Comparison of SV modulation methods for single phase on-line three-leg UPS

This paper presents a modulation method for single phase online three-leg uninterruptible power supplies. The proposed modulation method brings the flexibility of the well-known space vector modulation for the single-phase UPS considered and it can be easily implemented in digital controllers. Three switching sequences are presented and evaluated in terms of total harmonic distortion and the location of the harmonics in the frequency spectrum of the input current and output voltage. It is found out that first switching sequence would require simpler soft switching auxiliary circuit since one of the converter´s legs is commutated at low frequency. The second switching sequence is the best suited for hard switched implementation since it has the smallest switching losses, and third switching sequence has moderate switching losses while producing harmonics in well defined frequency for all operating conditions. Experiments of a DSP controlled 1 kW UPS are presented to validate the proposed method