Conceptual design for sub-sea power supplies for extremely long motor lead applications

This paper describes one concept for providing electrical power from a surface generated power source to a sub-sea, medium voltage motor(s) in water depths up to 10000 feet, and up to 10 miles away from the source. Continuous and instantaneous voltage regulation becomes an absolute necessity to maintain proper motor input voltages for these extremely long motor lead applications, due to the very large change in voltage drop between no-load and full load in the feeder cable. The heart of the design concept is a state of the art medium voltage VFD, which can handle voltages from 2.4 kV up to 7.2 kV. It also has the capability to continuously change both the frequency as well as voltage, on the output side upon command. Best of all, this unique medium voltage VFD technology eliminates most of the problems usually attributed to VFD´s, such as harmonics and load sensitive power factors on the front-end, plus “ringing” problems associated with the use of fast switching IGBTs in the inverter of pulse width modulation type VFDs, so no special filtering or isolation is required on either end, no matter how long the motor lead is. This VFD technology essentially turns an AC motor into a DC motor, which allows the use of a 1 to 1 ratio of generator capacity to motor(s) capacity, which is highly desirable for single motor applications. This will really help save on weight, space and the high costs associated with any offshore platform type of application