Rotary Frequency Converter for Long Step-out Subsea Applications

Abstract

Modern oil and gas field are in general discovered far from existing platforms or shore. Development of subsea processing systems are often a suitable alternative rather than platforms or floating vessels in an economical aspect. This is due to the fact that new fields are generally smaller than the old ones, they are located in deeper waters and some of the mature fields are in tail-production. As the step-out distances increases, the need for developing new technology and architectures for subsea equipment has become desirable to ensure reliable power supply to subsea processing equipment.This master thesis is more or less a feasibility study of a proposed innovative topology for a subsea power system for a long-step out application. The system is introducing a rotary frequency converter as a subsea application for supplying large power demanding loads, such as compressors and pumps. The motivation behind this topic is increasing the power transmission capability and performance of the subsea cable, accordingly also the step-out length. Two main power systems are presented for this analysis. A singly operated rotary converter feeding a high speed induction motor is modeled and analyzed. Secondly, a subsea distribution grid including parallel operated converters for supplying two induction motors. The systems has been partly divided and modeled and analyzed in different simulation tools.The simulation results indicate several promising results after introducing the rotary converter into the power system. Some of the challenges regarding long step-out is reduced or eliminated. Some of the key results from the power flow analysis are; the voltage drop and voltage amplification in the transmission cable during full-load and no-load is completely compensated from a synchronous generator equipped with a automatic voltage regulator (AVR). Furthermore, the reactive power production in the cable is considerable reduced. The voltage level can therefore relatively be increased for the same power level, due to the fact that the reactive power production is increasing squared with the voltage level. In addition, the AC resistance of the cable is some reduced, due to less influence of skin effect and proximity effect. Consequently, reducing $I^2R$ losses in the cable.A dynamic analysis has been performed regarding the stability during a disturbance in the system and the properties of the rotary converter. The objective is to examine how these disturbances affect the system. Additionally, a motor start-up analysis of the rotary converter has been executed. The results do not indicate any severe transient levels in the simulation cases.