Handling System Harmonic Propagation in a Diesel-Electric Ship with an Active Filter

Harmonic distortion is often a challenge in marine vessel power systems due to the presence of large motor drives containing diode or thyristor rectifiers. Governing standards impose requirements on bus voltage Total Harmonic Distortion (THD) which can be demanding to fulfil. An attractive measure to reduce THD is to use 12-pulse rectifiers, as they are known to cancel the 5th and the 7th harmonic. However, the added cost and space of this solution compared with standard 6-pulse rectifiers is significant. This paper proposes to base propulsion motor drives on 6-pulse rectifiers, and mitigate harmonic problems by means of power electronic based active filters. A detailed numeric simulation model is developed for evaluating the performance of the topology. A case study system is defined consisting of two propulsion motor drives and two synchronous generators. The active filter greatly improves the bus voltage THD in all defined simulation cases. The performance is also better than a corresponding system using 12-pulse rectifiers and no active filter.