Input–Output Small-Signal Stability Analysis of a PLL-Free Direct Power-Controlled Partially Power-Decoupled More-Electric Shipboard Propulsion System

With the partially power-decoupled system architecture, a doubly fed induction motor-based shipboard propulsion system (DFIM-SPS) has the potential of increasing the system safety level by decreasing the penetration of power electronics. In this article, a phase-locked-loop-free (PLL-free) direct power control (DPC) strategy is proposed for a DFIM-SPS to increase the system dynamic performance. The system small-signal stability analysis is carried out based on the source-load structure, where a salient-pole synchronous generator supplies power to the DFIM electric drivetrain. A detailed dq impedance model of DFIM-SPS is established with the power controller gains of the back-to-back power converter (BTBPC) considered. The proposed source-load dq impedance model of PLL-free DPC-based DFIM-SPS can easily reflect the equivalent impedance produced in the stator and rotor circuits. The effects on system stability caused by the equivalent impedance variations due to power controller gain changes are evaluated in detail. The frequency response of the elements in the DFIM impedance matrix is investigated first, and then, the minor loop gain matrix is obtained. The system input-output small-signal stability is analyzed based on the Nyquist stability criterion. Time-domain simulations are carried out in MATLAB/Simulink to validate the effectiveness of the proposed PLL-free DPC strategy for DFIM-SPS.