Selection of inverter power devices
At present, the domestic photovoltaic power generation system (Photovoltaic System, PVS for short) is mainly based on the DC system, but the most common electrical load is the AC load, which makes it difficult to popularize the DC-powered photovoltaic power supply as a commodity. At the same time, because solar photovoltaic grid-connected power generation does not require batteries and is simple to maintain, saving investment is the development trend of photovoltaic power generation. All of these must adopt AC power supply, so the application of inverters in PVS is becoming more and more important. Inverter is a power conversion device that converts direct current into alternating current. Inverter technology is relatively mature in power electronics technology. For example: inverters in UPS power supply, inverter technology in frequency conversion technology, inverter technology in special power supplies, and inverter technology in power conditioners, etc. These have all been introduced to the market in the form of products and have been well received by the society. Widely recognized.
In the solar PVS using battery energy storage, the nominal voltage of the battery pack is generally 12V, 24V or 48V. Therefore, the inverter circuit generally needs to be boosted to meet the power demand of 220V common AC loads. Inverters can be divided into power frequency and high frequency inverters according to the different boosting principles, and their performance varies greatly in applications.
(1) Power frequency inverter
It first inverts the direct current into a power frequency low voltage alternating current; then it is boosted into 220V, 50Hz alternating current for the load through a power frequency transformer. It has the advantage of simple structure and various protection functions can be realized under lower voltage. Because there is a power frequency transformer between the inverter power supply and the load, the inverter operates stably, reliably, has strong overload capacity and impact resistance, and can suppress high-order harmonic components in the waveform. However, the power frequency transformer also has the problem of being bulky and expensive, and its efficiency is relatively low. The small power frequency inverter made according to the current level generally has a rated load efficiency of not more than 90%. At the same time, the iron loss of the power frequency transformer is basically unchanged when operating under full load and light load, so it can run under light load. The no-load loss is larger and the efficiency is lower.
(2) High frequency inverter
It first uses high-frequency DC/DC conversion technology to invert low-voltage direct current into high-frequency low-voltage alternating current; then it is boosted by a high-frequency transformer, and then rectified by a high-frequency rectifier filter circuit to a high-voltage direct current usually above 300V; and finally Through the power frequency inverter circuit, 220V power frequency AC power is obtained for the load. Because the high-frequency inverter uses a small, light-weight high-frequency magnetic core material, the power density of the circuit is greatly increased, so that the no-load loss of the inverter power supply is small and the inverter efficiency is improved. Generally, high-frequency inverters used in small and medium-sized PVS have a peak conversion efficiency of more than 90%.
Inverter output waveform in PVS
(1) Modified Sine Wave Inverter
The traditional modified wave inverter is produced by stepwise superposition of square wave voltage. This method has complicated control circuits, more power switch tubes used in superimposed lines, and larger volume and weight of the inverter. Many problems. In recent years, with the rapid development of power electronics technology, the PWM pulse width modulation method has been widely used to generate the corrected wave output. At present, the modified sine wave inverter has been widely used in user systems in remote areas because these user systems do not require high power quality, and it can meet the needs of most electrical equipment, but it still has a 20% harmonic Wave distortion will cause problems when operating precision equipment and will also cause high-frequency interference to communication equipment. Therefore, a sine wave inverter must be used at this time.
(2) Sine wave inverter
Its advantage is that the output waveform is good, the distortion is very low, and its output waveform is basically the same as the AC waveform of the mains power grid. In fact, the AC power provided by a fine sine wave inverter is higher than the quality of the grid. The sine wave inverter has little interference to radios, communication equipment and precision equipment, low noise, strong load adaptability, can meet all AC load applications, and the overall efficiency is high; its shortcomings are line and relative correction wave inversion The converter is complex, requires high control chips and maintenance technology, and is more expensive. When solar power generation is connected to the grid, in order to avoid power pollution to the public grid, sine wave inverters must also be used.
All in all, the modified sine wave inverter provides a more economical power supply solution, suitable for lighting, TV, digital and other resistive load appliances. Pure sine wave inverter provides alternating current with the same waveform as commercial power, which can perfectly run inductive loads such as motors and motors.