VSG power current cooperative control method, system and equipment based on improved SOGI under unbalanced power grid and medium

Abstract

The invention relates to a VSG power current cooperative control method, system and device based on an improved SOGI under an unbalanced power grid and a medium, and the method comprises the steps: constructing a VSG control mathematical model, and carrying out the calculation to obtain the instantaneous active power and reactive power provided by a VSG; performing positive and negative sequence separation by adopting an improved SOGI to obtain a positive sequence component and a negative sequence component of voltage and current; during the normal operation state of the system, performing sequence-dividing control of the VSG under the voltage and current double closed loop based on the positive sequence component and the negative sequence component of the voltage and current; during a system fault period, a correlation coefficient is introduced to realize cooperative control of VSG output power constancy and grid-connected current balance, a negative sequence current instruction is obtained through calculation, and virtual impedance is introduced to carry out fault current limiting and carry out positive sequence current instruction calculation under a voltage outer loop. And realizing system stability according to the positive sequence current instruction, the negative sequence current instruction and the reactive power support. The method can be widely applied to the technical field of virtual synchronous generator control.

Description

Technical Field

[0001] This invention belongs to the field of virtual synchronous generator control technology, specifically relating to a method, system, device, and medium for VSG power and current coordinated control based on improved SOGI under unbalanced power grids. Background Technology

[0002] Wind power generation is one of the most technologically mature and scalable power generation methods among new energy power generation. Offshore wind power, with its abundant wind resources, scalable and sustainable development, and small footprint, has become an important support for accelerating the construction of a new power system. However, with the increasing penetration rate of wind power, the equivalent rotational inertia and damping of the power system continue to decline. Due to the lack of physical characteristics of traditional synchronous generators, power electronic equipment is unable to provide effective voltage and frequency support to the grid, threatening the safe and stable operation of the system.

[0003] Virtual synchronous generator (VSG) technology introduces virtual inertia and damping into grid-connected inverters by simulating the operation mechanism of synchronous generators, thereby effectively improving system stability. Constructing a new power system dominated by renewable energy sources requires that renewable energy grid-connected inverters continuously provide high-quality current and stable power to the grid. Under ideal grid conditions, this goal can be achieved by rationally designing VSG control parameters; however, in actual operation, factors such as asymmetrical line parameters and asymmetrical faults lead to grid voltage imbalance, which can easily cause problems such as second-harmonic oscillations in VSG output power and grid current imbalance, seriously affecting power quality and system stability. Therefore, in-depth research on VSG control strategies under unbalanced grid conditions to suppress power oscillations and balance output current is of great significance.

[0004] In existing technologies, VSG control strategies under unbalanced power grids mostly optimize single objectives such as power oscillation suppression or output current balance, making it difficult to achieve multi-objective coordinated optimization control of power and current. Furthermore, more efficient methods for positive / negative sequence separation and current limiting strategies are still needed to balance fast response and fault current suppression. The improved photovoltaic-storage VSG control strategy proposed by Li Shengqing et al. for unbalanced power grids utilizes a dual second-order generalized integrator (DSOGI) to achieve positive / negative sequence separation and employs a sequence control method to optimize active / reactive power oscillation suppression and output current balance. The control flowchart is shown below. Figure 1 As shown.

[0005] However, this control strategy still has the following drawbacks: 1) The response speed and separation effect of positive and negative sequence separation are poor when DC components and high-order harmonics are present in the power grid: The SOGI positive and negative sequence separation method cannot effectively eliminate both when DC components and high-order harmonics are present in the power grid. 2) Only the single objective of active / reactive power oscillation or output current balance is optimized and controlled, without establishing a unified expression to achieve coordinated optimization control of the three; 3) The problem of overcurrent and reactive power support during grid voltage imbalance faults has not been considered. Excessive current will affect the safe and stable operation of the grid.

[0006] The difficulty of existing technologies in solving these problems lies in the fact that when an asymmetrical fault occurs in the power grid, it causes many changes in state variables, such as active / reactive power oscillation, output current imbalance, and output current exceeding limits. It is difficult to achieve unified and coordinated control of power and current while ensuring that the current does not exceed the limits. Traditional positive and negative sequence separation methods are not ideal for power grids with DC components and high-order harmonics, and there is a certain time scale delay in the positive and negative sequence separation process, which affects the dynamic performance and safe and stable operation of the system. Summary of the Invention

[0007] To address the problems of power oscillation, output current imbalance, and current exceeding limits caused by grid voltage imbalance in offshore oil and gas field power grids, the present invention aims to provide a VSG power and current coordinated control method, system, device, and medium based on an improved SOGI for unbalanced power grids. The VSG power and current coordinated control strategy based on the improved SOGI improves the dynamic performance of the system, enhances the system's anti-interference capability, and increases the system's stability.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a VSG power-current coordinated control method based on an improved SOGI for unbalanced power grids, comprising: A mathematical model for VSG control is constructed to analyze the causes of VSG output power oscillation and grid current imbalance under unbalanced power grid. The instantaneous active and reactive power that VSG needs to provide is calculated based on the grid voltage and current, as well as the reference values ​​of active and reactive power. An improved SOGI positive and negative sequence separation method is used to separate the positive and negative sequence components of the grid voltage and current, obtaining the positive and negative sequence components of the voltage and current. During normal system operation, the VSG is controlled sequentially under dual closed-loop voltage and current based on the positive and negative sequence components of voltage and current. During system failures, correlation coefficients are introduced to achieve coordinated control of constant VSG output power and grid-connected current balance, and negative sequence current commands are calculated. At the same time, virtual impedance is introduced for fault current limiting and positive sequence current commands are calculated under the voltage outer loop. System stability is achieved by combining positive sequence current commands, negative sequence current commands and reactive power support.

[0009] Secondly, the present invention provides a VSG power-current coordinated control system based on an improved SOGI for unbalanced power grids, comprising: The model calculation module is used to construct the VSG control mathematical model, analyze the causes of VSG output power oscillation and grid current imbalance under unbalanced power grid, and calculate the instantaneous active and reactive power that the VSG needs to provide based on the grid voltage and current and active and reactive power reference values. The positive and negative sequence separation module is used to separate the positive and negative sequence of grid voltage and current using the improved SOGI positive and negative sequence separation method to obtain the positive and negative sequence components of voltage and current. The normal operation state control module is used to perform sequential control of the VSG under the dual closed loop of voltage and current based on the positive and negative sequence components of voltage and current during the normal operation state of the system. The fault operation status control module is used to achieve coordinated control of constant VSG output power and grid current balance by introducing correlation coefficients during system faults, and to calculate the negative sequence current command. At the same time, virtual impedance is introduced for fault current limiting and positive sequence current command is calculated under the voltage outer loop. The system stability is achieved by combining the positive sequence current command, the negative sequence current command and reactive power support.

[0010] Thirdly, the present invention provides a computer-readable storage medium for storing one or more programs, said one or more programs including instructions that, when executed by a computing device, cause the computing device to perform any method.

[0011] Fourthly, the present invention provides a computing device comprising: one or more processors and a memory, wherein the memory stores one or more programs and is configured to be executed by the one or more processors, the one or more programs including instructions for performing any method.

[0012] The present invention has the following advantages due to the adoption of the above technical solutions: 1. This invention employs a VSG power current coordinated control strategy based on an improved SOGI, which effectively suppresses DC components and higher harmonics through an improved SOGI positive and negative sequence separation method. 2. This invention achieves coordinated power and current control by establishing a unified expression under different control objectives; 3. This invention effectively suppresses fault current through virtual impedance. Simulation results show that compared with the traditional VSG control method under unbalanced power grids, the proposed method has a faster positive and negative sequence separation speed and better filtering effect. It can effectively suppress fault current while suppressing power oscillation and balancing output current, ensuring the safe and stable operation of the system.

[0013] Therefore, this invention is applicable to offshore wind power grid connection scenarios considering grid voltage imbalance. Attached Figure Description

[0014] Various other advantages and benefits will become apparent to those skilled in the art upon reading the following detailed description of preferred embodiments. The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. In the drawings: Figure 1 It is a flowchart of existing technology control. Figure 2 This is a schematic diagram of the VSG power current coordinated control process based on the improved SOGI provided in the embodiments of the present invention; Figure 3 This is a diagram of the system main circuit topology and VSG control structure provided in the embodiments of the present invention; Figure 4 It is a traditional SOGI structure diagram; Figure 5 This is provided by the embodiments of the present invention. and Bode plot; Figure 6 This is a block diagram of the improved SOGI structure provided in the embodiments of the present invention; Figure 7 This is an improved version provided by the embodiments of the present invention. and Bode plot; Figure 8 This is a diagram of the overall structure for separating positive and negative sequences provided in an embodiment of the present invention; Figure 9 This is an overall framework diagram provided in the embodiments of the present invention; Figure 10 This is a comparison of the effects of three positive and negative order separation methods provided in the embodiments of the present invention; Figures 11a-11b This is the high-order harmonic filtering effect based on the traditional VSG control strategy provided in the embodiments of the present invention; wherein, Figure 11a This is a waveform diagram of the power grid current. Figure 11b For FFT analysis plot; Figures 12a-12b This refers to the high-order harmonic filtering effect of the VSG control strategy based on the improved SOGI provided in the embodiments of the present invention; wherein, Figure 12a This is a waveform diagram of the power grid current. Figure 12b For FFT analysis plot; Figures 13a-13c These are waveforms of grid voltage and current, and VSG output active and reactive power under different control objectives provided in embodiments of the present invention; wherein, Figure 13a To balance the current;Figure 13b The reactive power is constant; Figure 13c The active power is constant; Figures 14a-14c This is a simulation diagram of the power-current coordinated control strategy provided in an embodiment of the present invention, wherein, Figure 14a This refers to the grid current. Figure 14b The waveforms of reactive and active power output from the VSG are shown. Figure 14c Correlation coefficient ; Figure 15a and Figure 15b This is a simulation diagram provided in an embodiment of the present invention without employing a current limiting strategy; wherein, Figure 15a These are the voltage and current waveforms of the power grid. Figure 15b The output waveforms of reactive and active power of the VSG; Figure 16a and Figure 16b This is a simulation diagram of a current limiting strategy provided in an embodiment of the present invention, wherein... Figure 16a These are the voltage and current waveforms of the power grid. Figure 16b The waveforms of reactive and active power output from the VSG are shown. Detailed Implementation

[0015] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention are within the scope of protection of the present invention.

[0016] It should be noted that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of exemplary embodiments according to the invention. As used herein, the singular form is intended to include the plural form as well, unless the context clearly indicates otherwise. Furthermore, it should be understood that when the terms "comprising" and / or "including" are used in this specification, they indicate the presence of features, steps, operations, devices, components, and / or combinations thereof.

[0017] When a voltage imbalance fault occurs in the power grid, it can cause problems such as VSG output power oscillation, grid current imbalance, and fault current exceeding the limit, which seriously affect the stable operation of the system.

[0018] Based on this, in some embodiments of the present invention, a VSG power current coordinated control based on improved SOGI is proposed under unbalanced power grids. The positive and negative sequence separation method of improved SOGI is used to effectively suppress DC components and high-order harmonics; and a unified expression under different control objectives is established to realize power current coordinated control; finally, the fault current is effectively suppressed through virtual impedance.

[0019] Correspondingly, in other embodiments of the present invention, a VSG power current coordinated control system, device, and medium based on improved SOGI is provided for unbalanced power grids.

[0020] Example 1 like Figure 2 As shown, this invention provides a VSG power-current coordinated control method based on improved SOGI under unbalanced power grids, which includes the following steps: 1) Construct a mathematical model for VSG control, analyze the causes of VSG output power oscillation and grid current imbalance under unbalanced power grid, and calculate the instantaneous active and reactive power that VSG needs to provide based on the grid voltage and current and the reference values ​​of active and reactive power. 2) An improved SOGI positive and negative sequence separation method is used to separate the positive and negative sequence components of the grid voltage and current, obtaining the positive and negative sequence components of the voltage and current; 3) During normal system operation, the VSG is controlled in sequence under dual closed-loop voltage and current based on the positive and negative sequence components of voltage and current. 4) During system faults, correlation coefficients are introduced to achieve coordinated control of constant VSG output power and grid-connected current balance, and negative sequence current commands are calculated. At the same time, virtual impedance is introduced for fault current limiting and positive sequence current commands are calculated under the voltage outer loop. System stability is achieved by combining positive sequence current commands, negative sequence current commands and reactive power support.

[0021] Furthermore, in step 1) above, such as Figure 3 As shown, when constructing the VSG control mathematical model, it is necessary to first establish the system main circuit topology, and then establish the VSG control mathematical model.

[0022] Traditional VSGs are primarily based on second-order synchronous generator models. Therefore, the VSG control mathematical model established in this embodiment includes two parts: an active-frequency loop and a reactive-voltage loop. The expression for the active-frequency loop is as follows: (1) In the formula: This is a virtual moment of inertia; and These are the angular frequency and rated angular frequency of the VSG internal potential, respectively. The phase of the internal potential of the VSG; The damping coefficient; This is a reference value for active power during normal operation. This represents the actual value of active power.

[0023] The expression for the reactive-voltage loop is: (2) In the formula: This represents the virtual internal potential. The inertia coefficient is used to simulate excitation regulation; This is the reactive power regulation coefficient; and These are the actual and reference values ​​of the VSG output voltage, respectively. This is a reference value for reactive power during normal operation. This is the actual value of reactive power; It is a differential operator.

[0024] Furthermore, step 2) above includes the following steps: 2.1) Analyze the transfer function of traditional SOGI to determine the sources of its detection error.

[0025] like Figure 4 The diagram shown is a block diagram of a traditional second-order generalized integrator (SOGI). The transfer function of SOGI is: (3) (4) In the formula: and These are a second-order bandpass filter and a low-pass filter, respectively. To track the angular frequency of a specific AC signal; The integrator gain of SOGI is determined by the filtering effect and response characteristics of SOGI, and is generally... The value of is in nearby; This is the input signal for SOGI. For SOGI and Output signals with the same amplitude and phase For SOGI and Output signals with the same amplitude but a phase lag of 90 degrees.

[0026] like Figure 5 As shown, this is SOGI and The Bode plot of the transfer function, where, This represents a second-order bandpass filter. This represents a low-pass filter. It can be seen that the traditional SOGI has a good filtering effect on higher harmonics, but when... When a DC component is present, It is susceptible to the influence of DC components, which can lead to certain errors in signal detection.

[0027] 2.2) Based on the analysis results, a difference node is introduced on the basis of the traditional SOGI, and a new SOGI is added to obtain the improved SOGI.

[0028] like Figure 6 As shown, based on the above analysis, the improved SOGI proposed in this embodiment introduces a difference node and adds a new SOGI on the basis of the traditional SOGI, enabling it to simultaneously suppress the DC component and higher harmonics of the input signal. Specifically, the transfer function of the improved SOGI is: (5) (6) In the formula: To improve the integrator gain of the SOGI; To increase the integrator gain of SOGI; To track the angular frequency of a specific AC signal; To increase the internal angular frequency of SOGI, under normal circumstances .

[0029] like Figure 7 As shown, this is the improved SOGI. and Bode plot of the transfer function. Comparison. Figure 5 and Figure 7 It can be seen that the improvements were made before and after. No difference, but By changing the original low-pass filter to a band-pass filter, the improved SOGI can simultaneously suppress the DC component and high-order harmonics of the input signal, and has a better filtering effect on high-order harmonics.

[0030] 2.3) As Figure 8 As shown, the improved SOGI is used to decouple the positive and negative sequence components of the grid voltage, current and grid connection point voltage and current, achieving rapid separation while effectively suppressing harmonics.

[0031] Furthermore, in step 3) above, when calculating the positive sequence current command under the voltage outer loop based on the instantaneous active and reactive power values ​​and the positive sequence components of voltage and current provided by VSG, conventional methods can be used, which will not be elaborated upon in this invention.

[0032] Furthermore, step 4) above includes the following steps: 4.1) By introducing correlation coefficients, we can achieve coordinated control of constant VSG output power and grid-connected current balance, and calculate the negative sequence current command. 4.2) Introduce virtual impedance for fault current limiting and calculate positive sequence current command under voltage outer loop. The system stability is achieved by combining positive sequence current command, negative sequence current command and reactive power support.

[0033] Furthermore, step 4.1 above includes the following steps: 4.1.1) Establish negative sequence current command expressions for suppressing power oscillations and balancing output current under different control objectives.

[0034] Figure 1 In the three-phase three-wire system shown, when an unbalanced grid voltage fault occurs, the expressions for the unbalanced grid voltage and current in the dq coordinate system are as follows: (7) In the formula: The grid voltage is represented by its dq-axis component. The positive-sequence grid voltage dq-axis component. This represents the dq-axis component of the negative-sequence grid voltage. The angular frequency of the power grid. The dq-axis component of the grid current. The positive-sequence grid current dq-axis component. This represents the dq-axis component of the negative sequence grid current.

[0035] According to instantaneous power theory, when a voltage imbalance fault occurs in the power grid, the instantaneous active power injected into the grid by the VSG is... and reactive power It can be represented as: (8) In the formula: and These are the DC components of instantaneous active power and reactive power, respectively. and The amplitude of active power fluctuation; and This represents the amplitude of reactive power fluctuations.

[0036] Writing formula (8) in vector form, we get: (9) In the formula: This represents the d-axis component of the positive-sequence grid voltage. This represents the d-axis component of the negative-sequence grid voltage. This represents the q-axis component of the positive-sequence grid voltage. This represents the q-axis component of the negative-sequence grid voltage. This represents the d-axis component of the positive-sequence grid current. This represents the d-axis component of the negative-sequence grid current. This represents the q-axis component of the positive-sequence grid current. This represents the q-axis component of the negative-sequence grid current.

[0037] As can be seen from equation (9), under unbalanced voltage conditions, the negative sequence components of the grid voltage and current are not zero, resulting in double frequency fluctuations in the instantaneous active and reactive power of the VSG and an imbalance in the output current.

[0038] In this embodiment, different control objectives need to be determined according to actual needs. For example, the control objectives include the following three types: suppressing active power oscillation, balancing output current, or suppressing reactive power oscillation; and then, different control strategies are adopted according to different control objectives. This will be explained in detail below: When the control objective is to suppress active power oscillations and maintain a constant active power, it is necessary to set the value in equation (9). ,Right now: (10) Assuming negative sequence current and It is used only to suppress power fluctuations and does not participate in the total power. and Given the given information, we can solve for the following: (11) When the control objective is to balance the three-phase output current, the negative sequence current reference value must be set to 0, i.e. .

[0039] When the control objective is to suppress reactive power oscillations and maintain a constant reactive power, it is necessary to set the value in equation (9). ,Right now: (12) Solving for the given information yields: (13) In the formula, The d-axis component of the negative sequence current command value; The q-axis component represents the negative sequence current command value.

[0040] 4.1.2) By introducing correlation coefficients, a connection is established between the negative sequence current command expressions for suppressing power oscillations and balancing output current under different control objectives, resulting in a unified expression for power current coordinated control.

[0041] The three control objectives mentioned above are independent of each other. To achieve coordinated control of current and power, this embodiment introduces a correlation coefficient. Its value ranges from [-1, 1]. A unified expression for the negative sequence current command under these three target controls is established: (14) From the above analysis, it can be seen that when When = -1, reactive power is kept constant; when When =0, three-phase current balance is achieved; when When the value is 1, constant active power is achieved. This can be achieved by adjusting... The value can adjust the continuous changes between the three objectives of constant output active and reactive power and three-phase current balance, so as to achieve a comprehensive and coordinated control effect.

[0042] 4.1.3) Based on the instantaneous active and reactive power values ​​provided by VSG and the negative sequence components of grid voltage and current, the negative sequence current command is calculated using a unified expression for power and current coordinated control.

[0043] In step 4.2) above, to maintain system stability during a fault, this embodiment employs two strategies: First, a virtual impedance is introduced into the system for fault current limiting. This is because a virtual impedance can increase the equivalent impedance in the system, thereby reducing overcurrent during a fault. However, since the increase in virtual impedance alters the system's impedance characteristics, it can easily lead to VSG instability; therefore, the virtual impedance is only connected when it is necessary to suppress overcurrent. Second, reactive power support is added to the system.

[0044] Specifically, such as Figure 9 As shown, it includes the following steps: 4.2.1) Determine the required reactive current reference value based on grid connection requirements, and calculate the active power reference value and reactive power reference value during the fault period; 4.2.2) Using the VSG control mathematical model, the instantaneous active power and reactive power are calculated based on the active power reference value and reactive power reference value during the fault period; 4.2.3) Introduce virtual impedance into the system and calculate the voltage drop caused by the virtual impedance to update the d-axis and q-axis components of the positive sequence grid voltage; The expression for calculating the voltage drop caused by the virtual impedance in the dq coordinate system is as follows: (15) In the formula: For virtual resistance, For virtual inductance; and These are the dq-axis components of the grid current, respectively. and These are the dq-axis components of the voltage drop caused by adding virtual impedance; 4.2.4) Based on the updated positive-sequence grid voltage d-axis and q-axis components and the instantaneous active and reactive power provided by VSG, calculate the positive-sequence current command under the voltage outer loop; 4.2.5) Based on the positive sequence current command, perform VSG control under dual closed-loop voltage and current control; 4.2.6) Based on the negative sequence current command, PI control with feedforward is used until the system stabilizes.

[0045] Furthermore, step 4.2.1 above includes the following steps: ① Determine the required reactive current reference value according to grid connection requirements, as shown in the following expression: (16) In the formula: This is the per-unit value of the grid connection point voltage; This is the rated value of the power grid current; This provides a reference value for reactive current.

[0046] ② Calculate the reference value of active current based on reactive current, using the following expression: (17) ③ Calculate the active power reference value and reactive power reference value based on the reactive current reference value and the active current reference value, input them into the VSG control mathematical model in step 1), and obtain the instantaneous active power value and reactive power value during the fault period.

[0047] The reference values ​​for active power and reactive power are as follows: (18) In the formula: The d-axis component of the VSG output voltage; and These are the reference values ​​for the active and reactive power output of the VSG during a fault.

[0048] Example 2 To verify the effectiveness of the control strategy proposed in this invention, a system was built using MATLAB / Simulink. Figure 3 The system parameters of the VSG simulation model shown are shown in Table 1.

[0049] Table 1 Simulation Parameters

[0050] To verify the effectiveness of the improved SOGI proposed in this invention for separating positive and negative sequence voltages, simulation condition one was set: during the period of 0.8s to 1.4s, the voltages of phases a and b dropped to 50% of their rated values, while the voltage of phase c remained at its rated value. Three methods—notch filter (NF), traditional SOGI, and improved SOGI—were used to separate the positive and negative sequence voltages of the grid. Figure 10 The graph compares the three separation methods. It shows that NF, SOGI, and the improved SOGI can all effectively separate the positive sequence of the grid voltage. During a fault, the separation times for NF, SOGI, and the improved SOGI are 26ms, 20ms, and 17.5ms, respectively; at the end of the fault, the separation times are 25ms, 19.8ms, and 18.7ms, respectively. It is evident that the improved SOGI has the shortest separation time and produces a smoother, more stable waveform during the separation process. This demonstrates the superiority of using the improved SOGI for positive and negative sequence separation during grid voltage dips.

[0051] To further verify the filtering effect of the improved SOGI on higher harmonics, simulation condition two was set up: -17th and +19th harmonics were injected into the grid at 0.8s. The grid current waveforms and FFT analyses under the traditional VSG control strategy and the improved SOGI VSG control strategy were compared. Figure 11a , Figure 11b , Figure 12a and Figure 12b As shown in the figure, under the traditional VSG control strategy, the current waveform exhibits some distortion, with the THD of the a-phase output current reaching 2.74% and the amplitudes of the -17th and +19th harmonics both exceeding 0.9%. However, with the improved SOGI VSG control strategy, the current waveform is smoother and exhibits no significant distortion. The THD of the a-phase output current decreases to 1.57%, and the amplitudes of the -17th and +19th harmonics are both below 0.6%. Simulation results demonstrate that the improved SOGI VSG control strategy provides better filtering of higher harmonics.

[0052] To verify the effectiveness of the proposed control strategy under different target control conditions, simulation condition three was set: during the period of 0.6s to 1s, the voltages of phases a and b dropped to 50% of their rated values, while the voltage of phase c remained at its rated value. Simulation waveforms under different control targets are shown below. Figures 13a-13c As shown.

[0053] Figure 13a The simulation results are presented to balance the grid current control objective. When the grid voltage is unbalanced, the output grid current is balanced, with a maximum current amplitude of 97.5A and a minimum current amplitude of 96.9A. At this time, the active power and reactive power still fluctuate at twice the grid fundamental frequency, and the peak-to-peak values ​​of the active power and reactive power fluctuations are 31.2 kW and 1.3 kvar, respectively.

[0054] Figure 13b The simulation results are for a constant reactive power control objective. When the grid voltage is unbalanced, the peak-to-peak value of reactive power fluctuation decreases to 0.72 kvar, while active power fluctuates with a peak-to-peak value of 36.5 kW; the output grid-connected current is unbalanced, with a maximum current amplitude of 119.5 A.

[0055] Figure 13c The simulation results are for a constant active power control objective. When the grid voltage is unbalanced, the peak-to-peak value of active power fluctuation decreases to 30.7 kW, while reactive power fluctuates with a peak-to-peak value of 7.5 kvar; the output grid-connected current is unbalanced, and the maximum current amplitude is 121.4 A.

[0056] To verify the comprehensive coordinated control effect under different target control conditions, simulation condition four was set up: the simulation duration was 3s, with 0.5s~1s for constant reactive power control, 1.2s~1.7s for grid current balancing control, and 1.9s~2.5s for constant active power control. The 1s~1.2s and 1.7s~1.9s periods represent the transition phases between different control targets. The simulation waveforms are shown below. Figures 14a-14c As shown. When the correlation coefficient When the range [-1,1] changes dynamically, the power / current coordinated control strategy can achieve continuous operation and smooth switching between three different control objectives: constant reactive power control, grid current control, and constant active power control.

[0057] To verify the current-limiting effect of virtual impedance and the supporting role of VSG output reactive power in the power grid, simulation condition five was set up based on the current control of the power grid: the simulation duration is 1.5s, during which the voltages of phases a and b drop to 50% of their rated values ​​from 0.5s to 0.9s, while the voltage of phase c remains at its rated value. The simulation waveforms with and without current-limiting strategies are compared. Figures 15a-15b and Figures 16a-16b As shown, without current limiting, during the 0.5s to 0.9s fault period, the steady-state overcurrent reaches 97.2A, which is 1.51 times the rated current amplitude; at 0.5s when the grid voltage just fails, the transient current reaches 321.2A, which is 5 times the rated current amplitude; and at 0.9s when the grid voltage recovers, the transient current reaches 395.3A, which is 6.1 times the rated current amplitude. Furthermore, both active and reactive power exhibit significant fluctuations and overshoot at both the fault occurrence and recovery times.

[0058] When the current limiting strategy is adopted, the transient current is 96.3A when the grid voltage fault occurs at 0.5s, and the fault duration is only 7ms. Moreover, the fault current is limited to below the current over-limit value during both the fault occurrence and recovery period. At the same time, the fluctuation and overshoot of active and reactive power are small at the time of fault occurrence and recovery, and the converter generates additional reactive power during the fault, ensuring the reactive power support function for the grid.

[0059] Example 3 The above-described Embodiment 1 provides a VSG power and current coordinated control method based on improved SOGI under unbalanced power grids. Correspondingly, this embodiment provides a VSG power and current coordinated control system based on improved SOGI under unbalanced power grids. The system provided in this embodiment can implement the VSG power and current coordinated control method based on improved SOGI under unbalanced power grids of Embodiment 1. This system can be implemented through software, hardware, or a combination of both. For example, the system may include integrated or separate functional modules or units to execute the corresponding steps in the methods of Embodiment 1. Since the system in this embodiment is basically similar to the method embodiment, the description process in this embodiment is relatively simple. Relevant details can be found in the description of Embodiment 1. The system embodiment provided in this embodiment is merely illustrative.

[0060] The VSG power-current coordinated control system based on improved SOGI under unbalanced power grids provided in this embodiment includes: The model calculation module is used to construct the VSG control mathematical model, analyze the causes of VSG output power oscillation and grid current imbalance under unbalanced power grid, and calculate the instantaneous active and reactive power that the VSG needs to provide based on the grid voltage and current and active and reactive power reference values. The positive and negative sequence separation module is used to separate the positive and negative sequence of grid voltage and current using the improved SOGI positive and negative sequence separation method to obtain the positive and negative sequence components of voltage and current. The normal operation state control module is used to perform sequential control of the VSG under the dual closed loop of voltage and current based on the positive and negative sequence components of voltage and current during the normal operation state of the system. The fault operation status control module is used to achieve coordinated control of constant VSG output power and grid current balance by introducing correlation coefficients during system faults, and to calculate the negative sequence current command. At the same time, virtual impedance is introduced for fault current limiting and positive sequence current command is calculated under the voltage outer loop. The system stability is achieved by combining the positive sequence current command, the negative sequence current command and reactive power support.

[0061] Example 4 This embodiment provides a processing device corresponding to the VSG power and current collaborative control method based on improved SOGI under unbalanced power grids provided in Embodiment 1. The processing device can be a processing device for a client, such as a mobile phone, laptop, tablet computer, desktop computer, etc., to execute the method of Embodiment 1.

[0062] The processing device includes a processor, a memory, a communication interface, and a bus. The processor, memory, and communication interface are connected via the bus to enable communication between them. The memory stores a computer program that can run on the processor. When the processor runs the computer program, it executes the VSG power and current coordinated control method based on improved SOGI under unbalanced power grids provided in Embodiment 1.

[0063] Preferably, the memory may be high-speed random access memory (RAM), and may also include non-volatile memory, such as at least one disk storage device.

[0064] Preferably, the processor can be any type of general-purpose processor such as a central processing unit (CPU) or a digital signal processor (DSP), and there is no limitation herein.

[0065] Example 5 The VSG power and current coordinated control method based on improved SOGI under unbalanced power grid in Embodiment 1 can be specifically implemented as a computer program product. The computer program product may include a computer-readable storage medium on which computer-readable program instructions are loaded for executing the VSG power and current coordinated control method based on improved SOGI under unbalanced power grid in Embodiment 1.

[0066] A computer-readable storage medium can be a tangible device that holds and stores instructions for use by an instruction execution device. A computer-readable storage medium can be, for example, but not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any combination thereof.

[0067] Those skilled in the art will understand that embodiments of the present invention can be provided as methods, systems, or computer program products. Therefore, the present invention can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, the present invention can take the form of a computer program product embodied on one or more computer-usable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. This invention is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of the invention. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, generate instructions for implementing the flowchart illustrations and / or block diagrams. Figure 1 One or more processes and / or boxes Figure 1 A device that provides the functions specified in one or more boxes. These computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to function in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The function specified in one or more boxes. These computer program instructions may also be loaded onto a computer or other programmable data processing equipment to cause a series of operational steps to be performed on the computer or other programmable equipment to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable equipment for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0068] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A VSG power-current coordinated control method based on improved SOGI under unbalanced power grids, characterized in that, include: A mathematical model for VSG control is constructed to analyze the causes of VSG output power oscillation and grid current imbalance under unbalanced power grid. The instantaneous active and reactive power that VSG needs to provide is calculated based on the grid voltage and current, as well as the reference values ​​of active and reactive power. An improved SOGI positive and negative sequence separation method is used to separate the positive and negative sequence components of the grid voltage and current, obtaining the positive and negative sequence components of the voltage and current. During normal system operation, the VSG is controlled sequentially under dual closed-loop voltage and current based on the positive and negative sequence components of voltage and current. During system failures, correlation coefficients are introduced to achieve coordinated control of constant VSG output power and grid-connected current balance, and negative sequence current commands are calculated. At the same time, virtual impedance is introduced for fault current limiting and positive sequence current commands are calculated under the voltage outer loop. System stability is achieved by combining positive sequence current commands, negative sequence current commands and reactive power support.

2. The VSG power-current coordinated control method based on improved SOGI under unbalanced power grids as described in claim 1, characterized in that, The improved SOGI positive and negative sequence separation method is used to separate the positive and negative sequence components of the grid voltage and current, obtaining the positive and negative sequence components of the voltage and current, including: The transfer function of traditional SOGI is analyzed to determine the sources of its detection error; Based on the analysis results, a difference node is introduced on the basis of the traditional SOGI, and a new SOGI is added to obtain the improved SOGI; The improved SOGI is used to decouple the positive and negative sequence components of grid voltage, current, and grid connection point voltage and current.

3. The VSG power-current coordinated control method based on improved SOGI under unbalanced power grids as described in claim 2, characterized in that, The transfer function of the improved SOGI is: In the formula: To improve the integrator gain of the SOGI; To increase the integrator gain of SOGI; To track the angular frequency of a specific AC signal; To increase the internal angular frequency of SOGI.

4. The VSG power-current coordinated control method based on improved SOGI under unbalanced power grids as described in claim 1, characterized in that, The introduction of correlation coefficients achieves coordinated control of constant VSG output power and grid-connected current balance, and calculates the negative sequence current command, including: Establish negative sequence current command expressions for suppressing power oscillations and balancing output current under different control objectives; By introducing correlation coefficients, a connection is established between the negative-sequence current command expressions for suppressing power oscillations and balancing output current under different control objectives, resulting in a unified expression for power current coordinated control. Based on the instantaneous active and reactive power values ​​provided by VSG and the negative sequence components of grid voltage and current, the negative sequence current command is calculated using a unified expression for power-current coordinated control.

5. The VSG power-current coordinated control method based on improved SOGI under unbalanced power grids as described in claim 4, characterized in that, The unified expression for the negative sequence current command is: In the formula, The d-axis component of the negative sequence current command value; The q-axis component of the negative sequence current command value; The correlation coefficient, This represents the d-axis component of the positive-sequence grid voltage. This represents the d-axis component of the negative-sequence grid voltage. This represents the q-axis component of the positive-sequence grid voltage. This represents the q-axis component of the negative-sequence grid voltage. and It represents the DC components of instantaneous active and reactive power.

6. The VSG power-current coordinated control method based on improved SOGI under unbalanced power grids as described in claim 1, characterized in that, The introduction of virtual impedance for fault current limiting and calculation of positive sequence current command under the voltage outer loop, along with the combined effect of positive sequence current command, negative sequence current command, and reactive power support to achieve system stability, includes: The required reactive current reference value is determined based on grid connection requirements, and the active power reference value and reactive power reference value during the fault period are calculated. Using the VSG control mathematical model, the instantaneous active and reactive power provided by the VSG are calculated based on the active power reference value and reactive power reference value during the fault period; A virtual impedance is introduced into the system, and the voltage drop caused by the virtual impedance is calculated to update the positive sequence component of the grid voltage. Based on the positive sequence component of the updated grid voltage and the instantaneous active and reactive power provided by VSG, the positive sequence current command under the voltage outer loop is calculated. Based on the positive sequence current command, VSG control with voltage and current dual closed loop is adopted; Based on the negative sequence current command, PI control with feedforward is used until the system stabilizes.

7. The VSG power-current coordinated control method based on improved SOGI under unbalanced power grids as described in claim 6, characterized in that, The voltage drop caused by the virtual impedance is expressed as follows: In the formula: For virtual resistance, For virtual inductance; and These are the dq-axis components of the grid current, respectively. and These are the dq-axis components of the voltage drop caused by adding virtual impedance.

8. A VSG power-current coordinated control system based on improved SOGI under unbalanced power grids, characterized in that, include: The model calculation module is used to construct the VSG control mathematical model, analyze the causes of VSG output power oscillation and grid current imbalance under unbalanced power grid, and calculate the instantaneous active and reactive power that the VSG needs to provide based on the grid voltage and current and active and reactive power reference values. The positive and negative sequence separation module is used to separate the positive and negative sequence of grid voltage and current using the improved SOGI positive and negative sequence separation method to obtain the positive and negative sequence components of voltage and current. The normal operation state control module is used to perform sequential control of the VSG under the dual closed loop of voltage and current based on the positive and negative sequence components of voltage and current during the normal operation state of the system. The fault operation status control module is used to achieve coordinated control of constant VSG output power and grid current balance by introducing correlation coefficients during system faults, and to calculate the negative sequence current command. At the same time, virtual impedance is introduced for fault current limiting and positive sequence current command is calculated under the voltage outer loop. The system stability is achieved by combining the positive sequence current command, the negative sequence current command and reactive power support.

9. A computer-readable storage medium for storing one or more programs, characterized in that, The one or more programs include instructions that, when executed by a computing device, cause the computing device to perform any of the methods described in claims 1 to 7.

10. A computing device, characterized in that, include: One or more processors and a memory, wherein the memory stores one or more programs and is configured to be executed by the one or more processors, the one or more programs including instructions for performing any of the methods described in claims 1 to 7.

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