Island microgrid control method and system based on adaptive virtual impedance droop control

Abstract

The invention discloses an island micro-grid control method and system based on adaptive virtual impedance droop control. The island micro-grid control method comprises the steps that active power and reactive power output by an inverter are measured in real time; inputting the active power and the reactive power into an improved droop controller, and generating a frequency reference value and a voltage amplitude reference value; calculating a difference value between the reactive power and a reference value, and dynamically adjusting the size of virtual impedance through an integral controller; adding the voltage amplitude reference value and the voltage compensation amount to obtain a voltage amplitude instruction, and adding the frequency reference value and the frequency compensation amount to obtain a frequency instruction; performing voltage synthesis by using the voltage amplitude instruction and the frequency instruction to generate a voltage reference value under a dq coordinate system, taking the voltage reference value as reference input of a double closed-loop control system, driving an inverter bridge after SPWM modulation, and regulating and controlling the output voltage; the system stability and reliability can be improved, reactive power loss and circulation can be reduced, the electric energy quality can be improved, and the load change coping capacity of the micro-grid can be enhanced.

Description

Technical Field

[0001] This invention relates to the field of power electronics, and in particular to a control method and system for an islanded microgrid based on adaptive virtual impedance droop control. Background Technology

[0002] In recent years, traditional power systems have gradually transformed into microgrid systems primarily powered by renewable energy sources such as solar and wind power. Microgrids can operate in both grid-connected and islanded modes. However, in microgrids, the characteristics and magnitude of the inverter's equivalent line impedance have a significant impact on the system's power distribution. If there are differences in the inverter's equivalent line impedance, the system's load power cannot be evenly distributed, leading to circulating currents. The larger the circulating current, the greater the system power loss. Moreover, when a microgrid operates in isolation, distributed generation typically employs droop control to achieve power distribution.

[0003] Traditional droop control lacks an integral component in the reactive power / voltage droop process. Therefore, even with identical equivalent output impedances, impedance mismatch between the inverter and the point of common coupling can lead to different output voltages, affecting the accurate distribution of reactive power. Existing improved droop control strategies for islanded microgrids only consider parallel operation of two inverters, limiting their application scope and neglecting frequency fluctuation compensation. This can negatively impact power transmission efficiency and reduce power quality. Therefore, there is an urgent need for an islanded microgrid control strategy based on adaptive virtual impedance droop control, aiming to improve system reactive power distribution, reduce circulating current, enhance operational stability, maintain DC bus voltage stability, and minimize frequency fluctuations. Summary of the Invention

[0004] In view of the aforementioned existing problems, this invention is proposed. Therefore, this invention provides a control method and system for islanded microgrids based on adaptive virtual impedance droop control to solve the problems of uneven reactive power distribution, bus voltage sag, and frequency deviation in multi-source networks.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0006] In a first aspect, embodiments of the present invention provide a control method for an islanded microgrid based on adaptive virtual impedance droop control, including: real-time measurement of the active power and reactive power output by the inverter;

[0007] The active and reactive power are input into the improved droop controller to generate frequency reference values ​​and voltage amplitude reference values.

[0008] The difference between the reactive power and the reference value is calculated, and the virtual impedance is dynamically adjusted through the integral controller.

[0009] The voltage amplitude reference value is added to the voltage compensation amount to obtain the voltage amplitude command, and the frequency reference value is added to the frequency compensation amount to obtain the frequency command.

[0010] Voltage is synthesized using the voltage amplitude command and frequency command to generate a voltage reference value in the dq coordinate system. This voltage reference value is used as the reference input of the dual closed-loop control system and, after SPWM modulation, drives the inverter bridge to regulate the output voltage.

[0011] As a preferred embodiment of the islanded microgrid control method based on adaptive virtual impedance droop control described in this invention, the active power and reactive power are input to the improved droop controller to generate frequency reference values ​​and voltage amplitude reference values, including:

[0012] The central controller calculates the reactive power reference value for each distributed power source based on the total load reactive power and the capacity of each distributed power source, and sends it to the local controller of each distributed power source.

[0013] When the capacity of distributed power sources is the same, the central controller will distribute the total load reactive power equally, and the reactive power reference value of each inverter is equal to the total load reactive power divided by the number of parallel power sources.

[0014] As a preferred embodiment of the islanded microgrid control method based on adaptive virtual impedance droop control described in this invention, the calculation of the difference between reactive power and a reference value, and the dynamic adjustment of the virtual impedance by the integral controller, includes: when the reactive power is less than the reactive power reference value and the difference between the reactive power and the reference value is greater than 0, the virtual impedance output by the integral controller increases; when the reactive power is greater than the reactive power reference value and the difference between the reactive power and the reference value is less than 0, the virtual impedance output by the integral controller decreases.

[0015] As a preferred embodiment of the islanded microgrid control method based on adaptive virtual impedance droop control described in this invention, it further includes: if N distributed power sources are connected in parallel, and the reactive power of the load is Q... L Then the inverter's given reactive power is

[0016] Q av =Q L / N

[0017] The improved expression for the inverter output voltage reference value in the dq coordinate system after droop control is:

[0018]

[0019] Among them, U dref and U qrefU represents the reference values ​​of the d-axis and q-axis components of the inverter output voltage in the dq coordinate system, respectively. d and U q Z represents the actual d-axis and actual q-axis components of the inverter output voltage in the dq coordinate system. v I represents the virtual impedance. Ld and I Lq These represent the d-axis and q-axis components of the load current in the dq coordinate system, respectively.

[0020] The beneficial effects of this preferred technical solution are as follows: the improved droop control constructs an integral term based on the difference between the DG's own frequency and the frequency reference value as a self-frequency recovery stage, and an integral term based on the difference between the DG's output active power and the average active power value as a frequency compensation stage, in order to compensate for frequency fluctuations.

[0021] As a preferred embodiment of the islanded microgrid control method based on adaptive virtual impedance droop control described in this invention, the method includes: adding the voltage amplitude reference value to the voltage compensation amount to obtain the voltage amplitude command; and adding the frequency reference value to the frequency compensation amount to obtain the frequency command, including:

[0022] The voltage amplitude command is expressed as:

[0023]

[0024] Among them, U i U represents the reference value of the output voltage amplitude of the i-th inverter in the dq coordinate system. * The system's desired reference voltage amplitude is represented by 'n', which represents the reactive power droop factor, and Q is the value of the reference voltage. i Z represents the reactive power output of the i-th inverter. vi This represents the virtual impedance of the i-th inverter;

[0025] Frequency commands are expressed as:

[0026] f i =f * -mP i +k f ∫(f * -f i )dt+k c ∫(P av -P i )dt

[0027] Among them, f i f represents the frequency output of the i-th inverter. * The system's desired reference frequency is represented by m, which represents the active power droop factor, and P is the active power droop factor. i k represents the active power output of the i-th inverter. fk represents the frequency deviation integral gain. c P represents the frequency compensation coefficient. av This represents the average value of the active power output by the distributed generation unit.

[0028] The beneficial effects of this preferred technical solution are that the improved droop control is based on adaptive virtual impedance control, which uses the reactive power information output by the distributed power source to construct an adaptive virtual impedance, eliminates the difference in equivalent line impedance, reduces the reactive power circulating current in the system, and thus improves the accuracy of reactive power distribution.

[0029] Secondly, the present invention provides an islanded microgrid control system based on adaptive virtual impedance droop control, comprising: a data acquisition device for real-time measurement of the active power and reactive power output by the inverter;

[0030] An adaptive virtual impedance control device is configured as follows:

[0031] The active and reactive power are input into the improved droop controller to generate frequency reference values ​​and voltage amplitude reference values.

[0032] The difference between the reactive power and the reference value is calculated, and the virtual impedance is dynamically adjusted through the integral controller.

[0033] The voltage amplitude reference value is added to the voltage compensation amount to obtain the voltage amplitude command, and the frequency reference value is added to the frequency compensation amount to obtain the frequency command.

[0034] Voltage is synthesized using the voltage amplitude command and frequency command to generate a voltage reference value in the dq coordinate system. This voltage reference value is used as the reference input of the dual closed-loop control system and, after SPWM modulation, drives the inverter bridge to regulate the output voltage.

[0035] As a preferred embodiment of the islanded microgrid control system based on adaptive virtual impedance droop control described in this invention, the active power and reactive power are input to the improved droop controller to generate frequency reference values ​​and voltage amplitude reference values, including:

[0036] The central controller calculates the reactive power reference value for each distributed power source based on the total load reactive power and the capacity of each distributed power source, and sends it to the local controller of each distributed power source.

[0037] When the capacity of distributed power sources is the same, the central controller will distribute the total load reactive power equally, and the reactive power reference value of each inverter is equal to the total load reactive power divided by the number of parallel power sources.

[0038] As a preferred embodiment of the islanded microgrid control system based on adaptive virtual impedance droop control described in this invention, the calculation of the difference between reactive power and a reference value, and the dynamic adjustment of the virtual impedance by the integral controller, includes: when the reactive power is less than the reactive power reference value and the difference between the reactive power and the reference value is greater than 0, the virtual impedance output by the integral controller increases; when the reactive power is greater than the reactive power reference value and the difference between the reactive power and the reference value is less than 0, the virtual impedance output by the integral controller decreases.

[0039] As a preferred embodiment of the islanded microgrid control system based on adaptive virtual impedance droop control described in this invention, it further includes: if N distributed power sources are connected in parallel, and the reactive power of the load is Q... L Then the inverter's given reactive power is

[0040] Q av =Q L / N

[0041] The improved expression for the inverter output voltage reference value in the dq coordinate system after droop control is:

[0042]

[0043] Among them, U dref and U qref U represents the reference values ​​of the d-axis and q-axis components of the inverter output voltage in the dq coordinate system, respectively. d and U q Z represents the actual d-axis and actual q-axis components of the inverter output voltage in the dq coordinate system. v I represents the virtual impedance. Ld and I Lq These represent the d-axis and q-axis components of the load current in the dq coordinate system, respectively.

[0044] As a preferred embodiment of the islanded microgrid control system based on adaptive virtual impedance droop control described in this invention, the following steps are taken: Adding the voltage amplitude reference value to the voltage compensation amount to obtain the voltage amplitude command; adding the frequency reference value to the frequency compensation amount to obtain the frequency command includes:

[0045] The voltage amplitude command is expressed as:

[0046]

[0047] Among them, U i U represents the reference value of the output voltage amplitude of the i-th inverter in the dq coordinate system. * The system's desired reference voltage amplitude is represented by 'n', which represents the reactive power droop factor, and Q is the value of the reference voltage. iZ represents the reactive power output of the i-th inverter. vi This represents the virtual impedance of the i-th inverter;

[0048] Frequency commands are expressed as:

[0049] f i =f * -mP i +k f ∫(f * -f i )dt+k c ∫(P av -P i )dt

[0050] Among them, f i f represents the frequency output of the i-th inverter. * The system's desired reference frequency is represented by m, which represents the active power droop factor, and P is the active power droop factor. i k represents the active power output of the i-th inverter. f k represents the frequency deviation integral gain. c P represents the frequency compensation coefficient. av This represents the average value of the active power output by the distributed generation unit.

[0051] Compared with existing technologies, the advantages of this invention are as follows: The improved droop control of this invention dynamically adjusts the reactive power distribution of the lines in an islanded microgrid by introducing virtual impedance, thereby improving the stability of the system's grid-connected operation. The improved droop control utilizes the reactive power information output by distributed generation sources to construct an adaptive virtual impedance, eliminating equivalent line impedance differences, reducing system reactive power circulating current, and thus improving the accuracy of reactive power distribution. The improved droop control adds a voltage compensation stage for secondary control to suppress voltage dips on the common bus. The improved droop control constructs a self-frequency recovery stage based on the integral term of the difference between the distributed generation's own frequency and the frequency reference value, and a frequency compensation stage based on the integral term of the difference between the distributed generation's output active power and the average active power value, to compensate for frequency fluctuations. Attached Figure Description

[0052] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. Wherein:

[0053] Figure 1 This is a schematic flowchart of an islanded microgrid control method based on adaptive virtual impedance droop control according to an embodiment of the present invention.

[0054] Figure 2 This is a schematic diagram of the main circuit of an islanded microgrid according to an embodiment of the islanded microgrid control method based on adaptive virtual impedance droop control according to an embodiment of the present invention.

[0055] Figure 3 This is a conventional droop control block diagram of an islanded microgrid control method based on adaptive virtual impedance droop control, as described in an embodiment of the present invention.

[0056] Figure 4 This is a schematic diagram of the islanded microgrid control strategy of the islanded microgrid control method based on adaptive virtual impedance droop control according to an embodiment of the present invention.

[0057] Figure 5 This is an adaptive virtual impedance control structure diagram of an islanded microgrid control method based on adaptive virtual impedance droop control according to an embodiment of the present invention.

[0058] Figure 6 The output active power waveform of three parallel distributed power sources in an islanded microgrid control method based on adaptive virtual impedance droop control, as described in an embodiment of the present invention, is shown.

[0059] Figure 7 The output reactive power waveform of three parallel distributed power sources in an islanded microgrid control method based on adaptive virtual impedance droop control, as described in an embodiment of the present invention, is shown.

[0060] Figure 8 This is a waveform diagram of the output bus voltage of an islanded microgrid control method based on adaptive virtual impedance droop control according to an embodiment of the present invention.

[0061] Figure 9 The output bus frequency waveform diagram is shown in one embodiment of the islanded microgrid control method based on adaptive virtual impedance droop control according to an embodiment of the present invention. Detailed Implementation

[0062] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of the present invention.

[0063] Terminology Explanation: DG: Distributed Generation.

[0064] Example 1, referring to Figures 1-5This is one embodiment of the present invention, which provides a control method for islanded microgrids based on adaptive virtual impedance droop control, such as... Figure 1 As shown, it includes:

[0065] S100: Real-time measurement of the active and reactive power output of the inverter;

[0066] S200: Input active and reactive power into the improved droop controller to generate frequency reference values ​​and voltage amplitude reference values;

[0067] S300: Calculates the difference between reactive power and reference value, and dynamically adjusts the virtual impedance through an integral controller;

[0068] S400: Add the voltage amplitude reference value to the voltage compensation amount to obtain the voltage amplitude command; add the frequency reference value to the frequency compensation amount to obtain the frequency command.

[0069] S500: Voltage is synthesized using voltage amplitude and frequency commands to generate a voltage reference value in the dq coordinate system. The voltage reference value is used as the reference input of the dual closed-loop control system. After SPWM modulation, it drives the inverter bridge to regulate the output voltage.

[0070] It should be noted that traditional droop control, which simulates the behavior of generators in a power system, adjusts its output power by controlling the frequency and voltage of distributed generation (DG), and is an effective control scheme. However, in islanded microgrids, inconsistent line parameters lead to uneven power output, making it difficult to maximize the efficiency of DG and even causing circulating current problems, affecting system stability. This invention proposes an improved droop control strategy for islanded microgrids, based on traditional DG droop control. Firstly, it introduces an adaptive virtual impedance. The inverter adjusts the virtual impedance value through an integral controller based on the difference between a given reactive power output reference value and the actual reactive power output, compensating for voltage drop caused by line impedance differences, thereby achieving even reactive power distribution. Secondly, to address the voltage drop problem of traditional droop control, a voltage compensation stage is added for secondary control. To address the frequency offset problem of traditional droop control, an integral term based on the difference between the DG's own frequency and the frequency reference value is constructed as a self-frequency recovery stage, and an integral term based on the difference between the DG's output active power and the average active power value is constructed as a frequency compensation stage.

[0071] For the main circuit of an isolated microgrid, such as Figure 2 As shown. Figure 3 As shown, the control improvement of the present invention is based on Figure 3Traditional droop control strategies, due to the introduction of the 1 / s integral term, ensure that active power is unaffected by the equivalent output impedance. However, reactive power / voltage droop lacks an integral term, meaning that even with identical inverter equivalent output impedances, impedance mismatch between the inverter and the point of common coupling can lead to different output voltages, thus affecting the accurate distribution of reactive power. Therefore, an improved droop control strategy based on adaptive virtual impedance is proposed.

[0072] like Figure 4 As shown, the improved droop control strategy based on adaptive virtual impedance adopted in this invention differs from the traditional droop control strategy in that it introduces virtual impedance, constructs an adaptive virtual impedance control structure, implements secondary control in the voltage compensation stage, and compensates for frequency fluctuations.

[0073] It should be noted that traditional droop control regulates the output frequency voltage through active and reactive power adjustments. However, due to the influence of line resistance, coupling exists between reactive and active power, affecting the system's stability and accuracy. This invention, by introducing virtual impedance, effectively controls droop performance and improves control accuracy.

[0074] In an optional embodiment, to address the problem of unreasonable reactive power allocation caused by differences in equivalent line impedance, an adaptive virtual impedance control strategy is proposed, such as... Figure 5 As shown. Figure 5 China Z v For adaptive virtual impedance, U d U q Both θ and Q are derived from the droop controller. av K represents the setpoint for the inverter's output reactive power, where Q is the actual value of the inverter's output reactive power; i Here, represents the integral coefficient. By constructing a virtual impedance related to reactive power information, the virtual impedance value is adaptively adjusted to reduce reactive power distribution errors. The expression for the adaptive virtual impedance is:

[0075]

[0076] Q av The reactive power is calculated by the central controller based on the load power and the power of each inverter, and then sent to the local controller of each inverter. The central controller allocates reactive power according to the DG capacity. For simplification, assuming all DG capacities are the same, the load power is then equally distributed among all inverters.

[0077] In this embodiment of the invention, step S200, which inputs active power and reactive power into an improved droop controller to generate frequency reference values ​​and voltage amplitude reference values, includes:

[0078] The central controller calculates the reactive power reference value for each distributed power source based on the total load reactive power and the capacity of each distributed power source, and sends it to the local controller of each distributed power source.

[0079] When the capacity of distributed power sources is the same, the central controller will distribute the total load reactive power equally, and the reactive power reference value of each inverter is equal to the total load reactive power divided by the number of parallel power sources.

[0080] In this embodiment of the invention, step S200 further includes: if N distributed power sources are connected in parallel, and the reactive power of the load is Q. L Then the inverter's given reactive power is

[0081] Q av =Q L / N

[0082] The improved expression for the inverter output voltage reference value in the dq coordinate system after droop control is:

[0083]

[0084] Among them, U dref and U qref U represents the reference values ​​of the d-axis and q-axis components of the inverter output voltage in the dq coordinate system, respectively. d and U q Z represents the actual d-axis and actual q-axis components of the inverter output voltage in the dq coordinate system. v I represents the virtual impedance. Ld and I Lq These represent the d-axis and q-axis components of the load current in the dq coordinate system, respectively.

[0085] It should be noted that after the introduction of virtual impedance, the reactive power difference of the parallel DG output is significantly reduced, and the power distribution accuracy is greatly improved.

[0086] In this embodiment of the invention, the step S300 of calculating the difference between reactive power and the reference value and dynamically adjusting the virtual impedance by the integral controller includes: when the reactive power is less than the reactive power reference value and the difference between the reactive power and the reference value is greater than 0, the virtual impedance output by the integral controller increases; when the reactive power is greater than the reactive power reference value and the difference between the reactive power and the reference value is less than 0, the virtual impedance output by the integral controller decreases.

[0087] In an optional embodiment, to further suppress voltage fluctuations, for Figure 3 The traditional droop control shown is improved and optimized. The DG of the i-th line... i In virtual impedance Z vi The pressure drop generated above is

[0088]

[0089] In this embodiment of the invention, step S400, which involves adding the voltage amplitude reference value to the voltage compensation amount to improve reactive voltage droop and obtain a voltage amplitude command, includes:

[0090] The voltage amplitude command is expressed as:

[0091]

[0092] Among them, U i U represents the reference value of the output voltage amplitude of the i-th inverter in the dq coordinate system. * The system's desired reference voltage amplitude is represented by 'n', which represents the reactive power droop factor, and Q is the value of the reference voltage. i Z represents the reactive power output of the i-th inverter. vi This represents the virtual impedance of the i-th inverter;

[0093] In one optional embodiment, conventional drooping can achieve reasonable distribution of active power, but the frequency will deviate from the rated value to some extent. Therefore, further... Figure 3 The traditional droop control shown is improved and optimized. To achieve better power transmission efficiency, an integral term based on the difference between the DG's own frequency and the frequency reference value is introduced as a self-frequency recovery stage:

[0094] Δf i =k f ∫(f * -f i )dt

[0095] Where, k f This is the self-frequency recovery coefficient.

[0096] Since even a small voltage phase angle difference can lead to different frequency recovery values, a frequency compensation stage based on the integral term of the deviation between the active power output and the mean active power output is introduced:

[0097] Δf i ′ =k c ∫(P av -P i )dt

[0098] Where, k c P is the frequency compensation coefficient. av This represents the average active power output of the DG.

[0099] In this embodiment of the invention, step S400, which involves adding the frequency reference value to the frequency compensation amount to improve the active frequency droop and obtain a frequency command, includes:

[0100] Frequency commands are expressed as:

[0101] f i =f * -mP i +k f ∫(f * -f i )dt+k c ∫(P av -P i )dt

[0102] Among them, f i f represents the frequency output of the i-th inverter. * The system's desired reference frequency is represented by m, which represents the active power droop factor, and P is the active power droop factor. i k represents the active power output of the i-th inverter. f k represents the frequency deviation integral gain. c P represents the frequency compensation coefficient. av This represents the average value of the active power output by the distributed generation unit.

[0103] It should be noted that the improved droop control of this invention dynamically adjusts the reactive power distribution of the microgrid lines by introducing adaptive virtual impedance, thereby improving the stability of grid-connected operation. The improved droop control is based on adaptive virtual impedance control, utilizing reactive power information from the distributed generation (DG) output to construct an adaptive virtual impedance, eliminating equivalent line impedance differences, reducing system reactive circulating current, and thus improving reactive power distribution accuracy. The improved droop control adds a voltage compensation stage for secondary control to suppress voltage dips on the common bus. The improved droop control constructs a self-frequency recovery stage based on the integral term of the difference between the DG's own frequency and the frequency reference value, and a frequency compensation stage based on the integral term of the difference between the DG's output active power and the average active power value, to compensate for frequency fluctuations. The improved droop control is flexible and can be adjusted according to the requirements of different power grids to adapt to various operating conditions.

[0104] Example 2, refer to Figures 2-5 This is one embodiment of the present invention. This embodiment differs from the first embodiment in that it provides an islanded microgrid control system based on adaptive virtual impedance droop control, including: a data acquisition device for real-time measurement of the active power and reactive power output by the inverter;

[0105] An adaptive virtual impedance control device is configured as follows:

[0106] Active and reactive power are input into an improved droop controller to generate frequency and voltage amplitude reference values.

[0107] The difference between the reactive power and the reference value is calculated, and the virtual impedance is dynamically adjusted through the integral controller.

[0108] The voltage amplitude reference value is added to the voltage compensation amount to obtain the voltage amplitude command, and the frequency reference value is added to the frequency compensation amount to obtain the frequency command.

[0109] Voltage is synthesized using voltage amplitude and frequency commands to generate a voltage reference value in the dq coordinate system. This voltage reference value is used as the reference input of the dual closed-loop control system and, after SPWM modulation, drives the inverter bridge to regulate the output voltage.

[0110] In this embodiment of the invention, active power and reactive power are input into an improved droop controller to generate frequency reference values ​​and voltage amplitude reference values, including:

[0111] The central controller calculates the reactive power reference value for each distributed power source based on the total load reactive power and the capacity of each distributed power source, and sends it to the local controller of each distributed power source.

[0112] When the capacity of distributed power sources is the same, the central controller will distribute the total load reactive power equally, and the reactive power reference value of each inverter is equal to the total load reactive power divided by the number of parallel power sources.

[0113] In this embodiment of the invention, calculating the difference between reactive power and a reference value, and dynamically adjusting the virtual impedance by an integral controller includes: when the reactive power is less than the reactive power reference value and the difference between the reactive power and the reference value is greater than 0, the virtual impedance output by the integral controller increases; when the reactive power is greater than the reactive power reference value and the difference between the reactive power and the reference value is less than 0, the virtual impedance output by the integral controller decreases.

[0114] In this embodiment of the invention, it further includes: if N distributed power sources are connected in parallel, and the reactive power of the load is Q... L Then the inverter's given reactive power is

[0115] Q av =Q L / N

[0116] The improved expression for the inverter output voltage reference value in the dq coordinate system after droop control is:

[0117]

[0118] Among them, U dref and U qref U represents the reference values ​​of the d-axis and q-axis components of the inverter output voltage in the dq coordinate system, respectively. d and U q Z represents the actual d-axis and actual q-axis components of the inverter output voltage in the dq coordinate system. v I represents the virtual impedance. Ldand I Lq These represent the d-axis and q-axis components of the load current in the dq coordinate system, respectively.

[0119] In this embodiment of the invention, the method includes: adding a voltage amplitude reference value to a voltage compensation amount to obtain a voltage amplitude command; and adding a frequency reference value to a frequency compensation amount to obtain a frequency command, including:

[0120] The voltage amplitude command is expressed as:

[0121]

[0122] Among them, U i U represents the reference value of the output voltage amplitude of the i-th inverter in the dq coordinate system. * The system's desired reference voltage amplitude is represented by 'n', which represents the reactive power droop factor, and Q is the value of the reference voltage. i Z represents the reactive power output of the i-th inverter. vi This represents the virtual impedance of the i-th inverter;

[0123] Frequency commands are expressed as:

[0124] f i =f * -mP i +k f ∫(f * -f i )dt+k c ∫(P av -P i )dt

[0125] Among them, f i f represents the frequency output of the i-th inverter. * The system's desired reference frequency is represented by m, which represents the active power droop factor, and P is the active power droop factor. i k represents the active power output of the i-th inverter. f k represents the frequency deviation integral gain. c P represents the frequency compensation coefficient. av This represents the average value of the active power output by the distributed generation unit.

[0126] Example 3, referring to Figures 6-9 This is one embodiment of the present invention. In this embodiment, a simulation experiment is conducted using three distributed power supplies connected in parallel to verify the beneficial effects of the present invention.

[0127] Assuming each distributed power source has the same capacity, the output active power waveform of the three parallel DGs in this embodiment is as follows: Figure 6As shown in the figure. The simulation conditions are set as follows: at 1s, the system load suddenly increases; at 2s, the sudden increase in load is cut off. It can be seen from the simulation waveform that, through the improved droop control strategy, the output active power of the three DGs can respond quickly and maintain even distribution when the load changes suddenly at 1s and 2s, and can remain stable within 0.1s.

[0128] The output reactive power waveform of three parallel DG units is shown below. Figure 7 As shown, from Figure 7 The simulation waveforms show that, through the improved droop control strategy, the reactive power output of the three DG units can respond quickly and remain evenly distributed when the load changes suddenly at 1s and 2s, and can remain stable within 0.1s.

[0129] Figure 8 This is a waveform diagram of the output bus voltage of the system applying the strategy proposed in this invention. The simulation waveform shows that the improved droop control strategy can stabilize the voltage within 0.1s when the load changes suddenly at 1s and 2s, effectively suppressing voltage drops. Figure 9 This is a waveform diagram of the bus frequency at the output end of the system applying the strategy proposed in this invention. The simulation waveform shows that the improved droop control strategy can stabilize the frequency within 0.1s when the load changes suddenly at 1s and 2s, effectively suppressing frequency fluctuations. The improved droop control of this invention introduces virtual impedance to dynamically adjust the reactive power distribution of the islanded microgrid lines, thereby improving the stability of the system's grid-connected operation. The improved droop control utilizes the reactive power information output by distributed generation sources to construct an adaptive virtual impedance, eliminating equivalent line impedance differences, reducing system reactive circulating current, and thus improving the accuracy of reactive power distribution. The improved droop control adds a voltage compensation stage for secondary control to suppress voltage dips on the common bus. The improved droop control constructs a self-frequency recovery stage based on the integral term of the difference between the distributed generation's own frequency and the frequency reference value, and a frequency compensation stage based on the integral term of the difference between the distributed generation's output active power and the average active power value, to compensate for frequency fluctuations.

[0130] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A control method for an islanded microgrid based on adaptive virtual impedance droop control, characterized in that, include: Real-time measurement of the active and reactive power output of the inverter; The active and reactive power are input into the improved droop controller to generate frequency reference values ​​and voltage amplitude reference values. The difference between the reactive power and the reference value is calculated, and the virtual impedance is dynamically adjusted through the integral controller. The voltage amplitude reference value is added to the voltage compensation amount to obtain the voltage amplitude command, and the frequency reference value is added to the frequency compensation amount to obtain the frequency command. Voltage is synthesized using the voltage amplitude command and frequency command to generate a voltage reference value in the dq coordinate system. This voltage reference value is used as the reference input of the dual closed-loop control system and, after SPWM modulation, drives the inverter bridge to regulate the output voltage.

2. The islanded microgrid control method based on adaptive virtual impedance droop control as described in claim 1, characterized in that, The active and reactive power are input into the improved droop controller to generate frequency reference values ​​and voltage amplitude reference values, including: The central controller calculates the reactive power reference value for each distributed power source based on the total load reactive power and the capacity of each distributed power source, and sends it to the local controller of each distributed power source. When the capacity of distributed power sources is the same, the central controller will distribute the total load reactive power equally, and the reactive power reference value of each inverter is equal to the total load reactive power divided by the number of parallel power sources.

3. The islanded microgrid control method based on adaptive virtual impedance droop control as described in claim 2, characterized in that, The difference between reactive power and the reference value is calculated, and the virtual impedance is dynamically adjusted by the integral controller. This includes: when the reactive power is less than the reactive power reference value and the difference between the reactive power and the reference value is greater than 0, the virtual impedance output by the integral controller increases; when the reactive power is greater than the reactive power reference value and the difference between the reactive power and the reference value is less than 0, the virtual impedance output by the integral controller decreases.

4. The islanded microgrid control method based on adaptive virtual impedance droop control as described in claim 2, characterized in that, Also includes: If N distributed power sources are connected in parallel, the reactive power of the load is Q. L Then the inverter's given reactive power is Q av =Q L / N The improved expression for the inverter output voltage reference value in the dq coordinate system after droop control is: Among them, U dref and U qref U represents the reference values ​​of the d-axis and q-axis components of the inverter output voltage in the dq coordinate system, respectively. d and U q Z represents the actual d-axis and actual q-axis components of the inverter output voltage in the dq coordinate system. v I represents the virtual impedance. Ld and I Lq These represent the d-axis and q-axis components of the load current in the dq coordinate system, respectively.

5. The islanded microgrid control method based on adaptive virtual impedance droop control as described in claim 4, characterized in that, The voltage amplitude reference value is added to the voltage compensation amount to obtain the voltage amplitude command. The frequency reference value is added to the frequency compensation amount to obtain the frequency command, which includes: The voltage amplitude command is expressed as: Among them, U i U represents the reference value of the output voltage amplitude of the i-th inverter in the dq coordinate system. * The system's desired reference voltage amplitude is represented by 'n', which represents the reactive power droop factor, and Q is the value of the reference voltage. i Z represents the reactive power output of the i-th inverter. vi This represents the virtual impedance of the i-th inverter; Frequency commands are expressed as: f i =f * -mP i +k f ∫(f * -f i )dt+k c ∫(P av -P i )dt Among them, f i f represents the frequency output of the i-th inverter. * The system's desired reference frequency is represented by m, which represents the active power droop factor, and P is the active power droop factor. i k represents the active power output of the i-th inverter. f k represents the frequency deviation integral gain. c P represents the frequency compensation coefficient. av This represents the average value of the active power output by the distributed generation unit.

6. An islanded microgrid control system based on adaptive virtual impedance droop control, characterized in that, include: Data acquisition device, used to measure the active and reactive power output of the inverter in real time; An adaptive virtual impedance control device is configured as follows: The active and reactive power are input into the improved droop controller to generate frequency reference values ​​and voltage amplitude reference values. The difference between the reactive power and the reference value is calculated, and the virtual impedance is dynamically adjusted through the integral controller. The voltage amplitude reference value is added to the voltage compensation amount to obtain the voltage amplitude command, and the frequency reference value is added to the frequency compensation amount to obtain the frequency command. Voltage is synthesized using the voltage amplitude command and frequency command to generate a voltage reference value in the dq coordinate system. This voltage reference value is used as the reference input of the dual closed-loop control system and, after SPWM modulation, drives the inverter bridge to regulate the output voltage.

7. The islanded microgrid control system based on adaptive virtual impedance droop control as described in claim 6, characterized in that, The active and reactive power are input into the improved droop controller to generate frequency reference values ​​and voltage amplitude reference values, including: The central controller calculates the reactive power reference value for each distributed power source based on the total load reactive power and the capacity of each distributed power source, and sends it to the local controller of each distributed power source. When the capacity of distributed power sources is the same, the central controller will distribute the total load reactive power equally, and the reactive power reference value of each inverter is equal to the total load reactive power divided by the number of parallel power sources.

8. The islanded microgrid control system based on adaptive virtual impedance droop control as described in claim 7, characterized in that, The difference between reactive power and the reference value is calculated, and the virtual impedance is dynamically adjusted by the integral controller. This includes: when the reactive power is less than the reactive power reference value and the difference between the reactive power and the reference value is greater than 0, the virtual impedance output by the integral controller increases; when the reactive power is greater than the reactive power reference value and the difference between the reactive power and the reference value is less than 0, the virtual impedance output by the integral controller decreases.

9. The islanded microgrid control system based on adaptive virtual impedance droop control as described in claim 8, characterized in that, Also includes: If N distributed power sources are connected in parallel, the reactive power of the load is Q. L Then the inverter's given reactive power is Q av =Q L / N The improved expression for the inverter output voltage reference value in the dq coordinate system after droop control is: Among them, U dref and U qref U represents the reference values ​​of the d-axis and q-axis components of the inverter output voltage in the dq coordinate system, respectively. d and U q Z represents the actual d-axis and actual q-axis components of the inverter output voltage in the dq coordinate system. v I represents the virtual impedance. Ld and I Lq These represent the d-axis and q-axis components of the load current in the dq coordinate system, respectively.

10. The islanded microgrid control system based on adaptive virtual impedance droop control as described in claim 9, characterized in that, include: The voltage amplitude reference value is added to the voltage compensation amount to obtain the voltage amplitude command. The frequency reference value is added to the frequency compensation amount to obtain the frequency command, which includes: The voltage amplitude command is expressed as: Among them, U i U represents the reference value of the output voltage amplitude of the i-th inverter in the dq coordinate system. * The system's desired reference voltage amplitude is represented by 'n', which represents the reactive power droop factor, and Q is the value of the reference voltage. i Z represents the reactive power output of the i-th inverter. vi This represents the virtual impedance of the i-th inverter; Frequency commands are expressed as: f i =f * -mP i +k f ∫(f * -f i )dt+k c ∫(P av -P i )dt Among them, f i f represents the frequency output of the i-th inverter. * The system's desired reference frequency is represented by m, which represents the active power droop factor, and P is the active power droop factor. i k represents the active power output of the i-th inverter. f k represents the frequency deviation integral gain. c P represents the frequency compensation coefficient. av This represents the average value of the active power output by the distributed generation unit.

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