Small Signal Analysis of a Subsea Drive System Based on a Permanent Magnet Machine

Permanent magnet synchronous machines (PMSMs) are now being considered for subsea variable speed drive (VSD) applications with very long cables, which were mainly driven by asynchronous motors in the past. There are many attractive features, but the stability of the new systems might prove an issue since encoderless operation should be achieved. This work presents a first effort in this direction and proposes a small signal analysis of such a subsea VSD system based on a PMSM. A system model utilizes physics-based models of the components of the subsea VSD architecture, including a new power cable model built as a distributed parameters model expressed in the dq reference frame. After linearizing the model, control analysis techniques evaluate the system's small signal stability. The analysis is carried out considering scalar control and reveals that the system may exhibit multiple points of instability. The mechanical load behavior has a significant influence on the stable operation points.