1. Introduction to Solar Energy Utilization
Solar energy, defined as the radiation energy reaching the earth in the form of electromagnetic waves, is abundant in China, especially in regions like Tibet, Qinghai, Xinjiang, Gansu, and Ningxia, where the annual radiation exceeds 5 × 10^6 kJ/m^2. However, in Sichuan Basin and Guizhou Province, the solar energy resources are relatively weak, with an annual radiation of less than 4.1 × 10^6 kJ/m^2. The main forms of solar energy utilization include photothermal, photochemical, and photovoltaic (PV) conversion. Among them, photovoltaic (PV) conversion has become the most desired form due to its potential in providing electrical energy, and has made significant progress in technology and market over the past 20 years. However, the high cost of PV systems still limits its widespread adoption in developing countries.
2. Structure of photovoltaic (PV) Grid-connected Systems
- Centralized Structure: This was the first structure used in photovoltaic (PV) systems, which connects a large number of solar panels in series to achieve a high voltage and then connects to the grid through a centralized inverter. However, it has drawbacks such as low efficiency of individual photovoltaic (PV) arrays, poor system reliability, and difficult maintenance.
- String and Multi-string Structures: These structures were developed to address the limitations of the centralized structure. In the string structure, each string of solar panels has its own maximum power point tracking (MPPT) function, while in the multi-string structure, the front stage is more expandable and suitable for installation in different orientations on buildings.
- AC Module Structure: In this structure, each solar panel is integrated with an MPPT function and a grid-connected inverter, which can ensure that each solar panel operates at its maximum power point, has strong anti-shading and component parameter mismatch capabilities, is easy to expand, supports plug-and-play and hot-swapping, and is convenient for maintenance. However, it has limitations such as complex connection lines, high cost, and large power loss in high-power systems.
3. Structure of Photovoltaic (PV) Grid-connected Inverters
- Frequency Isolation Type: This type of inverter uses a low-frequency isolation transformer to achieve electrical isolation between the grid and the solar panels, ensuring personal safety and providing voltage matching and suppression of the DC component in the grid-connected current. However, it increases the volume, weight, and cost of the system and reduces the conversion efficiency.
- High-frequency Isolation Type: By inserting a high-frequency transformer in the DC/AC conversion stage, this type of inverter can significantly reduce the volume, weight, and cost of the transformer, but it makes the power conversion more complex and has little improvement in the system efficiency.
- Non-isolation Type: Without a transformer (both high and low frequency), this type of inverter has the advantages of high conversion efficiency, small volume, low weight, and low cost, and has been rapidly developed and widely used in Europe. However, the elimination of the transformer leads to an electrical connection between the solar panels and the grid, which may cause a significant increase in common-mode current, posing a safety hazard.
4. Key Technical Issues and Current Status of Non-isolated Grid-connected Inverters
- Leakage Current Suppression: The non-isolated grid-connected inverter may cause a large increase in leakage current due to the parasitic capacitance of the solar panels to the ground. To suppress the leakage current, it is necessary to eliminate the switching frequency common-mode voltage source, which can be achieved by using an improved converter structure and switching modulation strategy.
- Common-mode Equivalent Circuit Modeling: A more comprehensive common-mode analysis model for single-phase non-isolated bridge PV grid-connected inverters is established, considering the influence of parasitic parameters. From the simplest common-mode equivalent model, two ways to eliminate leakage current are deduced: 1) In the case of matching circuits and parasitic parameters symmetry, the vCM voltage generated by the SPWM switching mode is a constant value; 2) By circuit parameter matching, vCM + vCM-DM = const is achieved.
- Full-bridge Inverter Topology Improvement: A new full-bridge circuit topology, oH5, is proposed by adding a bidirectional clamping branch in the H5 full-bridge inverter structure. This topology can ensure that the potential of the freewheeling circuit is at half of the battery voltage during the freewheeling stage, reduce the conduction loss, and lower the voltage stress of the high-frequency switch and the clamping switch.
- Half-bridge Inverter Topology Improvement: For the NPCTLI half-bridge inverter, three methods to suppress the leakage current by parameter matching are derived, and the full offset method is experimentally verified to be simple, reliable, and effective in enhancing the leakage current suppression performance.
- High-reliability and Low-leakage Inverter Topology: A split-inductor neutral point clamped three-level inverter (SI – NPCTLI) is proposed, which has the advantages of low device voltage stress, constant common-mode voltage, and anti-through structure, meeting the requirements of high efficiency, low cost, low leakage current, and high reliability for non-isolated photovoltaic (PV) grid-connected inverters.
- Grid-connected Current Quality: To meet the strict requirements of IEEE Std 929 – 2000 and UL1741 for the quality of grid-connected current, especially the suppression of switching frequency harmonic current, a third-order LCL filter is required. However, the application of the LCL filter faces challenges such as filter parameter design and resonance damping.
- LCL Filter Design Method: A filter parameter design method that considers both engineering experience and energy storage minimization is proposed to facilitate the rapid design of filter parameters.
- Active Damping Method: A systematic study of the active damping technology for the LCL filter is conducted, and a new active damping structure using the differential feedback of the grid-side inductance voltage is proposed.
- Maximum Power Point Tracking (MPPT): MPPT technology is crucial for improving the efficiency of photovoltaic (PV) systems. However, single MPPT algorithms have limitations, and a combination of multiple MPPT methods may be more effective to improve the MPPT performance.
- Anti-islanding: When the grid is disconnected due to faults or maintenance, the anti-islanding function of the grid-connected inverter must quickly detect the islanding state and disconnect the inverter from the grid to ensure the safety of users and equipment. Various detection methods, including remote and local detection methods, have been developed, but there is still room for improvement in the accuracy and reliability of these methods.
5. Distributed Power Generation Technology and DC Microgrid
- Distributed Power Generation: Integrating distributed power sources, such as solar, wind, and fuel cells, into a microgrid can effectively address the challenges of the intermittent and unstable output of these sources. By coordinating the control of these distributed power sources, the microgrid can provide high-quality and reliable power supply.
- DC Microgrid: A DC microgrid is proposed as a solution to improve the efficiency of photovoltaic (PV) power utilization. By directly supplying DC power to DC loads and using DC/DC converters for AC loads, the number of power conversion stages can be reduced, thereby reducing power losses. The DC microgrid can also operate in island mode, making full use of the output power of solar panels and providing uninterrupted power supply to users.
- System Architecture: The proposed DC microgrid system includes a bidirectional AC/DC converter for the interface with the main grid, a non-isolated bidirectional DC/DC converter for the interface with the energy storage device, a bidirectional DC/DC converter for the interface between the dual DC buses, and a non-isolated DC/DC converter for the interface with the photovoltaic cells.
- Key Technologies: The research focuses on the topology selection and implementation of these interface circuits, including the use of a multi-channel parallel Buck/Boost structure in the energy storage interface circuit, a current-source half-bridge and voltage-source half-bridge structure in the dual DC bus interface circuit, and an additional winding and two auxiliary diodes in the photovoltaic (PV) interface circuit to reduce the switching loss of the converter.
In conclusion, the first chapter of the paper provides a comprehensive overview of the background and key technologies related to photovoltaic (PV) grid-connected systems and DC microgrids, laying the foundation for further research and development in this field.