Under normal circumstances, grid-connected inverters are divided into single-phase and three-phase according to the number of output phases. The output power of single-phase grid-connected inverters is small, generally not exceeding 15kW. Therefore, it is suitable for small and medium-power grid-connected power generation systems. The three-phase method is mostly used in high-power energy conversion occasions. If it is divided according to the output power value, there are mainly four types of micro inverters, small power inverters, medium power inverters and high power inverters. At present, the most mature and fastest growing technology is the medium-power grid-connected converter, which has been widely used. Micro-inverters and high-power grid-connected inverters will gain broader market prospects. Micro-inverters are mostly used in charging systems for civil electrical appliances, and high-power photovoltaic grid-connected inverters are very suitable for photovoltaic power station systems because of their large-capacity power output. With the rapid development of the photovoltaic industry, grid-friendly inverters with large capacity, high efficiency and strong reliability will become the focus of research.
To realize the inverter with high efficiency and low current distortion rate, it is necessary to analyze and research from the following aspects.
① Topological structure of new inverter
According to the number of power stages, photovoltaic grid-connected inverters can be divided into two types: single-stage and two-stage structures. Although the single-stage type has a simple structure, it has many control objects and the coupling between these objects creates a complicated situation that is difficult to design. A power link is used to achieve the most MPPT (Maximum Power Point Tracking) control and inverter Grid-connected control, so high efficiency. The two-stage type has lower efficiency than the single-stage type, and can realize independent control of MPPT and grid connection, and is suitable for photovoltaic power generation systems.
②The driving mode of the inverter switching device
By controlling the switching devices of the inverter circuit, the method of outputting continuous pulses of equal amplitude but unequal width is called PWM (Pulse Width Modulation) control method. PWM adjusts the width of each pulse according to certain rules to achieve control inversion. The output voltage and output power function of the converter. In the PWM control method, there is a frequently used method, that is, sine wave PWM (SPWM method). People have optimized and upgraded the SPWM technology and improved its functions. Various new SPWM methods have been proposed. The triangular carrier modulation method is the most widely used. SPWM is the basic type of sine wave PWM, which has the characteristics of sensitive control and fast response.
③Power quality control method
The current fed into the grid has a great impact on the power quality of the grid, and its grid-connected quality has attracted much attention. Therefore, controlling the inverter output current waveform is particularly important in grid-connected control methods. PI control, hysteresis control, double closed-loop control, deadbeat control, repetitive control, neural network control, fuzzy control, etc. are used in many applications. The development of new control methods and the improvement of old control methods are always in progress.
④ MPPT
Maximum power point tracking is to control the output power of the photovoltaic array to always be the largest. There are many control methods that can achieve MPPT. They are fixed voltage method, disturbance observation method, increased conductance method, hysteresis comparison method, fuzzy control, sliding mode control, etc. Although these methods can achieve certain results, they have the disadvantage of low efficiency. Current research is still focused on how to better implement MPPT.
⑤Islanding effect
When the power grid fails, the grid-connected photovoltaic power generation system is still connected to the power grid and continues to provide power to various users. This system has an islanding phenomenon that is self-sufficient and cannot be controlled by the power company, which is called the islanding effect. The islanding phenomenon not only harms maintenance personnel, reduces the safety of the grid, and causes overload operation of the photovoltaic power generation system, but also damages electrical equipment due to the unstable output voltage and frequency of the inverter, and may even destroy the entire photovoltaic power generation system.
Under normal circumstances, there are two detection methods, active and passive. At the moment of power failure, the voltage and frequency of the connection point between the inverter and the grid, that is, the grid connection point, will change. Passive detection is the way to judge whether there is a change, but this method has a detection blind zone, because when the output power and the load realize the power When the special situation of balance occurs, the passive detection will fail. It cannot be used at the moment of a power outage of the power grid. Active detection overcomes the shortcomings of passive. It actively adds disturbance signals to the frequency and output current of the grid connection point, and detects them. The power grid has the function of updating disturbances and detects whether the signal has passed The real-time update to determine islands is active detection. Active detection is less effective or even invalid when there are multiple distributed energy systems in the local power grid. Therefore, the current research focus of islanding effect detection methods is high performance reliability and fast detection speed.