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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 interharmonic 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 interharmonic characteristics. This paper describes real case studies and compares three load-commutated-inverter VFD arrangements. Their advantages and limitations are analyzed; this highlights features that are different for each design for both the ldquonormalrdquo harmonics as well as the interharmonics. It is shown that, in islanded operation, the GTGs cannot provide any electrical damping to the torsional vibration which implies that strict interharmonic 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.
Date of Publication: Nov.-dec. 2008