Overriding individual harmonic current control with fast dynamics for UPS with nonlinear loads

When an UPS is loaded with nonlinear loads, significant voltage distortion in the output voltage might occur which is not within the limits of requirements specified by standards. A new control which was proposed for active-filters is adapted to UPS and it makes sure that the inverter supplies all the load harmonic currents so that the output filter capacitor is not loaded with these harmonics and thereby obtaining a very clean sinusoidal output voltage. In the proposed scheme, the PI current controller in the fundamental synchronous reference frame (FSRF) controls the fundamental DC currents and as well as the harmonic references which appear as AC quantities in this frame with fastest dynamics as possible. This controller has zero steady state error for the fundamental DC references and a finite magnitude and phase error for AC references which increases with the order of harmonics. Over-riding individual harmonic current controllers are implemented in their respective reference frames which correct the respective harmonic reference in FSRF such that the converter current is equal to that of load current at that harmonic. Thus excellent dynamic and steady state performance is achieved. Both closed loop and open loop approaches are presented. A 50 kVA lab prototype is built with both three and four leg inverter topologies and the results are given for the same.