Global warming caused by burning fossil fuels, rising prices of crude oil and natural gas, and political dilemmas caused by dependence on crude oil have led to continuous efforts to improve energy efficiency.
For those countries that are not self-sufficient in energy, solar energy and other alternative energy have indisputable advantages, which can help them achieve the goal of reducing fossil fuel consumption and achieving energy independence. Replacing fossil fuel energy with alternative energy system will have a significant impact on the global economy and human life. But the problem is that the cost of using alternative energy to generate electricity should be similar to or less than that of using fossil fuels to generate electricity, so as to really reduce the consumption of crude oil.
In the process of developing solar energy technology, people pay most attention to how to improve the efficiency of photovoltaic cells. But another important problem that can not be ignored is how to design the circuit to convert the direct current generated by the battery into alternating current. In order to compete with fossil fuel power generation methods such as burning medium and petroleum in cost, it is very important for designers to improve the efficiency of inverter by every percentage point.
Design of inverter
In the system based on photocurrent, the power inverter controls the current between the solar panel, the battery and the load, and converts the DC voltage of the solar panel into a clean sinusoidal current of 50 Hz or 60 Hz, which is output to the load or fed back to the power grid. Figure 1 shows the important role of inverter in solar power generation.
Figure 1 inverter plays an important role in improving the efficiency of solar energy conversion
As the output voltage of the solar panel is variable, it is very complicated to keep the power generation as high as possible. The key to accomplish this task is to detect the maximum power point (MPP). Figure 2 shows how the maximum power point varies with weather and voltage.
Figure 2 the output voltage of solar cells varies with voltage and weather
MPP tracking technology can be used to detect MPP and adjust DC / DC output voltage conversion to maximize output. MPP tracking can improve the overall efficiency of solar cell system by 1 / 3 or more in winter, when the power demand is the highest.
The most common algorithm to determine MPP is to interfere with the working voltage of the battery board and detect the output. The algorithm should leave a large enough oscillation range around the MPP point to avoid the controller’s wrong disturbance to the local power supply when the sky sweeps over the clouds.
The algorithm of battery
The efficiency of disturbance and detection algorithm is not high, because the output point will deviate from MPP in each cycle. Incremental induction algorithm can be used as an alternative. This method can solve the low efficiency caused by oscillation, but it will set a local peak instead of a real MPP, which will cause other problems. The combination of the two algorithms can maintain the high efficiency of incremental sensing algorithm, and at the same time, scan in a large range at a certain interval to avoid selecting the local peak.
Obviously, this will bring a lot of computing load to the controller of the inverter, and the controller must meet some real-time processing challenges.
The current digital signal controller can provide the high-speed computing ability required by real-time control algorithm. Integrated peripherals such as a / D converter and PWM enable the controller to directly detect the input signal and control the power MOSFET. On chip flash memory can be used for programming and data storage. The communication port simplifies the networking process of watt hour meter and other inverters.
The high efficiency of DSP controller in solar inverter has been proved, which can reduce the energy loss in the conversion process by up to 50%. The research of Distributed Generation Technology LLC by national renewable energy laboratory shows that the DSP based inverter can reduce the manufacturing and labor cost of a 10kW inverter by 56%, and reduce the size and weight of the inverter. For more information, please visit this link http://www.nrel.gov/ncpvprn/pdfs/33586076.pdf Download the file.
Texas Instruments’ tms320f28x digital signal controller is a very good example. Its performance is as high as 150 MIPs. It can use a DSP to control multiple conversion stages in the inverter, and it also has abundant processing capacity, which can be used to perform MPP tracking algorithm, battery charging monitoring, surge protection, data recording and communication and other additional functions. Figure 3 shows the block diagram of tms320f28x controlling multiple conversion stages.
Figure 3tms320f28x can control multiple conversion stages
The controller has a very fast 12 bit 16 channel A / D converter, which can detect the voltage and current accurately to achieve sine wave. For safety monitoring, the A / D converter also provides current detection function.
In addition, 12 independently controlled enhanced PWM channels on the chip have variable duty cycles, which provide high-speed switching for converter bridge and battery charging circuit.
Each enhanced PWM has its own timer and phase register, which can program the phase delay. All enhanced PWM can be synchronized to drive multiple stages at the same frequency. Multiple timers provide the required clock and fast interrupt management to support additional control tasks. Many standard communication ports including can bus provide easy-to-use interfaces for other components and systems.
Editor in charge: GT