Stability domain extension system and extension method of three-phase four-leg conversion unit, control equipment and storage medium

Abstract

The invention relates to a stability domain extension system and extension method of a three-phase four-leg conversion unit, control equipment and a storage medium. The method comprises a three-phase four-leg conversion unit; the positive and negative sequence loop control branch is used for acquiring the common access point voltage from the common power grid end of the three-phase four-leg conversion unit and acquiring the grid-connected current from the grid-connected current end, and performing conversion processing on the common access point voltage and the grid-connected current according to the reference current of an external voltage loop to generate a power grid modulation signal of a positive and negative sequence loop; a current loop control module; the common-mode loop stable interval expansion module is used for performing sampling frequency modulation processing on the modulation wave signal to generate a power grid modulation signal of a zero-sequence loop; a PWM generation module; according to the system, the expansion of the zero-sequence stability domain of the three-phase four-bridge-arm conversion unit is realized, and the stability of the three-phase four-bridge-arm conversion unit is improved; in addition, in combination with a common-mode loop stable interval extension module, the system is simple and easy to implement, and meanwhile, the robustness of the system can be improved.

Description

Technical Field

[0001] This application relates to the field of multi-source microgrid technology, and in particular to a stability domain extension system, extension method, control device and storage medium for a three-phase four-arm converter unit. Background Technology

[0002] The three-phase four-arm converter unit is the main node in the construction of AC microgrids. With the increasing number of microgrids built by various distributed energy sources, the three-phase four-arm converter unit will serve as the core network node for realizing three-phase four-wire power supply to the load side when the microgrid is disconnected from the main grid. Its unique fourth arm and neutral line structure provide a low-impedance path for zero-sequence current and provide unbalanced power between phases. At the same time, the three-phase four-arm converter unit also frequently switches to the mode of operation connected to the main grid. Since the microgrids composed of new energy distributed generation systems are all located at the end of the distribution network, the equivalent impedance of the grid connection point of the three-phase four-arm converter unit is significantly increased due to the extension of the line. At the same time, the random switching of distributed power sources in the microgrid and load fluctuations further cause the grid impedance to change dynamically over a wide range. Against this background, the three-phase four-arm converter unit faces its unique stability challenges.

[0003] The mainstream strategies employed in existing digital control systems include undamped grid-connected current control and active damping with capacitor current feedback. The necessary prerequisite for their stable operation is that the resonant frequency of the inductor-capacitor-inductor filter (LCL) in the system must be within acceptable limits. At sampling frequency Within a specific frequency band, taking the most widely used undamped control as an example, its necessary stability condition is: .

[0004] However, the wide range of grid impedance variations will cause the system's equivalent inductance value to drift, resulting in a significant shift in the resonant frequency. When the resonant frequency exceeds the stability boundary, the control system loses its phase margin, leading to continuous oscillations or even collapse. Summary of the Invention

[0005] Therefore, it is necessary to provide a stability domain extension system, extension method, control device and storage medium for a three-phase four-arm converter unit to address the above-mentioned technical problems.

[0006] In a first aspect, this application provides a stability domain extension system for a three-phase four-arm converter unit, comprising:

[0007] The positive and negative sequence loop control branch is used to obtain the common access point voltage from the common grid terminal of the three-phase four-arm converter unit, obtain the grid current from the grid current terminal of the three-phase four-arm converter unit, and generate the grid modulation signal of the positive and negative sequence loop after transforming the common access point voltage and the grid current according to the reference current of the external voltage loop.

[0008] The current loop control module is used to obtain the zero-axis current from the fourth arm of the three-phase four-arm converter unit and generate a modulation wave signal based on the zero-axis current after coordinate transformation and the zero-axis reference current.

[0009] The common-mode loop stability range extension module is used to modulate the sampling frequency of the zero-sequence reference voltage to generate the grid modulation signal of the zero-sequence loop.

[0010] The PWM generation module is used to generate PWM signals based on the grid modulation signals of the positive and negative sequence loops and the grid modulation signal of the zero sequence loop, and to control the three-phase four-arm converter unit based on the PWM signals.

[0011] In one embodiment, the positive and negative sequence loop control branch includes a first transformation branch, a second transformation branch, and a third transformation branch, comprising:

[0012] The first conversion branch is used to obtain the common access point voltage from the common grid terminal of the three-phase four-arm conversion unit and convert the common access point voltage into the fundamental positive sequence phase.

[0013] The second transformation branch is used to obtain the grid-connected current from the common current terminal of the three-phase four-arm transformation unit, and to perform coordinate transformation on the grid-connected current to obtain the grid-connected current component signal.

[0014] The third transformation branch is used to generate the grid modulation signal for the positive and negative sequence loops based on the reference current of the external voltage loop, the fundamental positive sequence phase, and the grid-connected current component signal.

[0015] In one embodiment, the first transformation branch includes a first coordinate transformation module and a phase-locked loop module, which include:

[0016] The first coordinate transformation module is used to obtain the common access point voltage from the common grid terminal of the three-phase four-arm transformation unit and obtain the components of the grid voltage.

[0017] The phase-locked loop module is used to decompose the components of the grid voltage to obtain the fundamental positive-sequence phase.

[0018] In one embodiment, the second transformation branch includes a second coordinate transformation module; the third transformation branch includes a third coordinate transformation module, a current loop control module, and a fourth coordinate transformation module, including:

[0019] The second coordinate transformation module is used to obtain the grid current from the grid current terminal of the three-phase four-arm conversion unit, and to perform coordinate transformation on the grid current to obtain the grid current component signal.

[0020] The third coordinate transformation module is used to generate the grid-connected current signal based on the reference current and fundamental positive sequence phase of the external voltage loop.

[0021] The current loop control module is used to generate the grid reference voltage based on the grid reference current and the zero-axis current sampling signal;

[0022] The fourth coordinate transformation module is used to perform coordinate transformation on the grid-connected reference voltage to obtain the grid modulation signal of the positive and negative sequence loops.

[0023] In one embodiment, the three-phase four-arm converter unit includes a three-phase four-arm circuit, a filter circuit, an impedance circuit, a common grid-connected probe line, and a common current sensing loop. The output terminal of the three-phase four-arm circuit is connected to the input terminal of the filter circuit, the output terminal of the filter circuit is connected to the input terminal of the common grid-connected probe line, the output terminal of the common grid-connected probe line is connected to the input terminal of the impedance circuit, the filter circuit is also connected to the input terminal of the common current sensing loop, the output terminal of the common current sensing loop is connected to the input terminal of the switch control unit, the input terminal of the switch control unit is also connected to the output terminal of the common grid-connected probe line, and the output terminal of the switch control unit is connected to the switch on the arm of the three-phase four-arm circuit.

[0024] In one embodiment, a stability domain extension method for a three-phase four-arm converter unit is applied to a stability domain extension system for the bridge arm converter unit, comprising:

[0025] Obtain the target resonant frequency and target phase margin of the three-phase four-arm converter unit, and configure specific parameters for the common-mode loop stability range extension module based on the target resonant frequency and target phase margin; the specific parameters are the additional discrete control cycles.

[0026] The common-mode loop stability range extension module is activated to optimize the zero-axis loop in the three-phase four-arm converter unit.

[0027] In one embodiment, the common-mode loop stability range extension module is configured with specific parameters based on the target resonant frequency and the target phase margin, including:

[0028] Based on the correspondence between phase margin and parameters, determine the upper limit value of the specific parameter corresponding to the target phase margin;

[0029] Based on the correspondence between the power grid impedance range and parameters, determine the lower limit value of the specific parameter corresponding to the target resonant frequency;

[0030] Configure specific parameters for the common-mode loop stability range extension module based on the upper and lower limits.

[0031] In one embodiment, a method for extending the stability domain of a three-phase four-arm converter further includes optimizing the inductance parameters of the fourth arm in the three-phase four-arm converter according to specific parameters.

[0032] Secondly, this application also provides a control device for a stability domain extension system of a three-phase four-arm converter, including a memory and a processor. The memory stores a computer program, and the processor executes the computer program to perform the following steps:

[0033] The positive and negative sequence loop control branch is used to obtain the common access point voltage from the common grid terminal of the three-phase four-arm converter unit, obtain the grid current from the grid current terminal of the three-phase four-arm converter unit, and generate the grid modulation signal of the positive and negative sequence loop after transforming the common access point voltage and the grid current according to the reference current of the external voltage loop.

[0034] The current loop control module is used to obtain the zero-axis current from the fourth arm of the three-phase four-arm converter unit and generate a modulation wave signal based on the zero-axis current after coordinate transformation and the zero-axis reference current.

[0035] The common-mode loop stability range extension module is used to modulate the sampling frequency of the zero-sequence reference voltage to generate the grid modulation signal of the zero-sequence loop.

[0036] The PWM generation module is used to generate PWM signals based on the grid modulation signals of the positive and negative sequence loops and the grid modulation signal of the zero sequence loop, and to control the three-phase four-arm converter unit based on the PWM signals.

[0037] Thirdly, this application also provides a storage medium for a stability domain extension system of a three-phase four-arm converter, on which a computer program is stored, and when the computer program is executed by a processor, it performs the following steps:

[0038] The positive and negative sequence loop control branch is used to obtain the common access point voltage from the common grid terminal of the three-phase four-arm converter unit, obtain the grid current from the grid current terminal of the three-phase four-arm converter unit, and generate the grid modulation signal of the positive and negative sequence loop after transforming the common access point voltage and the grid current according to the reference current of the external voltage loop.

[0039] The current loop control module is used to obtain the zero-axis current from the fourth arm of the three-phase four-arm converter unit and generate a modulation wave signal based on the zero-axis current after coordinate transformation and the zero-axis reference current.

[0040] The common-mode loop stability range extension module is used to modulate the sampling frequency of the zero-sequence reference voltage to generate the grid modulation signal of the zero-sequence loop.

[0041] The PWM generation module is used to generate PWM signals based on the grid modulation signals of the positive and negative sequence loops and the grid modulation signal of the zero sequence loop, and to control the three-phase four-arm converter unit based on the PWM signals.

[0042] The aforementioned three-phase four-arm converter unit's stability domain extension system, extension method, control device, and storage medium include a three-phase four-arm converter unit, a positive and negative sequence loop control branch, and a zero-sequence loop control branch, which includes a current loop control module, a common-mode loop stability range extension module, and a PWM generation module. The positive and negative sequence loop control branch is used to obtain the common connection point voltage from the common grid terminal of the three-phase four-arm converter unit, obtain the grid-connected current from the grid-connected current terminal of the three-phase four-arm converter unit, and generate the grid modulation signal for the positive and negative sequence loop after transforming the common connection point voltage and grid-connected current according to the reference current of the external voltage loop. The current loop control module is used to obtain the common connection point voltage from the common grid terminal of the three-phase four-arm converter unit, obtain the grid-connected current from the grid-connected current terminal of the three-phase four-arm converter unit, and generate the grid modulation signal for the positive and negative sequence loop after transforming the common connection point voltage and grid-connected current according to the reference current of the external voltage loop. The system acquires zero-axis current from the four bridge arms and generates a modulation wave signal based on the zero-axis current after coordinate transformation and the zero-axis reference current. A common-mode loop stability range extension module modulates the modulation wave signal at its sampling frequency to generate a grid modulation signal for the zero-sequence loop. A PWM generation module generates a PWM signal based on the grid modulation signals for the positive and negative sequence loops and the grid modulation signal for the zero-sequence loop, and controls the three-phase four-bridge arm converter unit based on the PWM signal. This system expands the zero-sequence stability domain of the three-phase four-bridge arm converter unit, improving its stability. Furthermore, combined with the common-mode loop stability range extension module, this system is simple to implement and improves its robustness. Attached Figure Description

[0043] To more clearly illustrate the technical solutions in the embodiments of this application or related technologies, the drawings used in the description of the embodiments of this application or related technologies will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0044] Figure 1 This is an application environment diagram of the stability domain extension system of the three-phase four-arm converter unit in the embodiments of this application;

[0045] Figure 2 This is a comparison diagram of the stability domain extension system of the three-phase four-bridge arm converter unit in the embodiments of this application before and after modulation;

[0046] Figure 3This is a block diagram of the zero-sequence loop control of the stable domain extension system of the three-phase four-arm converter unit in the embodiments of this application;

[0047] Figure 4 The zero-sequence resonant frequency f is the stable domain extension system of the three-phase four-arm converter unit in the embodiments of this application. r_CM Less than a specific resonant frequency f b Bode plots of system loop gain before and after compensation;

[0048] Figure 5 The zero-sequence resonant frequency f is the stable domain extension system of the three-phase four-arm converter unit in the embodiments of this application. r_CM Greater than a specific resonant frequency f b Bode plots of system loop gain before and after compensation;

[0049] Figure 6 This is one of the structural block diagrams of the stability domain extension system of the three-phase four-arm converter unit in the embodiments of this application;

[0050] Figure 7 This is the second structural block diagram of the stability domain extension system of the three-phase four-arm converter unit in the embodiments of this application;

[0051] Figure 8 This is the third structural block diagram of the stability domain extension system of the three-phase four-bridge arm converter unit in the embodiments of this application;

[0052] Figure 9 This is the fourth structural block diagram of the stable domain extension system of the three-phase four-bridge arm converter unit in the embodiments of this application;

[0053] Figure 10 This is a control block diagram of the stability domain extension system of the three-phase four-arm converter unit in the embodiments of this application;

[0054] Figure 11 This is one of the flowcharts illustrating the stability domain extension method for a three-phase four-arm converter unit in this application.

[0055] Figure 12 This is the second flowchart illustrating the zero-sequence loop stability domain extension method for the three-phase four-arm converter unit in this application.

[0056] Figure 13 This is a diagram showing the improved zero-sequence loop stability domain of the optimized stability domain extension system of the three-phase four-arm converter unit in this application embodiment.

[0057] Figure 14 This is the third flowchart illustrating the zero-sequence loop stability domain extension method for the three-phase four-arm converter unit in this application.

[0058] Figure 15This is a diagram showing the inductance range variation of the stability domain extension system of the three-phase four-arm converter unit in the embodiments of this application.

[0059] Figure 16 This is a system stability diagram of the stability domain extension system of the three-phase four-arm converter unit in the embodiments of this application;

[0060] Figure 17 This is a system instability diagram of the stability domain extension system of the three-phase four-arm converter unit in the embodiments of this application;

[0061] Figure 18 This is the optimized operating stability diagram of the three-phase four-bridge arm converter unit's stability domain extension system in the embodiments of this application.

[0062] Figure 19 This is an internal structure diagram of the stability domain extension system of a three-phase four-arm converter unit in one embodiment. Detailed Implementation

[0063] To make the objectives, technical solutions, and advantages of this application clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the scope of this application.

[0064] It should be noted that the terms "first," "second," etc., used in this application can be used to describe various elements, but these elements are not limited by these terms. These terms are only used to distinguish the first element from the second element. The terms "comprising" and "having," and any variations thereof, used in this application, are intended to cover non-exclusive inclusion. The term "multiple" used in this application refers to two or more. The term "and / or" used in this application refers to one of the embodiments, or any combination of multiple embodiments.

[0065] The three-phase four-arm converter unit is a key node in the construction of AC microgrids. With the increasing prevalence of microgrids built from various distributed energy sources, the three-phase four-arm converter unit serves as a core node for achieving three-phase four-wire power supply to the load side when the microgrid operates independently of the main grid. Its unique fourth arm and neutral line structure provide a low-impedance path for zero-sequence current, addressing phase-to-phase imbalance power. Simultaneously, the three-phase four-arm converter unit frequently switches to grid-connected operation. Since microgrids composed of distributed new energy generation systems are located at the end of the distribution network, the equivalent impedance at the grid connection point of the three-phase four-arm converter unit increases significantly due to line extension. Furthermore, the random switching of distributed power sources within the microgrid and load fluctuations further cause dynamic changes in grid impedance over a wide range. Against this backdrop, the three-phase four-arm converter unit faces unique stability challenges.

[0066] The mainstream strategies employed in existing digital control systems include undamped grid-connected current control and active damping with capacitor current feedback. The necessary prerequisite for their stable operation is that the resonant frequency of the inductor-capacitor-inductor filter (LCL) in the system must be within acceptable limits. At sampling frequency Within a specific frequency band, taking the most widely used undamped control as an example, its necessary stability condition is: .

[0067] However, a wide range of varying grid impedance will cause the system's equivalent inductance to drift, resulting in a significant shift in the resonant frequency. When the resonant frequency exceeds the stability boundary, the control system loses its phase margin, leading to continuous oscillations or even collapse.

[0068] In view of the above-mentioned technical problems, this application provides a stability domain extension system and method for a three-phase four-arm converter. The following embodiments will specifically illustrate the stability domain extension system and method for the three-phase four-arm converter.

[0069] In one exemplary embodiment, such as Figure 1 As shown, a stability domain extension system for a three-phase four-arm converter is provided. The stability domain extension system includes a three-phase four-arm converter and a switch control unit. The switch control unit includes a positive and negative sequence loop control branch, a zero sequence loop control branch, and a PWM generation module. The zero sequence loop control branch includes a current loop control module and a common-mode loop stability range extension module. The system includes a positive and negative sequence loop control branch, which obtains the common access point voltage from the common grid terminal of the three-phase four-arm converter unit, obtains the grid current from the grid current terminal of the three-phase four-arm converter unit, and generates a grid modulation signal for the positive and negative sequence loop after transforming the common access point voltage and the grid current according to the reference current of the external voltage loop; a current loop control module, which obtains the zero-axis current from the fourth arm of the three-phase four-arm converter unit and generates a modulation wave signal according to the zero-axis current after coordinate transformation and the zero-axis reference current; a common-mode loop stable interval extension module, which modulates the modulation wave signal by sampling frequency to generate a grid modulation signal for the zero-sequence loop; and a PWM generation module, which generates a PWM signal according to the grid modulation signals of the positive and negative sequence loops and the grid modulation signal of the zero-sequence loop, and controls the three-phase four-arm converter unit according to the PWM signal.

[0070] Among them, the positive and negative sequence loop control branch is used to handle the paths of positive and negative sequence components under symmetrical and asymmetrical operating conditions in a three-phase system. It is generally composed of multiple coordinate transformation modules and PWM generation modules. The zero sequence loop control branch is used to handle the paths of zero sequence components under asymmetrical conditions in a three-phase system, such as zero sequence voltage and zero sequence current.

[0071] Among them, the external voltage loop is the DC bus voltage loop, which can generate the command signal of the inner loop, and the grid-connected current represents the current injected into the public power grid by the three-phase four-bridge arm unit through the LCL filter.

[0072] Wherein, the zero-axis current i0 refers to the output current on the fourth arm of the three-phase four-arm converter unit, and the zero-axis reference current i 0_ref It is a current signal generated by the external voltage loop.

[0073] The working principle of the stability domain extension system of the three-phase four-arm converter unit described in this application embodiment includes: the system adopts an undamped grid-connected current loop control strategy, which consists of an external DC voltage loop and an internal grid-connected current loop. The external DC voltage loop generates a d-axis output current reference value I. d_ref q-axis output current reference value I q_ref and zero-axis reference current i 0_ref The positive and negative sequence loops acquire the common access point voltage from the common grid terminal and the grid-connected current from the grid-connected current terminal. Using the common access point voltage as the input source, combined with the first coordinate module and the phase-locked loop module, the output fundamental positive sequence phase, along with the reference current of the external voltage loop, serves as the input to the third coordinate transformation module. The acquired grid-connected current command signal, along with the grid-connected current component obtained through the second coordinate transformation module, serves as the input to the first current loop control module. Combined with the fourth coordinate transformation module, the grid modulation signal of the positive and negative sequence loops is obtained.

[0074] In the current loop module, based on the characteristics of the undamped grid-connected current loop control strategy, the cutoff frequency of the external voltage loop is much lower than that of the grid-connected current loop. Therefore, the system can design the internal grid-connected current loop separately, dividing it into two current loop modules. The first current loop module compares the grid-connected current (…). i 2_a i 2_b and i 2_c Feedback and reference values, for example, the αβ axis component of the grid-connected current after coordinate transformation. αβ And the grid-connected current reference value I after coordinate transformation αβ_ref The second current loop module compares the feedback value of the zero-axis current with a reference value, for example, the zero-axis current i0 and the zero-axis reference current i generated by the external voltage loop. 0_ref The first current loop module and the second current loop module generate error signals, which are then modulated to generate modulated wave signals.

[0075] In the common-mode loop stability interval extension module, this module exists only in the zero-sequence loop. A discrete control period is added independently to the control loop of the zero-sequence loop. After adding the discrete control period to the zero-sequence loop of the control system, the sampling time is advanced by λTs compared to the original time. The existence condition of the stability domain changes from relation (1) to relation (2):

[0076] (1);

[0077] in, The system resonant frequency, This is the system sampling frequency.

[0078] (2);

[0079] in, For a specific resonant frequency, the calculation method can be obtained from the relation (3). The system resonant frequency, This is the system sampling frequency.

[0080] (3);

[0081] Where λ is the number of additional discrete control cycles, f b For a specific resonant frequency, f s This is the system sampling frequency.

[0082] The control system can visualize the discrete regulation mechanism of the zero-sequence loop stability domain extension module by using the timing waveforms of the sampling and modulation processes, such as... Figure 2 As shown, where, The sampling frequency is determined by the switching cycle or the system's basic control cycle. ); It is the additional discrete control cycle introduced by the common-mode loop stability range extension module of the control system. (It is an adjustment parameter). It is the time interval between adjacent sampling times; the solid line in the sampling process represents the actual current, the real-time current state of the interaction between the grid and the inverter; the dashed line represents the sampling current, which is the discretized sampling result of the actual current; the sampling time represents the time point at which the actual current is collected, and the common-mode loop stability interval extension module changes the phase characteristics of the zero-sequence loop in the control system by adjusting the timing difference between this time and the subsequent modulation link; the sawtooth wave (carrier) in the modulation process is the reference signal of PWM modulation, which determines the switching frequency and duty cycle generation logic of the three-phase four-bridge arm switch tube; the dashed line (modulation wave) is the control reference signal of the zero-sequence loop; the modulation signal loading time refers to the time point at which the modulation wave is sent into the "carrier-modulation wave comparison link", and the common-mode loop stability interval extension module introduces time delay compensation in the control of the zero-sequence loop by shifting this time, that is, expressed by the transfer function, the relationship (4):

[0083] (4);

[0084] in: Let λ be the switching period, λ be the number of additional discrete control periods, and s be the complex variable of the Laplace transform. (See figure.) The time offset is equivalent to superimposing the transfer function in the zero-sequence loop of the three-phase four-arm converter unit in the control system, which directly changes the phase frequency characteristics of the loop gain.

[0085] The common-mode loop stability range extension module is shown in the control block diagram of the zero-sequence loop as follows. Figure 3 The purple part shows the calculation of the discrete period and the modulation discrete period, while the red part is the transfer function of the zero-sequence loop stability domain extension module. After adding the zero-sequence loop stability domain extension module, the loop gain expression of the original system becomes equation (5):

[0086] (5);

[0087] Where G0(s) is the transfer function of the proportional resonant controller, T s Where λ is the switching cycle, λ is the number of additional discrete control cycles, L1 is the inverter-side inductance, and C is the inductance. f L1 is the filter capacitor, L2 is the grid-side inductor, and L3 is the mains capacitor. 0_1 L is the filter inductance between the midpoint and neutral point of the fourth bridge arm. g Let be the equivalent impedance of the power grid, and s be the complex variable of the Laplace transform.

[0088] In the common-mode loop extension module, the existence condition of the stability region is modified by introducing an additional number of discrete periods λ. Increasing λ will decrease a specific resonant frequency f. b Make the zero-sequence resonant frequency Easier to fall in The stability region is expanded, and the lower limit of the region is lower, the minimum stability constraint frequency of the zero-sequence loop decreases, and the zero-sequence stability domain is expanded. For example, at the zero-sequence resonant frequency f... r_CM Less than a specific resonant frequency f b and zero-sequence resonant frequency f r_CM Greater than a specific resonant frequency f b In both cases, by introducing a common-mode loop stability range extension module into the control system for compensation, the Bode plots of the system loop gain before and after compensation can be obtained. Figure 4 It is the zero-sequence resonant frequency f r_CM Less than a specific resonant frequency f b That is, the system's instability region, when the resonant frequency is below the lower limit of the stability region f. b In the Bode plot, a negative phase crossing point appears at the resonance peak (the phase crosses from positive to negative). Furthermore, the loop gain is greater than 0 dB, directly leading to system instability. However, after introducing a common-mode loop stability range extension module into the control system, the additional discrete control period λ increases, thereby reducing the specific resonant frequency f. b Make the zero-sequence resonant frequency f r_CM Greater than or equal to a specific resonant frequency f b In other words, the resonant point re-enters the stable range, and the controller gain is adjusted so that the specific resonant frequency f is achieved. b The loop gain amplitude at this point is less than 0 dB, eliminating negative crossover, meaning the compensated amplitude-frequency curve at a specific resonant frequency f is... b The point moves downwards to below 0dB, thus cutting off the instability condition. Figure 5 It is the zero-sequence resonant frequency f r_CM Greater than a specific resonant frequency f b In other words, although the resonant frequency is within the stable region, the phase margin is close to the nearest value, making it prone to oscillation. Under these circumstances, after introducing a common-mode loop stability range extension module into the control system, a time delay is introduced through the transfer function, lowering the phase frequency characteristic curve and shifting the negative phase crossing point to the left, away from the zero-sequence resonant frequency f. r_CM The resonant peak is eliminated, and the amplitude frequency characteristics remain unchanged. Only the phase frequency characteristics are optimized to improve the phase margin.

[0089] In the PWM module, the zero-sequence loop in the three-phase four-bridge arm of the control system outputs a zero-sequence modulated wave through the PR regulator in the second current loop. The zero-sequence modulated wave signal then passes through the common-mode loop stable range extension module. The timing and phase are corrected to generate a zero-sequence loop grid modulation signal. This signal, combined with the positive and negative sequence grid modulation signals generated by the fourth current module, serves as the input for PWM modulation, outputting eight drive signals: six for the three-phase phases and two for the fourth bridge arm. This controls the switching action of the transistors on the three-phase four-bridge arm. The modulated wave is compared in real-time with the triangular carrier wave (frequency = switching frequency), and the intersection determines the pulse duty cycle. Optionally, the PWM modulation output can also be 12 drive signals, including six for the three-phase phases, two for the fourth bridge arm, and four extended redundant circuits. The control system needs to be compatible with different operating modes, such as grid-connected, off-grid, balanced, or unbalanced operating modes, and reserve spare drive channels. Since the main control DSP has limited PWM ports, external logic circuits, such as complex programmable logic devices (CPLDs), can be used to expand it to 12 signals.

[0090] The stability domain extension system of the three-phase four-arm converter described in the above embodiment achieves the effect of extending the stability domain of the zero-sequence circuit by adding a common-mode circuit stability range extension module to the zero-sequence control circuit. The common-mode circuit stability range extension module can correct the timing and phase of the modulation wave signal of the zero-sequence circuit, thereby extending the stability domain of the zero-sequence circuit. In addition, the stability of the three-phase four-arm converter is also improved as the stability of the zero-sequence circuit is improved.

[0091] In one exemplary embodiment, such as Figure 6 As shown, Figure 1 The positive and negative sequence loop control branches shown include a first transformation branch, a second transformation branch, and a third transformation branch. Specifically, the first transformation branch is used to obtain the common access point voltage from the common grid terminal of the three-phase four-arm converter unit and transform the common access point voltage into a fundamental positive sequence phase. The second transformation branch is used to obtain the grid-connected current from the common current terminal of the three-phase four-arm converter unit and perform coordinate transformation on the grid-connected current to obtain the grid-connected current component signal. The third transformation branch is used to generate the grid modulation signal for the positive and negative sequence loops based on the reference current of the external voltage loop, the fundamental positive sequence phase, and the grid-connected current component signal.

[0092] The first transformation branch consists of one or more coordinate transformation modules.

[0093] The coordinate transformation refers to the Clark transformation and the Park transformation.

[0094] Among them, the fundamental positive sequence phase is obtained by combining the public grid voltage (i.e., grid-side voltage) acquired by the control system with the first coordinate transformation module and the phase-locked loop module; the zero-axis current sampling signal is generated by the external voltage loop.

[0095] The working principle of the stability domain extension system of the three-phase four-arm converter unit described in this application embodiment includes: In the first converter branch, the control system collects the three-phase voltage from the common access point using a voltage sensor, including the phase a common grid voltage v. g_a Phase b public grid voltage v g_b and the voltage of the public power grid in phase c, V g_c As the input source, the data is fed into the first coordinate transformation module for Clark and Park transformations respectively. The control system obtains the α-axis component v of the public power grid voltage via the Clark transformation. α and the β-axis component of the public grid voltage v β The control system obtains the d-axis component v of the public power grid voltage via Park transformation. d and the q-axis component of the public grid voltage v q When the control system performs coordinate changes, it simultaneously inputs the three-phase voltage to the Decoupled Double Synchronous Reference Frame Phase Locked Loop (DDSRF_PLL) module to perform dual synchronous rotating coordinate system separation of the positive and negative sequence components of the grid voltage, obtaining the fundamental positive sequence phase θ`. The three-phase voltage acquired by the control system undergoes Clark transformation, Park transformation and the phase-locked loop module to complete the positive transformation of the voltage signal coordinates, transforming the voltage signal coordinates from the AC quantity in the αβ two-phase stationary coordinate system to the DC quantity in the dq two-phase rotating coordinate system, and obtaining the fundamental positive sequence phase θ`.

[0096] In the second conversion branch, the control system acquires the three-phase grid-connected current signal through current sensors, including the phase a grid-connected current. i 2_a Phase b grid current i 2_b and the grid-connected current i of phase c 2_c As the input source, the data is fed to the second coordinate transformation module for Clark and Park transformations respectively. The control system transforms the three variables in the abc three-phase stationary coordinate system to two variables in the αβ two-phase stationary coordinate system via Clark transformation, obtaining the α-axis component i of the grid-connected current. 2_α and grid-connected current β-axis component i 2_β The control system transforms the AC quantities in the αβ two-phase stationary coordinate system into DC quantities in the dq two-phase rotating coordinate system via Park transformation, thus obtaining the d-axis component i of the grid-connected current. 2_d and grid-connected current q-axis component i 2_q Therefore, the control system obtains the grid-connected current component i αβ .

[0097] In the third conversion branch, the overall control strategy of the control system adopts an undamped grid-connected current loop to control the three-phase four-arm converter unit. The undamped grid-connected current loop consists of an external DC bus voltage loop and an internal grid-connected current loop. The external voltage loop generates the d-axis output current reference value I. d_ref q-axis output current reference value I q_ref and zero-axis reference current i 0_ref The control system will use the d-axis output current reference value I d_ref q-axis output current reference value I q_ref In the current loop module within the positive and negative sequence circuits of the fundamental positive-sequence phase input, the control system obtains the error signals of the positive and negative sequence currents and modulates the current signals. The control system then converts the obtained modulated signals v of the positive and negative sequence currents. αβ_ref The input is sent to the fourth coordinate transformation module for coordinate transformation, and the control system obtains the grid modulation signal of the positive and negative sequence loops. .

[0098] In one exemplary embodiment, such as Figure 7 As shown, Figure 1 The first transformation branch shown includes a first coordinate transformation module and a phase-locked loop (PLL) module. The first coordinate transformation module is used to obtain the common access point voltage from the common grid terminal of the three-phase four-arm transformation unit, thus obtaining the grid voltage components. The PLL module is used to decompose the grid voltage components to obtain the fundamental positive-sequence phase.

[0099] Here, the common grid terminal refers to the common access point; the grid voltage components include the grid-connected current α-axis component i. 2_α , grid-connected current β-axis component i 2_β d-axis component of grid-connected current i 2_d and grid-connected current q-axis component i 2_q .

[0100] The working principle of the stability domain extension system of the three-phase four-arm converter unit described in this application embodiment includes: in the first coordinate transformation module, the control system collects the common access point voltage V through the common access point. abc The three variables in the three-phase stationary coordinate system abc are transformed into two variables in the two-phase stationary coordinate system αβ through Clark transformation. The transformation relationship is shown in equation (6):

[0101] (6);

[0102] Where v a Let V be the voltage of the public power grid in phase a. b The voltage of phase b of the public power grid is v. c The voltage of the C-phase public power grid is V. α For the α-axis component of the public grid voltage, v βThis represents the β-axis component of the public power grid voltage.

[0103] Then, the AC quantities in the αβ two-phase stationary coordinate system are transformed into DC quantities in the dq two-phase rotating coordinate system through the Park transformation. The transformation relationship is shown in equation (7):

[0104] (7);

[0105] Where v d For the d-axis component of the public power grid voltage, v q This represents the q-axis component of the public power grid voltage.

[0106] In the phase-locked loop module, the grid voltage signal acquired by the control system is at V abc During Clark transformation, the acquired point-of-compatibility voltage V is used. abc The input is fed into the decoupled double-synchronous rotating coordinate system phase-locked loop module (DDSRF_PLL) to perform double-synchronous rotating coordinate system separation of the positive and negative sequence components of the grid voltage and decouple and eliminate the coupling between them. The phase-locked loop module decomposes the voltage vector in the αβ coordinate system into a positive sequence voltage component rotating at angular frequency ω0 and a negative sequence voltage component rotating at angular frequency -ω0. The transformation relationship is shown in equation (8):

[0107] (8);

[0108] Where v α For the α-axis component of the public grid voltage, v β V represents the β-axis component of the public power grid voltage. + V is the positive sequence component of the grid voltage. - ω0 is the negative sequence component of the grid voltage, ω0 is the angular velocity of the positive sequence component of the voltage, and φ is the angular velocity of the positive sequence component of the voltage. + This represents the initial phase of the positive sequence voltage component.

[0109] Based on the vector relationship of the dual synchronization reference coordinate system, the dq coordinate transformations for positive-sequence synchronization and negative-sequence synchronization are performed respectively, resulting in relation (9) and relation (10):

[0110] (9);

[0111] (10);

[0112] After further approximation, we can obtain relations (11) and (12):

[0113] (11);

[0114] (12);

[0115] Among them, v d + For the positive sequence component of the d-axis voltage, v d - For the negative sequence component of the d-axis voltage, v q + The positive-sequence component of the q-axis voltage, v q - V is the negative sequence component of the q-axis voltage. + V is the positive sequence component of the grid voltage. - ω0 is the negative sequence component of the grid voltage, ω0 is the angular velocity of the positive sequence component of the voltage, and φ is the angular velocity of the positive sequence component of the voltage. + Let θ' be the initial phase of the positive sequence component of the voltage, and let θ' be the rotation angle of the positive sequence transformation. Thus, the control system obtains the phase and frequency of the fundamental positive sequence component.

[0116] In one exemplary embodiment, such as Figure 8 As shown, Figure 1 The second transformation branch shown includes a second coordinate transformation module; the third transformation branch includes a third coordinate transformation module, a current loop control module, and a fourth coordinate transformation module. Specifically, the second coordinate transformation module is used to obtain the grid-connected current from the grid-connected current terminal of the three-phase four-arm transformer unit and perform coordinate transformation on the grid-connected current to obtain the grid-connected current component signal; the third coordinate transformation module is used to generate the grid-connected current signal based on the reference current of the external voltage loop and the fundamental positive-sequence phase; the current loop control module is used to generate the grid-connected reference voltage based on the grid-connected reference current and the zero-axis current sampling signal; and the fourth coordinate transformation module is used to perform coordinate transformation on the grid-connected reference voltage to obtain the grid modulation signal of the positive and negative sequence loops.

[0117] The reference current of the external voltage loop refers to the d-axis output current reference value I. d_ref and q-axis output current reference value I q_ref The fundamental positive sequence phase θ is obtained from the first transformation branch.

[0118] Among them, the grid-connected reference voltage refers to the modulated wave signal v after being processed by the current loop module. αβ_ref and v 0_ref Coordinate transformation refers to the inverse transformation between Clark transformation and Park transformation.

[0119] The working principle of the stability domain extension system of the three-phase four-arm converter unit described in this application embodiment includes: In the second coordinate transformation module, the control system collects the grid-side current through the current sensor, and transforms the three variables in the abc three-phase stationary coordinate system to the two variables in the αβ two-phase stationary coordinate system through Clark transformation. The transformation relationship is shown in equation (13):

[0120] (13);

[0121] Where, i 2_a Let i be the grid-connected current of phase a. 2_b Let i be the grid-connected current of phase b. 2_c Let i be the c-phase grid-connected current. 2_α Let i be the α-axis component of the grid-connected current. 2_β This represents the β-axis component of the grid-connected current.

[0122] The control system then transforms the AC quantities in the αβ two-phase stationary coordinate system into DC quantities in the dq two-phase rotating coordinate system through the Park transformation. The transformation relationship is shown in equation (14):

[0123] (14);

[0124] Among them, i 2_d i represents the d-axis component of the grid-connected current. 2_q This represents the q-axis component of the grid-connected current.

[0125] The control system obtains the zero-axis current sampling signal, including the d-axis component i of the grid-connected current, through Clark and Park transforms. 2_d and grid-connected current q-axis component i 2_q Therefore, the control system obtains the grid-connected current component i αβ .

[0126] In the third coordinate transformation module, the control system combines the fundamental positive-sequence phase θ obtained from the first transformation branch and the d-axis output current reference value I obtained from the external voltage loop. d_ref and q-axis output current reference value I q_ref These three signals serve as the input sources for the third coordinate transformation module. The fundamental positive-sequence phase θ obtained from the first transformation branch, i.e., the phase of the public power grid, is multiplied by the reference current amplitude by the control system. Simultaneously, the control part is transformed from the dq axis to the αβ stationary coordinate system, thereby obtaining the grid-connected current command I under the αβ axis. αβ_ref .

[0127] The current loop control module is mainly divided into a first current loop module and a second current loop module. The first current loop control module processes the positive and negative sequence loop current signals, and the second current loop control module processes the zero sequence loop current signal. The grid-connected current command is subtracted from the sampled signal after coordinate transformation to obtain the error signal. The relationship between them can be expressed by equations (15) and (16):

[0128] (15);

[0129] (16);

[0130] Where i αβ Let i be the grid-connected current in a two-phase stationary coordinate system. αβ_ref Let i be the grid-connected reference current in a two-phase stationary coordinate system. * αβ Let i be the grid-connected error current in a two-phase stationary coordinate system, i0 be the zero-axis current, and i 0_ref For the zero-axis reference current, i * 0 represents the zero-axis error current.

[0131] The control device then sends the error signal into the proportional resonant controller to obtain the voltage reference signal. The relationship between them can be expressed by equations (17) and (18):

[0132] (17);

[0133] (18);

[0134] Where v αβ_ref Let v be the grid-connected reference voltage in a two-phase stationary coordinate system. 0_ref K is the zero-axis reference voltage in a two-phase stationary coordinate system. p K is the proportional gain of the controller. r ω is the controller resonant gain, ω0 is the resonant frequency, ω i is the angular frequency deviation value, and s is the complex variable of the Laplace transform.

[0135] The control device can obtain the grid reference voltage v in the two-phase stationary coordinate system by using the current loop control module through relation (15) to relation (18). αβ_ref and the zero-axis reference voltage v in a two-phase stationary coordinate system 0_ref .

[0136] In the fourth coordinate transformation module, the control system uses the signal processed by the third coordinate module as the input source to the first current loop module to obtain the processed modulated wave signal v. αβ_ref v αβ_refIt is the grid-connected reference voltage in a two-phase stationary coordinate system. The control system controls v αβ_ref A coordinate inverse transformation is performed to transform it into a three-phase modulated wave. Specifically, the control system first performs an inverse Park transform, converting the DC quantity in the dq two-phase rotating coordinate system into an AC quantity in the αβ two-phase stationary coordinate system. Then, the AC quantity in the αβ two-phase stationary coordinate system is transformed into a three-variable transformation in the abc three-phase stationary coordinate system through an inverse Clark transform. The control system then obtains the grid modulation signal for the positive and negative sequence loops. .

[0137] In one exemplary embodiment, such as Figure 9 As shown, Figure 1 The three-phase four-arm converter shown includes a three-phase four-arm circuit, a filter circuit, an impedance circuit, a common grid-connected probe line, and a common current sensing loop. The output of the three-phase four-arm circuit is connected to the input of the filter circuit, the output of the filter circuit is connected to the input of the common grid-connected probe line, the output of the common grid-connected probe line is connected to the input of the impedance circuit, the filter circuit is also connected to the input of the common current sensing loop, the output of the common current sensing loop is connected to the input of the switch control unit, the input of the switch control unit is also connected to the output of the common grid-connected probe line, and the output of the switch control unit is connected to the switch on the arm of the three-phase four-arm circuit.

[0138] The working principle of the stability domain extension system of the three-phase four-arm converter unit described in this application embodiment includes: the main power circuit includes a DC-side voltage source V dc For example, energy storage batteries or photovoltaic arrays; DC-side capacitor C dc For example, capacitance value A three-phase four-arm bridge circuit composed of IGBT modules, including three-phase bridge arms and a fourth bridge arm, with IGBT anti-series freewheeling diodes; filter circuits and impedance circuits; a common grid-connected probe line and the related common current sensing loop connected to the input terminal of the switch control unit, i.e., connected to the first coordinate module and the second coordinate module; the switching transistors on the bridge arms of the three-phase four-arm bridge circuit are connected to the output terminal of the switch control unit, i.e., the switching transistors are connected to the PWM generation module to control the pulse signals of the switching transistors, thereby forming a closed-loop control system, for example... Figure 10 As shown, V dc For DC side voltage, C dc L1 is the DC-side capacitor, connected to the three-phase LCL filter via a three-phase four-bridge topology. L1 is the inverter-side inductor of the filter, and C... f L1 is the filter capacitor of the filter, and L2 is the grid-side inductance of the filter; L g L is the equivalent impedance of the power grid. 0_1 This is the filter inductance between the midpoint and neutral point of the fourth bridge arm. i 1_aLet i be the inverter side current of phase a. 1_b For the b-phase inverter side current, i 1_c This refers to the current on the c-phase inverter side. i 2_a Let i be the grid-connected current of phase a. 2_b Let i be the grid-connected current of phase b. 2_c i0 is the c-phase grid-connected current, and i0 is the output current of the fourth bridge arm. PCC is the Point of Common Coupling (PCC). g_a Let V be the voltage of the public power grid in phase a. g_b The voltage of phase b of the public power grid is v. g_c V is the voltage of the C-phase public power grid. abc The voltage at the point of common access, v αβ Let α be the αβ axis component of the public grid voltage, and θ be the grid phase. αβ For the grid-connected current αβ-axis components, the control strategy of the control system adopts undamped grid-connected current loop control. The outer loop is the DC bus voltage loop, which can generate the command signal for the inner loop, such as I. d_ref I is the reference value for the d-axis output current. q_ref i is the reference value for the q-axis output current. 0_ref The zero-axis reference current is generated by an external voltage loop, and the control system obtains the grid-connected current from the grid-connected current terminal of the three-phase four-arm converter unit. i 2_a i 2_b and i 2_c The common grid terminal (PCC) of the three-phase four-arm converter unit obtains the common access point voltage V. abc and the reference current of the external voltage loop (I) d_ref and I q_ref The control system transforms and processes these three signals to generate a power grid modulation signal with positive and negative sequence loops.

[0139] In an exemplary embodiment, based on the stability domain extension system of the three-phase four-arm converter unit described in any of the foregoing embodiments, a method for extending the stability domain of the arm converter unit is also provided, such as... Figure 11 As shown, it includes:

[0140] S101: Obtain the target resonant frequency and target phase margin of the three-phase four-arm converter unit, and configure specific parameters for the common-mode loop stability range extension module based on the target resonant frequency and target phase margin; the specific parameter is the additional discrete control cycle.

[0141] The target resonant frequency refers to the specific resonant frequency f. b The target phase margin refers to the system phase margin.

[0142] In this embodiment, the control device configures the upper and lower limits of the additional discrete control period λ using a specific resonant frequency and the system phase margin. The lower limit is configured based on the specific resonant frequency, and the upper limit is configured based on the system phase margin. The control system operates at the minimum resonant frequency f that satisfies the specific resonant frequency, i.e., the zero-sequence loop stability. b This determines the lower limit of λ.

[0143] S102, activate the configured common-mode circuit stability range extension module to optimize the zero-sequence circuit in the three-phase four-arm converter unit.

[0144] In this embodiment, the control system configures the additional discrete control cycle number in the common-mode loop stability range extension module, then configures the fourth bridge arm side inductor in the three-phase four-bridge arm converter unit, and starts the configured common-mode loop stability range extension module. The control device then obtains the optimized zero-sequence loop in the three-phase four-bridge arm converter unit.

[0145] In an exemplary embodiment, the above-mentioned S101, "obtaining the target resonant frequency and target phase margin of the three-phase four-arm converter unit, and configuring specific parameters of the common-mode loop stability range extension module according to the target resonant frequency and target phase margin; the specific parameter is the additional discrete control cycle," is as follows: Figure 12 As shown, it includes:

[0146] S201, Based on the correspondence between phase margin and parameters, determine the upper limit value of a specific parameter corresponding to the target phase margin;

[0147] Here, the specific parameter refers to the number of additional discrete control cycles, λ.

[0148] In the embodiments of this application, the upper limit of a specific parameter, namely the number of additional discrete control cycles λ, is determined by the phase margin.

[0149] S202, Based on the correspondence between the power grid impedance range and the parameters, determine the lower limit value of the specific parameter corresponding to the target resonant frequency;

[0150] In this embodiment, the control device introduces a common-mode circuit stability range extension module to the zero-sequence circuit to extend the stability domain. The core of the common-mode circuit stability range extension module is based on the control device's setting of specific parameters of the transfer function. The control device can combine the grid impedance range and the correspondence between the parameters to determine the lower limit value of the specific parameter corresponding to the target resonant frequency. The correspondence is as shown in equation (19).

[0151] (19);

[0152] Where λ is the number of additional discrete control cycles, f s Where L1 is the system sampling frequency, C is the inverter-side inductance, and C is the inductance of the inverter side. f L1 is the filter capacitor, L2 is the grid-side inductor, and L3 is the mains capacitor. 0_1 L is the filter inductance between the midpoint and neutral point of the fourth bridge arm. g This is the equivalent impedance of the power grid.

[0153] The minimum resonant frequency of the control equipment can be obtained from the range of the power grid impedance. and be satisfied The minimum specific parameter λ is the lower limit of the specific parameter.

[0154] S203, configure specific parameters for the common-mode loop stability range extension module based on the upper and lower limits.

[0155] In this embodiment, the control device determines the upper limit value of a specific parameter corresponding to the target phase margin based on the correspondence between phase margin and parameters; and determines the lower limit value of a specific parameter corresponding to the target resonant frequency based on the correspondence between grid impedance range and parameters; the control device extends the stability domain by configuring specific parameters, for example, as... Figure 13 As shown, when the number of additional discrete control cycles is 0, the lower limit of the stability region is approximately When the number of additional discrete control cycles is 1, the lower limit of the stability region is approximately The lower limit of the stability region is reduced by about 40%, and the width of the zero-sequence loop stability region is increased.

[0156] In an exemplary embodiment, the above-mentioned S203 "specific parameter", such as Figure 14 As shown, it includes:

[0157] S301, optimize the inductance parameters of the fourth arm in the three-phase four-arm converter unit according to specific parameters.

[0158] The inductance parameter of the fourth bridge arm refers to the filter inductance L between the midpoint and neutral point of the fourth bridge arm. o_1 .

[0159] In this embodiment, the number λ of additional discrete control cycles is related to the inductance L on the inverter side of the fourth arm. o_1 Design limit L max There is a relationship, such as relation (20):

[0160] (20);

[0161] Where L1 is the inverter-side inductance, C f For the filter capacitor, λ is the number of additional discrete control cycles, and f is the number of cycles. sThis is the system sampling frequency.

[0162] When the control equipment increases λ, the inductance L on the inverter side of the fourth bridge arm... o_1 Design limit L max It will also increase, that is, widen the gap from the minimum value. and maximum value L max The distance increases the inductance L on the inverter side of the fourth bridge arm. o_1 Design scope, such as Figure 15 As shown, the control equipment follows a configuration process for specific parameters: determining the grid impedance range, calculating the minimum resonant frequency, calculating the phase margin boundary, and calculating the inductance constraint boundary, to obtain the optimized zero-sequence stability domain, as shown in equation (21):

[0163] (twenty one);

[0164] Where f s Where λ is the system sampling frequency, λ is the number of additional discrete control cycles, L1 is the inverter-side inductance, and C is the inductance of the inverter side. f L1 is the filter capacitor, L2 is the grid-side inductor, and L3 is the mains capacitor. 0_1 L is the filter inductance between the midpoint and neutral point of the fourth bridge arm. g This is the equivalent impedance of the power grid.

[0165] For example, a Kratzer high-voltage bidirectional power DC source was selected as the DC power supply in the three-phase four-arm converter unit, and a TMS320F28335 inverter was selected for control. Then, it was connected to the grid-side dry-type isolation transformer via a grid impedance module to achieve connection to the public power grid. A sampling board was used to sample the three-phase voltage and grid-connected current; an oscilloscope was used to observe the three-phase voltage and grid-connected current; and a host computer designed using LabVIEW was used to control the inverter. The three-phase output current was selected as 35A, and the same zero-sequence current as the A-phase current was injected into the fourth arm. The experimental results are as follows. Figure 16 As shown, the system operates normally when the microgrid's additional impedance is 0. However, when the microgrid's additional impedance is 2000µH, the system immediately loses stability upon startup and cannot achieve normal operation of the three-phase four-arm converter unit. The system's zero-sequence loop has only a finite stability region, such as... Figure 17 As shown, the control device introduces an optimized zero-sequence stability region, and the system transitions from an unstable state to a stable state, as follows. Figure 18 As shown.

[0166] It should be understood that although the steps in the flowcharts of the embodiments described above are shown sequentially according to the arrows, these steps are not necessarily executed in the order indicated by the arrows. Unless explicitly stated herein, there is no strict order restriction on the execution of these steps, and they can be executed in other orders. Moreover, at least some steps in the flowcharts of the embodiments described above may include multiple steps or multiple stages. These steps or stages are not necessarily completed at the same time, but can be executed at different times. The execution order of these steps or stages is not necessarily sequential, but can be performed alternately or in turn with other steps or at least some of the steps or stages in other steps. It is understood that the steps in different embodiments can be freely combined as needed, and all non-contradictory solutions formed by such combinations are within the scope of protection of this application.

[0167] In one exemplary embodiment, a control device for a stability domain extension system of a three-phase four-arm converter is provided. This control device can be a terminal, and its internal structure diagram can be as follows: Figure 19 As shown, the control device includes a processor, memory, input / output interface, communication interface, and input device. The processor, memory, and input / output interface are connected via a system bus, and the communication interface and input device are also connected to the system bus via the input / output interface. The processor provides computational and control capabilities. The memory includes a non-volatile storage medium and internal memory. The non-volatile storage medium stores the operating system and computer programs. The internal memory provides an environment for the operation of the operating system and computer programs stored in the non-volatile storage medium. The input / output interface is used for exchanging information between the processor and external devices. The communication interface is used for wired or wireless communication with external terminals; wireless communication can be achieved through Wi-Fi, mobile cellular networks, Near Field Communication (NFC), or other technologies. When executed by the processor, the computer program implements a method for extending the stable domain of a bridge arm converter unit.

[0168] Those skilled in the art will understand that Figure 19 The structure shown is merely a block diagram of a portion of the structure related to the present application and does not constitute a limitation on the control device to which the present application is applied. The specific control device may include more or fewer components than those shown in the figure, or combine certain components, or have different component arrangements.

[0169] In some exemplary embodiments, a control device is provided, including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to perform the following steps:

[0170] The target resonant frequency and target phase margin of the three-phase four-arm converter unit are obtained, and specific parameters are configured for the common-mode loop stability range extension module based on the target resonant frequency and the target phase margin; the specific parameters are additional discrete control cycles.

[0171] The common-mode loop stability range extension module is activated to optimize the zero-axis loop in the three-phase four-arm converter unit.

[0172] In some exemplary embodiments, a storage medium is provided that, when the processor executes a computer program, performs the following steps:

[0173] The target resonant frequency and target phase margin of the three-phase four-arm converter unit are obtained, and specific parameters are configured for the common-mode loop stability range extension module based on the target resonant frequency and the target phase margin; the specific parameters are additional discrete control cycles.

[0174] The common-mode loop stability range extension module is activated to optimize the zero-axis loop in the three-phase four-arm converter unit.

[0175] Those skilled in the art will understand that all or part of the processes in the methods of the above embodiments can be implemented by a computer program instructing related hardware. The computer program can be stored in a non-volatile computer-readable storage medium, and when executed, it can include the processes of the embodiments of the above methods. Any references to memory, databases, or other media used in the embodiments provided in this application can include at least one of non-volatile memory and volatile memory. Non-volatile memory can include read-only memory (ROM), magnetic tape, floppy disk, flash memory, optical memory, high-density embedded non-volatile memory, resistive random access memory (ReRAM), magnetic random access memory (MRAM), ferroelectric random access memory (FRAM), phase change memory (PCM), graphene memory, etc. Volatile memory can include random access memory (RAM) or external cache memory, etc. By way of illustration and not limitation, RAM can take many forms, such as Static Random Access Memory (SRAM) or Dynamic Random Access Memory (DRAM). The databases involved in the embodiments provided in this application may include at least one type of relational database and non-relational database. Non-relational databases may include, but are not limited to, blockchain-based distributed databases. The processors involved in the embodiments provided in this application may be general-purpose processors, central processing units, graphics processing units, digital signal processors, programmable logic devices, quantum computing-based data processing logic devices, artificial intelligence (AI) processors, etc., and are not limited to these.

[0176] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this application.

[0177] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this application should be determined by the appended claims.

Claims

1. A stable region extension system of a bridge arm conversion unit, characterized by, The system comprises a three-phase four-bridge arm conversion unit and a switch control unit; the switch control unit comprises a positive and negative sequence loop control branch, a zero sequence loop control branch and a PWM generation module; the zero sequence loop control branch comprises a current loop control module and a common mode loop stable interval expansion module; The positive and negative sequence loop control branch is configured to obtain a common access point voltage from a common grid end of the three-phase four-bridge arm conversion unit, obtain a grid-connected current from a grid-connected current end of the three-phase four-bridge arm conversion unit, and generate a grid modulation signal of a positive and negative sequence loop after transformation processing of the common access point voltage and the grid-connected current according to a reference current of an external voltage loop. The current loop control module is configured to obtain a zero-axis current from a fourth bridge arm of the three-phase four-bridge arm conversion unit, and generate a modulation wave signal according to the zero-axis current after coordinate transformation and a zero-axis reference current. The common mode loop stable interval expansion module is configured to perform sampling frequency modulation processing on the modulation wave signal to generate a grid modulation signal of a zero sequence loop. The PWM generation module is configured to generate a PWM signal according to the grid modulation signal of the positive and negative sequence loop and the grid modulation signal of the zero sequence loop, and control the three-phase four-bridge arm conversion unit according to the PWM signal.

2. The system of claim 1, wherein, The positive and negative sequence loop control branch comprises a first transformation branch, a second transformation branch and a third transformation branch. The first transformation branch is configured to obtain a common access point voltage from a common grid end of the three-phase four-bridge arm conversion unit, and transform the common access point voltage into a fundamental positive sequence phase. The second transformation branch is configured to obtain a grid-connected current from a common current end of the three-phase four-bridge arm conversion unit, and perform coordinate transformation on the grid-connected current to obtain a grid-connected current component signal. The third transformation branch is configured to generate the grid modulation signal of the positive and negative sequence loop according to a reference current of an external voltage loop, the fundamental positive sequence phase and the grid-connected current component signal.

3. The system of claim 2, wherein, The first transformation branch comprises a first coordinate transformation module and a phase-locked loop module. The first coordinate transformation module is configured to obtain a common access point voltage from a common grid end of the three-phase four-bridge arm conversion unit to obtain a component of a grid voltage. The phase-locked loop module is configured to decompose the component of the grid voltage to obtain the fundamental positive sequence phase.

4. The system of claim 2, wherein, The second transformation branch comprises a second coordinate transformation module; and the third transformation branch comprises a third coordinate transformation module, a current loop control module and a fourth coordinate transformation module. The second coordinate transformation module is configured to obtain the grid-connected current from a grid-connected current end of the three-phase four-bridge arm conversion unit, and perform coordinate transformation on the grid-connected current to obtain the grid-connected current component signal. The third coordinate transformation module is configured to generate a grid-connected current signal according to the reference current of the external voltage loop and the fundamental positive sequence phase. The current loop control module is configured to generate a grid-connected reference voltage according to the grid-connected reference current and a zero-axis current sampling signal. The fourth coordinate transformation module is configured to perform coordinate transformation on the grid reference voltage to obtain a grid modulation signal of the positive and negative sequence loops.

5. The system according to any of claims 1-4, characterized in that, The three-phase four-leg conversion unit comprises a three-phase four-leg circuit, a filter circuit, an impedance circuit, a common grid detection line and a common current detection ring; an output end of the three-phase four-leg circuit is connected with an input end of the filter circuit, an output end of the filter circuit is connected with an input end of the common grid detection line, an output end of the common grid detection line is connected with an input end of the impedance circuit, the filter circuit is also connected with an input end of the common current detection ring, an output end of the common current detection ring is connected with an input end of the switch control unit, an input end of the switch control unit is also connected with an output end of the common grid detection line, and an output end of the switch control unit is connected with switches on the bridge arms in the three-phase four-leg circuit.

6. A method for extending the stability region of a three-phase four-arm converter unit, characterized in that, The method applied to the stable region expansion system of the bridge arm conversion unit according to any one of claims 1-5, the method comprising: obtaining a target resonant frequency and a target phase margin of the three-phase four-leg conversion unit, and configuring specific parameters of the common-mode loop stable interval expansion module according to the target resonant frequency and the target phase margin; the specific parameters are additional discrete control periods; starting the configured common-mode loop stable interval expansion module to optimize the zero-axis loop in the three-phase four-leg conversion unit.

7. The method of claim 6, wherein, The method further comprises: optimizing an inductance parameter of a fourth bridge arm in the three-phase four-leg conversion unit according to the specific parameters. The processor executes the computer program to implement the steps of the method of any one of claims 6-8. The computer program is executed by the processor to implement the steps of the method of any one of claims 6-8.

8. The method of claim 7, wherein, ​ ​ 9. A control device comprising a memory and a processor, the memory storing a computer program, characterized in that, ​ 10. A storage medium having stored thereon a computer program, characterized in that ​

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