A multi-objective coordinated fault ride-through method for grid-connected converters

Abstract

The present invention discloses a multi-objective collaborative fault ride-through method suitable for a grid-type converter, which accurately limits the fault current through a ring current limiter and adjusts the power angle to the lower boundary of the transient stability domain during the fault, thereby realizing multi-objective collaborative optimization of transient synchronous stability, grid reactive support, and fault current suppression during the fault ride-through process. By conducting offline pre-rehearsal training on the system physical model, an optimal power angle collection that satisfies the multi-objective collaboration of fault ride-through under multiple fault scenarios is constructed; when a grid fault occurs, the grid-type converter performs an online table search based on the trained collection, and calculates the active power instruction that satisfies transient stability and optimal reactive support under the current fault scenario, thereby maximizing its support capacity for the grid while ensuring the safe and stable operation of the grid-type converter. The method proposed in the present invention can maintain the grid-type operation characteristics of the converter under a grid fault and enhance the multi-objective collaborative fault ride-through capability of the equipment.

Description

Technical Field

[0001] The present invention relates to the field of network control technology, and in particular to a multi-objective collaborative fault ride-through method suitable for a network-type converter. Background Art

[0002] As the proportion of traditional synchronous generators decreases, the system's short-circuit ratio decreases significantly, weakening the grid's inertia and voltage and frequency support capabilities. In recent years, grid control technology, as a method for proactively supporting the grid, has garnered significant industry attention. Through grid control technology, converters can simulate the inertia and damping effects of synchronous generators, externally representing the voltage source's operating characteristics and enhancing their support capabilities for the grid.

[0003] Grid-connected converters based on power electronic devices have limited short-term overload capabilities. To ensure operational safety during fault ride-through, fault current suppression is often required. The combination of virtual admittance control and a ring current limiter has become a popular current limiting method in recent years. This method enables the converter to maintain grid-connected characteristics during fault ride-through, and ensures that the dynamic current amplitude does not exceed its upper safety threshold in different fault scenarios. This method enables adaptive current limiting in grid-connected converters, and has therefore attracted widespread attention and application.

[0004] However, fault ride-through for grid-type converters requires them to meet requirements such as transient synchronous stability and optimal reactive power support in addition to current limiting protection. However, these control objectives conflict with each other, and currently there is still a lack of effective means to coordinate multiple control objectives during system transients. Therefore, a multi-objective coordinated fault ride-through method suitable for grid-type converters is urgently needed. Summary of the Invention

[0005] To solve the above problems, the present invention discloses a multi-objective collaborative fault ride-through method suitable for grid-type converters. While ensuring fault current limitation and transient synchronous stability, it maximizes the reactive support capability of the grid-type converter and realizes the hierarchical organic combination and collaborative optimization of multiple control objectives during the fault ride-through process.

[0006] To achieve the above object, the technical solution of the present invention is:

[0007] A multi-objective coordinated fault ride-through method applicable to a grid-type converter comprises the following steps:

[0008] Step 1: At the starting point of each sampling period, the grid-connected point voltage of the grid-connected converter is , AC inductor current Sampling is performed to obtain the output active power of the grid-type converter , reactive power 、 Grid connection point voltage Quantity , AC inductor current Quantity and the three-phase voltage amplitude at the grid connection point;

[0009] Step 2: Compare the three-phase voltage amplitude at the grid connection point with a preset low voltage threshold; if the three-phase voltage amplitude at the grid connection point is less than the preset low voltage threshold, it is determined that the grid connection point meets the low voltage fault ride-through condition, and the fault ride-through enable signal Flag is set to 1; otherwise, the low voltage fault ride-through condition is not met, and the fault ride-through enable signal Flag is set to 0;

[0010] Step 3: When the enable signal Flag is 0, the reactive power control link of the grid-type converter operates normally, and the original active power instruction of the current control cycle Remain unchanged; when the enable signal Flag is 1, the reactive power control link of the grid-type converter is frozen, and the fault transient active power instruction of the current control cycle is determined by the multi-objective collaborative fault ride-through algorithm , and replace the original active power instruction As a new reference instruction for the active power control link of the virtual synchronous generator, the current reference instruction is obtained And proceed to step 4;

[0011] The multi-objective collaborative fault-traversal algorithm is pre-built with Active power command with step length Full range of data sets, The grid voltage amplitude is the step size Full range data set and corresponding optimal power angle A collection of mapping relationships ; When Flag is 1, the three-phase voltage amplitude of the grid connection point obtained in the current sampling period is and active power command Input to mapping relationship collection , dynamically look up the table to search for the optimal power angle of the current control cycle ; Then according to the optimal power angle of the current control cycle Get the fault transient active power command of the current control cycle ;

[0012] Step 4: Output active power of the grid-connected inverter in the current control cycle , output reactive power , original active power instruction , Fault transient active power command , grid connection point voltage Quantity and AC inductor current Quantity , and obtain the modulation voltage of the grid-type converter Quantity , the modulation voltage Quantity conduct The coordinates are inversely transformed and subjected to SPWM modulation to output a PWM modulation signal to control the switching state of the power devices of each phase bridge arm in the grid-type converter.

[0013] As a further improvement, the topology of the grid-type converter is an ANPC three-level structure; the DC side of the grid-type converter is connected to a DC source, and the AC side of the converter is connected to an LC filter; the LC filter is connected to a common coupling point through a grid-connected switch to achieve connection to the grid.

[0014] As a further improvement, the step 1 includes the following steps:

[0015] 1.1) At the start of each sampling period, the sampling unit measures the grid-connected point voltage of the grid-connected converter. , AC inductor current Sampling and low-pass filtering are performed; ; ; is the terminal voltage of phase A, phase B and phase C of the grid-type converter, , are the three-phase inductor currents of phases A, B, and C of the grid-type converter; T represents the vector or matrix transpose symbol;

[0016] 1.2) Grid connection point voltage and AC inductor current The output active power of the grid-connected converter is obtained by sending it to the power calculation module of the control system. and reactive power ;

[0017] 1.3) Within the potential phase angle instruction The value of the last control cycle is used as the reference phase for coordinate transformation, and the grid voltage is and AC inductor current conduct Rotating coordinate transformation to obtain the grid connection point voltage of Quantity and AC inductor current of Quantity ; ; ; and are the d-axis component and q-axis component of the grid-connected point voltage, and are the d-axis component and q-axis component of the AC inductor current of the grid-type converter;

[0018] 1.4) According to the grid connection point voltage Quantity Determine the three-phase voltage amplitude at the grid connection point.

[0019] As a further improvement, in step 2, whether the grid at the grid connection point meets the low voltage fault ride-through condition is determined based on the three-phase voltage amplitude at the grid connection point and the preset low voltage threshold, and the fault ride-through enable signal Flag of the current control cycle is updated. The formula is:

[0020] .

[0021] As a further improvement, the step 4 includes the following steps:

[0022] 4.1) The output active power of the grid-connected converter and reactive power To the virtual synchronous generator active power control link, obtain the voltage phase angle instruction of the current control cycle and amplitude instructions , the calculation formula is:

[0023] ;

[0024] Where, is the moment of inertia; is the damping coefficient; is the reactive-voltage coefficient; is the rated voltage amplitude; is the rated voltage angular frequency; is the original active power instruction; There is a power command for the fault transient state; is the reactive power instruction; — It is a negation operator. When the enable signal Flag is 0, the reactive power control link operates normally. When the enable signal Flag is 1, the reactive power control link is frozen. is the Laplace operator;

[0025] 4.2) The voltage phase angle command and voltage amplitude command Sent to the voltage generation module to obtain the voltage reference instruction of the current control cycle ； and The voltage reference instructions are of Axis components and Axis component;

[0026] 4.3) Set the voltage reference instruction and grid connection point voltage Quantity Input to the virtual admittance control link of the control system to generate the initial current reference instruction ; ; and They are the initial current reference command Axis components and Axis component; initial current reference command The calculation formula is:

[0027] ;

[0028] Where, is the virtual resistance for virtual admittance control; is the virtual reactance for virtual admittance control;

[0029] 4.4) Set the initial current reference instruction Sent to the ring current limiter to obtain the current reference instruction after current limiting , the calculation formula is:

[0030] ;

[0031] Where, is the current limiting adjustment factor; is the maximum current limit value of the grid-type converter; Calculate identifier for the two-norm;

[0032] 4.5) Through the grid voltage Quantity , AC inductor current Quantity and current reference instructions Get the modulation voltage of the grid-type converter Quantity , the calculation formula is:

[0033] ;

[0034] Where, is the proportional coefficient of the inner current loop, is the integral coefficient of the inner current loop, is the imaginary unit, is the inductance value of the filter inductor of the grid-type converter;

[0035] 4.6) The modulation voltage Quantity conduct The coordinates are inversely transformed and modulated by SPWM to output PWM modulation signals to control the switching states of the power devices of each phase bridge arm in the grid-type converter.

[0036] Further improvement, in step 3, the relationship collection The build steps are as follows:

[0037] 3.1.1) Regularly storing the system parameters of the power grid, including the voltage amplitude , grid resistance and grid reactance , initialize system parameters;

[0038] 3.1.2) Active power command Set it to 0, and solve the initial power angle corresponding to the power command point according to the power angle characteristic model of the grid-type converter under normal working conditions ; The power angle characteristic model formula is:

[0039] ;

[0040] Where, Output active power Power Angle Abbreviated expression of the relationship between them;

[0041] 3.1.3) For a fixed step size, add active power instruction , and repeat step 3.1.2) to solve the initial power angle corresponding to each power command point , until the active power command Increase to ;

[0042] 3.1.4) Active power instructions calculated according to the cycle and its corresponding initial power angle As a result, a primary mapping relationship set of active power command and initial power angle is constructed. ;

[0043] 3.1.5) The grid voltage amplitude Set to 0, and solve the grid voltage amplitude under the fault drop depth according to the integral relationship of the multi-objective cooperative domain of the grid-type converter The optimal power angle corresponding to each power command point The integral relationship of the multi-objective collaborative domain is:

[0044] ;

[0045] Where, the integral lower limit power angle For collection Active power command The corresponding initial power angle ; Indicates the power angle; represents the differential operator;

[0046] is the relationship between the current limiting adjustment factor and the power angle, expressed as:

[0047] ;

[0048] Where, An operation to extract the real part of a complex number;

[0049] 3.1.6) is a fixed step size, increasing the grid voltage amplitude , and repeat step 3.1.5) to solve the optimal power angle corresponding to each power command point and each fault drop depth , until the grid voltage amplitude Increase to ;

[0050] 3.1.7) Active power instructions calculated according to the cycle , grid voltage amplitude and its corresponding optimal power angle As a result, a set of secondary mapping relationships among active power command, grid voltage amplitude and optimal power angle is constructed. .

[0051] Further improvement, in step 3, the optimal power angle under the current fault condition is searched dynamically The steps are as follows:

[0052] 3.2.1) In each control cycle, the three-phase voltage amplitude at the grid connection point and the preset low voltage threshold are detected to determine whether the grid at the grid connection point meets the conditions for entering low voltage fault ride-through;

[0053] 3.2.2) If the three-phase voltage amplitude at the grid connection point is less than the low voltage threshold, it is determined that the grid at the grid connection point has entered a low voltage fault ride-through condition, and a fault ride-through enable signal Flag is set to 1;

[0054] 3.2.3) The three-phase voltage amplitude of the grid connection point in the current sampling period and active power command Input to mapping relationship collection , dynamically look up the table to search for the optimal power angle of the current control cycle ;

[0055] 3.2.4) According to the optimal power angle of the current control cycle Calculate the current limiting adjustment factor , the calculation formula is:

[0056] ;

[0057] 3.2.5) Current limiting adjustment factor according to the current control cycle Calculate the fault transient active power command of the current control cycle , the calculation formula is:

[0058]

[0059] Where, The margin factor reserved to prevent active power flow from returning; Indicates taking the maximum value, Represents an operation that extracts the real part of a complex number.

[0060] Advantages of the present invention:

[0061] The present invention conducts offline pre-training through system parameters to construct an optimal power angle collection that satisfies multi-objective hierarchical coordination of fault crossing under multiple fault scenarios. Subsequently, based on the method proposed in the present invention, the grid-type converter can search the collection online under different faults, and calculate the active power instructions that meet transient synchronization stability and optimal reactive support, so as to achieve safe and stable operation of the grid-type converter during fault crossing while maximizing the support capacity of the grid-type converter for the power grid. The method proposed in the present invention can maintain the grid-type operation characteristics of the converter under a short-circuit fault in the power grid, and can be extended to various types of voltage source power electronic equipment in various application scenarios to enhance the multi-objective collaborative fault crossing capability of the equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 Schematic diagram of the structure of a grid-type converter according to an embodiment of the present invention;

[0063] Figure 2 is a control block diagram of a multi-objective collaborative fault traversal method according to an embodiment of the present invention;

[0064] Figure 3a The simulation waveform is shown when the grid voltage drops symmetrically to 0.6 pu in the three-phase grid converter according to the embodiment of the present invention using the traditional virtual synchronous generator control method, where A11 is the grid connection point voltage. , waveform, A12 is the AC inductor current Waveform, A13 is active power Waveform, A14 is reactive power Waveform, A15 is the power angle waveform;

[0065] Figure 3b for Figure 3a Simulation waveforms of the control method proposed by the present invention under the same fault conditions, where A21 is the grid connection point voltage , waveform, A22 is the AC inductor current Waveform, A23 is active power Waveform, A24 is reactive power Waveform, A25 is the power angle waveform;

[0066] Figure 4a The simulation waveform is shown when the grid voltage drops symmetrically to 0.2 pu in the traditional virtual synchronous generator control method for the grid-connected converter according to the embodiment of the present invention, where A31 is the grid connection point voltage. , waveform, A32 is the AC inductor current Waveform, A33 is active power Waveform, A34 is reactive power Waveform, A35 is the power angle waveform;

[0067] Figure 4b for Figure 4a Simulation waveforms of the control method proposed by the present invention under the same fault conditions, where A41 is the grid connection point voltage , waveform, A42 is the AC inductor current Waveform, A43 is active power Waveform, A44 is reactive power Waveform, A45 is the power angle waveform. DETAILED DESCRIPTION

[0068] To make the objectives, technical solutions, and advantages of the embodiments of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0069] See attached Figure 1, is a structural diagram of a grid-type converter according to an embodiment of the present invention, comprising a DC source, a DC capacitor, an ANPC three-level converter, an AC LC filter, and a control system; the DC source is connected in parallel to both sides of the DC capacitor; the DC capacitor is connected to the ANPC three-level converter; the ANPC three-level converter is connected to the AC LC filter; the AC LC filter is connected to the PCC point to achieve connection with the grid; the control system mainly consists of a multi-objective coordinated fault ride-through control link and a virtual synchronous generator control link.

[0070] See attached Figure 2 , which is a control block diagram of a multi-objective collaborative fault ride-through method according to an embodiment of the present invention, including two sub-links: offline design and online query calculation. The offline design link constructs an optimal power angle collection that satisfies the multi-objective hierarchical collaboration of fault ride-through under multiple fault scenarios by performing offline pre-rehearsal training on the physical model containing the system parameters, specifically including a system parameter initialization module, an initial power angle calculation module, and an optimal power angle acquisition module. The online query calculation link includes a fault detection module, a mapping relationship table lookup module, and a power instruction calculation module. This link performs an online table lookup search on the trained collection under a power grid fault, and calculates the active power instruction that meets the transient synchronous stability and optimal reactive support under the current fault scenario, thereby realizing the hierarchical organic combination and collaborative optimization of transient synchronous stability, grid reactive support, and fault current suppression during the fault ride-through process of the grid-type converter. The specific method includes:

[0071] 1) At the start of each sampling period, the sampling unit measures the grid-connected point voltage of the grid-connected converter. , AC inductor current Sampling and low-pass filtering are performed; ; ; is the three-phase terminal voltage of the grid-type converter, is the three-phase inductor current of the grid-type converter;

[0072] 2) The grid connection point voltage obtained by sampling in step 1) and AC inductor current The output active power of the grid-connected converter is obtained by sending it to the power calculation module of the control system. and reactive power ;

[0073] 3) Within the potential phase angle instruction The value of the last control cycle is used as the reference phase for coordinate transformation, and the grid voltage is and AC inductor current conduct Rotating coordinate transformation to obtain the grid connection point voltage Quantity and AC inductor current Quantity ; and The grid voltage Axis components and Axis component, and For grid-type converters Axis components and Axis component;

[0074] 4) According to the grid connection point voltage Quantity Determine the three-phase voltage amplitude at the grid connection point;

[0075] 5) The three-phase voltage amplitude at the grid connection point is sent to the fault detection module in the online query calculation link. Based on the three-phase voltage amplitude at the grid connection point and the preset low voltage threshold, it is determined whether the grid connection point meets the low voltage fault ride-through conditions, and the fault ride-through enable signal Flag of the current control cycle is updated. The formula is:

[0076] (1);

[0077] 6) If the fault ride-through enable signal Flag is 1, the active power command is determined through the multi-objective coordinated fault ride-through control link , and replace the original instruction New reference instructions for the active power control link of virtual synchronous generators;

[0078] 7) The output active power of the grid-type converter and reactive power To the virtual synchronous generator active power control link, obtain the voltage phase angle index of the current control cycle and amplitude instructions , the calculation formula is:

[0079] (2);

[0080] Where, is the moment of inertia; is the damping coefficient; is the reactive-voltage coefficient; is the rated voltage amplitude; is the rated voltage angular frequency; is the reactive power instruction; — It is a negation operator. When the enable signal Flag is 0, the reactive power control link operates normally. When the enable signal Flag is 1, the reactive power control link is frozen.

[0081] 8) The voltage phase angle command and voltage amplitude command Sent to the voltage generation module to obtain the voltage reference instruction of the current control cycle ; and For the voltage reference command Axis components and Axis component;

[0082] 9) Set the voltage reference instruction and grid connection point voltage Quantity Input to the virtual admittance control link of the control system to generate the initial current reference instruction ; and The initial current reference command Axis components and Axis component; the calculation formula of the initial current reference instruction is:

[0083] (3);

[0084] Where, is the virtual resistance for virtual admittance control; is the virtual reactance for virtual admittance control;

[0085] 10) Set the initial current reference instruction Sent to the ring current limiter to obtain the current reference instruction after current limiting , the calculation formula is:

[0086] (4);

[0087] Where, is the current limiting adjustment factor; is the maximum current limit value of the grid-type converter; Calculate identifier for the two-norm;

[0088] 11) Set the grid connection point voltage Quantity , AC inductor current Quantity and current reference instructions , and obtain the modulation voltage of the grid-type converter Quantity , the calculation formula is:

[0089] (5);

[0090] 12) The modulation voltage Quantity conduct The coordinates are inversely transformed and modulated by SPWM to output PWM modulation signals to control the switching states of the power devices of each phase bridge arm in the grid-type converter.

[0091] Furthermore, the offline design sub-link of the multi-objective collaborative fault ride-through control link in the present invention includes:

[0092] 1) Regularly adjust the voltage amplitude of the grid , grid resistance and grid reactance The system parameters are stored in the system parameter initialization module of the offline design link to initialize the system parameters;

[0093] 2) Send the system parameters to the initial power angle calculation module of the offline design link, so that the active power instruction Set it to 0, and solve the initial power angle corresponding to the power command point according to the power angle characteristic model of the grid-type converter under normal working conditions ; The power angle characteristic model formula is:

[0094] (6);

[0095] 3) For a fixed step size, increase the active power instruction , and repeat step 2) to solve the initial power angle corresponding to each power command point , until the active power instruction Increase to ;

[0096] 4) Each active power instruction calculated according to the cycle and its corresponding initial power angle As a result, a primary mapping relationship set of active power command and initial power angle is constructed. ;

[0097] 5) Collect the primary mapping relationships The optimal power angle acquisition module is sent to the offline design link, so that the grid voltage amplitude Set to 0, and solve the grid voltage amplitude under the fault drop depth according to the integral relationship of the multi-objective cooperative domain of the grid-type converter The optimal power angle corresponding to each power command point The integral relationship of the multi-objective collaborative domain is:

[0098] (7);

[0099] Where, the integral lower limit power angle For collection Active power command The corresponding initial power angle ; The relationship between the current limiting adjustment factor and the power angle can be expressed as:

[0100] (8);

[0101] Where, An operation to extract the real part of a complex number;

[0102] 6) is a fixed step size, increasing the grid voltage amplitude , and repeat step 5) to solve the optimal power angle corresponding to each power command point and each fault drop depth , until the grid voltage amplitude Increase to ;

[0103] 7) Each active power instruction calculated according to the cycle , grid voltage amplitude and its corresponding optimal power angle As a result, a set of secondary mapping relationships among active power command, grid voltage amplitude and optimal power angle is constructed. .

[0104] Furthermore, the online query calculation sub-link of the multi-objective collaborative fault ride-through control link in the present invention includes:

[0105] 1) The three-phase voltage amplitude of the grid connection point in the current control cycle is sent to the fault detection module of the online query calculation link, and compared with the preset low voltage threshold to determine whether the grid connection point meets the low voltage fault ride-through conditions;

[0106] 2) If the three-phase voltage amplitude of the grid connection point is less than the low voltage threshold, it is determined that the grid at the grid connection point enters a low voltage fault ride-through condition, and the fault ride-through enable signal Flag is set to 1;

[0107] 3) Collect the secondary mapping relationships , the three-phase voltage amplitude of the grid connection point in the current control cycle and the active power command of the current control cycle The mapping relationship table lookup module sent to the online query calculation link is based on the mapping relationship collection Dynamically look up the table to find the optimal power angle under the fault condition ;

[0108] 4) According to the optimal power angle of the current control cycle Calculate the current limiting adjustment factor , the calculation formula is:

[0109] (9);

[0110] 5) According to the current limiting adjustment factor of the current control cycle Calculate the active power command , the calculation formula is:

[0111] (10).

[0112] Where, The margin factor reserved to prevent active power flow from returning can be taken as 1.05.

[0113] The effectiveness and advancement of the control method proposed in the embodiment of the present invention are verified by MATLAB / Simulink software.

[0114] The rated capacity of the grid-type converter is 75kVA, the voltage level is DC 800V / AC 380V, the AC filter inductor is 0.5mH, the AC filter capacitor is 20μF, the DC capacitor is 4.2mF, and before the fault, the converter outputs active power 75kW and reactive power 0kVar; the grid voltage is AC 380V / 50Hz, and the grid impedance is 0.05+j0.47Ω.

[0115] See attached Figure 3a and Figure 3b , is the grid-connected point voltage after the grid-connected converter according to the embodiment of the present invention adopts the traditional virtual synchronous generator control and the proposed control method respectively, when a mild symmetrical fault with a voltage drop depth of 0.4 pu occurs in the grid. , AC inductor current , active power , reactive power , power angle The simulation waveform. Figure 3a In the example, a grid fault occurred at 1.5s and recovered at 3.5s. However, the fault current still exceeded the safety threshold of 1.2 pu using the traditional control method, and the grid connection point voltage was only supported to 0.75 pu. Figure 3b In the control method proposed by the present invention, the fault current amplitude is limited to 1.2 after the fault. pu, and the grid-connected converter always keeps synchronization with the power grid, ensuring the safety and stability of the grid-connected converter. In addition, this method maximizes the active support capability of the grid-connected converter during a fault, increasing the reactive emergency support from 0.55 pu to 1.05 pu, thereby supporting the grid connection point voltage to 0.92 pu.

[0116] See attached Figure 4a and Figure 4b , is the grid-connected point voltage after the grid-connected converter according to the embodiment of the present invention adopts the traditional virtual synchronous generator control and the proposed control method respectively, when a severe symmetrical fault with a voltage drop depth of 0.8 pu occurs in the grid. , AC inductor current , active power , reactive power , power angle The simulation waveform of . The simulation waveform of . Figure 4a In the simulation, a fault occurs in the power grid at 1.5s and is restored at 3.5s. Under this simulation condition, a fault overcurrent phenomenon occurs, and the current amplitude reaches a maximum of 2.58 pu, which is much higher than the overcurrent carrying limit of the converter. During the fault period, the voltage gradually diverges, resulting in transient synchronous instability, which seriously threatens the safety and stability of the grid-connected converter. Figure 4b In the embodiment of the present invention, the control method proposed in the embodiment of the present invention simultaneously realizes fault current suppression and transient synchronous stability, and at the same time maximizes the emergency reactive power support strength of the grid-connected converter, so that the grid connection point voltage is still maintained at the level of 0.52 pu under this serious fault, achieving the improvement of fault ride-through capability with multiple levels of goals such as coordinated current limiting safety, synchronous stability, and support reliability.

[0117] Although the preferred embodiments of the present application have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present application.

[0118] Although the embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the description and the embodiments. They can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily implemented. Therefore, without departing from the general concept defined by the claims and the scope of equivalents, the present invention is not limited to the specific details and shown here.

Claims

1. A multi-objective coordinated fault ride-through method for grid-connected converters, characterized in that: The steps include: Step 1: At the starting point of each sampling period, the grid-connected point voltage of the grid-connected converter is , AC inductor current Sampling is performed to obtain the output active power of the grid-type converter , reactive power 、 Grid connection point voltage Quantity , AC inductor current Quantity and the three-phase voltage amplitude at the grid connection point; Step 2: Compare the three-phase voltage amplitude at the grid connection point with a preset low voltage threshold; if the three-phase voltage amplitude at the grid connection point is less than the preset low voltage threshold, it is determined that the grid connection point meets the low voltage fault ride-through condition, and the fault ride-through enable signal Flag is set to 1; otherwise, the low voltage fault ride-through condition is not met, and the fault ride-through enable signal Flag is set to 0; Step 3: When the enable signal Flag is 0, the reactive power control link of the grid-type converter operates normally, and the original active power instruction of the current control cycle Remain unchanged; when the enable signal Flag is 1, the reactive power control link of the grid-type converter is frozen, and the fault transient active power instruction of the current control cycle is determined by the multi-objective collaborative fault ride-through algorithm , and replace the original active power instruction As a new reference instruction for the active power control link of the virtual synchronous generator, the current reference instruction is obtained And proceed to step 4; The multi-objective collaborative fault-traversal algorithm is pre-built with Active power command with step length Full range of data sets, The grid voltage amplitude is the step size Full range data set and corresponding optimal power angle A collection of mapping relationships ; When Flag is 1, the three-phase voltage amplitude of the grid connection point obtained in the current sampling period is and active power command Input to mapping relationship collection , dynamically look up the table to search for the optimal power angle of the current control cycle ; Then according to the optimal power angle of the current control cycle Get the fault transient active power command of the current control cycle ; Step 4: Output active power of the grid-connected inverter in the current control cycle , output reactive power , original active power instruction , Fault transient active power command , grid connection point voltage Quantity and AC inductor current Quantity , and obtain the modulation voltage of the grid-type converter Quantity , the modulation voltage Quantity conduct The coordinates are inversely transformed and subjected to SPWM modulation to output a PWM modulation signal to control the switching state of the power devices of each phase bridge arm in the grid-type converter.

2. The multi-objective coordinated fault ride-through method for a grid-connected converter according to claim 1, characterized in that: The topology of the grid-type converter is an ANPC three-level structure; the DC side of the grid-type converter is connected to a DC source, and the AC side of the converter is connected to an LC filter; the LC filter is connected to a common coupling point through a grid-connected switch to achieve connection to the grid.

3. The multi-objective coordinated fault ride-through method for a grid-connected converter according to claim 1, wherein: The step 1 includes the following steps: 1.1) At the start of each sampling period, the sampling unit measures the grid-connected point voltage of the grid-connected converter. , AC inductor current Sampling and low-pass filtering are performed; ; ; is the terminal voltage of phase A, phase B and phase C of the grid-type converter, are the three-phase inductor currents of phases A, B, and C of the grid-type converter; T represents the vector or matrix transpose symbol; 1.2) Grid connection point voltage and AC inductor current The output active power of the grid-connected converter is obtained by sending it to the power calculation module of the control system. and reactive power ; 1.3) Within the potential phase angle instruction The value of the last control cycle is used as the reference phase for coordinate transformation, and the grid voltage is and AC inductor current conduct Rotating coordinate transformation to obtain the grid connection point voltage of Quantity and AC inductor current of Quantity ; ; ; and The grid voltage Axis components and Axis component, and is the AC inductor current of the grid-type converter Axis components and Axis component; 1.4) According to the grid connection point voltage Quantity Determine the three-phase voltage amplitude at the grid connection point.

4. The multi-objective coordinated fault ride-through method for a grid-connected converter according to claim 3, wherein: In the second step, whether the grid connection point grid meets the low voltage fault ride-through condition is determined based on the three-phase voltage amplitude of the grid connection point and the preset low voltage threshold, and the fault ride-through enable signal Flag of the current control cycle is updated. The formula is: 。 5. The multi-objective coordinated fault ride-through method applicable to a grid-type converter according to claim 3, characterized in that: The step 4 includes the following steps: 4.1) The output active power of the grid-connected converter and reactive power Go to the virtual synchronous generator active power control link to obtain the voltage phase angle instruction of the current control cycle and amplitude instructions , the calculation formula is: ； Where, is the moment of inertia; is the damping coefficient; is the reactive-voltage coefficient; is the rated voltage amplitude; is the rated voltage angular frequency; is the original active power instruction; There is a power command for the fault transient state; is the reactive power instruction; — It is a negation operator. When the enable signal Flag is 0, the reactive power control link operates normally. When the enable signal Flag is 1, the reactive power control link is frozen. is the Laplace operator; 4.2) The voltage phase angle command and voltage amplitude command Sent to the voltage generation module to obtain the voltage reference instruction of the current control cycle ; and The voltage reference instructions are of Axis components and Axis component; 4.3) Set the voltage reference instruction and grid connection point voltage Quantity Input to the virtual admittance control link of the control system to generate the initial current reference instruction ; ; and They are respectively the initial current reference command Axis components and Axis component; initial current reference command The calculation formula is: ； Where, is the virtual resistance for virtual admittance control; is the virtual reactance for virtual admittance control; 4.4) Set the initial current reference instruction Sent to the ring current limiter to obtain the current reference instruction after current limiting , the calculation formula is: ； Where, is the current limiting adjustment factor; is the maximum current limit value of the grid-type converter; Calculate identifier for the two-norm; 4.5) Through the grid voltage Quantity , AC inductor current Quantity and current reference instructions Get the modulation voltage of the grid-type converter Quantity , the calculation formula is: ； Where, is the proportional coefficient of the inner current loop, is the integral coefficient of the inner current loop, is the imaginary unit, is the inductance value of the filter inductor of the grid-type converter; 4.6) The modulation voltage Quantity conduct The coordinates are inversely transformed and modulated by SPWM to output PWM modulation signals to control the switching states of the power devices of each phase bridge arm in the grid-type converter.

6. The multi-objective coordinated fault ride-through method for a grid-connected converter according to claim 5, characterized in that: In step 3, the relationship collection The build steps are as follows: 3.1.1) Regularly storing the system parameters of the power grid, including the voltage amplitude , grid resistance and grid reactance , initialize system parameters; 3.1.2) Active power command Set it to 0, and solve the initial power angle corresponding to the power command point according to the power angle characteristic model of the grid-type converter under normal working conditions ; The power angle characteristic model formula is: ； Where, Output active power Power Angle Abbreviated expression of the relationship between them; 3.1.3) For a fixed step size, add active power instruction , and repeat step 3.1.2) to solve the initial power angle corresponding to each power command point , until the active power command Increase to ; 3.1.4) Active power instructions calculated according to the cycle and its corresponding initial power angle As a result, a primary mapping relationship set of active power command and initial power angle is constructed. ; 3.1.5) The grid voltage amplitude Set to 0, and solve the grid voltage amplitude under the fault drop depth according to the integral relationship of the multi-objective cooperative domain of the grid-type converter The optimal power angle corresponding to each power command point The integral relationship of the multi-objective collaborative domain is: ； Where, the integral lower limit power angle For collection Active power command The corresponding initial power angle ; Indicates the power angle; represents the differential operator; is the relationship between the current limiting adjustment factor and the power angle, expressed as: ； Where, An operation to extract the real part of a complex number; 3.1.6) is a fixed step size, increasing the grid voltage amplitude , and repeat step 3.1.5) to solve the optimal power angle corresponding to each power command point and each fault drop depth , until the grid voltage amplitude Increase to ; 3.1.7) Active power instructions calculated according to the cycle , grid voltage amplitude and its corresponding optimal power angle As a result, a set of secondary mapping relationships among active power command, grid voltage amplitude and optimal power angle is constructed. .

7. The multi-objective coordinated fault ride-through method for a grid-connected converter according to claim 6, characterized in that: In step 3, the optimal power angle under the current fault condition is searched dynamically by looking up the table. The steps are as follows: 3.2.1) In each control cycle, the three-phase voltage amplitude at the grid connection point and the preset low voltage threshold are detected to determine whether the grid at the grid connection point meets the conditions for entering low voltage fault ride-through; 3.2.2) If the three-phase voltage amplitude at the grid connection point is less than the low voltage threshold, it is determined that the grid at the grid connection point has entered a low voltage fault ride-through condition, and a fault ride-through enable signal Flag is set to 1; 3.2.3) The three-phase voltage amplitude of the grid connection point in the current sampling period and active power command Input to mapping relationship collection , dynamically look up the table to search for the optimal power angle of the current control cycle ; 3.2.4) According to the optimal power angle of the current control cycle Calculate the current limiting adjustment factor , the calculation formula is: ； 3.2.5) Current limiting adjustment factor according to the current control cycle Calculate the fault transient active power command of the current control cycle , the calculation formula is: ； Where, The margin factor reserved to prevent active power flow from returning; Indicates taking the maximum value, Represents an operation that extracts the real part of a complex number.

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