A micro-grid trans-layer harmonic treatment method and system based on energy storage converter

Abstract

This invention relates to the field of distribution network technology, specifically including a method and system for cross-level harmonic mitigation in distribution microgrids based on energy storage converters. The method includes: determining the topology of a high-voltage side distribution network and a low-voltage side microgrid, and setting up an energy storage grid-connected converter. The energy storage grid-connected converter extracts the three-phase current from the low-voltage side and maps the three-phase current from the low-voltage side to current components in the synchronous rotating coordinate system of the high-voltage side. Harmonic information aligned with the harmonic phase of the high-voltage side is extracted from the current components to obtain a reference compensation current. A dual-vector modulation model predictive control strategy is used to achieve rapid tracking and accurate output of the reference compensation current. The energy storage converter of this invention can dynamically adjust the harmonic compensation current while completing power dispatch tasks, without requiring additional APF equipment, significantly reducing mitigation costs.

Description

Technical Field

[0001] This invention relates to the field of power distribution network technology, and in particular to a method and system for cross-level harmonic mitigation in distribution microgrids based on energy storage converters. Background Technology

[0002] As the penetration rate of new energy power generation in the power system continues to increase, the power quality problem of distribution microgrids is becoming increasingly prominent. Distributed power sources such as photovoltaic and wind power are connected to the grid through inverters. Their nonlinear characteristics and the impactful changes in load lead to aggravated harmonic pollution. Especially in complex systems with multiple voltage levels (such as 10kV distribution network and 380V microgrid), harmonics may be amplified through transformer coupling, affecting power quality and the safe operation of equipment. Traditional mitigation methods are often limited to a single voltage level, making it difficult to achieve cross-level coordinated mitigation. The intermittent power generation of new energy sources further exacerbates the difficulty of harmonic mitigation, necessitating a new solution that can balance system stability and power quality.

[0003] Existing harmonic suppression methods mainly rely on installing independent active power filters (APFs) or static var compensators (SVCs). While these methods can locally improve harmonic performance, they suffer from limitations such as high equipment investment and low capacity utilization. Especially in microgrids, power storage converters (PCSs) are typically used only for power balancing and frequency and voltage regulation, with their redundant capacity not being fully utilized. Furthermore, the coverage of independent APFs is limited by the voltage level of the installation point, making it impossible to achieve cross-level harmonic collaborative suppression between the 10kV distribution network and the 380V microgrid. In addition, when renewable energy output fluctuates, the dynamic response capability of traditional harmonic suppression equipment is insufficient, potentially leading to a decrease in harmonic suppression effectiveness and making it difficult to meet the power quality requirements of microgrids with a high proportion of renewable energy integration. Summary of the Invention

[0004] Purpose of the invention: Based on the problems existing in the prior art, the present invention proposes a method for cross-level harmonic mitigation of distribution microgrids based on energy storage converters, and a system for cross-level harmonic mitigation of distribution microgrids based on energy storage converters.

[0005] Technical solution: On the one hand, this invention provides a method for cross-level harmonic mitigation in distribution microgrids based on energy storage converters, the method comprising: The topology of a certain high-voltage side distribution network and low-voltage side microgrid is determined, and an energy storage grid-connected converter is set up. The energy storage grid-connected converter extracts the three-phase current of the low-voltage side and maps the three-phase current of the low-voltage side to the current component in the synchronous rotating coordinate system of the high-voltage side. Harmonic information aligned with the harmonic phase of the high-voltage side is extracted from the current component to obtain the reference compensation current. An improved dual-vector modulation model predictive control strategy is adopted to achieve rapid tracking and accurate output of the reference compensation current. This control strategy uses the reference compensation current as the reference target and constructs a value function using the reference compensation current. The switching signal of the converter is generated through prediction and optimization, thereby driving the energy storage grid-connected converter to output a compensation current that is opposite to the phase of the actual harmonics.

[0006] Furthermore, including: The process of mapping the three-phase current on the low-voltage side to the current components in the synchronous rotating coordinate system on the high-voltage side includes: A phase-locked loop structure is used to lock the high-voltage and low-voltage side voltages respectively, and the phase estimates of the high-voltage side and the low-voltage side under steady state are obtained respectively. Based on the dynamic tracking error of the phase-locked loop, the phase error of the high-voltage side phase-locked loop and the phase error of the low-voltage side phase-locked loop are determined, thereby obtaining the expression of the phase difference estimate; The estimated phase difference is low-pass filtered to obtain a smoothed corrected phase difference. The smoothed corrected phase difference is differentiated in the time domain, and the correction angle for coordinate transformation is determined based on the fixed phase shift of the transformer. The correction angle is substituted into the transformation formula, which is obtained based on the three-phase current on the low-voltage side and the corrected transformation matrix. The corrected transformation matrix is ​​determined by the correction angle.

[0007] Furthermore, including: The method employs a phase-locked loop structure to lock the high-voltage and low-voltage sides separately, obtaining steady-state phase estimates for the high-voltage side and the low-voltage side, respectively, including: Voltage signal models for the high-voltage side and the low-voltage side are constructed. Based on the three-phase voltage values ​​obtained from the high-voltage side voltage signal model, the voltage components of the α-axis and β-axis of the high-voltage side are obtained through Clark transformation, and then the voltage components of the d-axis and q-axis of the high-voltage side are obtained. Using the voltage component of the q-axis as the error signal, the output frequency deviation is adjusted so that the voltage component of the q-axis approaches zero. The high-voltage side phase estimate is obtained by integrating the output frequency deviation, thereby obtaining the high-voltage side phase estimate in steady state and determining the low-voltage side phase estimate in steady state.

[0008] Furthermore, including: The dynamic tracking error based on the phase-locked loop determines the phase error of the high-voltage side phase-locked loop and the phase error of the low-voltage side phase-locked loop, thereby obtaining the expression of the phase difference estimate, including: Considering the dynamic tracking error of the phase-locked loop, let the phase error of the high-voltage side phase-locked loop be expressed as... ,in, When it is unsteady t The estimated phase value of the high-voltage side at time [time]. The angular frequency of the high-voltage side phase-locked loop. This represents the initial phase of the high-voltage side phase-locked loop; The phase error of the low-voltage side phase-locked loop is expressed as: The phase difference estimate is expressed as: ; in, This represents the initial phase of the low-voltage side phase-locked loop. This is the phase difference between the high-voltage side and the low-voltage side phase-locked loop; When the system frequency is synchronized hour, It is a constant. Let be the angular frequency of the low-voltage side phase-locked loop; therefore, the estimated phase difference is expressed as: .

[0009] Furthermore, including: The process of low-pass filtering the estimated phase difference to obtain a smoothed corrected phase difference, performing time-domain differentiation on the smoothed corrected phase difference, and determining the correction angle for coordinate transformation based on the fixed phase shift of the transformer includes: Phase difference estimate Low-pass filtering is performed to obtain a smoothed corrected phase difference. : , The filtering time constant is Let be the complex frequency variable in the Laplace transform; the time-domain derivative of the smoothed corrected phase difference is expressed as: The correction angle is expressed as: ;in, This refers to the fixed phase shift present in a transformer.

[0010] Furthermore, including: Substituting the correction angle into the transformation formula includes: The transformation formula is expressed as follows: ;in, , These are the three-phase current components on the low-voltage side. This is the corrected transformation matrix, and ; Transformed These are the d-axis and q-axis current components in the synchronous rotating coordinate system of the high-voltage side, which contain harmonic information aligned with the phase of the high-voltage side harmonics. The corrected transformation matrix is ​​expressed as:

[0011] in, This is the low-pressure side phase. and These represent the two coefficients of the Park transformation matrix.

[0012] Furthermore, including: The method for obtaining the high-voltage side phase estimate in steady state includes: The output frequency deviation obtained using a PI controller is expressed as: ;in, and These are the proportional gain coefficient and integral gain coefficient of the PI control. The voltage component along the q-axis of the high-voltage side is integrated to obtain the phase estimate: ,in, The rated angular frequency, in steady state Then the phase estimate is expressed as , The angular frequency of the high-voltage side phase-locked loop. This represents the initial phase of the high-voltage side phase-locked loop.

[0013] Furthermore, including: The value function constructed using the reference compensation current is expressed as: ;in, For energy storage grid-connected converters in k+ The current value at time 1, This is the reference compensation current.

[0014] Secondly, the present invention also provides a cross-level harmonic mitigation system for distribution microgrids based on energy storage converters, the system comprising: The reference compensation current determination module is used to determine the topology of a certain high-voltage side distribution network and low-voltage side microgrid, and to set up an energy storage grid-connected converter. The energy storage grid-connected converter extracts the three-phase current of the low-voltage side and maps the three-phase current of the low-voltage side to the current component in the synchronous rotating coordinate system of the high-voltage side. Harmonic information aligned with the harmonic phase of the high-voltage side is extracted from the current component to obtain the reference compensation current. The prediction module is used to achieve fast tracking and accurate output of the reference compensation current by adopting an improved dual-vector modulation model predictive control strategy. This control strategy takes the reference compensation current command as the reference target and uses the reference compensation current to construct a value function. It generates the switching signal of the converter through prediction optimization, thereby driving the energy storage grid-connected converter to output a compensation current that is opposite to the phase of the actual harmonics.

[0015] Thirdly, the present invention also provides a computer-readable storage medium storing a computer program, which, when executed by a processor, implements the steps of the cross-level harmonic mitigation method for distribution microgrids based on energy storage converters as described in the first aspect.

[0016] Beneficial effects: Compared with the prior art, the present invention has the following advantages: In one aspect, the energy storage converter, while completing power dispatch tasks, also dynamically adjusts the harmonic compensation current without requiring additional APF equipment, significantly reducing governance costs. In another aspect, through harmonic detection methods and dual-vector modulation model predictive control strategies, the energy storage system maintains the power balance of the microgrid while utilizing its rapid response characteristics to accurately suppress multi-level harmonics, thereby improving power quality and operational stability at the system level. This provides a new, economical, and efficient approach to harmonic governance for distribution microgrids with a high proportion of renewable energy integration. The harmonic detection method is applicable to multi-level microgrids and distribution networks. In multi-level systems, there is a phase difference between the high-voltage and low-voltage sides, causing the detected harmonic phase to deviate from the actual harmonics on the high-voltage side. If compensation current is directly generated based on this, effective cancellation cannot be achieved, and it may even lead to harmonic amplification or resonance. Therefore, to extract high-voltage side harmonics, the low-voltage side current needs to be mapped to the high-voltage side coordinate system to accurately extract multi-level harmonic components. Attached Figure Description

[0017] Figure 1 This is a block diagram of the cross-level harmonic compensation principle structure of a distribution microgrid based on an energy storage converter, as described in an embodiment of the present invention. Figure 2 This is a flowchart of the cross-level harmonic mitigation method for distribution microgrids based on energy storage converters, as described in an embodiment of the present invention. Figure 3 This is as described in the embodiments of the present invention. Block diagram of the method for detecting harmonic currents in power grids; Figure 4 is a schematic diagram of the grid current waveform at a voltage level of 10kV according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the grid-connected current waveform after compensation by this method, as described in an embodiment of the present invention; Figure 6 This is a schematic diagram of the APF output current tracking effect according to an embodiment of the present invention. Detailed Implementation

[0018] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0019] Current harmonic mitigation methods rely on independent APF / SVC equipment, which suffers from high investment costs and low utilization rates. The redundant capacity of energy storage converters (PCS) is not fully utilized, and traditional methods do not consider cross-level mitigation (e.g., 10kV / 380V). Traditional equipment fails to respond adequately to renewable energy fluctuations, affecting harmonic suppression effectiveness. Existing methods are insufficient to meet the power quality requirements of high-proportion renewable energy integration.

[0020] The purpose of this embodiment is to achieve dynamic cross-level harmonic compensation through the redundant capacity of the PCS (Power Conversion System) in an energy storage system, thereby reducing the cost of grid harmonic mitigation and improving power quality. Specifically, this embodiment establishes a cross-level grid model of the energy storage converter containing an Active Power Filter (APF). It employs a finite control set-model predictive control (dual vector modulation model predictive control) algorithm to generate compensation current commands based on the target level harmonic signal, dynamically converting the remaining capacity of the PCS into the harmonic compensation capability of the APF. Compared with traditional methods, this scheme does not require additional independent active power filters. While meeting the basic power requirements of the energy storage system, it effectively mitigates grid harmonics by injecting harmonic compensation components, significantly improving the utilization rate of energy storage devices and achieving synergistic optimization and improvement of grid power quality. Therefore, this scheme has the following advantages: This solution can reduce harmonic mitigation costs: by utilizing the redundant capacity of the energy storage converter (PCS) to achieve cross-level harmonic compensation, there is no need to configure an additional independent active power filter (APF), thus reducing equipment investment and maintenance costs. This solution can also improve energy storage utilization: while meeting the basic power requirements of the energy storage system, it dynamically allocates remaining capacity for harmonic mitigation, improving the overall utilization rate of energy storage equipment and optimizing power grid quality.

[0021] In distribution microgrid systems, harmonic sources such as nonlinear loads and power electronic devices are widespread, and the harmonic currents they inject into the grid severely affect power quality. To achieve efficient harmonic mitigation, it is necessary to overcome the limitations of traditional single-voltage-level methods. The schematic diagram of a cross-level harmonic mitigation method for distribution microgrids using energy storage converters is shown below. Figure 1 As shown, this example uses two different grid voltage levels: a 10kV distribution network and a 380V microgrid.

[0022] like Figure 2 As shown, this embodiment provides a method for cross-level harmonic mitigation in distribution microgrids based on energy storage converters. This method is centered on a closed-loop logic of "detection-control-output-compensation". First, the power grid signal is acquired by the current sensor. The "current component extraction" module separates the fundamental wave, harmonics and reactive components, accurately identifies the harmonic current to be compensated, and generates a reference compensation current command in opposite phase. Subsequently, under the dual-vector modulation model predictive control drive, the energy storage converter performs real-time prediction and optimization of various switching states based on the system mathematical model, selects the optimal switching combination, and quickly generates accurate drive signals, enabling the converter output current to track the reference command with high dynamics and low error. The converter relies on the DC support of the energy storage system to output the required compensation current after completing the DC-AC conversion. The high-frequency switching ripple is filtered out by the LR filter to ensure that the current injected into the grid has high purity. Ultimately, the compensation current enters the high-voltage distribution network through a 380V / 10kV step-up transformer, at the line impedance... At the common coupling point, the distorted current generated by the harmonic source is superimposed and canceled out by the distorted current with similar amplitude and opposite phase, effectively suppressing harmonic propagation according to Kirchhoff's current law.

[0023] The specific implementation methods are as follows: Step S1: Determine the topology of a certain high-voltage side distribution network and low-voltage side microgrid, and set up an energy storage grid-connected converter. The energy storage grid-connected converter extracts the three-phase current of the low-voltage side and maps the three-phase current of the low-voltage side to the current component in the synchronous rotating coordinate system of the high-voltage side. Extract the harmonic information that is aligned with the harmonic phase of the high-voltage side from the current component, thereby obtaining the reference compensation current.

[0024] In this embodiment, the energy storage grid-connected converter only needs to collect the current and grid connection point voltage of its grid-connected branch, extract the harmonic current component at the grid connection point to generate the power quality compensation current, and then obtain the current signal of the energy storage system through predictive control current control using a dual-vector modulation model. This enables the energy storage converter PCS to achieve multi-functional grid connection and participate in the power quality management of the power grid. When the remaining capacity of the energy storage converter PCS is sufficient and the state of charge is within the normal range, the energy storage can fully compensate for the reactive power and harmonic current at the grid connection point. Even if the required compensation capacity is insufficient for complete compensation, the harmonic distortion of the power grid will still be improved, making full use of its remaining value.

[0025] In a preferred embodiment, it is assumed that multiple harmonics suddenly increase at the 380V grid connection point, and this node is not equipped with a dedicated harmonic compensation device. The harmonics will spread to the 10kV distribution network through the transformer. At this time, after the monitoring system detects the harmonics, the energy storage converter in another 380V microgrid responds dynamically. While ensuring the basic charging and discharging function of the energy storage, it uses its redundant capacity to perform harmonic mitigation, achieving cross-level compensation from 380V to 10kV.

[0026] To make the compensation current easier to understand, this embodiment first introduces the relevant content of compensation current detection. Compensation current detection is the foundation for realizing reactive power and harmonic compensation in PCS. Current detection is performed using a harmonic detection method based on instantaneous reactive power theory. The three-phase current can be expressed as:

[0027] In the formula, , k It is an integer. The effective value of each harmonic current. Let be the phase angle of each harmonic current.

[0028] By performing a Park transformation on the load current, we can obtain:

[0029] In the formula, This represents the instantaneous value of the active component. This represents the instantaneous value of the reactive component. and This represents the Park transformation matrix.

[0030] and The fundamental active and reactive currents can be obtained after passing through a low-pass filter:

[0031] The fundamental current component can be obtained as follows:

[0032] in, and Represents the inverse Park transformation matrix. and These are the three-phase fundamental current components.

[0033] Therefore, harmonic current components and It can be represented as:

[0034] To achieve harmonic and reactive power compensation in an energy storage converter, the compensation current and harmonic current must exactly cancel each other out, resulting in an ideal output waveform. In a rotating coordinate system, the reactive current is reversed to obtain the reactive current compensation command current. The active current passes through a low-pass filter to obtain the fundamental current. The harmonic active current is then extracted, reversed, and used to obtain the harmonic compensation command current. Finally, the compensation command current is synthesized. ,like Figure 3 As shown, it is about Schematic diagram of current detection principle.

[0035] In this embodiment, mapping the three-phase current on the low-voltage side to the current components in the synchronous rotating coordinate system on the high-voltage side includes: A phase-locked loop structure is used to lock the high-voltage and low-voltage side voltages respectively, and the phase estimates of the high-voltage side and the low-voltage side under steady state are obtained respectively. Based on the dynamic tracking error of the phase-locked loop, the phase error of the high-voltage side phase-locked loop and the phase error of the low-voltage side phase-locked loop are determined, thereby obtaining the expression of the phase difference estimate; The estimated phase difference is low-pass filtered to obtain a smoothed corrected phase difference. The smoothed corrected phase difference is differentiated in the time domain, and the correction angle for coordinate transformation is determined based on the fixed phase shift of the transformer. The correction angle is substituted into the transformation formula, which is obtained based on the three-phase current on the low-voltage side and the corrected transformation matrix. The corrected transformation matrix is ​​determined by the correction angle.

[0036] In this embodiment, a phase-locked loop (PLL) structure is used to lock the high-voltage and low-voltage sides separately, obtaining steady-state phase estimates for the high-voltage side and the low-voltage side, respectively, including: Voltage signal models for the high-voltage side and the low-voltage side are constructed. Based on the three-phase voltage values ​​obtained from the high-voltage side voltage signal model, the voltage components of the α-axis and β-axis of the high-voltage side are obtained through Clark transformation, and then the voltage components of the d-axis and q-axis of the high-voltage side are obtained. Using the voltage component of the q-axis as the error signal, the output frequency deviation is adjusted so that the voltage component of the q-axis approaches zero. The high-voltage side phase estimate is obtained by integrating the output frequency deviation, thereby obtaining the high-voltage side phase estimate in steady state and determining the low-voltage side phase estimate in steady state.

[0037] Specifically, in this embodiment, the traditional The method depends on the local voltage phase. Conduct testing.

[0038] In multi-level systems, there is a phase difference between the high-voltage and low-voltage sides. This causes the detected harmonic phase to deviate from the actual harmonics on the high-voltage side. If this is used directly to generate a compensation current, effective cancellation will not be achieved, and it may even lead to harmonic amplification or resonance. The dq transformation matrix of a traditional three-phase system is:

[0039] If the low-pressure side phase is directly adopted If the low-voltage side current is transformed, then the transformed current will be... d , q The shaft current component reflects the current component in the low-voltage side coordinate system, and it is in phase with the harmonics on the high-voltage side. The deviation.

[0040] To extract high-voltage side harmonics, the low-voltage side current needs to be mapped to the high-voltage side coordinate system.

[0041] Define the dq transformation matrix of the corrected three-phase system:

[0042] Right now: .

[0043] For the three-phase current on the low-voltage side Perform the transformation:

[0044] Therefore, after transformation These are the d-axis and q-axis current components in the synchronous rotating coordinate system of the high-voltage side, which contain harmonic information aligned with the phase of the high-voltage side harmonics.

[0045] In order to obtain To achieve dynamic phase correction, this embodiment employs a dual-phase-locked loop (DPLL) structure, which locks the high- and low-voltage sides separately to achieve phase synchronization and difference extraction: High-voltage side PLL: Input high-voltage side three-phase voltage Output phase ; Low-voltage side PLL: Input low-voltage side three-phase voltage Output phase .

[0046] High and low voltage side voltage signal models Assume the three-phase voltage on the high-voltage side (e.g., 10kV) is:

[0047] The three-phase voltage on the low-voltage side (e.g., 380V) is:

[0048] in, and These represent the voltage amplitudes on the high-voltage and low-voltage sides. The fundamental angular frequency, This is the initial phase.

[0049] During steady-state frequency synchronization But phase difference Caused by factors such as transformer connection group and line impedance, it may include fixed offset and slow time-varying components.

[0050] Phase-locked loop (PLL) dynamic model A software phase-locked loop (PLL) based on a Synchronous Reference System (SRF) is employed. Its structure includes a Clark transform, a Park transform, a PI controller, and an integrator. Taking the high-voltage side PLL as an example: Clark transformation (three-phase stationary abc → two-phase stationary αβ):

[0051] get:

[0052] in, and These are the voltage components along the α and β axes of the high-voltage side, respectively.

[0053] Park transformation (two-phase stationary αβ → two-phase rotating dq):

[0054] in, For phase estimation of PLL output, and These are the d-axis and q-axis voltage components on the high-voltage side, respectively. Substitute... :

[0055] Closed-loop control: PI controller As the error signal, adjust the output frequency deviation. ,make :

[0056] The phase estimate is obtained after integration:

[0057] in, The rated angular frequency, and These are the proportional gain coefficient and integral gain coefficient of the PI control, respectively. In steady state... ,but .

[0058] Similarly, low-voltage side PLL output phase estimation steady state .

[0059] Phase difference calculation and dynamic error analysis Define the phase difference estimate:

[0060] Under ideal steady state:

[0061] When the system frequency is synchronized hour, It is a constant.

[0062] Considering the dynamic tracking error of the PLL, let the phase error of the high-voltage side PLL be... The low-pressure side is ,but:

[0063] Error term Caused by voltage distortion, imbalance, and PLL bandwidth limitation.

[0064] Phase difference filtering and correction generation To suppress noise and transient fluctuations, Low-pass filtering is performed to obtain a smoothed corrected phase difference. :

[0065] The time-domain differential equation is:

[0066] Filtering time constant A trade-off needs to be struck between response speed and noise suppression capability. s is the complex frequency variable in the Laplace transform.

[0067] If the distribution transformer has a fixed phase shift If Yd11 introduces a 30° lag, then the actual correction angle used for coordinate transformation is:

[0068] in, It can be preset via offline measurement or nameplate data.

[0069] Corrected coordinate transformation When performing harmonic detection on the low-voltage side, use the corrected phase angle. Perform a Park transformation to map the low-voltage side current to the high-voltage side synchronous rotating coordinate system:

[0070] From this, we obtain Synchronized with high-voltage side harmonics, it can accurately extract cross-level harmonic components.

[0071] Because the relationship between the detected harmonics and the reference compensation current involves two signals with opposite amplitudes, the same phase, and the same frequency, the phase difference is known. When harmonics are detected on the low-voltage side (the waveform for mitigation; the reference current is used to cancel them out, and its amplitude is opposite to the reference current), the detection of harmonics on the low-voltage side means that a reference compensation current can be obtained. In order to mitigate harmonics on the high-voltage side, a phase bias is added in advance. (This ensures that the phase is the same as that of the high-voltage side, but the amplitude is opposite.)

[0072] The above improvements The method can extract harmonic current components in the power grid in real time and accurately, and generate corresponding reference compensation current commands. This command will serve as the input to the next stage of the control system, driving the energy storage converter to output the corresponding compensation current.

[0073] Step S2: An improved dual-vector modulation model predictive control strategy is adopted to achieve rapid tracking and accurate output of the reference compensation current. This control strategy uses the reference compensation current as the reference target and uses the reference compensation current to construct a value function. The switching signal of the converter is generated through prediction and optimization, thereby driving the energy storage grid-connected converter to output a compensation current that is opposite to the phase of the actual harmonics.

[0074] For the specific implementation of the dual vector modulation model predictive control strategy, please refer to "Guo Leilei, Li Guohao, Jin Nan, et al. Dual vector modulation model predictive control for two-level voltage source inverter: theoretical analysis, experimental verification and promotion [J]. Journal of Electrical Engineering, 2021, 36(1): 40-49".

[0075] Based on this paper, this application, in order to make the objective more explicit, modifies the original value function: , modified to ;in, For energy storage grid-connected converters in k+ The current value at time 1, This is used as a reference compensation current and applied in the current predictive control process. Furthermore, this modification is logically sound, and the derivation process is as follows: Because... ; therefore, ; Right now ; .

[0076] The motivation for this application is that in power systems, harmonics can propagate between grid connection points at different voltage levels, thus affecting the overall stability of the power grid. Due to the uncertainty of the location of harmonic sources, traditional centralized mitigation methods are insufficient to effectively address this issue. Therefore, it is necessary to rely on nearby grid-connected system equipment (such as energy storage converters with harmonic mitigation capabilities) for dynamic compensation to improve the coverage of harmonic mitigation and enhance the reliability of the power grid. Taking a 380V microgrid and a 10kV distribution network grid connection system as an example, such as... Figure 1 As shown.

[0077] Suppose a 5th harmonic suddenly appears at a 380V grid connection point, and this node is not equipped with a dedicated harmonic compensation device. The harmonics will spread to the 10kV distribution network through the transformer, as shown in Figure 4(a). At this time, after the monitoring system detects the harmonics, the energy storage converter in another 380V microgrid dynamically responds. While ensuring the basic charging and discharging functions of the energy storage, it utilizes its redundant capacity for harmonic mitigation, achieving cross-level compensation from 380V to 10kV. The effect after mitigation is shown in Figure 4(b), where the harmonic content of the 10kV distribution network is significantly reduced, leaving only the fundamental frequency component. Meanwhile, as... Figure 5 The total harmonic distortion (THD) was reduced to 0.22%, verifying the effectiveness of cross-level governance in energy storage converters. Although dynamic changes in the remaining capacity of energy storage converters can cause fluctuations in the compensation current amplitude, such as... Figure 6 As shown, it still plays a crucial role in harmonic suppression.

[0078] Therefore, this embodiment proposes a cross-level harmonic mitigation method based on the redundant capacity of energy storage converters. This method utilizes a local dynamic compensation mechanism to achieve harmonic mitigation at multiple voltage levels while ensuring the basic functions of the energy storage system, without requiring additional dedicated filtering equipment. Practical applications show that this method can stably control the harmonic content of the power grid within a safe range, significantly improving equipment utilization and grid operational reliability, while simultaneously reducing system upgrade costs.

[0079] Another invention provides a cross-level harmonic mitigation system for distribution microgrids based on energy storage converters, the system comprising: The reference compensation current determination module is used to determine the topology of a certain high-voltage side distribution network and low-voltage side microgrid, and to set up an energy storage grid-connected converter. The energy storage grid-connected converter extracts the three-phase current of the low-voltage side and maps the three-phase current of the low-voltage side to the current components in the synchronous rotating coordinate system of the high-voltage side. Harmonic information aligned with the harmonic phase of the high-voltage side is extracted from the current components to obtain the reference compensation current. The prediction module is used to achieve rapid tracking and accurate output of the reference compensation current by adopting a dual-vector modulation model predictive control strategy. This control strategy uses the reference compensation current as the reference target and uses the reference compensation current to construct a value function. It generates the switching signal of the converter through prediction optimization, thereby driving the energy storage grid-connected converter to output a compensation current that is opposite to the phase of the actual harmonics.

[0080] Other technical features of the cross-level harmonic mitigation system for distribution microgrids based on energy storage converters described in this embodiment are similar to the corresponding cross-level harmonic mitigation method for distribution microgrids based on energy storage converters, and will not be repeated here.

[0081] In the description of this invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of that feature. "A plurality of" means two or more, unless otherwise explicitly specified.

[0082] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0083] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0084] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0085] Any process or method description in the flowchart or otherwise herein can be understood as representing a module, segment, or portion of code comprising one or more executable instructions for implementing a particular logical function or process, and the scope of the preferred embodiments of the invention includes additional implementations in which functions may be performed not in the order shown or discussed, including substantially simultaneously or in reverse order depending on the functions involved, as will be understood by those skilled in the art to which embodiments of the invention pertain.

[0086] The logic and / or steps represented in the flowchart or otherwise described herein, for example, can be considered as a ordered list of executable instructions for implementing logical functions, and can be embodied in any computer-readable medium for use by, or in conjunction with, an instruction execution system, apparatus, or device (such as a computer-based system, a processor-included system, or other system that can fetch and execute instructions from, an instruction execution system, apparatus, or device). For the purposes of this specification, "computer-readable medium" can be any means that can contain, store, communicate, propagate, or transmit programs for use by, or in conjunction with, an instruction execution system, apparatus, or device. More specific examples (a non-exhaustive list) of computer-readable media include: an electrical connection having one or more wires (electronic device), a portable computer disk drive (magnetic device), random access memory (RAM), read-only memory (ROM), erasable and editable read-only memory (EPROM or flash memory), fiber optic devices, and portable optical disc read-only memory (CDROM). Alternatively, the computer-readable medium may be paper or other suitable media on which the program can be printed, since the program can be obtained electronically, for example, by optically scanning the paper or other medium, followed by editing, interpreting, or otherwise processing as necessary, and then stored in a computer memory.

[0087] It should be understood that various parts of the present invention can be implemented in hardware, software, firmware, or a combination thereof. In the above embodiments, multiple steps or methods can be implemented in software or firmware stored in memory and executed by a suitable instruction execution system. For example, if implemented in hardware, as in another embodiment, it can be implemented using any one or a combination of the following techniques known in the art: discrete logic circuits having logic gates for implementing logical functions on data signals, application-specific integrated circuits (ASICs) having suitable combinational logic gates, programmable gate arrays (PGAs), field-programmable gate arrays (FPGAs), etc.

[0088] Those skilled in the art will understand that all or part of the steps of the methods in the above embodiments can be implemented by a program instructing related hardware. The program can be stored in a computer-readable storage medium, and when executed, the program includes one or a combination of the steps of the method embodiments.

[0089] Furthermore, the functional units in the various embodiments of the present invention can be integrated into a processing module, or each unit can exist physically separately, or two or more units can be integrated into a module. The integrated module can be implemented in hardware or as a software functional module. If the integrated module is implemented as a software functional module and sold or used as an independent product, it can also be stored in a computer-readable storage medium.

[0090] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of the present invention.

Claims

1. A method for cross-level harmonic mitigation in distribution microgrids based on energy storage converters, characterized in that, The method includes: The topology of a certain high-voltage side distribution network and low-voltage side microgrid is determined, and an energy storage grid-connected converter is set up. The energy storage grid-connected converter extracts the three-phase current of the low-voltage side and maps the three-phase current of the low-voltage side to the current component in the synchronous rotating coordinate system of the high-voltage side. Harmonic information aligned with the harmonic phase of the high-voltage side is extracted from the current component to obtain the reference compensation current. An improved dual-vector modulation model predictive control strategy is adopted to achieve rapid tracking and accurate output of the reference compensation current. This control strategy uses the reference compensation current as the reference target and constructs a value function using the reference compensation current. The switching signal of the converter is generated through prediction and optimization, thereby driving the energy storage grid-connected converter to output a compensation current that is opposite to the phase of the actual harmonics.

2. The method for cross-level harmonic mitigation of distribution microgrids based on energy storage converters according to claim 1, characterized in that, The process of mapping the three-phase current on the low-voltage side to the current components in the synchronous rotating coordinate system on the high-voltage side includes: A phase-locked loop structure is used to lock the high-voltage and low-voltage side voltages respectively, and the phase estimates of the high-voltage side and the low-voltage side under steady state are obtained respectively. Based on the dynamic tracking error of the phase-locked loop, the phase error of the high-voltage side phase-locked loop and the phase error of the low-voltage side phase-locked loop are determined, thereby obtaining the expression of the phase difference estimate; The estimated phase difference is low-pass filtered to obtain a smoothed corrected phase difference. The smoothed corrected phase difference is differentiated in the time domain, and the correction angle for coordinate transformation is determined based on the fixed phase shift of the transformer. The correction angle is substituted into the transformation formula, which is obtained based on the three-phase current on the low-voltage side and the corrected transformation matrix. The corrected transformation matrix is ​​determined by the correction angle.

3. The method for cross-level harmonic mitigation of distribution microgrids based on energy storage converters according to claim 2, characterized in that, The method employs a phase-locked loop structure to lock the high-voltage and low-voltage sides separately, obtaining steady-state phase estimates for the high-voltage side and the low-voltage side, respectively, including: Voltage signal models for the high-voltage side and the low-voltage side are constructed. Based on the three-phase voltage values ​​obtained from the high-voltage side voltage signal model, the voltage components of the α-axis and β-axis of the high-voltage side are obtained through Clark transformation, and then the voltage components of the d-axis and q-axis of the high-voltage side are obtained. Using the voltage component of the q-axis as the error signal, the output frequency deviation is adjusted so that the voltage component of the q-axis approaches zero. The high-voltage side phase estimate is obtained by integrating the output frequency deviation, thereby obtaining the high-voltage side phase estimate in steady state and determining the low-voltage side phase estimate in steady state.

4. The method for cross-level harmonic mitigation of distribution microgrids based on energy storage converters according to claim 3, characterized in that, The dynamic tracking error based on the phase-locked loop determines the phase error of the high-voltage side phase-locked loop and the phase error of the low-voltage side phase-locked loop, thereby obtaining the expression of the phase difference estimate, including: Considering the dynamic tracking error of the phase-locked loop, let the phase error of the high-voltage side phase-locked loop be expressed as... ,in, When it is unsteady t The estimated phase value of the high-voltage side at time [time]. The angular frequency of the high-voltage side phase-locked loop. This represents the initial phase of the high-voltage side phase-locked loop; The phase error of the low-voltage side phase-locked loop is expressed as: The phase difference estimate is expressed as: ;in, This represents the initial phase of the low-voltage side phase-locked loop. It is the phase difference between the high-voltage and low-voltage phase-locked loops; when the system frequency is synchronized hour, It is a constant. ω is the angular frequency of the low-voltage side phase-locked loop.

5. The method for cross-level harmonic mitigation of distribution microgrids based on energy storage converters according to claim 4, characterized in that, The process of low-pass filtering the estimated phase difference to obtain a smoothed corrected phase difference, performing time-domain differentiation on the smoothed corrected phase difference, and determining the correction angle for coordinate transformation based on the fixed phase shift of the transformer includes: Phase difference estimate Low-pass filtering is performed to obtain a smoothed corrected phase difference. : , The filtering time constant is s Let be the complex frequency variable in the Laplace transform; the time-domain derivative of the smoothed corrected phase difference is expressed as: The correction angle is expressed as: ;in, This refers to the fixed phase shift present in a transformer.

6. The method for cross-level harmonic mitigation of distribution microgrids based on energy storage converters according to claim 5, characterized in that, Substituting the correction angle into the transformation formula includes: The transformation formula is expressed as follows: ;in, , These are the three-phase current components on the low-voltage side. This is the corrected transformation matrix, and ; Transformed These are the d-axis and q-axis current components in the synchronous rotating coordinate system of the high-voltage side, which contain harmonic information aligned with the phase of the high-voltage side harmonics. The corrected transformation matrix is ​​expressed as: ； in, This is the low-pressure side phase.

7. The method for cross-level harmonic mitigation of distribution microgrids based on energy storage converters according to claim 3, characterized in that, The method for obtaining the high-voltage side phase estimate in steady state includes: The output frequency deviation obtained using a PI controller is expressed as: ;in, and These are the proportional gain coefficient and integral gain coefficient of the PI control. The voltage component along the q-axis of the high-voltage side is integrated to obtain the phase estimate: ,in, The rated angular frequency, in steady state Then the phase estimate is expressed as , The angular frequency of the high-voltage side phase-locked loop. This represents the initial phase of the high-voltage side phase-locked loop.

8. The method for cross-level harmonic mitigation of distribution microgrids based on energy storage converters according to claim 1, characterized in that, The value function constructed using the reference compensation current is expressed as: ;in, For energy storage grid-connected converters in k+ The current value at time 1, This is the reference compensation current.

9. A multi-level harmonic mitigation system for distribution microgrids based on energy storage converters, characterized in that, The system includes: The reference compensation current determination module is used to determine the topology of a certain high-voltage side distribution network and low-voltage side microgrid, and to set up an energy storage grid-connected converter. The energy storage grid-connected converter extracts the three-phase current of the low-voltage side and maps the three-phase current of the low-voltage side to the current component in the synchronous rotating coordinate system of the high-voltage side. Harmonic information aligned with the harmonic phase of the high-voltage side is extracted from the current component to obtain the reference compensation current. The prediction module is used to achieve fast tracking and accurate output of the reference compensation current by adopting an improved dual-vector modulation model predictive control strategy. This control strategy uses the reference compensation current as the reference target and uses the reference compensation current to construct a value function. It generates the switching signal of the converter through prediction optimization, thereby driving the energy storage grid-connected converter to output a compensation current that is opposite to the phase of the actual harmonics.

10. A computer-readable storage medium, characterized in that, The storage medium stores a computer program, which, when executed by a processor, implements the steps of the cross-level harmonic mitigation method for distribution microgrids based on energy storage converters as described in any one of claims 1 to 8.

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