Multi-objective Dual-Sequence Current Injection Method under Three-Phase Unbalanced Faults for Maximizing Power Output

Abstract

The present invention discloses a multi-objective dual-sequence current injection method under three-phase unbalanced faults for maximizing power output, including 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 and currents at the PCC point in the dq coordinate system; S2. Calculate the positive- and negative-sequence voltages and their phase angle differences of the power grid under the current fault scenario according to and ; S3. Find the positive- and negative-sequence current reference values on the dq axis in the 3-D look-up table according to and ; S4. Generate the inverter modulation signal through positive- and negative-sequence separation control according to the found positive- and negative-sequence current reference values on the dq axis.

Description

Technical Field

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

[0002] In recent years, renewable energy generation with power electronic converters as the interface has been rapidly replacing fossil fuel 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 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 depth of voltage dip 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 surplus energy or installing an energy storage system to store the surplus 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. Generally, 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 oscillation, 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 the active power oscillation will increase the 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 adopt 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 a new energy grid-connected system under three-phase unbalanced faults aiming at 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 aiming at maximum power output, which is used to at least solve the problem that the prior art needs to adopt 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:

[0011] A multi-objective dual-sequence current injection method for three-phase unbalanced faults aiming at maximum power output includes the following steps:

[0012] S1. Obtain the three-phase voltages and currents , and correspondingly obtain dq the positive and negative sequence voltages and currents of the PCC point in the coordinate system ;

[0013] S2. According to and calculate the positive and negative sequence voltages of the power grid in the current fault scenario and their phase angle differences ;

[0014] S3. According to and find in the 3-D look-up table dq the reference values of the positive and negative sequence currents on the

[0015] axis; The method for obtaining the 3-D look-up table is as follows: dq When a fault occurs, obtain the three-phase voltages and currents of 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 differences, determine the fault type, and store them in the 3-D look-up table as the index for subsequent searching of

[0016] Set dq the initial value of the positive and negative sequence currents on the I N axis to - I N , and calculate the maximum value of the three-phase voltages corresponding to - and the maximum value of the three-phase currents based on the voltage support model, the active power fluctuation model, and the grid-connected inverter current model. With meeting the PCC point voltage constraint, not exceeding the active power fluctuation constraint, and not exceeding the current limit I 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 of dq axis positive and negative sequence current combinations that meet the constraint conditions are calculated, and the active power output of the inverter corresponding to each set of dq axis positive and negative sequence currents is obtained. The maximum inverter active power output and its corresponding dq axis positive and negative sequence currents are used as dqPositive and negative sequence current reference values of the shaft;

[0017] S4. According to the found dq Positive and negative sequence current reference values of the shaft generate the inverter modulation signal through positive and negative sequence separation control.

[0018] Preferably, the obtained three-phase voltage and current at the PCC point are passed through Park transformation and the second-order generalized integrator SOGI to obtain the corresponding dq Positive and negative sequence voltages and currents at the PCC point in the coordinate system.

[0019] Preferably, according to and Calculate the positive and negative sequence voltages of the power grid in the current fault scenario and their phase angle differences The specific method is:

[0020] Positive and negative sequence voltages at the PCC point and currents The relationships are respectively:

[0021] ;

[0022] ;

[0023] In the formula, and are respectively d Positive sequence voltages at the PCC point on the shaft and q the shaft, and are respectively d Negative sequence voltages at the PCC point on the shaft and q the shaft, and are respectively d Positive sequence currents at the PCC point on the shaft and q the shaft, and are respectively d Negative sequence currents at the PCC point on the shaft and q the shaft, and are respectively d Positive sequence voltages of the power grid on the shaft and q the shaft; and are respectively <{ d Negative sequence voltages of the power grid on the shaft and q the shaft; Is the power grid impedance, Represents the rotational angular velocity, Is the power grid inductance;

[0024] Then the initial phase angle difference between the positive and negative sequence voltages of the power grid is:

[0025] ;

[0026] In the formula, is the initial phase angle of the positive-sequence voltage, is the initial phase angle of the negative-sequence voltage.

[0027] Preferably, in S3, the voltage support model is:

[0028] ;

[0029] In the formula, and are the ABC-phase voltages at the PCC point respectively, and are respectively d axis and q axis positive-sequence voltages at the PCC point, and are respectively d axis and q axis negative-sequence voltages at the PCC point;

[0030] The active power fluctuation model is:

[0031] ;

[0032] In the formula, and are respectively d axis and q axis positive-sequence voltages at the PCC point;

[0033] The grid-connected inverter current model is:

[0034] ;

[0035] In the formula, and are the ABC-phase currents at the PCC point respectively.

[0036] Preferably, the specific content of obtaining the voltage support model and the grid-connected inverter current model includes:

[0037] In abc coordinates, the positive-sequence component expression of the three-phase voltage at the PCC point is:

[0038] ;

[0039] Among them:

[0040] ;

[0041] The negative sequence component expression of the three-phase voltage at the PCC point is as follows:

[0042] ;

[0043] Where:

[0044] ;

[0045] 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;

[0046] According to the symmetrical component method, we have:

[0047] ;

[0048] Where: k 1 = [0, -1, 1]; k 2 = [0, 1, -1];

[0049] After simplification, the voltage support model is:

[0050] ;

[0051] The current model of the grid-connected inverter is:

[0052] .

[0053] Preferably, the specific content of obtaining the active power fluctuation constraint includes:

[0054] According to the instantaneous power theory, dq the active and reactive power expressions of the GCI output in the

[0055] coordinate system are:

[0056] Where:

[0057] ;

[0058] ;

[0059] In the formula, and are the constant components of the active power and reactive power respectively, and are the second harmonic components of the active power, and are the second harmonic components of the reactive power;

[0060] Then the active power fluctuation value is:

[0061] 。

[0062] Preferably, taking meeting the PCC point voltage constraint, 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:

[0063] Set dq the initial value of the positive and negative sequence currents of the I N axis to be - active power fluctuation and the maximum value of the three-phase current , and judge successively according to the following constraint conditions and whether they meet the conditions:

[0064] ;

[0065] ;

[0066] ;

[0067] In the formula, is the rated voltage of the power grid; is the power fluctuation causing the capacitor voltage fluctuation; is the power fluctuation causing the 3rd harmonic content; is the rated voltage of the DC side; is the impedance of the DC side capacitor; 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 inductor of the power grid; is the DC side capacitor;

[0068] If the constraint conditions are met, store this set of dq axis current data, and update the positive and negative sequence currents of the current m axis according to the preset step size dq , and enter the loop judgment again;

[0069] If any one of the conditions is not met, update the positive and negative sequence currents of the current m axis according to the preset step size dq , and enter the loop judgment;

[0070] The loop judgment process is as follows:

[0071] Judge respectively whether the positive and negative sequence currents of the updated dq axis both fall within [- I N , I N . If within this range, then judge the current dq corresponding to the positive and negative sequence currents of the axis and according to the constraint conditions. If not, enter the loop judgment again. If it meets the conditions, store the current dq axis current data; if not within this range, then take all the stored dq axis current data corresponding to the inverter active output power to obtain the maximum inverter active output power and its corresponding dq axis positive and negative sequence currents as dq axis positive and negative sequence current reference values.

[0072] It can be seen from the above technical solutions that compared with the prior art, the present invention discloses a multi-objective dual-sequence current injection method for three-phase unbalanced faults for maximizing power output, which has the following beneficial effects:

[0073] The present invention proposes a multi-objective dual-sequence current injection method for 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 injection positive and negative sequence voltage phase angles of the axis current and the positive and negative sequence current phase angles are real-time variables, which will increase the complex trigonometric operations involved in calculating the positive and negative sequence voltage and current amplitudes. In view of this, the present invention converts the amplitude calculation to dq coordinate system for calculation and proposes a 3-D look-up table method, thereby solving the problems of time-varying phase angle problems and serious coupling relationship between various control objectives involved in amplitude problems. At the same time, the proposed strategy significantly improves the active output ability of the inverter under unbalanced conditions compared with the traditional strategy on the premise of effectively achieving each fault ride-through control objective. 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

[0074] 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, without creative efforts, other drawings can be obtained based on these drawings.

[0075] Figure 1 It is the overall flow block diagram of the multi-objective dual-sequence current injection method under three-phase unbalanced faults for maximizing power output disclosed by the present invention;

[0076] Figure 2 It is the dq Axis current acquisition flowchart in the 3-D data table of the multi-objective dual-sequence current injection method under three-phase unbalanced faults for maximizing power output disclosed by the present invention;

[0077] Figure 3 It is the schematic diagram of the grid-connected power electronic system structure provided by the embodiment of the present invention;

[0078] 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;

[0079] Figure 5 It is the grid voltage and current phasor diagram provided by the embodiment of the present invention;

[0080] Figure 6 It is provided by the embodiment of the present invention dq Four-dimensional visual diagram of axis current data, where: Figure 6 (a) is Four-dimensional visual diagram, Figure 6 (b) is Four-dimensional visual diagram, Figure 6 (c) is Four-dimensional visual diagram, Figure 6 (d) is Four-dimensional visual diagram;

[0081] Figure 7 It is the positive and negative sequence component vector diagram of voltage provided by the embodiment of the present invention, where: Figure 7 (a) is dq Voltage component under the axis, Figure 7 (b) is dq Synthesized three-phase voltage component under the axis;

[0082] Figure 8 It is the four-dimensional visual diagram of Pmax under different fault types provided by the embodiment of the present invention;

[0083] Figure 9 It is the schematic diagram of low voltage ride-through requirements provided by the embodiment of the present invention;

[0084] Figure 10 It is the 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;

[0085] Figure 11 This is the simulation result diagram of the method proposed in 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 in the present invention; Figure 11 (b) is the simulation result diagram of PCC current under the method proposed in the present invention; Figure 11 (c) is the simulation result diagram of the active power output by the inverter under the method proposed in the present invention;

[0086] Figure 12 This is the experimental waveform diagram of single-phase grounding 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 grounding fault; Figure 12 (b) is the waveform diagram of PCC current under single-phase grounding fault; Figure 12 (c) is the waveform diagram of the active power output by the inverter under single-phase grounding fault;

[0087] Figure 13 This is the experimental waveform diagram of two-phase grounding 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 grounding fault; Figure 13 (b) is the waveform diagram of PCC current under two-phase grounding fault; Figure 13 (c) is the waveform diagram of the active power output by the inverter under two-phase grounding fault. Specific implementation manner

[0088] 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.

[0089] 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, including the following steps:

[0090] 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 at the PCC point in the Sum current ;

[0091] S2. According to and calculate the positive and negative sequence voltages of the power grid in the current fault scenario and their phase angle differences ;

[0092] S3. According to and find the positive and negative sequence current reference values of the dq axis in the 3-D lookup table; The method for obtaining the 3-D lookup table is as follows:

[0093] When a fault occurs, obtain the three-phase voltages and currents at the PCC point without using 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 differences, determine the fault type, and store them in the 3-D lookup table as the index for subsequent searching for dq the positive and negative sequence current reference values of the

[0094] Set dq the initial value of the positive and negative sequence currents of the I N axis to - I N , and calculate the maximum value 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, active power fluctuation model and grid-connected inverter current model, and use not exceeding the active power fluctuation constraint and not exceeding the current limit I 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 satisfy the constraint conditions are found, calculate the active power output of the inverter corresponding to each set of dq axis positive and negative sequence currents, and obtain the maximum inverter active power output and its corresponding dq axis positive and negative sequence currents as dq the positive and negative sequence current reference values of the dq axis;

[0095] S4. According to the founddq The reference values of the positive and negative sequence currents of the shaft are used to generate the inverter modulation signal through positive and negative sequence separation control.

[0096] It should be noted that:

[0097] The overall block diagram of the proposed multi-objective double-sequence current injection strategy for power maximization is as Figure 1 shown, and the flowchart for obtaining the shaft current in the 3-D data table is as dq shown. Figure 2 shown.

[0098] 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 asymmetric faults. Therefore, a sequence-based control strategy is required to comprehensively control the inverter current and voltage.

[0099] The structure of the grid-connected power electronic system under study is as 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 a DC power supply is connected to the DC link of the inverter (e.g., 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.

[0100] The grid voltage conditions can be divided into symmetric 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 various unbalanced faults are as Figure 4 shown;

[0101] 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 equation:

[0102] ;

[0103] In the above equation: , , .

[0104] Further performing the prak transformation, the voltage-current relationship in its positive dq coordinate system can be obtained.

[0105] Analyzing the phase angle difference between the positive and negative sequence voltages of the grid under the fault scenario, the phase angle jump of the grid voltage during the fault is usually ignored in this analysis, and its phase angle relationship is as Figure 5 shown.

[0106] In the analysis of this invention, it is assumed that coincide with at the initial moment of the coordinate system, that is is zero. Further, from Figure 5 the initial phase angle difference between the positive and negative sequence voltages of the grid voltage can be obtained .

[0107] The present invention takes active power fluctuation suppression, voltage support, and inverter peak current as constraint conditions, and proposes a multi-objective two-sequence current injection method for a new energy grid-connected system under three-phase unbalanced conditions oriented to maximum power output. This method converts the voltage and current amplitudes and power calculations during the fault to dq coordinate system for analysis, avoiding abc the complex trigonometric operations caused by the phase angles of the three-phase time-varying voltages and currents in the

[0108] There is a coupling relationship among the three control objectives of voltage support, active power fluctuation suppression, inverter current limiting, and maximum output active power. Therefore, to determine the positive and negative sequence currents that meet each control objective, the control objectives need to be decoupled. 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 and dq axis positive and negative sequence currents as outputs to construct a dq axis positive and negative sequence current three-dimensional look-up table.

[0109] Figure 6 Figure 4-D shows the data points in the 3-D look-up table, which respectively represent the reference values of the dq axis current under different fault scenarios, where Figure 6 (a)-(d) are the four-dimensional visual diagrams of and in turn; the value of the dependent variable is shown as in the figure. A look-up table with more data points can expand the coverage of fault types, but requires more memory. The number of data points in the look-up table can be adjusted by reasonably selecting the judgment step according to the actual situation 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 is [-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.

[0110] Figure 8 The maximum active power output in different fault scenarios is shown. 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 systems 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

[0111] To further implement the above technical solution, the three-phase voltage and current of the PCC point obtained are passed through the Park transformation and the second-order generalized integrator SOGI to obtain the corresponding dq positive and negative sequence voltages and currents of the PCC point in the

[0112] To further implement the above technical solution, according to and the specific method for calculating the positive and negative sequence voltages of the power grid and their phase angle difference in the current fault scenario is as follows:

[0113] The relationships between the positive and negative sequence voltages and the current at the PCC point are respectively:

[0114] ;

[0115] ;

[0116] In the formula, and are respectively the positive sequence voltages of the PCC point on the d axis and the q axis, and respectively d axis and q the negative-sequence voltage at the PCC point on the axis, and respectively d axis and q the positive-sequence current at the PCC point on the axis, and respectively d axis and q the negative-sequence current at the PCC point on the axis, and respectively d axis and q the positive-sequence voltage of the power grid on the axis, and 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;

[0117] Then the initial phase angle difference between the positive- and negative-sequence voltages of the power grid voltage is:

[0118] ;

[0119] In the formula,

[0120] To further implement the above technical solution, in S3, the voltage support model is:

[0121] ;

[0122] In the formula, and are respectively the three-phase voltages of ABC at the PCC point, and respectively d axis and q the positive-sequence voltage at the PCC point on the axis, and respectively d axis and q the negative-sequence voltage at the PCC point on the axis;

[0123] The active power fluctuation model is:

[0124] ;

[0125] In the formula, and respectively d axis and q the positive-sequence voltage at the PCC point on the axis;

[0126] The current model of the grid-connected inverter is:

[0127] ;

[0128] Where, and They are the three-phase currents at points PCC, ABC, and F.

[0129] In order to further implement the above technical solution, the specific contents of obtaining the voltage support model and the grid-related inverter current model include:

[0130] exist abc Under the coordinates, the expression of the positive sequence component of the three-phase voltage at the PCC point is:

[0131] ;

[0132] in:

[0133] ;

[0134] The negative sequence component expression of the three-phase voltage at PCC point is:

[0135] ;

[0136] in:

[0137] ;

[0138] 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;

[0139] According to the symmetric component method, we can get:

[0140] ;

[0141] in: k 1=[0, -1, 1]; k 2=[0, 1, -1];

[0142] The simplified voltage support model is:

[0143] ;

[0144] The current model of the grid-connected inverter is:

[0145] .

[0146] In order to further implement the above technical solution, the specific contents of obtaining active power fluctuation constraints include:

[0147] Obtained according to the instantaneous power theory dq Active and reactive power expressions of the GCI output in the coordinate system:

[0148] ;

[0149] Where:

[0150] ;

[0151] ;

[0152] 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;

[0153] Then the active power fluctuation value is:

[0154] .

[0155] It should be noted that:

[0156] Combining the positive and negative sequence component voltage expressions of the PCC voltage and the symmetrical component method, can be simplified to obtain the relational expression between and , where the voltage vector diagram is as shown in Figure 7 .

[0157] From the relational expression between and and the relational expression between and , it can be seen that the three-phase voltage amplitude is 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 , amplitude is certain. Further combining the voltage and current relational expressions in the positive and negative sequence dq coordinate system and the relational expression between the active power double-frequency component and and , it can be obtained that at this time, the three-phase voltage, the output active power, and the active power fluctuation amplitude can all be represented by the positive and negative sequence currents of the dq axis:

[0158] .

[0159] In order to further implement the above technical solution, taking Meet the PCC point voltage constraint, do not exceed the active power fluctuation constraint and do not exceed the current limit I N The specific content of the logical judgment based on the constraint conditions includes:

[0160] Set dq the initial value of the positive and negative sequence currents of the I N axis to - , calculate the maximum value of the three-phase voltage, the active power fluctuation and the maximum value of the three-phase current based on the voltage support constraint, the active power fluctuation constraint and the grid-connected inverter current constraint , and judge in turn according to the following constraint conditions and whether they meet the conditions:

[0161] ;

[0162] ;

[0163] ;

[0164] 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 DC side capacitor voltage fluctuation value; is the effective value of the third harmonic; represents the rotational angular velocity; is the sum of the main circuit inverter inductor and the grid inductor; is the DC side capacitor;

[0165] If the constraint conditions are met, store this set of dq axis current data, and update the positive and negative sequence currents of the current m axis according to the preset step size dq , and enter the loop judgment again;

[0166] If any one of the conditions is not met, update the positive and negative sequence currents of the current m axis according to the preset step size dq and enter the loop judgment;

[0167] The loop judgment process is:

[0168] Judge the updated dqWhether the positive and negative sequence currents of the I N axis both fall within the range of [- 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 the conditions, store the current dq axis current data; if not within this range, then calculate the active power output of the inverter corresponding to all the stored dq axis current data to obtain the maximum inverter active power output and its corresponding dq axis positive and negative sequence currents as the dq axis positive and negative sequence current reference values.

[0169] It should be noted that:

[0170] 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.

[0171] However, with the increase in the capacity of renewable energy power generation systems, the disconnection of renewable energy power generation systems during faults may lead to voltage and frequency instability problems, and even cause large-scale power outages. Therefore, new grid codes require that renewable energy power generation systems remain connected during short-term and recoverable disturbances, as shown in Figure 9 .

[0172] 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. For the reactive current injection ratio, the German grid has regulations under 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.

[0173] (1) Grid-connected inverter current constraint

[0174] 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 inverter AC side current and increase the risk of electronic device damage. Therefore, a reasonable control strategy needs to be proposed to protect the safety of the devices. [[ID=--]]

[0175] (2) Active power fluctuation suppression

[0176] 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 the active power fluctuation is usually one of the control objectives during faults.

[0177] (3)Grid voltage support

[0178] When an asymmetric fault occurs, the voltage of the fault phase drops and the voltage of the healthy phase rises. As Figure 9 is known, it is desirable that the grid-connected inverter can remain connected to the grid within a certain range. At the same time, according to the voltage constraint at the PCC point, the voltage at the Point of Common Coupling (PCC) should be within [0.85 U N , 1.1 U N during the asymmetric fault. Therefore, the grid-connected inverter needs to have the ability to provide active / reactive power, i.e., voltage support ability, to improve the safety and economy of the large-capacity system.

[0179] The present invention will be further described through simulation experiments below:

[0180] 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:

[0181] Table 1 System parameters

[0182] ;

[0183] Table 2 Current reference values of the method proposed in the present invention and the traditional strategy

[0184] ;

[0185] Table 3 Current reference values of the method proposed in the present invention under different working conditions

[0186] ;

[0187] 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 the traditional strategy. The traditional fault ride-through strategy is through the introduction of a coefficientk p , k q To improve the control flexibility, in this embodiment, 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 the traditional strategy are shown in Table 2.

[0188] Figures 10 - 11 It 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 in the present invention; Figure 11 (b) is the simulation result diagram of the PCC current under the method proposed in the present invention; Figure 11 (c) is the simulation result diagram of the active power output by the inverter under the method proposed in the present invention; It can be seen from the figure that both the traditional strategy and the method proposed in the present invention can raise the PCC voltage to [0.85 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 in 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.

[0189] To further verify the effectiveness of the method proposed in the present invention, experimental verification was carried out. The method proposed in 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 and are shown in Table 1, and the positive and negative sequence current reference values under different working conditions are shown in Table 3. [[ID=!47]]

[0190] 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; Whenk 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 in the present invention, 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%, achieving the suppression of the third harmonic content of the current and the full utilization of the current capacity. The active power output under this condition is 37.2 kW, which is 20% higher than that of the traditional strategy, and the active power fluctuation is suppressed to 7.3 kW, not exceeding the allowable value of the maximum power fluctuation 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, meeting the voltage constraint at the PCC point. Under this 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 k = 0.5, the active power output increases by 13% to reach 42.1 kW at this time, and the active power fluctuation is 5.3 kW, meeting the requirements.

[0191] The experimental results of the two-phase grounding fault are shown as Figure 13 follows: Figure 13 (a) is the voltage waveform diagram at the PCC under the two-phase grounding fault; Figure 13 (b) is the current waveform diagram at the PCC under the two-phase grounding fault; Figure 13 (c) is the active power waveform diagram of the inverter output under the two-phase grounding fault; When k k = 0.5, the maximum phase voltage is controlled to 340.5 V and the minimum phase voltage is raised to 275 V by adopting the control strategy proposed in the present invention, 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%, achieving the full utilization of the current capacity and the suppression of the third harmonic content. The active power output of 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 k = 0.7, and there will be a certain overshoot but it quickly returns 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 voltage constraint at the PCC point. The inverter outputs a three-phase balanced current, and the maximum phase current amplitude is 100 A, and the third harmonic content and THD are both less than 4%. Compared with k k = 0.5, the active power output increases by 200% to reach 36.35 kW at this time, with a significant improvement effect, and the active power fluctuation is 4.8 kW, meeting the requirements.

[0192] As can be seen from the above analysis, the method proposed in the present invention has a good effect on improving the active power output under the premise of ensuring the fault ride-through requirements in the single-phase ground fault and two-phase ground fault scenarios, and the method proposed in 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 the remaining fault scenarios.

[0193] 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 on some of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to 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 dual-sequence current injection method under three-phase unbalanced faults for maximizing power output, characterized in that, It includes the following steps: S1. Obtain the three-phase voltages at the PCC point after the fault reaches a steady state and currents , and correspondingly obtain dq the positive and negative sequence voltages and currents at the PCC point in the coordinate system ; S2. Calculate, based on and the positive and negative sequence voltages of the power grid and their phase angle differences under the current fault scenario; S3. According to and find in the 3-D look-up table dq the positive and negative sequence current reference values of the axis; The method for obtaining the 3-D look-up table is: 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 and their initial phase angle differences at the current moment, determine the fault type, and store it in the 3-D lookup table as the index for subsequent searching for dq the positive and negative sequence current reference values of the shaft; set up dq The initial value of the axis 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 corresponding maximum value of the three-phase voltage , active power fluctuation value and the maximum value among the three-phase currents ,by Comply with PCC point voltage constraints, Do not exceed the active power fluctuation constraints and Do not exceed the current limit I N Perform logical judgment for the constraints and output the current dq If the axis positive and negative sequence current does not meet the constraint conditions, it will be gradually adjusted. I N Update and complete the iteration until [- I N , I N ] All the constraints that meet the conditions within the range dq The positive and negative sequence current combination of the shaft 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; Among them, the PCC point voltage constraint is: ； Wherein, and are the three-phase voltages of points A, B, and C of the PCC respectively, is the rated voltage of the power grid; S4. According to the found dq Positive and negative sequence current reference values of the shaft generate an inverter modulation signal through positive and negative sequence separation control.

2. The multi-objective dual-sequence current injection method under three-phase unbalanced faults for maximizing power output according to claim 1, wherein The three-phase voltage and current of the obtained PCC point are subjected to Park transformation and 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.

3. The multi-objective dual-sequence current injection method under three-phase unbalanced faults for maximizing power output according to claim 1, wherein According to and The specific method for calculating the positive and negative sequence voltages of the power grid and their phase angle differences under the current fault scenario is as follows: and their phase angle differences is as follows: Positive and negative sequence voltages at the PCC point and current The relationships are as follows: ； ； Wherein, 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 grid voltage is as follows: ； In the formula, is the initial phase angle of the positive-sequence voltage, is the initial phase angle of the negative-sequence voltage.

4. The multi-objective dual-sequence current injection method under three-phase unbalanced faults for maximizing power output according to claim 1, characterized in that In S3, the voltage support model is: ； Wherein, and are respectively d axis and q the positive-sequence voltages of the PCC points on the axis; and are respectively d axis and q the negative-sequence voltages of the PCC points on the axis; The active power fluctuation model is: ； Wherein, and are respectively d the positive-sequence voltages of the PCC points on the q axis and the axis; and d are respectively q the positive-sequence currents of the PCC points on the axis and the axis, d q andare respectively the negative-sequence currents of the PCC points on the axis and the axis; The grid-connected inverter current model is: ； Wherein, and are respectively the three-phase currents of points A, B, and C of the PCC.

5. The multi-objective dual-sequence current injection method under three-phase unbalanced faults for maximizing power output according to claim 4, characterized in that The specific content for obtaining the voltage support model and the grid-connected inverter current model includes: At abc coordinates, the positive sequence component expression of the three-phase voltage at the PCC point is: ； Among them: ； The negative-sequence component expression of the PCC point three-phase voltage is: ； Among them: ； 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 is obtained that: ； Wherein: k 1 = [0, -1, 1]; k 2 = [0, 1, -1]; After simplification, the voltage support model is: ； The grid-connected inverter current model is: 。 6. The multi-objective two-sequence current injection method under three-phase unbalanced faults for maximizing power output according to claim 4, characterized in that, The specific content for obtaining the active power fluctuation model includes: Obtained according to the instantaneous power theory dq Active and reactive power expressions of GCI output in the coordinate system: ； Among them: ； ； In the formula, and are the constant components of the active power and the reactive power respectively, and are the active double-frequency components, and are the reactive double-frequency components; Then the active power fluctuation value is as follows: 。 7. The multi-objective dual-sequence current injection method under three-phase unbalanced faults for maximizing power output according to claim 1, characterized in that With meeting the voltage constraint of the PCC point, not exceeding the active power fluctuation constraint, and not exceeding the current limit I N The specific content of the logical judgment with the above constraints includes: Settings dq The initial value of the positive and negative sequence currents of the shaft is - I N , and the maximum value among the three-phase voltages is calculated based on the voltage support constraint, the active power fluctuation constraint, and the grid-connected inverter current constraint , the active power fluctuation , and the maximum value among the three-phase currents , and judge in turn according to the following constraint conditions and whether it meets the conditions: ； ； ； Wherein, is the power fluctuation that causes the capacitor voltage fluctuation; is the power fluctuation that causes the third 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 m the current dq positive and negative sequence currents of the shaft will be updated, and the loop judgment will be entered again; If any one of the conditions is not satisfied, 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: Judge respectively whether the positive and negative sequence currents of the dq axis after update both fall within [- I N , I N . If within this range, then judge whether the dq corresponding to the positive and negative sequence currents of the and meet the conditions according to the constraint conditions. If not, enter the loop judgment again. If they meet the conditions, store the current dq axis current data; if not within this range, then the active power output of the inverters corresponding to all the dq axis current data stored is obtained to get the maximum active power output of the inverter and its corresponding dq axis positive and negative sequence currents as the dq axis positive and negative sequence current reference values.

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