In a photovoltaic power system, the efficiency of the inverter will directly affect the efficiency of the whole system, so it is of great significance to search for the control technology of the solar PV energy system inverter.
The analog control method is usually adopted In the design of an inverter. However, there are many defects in the analog control system, such as the aging of components and temperature drift effect, the sensitiveness to electromagnetic interference, and more components used.
An inverter is a power electronic circuit. It can convert the direct current from the solar cell array into an alternating current to power an alternating current load, which is the key component of the entire solar PV energy system.
On the one hand, it provides electrical energy to complete the DC/AC conversion to AC load. On the other hand, it is used to find the best working point to optimize the efficiency of the solar PV energy systems.
For specific solar radiation, temperature and solar cell type, the solar PV energy system has a unique optimal voltage and current, so that the photovoltaic system can produce the maximum power of electricity.
1. The inverter should have a reasonable circuit structure and a strict selection of components. And the inverter is required to prepare a variety of protection functions, such as input DC polarity reverse protection, AC output short circuit protection, overheating protection, overload protection, etc.
2. The inverter should have a wide DC input voltage range. Since the terminal voltage of the solar cell array varies with the load and sunshine intensity, the battery has a clamping effect on the voltage of the solar cell.
The voltage of the battery fluctuates with the change of the remaining capacity and internal resistance of the battery, especially when the battery is aging, the change range of the terminal voltage is very large, such as in the 12 V battery.
3. The inverter should reduce the intermediate link of power conversion as much as possible to save cost and improve efficiency.
4. The inverter should have high efficiency. Due to the high price of solar cells, to maximize the use of solar cells and improve the efficiency of the system, it is necessary to improve the efficiency of the inverter.
5. The inverter should have high reliability. At present, solar PV energy systems are mainly used in remote areas, so many solar PV energy systems are unattended and maintained, which requires the high reliability of the inverter.
6. The output voltage of the inverter should be the same frequency and amplitude as the Chinese market voltage, to be suitable for a load of general electric appliances.
7. In solar PV energy systems with medium and large capacity, the output of the inverter should be a sinusoidal wave with small distortion.
Because the solar PV energy systems with medium and large capacity, if a square wave power supply is adopted, the output will contain more harmonic components, and higher harmonics will produce additional losses. Many solar PV energy systems are loaded with communication or instrumentation equipment, which has high requirements on the quality of power supply.
For the inverter of a solar PV energy system, there are two requirements for high-quality waveform output: One is the high steady-state accuracy, including the small THD value, and the fundamental component has no error in phase and amplitude relative to the reference waveform; the other is the good dynamic performance, that is, in the external disturbance under the regulation of fast, the output waveform change is small.
8. The inverter has to have a certain overload capacity. The general inverter should overload 125% to 150%. When it overloads 150%, it should be able to last the 30s; when it overloads 125%, it should be able to last 60s.
The inverter shall guarantee the standard rated sinusoidal output of the solar PV energy system under any load condition (except overload condition) and transient condition.