Control strategy for a single phase bidirectional converter based UPS system using FPGA

Uninterruptible power supplies are widely used to supply uninterrupted power to the loads under power failure. Typical UPS systems consists of two power converters namely a rectifier/charger which charges the batteries and an inverter to provide an ac sinusoidal output. Employing a single converter for both rectification and inversion process reduces the hardware components. This paper presents a control strategy for the single phase bidirectional converter used in UPS systems. Under normal mode, the grid supplies the loads and charges the battery through the bidirectional converter. The converter is controlled using a hysteresis current controller and a constant current charging technique is used to charge the batteries. Upon grid failure, the battery supplies the load through the converter which operates in voltage control mode to maintain the load voltage. This system offers the advantage of reduced hardware, low harmonic distortion of line current and regulation of load voltage under grid failure. The control strategy is simulated in MATLAB/Simulink and implemented in an ALTERA cyclone II FPGA board. The steady state and the dynamic response of the system are simulated and verified experimentally on the prototype unit built in the laboratory.