LCL resonant current depression control strategy against pulse width modulated harmonic voltage under low switching freq
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ORIGINAL ARTICLE
LCL resonant current depression control strategy against pulse width modulated harmonic voltage under low switching frequency Zhijun Ma1 · Linyuan Zhou1 · Jinjun Liu1 · Sizhan Zhou1 Received: 19 August 2020 / Revised: 17 November 2020 / Accepted: 20 November 2020 © The Korean Institute of Power Electronics 2020
Abstract The LCL filter is widely used in grid connected converters for good harmonic mitigation performance. With pulse width modulation strategy, converter terminal voltage contains various sideband harmonic components together with expected components corresponding to the modulation wave, especially under a low switching frequency. Considerable resonant current can be generated if the undesired sideband harmonic voltage is located around the LCL resonant frequency. With the background of a medium voltage three-level wind power converter, where the switching frequency is as low as 1.5 kHz, a resonant current depression strategy is proposed against undesired sideband harmonic voltage under low switching frequency. Additional compensators are adopted in the converter current loop. Depending on the system mathematical model, two solutions, a Butterworth low pass filter and a notch filter, based on phase compensation and amplitude compensation, are proposed in this paper. It is shown that the Butterworth low pass filter exhibits better resonant current depression performance, while the notch filter performance is better in terms of system stability. Simulation and experimental results validated the proposed control method. Keywords Active damping · Resonant current depression · Anti-aliasing filter · Sideband harmonic voltage
1 Introduction When the grid penetration level and total installed capacity of wind power steadily increase, the size of the individual wind turbine dramatically increases to obtain a reduced price per generated kilowatt-hour. The medium voltage three-level wind power converter is a good choice for the next generation of 10 MW + wind turbines. An LCL-type filter is usually adopted to attenuate switching harmonics duo to its excellent harmonic mitigation capability. Despite its prominent advantages, the LCL filter also introduces resonance in systems, which affects stability. Passive damping that uses resistors can achieve currentloop stability at the cost of extra power loss. Various passive damped filter topologies were proposed in [1]. When compared with passive damping solutions, active damping (AD) has higher efficiency and greater flexibility [2–32]. Among the various active damping solutions, * Linyuan Zhou [email protected] 1
School of Electrical Engineering, Xi’an Jiaotong University, Xi’an, China
capacitor-current-feedback active damping has been widely used due to its simple implementation [2–9], and it is chosen in this paper. Without considering the delay effect, it has been shown that proportional feedback of capacitor current is equivalent to a virtual resistor connected in parallel with the filter capacitor [2]. However, in digitally controlled
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