A photovoltaic grid-connected inverter smooth switching control method, system and inverter

Abstract

The application provides a photovoltaic grid-connected inverter smooth switching control method, relates to the technical field of power systems, and comprises a phase angle smooth switching step, a current reference value smooth switching step and a power correction smooth switching step. The smooth switching of the grid-following control and the grid-forming control is realized under the premise that the external characteristics and the external control performance of the grid-connected inverter remain unchanged, the continuity of frequency support in the whole process can be maintained through correction of the reference value of active power, the mutation of frequency and power in the switching process is further weakened, and the ability of photovoltaic participation in system frequency modulation is improved.

Description

Technical Field

[0001] This invention relates to the field of power system technology, specifically to a smooth switching control method, system, and inverter for a photovoltaic grid-connected inverter. Background Technology

[0002] The statements in this section are merely background information related to the present invention and do not necessarily constitute prior art.

[0003] Photovoltaic power generation, as one of the most rapidly growing renewable energy sources in recent years, has many advantages in environmental protection and greenhouse gas emission reduction. However, it also has numerous drawbacks. Unlike traditional power sources, photovoltaic power generation is inevitably affected by natural conditions due to its working principle, resulting in intermittent and random power output. This leads to increased uncertainty in the power system as photovoltaic penetration increases. Furthermore, distributed photovoltaic systems are often connected to low-voltage distribution terminals. Due to the limited number of regulation devices in distribution systems, the grid's ability to withstand disturbances is weakened. Grids with a high proportion of integrated photovoltaic power generation will experience frequent and widespread system intensity fluctuations, seriously threatening the safe operation of the system.

[0004] Currently, most photovoltaic (PV) systems are connected to the grid via grid-connected inverters. However, this can lead to a failure to actively respond to changes in system frequency. Furthermore, when the AC system strength weakens, the phase-locked loop (PLL) can degrade the system's dynamic characteristics, resulting in decreased stability margins or even instability. To address this, some researchers have proposed grid-connected control, enabling grid-connected inverters to operate stably under weak AC system conditions. This control also allows for power reserve adjustments to PV output during system faults to regulate system frequency and provide frequency support. However, grid-connected inverters based on grid-connected control may experience a decrease in damping ratio under high AC system strength, leading to reduced operational stability. Therefore, for grids with significant strength fluctuations, a single control strategy cannot meet the stable operation requirements under different strengths. Adjustments and switching of the control mode for PV grid-connected inverters are necessary to improve their stability.

[0005] The inventors have found that although there is research on dual-mode smooth switching control strategies, the smooth switching technology between grid-type and network-structured control is still in its infancy. Considering that most fluctuations in system strength are caused by grid accidents, how to improve the system reliability service level throughout the entire accident process while switching control strategies to adapt to grid operating conditions still needs further research. Summary of the Invention

[0006] To address the aforementioned issues, this invention proposes a smooth switching control method, system, and inverter for photovoltaic grid-connected inverters. This method enables smooth switching between grid-following control and grid-connected control while maintaining the external characteristics and external control performance of the grid-connected inverter. Furthermore, by correcting the reference value of active power, it maintains the continuity of frequency support throughout the entire process, further reducing abrupt changes in frequency and power during the switching process and improving the photovoltaic system's ability to participate in frequency regulation.

[0007] To achieve the above objectives, in a first aspect, the present invention proposes a smooth switching control method for a photovoltaic grid-connected inverter, employing the following technical solution:

[0008] A method for smooth switching control of a photovoltaic grid-connected inverter includes:

[0009] Phase angle smooth switching steps: When the grid-connected inverter switches from current vector control mode to power synchronization control mode, the angular frequency output of the phase-locked loop is first switched to the angular frequency output of the power synchronization loop, and then integrated by the integrator.

[0010] Smooth switching steps for current reference value: When the grid-connected inverter switches its operating mode from current vector control mode to power synchronization control mode, the current reference value of power synchronization control tracks the current reference value of current vector control, and the current reference value remains unchanged before and after the mode switch; and

[0011] Power correction smooth switching steps: When the grid-connected inverter switches operating modes, the active power-frequency characteristic curve is shifted to correct the active power reference value so that the operating point remains unchanged before and after the mode switch.

[0012] In a further technical solution, the phase angle smoothing switching step also includes:

[0013] When the grid-connected inverter switches from power synchronization control mode to current vector control mode, the phase-locked loop tracks the grid phase, ensuring that the phase-locked loop and the power synchronization loop operate in a synchronized state, and the phase angle remains unchanged before and after the mode switch.

[0014] In a further technical solution, the smooth switching step of the current reference value also includes:

[0015] When the grid-connected inverter switches from power synchronous control mode to current vector control mode, the current reference value of current vector control tracks the current reference value of power synchronous control, and the current reference value remains unchanged before and after the mode switch.

[0016] In a further technical solution, the current reference value includes a d-axis current reference value and a q-axis current reference value.

[0017] Further technical solutions also include:

[0018] Power correction smooth switching steps: When the grid-connected inverter switches operating modes, the active power-frequency characteristic curve is shifted to correct the active power reference value so that the operating point remains unchanged before and after the mode switch.

[0019] A further technical solution involves shifting the active power-frequency characteristic curve and correcting the active power reference value when the grid-connected inverter switches operating modes, including:

[0020] Obtain the active power-frequency response curves for power synchronization control and current vector control;

[0021] If the slope of the active power-frequency characteristic curve of power synchronous control is greater than the slope of the active power-frequency characteristic curve of current vector control,

[0022] When the load increases and the grid-connected inverter switches from current vector control mode to power synchronous control mode, the active power-frequency characteristic curve of power synchronous control is shifted downward.

[0023] or,

[0024] When the load increases and the grid-connected inverter switches from power synchronous control mode to current vector control mode, the active power-frequency characteristic curve of current vector control is shifted upward.

[0025] or,

[0026] When the load decreases and the grid-connected inverter switches from current vector control mode to power synchronous control mode, the active power-frequency characteristic curve of power synchronous control is shifted upward.

[0027] or,

[0028] When the load decreases and the grid-connected inverter switches from power synchronous control mode to current vector control mode, the active power-frequency characteristic curve of current vector control is shifted downward.

[0029] A further technical solution is that if the slope of the active power-frequency characteristic curve of power synchronization control is greater than the slope of the active power-frequency characteristic curve of current vector control, the corrected active power reference values ​​for both current vector control and power synchronization control are:

[0030]

[0031] In the formula, k cvc and k pscThese are the slopes of the current vector control and power synchronization control characteristics, respectively; f0 is the steady-state system frequency; f1 is the system frequency after a sudden load increase when operating under current vector control; P1 is the active power after the system frequency drops to f1; f2 is the system frequency after a sudden load increase when operating under power synchronization control; P2 is the active power after the system frequency drops to f2; P cvc and P psc These are the corrected active power reference values ​​for current vector control and power synchronization control, respectively.

[0032] A further technical solution involves shifting the active power-frequency characteristic curve and correcting the active power reference value when the grid-connected inverter switches operating modes, including:

[0033] Obtain the active power-frequency response curves for power synchronization control and current vector control;

[0034] If the slope of the active power-frequency characteristic curve of power synchronous control is less than that of the active power-frequency characteristic curve of current vector control, when the load increases and the grid-connected inverter switches from current vector control mode to power synchronous control mode, the active power-frequency characteristic curve of power synchronous control will shift downwards and upwards.

[0035] or,

[0036] When the load increases and the grid-connected inverter switches from power synchronous control mode to current vector control mode, the active power-frequency characteristic curve of current vector control is shifted downward.

[0037] or,

[0038] When the load decreases and the grid-connected inverter switches from current vector control mode to power synchronous control mode, the active power-frequency characteristic curve of power synchronous control is shifted downward.

[0039] or,

[0040] When the load decreases and the grid-connected inverter switches from power synchronous control mode to current vector control mode, the active power-frequency characteristic curve of current vector control is shifted upward.

[0041] A further technical solution is that if the slope of the active power-frequency characteristic curve of power synchronization control is less than the slope of the active power-frequency characteristic curve of current vector control, the corrected active power reference values ​​for both current vector control and power synchronization control are:

[0042]

[0043] In the formula, kcvc and k psc These are the slopes of the current vector control and power synchronization control characteristics, respectively; f0 is the steady-state system frequency; f3 is the system frequency after a sudden load increase when operating under current vector control; P3 is the active power after the system frequency drops to f3; f4 is the system frequency after a sudden load increase when operating under power synchronization control; P4 is the active power after the system frequency drops to f4; P cvc and P psc These are the corrected active power reference values ​​for current vector control and power synchronization control, respectively.

[0044] A second aspect of the present invention provides a smooth switching control system for a photovoltaic grid-connected inverter, comprising:

[0045] The phase angle smoothing switching module is configured to switch the output angular frequency of the phase-locked loop to the output angular frequency of the power synchronization loop when the grid-connected inverter switches from the current vector control mode to the power synchronization control mode, and then integrates it through the integrator.

[0046] Current reference value smooth switching module: When the grid-connected inverter is configured to switch from current vector control mode to power synchronization control mode, the current reference value of power synchronization control tracks the current reference value of current vector control, and the current reference value remains unchanged before and after the mode switch.

[0047] The power correction smoothing switching module is configured to shift the active power-frequency characteristic curve and correct the active power reference value when the grid-connected inverter switches operating modes, so that the operating point remains unchanged before and after the mode switch.

[0048] In a third aspect, the present invention provides a photovoltaic grid-connected inverter that employs a photovoltaic grid-connected inverter smooth switching control method as described above or includes a photovoltaic grid-connected inverter smooth switching control system as described above.

[0049] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0050] 1. This invention can achieve smooth switching between grid-connected control and grid-connected control while maintaining the external characteristics and external control performance of the grid-connected inverter. Furthermore, by correcting the reference value of active power, it can maintain the continuity of frequency support throughout the process, further reducing the sudden changes in frequency and power during the switching process and improving the ability of photovoltaics to participate in system frequency regulation.

[0051] 2. When the system strength fluctuates significantly, the method proposed in this invention can be used to adjust the control mode of the photovoltaic grid-connected inverter, which can greatly improve the stability and reliability of photovoltaic grid connection. Moreover, this method has high versatility and can be used not only for photovoltaic grid-connected inverters, but also for VSC (voltage source converter) control mode switching in other scenarios.

[0052] 3. This invention can not only further improve switching performance, but also ensure the continuity of frequency support effect throughout the switching process, and has a good effect on improving the reliability of photovoltaics in providing frequency regulation services to the system when an accident occurs.

[0053] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, preferred embodiments are described below in detail with reference to the accompanying drawings. Attached Figure Description

[0054] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0055] Figure 1 This is a schematic diagram of a photovoltaic grid-connected structure based on current vector control.

[0056] Figure 2 This is a schematic diagram of a photovoltaic grid-connected structure based on power synchronization control.

[0057] Figure 3 A block diagram of the phase angle smooth switching structure;

[0058] Figure 4 Block diagram of the structure for smooth switching of current reference value;

[0059] Figure 5 The active power-frequency characteristics under different control modes during the switching process (the slope of the power synchronization control characteristic is greater than that of the current vector control);

[0060] Figure 6 The active power-frequency characteristics under different control modes during the switching process (the slope of the current vector control characteristic is greater than that of the power synchronization control);

[0061] Figure 7 This is the control block diagram for the power correction section when the slope of the power synchronization control characteristic is greater than that of the current vector control.

[0062] Figure 8 This is a simulation model diagram of a photovoltaic power generation system;

[0063] Figure 9 This is a comparison chart showing the effects of direct switching and smooth switching.

[0064] Figure 10 A comparison chart showing the effects of smooth switching with and without power correction. Detailed implementation method:

[0065] The present invention will be further described below with reference to the accompanying drawings and embodiments.

[0066] Definitions: Voltage Source Converter (VSC), Short Circuit Ratio (SCR), Phase-Locked Loop (PLL), Power Synchronization Loop (PSL), Current Vector Control (CVC), Power Synchronization Control (PSC).

[0067] This invention aims to solve the problem of stable operation of photovoltaic grid-connected inverters under conditions of large-scale continuous fluctuations in AC system intensity, while ensuring continuous frequency support from photovoltaics throughout the entire process. Since system intensity fluctuations are mainly caused by events in the power grid, control mode switching is required to adapt to changing operating conditions. However, the switching process will cause changes in the frequency regulation output of the photovoltaic system, potentially leading to a secondary decrease or increase in system frequency. Therefore, while reasonably adjusting and switching the grid-connected inverter control method to adapt to changes in power grid operating conditions, it is even more important to ensure the continuity of frequency support during the switching process. To address this issue, this invention proposes a smooth switching control method, system, and inverter for photovoltaic grid-connected inverters, maintaining continuous frequency support during the switching process between grid-following control and grid-connected control, thereby improving the photovoltaic system's ability to provide reliable services to the power grid.

[0068] Currently, the general control methods for photovoltaic grid-connected inverters can be divided into grid-following control and grid-connected control. Among them, the typical control method for grid-following control is current vector control, which is one of the most widely used basic control methods. Meanwhile, power synchronization control, as one of the representative control methods for grid-connected control, has received increasing attention due to its simple structure and strong robustness.

[0069] Current vector control:

[0070] The control block diagram of current vector control based on phase-locked loop is as follows: Figure 1 As shown in the figure. R s and L s For the system impedance, v a ref v b ref and vc ref The reference value for the three-phase voltage of a, b, and c is v. s abc It is the voltage at the grid connection point, i c abc This is the output current of the grid-connected inverter. Current vector control can be mainly divided into three parts: in the outer loop, P... ref P and Q represent the reference and actual values ​​of the active power transmitted by the grid-connected converter, respectively. ref Q and PI_cvc_outer represent the reference and actual values ​​of reactive power transmitted by the grid-connected converter, respectively. PI_cvc_outer represents the PI element of the outer loop control. d ref and i q ref These are the reference values ​​for the d-axis and q-axis currents, respectively; in the inner loop, v d v q and i d i q These are the d-axis and q-axis components of the AC voltage and AC current, respectively. PI_cvc_inner represents the PI element of the inner loop control; in the phase-locked loop, v q ref Here, q-axis voltage reference value is given, PI_pll is the PI element of the grid phase-locked loop, Δω is the angular frequency deviation, ω0 and f0 are the rated angular frequency and rated frequency, respectively, and θ is the angular frequency. pll This is the phase angle output by the phase-locked loop.

[0071] This invention employs a general two-stage photovoltaic grid-connected system, operating the photovoltaic system in a load-reducing mode to reserve a certain amount of power. In this mode, the grid-connected inverter controls the output active power of the photovoltaic system. The outer loop active power control uses constant active power control, while the outer loop reactive power control uses constant reactive power control. The outer loop control can achieve independent control based on the d- and q-axis decoupling characteristics of active and reactive power. Its output d- and q-axis current reference values ​​serve as the input to the inner loop, and the inner loop's function is to ensure that i... d and i q It quickly tracks its reference value. Furthermore, the phase-locked loop (PLL) is used to track the voltage phase of the power grid and provide the phase angle for coordinate transformations.

[0072] Although current vector control can ensure higher output efficiency and power quality under strong grid conditions, there is a coupling relationship between the phase-locked loop and the grid impedance. This coupling relationship will be aggravated when the grid strength is very weak. In addition, under weak grid conditions, the phase-locked loop will deteriorate the dynamic characteristics of the system and cause a decrease in system stability.

[0073] Power synchronization control:

[0074] As a representative control method of network-based control, power synchronization control possesses external characteristics similar to those of a synchronous generator and can provide inertia support for the system. The core of power synchronization control is to design an active power controller based on the swing equation of a synchronous generator, using its rotor angle as the output of the power synchronization loop. The equation of motion for a synchronous generator is:

[0075]

[0076] In the formula, T J Let P be the generator inertial time constant, D be the damping coefficient, Δω be the per-unit value of the speed deviation, and P be the generator inertial time constant. m and P e These are the per-unit values ​​for mechanical power and electromagnetic power, respectively; θ is the electrical angle of the generator rotor; and ω is the per-unit value for the actual angular frequency. N This is the angular frequency reference value. The power reference value P is used respectively. ref Replace P in the formula with the actual value P. m and P e You can get Figure 2 The control block diagram shown is as follows. Figure 2 As shown, power synchronization control consists of three main parts: a power synchronization loop, an inner loop, and an outer loop. In the outer loop, Q... ref Q and V represent the reference and actual values ​​of reactive power transmitted by the grid-connected converter, respectively. d ref and v q ref These are the reference values ​​for the d-axis and q-axis voltages, respectively. sm0 To output the no-load voltage when the reactive power is 0, PI_psc_outer1 represents the PI element of the outer loop reactive power control, and PI_psc_outer2 represents the outer loop voltage. d The PI control loop, PI_psc_outer3 represents the outer loop v q The PI control element; in the inner loop, PI_cvc_inner represents the PI control element in the inner loop; in the power synchronization loop, θ psl This refers to the phase angle output by the power synchronization loop. The inner loop of the power synchronization control has the same structure as the current vector control. The power synchronization loop generates a phase angle θ while controlling active power by simulating the characteristics of a synchronous generator, thus achieving two functions: active power control and the function of a phase-locked loop. The outer loop only needs to complete reactive power control, i.e., ii d ref It is obtained by tracking a reference value of reactive power or AC voltage. When a constant reactive power is used, v d ref It consists of two parts, one part being the open-circuit voltage U sm0 The other part is adjusting the voltage offset caused by reactive power, while i q refBy tracking the q-axis voltage reference value v q ref To obtain.

[0077] Power synchronization control does not require a phase-locked loop (PLL) to synchronize with the system, thus effectively avoiding the instability issues caused by PLLs in weak grid conditions. However, under strong grid conditions, this control mode reduces the system damping ratio, and grid-connected inverters based on power synchronization control may experience slow regulation speed and reduced AC fault ride-through capability when the AC system strength is high. Therefore, the applicable scope of the two control modes differs significantly. Reasonably adjusting and switching their control modes to improve the adaptability and stability of photovoltaic grid-connected inverters under different operating conditions is of great importance.

[0078] Under normal operating conditions, the grid strength of a system is generally stable, requiring no adjustment to the control method. However, if a grid fault occurs, it may cause the trip switch to operate, leading to changes in the grid structure and significant fluctuations in grid strength. Therefore, based on the applicability of current vector control and power synchronization control, it is necessary to consider switching the control of the photovoltaic grid-connected inverter when the AC system strength changes, in order to improve the versatility of the grid-connected inverter. However, due to the differences in the principles of the two control methods, direct switching may lead to switching failure. Therefore, it is necessary to analyze the key factors that cause disturbances or even switching failures during the switching process, and on this basis, to achieve smooth switching control, which can switch the internal control mode while ensuring that the external characteristics of the grid-connected inverter remain unchanged.

[0079] First, one of the most obvious differences between current vector control and power synchronization control lies in the phase-locked loop (PLL) and the power synchronization loop. For a given system, the two operate in parallel. If the grid-connected inverter uses current vector control at a certain moment, the control variables of the power synchronization loop will not form a closed loop, and due to the integral action of the integrator, the phase angle θ of the power synchronization loop output will be affected. psl There will be a deviation between the output phase angle of the current grid-connected inverter and the output phase angle of the current grid-connected inverter, and vice versa. Therefore, if the switching between the phase-locked loop and the power synchronization loop is performed at a certain moment, the phase angle will inevitably change abruptly, resulting in a large fluctuation in power.

[0080] Secondly, there are certain differences in the outer loops of the two control methods. Since their inner loop structures are identical, direct switching between the two control methods can be achieved by switching the dq-axis current reference values. However, this presents a problem: regardless of which control mode the grid-connected inverter operates in, the other control mode will not form a closed loop. Therefore, the dq-axis current reference value output in the non-operational control mode will not correspond to the current operating state, which will inevitably cause problems at the moment of switching. d ref and i q refSudden changes can lead to large fluctuations in power or even switching failures.

[0081] Therefore, based on the above differences, it is necessary to suppress the sudden changes in phase angle and d-axis and q-axis current reference values ​​during the switching process and the disturbances they cause.

[0082] First, in response to the abrupt change in phase angle, this invention proposes the following... Figure 3 The control method shown places the switching switch before the integrator, utilizing the continuity and non-abrupt nature of the integrator output to achieve smooth switching of the system phase angle θ. Specifically, a first switching switch and a second switching switch are set between the current vector control phase-locked loop and the power synchronization control power synchronization loop. The first switching switch is set at the output angular frequency ω of the phase-locked loop. pll and the output angular frequency ω of the power synchronization loop psl Between the two switches, the S11 terminal of the first switch is connected to the output angular frequency of the phase-locked loop (PLL), and the S12 terminal of the first switch is connected to the output angular frequency of the power synchronization loop. Simultaneously, the PLL's output angular frequency is normally input to its integrator. The second switch is positioned before the dq conversion of the PLL; its S21 terminal is connected to the system output phase angle θ, and its S22 terminal is connected to the PLL's integrator output. The switching of the first and second switches is synchronized, meaning that either S11 and S21 terminals are simultaneously connected, or S12 and S22 terminals are simultaneously connected.

[0083] The specific control logic for smooth phase angle switching is as follows:

[0084] (1) If the grid-connected inverter is operating under current vector control, the first switching switch is at terminal S11, the second switching switch is at terminal S21, and the phase angle of the system is θ = θ pll At this point, under stable operating conditions, P = P ref Therefore ω psl =ω0. When switching control modes, the first switch changes from S11 to S12, and the second switch changes from S21 to S22. Due to the presence of the integrator after the first switch and its continuous output, θ = θ at the instant of switching. pll =θ psl .

[0085] (2) If the grid-connected inverter is operating under power synchronization control, the first switching switch is at terminal S12, the second switching switch is at terminal S22, and the phase angle of the system is θ = θ psl Since the phase-locked loop (PLL) can track the grid phase, that is, when the PLL achieves phase-locked synchronization, θ pll It equals v sa The phase (cosine form) is given by θ, where vsa is the AC voltage of phase a. At this time, the phase-locked loop and the power synchronization loop are still in synchronous operation, therefore θ pll =θpsl and ω pll =ω psl When the control mode is switched, the first switch is switched from S12 to S11, and the second switch is switched from S22 to S21. There will be no sudden change in θ during the switching.

[0086] Secondly, to address the sudden changes in the reference values ​​of the d-axis and q-axis currents during the switching process, this invention constructs as follows: Figure 4 The image shown is for smooth switching i d ref and i q ref The control structure is as follows. Specifically, for the control of the d-axis, three switching switches are set: the third switching switch is set before the PI link PI_cvc_outer of the active power control, its Sd_11 terminal is connected to the active power reference value under normal current vector control with droop function, and its Sd_12 terminal is connected to P ref_switch This represents the active power reference value of the outer loop of the current vector control in power synchronization control mode; the fourth switching switch is set before PI_psc_outer1 of the reactive power control PI loop, and its Sd_21 terminal is connected to Q. ref_switch_psc This represents the reactive power reference value of the outer loop of power synchronization control under current vector control mode. The Sd_22 terminal is connected to Q under normal power synchronization control. ref The fifth switch is set at the system output i d ref Previously, its Sd_31 and Sd_32 terminals were connected to the d-axis current reference value i in the current vector control mode, respectively. d ref_cvc d-axis current reference value i in power synchronous control mode d ref_psc To achieve i d ref Switching between the two control modes. Similarly, three switches are set for q-axis control: the sixth switch is set before the reactive power control PI link PI_cvc_outer, and its Sq_11 terminal is connected to Q under normal current vector control. ref The Sq_12 end connects to Q. ref_switch_cvc This represents the reactive power reference value of the outer loop of the current vector control in power synchronous control mode; the seventh switching switch is set before PI_psc_outer3 of the q-axis voltage control PI link, and its Sq_21 terminal is connected to v q ref_switch This represents the q-axis voltage reference value of the power synchronous control outer loop under current vector control mode. The Sq_22 terminal is connected to the V value under normal power synchronous control. q ref The eighth switch is located at the system output iq ref Previously, its Sq_31 and Sq_32 terminals were connected to the q-axis current reference value i in the current vector control mode, respectively. q ref_cvc q-axis current reference value i in power synchronous control mode q ref_psc To achieve i q ref Switching between two control modes. PI_switch1, PI_switch2, PI_switch3, and PI_switch4 are all PI loops for switching control. All switches are controlled synchronously, that is, the Sd_11 terminal of the third switch, the Sd_21 terminal of the fourth switch, the Sd_31 terminal of the fifth switch, the Sq_11 terminal of the sixth switch, the Sq_21 terminal of the seventh switch, and the Sq_31 terminal of the eighth switch (hereinafter referred to as position 1) are simultaneously connected, or the Sd_12 terminal of the third switch, the Sd_22 terminal of the fourth switch, the Sd_32 terminal of the fifth switch, the Sq_12 terminal of the sixth switch, the Sq_22 terminal of the seventh switch, and the Sq_32 terminal of the eighth switch (hereinafter referred to as position 2) are simultaneously connected.

[0087] The specific control logic for smooth switching of the current reference value is as follows:

[0088] (1) If the grid-connected inverter is operating under current vector control, all switches are in position 1, and i d ref =i d ref_cvc and i q ref =i q ref_cvc The outer loop is equipped with active-frequency droop control, and the output i is obtained through a PI circuit. d ref_cvc At the same time, the reactive power Q is made to track its reference value Q. ref to obtain i q ref_cvc The d-axis and q-axis of the power synchronization control will also operate independently, enabling i d ref_psc and i q ref_psc Track i separately d ref_cvc and i q ref_cvc This is to achieve synchronization of the dq-axis current reference values ​​in power synchronization control mode and current vector control mode. When switching control modes, the switch moves from position 1 to position 2, and at the instant of switching, i d ref_cvc =i d ref_psc and i q ref_cvc =i q ref_psc Therefore i d ref and i q ref Neither of them will mutate.

[0089] (2) If the grid-connected inverter is operating under power synchronization control, all switches are in position 2, and i d ref =i d ref_psc and i q ref =iq ref_psc The outer loop causes reactive power Q to track its reference value Q. ref to obtain i d ref_psc and make the q-axis voltage v q Track its reference value v q ref to obtain i q ref_psc Similarly, at this time, the d-axis and q-axis of the current vector control will also operate independently, causing i d ref_cvc and i q ref_cvc Track i respectively d ref_psc and i q ref_psc When the control mode is switched, the switch moves from position 2 to position 1. Obviously, there is also an instantaneous change in the value of i. d ref_psc =i d ref_cvc and i q ref_psc =i q ref_cvc .

[0090] It should be noted that although the above method is designed with constant active power / constant reactive power control as an example, its principle is also applicable to the smooth switching of other control methods, such as constant active power / constant AC voltage control.

[0091] The aforementioned smooth switching control method solves the problem that a single control method cannot adapt to changes in AC system strength, achieving control mode switching while maintaining the external characteristics of the photovoltaic grid-connected inverter, thus effectively improving the stability of photovoltaic grid connection. However, since large fluctuations in grid strength are mostly caused by grid accidents, while the aforementioned smooth switching control method can achieve smooth switching between two control modes to ensure stable system operation, it cannot avoid the adverse effects of the switching process on the frequency support of photovoltaics. In fact, maintaining the continuity of the photovoltaic active frequency support effect during accidents is not only an urgent requirement for the future development of the grid for photovoltaics, but also a concrete manifestation of photovoltaics providing reliable services to the grid. Therefore, in order to achieve continuous frequency support throughout the switching process, this invention creatively adds a power correction component to the above method, further reducing the fluctuations in power and frequency at the moment of switching.

[0092] The active power-frequency characteristics of a photovoltaic power generation system under different control modes are as follows: Figure 5As shown in the figure, P0 corresponds to the photovoltaic load shedding operation point, f0 is the steady-state system frequency, the slope of the current vector control characteristic is the droop coefficient k, and the slope of the power synchronization control characteristic can be determined by the aforementioned equation of motion of the synchronous generator. Assuming that the derivative of Δω is 0 in steady state, then:

[0093] ΔP * =-DΔf *

[0094] Then the slope of the power synchronization control characteristic can be considered as its damping coefficient D, where ΔP * and Δf * These are the per-unit values ​​for power deviation and frequency deviation, respectively. It is assumed here that the slope of the power synchronization control characteristic is greater than that of the current vector control; that is, for the same frequency change, a photovoltaic power generation system based on power synchronization control can provide more output for frequency regulation than a photovoltaic power generation system based on current vector control with droop characteristics.

[0095] Figure 5 (a) and (b) correspond to the cases of switching from current vector control to power synchronization control and switching from power synchronization control to current vector control, respectively:

[0096] (a) Consider the initial stage of operation under current vector control, as shown by the solid line representing current vector control in (a). The operating point in the steady state is A1. Assume that at a certain moment, the load suddenly increases, the system frequency drops to f1, and due to the droop characteristic, the active power rises to P1 to provide frequency support. After the system reaches a new steady state, the operating point becomes B1. If the control mode is switched at this time, the operating point will abruptly change to C1 during the switch, which will lead to a sudden change in power and frequency. Since the power value P1' corresponding to point C1 is greater than P1, the original frequency regulation output cannot be maintained after the switch, that is, the photovoltaic will provide more active power, which may cause a reduction in power margin or even fail to provide sufficient support in the next accident.

[0097] (b) Considering the initial stage of operation under power synchronous control, as shown by the solid line representing power synchronous control in (b), the operating point in the steady state is A2. Suppose that at a certain moment the load suddenly increases, the system frequency drops to f2, and the operating point becomes B2. If the control mode is switched at this time, the operating point will suddenly change to C2 during the switch. Since P2' is less than P2, the original frequency regulation output cannot be maintained after the switch, the frequency support provided by photovoltaic is reduced, which may lead to a second drop in system frequency.

[0098] The above analysis only considers the case of increased load; the same applies to the case of decreased load.

[0099] To address the aforementioned issues caused by control mode switching, it's crucial to ensure the operating point doesn't abruptly change during the switching process. For switching from current vector control to power synchronization control, shifting the power synchronization control characteristic curve downwards until C1 coincides with B1, as shown by the dashed line in (a), will ensure the operating point remains at B1 during the switching process without causing disturbance. Similarly, for switching from power synchronization control to current vector control, shifting the current vector control characteristic curve upwards, as shown by the dashed line in (b), will ensure the operating point remains at B2 during the switching process.

[0100] Similarly, when the load decreases, for the case of switching from current vector control to power synchronous control, the characteristic curve of power synchronous control needs to be shifted upward so that the operating point remains unchanged at the moment of switching; for the case of switching from power synchronous control to current vector control, the characteristic curve of current vector control is shifted downward to ensure that the operating point remains unchanged at the moment of switching.

[0101] After the characteristic shift, the power reference value will also change accordingly. Under these circumstances, the corrected active power reference values ​​for current vector control and power synchronization control are:

[0102]

[0103] In the formula k cvc and k psc These are the slopes of the current vector control and power synchronization control characteristics, respectively. From the above equation, we can obtain... Figure 7 The control block diagram for the power correction section is shown below: When the control mode switches from current vector control to power synchronous control, the switch is switched from S1 to S2, and the active power reference value after the switch is P. psc Conversely, when the control mode switches from power synchronous control to current vector control, the switch is switched from S2 to S1, and the active power reference value after the switch is P. cvc Power correction can further reduce power and frequency fluctuations during switching, thereby achieving smooth switching control that maintains continuous frequency support throughout the entire process.

[0104] The following analysis addresses the case where the slope of the current vector control characteristic is greater than that of the power synchronous control.

[0105] Figure 6 (a) and (b) correspond to the cases of switching from current vector control to power synchronization control and switching from power synchronization control to current vector control, respectively:

[0106] (a) If the initial stage operates under current vector control, as shown by the solid line representing current vector control in (a), the steady-state operating point is A3. At a certain moment, a sudden increase in load causes the system frequency to drop to f3, and the active power to rise to P3 to provide frequency support. After the system reaches a new steady state, the operating point becomes B3. If the control mode is switched at this time, the operating point will abruptly change to C3, resulting in a sudden change in power and frequency. Since the power value P3' corresponding to point C3 is less than P3, the frequency support provided by photovoltaics decreases after the switch, which may lead to a secondary drop in system frequency.

[0107] (b) If the system is initially operating under power synchronization control, as shown by the solid line representing power synchronization control in (b), the steady-state operating point is A4. At a certain moment, a sudden increase in load causes the system frequency to drop to f4, and the operating point becomes B4. If the control mode is switched at this time, the operating point will abruptly change to C4. Since P4' is greater than P4, the photovoltaic system will provide more active power after the switch, which may lead to a reduction in the power margin.

[0108] The above analysis applies to the case of increased load; the same logic applies to the case of decreased load.

[0109] When switching from current vector control to power synchronous control, the characteristic curve of power synchronous control should be shifted upwards until C3 coincides with B3, as shown by the dashed line in (a). The operating point will then remain at B3 at the moment of switching without causing disturbance. Similarly, when switching from power synchronous control to current vector control, the characteristic curve of current vector control should be shifted downwards, as shown by the dashed line in (b). The operating point will then remain at B4 at the moment of switching.

[0110] Similarly, when the load decreases, for the case of switching from current vector control to power synchronous control, the characteristic curve of power synchronous control needs to be shifted downward so that the operating point remains unchanged at the moment of switching; for the case of switching from power synchronous control to current vector control, the characteristic curve of current vector control is shifted upward to ensure that the operating point remains unchanged at the moment of switching.

[0111] After the characteristic shift, the power reference value will also change accordingly. Under these circumstances, the corrected active power reference values ​​for current vector control and power synchronization control are:

[0112]

[0113] The control block diagram and principle of the power correction section are as follows: Figure 7 They are basically the same, so I won't go into details here.

[0114] To more intuitively verify the effectiveness of the method described in this invention, the switching effects of direct switching, smooth switching with phase angle and current reference values, and smooth switching with power correction were compared under the condition of system strength variation. Direct switching refers to directly switching at θ and i...d ref and i q ref Switching is performed at the location without any additional switching control measures. A system like this was built in PSCAD / EMTDC. Figure 8 The diagram shows a photovoltaic power generation system connected to the IEEE 33-node distribution network, with improvements made to the IEEE 33-node system by changing the voltage level from 12.66 kV to 10 kV. Specific system parameters are shown in Table 1.

[0115] Table 1 Parameters of each module in the system

[0116]

[0117]

[0118]

[0119] (1) Effectiveness analysis of smooth switching of phase angle and current reference value

[0120] Since grid faults may cause tie switches to trip, leading to significant fluctuations in system strength, this study considers a scenario where tie switches trip to transfer power to the de-energized area to verify the effectiveness of the proposed switching method. Assume the grid-connected inverter is connected to bus 27, and the AC system strength is SCR = 3.2. If lines 6-26 disconnect due to a fault after 45 seconds, the emergency measure is to close the tie switch between lines 25-29 to supply power to the load in the de-energized area. At this point, the AC system strength becomes SCR = 1.4. This low short-circuit ratio means current vector control may no longer be sufficient for stable system operation. Therefore, the control mode of the grid-connected inverter is switched to power synchronization control. Assuming the line fault is cleared after 75 seconds, the tie switch between lines 5-29 opens, lines 6-26 resume operation, the system short-circuit ratio returns to its original state, and the control mode switches back to current vector control. Considering the time required for control mode switching after detecting a change in system strength, the communication delay is set to 20 milliseconds.

[0121] Depend on Figure 9 It can be seen that if the direct switching method is used, due to the change in SCR and the instantaneous values ​​of θ and i during switching... d ref and i q ref Sudden changes in power and frequency can cause significant fluctuations when the control mode switches, especially in i d ref and i q refThis will reach its limit, causing switching failure. In contrast, the smooth switching control method, which suppresses the phase angle and dq axis current reference values, is only affected by SCR changes, resulting in very small power and frequency fluctuations and more stable and smooth switching performance.

[0122] As can be seen from the above, when the system strength fluctuates significantly, adjusting the control mode of the photovoltaic grid-connected inverter using the smooth switching control method proposed in this invention can greatly improve the stability and reliability of photovoltaic grid connection. It must be pointed out that this smooth switching control method has high versatility; it can be used not only for photovoltaic grid-connected inverters but also for VSC control mode switching in other scenarios.

[0123] (2) Frequency support continuity verification

[0124] The frequency support continuity of the smooth switching control method with added power correction was verified under the same scenario, but the action time of the tie switch was changed to 80 seconds and 130 seconds. Additionally, a 2MW load surge was set at 50 seconds and another 2MW load surge was set at 100 seconds; the load surges were designed to compare the frequency support effect of the photovoltaic system. The system's active power and frequency responses are as follows: Figure 10 As shown.

[0125] Depend on Figure 10 It is evident that adding power correction significantly improves switching performance. Whether switching from current vector control to power synchronization control or vice versa, the fluctuation amplitude after adding power correction is smaller. As analyzed earlier, after an 80-second switching control mode, the increase in photovoltaic active power output under the uncorrected smooth switching control method is greater than that under the smooth switching control method with added power correction, resulting in more pronounced power and frequency fluctuations. Similarly, differences in power and frequency fluctuations also exist during a 130-second switching. Therefore, the smooth switching control method proposed in this invention not only further improves switching performance but also ensures the continuity of frequency support throughout the switching process, effectively enhancing the reliability of photovoltaic systems providing frequency regulation services during accidents.

[0126] The present invention also provides the following product embodiments:

[0127] A smooth switching control system for a photovoltaic grid-connected inverter includes:

[0128] The phase angle smoothing switching module is configured to switch the output angular frequency of the phase-locked loop to the output angular frequency of the power synchronization loop when the grid-connected inverter switches from the current vector control mode to the power synchronization control mode, and then integrates it through the integrator.

[0129] Current reference value smooth switching module: When the grid-connected inverter is configured to switch from current vector control mode to power synchronization control mode, the current reference value of power synchronization control tracks the current reference value of current vector control, and the current reference value remains unchanged before and after the mode switch.

[0130] The power correction smoothing switching module is configured to shift the active power-frequency characteristic curve and correct the active power reference value when the grid-connected inverter switches operating modes, so that the operating point remains unchanged before and after the mode switch.

[0131] A photovoltaic grid-connected inverter employs a photovoltaic grid-connected inverter smooth switching control method as described above, or includes a photovoltaic grid-connected inverter smooth switching control system as described above.

[0132] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0133] While the specific embodiments of the present invention have been described above in conjunction with the accompanying drawings, this is not intended to limit the scope of protection of the present invention. Those skilled in the art should understand that various modifications or variations that can be made by those skilled in the art without creative effort based on the technical solutions of the present invention are still within the scope of protection of the present invention.

Claims

1. A method for smooth switching control of a photovoltaic grid-connected inverter, characterized in that, include: Phase angle smooth switching steps: When the grid-connected inverter switches from current vector control mode to power synchronization control mode, the angular frequency output of the phase-locked loop is first switched to the angular frequency output of the power synchronization loop, and then integrated by the integrator. Smooth switching steps for current reference value: When the grid-connected inverter switches its operating mode from current vector control mode to power synchronization control mode, the current reference value of power synchronization control tracks the current reference value of current vector control, and the current reference value remains unchanged before and after the mode switch; and Power correction smooth switching steps: When the grid-connected inverter switches operating modes, the active power-frequency characteristic curve is shifted to correct the active power reference value so that the operating point remains unchanged before and after the mode switch. If the slope of the active power-frequency characteristic curve of power synchronization control is greater than the slope of the active power-frequency characteristic curve of current vector control, the corrected active power reference values ​​for both current vector control and power synchronization control are: In the formula, k cvc and k psc These are the slopes of the current vector control and power synchronization control characteristics, respectively. f 0 represents the steady-state system frequency; f 1 represents the reduced system frequency when the load suddenly increases while the system is operating under current vector control. P 1 is when the system frequency drops to f At time 1, the active power after the lift; f 2 represents the reduced system frequency when the load suddenly increases while the system is operating under power synchronous control. P 2 is when the system frequency drops to f At 2 o'clock, the active power after the lift; P cvc and P psc These are the corrected active power reference values ​​for current vector control and power synchronization control, respectively. If the slope of the active power-frequency characteristic curve of power synchronization control is less than the slope of the active power-frequency characteristic curve of current vector control, the corrected active power reference values ​​for both current vector control and power synchronization control are: In the formula, f 3 represents the reduced system frequency when the load suddenly increases while the system is operating under current vector control. P 3 is when the system frequency drops to f At 3 o'clock, the active power after the lift; f 4 represents the reduced system frequency when the load suddenly increases while the system is operating under power synchronous control. P 4 is when the system frequency drops to f At 4 o'clock, the active power after the lift.

2. The smooth switching control method for a photovoltaic grid-connected inverter as described in claim 1, characterized in that, The phase angle smoothing switching step further includes: When the grid-connected inverter switches from power synchronization control mode to current vector control mode, the phase-locked loop tracks the grid phase, ensuring that the phase-locked loop and the power synchronization loop operate in a synchronized state, and the phase angle remains unchanged before and after the mode switch.

3. The method for smooth switching control of a photovoltaic grid-connected inverter as described in claim 1, characterized in that, The smooth switching step of the current reference value also includes: When the grid-connected inverter switches from power synchronous control mode to current vector control mode, the current reference value of current vector control tracks the current reference value of power synchronous control, and the current reference value remains unchanged before and after the mode switch.

4. The method for smooth switching control of a photovoltaic grid-connected inverter as described in claim 1, characterized in that, The current reference values ​​include d-axis current reference values ​​and q-axis current reference values.

5. The smooth switching control method for a photovoltaic grid-connected inverter as described in claim 1, characterized in that, When the grid-connected inverter switches operating modes, the active power-frequency characteristic curve is shifted to correct the active power reference value, including: Obtain the active power-frequency response curves for power synchronization control and current vector control; If the slope of the active power-frequency characteristic curve of power synchronous control is greater than that of the active power-frequency characteristic curve of current vector control, when the load increases and the grid-connected inverter switches from current vector control mode to power synchronous control mode, the active power-frequency characteristic curve of power synchronous control will be shifted downward. or, When the load increases and the grid-connected inverter switches from power synchronous control mode to current vector control mode, the active power-frequency characteristic curve of current vector control is shifted upward.

6. The smooth switching control method for a photovoltaic grid-connected inverter as described in claim 5, characterized in that, If the slope of the active power-frequency characteristic curve of power synchronous control is greater than that of the active power-frequency characteristic curve of current vector control, when the load decreases and the grid-connected inverter switches from current vector control mode to power synchronous control mode, the active power-frequency characteristic curve of power synchronous control will be shifted upward. or, When the load decreases and the grid-connected inverter switches from power synchronous control mode to current vector control mode, the active power-frequency characteristic curve of current vector control is shifted downward.

7. The smooth switching control method for a photovoltaic grid-connected inverter as described in claim 1, characterized in that, When the grid-connected inverter switches operating modes, the active power-frequency characteristic curve is shifted to correct the active power reference value, including: Obtain the active power-frequency response curves for power synchronization control and current vector control; If the slope of the active power-frequency characteristic curve of power synchronous control is less than that of the active power-frequency characteristic curve of current vector control, when the load increases and the grid-connected inverter switches from current vector control mode to power synchronous control mode, the active power-frequency characteristic curve of power synchronous control will shift downwards and upwards. or, When the load increases and the grid-connected inverter switches from power synchronous control mode to current vector control mode, the active power-frequency characteristic curve of current vector control is shifted downward.

8. The smooth switching control method for a photovoltaic grid-connected inverter as described in claim 7, characterized in that, If the slope of the active power-frequency characteristic curve of power synchronous control is less than that of the active power-frequency characteristic curve of current vector control, when the load decreases and the grid-connected inverter switches from current vector control mode to power synchronous control mode, the active power-frequency characteristic curve of power synchronous control will be shifted downward. or, When the load decreases and the grid-connected inverter switches from power synchronous control mode to current vector control mode, the active power-frequency characteristic curve of current vector control is shifted upward.

9. A smooth switching control system for a photovoltaic grid-connected inverter, characterized in that, include: The phase angle smoothing switching module is configured to switch the output angular frequency of the phase-locked loop to the output angular frequency of the power synchronization loop when the grid-connected inverter switches from the current vector control mode to the power synchronization control mode, and then integrates it through the integrator. The current reference value smoothing switching module is configured to ensure that when the grid-connected inverter switches from the current vector control mode to the power synchronization control mode, the current reference value of the power synchronization control tracks the current reference value of the current vector control, and the current reference value remains unchanged before and after the mode switch. The power correction smoothing switching module is configured to shift the active power-frequency characteristic curve and correct the active power reference value when the grid-connected inverter switches operating modes, so that the operating point remains unchanged before and after the mode switch. If the slope of the active power-frequency characteristic curve of power synchronization control is greater than the slope of the active power-frequency characteristic curve of current vector control, the corrected active power reference values ​​for both current vector control and power synchronization control are: In the formula, k cvc and k psc These are the slopes of the current vector control and power synchronization control characteristics, respectively. f 0 represents the steady-state system frequency; f 1 represents the reduced system frequency when the load suddenly increases while the system is operating under current vector control. P 1 is when the system frequency drops to f At time 1, the active power after the lift; f 2 represents the reduced system frequency when the load suddenly increases while the system is operating under power synchronous control. P 2 is when the system frequency drops to f At 2 o'clock, the active power after the lift; P cvc and P psc These are the corrected active power reference values ​​for current vector control and power synchronization control, respectively. If the slope of the active power-frequency characteristic curve of power synchronization control is less than the slope of the active power-frequency characteristic curve of current vector control, the corrected active power reference values ​​for both current vector control and power synchronization control are: In the formula, f 3 represents the reduced system frequency when the load suddenly increases while the system is operating under current vector control. P 3 is when the system frequency drops to f At 3 o'clock, the active power after the lift; f 4 represents the reduced system frequency when the load suddenly increases while the system is operating under power synchronous control. P 4 is when the system frequency drops to f At 4 o'clock, the active power after the lift.

10. A photovoltaic grid-connected inverter, characterized in that, The method employs a smooth switching control method for a photovoltaic grid-connected inverter as described in any one of claims 1-8, or includes a smooth switching control system for a photovoltaic grid-connected inverter as described in claim 9.

Images