Modeling variable frequency drive and motor systems in power systems dynamic studies

Variable frequency drives (VFD) are widely used in industrial facilities, however, dynamic models for VFD-motor systems suitable for power system dynamic studies are not available. In this paper, a generic VFD-motor system modeling technique is proposed for the case that VFDs are able to ride through the fault. To illustrate the proposed technique, a dynamic model for a low voltage 6-pulse voltage source inverter (VSI) based drive and induction motor system with voltage per Hz control is created. To verify the accuracy of the developed dynamic model, a case study is conducted using a sample VFD-motor system, both derived dynamic model and detailed switching model for the same system are simulated using Matlab/Simulink, and their dynamic responses are compared. It is found that there are good agreements between the two models, and thus the accuracy of the developed dynamic model is verified. For the case that VFDs will trip out of lines, a VFD trip characteristic curve is proposed, and a simple screening procedure is recommended for evaluating whether the VFD-motor system shall remain in service or trip out of lines in power systems dynamic studies.