Converter-fed subsea motor drives

A subsea adjustable-speed motor fed via a long cable in the range of several tens of kilometers between the converter and the motor are analyzed by simulations. Due to resonance, one critical frequency range occurs where significant generation of harmonics from the inverter should be avoided. A voltage source inverter is more feasible than a current source inverter since it is easier to modify the output waveform in order to avoid resonance problems. The resistive voltage drop in the long cable reduces the air-gap torque of the motor, particularly at low frequencies. This causes a problem for the start-up of the motor due to stiction torque. A start-up strategy is envisaged which is a compromise between voltage boost, inverter current, and transformer core dimensions. In normal operation mode the inverter voltage is proportional to the frequency. An open speed loop is used which keeps the system stable for potential load variations. The results from a 1-MW full-scale system test are summarised.