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The Review of Grid-connected Photovoltaic Power System and its nonlinear behavior

The Review of Grid-connected Photovoltaic Power System and its nonlinear behavior
The Review of Grid-connected Photovoltaic Power System and its nonlinear behavior

The Review of Grid-connected Photovoltaic Power System and

Its Nonlinear Behavior

Youjie Ma 1, a, Yingxue Cai 1,b and Xuesong Zhou 1,c 1Tianjin University of Technology, Tianjin Key Laboratory for Control Theory & Applications in Complicated Systems Nankai District, Tianjin 300384,China

a,c sjteam@https://www.doczj.com/doc/44296884.html,, b yingxue_tsai@https://www.doczj.com/doc/44296884.html,

Keywords:Grid-connected Photovoltaic Power System, Nonlinear behavior, DC/DC converter, DC/AC inverter, Bifurcation phenomenon.

Abstract. This article introduces the classification and composition of Grid-connected Photovoltaic Power System, and the research status of its nonlinear behavior. In the Photovoltaic Power System, photovoltaic array, DC/DC converter, DC/AC inverter and nonlinear load will appear lots of nonlinear dynamic behavior such as bifurcation and chaos phenomena. The paper elaborates the DC/DC converter and DC/AC inverter in the Photovoltaic Power System, and analyses their nonlinear phenomena.

Introduction

With the traditional energy resources exhausted continually, the serious problems of environmental pollution causing global warming restrict the sustainable development of the world. The growing demand for energy of all over the world makes the energy resource has become one of the most important strategic materials of the world. Therefore, the development of renewable energy is the only way for the development of global energy future. Hydropower and wind power in China have become the commercial power generation, but their resources are limited, they may not meet the needs of the future even if exploiting all the sources. Thus, Grid-connected Photovoltaic Power System is the most potential source of electricity.

Grid-connected Photovoltaic Power System, an inexhaustible and unlimited supply of energy, can directly transfer the sun lights into electrical energy. Grid-connected Photovoltaic Power System is an important solution to problems such as energy crisis and environment protection. Technologies related Grid-connected Photovoltaic Power System have been widely researched through out the world and great advances have been achieved[1]. The study of Grid-connected Photovoltaic Power System in China has been started, though not long ago. China has abundant solar energy resources. Therefore, there will be a great development for Grid-connected Photovoltaic Power System.

Grid-connected Photovoltaic Power System could thus make a considerable contribution to solving the energy problem that our society faces in the next generation.

The composition of Grid-connected Photovoltaic Power System

The categorization of Grid-connected Photovoltaic Power System is multitudinous, according to the distribution characteristics of Grid-connected Photovoltaic Power System, it can be divided into: distributed and centralized. According to the topological structure of grid-connected photovoltaic inverter, it can be divided into single Grid-connected Photovoltaic Power System and three-phase Grid-connected Photovoltaic Power System. While according to the current Grid-connected Photovoltaic Power System technology, in order to facilitate control and analysis, the researchers tend to divided the Grid-connected Photovoltaic Power System according to its structure:single stage Grid-connected Photovoltaic Power System and multiple-step type Grid-connected Photovoltaic Power System [1].

Grid-connected Photovoltaic Power System can be divided into three parts: PV array, DC-DC converter, DC-AC inverter and load.

Figure.1 is a schematic diagram of Grid-connected Photovoltaic Power System. The schematic diagram shows that, in the system, power electronic devices, such as DC-DC converter and DC-AC inverter, are the hinge of the generate electricity[2]. Therefore, the senior power electronic technology play a pivotal role in power transformation, power quality assurance, control and optimization

management.

Fig.1 The Grid-connected Photovoltaic Power Generation System Schematic Diagram

The nonlinear behaviors of Grid-connected Photovoltaic Power System mostly come from power electronics converter. The nonlinear behavior of Grid-connected Photovoltaic Power System comes from the sources: photovoltaic array, DC-DC converter, DC-AC inverter and nonlinear load part, etc, among which the biggest impact on the stability of the system is the DC-AC inverter. Actually DC-AC inverter is a nonlinear system, which may appear such as bifurcation phenomenon of nonlinear dynamic behavior. It is the core of the Grid-connected Photovoltaic Power System, its operation condition decide the whole system work.

The nonlinear behavior in Photovoltaic Power System

Due to the power electronics device, the system exist some nonlinear phenomena. Although the research of circuit theory has 100 years history, in the 1960s, the researches are still focus on the linear circuit theory. With highly wide application of the nonlinear electronic device, there appear a large number of nonlinear phenomena in the electronic circuit[3]. The existing linear circuit theory can neither explain the complex nonlinear behavior in the circuit, nor guide the analysis and comprehensive of the nonlinear circuits. But until the 1980s, the phenomena appeared to get the attention of scientific researchers. In recent decade, the nonlinear behavior of the power electronics has became one of the important research branches through the researchers’ efforts.

In 1984, Chua's Circuit raised by L. O. Chua and latter circuit experiments, computer simulation, theoretical analysis and other research tools are used to do the research of nonlinear behavior, setting a model for researching the bifurcation and chaos phenomena in the nonlinear circuit[3].

Applying the nonlinear dynamics theory to the power system will greatly promote the research of the stability of power system. Through numerical simulation and experimental methods, there are a lot of bifurcation and chaos phenomena in power electronic circuits. For example, in DC-DC converter, DC-AC inverter, there are quasi-periodic bifurcation, period doubling bifurcation, the border collision bifurcation phenomena, and chaos nonlinear behavior[4][5].

The nonlinear circuit theory provides a rationale for the application of the nonlinear electronics device. The chaos phenomenon has a huge application prospect in a large amount of fields. Avoiding, controlling, strengthening and using the chaos phenomenon purposefully have become a key research at present.

DC-DC convert circuit

The DC/DC transform circuit is widely used in Photovoltaic power generation system. Normally the photovoltaic cells can be regarded as a current mode power supply, whose voltage is generally unstable, because of the influence of the spectral characteristics, light intensity and temperature changes. Thus, it requires the DC/DC converter before the DC/AC inverter circuit to provide stability dc voltage.

Figure.2 is, Buck, Boost, Buck-Boost DC/DC circuit diagram. The current Grid-connected Photovoltaic Power System generally use these three converters. Buck converter is a step down transformer, generally it is used in the Maximum Power Point Tracking (MPPT) of the off-grid PV system rather than the Grid-connected Photovoltaic Power System [6]. Boost converter is a step-up

transformer, and can be used in the MPPT of the Grid-connected Photovoltaic Power System.

DC-DC converter and DC-AC inverter are switched regularly in the control system. Therefore, Grid-connected Photovoltaic Power System is a strong nonlinear circuit with particular nonlinear phenomena of power electronics converter. The results show that bifurcation phenomena are occurred as the parameters of the control system are changed. It will make the operation of the system properties become worse, even cause the system to lose stability[10]. So, bifurcation phenomena should be fully considered in the design process to prevent bifurcation behavior. And making it has enough stability margins.

Conclusion

The Grid-connected Photovoltaic Power System is a multilevel complex nonlinear system. The nonlinear behavior of each subsystem influence and restrain each other, making the analysis and control of the whole system more difficult. The bifurcation theory provides a new perspective for the further research of voltage instability and establishing a complete system of voltage stability study. References

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