An asymmetric fault multi-objective low voltage ride through control method and system for a virtual synchronous generator

Abstract

The present application relates to a kind of asymmetric fault multi-objective low voltage ride through control method and system of virtual synchronous generator, belong to new energy grid-connected converter control technical field.Real-time detection grid-connected point voltage, bypass virtual synchronous generator power ring when asymmetric voltage drop occurs, freeze the phase of A-phase grid voltage before fault and establish virtual reference coordinate system;Using the fourth bridge arm eliminates neutral point voltage offset, by B-phase, C-phase voltage vector pre-rotation establishes the direct mapping of three-phase output voltage and positive and negative sequence voltage;Under the constraint of three-phase instantaneous current limiting and converter capacity, according to the priority of positive sequence voltage support, negative sequence voltage suppression and positive sequence active power maximization, generate three-phase output voltage reference value during fault.The method can limit fault current and output capacity, improve grid-connected point positive sequence voltage, reduce negative sequence voltage and voltage unbalance degree.

Description

Technical Field

[0001] This invention belongs to the field of new energy grid-connected converter control technology, and relates to a method and system for multi-objective low voltage ride-through control of asymmetric faults of a virtual synchronous generator. Background Technology

[0002] With the continuous increase in grid-connected capacity of new energy power generation units such as wind power, photovoltaic power, and energy storage systems, the proportion of power electronic power sources in the power system continues to rise. The rotational inertia, damping, and voltage support capabilities provided by traditional synchronous generators are relatively weakened. New energy grid-connected converters need to have the ability to actively establish AC voltage and frequency, provide inertial response, and support grid voltage.

[0003] A Virtual Synchronous Generator (VSG) simulates the rotor motion characteristics, inertia characteristics, damping characteristics, and voltage source external characteristics of a synchronous generator, enabling grid-connected converters to exhibit operating characteristics similar to those of a synchronous generator, thereby enhancing the grid-connected converter's ability to support grid frequency and voltage.

[0004] However, VSGs still use power semiconductor devices as their power interface, and their short-time overcurrent withstand capability is significantly lower than that of synchronous generators. When a low-voltage fault occurs in the power grid, the grid voltage changes rapidly within a short period of time. The amplitude and phase of the VSG output voltage are limited by the dynamic characteristics of the active and reactive power loops, making it difficult to keep up with the grid voltage changes in a timely manner. This can easily lead to a large voltage difference between the inverter and the grid, resulting in fault current exceeding the limit.

[0005] Especially under asymmetrical grid fault conditions, the voltage at the grid connection point simultaneously contains positive-sequence, negative-sequence, and zero-sequence components. To increase the positive-sequence voltage at the grid connection point, a corresponding positive-sequence current needs to be injected into the grid; conversely, to suppress the negative-sequence voltage at the grid connection point, a corresponding negative-sequence current needs to be injected. The positive-sequence and negative-sequence currents jointly occupy the limited current capacity of the converter, leading to mutual coupling between positive-sequence voltage support, negative-sequence voltage suppression, and three-phase fault current limiting.

[0006] Existing asymmetric fault ride-through control methods typically first separate the positive-sequence and negative-sequence components, then generate positive-sequence and negative-sequence current commands respectively, and indirectly regulate the grid connection point voltage through a current closed loop. However, due to the limited current capacity of the converter and the achievable range of the current reference value, conflicts can easily arise between the positive-sequence voltage support target and the negative-sequence voltage suppression target.

[0007] In addition, asymmetrical grid faults can cause second-harmonic fluctuations in active and reactive power, which affect the generation of voltage reference values ​​through the VSG power feedback loop. Because the VSG power loop has dynamic elements such as virtual inertia, integration, and droop control, it is prone to dynamic hysteresis during faults, affecting the rapid adjustment of the three-phase output voltage reference values, and may even cause a further increase in fault current or power angle instability.

[0008] Three-phase four-arm inverters, compared to three-phase three-arm inverters, add a fourth arm, providing additional zero-sequence current paths and neutral point potential adjustment freedom, making them suitable for asymmetrical operating conditions such as single-phase voltage dips. However, current technology lacks a low-voltage ride-through control method that can fully utilize the fourth arm's zero-sequence adjustment freedom and directly coordinate the positive-sequence voltage, negative-sequence voltage, and positive-sequence active power at the grid connection point under unified three-phase current constraints and converter capacity constraints.

[0009] Therefore, it is necessary to propose a three-phase four-arm VSG asymmetrical fault low voltage ride-through control method and system, which can improve the positive sequence voltage at the grid connection point, suppress the negative sequence voltage and voltage imbalance, and take into account the positive sequence active power output, while ensuring that the three-phase instantaneous current and converter capacity do not exceed the limits. Summary of the Invention

[0010] In view of this, the purpose of this invention is to provide a multi-objective low-voltage ride-through control method and system for asymmetrical faults of virtual synchronous generators, so as to solve the following problems existing in the prior art under asymmetrical power grid fault conditions: The VSG power loop has a slow dynamic response speed, making it difficult to adjust the three-phase output voltage reference value in a timely manner during a fault. Positive sequence voltage support, negative sequence voltage suppression, and fault current limiting are mutually coupled; It is difficult to fully utilize the voltage and capacity regulation capabilities of the converter by indirectly adjusting the grid connection point voltage solely through sequence current commands. During asymmetrical faults, three-phase instantaneous current overruns and converter apparent power overruns are likely to occur. The three-phase, three-bridge arm structure lacks zero-sequence regulation freedom, making it difficult to eliminate voltage offset between the inverter neutral point and the grid neutral point.

[0011] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a multi-objective low-voltage ride-through control method for asymmetrical faults of a virtual synchronous generator, which is applied to a three-phase four-arm virtual synchronous generator grid-connected system and includes the following steps.

[0012] S1: Detect unbalanced grid faults and bypass the VSG power loop The grid connection point voltage of the three-phase four-arm virtual synchronous generator grid connection system is collected in real time, the amplitude or effective value of the grid connection point voltage is calculated, and it is compared with a preset safety threshold.

[0013] When the voltage at the grid connection point drops below the preset safety threshold, a low-voltage fault is determined to have occurred in the power grid, and a low-voltage ride-through trigger signal is generated.

[0014] In response to the low voltage ride-through trigger signal, the VSG rotor motion equation integrator and reactive voltage droop controller are bypassed, causing the phase angle and virtual electromotive force amplitude output by the power loop to stop being transmitted to the voltage loop during a fault.

[0015] During a fault, the internal state of the VSG power loop follows the grid voltage changes to reduce the difference between the power loop output and the low-voltage ride-through control output when the fault is recovered. After the grid connection point voltage is detected to have recovered, the reference input of the voltage loop is flexibly switched from the three-phase output voltage reference value generated by the low-voltage ride-through control back to the VSG power loop output.

[0016] The VSG active power control loop can adopt the following exemplary rotor motion equation: (1) In the formula, J For virtual rotational inertia, D This is the virtual damping coefficient. Given virtual mechanical power, The actual electromagnetic active power output by the inverter. For VSG virtual angular frequency, The rated angular frequency, This is the angular frequency of the power grid.

[0017] The VSG reactive power control loop can generate a virtual electromotive force amplitude reference value based on the grid connection point voltage deviation and reactive power deviation. Since the active power control loop includes a virtual inertia element and the reactive power control loop includes an integral or droop adjustment element, both of which have dynamic hysteresis, they are bypassed during faults.

[0018] S2: Construct a virtual reference coordinate system with phase angle memory function At the moment when low voltage ride-through control is triggered, the phase of the A-phase grid voltage before the fault occurs is frozen. The frozen A-phase grid voltage phasor direction is used as the horizontal axis direction, and the direction orthogonal to the horizontal axis direction is used as the vertical axis direction to construct a virtual reference coordinate system with phase angle memory function.

[0019] In the virtual reference coordinate system, the three-phase output voltage vectors of the inverter after the fault are represented as follows: (2) The three-phase voltage vectors of the power grid after the fault are expressed as follows: (3) in, These are the coordinates of the output voltage vectors of inverter phases A, B, and C in the horizontal axis of the virtual reference coordinate system, respectively. These are the coordinates of the inverter's output voltage vectors for phases A, B, and C in the vertical direction of the virtual reference coordinate system.

[0020] These are the coordinates of the voltage vectors of phases A, B, and C of the power grid after the fault, on the horizontal axis. These are the coordinates of the voltage vectors of phases A, B, and C of the power grid after the fault, along the vertical axis.

[0021] Therefore, the six-dimensional three-phase output voltage coordinate variables to be optimized are: (4) After a power grid failure, the three-phase power grid voltage vector can be obtained through the voltage sampling module. Therefore, each coordinate in equation (3) is a known quantity; the six coordinates in equation (4) are control variables to be determined.

[0022] S3: Eliminate neutral point voltage offset using the fourth bridge arm The fourth arm of a three-phase four-arm inverter is connected to the inverter neutral point. By adjusting the output of the fourth arm, zero-sequence voltage and zero-sequence current can be freely adjusted.

[0023] When the zero-sequence component of the inverter's three-phase output voltage is equal to the zero-sequence component of the grid's three-phase voltage, the voltage offset between the inverter's neutral point and the grid's neutral point is canceled out.

[0024] In the virtual reference coordinate system, the neutral point voltage offset elimination constraint is: (5) (6) Equations (5) and (6) represent the zero-sequence voltage balance conditions in the horizontal and vertical directions, respectively.

[0025] S4: Establish the mapping relationship between the three-phase output voltage and the positive and negative sequence voltages. In order to directly calculate the positive sequence voltage and negative sequence voltage without using the traditional positive and negative sequence separation control loop, the coordinates of the inverter's B-phase output voltage vector and C-phase output voltage vector are pre-rotated.

[0026] The output voltage vector of phase B is rotated 120° counterclockwise, and the output voltage vector of phase C is rotated 240° counterclockwise, so that both the output voltage vectors of phase B and phase C are rotated to the phase direction of phase A. These are then combined with the output voltage vector of phase A to obtain the positive sequence voltage component of the inverter. (7) in: (8) (9) The positive sequence voltage amplitude of the inverter is: (10) The output voltage vector of phase B is rotated 240° counterclockwise, and the output voltage vector of phase C is rotated 120° counterclockwise. These are then combined with the output voltage vector of phase A to obtain the negative sequence voltage component of the inverter. (11) in: (12) (13) The inverter negative sequence voltage amplitude is: (14) Following the same coordinate transformation method as equations (8) and (9), the horizontal and vertical components of the positive sequence voltage of the power grid in the virtual reference coordinate system can be obtained: (15) (16) By using the above coordinate pre-rotation and vector synthesis, there is no need to set up traditional positive sequence separation and negative sequence separation links in the control loop. The positive sequence voltage and negative sequence voltage can be directly calculated from the six three-phase output voltage coordinate variables.

[0027] S5: Constructing Three-Phase Instantaneous Current Limiting Constraints Let the line impedance between the inverter and the power grid be: (17) In the formula, R For line resistance, X For the line reactance, and: (18) For any phase i The difference between the inverter output voltage vector and the corresponding grid voltage vector is equal to the voltage drop across the line impedance of that phase line current, i.e.: (19) In the formula, For the first i Phase line current.

[0028] To ensure the safe operation of power semiconductor devices during asymmetrical faults, the maximum permissible current is set to the rated current. The preset multiple. In this embodiment, the maximum allowable current is set to 1.2 times the rated current, that is: (20) Transforming equations (19) and (20) into the virtual reference coordinate system, we obtain: (twenty one) (twenty two) (twenty three) Equations (21) to (23) geometrically correspond to three current limiting circles, respectively. For any phase, the endpoint of the inverter output voltage vector should be located at a point centered on the endpoint of the corresponding grid voltage vector. The inside of a current limiting circle with radius [missing information].

[0029] S6: Construct converter capacity constraints Inverter i The apparent power of a phase can be expressed as: (twenty four) The sum of the apparent power of the three phases is: |(25) Substituting equation (19) into equation (25), we get: (26) Assume the rated phase voltage amplitude is Rated current is The three-phase rated capacity corresponds to In this embodiment, the maximum apparent power output of the converter during low-voltage ride-through is set to 1.2 times its rated capacity, with the capacity constraint being: (27) The three-phase instantaneous current limiting constraint is used to limit the instantaneous current of any phase, and the capacity constraint is used to limit the overall output capacity of the inverter. Together, they constitute the hard constraints for the safe operation of the inverter.

[0030] S7: Construct a multi-objective control model with priority. Under the premise of satisfying the neutral point voltage offset elimination constraint, the three-phase instantaneous current limiting constraint and the converter capacity constraint, the positive sequence voltage support target, the negative sequence voltage suppression target and the positive sequence active power maximization target are set in order of priority from high to low.

[0031] 1. Positive sequence voltage supports the target The primary control objective is to increase the positive sequence voltage at the grid connection point. The supporting constraints for the positive sequence voltage are: (28) Under the condition of meeting the constraints of three-phase current and converter capacity, priority should be given to restoring the positive sequence voltage during the fault to the rated voltage amplitude or above.

[0032] 2. Negative sequence voltage suppression target Voltage unbalance is defined as: (29) Voltage imbalance is reduced by decreasing the magnitude of negative sequence voltage and increasing the magnitude of positive sequence voltage.

[0033] The target for negative sequence voltage suppression is: (30) In the formula, This is the preset voltage imbalance threshold.

[0034] In one implementation, the external control target can set the preset voltage imbalance threshold to 9%; when constructing the numerical optimization penalty term, an internal optimization threshold of 8% can be used to retain a certain control margin in the presence of constraint error and algorithm convergence error.

[0035] 3. The objective of maximizing positive-sequence active power After meeting the targets of positive sequence voltage support and negative sequence voltage suppression, the remaining capacity of the converter is used to increase the positive sequence active power output.

[0036] Positive sequence active power for: (31) In the formula, and These are the abscissa and ordinate components of the positive sequence voltage of the inverter, respectively. and These represent the abscissa and ordinate components of the positive sequence voltage of the power grid, respectively.

[0037] S8: Automatic optimization based on penalty function The neutral point voltage offset elimination constraint, three-phase instantaneous current limiting constraint, converter capacity constraint, and the three control objectives are converted into corresponding penalty terms.

[0038] The neutral point offsets in the horizontal and vertical directions are defined as follows: (32) (33) The normalized violations of the three-phase current limiting constraints are as follows: (34) (35) (36) The normalized violation of the converter capacity constraint is: (37) The normalized deviation of the positive sequence voltage support target is: (38) The normalized bias of the negative sequence voltage suppression target is: (39) The normalized deviation of the objective of maximizing positive-sequence active power can be expressed as: (40) In the formula, This is the positive-sequence active power reference value set according to the rated capacity of the converter.

[0039] Construct the total penalty function: (41) In the penalty function, the weight coefficients of different penalty terms represent the priority of different constraints and control objectives.

[0040] The weighting coefficient for neutral point voltage offset elimination constraint is The weighting coefficients for the three-phase instantaneous current limiting constraint and the converter capacity constraint are: The weighting coefficient for the positive sequence voltage support target is: The weighting coefficient for the negative sequence voltage suppression target is: The weighting coefficient for the objective of maximizing positive-sequence active power is: .

[0041] Since the optimization process aims to minimize the total penalty function, the larger the weight coefficient, the higher the priority of the corresponding constraint or control objective.

[0042] A numerical optimization algorithm capable of handling nonlinear constraints is employed, using six-dimensional three-phase output voltage coordinate variables. As the decision variables, minimize equation (41) to obtain the optimal three-phase output voltage coordinates: (42) The numerical optimization algorithm is used to calculate the reference value of the three-phase output voltage under given constraints and objectives. The specific type of numerical optimization algorithm does not constitute a limitation of this invention.

[0043] S9: Generate three-phase output voltage reference value Based on the optimal three-phase output voltage coordinates obtained through optimization, the amplitude of the output voltage of each phase of the inverter is calculated respectively: (43) And the additional phase angle of each phase output voltage: (44) in, This represents the arctangent operation that determines the phase angle based on the quadrants of the x and y coordinates.

[0044] Generate reference values ​​for the three-phase output voltage during a fault: (45) (46) (47) The three-phase output voltage reference values ​​obtained from equations (45) to (47) are directly input into the voltage loop, and the switching control signal of the three-phase four-bridge arm inverter is generated by the voltage loop and the modulation link.

[0045] III. Low Voltage Ride-Through Control System The present invention also provides a three-phase four-arm virtual synchronous generator asymmetric fault multi-target low voltage ride-through control system, including a three-phase four-arm inverter, a filter circuit, a grid connection line and a controller.

[0046] A three-phase four-arm inverter includes phase A, phase B, phase C, and a fourth arm. The fourth arm is connected to the inverter neutral point and is used to provide a zero-sequence current path and regulate the inverter neutral point potential.

[0047] The filter circuit includes a filter inductor and a filter capacitor. The three-phase four-arm inverter is connected to the power grid through the filter circuit and the grid connection line.

[0048] The controller includes a fault detection module, a power loop bypass module, a virtual reference coordinate system construction module, a sequence component mapping module, a constraint optimization module, a reference voltage generation module, and a switching control module.

[0049] The fault detection module is used to monitor the grid connection point voltage in real time and output a low voltage ride-through trigger signal when the grid connection point voltage drops below a preset safety threshold.

[0050] The power loop bypass module is used to respond to the low voltage ride-through trigger signal and bypass the VSG rotor motion equation integrator and reactive voltage droop controller.

[0051] The virtual reference coordinate system construction module is used to freeze the phase of the A-phase grid voltage before the fault and construct a virtual reference coordinate system with phase angle memory function.

[0052] The sequence component mapping module is used to establish a direct mapping relationship between the three-phase output voltage coordinate variables and the positive-sequence voltage components and negative-sequence voltage components by pre-rotating the coordinates of the B-phase and C-phase voltage vectors.

[0053] The constraint optimization module is used to optimize the three-phase output voltage coordinate variables in the order of priority, namely positive sequence voltage support, negative sequence voltage suppression, and positive sequence active power maximization, under the conditions of neutral point voltage offset elimination constraint, three-phase instantaneous current limiting constraint, and converter capacity constraint.

[0054] The reference voltage generation module is used to generate three-phase output voltage reference values ​​based on the optimal three-phase output voltage coordinates.

[0055] The switching control module is used to switch the reference input of the voltage loop to the output of the reference voltage generation module during a grid fault, and to flexibly switch the reference input of the voltage loop back to the output of the VSG power loop after the grid fault is restored.

[0056] The beneficial effects of this invention are as follows: (1) After detecting a low voltage fault, bypass the VSG conventional power loop to avoid the influence of virtual inertia, integral circuit and double frequency power fluctuation on the adjustment speed of the three-phase output voltage reference value.

[0057] (2) In a virtual reference coordinate system with phase angle memory function, a direct mapping between the three-phase output voltage coordinate variables and the positive and negative sequence voltages is established, without the need to indirectly adjust the grid connection point voltage through the positive and negative sequence current reference values.

[0058] (3) The zero-sequence voltage and zero-sequence current are provided by the fourth bridge arm, so that the inverter zero-sequence voltage matches the grid zero-sequence voltage and eliminates the voltage offset between the inverter neutral point and the grid neutral point.

[0059] (4) The three-phase instantaneous current limiting constraint and the overall capacity constraint of the converter are simultaneously incorporated into the optimization model to prevent the fault current of any phase from exceeding the limit and to prevent the overall apparent power of the converter from exceeding the allowable capacity.

[0060] (5) The regulation capacity is allocated according to the priority order of positive sequence voltage support, negative sequence voltage suppression and positive sequence active power output. On the basis of ensuring safety constraints, the positive sequence voltage is increased first, and the remaining capacity is used to suppress the negative sequence voltage and output positive sequence active power.

[0061] (6) Automatic optimization is performed by acquiring the three-phase grid voltage coordinates after the fault in real time. It does not depend on a fixed fault type and can be applied to single-phase voltage drop and other asymmetrical grid faults containing negative sequence and zero sequence components.

[0062] Other advantages, objectives, and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination, or may be learned from practice of the invention. The objectives and other advantages of the invention can be realized and obtained through the following description. Attached Figure Description

[0063] To make the objectives, technical solutions, and advantages of the present invention clearer, the preferred embodiments of the present invention will be described in detail below with reference to the accompanying drawings, wherein: Figure 1 This is a schematic diagram of the main circuit topology of the three-phase four-bridge arm virtual synchronous generator grid-connected system of the present invention; Figure 2 This is a schematic diagram of the inverter control system structure for the bypass power loop during a fault in this invention. Figure 3 This is a schematic diagram illustrating the vector relationship for calculating the positive sequence voltage under the condition of voltage amplitude drop in phase A single phase according to the present invention; Figure 4 This is a schematic diagram illustrating the vector relationship for calculating the negative sequence voltage under the condition of single-phase voltage amplitude drop in phase A according to the present invention; Figure 5 The simulation waveforms of this invention under asymmetrical power grid fault conditions. Figure 1 ; Figure 6 The simulation waveforms of this invention under asymmetrical power grid fault conditions. Figure 2 . Detailed Implementation

[0064] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the illustrations provided in the following embodiments are only schematic representations of the basic concept of the present invention. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0065] The accompanying drawings are for illustrative purposes only and are schematic diagrams, not actual pictures. They should not be construed as limiting the invention. To better illustrate the embodiments of the invention, some parts in the drawings may be omitted, enlarged, or reduced, and do not represent the actual product dimensions. It is understandable to those skilled in the art that some well-known structures and their descriptions may be omitted in the drawings.

[0066] In the accompanying drawings of the embodiments of the present invention, the same or similar reference numerals correspond to the same or similar components. In the description of the present invention, it should be understood that if terms such as "upper," "lower," "left," "right," "front," and "rear" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the drawings, they are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, the terms used to describe positional relationships in the drawings are only for illustrative purposes and should not be construed as limiting the present invention. For those skilled in the art, the specific meaning of the above terms can be understood according to the specific circumstances.

[0067] Example 1: Three-phase four-arm VSG grid-connected system like Figure 1 As shown, this embodiment uses a three-phase four-arm inverter as the power conversion device for the VSG.

[0068] The DC side of a three-phase four-arm inverter is connected to a DC power supply or energy storage unit, and the DC side voltage is denoted as... The AC side of a three-phase four-arm inverter includes the A-phase output terminal, the B-phase output terminal, the C-phase output terminal, and the neutral point output terminal.

[0069] Phase A arm, phase B arm, and phase C arm are used to adjust the output voltage of phases A, B, and C, respectively, while the fourth arm is used to adjust the neutral point potential of the inverter.

[0070] The three-phase four-arm inverter passes sequentially through a filter inductor L, a filter capacitor C, and a circuit consisting of a line resistor R and a line inductor. The resulting grid-connected lines are connected to the power grid.

[0071] The three-phase output voltages of the inverter are denoted as follows: , , The three-phase voltages of the power grid are denoted as follows: , , The inverter grid-connected current is denoted as follows: , , .

[0072] The fourth bridge arm is connected to the inverter neutral point. The neutral point of the power grid is denoted asN By adjusting the fourth bridge arm, the zero-sequence component of the inverter's three-phase output voltage is made equal to the zero-sequence component of the grid's three-phase voltage, thereby reducing or eliminating the neutral point. With neutral point N Voltage offset between.

[0073] Example 2: Fault Detection and Power Loop Bypass Control like Figure 2 As shown, during normal grid operation, the switching module selects the VSG conventional power loop output as the voltage loop input.

[0074] The fault detection module collects the three-phase voltage at the grid connection point in real time and calculates the voltage amplitude at the grid connection point. Let the fault detection coefficient be... k When the voltage amplitude at the grid connection point is lower than k When multiplied by the rated voltage, the fault detection module outputs a fault status signal.

[0075] The fault status signal controls the switching switch to disconnect the VSG power loop output from the voltage loop, while simultaneously connecting the three-phase output voltage reference value output by the multi-target low voltage ride-through control module of this invention to the voltage loop.

[0076] During a fault, the rotor motion equation integrator and reactive voltage droop controller no longer determine the voltage loop reference value, but their internal phase angle and amplitude status are updated based on the grid voltage.

[0077] When the grid connection point voltage recovers to above the recovery threshold and continues for a preset confirmation time, the switching control module determines that the fault has been eliminated. When the amplitude difference and phase angle difference between the VSG power loop output and the low voltage ride-through control output meet the switching conditions, the voltage loop reference value is gradually switched back to the VSG power loop output to avoid voltage and current surges caused by control mode switching.

[0078] Example 3: Direct mapping of positive and negative sequence voltages like Figure 3 As shown, when a single-phase voltage amplitude drop occurs in phase A, the phase of the grid voltage of phase A before the fault is frozen, and a virtual reference coordinate system is constructed with the phase direction.

[0079] The output voltage vector of phase B is rotated 120° counterclockwise, and the output voltage vector of phase C is rotated 240° counterclockwise. The rotated output voltage vectors of phase B and phase C are combined with the output voltage vector of phase A to obtain the positive sequence voltage component.

[0080] like Figure 4 As shown, the output voltage vector of phase B is rotated 240° counterclockwise and the output voltage vector of phase C is rotated 120° counterclockwise. The rotated output voltage vectors of phase B and phase C are combined with the output voltage vector of phase A to obtain the negative sequence voltage component.

[0081] Based on this, only the six-dimensional variables need to be addressed: ; By adjusting these parameters, the positive-sequence voltage amplitude, negative-sequence voltage amplitude, three-phase current, and apparent power of the converter can be changed simultaneously.

[0082] Example 4: Automatic Optimization with Multi-Objective Constraints In this embodiment, the three-phase grid voltage after the fault is first converted into the coordinates shown in equation (3), and the six three-phase output voltage coordinate variables are set as optimization variables.

[0083] Neutral point voltage offset elimination constraint is set according to equations (5) and (6), three-phase instantaneous current limiting constraint is set according to equations (21) to (23), and converter capacity constraint is set according to equation (27).

[0084] Based on satisfying the above safety constraints, the positive sequence voltage support target is set according to equation (28), the negative sequence voltage suppression target is set according to equations (29) and (30), and the positive sequence active power maximization target is set according to equation (31).

[0085] Each constraint and objective is converted into a normalized deviation as shown in equations (32) to (40), and different weight coefficients are set according to equation (41).

[0086] The optimization algorithm aims to minimize the total penalty function of equation (41). When the neutral point voltage offset is not eliminated, the corresponding The penalty term causes the optimization algorithm to prioritize adjusting the three-phase output voltage coordinates to meet the zero-sequence balance condition.

[0087] When any phase current exceeds 1.2 times the rated current, or the total apparent power of the converter exceeds 1.2 times the rated capacity, the corresponding The penalty term causes the optimization algorithm to preferentially revert to a safe operating region.

[0088] After satisfying safety constraints, the algorithm prioritizes bringing the positive sequence voltage to its rated value, then suppresses the negative sequence voltage and voltage imbalance, and finally utilizes the remaining capacity to increase the positive sequence active power.

[0089] After optimization, the amplitude and phase angle of the three-phase output voltage are obtained according to equations (43) and (44), and the reference value of the three-phase output voltage is generated according to equations (45) to (47).

[0090] Example 5: Simulation Verification To verify the effectiveness of the control method of the present invention, a three-phase four-arm VSG grid-connected system model was established in the Matlab / Simulink simulation environment.

[0091] The main simulation parameters of this embodiment are shown in Table 1.

[0092] Table 1

[0093] Set the grid to experience a single-phase voltage amplitude drop in phase A, with a voltage drop factor of 0.7.

[0094] like Figure 5 and Figure 6 As shown, after a fault occurs, the control method of this invention bypasses the VSG conventional power loop and automatically optimizes the six-dimensional three-phase output voltage coordinate variables based on the three-phase grid voltage coordinates after the fault.

[0095] Simulation results show that during the fault, the amplitude of the three-phase current is limited to within 1.2 per unit, the apparent power of the converter is about 1.18 per unit, which does not exceed the allowable capacity; the positive sequence voltage at the grid connection point is supported to about 1.08 per unit, the voltage imbalance is suppressed to about 8.5%, and the positive sequence active power is about 0.2 per unit.

[0096] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A multi-objective low-voltage ride-through control method for asymmetrical faults of a virtual synchronous generator, applied to a three-phase four-arm virtual synchronous generator grid-connected system, characterized in that, include: The grid connection point voltage of the three-phase four-arm virtual synchronous generator grid connection system is monitored in real time. When the voltage drop at the grid connection point exceeds the preset safety threshold caused by an asymmetrical grid fault, the low voltage ride-through control is triggered and the power loop of the virtual synchronous generator is bypassed, so that the phase angle and virtual electromotive force amplitude output by the power loop stop being transmitted to the voltage loop. The phase of the A-phase grid voltage before the low voltage ride-through control is frozen. The frozen A-phase grid voltage phasor direction is used as the horizontal axis to construct a virtual reference coordinate system with phase angle memory function. In the virtual reference coordinate system, the inverter three-phase output voltage vector and the grid three-phase voltage vector after the fault are represented by two-dimensional coordinates respectively. The zero-sequence adjustment degree is provided by the fourth bridge arm of the three-phase four-bridge inverter, so that the zero-sequence component of the inverter's three-phase output voltage is equal to the zero-sequence component of the grid's three-phase voltage, thereby eliminating the voltage offset between the inverter's neutral point and the grid's neutral point. The coordinates of the inverter's B-phase output voltage vector and C-phase output voltage vector are pre-rotated to establish a direct mapping relationship between the inverter's three-phase output voltage coordinate variables and the positive-sequence voltage components and negative-sequence voltage components; Using neutral point voltage offset elimination constraint, three-phase instantaneous current limiting constraint, and converter capacity constraint as safety hard constraints, and constructing a multi-objective optimization model according to the priority order of positive sequence voltage support, negative sequence voltage suppression, and positive sequence active power maximization, the three-phase output voltage coordinate variables of the inverter are optimized to obtain the optimal three-phase output voltage coordinates that satisfy the safety hard constraints. The amplitude and phase angle of each phase output voltage of the inverter are determined based on the optimal three-phase output voltage coordinates. A reference value for the three-phase output voltage during a fault is generated and directly input into the voltage loop to perform low-voltage ride-through control on the three-phase four-arm inverter.

2. The multi-objective low-voltage ride-through control method for asymmetric faults of a virtual synchronous generator according to claim 1, characterized in that: The power loop of the bypass virtual synchronous generator includes an integrator of the rotor motion equation of the bypass virtual synchronous generator and a reactive voltage droop controller, so that the phase angle generated by the rotor motion equation and the virtual electromotive force amplitude generated by the reactive voltage droop controller are no longer used as reference inputs of the voltage loop during a fault. During a fault, the state of the power loop is made to follow the changes in grid voltage, and after the grid connection point voltage is detected to have recovered, the reference input of the voltage loop is flexibly switched from the three-phase output voltage reference value back to the output of the power loop.

3. The multi-objective low-voltage ride-through control method for asymmetrical faults of a virtual synchronous generator according to claim 1, characterized in that: In the virtual reference coordinate system, the three-phase output voltage vectors of the inverter are respectively represented as: The three-phase voltage vectors of the power grid after the fault are expressed as follows: This constitutes the coordinate variables of the three-phase output voltage to be optimized: The neutral point voltage offset elimination constraint is: 。 4. The multi-objective low-voltage ride-through control method for asymmetrical faults of a virtual synchronous generator according to claim 3, characterized in that: The positive-sequence voltage component is obtained by rotating the B-phase output voltage vector counterclockwise by 120° and the C-phase output voltage vector counterclockwise by 240°, and then combining them with the A-phase output voltage vector. ,in: The negative sequence voltage component is obtained by rotating the B-phase output voltage vector counterclockwise by 240° and the C-phase output voltage vector counterclockwise by 120°, and then combining them with the A-phase output voltage vector. ,in: 。 5. The multi-objective low-voltage ride-through control method for asymmetric faults of a virtual synchronous generator according to claim 3, characterized in that: The three-phase instantaneous current limiting constraint is as follows: for any phase i All satisfy: in, For the first i Phase line current, For the first i Phase inverter output voltage vector, For the first i Phase grid voltage vector, For line impedance, R For line resistance, X For line reactance, This is the maximum allowable current; The maximum allowable current is the rated current. When the current is 1.2 times the rated current, the three-phase instantaneous current limiting constraint is expressed as: 。 6. The multi-objective low-voltage ride-through control method for asymmetric faults of a virtual synchronous generator according to claim 5, characterized in that: The converter capacity constraint is that the sum of the apparent power of the three phases of the inverter does not exceed 1.2 times the rated capacity, and is expressed as: in, S This is the sum of the apparent power of the three phases of the inverter. Rated phase voltage, This is the rated current.

7. The multi-objective low-voltage ride-through control method for asymmetrical faults of a virtual synchronous generator according to claim 4, characterized in that: The control objective of the positive sequence voltage support is: ; The control objective of the negative sequence voltage suppression is to ensure that the voltage imbalance does not exceed a preset imbalance threshold, wherein the voltage imbalance is: ; The control objective for maximizing positive-sequence active power is to increase positive-sequence active power. The positive sequence active power Determine according to the following formula: in, and These are the abscissa and ordinate components of the positive sequence voltage of the power grid in the virtual reference coordinate system, respectively, and: , and: .

8. The multi-objective low-voltage ride-through control method for asymmetrical faults of a virtual synchronous generator according to claim 7, characterized in that: The three-phase output voltage coordinate variables are optimized using a penalty function optimization algorithm. The penalty function is: in, and These represent the constraint violations of the neutral point voltage offset elimination constraint in the horizontal and vertical directions, respectively. , and These are the normalized constraint violations of the instantaneous current limiting constraints for phases A, B, and C, respectively. This is the normalized constraint violation amount of the converter capacity constraint. This is the normalized target deviation of the positive sequence voltage support target. The normalized target deviation of the voltage imbalance relative to a preset imbalance threshold is... The normalized target deviation of the positive-sequence active power relative to the capacity boundary is given by the penalty function. P Minimum is the direction for optimization; After obtaining the optimal three-phase output voltage coordinates, the amplitude of each phase output voltage is calculated according to the following formula. and phase angle : And generate three-phase output voltage reference values: in, This is the angular frequency of the power grid.

9. A multi-objective low-voltage ride-through control system for asymmetrical faults of a virtual synchronous generator, characterized in that, It includes a three-phase four-arm inverter, a filter circuit, a grid connection line, and a controller, wherein the controller includes: The fault detection module is used to monitor the grid connection point voltage in real time and output a low voltage ride-through trigger signal when the grid connection point voltage drops below a preset safety threshold due to an asymmetrical grid fault. The power loop bypass module is used to respond to the low voltage ride-through trigger signal, bypassing the rotor motion equation integrator and reactive voltage droop controller of the virtual synchronous generator, so that the phase angle and virtual electromotive force amplitude output by the power loop stop being transmitted to the voltage loop. The virtual reference coordinate system construction module is used to freeze the phase of the A-phase grid voltage before the fault, and construct a virtual reference coordinate system with phase angle memory function with the frozen A-phase grid voltage phasor direction as the horizontal axis. The sequence component mapping module is used to perform coordinate pre-rotation on the B-phase output voltage vector and C-phase output voltage vector of the three-phase four-bridge arm inverter to establish the mapping relationship between the three-phase output voltage coordinate variables and the positive sequence voltage components and negative sequence voltage components. The constraint optimization module is used to optimize the three-phase output voltage coordinate variables in the order of priority of positive sequence voltage support, negative sequence voltage suppression, and positive sequence active power maximization, under the conditions of satisfying the neutral point voltage offset elimination constraint, three-phase instantaneous current limiting constraint, and converter capacity constraint. The reference voltage generation module is used to generate a reference value for the three-phase output voltage during a fault based on the optimal three-phase output voltage coordinates obtained through optimization, and input the reference value for the three-phase output voltage into the voltage loop.

10. The asymmetric fault multi-objective low-voltage ride-through control system for a virtual synchronous generator according to claim 9, characterized in that: The three-phase four-arm inverter includes three phase arms corresponding to phases A, B, and C, respectively, and a fourth arm. The fourth arm is connected to the neutral point of the inverter and is used to provide a zero-sequence component path and adjust the potential of the neutral point of the inverter. The constraint optimization module uses penalty functions with different weighting coefficients for optimization. The weighting coefficient of the neutral point voltage offset elimination constraint is greater than that of the three-phase instantaneous current limiting constraint and the converter capacity constraint. The weighting coefficients of the three-phase instantaneous current limiting constraint and the converter capacity constraint are greater than that of the positive sequence voltage support target. The weighting coefficient of the positive sequence voltage support target is greater than that of the negative sequence voltage suppression target. The weighting coefficient of the negative sequence voltage suppression target is greater than that of the positive sequence active power maximization target.

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