Inverter and grid control switching method and system based on hybrid ratio regulation

Abstract

The present invention provides an inverter and grid-forming control switching method and system based on hybrid ratio regulation, which relates to the technical field of inverter switching control, including: constructing control equivalent circuits of a grid-following inverter and a grid-forming inverter respectively, and setting the hybrid inverter simulation equivalent to the parallel operation of the grid-following inverter and the grid-forming inverter; constructing a grid-connected virtual equivalent circuit of the hybrid inverter, respectively obtaining the current value of each branch under the grid-following control and the grid-forming control, calculating the current value and voltage value required for the operation of the grid-following control and the grid-forming control according to the voltage value of the main branch and the parameter value of the grid-connected virtual branch, and outputting respective voltage target values ​​through the grid-following control and the grid-forming control; switching the grid-following control and the grid-forming control in proportion, setting two proportional parameters, combining the proportional parameters, and adding the respective voltage target values ​​output by the grid-following control and the grid-forming control in proportion and inputting them into the VSC to realize control.

Description

Technical Field

[0001] The present disclosure relates to the technical field of inverter switching control, and in particular to an inverter and network control switching method and system based on hybrid ratio regulation. Background Art

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

[0003] At present, the large-scale access of renewable energy to the power grid has caused a decrease in the strength of the power grid and the inertia of the power system. The intermittent, volatile and random nature of renewable energy generation threatens the safe and stable operation of the power system. In recent years, power systems with large-scale renewable energy integration have experienced many accidents due to the reduction of inertia support level and anti-interference ability. This is because most renewable energy sites are currently unable to effectively support the power grid under system disturbances and failures, and even worsen the situation. Therefore, the development of new power systems has put forward an urgent need to improve the ability of renewable energy sites to adapt to the operating conditions of the AC system and provide transient support.

[0004] As an indispensable core equipment for renewable energy power generation, grid-connected inverters are the most important part in achieving efficient power conversion. The interconnection mode between them and the grid and the rationality of the control strategy determine whether new energy can be safely and reliably connected to the grid. At present, photovoltaic grid-connected inverters are usually divided into two types: GFL (grid-following) inverters and GFM (grid-forming) grid-forming inverters. The main difference between the two is the different ways of synchronizing with the grid. GFL synchronizes with the grid through a phase-locked loop (PLL), and GFM often achieves synchronous power synchronization control through frequency droop control. Grid-following control controls the power transmitted by the grid-connected inverter by adjusting the dq axis current. Therefore, the grid-following inverter can be regarded as a current source in the grid. The control target of grid-forming control is the common point voltage, so the grid-forming inverter can be regarded as a voltage source in the grid. In short, grid-following control can accurately control the output power, but its support capability for the system is weak. Grid-forming control has strong support capability for the system and will be more and more widely used in future new power systems.

[0005] The differences between GFL and GFM control methods lead to different applicable conditions for the two. For power grids with large system fluctuations, grid-connected inverters using a single control strategy are difficult to ensure stable system operation. Therefore, many scholars have proposed a grid-connected inverter control method that includes both grid-following control and grid-forming control. A study proposed a hybrid control method that simulates parallel grid-following control. Its main application is to connect multiple AC subgrids with a DC transmission system to achieve power transmission between AC subgrids. The inverter always operates in a single mode where both grid-following control and grid-forming control are in effect, without considering the application of the control system in an island system and the switching problem between grid-following control and grid-forming control.

[0006] Compared with pure grid-following and pure grid-forming control, the application effect of the above-mentioned hybrid control mode on the power system under various disturbances is still unclear and requires further research. The mainstream solution to this problem at this stage is to use a voltage source converter with multiple control modes and select the inverter control mode according to the operating conditions. Realizing smooth switching of control modes is a problem that must be solved for the safe and stable operation of the system.

[0007] Most of the existing smoothing measures are to reduce the waves in the switching process by compensating and suppressing phase angle and current mutations, or to reduce fluctuations through a phase-locked loop. However, its essence is to reduce fluctuations through compensation, but it does not really touch the control mode switching method. It cannot effectively reduce the fluctuations generated in the switching process and cannot achieve the effect of smooth switching. Summary of the invention

[0008] In order to solve the above problems, the present disclosure proposes an inverter and grid-forming control switching method and system based on hybrid ratio adjustment. By adjusting the ratio of grid-following control and grid-forming control in the inverter, smooth switching between grid-following control and grid-forming control is achieved, thereby reducing the fluctuation caused by the control switching process.

[0009] According to some embodiments, the present disclosure adopts the following technical solutions:

[0010] The inverter and grid control switching method based on hybrid ratio regulation includes:

[0011] Construct the control equivalent circuits of the grid-following inverter and the grid-forming inverter respectively, and set the hybrid inverter simulation equivalent to the parallel operation of the grid-following inverter and the grid-forming inverter;

[0012] Construct a grid-connected virtual equivalent circuit of the hybrid inverter, obtain the current value of each branch under grid-following control and grid-forming control respectively, calculate the current value and voltage value required for grid-following control and grid-forming control operation according to the voltage value of the main branch and the parameter value of the grid-connected virtual branch, and output the respective voltage target values ​​through grid-following control and grid-forming control;

[0013] Switch the grid-following control and the grid-forming control in proportion, set two proportional parameters, and combine the proportional parameters to proportionally add the respective voltage target values ​​output by the grid-following control and the grid-forming control and input them into the VSC to realize control.

[0014] According to some embodiments, the present disclosure adopts the following technical solutions:

[0015] The inverter and grid control switching system based on hybrid ratio regulation includes:

[0016] An initialization module is used to construct control equivalent circuits of the grid-following inverter and the grid-forming inverter respectively, and to set the hybrid inverter simulation equivalent to the parallel operation of the grid-following inverter and the grid-forming inverter;

[0017] The target output module is used to construct the grid-connected virtual equivalent circuit of the hybrid inverter, obtain the current value of each branch under the grid-following control and grid-forming control respectively, calculate the current value and voltage value required for the operation of the grid-following control and grid-forming control according to the voltage value of the main branch and the parameter value of the grid-connected virtual branch, and output the respective voltage target values ​​through the grid-following control and grid-forming control;

[0018] The switching control module is used to switch the grid-following control and the grid-forming control in proportion, set two proportional parameters, and combine the proportional parameters to proportionally add the respective voltage target values ​​output by the grid-following control and the grid-forming control and input them into the VSC to achieve control.

[0019] According to some embodiments, the present disclosure adopts the following technical solutions:

[0020] A non-transitory computer-readable storage medium is used to store computer instructions. When the computer instructions are executed by a processor, the inverter and grid control switching method based on hybrid ratio regulation is implemented.

[0021] According to some embodiments, the present disclosure adopts the following technical solutions:

[0022] An electronic device comprises: a processor, a memory and a computer program; wherein the processor is connected to the memory, the computer program is stored in the memory, and when the electronic device is running, the processor executes the computer program stored in the memory so that the electronic device executes the inverter and network control switching method based on hybrid ratio regulation.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The present invention discloses a method for switching between inverter and grid-forming control based on hybrid ratio regulation. On the basis of hybrid ratio control, an inverter is used to simulate a grid-following inverter and a grid-forming inverter to run in parallel. Then, it is proposed to switch between the grid-following control mode and the grid-forming control mode through ratio regulation. Two variables k are set in the hybrid ratio switching mode. gfl and k gfm When it is 1, the inverter is in a pure grid-following state. gfm When is 1, the converter is in pure network state. And the two variables satisfy k gfl +k gfm = 1. Therefore, when performing switching control, only one of the k values ​​needs to be controlled, which essentially changes the switching method of the control mode, greatly reduces the fluctuations generated during the switching process, and achieves a smooth switching effect.

[0025] The present invention discloses a method for switching inverter and grid-building control based on hybrid ratio regulation. When the control mode is converted from grid-building type to grid-following type, the switching rate is first selected according to the operation situation. When the system is running smoothly, a smaller rate is selected for mode switching. When the system strength changes suddenly, a larger switching rate is selected to achieve k gfm Increase at a constant speed according to the preset rate, and add an amplitude limit module to ensure that 0≤k gfm ≤1, during the entire operation process, it is always guaranteed to meet k gfm +k gfl =1, to achieve smooth switching of control with network. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The accompanying drawings constituting a part of the present disclosure are used to provide a further understanding of the present disclosure. The illustrative embodiments of the present disclosure and their descriptions are used to explain the present disclosure and do not constitute an improper limitation on the present disclosure.

[0027] Figure 1 This is a diagram of the network following control structure of an embodiment of the present disclosure;

[0028] Figure 2 This is a diagram of the network control structure of the embodiment of the present disclosure;

[0029] Figure 3 The equivalent circuit diagram of the hybrid ratio control inverter according to the embodiment of the present disclosure is as follows;

[0030] Figure 4 The mixing ratio control structure of the embodiment of the present disclosure;

[0031] Figure 5 It is a schematic diagram of the existing direct control switching;

[0032] Figure 6 A control switching diagram for adding phase angle compensation to an existing one;

[0033] Figure 7 This is a schematic diagram of the mixing ratio control switching according to an embodiment of the present disclosure;

[0034] Figure 8 This is a topology diagram of a new energy three-machine nine-node system according to an embodiment of the present disclosure;

[0035] Fig. 9 This is an active power comparison diagram in application 1 of the embodiment of the present disclosure;

[0036] Fig.10 A frequency comparison diagram is applied to an embodiment of the present disclosure;

[0037] Fig.11 A common point voltage comparison diagram is applied to an embodiment of the present disclosure;

[0038] Fig.12 A reactive power comparison diagram is applied to an embodiment of the present disclosure;

[0039] Fig.13 This is an active power comparison diagram in Application 2 of the embodiment of the present disclosure;

[0040] Fig.14 This is a frequency comparison diagram of the second application of the embodiment of the present disclosure;

[0041] Fig.15 This is a comparison diagram of common point voltages in application 2 of the embodiment of the present disclosure;

[0042] Fig.16 This is a reactive power comparison diagram of application 2 of the embodiment of the present disclosure;

[0043] Fig.17 This is a comparison diagram of active power in application three of the embodiments of the present disclosure;

[0044] Fig.18 Three frequency comparison diagrams are applied to the embodiments of the present disclosure;

[0045] Fig.19 This is a comparison diagram of three common point voltages applied in the embodiments of the present disclosure;

[0046] Fig. 20 Three reactive power comparison diagrams are applied in the embodiments of the present disclosure. DETAILED DESCRIPTION

[0047] The present disclosure is further described below in conjunction with the accompanying drawings and embodiments.

[0048] It should be noted that the following detailed descriptions are all illustrative and are intended to provide further explanation of the present disclosure. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present disclosure belongs.

[0049] It should be noted that the terms used herein are only for describing specific embodiments and are not intended to limit the exemplary embodiments according to the present disclosure. As used herein, unless the context clearly indicates otherwise, the singular form is also intended to include the plural form. In addition, it should be understood that when the terms "comprising" and / or "including" are used in this specification, it indicates the presence of features, steps, operations, devices, components and / or combinations thereof.

[0050] Example 1

[0051] In one embodiment of the present disclosure, a method for switching between inverter and grid-forming control based on hybrid ratio adjustment is provided. By adjusting the ratio of grid-forming control and grid-forming control in the inverter, smooth switching between grid-forming control and grid-forming control is achieved, and fluctuations caused by the control switching process are reduced. The method steps include:

[0052] Step 1: construct control equivalent circuits of the grid-following inverter and the grid-forming inverter respectively, and set the hybrid inverter simulation equivalent to the parallel operation of the grid-following inverter and the grid-forming inverter;

[0053] Step 2: Construct a grid-connected virtual equivalent circuit of the hybrid inverter, obtain the current value of each branch under the grid-following control and grid-forming control respectively, calculate the current value and voltage value required for the operation of the grid-following control and grid-forming control according to the voltage value of the main branch and the parameter value of the grid-connected virtual branch, and output the respective voltage target values ​​through the grid-following control and grid-forming control;

[0054] Step 3: Switch the grid-following control and grid-forming control in proportion, set two proportional parameters, and combine the proportional parameters to proportionally add the respective voltage target values ​​output by the grid-following control and grid-forming control and input them into the VSC to achieve control.

[0055] As an embodiment, the specific implementation process of the inverter and grid control switching method based on hybrid ratio adjustment disclosed in the present invention is as follows:

[0056] Step 1: Construct the control equivalent circuit of the grid-following inverter and the grid-forming inverter, and set the hybrid inverter simulation equivalent to the parallel operation of the grid-following inverter and the grid-forming inverter;

[0057] Specifically, it includes constructing the control structure and control method of the grid-following inverter and the grid-building inverter, wherein:

[0058] 1) Network control structure

[0059] The control method adopted by the grid-following inverter disclosed in the present invention is constant power control, and its control target is to input the set power into the system. Figure 1The inverter main circuit and its control module are displayed. The inverter is connected to the system through an LCL filter. Its main control structure can be divided into six parts: PLL, Parker transformation, droop control, power calculation, outer loop power controller, and inner loop current controller.

[0060] The control method includes: PLL tracks the voltage phase of the grid connection point, outputs the reference phase angle θ required for Park transformation and its inverse transformation, and the frequency f required for droop control. The three-phase AC quantity is converted into a two-phase DC quantity through Park transformation, which is easy to control. The power calculation is to calculate the instantaneous active power and reactive power input by the inverter to the system through the measured voltage and current. The instantaneous value is obtained through a low-pass filter to obtain the values ​​P and Q corresponding to its basic components and input to the power control outer loop. Droop control simulates the characteristics of the synchronous generator by changing the power set value through the change of system frequency, which is beneficial to the stable operation of the system. The power outer loop control receives the power command value (P sp , Q sp ), the power command value is subtracted from the power value obtained by the power calculation module, and the command value i required by the current inner loop is generated through the PI link d,ref and i q,ref After receiving the current command value, the current inner loop outputs the voltage target value of the dq axis through the PI controller, generates the three-phase voltage target value through the Park inverse transformation, inputs it into the inverter, and modulates the inverter to output the corresponding power. Since the grid-following control requires PLL to achieve synchronization, it relies on the existing and capable system, but does not have the ability to support the grid.

[0061] 2) Network control structure

[0062] The grid control adopts power synchronization control. The biggest difference between grid control and grid control is the different ways of generating θ, which also leads to differences in the operating characteristics of the two. In GFM, the reference phase angle θ is generated by the power controller, so that the inverter can simulate the characteristics of the synchronous generator. Since GFM is a constant voltage control and does not rely on PLL to generate the reference phase angle θ, it has the characteristics of building a grid and supporting the operation of the grid.

[0063] 3) Mixing ratio control

[0064] The hybrid proportional control disclosed in the present invention uses an inverter to simulate a grid-following inverter and a grid-forming inverter running in parallel, and sets the hybrid inverter to simulate the equivalent of the grid-following inverter and the grid-forming inverter running in parallel. The equivalent circuit diagram is as follows: Figure 3 shown.

[0065] At present, a hybrid ratio control has been proposed, which has the operating characteristics of both grid-following control and grid-forming control, but the selection method of its ratio has not been determined, and it is mainly used to connect the AC subgrid with the DC transmission system to realize the power flow between the AC subgrids. The present disclosure improves on this control method and applies it to the switching of grid-following control, which fundamentally solves the fluctuation problem of switching and realizes the smooth switching of grid-following control.

[0066] Step 2: Construct a grid-connected virtual equivalent circuit of the hybrid inverter, obtain the current value of each branch under grid-following control and grid-forming control respectively, calculate the current value and voltage value required for the operation of grid-following control and grid-forming control according to the voltage value of the main branch and the parameter value of the grid-connected virtual branch, and output their respective voltage target values ​​through grid-following control and grid-forming control.

[0067] The voltage and current equations are written according to Kirchhoff's laws. For the convenience of expression, the voltage and current are both expressed in the frequency domain.

[0068] V o,gfl =V g +I o,gfl (R c,gfl +sL c,gfl ) (1)

[0069] I o,gfl =I f,gfl -I c,gfl (2)

[0070]

[0071]

[0072] Substituting equations (2), (3) and (4) into equation (1), we obtain equation (5):

[0073]

[0074] Among them, V i,gfl is the output voltage of GFL, V o,gfl is the voltage of GFL to ground branch at the connection point of the main circuit in the equivalent circuit diagram, V g is the voltage at the grid connection point, I f,gfl is the total current of the GFL section, I c,gfl is the ground branch current of the GFL part, I o,gfl is the current input from the GFL to the grid, R c,gfl , L f,gfl , R f,gfl , R f2,gfl , C f,gfl , L c,gfl are the circuit parameters in the equivalent circuit. Figure 3shown.

[0075] Set the scale parameter k gfl , k gfm , respectively represent the proportion of GFL and GFM, k gfl , k gfm Self-set, no direct relationship with the parameters in the equivalent circuit diagram. gfl When it is 1, it means the inverter is running in pure grid-following mode, k gfm When it is 1, it indicates that the inverter operates in pure grid-building mode.

[0076] V ref,abc =k gfl V ref,gfl +k gfm V ref,gfm (6)

[0077] k gfl +k gfm =1 (7)

[0078] Its control structure is as follows Figure 4 As shown in the figure, the current values ​​of each branch of GFL and GFM are calculated by following the grid current calculation and forming the grid current, which are used for inverter control. According to the voltage measured in the main circuit and the parameter value of the virtual branch, the current value and voltage value required for the operation of the GFM control module and the GFL control module are calculated by equations (1)-(5), and the voltage target values ​​V of the outputs of GFM and GFL control are respectively obtained. ref,gfl , V ref,gfm , V is converted into ref,gfl and V ref,gfm The input is proportionally added to the VSC to achieve control.

[0079] Step 3: Switch the grid-following control and grid-forming control in proportion, set two proportional parameters, and combine the proportional parameters to proportionally add the respective voltage target values ​​output by the grid-following control and grid-forming control and input them into the VSC to achieve control.

[0080] As an embodiment, the existing methods include: 1. Direct switching method. 2. Switching method with phase angle compensation. Among them:

[0081] a. Direct switch

[0082] The inner loop controllers of the grid-following control mode and the grid-building control mode introduced in this disclosure are exactly the same, so direct switching is achieved by switching the command values ​​input to the inner loop in the two control modes. Since the reference phase angle θ of the grid-following control and the grid-building control is generated in different ways, when switching the control mode, the dq-axis electrical quantities obtained by the Park transformation also need to be switched.

[0083] When the grid-following control and the grid-building control coexist in the same inverter, when the inverter is in the grid-following control mode, the PLL tracks the grid voltage phase, and the grid-building control cannot form a closed-loop control with the controlled object, so the phase angle θ output by the PSL psl Mismatching the voltage phase angle of the current inverter output will result in large power fluctuations during the switching process.

[0084] b. Phase angle compensation switching

[0085] By comparing the network-following control and network-building control structures, it can be found that the main difference between the two is the different ways of generating the reference phase angle. psl With θ pll The difference will cause power fluctuation. To solve this problem, the phase angle compensation switching uses the principle that the output of the integral link does not change suddenly to construct the following Figure 6 When the switch is in position 2, the inverter is in grid-following control mode, θ = θ pll When the inverter receives the switching command, the switch position changes from 2 to 1. Due to the integrator after the switching mode, the reference phase angle will not produce a sudden change, and there is θ = θ at the switching moment. pll =θ psl .

[0086] Although this switching method solves the problem of phase angle mutation, due to the different outer loop outputs (current loop inputs) of the two controls, power fluctuations will still occur at the switching moment.

[0087] Therefore, the present invention makes an improvement. The present invention discloses a method for setting a mixing ratio switching according to a ratio parameter. Two variables k are set in the mixing ratio switching method. gfl and k gfm When it is 1, the inverter is in pure grid-following state. gfm When is 1, the converter is in pure network state. And the two variables satisfy k gfl +k gfm =1. Therefore, when switching control is performed, only one of the k values ​​needs to be controlled. Figure 7 shown.

[0088] Time is the running time, t 0 is the switching start time, g is the switching rate, k 0,gfm For k 0,gfl is the proportion of network control before switching, k gfm is the proportion of network control, k gflis the proportion of network-following control. When the control mode is converted from network-building type to network-following type, the switching rate g is first selected according to the operating conditions. For example, when the system is running smoothly, a smaller rate is selected for mode switching. When the system strength changes suddenly, a larger switching rate is selected. 0 After multiplying Δt by g and k 0,gfm Adding, we can achieve k gfm Increase at a constant speed according to the preset rate, and add an amplitude limit module to ensure that 0≤k gfm ≤1, during the entire operation process, it is always guaranteed to meet k gfm +k gfl =1. When the control mode is changed from the network-following type to the network-building type, the control method is the same as the above process, the difference is that the control object becomes k gfl .

[0089] Example 2

[0090] The present invention discloses a hybrid ratio-adjusted inverter and grid control switching method, based on the PSCAD open source 100% inverter-based generation 9-bus system, which changes all three inverters to hybrid ratio switching control. The system topology is as follows: Figure 8 Based on the above three-machine nine-node system, three application scenarios were set up. Each experiment used the three switching methods introduced in the previous article for comparison to verify the practicality and superiority of the hybrid ratio switching control. The specific application design is as follows.

[0091] Application 1: Control mode switching during system steady-state operation

[0092] The initial state of the system is: VSC 1 (voltage source inverter) is in grid-forming control operation, and VSC 2 and VSC 3 are in grid-following control operation. At the 7th second of system operation, three methods, direct switching, phase angle compensation switching, and mixed ratio switching, are used to convert VSC2 from grid-following control to grid-forming control, and the changes in active power, frequency, voltage, and reactive power during the switching process are observed and compared.

[0093] When the system is operating normally, the active power generated by VSC 3 is 43MW, the reactive power generated is 0, the system frequency is 60Hz, and the voltage at the PCC point is 1.06pu. The system is operating normally. At 7s, VSC 3 switches from GFL to GFM operation. The peak value of active power fluctuation caused by direct switching is as high as 105MW. Phase angle compensation switching is slightly better than direct switching, which is 89MW, while the peak value of active power fluctuation caused by mixed ratio switching is only 43.34MW. The peak value of system frequency fluctuation caused by direct switching is 62Hz, phase angle switching is 61.6Hz, and mixed ratio switching is 60.01Hz. The peak value of voltage fluctuation caused by direct switching is 1.13pu, phase angle switching is 1.12pu, and mixed ratio switching is 1.09pu. The peak value of reactive power fluctuation caused by direct switching is 25.34 MVar, and the valley value is -63.48 MVar. The peak value of reactive power fluctuation caused by switching after adding phase angle compensation is 20.7 MVar, and the valley value is -60 MVar. There is no overshoot in the mixed ratio switching, and it enters a stable operation state, and the steady-state value is 8.36 MVar.

[0094] From the results of Application 1, it can be found that directly switching the inverter control method will cause very large fluctuations in various electrical quantities. The fluctuations caused by adding phase angle compensation are slightly lower than those caused by direct switching, but the effect is not obvious. Compared with the first two switching methods, the fluctuations in various electrical quantities caused by mixed ratio switching can be ignored, achieving the effect of smooth switching control method.

[0095] Application 2: Control mode switching when system fails

[0096] The design system runs normally. At 7s, VSC 1 breaks the line and is out of system operation. At this time, only two GFLs are running in the system, so one GFL needs to be converted to GFM to support the system. Taking VSC 3 as an example, ignoring the signal transmission time, control switching starts at 7s. Observe and compare the changes in active power, frequency, voltage, and reactive power during the switching process.

[0097] The peak value of active power fluctuation caused by direct switching is 118.58MW, and the valley value is 33.51MW. The peak value of active power fluctuation caused by phase angle compensation switching is 117.41MW, and the valley value is 39.72MW. The peak value of active power fluctuation caused by mixed ratio switching is 108.95MW, and its overshoot is significantly smaller than the other two switching methods, and it reaches the steady-state value faster. The peak value of system frequency fluctuation caused by direct switching is 62.46Hz, and the valley value is 58.76Hz. The peak value of system frequency fluctuation caused by phase angle compensation switching is 62Hz, and the valley value is 58.79Hz. The valley value of system frequency fluctuation caused by mixed ratio switching is 59Hz. Due to the launch of VSC 1, the frequency of the system after stabilization is 59.03Hz. The peak value of the voltage at the PCC caused by direct switching is 1.13pu, and the valley value is 0.9pu. The peak value of the voltage at the PCC caused by phase angle compensation switching is 1.12pu, and the valley value is 0.9pu. The valley value of the voltage at the PCC caused by mixed ratio switching is 1.05pu. The peak value of reactive power fluctuation caused by direct switching is 92.28 MVar, and the valley value is -35.13 MVar. The peak value of reactive power fluctuation caused by phase angle compensation switching is 91.2 MVar, and the valley value is -30.33 MVar. The peak value of reactive power fluctuation caused by mixed ratio switching is 44.6 MVar.

[0098] Through the results of application 2, it can be concluded that even when a system failure occurs, the mixed ratio switching can still operate normally, and compared with the first two switching methods, the fluctuations of various electrical quantities caused by the switching process are smaller, and it enters a stable operating state faster, achieving a good smooth switching effect.

[0099] Application of three or two inverters switching control mode simultaneously

[0100] This experiment is designed to switch the control mode of two inverters at the same time. When the system is running normally for 5 seconds, VSC 2 switches from GFM to GFL, and VSC 3 switches from GFL to GFM, and compares the effects of the three different switching methods on system operation.

[0101] The peak value of active power fluctuation caused by direct switching is 128.69MW, and the valley value is 9.97MW. The peak value of active power fluctuation caused by phase angle compensation switching is 97.72MW, and the valley value is 24.24MW. The valley value of active power fluctuation caused by mixed ratio switching is 40.34MW. The peak value of system frequency fluctuation caused by direct switching is 62.65Hz, and the valley value is 58.46Hz. The peak value of system frequency fluctuation caused by phase angle compensation switching is 61.75Hz, and the valley value is 59.33Hz. The peak value of system frequency fluctuation caused by mixed ratio switching is 60.04Hz, and the valley value is 59.96Hz. The peak value of voltage at PCC caused by direct switching is 1.13pu, and the valley value is 0.91pu. The peak value of voltage at PCC caused by phase angle compensation switching is 1.11pu, and the valley value is 0.89pu. The peak value of voltage at PCC caused by mixed ratio switching is 1.07pu, and the valley value is 1.05pu. The peak value of the reactive power fluctuation caused by direct switching is 28.93 MVar, and the valley value is -27.61 MVar. The peak value of the reactive power fluctuation caused by phase angle compensation switching is 14.76 MVar, and the valley value is -182.39 MVar. The valley value of the reactive power fluctuation caused by mixed ratio switching is -31.49 MVar, and the steady-state value is -29.6 MVar.

[0102] Application 3 is the disclosed method, which innovatively proposes the situation of two inverters and switching the control modes at the same time. It can be seen from the simulation image that it is difficult to achieve such a scenario in the actual system through the first two control switching methods. This further highlights the wide application range of hybrid ratio switching.

[0103] Example 3

[0104] In one embodiment of the present disclosure, an inverter and grid control switching system based on hybrid ratio regulation is provided, including:

[0105] An initialization module is used to construct control equivalent circuits of the grid-following inverter and the grid-forming inverter respectively, and to set the hybrid inverter simulation equivalent to the parallel operation of the grid-following inverter and the grid-forming inverter;

[0106] The target output module is used to construct the grid-connected virtual equivalent circuit of the hybrid inverter, obtain the current value of each branch under the grid-following control and grid-forming control respectively, calculate the current value and voltage value required for the operation of the grid-following control and grid-forming control according to the voltage value of the main branch and the parameter value of the grid-connected virtual branch, and output the respective voltage target values ​​through the grid-following control and grid-forming control;

[0107] The switching control module is used to switch the grid-following control and the grid-forming control in proportion, set two proportional parameters, and combine the proportional parameters to proportionally add the respective voltage target values ​​output by the grid-following control and the grid-forming control and input them into the VSC to achieve control.

[0108] Example 4

[0109] In one embodiment of the present disclosure, a non-transitory computer-readable storage medium is provided, wherein the non-transitory computer-readable storage medium is used to store computer instructions. When the computer instructions are executed by a processor, the inverter and grid control switching method based on hybrid ratio regulation is implemented.

[0110] Example 5

[0111] In one embodiment of the present disclosure, an electronic device is provided, including: a processor, a memory, and a computer program; wherein the processor is connected to the memory, the computer program is stored in the memory, and when the electronic device is running, the processor executes the computer program stored in the memory, so that the electronic device executes the inverter and network control switching method based on hybrid ratio regulation.

[0112] The present disclosure is described with reference to the flowcharts and / or block diagrams of the methods, devices (systems), and computer program products according to the embodiments of the present disclosure. It should be understood that each process and / or box in the flowchart and / or block diagram, as well as the combination of the processes and / or boxes in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to generate a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0113] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 A step that specifies a function in one or more boxes.

[0114] Although the above describes the specific implementation methods of the present disclosure in conjunction with the accompanying drawings, it is not intended to limit the scope of protection of the present disclosure. Technical personnel in the relevant field should understand that on the basis of the technical solution of the present disclosure, various modifications or variations that can be made by those skilled in the art without creative work are still within the scope of protection of the present disclosure.

Claims

1. The inverter and grid control switching method based on hybrid ratio regulation is characterized in that: include: Construct the control equivalent circuits of the grid-following inverter and the grid-forming inverter respectively, and set the hybrid inverter simulation equivalent to the parallel operation of the grid-following inverter and the grid-forming inverter; Construct a grid-connected virtual equivalent circuit of the hybrid inverter, obtain the current value of each branch under grid-following control and grid-forming control respectively, calculate the current value and voltage value required for grid-following control and grid-forming control operation according to the voltage value of the main branch and the parameter value of the grid-connected virtual branch, and output the respective voltage target values ​​through grid-following control and grid-forming control; Write the voltage and current equations according to Kirchhoff's law. Both voltage and current are expressed in the frequency domain. (1) (2) (3) (4) Substituting equations (2), (3), and (4) into equation (1), we obtain equation (5): (5) in, V i,gfl is the output voltage of GFL, V o,gfl is the voltage of GFL to ground branch at the connection point of the main circuit in the equivalent circuit diagram, V g is the voltage at the grid connection point, I f,gfl is the total current of the GFL section, I c,gfl is the ground branch current of the GFL part, I o,gfl is the current input from the GFL part to the grid, R c,gfl 、L f,gfl 、R f,gfl 、R f2,gfl 、C f,gfl 、L c,gfl These are all line parameters in the equivalent circuit; The control method adopted by the grid-connected inverter is constant power control. The control target is to input the set power into the control system, use PLL to track the voltage phase of the grid-connected point, output the reference phase angle required for Park transformation and inverse transformation and the frequency required for droop control, and convert the three-phase AC quantity into two-phase DC quantity through Park transformation for control; The power control process in the grid control includes: calculating the instantaneous active power and reactive power input by the inverter to the control system through the measured voltage and current, obtaining the value corresponding to its basic component through a low-pass filter and inputting it into the power control outer loop, and the droop control simulates the characteristics of the synchronous generator by changing the power setting value through the change of the system frequency. The power outer loop control receives the power command value output by the droop control, subtracts the power command value from the power value obtained by the power calculation module, and generates the command value required by the current inner loop through the PI link. After receiving the current command value, the current inner loop outputs the voltage target value of the dq axis through the PI controller, generates the three-phase voltage target value through the Park inverse transformation, and inputs it into the inverter, so that the inverter outputs the corresponding power through modulation; The grid control adopts power synchronous control. In the grid control, the reference phase angle is generated by the power controller so that the inverter can simulate the characteristics of the synchronous generator. The GFM is a constant voltage control and does not rely on the PLL to generate the reference phase angle. The grid control and grid control are switched proportionally, and two proportional parameters are set. Combined with the proportional parameters, the voltage target values ​​output by the grid control and the grid control are added proportionally and input into the VSC to realize control. Set scale parameters k gfl , k gfm , respectively represent the proportion of GFL and GFM, k gfl , k gfm Self-set, no direct relationship with the parameters in the equivalent circuit diagram; k gfl When it is 1, it means the inverter is running in pure grid-following mode. k gfm When it is 1, it indicates that the inverter operates in pure grid-building mode; (6) (7) The current values ​​of each branch of GFL and GFM are calculated by following the grid current calculation and forming the grid current, which are used for inverter control. According to the voltage measured in the main circuit and the parameter value of the virtual branch, the current value and voltage value required for the operation of the GFM control module and the GFL control module are calculated by equations (1)-(5), and the respective voltage target values ​​are output by GFM and GFL control. V ref,gfl , V ref,gfm , through (6) V ref,gfl and V ref,gfm The input is added proportionally to the VSC to achieve control; When the ratio parameter is used for mixing ratio switching control, two variables are set in the mixing ratio switching mode. k gfl and k gfm ,when k gfl When it is 1, the inverter is in pure grid-following state. k gfm When it is 1, the converter is in a pure networking state, and the two variables satisfy k gfl1 + k gfm =1, then when switching control is performed, only one of the k values ​​needs to be controlled; When the control mode is switched from network control to network following control, the switching rate is first selected according to the operating conditions. When the system is running smoothly, a small rate is selected for mode switching. When the system strength changes suddenly, a large switching rate is selected. The difference between the running time and the initial switching time is obtained. The difference is multiplied by the switching rate and then added to the network control ratio before the switching, so that the network control ratio increases uniformly at the preset rate and ensures that 0≤ k gfm ≤1, during the entire operation process, it is always guaranteed to meet k gfm + k gfl =1.

2. The inverter and grid control switching system based on hybrid ratio regulation is characterized by: include: An initialization module is used to construct control equivalent circuits of the grid-following inverter and the grid-forming inverter respectively, and to set the hybrid inverter simulation equivalent to the parallel operation of the grid-following inverter and the grid-forming inverter; The target output module is used to construct the grid-connected virtual equivalent circuit of the hybrid inverter, obtain the current value of each branch under the grid-following control and grid-forming control respectively, calculate the current value and voltage value required for the operation of the grid-following control and grid-forming control according to the voltage value of the main branch and the parameter value of the grid-connected virtual branch, and output the respective voltage target values ​​through the grid-following control and grid-forming control; Write the voltage and current equations according to Kirchhoff's law. Both voltage and current are expressed in the frequency domain. (1) (2) (3) (4) Substituting equations (2), (3), and (4) into equation (1), we obtain equation (5): (5) in, V i,gfl is the output voltage of GFL, V o,gfl is the voltage of GFL to ground branch at the connection point of the main circuit in the equivalent circuit diagram, V g is the voltage at the grid connection point, I f,gfl is the total current of the GFL section, I c,gfl is the ground branch current of the GFL part, I o,gfl is the current input from the GFL part to the grid, R c,gfl 、L f,gfl 、R f,gfl 、R f2,gfl 、C f,gfl 、L c,gfl These are all line parameters in the equivalent circuit; The control method adopted by the grid-connected inverter is constant power control. The control target is to input the set power into the control system, use PLL to track the voltage phase of the grid-connected point, output the reference phase angle required for Park transformation and inverse transformation and the frequency required for droop control, and convert the three-phase AC quantity into two-phase DC quantity through Park transformation for control; The power control process in the grid control includes: calculating the instantaneous active power and reactive power input by the inverter to the control system through the measured voltage and current, obtaining the value corresponding to its basic component through a low-pass filter and inputting it into the power control outer loop, and the droop control simulates the characteristics of the synchronous generator by changing the power setting value through the change of the system frequency. The power outer loop control receives the power command value output by the droop control, subtracts the power command value from the power value obtained by the power calculation module, and generates the command value required by the current inner loop through the PI link. After receiving the current command value, the current inner loop outputs the voltage target value of the dq axis through the PI controller, generates the three-phase voltage target value through the Park inverse transformation, and inputs it into the inverter, so that the inverter outputs the corresponding power through modulation; The grid control adopts power synchronous control. In the grid control, the reference phase angle is generated by the power controller, so that the inverter can simulate the characteristics of the synchronous generator. The GFM is a constant voltage control and does not rely on the PLL to generate the reference phase angle. The switching control module is used to switch the grid-following control and the grid-forming control in proportion, set two proportional parameters, and combine the proportional parameters to proportionally add the respective voltage target values ​​output by the grid-following control and the grid-forming control and input them into the VSC to realize control; Set scale parameters k gfl , k gfm , respectively represent the proportion of GFL and GFM, k gfl , k gfm Self-set, no direct relationship with the parameters in the equivalent circuit diagram; k gfl When it is 1, it means the inverter is running in pure grid-following mode. k gfm When it is 1, it indicates that the inverter operates in pure grid-building mode; (6) (7) The current values ​​of each branch of GFL and GFM are calculated by following the grid current calculation and forming the grid current, which are used for inverter control. According to the voltage measured in the main circuit and the parameter value of the virtual branch, the current value and voltage value required for the operation of the GFM control module and the GFL control module are calculated by equations (1)-(5), and the respective voltage target values ​​are output by GFM and GFL control. V ref,gfl , V ref,gfm , through (6) V ref,gfl and V ref,gfm The input is added proportionally to the VSC to achieve control; When the ratio parameter is used for mixing ratio switching control, two variables are set in the mixing ratio switching mode. k gfl and k gfm ,when k gfl When it is 1, the inverter is in pure grid-following state. k gfm When it is 1, the converter is in a pure networking state, and the two variables satisfy k gfl1 + k gfm =1, then when switching control is performed, only one of the k values ​​needs to be controlled; When the control mode is switched from network control to network following control, the switching rate is first selected according to the operating conditions. When the system is running smoothly, a small rate is selected for mode switching. When the system strength changes suddenly, a large switching rate is selected. The difference between the running time and the initial switching time is obtained. The difference is multiplied by the switching rate and then added to the network control ratio before the switch, so that the network control ratio increases uniformly at the preset rate and ensures that 0≤ k gfm ≤1, during the entire operation process, it is always guaranteed to meet k gfm + k gfl =1.

3. A non-transitory computer-readable storage medium, characterized in that: The non-transitory computer-readable storage medium is used to store computer instructions. When the computer instructions are executed by the processor, the inverter and grid control switching method based on hybrid ratio regulation as described in claim 1 is implemented.

4. An electronic device, characterized in that: include: A processor, a memory and a computer program; wherein the processor is connected to the memory, the computer program is stored in the memory, and when the electronic device is running, the processor executes the computer program stored in the memory to enable the electronic device to implement the inverter and network control switching method based on hybrid ratio regulation as described in claim 1.

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