Novel control techniques for three-phase three-wire series-parallel compensated line-interactive UPS system

A three-phase three-wire series-parallel compensated line-interactive UPS consisting of two voltage-source converters controlled separately by two digital signal processors is studied in this paper. The series converter can be controlled as non-sinusoidal voltage source as series active power filter to compensate harmonic voltage at the utility, while the parallel converter can be controlled as non-sinusoidal current source as shunt active power filter to compensate reactive and harmonic currents deriving from inductive and non-linear loads. However, controlling the series converter as fundamental sinusoidal current source and the parallel converter as fundamental sinusoidal voltage source is a much better choice for this kind of system to facilitate the transition from stand-by to back-up mode or vice versa. Additionally, state feedback decoupling terms are used in the current regulator of the series converter to reduce the response time, while repetitive controllers are plugged in the voltage regulator of the parallel converter to reduce steady-state errors and distortion caused by unknown periodic disturbances. Both algorithms are based on the synchronous rotating frame and implemented respectively by two digital signal processors. These novel control techniques are proposed firstly for this kind of UPS and are verified by detailed experimental results.