Transient performance of UPS system with synchronous-frame digital controller

This paper proposes a voltage controlled voltage source type nonlinear regulator, based on the state variable approach, for the control of three-phase UPS inverters. The purposes are to improve dynamic response, incorporate the output LC filter as part of the output voltage control system and reduce the number of sensors. The design and analysis are based on the system state-space models, with state variable linearization and decoupling performed through on-line calculation. An output voltage estimator is proposed, using the instantaneous power as the basis for the algorithm, with data sampling executed only at the zero switching states, thereby eliminating the filter. In addition, a full-order load current observer is used, which makes the proposed control system load parameter insensitive, and results in fast transient response. The steady state and transient performance of the UPS system are evaluated. The dynamic response time is around 4 ms. Experimental results are obtained on a 4 kVA DSP controlled prototype UPS.