Multi-target double-sequence current injection method for power maximization output under three-phase imbalance fault

Abstract

The invention discloses a power maximization output-oriented multi-target double-sequence current injection method under a three-phase imbalance fault, and the method comprises the following steps: S1, obtaining a three-phase voltage # imgabs0 # and a current # imgabs1 # of a PCC point after a fault steady state, and correspondingly obtaining a positive and negative sequence voltage # imgabs2 # and a current # imgabs3 # of the PCC point under a dq coordinate system; s2, according to the # imgabs 4 # and the # imgabs 5 #, calculating to obtain positive and negative sequence voltage # imgabs 6 # and phase angle difference # imgabs 7 # of the power grid in the current fault scene; s3, according to # imgabs8 # and # imgabs9 #, finding out positive and negative sequence current reference values of a dq axis in a 3-D lookup table; and S4, generating an inverter modulation signal through positive and negative sequence separation control according to the found dq-axis positive and negative sequence current reference values.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and particularly to a multi-objective dual-sequence current injection method under three-phase unbalanced faults for maximizing power output. Background Art

[0002] In recent years, renewable energy power generation with power electronic converters as the interface has been rapidly replacing fossil fuel power generation with large synchronous generators as the interface. However, due to the extensive use of voltage-sensitive devices, the voltage quality problem of the distribution network has become increasingly prominent. Among them, voltage unbalance is one of the more serious disturbances in the current distribution network, which will cause a series of hazards such as increased line losses and equipment losses. To ensure the reliability and security of the future power grid under a high proportion of renewable energy, the grid-connected inverter (GCI), as the interface between distributed new energy and the power grid, is expected to provide auxiliary functions similar to those of large synchronous generators. Among them, fault ride-through (FRT) is one of the key auxiliary functions.

[0003] Voltage faults are mainly divided into three-phase balanced fault conditions and unbalanced faults. Unbalanced faults are the most common power grid faults caused by natural reasons, conductor aging, and human errors. In terms of the actual occurrence probability, unbalanced faults account for more than 90% of the total distribution network faults. For traditional power grids, during unbalanced faults, large synchronous generators can inject short-circuit currents of 6 - 8 pu to support the grid voltage and activate relay protection actions. However, the GCI based on power devices can only provide a limited current-carrying capacity of 1 - 2 pu, which increases the complexity of FRT. Therefore, how to achieve the optimal operation of the GCI during unbalanced faults with limited capacity is one of the key challenges faced by power electronic power systems.

[0004] Reliable operation and high-quality power supply are the key objectives of GCI operation during unbalanced faults. "Reliable operation" means that when a fault occurs, the grid-connected system should be ensured to remain connected to the power grid, and the inverter should be protected from overcurrent on the AC side and overvoltage on the DC side. "High-quality power supply" expects the GCI to inject reasonable reactive current according to the voltage drop depth and relevant standard requirements to support the grid voltage, reduce active / reactive power oscillation, and suppress harmonics.

[0005] Early studies achieved the above goals by installing additional equipment. On the DC side, power imbalance can cause the DC bus voltage to rise, affecting the reliability of power electronic devices. Using a braking chopper to absorb the remaining energy or installing an energy storage system to store the remaining energy helps balance the energy on the AC and DC sides and protects the inverter from DC overvoltage. On the AC side, flexible AC transmission system devices such as dynamic voltage restorers and static synchronous compensators are used to maintain the connection between the power grid and the renewable energy grid-connected system during faults and inject the necessary reactive power. In addition, a fault current limiter is used to limit the overcurrent on the AC side. However, the additional equipment will significantly increase the system installation and operation and maintenance costs.

[0006] To achieve reliable operation and high-quality power supply of the grid-connected system during faults while ensuring economy, some literature has proposed a dual-sequence current regulation technology. Its basic idea is to reshape the fault current characteristics by flexibly adjusting the positive- and negative-sequence current references to achieve specific control objectives. Usually, the positive-sequence current aims to inject reasonable reactive current and keep the DC link voltage of the inverter constant; the negative-sequence current is mainly used to achieve power quality objectives, such as minimizing power oscillations, flexible voltage support, and harmonic suppression.

[0007] During grid faults, there are complex coupling relationships among control objectives such as power oscillation, voltage support, and current limitation. Achieving one objective usually comes at the expense of another. For example, reducing active power oscillation will increase reactive power oscillation. To solve this problem, some literature has focused on achieving multiple control objectives to improve grid stability and ensure the optimal utilization of the inverter power capacity. However, it is necessary to use complex control strategies and calculation methods to achieve the trade-off among multiple control objectives.

[0008] Therefore, how to propose a multi-objective dual-sequence current injection strategy for new energy grid-connected systems under three-phase unbalanced faults oriented to maximum power output while comprehensively considering various control objectives of the GCI under three-phase unbalanced faults. Summary of the Invention

[0009] In view of this, the present invention provides a multi-objective dual-sequence current injection method for three-phase unbalanced faults oriented to maximum power output, which is used to at least solve the problem that the prior art needs to use complex control strategies and calculation methods to achieve the trade-off among multiple control objectives.

[0010] To achieve the above object, the present invention adopts the following technical solutions: A multi-objective dual-sequence current injection method for three-phase unbalanced faults oriented to maximum power output includes the following steps: S1. Obtain the three-phase voltages and currents at the PCC point after the fault reaches a steady state, and correspondingly obtaindq Positive and negative sequence voltages at PCC point in the coordinate system and current ; S2. According to and Calculate the positive and negative sequence voltages of the power grid under the current fault scenario and its phase angle difference ; S3. According to and Find in 3-D lookup table dq Axis positive and negative sequence current reference value; the method for obtaining the 3-D lookup table is: When a fault occurs, the three-phase voltage and current of the PCC point when the fault ride-through strategy is not adopted are obtained, the positive and negative sequence voltages of the power grid at the current moment and their initial phase angle difference are obtained, the fault type is determined, and stored in the 3-D lookup table for subsequent search dq Index of the axis positive and negative sequence current reference value; set up dq The initial value of the shaft positive and negative sequence current is - I N , calculated based on the voltage support model, active power fluctuation model and grid-related inverter current model - I N The maximum value of the corresponding three-phase voltage , Active power fluctuation value and the maximum value among the three-phase currents ,by Conform to the national standard of PCC point voltage, Does not exceed active power fluctuation constraints and Do not exceed the current limit I N Perform logical judgment for the constraints and output the current dq The positive and negative sequence current of the shaft, if it does not meet the constraint conditions, will be gradually adjusted. I N Update and complete the iteration until [- I N , I N ] All the constraints that meet the conditions dq The shaft positive and negative sequence current combination is calculated for each group dq The inverter active output power corresponding to the positive and negative sequence current of the shaft is obtained to obtain the maximum inverter active output power and its corresponding dq The positive and negative sequence currents of the shaft are dq Reference value of shaft positive and negative sequence current; S4. Based on what is found dq The shaft positive and negative sequence current reference values ​​generate inverter modulation signals through positive and negative sequence separation control.

[0011] Preferably, the three-phase voltage and current of the obtained PCC point are subjected to Park transformation and the second-order generalized integrator SOGI to obtain the positive and negative sequence voltages and currents of the PCC point in the corresponding dq coordinate system.

[0012] Preferably, according to and calculate the positive and negative sequence voltages of the power grid and their phase angle difference in the current fault scenario. The specific method is as follows: The relationships between the positive and negative sequence voltages and currents of the PCC point are respectively: ; ; In the formula, and are respectively the positive sequence voltages of the PCC point on the d axis and the q axis, and are respectively the negative sequence voltages of the PCC point on the d axis and the q axis, and are respectively the positive sequence currents of the PCC point on the d axis and the q axis, and are respectively the negative sequence currents of the PCC point on the d axis and the q axis, and are respectively the positive sequence voltages of the power grid on the d axis and the q axis, and are respectively the negative sequence voltages of the power grid on the d axis and the q axis; is the power grid impedance, represents the rotational angular velocity, is the power grid inductance; Then the initial phase angle difference between the positive and negative sequence voltages of the power grid voltage is: ; In the formula, is the initial phase angle of the positive sequence voltage, is the initial phase angle of the negative sequence voltage.

[0013] Preferably, in S3, the voltage support model is: ; Wherein, and are respectively the three-phase voltages of points A, B, and C of the PCC point, and are respectively d axis and q axis positive sequence voltages of the PCC point; and are respectively d axis and q axis negative sequence voltages of the PCC point; The active power fluctuation model is: ; Wherein, and are respectively d axis and q axis positive sequence voltages of the PCC point; The current model of the grid-connected inverter is: ; Wherein, and are respectively the three-phase currents of points A, B, and C of the PCC point.

[0014] Preferably, the specific content of obtaining the voltage support model and the current model of the grid-connected inverter includes: In abc coordinates, the positive sequence component expression of the three-phase voltage of the PCC point is: ; Wherein: ; The negative sequence component expression of the three-phase voltage of the PCC point is: ; Wherein: ; Wherein, is the initial phase angle of the positive sequence voltage of phase a, is the initial phase angle of the negative sequence voltage of phase a; According to the symmetrical component method, it can be obtained that: ; Wherein: k 1 = [0, 1, -1]; k 2 = [0, -1, 1]; After simplification, the voltage support model is: ; The current model of the grid-connected inverter is as follows: .

[0015] Preferably, the specific content of obtaining the active power fluctuation constraint includes: Obtained according to the instantaneous power theory dq The expressions of the active and reactive powers output by the GCI in the coordinate system: ; Where: ; ; In the formula, and are respectively the constant components of the active power and the reactive power, and are the active double-frequency components, and are the reactive double-frequency components; Then the active power fluctuation value is: .

[0016] Preferably, with meeting the national standard of the PCC point voltage, not exceeding the active power fluctuation constraint and not exceeding the current limit I N as the constraint conditions, the specific content of the logical judgment includes: Set dq the initial values of the positive and negative sequence currents on the I N axis to - The maximum value of the three-phase voltage, the active power fluctuation and the maximum value of the three-phase current are calculated based on the voltage support constraint, the active power fluctuation constraint and the grid-connected inverter current constraint, and and are judged in turn according to the following constraint conditions whether they meet the conditions: ; ; ; In the formula, is the rated voltage of the power grid; is the power fluctuation that causes the capacitor voltage fluctuation; is the power fluctuation that causes the 3rd harmonic content; is the rated voltage of the DC side; is the DC - side capacitor impedance; is the DC - side capacitor voltage fluctuation value; is the effective value of the third - harmonic; represents the rotational angular velocity; is the sum of the inductor of the main - circuit inverter and the grid inductor; is the DC - side capacitor; If the constraint conditions are met, this set of dq axis - current data will be stored, and according to the preset step - size m the current dq axis positive - and negative - sequence currents will be updated, and then enter the loop judgment again; If any one of the conditions is not met, according to the preset step - size m the current dq axis positive - and negative - sequence currents will be updated, and enter the loop judgment; The loop judgment process is as follows: Respectively judge whether the updated dq axis positive - and negative - sequence currents both fall within [- I N , I N . If within this range, then according to the constraint conditions, judge whether the dq axis positive - and negative - sequence currents corresponding to and meet the conditions. If not, enter the loop judgment again. If they meet, store the current dq axis - current data; if not within this range, then for all the stored dq axis - current data, calculate the active power output of the inverter, obtain the maximum active power output of the inverter and its corresponding dq axis positive - and negative - sequence currents as dq the reference values of the axis positive - and negative - sequence currents.

[0017] As can be seen from the above - mentioned technical solutions, compared with the prior art, the present invention discloses a multi - objective dual - sequence current injection method under three - phase unbalanced faults for maximizing power output, and has the following beneficial effects: The present invention proposes a multi - objective dual - sequence current injection method under three - phase unbalanced faults for maximizing power output, providing technical support for fault - ride - through under unbalanced conditions. Under unbalanced conditions, the coupling relationship between various control objectives makes the analysis complex. Existing research usually analyzes each constraint condition in the ab coordinate system. However, as dq the phase angles of the positive - and negative - sequence voltages injected by the axis current and the phase angles of the positive - and negative - sequence currents are real - time variables, this will increase the complex trigonometric operations involved in calculating the amplitudes of the positive - and negative - sequence voltages and currents. In view of this, the present invention converts the amplitude calculation to dqUnder the coordinate system, a 3-D look-up table method is calculated and proposed, which further solves the problems of serious coupling relationship between the phase angle problem and each control target and complex analysis involved in the amplitude problem. At the same time, on the premise of effectively realizing each fault ride-through control target, the proposed strategy greatly improves the active power output ability of the inverter under unbalanced conditions compared with the traditional strategy. Finally, the correctness of the theoretical analysis and the effectiveness of the proposed strategy are verified through simulation and experiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0019] Figure 1 It is the overall flowchart of the multi-objective dual-sequence current injection method for three-phase unbalanced faults oriented to maximum power output disclosed in the present invention; Figure 2 It is for the 3-D data table in the multi-objective dual-sequence current injection method for three-phase unbalanced faults oriented to maximum power output disclosed in the present invention dq Axis current acquisition flowchart; Figure 3 It is the schematic diagram of the grid-connected power electronic system structure provided by the embodiment of the present invention; Figure 4 It is the system schematic diagram under each unbalanced fault provided by the embodiment of the present invention, where: Figure 4 (a) is single-phase grounding fault, Figure 4 (b) is two-phase short-circuit fault, Figure 4 (c) is three-phase short-circuit fault; Figure 5 It is the grid voltage and current phasor diagram provided by the embodiment of the present invention; Figure 6 It is provided by the embodiment of the present invention dq Axis current data four-dimensional visualization diagram, where: Figure 6 (a) is Four-dimensional visualization diagram, Figure 6 (b) is Four-dimensional visualization diagram, Figure 6 (c) is Four-dimensional visualization diagram, Figure 6 (d) is Four-dimensional visualization diagram; Figure 7 It is the voltage positive and negative sequence component vector diagram provided by the embodiment of the present invention, where: Figure 7 (a) is dqUnder-axis voltage component Figure 7 (b) is dq Under-axis synthetic three-phase voltage component Figure 8 Pmax four-dimensional visualization diagram under different fault types provided by the embodiment of the present invention Figure 9 Schematic diagram of low voltage ride-through requirements provided by the embodiment of the present invention Figure 10 Simulation result diagram of the traditional strategy provided by the embodiment of the present invention, where: Figure 10 (a) is the simulation result diagram of PCC voltage under the traditional strategy Figure 10 (b) is the simulation result diagram of PCC current under the traditional strategy Figure 10 (c) is the simulation result diagram of the active power output by the inverter under the traditional strategy Figure 11 Simulation result diagram of the method proposed by the present invention provided by the embodiment of the present invention, where: Figure 11 (a) is the simulation result diagram of PCC voltage under the method proposed by the present invention Figure 11 (b) is the simulation result diagram of PCC current under the method proposed by the present invention Figure 11 (c) is the simulation result diagram of the active power output by the inverter under the method proposed by the present invention Figure 12 Experimental waveform diagram of single-phase ground fault k = 0.5, -k = 0.7 provided by the embodiment of the present invention; where: Figure 12 (a) is the waveform diagram of PCC voltage under single-phase ground fault Figure 12 (b) is the waveform diagram of PCC current under single-phase ground fault Figure 12 (c) is the waveform diagram of the active power output by the inverter under single-phase ground fault Figure 13 Experimental waveform diagram of two-phase ground fault k = 0.5, -k = 0.7 provided by the embodiment of the present invention; where: Figure 13 (a) is the waveform diagram of PCC voltage under two-phase ground fault Figure 13 (b) is the waveform diagram of PCC current under two-phase ground fault Figure 13 (c) is the waveform diagram of the active power output by the inverter under two-phase ground fault Detailed implementation manners

[0020] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0021] The present invention provides a multi-objective dual-sequence current injection method under three-phase unbalanced faults for maximizing power output, as Figure 1 shown, which includes the following steps: S1. Obtain the three-phase voltages and currents at the PCC point after the fault reaches a steady state, and correspondingly obtain the positive and negative sequence voltages dq and currents at the PCC point in the coordinate system; S2. Calculate the positive and negative sequence voltages and of the power grid in the current fault scenario and their phase angle difference ; S3. Find the reference values of the positive and negative sequence currents on the axis in the 3-D lookup table according to and ; The method for obtaining the 3-D lookup table is as follows: dq When a fault occurs, obtain the three-phase voltages and currents at the PCC point without adopting the fault ride-through strategy, obtain the positive and negative sequence voltages of the power grid at the current moment and their initial phase angle difference, determine the fault type, and store them in the 3-D lookup table as the index for subsequent searching for the reference values of the positive and negative sequence currents on the dq axis; Set the initial values of the positive and negative sequence currents on the dq axis to - I N , and calculate the maximum value I N of the three-phase voltages, the active power fluctuation value , and the maximum value of the three-phase currents corresponding to - based on the voltage support model, the active power fluctuation model, and the grid-connected inverter current model. Take meeting the national standard of the PCC point voltage, not exceeding the active power fluctuation constraint, and I not exceeding the current limit N as the constraint conditions for logical judgment. If the constraint conditions are met, output the current dq axis positive and negative sequence currents. If the constraint conditions are not met, gradually update - I N to complete the iteration until all I N , I N within the range that satisfies the constraint conditions are obtained for the dq axis positive and negative sequence current combinations, and calculate each group dqThe active output power of the inverter corresponding to the positive and negative sequence currents of the axis is obtained to get the maximum active output power of the inverter and its corresponding dq positive and negative sequence currents of the axis are used as dq the reference values of the positive and negative sequence currents of the axis; S4. According to the found dq reference values of the positive and negative sequence currents of the axis, the inverter modulation signal is generated through positive and negative sequence separation control.

[0022] It should be noted that: The overall block diagram of the proposed multi-objective dual-sequence current injection strategy for power maximization is as shown in Figure 1 shown, and the flowchart for obtaining the axis current in the 3-D data table is as shown in dq the axis current acquisition flowchart is as shown in Figure 2 shown.

[0023] Three-phase three-leg inverters are the majority of inverters integrated into low-voltage power grids. They can generate positive and negative sequence components under unbalanced conditions (including asymmetrical faults). Therefore, a sequence-based control strategy is required to comprehensively control the inverter current and voltage.

[0024] The structure of the grid-connected power electronic system under study is as shown in Figure 3 shown. There is a three-phase three-wire inverter connected to the grid through an LCL filter. In this study, it is assumed that the DC power supply is connected to the DC link of the inverter (such as a rectifier in wind power plant control or a DC / DC converter in photovoltaic (PV) applications); therefore, the dynamic characteristics of the DC link are not studied in this invention.

[0025] The grid voltage conditions can be divided into symmetrical conditions and unbalanced faults. Among them, the single-phase grounding fault of the unbalanced fault is the most common fault condition in practice, and there is a phase shift in the fault voltage. The schematic diagrams of the system under each unbalanced fault are as shown in Figure 4 shown; In this invention, the expressions of voltage, current, and power in the dq coordinate system are derived to establish a positive and negative sequence equivalent model (the zero-sequence voltage can be suppressed by a star-delta transformer). By analyzing the circuit, the positive-sequence voltage can be obtained as shown in the following formula: ; In the above formula: , , .

[0026] Further performing the prak transformation, the voltage-current relationship in its positive-sequence dq coordinate system can be obtained.

[0027] Analyze the phase angle difference between the positive and negative sequence grid voltages in the fault scenario. Usually, the phase angle jump of the grid voltage during the fault is ignored in this analysis, and its phase angle relationship is as shown inFigure 5 as shown

[0028] In the analysis of the present invention, it is assumed that and coincide with the initial moment of the coordinate system, that is is zero. Further, from Figure 5 the initial phase angle difference of the positive and negative sequence voltages of the power grid voltage can be obtained .

[0029] The present invention takes active power fluctuation suppression, voltage support, and inverter peak current as constraint conditions, and proposes a multi-objective dual-sequence current injection method for a new energy grid-connected system under a three-phase unbalanced condition aiming at maximizing power output. This method converts the voltage and current amplitudes and power calculations during a fault to dq coordinate system for analysis, avoiding abc the complex trigonometric operations caused by the phase angles of the three-phase time-varying voltage and current in the

[0030] There is a coupling relationship among the three control objectives of voltage support, active power fluctuation suppression, inverter current limiting, and maximizing active power output. Therefore, in order to determine the positive and negative sequence currents that meet each control objective, it is necessary to decouple the control objectives. Since the relationship between the positive and negative sequence voltages of the voltage and the voltage drop degree is different for different fault types, the present invention adopts the look-up table method commonly used in the field of power electronics when analyzing complex situations, and stores the reference values of the positive and negative sequence currents under different fault conditions in the look-up table offline. Similar to the existing voltage research, the present invention uses , and to represent the fault conditions, where is the phase angle difference between the positive and negative sequence voltages. Given a set of and the fault type can be uniquely determined. The present invention uses and as inputs, dq axis positive and negative sequence currents as outputs, and constructs dq axis positive and negative sequence current three-dimensional look-up table.

[0031] Figure 6 The 4-D graph is used to display the data points in the 3-D look-up table, which respectively represent the dq axis current reference values under different fault scenarios, where Figure 6 (a)-(d) are and four-dimensional visualizations; the dependent variable The magnitude of the value is as shown in the figure. A query table with more data points can expand the fault type coverage, but requires more memory. The number of data points in the query table can be reasonably selected according to the actual situation by adjusting the judgment step size to make full use of the memory. Select and The range is to cover all possible unbalanced fault types. In the embodiments of the present invention, the range of is selected as [-3.14, 3.14], which is sufficient to cover all possible phase angle differences between the positive and negative sequence voltages of the power grid.

[0032] Figure 8 Shows the maximum active power output under different fault scenarios. Figure 7 and Figure 8 Each query table stores 1,656 data points. It can be seen from Figure 8 that the smaller the fault degree, the stronger the active power output ability. Correspondingly, I d + The proportion in the current is larger, as shown in Figure 7 (a). When the fault is severe, the positive sequence voltage of the power grid decreases and the negative sequence voltage increases. At this time, the inverter needs to inject reactive power. Therefore, q the shaft current proportion increases and the active power output ability decreases. By combining the voltage-current relationship in the positive and negative sequence dq coordinate system and the relationship between the second harmonic component of the active power and and analysis shows that when the positive sequence current proportion increases, the positive sequence voltage rises, the negative sequence voltage decreases, and the active power output increases. , The value of

[0033] To further implement the above technical solution, the three-phase voltage and current at the PCC point obtained are passed through the Park transformation and the second-order generalized integrator SOGI to obtain the corresponding positive and negative sequence voltages and currents at the PCC point in the dq coordinate system.

[0034] To further implement the above technical solution, according to and calculate the specific method of the positive and negative sequence voltages of the power grid and their phase angle difference in the current fault scenario is as follows: The relationships between the positive and negative sequence voltages and the current at the PCC point are respectively: ; ; In the formula, and are respectively d axis and q the positive-sequence voltage of the PCC point on the axis, and are respectively d axis and q the negative-sequence voltage of the PCC point on the axis, and are respectively d axis and q the positive-sequence current of the PCC point on the axis, and are respectively d axis and q the negative-sequence current of the PCC point on the axis, and are respectively d axis and q the positive-sequence voltage of the power grid on the axis, and are respectively d axis and q the negative-sequence voltage of the power grid on the axis, is the power grid impedance, represents the rotational angular velocity, is the power grid inductance; then the initial phase angle difference between the positive and negative sequence voltages of the power grid voltage is: ; In the formula, To further implement the above technical solution, in S3, the voltage support model is: ; In the formula, and are respectively the three-phase voltages of ABC at the PCC point, and are respectively d axis and q the positive-sequence voltage of the PCC point on the axis, and are respectively d axis and q the negative-sequence voltage of the PCC point on the axis; The active power fluctuation model is: ; In the formula, and are respectively d axis and q the positive-sequence voltage of the PCC point on the axis; The current model of the grid-connected inverter is: ; In the formula, and are the three-phase currents of points A, B, and C at the PCC, respectively.

[0035] To further implement the above technical solution, the specific content for obtaining the voltage support model and the grid-connected inverter current model includes: In abc coordinates, the positive-sequence component expression of the three-phase voltage at the PCC is: ; where: ; The negative-sequence component expression of the three-phase voltage at the PCC is: ; where: ; In the formula ， is the initial phase angle of the positive-sequence voltage of phase a, is the initial phase angle of the negative-sequence voltage of phase a; According to the symmetrical component method: ; where: k 1 = [0, 1, -1]; k 2 = [0, -1, 1]; After simplification, the voltage support model is: ; The grid-connected inverter current model is: .

[0036] To further implement the above technical solution, the specific content for obtaining the active power fluctuation constraint includes: According to the instantaneous power theory, dq the expressions for the active and reactive powers output by the GCI in the coordinate system are: ; where: ; ; In the formula, and are the constant components of the active and reactive powers, respectively, and are the active second-harmonic components, and are the reactive second-harmonic components; Then the active power fluctuation value is: .

[0037] It should be noted that: Combining the positive and negative sequence component voltage expressions of the PCC voltage and the symmetrical component method, for simplification can obtain the relational expression between and , where the voltage vector diagram is as shown in Figure 7 .

[0038] From the relational expression between and and the relational expression between and , it can be seen that the three-phase voltage amplitudes are only related to the positive and negative sequence voltage and current amplitudes of the dq axis at the PCC point. When the fault type is certain , amplitudes are certain. Further combining the voltage and current relational expressions in the positive and negative sequence dq coordinate systems and the relational expression between the second harmonic component of the active power and and , it can be obtained that at this time, the three-phase voltage, the output active power, and the amplitude of the active power fluctuation can all be expressed by the positive and negative sequence currents of the dq axis: .

[0039] To further implement the above technical solution, taking meeting the national standard of the PCC point voltage, not exceeding the active power fluctuation constraint, and not exceeding the current limit I N as the constraint conditions, the specific content of the logical judgment includes: Set the initial value of the positive and negative sequence currents of the dq axis to - I N . Based on the voltage support constraint, the active power fluctuation constraint, and the grid-connected inverter current constraint, calculate the maximum value of the three-phase voltage, the active power fluctuation , and the maximum value of the three-phase current, and sequentially judge whether and meet the conditions according to the following constraint conditions: ; ; ; In the formula, is the rated voltage of the power grid; is the power fluctuation that causes the capacitor voltage fluctuation; is the power fluctuation that causes the 3rd harmonic content; is the rated voltage of the DC side; is the impedance of the DC side capacitor; is the fluctuation value of the DC side capacitor voltage; is the effective value of the third harmonic; represents the rotational angular velocity; is the sum of the inductor of the main circuit inverter and the grid inductor; is the DC side capacitor; If the constraint conditions are met, this set of dq shaft current data will be stored, and according to the preset step size m for the current dq positive and negative sequence currents of the shaft are updated, and then enter the loop judgment again; If any one of the conditions is not met, then according to the preset step size m for the current dq positive and negative sequence currents of the shaft are updated, and enter the loop judgment; The loop judgment process is as follows: Judge respectively whether the updated dq positive and negative sequence currents of the shaft all fall within [- I N , I N . If within this range, then judge according to the constraint conditions whether the current dq positive and negative sequence currents of the shaft corresponding and meet the conditions. If not, enter the loop judgment again. If they meet, store the current dq shaft current data; if not within this range, then for all the dq shaft current data stored, obtain the inverter active output power corresponding to it, and get the maximum inverter active output power and its corresponding dq positive and negative sequence currents of the shaft as dq the reference values of the positive and negative sequence currents of the shaft.

[0040] It should be noted that: In the early stage, the proportion of renewable energy power generation in the total power generation was relatively small, and the impact caused by its disconnection could be ignored. Therefore, most standards require that renewable energy power generation systems need to be disconnected during grid disturbances, which is also called anti-islanding protection.

[0041] However, with the increase in the capacity of renewable energy power generation systems, the disconnection of renewable energy power generation systems from the grid during faults may lead to voltage and frequency instability problems, and even cause large-scale power outages. Therefore, new grid codes require renewable energy power generation systems to remain connected during short-term and recoverable disturbances, such as Figure 9 as shown

[0042] In addition, renewable energy power generation systems are also required to inject reactive current to minimize the voltage drop during faults and ensure a rapid voltage recovery after faults. The German grid has specified the proportion of reactive current injection for different voltage drop depths. When the voltage change is within the dead zone (±10%), no reactive current needs to be injected. When the voltage exceeds the dead zone, reactive current must be provided according to the slope k ( k ≥2 pu). When the voltage drop exceeds 50%, the reactive current needs to provide at least 100% of the rated current. According to GB / T37048-2019, China also requires that when a fault occurs, the inverter injects reactive power to remain connected during the fault and improve the stability of the system, but does not specify the proportion of reactive current

[0043] (1) Grid-connected inverter current constraint Under unbalanced faults, there are positive and negative sequence components in the grid voltage and current. If traditional control strategies are used, the asymmetric phase current problem cannot be effectively solved, which will lead to overcurrent in the AC side current of the inverter and increase the risk of damage to electronic devices. Therefore, a reasonable control strategy needs to be proposed to protect the safety of the devices

[0044] (2) Suppression of active power fluctuation The interaction between the positive and negative sequence components of voltage and current generates a second harmonic component of active power, causing oscillations in the DC side capacitor voltage and the generation of 3rd harmonic current. This will lead to the deterioration of the DC side voltage stability and seriously affect the life of the DC capacitor. At the same time, the increase in harmonics will cause local parallel resonance or series resonance in the power system, resulting in damage to equipment such as capacitors. Therefore, suppressing active power fluctuations is usually one of the control objectives during faults. According to the provisions of GB / T37048-2019, the 3rd harmonic content should be less than 4% of the fundamental current, and the fluctuation of the DC side capacitor voltage should be less than 1% of the rated voltage of the DC side V dc of the DC side

[0045] (3) Grid voltage support When an asymmetric fault occurs, the voltage of the fault phase drops, and the voltage of the healthy phase rises. From Figure 9It can be seen that within a certain range, it is desired that the grid-connected inverter can maintain connection with the power grid. At the same time, according to the national standard GB / T37048-2019, during an asymmetric fault, the voltage at the Point of Common Coupling (PCC) should be within U N , 1.1 U N . Therefore, the grid-connected inverter needs to have the ability to provide active / reactive power, that is, voltage support ability, to improve the safety and economy of the large-capacity system.

[0046] The present invention will be further illustrated through simulation experiments below: Since different inverters have different parameters, according to the simulation and experimental parameters given in Table 1, it is calculated that = 7.3 kw, = 34.3 kw, and take = 7.3 kw. In the subsequent simulation experiment verification, let , = 7.3 kw: Table 1 System parameters ; Table 2 Current reference values of the method proposed in the present invention and traditional strategies ; Table 3 Current reference values of the method proposed in the present invention under different working conditions ; Considering the scenario where the grid A-phase voltage drops by 50% ( k = 0.5), a simulation analysis and comparison are carried out on the method proposed in the present invention and traditional strategies. The traditional fault ride-through strategy improves the control flexibility by introducing coefficients k p , k q . In this embodiment, take k p = -1.268, k q = 1.268. The specific numerical values of the positive and negative sequence current reference values of the method proposed in the present invention and traditional strategies are shown in Table 2.

[0047] Figures 10 - 11 shows the simulation results of the two strategies when k = 0.5, where Figure 10 is the simulation result diagram of the traditional strategy, Figure 11 is the simulation result diagram of the method proposed in the present invention, Figure 10 (a) is the simulation result diagram of the PCC voltage under the traditional strategy; Figure 10(b) is the simulation result diagram of the PCC current under the traditional strategy; Figure 10 (c) is the simulation result diagram of the active power output by the inverter under the traditional strategy; Figure 11 (a) is the simulation result diagram of the PCC voltage under the method proposed by the present invention; Figure 11 (b) is the simulation result diagram of the PCC current under the method proposed by the present invention; Figure 11 (c) is the simulation result diagram of the active power output by the inverter under the method proposed by the present invention. It can be seen from the figure that both the traditional strategy and the method proposed by the present invention can raise the PCC voltage to U N , 1.1 U N , and limit the current to I N . However, compared with the traditional strategy, when achieving the same fault ride-through control goal, the active power output by the method proposed by the present invention increases by 20%. However, the active power fluctuation is 7.3 kw, which is 17% less than that of the traditional strategy.

[0048] To further verify the effectiveness of the method proposed by the present invention, experimental verification was carried out. The method proposed by the present invention is applicable to various fault types. In this experiment, two fault types, single-phase grounding fault and two-phase grounding fault, were verified respectively, and the fault depth was changed at 0.6 s. The experimental parameters are the same as the simulation parameters, as shown in Table 1, and the positive and negative sequence current reference values under different working conditions are shown in Table 3.

[0049] The experimental results of the single-phase grounding fault are as Figure 12 shown, Figure 12 (a) is the waveform diagram of the PCC voltage under the single-phase grounding fault; Figure 12 (b) is the waveform diagram of the PCC current under the single-phase grounding fault; Figure 12 (c) is the waveform diagram of the active power output by the inverter under the single-phase grounding fault; When k = 0.5, the maximum phase voltage is controlled to 340.1 V and the minimum phase voltage is raised to 270.6 V by adopting the control strategy proposed by the present invention, which meets the voltage control goal. The maximum phase output current of the inverter is 100 A, and the third harmonic content and THD are both less than 4%, realizing the suppression of the third harmonic content of the current and the full application of the current capacity. The active power output under this working condition is 37.2 kw. As analyzed in Section 5.1, its active power output is 20% higher than that of the traditional strategy, and the active power fluctuation is suppressed to 7.3 kw, which does not exceed the maximum power fluctuation allowable value obtained from the previous analysis. When the fault degree changes to k = 0.7, the current reference value changes. At this time, the maximum phase voltage is 339 V and the minimum phase voltage is 265 V, which meets the national standard. Under this working condition, the inverter outputs a three-phase balanced current with an amplitude of 100 A, and the third harmonic content and THD are both less than 4%. Compared with k= 0.5, at this time, the output active power increases by 13% to reach 42.1 kW, and the active power fluctuation is 5.3 kW, meeting the requirements.

[0050] The experimental result diagram of the two-phase grounding fault is as Figure 13 shown. Figure 13 (a) is the PCC voltage waveform diagram under the two-phase grounding fault; Figure 13 (b) is the PCC current waveform diagram under the two-phase grounding fault; Figure 13 (c) is the waveform diagram of the active power output by the inverter under the two-phase grounding fault; when k = 0.5, adopting the control strategy proposed by the present invention, the maximum phase voltage is controlled to 340.5 V, and the minimum phase voltage is raised to 275 V, meeting the voltage control target. The maximum phase output current of the inverter is 100 A, and the third harmonic content and THD are both less than 4%, realizing the full application of the current capacity and the suppression of the third harmonic content. The active power output by the inverter is 18.72 kW, and the power fluctuation is 4.28 kW, realizing the maximum power output on the premise of meeting the fluctuation suppression target. At 0.6 s, the fault degree changes to k = 0.7, which will cause a certain overshoot but quickly return to the steady state. At this time, the maximum phase voltage is controlled to 338 V, and the minimum phase voltage is raised to 280 V, meeting the national standard voltage requirements. The inverter outputs a three-phase balanced current, the maximum phase current amplitude is 100 A, and the third harmonic content and THD are both less than 4%. Compared with k = 0.5, at this time, the output active power increases by 200% to reach 36.35 kW, and the improvement effect is significant. The active power fluctuation is 4.8 kW, meeting the requirements.

[0051] It can be seen from the above analysis that the method proposed by the present invention has a good effect on improving the active power output capacity under the single-phase grounding fault and two-phase grounding fault scenarios on the premise of ensuring the fault ride-through requirements, and the method proposed by the present invention can quickly respond to the change of the fault depth and has good adaptability. The conclusions obtained from the above analysis still apply in other fault scenarios.

[0052] The above embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application, and should all be included in the protection scope of the present application.

Claims

1. A multi-objective double-sequence current injection method for three-phase unbalanced fault with maximum power output, characterized by: The following steps are involved: S1. Obtain the three-phase voltage of PCC point after fault steady state and current , and correspondingly obtain dq Positive and negative sequence voltages at PCC point in the coordinate system and current ; S2. According to and Calculate the positive and negative sequence voltages of the power grid under the current fault scenario and its phase angle difference ; S3. According to and Find in 3-D lookup table dq Reference value of shaft positive and negative sequence current; The method for obtaining the 3-D lookup table is: When a fault occurs, the three-phase voltage and current of the PCC point when the fault ride-through strategy is not adopted are obtained, the positive and negative sequence voltages of the power grid at the current moment and their initial phase angle difference are obtained, the fault type is determined, and stored in the 3-D lookup table for subsequent search dq Index of the axis positive and negative sequence current reference value; set up dq The initial value of the shaft positive and negative sequence current is - I N , calculated based on the voltage support model, active power fluctuation model and grid-related inverter current model - I N The maximum value of the corresponding three-phase voltage , Active power fluctuation value and the maximum value among the three-phase currents ,by Conform to the national standard of PCC point voltage, Does not exceed active power fluctuation constraints and Do not exceed the current limit I N Perform logical judgment for the constraints and output the current dq The positive and negative sequence current of the shaft, if it does not meet the constraint conditions, will be gradually adjusted. I N Update and complete the iteration until [- I N , I N ] All the constraints that meet the conditions dq The shaft positive and negative sequence current combination is calculated for each group dq The inverter active output power corresponding to the positive and negative sequence current of the shaft is obtained to obtain the maximum inverter active output power and its corresponding dq The positive and negative sequence current of the shaft is dq Reference value of shaft positive and negative sequence current; S4. Based on what is found dq The shaft positive and negative sequence current reference values ​​generate inverter modulation signals through positive and negative sequence separation control.

2. The multi-objective double-sequence current injection method under three-phase unbalanced fault for maximizing power output according to claim 1 is characterized in that: The obtained three-phase voltage and current of PCC point are transformed by Park and second-order generalized integrator SOGI to obtain the corresponding dq Positive and negative sequence voltage and current at the PCC point in the coordinate system.

3. The multi-objective double-sequence current injection method under three-phase unbalanced fault for maximizing power output according to claim 1 is characterized in that: according to and Calculate the positive and negative sequence voltages of the power grid under the current fault scenario and its phase angle difference The specific method is: Positive and negative sequence voltage at PCC point and current The relationships are: ； ； In the formula, and They are d Axis and q Positive sequence voltage at the PCC point on the axis, and They are d Axis and q Negative sequence voltage at the PCC point on the axis, and They are d Axis and q Positive sequence current at the PCC point on the axis, and They are d Axis and q Negative sequence current at the PCC point on the axis, and They are d Axis and q Positive sequence voltage of the grid on the axis, and They are d Axis and q Negative sequence voltage of the grid on the axis; is the grid impedance, represents the angular velocity of rotation, is the grid inductance; Then the initial phase angle difference between the positive and negative sequence voltages of the power grid is for: ； In the formula, is the initial phase angle of the positive sequence voltage, is the initial phase angle of negative sequence voltage.

4. The multi-objective double-sequence current injection method under three-phase unbalanced fault for maximizing power output according to claim 1 is characterized in that: In S3, the voltage support model is: ； In the formula, and They are the three-phase voltages at point PCC, ABC, and They are d Axis and q Positive sequence voltage at the PCC point on the axis, and They are d Axis and q Negative sequence voltage at the PCC point on the axis; The active power fluctuation model is: ； In the formula, and They are d Axis and q Positive sequence voltage at PCC point on the axis; The grid-related inverter current model is: ； In the formula, and They are the three-phase currents at points PCC, ABC and PCC respectively.

5. The multi-objective double-sequence current injection method under three-phase unbalanced fault for maximizing power output according to claim 4 is characterized in that: The specific contents of obtaining the voltage support model and the grid-related inverter current model include: exist abc Under the coordinates, the expression of the positive sequence component of the three-phase voltage at the PCC point is: ； in: ； The negative sequence component expression of the three-phase voltage at PCC point is: ； in: ； In the formula, is the initial phase angle of the positive sequence voltage of phase a, is the initial phase angle of negative sequence voltage of phase a; According to the symmetric component method: ； in: k 1=[0, 1, -1]; k 2=[0, -1, 1]; The simplified voltage support model is: ； The current model of the grid-related inverter is: 。 6. The multi-objective double-sequence current injection method under three-phase unbalanced fault for maximizing power output according to claim 4 is characterized in that: The specific contents of obtaining the active power fluctuation model include: According to the instantaneous power theory, dq The expression of GCI output active and reactive power in the coordinate system is: ； in: ； ； In the formula, and are the constant components of active power and reactive power, respectively, and is the active double frequency component, and It is the reactive double frequency component; The active power fluctuation value for: 。 7. The multi-objective double-sequence current injection method under three-phase unbalanced fault for maximizing power output according to claim 1 is characterized in that: by Conform to the national standard of PCC point voltage, Does not exceed active power fluctuation constraints and Do not exceed the current limit I N The specific contents of logical judgment for constraint conditions include: set up dq The initial value of the shaft positive and negative sequence current is - I N The maximum value of the three-phase voltage is calculated based on the voltage support constraint, active power fluctuation constraint and grid-related inverter current constraint. , Active power fluctuation and the maximum value among the three-phase currents , and judge according to the following constraints and Eligibility: ； ； ； In the formula, is the rated voltage of the power grid; To account for power fluctuations that cause capacitor voltage fluctuations; To cause power fluctuations with 3rd harmonic content; is the rated voltage of the DC side; is the DC side capacitor impedance; is the voltage fluctuation value of the DC side capacitor; is the effective value of the third harmonic; represents the angular velocity of rotation; The sum of the main circuit inverter inductance and the grid inductance; is the DC side capacitance; If the constraints are met, then this group dq The shaft current data is stored and read according to the preset step size. m For the current dq The positive and negative sequence currents of the shaft are updated and the cycle judgment is entered again; If any of the conditions is not met, the preset step size m For the current dq The positive and negative sequence currents of the shaft are updated and enter into a loop judgment; The cycle judgment process is: Determine the updated dq Are the positive and negative sequence currents of the shaft both within [- I N , I N ] range, if it is within this range, the current dq The positive and negative sequence currents of the shaft correspond to and Whether it meets the conditions, if not, it will enter the loop judgment again, if it meets the conditions, it will store the current dq If the shaft current data is not within this range, all stored dq The inverter active output power corresponding to the shaft current data is used to obtain the maximum inverter active output power and its corresponding dq The positive and negative sequence current of the shaft is dq Reference value of shaft positive and negative sequence current.

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