Design and Implementation of a Power Converter to Process Renewable Energy for Step-down Voltage Applications

In this study a power converter to process renewable energy is proposed, which can not only process solar energy but deal with wind power. The proposed converter is derived from two series modified forwards to step down voltage for charger system or dc distribution application, so as called Modified-Forward Dual-Input Converter (MFDIC). The MFDIC mainly contains an upper Modified Forward (MF), a lower MF, a common output inductor and a DSP-based system controller. The upper and lower MFs can operate individually or simultaneously to accommodate the variation of atmospheric conditions. Since the MFDIC can process renewable power with interleaved operation, the ripple of output current is suppressed significantly and thus better performance is achieved. In the MFDIC only a common output inductor is needed, instead of two separated inductors, so that the volume of the converter is reduced significantly. To draw maximum power from PV panel and wind turbine, perturb-and-observe method is adopted to achieve the feature of Maximum Power Point Tracking (MPPT). The MFDIC is constructed, designed, analyzed, simulated and tested. Simulations and practical measurements have demonstrated the validity and the feasibility of the proposed dual-input converter.


INTRODUCTION
With rapid development of industry and commerce, requirement for electricity has been growing dramatically.Conventionally, electric power is mainly generated from fossil fuels.However, this kind of energy resources is highly limited and will exhaust in the near future.Therefore, adopting renewable and clean energy resources to replace fossil fuels for electric power generation is a research of great urgency.Among renewable resources, solar energy and wind power attract a great deal of interest owing to their easy acquirement.
In Photovoltaic (PV) or wind power generation system, a power converter is needed to process renewable energy.In literature (Wai and Wang, 2008;Gules et al., 2008;Vazquez et al., 2008;Lohmeier et al., 2011;Shen and Chau, 2012) PV converters are presented while wind power converters are discussed in Yazdani and Iravani (2006) and Ullah and Thiringer (2007).However, these power converters only handle single kind of renewable energy, that is, which cannot deal with multi-input power.Therefore, some researchers propose multi-input converters for solar/wind hybrid power generation system (Sedaghati and Babaei, 2011;Cacciato et al., 2008;Fang and Ma, 2010).Even though these multi-input converters can process hybrid renewable energy, two separated converters in series or in parallel are required.A series double-boost converter is presented to process PV power and wind energy simultaneously, in which, as compared with single-boost configuration, power component only imposes one-half of the voltage stress (Solero et al., 1996).Because boost-type converter steps up voltage, it is not suitable for low voltage and galvanic isolated applications.Double-input buck-boost converter is capable of processing high-/low-voltage sources (Chen et al., 2007); however, this type of configuration is non-isolated electrically.Besides, the output voltage has opposite polarity from the input voltage.Instead of combining renewable energy in electricity, the concept of magnetic flux additivity is proposed for the design of multi-input converter but complicated control low and complex structure are required (Chen et al., 2002).
In this study, a Modified-Forward Dual-Input Converter (MFDIC) is proposed, which can deal with PV power and wind energy simultaneously or individually.The converter is composed of two modified series forward converters with a common output filter inductor, which simplifies multi-input structure and lowers cost.Furthermore, the converter can draw maximum power from PV arrays and wind turbine by perturb-and-observe method.The system controller of the converter can be implemented in a DSP chip to lower the total number of discrete devices, which reduces the volume of the converter and promotes its reliability.A prototype of the MFDIC is built for demonstration.Key waveforms measured from the prototype are shown in this study to verify the validity and the feasibility of the proposed converter.

METHODOLOGY
System architecture: A block diagram to illustrate a PV-wind power generation system is shown in Fig. 1, which mainly contains a PV panel, a wind turbine, a multi-input converter and a system controller.In order to process PV power and wind-turbine energy, in this study a modified-forward dual-input converter is derived.(2) In which f s is the switching frequency of the MFDIC, D wind expresses the duty ratio of the upper MF, T denotes switching period and V r,wind2 and V r,pv2 stand for the cut-in voltages of D wind2 and D pv2 , respectively.In mode 3, the magnetizing inductor in the upper modified forward discharges via the path of N wind3 -D wind3 -C wind .
Mode 4 (Fig. 4d, t 3 ≤t<t 4 ): The SW pv is turned on at t 3 and thus PV energy is dealt with by the lower modified forward.The inductor current i L,mic increases linearly.In Fig. 5: Flowchart of the adopted MPPT algorithm Σ Σ (3) in which v pv is the terminal voltage of PV arrays.
Mode 6 (Fig. 4f, t 5 ≤t<t 6 ): At time t 5 , the switch SW pv is turned off and the operation of MFDIC enters into mode 6.The magnetizing inductor L m,pv releases energy to capacitor C pv via N pv1 , N pv3 and D pv3 .Meanwhile, the current i L,mic decreases linearly.A complete switching cycle is terminated at t = t 6 , at which SW wind is turned on again.
To draw maximum power from PV arrays and wind generator, the perturb-and-observe method is adopted to accomplish MPPT feature, of which flowchart is illustrated in Fig. 5.In addition, a simplified control block diagram is also shown in Fig. 6.According to various atmospheric conditions, the maximum-powerpoint trackers determine optimal terminal voltages (v wind,ref and v pv,ref ) for PV arrays and wind turbine so as to draw maximum power from the two renewable generators.Then, the commands are compared with practical voltages.An error is fed to the K-factor controllers for the determination of control signals.The total amount of the converter output power P o is calculated by: where I L,mic stands for the average current of the output inductor.

SIMULATED AND EXPERIMENTAL RESULTS
An MFDIC system is constructed, designed, analyzed, simulated and measured to verify its feasibility and validity.Some important parameters are listed as follows: • During the interval of PV arrays and wind turbine providing power to load simultaneously, Fig. 7 is the measured control signals and Fig. 8 shows the corresponding output inductor current.It can be seen that the MFDIC drives SW pv and SW wind in interleaving mode.The waveform of v po is also shown in Fig. 9, which affects output inductor current and output power directly.While 100 W PV power supplies the MFDIC and then 250 W wind energy cuts in the generation system, the output power variation is shown

CONCLUSION
This study proposed a modified-forward multiinput converter to deal with renewable energy.The converter is derived from two series forward converters, which is able to process PV power and wind energy simultaneously or individually under various atmospheric conditions.Instead of two separated converters, the proposed MFDIC has lower volume and is cost-effective.In this study, the MFDIC is designed, analyzed and measured.Key waveforms have demonstrated the feasibility of the proposed converter.

Fig. 1 :
Fig. 1: A block diagram to illustrate a PV-wind power generation system