Isolation techniques for various topologies of medium voltage adjustable speed drives

Compression and pumping applications represent a majority of all rotating machinery in modern industrial plants. Increasingly in the past two decades, electric motors are replacing turbines as the prime mover of choice for large rotating equipment. The use of electrically driven equipment is now widespread even for critical applications. As semiconductor technology evolves, adjustable speed drives (ASD) gain favor as the best option for both; motor starting and speed control. High volume, pressure and flow is easily adjusted by varying the speed of electric motors even larger than 50,000HP. Multiple medium voltage ASD topologies exist today which offer excellent motor performance. However, one of the key topics of discussion among specifiers and end users concerns the influence of ASD to the utility line. Several methods for improving line-side performance use incoming isolation transformers, but some do not. It is often not clear when drive isolation transformers and which converter topology should be employed. This paper is intended to be a guide on drive isolation techniques and major types of converters for various topologies of medium voltage ASD, exploring brief theory, advantages, disadvantages of each configuration and application considerations for selecting the optimal drive isolation technique.