How to select inverters for off grid systems


How to select inverters for off grid systems

For the off grid photovoltaic power generation systems, the efficiency of the inverter will directly affect the efficiency of the entire system. Therefore, the control technology of the inverter in solar photovoltaic power generation systems has important research significance. In the design of inverters, analog control methods are usually used. However, there are many defects in analog control systems, such as aging and temperature drift effects of components, sensitivity to electromagnetic interference, and the use of a large number of components. The typical analog PWM inverter control system uses natural sampling method to compare the sinusoidal modulation wave with the triangular carrier wave to control the trigger pulse. However, the triangular wave generation circuit is vulnerable to interference from temperature, device characteristics and other factors at high frequency (20kHz), resulting in DC offset in the output voltage, increased harmonic content, Dead time change and other adverse effects. The development of high-speed digital signal processors (DSP) has made digital control of inverters in solar photovoltaic power generation systems possible. Because most of its instructions can be completed in a Instruction cycle, it can realize more complex advanced control algorithms, further improve the dynamic and steady performance of the output waveform, and simplify the design of the entire system, so that the system has good consistency.

Inverter is a power electronic circuit that can convert direct current from a solar cell array into alternating current to supply AC loads. It is a key component of the entire solar power generation system. The off grid Solar inverter has two basic functions: on the one hand, it provides power for completing DC/AC conversion to AC load, and on the other hand, it finds the best working point to optimize the efficiency of the solar photovoltaic system. For specific solar radiation, temperature, and solar cell types, solar photovoltaic systems have a unique optimal voltage and current, allowing the photovoltaic power generation system to emit maximum power. Therefore, the following basic requirements are proposed for inverters in off grid solar photovoltaic power generation systems:

1) The inverter should have a reasonable circuit structure, strict component selection, and various protection functions, such as input DC polarity reversal protection, AC output short circuit protection, overheating, overload protection, etc.

2) It has a wide range of DC input voltage adaptation. Due to the variation of the terminal voltage of the solar cell array with load and sunlight intensity, although the battery has a clamping effect on the voltage of the solar cell, the voltage of the battery fluctuates with changes in the remaining capacity and internal resistance of the battery, especially when the battery ages, the range of terminal voltage variation is large, such as in a 12V battery, The terminal voltage can vary between 10V and 16V, which requires the inverter to ensure normal operation within a wide DC input voltage range and ensure that the AC output voltage is stable within the voltage range required by the load.

3) The inverter should minimize the intermediate stages of electrical energy conversion to save costs and improve efficiency.

4) Inverters should have high efficiency. Due to the current high price of solar cells, in order to maximize the utilization of solar cells and improve system efficiency, it is necessary to improve the efficiency of inverters.

5) Inverters should have high reliability. Currently, off grid solar photovoltaic power generation systems are mainly used in remote areas, and many off grid solar photovoltaic power generation systems are unmanned and maintained. This requires the inverter to have high reliability.

6) The output voltage of the inverter is of the same frequency and amplitude as the domestic mains voltage, suitable for general electrical loads.

7) In medium to large capacity off grid solar photovoltaic power generation systems, the output of the inverter should be a sine wave with low distortion. Due to the use of square wave power supply in medium to large capacity systems, the output will contain more harmonic components, and higher harmonics will generate additional losses. Many off grid solar photovoltaic power generation systems are loaded with communication or instrument equipment, which have high requirements for power quality. For inverters in off grid solar photovoltaic power generation systems, there are two requirements for high-quality output waveform: firstly, high steady-state accuracy, including small THD values, and no static differences in phase and amplitude between the fundamental component and the reference waveform; The second is good dynamic performance, which means fast adjustment under external disturbances and small changes in output waveform.

Power Frequency Solar Inverter

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