Studies on the perfomances of parallel-running UPS systems.

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The UPS (uninterruptible power supply) systems keep computers, communication, or medical systems continuously supplied with electric energy in spite of disturbances and interruptions in the primary power supply to the facilities. In order to increase power capacity of the system UPSs are often used in parallel.In this project, the performances of a parallel-running UPS system loaded with a capacitor-filtered rectifier are investigated in detail, and the effects of various parameters of the system on the cross current, which circulates through the UPSs, as well as the active and reactive powers are theoretically clarified based on the state space method. It is shown that the cross current increases with the difference in the magnitudes and phases of the source voltages of the individual UPSs, and this increase in the cross current makes the reactive power of the whole system increase, reducing the power factor. It is also shown that there is a considerable effect of the capacitor in the output filter of the UPSs on the cross current. A new control algorithm with observer and voltage drop compensation for a digitally controlled UPS is then proposed, and it is shown that an excellent voltage waveform is obtained with this new control when compared with the system with conventional PID with additional feed-forward loop. A new algorithm for suppressing the cross current of the system is finally proposed.