Specific order harmonics compensation algorithm and digital implementation for multi-level active power filter

A specific harmonic compensation control based on the nth specific order d–q transformation in the nth synchronous coordinates is proposed in this study. The distorted load currents are transformed into the nth– dq components in the separate nth harmonic frequency channels. These components corresponding to the specific nth harmonics are individually controlled in closed-loop by the decoupling regulators, which can track the amplitude and phase angle of the specific order harmonic with zero steady-state error. Meanwhile the current references would be transformed to voltage references. Therefore, the voltage references generated by these separate harmonic control channels could be accumulated together to form the final real-time modulation voltage references to drive the voltage-source inverter including two-level and multi-level structures. The control algorithm is analysed and its parameters are designed. The three-level active power filter simulation and a small-scale prototype are established. The simulation and experimental results indicate that the harmonics can be specific eliminated and controlled. The total harmonic distortion of source currents could be reduced from 20 to 0.5% with the specific filtering of 5th, 7th, …, 31st harmonics.