Mitigating In-rush Currents for Induction Motor Loads

Power loads can have high starting. These can be a serious source of concern in weak grids. This problem is envisaged to be exacerbated by the rollout of smart microgrids. When a high power induction motor is turned on, its inrush current drawn can be more than ten times the full-load current. This transient current can cause problems in weak grid connections. The increased current is due to both power required to start the load and the increased reactive power demand during the starting process. To protect the grid connection as well as the load, energy storage units can be used to compensate for the increased power requirement. A more pragmatic approach is to reduce the reactive power requirement using tuned compensation capacitors in order to reduce the inrush current. The aim of this work is to address the selection, calculation and switching of the capacitor bank for reactive power compensation. This first requires the study of the reactive power drawn by an induction motor during starting. The capacitances are calculated and switched on to compensate the starting transient and disconnected when the machine has run up to speed using a point-on switching approach that reduces the switching transient.