CONVERTER-FED SUBSEA MOTOR-DRIVES

A subsea adjustable-speed motor fed via a long cable in range of sever al 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 so urce inverter since it is easier to modify the output waveform in orde r to avoid resonance problems. The resistive voltage drop in the long cable reduces the air-gap torque of the motor particular at low freque ncies. This causes a problem for the start-up of the motor due to stic tion torque. A start-up strategy is envisaged which is a compromise be tween 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 f or potential load variations. The results from a 1-MW full-scale syste m test are summaried.