Key Issues Research On Paralleled UPS Systems

UPS (uninterruptible power supply) has been widely used in various applications, providing power supply to critical load. Nowadays, paralleled UPS systems become more and more attractive for their merits on reliability, redundancy, expandability, and maintenance.This paper proposes a control strategy of the economical operation mode of DSP controlled online UPS. When the grid is abnormal, UPS switches to inverter working mode. Until the grid is normal again, then UPS back to bypass working mode. The proposed control strategy realizes the reliable working mode change of UPS system. When putting the rectified and motor load into UPS system, the initiation current is very large. In order to ensure the normal operation of UPS in these conditions, a control strategy is proposed.The most important part of this paper is load-sharing control strategy. Through comparing the current-sharing control strategies of instant current control with current sharing bus, output droop control scheme, active and reactive power control scheme, the active and reactive power control scheme is used to achieve the load-sharing.Averaged power control method needs to calculate UPS output active and reactive powers accurately for control of the phase and amplitude of output voltage. In light or no load conditions, UPS output current is very small; the sampling value contains many measured errors, which will affect the active and reactive power calculation accuracy. This paper suggests an improved power calculation method of using the UPS output filter inductor current instead of the output current when calculating the UPS output active and reactive powers. The sampling time of the inductor current is chosen to ensure high accuracy of the power calculation. Theory analysis and experimental results of power calculation is presented.Based on the equivalent circuit of N paralleled UPS inverters, an averaged power control strategy is proposed. Because the pure active and reactive power sharing control can not guarantee the DC current sharing among the paralleled UPS system. Hence, a combination of the power control and the filter DC circulation current control is proposed. In actual UPS systems, there exist error regulations of active, reactive and DC circulation current because of detecting error. To solve these problems, an improved load-sharing control strategy is proposed in this paper. In paralleled UPS system, because the active and reactive power control loops are quite slow, when there are disturbances in the paralleled systems, the device parameters variation, UPS output impedance mismatch have influence on the control precision of the UPS output voltage amplitude and phase. In order to increase the control precision of the output power control loop of paralleled UPS system and further minimize the circulation current, an intermediate droop control loop to program a virtual resistance is added to the output voltage and inductor current double loop controller. The influence of virtual resistance on circulation current caused by none idea situation of UPS inverter is analyzed. Also the influence of virtual resistance on load regulation and source regulation of UPS inverter is analyzed.The small signal model of two paralleled UPS inverters based on averaged power load-sharing control strategy is derived; the influence of active and reactive power controller PI parameters on the transition characteristics of paralleled UPS systems is analyzed.Using above mentioned control strategies, the load-sharing quality and stability of paralleled UPS system can be greatly increased. The related experimental results are presented to verify this control method with digital controlled three-phase four-wire 10-40KVA paralleled UPS systems.