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Hybrid solutions for improving passive filter performance in high power applications

This paper presents a new control scheme for a parallel hybrid active filter system intended for harmonic compensation of large nonlinear loads upto 20 MVA to meet IEEE 519 recommended harmonic standards. The control scheme is based on the concept of synthesizing a dynamically variable inductance and is used for an active filtering application. A synchronous reference frame based controller implements the dynamically varying, negative or positive inductance, by generating active filter inverter voltage commands. This controller based parallel hybrid active filter system can selectively synthesize multiple active inductances at dominant harmonic frequencies without affecting passive filter impedances at all other frequencies. The controller can be used to provide 'current limiting' function to prevent passive filter overloading under ambient harmonic loads and/or supply voltage distortions. Three implementation variations of parallel hybrid active filter system are presented. This paper also proposes the use of power factor correction capacitors as passive filters for parallel hybrid active filter system, controlled to provide multiple tuned harmonic sinks and to increase cost-effectiveness for high power applications. Simulation results with both PWM and square-wave inverters validate the controller operation for mis-tuned passive filters, single and multiple frequency tuning, to achieve harmonic compensation of a 325 kVA harmonic load under supply voltage harmonics and ambient harmonic loads.