The modeling of solar photovoltaic grid connected power generation system is the basis for conducting operation control and networking performance analysis of multi parallel solar photovoltaic power generation units. Around the modeling of solar photovoltaic arrays, control of solar photovoltaic grid connected inverters, design of LCL filtering circuits, and stability analysis, various countries have started a large amount of related research. As a core electrical component in solar photovoltaic power generation systems, solar inverters are crucial interface devices for grid source connections. The design of the inverter device has a significant impact on the operational performance of the system. With the continuous increase of power generation capacity and the rapid development of grid connected inverter technology, some new and efficient cascaded topology structures and modular grid connected structures are constantly proposed and practiced. Inverter devices are rapidly advancing towards the goal of improving utilization, automation, and high safety.
The key to improving the performance of solar photovoltaic grid connected systems lies in selecting reasonable and appropriate control technologies. With the increasing progress of science and technology, the understanding and recognition of solar photovoltaic grid connected systems are becoming more mature. People are gradually able to apply some advanced control algorithms to the control of solar photovoltaic grid connected systems. The solar photovoltaic grid connected control system mainly consists of solar photovoltaic modules, converters, solar inverters, and controllers. According to the principle of photovoltaic effect, solar photovoltaic cells will generate photocurrent due to external sunlight exposure. By adjusting the duty cycle of the converter through reasonable control and impedance matching, the system can operate at the maximum power point, and the solar photovoltaic module can output the maximum power; The controller can reasonably control the solar grid connected inverter, allowing the output current of the solar inverter to track the voltage waveform of the grid. Therefore, the control of solar photovoltaic grid connected systems is mainly divided into maximum power point tracking control and solar grid connected inverter control.
1) Maximum power point tracking control for solar photovoltaic grid connection
Maximum power point tracking control is the use of certain control techniques to seek the optimal working state of solar photovoltaic cells, so that the system can maintain maximum power output. To achieve maximum power point tracking control of solar photovoltaics, two conditions need to be met: (1) selecting an appropriate transformation circuit as the main circuit for maximum power point tracking control of solar photovoltaics; (2) By using appropriate maximum power tracking control algorithms to control the duty cycle of the converter, the goal of tracking the maximum power point is achieved. The commonly used MPPT control algorithms at home and abroad mainly include: fixed voltage tracking method, short-circuit current proportional coefficient method, disturbance observation method, conductivity increase method, current scanning method, difference calculation method, neural network method, fuzzy logic control method, sliding mode control method, and comprehensive control calculation method, These methods have achieved significant results.
2) Control of solar grid connected inverters
Solar grid connected inverters are the core components of solar photovoltaic grid connected systems. The main condition for achieving grid connection of solar grid inverters is that the solar inverter outputs blue phase AC power, whose harmonic content is within the specified range, can track the grid voltage waveform well, maintain the same frequency and phase as the grid voltage, and transmit active power to the grid at a unit power factor. Therefore, whether friendly grid connection can be achieved mainly depends on the control of solar grid connected inverters. How to control solar grid connected inverters to obtain good output current waveforms has become a hot research topic in the field of solar inverters. People have applied various control theories, such as PID control, hysteresis current control, adaptive control, fuzzy control, repetitive control, etc., to the research of solar grid connected inverters. Due to the advantages and limitations of each control method, the organic combination of two or more control methods, learning from each other’s strengths and weaknesses, and achieving algorithmic complementarity, has become a trend in current research on solar photovoltaic grid connected system control. The reactive power capability generated by solar inverters is limited and also limited by various factors, and the reactive power capability of solar inverters with different structures is also different. In the reactive power control strategy of solar inverters for solar photovoltaic power plants, the solar photovoltaic power plant needs to allocate reactive power to the solar inverters, and reasonably allocate the total reactive power reference required by the power grid to each solar energy inverter according to the reactive power limit of different solar inverters, It can effectively improve the utilization rate of reactive power capacity of solar inverters and prevent damage to power electronic devices caused by reactive power output exceeding the line. Therefore, it is very meaningful to conduct in-depth research and explore the range of reactive power that can be adjusted when different structures of solar inverters are used to transmit active power.
To ensure stable and safe operation of the system voltage, the reactive power of the power grid is maintained in balance. Each solar photovoltaic power station is equipped with reactive power compensation equipment with rapid dynamic adjustment function, and the power grid requires a large compensation capacity (about 10-30% of the total power plant capacity). However, these devices are all expensive, and as the capacity of solar photovoltaic power stations increases, the capacity of reactive power compensation devices also increases accordingly, resulting in a large investment in reactive power compensation in the early stage of solar photovoltaic power stations, which will also cause significant active power loss and adverse effects such as high-order harmonic pollution. On the other hand, in order to compensate for the reactive power required by the system when the solar photovoltaic power station is connected to the grid, the compensation speed is relatively low. The reactive power regulation ability of the solar inverter itself can be used to compensate for the actual reactive power problem required by the solar photovoltaic power station, which can save the cost of the reactive power compensation device equipped in the solar photovoltaic power station, At the same time, it can also reduce line losses.
At present, research on modeling large-scale solar photovoltaic power generation systems mainly focuses on the overall equivalent modeling and operational control strategies of solar photovoltaic power plants. Based on the research of traditional control strategies for solar inverters, a reasonable reactive power control strategy is adopted to fully tap into the reactive power potential of solar inverters and make use of it. The utilization of the reactive power potential of solar inverters involves power control issues, reactive power regulation range issues, and the selection of control strategies. Many literature has already conducted some basic research on this, mainly reflected in:
A study was conducted on the topology and working principle of the main circuit for solar inverters with different topologies and power capacities, and the composition of the inverter device was designed. In order to maintain the maximum power state of the system, real-time monitoring of its state can be used to adjust the maximum operating point of the array. This method of real-time control and adjustment of power points is called MPPT.
The solar grid connected inverter is the most important control equipment in the solar photovoltaic grid connected system. The traditional control strategy aims to achieve sinusoidal current grid connection, while maintaining that the grid connected current and grid voltage are of the same frequency and phase. Domestic and foreign researchers have conducted extensive research on two traditional control strategies for solar inverters: dual closed-loop decoupling control and nonlinear vector decoupling control.
By using a vector control scheme of current inner loop and voltage outer loop, feedforward control is adopted to decouple the d-axis and q-axis currents in a vertical rotating coordinate system. The stable DC side voltage of the voltage outer loop and the current tracking characteristics of the current inner loop are analyzed.
Based on differential geometry theory and the nonlinear model of solar inverters, a state feedback precise linearization nonlinear control strategy is proposed to achieve decoupling control of reactive power and active power of solar inverters.
With different practical background issues and different requirements for the role of solar inverters, some new control strategies for solar inverters have been proposed by domestic and foreign personnel.
Using dual-purpose solar inverters as generators and battery energy storage in remote areas, and improving power quality and reliability of weak grid systems under weak grid conditions.
Propose a control algorithm based on space vector theory, which is suitable for effectively reducing the output voltage and current harmonics of solar photovoltaic grid connected side under weak grid conditions. The research on using solar inverters for reactive power regulation to provide reactive power support for the connected power grid and improve the quality of power grid operation is gradually receiving attention. A large amount of literature has provided a very detailed introduction to reactive power regulation.
A comparative analysis was conducted on the working mechanism and characteristics of two typical reactive power compensation devices, SVC and SVG. By establishing a steady-state analysis model for the integration of solar inverters into the distribution network, this paper analyzes the reactive power regulation capability and its influencing factors of solar inverters, and proposes a power control strategy for grid voltage regulation.
A reactive power control strategy for solar inverters applied in low voltage ride through is described, which can prevent solar photovoltaic power plants from disconnecting from the grid in the event of grid faults, and can support the grid voltage through the reactive power of the solar inverter.
Propose a unified control strategy that can achieve active filtering, reactive power compensation, and photovoltaic grid connected power generation, so that solar photovoltaic can provide the required reactive power while providing active power to the grid, which can improve the voltage quality of the power network.
Considering the reactive power regulation capability of solar photovoltaic power plants, a stochastic power flow algorithm is adopted to study the reactive power optimization of the distribution network of solar photovoltaic power plants.
When the proportion of photovoltaic power generation in the power network is greater than 30%, the voltage regulating capacitors in the substation can be completely replaced by their voltage regulating capacity. For the load of solar inverters, their reactive power control can compensate for it, but its reactive power control cannot control the reactive power of the substation separately.