Unlike traditional passive harmonic filters, active harmonic filters reduce the harmonic distortion in the system by cancellation of harmonic current injections.

Active harmonic filters inject harmonic currents in phase opposition to the distorting element. By means of such an active filter (that needs to be installed in close vicinity to the distorting element), the harmonic currents injections are canceled out.

For the cancellation, the active harmonic filter needs to inject a current with equivalent magnitude and opposite phase angle of the distorting element for each harmonic order.

The active filter can be modeled with the current source (ElmIac) as shown in the attached example. The active filters (ElmIac) in this example is tuned to reduce the distortion for 5th and 7th order harmonics. The tuning is done as follows:

Tuning of 5th order harmonic:

• Current angle for fundamental frequency is 30.8° for the source (WTG) and -59.2° for the active filter. The overall phase shift is therefore 90°
• The required phase rotation between WTG and active filter needs to be 180°. Thus, the relative harmonic angle for the 5th harmonic current injection of the filter is phi_h - h * phi_1 = 180° - 5 * 90° = - 270° => 90° 
• The 5th order harmonic current injection of the WTG is 8.57 A, thus the active filter needs to inject 8.57 A as well. The base current (50 Hz) of the active filter is 1 A. Thus, the magnitude in the TypHmccr linked to the ElmIac needs to be 857%

Tuning of 7th order harmonic:

• Current angle for fundamental frequency is 30.8° for the source (WTG) and -59.2° for the active filter. The overall phase shift is therefore 90°
• The required phase rotation between WTG and active filter needs to be 180°. Thus, the relative harmonic angle for the 5th harmonic current injection of the filter is phi_h - h * phi_1 = 180° - 7 * 90° = - 450° => -90° 
• The 5th order harmonic current injection of the WTG is 4.29 A, thus the active filter needs to inject 4.29 A as well. The base current (50 Hz) of the active filter is 1 A. Thus, the magnitude in the TypHmccr linked to the ElmIac needs to be 429%

The attached example contains two study cases:

• 01 Base Case: active harmonic filter is switched off; the voltage is distorted for 5th and 7th order harmonic
• 02 Active Filter: active harmonic filter is switched on; the voltage distortion for 5th and 7th order harmonic is canceled out