DC-link selectivity analysis of variable frequency drives system

Abstract

DC-based shipboard power systems are increasingly adopted due to their operational advantages and the growing integration of energy storage systems. However, short-circuit faults in DC systems are particularly severe, as fault currents rise rapidly and can reach damaging levels within a few milliseconds. Therefore, the reliable implementation of DC systems in shipboard applications requires a robust protection scheme, with particular emphasis on DC fuse selectivity, to ensure fast and accurate fault isolation.

DC selectivity is a critical protection mechanism that ensures only the faulty section of the system is disconnected while maintaining uninterrupted operation of healthy sections. This thesis presents a comprehensive analysis of DC fuse selectivity for a shipboard power system incorporating variable frequency drives. Initial selectivity studies are conducted using the Danfoss Drive Selectivity Toolbox implemented in the Simscape Electrical environment of MathWorks, which enables visual identification of isolated fuses during fault events. Three distinct methods for DC selectivity analysis based on fuse time–current characteristics are investigated and compared under various fault scenarios. The accuracy and effectiveness of each method are evaluated, and the most reliable approach for DC selectivity assessment is identified. A detailed sensitivity analysis is also carried out to identify the critical parameters which affect the fault current during the short-circuit.

Furthermore, a customized Simscape-based toolbox is developed to perform DC selectivity analysis independently of third-party software. The drive and fuse models are implemented using manufacturer data sheets and technical manuals. Simulation results obtained from the customized toolbox are validated through comparison with the Danfoss selectivity toolbox, and the resulting fault current waveforms are analyzed. The developed toolbox also enables automatic generation of project-specific selectivity reports, including relevant system parameters and simulation results.