Patent ID: 7057908

Claim:
A method in connection with a network inverter, an LCL network filter being connected between the network inverter and network, and the network inverter being arranged to act as an active converter of harmonic components of electricity, the method comprising a step of forming a current reference (i 1, ref ) for the network inverter for producing a desired voltage, the method comprising: selecting a harmonic electric component, the proportion of which needs to be changed, forming a unit vector ( s v ), which rotates at the speed of the selected harmonic component, attaching a rectangular coordinate system to the unit vector, determining the magnitude of network current (i 2 ), transforming a space vector of the network current into said rotating coordinate system, dividing the space vector of the network current into a rectangular d and q components (i 2d v , i 2q v ), low-pass filtering the network current components to obtain low-pass filtered components (i 2d , f v , i 2q, v ), forming reference values (i 2d , ref v , i 2q,ref v ) for the magnitude of the selected harmonic current, subtracting the low-pass filtered components from the corresponding reference values to provide difference signals (i 2d , e v , i 2q,e v ), applying the difference signals to PI algorithm to obtain reference signals (i 1d , ref v , i 1q,ref v ), converting the component-form current reference signals to a current reference vector i ( i 1, ref v ), summing the formed current reference (i 1, ref v ) altering the harmonic component with the current reference ( i 1, ref ) of the network inverter to produce a summed current reference ( i 1 , ref, tot), and controlling the network inverter with the summed current reference ( i 1 , ref, tot) in a coordinate system linked to the virtual flux linkage of the network inverter by using DTC control.