Line Interactive UPS

Line interactive Uninterruptable Power Supply (UPS) systems are good candidates for providing energy storage within a microgrid to help improve its reliability, economy and efficiency. In grid-connected mode, power can be imported from the grid by the UPS to charge its battery; power can also be exported when required, e.g., when the tariffs are advantageous. In stand-alone mode, the UPS supplies local distributed loads in parallel with other sources. A complete line interactive UPS and its control system are presented and discussed in this chapter. Power flow is controlled using the frequency and voltage drooping technique to ensure seamless transfer between grid-connected and stand-alone parallel modes of operation. The traditional way of measuring active and reactive power, which is based on using a low pass filter, is replaced by a real time integration method which is shown to improve the speed of response of the controller. A virtual impedance is utilized to improve the output current THD in grid-connected mode. When power is imported from the grid, an outer voltage controller that regulates the DC-link voltage sets the active power demand for an inner power flow controller. This facilitates smooth transition between battery charging and discharging modes. This controller is designed with the aid of small signal analysis, which is used to model the dynamics of the inner power flow controller. The drooping coefficients are chosen to limit the energy imported by the USP when re-connecting to the grid and to give good transient response. Experimental results of a microgrid consisting of two 60kW line interactive UPS systems are provided to validate the design.