Practical Application of Transient-Free Capacitor Switch (TFCS) Technology for Starting Large Induction Motors at a Texas Natural Gas Processing Plant

Abstract

Starting large medium-voltage motors on relatively weak power distribution systems leads to voltage sags which may compromise the power quality of the electrical system or (worse yet) prevent the starting of the motors themselves. In some cases, the utility in-charge has stringent voltage drop requirements (typically <5%) during a motor start event. The industry standard, when direct-on-line (DOL) motor starting is not possible, is to use adjustable speed drives (ASD) or reduced- voltage soft starters (RVSS). These systems are complex, costly, and can be difficult to install and maintain.This paper presents a potentially cost-effective application of transient-free capacitor switch technology (TFCS) and large multi-stage capacitor banks being controlled by modern high-speed digital relays for DOL starting of 5000 HP induction motors at a Texas natural gas processing plant. The paper first reviews the application from a design perspective. Power quality targets are detailed, and the pros and cons of ASD technology, RVSS technology, and TFCS capacitor start (TFCS-CS) technology are each discussed. Secondly, the paper reviews details of the TFCS-CS scheme employed to deliver on power quality targets, inclusive of digital power system simulations. Thirdly, test results from the TFCS-CS application are reviewed and compared to simulation runs. Based on the outcome achieved, TFCS-CS technology has proven to be a low cost, reliable, and effective means of starting large motors when voltage sag mitigation is required. The success of this application validates the TFCS-CS technology as an attractive choice for medium-voltage DOL motor starting that could find application across many industries.