This paper presents an improved parallel processing uninterruptible power supply (UPS) for a strong grid using a bi-directional voltage-controlled voltage source inverter (VCVSI). To maintain the load voltage at the desired value and to control the active power flow between the VCVSI and grid, the amplitude and phase angle of the inverter output voltage (power angle) must be controlled. Selecting the power angle operating range is an important factor which has a direct effect on various parameters, such as the size of a decoupling inductor, grid power factor and the power ratings of VCVSI components. It is shown how the optimum power angles can be chosen and by restricting the operation of this, the power factor can be maintained above 0.9 at different loads and operating conditions. The paper examines the steady state modeling and analysis of a single phase parallel processing UPS while maintaining a high system power factor under different conditions. Experimental and simulation results of a prototyped 1KVA VCVSI confirms the validity of the proposed method.