Feasibilities for power quality problem mitigation in microgrid

Different electric loads can be problematic for power quality in microgrids but also microgrid can offer more options for power quality management. Type of problems can be divided to either voltage level problems or harmonic problems. Devices emitting harmonics can be usually filtered to comply with power quality norms but more active approach might have to be taken if the load just draws too much current. This research report investigates how different load types affect the power quality in microgrid. Transient electric simulation software, PSCAD and microgrid simulation model was used for the research. Load types in the microgrid simulation were compact fluorescent lamp CFL, single phase rectifier load, three phase thyristor load, and single phase motor load. Also 20 kW static three phase load was used for base case. A good filtering solution was found for CFL using LC-filter configuration and values for components were found using simplex optimization algorithm. There was a small resonance with CFL and inverter without filter in CFL. Three phase thyristor load was more difficult because it had high RMS value of current and high harmonic distortion. Selected filtering solution was to use LCL filter and cut off switch. Simplex algorithm did not work in this case for finding inductor and capacitor values. Values were set so that adequate current filtering and reactive power compensation were achieved. Problem for the single phase motor load was that it draws high current at start and also that there is very limited capacity of power per phase in inverter driven microgrid. Solution for to problem was a device which selects a phase for the single phase load so that single phase loads are spread evenly among all three phases. The device was named a phase jumper. Also virtual impedance method was used to compensate voltage sag from isolation transformer; raising the voltage reference of the feeding inverter respect to current.