Microgrid electric energy quality global optimization method

Abstract

The invention discloses a micro-grid electric energy quality global optimization method, which comprises the steps of collecting real-time data, dividing the real-time data into a plurality of segments according to frequency, and calculating harmonic compensation capacity of each frequency band according to the real-time data; according to the harmonic compensation capacity obtained through calculation, priority distribution is carried out on the compensation sequence of the polluted nodes in the micro-grid; the virtual impedance and the injection damping resistance of the inverter are adjusted, and the harmonic current and the resonance phenomenon in the micro-grid are suppressed in different frequency bands; and according to the adjusted virtual impedance and damping resistance, updating a node admittance matrix, monitoring the compensation capacity of the inverter in real time, comparing real-time monitoring data with a threshold value, if the real-time monitoring data exceed the threshold value, correcting the inverter, otherwise, not changing. Through frequency-band-divided control, pertinence and effect of harmonic suppression are improved, frequency-band-divided suppression of micro-grid harmonic and high-frequency resonance is realized, residual capacity of an inverter and harmonic compensation requirements are dynamically matched, and safe and stable operation of a system is ensured.

Description

Technical Field

[0001] The present invention relates to the field of power supply technology, and in particular to a method for global optimization of power quality in a microgrid. Background Art

[0002] With the transformation of energy structures and the rapid development of distributed generation technology, microgrids, as local power supply networks integrating distributed power sources, energy storage devices, loads, and related protection and monitoring equipment, have shown significant advantages in improving energy efficiency, enhancing grid reliability, and increasing flexibility. However, the integration of a large number of power electronic devices in microgrids, such as inverters, rectifiers, switching power supplies, and LED lighting, has led to increasingly prominent harmonic problems, seriously affecting the power quality of microgrids.

[0003] Traditional harmonic control methods mainly target a single voltage level and use centralized or decentralized filtering devices, such as passive filters and active power filters (APFs). Passive filters have a simple structure and low cost, but can only filter out harmonics of a specific order and are prone to resonating with the power grid. Although active filters can dynamically compensate for harmonics, they are expensive and face technical bottlenecks when compensating for large-capacity harmonics. Traditional single-voltage level control methods cannot effectively address the problem of cross-regional harmonic propagation, resulting in poor harmonic control results. In addition, the propagation paths of harmonics in microgrids are complex and diverse, and the harmonic characteristics (such as frequency, amplitude, phase, etc.) on different paths vary. Traditional control methods make it difficult to accurately control different types of harmonic paths. Summary of the Invention

[0004] This application provides a global optimization method for microgrid power quality. Through frequency band control, it improves the pertinence and effect of harmonic suppression, realizes frequency band suppression of microgrid harmonics and high-frequency resonance, dynamically matches the remaining capacity of the inverter with the harmonic compensation demand, and ensures safe and stable operation of the system.

[0005] This application provides a method for global optimization of microgrid power quality, including: S101, collecting real-time data, dividing the real-time data into several segments according to frequency, and calculating the harmonic compensation capacity of each frequency segment based on the real-time data; S102, prioritizing the compensation order of polluted nodes in the microgrid based on the calculated harmonic compensation capacity; S103, adjusting the virtual impedance and injection damping resistance of the inverter to suppress harmonic currents and resonance phenomena in the microgrid in different frequency bands; S104, based on the adjusted virtual impedance and damping resistance, the node admittance matrix is ​​updated, the compensation capacity of the inverter is monitored in real time, and the real-time monitoring data is compared with the threshold. If the threshold is exceeded, the inverter is corrected, otherwise, no change is made.

[0006] Preferably, the formula for calculating the total harmonic compensation capacity based on the collected real-time data is: Among them, S H is the total harmonic compensation capacity of the node, indicating the apparent power to be compensated. is the three-phase system coefficient, which is derived from the geometric relationship between the line voltage and the phase voltage in the three-phase balanced circuit. J is the effective value of the microgrid fundamental voltage, I i is the amplitude of the node i-th harmonic current, i is the harmonic order, n is the highest harmonic order involved in the calculation, θ i is the phase angle of the i-th harmonic current relative to the fundamental voltage.

[0007] Preferably, the apparent power is the product of the effective value of the fundamental voltage and the effective value of the harmonic current in the microgrid, reflecting the equivalent power demand generated by the harmonic current under the action of the fundamental voltage, including both active power and reactive power.

[0008] Preferably, the node admittance matrix is ​​a square matrix representing the relationship between node voltage and current in the power system, and its elements reflect the admittance between nodes. The power network model is constructed by using diagonal elements and off-diagonal elements.

[0009] Preferably, S201, drawing a connection diagram according to the collected real-time data, and constructing a coupling admittance matrix based on the connection diagram; S202, using dual channels to adjust the impedance so that the fundamental current presents low impedance and the harmonic current presents high impedance, detecting the harmonic spectrum in the microgrid in real time, and adjusting the impedance value according to the detection result; S203: Use a sensor to detect the power flow and adjust the impedance according to the direction of the power flow.

[0010] Preferably, the formula for identifying cross-region propagation of harmonics based on the coupling admittance matrix is: Among them, A h is the harmonic voltage magnification factor, Y AC-DC is the coupling admittance between the AC side and the DC side, Y′ DC is the corrected coupling admittance matrix, m is the transformer ratio, and a threshold is set according to the calculated harmonic voltage amplification factor. If the harmonic voltage amplification factor is greater than the preset threshold, it indicates that the harmonics have a cross-zone phenomenon. Conversely, if the harmonic voltage amplification factor is less than or equal to the preset threshold, it indicates that the harmonics do not have a cross-zone phenomenon.

[0011] Preferably, the dual channels include a fundamental channel and a harmonic channel. The fundamental channel uses an algorithm to synchronously track the fundamental signal to obtain a synchronization signal, and sets a low impedance that allows the fundamental current to pass based on the synchronization signal; the harmonic channel uses a filter to screen the harmonic current, and sets a high impedance that can hinder the propagation of the harmonic current according to the harmonic current.

[0012] Preferably, the active power of the AC side and the DC side is calculated based on the data measured by the sensor. If the active power is positive, the flow from the AC side to the DC side is forward flow; if the active power is negative, the flow from the AC side to the DC side is reverse flow.

[0013] Preferably, S301, identifying multiple paths based on data collected in real time; S302, generating a blocking strategy based on the identified multi-paths; S303: Construct an electric energy monitoring network according to the identified multi-paths.

[0014] Preferably, the identified multipaths are classified into high-frequency paths, low-frequency paths and resonant paths, the high-frequency paths are blocked using active filters, the low-frequency paths are blocked using passive filters, and the resonant paths are blocked using active damping resistors.

[0015] One or more technical solutions provided in this application have at least the following technical effects or advantages: through frequency band control, the pertinence and effect of harmonic suppression are improved, the frequency band suppression of microgrid harmonics and high-frequency resonance is achieved, the remaining capacity of the inverter is dynamically matched with the harmonic compensation requirements, the safe and stable operation of the system is ensured, the governance priority of key nodes is improved, resource allocation is optimized, the pollution of harmonics to the power grid is reduced, and the power quality is improved.

[0016] By adjusting the impedance through the coupled admittance matrix and dual channels, the cross-zone propagation path of harmonics is successfully blocked, reducing the harmonic pollution level on the AC and DC sides. While blocking harmonics, the normal transmission of fundamental power is ensured, improving the overall operating efficiency of the system. The blocking impedance value is dynamically adjusted according to the real-time detected harmonic spectrum and power flow direction, achieving more accurate harmonic suppression. Setting up special monitoring points for different types of harmonic paths (high frequency, low frequency, resonance) can more accurately capture power quality problems on each path. When power quality problems or faults occur in the power grid, appropriate blocking devices and parameters are selected according to the harmonic characteristics and power quality problems on different paths to achieve precise management, effectively improve harmonic blocking efficiency, reduce equipment losses and impact on power, and thus improve the power quality of the entire power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1A schematic flow chart of a method for global optimization of microgrid power quality according to the present invention; Figure 2 A schematic diagram of the process of constructing a coupling admittance matrix according to the present invention; Figure 3 This is a schematic diagram of the process of constructing an electric energy monitoring network according to the present invention. DETAILED DESCRIPTION

[0018] To facilitate understanding of the present invention, the present application will be described more comprehensively below with reference to the relevant drawings; the drawings show preferred embodiments of the present invention, but the present invention can be implemented in many different forms and is not limited to the embodiments described herein; on the contrary, the purpose of providing these embodiments is to enable a more thorough and comprehensive understanding of the disclosed content of the present invention.

[0019] It should be noted that the terms “vertical”, “horizontal”, “up”, “down”, “left”, “right” and similar expressions used in this document are for illustrative purposes only and do not represent the only implementation method.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which this invention pertains; the terms used herein in the specification of the present invention are for the purpose of describing specific embodiments only and are not intended to limit the present invention; the term "and / or" used herein includes any and all combinations of one or more of the associated listed items.

[0021] Example 1: Figure 1 1 is a flow chart of a method for global optimization of microgrid power quality according to an embodiment of the present invention, comprising: S101, collecting real-time data, dividing the real-time data into several segments according to frequency, and calculating the harmonic compensation capacity of each frequency segment based on the real-time data; Furthermore, frequency information of harmonic currents and fundamental voltages in the microgrid and the dynamic frequency change of the inverter output power are collected. The harmonic frequencies and fundamental frequencies are monitored using a power quality analyzer (Fluke 435). The current and voltage signals are decomposed in real time using a fast Fourier transform (FFT) to decompose the frequency components of the 3rd to 25th harmonics. A power quality analyzer (e.g., Fluke 435) is used to set the sampling frequency and frequency resolution of the power quality analyzer. The fundamental frequency is tracked in real time using a zero-crossing detection method. The zero-crossing detection method captures the zero-crossing points of the fundamental voltage or current signal (the moment when the waveform changes from positive to negative or from negative to positive) in real time, uses a timer to record the time interval between adjacent zero-crossing points, and calculates the fundamental frequency based on the time interval. The inverter output power is collected using a DC side current sensor and an AC side voltage transformer. A smart meter is installed next to the DC side current sensor and the AC side voltage transformer. The collected data is converted by an analog-to-digital converter to calculate the instantaneous power. The collected data is transmitted to a central controller via a wireless communication device and stored in a database.

[0022] The total harmonic compensation capacity is calculated based on the received real-time data. The calculation formula is: Among them, S H is the total harmonic compensation capacity of the node, indicating the apparent power, is the three-phase system coefficient, which is derived from the geometric relationship between the line voltage and the phase voltage in the three-phase balanced circuit. J is the effective value of the microgrid fundamental voltage, I i is the amplitude of the node i-th harmonic current, i is the harmonic order, usually odd, even harmonics are usually small due to symmetry and can be ignored, n is the highest harmonic order involved in the calculation, θ i is the phase angle of the i-th harmonic current relative to the fundamental voltage. The apparent power generated by the harmonic current under the fundamental voltage calculated by the above formula reflects the total power that the harmonic control device needs to output. The apparent power is the product of the effective value of the fundamental voltage and the effective value of the harmonic current in the microgrid, reflecting the equivalent power demand generated by the harmonic current under the action of the fundamental voltage, including both the active power that actually does work and the reactive power used to establish the electromagnetic field but does not consume energy. The apparent power is the total capacity that the harmonic control device (such as APF) needs to output. Its value determines the ability of the compensation equipment to suppress voltage distortion and reduce line losses. Harmonics are divided into three segments according to frequency: low frequency, medium frequency and high frequency. The compensation capacity of each frequency band is calculated. The low frequency band contains harmonic components (such as 3rd, 5th, 7th and 9th) generated by nonlinear loads (such as inverters and rectifiers). The low frequency band compensation capacity calculation formula is: Among them, S H低is the harmonic compensation capacity of the low-frequency node, is the three-phase system coefficient, V J is the effective value of the microgrid fundamental voltage, I i is the amplitude of the node i-th harmonic current, i is the harmonic order, n is the highest harmonic order involved in the calculation, θ i is the phase angle of the i-th harmonic current relative to the fundamental voltage. Low-frequency harmonics have large energy and have a significant impact on equipment such as transformers and motors, and need to be compensated first. The intermediate frequency band contains intermediate frequency harmonics (such as 11th, 13th, 15th, 17th, and 19th), which are generated by equipment such as electric arc furnaces and intermediate frequency induction furnaces. The calculation formula for the intermediate frequency band compensation capacity is: Among them, S H中 is the harmonic compensation capacity of the mid-frequency band node, is the three-phase system coefficient, V J is the effective value of the microgrid fundamental voltage, I i is the amplitude of the node i-th harmonic current, i is the harmonic order, n is the highest harmonic order involved in the calculation, θ i is the phase angle of the i-th harmonic current relative to the fundamental voltage. Intermediate-frequency harmonics have a significant impact on equipment such as cable insulation and capacitors, requiring targeted compensation. The high-frequency band includes high-frequency harmonics (such as 21st, 23rd, and 25th harmonics), which are generated by switching power supplies, LED lighting, and other equipment. The high-frequency band compensation capacity calculation formula is: Among them, S H高 is the harmonic compensation capacity of the high frequency node, is the three-phase system coefficient, V J is the effective value of the microgrid fundamental voltage, I i is the amplitude of the node i-th harmonic current, i is the harmonic order, n is the highest harmonic order involved in the calculation, θ i is the phase angle of the i-th harmonic current relative to the fundamental voltage. High-frequency harmonics decay quickly, but they significantly interfere with precision electronic equipment and require appropriate compensation.

[0023] S102, prioritizing the compensation order of polluted nodes in the microgrid based on the calculated harmonic compensation capacity; Specifically, based on the calculated harmonic compensation capacity, when the harmonic compensation capacity is limited, the priority of power quality management is allocated by analyzing the harmonic pollution degree of the node and the remaining capacity of the inverter. At the same time, it is ensured that the inverter operates within the safety threshold (avoiding overload) to achieve efficient utilization of compensation resources. The harmonics used for compensation are transmitted to the inverter, and the inverter generates a compensation current with equal amplitude and opposite phase to the harmonic current, thereby offsetting the harmonic pollution generated by the load. Constraints are set so that the inverter reserves a safety margin to deal with dynamic harmonic fluctuations or sudden load changes. The constraint formula is: S H ≤k1×S R , where S H is the total harmonic compensation capacity of the node, S R is the remaining available capacity of the inverter at the node, that is, the rated capacity of the inverter minus the currently compensated capacity. k1 is the safety factor. In this embodiment, k1=0.8, which is used to prevent inverter overload. If the node does not meet the constraint conditions, it is necessary to adjust the filter parameters or perform compensation in stages (first compensate the low frequency band, then the high frequency band). The ratio of the compensation demand of each node to the remaining capacity of the inverter is calculated to reflect the relative severity of the node harmonic pollution. The formula for calculating the ratio is: Among them, β j To compensate for the ratio of the demand to the remaining capacity of the inverter, β j The larger the value, the more serious the harmonic pollution of node j is relative to the inverter capacity, and it needs to be treated first. H is the total harmonic compensation capacity of the node, S R is the remaining available capacity of the inverter at the node, and all nodes β j Normalization is performed to obtain dynamic weights. According to the dynamic weights and load types, a threshold range is set. The threshold range includes a maximum threshold and a minimum threshold. The nodes are divided into three levels, namely high-priority nodes, medium-priority nodes and low-priority nodes. High-priority nodes have dynamic weights greater than the maximum threshold, serious harmonic pollution and sensitive loads (such as hospital CT machines). Sufficient compensation capacity must be allocated immediately, even at the expense of the governance effect of some low-priority nodes; medium-priority nodes have dynamic weights greater than the minimum threshold and less than or equal to the maximum threshold. After meeting the high-priority demand, the remaining capacity is allocated according to the weight ratio; low-priority nodes have dynamic weights less than or equal to the minimum threshold. Compensation is only performed when the inverter capacity is sufficient, or through time-sharing compensation (such as low load period at night).

[0024] S103, adjusting the virtual impedance and injection damping resistance of the inverter to suppress harmonic currents and resonance phenomena in the microgrid in different frequency bands; Furthermore, for the suppression of harmonics in the low-frequency band, low-frequency harmonics (such as the 3rd, 5th, 7th, and 11th) are the most common harmonic components in the power grid, which are mainly generated by nonlinear loads. By adjusting the virtual impedance of the inverter, the impedance characteristics of the inverter to specific frequency harmonics are changed, making it present a high impedance state, thereby reducing the harmonic current flowing into the power grid. The harmonic current and harmonic voltage in the power grid are collected through current transformers and voltage transformers, and the particle swarm optimization (PSO) algorithm is used to dynamically adjust the virtual impedance of the inverter. For nodes with serious harmonic pollution, its virtual impedance is increased to reduce the harmonic current flowing into the power grid. Rapidly suppress harmonics; for high-frequency resonance damping, high-frequency resonance is usually caused by the interaction between capacitive elements (such as long cables, capacitor banks) and inductive elements (such as transformer leakage inductance) in the power grid, forming an impedance peak at a specific frequency (such as 5.5kHz), causing the voltage or current to be sharply amplified, threatening equipment safety. By injecting a damping resistor, the resonant energy can be consumed, the impedance peak can be reduced, and thus the resonance can be suppressed. The impedance characteristics of the power grid in the high frequency band are identified through the small signal injection model. According to the size of the harmonic current at the resonant frequency, the formula for calculating the injection damping resistor size is: Among them, R d is the injected damping resistance value, used to suppress the high-frequency resonance of the power grid, R max is the hard upper limit of the damping resistor, which is set according to the system capacity and the voltage resistance of the equipment. K2 is the adjustment coefficient used to calibrate the calculated value of the damping resistor. V r is the harmonic voltage spectrum, S j The damping resistance value is added to the virtual impedance of the inverter for the system rated capacity corresponding to the resonant frequency point, which is equivalent to adding a resistive load at the resonant frequency point to consume the resonant energy. The low-frequency band is mainly based on harmonic suppression, and active blocking is achieved through virtual impedance adjustment. The high-frequency band is mainly based on resonant damping, and passive consumption is achieved through damping resistance injection.

[0025] S104, updating the node admittance matrix based on the adjusted virtual impedance and damping resistance, monitoring the compensation capacity of the inverter in real time, comparing the real-time monitoring data with a threshold, and correcting the inverter if the threshold is exceeded, otherwise, no change is made; Specifically, the node admittance matrix is ​​a square matrix that represents the relationship between node voltage and current in the power system. Its elements reflect the admittance between nodes (i.e., the inverse of impedance). The power network model is constructed by using diagonal elements (self-admittance, including the equivalent virtual impedance of the line, load, and inverter) and off-diagonal elements (mutual admittance, reflecting the coupling between nodes). The calculated virtual impedance and damping resistance are input into the node admittance matrix, and the node admittance matrix is ​​updated. The formula is: Among them, Y n It represents the node admittance matrix after the virtual impedance is superimposed, Y o is the original node admittance matrix, R ref,kis the equivalent virtual resistance of the inverter at node k, which is used to adjust the active power distribution, X ref,k is the equivalent virtual reactance of the inverter at node k, which is used to adjust the reactive power distribution. Among them, Y r is the node admittance matrix updated by virtual impedance and damping resistance, Y n It represents the node admittance matrix after the virtual impedance is superimposed, R d For the damping resistance value that needs to be injected, based on the above formula, sparse matrix storage technology is used to update the global admittance matrix to ensure computational efficiency; the overload threshold is set according to the inverter parameters, and the compensation capacity of the inverter is monitored and compared with the overload threshold. If the compensation capacity exceeds the overload threshold, the inverter is dynamically corrected by transferring 20% ​​of the compensation task (decomposed by harmonic order) of the overloaded inverter to the redundant inverter.

[0026] The technical solutions in the above-mentioned embodiments of the present application have at least the following technical effects or advantages: through frequency band control, the pertinence and effect of harmonic suppression are improved, frequency band suppression of microgrid harmonics and high-frequency resonance is achieved, the remaining capacity of the inverter is dynamically matched with the harmonic compensation requirements, the safe and stable operation of the system is ensured, the governance priority of key nodes is improved, resource allocation is optimized, the pollution of harmonics to the power grid is reduced, and the quality of power is improved.

[0027] Example 2: Based on the above example 1, harmonics in the microgrid will propagate between different voltage level areas through transformers. Example 1 only targets single voltage level governance, resulting in cross-region harmonic amplification effects, such as Figure 2 shown.

[0028] S201, drawing a connection diagram according to the collected real-time data, and constructing a coupling admittance matrix based on the connection diagram; Based on the collected data, Visio is used to draw a connection diagram between the AC and DC sides of the microgrid. On the AC side, the connection points of the two voltage levels of 10kV and 400V are marked, and the connection positions of ±750V and 380V are marked on the DC side. At the same time, the specific positions of the power electronic transformer (PET), distributed power supply (including photovoltaic power generation equipment and energy storage devices) and load (distinguishing between critical loads and non-critical loads) are marked in the diagram, so that the connection relationship between each device is clear at a glance; in the microgrid, the AC side and the DC side are connected through a power electronic transformer (PET). The transformer ratio and leakage inductance have an impact on the propagation of harmonics. The transformer ratio determines the conversion relationship between the AC and DC side voltages, while the leakage inductance will generate inductive reactance when the harmonic current passes through, hindering the flow of current. In order to reflect the coupling relationship between the AC side and the DC side in the admittance matrix, based on the updated node admittance matrix, the transformer ratio and leakage inductance are added to form a coupled admittance matrix. The coupled admittance matrix connects the inductive reactance of the leakage inductance in parallel with the DC side admittance converted by the ratio, and then takes the inverse to obtain the coupled admittance value. Through the coupled admittance value, the coupled admittance matrix can quantify the influence of the physical parameters of the transformer on the cross-region propagation of harmonics. The degree of obstruction of the propagation of harmonics from the AC side to the DC side is increased, thereby providing a theoretical basis for the formulation of harmonic blocking strategies. The admittance component corresponding to the distributed capacitance at the harmonic frequency is superimposed on the coupled admittance matrix to form a revised coupled admittance matrix. The admittance component is the imaginary unit multiplied by the harmonic angular frequency and then multiplied by the distributed capacitance value. The cross-region propagation of harmonics is identified according to the coupled admittance matrix. The formula is: Among them, A h Y is the harmonic voltage amplification factor, which is used to measure the degree to which the voltage amplitude is amplified when the harmonic of a specific order h propagates from the AC side to the DC side. AC-DC is the coupling admittance between the AC side and the DC side, Y′ DC is the corrected coupling admittance matrix, m is the transformer ratio, and the threshold is set according to the calculated harmonic voltage amplification factor. If the harmonic voltage amplification factor is greater than the preset threshold, it indicates that the harmonics have a cross-zone phenomenon. Conversely, if the harmonic voltage amplification factor is less than or equal to the preset threshold, it indicates that the harmonics do not have a cross-zone phenomenon.

[0029] S202, using dual channels to adjust the impedance so that the fundamental current presents low impedance and the harmonic current presents high impedance, detecting the harmonic spectrum in the microgrid in real time, and adjusting the impedance value according to the detection result; further, using dual channels for adjustment, the dual channels include a fundamental channel and a harmonic channel, for the fundamental channel, using a phase-locked loop (PLL) to synchronously track the input fundamental signal, the phase-locked loop can track the phase and frequency of the input signal in real time, ensuring that the output signal is synchronized with the fundamental signal, setting a low impedance according to the obtained synchronization signal, the set low impedance can make the fundamental current pass smoothly, reduce the energy loss of the fundamental during the transmission process, and improve the fundamental power transmission efficiency, for the harmonic channel, A bandpass filter is used to filter out harmonic signals in a specified frequency range, thereby eliminating interference from the fundamental wave and other irrelevant frequency signals. A high impedance is output based on the detected harmonic current, which can hinder the propagation of the harmonic current. A threshold for the harmonic current amplitude is set. When the detected harmonic current amplitude exceeds the threshold, the impedance switching mechanism is triggered, and the proportional-integral (PI) control algorithm is used to increase the equivalent impedance from the fundamental channel to the harmonic channel. After the equivalent impedance is switched to the high impedance value of the harmonic channel, according to Ohm's law, under the condition of a certain harmonic voltage, the high impedance will limit the passage of the harmonic current, thereby effectively suppressing the further propagation of the harmonic current and preventing the harmonic current from causing greater damage to the equipment in the system.

[0030] The Fast Fourier Transform (FFT) algorithm is used to detect the harmonic spectrum in real time, identify the dominant harmonic order, and obtain its amplitude information. If the amplitude of a harmonic of a specific order is detected to exceed a preset threshold during system operation, this indicates that the harmonic poses a serious threat to the system. At this time, the system will increase the blocking impedance for this specific harmonic order to a higher level, thereby further strengthening the suppression of this harmonic, preventing it from damaging system equipment and ensuring stable system operation. When the amplitude of another harmonic of a different order is detected to exceed its preset threshold, the system will increase the blocking impedance of this harmonic order to a specific high impedance value, specifically suppressing this harmonic order and reducing its propagation and impact in the system. For other harmonic orders that do not exceed the preset threshold, the system maintains their blocking impedance at a basic and fixed impedance value.

[0031] S203, using a sensor to detect power flow and adjusting impedance according to the direction of power flow; Specifically, current sensors and voltage sensors are installed on the AC side and DC side of the power electronic transformer (PET), respectively. The current and voltage data of the AC side and the DC side are acquired in real time using the current sensors and voltage sensors. The active power of the AC side and the DC side is calculated according to the active power calculation formula. The direction of power flow is determined according to the positive and negative value of the active power. Flow from the AC side to the DC side is forward flow, and vice versa. When the power flow direction is detected to be forward, the system issues an impedance adjustment instruction to increase the blocking impedance of the DC side while keeping the blocking impedance of the AC side unchanged. When the power flow direction is detected to be reverse, the blocking impedance of the AC side is increased, while the blocking impedance of the DC side remains unchanged.

[0032] The technical solutions in the above-mentioned embodiments of the present application have at least the following technical effects or advantages: by adjusting the impedance through the coupling admittance matrix and the dual channel, the cross-region propagation path of the harmonics is successfully blocked, and the harmonic pollution level on the AC side and the DC side is reduced. While blocking the harmonics, the normal transmission of the fundamental power is ensured, and the overall operation efficiency of the system is improved. According to the real-time detected harmonic spectrum and power flow direction, the blocking impedance value is dynamically adjusted to achieve more accurate harmonic suppression.

[0033] Example 3: Based on the above-mentioned Example 1 and Example 2, this embodiment identifies multipath harmonic pollution and performs multipath blocking on it. Based on the multipath blocking and power quality global optimization framework, an electric energy monitoring network is constructed, such as Figure 3 shown.

[0034] S301, identifying multiple paths based on real-time collected data; Furthermore, the real-time data collected includes harmonics, voltage, frequency, three-phase imbalance, and reactive power. For the collection of three-phase imbalance, a three-phase imbalance monitoring device is deployed at the feeder outlet. The collected data is pre-processed to remove noise and abnormal data. The collected data is input into the intelligent quality analysis software Fluke 435. Fluke 435 calculates the Poynting energy flow density of each branch. For each branch, the energy proportion of its Poynting energy flow density in the entire microgrid is calculated. The formula is: Among them, P i is the energy proportion of the i-th branch, S i is the Poynting energy flow density of the i-th branch, S jTo calculate the total energy flow density, for each item when summing the Poynting energy flow density of all branches (from the 1st to the nth) in the microgrid, a proportion threshold is set, the calculated energy proportion is compared with the proportion threshold, and the path with an energy proportion greater than the proportion threshold is marked as a critical path. The impedance matching is obtained using the node admittance matrix according to the marked critical path, and the coherence between different paths is calculated using the cross-correlation function. The coherence is used to describe the similarity of harmonic signals between different paths. The cross-correlation function is an existing technology and will not be repeated here. The formula for calculating the path weight based on the energy proportion, coherence and impedance matching is: w i =k3×P i +k4×C ij +k5×M i , where w i is the path weight, P i is the energy proportion of the i-th branch, C ij is the coherence between the i-th branch and the j-th branch, M i is the impedance matching, k3, k4 and k5 are the energy share coefficient, average coherence coefficient and impedance matching coefficient respectively, and k3+k4+k5=1.

[0035] S302, generating a blocking strategy based on the identified multi-paths; Specifically, the multiple identified paths are classified into high-frequency paths, low-frequency paths and resonant paths. The high-frequency path has the characteristics of high frequency and rapid change, and the electrical parameters on its propagation path are also relatively complex, and the response speed and filtering performance of the blocking device are required to be high. An active filter is used as a blocking device for the high-frequency path. The low-frequency path is relatively stable, with small frequency changes, but its energy is large. A passive filter is used as a blocking device for the low-frequency path. The resonant path is a path where resonance occurs in the power grid. When the harmonic frequency is close to the natural frequency of the power grid, resonance will occur, resulting in harmonic current amplification, causing serious damage to the power grid equipment. An active damping resistor is used to block the resonant path, and the parameters of the selected blocking device are optimized to improve the blocking effect, reduce equipment loss and the impact on power.

[0036] S303, constructing an electric energy monitoring network based on the identified multi-paths; Furthermore, according to the identified path type, monitoring points are set at the key nodes of each path, and monitoring points are set at the starting end, middle node and end of the high-frequency path respectively to comprehensively monitor the propagation and changes of high-frequency harmonics in the path; for resonant paths, monitoring points are added at the nodes where resonance occurs and nearby areas to detect resonance phenomena in a timely manner. Combined with the topological structure of the microgrid, monitoring points are set at locations such as the busbar, feeder, and transformer output terminal to fully understand the power quality status of various parts of the power grid. At the same time, avoid excessive concentration or dispersion of monitoring points, improve the coverage efficiency and data representativeness of the monitoring network, install monitoring equipment at the monitoring points, and coordinate the system with the installed monitoring equipment to build an electric energy monitoring network.

[0037] The technical solutions in the above-mentioned embodiments of the present application have at least the following technical effects or advantages: setting special monitoring points for different types of harmonic paths (high frequency, low frequency, resonance) can more accurately capture power quality problems on each path. When power quality problems or faults occur in the power grid, appropriate blocking devices and parameters are selected according to the harmonic characteristics and power quality problems on different paths to achieve precise management, effectively improve harmonic blocking efficiency, reduce equipment losses and impact on power, and thus improve the power quality of the entire power grid.

[0038] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Various modifications and variations are readily apparent to those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. A global optimization method for microgrid power quality, characterized in that: include: S101, collecting real-time data, dividing the real-time data into several segments according to frequency, and calculating the harmonic compensation capacity of each frequency segment based on the real-time data; S102, prioritizing the compensation order of polluted nodes in the microgrid based on the calculated harmonic compensation capacity; S103, adjusting the virtual impedance and injection damping resistance of the inverter to suppress harmonic currents and resonance phenomena in the microgrid in different frequency bands; S104, based on the adjusted virtual impedance and damping resistance, the node admittance matrix is ​​updated, the compensation capacity of the inverter is monitored in real time, and the real-time monitoring data is compared with the threshold. If the threshold is exceeded, the inverter is corrected, otherwise, no change is made.

2. A microgrid power quality global optimization method according to claim 1, characterized in that: According to the collected real-time data, the formula for calculating the total harmonic compensation capacity is: Among them, S H is the total harmonic compensation capacity of the node, indicating the apparent power to be compensated. is the three-phase system coefficient, which is derived from the geometric relationship between the line voltage and the phase voltage in the three-phase balanced circuit. J is the effective value of the microgrid fundamental voltage, I i is the amplitude of the node i-th harmonic current, i is the harmonic order, n is the highest harmonic order involved in the calculation, θ i is the phase angle of the i-th harmonic current relative to the fundamental voltage.

3. A microgrid power quality global optimization method according to claim 2, characterized in that: The apparent power is the product of the effective value of the fundamental voltage and the effective value of the harmonic current in the microgrid, reflecting the equivalent power demand generated by the harmonic current under the action of the fundamental voltage, including both active power and reactive power.

4. A microgrid power quality global optimization method according to claim 1, characterized in that: The node admittance matrix is ​​a square matrix that represents the relationship between node voltage and current in the power system. Its elements reflect the admittance between nodes, and the power network model is constructed through diagonal elements and off-diagonal elements.

5. The method for global optimization of microgrid power quality according to claim 1, wherein: S201, drawing a connection diagram according to the collected real-time data, and constructing a coupling admittance matrix based on the connection diagram; S202, using dual channels to adjust the impedance so that the fundamental current presents low impedance and the harmonic current presents high impedance, detecting the harmonic spectrum in the microgrid in real time, and adjusting the impedance value according to the detection result; S203: Use a sensor to detect the power flow and adjust the impedance according to the direction of the power flow.

6. A microgrid power quality global optimization method according to claim 5, characterized in that: The formula for identifying cross-region propagation of harmonics based on the coupling admittance matrix is: Among them, A h is the harmonic voltage magnification factor, Y AC-DC is the coupling admittance between the AC side and the DC side, Y′ DC is the corrected coupling admittance matrix, m is the transformer ratio, and a threshold is set according to the calculated harmonic voltage amplification factor. If the harmonic voltage amplification factor is greater than the preset threshold, it indicates that the harmonics have a cross-zone phenomenon. Conversely, if the harmonic voltage amplification factor is less than or equal to the preset threshold, it indicates that the harmonics do not have a cross-zone phenomenon.

7. A microgrid power quality global optimization method according to claim 5, characterized in that: The dual channels include a fundamental channel and a harmonic channel. The fundamental channel uses an algorithm to synchronously track the fundamental signal to obtain a synchronization signal, and sets a low impedance based on the synchronization signal to allow the fundamental current to pass through; the harmonic channel uses a filter to screen the harmonic current, and sets a high impedance based on the harmonic current to prevent the propagation of the harmonic current.

8. A microgrid power quality global optimization method according to claim 5, characterized in that: The active power on the AC and DC sides is calculated based on the data measured by the sensor. If the active power is positive, the flow from the AC side to the DC side is forward flow. If the active power is negative, the flow from the AC side to the DC side is reverse flow.

9. A microgrid power quality global optimization method according to claim 5, characterized in that: S301, identifying multiple paths based on real-time collected data; S302, generating a blocking strategy based on the identified multi-paths; S303: Construct an electric energy monitoring network according to the identified multi-paths.

10. A microgrid power quality global optimization method according to claim 9, characterized in that: The identified multipaths are classified into a high-frequency path, a low-frequency path, and a resonant path. The high-frequency path is blocked using an active filter, the low-frequency path is blocked using a passive filter, and the resonant path is blocked using an active damping resistor.

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