonomous self-synchronized voltage source string-type photovoltaic power generation system according to claim 3, characterized in that, The self-synchronizing voltage source control strategies for constant power operation range and low-voltage grid operation range include: The multi-channel Boost circuit in the machine-side converter controls the DC bus voltage. Each channel controls the same DC bus voltage setpoint. Switch S... M In M1 mode; The grid-side inverter employs virtual synchronous control, introducing the error signal between the power reference command and the actual value of the grid output active power into the grid-connected frequency of the grid-side converter through a virtual synchronous transfer function. The phase angle of its output modulation voltage is: In the formula: ω p Indicates the virtual synchronous output grid angular frequency error value; P gref Indicates the power grid active power reference command; P g J represents the actual active power output of the grid-side converter; P D represents the virtual inertia coefficient of the virtual synchronization link; P This represents the virtual damping coefficient of the virtual synchronization link.

5. The dual-mode parallel competitive autonomous self-synchronized voltage source string-type photovoltaic power generation system according to claim 4, characterized in that, When using virtual synchronous control, damped stabilization already exists in the photovoltaic power generation system. The output of the stabilization stage is then: D pss ＝0 (36) Grid-side synchronous switching switch S ω In M2 mode, voltage stabilization switch S D In M2 mode, the active power output of the photovoltaic inverter is adjusted while grid frequency synchronization is achieved. Under conditions of enhanced solar illumination and no grid faults, the grid-side converter participates in the grid frequency response. Its virtual synchronization control active power reference command is: P gref = P GN + ΔP g (37) = P GN + (-K tc · ω p -K gc · s · ω p ) Where: K tc K represents the primary frequency regulation coefficient of the grid-side converter. gc This represents the inertia response coefficient of the grid-side converter; -0.2P GN <ΔP g <0.1P GN ; Machine-side switching switch S M In M2 mode, the reference value V of the DC bus voltage on the machine side is given. dcrefM The actual value of DC bus voltage V dc The error is converted into a modulation signal PI by the PI controller. Vdc for: wherein: k pvdc represents the proportional coefficient of the PI controller in the constant voltage control, k ivdc represents the integral coefficient of the PI controller in the constant voltage control.

6. The dual-mode parallel competitive autonomous self-synchronized voltage source string-type photovoltaic power generation system according to claim 5, characterized in that, During steady-state operation of the power grid, in the MPPT operating range, the generator-side switch S M Operating in M1 mode, the network-side synchronization loop switching switch S ω In M1 mode, with enhanced illumination and increased generator-side output power, the grid-side active power output lags due to the smaller DC capacitor, leading to an increase in DC bus voltage and affecting the generator-side converter PI. Vdc As the voltage decreases, the photovoltaic (PV) operating point jumps to a new PV curve, and its output V... PVref Slightly increase the MPPT control output modulation signal PI Vpv Increase, dual-mode parallel competitive autonomous strategy selection final machine-side output is PI Vdc Switch S M When switched to M2 mode, the grid-side converter outputs active power P. g Get bigger, P gref ω remains unchanged p The output of the network side is reduced to ω, representing the dual-mode parallel competitive autonomous strategy selection. p Switch S D When operating in M2 mode, the system automatically switches to the constant power operating range.

7. The dual-mode parallel competitive autonomous self-synchronizing voltage source string photovoltaic power generation system according to claim 6, characterized in that, During constant power operation, the machine-side switch S M Operating in M2 mode, the grid-side synchronization loop switching switch S ω In M2 mode, as illumination decreases, the output power on the machine side decreases, the DC bus voltage gradually decreases, and PI... Vdc As the voltage increases, the operating point of the photovoltaic (PV) system shifts to the lower left, and its output V... PVref Slightly reduced, PI Vpv Reduce, dual-mode parallel competitive autonomous strategy selection machine-side output is PI Vpv Switch S M Switching to M1 mode; the active power output of the grid-side converter is greater than the active power input of the grid side, the DC bus voltage decreases, ω dc As the grid-side output active power decreases, it gradually decreases until it equals the grid-side input active power, ω. p Increase, dual-mode parallel competitive autonomous strategy selection network-side output is ω dc Switch S D When operating in M1 mode, the system automatically switches to the MPPT operating range.

8. The dual-mode parallel competitive autonomous self-synchronizing voltage source string photovoltaic power generation system according to claim 7, characterized in that, During a low-voltage grid fault, the active power output of the grid-side converter is blocked, the DC bus voltage rises, and the photovoltaic operating point shifts to the right on the original PV curve, resulting in a decrease in output V. PVref Increase, PI Vpv Increase, PI Vdc Reduce, dual-mode parallel competitive autonomous strategy selection machine-side output is PI Vdc Switch S M It will switch to M2 mode, network side ω dc Increase, P gref It will decrease as the grid voltage decreases, and is related to P. g Keep it consistent, ω p The dual-mode parallel competitive autonomous strategy selection network-side output remains unchanged at ω. p Switch S D It operates in M2 mode.

9. The dual-mode parallel competitive autonomous self-synchronizing voltage source string photovoltaic power generation system according to claim 8, characterized in that, When the fault is recovered, the output power of the grid-side converter is greater than the input power of the generator-side converter, the DC bus voltage decreases, and PI... Vdc Increase, the photovoltaic returns to its original maximum power point, outputting V PVref Increase, PI Vpv Reduce, dual-mode parallel competitive autonomous strategy selection machine-side output is PI Vpv Switch S M It will automatically switch to M1 mode, network side ω dc Decrease, P gref With P g To maintain consistency, the dual-mode parallel competitive autonomous strategy selects the network-side output as ω. dc Switch S D It operates in M2 mode and automatically switches when a fault occurs or ends.

10. The dual-mode parallel competitive autonomous self-synchronizing voltage source string photovoltaic power generation system according to claim 9, characterized in that, The inverter power reference command design during low-voltage grid faults includes: During low-voltage ride-through, the photovoltaic inverter injects a dynamic reactive current into the power system that varies with the grid connection point voltage. The command for this dynamic reactive current is expressed as: I Tq = K i (0.9 - V orms )I N , 0.2≤ V orms ≤0.9 (39) In the formula: I Tq The command K represents dynamic reactive current. i I represents the dynamic reactive current proportional gain of a photovoltaic inverter. N K represents the rated current of the photovoltaic inverter; under conditions of high reactive current output, K... i The inverter's reactive current varies with the terminal voltage, ensuring it outputs three times the normal current regardless of any percentage drop in grid voltage. orms Indicates the effective value of the grid voltage at the grid connection point; Due to inverter capacity limitations, the active current reference command is as follows: In the formula: I Td Indicates the variable of active current that the inverter needs to output; I lim This indicates the maximum current limit that the inverter can output; Under current source control, because the phase-locked loop locks the inverter phase angle with the grid phase angle, its active and reactive power are decoupled. Active power has a linear relationship with the d-axis current, and reactive power has a linear relationship with the q-axis current. Under voltage source control, there is a phase difference between the inverter and the grid, making decoupling impossible. In this case, the relationships between active and reactive power and the d-axis and q-axis currents are as follows: From equation (27), the active and reactive power reference commands during low-voltage ride-through are obtained: The voltage support function is achieved by modifying the power command as described above. Among them, V od Indicates the D-axis component of the generated modulated wave voltage; I fd V represents the D-axis component of the generated output current; oq Indicates the Q-axis component of the generated modulated wave voltage; I fq This represents the Q-axis component of the generated output current.

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