An on-grid / off-grid switching control method and system for an islanded microgrid

Abstract

The present invention discloses a grid-connected and off-grid switching control method and system for an island microgrid. The method includes constructing a circuit topology structure composed of energy storage converters in the island microgrid system; using the deep deterministic policy gradient algorithm to analyze the circuit topology structure to perform a primary adjustment on the response characteristics of the energy storage converters; if it is detected that the microgrid system switches to the off-grid mode, controlling the droop controller and the differential controller to act cooperatively, and based on the first gain parameter of the adjusted droop controller, performing a secondary adjustment on the response characteristics; when a grid-connected mode switching signal is detected, tracking and adjusting the real-time power of the island microgrid system with a set target reference power, and when it is detected that the island microgrid system switches to the grid-connected mode, controlling the droop controller and the integral controller to act cooperatively, and based on the second gain parameter of the adjusted droop controller, performing a tertiary adjustment on the response characteristics.

Description

Technical Field

[0001] The present invention relates to the technical field of microgrids, and in particular, to a method and system for controlling the grid-connected and islanded mode switching of an island microgrid. Background Art

[0002] In the modernization process of the power system, with the wide application of renewable energy and the rapid development of smart grid technology, the rise of distributed generation and microgrids has become an indispensable part of the power system.

[0003] Energy storage technology, with its advantages in improving energy utilization efficiency and enhancing system stability, is widely regarded as a key technology to ensure the safe and stable operation of the power grid. Especially in special environments such as island microgrids, the role of energy storage converters is particularly prominent. These environments usually face more technical challenges. For example, when the power grid experiences abnormalities due to natural disasters or equipment failures, the energy storage system must respond quickly and switch the energy storage converter from the grid-connected mode to the islanded mode to ensure continuous power supply to sensitive loads within the microgrid. However, during the mode switching process, existing energy storage converters often encounter problems such as voltage shocks, power oscillations, and frequency oscillations, which affect the grid-connected and islanded mode switching effect of the island microgrid, and further disrupt the overall stability of the power grid system.

[0004] Therefore, how to effectively control the switching of the island microgrid between grid-connected and islanded modes has become a technical problem that needs to be urgently solved by those skilled in the art. Summary of the Invention

[0005] The present invention provides a method for controlling the grid-connected and islanded mode switching of an island microgrid to solve the problem of how to effectively control the switching process of the island microgrid between grid-connected and islanded modes to achieve smooth switching and provide strong guarantee for the stable operation of the microgrid.

[0006] To solve the above technical problem, an embodiment of the present invention provides a method for effectively controlling the switching of the island microgrid between grid-connected and islanded modes, including:

[0007] In the island microgrid system, construct a circuit topology structure composed of energy storage converters;

[0008] When performing PID control on the island microgrid system, use the deep deterministic policy gradient algorithm to analyze the circuit topology structure, and perform a primary adjustment on the response characteristics of the energy storage converter based on the analysis results;

[0009] If it is detected that the island microgrid system switches to the islanded mode, control the droop controller and the differential controller to act in coordination, and perform a secondary adjustment on the response characteristics after the primary adjustment based on the first gain parameter of the adjusted droop controller; and,

[0010] When a grid-connected mode switching signal is detected, the real-time power of the island microgrid system is tracked and adjusted with a set target reference power. When it is detected that the island microgrid system switches to the grid-connected mode, the droop controller and the integral controller are controlled to act cooperatively, and based on the second gain parameter of the adjusted droop controller, a tertiary adjustment is performed on the response characteristics after the primary adjustment.

[0011] Further, the construction of the circuit topology of the energy storage converter includes:

[0012] Using the silicon carbide semiconductor as the power device of the energy storage converter, and performing a low-inductance design on the silicon carbide semiconductor power device to construct a preliminary circuit topology of the energy storage converter;

[0013] Performing a modular cascade design on the preliminary circuit topology to obtain the circuit topology structure.

[0014] Further, the response characteristics include voltage response characteristics and frequency response characteristics;

[0015] The primary adjustment of the response characteristics of the energy storage converter based on the analysis result by using the deep deterministic policy gradient algorithm for analyzing the circuit topology structure includes:

[0016] When the energy storage converter is in the charging state, the proportional-integral controller parameters of the voltage outer loop of the energy storage converter are controlled by using the deep deterministic policy gradient algorithm to adjust the voltage response characteristics;

[0017] When the energy storage converter is in the discharging state, the virtual moment of inertia and damping coefficient of the power outer loop of the energy storage converter are controlled by using the deep deterministic policy gradient algorithm to adjust the frequency response characteristics.

[0018] Further, the primary adjustment of the response characteristics of the energy storage converter based on the analysis result by using the deep deterministic policy gradient algorithm for analyzing the circuit topology structure further includes:

[0019] Using the deep deterministic policy gradient algorithm to analyze the circuit topology structure to determine the target state space parameters and target action space parameters of the energy storage converter;

[0020] Designing a reward function according to the target state space parameters and the target action space parameters, and performing a primary adjustment on the response characteristics of the energy storage converter based on the reward function.

[0021] Further, when detecting the grid connection mode switching signal, tracking and adjusting the real-time power of the island microgrid system with a set target reference power includes:

[0022] In the off-grid case, setting the target reference power according to the frequency response characteristic;

[0023] When detecting the grid connection mode switching signal, by real-time monitoring the actual power output by the energy storage system inverter and using it as a new reference power to perform the tracking and adjustment of the real-time power.

[0024] Further, when detecting the grid connection mode switching signal, tracking and adjusting the real-time power of the island microgrid system with a set target reference power further includes:

[0025] During the tracking and matching process, introducing an inertia link in the energy storage converter;

[0026] By adjusting the time parameter and gain coefficient of the inertia link, controlling the target reference power to be adjusted to the actual power in the grid-connected case to achieve smooth switching between grid connection and off-grid.

[0027] Further, introducing an inertia link in the energy storage converter includes:

[0028] The inertia equation of the inertia link is expressed by the following formula:

[0029]

[0030] In the formula, is the time parameter, is the gain coefficient of the inertia link, is the input signal varying with time, is the output signal processed by the inertia link;

[0031] Performing Laplace transform on the inertia equation to obtain the transfer function, and the transfer function is expressed by the following formula:

[0032]

[0033] Performing inverse Laplace transform on the transfer function to convert the output signal from the complex frequency domain to the time domain, which is expressed by the following formula:

[0034]

[0035] Further, the control droop controller cooperates with the differential controller and the integral controller respectively, including:

[0036] The droop controller introduces the active power differential term and the reactive power integral term in different modes respectively, which is expressed by the following formula:

[0037]

[0038] In the formula, and are the output frequency and output voltage of inverter n respectively; and are the rated frequency and rated voltage of inverter n respectively; and are the rated active power and rated reactive power of inverter n; and are the rated reactive power of the power grid and the actually output reactive power; and are the differential coefficient and the integral coefficient respectively; and are the active droop coefficient and the reactive droop coefficient of the droop controller respectively.

[0039] Furthermore, it further includes:

[0040] When detecting the grid-connected mode switching signal, detect the voltage phase and voltage amplitude on both sides of the point of common coupling, and calculate the voltage phase difference and voltage amplitude difference on both sides of the point of common coupling;

[0041] According to the voltage phase difference and the voltage amplitude difference, adopt a synchronous control algorithm to adjust the voltage of the island microgrid system.

[0042] Another embodiment of the present invention provides a grid-connected and islanded switching control system for an island microgrid, including:

[0043] A topology construction module, used to construct a circuit topology structure composed of energy storage converters in the island microgrid system;

[0044] A primary adjustment module, used to adopt a deep deterministic policy gradient algorithm to analyze the circuit topology structure when performing PID control on the island microgrid system, and perform a primary adjustment on the response characteristics of the energy storage converter based on the analysis result;

[0045] A secondary adjustment module, used to control the coordinated action of the droop controller and the differential controller if it is detected that the island microgrid system switches to the islanded mode, and perform a secondary adjustment on the response characteristics after the primary adjustment based on the first gain parameter of the adjusted droop controller;

[0046] The three - level adjustment module is used to track and adjust the real - time power of the island micro - grid system with a set target reference power when a grid - connection mode switching signal is detected, and when it is detected that the island micro - grid system switches to the grid - connection mode, it controls the droop controller and the integral controller to act cooperatively, and based on the second gain parameter of the adjusted droop controller, performs a three - level adjustment on the response characteristics after the first - level adjustment.

[0047] Compared with the prior art, the beneficial effects of the embodiments of the present invention are at least one of the following:

[0048] (1) The present invention designs an adaptive control and parameter optimization mechanism for the energy storage converter based on the DPPG algorithm, which can adjust the control response parameters of the energy storage converter in real time according to the operating state of the micro - grid, thereby ensuring the stable operation of the micro - grid.

[0049] (2) The present invention also improves the traditional droop controller, which is combined with the differential controller and the integral controller respectively in the island mode and the grid - connected operation mode, and optimizes the system dynamic response by adjusting the control parameters and gains. And a power tracking mechanism is introduced to reduce the transient impact caused by the switching between the grid - connected and islanding modes, thereby improving the stability of the island micro - grid. Description of the Drawings

[0050] Figure 1 It is a schematic diagram of the grid - connected and islanding switching control method flow of the island micro - grid in one embodiment of the present invention;

[0051] Figure 2 It is a schematic diagram of the circuit topology of the energy storage converter in one embodiment of the present invention;

[0052] Figure 3 It is a schematic diagram of the PCS voltage outer - loop PI parameter adaptive control structure based on the DPPG algorithm in one embodiment of the present invention;

[0053] Figure 4 It is a schematic diagram of the PCS frequency response characteristic adaptive control structure based on the DPPG algorithm in one embodiment of the present invention;

[0054] Figure 5 It is a comparison diagram of the voltage response parameter fluctuation states in one embodiment of the present invention;

[0055] Figure 6 It is a comparison diagram of the voltage response parameter steady - state states in one embodiment of the present invention;

[0056] Figure 7 It is a schematic diagram of the frequency response parameter fluctuation states in one embodiment of the present invention;

[0057] Figure 8It is a schematic diagram of the fixed and adaptive parameter fluctuation states in one embodiment of the present invention;

[0058] Figure 9 It is a schematic diagram of the frequency response of the output impedance transfer function in one embodiment of the present invention;

[0059] Figure 10 It is a schematic diagram of the step response of the controller in one embodiment of the present invention;

[0060] Figure 11 It is a schematic diagram of the parameter oscillation condition of the inverter in one embodiment of the present invention;

[0061] Figure 12 It is a structural block diagram of the grid-connected and off-grid switching control system of the island microgrid in one embodiment of the present invention. Detailed implementation manners

[0062] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. The purpose of providing these embodiments is to make the disclosure of the present invention more thorough and comprehensive. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative efforts shall fall within the protection scope of the present invention.

[0063] In the description of the present application, the terms "first", "second", "third", etc. are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first", "second", "third", etc. may explicitly or implicitly include one or more of such features. In the description of the present application, unless otherwise stated, the meaning of "a plurality" is two or more.

[0064] In the description of the present application, it should be noted that unless otherwise clearly specified and defined, the terms "installed", "connected", and "coupled" should be understood in a broad sense. For example, it may be a fixed connection, a detachable connection, or an integral connection; it may be a mechanical connection or an electrical connection; it may be directly connected or indirectly connected through an intermediate medium, and it may be the communication inside two components. The terms "vertical", "horizontal", "left", "right", "upper", "lower" and similar expressions used herein are only for the purpose of illustration, rather than indicating or implying that the device or component referred to must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation on the present invention. The term "and / or" used herein includes any and all combinations of one or more of the related listed items. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to specific circumstances.

[0065] In the description of the present application, it should be noted that unless otherwise defined, all technical and scientific terms used in the present invention have the same meanings as those commonly understood by those skilled in the technical field to which the present invention belongs. The terms used in the specification of the present invention are only for the purpose of describing specific embodiments, and are not intended to limit the present invention. For those of ordinary skill in the art, the specific meanings of the above terms in the present application can be understood according to specific circumstances.

[0066] An embodiment of the present invention provides a grid-connected and off-grid switching control method for an island microgrid. Specifically, please refer to Figure 1 , Figure 1 which shows a schematic flow chart of the grid-connected and off-grid switching control method for an island microgrid in one of the embodiments of the present invention, and includes the following steps:

[0067] S1. In the island microgrid system, construct a circuit topology structure composed of energy storage converters.

[0068] It is understandable that the silicon carbide metal oxide semiconductor field effect transistor (SiCMOSFET) is a new type of power semiconductor device with high switching speed and low conduction loss. Compared with traditional silicon-based IGBTs, it can significantly improve the efficiency of the power conversion system (PCS). Considering the excellent characteristics of SiC in the power grid system, in some embodiments of the present invention, silicon carbide semiconductors are used as the power devices of the power conversion system. And in order to cope with the sensitivity to stray inductance that may be brought about by the fast switching characteristics of such SiC devices, a low-inductance design is carried out on the silicon carbide semiconductor power devices in the circuit design to optimize the performance of the SiC devices, thereby obtaining a preliminary circuit topology of the power conversion system. As an example, the low-inductance design structure of the laminated busbar adopted in the embodiments of the present invention is composed of positive and negative copper busbar conductors in a laminated manner, and an insulating material is added between the conductors. And by connecting multiple capacitors in parallel, the number of branches of the stray inductance can be increased and its uniformity can be improved. At the same time, the mutual cancellation effect of the magnetic fields is used to reduce the loop inductance. To prevent the reduction of the inductance reduction amplitude caused by the increase in the number of absorption capacitors, preferably, a 4-absorption-capacitor structure can be selected.

[0069] In this step, taking the SiCMOSFET of model CAS300M12BM2 as an example, by extracting the stray inductance of the laminated busbar, the stray inductance of the laminated busbar of the high-voltage AC module is obtained as 734 nH, while that of the low-voltage DC module is 175 nH. And the high-frequency parasitic inductances of the SiCMOSFETs of models CAS300M12BM2 and CAS120M12BM2 and the diodes are all 15 nH.

[0070] The total stray inductance of the commutation loop of the high-voltage AC module and the total stray inductance of the commutation loop of the low-voltage DC module are represented by the following formula:

[0071]

[0072]

[0073] where and are the high-frequency parasitic inductance and stray inductance of the SiCMOSFET respectively.

[0074] It can be seen that the low-inductance design helps to reduce the switching oscillation of the power conversion system during the operation mode switching, and improve the stability and performance of the system. This helps to reduce electromagnetic interference and reduce the impact on surrounding equipment and the power grid.

[0075] Furthermore, a modular cascade design is carried out on the preliminary circuit topology to obtain the circuit topology structure. Specifically, please refer to Figure 2 , Figure 2The figure shows a schematic diagram of the circuit topology of the energy storage converter in one embodiment of the present invention. It can be understood that with a modular cascaded design, it is allowed to easily achieve a higher voltage level output by connecting power units in series. This design enables the energy storage converter to flexibly adapt to energy storage systems of different scales and voltage requirements.

[0076] S2. When performing PID control on the island microgrid system, the deep deterministic policy gradient algorithm is used to analyze the circuit topology structure, and based on the analysis results, the response characteristics of the energy storage converter are adjusted at the first level.

[0077] It can be understood that when the operation mode of the energy storage converter is switched, especially in the scenario of unplanned off-grid / island mode switching, the response speed and stability of the energy storage converter system largely depend on the timely adjustment of the control parameters of the pre-stage converter. The DDPG algorithm, namely the Deep Deterministic Policy Gradient algorithm, is an online deep reinforcement learning algorithm under the Actor-Critic framework, which can effectively handle continuous power adjustment and achieve real-time control, and is applicable to the optimization of the control parameters of the energy storage converter.

[0078] In this embodiment, the deep deterministic policy gradient algorithm is used to adjust the response characteristics of the energy storage converter when it is in different charge / discharge states. Specifically, when performing PID control on the island microgrid system, the deep deterministic policy gradient algorithm is used to analyze the circuit topology structure, so as to determine the target state space parameters and target action space parameters of the energy storage converter. In the embodiment of the present invention, the state space parameters include parameters such as voltage, current, state of charge (SOC), charge / discharge power, total harmonic content, and grid parameters (active power, reactive power, power factor, etc.), while the action space parameters include but are not limited to: charge / discharge commands, voltage / current regulation commands, and power factor correction commands.

[0079] Furthermore, a reward function is designed according to the target state space parameters and target action space parameters, and the response characteristics of the energy storage converter are adjusted at the first level based on the reward function. It can be understood that the response characteristics include voltage response characteristics and frequency response characteristics.

[0080] As an example, when the energy storage converter is in the charging state, the proportional-integral controller parameters (PI parameters) of the voltage outer loop of the energy storage converter are controlled by the deep deterministic policy gradient algorithm to adjust the voltage response characteristics. Specifically, please refer to Figure 3 , Figure 3 The figure shows a schematic diagram of the adaptive control structure of the PCS voltage outer loop PI parameters based on the DPPG algorithm in one embodiment of the present invention.

[0081] When the energy storage converter is in the discharging state, the virtual moment of inertia and damping coefficient of the power outer loop of the energy storage converter are controlled by the deep deterministic policy gradient algorithm to adjust the frequency response characteristics. Specifically, please refer to Figure 4 , Figure 4 which shows a schematic diagram of the adaptive control structure of the PCS frequency response characteristics based on the DPPG algorithm in one embodiment of the present invention.

[0082] The following provides a specific embodiment to refine and describe the above first-level adjustment process:

[0083] When the battery is in the charging state, the front-stage three-phase converter operates as a rectifier to provide a stable DC bus voltage for the rear-stage converter. The DPPG algorithm adaptively adjusts the PI parameters of the voltage outer loop according to the incoming bus voltage and voltage difference to maintain good steady-state characteristics of the DC bus voltage. The specific implementation process is as follows:

[0084] The DPPG algorithm evaluates the performance of the current PI parameters according to the DC bus voltage and its deviation , and then adjusts the policy network (Actor) and value network (Critic) according to the evaluation results. Then, according to the real-time DC bus voltage deviation , a new set of PI parameters is output, and a reward function is designed accordingly to optimize and adjust the voltage characteristic response. The reward function is expressed as follows:

[0085]

[0086] where is the Gaussian function parameter.

[0087] In some embodiments of the present invention, after introducing the DPPG algorithm, a three-phase converter simulation model is built. At 0.3 s after the simulation starts, a sudden scenario is simulated, and the energy storage converter enters the island operation mode. At this time, the bus load changes suddenly, and then the DPPG algorithm is used to control the PI parameters of the voltage outer loop for adaptive training and adjustment. The trained DDPG adaptive PI controller will determine a set of suitable PI parameters according to the DC bus voltage deviation collected every 0.02 s and Figure 5 parameters. The comparison of the optimized parameters and the non-optimized parameters in terms of DC bus voltage fluctuation and steady state is shown in Figure 6 respectively. It can be seen that the response speed of the system after the DPPG adaptive optimization of the parameters is faster, and the overshoot is reduced; the steady-state performance has also been greatly improved.

[0088] When the battery is in the discharge state, the front-stage three-phase converter is in the inverter operating state, and the output three-phase AC voltage provides voltage and frequency support for the power grid. When the energy storage converter switches to the non-almost island mode, there is a sudden change in active power, and its frequency also oscillates accordingly. The frequency response of the energy storage converter is closely related to the virtual inertia J and damping coefficient D of the power loop.

[0089] During the process of optimizing and adjusting the frequency response, the DDPG algorithm adjusts the power loop parameters of the front-stage three-phase PWM converter, namely the virtual inertia J and damping coefficient D. In some embodiments of the present invention, the quality of the parameter settings is also evaluated by real-time sampling and analyzing the performance corresponding to the frequency f, and a reward function is constructed to update the Actor and Critic networks in the DDPG controller accordingly.

[0090] When constructing the reward function, it is necessary to consider suppressing the frequency fluctuation when the active power of the energy storage converter changes, and at the same time ensure the dynamic performance of the output frequency of the energy storage converter. In addition, it is also necessary to prevent the steady-state values of J and D from being too large or too small to avoid affecting the stability of the energy storage converter. Then the reward function is represented by the following formula:

[0091]

[0092] where is the parameter of the Gaussian function, representing the standard deviation of the frequency fluctuation.

[0093] It should be noted that to balance the system dynamic performance and frequency response, only when the power deviation is large, the DDPG algorithm is used to adaptively adjust the J and D parameters. After the power is stabilized, the parameters are restored to the preset values to maintain the stability of the weak power grid.

[0094] When the training scenario of the simulation algorithm is 0.5 s, the energy storage converter encounters a sudden situation and enters the island operation mode. The output active power of the energy storage converter changes from 10 kW to 5 kW. The change curves of the power loop parameters J and D after training are as Figure 7 shown, and the frequency fluctuation curves of the energy storage converter with fixed parameters and adaptively changed parameters are as Figure 8 shown. It can be found that by using the DDPG algorithm to adaptively adjust the J and D parameters in the power loop, the frequency fluctuation can be effectively suppressed, and to a certain extent, the dynamic response speed of the energy storage converter system can be improved.

[0095] In this step, generally speaking, introducing the DPPG reinforcement learning adaptive adjustment mechanism enables the energy storage converter to achieve a faster response and better steady-state accuracy after being disturbed.

[0096] S3 - S4. If it is detected that the island micro - grid system switches to the off - grid mode, control the droop controller and the differential controller to act cooperatively, and based on the adjusted first gain parameter of the droop controller, perform a secondary adjustment on the response characteristics after the primary adjustment. It can be understood that the primary adjustment is the process of adjusting the response characteristics of the energy storage converter by the above - mentioned deep deterministic policy gradient algorithm. On this basis, further, by improving the droop controller in different modes (parallel / off - grid and the process of parallel - off - grid switching), optimize the response characteristics of the energy storage converter (i.e., the so - called secondary adjustment and tertiary adjustment).

[0097] When a grid - connected mode switching signal is detected, track and adjust the real - time power of the island micro - grid system with a set target reference power, and when it is detected that the island micro - grid system switches to the grid - connected mode, control the droop controller and the integral controller to act cooperatively, and based on the adjusted second gain parameter of the droop controller, perform a tertiary adjustment on the response characteristics after the primary adjustment.

[0098] It can be understood that the embodiments of the present invention optimize and adjust the characteristic parameters of the energy storage converter by improving the droop controller. In some embodiments of the present invention, first, a parallel model of the droop controller is constructed. Specifically, the parallel model of the droop control is represented by the following formula

[0099]

[0100] In the formula, is the voltage frequency of the inverter, is the voltage amplitude of the inverter; and are respectively the rated power and the rated voltage amplitude of the inverter output voltage; and are respectively the active power output and the reactive power output of the inverter; and are respectively the rated active power output and the rated reactive power output of the inverter; and are respectively the active droop coefficient and the reactive droop coefficient of the droop control equation.

[0101] In some embodiments of the present invention, the parameters and are calculated as:

[0102]

[0103] In the formula, and are respectively the minimum voltage frequency and the minimum voltage amplitude limited at the maximum power output; and are the maximum active power output limit when the generation frequency drops and the maximum reactive power output limit when the voltage drops, respectively.

[0104] It should be noted that when the grid-connected mode switching signal is detected (at this time, the microgrid is in the island / off-grid mode), to achieve the grid-connected operation of the microgrid, it is necessary to ensure that the voltage phase and voltage amplitude at the point of common coupling (PCC) are consistent with those on the grid side.

[0105] In the embodiment of the present invention, the voltage phase and amplitude differences on both sides of the PCC point (i.e., the microgrid side and the grid side) are detected, and based on this, the voltage on the microgrid side is adjusted to achieve synchronization. In some embodiments of the present invention, voltage sensors or measuring devices can be used to detect the voltage phase and voltage amplitude on both sides of the point of common connection, and then the voltage phase difference and voltage amplitude difference on both sides of the point of common connection are calculated according to the detected voltage data.

[0106] According to the voltage phase difference and the voltage amplitude difference, a synchronization control algorithm is used to adjust the voltage of the island microgrid system. The synchronization control algorithm can output an adjustment signal to adjust the voltage phase and amplitude on the microgrid side, making it gradually approach the voltage phase and amplitude on the grid side until synchronization is achieved. Specifically, the above-mentioned phase and amplitude adjustment processes are represented by the following formulas:

[0107]

[0108] Among them, and are the reference values of the voltage frequency and amplitude after signal superposition, respectively; and are the reference values of the voltage frequency and amplitude under droop control, respectively; and are the phase and amplitude of the voltage on the distribution network side, respectively; and are the phase and amplitude of the voltage on the microgrid side, respectively; and are both parameters of the PI controller in the voltage phase synchronization control link; and are both parameters of the PI controller in the voltage amplitude synchronization control link.

[0109] It can be understood that the core of the synchronous grid-connection control lies in being able to ensure that the voltage phase and amplitude of the microgrid match those on the grid side before grid connection, so as to avoid the impact current and voltage fluctuations generated during grid connection.

[0110] In order to achieve a smooth transition of the system in the energy storage converter of the island microgrid, the above droop controller is further improved accordingly.

[0111] In the embodiment of the present invention, the droop controller introduces the active power differential term and the reactive power integral term in different modes respectively. As an example, if it is detected that the microgrid system switches to the off-grid mode, the active power differential term is introduced into the droop controller, and if it is detected that the microgrid system switches to the grid-connected mode, the reactive power integral term is introduced, so as to obtain the improved droop control, which is specifically represented by the following formula:

[0112]

[0113] In the formula, and are respectively the output frequency and output voltage of the inverter n; and are respectively the rated frequency and rated voltage of the inverter n; and are the rated active power and rated reactive power of the inverter n; and are the rated reactive power of the power grid and the actually output reactive power; and are respectively the differential coefficient and the integral coefficient; and are respectively the active droop coefficient and the reactive droop coefficient of the droop controller.

[0114] Under normal circumstances, the rated reactive power can be set to 0, that is, is satisfied, and thus the droop control of the parallel system of the improved energy storage converter is as follows:

[0115]

[0116] According to the above formula, through the improved droop control, the gain parameters of the droop controller are adjusted to further optimize and adjust the response characteristics of the energy storage converter. The parameters and gains of the droop controller reflect the adjustment range of the response characteristics.

[0117] In this embodiment, when the grid-connected mode switching signal is detected, that is, during the off-grid process, a power tracking mechanism is introduced. When the microgrid operates in island mode, the reference power is set to a fixed value. Before active grid connection, it is switched from the fixed value to track the actual power, and an inertia link is constructed in the controller and connected to the current output loop. When the output signal of the inertia link reaches the value required for grid connection, the microgrid is grid-connected. Specifically, the implementation process of power tracking adjustment is as follows:

[0118] In the off-grid situation, the target reference power is set according to the frequency response characteristics. In some embodiments of the present invention, if it is detected that the microgrid is in the islanded situation, the target reference power is set according to the real-time frequency response characteristics. Since in the island mode, the microgrid operates independently and does not depend on the external power grid. To ensure the stability of the system and the power quality, as an example, the reference power value can be set to 30 kW.

[0119] By setting the reference power, the droop controller can effectively adjust the system frequency and voltage to cope with the load changes and ensure the stable operation of the microgrid.

[0120] When the grid-connected mode switching signal is detected, that is, when the microgrid is ready to be grid-connected, to avoid the transient impact caused by grid connection, the actual power output by the energy storage system inverter is monitored in real time and used as the new reference power to perform the tracking adjustment of the real-time power.

[0121] In the embodiments of the present invention, in order to further reduce the transient surge of the converter output current, an inertia link will be introduced in the energy storage converter during the tracking and matching process. Through the adjustment of the inertia link, the output power of the inverter will gradually transition from the fixed value in the island mode to the actual value in the grid-connected mode, thus achieving a smooth transition.

[0122] Specifically, in some embodiments of the present invention, the target reference power is adjusted to the actual power in the grid-connected situation by adjusting the time parameter and gain coefficient of the inertia link to achieve a smooth switching between grid-connected and off-grid. It can be understood that the time constant and gain coefficient determine the response speed and amplification degree of the inertia link to the change of the input signal. Therefore, when the microgrid is ready to be grid-connected, introducing an inertia link and adjusting its corresponding parameters can ensure a smooth switching between grid-connected and off-grid.

[0123] In this embodiment, the inertia equation of the inertia link is expressed by the following formula:

[0124]

[0125] In the formula, is the time parameter, is the gain coefficient of the inertia link, is the input signal changing with time, is the output signal processed by the inertia link;

[0126] Performing a Laplace transform on the inertia equation to obtain the transfer function, the transfer function is expressed by the following formula:

[0127]

[0128] Then, from the above formula, it can be obtained that:

[0129]

[0130] The Laplace inverse transform is performed on the transfer function, and the output signal is converted from the complex frequency domain to the time domain. Then, the output signal is represented by the following formula:

[0131]

[0132] Through the above operations, the embodiments of the present invention can achieve the matching of the output power of the inverter with the grid demand, thereby reducing the transient impact during grid connection. At the moment of grid connection, the inertia link can continue to finely adjust the output signal to ensure that the output of the microgrid is synchronized with the input of the large grid, which helps to reduce the current impact and voltage fluctuation that may occur during the grid connection process.

[0133] It should be noted that when the grid connection is completed, the stable clamping effect of the large grid can be utilized to stabilize the frequency and voltage of the microgrid. At this time, through the above operations, the output power of the inverter is made equal to the reference power to meet the power balance of the system.

[0134] To verify the applicability of the proposed improved droop control algorithm, the present invention provides a specific embodiment to conduct a series of comparative simulation experiments to analyze the impact of the above operations on the operation of the microgrid system. Specifically, the key parameters of the simulation are shown in Table 1:

[0135] Table 1

[0136]

[0137] In the first group of experiments, the improvement effect is evaluated by comparing the frequency responses before and after the system enhancement. Specifically, the effects of introducing the voltage control loop and the inertia link are analyzed.

[0138] Output impedance transfer function and The frequency response curves of Figure 9 are shown in and are the output impedance transfer functions of the traditional droop control algorithm and the improved droop control algorithm respectively. It can be seen that compared with the traditional droop control, the improved droop control effectively reduces the peak amplitude of the frequency response, while increasing the system bandwidth and the phase margin.

[0139] Furthermore, in the second group of experiments, step response is used for comparative experiments to analyze and evaluate the effectiveness of the improved droop control. As shown in Figure 10As shown, it can be seen that the traditional controller has a large oscillation peak in the step response, and it takes about 0.08 seconds for the system to stabilize. In contrast, the improved droop controller has less oscillation and a shorter stabilization time, showing the ability to reach the steady state faster.

[0140] Finally, in the third group of experiments, the voltage oscillation will be tested. Specifically, as shown in Figure 11, it can be seen that in the island microgrid, the energy storage converter is first in the grid-connected operation state. At the 4-second mark, i.e., at time t1 in the figure, a grid fault occurs, and the energy storage converter is forced to enter the island operation state. Figure 11 The results of the improved control method and the traditional control method are given from top to bottom. After time t2 when the STS (Static Transfer Switch) is turned on, under the traditional control method, the output voltage and frequency of the inverter oscillate for about 300 ms. In contrast, with the improved control method, the oscillation duration of the inverter output voltage is significantly reduced to about 60 ms, greatly improving the voltage recovery process.

[0141] In summary, the present invention first introduces the idea of deep reinforcement learning, constructs an adaptive optimization mechanism for the control parameters of the energy storage converter based on the DDPG algorithm, optimizes and adjusts the core control response parameters in the voltage outer loop and power loop of the energy storage converter respectively, improves the system stability and suppresses the fluctuations of the system frequency and voltage; and improves the droop controller, combines the droop control with the integral controller and the differential controller to further optimize the response parameters of the energy storage converter to improve the dynamic performance of the microgrid system; also introduces a power tracking mechanism and combines the inertia link to suppress the transient impact; thus realizing the smooth switching of the island microgrid between grid-connected and islanded modes.

[0142] An embodiment of the present invention provides a grid-connected and islanded switching control system for an island microgrid. Specifically, please refer to Figure 12 , Figure 12 which shows the block diagram of the grid-connected and islanded switching control system of the island microgrid in one of the embodiments of the present invention, including:

[0143] A topology construction module M1, configured to construct a circuit topology structure composed of energy storage converters in the island microgrid system;

[0144] A first-level adjustment module M2, configured to use the deep deterministic policy gradient algorithm to analyze the circuit topology structure when performing PID control on the island microgrid system, and perform a first-level adjustment on the response characteristics of the energy storage converter based on the analysis results;

[0145] The secondary adjustment module M3 is configured to, if it detects that the island microgrid system switches to the off-grid mode, control the droop controller and the differential controller to act cooperatively, and perform secondary adjustment on the response characteristics after the primary adjustment based on the first gain parameter of the adjusted droop controller;

[0146] The tertiary adjustment module M4 is configured to, when detecting a grid connection mode switching signal, perform tracking adjustment on the real-time power of the island microgrid system with a set target reference power, and when detecting that the island microgrid system switches to the grid connection mode, control the droop controller and the integral controller to act cooperatively, and perform tertiary adjustment on the response characteristics after the primary adjustment based on the second gain parameter of the adjusted droop controller.

[0147] The above embodiments merely represent several implementation manners of the present invention. The description thereof is relatively specific and detailed, but it should not be construed as a limitation to the scope of the patent of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the appended claims.

Claims

1. A method for controlling on-grid and off-grid switching of an island microgrid, characterized in that: include: In the island microgrid system, a circuit topology consisting of energy storage converters is constructed; When the island microgrid system is PID controlled, a deep deterministic policy gradient algorithm is used to analyze the circuit topology structure, and a primary adjustment is performed on the response characteristics of the energy storage converter based on the analysis results, and the response characteristics include voltage response characteristics and frequency response characteristics; the primary adjustment specifically includes: when the energy storage converter is in a charging state, the proportional-integral controller parameters of the voltage outer loop of the energy storage converter are controlled by a deep deterministic policy gradient algorithm to adjust the voltage response characteristics; when the energy storage converter is in a discharging state, the virtual moment of inertia and damping coefficient of the power outer loop of the energy storage converter are controlled by a deep deterministic policy gradient algorithm to adjust the frequency response characteristics; if it is detected that the island microgrid system is switched to an off-grid mode, the droop controller is controlled to work in coordination with the differential controller, and based on the adjusted first gain parameter of the droop controller, the response characteristics after the primary adjustment are adjusted at a secondary level; and, When a grid-connected mode switching signal is detected, the real-time power of the island microgrid system is tracked and adjusted with the set target reference power, and when it is detected that the island microgrid system switches to the grid-connected mode, the droop controller and the integral controller are controlled to work together, and based on the adjusted second gain parameter of the droop controller, the response characteristic after the first-stage adjustment is adjusted to a third level; specifically, in an off-grid state, the target reference power is set according to the frequency response characteristic; when a grid-connected mode switching signal is detected, the real-time power is tracked and adjusted by monitoring the actual power output by the energy storage system inverter in real time and using it as a new reference power, wherein in the tracking and matching process, an inertia link is introduced into the energy storage converter; the target reference power is controlled to be adjusted to the actual power in the grid-connected state by adjusting the time parameter and gain coefficient of the inertia link, so as to achieve smooth switching between on-grid and off-grid.

2. The on-grid and off-grid switching control method of the island microgrid according to claim 1, characterized in that: The construction of the circuit topology structure of the energy storage converter includes: Using silicon carbide semiconductor as the power device of the energy storage converter, and performing low-inductance design on the silicon carbide semiconductor power device to construct a preliminary circuit topology of the energy storage converter; The circuit topology structure is obtained by performing modular cascade design on the preliminary circuit topology.

3. The on-grid and off-grid switching control method of the island microgrid according to claim 1, characterized in that: The method of analyzing the circuit topology structure by using a deep deterministic policy gradient algorithm and adjusting the response characteristics of the energy storage converter based on the analysis result at a first level also includes: Using a deep deterministic policy gradient algorithm to analyze the circuit topology structure, and determine the target state space parameters and target action space parameters of the energy storage converter; A reward function is designed according to the target state space parameters and the target action space parameters, and a primary adjustment is performed on the response characteristics of the energy storage converter based on the reward function.

4. The on-grid and off-grid switching control method of the island microgrid according to claim 1, characterized in that: The inertia link is introduced into the energy storage converter, including: The inertia equation of the inertia link is expressed by the following formula: In the formula, is the time parameter, is the gain coefficient of the inertia link, is the time-varying input signal, is the output signal after being processed by the inertia link; The inertia equation is subjected to Laplace transformation to obtain a transfer function, which is expressed by the following formula: The transfer function is subjected to an inverse Laplace transform to convert the output signal from the complex frequency domain to the time domain, which is expressed by the following formula: 。 5. The on-grid and off-grid switching control method of the island microgrid according to claim 1, characterized in that: The control droop controller cooperates with the differential controller and the integral controller respectively, and includes: The droop controller introduces active power differential term and reactive power integral term in different modes, which are expressed by the following formulas: In the formula, and are the output frequency and output voltage of inverter n respectively; and are the rated frequency and rated voltage of inverter n respectively; and is the rated active power and rated reactive power of inverter n; and is the rated reactive power of the power grid and the actual output reactive power; and are the differential coefficient and the integral coefficient respectively; and are the active droop coefficient and reactive droop coefficient of the droop controller respectively.

6. The on-grid and off-grid switching control method of the island microgrid according to claim 1, characterized in that: Also includes: When the grid-connected mode switching signal is detected, the voltage phase and voltage amplitude on both sides of the common connection point are detected, and the voltage phase difference and voltage amplitude difference on both sides of the common connection point are calculated; According to the voltage phase difference and the voltage amplitude difference, a synchronous control algorithm is used to adjust the voltage of the island microgrid system.

7. A grid-connected and off-grid switching control system for an island microgrid, characterized in that: include: A topology building module is used to build a circuit topology structure consisting of energy storage converters in an island microgrid system; A first-level adjustment module is used to analyze the circuit topology structure using a deep deterministic policy gradient algorithm when performing PID control on the island microgrid system, and perform a first-level adjustment on the response characteristics of the energy storage converter based on the analysis results, wherein the response characteristics include voltage response characteristics and frequency response characteristics; the first-level adjustment specifically includes: when the energy storage converter is in a charging state, controlling the proportional-integral controller parameters of the voltage outer loop of the energy storage converter by a deep deterministic policy gradient algorithm to adjust the voltage response characteristics; when the energy storage converter is in a discharging state, controlling the virtual moment of inertia and damping coefficient of the power outer loop of the energy storage converter by a deep deterministic policy gradient algorithm to adjust the frequency response characteristics; A secondary adjustment module, for controlling the droop controller and the differential controller to work together if it is detected that the island microgrid system switches to an off-grid mode, and performing secondary adjustment on the response characteristic after the primary adjustment based on the adjusted first gain parameter of the droop controller; The three-stage adjustment module is used to track and adjust the real-time power of the island microgrid system with the set target reference power when a grid-connected mode switching signal is detected, and to control the droop controller and the integral controller to work together when it is detected that the island microgrid system switches to the grid-connected mode, and to perform a three-stage adjustment on the response characteristic after the first-stage adjustment based on the adjusted second gain parameter of the droop controller; specifically, in an off-grid situation, the target reference power is set according to the frequency response characteristic; when a grid-connected mode switching signal is detected, the real-time power is tracked and adjusted by monitoring the actual power output by the inverter of the energy storage system in real time and using it as a new reference power, wherein, in the tracking and matching process, an inertia link is introduced into the energy storage converter; the target reference power is controlled to be adjusted to the actual power in the grid-connected situation by adjusting the time parameter and gain coefficient of the inertia link, so as to achieve smooth switching between grid and off-grid.

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