Energy is the source of power for the development of human society. With the rapid development of the world economy, the demand for energy continues to increase. From the perspective of long-term social and economic development, traditional fossil fuels have become increasingly uncompetitive due to the disadvantages of being non-renewable and easily polluting the environment.
In recent years, more and more countries have begun to pay attention to, develop and use clean energy such as photovoltaic power generation, wind power generation and biomass energy. Clean energy is also due to its environmental protection, economic and renewable advantages. It will inevitably become a powerful weapon to deal with environmental pollution and energy crises. As the most abundant renewable energy on earth, solar energy does not require fuel supply in the process of using photovoltaic cells to convert solar energy into electric energy, and there is no problem of pollutant emission, which is environmentally friendly and economical. sex.
Hydrogen energy , as an emerging green energy, has the advantages of abundant reserves, high energy density, non-toxic and pollution-free. The conversion of hydrogen energy into electric energy is usually achieved through fuel cells. Since fuel cell power generation does not go through a heat engine process, it is not limited by the Carnot cycle , and the energy conversion rate is high.
In practical applications, photovoltaic power generation has the characteristics of fast dynamic response but greatly affected by the weather, while fuel cells have the problem of stable but slow dynamic response. The output characteristics of the two have good complementarity, so it can be considered Combining the advantages of the two to establish a joint power generation system to obtain continuous and stable electrical energy, an electric-hydrogen multi-energy complementary microgrid architecture has gradually entered people's field of vision.
The electric-hydrogen multi-energy complementary micro-grid architecture is shown in Figure 1. It includes an electric power system composed of photovoltaic power generation, storage batteries, and hydrogen composed of electrolyzers, hydrogen storage tanks, and fuel cells. Can system.The photovoltaic power generation unit and the storage battery are connected to the DC bus through the DC-DC converter; the DC bus is connected to the AC grid by the DC-AC converter to deliver power to the grid, and also needs to supply power to the electrolyzer to complete the electrolysis of hydrogen. The hydrogen obtained from the electrolysis cell is supplied to the fuel cell for power generation, and the electrical energy generated by the fuel cell is fed back to the DC bus through the DC-DC converter.
Figure 1 Electric-hydrogen multi-energy complementary microgrid architecture
For the photovoltaic power generation part of the architecture, how to reduce power generation costs and improve power generation efficiency is the core issue of its future development. For the photovoltaic industry to reduce costs and increase efficiency, increasing voltage is one of the effective measures. With the vigorous development of my country's photovoltaic industry, the current 1000V bus voltage level can no longer meet the requirements. In 2014, the world's first 1500V power station was jointly established by Firstsolar and GE. Since then, 1500V photovoltaic systems have gradually been used in overseas and domestic front-runner projects.
Compared with the 1000V system, the number of modules in a single string in the 1500V system has increased by 50%, and the number of strings in the sub-PV array has been reduced by 33%, reducing the number of cables, combiner boxes and brackets in the sub-PV array. In addition, the 1500V system supports larger photovoltaic arrays, which can reduce the number of box transformers and inverters, thereby reducing installation and maintenance costs; in addition, after the voltage is increased, AC and DC The cable loss can be effectively reduced, thereby realizing the improvement of power generation efficiency. Therefore, increasing the voltage level will inevitably become the development trend of the future electricity-hydrogen multi-energy complementary microgrid architecture.
However, increasing the voltage of brings benefits to the system but also brings many challenges, such as reduced electrical safety, higher requirements for the withstand voltage and reliability of raw materials, and AC-DC, DC-DC The converter input voltage level is increased, etc.
From the point of view of the DC-DC link at the junction of 1500V and higher voltage levels of the bus and the electrolytic cell that may appear in the future, the DC-DC converter needs to reduce the DC bus voltage to match the electrolytic cell Low voltage, but due to the limited withstand voltage level of power electronic devices, most of the existing single-topology Buck circuits have been unable to meet the system operation requirements after the voltage level is increased. Therefore, the research on DC-DC converters that can withstand high input voltage levels, have step-down characteristics, and have high electrical safety is very important.
- Some scholars proposed to connect the DC-DC converter in series on the input side to increase its voltage level and parallel on the output side to increase the power level. Input-Series Output-Parallel (ISOP) structure , And pointed out that the prerequisite for the structure to work normally is to ensure the voltage balance of each sub-module on the series side.
- Some scholars proposed a DC Solid State Transformer (DC Solid State Transformer, DC SST) for flexible DC power distribution based on the series-parallel combination of Dual Active Bridge (DAB) converters. Its structure is shown in Figure 2, and the corresponding control and management strategies are introduced. However, due to the existence of the centralized capacitor, this structure cannot isolate the faulty module when the sub-module fails, which reduces the safety and reliability of system operation.
- Some scholars have proposed a DC solid-state transformer with fault removal capability by cascading the Boost circuit and DAB circuit as a sub-module of the DC solid-state transformer. When a DAB unit fails, the upper tube of the corresponding Boost circuit Turn off, the down tube is always on, and all the switch tubes of the fault DAB are blocked to remove them from the system, but the number of switch tubes of this topology is large.
Figure 2 DCSST topology based on DAB converter
Based on the above analysis, Yanshan University Researchers from Hebei Province Key Laboratory of Power Electronics Energy Conservation and Transmission Control proposed a dual active bridge (DAB) integrated Boost converter topology, and its working principle and software under the PWM+ phase shift control mode The switch situation is analyzed.
Figure 3 Experimental prototype
Researchers combined multiple converters in series and parallel to obtain a high-voltage DC bus that can be used in the electric-hydrogen multi-energy complementary microgrid and hydrogen production electrolysis The input series-output parallel (ISOP) type DC transformer of the DC-DC link between the tanks. Through the joint control of the output voltage loop, the input voltage stabilization loop and the input voltage equalization loop, the ISOP DC transformer can achieve voltage balance on the series side, voltage stabilization on the parallel side, and the primary and secondary voltage matching of each sub-module transformer. On the basis of theoretical analysis, an experimental prototype with STM32 + FPGA as the core controller was built to verify the correctness of the proposed circuit topology and the effectiveness of the control strategy.
.This article is compiled from the 10th issue of 2021 " Journal of Electrotechnical Engineering", the title of the paper is "A dual active bridge integrated Boost topology and its control for electric-hydrogen multi-energy complementary microgrids" , The authors are Sun Xiaofeng, Zhang Huixin, etc.
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