Torsional Resonance Risk Management in Islanded Industrial Power Systems Supplying Large VFDs

This paper describes a design study for the power system for an industrial plant with islanded gas turbine generators (GTGs) and large variable frequency drives (VFDs). Examples discussed are based on the authors\´ experience of liquefied natural gas plants where VFDs are used as compressor drivers. The shaft of a GTG has mechanical torsional resonant modes which may be excited by the inter-harmonic currents produced by the VFDs. At the power levels considered, up to 90 MW for each compressor, the VFD converters should be arranged to give suitable harmonic and inter- harmonic characteristics. This paper describes real case studies and compares three LCI (load commutated inverter) VFD arrangements. Their advantages and limitations are analyzed; this highlights features that are different for each design for both the "normal" harmonics as well as for the inter-harmonics. It is shown that in islanded operation the GTGs cannot provide any electrical damping to the torsional vibration which implies that strict inter- harmonic limits should be applied in the design. PSCAD-EMTDC time domain simulation results are used to confirm the analysis and justify the considerations that are important in the design exercise.