Reduction of the output impedance of PWM inverters for uninterruptible power supply

Inverters in UPS (uninterruptible power supply) systems feeding nonlinear or step-changing loads must be capable of providing good output-voltage-waveform quality in spite of all load and line disturbances. A switching algorithm applying output filter state feedback in order to control the output-voltage instantaneous value is presented. The algorithm requires a synchronous pulse-width modulator to be placed in the feedback loop. The modulator introduces nonlinearity, and special attention is paid to stability limits of the resulting nonlinear discrete-time system. Results are applicable to any converter controlled by the pulse-width modulator in the closed loop.