At present, China's photovoltaic power generation system is mainly a DC system, that is, the electric energy generated by the solar battery charges the battery, and the battery directly supplies power to the load. For example, the solar household lighting system used in the northwest of China and the microwave station power supply system far from the grid DC system. This type of system has a simple structure and low cost, but due to the different DC voltage of the load (such as 12V, 24V, 48V, etc.), it is difficult to achieve system standardization and compatibility, especially for civilian power. Since most of them are AC loads, DC power is used. It is difficult to supply photovoltaic power supply as a commodity to enter the city. In addition, photovoltaic power generation will eventually be connected to the grid. This must adopt a mature market model. In the future, AC photovoltaic power generation systems will become the mainstream of photovoltaic power generation.
Requirements for the inverter in the application:
1. Require high efficiency. Due to the current high price of solar cells, in order to maximize the use of solar cells and improve system efficiency, we must try to improve the efficiency of the inverter.
2. High reliability is required. At present, photovoltaic power generation systems are mainly used in remote areas, and many power stations are unattended and maintained. This requires the inverter to have a reasonable circuit structure, strict component screening, and requires the inverter to have various protection functions, such as input DC Reverse polarity protection, AC output short circuit protection, overheating, overload protection, etc.
3. The DC input voltage is required to have a wide adaptation range. Since the terminal voltage of the solar cell changes with the load and the intensity of the sunlight, although the battery has an important effect on the voltage of the solar cell, because the battery voltage varies with the remaining capacity and internal resistance of the battery Fluctuates due to the change of the battery voltage, especially when the battery ages, the terminal voltage has a large range, such as 12V battery, the terminal voltage can vary between 10V ~ 16V, which requires the inverter must be at a large DC input voltage Normal operation is guaranteed within the range, and the stability of the AC output voltage is guaranteed.
4. In medium and large-capacity photovoltaic power generation systems, the output of the inverter power supply should be a sine wave with low distortion. This is because in medium and large capacity systems, if the square wave power supply is used, the output will contain more harmonic components, and higher harmonics will cause additional losses. Many photovoltaic power generation systems are loaded with communications or instrumentation equipment. These The equipment has high requirements on the quality of the power grid. When the medium and large-capacity photovoltaic power generation system is connected to the grid, in order to avoid power pollution with the public power grid, the inverter is also required to output a sine wave current.
The inverter converts DC power into AC power. If the DC voltage is low, it is boosted by an AC transformer to obtain a standard AC voltage and frequency. For large-capacity inverters, due to the high DC bus voltage, the AC output can generally reach 220V without transformer boosting. In medium and small-capacity inverters, due to the low DC voltage, such as 12V, 24V, It is necessary to design a boost circuit.
There are three types of medium and small capacity inverters: push-pull inverter circuit, full-bridge inverter circuit and high-frequency boost inverter circuit. The push-pull circuit connects the neutral plug of the booster transformer to the positive power supply and two power tubes Alternating work, the output gets AC power. Due to the common ground connection of the power transistors, the driving and control circuits are simple. In addition, because the transformer has a certain leakage inductance, it can limit the short circuit current, thus improving the reliability of the circuit. The disadvantage is that the transformer utilization rate is low, and the ability to drive inductive loads is poor.
The full-bridge inverter circuit overcomes the shortcomings of the push-pull circuit. The power transistor adjusts the output pulse width, and the effective value of the output AC voltage changes accordingly. Because this circuit has a freewheeling circuit, even for inductive loads, the output voltage waveform will not be distorted. The disadvantage of this circuit is that the power transistors of the upper and lower bridge arms do not share ground, so a special driving circuit or an isolated power supply must be used. In addition, in order to prevent the upper and lower bridge arms from conducting together, it is necessary to design a circuit after turning off first, that is, a dead time must be set, and the circuit structure is more complicated.
Control circuit of inverter circuit
The main circuits of the above-mentioned inverters all need to be implemented by a control circuit. Generally, there are two control methods of square wave and positive weak wave. The inverter power supply circuit with square wave output is simple, low cost, but low in efficiency and large in harmonic content. . The sine wave output is the development trend of the inverter. With the development of microelectronics technology, the microprocessor with PWM function has also come out, so the sine wave output inverter technology has matured.
1. The inverter with square wave output currently uses pulse width modulation integrated circuits, such as SG3525, TL494, etc. Practice has proved that the use of SG3525 integrated circuit and the use of power field effect transistors as switching power components can achieve a relatively high-performance inverter. Because SG3525 has the ability to directly drive power field effect transistors and has an internal reference source and operational amplifier and Undervoltage protection function, so its peripheral circuit is very simple.
2. Sine wave output inverter control integrated circuit, sine wave output inverter, the control circuit can be controlled by microprocessor, such as 80C196MC produced by INTEL company, MP16 produced by Motorola company and produced by MI-CROCHIP company PIC16C73, etc., these single-chip microcomputers all have multi-channel PWM generators, and can set the dead time between the upper and upper bridge arms. The INTE company 80C196MC is used to realize the sine wave output circuit. AC output voltage to achieve voltage regulation.
Inverter main circuit power device selection
The selection of the main power components of the inverter is very important. At present, the more used power components are Darlington power transistors (BJT), power field effect transistors (MOS-FET), insulated gate transistors (IGBT) and turn-off. Thyristor (GTO), etc., more devices are used in low-capacity low-voltage systems as MOSFETs, because MOSFETs have lower on-state voltage drop and higher switching frequency, IGBT modules are generally used in high-voltage large-capacity systems. This is because the on-resistance of the MOSFET increases as the voltage rises, and the IGBT occupies a large advantage in the medium-capacity system. In the large-capacity (above 100kVA) system, GTO is generally used as the power element .
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