By Van Wagner, P.E., Schneider Electric Water Wastewater Competency Center
Traditional power system design practice suggests that the motor crossover from low voltage to medium voltage is in the 200 to 500 HP range. A major reason is due to the fact that the starting current at low voltage in this range can abnormally depress facility voltage. The addition of reduced-voltage motor starting can alleviate the voltage dip but at considerable extra cost.
Adjustable speed drives may have increased the upper limit of the crossover. Drives have inherent soft starting capability, and combined with increased process efficiencies at lower speeds, may have moved the crossover to a greater horsepower rating. Has the crossover changed?
This paper attempts to answer that question with a focus on water and wastewater applications. An example of low voltage and medium voltage application is compared for 300, 500 and 800 HP drives. Since installation of a drive in an existing pumping application is more commonly encountered than new applications, the example given is for a retrofit in an existing facility. The results are sensitive to the assumptions of the analysis and the assumptions are framed in a realistic scenario that could be projected to other situations.
The paper compares the installed cost, reliability and operating costs for the medium voltage and low voltage applications. Other considerations also are discussed.more...