Over the last few decades, the production of liquefied natural gas (LNG) has been pushing the development of electric drives with increasingly high power ratings, up to several tens of megawatts. A consolidated technology in this field entails dual-star two-pole synchronous motors fed by load-commutated inverters with supply frequencies between 50 and 80 Hz. This paper presents a novel drive concept for very high-power and high-performance LNG applications based on a 45-MW four-pole 100-Hz quadruple-star synchronous motor supplied by four pulsewidth modulation multilevel voltage source inverters. The genesis, development, and industrial implementation of this new design concept are outlined. Full-load drive system testing and commissioning results are presented which successfully validate the proposed solution.

Design and Testing of a 45-MW 100-Hz Quadruple-Star Synchronous Motor for a Liquefied Natural Gas Turbo-Compressor Drive

Mechanical damage of rotating shafts has been reported for several years from various high-power applications. This paper shows that the variable frequency drive incorporated in a rotating shaft is one of the main root causes of mechanical-shaft failures. Simple analytical relationships show that the frequencies of the motor air-gap torque have a more significant impact on the mechanical-shaft failure than their magnitudes. Effects of mechanical damping are analytically derived and analyzed. Motor air-gap torque is successfully reconstructed using only the motor's voltage and current, thus avoiding torque sensors, which are subject to failure and errors. Simple relationships between frequencies of current harmonics and frequencies of motor pulsating torques are proposed. For pulsewidth-modulated inverters (two and multilevel), possible drive operating points that might excite the shaft's eigenmodes are predicted. Simulation results of four interleaved three-level neutral-point-clamped converters are analyzed for validation purposes. Experimental tests up to 35 MW are performed on a compressor test bed. The presented results confirm the accuracy of the proposed approach, which is particularly valuable for multimegawatt drive applications.

Prediction of Mechanical Shaft Failures Due to Pulsating Torques of Variable-Frequency Drives

The paper describes experimental results of a medium-voltage, modular, 35 MW drive for oil & gas applications realized by interconnecting voltage-source three-level converters equally sharing the motor current. Drives rated for tens of Megawatts are increasingly needed as torque helpers for gas turbines in the oil & gas industry and one envisions that they will replace them completely in the future. These drives must be very reliable, often capable of reversible power flow and high power quality when connected to weak grids in remote areas. The pronounced torsional eigenmodes of large compressors and gas turbines demand for very low torque harmonic content.

Modular High-Power Shunt-Interleaved Drive System: A Realization up to 35 MW for Oil & Gas Applications

This paper describes the experimental results of a medium-voltage modular 35-MW drive for oil and gas applications realized by interconnecting voltage-source three-level converters equally sharing the motor current. Drives rated for tens of megawatts are increasingly needed as torque helpers for gas turbines in the oil and gas industry, and one envisions that they will replace them completely in the future. These drives must be very reliable, often capable of reversible power flow and high power quality when connected to weak grids in remote areas. The pronounced torsional eigenmodes of large compressors and gas turbines demand for very low torque harmonic content.

Modular High-Power Shunt-Interleaved Drive System: A Realization up to 35 MW for Oil and Gas Applications

Shunt-interleaved electrical drive systems consisting of several parallel medium-voltage back-to-back converters enable power ratings of tens of MVA, low current distortions, and a very smooth air-gap torque. To meet stringent reliability and availability goals despite the large parts count, the modularity of the drive system needs to be exploited and a suitable fault-handling strategy that allows the exclusion and isolation of faulted threads is required. This avoids the shutdown of the complete system and enables the drive system to continue operation. If full power capability is also required in degraded mode operation, redundancy on a thread level needs to be added. Experimental results confirm that thread exclusion allows the isolation of the majority of faults without affecting the mechanical load. As the drive system continues to run, faulted threads can be repaired and then added on-the-fly to the running system by thread inclusion. As a result, the downtime of such a modular drive system is expected to not exceed a few hours per year.

Reliability Considerations and Fault-Handling Strategies for Multi-MW Modular Drive Systems

This paper presents the development process for a new variable frequency drive (VFD) system for large size compressor trains for natural gas and liquefied natural gas (LNG) applications. The paper covers the multiple steps of the product development process, from identification of applications requirements, definition of design 61 ELECTRIC SYSTEMS FOR HIGH POWER COMPRESSOR TRAINS IN OILAND GASAPPLICATIONS— SYSTEM DESIGN,VALIDATIONAPPROACH,AND PERFORMANCE

Tommaso Toma

Papers

Electric Systems For High Power Compressor Trains In Oil And Gas Applications - System Design, Validation Approach, And Performances.