Study of Torsional Sub-synchronous Interactions of Drive Systems with Hybrid Converters

Electrical drives for large capacity compressors in a liquefied natural gas (LNG) plant have been investigated. There are two topologies for such an electrical drive; load commutated inverters (LCIs) and voltage source inverters (VSIs). Due to commutation of the thyristors, LCIs produce current harmonics with variable frequencies, and their interharmonics are significant. Therefore, torsional interactions should strongly be concerned. On the other hand, the VSI systems have lower harmonics in voltage and current. In addition to its performance, this technology has become mature and reliable. For this reason, VSI technology is getting attractive to large capacity motor drives. There is a configuration in which an electrical drive with a voltage source converter is connected to a compressor driven by a gas turbine drive, and an operation mode that the electrical drive supplies power to the distribution system. The power is usually smaller than the rated capacity of the electrical drive; therefore, the voltage source converter can be designed with a smaller capacity and cooperate with a full-rated diode rectifier, which is called a hybrid converter, for power supply to the electrical drive. This paper shows sub-synchronous tortional interactions characteristics of the VSI with the hybrid-converter for variable speed drive system for LNG plant. This study shows that the SSTI is less concerned with the VSI-VSDS. Therefore, the system is suitable to apply large capacity drive application in LNG plants.