A network configuration type inverter control method and system for improving short circuit ratio

Abstract

The application discloses a network-constructed inverter control method and system for improving short-circuit ratio, and the control method comprises the following steps: establishing a steady-state power transmission equivalent model of the network-constructed inverter and a power grid, and deducing active power and maximum transmissible active power; combining the definition of the short-circuit ratio to establish a rated active power transmission condition of the network-constructed inverter, and obtaining a short-circuit ratio boundary corresponding to the rated power operation; introducing a virtual impedance into a voltage reference instruction, reducing an equivalent impedance modulus value by configuring a virtual negative inductance, or adjusting the impedance modulus value and the impedance angle in combination with a positive resistance; establishing a quantitative relationship between the virtual impedance parameters and the short-circuit ratio boundary according to a target short-circuit ratio, and determining a virtual resistance and a virtual inductance parameter line impedance. The application can realize quantitative design of the virtual impedance of the network-constructed inverter from the steady-state power transmission mechanism, expand the rated power operation range under the condition of low short-circuit ratio, and improve the adaptability of the network-constructed inverter to weak power grids and super-weak power grids.

Description

Technical Field

[0001] This invention relates to the field of new energy grid connection and power electronic converter control technology, and in particular to a grid-connected inverter control method and system for improving the short-circuit ratio. Background Technology

[0002] With the large-scale integration of new energy power generation units such as wind power, photovoltaics, and energy storage into the power system, the proportion of power sources using power electronic converters as interfaces continues to increase, while the proportion of traditional synchronous generators is relatively decreasing. The power system is gradually exhibiting characteristics such as low inertia, low damping, and wide-range variations in grid strength. At the same time, large-scale new energy bases typically connect to the main grid via long transmission lines, further increasing the line impedance at the new energy grid connection point, reducing short-circuit capacity, and subjecting grid-connected converters to operating environments with low or even ultra-low short-circuit ratios.

[0003] Existing grid-connected converters for new energy sources mainly include grid-following converters and grid-connecting converters. Grid-following converters typically rely on phase-locked loops (PLLs) to obtain the grid phase and output power to the grid as a controlled current source. They exhibit good power point tracking performance when the grid is strong. However, as the grid short-circuit ratio decreases, the PLL and current control loop are prone to coupling with line impedance, thereby reducing the system stability margin. Grid-connecting converters, on the other hand, generate internal voltage amplitude and phase through power synchronization control, droop control, or virtual synchronous generator control. Externally, they exhibit controlled voltage source characteristics, actively establishing AC voltage and providing inertia, frequency, and voltage support. Therefore, they are considered an important technical means to improve the weak grid operation capability of new energy power systems.

[0004] However, grid-based control does not mean that a grid-based inverter can operate stably at a given power under any low short-circuit ratio conditions. Essentially, a grid-based inverter can be considered equivalent to a controlled voltage source transmitting power to the grid through line impedance. Its active power is determined by the inverter's internal electromotive force, the grid voltage, the power angle between them, and the transmission impedance. When the grid short-circuit ratio decreases, the equivalent line impedance increases, and the range of achievable steady-state active power transmission between the inverter and the grid changes accordingly. When the rated active power command exceeds the maximum steady-state active power achievable under current grid conditions, the system will not have a steady-state operating point corresponding to that power command. Adjusting only the virtual inertia, damping coefficient, and voltage-current loop parameters is insufficient to fundamentally change these steady-state power transmission constraints.

[0005] To address the operational issues of grid-connected converters under varying grid strengths, existing technologies have proposed several improvement methods, including grid control parameter optimization, output impedance reshaping, virtual impedance or virtual admittance control, fault current limiting, and adaptive control.

[0006] Patent CN117254527B discloses a method and apparatus for optimizing control parameters of a grid-connected inverter. Under given grid strength, stability margin, and control bandwidth requirements, the feasible domain of each control parameter is obtained by solving the frequency domain equations of the current loop, voltage loop, and power loop, thus balancing the stability of the grid-connected inverter under strong grid conditions and its dynamic response performance under weak grid conditions. This method primarily improves dynamic stability under different grid strengths by optimizing control loop parameters. Its analysis focuses mainly on the frequency domain characteristics and stability margin of the control loop, without establishing an analytical relationship between rated power operating conditions and short-circuit ratio based on the steady-state maximum active power transmission capacity of the grid-connected inverter.

[0007] Patent CN118971218A discloses a method and device for improving the adaptability of grid-connected new energy sources across a wide short-circuit ratio range. This method first establishes a full-order impedance model of the grid-connected converter, including power control loops, voltage and current control loops, and filtering stages. It then uses the Bode plot of the converter impedance and line impedance to determine the stability of the grid-connected system and locates the frequency bands where oscillations and instability may occur under wide short-circuit ratio variations. Based on this, a hybrid virtual impedance method is used to improve the converter impedance characteristics, thereby enhancing stable operation across a wide short-circuit ratio range. This method primarily addresses the small-signal oscillations and frequency domain stability issues caused by changes in the grid short-circuit ratio. Its virtual impedance parameters are mainly used to change the impedance characteristics and phase margin within specific frequency bands, but it does not analyze whether a feasible steady-state operating point exists for the rated active power under low short-circuit ratios.

[0008] Patent CN119994949A discloses a control method and system for a grid-type converter based on a virtual impedance feedforward controller. It establishes a small-signal model considering the coupling relationship between active and reactive power, analyzes the resonance problem caused by conjugate poles near the synchronous frequency, and designs a feedforward controller with virtual impedance to increase system damping. Simultaneously, it comprehensively considers system damping and control bandwidth to determine the virtual impedance tuning conditions. In this scheme, the virtual impedance mainly serves as dynamic damping and synchronous frequency resonance suppression. Its parameter design aims to improve the frequency response, damping, and dynamic stability of the closed-loop system, which differs from virtual impedance design based on the steady-state maximum active power and rated power transmission boundary.

[0009] In summary, existing virtual impedance technologies for grid-connected inverters can be used for output impedance reshaping, oscillation suppression, damping enhancement, fault current limiting, voltage support, and dynamic stability improvement over wide short-circuit ratios. However, existing technologies mainly design virtual impedance from the perspectives of dynamic stability, frequency response, power distribution accuracy, or fault transient control, lacking a targeted quantitative design method for the steady-state active power transmission limit of grid-connected inverters under low short-circuit ratio operating conditions. In particular, there is a lack of a method that unifies the maximum transmittable active power of the grid-connected inverter, the grid short-circuit ratio, the equivalent transmission impedance magnitude, and the impedance angle, and further directly determines the virtual impedance parameters based on the target short-circuit ratio and rated power operating requirements. Therefore, it is necessary to establish an analytical relationship between the steady-state maximum active power and the equivalent transmission impedance of a grid-connected inverter, further obtain the short-circuit ratio boundary corresponding to the rated active power operation, and use the short-circuit ratio boundary as the design basis. By using virtual negative inductance and the combination of virtual negative inductance and positive resistance, the equivalent transmission impedance between the grid-connected inverter and the grid can be actively adjusted. This transforms the virtual impedance parameter from a traditional damping, current limiting, or power distribution parameter into a control degree of freedom for adjusting the steady-state power transmission boundary, thereby expanding the rated active power operating range of the grid-connected inverter under low short-circuit ratio conditions. Summary of the Invention

[0010] The purpose of this invention is to address the problems of grid-connected inverters being constrained by steady-state power transmission limits under low short-circuit ratio conditions, the difficulty of directly exceeding the rated power transmission boundary through traditional control parameter adjustments, and the fact that existing virtual impedance parameters mainly rely on empirical tuning. This invention proposes a control method and system for grid-connected inverters that improves the short-circuit ratio. By establishing a quantitative relationship between the maximum transmittable active power of the grid-connected inverter and the equivalent impedance magnitude and impedance angle, and further establishing an analytical relationship between the short-circuit ratio boundary and virtual impedance parameters, the virtual impedance can be quantitatively configured according to the target short-circuit ratio.

[0011] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0012] A control method for a grid-connected inverter that improves the short-circuit ratio includes the following steps:

[0013] S1. The grid-type inverter is equivalent to a controlled voltage source under the fundamental steady state. The controlled voltage source is connected to the grid voltage source through the line impedance. Based on the internal voltage of the grid-type inverter, the grid voltage and the line impedance, the output current and active power expressions of the inverter are established, and the maximum transmittable active power is further obtained.

[0014] S2. Define the short-circuit ratio (SCR) based on the grid short-circuit capacity, inverter rated capacity, and line impedance. Compare the maximum transmittable active power obtained in step S1 with the inverter rated active power to obtain the short-circuit ratio condition that the grid-type inverter needs to meet to maintain rated active power operation.

[0015] S3. Introduce a virtual impedance consisting of a virtual resistor and a virtual inductor into the voltage reference command of the grid-type inverter, so that the virtual impedance and the line impedance together constitute the equivalent transmission impedance between the internal voltage source of the inverter and the grid, and redetermine the maximum transmittable active power and the short-circuit ratio boundary based on the equivalent transmission impedance.

[0016] S4. Determine the virtual negative inductance compensation amount based on the target operating short-circuit ratio and the inverter's rated power requirement to reduce the equivalent transmission reactance and lower the minimum short-circuit ratio required for rated power transmission. On the basis of satisfying the short-circuit ratio constraint, configure a positive virtual resistor according to the dynamic damping requirement and adjust the virtual negative inductance compensation amount accordingly, so that the system can improve dynamic response performance while meeting the rated power transmission conditions under the target short-circuit ratio.

[0017] As a preferred technical solution of the present invention: In step S1, if the grid voltage phase is used as the phase angle reference, then:

[0018] The internal voltage of a grid-connected inverter is expressed as: ;

[0019] The grid voltage is expressed as: ;

[0020] Line impedance is expressed as: ;

[0021] in, The output voltage amplitude of the grid-connected inverter. The voltage amplitude of the power grid. For the voltage phase angle of the grid-connected inverter, The phase angle of the grid voltage. The imaginary unit, It is a natural constant. Used to represent the phase of an AC phasor, defined This is the power angle of the inverter's internal voltage relative to the grid voltage. and These are the line resistance and equivalent reactance, respectively. The line impedance angle.

[0022] The inverter output current for:

[0023] (1);

[0024] Complex power output to the grid by a grid-connected inverter for:

[0025] (2);

[0026] in, Active power For martial arts power, For output current conjugate, The inverter output voltage vector is used to obtain the active power. expression:

[0027] (3);

[0028] Regarding active power with respect to the power angle Find the extreme value when At that time, the maximum transmittable active power is obtained:

[0029] (4).

[0030] As a preferred technical solution of the present invention: when both the internal voltage of the grid-connected inverter and the grid voltage are maintained at the rated voltage. When nearby, and Then we have:

[0031] (5).

[0032] As a preferred technical solution of the present invention: in step S2, the power grid short-circuit capacity and short-circuit ratio They are defined as follows:

[0033] (6);

[0034] (7);

[0035] in Given the inverter's rated capacity, under rated operating conditions with unity power factor, let the rated active power be... = Then we have:

[0036] (8);

[0037] Therefore, when a grid-connected inverter can maintain the conditions required for rated active power operation, that is... Then:

[0038] (9);

[0039] in For grid-connected inverters, the SCR range within which they can operate normally is defined as follows: For a grid dominated by inductive impedance, when... At that time, there is .

[0040] As a preferred technical solution of the present invention: In step S3, the virtual impedance is realized by the digital controller of the grid-type inverter, and the virtual impedance transfer function is expressed as:

[0041] (10);

[0042] Where s is the Laplace complex frequency operator, a virtual impedance voltage drop is added to the original voltage reference command of the grid-connected inverter. The voltage reference command of the grid-connected inverter after virtual impedance correction is expressed as:

[0043] (11);

[0044] in The original voltage reference command output by the grid controller is the inverter output current. and These are virtual resistance and virtual inductance, respectively;

[0045] At the rated angular frequency Below, the virtual impedance is expressed as:

[0046] (12);

[0047] The virtual impedance and the line impedance together form the equivalent transmission impedance, expressed as:

[0048] (13);

[0049] in The equivalent line impedance angle after adding virtual impedance is given. After introducing virtual impedance, the maximum active power that a grid-connected inverter can transmit is expressed as:

[0050] (14);

[0051] Operating conditions based on rated power The short-circuit ratio boundary after introducing virtual impedance is obtained, and is expressed as:

[0052] (15);

[0053] in This refers to the range of SCRs within which a grid-connected inverter can operate normally after adding virtual impedance.

[0054] As a preferred technical solution of the present invention: In step S4, for a weak power grid that is approximately purely inductive, let ,in , Line reactance value, The line inductance is given by the virtual impedance. Define normalized virtual resistance parameters and virtual negative reactance compensation system for:

[0055] (16);

[0056] (17);

[0057] in If the virtual reactance is used, then the equivalent transmission impedance is:

[0058] (18);

[0059] The corresponding rated power short-circuit ratio boundary is:

[0060] (19);

[0061] When only virtual negative inductance is used, that is When = 0, then:

[0062] (20);

[0063] When the virtual negative reactance does not exceed the line reactance, i.e. When, then:

[0064] (twenty one);

[0065] For a given target short-circuit ratio The virtual negative reactance compensation coefficient satisfies The corresponding virtual inductance is When using both positive virtual resistors and negative virtual inductors, by selecting... >0 and <0, while reducing the equivalent inductive reactance, changes the equivalent impedance angle to extend the rated power operating range of the grid-type inverter under low short-circuit ratio conditions.

[0066] A grid-connected inverter control system for improving the short-circuit ratio includes a processor and a computer-readable storage medium interconnected with each other. The computer-readable storage medium stores a computer program that, when executed by the processor, implements the steps of a grid-connected inverter control method for improving the short-circuit ratio.

[0067] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0068] This invention, starting from the steady-state power transmission mechanism of grid-connected inverters, directly correlates the rated power operating capability under low short-circuit ratios with the maximum transferable active power. This explains why, under some low SCR conditions, adjusting dynamic control parameters such as inertia and damping alone cannot maintain rated power operation. An analytical relationship is established between the maximum transferable active power and the equivalent transmission impedance magnitude and impedance angle, transforming the virtual impedance from a traditional empirical dynamic adjustment parameter into a control degree of freedom that can directly adjust the steady-state power transmission boundary. A direct relationship is established between the target short-circuit ratio and the virtual negative inductance compensation ratio, enabling quantitative design of virtual impedance parameters based on the target SCR, reducing repeated parameter testing. Using a virtual negative inductance can offset part of the line inductive reactance and reduce the equivalent transmission impedance magnitude, thereby increasing the maximum transferable active power and expanding the rated power operating range of grid-connected inverters under low short-circuit ratio conditions. Attached Figure Description

[0069] Figure 1 This is a schematic diagram of the grid-connected inverter system and VSG control structure in an embodiment of the present invention.

[0070] Figure 2 This is a schematic diagram of the equivalent circuit for steady-state power transmission of a grid-type inverter in an embodiment of the present invention.

[0071] Figure 3 This is a schematic diagram of the power angle characteristic curves under different equivalent transmission impedance conditions in an embodiment of the present invention.

[0072] Figure 4 This is a schematic diagram of the virtual impedance control structure in an embodiment of the present invention.

[0073] Figure 5 This is a schematic diagram of the operating waveform of the system changing from SCR=1 to SCR=0.8 without adding virtual impedance in an embodiment of the present invention.

[0074] Figure 6 The virtual negative reactance compensation coefficient is used in this embodiment of the invention. The waveform showing the SCR decreasing from 1 to 0.8;

[0075] Figure 7 The virtual negative inductance and virtual negative reactance compensation coefficient are used in this embodiment of the invention. Time, normalized virtual resistance The waveform of the SCR decreasing from 1 to 0.8. Detailed Implementation

[0076] The present invention will be further illustrated below with reference to the accompanying drawings and specific embodiments. It should be understood that the following specific embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0077] like Figure 1 The embodiment shown uses a three-phase grid-type inverter based on virtual synchronous generator control.

[0078] The present invention proposes a control method for a grid-connected inverter to improve the short-circuit ratio, which specifically includes the following steps:

[0079] S1: The grid-type inverter is equivalent to a controlled voltage source under the fundamental steady state. The controlled voltage source is connected to the grid voltage source through the line impedance. Based on the internal voltage of the grid-type inverter, the grid voltage, and the line impedance, the expressions for the inverter output current and active power are established, and the maximum transmittable active power is further obtained.

[0080] Specifically as follows:

[0081] The grid controller generates frequency and phase angle references based on the active power reference value and the measured active power, and generates the internal voltage reference signal of the inverter, which drives the power switching devices through the voltage controller and PWM.

[0082] Under fundamental steady-state conditions, the inverter can be further equivalent to... Figure 2 The controlled voltage source shown is specifically connected as follows: the internal voltage of the grid-type inverter is connected in series with inductor Lf, resistor Rg, inductor Lg and grid voltage, and capacitor Cf is connected in parallel between inductor Lf and resistor Rg.

[0083] The internal voltage of a grid-connected inverter is expressed as: The grid voltage is expressed as The line impedance is expressed as , in, The output voltage amplitude of the grid-connected inverter. The voltage amplitude of the power grid. For the voltage phase angle of the grid-connected inverter, The phase angle of the grid voltage. The imaginary unit, It is a natural constant. Used to represent the phase of an AC phasor, defined This is the power angle of the inverter's internal voltage relative to the grid voltage. and These are the line resistance and equivalent reactance, respectively. This is the line impedance angle. The inverter output current is obtained from the circuit relationships. for:

[0084] (1);

[0085] Complex power output to the grid by a grid-connected inverter for:

[0086] (2);

[0087] in, Active power For martial arts power, For output current conjugate, The inverter output voltage vector is used to obtain the active power. expression:

[0088] (3);

[0089] The active power with respect to the power angle Find the extreme value when At that time, the maximum transmittable active power is obtained:

[0090] (4);

[0091] When the internal voltage of the grid-connected inverter and the grid voltage are both maintained at the rated voltage When nearby, and ,have:

[0092] (5);

[0093] Therefore, the maximum steady-state active power output of a grid-connected inverter is determined by both the magnitude of the equivalent transmission impedance and the impedance angle. For example... Figure 3 As shown, under approximately inductive impedance conditions, the peak value of the power angle curve decreases as the transmission impedance magnitude increases; however, when the impedance magnitude is the same, reducing the impedance angle can increase the peak value of the power angle curve.

[0094] S2: Define the short-circuit ratio (SCR) based on the grid short-circuit capacity, inverter rated capacity, and line impedance. Compare the maximum transmittable active power obtained in step S1 with the inverter rated active power to obtain the short-circuit ratio condition that the grid-connected inverter needs to meet to maintain rated active power operation.

[0095] Specifically as follows:

[0096] Grid short-circuit capacity and short-circuit ratio They are defined as follows:

[0097] (6);

[0098] (7);

[0099] in This is the rated capacity of the grid-connected inverter. Under the rated operating conditions of unity power factor, let the rated active power be... = ,but:

[0100] (8);

[0101] Therefore, when a grid-connected inverter can maintain the conditions required for rated active power operation, that is... hour:

[0102] (9);

[0103] in For grid-connected inverters, the SCR range within which they can operate normally is defined as follows: For a grid dominated by inductive impedance, when... Sometimes, .

[0104] S3: In the voltage reference command of the grid-type inverter, a virtual impedance consisting of a virtual resistor and a virtual inductor is introduced, so that the virtual impedance and the line impedance together constitute the equivalent transmission impedance between the voltage source inside the inverter and the grid, and the maximum transmittable active power and the short-circuit ratio boundary are re-determined based on the equivalent transmission impedance.

[0105] Specifically as follows:

[0106] Virtual impedance is realized through the digital controller of the grid-type inverter, and the virtual impedance transfer function is expressed as:

[0107] (10);

[0108] Where s is the Laplace complex frequency operator.

[0109] like Figure 4 As shown, by adding a virtual impedance voltage drop to the original voltage reference command of the grid-connected inverter, the voltage reference command of the grid-connected inverter after virtual impedance correction is expressed as follows:

[0110] (11);

[0111] in, The original voltage reference command output by the grid controller. For inverter output current, and These are virtual resistance and virtual inductance, respectively.

[0112] At the rated angular frequency Below, the virtual impedance is expressed as:

[0113] (12);

[0114] The virtual impedance and the line impedance together form the equivalent transmission impedance:

[0115] (13);

[0116] After introducing virtual impedance, the maximum active power that a grid-connected inverter can transmit is expressed as:

[0117] (14);

[0118] Operating conditions based on rated power The short-circuit ratio boundary after introducing virtual impedance is obtained:

[0119] (15);

[0120] in To ensure the SCR range within which the grid-connected inverter can operate normally after adding virtual impedance, the equivalent transmission impedance magnitude is reduced. or increase This can reduce the short-circuit ratio required for grid-connected inverters to maintain rated power operation.

[0121] S4: Determine the virtual negative inductance compensation amount based on the target operating short-circuit ratio and the inverter's rated power requirement to reduce the equivalent transmission reactance and lower the minimum short-circuit ratio required for rated power transmission; on the basis of satisfying the short-circuit ratio constraint, configure a positive virtual resistor according to the dynamic damping requirements and adjust the virtual negative inductance compensation amount accordingly, so that the system can improve dynamic response performance while meeting the rated power transmission conditions under the target short-circuit ratio.

[0122] Specifically as follows:

[0123] For a weak power grid that is approximately purely inductive, let ,in , Line reactance value, The line inductance is given by the virtual impedance. Define normalized virtual resistance parameters and virtual negative reactance compensation system for:

[0124] (16);

[0125] (17);

[0126] in If the virtual reactance is used, then the equivalent transmission impedance is:

[0127] (18);

[0128] The corresponding rated power short-circuit ratio boundary is:

[0129] (19);

[0130] When only virtual negative inductance is used, that is When = 0, we have:

[0131] (20);

[0132] When the virtual negative reactance does not exceed the line reactance, i.e. At that time, there were:

[0133] (twenty one);

[0134] For a given target short-circuit ratio The virtual negative reactance compensation coefficient satisfies The corresponding virtual inductance is When using both positive virtual resistance and negative virtual inductance, by selecting... >0 and <0, while reducing the equivalent inductive reactance, changes the equivalent impedance angle to extend the rated power operating range of the grid-type inverter under low short-circuit ratio conditions.

[0135] Without adding virtual impedance, such as Figure 5 As shown, when the SCR drops to 0.8, the grid-connected inverter can no longer stably maintain the rated active power, and the power, voltage and current oscillate continuously.

[0136] To enable grid-connected inverters to Under stable operation conditions, the theoretical minimum virtual negative reactance compensation coefficient required is: At this time, the corresponding virtual reactance and virtual inductance are respectively At this point, the rated power operating point is near the theoretical maximum power transmission boundary, and the power transmission margin is relatively small. Considering that actual converters also have factors such as voltage and current inner loop dynamics, filtering stages, digital control delays, and parameter deviations, this embodiment retains a certain design margin based on the theoretical minimum compensation amount, taking... After adding a virtual negative inductance, such as Figure 6 As shown, the system can regain its rated active power under the condition of SCR=0.8, indicating that the rated power operating range of the grid-type inverter can be extended by reducing the equivalent transmission reactance.

[0137] While using only a virtual negative inductor can reduce the equivalent transmission impedance magnitude, a large negative inductance compensation can alter the inverter port voltage characteristics and potentially lead to a significant voltage rise and weaker dynamic damping. Therefore, this embodiment further introduces a positive virtual resistor, which works together with the virtual negative inductor to shape the equivalent impedance. According to the boundary expression for the rated power short-circuit ratio, we have Therefore, this parameter combination can theoretically also meet the rated power transmission requirements under the condition of SCR=0.8, and has a larger steady-state power transmission margin.

[0138] like Figure 7 As shown, when the SCR decreases from 1 to 0.8, the grid-connected inverter can still stably maintain its rated active power. Figure 6 Compared to the case using only a virtual negative inductor, adding a positive virtual resistor reduces voltage transient rise and increases the rate of decay of transient oscillations in frequency and output current. This is because the virtual negative inductor primarily increases the maximum transferable active power by reducing the equivalent inductive reactance and equivalent impedance magnitude, while the positive virtual resistor further alters the equivalent impedance angle and increases damping. Therefore, the combination of a virtual negative inductor and a positive resistor can improve the system's dynamic response while meeting the requirement of low short-circuit ratio rated power transmission.

[0139] The above results show that the present invention can quantitatively configure the virtual resistance and virtual inductance according to the target SCR, and improve the steady-state power transmission capability under low short-circuit ratio by simultaneously adjusting the magnitude and impedance angle of the equivalent transmission impedance.

[0140] This embodiment also proposes a grid-connected inverter control system for improving the short-circuit ratio, including a processor and a computer-readable storage medium interconnected with each other. The computer-readable storage medium stores a computer program, and when the computer program is executed by the processor, it implements the steps of a grid-connected inverter control method for improving the short-circuit ratio.

[0141] This embodiment also includes a computer-readable storage medium storing a computer program that, when executed by a processor, implements the steps of the method described above.

[0142] This embodiment also includes a computer program product, comprising a computer program that, when executed by a processor, implements the steps of the method described above.

[0143] Those skilled in the art will understand that embodiments of this application can be provided as methods, systems, or computer program products. Therefore, this application can take the form of a completely hardware embodiment, a completely software embodiment, or an embodiment combining software and hardware aspects. Furthermore, this application can take the form of a computer program product embodied on one or more computer-readable storage media (including, but not limited to, disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code. This application is described with reference to flowchart illustrations and / or block diagrams of methods, apparatus (systems), and computer program products according to embodiments of this application. It will be understood that each block of the flowchart illustrations and / or block diagrams, and combinations of blocks in the flowchart illustrations and / or block diagrams, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, special-purpose computer, embedded processor, or other programmable data processing apparatus to produce a machine, such that the instructions, which execute via the processor of the computer or other programmable data processing apparatus, create a machine for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The computer program instructions may also be stored in a computer-readable storage medium that can direct a computer or other programmable data processing device to operate in a particular manner, such that the instructions stored in the computer-readable storage medium produce an article of manufacture including instruction means, which are implemented in a process Figure 1 One or more processes and / or boxes Figure 1 The functions specified in one or more boxes. These computer program instructions may also be loaded onto a computer or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer or other programmable apparatus to produce a computer-implemented process, thereby providing instructions that execute on the computer or other programmable apparatus for implementing the process. Figure 1 One or more processes and / or boxes Figure 1 The steps of the function specified in one or more boxes.

[0144] It should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any other way. Any modifications or equivalent changes made based on the technical essence of the present invention shall still fall within the scope of protection claimed by the present invention.

Claims

1. A control method for a grid-connected inverter to improve the short-circuit ratio, characterized in that, Includes the following steps: S1. The grid-type inverter is equivalent to a controlled voltage source under the fundamental steady state. The controlled voltage source is connected to the grid voltage source through the line impedance. Based on the internal voltage of the grid-type inverter, the grid voltage and the line impedance, the output current and active power expressions of the inverter are established, and the maximum transmittable active power is further obtained. S2. Define the short-circuit ratio (SCR) based on the grid short-circuit capacity, inverter rated capacity, and line impedance. Compare the maximum transmittable active power obtained in step S1 with the inverter rated active power to obtain the short-circuit ratio condition that the grid-type inverter needs to meet to maintain rated active power operation. S3. Introduce a virtual impedance consisting of a virtual resistor and a virtual inductor into the voltage reference command of the grid-type inverter, so that the virtual impedance and the line impedance together constitute the equivalent transmission impedance between the internal voltage source of the inverter and the grid, and redetermine the maximum transmittable active power and the short-circuit ratio boundary based on the equivalent transmission impedance. S4. Determine the virtual negative inductance compensation amount based on the target operating short-circuit ratio and the inverter's rated power requirements to reduce the equivalent transmission reactance and lower the minimum short-circuit ratio required for rated power transmission. On the basis of meeting the short-circuit ratio constraint, configure the positive virtual resistor according to the dynamic damping requirements and adjust the virtual negative inductance compensation amount accordingly to improve the dynamic response performance of the system while meeting the rated power transmission conditions under the target short-circuit ratio.

2. The grid-type inverter control method for improving the short-circuit ratio according to claim 1, characterized in that, In step S1, using the grid voltage phase as the phase angle reference, then: The internal voltage of a grid-connected inverter is expressed as: ; The grid voltage is expressed as: ; Line impedance is expressed as: ; in, The output voltage amplitude of the grid-connected inverter. The voltage amplitude of the power grid. For the voltage phase angle of the grid-connected inverter, The phase angle of the grid voltage. The imaginary unit, It is a natural constant. Used to represent the phase of an AC phasor, defined This is the power angle of the inverter's internal voltage relative to the grid voltage. and These are the line resistance and equivalent reactance, respectively. The line impedance angle. The inverter output current for: （1）； Complex power output to the grid by a grid-connected inverter for: （2）； in, Active power For martial arts power, For output current conjugate, The inverter output voltage vector is used to obtain the active power. expression: （3）； Regarding active power with respect to the power angle Find the extreme value when At that time, the maximum transmittable active power is obtained: （4）。 3. The grid-type inverter control method for improving the short-circuit ratio according to claim 2, characterized in that, When the internal voltage of the grid-connected inverter and the grid voltage are both maintained at the rated voltage When nearby, and Then we have: （5）。 4. The grid-type inverter control method for improving the short-circuit ratio according to claim 1, characterized in that, In step S2, the grid short-circuit capacity and short-circuit ratio They are defined as follows: （6）； （7）； in Given the inverter's rated capacity, under rated operating conditions with unity power factor, let the rated active power be... = Then we have: （8）； Therefore, when a grid-connected inverter can maintain the conditions required for rated active power operation, that is... Then: （9）； in For grid-connected inverters, the SCR range within which they can operate normally is defined as follows: For a grid dominated by inductive impedance, when... At that time, there is .

5. The grid-type inverter control method for improving the short-circuit ratio according to claim 1, characterized in that, In step S3, the virtual impedance is realized through the digital controller of the grid-type inverter, and the virtual impedance transfer function is expressed as: （10）； Where s is the Laplace complex frequency operator, a virtual impedance voltage drop is added to the original voltage reference command of the grid-connected inverter. The voltage reference command of the grid-connected inverter after virtual impedance correction is expressed as: （11）； in The original voltage reference command output by the grid controller is the inverter output current. and These are virtual resistance and virtual inductance, respectively; At the rated angular frequency Below, the virtual impedance is expressed as: （12）； The virtual impedance and the line impedance together form the equivalent transmission impedance, expressed as: （13）； in The equivalent line impedance angle after adding virtual impedance is given. After introducing virtual impedance, the maximum active power that a grid-connected inverter can transmit is expressed as: （14）； Operating conditions based on rated power The short-circuit ratio boundary after introducing virtual impedance is obtained, and is expressed as: （15）； in This refers to the range of SCRs within which a grid-connected inverter can operate normally after adding virtual impedance.

6. The grid-type inverter control method for improving the short-circuit ratio according to claim 1, characterized in that, In step S4, for a weak grid that is approximately purely inductive, let ,in , Line reactance value, The line inductance is given by the virtual impedance. Define normalized virtual resistance parameters and virtual negative reactance compensation system for: （16）； （17）； in If the virtual reactance is used, then the equivalent transmission impedance is: （18）； The corresponding rated power short-circuit ratio boundary is: （19）； When only virtual negative inductance is used, that is When = 0, then: （20）； When the virtual negative reactance does not exceed the line reactance, i.e. When, then: （21）； For a given target short-circuit ratio The virtual negative reactance compensation coefficient satisfies The corresponding virtual inductance is When using both positive virtual resistors and negative virtual inductors, by selecting... >0 and <0, while reducing the equivalent inductive reactance, changes the equivalent impedance angle to extend the rated power operating range of the grid-type inverter under low short-circuit ratio conditions.

7. A grid-connected inverter control system for improving the short-circuit ratio, characterized in that, The method includes an interconnected processor and a computer-readable storage medium storing a computer program that, when executed by the processor, implements the steps of the method according to any one of claims 1 to 6.

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