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The principle and application of the photovoltaic inverter in the system

Photovoltaic power generation system in China at present is mainly dc system, the energy from the solar cell battery, the battery directly to the load power supply, such as our country northwest use more solar home lighting system and away from the grid power supply system of microwave station are in dc system. Such a system has simple structure, low cost, but due to the different load dc voltage (12 v, 24 v, 48 v, etc.), it is difficult to implement system of standardization and compatibility, especially the civil power, because most of the ac load, with dc power supply of photovoltaic (pv) power is hard to enter the market as commodity.

In addition, photovoltaic power generation will eventually achieve parallel operation, it must adopt mature market pattern, communication in the future will become the mainstream of the photovoltaic photovoltaic power generation system. Photovoltaic power generation system to the requirements of the inverter power supply adopt ac power output of photovoltaic power generation system, by the solar array, charge and discharge controller, storage battery and inverter of four parts (grid generation system can save battery commonly), and the inverter is the key part. Photovoltaic power generation system of inverter to demand higher:

1. Requirements with high efficiency. Because now the price of solar cells on the high side, in order to maximize the use of solar cells, improve the efficiency of the system, must try to improve the efficiency of the inverter.

2. The request has higher reliability. The photovoltaic power generation system is mainly used in remote areas, many power plants unattended and maintenance, which requires the inverter has the reasonable structure of circuit, strict screening of electronic components, instruments and demanded that the inverter for various protective functions, such as the input dc polarity reverse protection, ac output short circuit protection, overheating protection, overload protection, etc.

3. The dc input voltage has wide adaptation range, as a result of the solar battery terminal voltage with load and sunshine intensity, although battery voltage plays an important role on the solar battery, but because of the battery voltage fluctuates according to the battery residual capacity and the change of the internal resistance, especially when the voltage of battery aging change range is very large, such as the 12 v battery, its voltage may change between 10 v ~ 16 v, which requires the inverter dc input voltage must be in a larger range to ensure the normal work, and ensure the stability of the ac output voltage.

4. In the medium and large capacity of photovoltaic power generation systems, the output of the inverter power supply should be smaller distortion of sine wave. This is because in the medium and large capacity system, with a square wave power supply, the output will contain more harmonic component and high harmonic will generate additional loss, many of the photovoltaic power generation system load for communication or instrument equipment, these devices have higher request for power quality, and large capacity of parallel operation of photovoltaic power generation system, in order to avoid pollution and public power grid electricity, also calls for sine wave inverter output current. If convert dc to ac inverter, the dc voltage is low, the step-up transformer through communication, namely the standard ac voltage and frequency. Inverter for large capacity, because of the dc bus voltage is higher, ac output generally don't need a step-up transformer can be up to 220 v, in the medium and small capacity of the inverter, due to the low dc voltage, such as 12 v, 24 v, must design the boost circuit. Medium and small capacity inverter generally have push-pull inverter circuit, full bridge inverter circuit and high frequency inverter circuit of pressure three, push-pull circuit, the step-up transformer neutral plug connected into the power, the two power tube work alternately, ac power output, because of the power transistor were juggle, drive and control circuit is simple, because the transformer leakage inductance of the has certain, can limit short circuit current, thus improving the reliability of the circuit. Its defect is low utilization rate of transformer, drive the perceptual load ability is poor. Full bridge inverter circuit overcomes the drawback of push-pull circuit, power transistor to adjust the output pulse width, the effective value of ac voltage output is changed. Because the circuit has a flow loop, even to the perceptual load, the output voltage waveform will not distortion. The disadvantage of this circuit is upper and lower bridge arm of power transistor is not altogether, so must adopt special driving circuit or use isolated power supply. In addition, to prevent the upper and lower bridge arm joint conduction, must first break current road design, namely must set up the dead zone, its circuit structure is relatively complex.

Push-pull circuit and the output of the whole bridge circuit must add a step-up transformer, due to the step-up transformer is big, efficiency is low, the price is more expensive, with the development of power electronics technology and microelectronic technology, inverter is realized by using high frequency transformation technology of pressure, can achieve high power density inverter, the inverter circuit level before the booster circuit adopts the push-pull structure, but the working frequency are in more than 20 KHZ, step-up transformer adopts high frequency magnetic core materials, and small volume, light weight, high frequency inverter after high-frequency transformer into a high frequency alternating current, and the high-frequency rectifier filter circuit for high voltage direct current (generally in more than 300 v) and then by power frequency inverter circuit to realize the inverter. By the circuit structure, make the inverter power is greatly increased, inverter no-load loss is reduced, efficiency was improved, the disadvantage is that the circuit is complex, the reliability is lower than the above two circuits.

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