Effect of Source Impedance on Hybrid Wind and Solar Power System

Mu-Kuen Chen and Chao-Yuan Cheng

Additional information is available at the end of the chapter http://dx. doi. org/10.5772/54495

1. Introduction

Large wind turbines use mechanical systems, such as geared or gearless devices to increase the speed of the generator. In addition, an inverter is employed to adjust the output voltage to exceed the grid value. With its phase leading the bus phase, wind power can be integrated into the grid bus. The integration can be easily realized owing to negligible impedance of the utility bus. The main issues for wind-power generating systems include fluctuations in output voltage and quality of power supplied to the utility power system. In small renewable energy systems, wind power and solar energy are integrated to improve the reliability of the indi­vidual power system. Conventionally, the AC output voltage of the wind turbine is rectified, and then combined with the output voltage of the solar cell to charge the battery and provide power supply to the load. To characterize the battery, the Thevenin battery model considering the nonlinear effect of source impedances was proposed. A battery evaluation test system was employed to validate this model. The curve of test results follows entirely the theoretical calculation [1]-[3]. For photovoltaic application, the inner resistance of solar panel was also included in the theoretical analysis. The maximum power point tracking (MPPT) of solar panel for different insolation levels verified the proposed solar cell model. The MPPT technique adjusted continuously the battery-charging rate and obtained shorter charge time [4]. It was reported that a dual battery configuration with deep-cycle batteries can increase the available capacity. Moreover, the system may achieve optimum utilization of the PV array and proper maintenance of the storage battery [5]. In another research, a microcontroller was employed to adjust the maximum charging current according to the PV power production and battery voltage level. Using this method, better exploitation of the power produced by the PV power source can be achieved. Moreover, battery lifetime can be increased by restoring high state of charge (SOC) in short charging time [6]. Another study compared the performance of equal

rate charging, proportional rate and pulse current charging in charging multiple batteries. The total charging time is shortest when using pulse current charging strategy. All the batteries become fully charged almost simultaneously when they are charged with proportional rate or pulse current method [7]. The optimum size of the PV module for a specific wind turbine to meet the load requirement for the hybrid wind/PV system was investigated in order to minimize the overall cost of the system [8] – [9]. Nevertheless, the effect of source impedance in a small hybrid wind/PV system has not yet been investigated. In this study, theoretical analysis shows that it is difficult to obtain both wind power and solar energy at the same time by traditional methods, which is verified by field test. To overcome such problem, a micro­processor-based controller design for detecting instantaneous voltage variations of both energy sources is proposed, and a charge controller is employed to optimize the charging operation.

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