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Kalman filter based grid impedance estimating for harmonic order scheduling method of active power filter with output LCL filter

The control of an active power filter (APF) with LCL-filter is affected by variable inductance present in the power line of the grid. The variation of impedance will impacts the resonance frequency and system bandwidth. Therefore, the exact measurement of the grid impedance is necessary. Real-time estimation of the equivalent grid impedance by means of extended Kalman filter is addressed. The theoretical background of the extended Kalman filter used for equivalent grid impedance estimation is introduced. Using the estimated impedance and other known parameters to quantitative calculate the system cross over frequency and system resonant frequency caused by LCL-filter. Taking these limitations into account, this paper presents a reliable method to determine the maximum order of harmonic current need to be compensated. The overall system stability and harmonic compensation under different power grid conditions are guaranteed through this method. The theoretical analysis is verified on a 65kVA three-phase shunt APF.