Grid-connected and off-grid switching control method and system for island microgrid

Abstract

The invention discloses a grid-connected and off-grid switching control method and system for an island micro-grid, and the method comprises the steps: constructing a circuit topological structure composed of an energy storage converter in an island micro-grid system; analyzing the circuit topological structure by adopting a depth deterministic strategy gradient algorithm so as to perform primary adjustment on response characteristics of the energy storage converter; if it is detected that the micro-grid system is switched to the off-grid mode, controlling the droop controller and the differential controller to cooperatively act, and performing secondary adjustment on the response characteristic based on the adjusted first gain parameter of the droop controller; when a grid-connected mode switching signal is detected, tracking adjustment is performed on 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 is switched to a grid-connected mode, the droop controller and the integral controller are controlled to act cooperatively, and on the basis of a second gain parameter of the adjusted droop controller, the grid-connected mode of the island micro-grid system is switched to the grid-connected mode. And performing three-stage 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 on-grid and off-grid switching of an island microgrid. Background Art

[0002] In the modernization process of the power system, with the widespread 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 efficiency and enhancing system stability, is widely regarded as a key technology to ensure the safe and stable operation of the power grid. In particular, in special environments such as island microgrids, the role of energy storage inverters is particularly prominent. These environments usually face more technical challenges. For example, when the power grid is abnormal due to natural disasters or equipment failures, the energy storage system must respond quickly and switch the energy storage inverter from grid-connected mode to island mode to ensure continuous power supply to sensitive loads in the microgrid. However, during the mode switching process, existing energy storage inverters often encounter problems such as voltage shocks, power oscillations, and frequency oscillations during mode switching, which affects the grid-connected and off-grid switching effect of the island microgrid, and further disrupts the overall stability of the power grid system.

[0004] It can be seen that how to effectively control the switching of the island microgrid on and off the grid 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 an on-grid and off-grid switching control method for an island microgrid, so as to solve the problem of how to effectively control the on-grid and off-grid switching process of the island microgrid to achieve smooth on-grid and off-grid switching, thereby providing a strong guarantee for the stable operation of the microgrid.

[0006] In order to solve the above technical problems, an embodiment of the present invention provides a method for effectively controlling the switching of an island microgrid on and off the grid, including: 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 made to the response characteristics of the energy storage converter based on the analysis result; If it is detected that the island microgrid system switches to the 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 characteristic after the first-level adjustment is adjusted at a second 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 act in coordination, and based on the adjusted second gain parameter of the droop controller, the response characteristic after the first-level adjustment is adjusted to a third level.

[0007] Furthermore, the construction of the circuit topology structure of the energy storage converter includes: The silicon carbide semiconductor is used as a power device of an energy storage converter, and a low-inductance design is performed 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.

[0008] Further, the response characteristics include voltage response characteristics and frequency response characteristics; 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 comprises: When the energy storage converter is in a charging state, a proportional-integral controller parameter of a voltage outer loop of the energy storage converter is controlled by a deep deterministic policy gradient algorithm to adjust the voltage response characteristic; 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.

[0009] Furthermore, 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 may also include: 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.

[0010] Furthermore, when the 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, including: In an off-grid situation, setting the target reference power according to the frequency response characteristics; When the grid-connected mode switching signal is detected, the actual power output by the energy storage system inverter is monitored in real time and used as a new reference power to track and adjust the real-time power.

[0011] Furthermore, when the 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 further includes: 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 the gain coefficient of the inertia link, so as to achieve smooth switching between grid connection and off-grid connection.

[0012] Furthermore, the introduction of an inertia link into the energy storage converter includes: 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: Furthermore, the control droop controller cooperates with the differential controller and the integral controller respectively, including: 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.

[0013] Furthermore, it 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.

[0014] Another embodiment of the present invention provides a grid-connected and off-grid switching control system for an island microgrid, comprising: A topology building module is used to build a circuit topology structure consisting of energy storage converters in an island microgrid system; A primary adjustment module, used to analyze the circuit topology structure using a deep deterministic policy gradient algorithm when performing PID control on the island microgrid system, and to make a primary adjustment to the response characteristics of the energy storage converter based on the analysis result; 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 act in coordination 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.

[0015] Compared with the prior art, the embodiments of the present invention have the following advantages: (1) The present invention designs an adaptive control and parameter optimization mechanism for an 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 status of the microgrid, thereby ensuring the stable operation of the microgrid.

[0016] (2) The present invention also improves the traditional droop controller, which is combined with the differential controller and the integral controller in the island mode and grid-connected operation, respectively, to optimize the system dynamic response by adjusting the control parameters and gain. A power tracking mechanism is introduced to reduce the transient impact caused by the switching between grid-connected and off-grid modes, thereby improving the stability of the island microgrid. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] FIG1 is a schematic flow chart of a method for controlling on-grid and off-grid switching of an island microgrid in one embodiment of the present invention; FIG2 is a schematic diagram of a circuit topology structure of an energy storage converter in one embodiment of the present invention; 3 is a schematic diagram of a PCS voltage outer loop PI parameter adaptive control structure based on a DPPG algorithm in one embodiment of the present invention; 4 is a schematic diagram of a PCS frequency response characteristic adaptive control structure based on a DPPG algorithm in one embodiment of the present invention; FIG5 is a comparison diagram of voltage response parameter fluctuation states in one embodiment of the present invention; FIG6 is a comparison diagram of steady-state voltage response parameters in one embodiment of the present invention; FIG7 is a schematic diagram of a frequency response parameter fluctuation state in one embodiment of the present invention; FIG8 is a schematic diagram of fixed and adaptive parameter fluctuation states in one embodiment of the present invention; FIG9 is a frequency response diagram of an output impedance transfer function in one embodiment of the present invention; FIG10 is a schematic diagram of a step response of a controller in one embodiment of the present invention; FIG11 is a schematic diagram of parameter oscillation of an inverter in one embodiment of the present invention; FIG. 12 is a structural block diagram of an on-grid and off-grid switching control system of an island microgrid in one embodiment of the present invention. DETAILED DESCRIPTION

[0018] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of 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 ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0019] In the description of this application, the terms "first", "second", "third", etc. are used for descriptive purposes only and are not to be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first", "second", "third", etc. may explicitly or implicitly include one or more of the feature. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0020] In the description of the present application, it should be noted that, unless otherwise clearly specified and limited, the terms "installed", "connected", and "connected" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be a connection between the two elements. The terms "vertical", "horizontal", "left", "right", "upper", "lower" and similar expressions used in this article are only for illustrative purposes, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as a limitation on the present invention. The term "and / or" used in this article includes any and all combinations of one or more related listed items. For those of ordinary skill in the art, the specific meanings of the above terms in this application can be understood according to specific circumstances.

[0021] In the description of this application, it should be noted that, unless otherwise defined, all technical and scientific terms used in the present invention have the same meaning as those commonly understood by those skilled in the art. 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 this application can be understood by specific circumstances.

[0022] An embodiment of the present invention provides a method for controlling the on-grid and off-grid switching of an island microgrid. Specifically, please refer to FIG. 1 , which is a flow chart of a method for controlling the on-grid and off-grid switching of an island microgrid in one embodiment of the present invention, including the following steps: S1. In the island microgrid system, a circuit topology consisting of energy storage inverters is constructed.

[0023] It can be understood that silicon carbide metal oxide semiconductor field effect transistor (SiCMOSFET) is a new type of power semiconductor component with high switching speed and low conduction loss. Compared with traditional silicon-based IGBT, it can significantly improve the efficiency of energy storage converter (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 power devices of energy storage converters. And in order to cope with the sensitivity to stray inductance that may be caused by the fast switching characteristics of such SiC devices, low-inductance design is performed on silicon carbide semiconductor power devices in circuit design to optimize the performance of SiC devices and thus construct the preliminary circuit topology of the energy storage converter. As an example, the low-inductance design structure of the laminated busbar adopted in the embodiment of the present invention is composed of positive and negative copper busbar conductors in a laminated manner, and insulating materials are added between the conductors. By connecting multiple capacitors in parallel, the number of branches of stray inductance can be increased and its consistency can be improved, while the mutual cancellation of magnetic fields is used to reduce the loop inductance. In order to prevent the increase in the number of absorption capacitors from causing a decrease in the effective amplitude of inductance, preferably, a 4-absorption capacitor structure can be selected.

[0024] In this step, taking the SiC MOSFET of model CAS300M12BM2 as an example, by extracting the stray inductance of the laminated busbar, it is found that the stray inductance of the laminated busbar of the high-voltage AC module is 734nH, while that of the low-voltage DC module is 175nH. The high-frequency parasitic inductance of the SiC MOSFET and diode of models CAS300M12BM2 and CAS120M12BM2 is 15nH.

[0025] Total stray inductance of the commutation circuit of the high voltage AC module Total stray inductance of the commutation circuit of the low voltage DC module It is expressed by the following formula: in, and They are the high-frequency parasitic inductance and stray inductance of SiC MOSFET respectively.

[0026] It can be seen that the low-inductance design helps reduce the switching oscillation of the energy storage converter when switching the operating mode, improving the stability and performance of the system. This helps reduce electromagnetic interference and reduce the impact on surrounding equipment and the power grid.

[0027] Furthermore, the circuit preliminary topology is modularly cascaded to obtain the circuit topology. Specifically, please refer to FIG2, which 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 the modular cascade design allows the output of a higher voltage level to be easily achieved by connecting power units in series. This design enables the energy storage converter to flexibly adapt to energy storage systems of different sizes and voltage requirements.

[0028] S2. When performing PID control on the island microgrid system, a deep deterministic policy gradient algorithm is used to analyze the circuit topology structure, and a primary adjustment is made to the response characteristics of the energy storage converter based on the analysis results.

[0029] It is understandable that when the energy storage converter operating mode is switched, especially in the unplanned off-grid / island mode switching scenario, the response speed and stability of the energy storage converter system largely depend on the timely adjustment of the control parameters of the previous converter. The DDPG algorithm, namely the Deep Deterministic Policy Gradient algorithm, is an online deep reinforcement learning algorithm under the Actor-Critic framework. It can effectively handle continuous power adjustment and realize real-time control, and is suitable for energy storage converter control parameter optimization.

[0030] In this embodiment, a deep deterministic policy gradient algorithm is used to adjust the response characteristics of the energy storage converter when it is in different states of charge / discharge. Specifically, when performing PID control on the island microgrid system, a deep deterministic policy gradient algorithm is used to analyze the circuit topology structure to determine the target state space parameters and target action space parameters of the energy storage converter. In an embodiment of the present invention, the state space parameters include parameters such as voltage, current, state of charge (SOC), charge and 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 and discharge instructions, voltage / current regulation instructions and power factor correction instructions.

[0031] Furthermore, a reward function is designed according to the target state space parameters and the target action space parameters, and the response characteristics of the energy storage converter are adjusted in a first level based on the reward function. It can be understood that the response characteristics include voltage response characteristics and frequency response characteristics.

[0032] As an example, when the energy storage converter is in a charging state, the proportional-integral controller parameters (PI 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. Specifically, please refer to Figure 3, which shows a schematic diagram of a PCS voltage outer loop PI parameter adaptive control structure based on a DPPG algorithm in one embodiment of the present invention.

[0033] 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. Specifically, please refer to FIG4 , which shows a schematic diagram of a PCS frequency response characteristic adaptive control structure based on a DPPG algorithm in one embodiment of the present invention.

[0034] The following provides a specific embodiment to describe the above-mentioned first-level adjustment process in detail: When the battery is in the charging state, the front-stage three-phase converter works as a rectifier to provide a stable DC bus voltage for the rear-stage converter. The DPPG algorithm is used to adaptively adjust the PI parameters of the voltage outer loop according to the incoming bus voltage and voltage difference to maintain the good steady-state characteristics of the DC bus voltage. The specific execution process is as follows: The DPPG algorithm is based on the DC bus voltage and its deviation To evaluate the performance of the current PI parameters, and then adjust the strategy network (Actor) and the 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 the voltage characteristic response. The reward function is expressed as follows: in, is the Gaussian function parameter.

[0035] In some embodiments of the present invention, after the DPPG algorithm is introduced, a three-phase converter simulation model is built. At 0.3s after the simulation is started, a sudden scenario is simulated, and the energy storage converter enters the island operation mode. At this time, the bus load suddenly changes, and the DPPG algorithm is used to control the voltage outer loop PI parameters for adaptive training and adjustment. The trained DDPG adaptive PI controller will be based on the DC bus voltage deviation collected every 0.02 seconds To determine a suitable set of PI parameters and Parameters. The comparison of the optimized parameters and the non-optimized parameters on the DC bus voltage fluctuation and steady-state comparison are shown in Figure 5 and Figure 6 respectively. It can be seen that the response speed of the system after the DPPG adaptive optimization parameters is faster and the overshoot is reduced; the steady-state performance is also greatly improved.

[0036] When the battery is in a discharging state, the preceding three-phase converter is in an inverter working state, and the output three-phase AC voltage provides voltage and frequency support for the power grid. When the energy storage converter switches to a non-almost islanding mode, the active power changes suddenly, and its frequency also oscillates. The frequency response of the energy storage converter is closely related to the virtual moment of inertia J and the damping coefficient D of the power loop.

[0037] In the process of optimizing and adjusting the frequency response, the DDPG algorithm adjusts the power loop parameters of the preceding three-phase PWM converter, namely, the virtual moment of inertia J and the damping coefficient D. In some embodiments of the present invention, the performance of the corresponding frequency f is also sampled and analyzed in real time to evaluate the quality of the parameter setting, and a reward function is constructed to update the Actor and Critic networks in the DDPG controller accordingly.

[0038] When constructing the reward function, it is necessary to consider the frequency fluctuation suppression of the energy storage converter when the active power changes, while ensuring 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, so as not to affect the stability of the energy storage converter. Then the reward function It is expressed by the following formula: in, is the shape parameter of the Gaussian function, which represents the standard deviation of frequency fluctuation.

[0039] It is worth noting that in order to balance the system dynamic performance and frequency response, the J and D parameters are adaptively adjusted through the DDPG algorithm only when the power deviation is large. After the power is stable, the parameters are restored to the preset values ​​to maintain the stability of the weak power grid.

[0040] When the training scenario of the simulation algorithm is 0.5s, the energy storage converter encounters an emergency and enters the island operation mode. The output active power of the energy storage converter changes from 10kW to 5kW. The change curves of the power loop parameters J and D after training are shown in Figure 7, and the frequency fluctuation curves of the energy storage converter after fixed parameters and adaptive parameter changes are shown in Figure 8. It can be found that the adaptive adjustment of the J and D parameters in the power loop by the DDPG algorithm can effectively suppress the frequency fluctuation and improve the dynamic response speed of the energy storage converter system to a certain extent.

[0041] In this step, in general, the DPPG reinforcement learning adaptive adjustment mechanism is introduced to enable the energy storage converter to achieve a faster response and better steady-state accuracy after being disturbed.

[0042] S3~S4, if it is detected that the island microgrid system switches to the 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 characteristic after the first-level adjustment is adjusted at the second level. It can be understood that the first-level 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 (on / off-grid and on / off-grid switching process), the response characteristics of the energy storage converter are optimized (i.e., the second-level adjustment and the third-level adjustment).

[0043] 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 act in coordination, and based on the adjusted second gain parameter of the droop controller, the response characteristic after the first-level adjustment is adjusted to a third level.

[0044] 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, a parallel model of the droop controller is first constructed. Specifically, the parallel model of the droop control is expressed by the following formula: In the formula, is the voltage frequency of the inverter, is the voltage amplitude of the inverter; and are the rated power and rated voltage amplitude of the inverter output voltage respectively; and are the output active power and reactive power of the inverter respectively; as well as are the rated output active power and reactive power of the inverter respectively; and are the active power droop coefficient and reactive power droop coefficient of the droop control equation respectively.

[0045] In some embodiments of the present invention, the parameter and Calculated as: In the formula, and They are respectively the limited minimum voltage frequency and the limited minimum voltage amplitude at maximum power output; and They are respectively the maximum output value of active power limited when the frequency drops and the maximum output value of reactive power limited when the voltage drops.

[0046] It is worth noting that when the grid-connected mode switching signal is detected (the microgrid is in isolated / off-grid mode), in order to achieve grid-connected operation of the microgrid, it is necessary to ensure that the voltage phase and voltage amplitude at the common connection node (PCC) are consistent with the grid side.

[0047] In the embodiment of the present invention, the voltage phase and amplitude difference on both sides of the PCC point (i.e., the microgrid side and the grid side) is detected, and the voltage on the microgrid side is adjusted accordingly to achieve synchronization. In some embodiments of the present invention, a voltage sensor or a measuring device can be used to detect the voltage phase and voltage amplitude on both sides of the common connection point, and then the voltage phase difference and voltage amplitude difference on both sides of the common connection point are calculated based on the detected voltage data.

[0048] 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. The synchronous control algorithm can output an adjustment signal to adjust the voltage phase and amplitude on the microgrid side so that it gradually approaches the voltage phase and amplitude on the grid side until synchronization is achieved. Specifically, the above phase and amplitude adjustment process is represented by the following formula: in, and They are respectively the voltage frequency reference value and the amplitude reference value after signal superposition; and They are the voltage frequency reference value and amplitude reference value 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 These are the parameters of the PI controller in the voltage phase synchronization control link; and These are the parameters of the PI controller in the voltage amplitude synchronization control link.

[0049] It can be understood that the core of synchronous grid-connected control is to ensure that the voltage phase and amplitude of the microgrid match those of the grid side before grid connection, so as to avoid the impact current and voltage fluctuations generated during grid connection.

[0050] In order to achieve a smooth transition of the system in the energy storage converter of the island microgrid, the above-mentioned droop controller is further improved accordingly.

[0051] In an embodiment of the present invention, the droop controller introduces active power differential terms and reactive power integral terms in different modes. As an example, if it is detected that the microgrid system switches to an off-grid mode, an active power differential term is introduced into the droop controller, and if it is detected that the microgrid system switches to a grid-connected mode, a reactive power integral term is introduced, thereby obtaining an improved droop control, which is specifically represented by the following formula: 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.

[0052] Normally, the rated reactive power can be set to 0, which satisfies , so the improved parallel system droop control of energy storage converter is as follows: According to the above formula, by improving the droop control, the gain parameter of the droop controller is adjusted to achieve further optimization and adjustment of the response characteristics of the energy storage converter. The parameters and gain of the droop controller reflect the adjustment range of the response characteristics.

[0053] 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 is operating in an isolated island, the reference power is set to a fixed value. Before actively connecting to the grid, it is switched from a fixed value to tracking the actual power. 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 connected to the grid. Specifically, the implementation process of power tracking adjustment is as follows: 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 an isolated grid 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 rely on the external grid. In order to ensure the stability and power quality of the system, as an example, the reference power value can be set to 30kW.

[0054] By setting the reference power, the droop controller can effectively adjust the system frequency and voltage to cope with load changes and ensure the stable operation of the microgrid.

[0055] When the grid-connected mode switching signal is detected, that is, when the microgrid is preparing to be connected to the grid, in order to avoid transient shocks caused by the 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 track and adjust the real-time power.

[0056] In the embodiment of the present invention, in order to further reduce the transient surge of the converter output current, an inertia link is introduced into 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, thereby achieving a smooth transition.

[0057] Specifically, in some embodiments of the present invention, the target reference power is adjusted to the actual power in the case of grid connection by adjusting the time parameter and gain coefficient of the inertia link to achieve smooth switching between grid connection and off-grid connection. It can be understood that the time constant and gain coefficient determine the response speed and amplification degree of the inertia link to the input signal change. Therefore, when the microgrid is preparing to be grid-connected, the introduction of the inertia link to adjust its corresponding parameters can ensure smooth switching between grid connection and off-grid connection.

[0058] In this embodiment, 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: From the above formula we can get: The transfer function is subjected to an inverse Laplace transform to convert the output signal from the complex frequency domain to the time domain. The output signal is then expressed by the following formula: The embodiment of the present invention can achieve the matching of the output power of the inverter with the grid demand through the above operation, thereby reducing the transient impact when connecting to the grid. At the moment of connecting to the grid, the inertia link can continue to fine-tune 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.

[0059] It is worth noting that when the grid connection is completed, the stable clamping effect of the large power grid can be used to stabilize the frequency and voltage of the microgrid. At this time, through the above operation, the inverter output power is equal to the reference power to meet the power balance of the system.

[0060] In order to verify the applicability of the proposed improved droop control algorithm, the present invention provides a specific embodiment and conducts 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: Table 1 In the first set of experiments, the improvement effect was evaluated by comparing the frequency response of the system before and after enhancement. Specifically, the effect of introducing the voltage control loop and the inertia link was analyzed.

[0061] Output Impedance Transfer Function and The frequency response curve of is shown in Figure 9, where and The output impedance transfer functions of the traditional droop control algorithm and the improved droop control algorithm are shown 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 improving the system bandwidth and increasing the phase margin.

[0062] Furthermore, in the second set of experiments, a comparative experiment was conducted using step response to analyze and evaluate the effectiveness of the improved droop control. As shown in Figure 10, it can be seen that the conventional controller has a large oscillation peak in the step response, and the system takes about 0.08 seconds to stabilize. In contrast, the improved droop controller has smaller oscillations and a shorter stabilization time, showing the ability to reach steady state faster.

[0063] Finally, in the third set 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, that is, at time t1 in the figure, a grid failure occurs, and the energy storage converter is forced to enter the island operation state. Figure 11 shows the results of the improved control method and the traditional control method from top to bottom. After the STS (static transfer switch) is turned on at time t2, under the traditional control method, the output voltage and frequency of the inverter oscillate for about 300ms. In contrast, through the improved control method, the oscillation duration of the inverter output voltage is significantly reduced to about 60ms, which greatly improves the voltage recovery process.

[0064] In summary, the present invention firstly introduces the idea of ​​deep reinforcement learning, constructs an adaptive optimization mechanism for the control parameters of the energy storage inverter 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 inverter respectively, improves the system stability and suppresses the frequency and voltage fluctuations of the system; and improves the droop controller, combining the droop control with the integral controller and the differential controller to further optimize the response parameters of the energy storage inverter, so as to improve the dynamic performance of the microgrid system; and also introduces a power tracking mechanism, which is combined with the inertia link to suppress transient shocks; thereby achieving smooth switching of the island microgrid on and off the grid.

[0065] An embodiment of the present invention provides an on-grid and off-grid switching control system for an island microgrid. Specifically, please refer to FIG. 12 , which shows a structural block diagram of an on-grid and off-grid switching control system for an island microgrid in one embodiment of the present invention, including: The topology building module M1 is used to build a circuit topology structure composed of energy storage converters in the island microgrid system; The primary adjustment module M2 is used to analyze the circuit topology structure by using a deep deterministic policy gradient algorithm when performing PID control on the island microgrid system, and to make a primary adjustment to the response characteristics of the energy storage converter based on the analysis result; The secondary adjustment module M3 is used for controlling the droop controller and the differential controller to work together if it is detected that the island microgrid system switches to the 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-level adjustment module M4 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 act in coordination when it is detected that the island microgrid system switches to the grid-connected mode, and to perform a three-level adjustment on the response characteristics after the first-level adjustment based on the adjusted second gain parameter of the droop controller.

[0066] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which 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 attached 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 made to the response characteristics of the energy storage converter based on the analysis result; If it is detected that the island microgrid system switches to the 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 characteristic after the first-level adjustment is adjusted at the second level; as well as, 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 act in coordination, and based on the adjusted second gain parameter of the droop controller, the response characteristic after the first-level adjustment is adjusted in three levels.

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 response characteristics include voltage response characteristics and frequency response characteristics; 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 comprises: When the energy storage converter is in a charging state, a proportional-integral controller parameter of a voltage outer loop of the energy storage converter is controlled by a deep deterministic policy gradient algorithm to adjust the voltage response characteristic; 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.

4. 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.

5. The on-grid and off-grid switching control method of the island microgrid as claimed in claim 3, characterized in that: When the 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, including: In an off-grid situation, setting the target reference power according to the frequency response characteristics; When the grid-connected mode switching signal is detected, the actual power output by the energy storage system inverter is monitored in real time and used as a new reference power to track and adjust the real-time power.

6. The on-grid and off-grid switching control method of the island microgrid as claimed in claim 5, characterized in that: When the 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 further includes: 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 the gain coefficient of the inertia link, so as to achieve smooth switching between grid connection and off-grid connection.

7. The on-grid and off-grid switching control method of the island microgrid as claimed in claim 6, 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: 。 8. The on-grid and off-grid switching control method of the island microgrid as claimed in 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.

9. The on-grid and off-grid switching control method of the island microgrid as claimed in 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.

10. 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 primary adjustment module, used to analyze the circuit topology structure using a deep deterministic policy gradient algorithm when performing PID control on the island microgrid system, and to make a primary adjustment to the response characteristics of the energy storage converter based on the analysis result; 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 act in coordination 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.

Images