Many critical load applications depend upon UPS system to uphold power in the course of outages as well as during under- and over-voltage grid conditions. Once any disruption occurs at the grid, a UPS system either takes over the load completely or injects the compensation voltage to the load to avoid interruption. For an off-line and line-interactive UPS systems, this transfer of load takes a minimum of 1?5 msec and can be as much as 20 msec, mainly depending upon the time required for the fault detection and the operating mechanisms of the transfer switches. Throughout this time duration, the load transformer, accompanied by the sensitive load is probable to be exposed to the distorted grid voltage. As a result, a flux-offset of the load transformer is established. Thus, as soon as the UPS system takes over the load and reinstates its voltage, the transformer flux might increase above its saturation level and generates substantial inrush current.
In the case of an on-line UPS topology, the load transfer time period is zero. The phenomenon of inrush current for an on-line UPS system is observed when it powers multiple transformer-coupled loads. Under such a condition, the switching-in of a load when other loads are already on-line can cause a large magnitude of inrush current because of the energizing of the load transformer. This inrush current can take several cycles, which reduces the line voltages and triggers over-current protecting devices of the UPS systems.
In this research, an off-line UPS system based on current regulated voltage source inverter (CRVSI) instead of conventional voltage source inverter is proposed. The inverter of the UPS system utilizes a current control algorithm implemented in the stationary frame. This enables an off-line UPS system to eliminate the inrush current during the transition while powering a transformer-coupled load.
To eliminate the inrush current phenomenon related with a line-interactive UPS system and to achieve fast current injection by the inverter during any abnormal grid power condition, a new line-interactive UPS system is proposed in this research that uses a standard current regulated inverter coupled with the secondary of the load transformer. The possibility of inrush current is eliminated with an instantaneous compensation of load current and a seamless transition of a load.
This research also proposes an on-line UPS topology with eliminated inrush current while operating with multiple transformer-coupled loads, achieved through a swift performing current regulated inverter operating with a control scheme consists of two control loops to supply the additional current caused by the switching-in of the loads. The outer voltage control loop controls the load voltage. However, the inner current loop controls the load current to vary it sinusoidally as per the supplementary load demand during the engagement of other load transformers, leaving behind any possibility of inrush current generation.
All of the above mentioned UPS topologies are well-analyzed and implemented through simulation and experimental results. A comparison between the conventional static UPS topologies and the proposed UPS topologies is also provided in this research.