A grid-connected inverter stability control method and system suitable for very weak grids

Abstract

The application provides a grid-connected inverter stability control method and system suitable for extremely weak power grids, comprising: for the extremely weak power grid environment, a reactive power synchronization link is added to the phase-locked loop of the grid-connected inverter control system; the additional reactive power synchronization link comprises: obtaining the reactive power measurement value of the inverter and the set reactive power reference value, and calculating the difference; inputting the difference into a proportional controller or a proportional integral controller to output an angular frequency change; superimposing the angular frequency change on the angular frequency output by the phase-locked loop; and outputting the synchronization phase angle of the phase-locked loop through the integral link of the integrator. The application adds a reactive power synchronization link to the phase-locked loop of the grid-connected inverter control system under the extremely weak power grid environment, which significantly improves the oscillation stability of the grid-connected inverter under the extremely weak power grid condition, has small changes to the original control structure, does not affect the basic dynamic performance, and provides a strong guarantee for the safe and reliable operation of the new energy power generation system.

Description

Technical Field

[0001] The present application relates to the field of new energy power generation technology, and in particular to a method and system for stabilizing control of a grid-connected inverter suitable for an extremely weak power grid. Background Art

[0002] Against the backdrop of energy restructuring and sustainable development, renewable energy generation technologies, represented by wind and solar power, have garnered unprecedented attention and development. These renewable energy generation systems are typically connected to the grid through grid-connected inverters to achieve power conversion and transmission. However, as the scale of renewable energy grid integration continues to expand, the interaction between grid-connected inverters and the grid has become increasingly problematic, particularly in extremely weak grid environments.

[0003] The oscillation stability of grid-connected inverters for renewable energy power generation units is directly related to the overall system's operational efficiency and safety. In strong power grids, traditional control strategies are effective in maintaining stable wind turbine operation. However, in extremely weak power grids with a short-circuit ratio of 1.1, power electronics devices using existing control strategies can experience various oscillation instabilities.

[0004] Under extremely weak grid conditions, fluctuations in grid voltage and frequency can cause phase-locked loop (PLL) instability, impacting the overall stability of renewable energy power generation units. Optimizing the design of PLLs and improving their anti-disturbance capabilities has become a key area of ​​control strategy research. Summary of the Invention

[0005] In response to the defects in the existing technology, the purpose of this application is to provide a grid-connected inverter stability control method and system suitable for extremely weak power grids, which can improve the stable operation capability of new energy power generation units in harsh power grid environments and provide strong guarantees for the safe and reliable operation of new energy power generation systems.

[0006] In one aspect of the present application, a method for stabilizing a grid-connected inverter applicable to an extremely weak power grid is provided, comprising:

[0007] For extremely weak power grid environments, a reactive power synchronization link is added to the phase-locked loop of the grid-connected inverter control system;

[0008] The additional reactive power synchronization step includes: obtaining the reactive power measurement value of the inverter and the set reactive power reference value, and calculating the difference;

[0009] The difference is input into a proportional controller or a proportional integral controller to output an angular frequency change;

[0010] Superimposing the angular frequency variation onto the angular frequency output by the phase-locked loop;

[0011] The synchronous phase angle of the phase-locked loop is output through the integral link of the integrator.

[0012] Furthermore, the extremely weak electrical environment is a power grid condition with a short circuit ratio of not less than 1.1.

[0013] Furthermore, the difference is input into a proportional controller or a proportional-integral controller, and among the output angular frequency variation, the proportional-integral controller is selected to output the angular frequency variation.

[0014] Furthermore, the difference is input into a proportional-integral controller to output the angular frequency change, which is expressed as:

[0015]

[0016] Where, ω Q is the angular frequency change; Q Gref is the reference value of reactive power; Q G is the reactive power measurement value; k pRPHSC and k iRPHSC are the proportional and integral coefficients of the reactive power loop PI controller, respectively, and S is the rated capacity of the grid-connected inverter;

[0017] Wherein, the expression of the proportional integral controller is: H RPHSC (s) = k pRPHSC +k iRPHSC / s.

[0018] Furthermore, the proportional and integral coefficients of the reactive power PI controller are expressed as follows:

[0019]

[0020] Where k ppll and k ipll are the proportional and integral coefficients of the phase-locked loop PI control, u sd0 and u sq0 represents the dq-axis component of the AC voltage, S represents the rated capacity of the grid-connected inverter, and PF represents the power factor of the grid-connected inverter.

[0021] Furthermore, the angular frequency variation is superimposed on the angular frequency output by the phase-locked loop, and the expression is:

[0022]

[0023] Where, is the component of the AC voltage at the common connection point on the q axis, the superscript "c" is the component in the control coordinate system, ω0 is the fundamental angular frequency, ω PLL is the angular frequency output by the hybrid synchronization link, H pll (s) = kppll +k ipll / s.

[0024] Furthermore, the synchronous phase angle of the phase-locked loop output by the integration link of the integrator is expressed as:

[0025]

[0026] Where 1 / s represents the integral link, θ is the synchronous phase angle output by the phase-locked loop, ω PLL It is the angular frequency output by the hybrid synchronization link.

[0027] Furthermore, the grid-connected inverter includes at least a photovoltaic power generation unit, an energy storage unit, an SVG, a doubly-fed wind turbine generator set or a grid-connected inverter of a direct-drive wind turbine generator set.

[0028] A second aspect of the present application provides a grid-connected inverter stability control system applicable to an extremely weak power grid, comprising:

[0029] The outer loop control module is used to control the DC bus voltage or active power, reactive power or AC voltage amplitude of the grid-connected inverter;

[0030] The inner loop control module is used to control the AC side current of the grid-connected inverter and generate the modulation voltage;

[0031] The hybrid synchronization control module is used to synchronize the grid-connected inverter with the AC power grid.

[0032] Furthermore, a reactive power synchronization link is additionally provided in the hybrid synchronization module, which is used to improve the stability of the grid-connected inverter under extremely weak grid conditions through the reactive power synchronization link.

[0033] Compared with the prior art, the present invention has at least one of the following beneficial effects:

[0034] This application adds a reactive power synchronization link to the phase-locked loop of the grid-connected inverter control system in an extremely weak grid environment, so that when the grid strength is extremely low, the phase-locked loop can be prevented from being unstable due to fluctuations in grid voltage and frequency. The oscillation stability of the grid-connected inverter is significantly improved under extremely weak grid conditions, with minimal changes to the original control structure and without affecting its basic dynamic performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Other features, objects and advantages of the present application will become more apparent upon reading the detailed description of non-limiting embodiments with reference to the following drawings:

[0036] Figure 1 This is a control block diagram of a grid-connected inverter stability control method suitable for an extremely weak power grid in one embodiment of the present application.

[0037] Figure 2 This is a power waveform diagram of a phase-locked loop with and without an additional reactive power synchronization link in an extremely weak power grid in one embodiment of the present application.

[0038] Figure 3 This is a flow chart of a grid-connected inverter stability control method applicable to an extremely weak power grid in one embodiment of the present application. DETAILED DESCRIPTION

[0039] The present application is described in detail below with reference to specific embodiments. The following embodiments will help those skilled in the art to further understand the present application, but are not intended to limit the present application in any form. It should be noted that those skilled in the art may make several variations and improvements without departing from the scope of the present application. These all fall within the scope of protection of the present application.

[0040] Reference Figure 1 and Figure 3 As shown, a grid-connected inverter stability control method applicable to an extremely weak power grid according to an embodiment of the present application includes: for an extremely weak power grid environment, adding a reactive power synchronization link in the phase-locked loop of the grid-connected inverter control system.

[0041] The additional reactive power synchronization link includes: S100, obtaining the reactive power measurement value of the inverter and the set reactive power reference value, and calculating the difference; S200, inputting the difference into a proportional controller or a proportional integral controller, and outputting an angular frequency change; S300, superimposing the angular frequency change on the angular frequency output by the phase-locked loop; S400, outputting the synchronous phase angle of the phase-locked loop through the integral link of the integrator.

[0042] This application adds a reactive power synchronization link to the phase-locked loop of the grid-connected inverter control system in an extremely weak grid environment, so that when the grid strength is extremely low, the phase-locked loop can be prevented from being unstable due to fluctuations in grid voltage and frequency. The oscillation stability of the grid-connected inverter is significantly improved under extremely weak grid conditions, with minimal changes to the original control structure and without affecting its basic dynamic performance.

[0043] In the reactive power synchronization link, specifically, the reactive power measurement value of the inverter is first obtained and compared with the set reactive power reference value, and the difference between the two is calculated; then the difference is input into the proportional controller or proportional integral controller, and the angular frequency change that needs to be adjusted is calculated through the control algorithm; then this angular frequency change is superimposed on the angular frequency originally output by the phase-locked loop to achieve fine-tuning of the angular frequency; finally, through the integration link of the integrator, the adjusted phase-locked loop synchronization phase angle is output to achieve the stability of the grid-connected inverter operating under extremely weak grid conditions.

[0044] The control method of the present application can be used for photovoltaic power generation units, doubly-fed wind turbines, direct-drive wind turbines, energy storage, SVG and other grid-connected power electronic equipment.

[0045] In some specific embodiments, the extremely weak electrical environment is a power grid condition with a short circuit ratio of not less than 1.1.

[0046] In some specific embodiments, the difference is input into a proportional controller or a proportional-integral controller to output the angular frequency change. In this application, the proportional-integral controller is selected to output the angular frequency change.

[0047] Among them, the proportional controller is mainly adjusted according to the current difference, and there is a steady-state static difference between the reference value and the actual value, while the proportional-integral controller takes into account the cumulative effect of the difference to achieve smoother control, and there is no steady-state static difference between the reference value and the actual value.

[0048] In this application, a proportional-integral controller is selected to output the angular frequency change. In an extremely weak grid environment, the fluctuation of the grid will cause challenges to the operation capability of the grid-connected inverter. The PI controller can more effectively respond to these disturbances and enhance the stability of the system through its comprehensive control strategy. In contrast, the P controller may exhibit large fluctuations or unstable behavior in an extremely weak grid environment. During long-term operation, the PI controller can continuously accumulate the deviation signal of the control system through its integral link, and gradually adjust the control output, thereby eliminating the steady-state error of the system and making the system reach a more accurate and stable operating state. Although the P controller can quickly respond to transient changes in the system, it cannot completely eliminate the steady-state error.

[0049] In some specific embodiments, the difference is input into a proportional-integral controller, and the output angular frequency change is expressed as:

[0050]

[0051] Where, ω Q is the angular frequency change; Q Gref is the reference value of reactive power; Q G is the reactive power measurement value; k pRPHSC and k iRPHSC are the proportional and integral coefficients of the reactive power loop PI controller, respectively, and S is the rated capacity of the grid-connected inverter;

[0052] Among them, the expression of proportional integral controller is: H RPHSC (s) = k pRPHSC +k iRPHSC / s.

[0053] Among them, the reactive power ratio and integral coefficient are used to adjust the reactive power output of the inverter; the phase-locked loop ratio and integral coefficient are used to adjust the angular frequency of the inverter output;

[0054] In a specific embodiment, the proportional and integral coefficients of the reactive power PI controller are expressed as follows:

[0055]

[0056] Where k ppll and k ipll are the proportional and integral coefficients of the phase-locked loop PI control, u sd0 and u sq0 represents the dq-axis component of the AC voltage, S represents the rated capacity of the grid-connected inverter, and PF represents the power factor of the grid-connected inverter.

[0057] In some specific embodiments, the angular frequency variation is added to the angular frequency output by the phase-locked loop, and the expression is:

[0058]

[0059] Where, is the component of the AC voltage at the common connection point on the q axis, the superscript "c" is the component in the control coordinate system, ω0 is the fundamental angular frequency, ω PLL is the angular frequency output by the hybrid synchronization link, the phase-locked loop H pll (s) = k ppll +k ipll / s.

[0060] In some specific embodiments, the synchronous phase angle of the phase-locked loop is output through the integral link of the integrator, and the expression is:

[0061]

[0062] Where 1 / s represents the integral link, θ is the synchronous phase angle output by the phase-locked loop, ω PLL It is the angular frequency output by the hybrid synchronization link.

[0063] like Figure 2 As shown in the power waveform diagram (a) without adding a reactive power synchronization link in a weak power grid, it can be seen that the system is unstable; and as shown in the power waveform diagram (b) with adding a reactive power synchronization link in the phase-locked loop in an extremely weak power grid, it can be seen that the system is stable.

[0064] In the above specific embodiment, one of the grid-connected inverter parameters is shown in Table 1 below.

[0065] Table 1 Grid-connected inverter parameters

[0066]

[0067] In some specific embodiments, the grid-connected inverter includes at least a photovoltaic power generation unit, an energy storage unit, an SVG, a doubly-fed wind turbine generator set, or a grid-connected inverter of a direct-drive wind turbine generator set.

[0068] Exemplarily, the grid-connected inverter may also include grid-connected equipment in a distributed energy system (DES). In a distributed energy system, there are various types of grid-connected inverters. The electricity generated by, for example, solar photovoltaic panels, small wind turbines, fuel cells, etc. needs to be connected to the power grid through grid-connected inverters. Through the control method of the present application, the stability of the operation of the distributed energy system in an extremely weak power grid environment can be improved.

[0069] A second aspect of the present application provides a grid-connected inverter stability control system suitable for an extremely weak power grid, comprising: an outer loop control module, an inner loop control module and a hybrid synchronous control module.

[0070] The outer loop control module is used to control the DC bus voltage or active power, reactive power or AC voltage amplitude of the grid-connected inverter; the inner loop control module is used to control the AC side current of the grid-connected inverter and generate the modulation voltage; the hybrid synchronization control module is used to synchronize the grid-connected inverter with the AC grid.

[0071] By integrating the outer-loop control module, the inner-loop control module and the hybrid synchronous control module, precise and stable control of the grid-connected inverter under extremely weak grid conditions is achieved. The outer-loop control module ensures the stable output of the grid-connected inverter in terms of DC bus voltage, active power, reactive power or AC voltage amplitude. The inner-loop control module effectively controls the AC side current of the grid-connected inverter by generating a modulation voltage, reducing harmonics and current distortion and avoiding phase-locked loop instability. The hybrid synchronous control module synchronizes the grid-connected inverter with the AC grid, avoiding shocks and instability during the grid connection process and improving system stability.

[0072] Specifically, when the system is running, the outer loop control module first calculates the current reference value that the grid-connected inverter needs to output based on the set target values ​​such as DC bus voltage, active power, reactive power or AC voltage amplitude; then, the inner loop control module receives the current reference value and monitors the AC side current of the grid-connected inverter in real time, and generates a modulated voltage signal through the current control algorithm to control the AC side current; at the same time, the reactive power hybrid synchronous control module continuously monitors the reactive power emitted by the grid-connected inverter, and by adjusting the PI controller parameters of the reactive power hybrid synchronous control, the system damping is improved to ensure the smoothness and reliability of the grid-connected process, thereby achieving stable control of the grid-connected inverter under extremely weak grid conditions.

[0073] In some specific embodiments, a reactive power synchronization link is additionally provided in the hybrid synchronization module, for improving the stability of the grid-connected inverter under extremely weak grid conditions through the reactive power synchronization link.

[0074] By adding a reactive power synchronization link in the hybrid synchronization module, the reactive power exchange between the grid-connected inverter and the grid under extremely weak grid conditions is controlled, thereby improving the dynamic response capability and stability of the grid-connected inverter. This allows it to avoid phase-locked loop instability caused by fluctuations in grid voltage and frequency when the grid strength is extremely low, thereby improving the overall stability of the new energy power generation unit.

[0075] The above describes the specific embodiments of the present application. It should be understood that the present application is not limited to the specific embodiments described above, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the substantive content of the present application. The above preferred features may be used in any combination as long as they do not conflict with each other.

Claims

1. A grid-connected inverter stabilization control method applicable to extremely weak power grids, characterized in that: include: For extremely weak power grid environments, a reactive power synchronization link is added to the phase-locked loop of the grid-connected inverter control system; The additional reactive power synchronization step includes: obtaining the reactive power measurement value of the inverter and the set reactive power reference value, and calculating the difference; The difference is input into a proportional controller or a proportional integral controller to output an angular frequency change; Superimposing the angular frequency variation onto the angular frequency output by the phase-locked loop; The synchronous phase angle of the phase-locked loop is output through the integral link of the integrator; The difference is input into a proportional controller or a proportional-integral controller, and among the output angular frequency variation, the proportional-integral controller is selected to output the angular frequency variation; The difference is input into the proportional-integral controller, and the output angular frequency change is expressed as: Where, ω Q is the angular frequency change; Q Gref is the reference value of reactive power; Q G is the reactive power measurement value; k pRPHSC and k iRPHSC are the proportional and integral coefficients of the reactive power loop PI controller, respectively, and S is the rated capacity of the grid-connected inverter; Wherein, the expression of the proportional integral controller is: H RPHSC (s) = k pRPHSC +k iRPHSC / s; The proportional and integral coefficients of the reactive power loop PI controller are expressed as follows: Where k ppll and k ipll are the proportional and integral coefficients of the phase-locked loop PI control, u sd0 and u sq0 represents the dq-axis component of the AC voltage, S represents the rated capacity of the grid-connected inverter, and PF represents the power factor of the grid-connected inverter.

2. A grid-connected inverter stabilization control method applicable to an extremely weak power grid according to claim 1, characterized in that: The extremely weak power grid environment refers to a power grid condition with a short circuit ratio of not less than 1.

1.

3. The method for stabilizing a grid-connected inverter suitable for an extremely weak power grid according to claim 1, characterized in that: The angular frequency variation is superimposed on the angular frequency output by the phase-locked loop, and the expression is: Where, is the component of the AC voltage at the common connection point on the q axis, the superscript "c" is the component in the control coordinate system, ω0 is the fundamental angular frequency, ω PLL is the angular frequency output by the hybrid synchronization link, the phase-locked loop H pll (s) = k ppll +k ipll / s.

4. The method for stabilizing a grid-connected inverter suitable for an extremely weak power grid according to claim 1, characterized in that: The synchronous phase angle of the phase-locked loop is output through the integral link of the integrator, and the expression is: Where 1 / s represents the integral link, θ is the synchronous phase angle output by the phase-locked loop, ω PLL It is the angular frequency output by the hybrid synchronization link.

5. The method for stabilizing a grid-connected inverter suitable for an extremely weak power grid according to claim 1, characterized in that: The grid-connected inverter at least includes a photovoltaic power generation unit, an energy storage unit, an SVG, a double-fed wind turbine generator set or a grid-connected inverter of a direct-drive wind turbine generator set.

6. A control system using the grid-connected inverter stabilization control method applicable to an extremely weak power grid according to any one of claims 1 to 5, characterized in that: include: The outer loop control module is used to control the DC bus voltage or active power, reactive power or AC voltage amplitude of the grid-connected inverter; The inner loop control module is used to control the AC side current of the grid-connected inverter and generate the modulation voltage; The hybrid synchronization control module is used to synchronize the grid-connected inverter with the AC power grid.

7. The control system according to claim 6, characterized in that: The hybrid synchronous control module is additionally provided with a reactive power synchronization link, which is used to improve the stability of the grid-connected inverter under extremely weak power grid conditions through the reactive power synchronization link.

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