The oil crisis, environmental pollution, and energy shortages that occurred in the 1970s have gradually made people realize that relying solely on conventional fossil fuels cannot meet human energy needs. Therefore, countries have started to develop clean energy power generation, among which solar power generation has developed rapidly due to its unique advantages. At the same time, in order to better develop solar photovoltaic power generation, various countries have also proposed a series of corresponding support policies. China has a vast territory and abundant solar energy resources, and the development of solar power generation has obvious advantages.
At present, China is still composed of traditional energy structures, mainly dominated by fossil fuels such as coal and oil, and solar power generation has broad development space. In recent years, the Chinese government has attached increasing importance to photovoltaic power generation, gradually implementing and improving the subsidy policy for grid electricity prices, further promoting the rapid development of solar photovoltaic power generation technology and making China more competitive in the solar photovoltaic power generation industry. As the core converter component of photovoltaic power generation systems, inverters will inevitably become a competitive and key research object in China’s photovoltaic power generation market, and their research and development, application, and promotion have extremely profound significance.
1. Current Development of Photovoltaic Inverters
In the past 10 years of research and development, the average increase in output value of the photovoltaic industry worldwide has been 57.1%, making it one of the fastest growing industries in the world. As the most core inverter technology in photovoltaic power generation systems, it has also achieved rapid development in recent years. The main principle of inverter control technology is to transform and output signals from DC to AC, combined with relevant modulation techniques. In the 1960s, German A. Schonung first proposed the application of pulse width modulation technology. Afterwards, the sine wave PWM (SPWM) control technology was proposed by Dr. S.R. Bowes of the UK. This innovation is a revolutionary leap in pulse width modulation technology and even power electronics technology, and has also become the theoretical basis for the transformation of inverter technology. The continuous development of new energy power generation technology has led many domestic and foreign research institutions to extensively and deeply carry out research on photovoltaic inverter controllers, especially for related topics.
As early as the 1980s, China began to conduct research and development in inverter control and manufacturing technology. In terms of demand in the photovoltaic market, with the country’s strong regulation of photovoltaic power generation and the introduction of relevant subsidy policies, as of the end of 2014, China’s installed capacity of photovoltaic power generation exceeded 4GW. With the vigorous promotion of the “Golden Sun Demonstration Project” by the country and the implementation of a large number of photovoltaic grid connection subsidies and policies benefiting the people, the environment and policies will stimulate the vigorous development of inverter technology research in China. At present, China’s scale and key technologies in the field of photovoltaic power generation are at the forefront of the world.
2. Control strategy for solar photovoltaic inverters
2.1 Classification of solar photovoltaic inverters
The main function of inverters in solar photovoltaic power generation systems is to convert the direct current generated by solar cells into alternating current, while changing the original voltage, amplitude, frequency, and waveform, thereby providing electricity for various AC power devices and equipment, and meeting the requirements of grid connected power generation. At present, there are multiple types of inverters on the market, so special attention should be paid when selecting models and capacities. Especially in solar power generation systems, the efficiency of inverters has a decisive impact on the capacity of solar cells and batteries. The classification of commonly used inverters in the current market is shown in the table.
| Classification basis | Type |
| Purpose | ① Ordinary inverter ② Inverter/control integrated machine ③ Special inverter for postal and telecommunications communication ④ Aerospace and military specific inverters |
| Output frequency | ① Power frequency inverter ② Intermediate frequency inverter ③ High frequency inverter |
| Output phase number | ① Single-phase inverter ② Three-phase inverter ③ Polyphase inverter |
| The direction of output electrical energy | ① Active inverter ② Passive inverter |
| The form of the main circuit | ① Single ended inverter ② Push-pull inverter ③ Half bridge inverter ④ Full bridge inverter |
| Type of main switch device | ① Thyristor inverter ② Transistor inverter ③ Field effect inverter ④ Insulated gate bipolar transistor (IGBT) inverter |
| Power supply type | ① Voltage source inverter (VSI) ② Current source inverter (CSI) |
| Inverter control mode | ① Frequency modulated (PFM) inverter ② Pulse width modulation (PWM) inverter |
| Switching circuit operation mode | ① Resonant inverter ② Fixed frequency hard switching inverter ③ Fixed frequency soft switching inverter |
| The waveform of the output voltage or current | ① Square wave inverter ② Stepped wave inverter ③ Sine wave inverter |
| Isolation method | ① Independent photovoltaic system inverter ② Grid connected photovoltaic system inverter |
2.2 Inverter control strategy
At present, the control methods used by most inverters can be divided into two categories based on control principles: ① inverters using classical control strategies; ② Inverters using modern control strategies.
2.2.1 Classic control strategy.
① Voltage mean feedback control. The main control method of this control strategy is to give a target voltage mean, and feedback the sampling value of the voltage mean provided by the sampling device. The difference between the sampling value and the target value is used to obtain an error value. Based on this error value, a new feedback system is established for PI adjustment to obtain a controllable output. As a constant value regulation system, the biggest advantage of voltage mean feedback control is that there is no net difference output, while the biggest disadvantage is the slow response speed of the system.
② Voltage single closed-loop instantaneous value feedback control. This control mainly establishes feedback by taking the instantaneous voltage value as the given target value, and then establishes feedback based on the instantaneous voltage value provided by the feedback sampling device, and adjusts PI for the error between the two, thereby obtaining a controllable output. As a servo control system, the integral part of its control strategy has a certain phase lag, and the system will inevitably have a net error. Therefore, the steady-state error of this control strategy is not good, but the system response is very fast.
③ A voltage single closed-loop instantaneous value control method based on voltage mean. If a voltage instantaneous value single closed-loop control system is used, the steady-state error is not good, but the voltage mean feedback can just make up for this disadvantage. Therefore, based on the PI adjustment of the voltage instantaneous value single closed-loop control system, a mean voltage feedback control link can be added to greatly reduce the steady-state error of the original system.
④ Double closed-loop instantaneous control combining voltage and current. The voltage single closed-loop control system is similar to the speed single closed-loop control of a DC motor, but has a significant error in resisting load disturbances. The controller can only respond by first outputting the impact of load disturbances on the system’s output voltage, speed, and other physical quantities. Therefore, when designing, a current inner loop can be added to the voltage outer loop for compensation. When the load is disturbed by a signal, the current inner loop can quickly Timely suppressing the impact of load fluctuations can greatly improve anti-interference and control performance by adjusting the voltage outer loop.
2.2.2 Modern control strategies.
① State feedback control based on multiple variables. By adopting this strategy, the poles of the system can be arbitrarily configured to improve the dynamic characteristics of the system. However, when the control strategy initially established the system state variable model, it was difficult to estimate the actual dynamic characteristics of the load, so the actual control scheme can only assume no load or assumed load. To address this drawback, a load current feedforward compensation link can be added to the control system, and robust analysis of the system can be carried out in advance to greatly improve the dynamic quality of the system, resulting in better steady-state and dynamic performance.
② No beat control. This control strategy is to divide the given sine wave reference waveform into several cycles at equal intervals, use prediction algorithms to calculate the starting value of each sampling cycle, and then calculate the value that the load should output at the end of the sampling cycle. By reasonably calculating the starting value of the sampling period, the output waveform of the system will completely coincide with the reference waveform, thus avoiding any phase or amplitude deviation.
③ Sliding mode variable structure control. This control strategy is essentially a nonlinear control method that utilizes a discontinuous switching control strategy to force the system’s state variables to move along a designed sliding surface. The advantage of this control strategy is that it is not sensitive to system parameter changes and external disturbances, and has strong robustness. However, it is difficult to determine an ideal sliding surface and has a high requirement for switching frequency.
④ Fuzzy control. This control strategy belongs to intelligent control and is an advanced stage in the development of control theory compared to traditional control schemes. The greatest advantage of fuzzy control is that it does not rely on the mathematical model of the system. For systems with highly nonlinear and uncertain object problems, this control strategy can usually be adopted.
⑤ Repetitive control. The principle of endometrial control is the fundamental idea of repetitive control strategies. The control strategy can assume an inner layer for controlling instructions and disturbance signals, thus achieving output zero net error control. However, this control scheme has poor dynamic response and requires a large amount of memory.
3. Conclusion
Solar photovoltaic power generation, as an emerging energy source, has developed rapidly in recent years. As the control core of solar photovoltaic power generation, inverters are key factors for the economic, reliable, safe, and high-quality operation of photovoltaic power generation systems. They have decisive significance for the energy conversion efficiency and output power quality of the entire photovoltaic power generation system. Therefore, this article categorizes and organizes the current inverters, briefly introduces the control strategies of photovoltaic inverters, in order to lay a certain foundation for the research and development of photovoltaic inverters.