Photovoltaic voltage support method based on adaptive mode switching in direct current micro-grid

Abstract

The invention discloses a photovoltaic voltage support method based on adaptive mode switching in a DC micro-grid, and the method comprises the steps: designing a photovoltaic power representation variable according to the output characteristics of the photovoltaic, enabling the power representation variable to unify an MPPT mode and a voltage regulation mode, and enabling the power representation variable to quantitatively represent the proportion of the photovoltaic output to the maximum available power; the influence of the number and type parameters of the photovoltaic modules is basically avoided; adaptive smooth operation mode switching is realized through droop control of a direct current voltage-power representation variable, photovoltaic output power is adjusted through a power representation variable adjusting ring and a photovoltaic voltage control ring, and active voltage support is realized. According to the invention, communication between photovoltaic systems is not needed, the voltage support of adaptive mode switching can be realized by using a unified photovoltaic controller, the capacity pressure of the energy storage system is reduced in the operation process, and the investment and maintenance cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic power generation technology in DC microgrids, and specifically to a voltage support method for photovoltaics in DC microgrids based on adaptive mode switching. Background Technology

[0002] With the rapid development of renewable energy, photovoltaic (PV) power generation, as an important form of clean energy, is increasingly being widely applied in microgrids. In DC-connected scenarios such as DC microgrids or distribution networks, stable control of DC voltage mainly relies on the public grid or energy storage within the system. However, as the penetration rate of PV power increases, its inherent intermittent and random characteristics will lead to problems such as overvoltage and battery overcharging, posing challenges to the stable operation of weak or isolated microgrids and placing enormous pressure on energy storage systems. Therefore, constructing PV systems with active voltage support capabilities has become an inevitable choice to ensure the stable operation of microgrids in high-penetration PV scenarios. For this reason, PV operating modes should not only have maximum power point tracking (MPPT) mode but also voltage regulation mode. Traditionally, these two operating modes correspond to different controllers and follow a dual-controller hard-switching paradigm. Direct switching between the two operating modes will cause a sudden change in the internal current loop reference value, leading to drastic fluctuations in bus voltage and PV output. Although adding an additional compensator can enable the inactive controller to track the current output of the active controller in real time, preventing sudden changes in duty cycle, this also increases the complexity of controller design and reliability risks. Therefore, developing an adaptive smooth switching algorithm based on a unified control framework and constructing a new control system with autonomous mode recognition and seamless transition capabilities has become a core technical requirement for achieving dynamic and stable operation of high-penetration photovoltaic systems. Summary of the Invention

[0003] The purpose of this invention is to address the shortcomings of existing technologies by providing a voltage support method for photovoltaic (PV) systems in DC microgrids based on adaptive mode switching. This invention constructs a PV power characterization variable S to quantify the proportion of PV output to the maximum available power, and adaptively achieves smooth and seamless mode switching and transition between MPPT (Multi-Level Photovoltaic Power Transmission) mode and voltage regulation mode based on real-time DC voltage changes. Furthermore, during operation, it reduces the capacity pressure on the energy storage system, thereby lowering investment and maintenance costs.

[0004] The objective of this invention is achieved through the following technical solution: a voltage support method for photovoltaic systems in a DC microgrid based on adaptive mode switching, comprising the following steps: (1) Collect the photovoltaic voltage and photovoltaic current output by the photovoltaic array and the DC microgrid bus voltage; (2) In the power characterization variable adjustment loop, the power characterization variable is calculated based on the collected photovoltaic voltage and photovoltaic current, and the power characterization variable at the current sampling time and the previous N-1 historical time is subjected to moving average filtering to generate the average value of the smoothed power characterization variable. (3) Based on the collected DC microgrid bus voltage, rated DC voltage value, rated power characterization variable value and droop coefficient, droop control is carried out through the droop control equation of DC voltage-power characterization variable to obtain the reference value of power characterization variable and impose an upper limit constraint on it to realize adaptive operation mode switching. (4) Subtract the average value of the power characterization variable from the reference value of the power characterization variable after the upper limit constraint to obtain the deviation value, and then obtain the photovoltaic voltage reference value through the PI controller; (5) In the photovoltaic voltage control loop, the error is obtained by subtracting the collected photovoltaic voltage from the photovoltaic voltage reference value. Then, the PWM signal is generated by the PI controller to control the switching transistor in the DC / DC converter to adjust the photovoltaic voltage, forming a closed-loop control and realizing active voltage support.

[0005] Furthermore, the formula for calculating the power characterization variable is as follows:

[0006] In the formula, The instantaneous value of the power characterization variable. The photovoltaic voltage output by the photovoltaic array. The photovoltaic current output by the photovoltaic array. This represents the change in photovoltaic voltage at the current sampling time relative to the previous sampling time. This represents the change in photovoltaic current at the current sampling time relative to the previous sampling time.

[0007] Furthermore, the average value of the power characterization variable is generated according to the following formula:

[0008] In the formula, S is the average value of the power characterization variable, and N is the preset sampling window length. Let be the power representation variable relative to the previous k sampling times i.

[0009] Furthermore, the reference value of the power characterization variable is obtained according to the following formula:

[0010] In the formula, Here, m is the reference value for the power characterization variable, and m is the droop coefficient. This is the rated DC voltage value. This is the bus voltage of the DC microgrid. The value of the variable is the set rated power.

[0011] Furthermore, the upper limit constraint is specifically implemented through the following method: The power characterization variable reference value is input into a saturator with an upper limit of -1 for upper limit constraint.

[0012] Furthermore, the strategy for switching operating modes specifically includes: When the DC microgrid bus voltage equals the rated DC voltage, it is determined to be in grid-connected operation. The DC microgrid bus voltage is controlled by the main grid, and the photovoltaic array does not need to participate in voltage regulation. At this time, the power characterization variable reference value is -1, the photovoltaic array outputs maximum power, and operates in maximum power point tracking mode. When the DC microgrid bus voltage is less than the rated DC voltage, it is determined to be islanded undervoltage. At this time, the power characterization variable reference value is greater than -1, and the upper limit constraint makes the power characterization variable reference value -1. The photovoltaic array outputs maximum power and operates in maximum power point tracking mode. When the DC microgrid bus voltage is greater than the rated DC voltage, it is determined to be islanded overvoltage. The photovoltaic array needs to participate in voltage support regulation and operates in voltage regulation mode. By activating droop control to reduce the power characterization variable reference value, the output power of the photovoltaic array is guided to decrease until the system is balanced and stable.

[0013] Furthermore, the control law for the photovoltaic voltage reference value is:

[0014]

[0015] In the formula, This is a reference value for photovoltaic voltage. and Let represent the proportional and integral control parameters of the PI controller in the power characterization variable control loop, respectively, and s be the Laplace operator. Let S be the deviation value of the power characterization variable, and S be the average value of the power characterization variable. These are reference values ​​for the power characterization variables after the upper limit constraint.

[0016] Furthermore, the control law of the PWM signal is:

[0017]

[0018] In the formula, This is a PWM signal used to control the on / off state of the switching transistors in a DC / DC converter. and Let represent the proportional and integral control parameters of the PI controller in the photovoltaic voltage control loop, respectively, and s be the Laplace operator. For the error of photovoltaic voltage, This is a reference value for photovoltaic voltage. This is the photovoltaic voltage.

[0019] Compared with the prior art, the beneficial effects of the present invention are: (1) The photovoltaic power characterization variable designed according to the photovoltaic power output characteristics of the photovoltaic itself unifies the MPPT mode and voltage regulation mode. It adopts the droop control of DC voltage-characterization variable and adaptively realizes the smooth mode switching and seamless connection between MPPT mode and voltage regulation mode based on the continuously changing characterization variable command value.

[0020] (2) The photovoltaic power characterization variable of the present invention can quantitatively characterize the proportion of photovoltaic output to the maximum available power, and is basically unaffected by the number and type parameters of photovoltaic modules. This feature enables the present invention to be widely applied to different types of photovoltaic systems, enhancing its applicability and flexibility.

[0021] (3) The present invention does not rely on the integrated energy storage device inside the photovoltaic system to achieve voltage regulation, but works efficiently in conjunction with the system-level energy storage system, and can reduce the capacity and cost pressure of the energy storage system. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the overall voltage support method for photovoltaics based on adaptive mode switching in DC microgrids; Figure 2 This is a schematic diagram of the photovoltaic power-power characterization variable characteristic curves under different component numbers; Figure 3 This is a schematic diagram of the photovoltaic power-power characterization variable characteristic curves under different component types; Figure 4 This is a schematic diagram comparing the output power changes of photovoltaic arrays under different control strategies after load changes; Figure 5 This is a schematic diagram comparing the output power changes of energy storage using different control strategies after load changes; Figure 6 This is a schematic diagram comparing the bus voltage of a DC microgrid using different control strategies after load changes. Detailed Implementation

[0023] Exemplary embodiments will now be described in detail, examples of which are illustrated in the accompanying drawings. When the following description relates to the drawings, unless otherwise indicated, the same numerals in different drawings denote the same or similar elements. The embodiments described in the following exemplary embodiments do not represent all embodiments consistent with the present invention. Rather, they are merely examples of apparatuses and methods consistent with some aspects of the invention as detailed in the appended claims.

[0024] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The singular forms “a,” “the,” and “the” used in this invention and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise. It should also be understood that the term “and / or” as used herein refers to and includes any or all possible combinations of one or more of the associated listed items.

[0025] It should be understood that although the terms first, second, third, etc., may be used in this invention to describe various information, this information should not be limited to these terms. These terms are only used to distinguish information of the same type from one another. For example, first information may also be referred to as second information without departing from the scope of this invention, and similarly, second information may also be referred to as first information. Depending on the context, the word "if" as used herein may be interpreted as "when," "when," or "in response to a determination."

[0026] The present invention will now be described in detail with reference to the accompanying drawings. Unless otherwise specified, the features of the following embodiments and implementations can be combined with each other.

[0027] This invention provides a voltage support method for photovoltaics in a DC microgrid based on adaptive mode switching. A photovoltaic power characterization variable is designed according to the photovoltaic's own output characteristics. This power characterization variable unifies the MPPT mode and voltage regulation mode, and it can quantify the proportion of photovoltaic output to the maximum available power, largely unaffected by the number and type of photovoltaic modules. Adaptive and smooth operation mode switching is achieved through the droop control of the DC voltage-power characterization variable. Active voltage support is realized by adjusting the photovoltaic output power through a power characterization variable adjustment loop and a photovoltaic voltage control loop. Figure 1 This demonstrates the overall architecture and control scheme of a photovoltaic power generation system in a DC microgrid. In the DC microgrid, on the DC bus side, the photovoltaic array is connected to the DC bus capacitor. It is connected to a DC / DC converter and then connected to the DC bus network through the DC / DC converter.

[0028] like Figure 1 As shown, the voltage support method specifically includes the following steps: (1) Acquire the photovoltaic voltage output of the photovoltaic array and photovoltaic current and DC microgrid bus voltage .

[0029] (2) In the power characterization variable adjustment loop, based on the photovoltaic voltage collected in step (1) and photovoltaic current Calculate power characterization variables And the power characterization variables at the current sampling time and the previous N-1 historical times. A moving average filter is applied to generate a smoothed average value S of the power representation variable, which is then fed into the subsequent control loop.

[0030] It should be understood that for power characterization variables Smoothing can effectively suppress noise and ensure the stability and robustness of the control system.

[0031] Furthermore, power characterization variables The calculation formula is:

[0032] In the formula, The instantaneous value of the power characterization variable. The photovoltaic voltage output by the photovoltaic array. The photovoltaic current output by the photovoltaic array. This represents the change in photovoltaic voltage at the current sampling time relative to the previous sampling time. This represents the change in photovoltaic current at the current sampling time relative to the previous sampling time.

[0033] Figure 2 and Figure 3 The photovoltaic power output is shown for different numbers and types of modules. and The characteristic relationship curve, where the component refers to the photovoltaic array, This indicates the actual photovoltaic power output of the photovoltaic array. This indicates the actual maximum usable power of photovoltaic power. The photovoltaic output represents the proportion of maximum available power, characterizing the photovoltaic operating status, and its range is [0,1]. Variable The value at the maximum power point is always -1. and The relationship between components is not affected by the number of components, and different numbers of components and The characteristic curves completely overlap; and The relationship between them is minimally affected by component parameters, remaining within an acceptable range, thus... Becoming a quantitative representation of real-time photovoltaic power output An excellent variable, defined This is a variable characterizing photovoltaic power. Because... and Characteristic curves at The wide region with a value of <-1 covers the entire power range of photovoltaics, so this region is selected for photovoltaic power adjustment. Therefore, subsequent steps use a saturator with an upper limit of -1 to constrain the upper limit.

[0034] Furthermore, the average value S of the power characterization variable is generated according to the following formula:

[0035] In the formula, S is the average value of the power characterization variable, and N is the preset sampling window length. Let be the power representation variable relative to the previous k sampling times i.

[0036] (3) Based on the DC microgrid bus voltage collected in step (1) and rated DC voltage value Rated power characterization variable value The droop coefficient m is used to control the droop through the droop control equation of the DC voltage-power characterization variable, so as to obtain the reference value of the power characterization variable. It also imposes upper limit constraints on it to achieve adaptive switching of operating modes.

[0037] Furthermore, the reference value of the power characterization variable Obtained using the following formula:

[0038] In the formula, Here, m is the reference value for the power characterization variable, and m is the droop coefficient. This is the rated DC voltage value. The DC microgrid bus voltage acquired in step (1) is... The value of the variable is the set rated power.

[0039] Furthermore, the upper limit constraint is specifically implemented through the following method: the power characterization variable reference value obtained from droop control is used... The upper limit constraint applied to the input to a saturator with an upper limit of -1 can be expressed by the following formula:

[0040] In the above formula, the reference value for the power characterization variable is... Apply upper limit constraints: if the power characterization variable reference value If the value is greater than -1, then the power characterization variable reference value is... Set to 1; if the power characterization variable reference value If the value is less than or equal to -1, then the power characterization variable reference value is... Set it to its own value.

[0041] Furthermore, the strategy for switching operating modes specifically includes: when When the system is in grid-connected operation, the DC microgrid bus voltage is controlled by the main grid, and the photovoltaic array does not need to participate in voltage regulation. At this time, the power characterization variable reference value... A value of -1 indicates that the photovoltaic array outputs maximum power to maximize power generation efficiency, operating in maximum power point tracking mode; when the light intensity decreases or the load increases, it causes... When this occurs, it is determined to be an islanded undervoltage situation. At this time, the load power is too high, and the calculated power characteristic variable reference value is... Subsequently, upper limit constraints were applied to ensure that the reference value of the power characterization variable was... When the value is -1, the photovoltaic array outputs maximum power, cooperating with the remaining energy storage in the DC microgrid to balance the load as much as possible; at this time, it operates in maximum power point tracking mode. When the light intensity increases or the load decreases, the photovoltaic array outputs maximum power, cooperating with the remaining energy storage in the DC microgrid to balance the load as much as possible; at this time, it operates in maximum power point tracking mode. When an islanded overvoltage condition is detected, the photovoltaic array needs to participate in voltage support regulation and operate in voltage regulation mode. At this time, droop control is activated, and the power characterization variable reference value is adjusted. The droop control value is reduced to guide the photovoltaic array output power to decrease until the system is balanced and stable.

[0042] When the power characterization variable reference value A change from -1 to a value less than -1 signifies a transition from MPPT mode to voltage regulation mode; conversely, a change back to MPPT mode signifies the loss of voltage regulation capability. The switching between the two modes is achieved through a unified control structure and continuous... The value is determined, thus avoiding drastic fluctuations in bus voltage and photovoltaic output power caused by current step jumps during controller switching.

[0043] (4) Combine the average value S of the power characterization variable obtained in step (2) with the reference value of the power characterization variable after upper limit constraint obtained in step (3). The deviation value is obtained by subtraction, and then the photovoltaic voltage reference value is obtained through the PI controller. .

[0044] Furthermore, photovoltaic voltage reference value The control law is:

[0045]

[0046] In the formula, This is a reference value for photovoltaic voltage. and Let represent the proportional and integral control parameters of the PI controller in the power characterization variable control loop, respectively, and s be the Laplace operator. Let S be the deviation value of the power characterization variable, and S be the average value of the power characterization variable obtained in step (2). The reference value for the power characterization variable after the upper limit constraint obtained in step (3) is.

[0047] (5) In the photovoltaic voltage control loop, the photovoltaic voltage collected in step (1) is... And the photovoltaic voltage reference value obtained in step (4) The error is obtained by subtraction, and then a PWM signal is generated by the PI controller. Used to control the switching transistors (IGBTs) in the DC / DC converter to regulate the photovoltaic voltage, forming a closed-loop control and achieving active voltage support.

[0048] Furthermore, the PWM signal The control law is:

[0049]

[0050] In the formula, This is a PWM signal used to control the on / off state of the switching transistors in a DC / DC converter. and Let represent the proportional and integral control parameters of the PI controller in the photovoltaic voltage control loop, respectively, and s be the Laplace operator. For the error of photovoltaic voltage, The photovoltaic voltage reference value obtained in step (4) is... The photovoltaic voltage is the voltage collected in step (1).

[0051] For example, to demonstrate the control effect of the method described in this invention, a simulation model of a DC microgrid consisting of three photovoltaic power sources, one energy storage battery, and multiple DC loads was established using Matlab / Simulink, and simulation experiments were conducted. The specific parameters of the three photovoltaic arrays are shown in Table 1.

[0052] Table 1: Photovoltaic Array Parameters

[0053] In this embodiment, the MPPT method and the voltage support method based on adaptive mode switching proposed in this invention are compared and analyzed. Both control schemes use the same solar radiation intensity (1000W / m²), temperature (25°C), and load conditions. The load settings for each time period are 0.51 ohms, 0.635 ohms, and 0.84 ohms, respectively. Figure 4 , Figure 5 , Figure 6 The paper compares the output power of three photovoltaic arrays, the output power of energy storage, and the changes in DC microgrid bus voltage under different control strategies after load changes. Before t=5s, all three photovoltaic arrays were in MPPT mode, the energy storage system power was 0kW, and the bus voltage remained at 550V. From t=5 to 8s, after the load decreased, the photovoltaic array using the method proposed in this invention automatically exited MPPT mode and switched to voltage regulation mode. The mode switching was smooth, the output power decreased, the energy storage system power increased to 62.9kW, and the DC voltage rose to 559V. In contrast, when only MPPT control was used, the energy storage system power increased to 93.7kW, and the DC voltage exceeded 563V. In the 8-10s when the load decreased again, when only MPPT control was used, the energy storage system power increased to 196.7kW, and the DC voltage rose to 567.6V. The method proposed in this invention only increased the energy storage system power to 123.5kW, which is 73kW lower than MPPT control. This reduces the burden on the energy storage system and thus reduces the required energy storage capacity. Furthermore, the DC voltage was increased to only 577.3V, which is nearly 10V lower than that controlled by MPPT, thus further realizing the function of supporting the bus voltage.

[0054] 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 the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for voltage support based on adaptive mode switching of photovoltaic in DC microgrid, characterized in that, The method comprises the following steps: (1) collecting photovoltaic voltage and photovoltaic current output by a photovoltaic array and a DC microgrid bus voltage; (2) in a power characteristic variable adjusting loop, calculating a power characteristic variable according to the collected photovoltaic voltage and photovoltaic current, and performing sliding average filtering processing on the power characteristic variable at a current sampling time and N-1 historical times to generate a power characteristic variable average value after smoothing processing; (3) performing droop control through a DC voltage-power characteristic variable droop control equation according to the collected DC microgrid bus voltage, a rated DC voltage value, a rated power characteristic variable value and a droop coefficient to obtain a power characteristic variable reference value, and performing upper limit constraint to realize adaptive operation mode switching; (4) obtaining a photovoltaic voltage reference value by subtracting the power characteristic variable average value from the power characteristic variable reference value after the upper limit constraint and then through a PI controller; (5) in a photovoltaic voltage control loop, obtaining an error by subtracting the collected photovoltaic voltage from the photovoltaic voltage reference value, and then generating a PWM signal through a PI controller to control a switch tube in a DC / DC converter to adjust the photovoltaic voltage, forming a closed-loop control and realizing active voltage support.

2. The method of claim 1, wherein the PV voltage support based on adaptive mode switching in a DC microgrid is characterized by, The calculation formula of the power characteristic variable is: wherein is the instantaneous value of the power characterization variable, is the photovoltaic voltage output by the photovoltaic array, is the photovoltaic current output by the photovoltaic array, is the photovoltaic voltage variation quantity with respect to the previous sampling instant, is the photovoltaic current variation quantity with respect to the previous sampling instant.

3. The method of claim 1, wherein the PV voltage support based on adaptive mode switching in a DC microgrid is characterized by, The power characteristic variable average value is generated according to the following formula: In the formula, S is the average value of the power characteristic variable, N is the preset sampling window length, is the power characteristic variable relative to the previous k sampling time points of the current sampling time i.

4. The method of claim 1, wherein the PV voltage support based on adaptive mode switching in a DC microgrid is characterized by, The power characteristic variable reference value is obtained according to the following formula: In the formula, is a power characteristic variable reference value, m is a droop coefficient, is a rated DC voltage value, is a DC microgrid bus voltage, is a set rated power characteristic variable value.

5. The method for PV voltage support based on adaptive mode switching in DC microgrid of claim 1, wherein, The upper limit constraint is realized by the following method: The power characteristic variable reference value is input into a saturator with an upper limit value of -1 for upper limit constraint.

6. The method of claim 1, wherein the PV voltage support based on adaptive mode switching in a DC microgrid is characterized by, The strategy of the operation mode switching specifically comprises: When the DC microgrid bus voltage is equal to the rated DC voltage value, it is determined that the system is in grid-connected operation, the DC microgrid bus voltage is controlled by the main grid, and the photovoltaic array does not need to participate in voltage adjustment, at this time, the power characteristic variable reference value is -1, the photovoltaic array outputs maximum power, and operates in a maximum power point tracking mode; when the DC microgrid bus voltage is less than the rated DC voltage value, it is determined that the system is in islanded under-voltage, at this time, the power characteristic variable reference value is greater than -1, and is constrained to -1 through the upper limit constraint, the photovoltaic array outputs maximum power, and operates in the maximum power point tracking mode; when the DC microgrid bus voltage is greater than the rated DC voltage value, it is determined that the system is in islanded over-voltage, and the photovoltaic array needs to participate in voltage support adjustment, operates in a voltage adjustment mode, and the power characteristic variable reference value is reduced through activation of the droop control to guide the photovoltaic array to output power, until the system is balanced and stable.

7. The method for PV voltage support based on adaptive mode switching in DC microgrid of claim 1, wherein, The control law of the photovoltaic voltage reference value is: wherein is the photovoltaic voltage reference value, and respectively represent the proportional and integral control parameters of the PI controller in the power characterization variable regulation loop, s is the Laplace operator, is the power characterization variable deviation value, S is the power characterization variable average value, is the power characterization variable reference value after the upper limit constraint.

8. The method for PV voltage support based on adaptive mode switching in DC microgrid of claim 1, wherein, The control law of the PWM signal is: wherein is a PWM signal for controlling the on-off of the switching tube in the DC / DC converter, and respectively represent the proportional and integral control parameters of the PI controller in the photovoltaic voltage control loop, s is the Laplace operator, is the error of the photovoltaic voltage, is the reference value of the photovoltaic voltage, is the photovoltaic voltage.

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