Negative voltage suppression method and device for network-forming inverter, and storage medium

Abstract

The invention discloses a negative voltage suppression method and device for a network-forming inverter and a storage medium. The method comprises the following steps: executing signal splitting according to a three-phase electric signal at a grid connection point, and determining a positive sequence component and a negative sequence component; executing droop control according to the positive sequence component, and determining a positive sequence voltage modulation instruction; according to the negative-sequence component, coordinate transformation and negative-sequence current closed-loop control are executed, and a negative-sequence voltage modulation instruction is determined; and determining a comprehensive modulation signal according to the positive sequence voltage modulation instruction and the negative sequence voltage modulation instruction. According to the invention, positive and negative sequence separation is carried out on the three-phase electric signal, the suppression strategy taking the negative-sequence current as an object is constructed, and the power grid angle and the transmission line impedance angle are combined to realize negative-sequence current control, so that the negative-sequence component is weakened. Meanwhile, the positive sequence signal is independently used for droop control, voltage and frequency support of the inverter is guaranteed, correction for frequency reference value oscillation is introduced into droop control, and the transient performance of the grid-forming inverter under the unbalanced power grid is improved.

Description

Technical Field

[0001] This application relates to a negative voltage suppression method, electronic device, and computer-readable storage medium for grid-connected inverters, belonging to the field of power system control. Background Technology

[0002] Under conditions of grid voltage imbalance, the output voltage of grid-connected inverters is easily affected by negative sequence components, resulting in voltage asymmetry at the grid connection point, which in turn leads to problems such as power fluctuations, increased current stress, and decreased operational stability.

[0003] Currently, in related technologies, positive and negative sequence component separation methods based on time delay and equivalent phase shift are generally used to overcome the above situation. However, the former requires storing sampling data for a certain period of time, which is not conducive to transient control. The latter's phase shift mitigation usually has dynamic characteristics of a certain order, and it takes a certain amount of time to establish the separation result during transient disturbance, which objectively limits the response speed of positive and negative sequence separation.

[0004] Regarding negative-sequence control, the solutions in related technologies follow the conventional voltage and current dual-loop control structure in grid-connected inverters, achieving suppression by introducing negative-sequence component feedback. However, since the feedback signal needs to undergo positive-to-negative-sequence separation processing first, the signal lag introduced in this processing stage is located in the feedback channel of the control system. This makes it difficult for the inner current loop to fully suppress the phase lag of the controlled object. Even with the introduction of a dual-loop structure, it actually increases the complexity of the control structure and the difficulty of parameter tuning, reducing the overall stability of the system. Summary of the Invention

[0005] This application discloses a negative voltage suppression method, electronic device, and computer-readable storage medium for grid-type inverters.

[0006] The negative voltage suppression method for grid-connected inverters in this application includes:

[0007] Based on the three-phase electrical signal at the grid connection point between the grid-connected inverter and the external power grid, signal splitting is performed to determine the positive-sequence component and the negative-sequence component. The grid-connected inverter is connected to the external power grid via a filter, and the grid connection point is the connection point between the filter and the external power grid. Based on the positive sequence component, droop control is executed to determine the positive sequence voltage modulation command in order to maintain the voltage and frequency stability of the external power grid. Based on the negative sequence component, coordinate transformation and negative sequence current closed-loop control are performed to determine the negative sequence voltage modulation command in order to suppress the negative sequence voltage at the grid connection point. Based on the positive-sequence voltage modulation command and the negative-sequence voltage modulation command, superposition processing is performed to determine the comprehensive modulation signal, so as to control the voltage balance at the grid connection point.

[0008] In some implementations, the step of performing signal splitting and determining the positive-sequence and negative-sequence components based on the three-phase electrical signal at the grid connection point between the grid-connected inverter and the external power grid includes: Based on the three-phase electrical signals, perform Clarke transformation to determine the two-phase stationary electrical signals; Based on the two-phase static electrical signals and the fundamental period of the external power grid, signal splitting is performed to determine the positive sequence component and the negative sequence component.

[0009] In some implementations, the step of performing droop control and determining a positive-sequence voltage modulation command based on the positive-sequence component includes: Based on the positive sequence components, perform the Park transform to determine the direct-axis positive sequence components and the quadrature-axis positive sequence components. Based on the direct-axis positive-sequence component and the quadrature-axis positive-sequence component, the positive-sequence active power and positive-sequence reactive power output by the grid-type inverter are determined. Based on the positive sequence active power, the positive sequence reactive power, and the reference active power and reference reactive power of the grid-type inverter, determine the frequency reference value and the positive sequence voltage amplitude reference value. The positive sequence voltage modulation command is determined based on the frequency reference value and the positive sequence voltage amplitude reference value.

[0010] In some embodiments, a transmission line is provided between the grid-connected inverter and the external power grid; The step of performing coordinate transformation and negative sequence current closed-loop control based on the negative sequence component to determine the negative sequence voltage modulation command includes: Based on the negative order components, perform the Park transform to determine the direct-axis negative order components and the quadrature-axis negative order components. The negative sequence current reference value is determined based on the negative sequence voltage amplitude of the external power grid and the equivalent impedance amplitude of the transmission line; The original negative sequence voltage control quantity is determined based on the direct-axis negative sequence component, the quadrature-axis negative sequence component, and the negative sequence current reference value. Based on the impedance angle of the transmission line and the voltage phase of the external power grid, the original negative sequence voltage control quantity is corrected to determine the negative sequence voltage modulation command.

[0011] In some embodiments, the direct-axis negative sequence component includes a direct-axis negative sequence current, and the quadrature-axis negative sequence component includes a quadrature-axis negative sequence current; The step of determining the original negative sequence voltage control quantity based on the direct-axis negative sequence component, the quadrature-axis negative sequence component, and the negative sequence current reference value includes: Based on the negative sequence current reference value, perform Park transformation to determine the direct-axis negative sequence current reference value and the quadrature-axis negative sequence current reference value; The first error signal is determined based on the direct-axis negative sequence current and the direct-axis negative sequence reference value; The second error signal is determined based on the cross-axis negative sequence current and the cross-axis negative sequence reference value; Based on the first error signal and the second error signal, amplification and integral correction are performed to determine the original negative sequence voltage control quantity.

[0012] In some embodiments, determining the negative sequence voltage modulation command by correcting the original negative sequence voltage control quantity based on the impedance angle of the transmission line and the voltage phase of the external power grid includes: The impedance angle compensation offset is determined based on the impedance angle of the transmission line and the voltage phase of the external power grid. The current offset angle is determined based on the impedance angle compensation offset. Based on the coupling characteristics of the filter, determine the cross-coupling compensation amount of the filter inductor; Based on the current offset angle and the cross-coupling compensation amount of the filter inductor, the original negative sequence voltage control amount is corrected, and the negative sequence voltage modulation command is determined.

[0013] In some embodiments, the grid-connected inverter includes multiple bridge arms arranged in parallel, each bridge arm having at least two switching transistors, the gates of which are connected to the integrated modulation signal; The step of performing superposition processing based on the positive-sequence voltage modulation command and the negative-sequence voltage modulation command to determine the comprehensive modulation signal in order to control the voltage balance at the grid connection point includes: Verify the positive-sequence voltage modulation instruction and the negative-sequence voltage modulation instruction to make the positive-sequence voltage modulation instruction and the negative-sequence voltage modulation instruction adapt to the driving format of the switching transistor; The positive-sequence voltage modulation command and the negative-sequence voltage modulation command are linearly superimposed to determine the composite modulation signal; The integrated modulation signal is input to the gate of the switching transistor to control the operating state of the switching transistor, thereby controlling the voltage balance at the grid connection point.

[0014] In some embodiments, the method further includes: The negative sequence current reference value is controlled to be less than a first limit and less than or equal to a second limit to protect the power safety of the grid-connected inverter and the transmission line, and to make the negative sequence voltage at the grid connection point positively correlated with the negative sequence voltage of the external power grid. The first limit is determined based on the rated reactive power of the grid-connected inverter and the rated voltage of the external power grid, and the second limit is determined based on the negative sequence voltage amplitude of the external power grid and the equivalent impedance amplitude of the transmission line.

[0015] The electronic device in this application includes a memory and a processor. The memory stores a computer program, and when the computer program is executed by the processor, the negative voltage suppression method for grid-connected inverters described in the above embodiments is implemented.

[0016] The computer-readable storage medium in this application embodiment stores a computer program that, when executed by one or more processors, implements the negative voltage suppression method for grid-connected inverters described above.

[0017] The beneficial effects of this application are as follows: By separating the positive and negative sequences of the three-phase electrical signals, this application constructs a negative sequence voltage suppression strategy with the negative sequence current as the control object. Combined with the grid electrical angle and transmission line impedance angle, it achieves directional closed-loop control of the grid-connected negative sequence current, thereby effectively reducing the negative sequence component. Simultaneously, the separated positive sequence current and positive sequence voltage are used independently for droop control to ensure normal voltage and frequency support for the grid-connected inverter. Furthermore, a correction mechanism for frequency reference value oscillation is introduced into the droop control, thereby improving the transient performance of the grid-connected inverter under unbalanced grid conditions. Attached Figure Description

[0018] Figure 1 This is one of the flowcharts illustrating the negative voltage suppression method for grid-connected inverters in the embodiments of this application; Figure 2 This is a schematic diagram of the power system circuit in the embodiments of this application; Figure 3 This is a schematic diagram of the circuit structure of the power system circuit in the embodiments of this application; Figure 4 This is the second flowchart illustrating the negative voltage suppression method for grid-connected inverters in the embodiments of this application. Figure 5 This is the third flowchart illustrating the negative voltage suppression method for grid-connected inverters in the embodiments of this application; Figure 6 This is the fourth flowchart illustrating the negative voltage suppression method for grid-connected inverters in the embodiments of this application; Figure 7 This is the fifth flowchart illustrating the negative voltage suppression method for grid-connected inverters in the embodiments of this application; Figure 8 This is the sixth flowchart illustrating the negative voltage suppression method for grid-connected inverters in the embodiments of this application. Detailed Implementation

[0019] Please see Figure 1 The negative voltage suppression method for grid-connected inverters in this application includes: Step 01: Based on the three-phase electrical signal at the grid connection point between the grid-connected inverter and the external power grid, perform signal splitting to determine the positive-sequence and negative-sequence components. Among them, the grid-connected inverter is connected to the external power grid by a filter, and the grid connection point is the connection point between the filter and the external power grid; Step 02: Based on the positive sequence component, execute droop control to determine the positive sequence voltage modulation command in order to maintain the voltage and frequency stability of the external power grid; Step 03: Based on the negative sequence component, perform coordinate transformation and negative sequence current closed-loop control to determine the negative sequence voltage modulation command in order to suppress the negative sequence voltage at the grid connection point; Step 04: Perform superposition processing according to the positive sequence voltage modulation command and the negative sequence voltage modulation command to determine the comprehensive modulation signal in order to control the voltage balance at the grid connection point.

[0020] Specifically, the negative voltage suppression method for grid-connected inverters in this application mainly achieves negative voltage control based on the power system circuit formed by the connection between the grid-connected inverter and the external power grid. The module diagram of the aforementioned power system circuit is shown below. Figure 2 As shown, a voltage source drives a three-phase grid-connected inverter. When the grid-connected inverter receives a control signal, it outputs DC power as three-phase AC power signals, which generally include the output voltage signals and output current signals of each of the three phases. These signals are then input into a filter for filtering. The filtered electrical signals are transmitted to the external power grid through a transmission line. The connection point between the filter and the transmission line is the grid connection point (PCC) of the grid-connected inverter.

[0021] Further, please refer to Figure 3 , Figure 3This diagram illustrates a specific circuit structure for the aforementioned power system circuit. Specifically, the grid-connected inverter comprises three parallel bridge arms. Each bridge arm includes an upper arm switch and a lower arm switch. Both the upper and lower arm switches are N-type switches, such as NMOS or N-type IGBT devices. The two switches in the same bridge arm are arranged in the same direction, with the source of the upper arm switch connected to the drain of the lower arm switch. This connection point is the corresponding electrical signal output point for that bridge arm. As for the filter, it employs an LC filter circuit structure, with a filter inductor provided for any one of the three-phase output phases. L f and filter capacitors C f Based on this, the filter inductor L f It also has an additional passive damping resistor connected in series. R f This is used to suppress the resonant spikes in the current of the LC filter circuit. The filter is connected to the transmission line via the grid connection point PCC, and the transmission line has an equivalent inductance. L g and equivalent resistance R g Ultimately, it is connected to the external power grid.

[0022] Based on the aforementioned power system circuit, the main logic of the negative voltage suppression method for grid-connected inverters in this application is as follows: Based on the three-phase electrical signal obtained at the grid connection point (PCC), positive and negative sequence separation is performed to obtain positive and negative sequence components. The positive sequence component is used to calculate the active and reactive power of the grid-connected inverter using the instantaneous power formula. Then, based on the calculated active and reactive power, a positive sequence voltage modulation command suitable for droop control is determined to ensure the grid-connected inverter's voltage and frequency support to the external power grid. The negative sequence component and a negative sequence reference value obtained from the attribute parameters of the power grid and transmission lines are used to determine the negative sequence voltage control command. Furthermore, the control quantity is corrected based on the attribute parameters of the power grid, transmission lines, and filters to eliminate the influence of inductive coupling on control accuracy, thereby weakening the negative sequence component. Finally, the positive-sequence voltage control command and the negative-sequence voltage control command are superimposed and sent to the gate of each switch of the grid-type inverter in the form of a control signal to control the operating state of each switch, thereby controlling the overall operating state of the grid-type inverter and achieving the suppression effect on negative-sequence voltage.

[0023] Please see Figure 4 In some implementations, step 01 includes: Step 011: Based on the three-phase electrical signals, perform Clarke transformation to determine the two-phase stationary electrical signals; Step 012: Based on the two-phase static electrical signal and the fundamental period of the external power grid, perform signal splitting to determine the positive sequence component and the negative sequence component.

[0024] Specifically, for positive and negative sequence separation, in some examples, voltage and current sensors are first used to acquire three-phase AC voltage and current signals (corresponding to three-phase electrical signals) at the grid connection point (PCC). These signals include both positive and negative sequence components. When there is an imbalance in the external power grid, the aforementioned three-phase AC voltage and current signals will also exhibit imbalance, which forms the basis for subsequent control of the power system circuit in the above implementation. Let's assume the three phases of the grid-connected inverter are phase a, phase b, and phase c; then, three-phase voltage signals can be acquired at the grid connection point (PCC). , and and three-phase current signals , and .

[0025] Then, for ease of data processing, based on the aforementioned three-phase voltage and current signals, a Clarke transform is performed to convert the three-phase AC signals into two-phase stationary electrical signals in a two-phase stationary coordinate system (i.e., the αβ coordinate system), wherein each of the two-phase stationary electrical signals includes an α-axis voltage component. α-axis current component β-axis voltage component and β-axis current components .

[0026] Next, taking the voltage component as an example, based on the above-mentioned α-axis voltage component... and β-axis voltage component By incorporating the fundamental period of the voltage signal from the external power grid, a quarter-cycle time delay is introduced, and the voltage signal obtained after introducing the time delay is further compared with the aforementioned α-axis voltage component. and β-axis voltage component Linear combination can achieve equivalent separation of positive-sequence and negative-sequence components, as shown in Equation 1. The method for separating the positive and negative sequences of current components is similar and will not be elaborated here.

[0027] ………… Formula 1 in At time t, the voltage component along the α axis and β-axis voltage component All are moments The function, The fundamental period of a two-phase stationary electrical signal. This represents the positive-sequence α-axis voltage component. The positive-sequence β-axis voltage component. The negative-sequence α-axis voltage component. It represents the negative-sequence β-axis voltage component.

[0028] Based on the positive and negative sequence separation method of Formula 1 above, the following positive and negative sequence components can be obtained: The positive sequence component includes the positive sequence α-axis voltage component. Positive sequence β-axis voltage component Positive sequence α-axis current component and the positive sequence β-axis current component The negative sequence component includes the negative sequence α-axis voltage component. Negative sequence β-axis voltage component Negative sequence α-axis current component and negative sequence β-axis current components Subsequently, the positive sequence components mentioned above can be used to determine the positive sequence control command for the grid-type inverter, thereby performing droop control on the circuit to ensure the grid-type inverter supports voltage and frequency, and the negative sequence components can be used to determine the negative sequence control command for the grid-type inverter to weaken the negative sequence components.

[0029] Please see Figure 5 In some implementations, step 02 includes: Step 021: Perform Park transform based on the positive sequence components to determine the direct-axis positive sequence components and the quadrature-axis positive sequence components; Step 022: Determine the positive-sequence active power and positive-sequence reactive power output by the grid-type inverter based on the direct-axis positive-sequence component and the quadrature-axis positive-sequence component; Step 023: Determine the frequency reference value and the positive sequence voltage amplitude reference value based on the positive sequence active power, positive sequence reactive power, and the reference active power and reference reactive power of the grid-type inverter. Step 024: Determine the positive sequence voltage modulation command based on the frequency reference value and the positive sequence voltage amplitude reference value.

[0030] Specifically, based on the above implementation method, for the specific method of determining the positive sequence voltage modulation command using the positive sequence component, for example, firstly based on the above-mentioned positive sequence α-axis voltage component... Positive sequence β-axis voltage component Positive sequence α-axis current component and the positive sequence β-axis current component Performing the Park transformation converts the aforementioned positive-sequence components in the αβ coordinate system to a positive-sequence synchronous rotating coordinate system (positive-sequence dq coordinate system), forming a direct-axis (d-axis) positive-sequence component and a quadrature-axis (q-axis) positive-sequence component, wherein the direct-axis positive-sequence component includes the positive-sequence d-axis voltage component. and the positive sequence d-axis current component The quadrature-axis positive-sequence component includes the positive-sequence q-axis voltage component. and the positive sequence q-axis current component .

[0031] Based on this, the instantaneous power formula is used to calculate the positive sequence active power and positive sequence reactive power of the grid-type inverter according to the above direct-axis positive sequence components and quadrature-axis positive sequence components, as shown in Formula 2.

[0032] ………… Formula 2 in This represents positive-sequence active power. This represents positive-sequence reactive power.

[0033] In this way, calculating active and reactive power based solely on positive-sequence components can directly avoid the interference of negative-sequence components with the power regulation of the power system circuit.

[0034] Furthermore, based on the active power reference value and reactive power reference value in the attribute parameters of the grid-connected inverter, and combined with the positive-sequence active power and positive-sequence reactive power mentioned above, the active power error and reactive power error can be obtained. These two sets of errors can serve as the data basis for further execution of droop control. The error calculation is specifically shown in Formula 3.

[0035] ………… Formula 3 in The above refers to the active power error. The aforementioned reactive power error, This is a reference value for active power. This is a reference value for reactive power.

[0036] Next, based on a droop control algorithm capable of simulating the characteristics of a synchronous generator, the active power error is... Based on this, frequency reference values ​​can be calculated. Meanwhile, reactive power error Based on this, a reference value for the positive sequence voltage amplitude is generated. Among them, active power error The larger the value, the higher the frequency reference value. The greater the difference between the frequency and the current frequency of the electrical signal, the greater the frequency adjustment required. Finally, the frequency reference value... Reference value of positive sequence voltage amplitude Together, they are converted into positive sequence voltage modulation commands that can be used to control the switching transistors in a grid-type inverter.

[0037] In this way, the droop control process can correct for oscillations in the aforementioned frequency reference value, improving the transient performance of the grid-connected inverter under unbalanced grid conditions. Furthermore, when the positive sequence voltage modulation command is sent to the grid-connected inverter, each switch achieves overall circuit droop control under the control of the positive sequence voltage modulation command, thereby ensuring the grid-connected inverter's stable support function for external grid voltage and frequency.

[0038] Please see Figure 6 In some implementations, step 03 includes: Step 031: Perform Park transform based on the negative sequence components to determine the direct-axis negative sequence components and the quadrature-axis negative sequence components; Step 032: Determine the reference value of the negative sequence current based on the magnitude of the negative sequence voltage of the external power grid and the magnitude of the equivalent impedance of the transmission line; Step 033: Determine the original negative sequence voltage control quantity based on the direct-axis negative sequence component, the quadrature-axis negative sequence component, and the negative sequence current reference value; Step 034: Based on the impedance angle of the transmission line and the voltage phase of the external power grid, correct the original negative sequence voltage control quantity and determine the negative sequence voltage modulation command.

[0039] Furthermore, the direct-axis negative sequence component includes the direct-axis negative sequence current, and the quadrature-axis negative sequence component includes the quadrature-axis negative sequence current. Based on this, please refer to [link to relevant documentation]. Figure 7 In some implementations, step 033 includes: Step 0331: Based on the negative sequence current reference value, perform Park transformation to determine the direct-axis negative sequence current reference value and the quadrature-axis negative sequence current reference value; Step 0332: Determine the first error signal based on the direct-axis negative sequence current and the direct-axis negative sequence reference value; Step 0333: Determine the second error signal based on the quadrature-axis negative sequence current and the quadrature-axis negative sequence reference value; Step 0334: Based on the first error signal and the second error signal, perform amplification and integral correction to determine the original negative sequence voltage control quantity.

[0040] Specifically, based on the above implementation method, for the specific method of determining the negative sequence voltage modulation command using the negative sequence component, for example, firstly based on the above-mentioned negative sequence α-axis voltage component... Negative sequence β-axis voltage component Negative sequence α-axis current component and negative sequence β-axis current components Performing the Park transformation converts the aforementioned negative-sequence components in the αβ coordinate system to a negative-sequence synchronously rotating coordinate system (negative-sequence dq coordinate system), forming a direct-axis (d-axis) negative-sequence component and a quadrature-axis (q-axis) negative-sequence component, wherein the direct-axis negative-sequence component includes the negative-sequence d-axis voltage component. and negative sequence d-axis current components (Corresponding to the direct-axis negative-sequence current), while the quadrature-axis negative-sequence component includes the negative-sequence q-axis voltage component. and negative sequence q-axis current components (Corresponding to the quadrature-axis negative sequence current). Regarding the determination process of the negative sequence voltage modulation command, the aforementioned negative sequence d-axis current component... With negative sequence q-axis current components More importantly, please refer to the following example for details.

[0041] However, it should be noted that the negative-sequence dq coordinate system rotates in the opposite direction relative to the αβ coordinate system compared to the positive-sequence dq coordinate system. That is, if the angular velocity corresponding to the positive-sequence dq coordinate system is... Then the angular velocity corresponding to the negative-order dq coordinate system is For example, the positive-order dq coordinate system is obtained by rotating the αβ coordinate system counterclockwise, while the negative-order dq coordinate system is obtained by rotating the αβ coordinate system clockwise.

[0042] Next, based on the aforementioned negative sequence d-axis current components... With negative sequence q-axis current components , and the negative sequence d-axis current reference component and negative sequence q-axis current reference component A comparison calculation is performed to obtain the direct-axis current error (corresponding to the first error signal) and the quadrature-axis current error (corresponding to the second error signal). These direct-axis and quadrature-axis current errors form the data basis for the negative-sequence voltage modulation command. Specifically, for the negative-sequence d-axis current reference component... and negative sequence q-axis current reference component The calculations for both are primarily based on the negative sequence voltage amplitude of the external power grid. and the equivalent impedance magnitude of the transmission line This is determined by the fact that the negative sequence voltage amplitude of the external power grid needs to be pre-calculated. Transforming to the aforementioned negative-sequence dq coordinate system, the negative-sequence d-axis voltage amplitude of the external power grid is formed. and negative sequence q-axis voltage amplitude Then, further calculations are performed. See Formula 4 for details.

[0043] ………… Formula 4 in The equivalent inductance of the transmission line, This is the equivalent resistance of the transmission line.

[0044] After obtaining the negative sequence d-axis current reference component and negative sequence q-axis current reference component In the case of [condition], the specific process of performing the comparison calculation is shown in Formula 5.

[0045] ………… Formula 5 in For direct-axis current error, This refers to the quadrature-axis current error.

[0046] After obtaining the direct-axis current error and quadrature axis current error Based on this, the two sets of error signals mentioned above are further input into a preset negative sequence current regulator, such as a PI regulator. Combined with the dynamic response characteristics of filters in the power system circuit, the direct-axis current error is addressed. and quadrature axis current error Amplification and integration correction yield the original negative-sequence voltage control quantity. However, since the filter inductor in the filter still has coupling characteristics, it negatively impacts the overall control accuracy of the circuit. Therefore, further correction is needed for the original negative-sequence voltage control quantity.

[0047] For example, the above correction process first takes into account the impedance angle of the transmission line. and the phase angle of the external grid voltage The impedance angle compensation offset can be set according to the actual operating state of the grid-type inverter. Furthermore, the impedance angle compensation offset will be increased. Combined with phase angle This allows for the calibration of the reference angle in the negative-sequence dq coordinate system, ensuring that the negative-sequence current enters the transmission line from the grid-connected inverter along the optimal direction that can offset the negative-sequence voltage at the grid connection point. The impedance angle compensation offset is included. The setup process is generally adjusted according to the actual situation and can be implemented in accordance with the methods currently available in related technologies; this application does not impose specific limitations. The above process is specifically shown in Formula 6.

[0048] ………… Formula 6 in The reference angle for the negative-order dq coordinate system.

[0049] Finally, using the reference angles mentioned above Cross-coupling compensation term of filter inductor in superimposed dq coordinate system The original negative sequence voltage control quantity is corrected and converted into a negative sequence voltage regulation command. In this way, when the negative sequence voltage modulation command is sent to the grid-connected inverter, each switch can perform directional closed-loop control of the negative sequence current under the control of the negative sequence voltage modulation command, thereby adjusting the direction of the negative sequence current entering the transmission line, eliminating the influence of filter inductive coupling on control accuracy, and suppressing the negative sequence voltage at the grid connection point.

[0050] Please see Figure 8 In some embodiments, step 04 further includes: Step 041: Verify the positive-sequence voltage modulation instruction and the negative-sequence voltage modulation instruction to make the positive-sequence voltage modulation instruction and the negative-sequence voltage modulation instruction compatible with the driving format of the switching transistor; Step 042: Linearly superimpose the positive-sequence voltage modulation command and the negative-sequence voltage modulation command to determine the comprehensive modulation signal; Step 043: Input the integrated modulation signal to the gate of the switching transistor to control the operating state of the switching transistor, so as to control the voltage balance at the grid connection point.

[0051] Specifically, based on the above implementation method, after determining the positive-sequence voltage modulation command and the negative-sequence voltage modulation command, the two can be combined and input into the gates of each switch included in the grid-type inverter, thereby controlling the operating state of each switch. Generally, the two can be superimposed linearly to form a comprehensive modulation signal. This superposition does not change the control logic of the two channels, and the fusion is only performed at the final output command level, avoiding mutual interference between the positive-sequence support and negative-sequence suppression ends.

[0052] In addition, for example, the signal formats of the positive-sequence voltage modulation command and the negative-sequence voltage modulation command are verified before superposition to ensure that both are three-phase voltage modulation signals (such as SPWM signals) that can be adapted to each switch in the grid inverter, that the amplitude and frequency range meet the operating parameters of each switch, and that there is no phase conflict between them.

[0053] In some embodiments, the negative voltage suppression method for grid-connected inverters further includes: The negative sequence current reference value is controlled to be less than the first limit and less than or equal to the second limit to protect the electrical safety of the grid-connected inverter and transmission line, and to make the negative sequence voltage at the grid connection point positively correlated with the negative sequence voltage of the external power grid. The first limit is determined based on the rated reactive power of the grid-connected inverter and the rated voltage of the external power grid, while the second limit is determined based on the magnitude of the negative sequence voltage of the external power grid and the magnitude of the equivalent impedance of the transmission line.

[0054] Specifically, in the power system circuit described above, the current at the grid connection point is not necessarily better the larger it is. On the one hand, excessive current will cause reactive power to exceed the rated value. On the other hand, when the negative sequence grid voltage is low, excessive grid connection negative sequence current may cause the negative sequence voltage at the grid connection point to reverse with the voltage of the external grid. In this case, the larger the negative sequence current, the larger the amplitude of the negative sequence voltage will be, which will lead to a violation of the control objective of suppressing negative sequence voltage and result in a situation where the more compensation is made, the more unbalanced the situation becomes.

[0055] Therefore, a dual limiting condition is required to restrict the magnitude of the negative sequence current reference value. This is illustrated in Formula 7.

[0056] ………… Formula 7 in This refers to the rated reactive power of the grid-connected inverter. The rated voltage of the external power grid. This represents the negative sequence voltage amplitude of the external power grid. This represents the equivalent impedance magnitude of the transmission line.

[0057] Based on Formula 7, when the rated reactive power is the main constraint, the negative sequence current reference value should satisfy Equation (1) in Formula 7, while when the negative sequence voltage suppression is the main constraint, the negative sequence current reference value should satisfy Equation (2) in Formula 7.

[0058] Thus, this application constructs a negative-sequence voltage suppression strategy with negative-sequence current as the control object by separating the positive and negative sequences of the three-phase electrical signals. Combined with the grid electrical angle and transmission line impedance angle, it achieves directional closed-loop control of the grid-connected negative-sequence current, thereby effectively reducing the negative-sequence component. Simultaneously, the separated positive-sequence signal is used independently for droop control to ensure normal voltage and frequency support for the grid-connected inverter. Furthermore, a correction mechanism for frequency reference value oscillations is introduced into the droop control, thereby improving the transient performance of the grid-connected inverter under unbalanced grid conditions.

[0059] The electronic device in this application includes a memory and a processor. The memory stores a computer program, and when the computer program is executed by the processor, the negative voltage suppression method for grid-connected inverters described in the above embodiments is implemented.

[0060] The computer-readable storage medium in the embodiments of this application stores a computer program that, when executed by one or more processors, implements the negative voltage suppression method for grid-connected inverters described above.

[0061] The above description is merely a preferred embodiment of this application and is not intended to limit this application in any way. Although this application has disclosed the preferred embodiment as above, it is not intended to limit this application. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the technical solution of this application. Any simple modifications, equivalent substitutions, and improvements made to the above embodiments without departing from the technical solution of this application, based on the technical essence of this application and within the spirit and principles of this application, shall still fall within the protection scope of the technical solution of this application.

Claims

1. A method for suppressing negative voltage in a grid-connected inverter, characterized in that, The method includes: Based on the three-phase electrical signal at the grid connection point between the grid-connected inverter and the external power grid, signal splitting is performed to determine the positive-sequence component and the negative-sequence component. The grid-connected inverter is connected to the external power grid via a filter, and the grid connection point is the connection point between the filter and the external power grid. Based on the positive sequence component, droop control is executed to determine the positive sequence voltage modulation command in order to maintain the voltage and frequency stability of the external power grid. Based on the negative sequence component, coordinate transformation and negative sequence current closed-loop control are performed to determine the negative sequence voltage modulation command in order to suppress the negative sequence voltage at the grid connection point. Based on the positive-sequence voltage modulation command and the negative-sequence voltage modulation command, superposition processing is performed to determine the comprehensive modulation signal, so as to control the voltage balance at the grid connection point.

2. The method according to claim 1, characterized in that: The step of performing signal splitting and determining the positive-sequence and negative-sequence components based on the three-phase electrical signal at the grid connection point between the grid-connected inverter and the external power grid includes: Based on the three-phase electrical signals, perform Clarke transformation to determine the two-phase stationary electrical signals; Based on the two-phase static electrical signals and the fundamental period of the external power grid, signal splitting is performed to determine the positive sequence component and the negative sequence component.

3. The method according to claim 1, characterized in that, The step of performing droop control based on the positive sequence component to determine the positive sequence voltage modulation command includes: Based on the positive sequence components, perform the Park transform to determine the direct-axis positive sequence components and the quadrature-axis positive sequence components. Based on the direct-axis positive-sequence component and the quadrature-axis positive-sequence component, the positive-sequence active power and positive-sequence reactive power output by the grid-type inverter are determined. Based on the positive sequence active power, the positive sequence reactive power, and the reference active power and reference reactive power of the grid-type inverter, determine the frequency reference value and the positive sequence voltage amplitude reference value. The positive sequence voltage modulation command is determined based on the frequency reference value and the positive sequence voltage amplitude reference value.

4. The method according to claim 1, characterized in that, A transmission line is provided between the grid-type inverter and the external power grid; The step of performing coordinate transformation and negative sequence current closed-loop control based on the negative sequence component to determine the negative sequence voltage modulation command includes: Based on the negative order components, perform the Park transform to determine the direct-axis negative order components and the quadrature-axis negative order components. The negative sequence current reference value is determined based on the negative sequence voltage amplitude of the external power grid and the equivalent impedance amplitude of the transmission line; The original negative sequence voltage control quantity is determined based on the direct-axis negative sequence component, the quadrature-axis negative sequence component, and the negative sequence current reference value. Based on the impedance angle of the transmission line and the voltage phase of the external power grid, the original negative sequence voltage control quantity is corrected to determine the negative sequence voltage modulation command.

5. The method according to claim 4, characterized in that, The direct-axis negative sequence component includes the direct-axis negative sequence current, and the quadrature-axis negative sequence component includes the quadrature-axis negative sequence current. The step of determining the original negative sequence voltage control quantity based on the direct-axis negative sequence component, the quadrature-axis negative sequence component, and the negative sequence current reference value includes: Based on the negative sequence current reference value, perform Park transformation to determine the direct-axis negative sequence current reference value and the quadrature-axis negative sequence current reference value; The first error signal is determined based on the direct-axis negative sequence current and the direct-axis negative sequence reference value; The second error signal is determined based on the cross-axis negative sequence current and the cross-axis negative sequence reference value; Based on the first error signal and the second error signal, amplification and integral correction are performed to determine the original negative sequence voltage control quantity.

6. The method according to claim 4, characterized in that, The step of correcting the original negative sequence voltage control quantity based on the impedance angle of the transmission line and the voltage phase of the external power grid, and determining the negative sequence voltage modulation command, includes: The impedance angle compensation offset is determined based on the impedance angle of the transmission line and the voltage phase of the external power grid. The current offset angle is determined based on the impedance angle compensation offset. Based on the coupling characteristics of the filter, determine the cross-coupling compensation amount of the filter inductor; Based on the current offset angle and the cross-coupling compensation amount of the filter inductor, the original negative sequence voltage control amount is corrected, and the negative sequence voltage modulation command is determined.

7. The method according to claim 1, characterized in that, The grid-type inverter includes multiple bridge arms arranged in parallel, each bridge arm is provided with at least two switching transistors, and the gate of the switching transistors is connected to the integrated modulation signal; The step of performing superposition processing based on the positive-sequence voltage modulation command and the negative-sequence voltage modulation command to determine the comprehensive modulation signal in order to control the voltage balance at the grid connection point includes: Verify the positive-sequence voltage modulation instruction and the negative-sequence voltage modulation instruction to make the positive-sequence voltage modulation instruction and the negative-sequence voltage modulation instruction adapt to the driving format of the switching transistor; The positive-sequence voltage modulation command and the negative-sequence voltage modulation command are linearly superimposed to determine the composite modulation signal; The integrated modulation signal is input to the gate of the switching transistor to control the operating state of the switching transistor, thereby controlling the voltage balance at the grid connection point.

8. The method according to any one of claims 4-6, characterized in that, The method further includes: The negative sequence current reference value is controlled to be less than a first limit and less than or equal to a second limit to protect the power safety of the grid-connected inverter and the transmission line, and to make the negative sequence voltage at the grid connection point positively correlated with the negative sequence voltage of the external power grid. The first limit is determined based on the rated reactive power of the grid-connected inverter and the rated voltage of the external power grid, and the second limit is determined based on the negative sequence voltage amplitude of the external power grid and the equivalent impedance amplitude of the transmission line.

9. An electronic device, characterized in that, The electronic device includes a memory and a processor. The memory stores a computer program that, when executed by the processor, implements the negative voltage suppression method for a grid-connected inverter as described in any one of claims 1-8.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a computer program that, when executed by one or more processors, implements the negative voltage suppression method for a grid-connected inverter as described in any one of claims 1-8.

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