Off-grid charging micro-grid system based on dual-mode photovoltaic

Abstract

The invention belongs to the technical field of power electronics and new energy micro-grids, particularly relates to an off-grid charging micro-grid system based on dual-mode photovoltaic, and aims to solve the problems of unstable voltage frequency caused by photovoltaic fluctuation in an off-grid scene and system collapse caused by high transient charging load of an electric vehicle. The system comprises a dual-mode photovoltaic power generation unit, a hybrid energy storage buffer unit, an intelligent charging load management unit and an off-grid central control system, and high-stability, long-life and efficient energy scheduling is realized through dynamic switching of photovoltaic working modes, power frequency division cooperative energy storage, flexible power distribution and virtual synchronous machine control.

Description

Technical Field

[0001] This invention belongs to the field of power electronics and new energy microgrid technology, specifically relating to an off-grid charging microgrid system based on dual-mode photovoltaics. Background Technology

[0002] Against the backdrop of the rapid evolution of the energy internet and distributed energy systems, microgrids, as an important technological means to improve the penetration rate of renewable energy and the reliability of power supply, are playing an increasingly crucial role in the power system. By integrating distributed generation, energy storage systems, and electricity loads, microgrids achieve local energy balance and flexible dispatch, demonstrating enormous application potential, particularly in enhancing the resilience of distribution networks and promoting green energy consumption. With the widespread adoption of electric vehicles and mobile electrical devices, microgrid systems with autonomous operation capabilities have become a core solution for energy supply in special geographical environments or emergency scenarios.

[0003] Among them, an off-grid charging microgrid system based on dual-mode photovoltaics, serving as an independent power unit, is a key technological direction for ensuring energy self-sufficiency in remote areas or regions without grid coverage. This type of system utilizes dual-mode photovoltaic conversion technology, aiming to flexibly switch between different energy conversion modes according to dynamic changes in irradiance, environmental conditions, and load demand, thereby providing continuous and stable power support for energy storage facilities and charging loads.

[0004] Current technologies still face multiple challenges in energy flow control and operational stability of off-grid charging microgrids. Due to the lack of voltage and frequency support from the main grid, the extreme intermittency of photovoltaic (PV) output and random fluctuations at the load end can easily cause instantaneous oscillations in the system bus voltage, leading to power quality degradation or even triggering system protection shutdowns. Existing multi-mode switching logic often suffers from control dead zones or response lags, easily generating severe inrush currents during mode transitions, damaging the hardware lifespan of power converters and energy storage batteries. Furthermore, the collaborative optimization capabilities between internal system units are insufficient, failing to achieve globally optimal scheduling for multi-dimensional variables of photovoltaic, energy storage, and charging, resulting in a high PV curtailment rate and limited overall system energy conversion efficiency. These technical deficiencies are particularly pronounced under extreme environments or high-intensity charging demands, becoming significant technical challenges hindering the high-reliability operation of off-grid microgrid systems. Summary of the Invention

[0005] The purpose of this invention is to provide an off-grid charging microgrid system based on dual-mode photovoltaics to solve the problems of poor system voltage and frequency stability caused by the severe fluctuations in ambient light in off-grid environments, as well as the problems of system frequency collapse and energy storage life decay caused by traditional microgrids when dealing with high transient charging loads of electric vehicles.

[0006] The technical solution of the present invention includes: The dual-mode photovoltaic power generation unit is used to dynamically switch between maximum power point tracking mode and voltage support mode to adjust the output power according to the system power surplus / deficit status. The hybrid energy storage buffer unit, connected to the DC bus, is used to mitigate the transient processes and steady-state energy of the system through the power frequency division coordination of the lithium battery pack and the supercapacitor. The intelligent charging load management unit is connected to the system output terminal and is used to sense the charging pile access status in real time and execute priority-based flexible power allocation logic. The off-grid central control system is connected to the dual-mode photovoltaic power generation unit, the hybrid energy storage buffer unit, and the intelligent charging load management unit via a communication network. It is used to execute global energy dispatch commands and control the system to switch between black start, steady-state operation, and fault recovery modes.

[0007] Furthermore, the dual-mode photovoltaic power generation unit includes a photovoltaic array, a multi-channel interleaved parallel DC-DC converter, and a dual-mode inverter. The multi-channel interleaved parallel DC-DC converter acquires the output voltage and current signals of the photovoltaic array through a sampling circuit and feeds back real-time power data to the off-grid central control system. The dual-mode inverter includes a voltage-frequency control module and a maximum power point tracking (MPPT) control module. When the off-grid central control system determines that the system's energy storage capacity is lower than a preset 30% threshold and the load demand is greater than 50% of the photovoltaic rated power, it instructs the dual-mode inverter to enter the MPPT mode to maximize the extraction of photovoltaic energy. When the off-grid central control system determines that the system's energy storage capacity is higher than 90% and the load demand is extremely low, it instructs the dual-mode inverter to switch to voltage support mode and participate in system frequency regulation by adjusting the modulation ratio and phase angle.

[0008] In one embodiment of the present invention, the hybrid energy storage buffer unit includes a high-rate lithium battery pack, a supercapacitor pack, a bidirectional DC-DC converter array, and an energy storage monitoring module. The bidirectional DC-DC converter array adopts a dual closed-loop control architecture, with the outer loop being a bus voltage control loop and the inner loop being a current tracking loop. The energy storage monitoring module acquires the fluctuation frequency of the DC bus voltage in real time. When the voltage fluctuation frequency is higher than 10 Hz, the off-grid central control system allocates the supercapacitor pack to handle the transient power demand of that frequency band. When the voltage fluctuation frequency is lower than 1 Hz, the lithium battery pack responds to the steady-state power compensation command. Through this frequency decomposition technology, the lithium battery pack is protected from high-frequency current impacts.

[0009] Furthermore, the intelligent charging load management unit includes multiple DC charging piles, a vehicle identification module, a power regulation controller, and a load prediction module. The load prediction module calculates the estimated charging load demand for the next 30 minutes based on historical charging behavior data and the remaining battery power of currently connected vehicles. The power regulation controller allocates the output current of each DC charging pile to a limit according to the available power quota issued by the off-grid central control system. When the total system power is insufficient, the intelligent charging load management unit executes a tiered load reduction strategy based on the priority of battery health status and remaining driving range.

[0010] As one embodiment of the present invention, the off-grid central control system has a built-in virtual synchronous machine algorithm module. This module collects the voltage and current signals of the AC bus and simulates the rotor motion equation and excitation regulation characteristics of the synchronous generator in the control algorithm, thereby providing inertial support and damping characteristics for the off-grid system. When the charging load undergoes a sudden change, the virtual synchronous machine algorithm module adjusts the output power angle of the dual-mode inverter to offset the drastic change in system frequency.

[0011] Furthermore, the switching logic of the dual-mode photovoltaic power generation unit is as follows: The off-grid central control system periodically collects the DC bus voltage change rate; when the absolute value of the change rate is less than 0.05 times the nominal voltage per second and the state of charge of the energy storage system is in the range of 40% to 85%, the system maintains the maximum power point tracking mode; when the absolute value of the change rate exceeds 0.1 times the nominal voltage per second for 3 consecutive cycles and the energy storage system discharge rate is detected to reach 2 times the rated current, the off-grid central control system forces the photovoltaic inverter to switch to the voltage source control mode and participates in the voltage distribution of the system through the droop control characteristics.

[0012] As one embodiment of the present invention, the off-grid central control system also has a black start management function; after the system experiences a power outage due to a fault or energy depletion, the system enters a standby recovery state; when the photovoltaic array detects that the effective light intensity reaches more than 200 watts per square meter and lasts for more than 300 seconds, the system instructs the supercapacitor bank to establish an initial DC bus voltage through the bidirectional DC converter; subsequently, the dual-mode inverter starts up under zero load and establishes a 50 Hz, 220 V reference voltage AC field; after the AC field has been running stably for 120 seconds, the system's key loads are gradually put into operation and the lithium battery bank is guided to be connected to the bus.

[0013] Furthermore, the flexible power allocation logic of the intelligent charging load management unit is specifically implemented as follows: obtain the total target power demand of all online charging piles; compare the difference between the total target power demand and the current available remaining power of the system; if the difference is greater than 0, then allocate full power to vehicles with less than 20% battery level, reduce power proportionally for vehicles with 20% to 80% battery level, and implement trickle charging strategy for vehicles with more than 80% battery level; if the difference is less than or equal to 0, then all charging piles operate at their maximum rated power.

[0014] In one embodiment of the present invention, the internal filtering component of the dual-mode inverter adopts an LCL-type filter structure, and its inductance parameters are matched based on 10 times the system switching frequency to suppress high-frequency carrier interference; the inverter bridge arm adopts silicon carbide power devices, and the switching frequency is set to 20 kHz to reduce switching losses and reduce the size of passive components; in voltage-supported mode, the internal resistance characteristics of the inverter are simulated as inductive through software algorithm to optimize the power sharing effect under multi-source parallel state.

[0015] Furthermore, the bidirectional DC-DC converter in the hybrid energy storage buffer unit has an autonomous current sharing function; when multiple energy storage modules are running in parallel, each converter automatically fine-tunes the output voltage reference by detecting the DC component of the output current to ensure that the current deviation between each energy storage branch is less than 5%; in addition, the energy storage monitoring module continuously monitors the voltage range of the individual cells of the lithium battery pack, and when the range exceeds 50 millivolts, it automatically starts the equalization circuit to transfer energy.

[0016] As one embodiment of the present invention, the system also includes an auxiliary energy-consuming heat dissipation subsystem; when photovoltaic output is extremely excessive and energy storage is fully loaded and charging load is extremely low, the off-grid central control system connects the auxiliary energy-consuming resistor by adjusting the duty cycle signal to convert excess energy into heat energy for discharge, so as to maintain the energy balance of the system and prevent the DC bus voltage from exceeding the protection threshold of 450 volts.

[0017] Furthermore, the communication between the off-grid central control system and each unit adopts a redundant CAN bus structure, with a communication rate set at 500 kilobits per second. The system performs a full-network status scan every 10 milliseconds. If no feedback frame is received from a specific unit for three consecutive times, the communication link of that unit is determined to be faulty, and the off-grid central control system then instructs the affected unit to enter a preset safe independent operation mode.

[0018] As one embodiment of the present invention, the maximum power point tracking control module in the dual-mode photovoltaic power generation unit adopts an improved conductivity incremental method. This algorithm introduces a variable step size factor, which allows the maximum power point to be quickly approached by a large step size when the ambient light changes rapidly, and the power oscillation is reduced by a small step size during the steady-state operation phase. The sampling period is set to 1 millisecond to ensure that the photovoltaic extraction efficiency is not less than 98% under complex weather conditions such as passing clouds.

[0019] Furthermore, the intelligent charging load management unit also includes an environmental sensing sensor to monitor the temperature and humidity of the charging area; when the ambient temperature exceeds 45 degrees Celsius, the load management unit automatically reduces the maximum output current of all charging piles to 70% of the rated value to prevent the charging cables and vehicle batteries from overheating.

[0020] As one embodiment of the present invention, the main circuit structure of the system adopts a three-phase five-wire output, which supports the mixed access of balanced and unbalanced loads; the neutral line is suppressed by a dedicated compensation module to ensure that the distortion rate of the AC output voltage is less than 3% under unbalanced conditions where the single-phase charging load accounts for 40%.

[0021] Furthermore, the off-grid central control system calculates the system's operational economic indicators in real time, including the average cost per kilowatt-hour and the converted value of equipment losses. Based on the calculation results, the system dynamically adjusts the charging pile's billing rate, using price leverage to guide users to charge during peak photovoltaic output periods, thereby further optimizing the system's energy self-sufficiency rate.

[0022] As one embodiment of the present invention, the supercapacitor bank in the hybrid energy storage buffer unit is equipped with a dedicated pre-charging circuit; at the initial stage of system power-on, the supercapacitor voltage is steadily increased to more than 95% of the bus voltage through the current-limiting resistor, and then the main contactor is closed to avoid system startup surge current caused by capacitive load.

[0023] Furthermore, the dual-mode inverter has a seamless switching function in voltage source mode; when external communication is interrupted or the central control system fails, the inverter relies on the bus signal collected by the local controller to autonomously switch to islanded operation mode based on droop characteristics to maintain the power supply continuity of critical loads.

[0024] As one embodiment of the present invention, the energy flow control strategy of the system is based on first derivative prediction logic; by calculating the first derivative of the load change in the past 5 minutes, the power gap trend is predicted, and the pre-response mechanism of the energy storage system is triggered 10 seconds in advance, thereby effectively reducing the peak value of bus voltage fluctuation caused by control delay.

[0025] Furthermore, the intelligent charging load management unit supports V2G functionality; during nighttime periods of extreme power shortage and insufficient lighting, with user authorization, the system can call upon the battery energy of connected vehicles to support the microgrid in reverse, feeding back the vehicle's power battery energy to the microgrid's AC bus via bidirectional charging piles to ensure the operational needs of primary loads such as lighting and monitoring.

[0026] As one embodiment of the present invention, the converter of the dual-mode photovoltaic power generation unit has high-temperature derating protection logic; when the temperature of the power module heat sink reaches 85 degrees Celsius, the system automatically performs power limiting operation at a slope of 10% power reduction for every 1 degree Celsius increase, until the temperature drops to the safe range of 70 degrees Celsius.

[0027] Furthermore, the off-grid central control system stores a database of response plans for various standard fault scenarios. When an AC side short-circuit fault is detected, the system shuts down all inverter outputs within 20 milliseconds and maintains a 3-second monitoring period. After the fault current disappears, it attempts to perform one automatic reclosing. If the reclosing fails, the fault is locked and a manual intervention alarm is issued.

[0028] Compared with the prior art, the advantages and positive effects of the present invention are as follows: To address the lack of voltage / frequency support in off-grid systems, the dual-mode photovoltaic (PV) power generation unit of this invention abandons the single mode of traditional PV inverters, which operate only in Maximum Power Point Tracking (MPPT). When the system's energy storage state of charge is below 30% and the load demand exceeds 50% of the PV's rated power, the inverter operates in MPPT mode to maximize energy capture. When the energy storage capacity is above 90% and the load is very light, it automatically switches to voltage support mode, actively participating in the stable control of the AC bus voltage and frequency by adjusting the modulation ratio and phase angle. This mechanism transforms the PV system from a passive energy source into a dispatchable voltage source. In actual measurements, it suppresses frequency fluctuations caused by a 50kW step load from ±1.2Hz to ±0.3Hz, eliminating the need for spinning standby equipment such as diesel generators.

[0029] This solution employs a hybrid energy storage system consisting of lithium batteries and supercapacitors, coupled with a high-frequency frequency division control strategy, which fundamentally solves the problem of high transient current surges during electric vehicle charging. The supercapacitors respond to millisecond-level power surges, protecting the lithium batteries from lifespan loss caused by frequent charge-discharge switching, thus extending the overall service life of the energy storage system by more than 30%. At the same time, it reduces the voltage drop of the DC bus during charging, ensuring power supply quality.

[0030] This solution introduces a central control logic based on a virtual synchronous machine algorithm, giving the off-grid microgrid mechanical inertia similar to that of a traditional large power grid. This enables the system to simulate rotor kinetic energy release and buffer frequency shift when facing drastic step loads such as the start-up and shutdown of high-power charging piles. This avoids low-frequency load shedding protection triggered by excessive frequency change rate and enhances the system's load-bearing limit for high-power pulse loads.

[0031] This solution achieves optimal energy utilization through an intelligent charging load management unit; through flexible power allocation logic, the system can dynamically adjust the output of each charging pile according to the photovoltaic output and energy storage status, realizing the transformation from source following load to load following source; this priority-based scheduling strategy ensures the charging needs of critical emergency vehicles and low-battery vehicles, improving the system's social benefits and survivability in extreme environments.

[0032] This solution features a robust black start and fault recovery mechanism, enabling fully automated system reconstruction using extremely limited light resources through multi-stage pre-charging and gradual pressurization after a complete power outage. This highly automated and intelligent self-repair capability has significant engineering application value for off-grid charging scenarios in remote areas and islands where manual maintenance is lacking, ensuring the long-term availability of charging services.

[0033] The hardware structure of this solution adopts silicon carbide wide-bandgap semiconductor devices and interleaved parallel topology, combined with advanced control algorithms, which improves the power conversion efficiency of the entire system to over 96%. By reducing heat loss and optimizing heat dissipation logic, the reliability of the system is enhanced in harsh environments such as high humidity and high temperature, and the system achieves efficient and green operation throughout its entire life cycle. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the overall technical solution architecture proposed in this invention; Figure 2 This is a schematic diagram of the core principle framework of the dual-mode photovoltaic power generation unit in this invention, which switches between maximum power point tracking and voltage support modes. Figure 3 This is a schematic diagram of the power frequency division coordinated control principle framework for achieving transient and steady-state energy smoothing in the hybrid energy storage buffer unit of this invention; Figure 4 This is a flowchart illustrating the priority-based flexible power allocation logic executed by the intelligent charging load management unit in this invention. Figure 5 This is a schematic diagram of the core principle framework of the off-grid central control system in this invention, which provides inertial support through a virtual synchronous machine algorithm. Detailed Implementation Example

[0035] Please refer to Figures 1 to 5 This invention provides an off-grid charging microgrid system based on dual-mode photovoltaics. Its core architecture consists of a dual-mode photovoltaic power generation unit, a hybrid energy storage buffer unit, an intelligent charging load management unit, and an off-grid central control system. The system is designed to establish a highly self-sufficient, highly stable, and long-lifespan electric vehicle charging infrastructure in extreme environments completely disconnected from the public power grid. Through precise control of energy flow, the system solves the problem of voltage and frequency instability caused by drastic fluctuations in photovoltaic output due to ambient light intensity, while effectively mitigating the extremely high transient loads generated during electric vehicle charging.

[0036] In the physical layer layout of the system, a three-phase five-wire output architecture is adopted. This design supports mixed access of balanced and unbalanced loads, making it particularly suitable for actual working conditions where single-phase and three-phase charging piles coexist. To suppress excessive neutral line current caused by a high proportion of single-phase load, a dedicated compensation module is configured on the neutral line to suppress zero-sequence current by actively injecting offset current. Even under severe imbalance conditions where the single-phase charging load accounts for 40%, the system can still ensure that the distortion rate of the AC output voltage remains at an extremely low level of less than 3%, ensuring the electrical safety of precision electronic equipment.

[0037] The dual-mode photovoltaic (PV) power generation unit is the main energy source of the system, integrating a PV array, a multi-channel interleaved parallel DC-DC converter, and a dual-mode inverter with complex control logic. The PV array, composed of multiple sets of monocrystalline silicon PV modules connected in series and parallel, is responsible for converting solar radiation energy into raw DC power. The multi-channel interleaved parallel DC-DC converter serves as the first stage of power conversion. Its core technology lies in reducing input current ripple, extending the lifespan of the PV modules, and improving overall conversion efficiency through the operation of multiple phase-interleaved switches. This converter integrates a high-precision sampling circuit capable of acquiring the output voltage and current signals of the PV array at 1-millisecond intervals, and feeding this real-time power data back to the off-grid central control system via the controller area network bus.

[0038] Dual-mode inverters are key to realizing multi-dimensional utilization of photovoltaic energy. Internally, they include a voltage-frequency control module and a maximum power point tracking (MPPT) control module, which function under different system conditions. The inverter arms utilize advanced silicon carbide (SiC) power devices. This wide-bandgap semiconductor material allows the switching frequency to be set at a high level of 20 kHz, reducing energy loss during switching and allowing for a smaller size of passive components such as filters. The inverter output employs an LCL-type filter structure, with its inductor parameters precisely matched to 10 times the system switching frequency, thereby suppressing high-frequency carrier interference at the source.

[0039] In terms of operational logic, the off-grid central control system periodically collects the rate of change of the DC bus voltage and the state of charge of the energy storage system as the basis for mode switching decisions. When the off-grid central control system determines that the system's energy storage capacity is lower than the preset 30% threshold and the current load demand has exceeded 50% of the photovoltaic rated power, the system is in an energy shortage state. At this time, the off-grid central control system instructs the dual-mode inverter to enter maximum power point tracking (MPPT) mode. In this mode, the MPPT control module adopts an improved incremental conductance method, introducing a variable step size factor to balance tracking speed and steady-state accuracy. When the ambient light changes rapidly, the algorithm uses a large step size to quickly approach the maximum power point; while in the steady-state operation phase, it switches to a small step size to reduce power oscillations and ensure that the photovoltaic extraction efficiency is always not lower than 98%.

[0040] Conversely, when the off-grid central control system determines that the system's energy storage capacity is already above 90% and the current charging load is extremely low, the system faces the risk of excess energy. At this time, the off-grid central control system instructs the dual-mode inverter to switch to voltage-supported mode. In voltage-supported mode, the inverter no longer pursues maximum power output but instead simulates voltage source characteristics. By adjusting the modulation ratio and phase angle, the inverter can actively participate in system frequency regulation. Its internal control algorithm simulates the inverter's internal resistance characteristics as inductive. This technique optimizes the power sharing effect in multi-source parallel operation and prevents circulating currents between inverters.

[0041] The hybrid energy storage buffer unit, acting as the system's energy reservoir, is connected to the DC bus and undertakes the dual tasks of smoothing transient fluctuations and maintaining steady-state energy balance. This unit consists of a high-rate lithium-ion battery pack, a supercapacitor bank, a bidirectional DC-DC converter array, and an energy storage monitoring module. The lithium-ion battery pack provides high-energy-density, long-term power supply, while the supercapacitor bank utilizes its high power density and fast charging / discharging speed to handle millisecond-level transient impacts. The bidirectional DC-DC converter array employs a dual-closed-loop control architecture. Its outer loop is the bus voltage control loop, used to stabilize the DC voltage; the inner loop is the current tracking loop, responsible for precisely controlling the throughput current of the energy storage medium.

[0042] To achieve precise management of power disturbances across different frequency bands, the hybrid energy storage buffer unit incorporates frequency decomposition technology. The energy storage monitoring module acquires the fluctuation frequency of the DC bus voltage in real time. When a voltage fluctuation frequency higher than 10 Hz is detected, typically caused by frequent switching of high-power electronic devices or the instantaneous startup of charging piles, the off-grid central control system instructs the bidirectional DC-DC converter array to allocate supercapacitor banks to handle the transient power demand in that frequency band. The supercapacitors, through rapid response, absorb or compensate for high-frequency currents, preventing these surge currents from entering the lithium battery. When the voltage fluctuation frequency is lower than 1 Hz, manifesting as slow power gains and losses, the lithium battery bank responds with steady-state power compensation commands. Through this power frequency division coordination, the lithium battery bank is protected from high-frequency current surges, reducing temperature rise and extending the overall service life of the energy storage system by more than 30%.

[0043] Furthermore, the bidirectional DC-DC converter also features autonomous current sharing. When multiple energy storage modules are operating in parallel, each converter automatically fine-tunes its internal output voltage reference by detecting the DC component of the output current. This fine-tuning mechanism ensures that the current deviation between each energy storage branch is controlled within 5%, preventing uneven load distribution caused by inconsistent internal resistance of each module. The energy storage monitoring module also continuously monitors the voltage range of individual lithium battery cells. Once a range difference exceeds 50 millivolts, it automatically activates the balancing circuit to transfer energy, ensuring the overall consistency of the battery array.

[0044] The intelligent charging load management unit is connected to the AC output of the microgrid and serves as the direct interface for interaction between the system and charging vehicles. This unit includes multiple DC charging piles, a vehicle identification module, a power regulation controller, and a load prediction module. The load prediction module is the cornerstone of intelligent management. Through in-depth analysis of historical charging behavior data, combined with the remaining battery power of currently connected vehicles, it uses prediction algorithms to calculate the estimated charging load demand for the next 30 minutes.

[0045] In the specific power allocation process, the intelligent charging load management unit executes priority-based flexible power allocation logic. This process begins by obtaining the total target power demand of all online charging piles and comparing it with the current available remaining power issued by the off-grid central control system. If the difference between the total target power demand and the remaining power is greater than 0, it means that the system is in a power shortage state. At this time, the power regulation controller will allocate power according to a tiered load reduction strategy. The system will prioritize obtaining the battery health status and remaining driving range of each vehicle, sorting them in order of battery level from low to high. For emergency vehicles with less than 20% battery level, the system allocates full power to ensure their rapid recovery of driving capability; for ordinary vehicles with battery level between 20% and 80%, the charging current is reduced proportionally; for vehicles with battery level above 80%, a trickle charging strategy is enforced. This load shift based on the source ensures that the microgrid system does not collapse due to overload when photovoltaic output is insufficient or energy storage is low.

[0046] The off-grid central control system is the nerve center of the entire microgrid, and one of its core technologies is the built-in virtual synchronous machine algorithm module. This module acquires voltage and current signals from the AC bus at high speed and simulates the rotor motion equations and excitation regulation characteristics of a synchronous generator in the control program. This algorithm-level simulation endows the off-grid system based on power electronic devices with physical inertial support and damping characteristics.

Claims

1. An off-grid charging microgrid system based on dual-mode photovoltaics, characterized in that, include: The dual-mode photovoltaic power generation unit is used to dynamically switch between maximum power point tracking mode and voltage support mode to adjust the output power according to the system power surplus / deficit status. The hybrid energy storage buffer unit, connected to the DC bus, is used to mitigate the transient processes and steady-state energy of the system through the power frequency division coordination of the lithium battery pack and the supercapacitor. The intelligent charging load management unit is connected to the system output terminal and is used to sense the charging pile access status in real time and execute priority-based flexible power allocation logic. The off-grid central control system is connected to the dual-mode photovoltaic power generation unit, the hybrid energy storage buffer unit, and the intelligent charging load management unit via a communication network. It is used to execute global energy dispatch commands and control the system to switch between black start, steady-state operation, and fault recovery modes.

2. The off-grid charging microgrid system based on dual-mode photovoltaics according to claim 1, characterized in that, The dual-mode photovoltaic power generation unit includes a photovoltaic array, a multi-channel interleaved parallel DC-DC converter, and a dual-mode inverter. The multi-channel interleaved parallel DC-DC converter acquires the output voltage and current signals of the photovoltaic array through a sampling circuit and feeds back real-time power data to the off-grid central control system. The dual-mode inverter includes a voltage and frequency control module and a maximum power point tracking control module, which are used to switch operating modes under different system conditions.

3. The off-grid charging microgrid system based on dual-mode photovoltaics according to claim 1, characterized in that, The hybrid energy storage buffer unit includes a high-rate lithium battery pack, a supercapacitor pack, a bidirectional DC-DC converter array, and an energy storage monitoring module. The bidirectional DC-DC converter array adopts a dual closed-loop control architecture, with the outer loop being the bus voltage control loop and the inner loop being the current tracking loop; The energy storage monitoring module is used to acquire the fluctuation frequency of the DC bus voltage in real time, and allocate lithium battery packs or supercapacitor packs to respond to the corresponding power demand based on the frequency characteristics.

4. The off-grid charging microgrid system based on dual-mode photovoltaics according to claim 1, characterized in that, The intelligent charging load management unit includes multiple DC charging piles, a vehicle identification module, a power regulation controller, and a load prediction module. The load prediction module calculates the estimated charging load demand within a preset time period based on historical charging behavior data and the remaining battery power of currently connected vehicles. The power regulation controller allocates the output current of each DC charging pile according to the available power quota issued by the off-grid central control system.

5. The off-grid charging microgrid system based on dual-mode photovoltaics according to claim 1, characterized in that, The off-grid central control system has a built-in virtual synchronization machine algorithm module; The virtual synchronous machine algorithm module collects voltage and current signals from the AC bus and simulates the rotor motion equation and excitation regulation characteristics of the synchronous generator in the control algorithm, thereby providing inertial support and damping characteristics for the off-grid system.

6. The off-grid charging microgrid system based on dual-mode photovoltaics according to claim 1, characterized in that, The switching logic of the dual-mode photovoltaic power generation unit is determined based on the DC bus voltage change rate and the state of charge of the energy storage system. When the voltage change rate is lower than the preset threshold and the energy storage state of charge is in the middle range, the maximum power point tracking mode is maintained. When the voltage change rate continuously exceeds the threshold and the energy storage system discharge rate reaches the set level, the system is forced to switch to voltage source control mode.

7. The off-grid charging microgrid system based on dual-mode photovoltaics according to claim 1, characterized in that, The off-grid central control system has a black start management function; After a complete power outage of the system, when the effective light intensity is detected to meet the start-up conditions, the supercapacitor bank is instructed to establish the initial DC bus voltage. Subsequently, the dual-mode inverter was started to establish a reference voltage AC field, and after stabilization, key loads were gradually put into operation and the lithium battery pack was guided to be connected to the bus.

8. The off-grid charging microgrid system based on dual-mode photovoltaics according to claim 1, characterized in that, The flexible power allocation logic of the intelligent charging load management unit executes a tiered load reduction strategy based on the current battery level of the vehicle. When the total power of the system is insufficient, priority is given to ensuring the charging needs of vehicles with low battery levels, while trickle charging or power reduction strategies are implemented for vehicles with high battery levels.

9. The off-grid charging microgrid system based on dual-mode photovoltaics according to claim 2, characterized in that, The dual-mode inverter adopts an LCL filter structure, and its inductance parameters are matched with the system switching frequency to suppress high-frequency interference. The inverter arms use wide-bandgap semiconductor devices to reduce switching losses; In voltage-supported mode, the inverter's internal resistance characteristics are simulated as inductive through software algorithms to optimize the power sharing effect under multi-source parallel operation.

10. The off-grid charging microgrid system based on dual-mode photovoltaics according to claim 1, characterized in that, The bidirectional DC-DC converter in the hybrid energy storage buffer unit has an autonomous current sharing function; When multiple energy storage modules are connected in parallel, each converter automatically fine-tunes the output voltage reference by detecting the DC component of the output current to ensure that the current deviation of each branch is within the allowable range.

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