Study On Three Phase On-Line UPS And Its Parallel Operation

As an important power electronics device, Uninterruptible Power Supply (UPS) systemis widely used for supplying critical loads which can not afford utility power failure. In orderto improve the performance and practicability of the UPS, this paper focuses on thethree-phase on line UPS, including control of single device and parallel operation techniques. Firstly, the structure of the system is studied, laying emphasis on the rectifier andinverter topology. To improve the power factor of SCR rectifier, some passive Power FactorCorrection (PFC) measures are used. System simulation is taken based on MATLAB to provethe correction effect. Inverter is the core of the UPS. A mathematical model of PWM inverteris established after theoretical analysis. With the load current treated as measurabledisturbance input, the model is linear which has two inputs and a single output. This model isvery simple and easy to deal with. In order to meet the practical requirement, the controller ofinverter is designed with a digital controlled average voltage outer loop and an analogcontrolled instantaneous voltage inner loop. The parameters of inner loop are designed bysimulation software. To improve the capacity and reliability of UPS, it's a good choice to applying paralleloperation techniques. Models of parallel system are established to analyze the principle of theparallel operation. The existence of circumfluence in the paralleling operation system makesthe system work abnormally. Analyzing the characters of circumfluence, a control strategy ispresented which using active info to regulate the amplitude of voltage to achieve active powerequilibrium. At the same time, a Digital Phase Locked Loop (DPLL) is adopted to achieve thefrequency and phase synchronization of UPS output voltage. Re-modulation technique isemployed to improve the precision of PLL and restrain the reactive circumfluence.The 50KVA on line UPS prototypes are organized based on the principle described in this paper. Theexperimental results of output voltage, PLL,parallel of two UPS and three UPS are given and analyzed atthe end of this paper. The results prove that the UPS has good static and transient performance, and canparallel with high reliability.