1. needs high efficiency. Due to the high price of solar cells at present, in order to maximize the use of solar cells and improve the efficiency of the system, we must try to improve the efficiency of the inverter.
2. Require high reliability. At present, photovoltaic power generation system is mainly used in remote areas, and many power stations are unattended for maintenance, which requires the inverter to have a reasonable circuit structure, strict component screening, and a variety of protection functions, such as input DC polarity reverse connection protection, AC output short circuit protection, overheating overload protection and so on.
3. DC input voltage is required to have a wide adaptive range. Because the terminal voltage of solar cells changes with the change of load and sunshine intensity, although the battery plays an important role in the voltage of solar cells, the battery voltage fluctuates with the change of battery residual capacity and internal resistance, especially when the battery is aging, its terminal voltage changes greatly, such as 12V battery, which can change between 10V and16v.
4. In the medium and large capacity photovoltaic power generation system, the output of inverter power supply should be a sine wave with small distortion. This is because in the medium and large capacity system, if the square wave power supply is used, the output will contain more harmonic components, and higher harmonics will produce additional losses. Many photovoltaic power generation systems are loaded with communication or instrumentation equipment, which requires higher grid quality. When medium and large capacity photovoltaic power generation systems are connected to the grid, in order to avoid power pollution with the public power grid, the inverter is also required to output sine wave current.
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principle of operation
The inverter converts DC into AC, and if the DC voltage is low, it is boosted by AC transformer to get the standard AC voltage and frequency. For large-capacity inverter, due to the high DC bus voltage, the AC output can usually reach 220V without transformer boosting. For small and medium capacity inverters, the DC voltage is low, such as 12V, 24V, so it is necessary to design a boost circuit.
Generally, there are three kinds of inverters with small and medium capacity: push-pull inverter, full-bridge inverter and high-frequency boost inverter. In the push-pull inverter, the neutral plug of the step-up transformer is connected to the positive power supply, and the two power tubes work alternately to output alternating current. Because the rate transistor of photovoltaic grid-connected inverter is grounded, the driving and control circuit is simple. In addition, the transformer has a certain leakage inductance, which can limit the short-circuit current, thus improving the reliability of the circuit. Its disadvantages are low utilization rate of transformer and poor ability to drive inductive load.
The full-bridge inverter circuit overcomes the shortcomings of push-pull circuit. When the power transistor adjusts the output pulse width, the effective value of the output AC voltage changes. Because the circuit has a free-wheeling circuit, the output voltage waveform will not be distorted even for inductive loads. The disadvantage of this circuit is that the power transistors of the upper and lower bridge arms are not grounded, so it is necessary to adopt a special driving circuit or an isolated power supply. In addition, in order to prevent the upper and lower bridge arms from conducting at the same time, it is necessary to design a circuit that is turned off first and then turned on, that is, a dead time must be set, and its circuit structure is complicated.
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Control circuit work
The main circuit of the inverter needs to be realized by the control circuit. Generally, there are two control methods: square wave and sine wave. Square wave output inverter power supply has simple circuit and low cost, but low efficiency and large harmonic component. Sine wave output is the development trend of inverter. With the development of microelectronics technology, microprocessors with PWM function have come out one after another, so the inverter technology of sine wave output has matured.
1. Square wave output inverter
At present, PWM integrated circuits, such as SG3525 and TL494, are widely used in square wave output inverters. Practice has proved that the inverter with high performance-price ratio can be realized by using SG3525 integrated circuit and power field effect transistor as switching power components. Because SG3525 has the ability to directly drive power field effect transistor, and has the functions of internal reference source, operational amplifier and under-voltage protection, its peripheral circuit is very simple.
2. Sine wave output inverter
2. Sine-wave output inverter control integrated circuit, sine-wave output inverter, whose control circuit can be controlled by microprocessor, such as 80C 196MC produced by INTEL, MP 16 produced by Motorola, PIC 16C73 produced by Mi-Crochip, etc., all of which have multi-channel PWM generators. The dead time between the upper and lower bridge arms can be set. The sine wave output circuit is realized by INTEL company 80C 196MC, and the sine wave signal is generated by 80C 196MC. The AC output voltage is detected to realize voltage stabilization.
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Selection of main circuit power devices
It is very important to choose the main power components of the inverter. At present, many power components are used, such as Linton power transistor (BJT), power field effect transistor (MOS-FET), insulated gate transistor (IGBT) and turn-off thyristor (GTO). MOS-FET is the most widely used device in small capacity and low voltage systems. Because MOSFET has low on-state voltage drop and high switching frequency, IGBT module is usually used in high voltage and large capacity systems. This is because the on-state resistance of MOSFET increases with the increase of voltage, IGBT has greater advantages in medium-capacity systems, while GTO is generally used as a power element in ultra-large-capacity systems (above 100kVA).
Grid-connected photovoltaic inverter Solar inverter SolarMax photovoltaic inverter has complete specifications, including low-power series inverter and high-power centralized inverter. With the rapid development of photovoltaic power generation market in China, SolarMax inverter will be used by more and more customers in China.