Smart Grid Connection of an Induction Motor Using a Three-Phase Floating H-bridge System as a Series Compensator

Electrical grid voltage sags are a significant industrial power quality concern. According to a survey result across the US, voltage sags and short-duration power outages are responsible for 92% of power quality problems faced by industrial customers. These power interruptions often impose severe cost penalties in plant shutdowns for many industries. A series compensation scheme for an induction motor is presented with inherent voltage sag ride through capability. The system utilizes a system of three-phase floating capacitor H-bridge converters located in each phase between the utility grid and a squirrel-cage induction motor. By injecting a series voltage in each phase, the proposed system can manipulate the voltage supplied to a motor, increasing its tolerance of grid voltage sags. The voltage injection scheme has an inherently leading grid power factor under steady state and, hence, generates VARs into the grid, over a wide range of load conditions. The paper develops mathematical analysis of the proposed system to quantify both the voltage sag ride-through and the reactive power generation that results. The analysis shows that voltage sag tolerance of the proposed system is closely related to the motor fundamental power factor. Moreover, it is found that unity power factor operation of the system, as seen from the grid, is possible and that the reactive power generation capability can also be accurately quantified. A 5-hp experimental testbed is used to validate both the grid voltage ride-through capability feature and the reactive power generation characteristics.