The quality and security of a vessel’s power supply system is crucial to its safety. Harmonic distortion of the electrical power on board ships has become an ever increasing concern as the use of AC variable frequency drives (VFD’s) for main propulsion, thrusters and other shipboard functions has grown. By generating high levels of harmonic current, VFD’s can badly distort the voltage waveform resulting in equipment operational problems and premature failure. As a result, classification bodies, such as Det Norske Veritas (DnV), have begun to apply limits on acceptable levels of harmonic distortion on board ships. To meet the DnV’s 5% limit for voltage distortion, a Norwegian cable laying vessel known as the Ocean Challenger, was equipped with passive, wide spectrum harmonic filters.
The Ocean Challenger is capable of operating as both a cable lay and repair/ maintenance vessel. The trenching operation is performed by a 2MW Remotely Operated Pipe-line Trenching Vehicle, referred to as the ROV PT1. The PT1 is equipped with ten 30kW electric thrusters for maneuvering and four 300kW Jet Sword high volume flow rate electric pumps. The speed of electric thrusters and pumps is controlled via dedicated AC PWM VFD’s mounted in the surface module.
Harmonic distortion resulting from the operation of these VFD’s was preventing the ship from sailing without the significant added cost of rented generators. When the VFD’s were fed from the main power shaft generators, total harmonic voltage distortion (VTHD) exceeded the 5% level permitted by DnV. By supplying all harmonic generating VFD’s from the rented generators, the voltage distortion that they were creating was isolated from the rest of the equipment on the ship. The ship’s operators wanted to find a cost effective solution for treating the VFD harmonics so that they could be returned to the ship’s main distribution supply, eliminating the need for the rented generators.
Two 750kW Lineator AUHF wide spectrum harmonic filters, manufactured by Mirus International (Canada), were used to supply the 1.5 MW VFD load. Each filter was supplied from one of the two 2880kVA main power shaft generators (one starboard and one port). Total harmonic current distortion (ITHD) was substantially reduced to 6.4% by the starboard filter and 7.3% by the port filter. During the sea trials, ship staff monitored both the operation of the two shaft generators and the VTHD on the main switchboards. The generators operated flawlessly and at no time did the VTHD rise above 1.4% and 1.6% on their respective switchboards. The 5% voltage distortion limit of the DnV was easily met without the need for the rented generators and their associated cost both financially and in valuable deck space.