A split-phase high-low voltage ride-through control method for a three-phase four-bridge-arm grid-connected converter

Abstract

The application belongs to the technical field of grid fault ride-through control of power electronic grid-connected converters, and particularly relates to a split-phase high-low voltage ride-through control method for a three-phase four-bridge-arm grid-connected converter. A split-phase independent high-low voltage ride-through control system is constructed, and independent extraction of three-phase voltage amplitude and phase is realized based on three independent DSOGIs; high-low state determination and current command calculation are independently completed for each phase, and three-phase control is completely decoupled; split-phase amplitude current limiting in accordance with "reactive power priority" is performed to ensure that a single bridge arm does not overcurrent; split-phase multi-resonant PR current inner loops are matched to improve the quality of current waveforms under faults; finally, the coordinated output of the first three bridge arms and the fourth bridge arm is realized through unified zero sequence injection modulation, the topological advantages of the three-phase four-bridge-arm are fully utilized, and smooth high-low voltage ride-through under any asymmetric fault is realized.

Description

Technical Field

[0001] This invention belongs to the field of power electronic grid-connected converter grid fault ride-through control technology, specifically relating to a phase-separated high and low voltage ride-through control method for a three-phase four-arm grid-connected converter. Background Technology

[0002] With the large-scale grid connection of new energy power generation and energy storage systems, grid fault ride-through capability has become a mandatory requirement for grid-connected converters. In actual grid faults, asymmetrical voltage faults such as single-phase grounding and phase-to-phase short circuits account for a high proportion, and both voltage sag (low voltage ride-through, LVRT) and voltage swell (high voltage ride-through, HVRT) conditions exist simultaneously. The three-phase four-arm topology, due to its neutral line path, can directly carry zero-sequence current and is widely used in scenarios prone to asymmetrical faults, such as low-voltage distribution networks and microgrids.

[0003] The existing high and low voltage ride-through control schemes for three-phase four-arm grid-connected converters have the following main technical defects: (1) The control architecture has high coupling and slow response to asymmetric faults. The mainstream solution adopts an architecture of positive and negative zero sequence component separation + multi-synchronous rotating coordinate system PI control. Under asymmetric faults, the dq axis current will generate second and third harmonic pulsating components, which require additional resonant controllers or complex filtering circuits. The parameter tuning is difficult, and the fault response speed and reactive power support accuracy are limited.

[0004] (2) The phase decoupling degree is low and the advantages of the four-arm topology are not fully utilized. Most of the existing phase control schemes are designed for three-phase three-wire systems. They require additional zero-sequence current suppression circuits to meet the constraint that the sum of the three-phase currents is 0. They cannot use the neutral path of the four-arm to achieve completely independent phase control, and the topological advantages are wasted. Moreover, most schemes only achieve low voltage ride-through and are not compatible with phase control for high voltage ride-through.

[0005] (3) Three-phase unified current limiting results in low capacity utilization under extreme faults. Existing solutions mostly use the total capacity of the three phases as a constraint for unified current limiting. Under extreme asymmetrical faults such as single-phase deep drop / rise, the reactive power support strength of the faulty phase is insufficient, the capacity of the non-faulty phase cannot be fully utilized, and the overall ride-through performance of the equipment is limited.

[0006] (4) The fourth arm modulation algorithm is complex and has poor compatibility with phase-splitting control. Existing four-arm modulation methods mostly adopt three-dimensional space vector modulation or independent zero-sequence current / midpoint voltage closed loop, which has high algorithm complexity, large controller resource consumption, and poor compatibility with phase-splitting control architecture, making it difficult to achieve efficient coordination with phase-splitting commands. Summary of the Invention

[0007] To overcome the problems in the prior art, the present invention is proposed.

[0008] The technical solution of the present invention to solve the above-mentioned technical problems is as follows: In a first aspect, the present invention provides a phase-by-phase high and low voltage ride-through control method for a three-phase four-arm grid-connected converter, comprising the following steps: Step 100: Independently detect the instantaneous three-phase grid voltage phase by phase, and extract the voltage amplitude and unit voltage component of each phase; Step 200: Based on the voltage amplitude of each phase, independently determine the ride-through operation state of each phase, which includes low voltage ride-through state, normal operation state and high voltage ride-through state; Step 300: Based on the cross-operation status of each phase, calculate the current command for each phase, which includes the active current amplitude and the reactive current amplitude. Step 400: Based on the current command of each phase, perform phase-separated reactive power priority current amplitude limiting to obtain the current command after limiting. Step 500: Based on the current command after limiting and the unit voltage component of the corresponding phase, synthesize the instantaneous current command of the three phases; Step 600: Based on the instantaneous current command of the three phases, perform phase-separated multi-resonance proportional resonant current closed loop and voltage feedforward to obtain the initial modulation wave of the three phases. Step 700: Based on the initial modulation wave of the three phases, perform zero-sequence injection four-arm coordinated modulation to obtain the modulation wave of the four arms; Step 800: Generate four-phase independent NPC three-level unipolar SPWM pulses based on the modulation wave of the four bridge arms.

[0009] Further, step 100 includes: The sampled instantaneous three-phase grid voltages are input into three independent dual second-order generalized integrators for processing. Each dual second-order generalized integrator outputs the in-phase voltage component and the quadrature voltage component of the corresponding phase. The voltage amplitude of each phase is calculated independently based on the in-phase voltage component and the quadrature voltage component; a minimum voltage amplitude limit is set to clamp the voltage amplitude of each phase to above the minimum voltage amplitude limit; Divide the in-phase voltage component and the quadrature voltage component by the corresponding phase voltage amplitude to obtain the unit voltage component, which is the unit in-phase component and the unit quadrature component of each phase voltage.

[0010] Furthermore, in step 200, independently determining the crossover operation state of each phase based on the voltage amplitude of each phase includes: Define three independent high-low current operation status flags : ; In the formula, Indicates the low-voltage ride-through trigger threshold. Indicates the high voltage ride-through trigger threshold; This indicates the voltage amplitude of each phase.

[0011] Further, step 300 includes: Under normal operating conditions, each phase uses an independent PQ power outer loop to generate current commands; During high-voltage ride-through and low-voltage ride-through conditions, reactive current amplitude is allocated first; active current amplitude is allocated to each phase based on the total active power command of the system and the voltage amplitude of each phase.

[0012] Further, step 400 includes: Calculate the active current limit value based on the active current amplitude; Based on the active current limit value, the bidirectional active current limit is calculated to obtain the current command after the limit is obtained.

[0013] Further, step 600 includes: Obtain the output current sample value, and calculate the three-phase current error value based on the instantaneous current command of the three phases and the output current sample value of the three phases; Based on the current error value, and combined with the proportional coefficient and the output of the resonant controller, the output voltage of the proportional resonant controller is calculated; The output voltage of the proportional resonant controller is superimposed with the grid voltage feedforward term to obtain the initial modulation wave of the three phases.

[0014] Further, step 700 includes: Extract the maximum and minimum values ​​of the initial modulation wave from the initial modulation wave of the three phases; The zero-sequence injection component is calculated based on the initial maximum and minimum values ​​of the modulated wave. The zero-sequence injection component is superimposed on the initial modulation wave of the first three phases to obtain the final modulation wave of the first three bridge arms: the zero-sequence injection component is used as the modulation wave of the fourth bridge arm.

[0015] Compared with the prior art, the present invention has the following technical effects: (1) Phase-separated independent control, strong adaptability to asymmetrical faults. From voltage detection and status determination to command calculation, the entire process is phase-separated and independent, without the need for positive and negative zero sequence separation or coordinate transformation. It eliminates current coupling pulsation under asymmetrical faults, has a fast fault response speed, and can adapt to any asymmetrical fault conditions such as single-phase grounding, phase-to-phase short circuit, and high and low voltage mixed drop.

[0016] (2) Unified architecture for high and low voltage ride-through, smooth mode switching. The same set of phase-separated control architecture covers three states: low voltage ride-through, high voltage ride-through and normal operation. The output format is consistent and the phase reference is unified. There is no need for complex mode switching logic and the transient current impact is small.

[0017] (3) High capacity utilization of phase-specific current limiting. Each phase independently performs amplitude current limiting with "reactive power priority", and the capacity of each phase does not encroach on each other. Under extreme asymmetrical faults, the faulty phase can output reactive power at full capacity to support the fault, while the non-faulty phase can maximize the retention of active power transmission, and the equipment's ride-through performance and capacity utilization are significantly improved.

[0018] (4) Deep matching between the four-arm topology and phase-separated control. Relying on the neutral line path of the fourth arm, the phase-separated control is not constrained by the topology that the sum of the three-phase currents is zero; the zero-sequence injection cooperative modulation is adopted, which does not require complex three-dimensional vector modulation or independent zero-sequence closed loop. The algorithm is simple and efficient, and the controller resource consumption is low.

[0019] (5) Phase-by-phase multi-resonance suppression, resulting in superior waveform quality under fault conditions. Each phase is independently configured with a fundamental and multi-harmonic resonant controller, which can accurately suppress different harmonic distortion levels in each phase under asymmetrical fault conditions. Compared with centralized harmonic control schemes, the current waveform quality under fault conditions is superior. Attached Figure Description

[0020] To more clearly illustrate the technical solutions and advantages in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0021] Figure 1 This is the main circuit topology diagram of a three-phase four-bridge-arm NPC three-level grid-connected converter; Figure 2 This is a block diagram of the overall control architecture for a phase-splitting high and low voltage ride-through system. Figure 3 The flowchart shows the logic for determining the high-low voltage state and generating instructions for a single phase. Detailed Implementation

[0022] To further illustrate the technical means and effects adopted by the present invention to achieve its intended purpose, the specific implementation methods, structures, features, and effects of the technical solutions proposed according to the present invention are described in detail below with reference to the accompanying drawings and preferred embodiments. Specific features, structures, or characteristics in one or more embodiments may be combined in any suitable form. Unless otherwise defined, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0023] To address the technical challenges of high-voltage ride-through (HVRT) and low-voltage ride-through (LVRT) control architectures in three-phase four-arm grid-connected converters under asymmetrical grid faults, including high coupling, slow response, insufficient phase decoupling, low equipment capacity utilization, and complex modulation of the fourth arm, this paper proposes a HVRT and LVRT control method that features a fully independent phase-by-phase architecture, a unified HVRT and LVRT architecture, and deep topological matching with the four-arm topology. The method constructs a phase-by-phase independent HVRT and LVRT control system, using three independent DSOGI channels to independently extract the amplitude and phase of the three-phase voltages. Each phase independently performs HVRT and LVRT status determination and current command calculation, achieving complete decoupling of the three-phase control. Phase-by-phase "reactive power priority" amplitude current limiting ensures no current overflow in any single arm. A phase-by-phase multi-resonant PR current inner loop improves the current waveform quality under fault conditions. Finally, unified zero-sequence injection modulation achieves coordinated output between the first three arms and the fourth arm, fully leveraging the topological advantages of the three-phase four-arm system to achieve smooth HVRT and LVRT under any asymmetrical fault.

[0024] In this embodiment, the reference Figures 1-3 A phase-by-phase high and low voltage ride-through control method for a three-phase four-arm grid-connected converter is provided, comprising the following steps: Step 100: Independently detect the instantaneous three-phase grid voltage phase by phase, and extract the voltage amplitude and unit voltage component of each phase; Step 200: Based on the voltage amplitude of each phase, independently determine the ride-through operation state of each phase, which includes low voltage ride-through state, normal operation state and high voltage ride-through state; Step 300: Based on the cross-operation status of each phase, calculate the current command for each phase, which includes the active current amplitude and the reactive current amplitude. Step 400: Based on the current command of each phase, perform phase-separated reactive power priority current amplitude limiting to obtain the current command after limiting. Step 500: Based on the current command after limiting and the unit voltage component of the corresponding phase, synthesize the instantaneous current command of the three phases; Step 600: Based on the instantaneous current command of the three phases, perform phase-separated multi-resonance proportional resonant current closed loop and voltage feedforward to obtain the initial modulation wave of the three phases. Step 700: Based on the initial modulation wave of the three phases, perform zero-sequence injection four-arm coordinated modulation to obtain the modulation wave of the four arms; Step 800: Generate four-phase independent NPC three-level unipolar SPWM pulses based on the modulation wave of the four bridge arms.

[0025] The following is a detailed explanation of each of the above steps: Step 100: Perform independent phase detection of the three-phase instantaneous grid voltage and extract the voltage amplitude and unit voltage component of each phase.

[0026] For the sampled three-phase instantaneous grid voltage , , The inputs are processed by three independent dual second-order generalized integrators (DSOGIs), and each DSOGI outputs the in-phase voltage component of the corresponding phase. With orthogonal voltage components ,in, , They represent the three phases respectively.

[0027] The voltage amplitude of each phase is calculated independently based on the in-phase voltage component and the quadrature voltage component: ; In the above formula, This indicates the voltage amplitude of each phase.

[0028] Set minimum voltage amplitude limit (Take the rated phase voltage amplitude) (1%), the voltage amplitude of each phase Clamp to minimum voltage amplitude limit The above measures are to avoid division by zero errors in subsequent calculations during deep drops.

[0029] ; Dividing the in-phase voltage component and the quadrature voltage component by the corresponding phase voltage amplitude yields the unit voltage component, which is the unit in-phase component of each phase voltage. orthogonal components , serving as the phase reference for current synthesis: ; ; In this step, the three-phase voltage calculations are completely independent. The sampling fluctuations and fault transients of any one phase will not be transmitted to other phases, achieving complete decoupling of voltage information extraction and providing a precise phase and amplitude basis for phase-separated high-low voltage control.

[0030] Step 200: Based on the voltage amplitude of each phase, independently determine the ride-through operating state of each phase, which includes low voltage ride-through state, normal operation state and high voltage ride-through state.

[0031] against a , b , c Each phase, based solely on the voltage amplitude of that phase. The operating status is determined independently, and the three-phase states do not affect or couple with each other.

[0032] Define three independent high-low current operation status flags : ; In the formula, Indicates the low-voltage ride-through trigger threshold. This indicates the high voltage ride-through trigger threshold, which can be adjusted according to the corresponding grid connection standard requirements.

[0033] Three-phase high / low voltage operation status indicator , , They are independent of each other and can be combined arbitrarily to adapt to all asymmetrical mixed fault conditions such as single-phase grounding faults, phase-to-phase short-circuit faults, and high- and low-voltage mixed drops, breaking through the limitations of traditional three-phase unified judgment for condition adaptation.

[0034] Step 300: Based on the cross-operation status of each phase, calculate the current command for each phase, which includes the active current amplitude and the reactive current amplitude.

[0035] Under normal operating conditions, each phase generates current commands using an independent PQ power outer loop. When a corresponding phase is determined to be in a low-voltage ride-through or high-voltage ride-through state, that phase automatically switches to the high / low ride-through current command calculation mode, while the remaining phases maintain their original operating states. Both modes output active current amplitude commands in a unified format. With reactive current amplitude command They share subsequent flow limiting, synthesis, and inner loop control links.

[0036] Step 310: Normal operating state: Separate phase independent PQ power outer loop.

[0037] Independent active power PI controllers and reactive power PI controllers are configured for each of the three phases, and the three-phase control is completely decoupled, providing a unified basic control architecture for high-low power transmission mode.

[0038] by x Taking one phase as an example, calculate the power error: ; In the formula, express There is always active power error; express Real-time reactive power error; express time x Phase active power command; express time x Phase reactive power command; express time x Instantaneous active power measured in phase; express time x Phase instantaneous reactive power detection value.

[0039] Based on the power error, an incremental PI controller is used to calculate... x Active current amplitude command, reactive current amplitude command: ; In the formula, This represents the proportional gain of the power loop; Indicates the integral coefficient of the power loop; express time x Phase active current amplitude components; express time x Phase active current amplitude components; express time x Phase reactive current amplitude components; express time x Phase reactive current amplitude components; express There is always active power error; express Real-time reactive power error.

[0040] Step 320: Low Voltage Ride-Through State: Calculation of Phase-by-Phase Inductive Reactive Power Support.

[0041] When the high / low cross operation status flag bit When this phase enters a low-voltage ride-through state, the inductive reactive current is calculated based on the voltage drop depth of this phase to support the grid voltage.

[0042] The magnitude of reactive current is directly proportional to the voltage sag depth of the current phase: ; In the formula, Indicates the low-voltage reactive support coefficient; Indicates the rated phase voltage amplitude; This indicates the rated phase current amplitude of the converter.

[0043] Step 330: High voltage ride-through state: Phase compatibility reactive power absorption.

[0044] When the high / low cross operation status flag bit When this phase enters high-voltage ride-through mode, the capacitive reactive current is calculated based on the voltage rise depth of this phase to suppress the rise in grid voltage.

[0045] The magnitude of reactive current is directly proportional to the depth of voltage rise in the current phase: ; In the formula, It is the high-throughput reactive power absorption coefficient.

[0046] Step 340: Under high-voltage ride-through and low-voltage ride-through conditions, the active current amplitude of each phase is allocated based on the total active power command of the system and the voltage amplitude of each phase.

[0047] The active current amplitude is allocated to each phase according to the total active power command of the system and weighted by the voltage amplitude of each phase. The phase with higher voltage bears more active power to avoid overcurrent in low-voltage phases. ; ; ; ; In the formula, This is the total active power command for the three phases of the system; Indicates the total three-phase voltage amplitude; express x High-low crossover active command; This indicates the amplitude of the active current.

[0048] Step 350: Seamless mode switching mechanism.

[0049] The output in both normal and high / low penetration states is a phase current amplitude command (active current amplitude component). reactive current amplitude component Furthermore, they share the same voltage phase reference for current synthesis. The switching process only replaces the source of current amplitude generation. The phase reference is completely unified, with no instruction step, achieving a smooth transition.

[0050] Step 400: Based on the current command of each phase, perform phase-separated reactive power priority current amplitude limiting to obtain the current command after limiting.

[0051] Each phase independently implements current amplitude limiting, prioritizing reactive current output and allocating the remaining capacity to active current to ensure that no single bridge arm experiences overcurrent.

[0052] Step 410: Calculate the active current limit based on the active current amplitude: ; ; In the formula, This is the active current limiting value; for d Remaining available current amplitude on the shaft.

[0053] Step 420: Based on the active current limiting value, calculate the bidirectional active current limiting, i.e., the active current amplitude after limiting:

[0054] After current limiting, the current amplitude of each phase satisfies Each phase current limiting constraint is independent of the others, and the faulty phase will not encroach on the current capacity of the non-faulty phase, thus maximizing the reactive power support capacity of the faulty phase under the rated capacity constraint.

[0055] Step 500: Based on the current command after limiting and the unit voltage component of the corresponding phase, synthesize the instantaneous current command of the three phases.

[0056] By combining the active current amplitude and reactive current amplitude of each phase with the unit voltage component of the corresponding phase, a three-phase instantaneous current command is synthesized: ; In the formula, and In phase, corresponding to the active current amplitude component; and In phase, corresponding to the inductive reactive current component; This represents the amplitude of the reactive current; when its magnitude is negative, it corresponds to the capacitive reactive component.

[0057] Step 600: Based on the instantaneous current command of the three phases, perform phase-separated multi-resonance proportional resonant current closed loop and voltage feedforward to obtain the initial modulation wave of the three phases.

[0058] The first three phases adopt independent multi-resonance proportional resonant (PR) controllers. Each phase controller operates independently, and the parameters are set independently. There is no cross-operation between phases, which is suitable for working conditions with different harmonic contents in each phase under asymmetrical faults.

[0059] Obtain the output current sample value, and calculate the three-phase current error value based on the three-phase instantaneous current command and the three-phase output current sample value: ; In the formula, for x Phase output current sampling value; Indicates the first k time x Phase output current sampling value; Indicates the first k Instantaneous current command for three phases at any given time; Indicates the first k time x Phase current error value.

[0060] Based on the current error value, and combined with the proportional coefficient and the resonant controller output, calculate the output voltage of the proportional resonant controller: ; In the formula, This is the proportionality coefficient; Indicates the first k The output voltage of the proportional resonant controller at any given moment; Indicates the first k Time of the first n The output of the resonant controller for the subharmonic. Corresponding to the fundamental frequency, Corresponding to each harmonic.

[0061] The output voltage of the proportional resonant controller is superimposed with the grid voltage feedforward term to obtain the first... x The initial modulation wave of the phase: ; In the formula, This refers to the voltage feedforward coefficient; For the first x Instantaneous sampling values ​​of the phase grid voltage are used to improve the current response speed under grid voltage disturbances; Indicates the k-th time. x The initial modulation wave of the phase.

[0062] Step 700: Based on the initial modulation wave of the three phases, perform zero-sequence injection four-arm coordinated modulation to obtain the modulation wave of the four arms.

[0063] The DC voltage utilization rate is improved by using the third harmonic zero-sequence injection method, while simultaneously achieving coordinated control of the fourth bridge arm. The specific steps are as follows: Step 710: Extract the maximum and minimum values ​​of the initial modulation wave from the initial modulation wave of the three phases: ; ; In the formula, This represents the initial maximum value of the modulated wave; This represents the minimum value of the initial modulated wave.

[0064] Step 720: Based on the initial maximum and minimum values ​​of the modulated wave, calculate the zero-sequence injection component (the common-mode component characteristic of the third harmonic): ; In the formula, This represents the zero-order injection component.

[0065] Step 730: Superimpose the zero-sequence injection component onto the initial modulation wave of the first three phases to obtain the final modulation wave of the first three bridge arms: ; In the formula, This indicates the final modulated wave of the first three bridge arms.

[0066] Step 740: Directly use the zero-sequence injection component as the modulation wave of the fourth bridge arm: ; In this approach, the fourth bridge arm does not require an independent current or voltage closed loop. It can provide a zero-sequence path for the three-phase unbalanced current generated by phase-splitting control simply by following the zero-sequence injection component. At the same time, it can achieve synchronous modulation with the first three bridge arms. The algorithm is extremely simple, has a fast dynamic response, and is naturally compatible with the phase-splitting high-low ride control architecture.

[0067] Step 800: Generate four-phase independent NPC three-level unipolar SPWM pulses based on the modulation wave of the four bridge arms.

[0068] For the NPC three-level topology, a four-phase independent unipolar SPWM modulation method is adopted. A, B, C, and N each calculate the switching pulse independently, with no phase coupling operation, which is fully matched with the phase-separated control architecture.

[0069] Each phase of the NPC bridge arm includes two series-connected switches in both the upper and lower arms. These switches work together to achieve a three-level output. x Taking one phase as an example, the switching cycle is T, and the total DC bus voltage is... According to the final modulation wave of the first three bridge arms Calculate the conduction time of the bridge arm switch based on the polarity half-cycle: Step 810: Positive half-cycle modulation ( ): Based on the final modulation waveform of the first three bridge arms, the first switch of the upper bridge arm turns on according to the modulation ratio, and the second switch of the upper bridge arm turns off. The conduction time of the upper bridge arm is... : ; The conduction time of the second switch tube in the upper bridge arm is set to an invalid value that exceeds the switching cycle, and it remains in the off state.

[0070] Step 820: Negative half-cycle modulation ( ): The second switch of the upper bridge arm is turned on according to the modulation ratio, and the first switch of the upper bridge arm is turned off. The on-time of the second switch of the upper bridge arm is... : ; The conduction time of the first switch tube of the upper bridge arm is set to an invalid value, and it remains in the off state.

[0071] Furthermore, the conduction times of the first switch transistor of the upper arm and the first switch transistor of the lower arm are complementary, and the conduction times of the second switch transistor of the upper arm and the second switch transistor of the lower arm are also complementary.

[0072] The four-phase modulation is computed in parallel and independently, without the need for complex vector partitioning and zero-sequence component allocation. The algorithm is simple and easy to implement in engineering.

[0073] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention, and should all be included within the protection scope of the present invention.

Claims

1. A phase-by-phase high and low voltage ride-through control method for a three-phase four-arm grid-connected converter, characterized in that, Includes the following steps: Step 100: Independently detect the instantaneous three-phase grid voltage phase by phase, and extract the voltage amplitude and unit voltage component of each phase; Step 200: Based on the voltage amplitude of each phase, independently determine the ride-through operation state of each phase, which includes low voltage ride-through state, normal operation state and high voltage ride-through state; Step 300: Based on the cross-operation status of each phase, calculate the current command for each phase, which includes the active current amplitude and the reactive current amplitude. Step 400: Based on the current command of each phase, perform phase-separated reactive power priority current amplitude limiting to obtain the current command after limiting. Step 500: Based on the current command after limiting and the unit voltage component of the corresponding phase, synthesize the instantaneous current command of the three phases; Step 600: Based on the instantaneous current command of the three phases, perform phase-separated multi-resonance proportional resonant current closed loop and voltage feedforward to obtain the initial modulation wave of the three phases. Step 700: Based on the initial modulation wave of the three phases, perform zero-sequence injection four-arm coordinated modulation to obtain the modulation wave of the four arms; Step 800: Generate four-phase independent NPC three-level unipolar SPWM pulses based on the modulation wave of the four bridge arms.

2. The phase-by-phase high and low voltage ride-through control method for a three-phase four-arm grid-connected converter according to claim 1, characterized in that, Step 100 includes: The sampled instantaneous three-phase grid voltages are input into three independent dual second-order generalized integrators for processing. Each dual second-order generalized integrator outputs the in-phase voltage component and the quadrature voltage component of the corresponding phase. The voltage amplitude of each phase is calculated independently based on the in-phase voltage component and the quadrature voltage component; a minimum voltage amplitude limit is set to clamp the voltage amplitude of each phase to above the minimum voltage amplitude limit; Divide the in-phase voltage component and the quadrature voltage component by the corresponding phase voltage amplitude to obtain the unit voltage component, which is the unit in-phase component and the unit quadrature component of each phase voltage.

3. The phase-by-phase high and low voltage ride-through control method for a three-phase four-arm grid-connected converter according to claim 1, characterized in that, In step 200, independently determining the cross-running status of each phase based on the voltage amplitude of each phase includes: Define three independent high-low current operation status flags : ； In the formula, Indicates the low-voltage ride-through trigger threshold; Indicates the high voltage ride-through trigger threshold; This indicates the voltage amplitude of each phase.

4. The phase-by-phase high and low voltage ride-through control method for a three-phase four-arm grid-connected converter according to claim 1, characterized in that, Step 300 includes: Under normal operating conditions, each phase uses an independent PQ power outer loop to generate current commands; During high-voltage ride-through and low-voltage ride-through conditions, reactive current amplitude is allocated first; active current amplitude is allocated to each phase based on the total active power command of the system and the voltage amplitude of each phase.

5. The phase-by-phase high and low voltage ride-through control method for a three-phase four-arm grid-connected converter according to claim 1, characterized in that, Step 400 includes: Calculate the active current limit value based on the active current amplitude; Based on the active current limit value, the bidirectional active current limit is calculated to obtain the current command after the limit is obtained.

6. The phase-by-phase high and low voltage ride-through control method for a three-phase four-arm grid-connected converter according to claim 1, characterized in that, Step 600 includes: Obtain the output current sample value, and calculate the three-phase current error value based on the instantaneous current command of the three phases and the output current sample value of the three phases; Based on the current error value, and combined with the proportional coefficient and the output of the resonant controller, the output voltage of the proportional resonant controller is calculated; The output voltage of the proportional resonant controller is superimposed with the grid voltage feedforward term to obtain the initial modulation wave of the three phases.

7. The phase-by-phase high and low voltage ride-through control method for a three-phase four-arm grid-connected converter according to claim 1, characterized in that, Step 700 includes: Extract the maximum and minimum values ​​of the initial modulation wave from the initial modulation wave of the three phases; The zero-sequence injection component is calculated based on the initial maximum and minimum values ​​of the modulated wave. The zero-sequence injection component is superimposed on the initial modulation wave of the first three phases to obtain the final modulation wave of the first three bridge arms: the zero-sequence injection component is used as the modulation wave of the fourth bridge arm.

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