Improvement of Power Quality in Microgrids
A microgrid is a group of interconnected Distributed Generation (DG) sources, loads and intermediate storage units that cooperate with each other to be collectively treated by the grid. In a microgrid, some common DG sources are renewable generation such as wind turbine and PV energy systems. Due to the unstable and intermittency nature of renewable energy sources, power reliability and quality problems are more acute. The power quality issues in a microgrid are important since all the renewable sources come with power electronic interfaces which generate undesirable voltage and current harmonics.
In order to solve the above issues in microgrids, a new type of Unified Power Quality Conditioner (UPQC) is proposed in Project 2. The conventional UPQC is an integration of series and shunt active power filters and has the capability of improving power quality at both the microgrid side and load side. Fig. 3 shows the circuit configuration for the proposed UPQC. It has two converters connected back-to-back on the dc side via a common DC capacitor similar to the conventional UPQC, but a DG source is connected to the DC capacitor via a converter. The proposed UPQC has two major components: the series and shunt components. The series component is employed to compensate the microgrid side disturbances such as voltage sags/swells, frequency drifts, voltage unbalance, flickers and harmonics. It injects a compensating voltage in series with the microgrid voltage to maintain a balanced and distortion free load voltage. The shunt component is employed to improve the current quality at the load side by injecting command currents into the microgrid system to keep the current in phase with the microgrid voltage for power factor improvement and to ensure that the source currents are balanced during unbalanced operations.
