Research hotspots of photovoltaic grid-connected inverter technology
Under normal circumstances, grid-connected inverters are divided into two types: 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 comes according to the output power value, there are mainly four types of micro-inverters, low-power inverters, medium-power inverters and high-power inverters. At present, the most mature and fastest developing technology is the medium-power grid-connected inverter, which has been widely used. Micro-inverters and high-power grid-connected inverters will gain broader market prospects. Micro-inverters are mostly used in residential electrical charging systems, while high-power photovoltaic grid-connected inverters have large-capacity power output. Therefore, it is very suitable for photovoltaic power plant systems. 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 study from the following aspects.
New Inverter Topology
Photovoltaic grid-connected inverters can be divided into single-stage and two-stage structures according to the number of power stages. Although the single-stage type has a simple structure, because it has many control objects and the objects are coupled with each other, the design is difficult and complicated. The most MPPT (maximum power point tracking) control and inverter are realized through a power link. Grid-connected control, so the efficiency is high. Compared with the single-stage type, the two-stage type has lower efficiency, and can realize independent control of MPPT and grid connection, which is suitable for photovoltaic power generation systems.
Inverter switching device driving method
By controlling the on-off of the switching device of the inverter circuit, the method of outputting continuous pulses with equal amplitude and unequal width is called PWM (pulse width modulation) control mode. PWM adjusts the width of each pulse according to certain rules to achieve control The functions of the output voltage and output power of the inverter are frequently used in the PWM control method, that is, the sine wave PWM (SPWM method). It is a new type of SPWM method, and 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.
Power Quality Control Methods
The current fed into the power grid has a great impact on the power quality of the power grid, and its grid-connected quality has attracted much attention. Therefore, controlling the output current waveform of the inverter is particularly important in the grid-connected control method. There are many applications such as PI control, hysteresis control, double closed-loop control, deadbeat control, repetitive control, neural network control, and fuzzy control. The development of new control methods and the improvement of old control methods have been in progress.
The maximum power point tracking is to control the output power of the photovoltaic array to always be the largest. There are many control methods to realize MPPT, they are fixed voltage method, disturbance observation method, increasing conductance method, hysteresis comparison method, fuzzy control, sliding mode control, etc. Although these methods can achieve certain effects, they have the disadvantage of low efficiency. Current research still focuses on how to better implement MPPT.
When the power grid fails, the photovoltaic grid-connected power generation system is still connected to the power grid and continues to provide electrical energy for each user. The islanding phenomenon will not only harm the maintenance personnel, reduce the safety of the power grid, and cause the overload operation of the photovoltaic power generation system, but also damage the electrical equipment due to the unstable output voltage and frequency of the inverter, and may even destroy the entire photovoltaic power generation system.
In general, 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. The method of judging whether there is a change is passive detection, but this method has a detection blind spot, because when the output power and the load achieve power Passive detection fails when the special case of balance arises. It cannot be used at the moment of power outage. Active detection overcomes the shortcomings of passive type. 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 to detect whether the signal has passed The way to judge islanding by real-time update 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 research focus of the islanding effect detection method is high performance reliability, fast detection speed and so on.