Generators operating with variable frequency drives in an offshore facility

Due to wide application of Variable Frequency Drives (VFDs), it becomes common that a dedicated generator supplies power to a VFD in the offshore oil field. In this paper, a case study was conducted and analyzed for a generator instability issue when operating with a VFD. Serious current oscillation happens between the generator output and the drive input at two electrical submersible pump (ESP) systems consisting of a dedicated generator and a VFD in each well. Minor changes of certain parameters and settings of the two ESP systems could trigger current oscillation at the input of the drives, also a current fluctuation at the ESP motors connected to the output of the drive and thus serious vibration of the ESP pumps. The analysis indicates that the low DC link voltage at the drives and the low displacement power factor at the drive input are the root causes of the problem. Effective solutions for this type of instability problem are proposed in this paper.