Power Smoothing of a Variable-speed Wind Turbine Generator
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ISSN:1598-6446 eISSN:2005-4092 http://www.springer.com/12555
Power Smoothing of a Variable-speed Wind Turbine Generator Dejian Yang, Yong Cheol Kang*, Jung-Wook Park, Young Il Lee, and Seung-Ho Song Abstract: This paper presents a power-smoothing scheme of a variable-speed wind turbine generator (VSWTG) that employs separate control gains for the over-frequency section (OFS) and under-frequency section (UFS). In the proposed scheme, an additional proportional control loop based on the system frequency deviation operating in conjunction with maximum power point tracking operation is used. In the OFS, to improve the energy-storing capability, the scheme suggests the gain of the frequency deviation control loop, which is set to be monotonously decreasing with the rotor speed while being significantly larger than that in the UFS. In the UFS, to improve the energy-releasing capability while preventing over-deceleration, the gain of the frequency deviation control loop is set to be a linear function of the rotor speed. The simulation results under continuously varying wind speeds with different wind patterns and wind speeds clearly demonstrate that the proposed scheme significantly mitigates the output power fluctuations of a VSWTG. The proposed scheme keeps the frequency within a narrow range, thereby reducing the required primary frequency control reserve for regulating the frequency under normal operations. Keywords: Control gain, energy-releasing capability, energy-storing capability, over-frequency section, power smoothing, under-frequency section, variable-speed wind turbine generator.
1. INTRODUCTION In an electric power grid that has high penetration levels of wind power generation, the severe variation of its output power caused by continuously varying wind speeds results in a crucial challenge for maintaining the system frequency within a narrow range under normal operations [1, 2]. This is because a variable-speed wind turbine generators (VSWTG) such as a doubly-fed induction generators (DFIG) and a fully-rated converter-based VSWTG perform maximum power point tracking (MPPT) operation to extract the maximum energy from the wind; this causes VSWTGs not to respond to the system frequency variation caused by continuously varying wind speeds [3]. To keep the system frequency within a narrow range, more and faster regulating reserve are required, thereby causing extra operating costs. To avoid this, it is essential to mitigate the VSWTG output power fluctuations caused by continuously varying wind speeds [4]. The requirements on the ramp rates of the output power of a wind power plant (WPP) have been specified in some countries [5, 6]. To mitigate the output power fluctuations from a
VSWTG caused by varying wind speeds, when the system frequency lies in the over-frequency section (OFS)– which means the system frequency exceeds the nominal frequency−the output power generated from a VSWTG should be decreased by storing energy into the rotating masses; in contrast, the VSWTG output power in the under-frequency se
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