A network-constructed energy storage converter active stability evaluation method

Abstract

The present disclosure relates to a method for active stability evaluation of grid-connected energy storage converter. Firstly, the positive and negative sequence coupled impedance matrix of the grid-connected energy storage converter considering the frequency coupling effect is established, and is converted into equivalent single-input single-output sequence impedance. Secondly, an online impedance scanning method based on current disturbance injection and phase-locked synchronous detection is designed to realize online acquisition of equivalent impedance of the point of common coupling. Then, based on the sequence impedance model and the online scanning results, the secondary loop gain is constructed, and the Nyquist criterion or the phase margin criterion is used to evaluate the small signal stability of the system. Finally, according to the evaluation results, the minimum virtual impedance of single-machine grid-connected, the maximum number of parallel machines and the stability boundary of the newly added grid-connected source of the black-box microgrid are determined. The present disclosure is suitable for weak grid connection, parallel expansion and engineering parameter design of grid-connected energy storage converter, and can be realized in a DSP controller without relying on an external impedance analyzer.

Description

Technical Field

[0001] This disclosure relates to the field of grid-type energy storage converter control, and more specifically, to a method and apparatus for active stability assessment of grid-type energy storage converters. Background Technology

[0002] As the penetration rate of new energy and energy storage systems in distribution networks, microgrids, and islanded power supply systems continues to increase, the dynamic characteristics of power systems are gradually shifting from being dominated by traditional synchronous machines to being dominated by power electronic devices. Unlike grid-following converters that rely on phase-locked loops to follow the grid voltage, grid-based energy storage converters can actively establish voltage amplitude and phase angle, providing voltage and frequency support for weak grids, islanded microgrids, and new energy bases. Therefore, grid-based control has become one of the key technologies in high-proportion new energy power systems.

[0003] However, grid-connected energy storage converters do not inherently guarantee system stability under all connection conditions. Their internal control includes multiple dynamic components such as active-frequency control, reactive-voltage control, voltage-current dual-loop control, virtual impedance, coordinate transformation, and modulation. Their port characteristics vary significantly with control parameters, operating power, grid connection point short-circuit capacity, and line impedance. When a grid-connected energy storage converter is connected to a weak grid, multiple converters operate in parallel, or it is connected to a microgrid composed of multiple unknown power electronic sources, the impedance matching between the converter and the external network may deteriorate, leading to low-frequency oscillations, mid-frequency resonances, grid current divergence, or even system instability.

[0004] Traditional state-space small-signal analysis methods can provide system eigenvalues ​​and participation factors, but require a complete understanding of the control and electrical parameters of the converter and external network. When the system includes equipment from multiple vendors, black-box power supplies, or complex microgrid structures, complete modeling is often difficult to achieve. While time-domain disturbance testing can directly observe the system response, it is not convenient for providing stability margins and parameter optimization directions during the design phase. In contrast, impedance stability analysis only requires obtaining the impedance characteristics of the interconnected system at the common port. It can then construct the subloop gain based on the impedance ratio and use the Nyquist criterion or phase margin criterion to evaluate the stability of the interconnected system. Therefore, this method is suitable for weak grid access assessment and engineering parameter design of grid-connected energy storage converters.

[0005] Existing research has extended impedance analysis methods from DC source-load systems to three-phase AC systems, and proposed analytical tools such as direct-axis-quadrature-axis (dq) impedance, sequence impedance, and the generalized Nyquist criterion. Research on virtual synchronous generators, synchronizers, and microgrid converter control provides a foundation for modeling grid-connected energy storage converters. Furthermore, it is often difficult to perform frequency sweep tests in engineering fields using external impedance analyzers or programmable power supplies; therefore, an online impedance measurement method that can be embedded in the energy storage converter controller is needed. For grid-connected energy storage converters, due to the coordinate transformation and power synchronization links in their control structure, positive-sequence disturbances will generate responses at negative-sequence coupling frequencies, and negative-sequence disturbances will also couple to positive-sequence frequencies. Further research on weak grid frequency coupling, multi-converter network impedance, and GFM / GFL interactions indicates that if this frequency coupling effect is ignored, the resulting impedance model may not accurately reflect the port characteristics of the grid-connected converter over a wide frequency range, thus affecting stability determination and the selection of virtual impedance parameters.

[0006] Therefore, one or more methods are needed to solve the above problems.

[0007] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this disclosure, and therefore may include information that does not constitute prior art known to those skilled in the art. Summary of the Invention

[0008] The purpose of this disclosure is to provide a method and apparatus for active stability assessment of grid-type energy storage converters, thereby overcoming, to at least some extent, one or more problems caused by the limitations and defects of related technologies.

[0009] According to one aspect of this disclosure, an active stability assessment method for a grid-type energy storage converter is provided, comprising: A sequence impedance model considering frequency coupling effect is established, the mapping relationship between disturbance frequency and coupling frequency of grid-type energy storage converter is analyzed, positive and negative sequence coupling impedance matrices are obtained, and the coupling impedance matrix is ​​converted into equivalent single-input single-output sequence impedance. Based on current disturbance injection and phase-locked synchronous detection, online impedance scanning is performed on the common coupling point. The equivalent port impedance of the object under test at the common coupling point is obtained by reference phasor subtraction, positive and negative sequence phasor extraction and complex impedance calculation. Based on the equivalent single-input single-output sequence impedance and the equivalent port impedance, the sub-loop gain of the positive-sequence channel and the negative-sequence channel are constructed respectively, and the small-signal stability of the interconnect system is evaluated using the stability criterion. Based on the stability assessment results, determine the virtual impedance parameter values ​​of the grid-type energy storage converter, the maximum number of units that can be accepted in a multi-unit parallel system, or the stability boundary for the access of new grid-type sources in a black-box microgrid.

[0010] In one exemplary embodiment of this disclosure, the mapping relationship between the disturbance frequency and the coupling frequency is as follows:

[0011] in, f dist Let be the perturbation frequency in the stationary coordinate system. f 1 represents the fundamental frequency. f cpl The corresponding coupling frequency is; the positive and negative sequence coupling impedance matrix is:

[0012] in, s p = j ωp Let be the positive-sequence perturbation complex frequency in the stationary coordinate system. s n = s p 2 j ωn This corresponds to the negative-sequence coupling frequency. ω n The rated angular frequency, Z pp ( s p ) Z pn ( s p ) Z np ( s n ) Z nn ( s n ) are the four elements of the positive and negative order coupling impedance matrix, respectively.

[0013] In an exemplary embodiment of this disclosure, the equivalent single-input single-output sequence impedance includes an equivalent positive-sequence output impedance and an equivalent negative-sequence output impedance, wherein the equivalent positive-sequence output impedance is:

[0014] The equivalent negative sequence output impedance is:

[0015] in,Z g ( s ) represents the equivalent impedance of the external power grid or microgrid at the common coupling point.

[0016] In one exemplary embodiment of this disclosure, the phase-locked synchronization detection includes: The complex variables obtained after Clarke transformation of the three-phase sampled quantities X αβ = x α + jx β For the current testing frequency f test Synchronization angle is θ =2 πf test t The positive-sequence perturbation phasor and the negative-sequence perturbation phasor are respectively passed through e jθ and e jθ Rotational extraction:

[0017] in, X p The real and imaginary parts correspond to the in-phase and quadrature components of the positive sequence quantity, respectively. X n The real and imaginary parts correspond to the in-phase and quadrature components of the negative sequence quantity, respectively; the formula for calculating the sequence impedance of the measured object is:

[0018] Where, Δ V and Δ I These are the disturbance voltage and disturbance current after deducting the reference value.

[0019] In one exemplary embodiment of this disclosure, the secondary loop gain of the positive sequence channel of the method is:

[0020] The secondary loop gain of the negative sequence channel is:

[0021] The stability criterion adopted is the Nyquist criterion, when... T p ( s ) and T n ( s The Nyquist curves of ) do not enclose ( 1,j At point 0), the system meets the impedance stability requirements; Bode diagrams are used for project evaluation, in | Z vsg ( jω )∣=∣ Z g ( jω At the frequency of |, the phase difference between the two satisfies:

[0022] In one exemplary embodiment of this disclosure, for a stand-alone grid-connected system, the minimum virtual impedance that satisfies the stability margin requirement is determined by changing the virtual impedance parameter and repeating the stability assessment; wherein the virtual impedance is a virtual resistance. R v and virtual reactance X v The combination of the elements of the positive and negative order coupling impedance matrix Z pp ( s p In the expression, intermediate variables H vp ( s p ) for:

[0023] in, G vPI ( λ p ) is the transfer function for the voltage loop PI controller. G iPI ( λ p ) is the transfer function for the current loop PI controller. λ p = s p jω n This is the frequency after the positive-sequence disturbance enters the synchronous rotating coordinate system.

[0024] In one exemplary embodiment of this disclosure, for a multi-machine parallel system, assuming N grid-type energy storage converters with identical parameters are connected in parallel, the equivalent port impedance of the parallel system is converted to the impedance of a single unit. By changing the number of parallel units N and recalculating the sub-loop gain, the maximum number of grid-type energy storage converters that the system can be connected to under a given grid connection point short-circuit capacity is evaluated.

[0025] In one exemplary embodiment of this disclosure, for a black-box microgrid expansion scenario, the object under test is an existing power electronic power supply network with unknown control structure and line parameters; The grid-type energy storage converter first uses the grid-following mode as the current disturbance source to perform online scanning of the common coupling point to obtain the equivalent impedance of the black-box network. Then, combined with the sequence impedance model of the grid-type energy storage converter, the stability after the addition of a new grid source is determined.

[0026] In one exemplary embodiment of this disclosure, the grid-type energy storage converter employs virtual synchronous generator control, including active-frequency control, reactive-voltage control, voltage-current dual-loop control, virtual impedance link, coordinate transformation, and modulation link; The main circuit consists of a DC-side energy storage unit, a three-phase bridge arm, an inductor-capacitor filter, and a grid connection port. The online impedance scanning is implemented within a digital signal processor controller and includes perturbation generation, reference phasor subtraction, positive and negative sequence phasor extraction, and impedance calculation.

[0027] In one aspect of this disclosure, an active stability assessment device for a grid-type energy storage converter is provided, comprising: The modeling module is used to establish a sequence impedance model that considers the frequency coupling effect, analyze the mapping relationship between the disturbance frequency and the coupling frequency of the grid-type energy storage converter, obtain the positive and negative sequence coupling impedance matrices, and convert the coupling impedance matrix into an equivalent single-input single-output sequence impedance. The scanning module is used to perform online impedance scanning of the common coupling point based on current disturbance injection and phase-locked synchronization detection. It obtains the equivalent port impedance of the object under test at the common coupling point by means of reference phasor subtraction, positive and negative sequence phasor extraction and complex impedance calculation. The evaluation module is used to construct the sub-loop gain of the positive-sequence channel and the negative-sequence channel based on the equivalent single-input single-output sequence impedance and the equivalent port impedance, and evaluate the small-signal stability of the interconnect system using stability criteria. The decision module is used to determine the virtual impedance parameter value of the grid-type energy storage converter, the maximum number of units that can be accepted in a multi-unit parallel system, or the stability boundary of the newly added grid source access in a black-box microgrid, based on the stability assessment results.

[0028] An exemplary embodiment of this disclosure discloses an active stability assessment method for a grid-connected energy storage converter. The method first establishes a positive and negative sequence coupling impedance matrix for the grid-connected energy storage converter considering frequency coupling effects, and converts it into an equivalent single-input single-output sequence impedance. Second, it designs an online impedance scanning method based on current disturbance injection and phase-locked loop synchronization detection to achieve online acquisition of the equivalent impedance at the common coupling point. Then, it constructs a secondary loop gain based on the sequence impedance model and the online scanning results, and evaluates the small-signal stability of the system using the Nyquist criterion or phase margin criterion. Finally, it determines the minimum virtual impedance for single-unit grid connection, the maximum number of units that can be accepted in parallel multi-unit grid connection, and the stability boundaries for new grid-connected sources in a black-box microgrid based on the evaluation results. This invention is applicable to weak grid access, parallel expansion, and engineering parameter design of grid-connected energy storage converters, and can be implemented within a DSP controller without relying on an external impedance analyzer.

[0029] It should be understood that the above general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure. Attached Figure Description

[0030] The above and other features and advantages of this disclosure will become more apparent from the detailed description of exemplary embodiments thereof with reference to the accompanying drawings.

[0031] Figure 1 A flowchart of an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure is shown; Figure 2 A VSG control structure diagram of a grid-type energy storage converter is shown, which is part of an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure. Figure 3 A flowchart illustrating the frequency coupling sequence impedance modeling of a grid-type energy storage converter is shown in an exemplary embodiment of the present disclosure, which is part of an active stability assessment method for a grid-type energy storage converter. Figure 4 A comparison diagram of positive and negative sequence impedance models considering frequency coupling and simulation results is shown for an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure. Figure 5 A schematic diagram illustrating the stability assessment based on sequence impedance under different short-circuit ratios of an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure is shown. Figure 6 The diagram illustrates a stability assessment based on sequence impedance under different virtual impedance conditions according to an exemplary embodiment of the present disclosure of an active stability assessment method for a grid-type energy storage converter. Figure 7A schematic diagram of an online impedance scanning state machine for an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure is shown. Figure 8 A flowchart of phase-locked synchronization detection for an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure is shown. Figure 9 A schematic diagram of a parallel PCS real-time control matrix semi-physical platform for an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure is shown. Figure 10 A schematic diagram of the frequency sweep verification results of grid impedance measurement for an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure is shown. Figure 11 The Nyquist plot and semi-physical waveform diagram of a single-unit grid-connected system of an active stability assessment method for a grid-connected energy storage converter according to an exemplary embodiment of the present disclosure are shown under different virtual impedances. Figure 12 The diagram shows a semi-physical experimental waveform of a single grid-connected VSG under different virtual impedances in Case 1 of an active stability evaluation method for a grid-connected energy storage converter according to an exemplary embodiment of the present disclosure. Figure 13 Example 2 shows a system topology diagram of an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure; Figure 14 The positive-sequence Nyquist plot of the system secondary loop gain is shown in Case 2 of an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure. Figure 15 Semi-physical waveforms are shown for different virtual impedances and the number of parallel units under an exemplary embodiment of the present disclosure of an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure. Figure 16 A black-box microgrid expansion system topology diagram is shown, illustrating an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of this disclosure. Figure 17 The diagram illustrates the black-box network PCC impedance scan results and VSG impedance model results of an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure. Figure 18 The figure shows a semi-physical waveform diagram after the addition of a new grid source in an active stability assessment method for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure; Figure 19A schematic block diagram of an active stability assessment device for a grid-type energy storage converter according to an exemplary embodiment of the present disclosure is shown. Detailed Implementation

[0032] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the embodiments set forth herein; rather, they are provided so that this disclosure will be thorough and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art. The same reference numerals in the drawings denote the same or similar parts, and therefore repeated descriptions of them will be omitted.

[0033] Furthermore, the described features, structures, or characteristics can be combined in any suitable manner in one or more embodiments. Numerous specific details are provided in the following description to give a thorough understanding of embodiments of this disclosure. However, those skilled in the art will recognize that the technical solutions of this disclosure can be practiced without one or more of the specific details described, or other methods, components, materials, apparatuses, steps, etc., can be employed. In other instances, well-known structures, methods, apparatuses, implementations, materials, or operations are not shown or described in detail to avoid obscuring various aspects of this disclosure.

[0034] The block diagrams shown in the accompanying drawings are merely functional entities and do not necessarily correspond to physically independent entities. That is, these functional entities can be implemented in software, or in one or more software-hardened modules, or in different network and / or processor devices and / or microcontroller devices.

[0035] In this example embodiment, an active stability assessment method for grid-type energy storage converters is first provided; refer to Figure 1 As shown, the active stability assessment method for a grid-type energy storage converter may include the following steps: Step S110: Establish a sequence impedance model considering frequency coupling effect, analyze the mapping relationship between disturbance frequency and coupling frequency of grid-type energy storage converter, obtain positive and negative sequence coupling impedance matrix, and convert the coupling impedance matrix into equivalent single-input single-output sequence impedance. Step S120: Based on current disturbance injection and phase-locked synchronization detection, online impedance scanning is performed on the common coupling point. The equivalent port impedance of the object under test at the common coupling point is obtained by reference phasor subtraction, positive and negative sequence phasor extraction and complex impedance calculation. Step S130: Based on the equivalent single-input single-output sequence impedance and the equivalent port impedance, construct the sub-loop gain of the positive-sequence channel and the negative-sequence channel respectively, and use the stability criterion to evaluate the small-signal stability of the interconnect system. Step S140: Based on the stability assessment results, determine the virtual impedance parameter values ​​of the grid-type energy storage converter, the maximum number of units that can be accepted in the multi-machine parallel system, or the stability boundary for the access of new grid-type sources in the black-box microgrid.

[0036] An exemplary embodiment of this disclosure discloses an active stability assessment method for a grid-connected energy storage converter. The method first establishes a positive and negative sequence coupling impedance matrix for the grid-connected energy storage converter considering frequency coupling effects, and converts it into an equivalent single-input single-output sequence impedance. Second, it designs an online impedance scanning method based on current disturbance injection and phase-locked loop synchronization detection to achieve online acquisition of the equivalent impedance at the common coupling point. Then, it constructs a secondary loop gain based on the sequence impedance model and the online scanning results, and evaluates the small-signal stability of the system using the Nyquist criterion or phase margin criterion. Finally, it determines the minimum virtual impedance for single-unit grid connection, the maximum number of units that can be accepted in parallel multi-unit grid connection, and the stability boundaries for new grid-connected sources in a black-box microgrid based on the evaluation results. This invention is applicable to weak grid access, parallel expansion, and engineering parameter design of grid-connected energy storage converters, and can be implemented within a DSP controller without relying on an external impedance analyzer.

[0037] The active stability assessment method for a grid-type energy storage converter in this example embodiment will be further explained below.

[0038] Example 1: In step S110, a sequence impedance model considering frequency coupling effects can be established, the mapping relationship between the disturbance frequency and the coupling frequency of the grid-type energy storage converter can be analyzed, the positive and negative sequence coupling impedance matrices can be obtained, and the coupling impedance matrix can be converted into an equivalent single-input single-output sequence impedance.

[0039] In this example embodiment, the mapping relationship between the perturbation frequency and the coupling frequency in the method is as follows:

[0040] in, f dist Let be the perturbation frequency in the stationary coordinate system. f 1 represents the fundamental frequency. f cpl The corresponding coupling frequency is; the positive and negative sequence coupling impedance matrix is:

[0041] in, s p = j ωp Let be the positive-sequence perturbation complex frequency in the stationary coordinate system. s n = s p 2 j ωn This corresponds to the negative-sequence coupling frequency. ω n The rated angular frequency, Z pp ( s p ) Z pn ( s p ) Z np ( s n ) Z nn ( s n ) are the four elements of the positive and negative order coupling impedance matrix, respectively.

[0042] In this example embodiment, the equivalent single-input single-output sequence impedance includes an equivalent positive-sequence output impedance and an equivalent negative-sequence output impedance, wherein the equivalent positive-sequence output impedance is:

[0043] The equivalent negative sequence output impedance is:

[0044] in, Z g ( s ) represents the equivalent impedance of the external power grid or microgrid at the common coupling point.

[0045] In step S120, based on current disturbance injection and phase-locked synchronization detection, an online impedance scan can be performed on the common coupling point. By subtracting the reference phasor, extracting the positive and negative sequence phasors, and calculating the complex impedance, the equivalent port impedance of the object under test at the common coupling point can be obtained.

[0046] In this example embodiment, the phase-locked synchronization detection includes: The complex variables obtained after Clarke transformation of the three-phase sampled quantities X αβ = x α + jx β For the current testing frequency f test Synchronization angle is θ =2 πf test t The positive-sequence perturbation phasor and the negative-sequence perturbation phasor are respectively passed through e jθ and e jθ Rotational extraction:

[0047] in, X p The real and imaginary parts correspond to the in-phase and quadrature components of the positive sequence quantity, respectively. X n The real and imaginary parts correspond to the in-phase and quadrature components of the negative sequence quantity, respectively; the formula for calculating the sequence impedance of the measured object is:

[0048] Where, Δ V and Δ I These are the disturbance voltage and disturbance current after deducting the reference value.

[0049] In step S130, the sub-loop gains of the positive-sequence channel and the negative-sequence channel can be constructed based on the equivalent single-input single-output sequence impedance and the equivalent port impedance, respectively, and the small-signal stability of the interconnect system can be evaluated using stability criteria.

[0050] In this example embodiment, the secondary loop gain of the positive sequence channel of the method is:

[0051] The secondary loop gain of the negative sequence channel is:

[0052] The stability criterion adopted is the Nyquist criterion, when... T p ( s ) and T n ( s The Nyquist curves of ) do not enclose ( 1, j At point 0), the system meets the impedance stability requirements; Bode diagrams are used for project evaluation, in | Z vsg ( jω )∣=∣ Z g ( jω At the frequency of |, the phase difference between the two satisfies: .

[0053] In step S140, the virtual impedance parameter value of the grid-type energy storage converter, the maximum number of units that can be accepted in a multi-machine parallel system, or the stability boundary of the newly added grid source access in a black-box microgrid can be determined based on the stability assessment results.

[0054] In this example embodiment, for a stand-alone grid-connected system, the minimum virtual impedance that satisfies the stability margin requirement is determined by changing the virtual impedance parameter and repeating the stability assessment; wherein the virtual impedance is a virtual resistance. R v and virtual reactance X v The combination of the elements of the positive and negative order coupling impedance matrix Z pp ( s p In the expression, intermediate variables H vp ( s p )for:

[0055] in, G vPI ( λ p ) is the transfer function for the voltage loop PI controller. G iPI ( λ p ) is the transfer function for the current loop PI controller. λ p = s p jω n This is the frequency after the positive-sequence disturbance enters the synchronous rotating coordinate system.

[0056] In this example embodiment, for a multi-machine parallel system, it is assumed that N grid-type energy storage converters with identical parameters are connected in parallel. The equivalent port impedance of the parallel system is converted to the impedance of a single unit. By changing the number of parallel units N and recalculating the sub-loop gain, the maximum number of grid-type energy storage converters that the system can connect under a given grid connection point short-circuit capacity is evaluated.

[0057] In this example embodiment, for the black-box microgrid expansion scenario, the object under test is an existing power electronic power network with unknown control structure and line parameters; The grid-type energy storage converter first uses the grid-following mode as the current disturbance source to perform online scanning of the common coupling point to obtain the equivalent impedance of the black-box network. Then, combined with the sequence impedance model of the grid-type energy storage converter, the stability after the addition of a new grid source is determined.

[0058] In this example embodiment, the grid-type energy storage converter adopts virtual synchronous generator control, including active-frequency control, reactive-voltage control, voltage-current dual loop, virtual impedance link, coordinate transformation and modulation link; The main circuit consists of a DC-side energy storage unit, a three-phase bridge arm, an inductor-capacitor filter, and a grid connection port. The online impedance scanning is implemented within a digital signal processor controller and includes perturbation generation, reference phasor subtraction, positive and negative sequence phasor extraction, and impedance calculation.

[0059] Example 2: In this example embodiment, the stability criterion based on order impedance includes: The core idea of ​​impedance stability analysis is to treat both sides of the interconnected system as equivalent port impedances and construct the secondary loop gain through the impedance ratio. For grid-connected systems with grid-connected energy storage converters, the grid-connected converter can be regarded as a voltage source device, and its output impedance is denoted as... Z vsg (s), where the equivalent impedance of the external power grid or microgrid at the point of common coupling is denoted as . Z g ( s Under the premise that each subsystem operates stably and independently, the small-signal stability of the interconnected system can be determined by the port impedance ratio.

[0060] For the positive sequence channel, the secondary loop gain can be written as:

[0061] For the negative-sequence channel, the secondary loop gain can be written as:

[0062] when T p (s) and T n When none of the Nyquist curves of (s) enclose the point (-1, j0), the system meets the impedance stability requirement. If a Bode plot is used for engineering evaluation, then in | Z vsg (jω)|=| Z g At the frequency of (jω)|, the phase difference between the two should be less than 180°, that is...

[0063] It should be noted that for three-phase AC systems, the positive and negative sequence impedances are not completely decoupled. Coordinate transformations and power control in network-based control can introduce frequency coupling between the positive-sequence disturbance and the negative-sequence response. Therefore, directly using the positive and negative sequence impedances without coupling terms for stability determination may lead to inaccurate stability margin estimates. This invention explicitly considers this coupling effect in subsequent modeling.

[0064] For a parallel system of N grid-connected energy storage converters with identical parameters, the equivalent port impedance of the parallel system can be calculated based on the impedance of a single converter, ignoring device variability, communication delay, and line differences. For systems with multiple control loops and multiple converters, port interaction stability can be further analyzed using multiple-input multiple-output (MIMO) impedance or network impedance criteria. Therefore, by changing the number of parallel converters N and recalculating the subloop gain, the maximum number of grid-connected energy storage converters that can be connected to the system under a given grid connection point short-circuit capacity can be evaluated.

[0065] In this example embodiment, the system architecture and modeling assumptions for the sequence impedance model of a grid-type energy storage converter considering frequency coupling include: This invention focuses on a three-phase grid-type energy storage converter controlled by a virtual synchronous generator (VSG). The main circuit consists of a DC-side energy storage unit, three-phase bridge arms, an inductor-capacitor (LC) filter, and a grid connection port. The control system includes active-frequency control, reactive-voltage control, a voltage-current dual-loop control, a virtual impedance element, coordinate transformation, and modulation elements, as shown in the diagram. Figure 2 As shown.

[0066] To highlight the main mechanism of sequence impedance modeling, this invention adopts the following assumptions: 1. The DC side voltage of the converter is stable, and the equivalent gain of the modulation stage is 1.

[0067] 2. The energy storage converter satisfies the small-signal linearization condition near the operating point under test.

[0068] 3. The sampling and pulse width modulation (PWM) delays have little impact on the main conclusions of the frequency band of interest in this invention; if used for high-frequency stability analysis, the delay can be added to the subsequent model.

[0069] 4. The three-phase system has symmetrical parameters, and the positive and negative sequence coupling is mainly introduced by the control loop and coordinate transformation.

[0070] The injection frequency at the common coupling point is... f dist After a small-signal disturbance, the grid-type energy storage converter will not only respond at the disturbance frequency but also at the coupling frequency. For a fundamental frequency... f For a running system, the coupling frequency can be expressed as:

[0071] This frequency coupling relationship is a key factor to consider in the sequence impedance modeling of grid-type energy storage converters. Its physical meaning is consistent with the frequency shifting phenomenon in the research on sequence impedance and frequency coupling modeling.

[0072] The key to impedance modeling is establishing the transfer function relationship between the PCC disturbance voltage and the corresponding response current. For example... Figure 3 As shown, the main circuit describes the relationship between the disturbance voltage, response current, and bridge arm voltage; the outer and inner loop control circuits of the grid-type energy storage converter describe the transfer function relationship between the disturbance voltage, response current, and modulation signal. The modulation signal is related to the bridge arm voltage via the DC side voltage. When the 1.5 sampling period delay caused by ADC sampling and PWM generation is ignored, the modulation gain can be approximated as 1. By jointly solving the above relationships, the transfer function between the disturbance voltage and response current can be obtained, and the sequence impedance of the grid-type energy storage converter can be further derived.

[0073] In this example embodiment, the positive and negative order coupling impedance matrix includes: The goal of impedance modeling is to establish the transfer function relationship between the disturbance voltage and response current at the common coupling point. Since the derivation process involves the stationary coordinate system frequency, the synchronous rotating coordinate system frequency, and the positive and negative sequence coupling frequencies simultaneously, directly using a single variable `s` can easily lead to confusion between the controller frequency and the main circuit frequency. Therefore, a unified convention for the frequency variable is first established.

[0074] Let the complex frequency of the positive-sequence perturbation in the stationary coordinate system be...

[0075] The corresponding negative-order coupling frequency is

[0076] in ω n The rated angular frequency. The frequency after the positive-sequence disturbance enters the synchronous rotating coordinate system is...

[0077] The frequency of the negative-sequence coupled component after entering the synchronous rotating coordinate system is

[0078] Therefore, the positive-sequence disturbance component and the negative-sequence coupling component fall at the same frequency point in the dq coordinate system. This is the fundamental reason why the sequence impedance of a grid-type energy storage converter has positive and negative-sequence coupling terms.

[0079] The main circuit relationship can be obtained from the single-phase equivalent circuit of the LC filter:

[0080] The admittance of the filter capacitor branch is:

[0081] Substituting these values, we obtain the relationship between the bridge arm voltage, the PCC voltage, and the output current:

[0082] in

[0083] After adding a positive-sequence small-signal disturbance to the A-phase PCC voltage, the voltage and current can be written as follows:

[0084] The frequency of the positive sequence component after Park transform is reduced by ω n The frequency of the negative-order component after Park transform is increased. ω n Therefore, there is

[0085] In the dq coordinate system, the disturbance component and the coupling component are located at the same frequency point, so they can be linearly superimposed:

[0086] The upper and lower signs correspond to positive and negative frequency spectral lines. The output active and reactive power are...

[0087] Substituting the small-signal components of voltage and current into dq, the active and reactive components at the disturbance frequency can be written as:

[0088] in

[0089] The Pf element corresponds to the oscillation equation. If an electromagnetic torque disturbance is used in the oscillation equation, the power disturbance needs to be passed through...

[0090] This is converted into torque disturbance. Therefore, the small-signal phase angle is...

[0091] In the formula 1 / ω n Source T e = P e / ω n Power-torque conversion. Q - V The circuit obtains a voltage amplitude reference disturbance:

[0092] in

[0093] make θ = θ 1+ θ hat By performing first-order linearization on the abc / dq transformation matrix, we can obtain...

[0094] Similarly, we can obtain and After ignoring the second-order small-signal terms, the corrected voltage and current quantities are substituted into the virtual impedance, voltage loop, and current loop:

[0095] From this, we can obtain m dref , m qref about V Dist , V Cpl , I Dist and I Cpl The linear expression for dq modulated wave.

[0096] Transform back to the abc coordinate system. Since cosθ and sinθ contain the fundamental frequency... ω n In the dq coordinate system λ p = s p -j ω n The components will shift to two stationary coordinate frequencies:

[0097] Therefore, the A-phase modulated wave in s p and s n The two frequency points contain positive-sequence perturbation terms and negative-sequence coupling terms, respectively. Setting the modulation element and DC-side gain to 1, the bridge arm voltage can be approximated as...

[0098] exist s p and s n At both frequencies, compare this equation with the main circuit equation.

[0099] After combining the equations, the MIMO order impedance matrix can be written as follows:

[0100] To simplify the expression, the intermediate variable of the forward channel is defined as...

[0101] The intermediate variable of the negative-order channel is defined as

[0102] The four elements of the MIMO order impedance matrix are

[0103]

[0104]

[0105]

[0106] For positive-sequence stability analysis, we take s = s_p and the negative-sequence coupling frequency as s - 2jω_n. After eliminating the coupling current, the equivalent positive-sequence output impedance is...

[0107] For negative-sequence stability analysis, taking s = s_n, the corresponding positive-sequence coupling frequency is s + 2jω_n. The equivalent negative-sequence output impedance is...

[0108] In this example embodiment, model validation and feature analysis include: Based on the established sequence impedance model, the amplitude and phase characteristics of the positive and negative sequence impedances of the grid-type energy storage converter can be calculated using MATLAB and compared with the simulation frequency sweep results, such as... Figure 4 As shown. Similar to existing studies on GFM / GFL converter impedance, MMC-HVDC network impedance, and modeling equivalence, the results show that the model considering frequency coupling can accurately characterize the amplitude and phase variation trends of the sequence output impedance within the main frequency band of concern, thus verifying the effectiveness of the established model.

[0109] The stability evaluation results under different line impedances and virtual impedances are as follows: Figure 5 and Figure 6 As shown in the figure, the impedance characteristics reveal that the positive-sequence impedance of the grid-type energy storage converter is highly sensitive to the virtual impedance parameter near the fundamental frequency. As the virtual impedance increases, the positive-sequence impedance curve changes significantly within the interaction frequency band, improving the system phase margin. In contrast, the negative-sequence impedance is less sensitive to changes in virtual impedance. Therefore, within the system and parameter range studied in this invention, virtual impedance design primarily enhances small-signal stability by improving positive-sequence impedance matching. This conclusion is consistent with related studies on virtual impedance, frequency coupling, and VSG sequence impedance stability.

[0110] In this example embodiment, the current perturbation injection scheme of the online impedance scanning method includes: To obtain the equivalent impedance of an external power grid or an unknown microgrid at the point of common coupling, this invention employs a current disturbance injection method. Specifically, a grid-connected energy storage converter is used as the disturbance source, and a small-signal disturbance current of a specified frequency is superimposed on its current setpoint. The object under test can be an actual power grid, an unknown voltage source network, a grid-connected energy storage converter, or a microgrid system. By sampling the three-phase voltage of the PCC and the output current of the disturbance source, the port impedance of the object under test at the specified frequency can be calculated.

[0111] Compared to voltage disturbance injection, current disturbance injection does not require an additional programmable voltage source, making it easier to implement in existing energy storage converter control systems. Disturbance generation, data sampling, synchronous detection, and impedance calculation can all be completed online by the controller, thereby reducing the reliance on external impedance analyzers for field testing.

[0112] In this example embodiment, phasor extraction based on phase-locked synchronization detection includes: The online impedance scanning algorithm is called periodically within the controller interrupt service routine. For each frequency to be tested, the algorithm first extracts the reference phasor without injected disturbance; then, disturbance injection is initiated, and the measurement phasor is extracted after a warm-up period. Subtracting the reference phasor from the measurement phasor suppresses the influence of the fundamental component, background harmonics, and steady-state operating components on the impedance calculation. The online impedance scanning state machine is as follows: Figure 7 As shown.

[0113] Impedance calculation employs a phase-locked synchronous detection method, such as... Figure 8 As shown. Let the complex variables of the three-phase sampled quantities after Clarke transformation be...

[0114] For the current test frequency f_test, the synchronization angle is...

[0115] Positive and negative order perturbation phasors can be obtained respectively through e -jθ and e jθ Rotate to extract. Let X αβ = x α +j x β Then there is

[0116] in, X The real and imaginary parts of p correspond to the in-phase and quadrature components of the positive sequence quantity, respectively. X The real and imaginary parts of n correspond to the in-phase and quadrature components of the negative sequence quantity, respectively. Voltage and current signals can be calculated using the same method. By averaging in a synchronously rotating coordinate system, asynchronous components can be filtered out, yielding the positive and negative sequence voltage and current phasors at the current frequency.

[0117] Where ΔV and ΔI are the disturbance voltage and disturbance current after deducting the reference.

[0118] In this example embodiment, the hardware-in-the-loop (HIL) platform includes: This invention employs a real-time control matrix platform for parallel grid-connected energy storage converters to conduct hardware-in-the-loop (HIL) experimental verification. This platform adopts a modular control board structure, expandable to multiple control nodes, and runs in conjunction with a hardware-in-the-loop real-time simulator. Compared to a rapid control prototype system, the platform's control algorithm, task scheduling, interrupt service routines, sampling processing flow, and low-level peripheral drivers are consistent with the actual energy storage converter controller. Therefore, the HIL experimental results can more realistically reflect the timing characteristics and engineering implementation constraints of the actual controller, providing support for verifying the stability of grid-connected and multi-unit parallel operation.

[0119] The experimental platform and parameters are as follows: Figure 9 As shown in Table 1.

[0120] Table 1 Parameters of 15 kVA grid-type energy storage converter.

[0121]

[0122] In this example embodiment, the scanning algorithm verification includes: To verify the accuracy of the online impedance scanning algorithm, the simulated grid impedance with known parameters was first tested. The equivalent grid impedance is...

[0123] inR g = 0.36 Ω L g=2 mH.

[0124] The frequency sweep verification results of PCS-based power grid impedance measurement are as follows: Figure 10 As shown, the comparison between the DSP online calculation results and the theoretical curves shows that the positive and negative sequence impedance amplitudes and phases are in good agreement with the theoretical values. As the frequency increases, the impedance amplitude exhibits an inductive increase, and the phase gradually approaches 90°, consistent with the resistor-inductor (RL) impedance model. These results demonstrate that the proposed online scanning algorithm can accurately extract the response components at the perturbation frequency and can be used for subsequent impedance acquisition and stability assessment of black-box networks.

[0125] In this example embodiment, the active stability assessment and parameter design based on order impedance includes: Case 1: Minimum Virtual Impedance of a Single-Unit Grid-Connected System First, consider the scenario of a single grid-connected energy storage converter connected to a weak power grid. The short-circuit capacity at the grid connection point is 50 kVA, the rated line voltage is 380 V, the system frequency is 50 Hz, and the rated capacity of the grid-connected energy storage converter is 15 kVA. Neglecting line resistance, the grid connection point can be equivalent to a series reactance of approximately 9.2 mH.

[0126] Given the equivalent grid impedance and converter control parameters, calculate the virtual inductance. L The positive and negative sequence output impedances at v=0 mH, 3 mH, and 6 mH were calculated, and the secondary loop gain was constructed. Since a phase transition exists near the fundamental frequency, the stability boundary cannot be intuitively determined using only the Bode plot. Therefore, a Nyquist plot was further plotted to observe whether the secondary loop gain approaches or surrounds the point (-1, j0). Figure 11 As shown.

[0127] The analysis results show that when L When v=0 mH, the positive-sequence loop gain is close to the danger zone, and the system stability margin is insufficient. When Lv=3 mH, the Nyquist curve moves away from (-1,j0), and the system changes from an unstable or critically stable state to a stable state. When Lv=6 mH, the stability margin is further improved, but the increased virtual impedance will also reduce the dynamic response speed of the system.

[0128] The semi-physical experimental results are consistent with the order impedance analysis, such as... Figure 12As shown, when Lv = 0 mH, the system exhibits divergent oscillations after grid connection; when Lv = 3 mH, the oscillations change from divergent to convergent, and the system can achieve stable grid connection; when Lv = 6 mH, the system stability margin is further improved, but the grid-connected current build-up process slows down, and the transient settling time increases. This demonstrates that virtual impedance design involves a trade-off between improved stability and degraded dynamic performance. Under this short-circuit capacity condition, a conservative selection of Lv ≥ 3 mH can meet the system stability requirements.

[0129] Case 2: Maximum Number of Units Accepted in a Multi-Unit Parallel System Further consider scenarios where multiple grid-connected energy storage converters are connected in parallel to the same common coupling point. The system topology in Case 2 is as follows: Figure 13 As shown. The grid connection point short-circuit capacity is 100 kVA, the rated line voltage is 380 V, and the system frequency is 50 Hz. All converters use the same VSG control parameters, and each converter has a rated capacity of 15 kVA. Based on the short-circuit capacity calculation, the equivalent series reactance of the external power grid is approximately 4.6 mH.

[0130] Ignoring device differences and communication delays, the equivalent port impedance of a multi-unit parallel system can be calculated based on the sequence impedance of a single grid-connected energy storage converter. By changing the number of parallel converters N and the virtual impedance Lv, the Nyquist curve of the system's secondary loop gain can be calculated, allowing for the evaluation of stability margins under different configurations. For parallel systems of multiple GFM / GFL converters, existing research has also shown that different control structures and network impedances significantly affect the system's interactive stability.

[0131] The positive-sequence Nyquist plot of the system secondary loop gain in Case 2 is shown below. Figure 14 As shown in the figure. The analysis results show that when Lv=0mH, the system can hardly meet the stability requirements under the condition of a single unit connected to the grid; when Lv=6mH, the single unit connected to the grid is near the critical stability; when Lv=9mH, the single unit connected to the grid is stable, but as the number of parallel units increases, the system stability margin decreases, and the two units connected in parallel are close to the critical state; when Lv is further increased to 12mH, the system can remain stable under more parallel configurations.

[0132] Semi-physical experiments further verified the above stability boundary, such as Figure 15 As shown. For a two-unit parallel system, the smaller virtual impedance ( Figure 15 (a) can lead to instability or low-frequency oscillations. Figure 15 (b) Increasing the virtual impedance improves the system stability margin. For a three-unit parallel system, the stability margin is insufficient when Lv = 9 mH. Figure 15(c) The output current and power exhibit significant oscillations; when Lv=12 mH, the three grid-type energy storage converters can achieve stable parallel operation. Figure 15 (d) This result shows that as the scale of parallel grid-connected energy storage converters expands, simply ensuring the stability of a single unit connected to the grid is not enough to guarantee the stability of the system after expansion. It is necessary to redesign the virtual impedance in combination with the short-circuit capacity of the grid connection point and the number of units connected in parallel.

[0133] In this example embodiment, the stability analysis of newly added grid-connected sources under the black-box microgrid expansion condition includes: In practical microgrid expansion scenarios, the external network is often not simply line impedance plus an ideal grid, but rather composed of multiple power electronic sources, line impedances, and loads. If the internal control structure, line parameters, and load parameters of the existing power sources are unknown, traditional modeling methods struggle to accurately obtain their equivalent models. In this case, the online impedance scanning method proposed in this invention can be used to directly measure the port impedance of the black-box network at the point of common coupling, and then combined with the sequence impedance model of the newly added grid-connected energy storage converter for active stability assessment. This approach shares the same engineering goals as impedance estimation methods for unknown or data-driven inverter resource scenarios.

[0134] This invention considers a microgrid consisting of two power electronic power sources, such as... Figure 16 As shown. Initially, the system consists only of power sources G1 and G2, which are connected to the microgrid through unknown line impedances. To improve power supply capacity, a new grid-connected energy storage converter is planned to be added near the common coupling point. Since the control structures and line parameters of G1 and G2 are unknown, the energy storage converter first uses grid-connected mode as a current disturbance source to perform an online scan of the PCC, obtaining the equivalent PCC impedance of the black-box network; then, this impedance is compared with the known sequence impedance of the grid-connected energy storage converter to determine the stability after the new grid source is connected.

[0135] Scan results as follows Figure 17 As shown, within the main impedance interaction frequency band, the phase difference between the PCC impedance and the grid-type energy storage converter impedance does not exceed 180°, indicating that the system has sufficient stability margin. Subsequently, the energy storage converter is switched or connected to grid mode.

[0136] Figure 18 The semi-physical waveforms shown indicate that neither the PCC voltage nor the energy storage converter output current exhibited divergent oscillations at the moment of connection, demonstrating stable system operation. This result verifies the applicability of the proposed online impedance measurement method and the sequence impedance-based active stability assessment method in black-box microgrid expansion scenarios.

[0137] In this example embodiment, the active stability assessment and engineering design process includes: Based on the aforementioned modeling, scanning, and experimental results, the active stability assessment and parameter design process for grid-type energy storage converters can be summarized as follows: 1. Determine the main circuit parameters, control parameters, operating power, and candidate virtual impedance range of the grid-type energy storage converter.

[0138] 2. Establish positive and negative sequence impedance models that take into account frequency coupling effects, and verify the accuracy of the models through simulation or sweep tests.

[0139] 3. If the external power grid parameters are known, calculate Z_g(s) based on the short-circuit capacity and line impedance; if the external network parameters are unknown, use the online impedance scanning method to measure the PCC equivalent impedance.

[0140] 4. Construct positive-sequence and negative-sequence second-loop gains respectively, and conduct active stability assessment using the Nyquist criterion or phase margin criterion.

[0141] 5. For a single-unit grid-connected system, scan the virtual impedance parameters to determine the minimum virtual impedance that meets the stability margin requirements.

[0142] 6. For multi-machine parallel systems, repeat stability tests under different numbers of parallel units to determine the maximum number of units that can be accepted under a given grid connection point short-circuit capacity.

[0143] 7. Verify boundary conditions by combining semi-physical experiments or field small-disturbance tests to avoid overly optimistic stability margin estimates due to model simplification, sampling delay, or parameter deviation.

[0144] The advantage of this process is that it can provide parameter design direction using analytical models and handle black-box network access problems through online scanning, making it suitable for grid connection evaluation of energy storage power stations, microgrid expansion, and capacity planning of multi-unit parallel operation.

[0145] In this example embodiment, the present invention proposes an active stability assessment method for grid-type energy storage converters that considers frequency coupling effects and online impedance measurement, and verifies it by combining sequence impedance modeling, online impedance scanning, and semi-physical experiments. The main conclusions are as follows: 1. When a grid-type energy storage converter is subjected to a single-frequency disturbance, it will generate disturbance frequency and coupling frequency responses, and there is a coupling relationship between the positive and negative sequence impedances. The sequence impedance model established after considering frequency coupling can more accurately describe the port characteristics of the grid-type energy storage converter and can be used for subsequent stability assessment and parameter optimization.

[0146] 2. Virtual impedance has a significant impact on the positive sequence impedance characteristics of grid-connected energy storage converters. Appropriately increasing the virtual impedance can improve the stability margin under weak grid conditions, but excessive virtual impedance will reduce the dynamic response speed. Therefore, a compromise design is required based on the short-circuit capacity and stability margin requirements at the grid connection point.

[0147] 3. For multi-unit parallel systems, the stability of a single unit connected to the grid does not guarantee stability after parallel expansion. Sequence impedance-based analysis methods can quantitatively assess the constraints between virtual impedance, the number of units connected in parallel, and the short-circuit capacity at the grid connection point, providing a basis for planning the parallel capacity of energy storage power stations.

[0148] 4. The online impedance scanning method based on current disturbance injection and phase-locked loop synchronization detection can accurately obtain the PCC equivalent impedance of unknown networks. For black-box microgrid expansion scenarios, this method can be combined with the sequence impedance model of grid-connected energy storage converters to achieve active stability assessment before the addition of new grid sources.

[0149] It should be noted that although the steps of the method in this disclosure are described in a specific order in the accompanying drawings, this does not require or imply that the steps must be performed in that specific order, or that all the steps shown must be performed to achieve the desired result. Additional or alternative steps may be omitted, multiple steps may be combined into one step, and / or a step may be broken down into multiple steps.

[0150] Furthermore, in this example embodiment, an active stability assessment device for a grid-type energy storage converter is also provided. (Refer to...) Figure 19 As shown, the active stability assessment device 200 for a grid-type energy storage converter may include: a modeling module 210, a scanning module 220, an assessment module 230, and a decision module 240. Wherein: Modeling module 210 is used to establish a sequence impedance model considering frequency coupling effect, analyze the mapping relationship between disturbance frequency and coupling frequency of grid-type energy storage converter, obtain positive and negative sequence coupling impedance matrix, and convert the coupling impedance matrix into equivalent single-input single-output sequence impedance. The scanning module 220 is used to perform online impedance scanning of the common coupling point based on current disturbance injection and phase-locked synchronization detection. It obtains the equivalent port impedance of the object under test at the common coupling point by means of reference phasor subtraction, positive and negative sequence phasor extraction and complex impedance calculation. Evaluation module 230 is used to construct the sub-loop gain of the positive sequence channel and the negative sequence channel based on the equivalent single-input single-output sequence impedance and the equivalent port impedance, and evaluate the small-signal stability of the interconnect system using stability criteria. The decision module 240 is used to determine the virtual impedance parameter value of the grid-type energy storage converter, the maximum number of units that can be accepted in the multi-machine parallel system, or the stability boundary of the newly added grid source access in the black box microgrid, based on the stability assessment results.

[0151] The specific details of each of the above-mentioned active stability assessment device modules for grid-type energy storage converters have been described in detail in the corresponding active stability assessment method for grid-type energy storage converters, so they will not be repeated here.

[0152] It should be noted that although several modules or units of a grid-type energy storage converter active stability assessment device 200 have been mentioned in the detailed description above, this division is not mandatory. In fact, according to embodiments of this disclosure, the features and functions of two or more modules or units described above can be embodied in one module or unit. Conversely, the features and functions of one module or unit described above can be further divided and embodied by multiple modules or units.

[0153] Furthermore, the above figures are merely illustrative of the processes included in the method according to exemplary embodiments of the present invention, and are not intended to be limiting. It is readily understood that the processes shown in the above figures do not indicate or limit the temporal order of these processes. Additionally, it is readily understood that these processes may be executed synchronously or asynchronously, for example, in multiple modules.

[0154] Other embodiments of this disclosure will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This application is intended to cover any variations, uses, or adaptations of this disclosure that follow the general principles of this disclosure and include common knowledge or customary techniques in the art not disclosed herein. The specification and embodiments are to be considered exemplary only, and the true scope and spirit of this disclosure are indicated by the claims.

[0155] It should be understood that this disclosure is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of this disclosure is limited only by the appended claims.

Claims

1. A method for active stability assessment of grid-type energy storage converters, characterized in that, The method includes: A sequence impedance model considering frequency coupling effect is established, the mapping relationship between disturbance frequency and coupling frequency of grid-type energy storage converter is analyzed, positive and negative sequence coupling impedance matrices are obtained, and the coupling impedance matrix is ​​converted into equivalent single-input single-output sequence impedance. Based on current disturbance injection and phase-locked synchronous detection, online impedance scanning is performed on the common coupling point. The equivalent port impedance of the object under test at the common coupling point is obtained by reference phasor subtraction, positive and negative sequence phasor extraction and complex impedance calculation. Based on the equivalent single-input single-output sequence impedance and the equivalent port impedance, the sub-loop gain of the positive-sequence channel and the negative-sequence channel are constructed respectively, and the small-signal stability of the interconnect system is evaluated using the stability criterion. Based on the stability assessment results, determine the virtual impedance parameter values ​​of the grid-type energy storage converter, the maximum number of units that can be accepted in a multi-unit parallel system, or the stability boundary for the access of new grid-type sources in a black-box microgrid.

2. The method as described in claim 1, characterized in that, In the method, the mapping relationship between the disturbance frequency and the coupling frequency is as follows: in, f dist Let be the perturbation frequency in the stationary coordinate system. f 1 represents the fundamental frequency. f cpl The corresponding coupling frequency is; the positive and negative sequence coupling impedance matrix is: in, s p = j ωp Let be the positive-sequence perturbation complex frequency in the stationary coordinate system. s n = s p 2 j ωn This corresponds to the negative-sequence coupling frequency. ω n The rated angular frequency, Z pp ( s p ) Z pn ( s p ) Z np ( s n ) Z nn ( s n ) are the four elements of the positive and negative order coupling impedance matrix, respectively.

3. The method as described in claim 2, characterized in that, The equivalent single-input single-output sequence impedance includes the equivalent positive-sequence output impedance and the equivalent negative-sequence output impedance, wherein the equivalent positive-sequence output impedance is: The equivalent negative sequence output impedance is: in, Z g ( s ) represents the equivalent impedance of the external power grid or microgrid at the common coupling point.

4. The method as described in claim 1, characterized in that, The phase-locked synchronization detection includes: The complex variables obtained after Clarke transformation of the three-phase sampled quantities X αβ = x α + jx β For the current testing frequency f test Synchronization angle is θ =2 πf test t The positive-sequence perturbation phasor and the negative-sequence perturbation phasor are respectively passed through e jθ and e jθ Rotational extraction: in, X p The real and imaginary parts correspond to the in-phase and quadrature components of the positive sequence quantity, respectively. X n The real and imaginary parts correspond to the in-phase and quadrature components of the negative sequence quantity, respectively; the formula for calculating the sequence impedance of the measured object is: Where, Δ V and Δ I These are the disturbance voltage and disturbance current after deducting the reference value.

5. The method as described in claim 1, characterized in that, The secondary loop gain of the positive sequence channel in the method is: The secondary loop gain of the negative sequence channel is: The stability criterion adopted is the Nyquist criterion, when... T p ( s ) and T n ( s The Nyquist curves of ) do not enclose ( 1, j At point 0), the system meets the impedance stability requirements; Bode diagrams are used for project evaluation, in | Z vsg ( jω )∣=∣ Z g ( jω At the frequency of |, the phase difference between the two satisfies: 。 6. The method as described in claim 1, characterized in that, For a stand-alone grid-connected system, the minimum virtual impedance that meets the stability margin requirement is determined by changing the virtual impedance parameter and repeating the stability assessment; wherein the virtual impedance is a virtual resistance. R v and virtual reactance X v The combination of the elements of the positive and negative order coupling impedance matrix Z pp ( s p In the expression, intermediate variables H vp ( s p ) for: in, G vPI ( λ p ) is the transfer function for the voltage loop PI controller. G iPI ( λ p ) is the transfer function for the current loop PI controller. λ p = s p jω n This is the frequency after the positive-sequence disturbance enters the synchronous rotating coordinate system.

7. The method as described in claim 1, characterized in that, For a multi-unit parallel system, assuming N grid-type energy storage converters with identical parameters are connected in parallel, the equivalent port impedance of the parallel system is converted to the impedance of a single unit. By changing the number of parallel units N and recalculating the sub-loop gain, the maximum number of grid-type energy storage converters that the system can be connected to under a given grid connection point short-circuit capacity is evaluated.

8. The method as described in claim 1, characterized in that, For the black-box microgrid expansion scenario, the object under test is an existing power electronic power supply network with unknown control structure and line parameters; The grid-type energy storage converter first uses the grid-following mode as the current disturbance source to perform online scanning of the common coupling point to obtain the equivalent impedance of the black-box network. Then, combined with the sequence impedance model of the grid-type energy storage converter, the stability after the addition of a new grid source is determined.

9. The method as described in claim 1, characterized in that, The grid-type energy storage converter adopts virtual synchronous generator control, including active-frequency control, reactive-voltage control, voltage and current dual loop, virtual impedance link, coordinate transformation and modulation link; The main circuit consists of a DC-side energy storage unit, a three-phase bridge arm, an inductor-capacitor filter, and a grid connection port. The online impedance scanning is implemented within a digital signal processor controller and includes perturbation generation, reference phasor subtraction, positive and negative sequence phasor extraction, and impedance calculation.

10. An active stability assessment device for a grid-type energy storage converter, characterized in that, The device includes: The modeling module is used to establish a sequence impedance model that considers the frequency coupling effect, analyze the mapping relationship between the disturbance frequency and the coupling frequency of the grid-type energy storage converter, obtain the positive and negative sequence coupling impedance matrices, and convert the coupling impedance matrix into an equivalent single-input single-output sequence impedance. The scanning module is used to perform online impedance scanning of the common coupling point based on current disturbance injection and phase-locked synchronization detection. It obtains the equivalent port impedance of the object under test at the common coupling point by means of reference phasor subtraction, positive and negative sequence phasor extraction and complex impedance calculation. The evaluation module is used to construct the sub-loop gain of the positive-sequence channel and the negative-sequence channel based on the equivalent single-input single-output sequence impedance and the equivalent port impedance, and evaluate the small-signal stability of the interconnect system using stability criteria. The decision module is used to determine the virtual impedance parameter value of the grid-type energy storage converter, the maximum number of units that can be accepted in a multi-unit parallel system, or the stability boundary of the newly added grid source access in a black-box microgrid, based on the stability assessment results.

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