Dynamic virtual impedance control method for multi-machine parallel connection of network construction type energy storage converters

Abstract

The invention discloses a dynamic virtual impedance control method for multi-machine parallel connection of a network construction type energy storage converter, and belongs to the technical field of power electronics. The method aims to solve the problem that accurate reactive power distribution, circulating current suppression, dynamic response and voltage precision are difficult to consider in a multi-machine parallel system through fixed parameter virtual impedance. A dynamic adjustable virtual impedance module is introduced into the voltage outer loop output end of each converter, and virtual resistance and virtual inductance are calculated in real time based on locally measured output current, bus voltage change rate and adjacent machine reactive power deviation information; and the average reactive power of the system is estimated through a phase synchronization mechanism under the condition of no communication so as to realize multi-machine cooperation. According to the method, the reactive power distribution error is less than 5%, the bus voltage drop is less than 3%, the high-frequency circulating current is controlled within 5% of the rated current under all working conditions, and the dynamic performance and robustness of the system are remarkably improved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of power electronics and new energy power generation, and particularly relates to a dynamic virtual impedance control method for multi-machine parallel connection of grid-constructing energy storage converters. BACKGROUND

[0002] With the rapid evolution of new power systems towards high-proportion renewable energy access, high-elasticity flexible regulation and high-autonomous cooperative operation, grid-constructing energy storage converters, as key equipment for supporting stable operation of power grids, have increasingly prominent technical status. Different from the passive response mode of traditional follow-grid converters that rely on grid voltage and frequency for synchronous control, grid-constructing energy storage converters actively construct and maintain the voltage and frequency reference of the local power grid by simulating the external characteristics of synchronous generators, thereby still providing reliable power support in weak grid or even island conditions. Under this background, to meet the needs of large capacity, high redundancy and modular deployment, multi-machine parallel operation of grid-constructing energy storage converters has become the mainstream architecture in engineering practice. However, while multi-machine parallel systems improve power level and system reliability, they also introduce multiple challenges such as complex power distribution, circulating current suppression and dynamic stability, which need to be deeply optimized at the control strategy level.

[0003] In the prior art, to realize reasonable distribution of reactive power among multiple machines and suppress circulating current problems caused by line impedance differences, a virtual impedance control method is generally used. This method introduces a controllable virtual impedance at the output port of the converter to reconfigure the output impedance characteristics, so that the system as a whole presents an approximately inductive or resistive external characteristic, thereby decoupling the coupling relationship between active and reactive power and improving power distribution accuracy. A typical scheme usually sets a fixed value of virtual resistance or virtual inductance, and the parameters are conservatively adjusted according to the line impedance in the worst case to ensure stability under various operating conditions. Specifically, let the output voltage of the i-th converter be , the output current be , and the local control instruction voltage be After the virtual impedance link, it is revised as:

[0004]

[0005] wherein, is the preset virtual impedance. This method can effectively improve the reactive power sharing performance under steady-state conditions, especially under symmetric line parameter configuration.

[0006] However, with the continuous development of related technologies and the increasingly stringent requirements for dynamic response speed, power quality, and system robustness in application scenarios, some inherent characteristics of the aforementioned fixed-parameter virtual impedance control scheme at the principle level have gradually revealed its deep-seated limitations in addressing new challenges. The reason for this is that while the introduction of fixed virtual impedance can improve steady-state power distribution, it inevitably reduces the system's dynamic response capability and voltage regulation accuracy. On the one hand, excessively large virtual resistance, while beneficial for suppressing high-frequency circulating currents, significantly weakens the converter's ability to support the bus voltage, leading to aggravated voltage drops during load surges. On the other hand, if the virtual impedance value is reduced to improve dynamic performance, under conditions of multi-machine parameter mismatch or line asymmetry, even a small voltage phase difference will trigger significant reactive circulating currents, thereby inducing system oscillations or even instability. Furthermore, in practical engineering, line impedance dynamically changes with ambient temperature, connection status, and aging, while fixed parameters cannot adaptively adjust, resulting in drastic fluctuations in control performance under different operating conditions. Experimental data shows that in a parallel system of three 50kW grid-type converters, when a fixed virtual resistance of 0.1 ohms is used, the instantaneous voltage drop of the bus during the step transition from no-load to full-load reaches 8.2%. If the virtual resistance is reduced to 0.03 ohms to improve dynamic performance, the reactive power distribution error can exceed 15% under asymmetrical circuit conditions, and the effective value of the high-frequency circulating current rises to more than 12% of the rated current, seriously threatening equipment safety and power quality.

[0007] Therefore, how to ensure accurate reactive power distribution and effective suppression of circulating current in multi-machine systems, while taking into account dynamic response speed and voltage regulation accuracy, and achieving adaptive compensation for time-varying characteristics of line parameters, has become a key challenge and an urgent technical problem for those skilled in the art. Summary of the Invention

[0008] The purpose of this invention is to provide a dynamic virtual impedance control method for multiple parallel grid-connected energy storage converters, aiming to solve the technical contradiction that existing fixed-parameter virtual impedance control strategies are difficult to balance in multi-converter parallel systems, such as accurate reactive power distribution, effective circulating current suppression, dynamic response speed and voltage regulation accuracy.

[0009] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0010] A dynamic virtual impedance control method for multiple grid-connected energy storage converters in parallel is provided, applicable to a system in which N grid-connected energy storage converters are connected in parallel to a common AC bus, with each converter configured with an independent local controller. The method includes the following steps:

[0011] S1, introduce a dynamically adjustable virtual impedance module at the voltage outer loop control command output of each converter;

[0012] S2, the virtual impedance module generates dynamic virtual impedance in real time based on the locally measured output current, bus voltage amplitude and its rate of change, and the local estimate of the system's average reactive power;

[0013] S3, the voltage drop vector obtained by multiplying the dynamic virtual impedance by the output current is subtracted from the original voltage command to form a corrected voltage command for driving the converter; wherein, the dynamic virtual impedance is in complex form and includes a dynamic virtual resistance component and a dynamic virtual inductance component.

[0014] Furthermore, in this invention, the dynamic virtual resistance component is composed of the superposition of a basic damping term, a voltage sag compensation term, and a reactive current circulation suppression term; the basic damping term is a constant positive value; the voltage sag compensation term is inversely proportional to the absolute value of the rate of change of the bus voltage amplitude; and the reactive current circulation suppression term is proportional to the absolute value of the difference between the estimated local reactive power and the estimated average reactive power of the system.

[0015] Furthermore, in this invention, the expression for the voltage sag compensation term is:

[0016]

[0017] in, and To preset positive real number parameters, This refers to the amplitude of the common bus voltage.

[0018] Furthermore, in this invention, the expression for the reactive power circulation suppression term is:

[0019]

[0020] in, Let be the local reactive power of the i-th converter. This is its local estimate of the system's average reactive power. This is a preset proportionality coefficient.

[0021] Furthermore, in this invention, the energy density of the dynamic virtual inductance component is positively correlated with that of the high-frequency component of the output current; the high-frequency component is obtained by subtracting the low-frequency component of the output current signal after passing through a low-pass filter with a cutoff frequency of 1.5 times the system fundamental frequency, and its root mean square value is calculated as an energy density index within a preset time window.

[0022] Furthermore, in this invention, the expression for the dynamic virtual inductance component is:

[0023]

[0024] in, As a reference virtual inductance value, The preset positive real gain coefficient, This is the root mean square value of the high-frequency component of the output current.

[0025] Furthermore, in this invention, the local estimate of the system's average reactive power... Obtained through the following methods:

[0026] Local voltage phase extracted based on phase-locked loop With preset virtual synchronization signal Calculate phase difference ;

[0027] Multiply the phase difference by the preset phase-reactive power conversion coefficient The reactive power deviation estimate is obtained;

[0028] Subtract the deviation estimate from the local reactive power to obtain the local estimate of the system's average reactive power.

[0029] Furthermore, in this invention, the dynamic virtual resistance component and the dynamic virtual inductance component are each provided with an upper limit threshold; when the calculated value exceeds the corresponding upper limit threshold, it is forcibly clamped at the upper limit threshold to ensure that the system retains the minimum voltage regulation capability.

[0030] Furthermore, in this invention, the original voltage command is generated by an improved droop control structure, and its expression is:

[0031]

[0032] in, This is the rated voltage amplitude. , Let represent the active and reactive power outputs of the i-th converter. , For the corresponding power reference value , This is the droop coefficient.

[0033] Furthermore, in this invention, the local controller acquires three-phase voltage and current signals at a frequency of not less than 10kHz, calculates the power after synchronous rotating coordinate transformation, and performs the calculation within a control cycle of not more than 100... The virtual impedance calculation and voltage command correction are performed in the digital signal processor. Fixed-point arithmetic is used and dead-time compensation and DC bus voltage feedforward are integrated.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] (1) This invention uses the reactive circulating current suppression term of the dynamic virtual resistor to respond in real time to the deviation between the local reactive power and the system average reactive power, and combines it with the non-communication phase synchronization mechanism to achieve multi-machine collaboration. In scenarios such as load change, line parameter asymmetry, or even communication interruption, the reactive power distribution error can be controlled within 5%, and the high-frequency circulating current can be suppressed to less than 5% of the rated current. This completely solves the technical pain point of excessive reactive power deviation and excessive circulating current when the fixed parameter virtual impedance is mismatched with the parameters of multiple machines.

[0036] (2) The voltage drop compensation term in the dynamic virtual resistor of the present invention is inversely proportional to the bus voltage change rate. When the load switching causes a voltage drop, the virtual resistance is rapidly reduced to enhance the voltage support capability, so that the bus voltage drop is less than 3% and quickly recovers to the rated value. In steady state, the virtual resistance is automatically increased to maintain the circulating current suppression performance. At the same time, the dynamic virtual inductance is adaptively increased only when the high-frequency circulating current is significant, avoiding the contradiction of "paying attention to one thing but losing attention to another" of the fixed virtual impedance, and realizing the dual improvement of dynamic response speed and voltage regulation accuracy.

[0037] (3) The dynamic virtual impedance of the present invention can adapt to the time-varying characteristics of line parameters (such as temperature changes and line aging), and ensure the voltage regulation capability under extreme conditions through the upper limit constraint mechanism; it does not rely on high-speed communication networks and can achieve multi-machine collaboration through local measurement; at the same time, it is compatible with the existing droop control architecture, which is convenient for upgrading and deployment in existing systems, and significantly enhances the stability and engineering applicability of the system under complex conditions. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the process of the present invention. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings and embodiments. The embodiments of the present invention include, but are not limited to, the following embodiments.

[0040] like Figure 1As shown, this invention discloses a dynamic virtual impedance control method for multiple grid-connected energy storage converters in parallel. The system architecture applicable to this invention consists of N grid-connected energy storage converters connected in parallel. Each converter is connected to the same common AC bus and connected to the power grid or an independent microgrid through its own LC filter. Each converter is equipped with an independent local controller. The controllers can exchange limited information through a low-bandwidth communication link, or they can achieve multi-converter coordination by relying solely on local electrical quantities when no communication is needed. All converters in the system share the same bus voltage signal, but due to differences in line impedance, the actual impedance between the output end of each converter and the bus is inconsistent, leading to reactive power distribution deviation and circulating current problems. To solve this problem, this invention introduces a dynamically adjustable virtual impedance module at the voltage outer loop control command output end of each converter. This module generates a virtual impedance value in real time based on locally measured output current, bus voltage amplitude and its rate of change, and reactive power deviation information of adjacent converters, and injects it into the voltage command correction stage to complete the dynamic compensation of the converter output voltage.

[0041] In this embodiment, the control system of a single converter includes an analog-to-digital conversion module, a coordinate transformation module, a power calculation module, a virtual impedance calculation module, a voltage correction module, and a pulse width modulation drive module. The analog-to-digital conversion module acquires the three-phase voltage and current signals of the converter's AC side at a sampling frequency of no less than 10kHz; the coordinate transformation module uses a synchronous rotating coordinate system transformation based on a software phase-locked loop to convert the three-phase signals into... Shaft components; power calculation module based on Real-time calculation of active power based on axis voltage and current components With reactive power The virtual impedance calculation module generates dynamic virtual resistance in real time based on the system's operating status. With dynamic virtual inductance The voltage correction module will change the virtual impedance voltage drop vector from... The shaft voltage command is subtracted to generate the final modulation command; the pulse width modulation drive module uses a space vector modulation strategy to generate the drive signals for the switching devices. The entire control process is embedded in the converter's digital signal processor, with a control cycle of no more than 100 seconds. All calculation modules employ fixed-point arithmetic to improve real-time performance, and dead-time compensation and DC bus voltage feedforward are implemented to eliminate the impact of non-ideal factors on control accuracy.

[0042] In a preferred embodiment of the present invention, the output voltage uiui of the dynamic virtual impedance module is determined by the following formula:

[0043]

[0044] in, This is the initial voltage command for the i-th converter. Its output current vector, Let be the dynamic virtual impedance of the i-th converter at time t, and its expression is:

[0045]

[0046] in, For dynamic virtual resistance components, Both are dynamic virtual inductance components, which are calculated and generated in real time by the local controller based on the system's operating status.

[0047] Specifically, dynamic virtual resistance It consists of three superimposed parts: a basic damping term, a voltage sag compensation term, and a reactive power circulation suppression term. The basic damping term... This is a constant positive value used to provide minimum circulating current suppression capability, typically ranging from 0.01 to 0.05 Ω. The specific value is calibrated based on the system's rated capacity and line impedance characteristics. Voltage sag compensation item. It is inversely proportional to the instantaneous voltage drop depth of the bus, and its mathematical expression is:

[0048]

[0049] in, The amplitude of the common bus voltage. and These are preset positive real parameters used to adjust the dynamic response sensitivity. In engineering practice, It is usually set to 0.02 to 0.08 Ω·s. The time is set to 10 to 50 seconds to ensure that the virtual resistance increases rapidly when the bus voltage drops quickly, enhancing voltage support, and automatically decreases when the voltage stabilizes, avoiding excessive weakening of voltage regulation performance. Reactive current circulation suppression term. Based on local reactive power With system average reactive power The absolute value of the deviation is used for nonlinear gain adjustment, and its expression is:

[0050]

[0051] in, , This is a proportionality coefficient, typically ranging from 0.1 to 0.5 Ω / kvar. Ultimately, the dynamic virtual resistance is:

[0052]

[0053] Furthermore, dynamic virtual inductance This is used to enhance the suppression capability of high-frequency circulating current, and its value is positively correlated with the energy density of the high-frequency component of the output current. Specifically, by adjusting the output current... To calculate the root mean square value after high-pass filtering. And set:

[0054]

[0055] in, This serves as a reference virtual inductance value, typically ranging from 0.1 to 0.5 mH. It is a positive real gain coefficient, typically ranging from 0.5 to 2 mH / A. Obtained from the following formula:

[0056]

[0057] in, for With cutoff frequency The output after the low-pass filter, The length of the high-frequency energy integration window is typically set to 1 to 5 ms. Set to 1.5 times the system fundamental frequency, that is, for a 50Hz system, =75Hz. This design allows the virtual inductance to automatically increase when there is significant high-frequency circulating current, effectively suppressing high-frequency oscillations, while maintaining a low value under steady-state conditions to avoid affecting the dynamic response speed.

[0058] As another key feature of the present invention, the average reactive power The acquisition of the voltage phase does not rely on high-speed communication networks, but is indirectly estimated through a local voltage phase synchronization mechanism. Specifically, each converter extracts the local voltage phase based on a phase-locked loop. Combined with the preset rated frequency Constructing a virtual synchronization signal And through phase difference This reflects its deviation relative to the system's average state. Based on this, an estimate of the reactive power deviation is constructed using the approximately linear relationship between the phase difference and the reactive power deviation. ,in The phase-to-reactive power conversion coefficient, whose value can be determined offline or through an online adaptive algorithm, typically ranges from 100 to 500 kvar / rad. The average reactive power of the system is then estimated using a local moving average filter.

[0059]

[0060] The estimated value That is, as The local substitution amount is used to calculate This enables implicit collaboration among multiple machines even without direct communication. This mechanism effectively avoids the reliance on high-speed communication found in traditional solutions, thus improving the system's robustness and scalability.

[0061] Furthermore, to prevent the virtual impedance from increasing excessively under extreme operating conditions and causing a loss of voltage support capability, this invention sets up a virtual impedance upper limit constraint mechanism. The dynamic virtual resistance... With dynamic virtual inductance Each is limited by a preset threshold. With L max ,Right now:

[0062] ,

[0063] When the calculated value exceeds the upper limit, it is forcibly clamped to the corresponding threshold to ensure that the system always retains a minimum voltage regulation capability. In engineering implementation, It is usually set to 0.2 to 0.5 Ω. The value is set to 2 to 5 mH, and the specific value is adjusted according to the system's short-circuit capacity and voltage stability margin requirements.

[0064] In another embodiment of the present invention, the voltage outer loop controller adopts an improved droop control structure, and its voltage command... It is generated by the following formula:

[0065]

[0066] in, This is the rated voltage amplitude. and These represent the active and reactive power outputs of the i-th converter, respectively. and This is a power reference value. and The droop coefficient is defined as follows: Based on this, the introduction of dynamic virtual impedance does not change the basic structure of droop control, but rather adds an impedance voltage drop term to its output, thereby enhancing the system's impedance matching capability while maintaining the power distribution mechanism. This design ensures the compatibility of the method of this invention with existing droop control strategies, facilitating upgrades and deployments in existing systems.

[0067] In one specific embodiment, an experimental platform consisting of four 500kW grid-type energy storage converters connected in parallel was constructed. The system's rated voltage was 400V and rated frequency was 50Hz. Each converter was connected to a common bus via cables of varying lengths, resulting in significant differences in line impedance; the longest cable had an impedance 2.3 times that of the shortest cable. The system parameters were set as follows: =0.02Ω, =0.05Ω·s, =30s, =0.3Ω / kvar =0.2mH, =1.2mH / A, T h =2ms, =75Hz, =300kvar / rad, =0.3Ω, =3mH. The sag factor is set to... =0.001V / W, =0.005V / var.

[0068] In this embodiment, the system initially operates under no-load conditions, followed by the application of a 300kW step active load and a 150kvar step reactive load at 1.0 second. Experimental records show that at the moment of load switching, the bus voltage instantaneously drops by 2.8% and recovers to over 99.5% of its rated value within 150ms. The reactive power outputs of the four converters are 37.2kvar, 38.1kvar, 36.9kvar, and 37.8kvar, respectively, with an average of 37.5kvar. The maximum distribution error is 0.8%, far below the design target of 5%. The effective value of the high-frequency circulating current (defined as components above 150Hz) is 3.2% of the rated current, meeting the requirement of less than 5%.

[0069] In comparison, a comparative example was constructed using a fixed virtual impedance scheme, where the virtual resistance was fixed at 0.1Ω and the virtual inductance at 0.5mH, with all other system parameters identical to the embodiment. Under the same load step conditions, the instantaneous voltage drop of the bus reached 4.7%, and the recovery time was extended to 280ms. The reactive power distribution results were 32.1kvar, 41.5kvar, 30.8kvar, and 45.6kvar, with an average of 37.5kvar. The maximum distribution error was as high as 21.6%, significantly exceeding the allowable range. The effective value of the high-frequency circulating current was 8.7% of the rated current, also significantly worse than the scheme of this invention.

[0070] The performance data of the above embodiments and comparative examples are summarized in Table 1.

[0071] Table 1 Summary of Performance Data

[0072]

[0073] The communication-independent coordination capability of this invention was verified under a communication interruption scenario. In the embodiment system, the communication links between all converters were manually disconnected, retaining only local measurements. An unbalanced load of 200kW active power and 100kvar reactive power (connected only near the first converter) was applied at 0.5 seconds. Experimental results show that the system can still achieve accurate reactive power allocation, with the reactive power outputs of the four converters being 25.3kvar, 24.8kvar, 25.1kvar, and 24.8kvar, respectively, and a maximum error of 1.0%. This indicates that the average reactive power estimation method based on the phase synchronization mechanism effectively achieves implicit multi-machine coordination, maintaining system stability without relying on communication.

[0074] In another test scenario, a sudden change in line parameters was simulated. During steady-state operation of the system, the output line impedance of the third converter was suddenly increased by 50%. The proposed solution completed the adaptive adjustment of the virtual impedance within 200ms, and the reactive power was redistributed to a balanced state, with the maximum deviation never exceeding 3%. In contrast, under the same disturbance, the reactive power deviation of the fixed parameter solution continued to expand, eventually triggering overcurrent protection.

[0075] In summary, this invention achieves an adaptive balance between steady-state high-precision allocation and transient rapid response of virtual impedance values ​​by constructing a virtual impedance tuning mechanism strongly coupled with the system's dynamic state. Under typical operating conditions such as load step, line parameter asymmetry, and communication interruption, this method can automatically adjust the virtual impedance magnitude, ensuring that the reactive power allocation error is less than 5%, while suppressing the instantaneous voltage drop of the bus to within 3%, and controlling the effective value of the high-frequency circulating current to below 5% of the rated current, significantly outperforming existing fixed-parameter schemes. This method does not rely on high-speed communication networks and can achieve multi-machine collaboration based solely on local measurements, exhibiting good engineering applicability and robustness, and is suitable for grid-based energy storage systems in scenarios with a high proportion of renewable energy access.

[0076] The above embodiments are merely one of the preferred embodiments of the present invention and should not be used to limit the scope of protection of the present invention. Any modifications or refinements made to the main design concept and spirit of the present invention that are not of substantial significance, but solve the same technical problem as the present invention, should be included within the scope of protection of the present invention.

Claims

1. A dynamic virtual impedance control method for multiple grid-connected energy storage converters in parallel, applied to a system consisting of N grid-connected energy storage converters connected in parallel to a common AC bus, wherein each converter is equipped with an independent local controller, characterized in that... The method comprises the following steps: S1, introducing a dynamic adjustable virtual impedance module at the voltage outer loop control instruction output end of each converter; S2, the virtual impedance module generates a dynamic virtual impedance in real time according to the locally measured output current, bus voltage amplitude and its change rate, and the local estimation value of the system average reactive power; S3, the voltage drop vector obtained by multiplying the dynamic virtual impedance by the output current is deducted from the original voltage instruction to form a corrected voltage instruction for driving the converter; wherein the dynamic virtual impedance is in complex form, including a dynamic virtual resistance component and a dynamic virtual inductance component.

2. The dynamic virtual impedance control method for multi-machine parallel connection of network-constructed energy storage converters according to claim 1, characterized in that, The dynamic virtual resistance component is composed of a basic damping term, a voltage drop compensation term and a reactive current suppression term; the basic damping term is a constant positive value; the voltage drop compensation term is inversely proportional to the absolute value of the change rate of the bus voltage amplitude; the reactive current suppression term is proportional to the absolute value of the difference between the local reactive power and the estimated value of the system average reactive power.

3. The dynamic virtual impedance control method for multi-machine parallel connection of network-constructed energy storage converters according to claim 2, characterized in that, The expression of the voltage drop compensation term is: wherein, with is a predetermined positive real number parameter, is the common bus voltage amplitude.

4. The dynamic virtual impedance control method for multi-machine parallel connection of network-constructed energy storage converters according to claim 3, characterized in that, The expression of the reactive current suppression term is: wherein, Qi is the local reactive power for the ith converter, Qi is its local estimate of the system average reactive power, Kp is a preset positive proportionality coefficient.

5. The dynamic virtual impedance control method for multi-machine parallel connection of network-constructed energy storage converters according to claim 4, characterized in that, The dynamic virtual inductance component is positively correlated with the energy density of the high frequency component of the output current; the high frequency component is obtained by subtracting the low frequency component of the output current signal after passing through a low-pass filter with a cutoff frequency of 1.5 times the system fundamental frequency, and the root mean square value thereof is calculated within a preset time window as an energy density indicator.

6. The dynamic virtual impedance control method for multi-machine parallel connection of network-constructed energy storage converters according to claim 5, characterized in that, The expression of the dynamic virtual inductance component is: wherein, is a reference virtual inductance value, is a preset positive real gain coefficient, is a root mean square value of the high-frequency component of the output current.

7. The dynamic virtual impedance control method for multi-machine parallel connection of network-constructed energy storage converters according to claim 6, characterized in that, a local estimate of the system average reactive power by Local voltage phase extraction based on phase-locked loop With preset virtual synchronization signal Computing phase difference ; multiplying the phase difference by a preset phase-reactive power conversion coefficient obtaining a reactive power deviation estimate The local estimation value of the system average reactive power is obtained by subtracting the deviation estimate from the local reactive power.

8. The dynamic virtual impedance control method for multi-machine parallel connection of network-constructed energy storage converters according to claim 7, characterized in that, The dynamic virtual resistance component and the dynamic virtual inductance component are respectively provided with upper threshold values; when the calculated value exceeds the corresponding upper threshold value, the upper threshold value is forced to be clamped to ensure that the system retains the lowest voltage regulation capability.

9. The dynamic virtual impedance control method for multi-machine parallel connection of network-constructed energy storage converters according to claim 8, characterized in that, The original voltage instruction is generated by an improved droop control structure, and the expression is: wherein, is the rated voltage amplitude, , is the active and reactive power output of the i-th converter, , is the corresponding power reference value , is the droop coefficient.

10. The dynamic virtual impedance control method for multi-machine parallel connection of network-constructed energy storage converters according to claim 9, characterized in that, The local controller collects three-phase voltage and current signals at a frequency not less than 10 kHz, calculates power after synchronous rotating coordinate transformation, and controls the period not more than 100 The virtual impedance calculation and voltage instruction correction are completed in the digital signal processor, fixed-point number operation is adopted, and dead time compensation and DC bus voltage feedforward link are integrated.

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