Multi-target cooperative fault ride-through method suitable for network-forming converter

Abstract

The invention discloses a multi-target cooperative fault ride-through method suitable for a network-forming converter, and the method comprises the steps: carrying out the precise amplitude limiting of a fault current through an annular current limiter, and adjusting a power angle to a lower boundary of a transient stability domain during a fault period; and multi-target collaborative optimization of transient synchronous stability, power grid reactive power support and fault current suppression in the fault ride-through process is realized. An optimal power angle set meeting fault ride-through multi-target cooperation in a multi-element fault scene is constructed by performing off-line beforehand rehearsal training on a system physical model; when the power grid breaks down, the network construction type converter carries out online table look-up search based on the trained set, an active power instruction meeting the transient stability and the optimal reactive power support in the current fault scene is calculated, and the supporting capacity of the network construction type converter to the power grid is mined to the maximum extent while the network construction type converter operates safely and stably. According to the method provided by the invention, the network construction operation characteristics of the converter can be maintained under the power grid fault, and the multi-target collaborative fault ride-through capability of equipment is enhanced.

Description

Technical Field

[0001] The present invention relates to the technical field of grid-forming control, and in particular to a multi-objective collaborative fault ride-through method applicable to grid-forming converters. Background Art

[0002] With the decreasing proportion of traditional synchronous generators, the short-circuit ratio of the system has decreased significantly, while weakening the inertia of the power grid and the voltage and frequency support capabilities. In recent years, grid-forming control technology, as a method with the ability to actively support the power grid, has received great attention in the industry. Through grid-forming control technology, the converter can simulate the inertia and damping effects of synchronous generators and externally exhibit the operating characteristics of a voltage source, enhancing its support for the power grid.

[0003] The grid-forming converter based on power electronic devices has limited short-term overload capacity. To ensure the operating safety during fault ride-through, it is usually necessary to suppress the fault current. The combination of virtual admittance control and a loop current limiter is a relatively popular current-limiting method in recent years. This method can enable the converter to maintain the grid-forming characteristics during fault ride-through, and the dynamic current amplitude never exceeds its safety upper limit threshold under different fault scenarios, enabling adaptive current limiting of the grid-forming converter, and thus has received wide attention and application.

[0004] However, the fault ride-through of grid-forming converters requires that in addition to current-limiting protection, it also needs to meet requirements such as transient synchronous stability and optimal reactive power support. However, these control objectives conflict with each other, and there is still a lack of effective means to coordinate multiple control objectives during the transient process of the system. Therefore, there is an urgent need for a multi-objective collaborative fault ride-through method applicable to grid-forming converters. Summary of the Invention

[0005] To solve the above problems, the present invention discloses a multi-objective collaborative fault ride-through method applicable to grid-forming converters, which maximizes the reactive power support ability of the grid-forming converter while ensuring fault current limitation and transient synchronous stability, 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 as follows: A multi-objective collaborative fault ride-through method applicable to grid-forming converters, comprising the following steps: Step 1: At the start of each sampling period, sample the grid connection point voltage and the AC inductor current of the grid-forming converter to obtain the output active power and reactive power 、 of the grid-forming converter, the grid connection point voltage component , AC inductor current component and the three-phase voltage amplitudes at the grid connection point; Step 2: Compare the three-phase voltage amplitudes at the grid connection point with a preset low voltage threshold; if the three-phase voltage amplitudes at the grid connection point are less than the preset low voltage threshold, it is determined that the grid at 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 it does not meet the low voltage fault ride-through condition, 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-forming converter operates normally, and the original active power command in the current control period remains unchanged; when the enable signal Flag is 1, the reactive power control link of the grid-forming converter is frozen, and the fault transient active power command in the current control period is determined through a multi-objective collaborative fault ride-through algorithm and replaces the original active power command as the new reference command for the active power control link of the virtual synchronous generator to obtain the current reference command and proceed to Step 4; The multi-objective collaborative fault ride-through algorithm pre-constructs an active power command full range data set with as the step size, a grid voltage amplitude full range data set with as the step size, and a mapping relationship set of the corresponding optimal power angle; when Flag is 1, the three-phase voltage amplitudes at the grid connection point obtained in the current sampling period and the active power command are input into the mapping relationship set to dynamically search the table to find the optimal power angle in the current control period; then, based on the optimal power angle in the current control period, the fault transient active power command in the current control period is obtained; Step 4: Based on the output active power , output reactive power , original active power command , fault transient active power command , grid connection point voltage component and AC inductor current component of the grid-forming converter, the modulation voltage component of the grid-forming converter is obtained, and the modulation voltage component Perform coordinate inverse transformation, and through SPWM modulation, output a PWM modulation signal to control the switching states of the power devices of each phase leg in the network-forming converter.

[0007] For further improvement, the topology of the network-forming converter is an ANPC-type three-level structure; the DC side of the network-forming 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 the point of common coupling through a grid connection switch to realize connection to the power grid.

[0008] For further improvement, step one includes the following steps: 1.1) At the starting point of each sampling period, the sampling unit samples the grid connection point voltage and the AC inductor current of the network-forming converter, and performs low-pass filtering. ; ; are the terminal voltages of phases A, B, and C of the network-forming converter, , are the three-phase inductor currents of phases A, B, and C of the network-forming converter; T represents the vector or matrix transpose symbol; 1.2) The grid connection point voltage and the AC inductor current are sent to the power calculation module of the control system to obtain the output active power and reactive power of the network-forming converter; 1.3) Using the value of the internal potential phase angle command in the previous control period as the coordinate transformation reference phase, perform a rotational coordinate transformation on the grid connection point voltage and the AC inductor current to obtain the component of the grid connection point voltage and the component of the AC inductor current ; ; ; and are the d-axis and q-axis components of the grid connection point voltage, and are the d-axis and q-axis components of the AC inductor current of the network-forming converter; 1.4) Determine the three-phase voltage amplitudes of the grid connection point according to the component of the grid connection point voltage.

[0009] For further improvement, in step 2, it is determined whether the grid connection point grid meets the condition of entering the low-voltage fault ride-through according to the three-phase voltage amplitude at the grid connection point and a preset low-voltage threshold, and the fault ride-through enable signal Flag for the current control period is updated. The formula is: 。

[0010] For further improvement, step 4 includes the following steps: 4.1) Feed the output active power and reactive power of the grid-forming converter to the active power control link of the virtual synchronous generator to obtain the voltage phase angle command and amplitude command for the current control period. The calculation formulas are: ; In the formula, is the moment of inertia; is the damping coefficient; is the reactive power-voltage coefficient; is the rated voltage amplitude; is the rated voltage angular frequency; is the original active power command; is the transient active power command during the fault; is the reactive power command; — is a non-operation 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) Feed the voltage phase angle command and voltage amplitude command to the voltage generation module to obtain the voltage reference command ； and are the axis component and axis component of the voltage reference command respectively; 4.3) Feed the voltage reference command and the grid connection point voltage component to the virtual admittance control link of the control system to generate the initial current reference command ; ; and are the axis component and axis component of the initial current reference command respectively; The initial current reference command The calculation formula is: ; In the formula, is the virtual resistance of the virtual admittance control; is the virtual reactance of the virtual admittance control; 4.4) Send the initial current reference command to the loop current limiter to obtain the current reference command after current limiting processing. The calculation formula is: ; In the formula, is the current limiting adjustment factor; is the maximum current limiting value of the network-forming converter; is the two-norm calculation identifier; 4.5) Obtain the component of the grid-connected point voltage, the component of the AC inductor current, and the current reference command to get the component of the modulation voltage of the network-forming converter. The calculation formula is: ; In the formula, is the proportional coefficient of the current inner loop, is the integral coefficient of the current inner loop, is the imaginary unit, is the inductance value of the filter inductor of the network-forming converter; 4.6) Perform inverse coordinate transformation on the component of the modulation voltage, and after SPWM modulation, output the PWM modulation signal to control the switching states of the power devices of each phase bridge arm in the network-forming converter.

[0011] For further improvement, in step three, the relationship set is constructed as follows: 3.1.1) Regularly store the system parameters of the power grid. The system parameters include the voltage amplitude , the grid resistance , and the grid reactance , and perform system parameter initialization; 3.1.2) Set the active power command to 0, and according to the power angle characteristic model of the network-forming converter under normal operating conditions, solve the initial power angle corresponding to the power command point. The power angle characteristic model formula is: ; Wherein, is the abbreviated expression of the relational formula between the output active power and the power angle ; 3.1.3) With as the fixed step size, increase the active power command , and repeat step 3.1.2) to solve the initial power angle corresponding to each power command point until the active power command increases to ; 3.1.4) According to the results of each active power command calculated in the loop and its corresponding initial power angle , construct a collection of primary mapping relationships between the active power command and the initial power angle ; 3.1.5) Set the grid voltage amplitude to 0, and according to the integral relational formula of the multi-objective cooperation domain of the grid-forming converter, solve the optimal power angle corresponding to the grid voltage amplitude at each power command point under the fault dip depth; the integral relational formula of the multi-objective cooperation domain is: ; Wherein, the lower limit power angle of the integral is the initial power angle corresponding to the active power command in the collection ; represents the power angle; represents the differential operator; is the relational formula between the current limiting adjustment factor and the power angle, expressed as: ; Wherein, is the operation of extracting the real part of the complex number; 3.1.6) With as the fixed step size, increase 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 dip depth until the grid voltage amplitude increases to ; 3.1.7) According to each active power command , grid voltage amplitude calculated in the loop and its corresponding optimal power angle As a result, a secondary mapping relationship set of active power commands, grid voltage amplitudes, and optimal power angles is constructed. 。

[0012] For further improvement, in step three, the optimal power angle under the current fault condition is dynamically searched through a look-up table. The steps are as follows: 3.2.1) Detect the three-phase voltage amplitude at the grid connection point and a preset low-voltage threshold in each control cycle to determine whether the grid at the grid connection point meets the condition 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 enters the low-voltage fault ride-through condition, and the fault ride-through enable signal Flag is set to 1. 3.2.3) Input the three-phase voltage amplitude at the grid connection point and the active power command at the current sampling period into the mapping relationship set , and dynamically search through the look-up table to find the optimal power angle at the current control cycle ; 3.2.4) Calculate the current-limiting regulation factor according to the optimal power angle at the current control cycle , and the calculation formula is: ; 3.2.5) Calculate the transient fault active power command at the current control cycle according to the current-limiting regulation factor at the current control cycle , and the calculation formula is:

[0013] In the formula, is the margin coefficient reserved to prevent the reverse flow of active power; represents taking the maximum value, represents the operation of extracting the real part of a complex number.

[0014] Advantages of the present invention: Through offline pre-training with system parameters, the present invention constructs an optimal power angle set that meets the multi-objective hierarchical coordination of fault ride-through under multiple fault scenarios. Subsequently, based on the method proposed in the present invention, the grid-forming converter can search the set through an online look-up table under different faults and calculate the active power command that meets the transient synchronous stability and optimal reactive power support, realizing the safe and stable operation of the grid-forming converter during fault ride-through while maximizing the support capacity of the grid-forming converter for the power grid. The method proposed in the present invention can maintain the grid-forming operation characteristics of the converter under grid short-circuit faults and be extended to various voltage-source power electronic equipment in various application scenarios to enhance the multi-objective collaborative fault ride-through ability of the equipment. Description of the Drawings

[0015] Figure 1 Schematic diagram of the structure of the network-forming converter according to an embodiment of the present invention; Figure 2 Control block diagram of the multi-objective collaborative fault ride-through method according to an embodiment of the present invention; Figure 3a Under the traditional virtual synchronous generator control method of the network-forming converter according to an embodiment of the present invention, the simulation waveforms when the three-phase grid voltage symmetrically drops to 0.6 p.u., where A11 is the grid connection point voltage waveform, A12 is the AC inductor current waveform, A13 is the active power waveform, A14 is the reactive power waveform, A15 is the power angle waveform; Figure 3b is Figure 3a The simulation waveforms under the same fault conditions using the control method proposed by the present invention, where A21 is the grid connection point voltage waveform, A22 is the AC inductor current waveform, A23 is the active power waveform, A24 is the reactive power waveform, A25 is the power angle waveform; Figure 4a Under the traditional virtual synchronous generator control method of the network-forming converter according to an embodiment of the present invention, the simulation waveforms when the three-phase grid voltage symmetrically drops to 0.2 p.u., where A31 is the grid connection point voltage waveform, A32 is the AC inductor current waveform, A33 is the active power waveform, A34 is the reactive power waveform, A35 is the power angle waveform; Figure 4b is Figure 4a The simulation waveforms under the same fault conditions using the control method proposed by the present invention, where A41 is the grid connection point voltage waveform, A42 is the AC inductor current waveform, A43 is the active power waveform, A44 is the reactive power waveform, A45 is the power angle waveform. Detailed Embodiments

[0016] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the protection scope of the present invention.

[0017] See the attached Figure 1 , which is a schematic structural diagram of a grid-forming converter according to an embodiment of the present invention, including a DC source, a DC capacitor, an ANPC-type 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-type three-level converter; the ANPC-type three-level converter is connected to the AC LC filter; the AC LC filter is connected to the PCC point to realize connection to the power grid; the control system is mainly composed of a multi-objective collaborative fault ride-through control link and a virtual synchronous generator control link.

[0018] See the 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 pre-trains a physical model containing system parameters offline in advance to construct an optimal power angle set that meets the hierarchical collaboration of multiple objectives for fault ride-through under multiple fault scenarios, 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 look-up table module, and a power command calculation module. This link searches the pre-trained set online by table look-up under grid faults and calculates the active power command that meets transient synchronous stability and optimal reactive power support under the current fault scenario, realizing the hierarchical organic combination and collaborative optimization of transient synchronous stability, grid reactive power support, and fault current suppression during the fault ride-through process of the grid-forming converter. The specific method includes: 1) At the start point of each sampling period, the sampling unit samples the grid connection point voltage of the grid-forming converter and the AC inductor current , and performs low-pass filtering processing; ; ; is the three-phase terminal voltage of the grid-forming converter, and is the three-phase inductor current of the grid-forming converter; 2) Send the grid connection point voltage and the AC inductor current sampled in step 1) to the power calculation module of the control system to obtain the output active power and reactive power of the grid-forming converter ; 3) Inner potential phase angle command Using the value in the previous control period as the coordinate transformation reference phase, transform the grid connection point voltage and the AC inductor current by rotating coordinate transformation to obtain the grid connection point voltage component and the AC inductor current component ; and are the axis component and axis component of the grid connection point voltage, and are the axis component and axis component of the grid-forming converter; 4) Determine the three-phase voltage amplitude of the grid connection point according to the grid connection point voltage component ; 5) Send the three-phase voltage amplitude of the grid connection point to the fault detection module of the online query calculation link, and determine whether the grid at the grid connection point meets the condition of entering the low-voltage fault ride-through according to the three-phase voltage amplitude of the grid connection point and the preset low-voltage threshold, and update the fault ride-through enable signal Flag of the current control period. The formula is: (1); 6) If the fault ride-through enable signal Flag is 1, determine the active power command through the multi-objective collaborative fault ride-through control link, and replace the original command as the new reference command for the active power control link of the virtual synchronous generator; 7) Feed the output active power and reactive power of the grid-forming converter to the active power control link of the virtual synchronous generator to obtain the voltage phase angle instruction and amplitude instruction of the current control period. The calculation formula is: (2); In the formula, is the moment of inertia; is the damping coefficient; is the reactive power-voltage coefficient; is the rated voltage amplitude; is the rated voltage angular frequency; is the reactive power command; —is a NOT operation 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; 8) Send the voltage phase angle command and the voltage amplitude command to the voltage generation module to obtain the voltage reference command for the current control period; and are the axis component and axis component of the voltage reference command; 9) Input the voltage reference command and the grid connection point voltage component to the virtual admittance control link of the control system to generate the initial current reference command ; and are the axis component and axis component of the initial current reference command. The calculation formula for the initial current reference command is: (3); In the formula, is the virtual resistance of the virtual admittance control; is the virtual reactance of the virtual admittance control; 10) Send the initial current reference command to the ring current limiter to obtain the current reference command after current limiting processing. The calculation formula is: (4); In the formula, is the current limiting adjustment factor; is the maximum current limiting value of the grid-forming converter; is the two-norm calculation identifier; 11) Input the grid connection point voltage component , the AC inductor current component and the current reference command to obtain the modulation voltage component of the grid-forming converter. The calculation formula is: (5); 12) Perform component of the modulation voltage Perform coordinate inverse transformation, and through SPWM modulation, output PWM modulation signals to control the switching states of the power devices of each phase bridge arm in the network-forming converter.

[0019] Furthermore, the offline design sub-link of the multi-objective collaborative fault ride-through control link in the present invention includes: 1) Regularly store system parameters such as the voltage amplitude of the power grid , the grid resistance and the grid reactance into the system parameter initialization module of the offline design link for system parameter initialization; 2) Send the system parameters to the initial power angle calculation module of the offline design link, set the active power command to 0, and solve the initial power angle corresponding to this power command point according to the power angle characteristic model under the normal operating conditions of the network-forming converter; the formula of the power angle characteristic model is: (6); 3) With as the fixed step size, increase the active power command , and repeat step 2) to solve the initial power angle corresponding to each power command point until the active power command increases to ; 4) According to the results of each active power command calculated in a loop and its corresponding initial power angle , construct a collection of primary mapping relationships between the active power command and the initial power angle ; 5) Send the collection of primary mapping relationships to the optimal power angle acquisition module of the offline design link, set the voltage amplitude of the power grid to 0, and solve the optimal power angle corresponding to the voltage amplitude of the power grid at each power command point according to the integral relationship formula of the multi-objective collaborative domain of the network-forming converter; the integral relationship formula of the multi-objective collaborative domain is: (7); In the formula, the integral lower limit power angle is the initial power angle corresponding to the active power command in the collection ; is the relationship formula between the current limiting adjustment factor and the power angle, which can be expressed as: (8); Wherein, is the operation of extracting the real part of a complex number; 6) With as the fixed step size, increase the grid voltage amplitude , and repeat step 5) to solve the optimal power angle corresponding to each power command point and each fault ride-through depth , until the grid voltage amplitude increases to ; 7) According to the active power commands calculated by cyclic calculation, the grid voltage amplitude and their corresponding optimal power angles results, construct a secondary mapping relationship set of active power commands, grid voltage amplitudes and optimal power angles .

[0020] Furthermore, the online query calculation sub-link of the multi-objective collaborative fault ride-through control link in the present invention includes: 1) Send the three-phase voltage amplitudes of the grid connection point in the current control period to the fault detection module of the online query calculation link, and determine whether the grid of the grid connection point meets the conditions for entering the low-voltage fault ride-through with a preset low-voltage threshold; 2) If the three-phase voltage amplitudes of the grid connection point are less than the low-voltage threshold, determine that the grid of the grid connection point enters the low-voltage fault ride-through condition, and set the fault ride-through enable signal Flag to 1; 3) Send the secondary mapping relationship set , the three-phase voltage amplitudes of the grid connection point in the current control period and the active power command in the current control period to the mapping relationship look-up table module of the online query calculation link, and dynamically look up the table based on the mapping relationship set to search out the optimal power angle under this fault condition; 4) Calculate the current-limiting adjustment factor according to the optimal power angle in the current control period, and the calculation formula is: (9); 5) Calculate the active power command according to the current-limiting adjustment factor in the current control period, and the calculation formula is: (10).

[0021] Wherein, is the margin coefficient reserved to prevent the reverse transmission of active power flow, and 1.05 can be taken.

[0022] Verify the effectiveness and advancement of the control method proposed in the embodiments of the present invention through MATLAB / Simulink software.

[0023] The rated capacity of the network-forming converter is 75 kVA, the voltage level is DC 800V / AC 380V, the AC filter inductor is 0.5 mH, the AC filter capacitor is 20 μF, the DC capacitor is 4.2 mF, the active power output by the converter before the fault is 75 kW, and the reactive power is 0 kVar; the grid voltage is AC 380V / 50Hz, and the grid impedance is 0.05 + j0.47 Ω.

[0024] See Attachment Figure 3a and Figure 3b , which are the simulation waveforms of the grid-connected point voltage , AC inductor current , active power , reactive power , power angle when the network-forming converter according to the embodiment of the present invention adopts the traditional virtual synchronous generator control and the proposed control method respectively under a mild symmetrical fault with a voltage dip depth of 0.4 p.u. in the power grid. Figure 3a In Figure 3b , a fault occurs in the power grid at 1.5 s, and the fault is restored at 3.5 s. In the case of adopting the traditional control method, the fault current still exceeds the safety threshold of 1.2 p.u., and at the same time, the grid-connected point voltage is only supported to 0.75 p.u. In Figure 3b , through the control method proposed by the present invention, the amplitude of the fault current is limited to 1.2 p.u. after the fault, and the network-forming converter and the power grid always remain synchronized, ensuring the safety and stability of the network-forming converter; in addition, this method maximally improves the active support ability of the network-forming converter during the fault, and the reactive power emergency support amount is increased from 0.55 p.u. to 1.05 p.u., thereby supporting the grid-connected point voltage to 0.92 p.u.

[0025] See Attachment Figure 4a and Figure 4b , which are the simulation waveforms of the grid-connected point voltage , AC inductor current , active power , reactive power , power angle when the network-forming converter according to the embodiment of the present invention adopts the traditional virtual synchronous generator control and the proposed control method respectively under a severe symmetrical fault with a voltage dip depth of 0.8 p.u. in the power grid. The simulation waveforms. Figure 4aAmong them, a grid fault occurs at 1.5 s, and the fault is restored at 3.5 s. Under this simulation condition, a fault overcurrent phenomenon occurs, and the current amplitude reaches up to 2.58 p.u., which is much higher than the overcurrent carrying limit of the converter; in addition, the power angle gradually diverges during the fault, resulting in transient synchronous instability, and the safety and stability of the grid-forming converter are severely threatened. In Figure 4b Among them, 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 of the grid-forming converter, so that the grid-connected point voltage remains at the level of 0.52 p.u. under this severe fault, achieving the improvement of the fault crossing ability of multi-level objectives such as coordinated current limiting safety, synchronous stability, and support reliability.

[0026] Although the preferred embodiments of the present application have been described, those skilled in the art can make additional changes and modifications once they know the basic creative concept. Therefore, the appended claims are intended to be construed to include the preferred embodiments as well as all changes and modifications falling within the scope of the present application.

[0027] Although the embodiments of the present invention have been disclosed as above, they are not limited to only the applications listed in the specification and embodiments. It can be fully applied to various fields suitable for the present invention. For those familiar with the art, additional modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and those shown here.

Claims

1. A multi-objective collaborative fault ride-through method applicable to grid-forming converters, characterized in that, It includes the following steps: Step 1: At the start of each sampling period, sample the grid-connected point voltage of the grid-forming converter , the AC inductor current to obtain the output active power , reactive power 、 of the grid-connected point voltage component , the AC inductor current component and the three-phase voltage amplitudes of the grid-connected point; Step 2: Compare the three-phase voltage amplitudes at the grid connection point with a preset low voltage threshold; if the three-phase voltage amplitudes at the grid connection point are less than the preset low voltage threshold, it is determined that the grid at 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, it is determined that 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-forming converter operates normally, and the original active power command of the current control period remains unchanged; when the enable signal Flag is 1, the reactive power control link of the grid-forming converter is frozen, and the fault transient active power command of the current control period is determined through the multi-objective collaborative fault ride-through algorithm , and replaces the original active power command as the new reference command for the active power control link of the virtual synchronous generator, obtains the current reference command and proceeds to Step 4; The multi-objective collaborative fault ride-through algorithm pre-constructs an active power command with a step size of and a full-range data set of grid voltage amplitudes with a step size of as well as a mapping relationship set corresponding to the optimal power angle of and the full-range data set; ​​​ When the Flag is 1, the three-phase voltage amplitudes of the grid connection point obtained in the current sampling period and the active power command are input into the mapping relationship set , and the optimal power angle of the current control period is searched by dynamic table look-up ; Then, according to the optimal power angle of the current control period , the fault transient active power command of the current control period is obtained ; 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 collaborative fault ride-through method for a grid-forming converter according to claim 1, characterized in that, The topology of the grid-forming converter is an ANPC-type three-level structure; the DC side of the grid-forming 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 the point of common coupling through a grid connection switch to realize connection to the grid.

3. The multi-objective collaborative fault ride-through method applicable to a network-forming converter according to claim 1, wherein Step 1 includes the following steps: 1.1) At the starting point of each sampling period, the sampling unit samples the grid-connected point voltage of the grid-forming converter , the AC inductor current , and performs low-pass filtering; ; ; are the terminal voltages of the A, B, and C phases of the grid-forming converter, are the three-phase inductor currents of the A, B, and C phases of the grid-forming converter; T represents the vector or matrix transpose symbol; 1.2) Point of common coupling voltage and AC inductor current are sent to the power calculation module of the control system to obtain the output active power and reactive power ; 1.3) Inner potential angle command Using the value in the previous control period as the reference phase for coordinate transformation, the grid connection point voltage and the AC inductor current are subjected to a rotation coordinate transformation to obtain the component of the grid connection point voltage and the component of the AC inductor current ; ; ; and are the axis component and axis component of the grid connection point voltage, and are the axis component and axis component of the AC inductor current of the grid-forming converter; 1.4) According to the component to determine the three-phase voltage amplitudes at the point of common coupling.

4. The multi-objective collaborative fault ride-through method for a grid-forming converter according to claim 3, characterized in that In Step 2, it is determined whether the grid at the grid connection point meets the condition for entering the low voltage fault ride-through according to the three-phase voltage amplitudes at the grid connection point and the preset low voltage threshold, and the fault ride-through enable signal Flag for the current control period is updated. The formula is: 。 5. The multi-objective collaborative fault ride-through method for a grid-forming converter according to claim 3, wherein Step 4 includes the following steps: 4.1) The active power output of the grid-forming converter and the reactive power are sent to the active power control link of the virtual synchronous generator to obtain the voltage phase angle command and the amplitude command in the current control cycle. The calculation formulas are as follows: ； Wherein, is the moment of inertia; is the damping coefficient; is the reactive power-voltage coefficient; is the rated voltage amplitude; is the rated voltage angular frequency; is the original active power command; is the fault transient active power command; is the reactive power command; — is a NOT operation 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) Send the voltage phase angle command and the voltage amplitude command to the voltage generation module to obtain the voltage reference command for the current control period; and are the axis component and axis component of the voltage reference command respectively; 4.3) Input the voltage reference command and the grid connection point voltage components into the virtual admittance control link of the control system to generate the initial current reference command ; ; and are respectively the axis component and the axis component of the initial current reference command; the calculation formula of the initial current reference command is: ； In the formula, is the virtual resistance of the virtual admittance control; is the virtual reactance of the virtual admittance control; 4.4) Send the initial current reference command to the ring current limiter to obtain the current reference command after current limiting processing , and the calculation formula is: ； Wherein, is the current limiting adjustment factor; is the maximum current limiting value of the network-forming converter; is the two-norm calculation identifier; 4.5) Through the component of the grid-connected point voltage, the component of the AC inductor current, and the current reference command to obtain the component of the modulation voltage of the grid-forming converter. The calculation formula is: ； Wherein, 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 network-forming converter; 4.6) Apply the modulation voltage component to perform inverse coordinate transformation, and through SPWM modulation, output a PWM modulation signal to control the switching states of the power devices of each phase leg in the network-forming converter.

6. The multi-objective collaborative fault ride-through method for a grid-forming converter according to claim 5, characterized in that, In step 3, the relationship set is constructed as follows: 3.1.1) Regularly store the system parameters of the power grid, where the system parameters include voltage amplitude , grid resistance and grid reactance , and perform system parameter initialization; 3.1.2) Set the active power command to 0, and solve the initial power angle corresponding to the power command point according to the power angle characteristic model of the network-forming converter under normal operating conditions ; The formula of the power angle characteristic model is: ； In the formula, is the abbreviated expression of the relational formula between the output active power and the power angle; 3.1.3) With as the fixed step size, increase the active power command , and repeat step 3.1.2) to solve the initial power angle corresponding to each power command point , until the active power command increases to ; 3.1.4) Each active power command calculated according to the loop and its corresponding initial power angle results to construct a collection of primary mapping relationships between the active power command and the initial power angle ; 3.1.5) Set the grid voltage amplitude to 0, and solve for the grid voltage amplitude at the fault ride-through depth according to the integral relationship of the multi-objective cooperation domain of the grid-forming converter at the optimal power angle corresponding to each power command point ; the integral relationship of the multi-objective cooperation domain is: ； wherein, the lower integration limit power angle is the set and the active power command corresponds to the initial power angle ; represents the power angle; represents the differential operator; The relationship formula between the current-limiting adjustment factor and the power angle is expressed as: ； In the formula, is the operation of extracting the real part of a complex number; 3.1.6) With as the fixed step size, increase the amplitude of the grid voltage , and repeat step 3.1.5) to solve the optimal power angle corresponding to each power command point and each fault ride-through depth , until the amplitude of the grid voltage increases to ; 3.1.7) Each active power command calculated according to the cycle , the grid voltage amplitude and its corresponding optimal power angle results to construct a secondary mapping relationship set of the active power command, the grid voltage amplitude and the optimal power angle .

7. The multi-objective collaborative fault ride-through method for a grid-forming converter according to claim 6, characterized in that In the third step described above, the optimal power angle under the current fault condition is searched through dynamic look-up table The steps are as follows: 3.2.1) Detect the three-phase voltage amplitudes at the grid connection point and the preset low voltage threshold in each control period to determine whether the grid at the grid connection point meets the condition for entering the low voltage fault ride-through; 3.2.2) If the three-phase voltage amplitudes at the grid connection point are less than the low voltage threshold, it is determined that the grid at the grid connection point enters the low voltage fault ride-through condition, and the fault ride-through enable signal Flag is set to 1; 3.2.3) Input the three-phase voltage amplitudes at the grid connection point in the current sampling period and the active power command into the mapping relationship set , and dynamically search the table to find the optimal power angle in the current control period ; 3.2.4) According to the optimal power angle of the current control period calculate the current-limiting regulation factor , and the calculation formula is: ； 3.2.5) According to the current control cycle's current limiting adjustment factor Calculate the fault transient active power command for the current control cycle , and the calculation formula is: ； wherein, is the margin coefficient reserved to prevent the reverse transmission of active power flow; represents taking the maximum value, represents the operation of extracting the real part of a complex number.

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