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Publication Title | Power Flow Control of Renewable Energy Based Distributed Generators Using Advanced Power Converter Technologies

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Abstract—This paper describes a design concept in which a digital controlled power interface embedded with a battery energy storage system (BESS) named battery interfaced static synchronous generator (BISSG) is constructed to improve the power quality of micro-grid (MG) systems. Some distributed generation (DG) systems, e.g., the wind turbine generator (WTG) and the Photovoltaic (PV) systems conventionally generate real power based on natural conditions thus their output power are fluctuating from time to time. To eliminate this shortcoming, the proposed BISSG attempts to smooth the output power of DG with fast charging and discharging its BESS. To achieve a cost-effective design, the proposed BISSG is designed to maximize its control capability in terms of bilateral real power regulation and reactive power compensation for MG voltage support or power factor correction. It is important to note that the proposed BISSG is able to fulfill various real power dispatching functionalities required by the system operator. In this paper, the mathematical model of BISSG and its related controllers are firstly addressed. Then, simulation studies and hardware tests on a simplified MG network are carried out. Typical results are presented with brief discussions to demonstrate the feasibility and effectiveness of the proposed control scheme.

Index Terms—Renewable energy, wind turbine generator, battery interfaced static synchronous generator, distributed generation, micro-grid.

I. INTRODUCTION

With the rapid development of new energy technologies, more and more renewable energy sources are being integrated into power distribution systems in the form of distributed generations (DG). In recent years, power converter interfaced DG systems, renewable energy sources (RES) based micro-grids (MG) and state-of-the-art communication and control technologies have been recognized to play important roles in the development of future energy policies. Short-term goals of these energy policies include reduction in high-polluting power generations and global greenhouse gas emissions, improved diversity and security of energy supply, and exploitation of possible incentives for creating local value added opportunities for the related industrial sectors. Based on the related technical reports in the open literature, potential

Manuscript received September 25, 2013; revised January 16, 2014. This work was supported in part by the National Science Council of Taiwan, R.O.C. under Grant NSC102-2221-E-239-018.

Chao-Tsung Ma and Tzung-Han Shr are with the Electrical Engineering Department, National United University, Miao-Li City 36003, Taiwan,

RES based power generations may include wind turbine generator, photovoltaic and fuel cell systems [1]-[3]. Of these power generating methods, the interests in photovoltaic (PV) and wind turbine generator (WTG) are growing worldwide. The annual growth rate is counted at 25-35% for PV and about 20-23% for WTG over last five years. It is recorded roughly 25 GW of total commercial PV installations and about 270 GW for WTG at the beginning of 2013 and the increasing trend of using PV and WTG systems still continues [4]. In fact, a number of different PV and WTG incentive programs have been introduced in Taiwan since 2008 and most of the developed countries are currently promoting residential and commercial uses of PV and WTG generation systems [5]-[7]. Based on the standards such as IEEE1547, IEEE 929 and UL1741, DG inverter systems should operate at unity power factor [8]; however, this regulation has some limitations in practical applications, especially when some system-level control functions are to be performed. On the other hand, considering the installation quantity of PV and WTG has been continuously increased in recent years and the WTG and the PV systems generate real power based on nature conditions thus their output powers are not stable; this leads to a problem that fast output power fluctuation of the DG with high capacity can cause network frequency and voltage variations, especially in isolated power systems, e.g., a MG operated in autonomous mode and thus impairing the power quality of the local power systems [9]. This problem becomes even more crucial in WTG as wind is normally intermittent and stochastic in fast dynamic feature.

In the open literature, there have been a number of methods proposed to smooth the wind farm output power flow. Most of the reported methods are using an energy storage system (BESS) to absorb or provide real power on various control algorithms. Potential technical issues, concerning different aspects related with the management of WTG systems and their integration into the distributed network, have been discussed in [10]-[12]. In those reported methods, the major difficulty in WTG output power smoothing control is the setting of its output power command. Using a constant output power reference based on historical WTG power generation data is easy but not reliable because there can be some cases where wind speed is very low and then the logged power command cannot be achieved. Theoretically, the BESS is able to solve the problem but large energy capacity will be needed. To give flexibility in designing the control scheme, this paper proposes an easy control scheme to generate the reference output power for the MG feeder connected with WTG and thus the energy capacity of BESS unit in the BISSG can be

R.O.C. (tel.: 886-37-381369; fax: 886-37-327887; ctma@nuu.edu.tw).

e-mail:

DOI: 10.7763/JOCET.2015.V3.167

48

Journal of Clean Energy Technologies, Vol. 3, No. 1, January 2015

Power Flow Control of Renewable Energy Based Distributed Generators Using Advanced Power Converter Technologies

Chao-Tsung Ma and Tzung-Han Shr

Image | Power Flow Control of Renewable Energy Based Distributed Generators Using Advanced Power Converter Technologies



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