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Offshore installations have for many years used large and multiple AC and DC variable speed drives for draw-works, mud pumps, submersible pumps, etc. There has been in the marine sector, over the last 10 years, a large increase in vessels using large variable speed drives, especially AC drives. Applications include thruster drives (>8MW per unit rating), cargo pumps (>1000kW) and main propulsion drives (up 40MW per unit rating).
The effects on generators and other marine equipment due to harmonic currents is significant and more serious due the fact that the value of the generator subtransient reactance (Xd”) is much higher compared to the impedance apparent on shore based power transformers (e.g. typical transformer impedance (Z) is 5%-6%; a typical marine generator is 14-18% Xd”); for a given harmonic current, the subsequent voltage distortion can be up to three to four times greater on generators.
Due to safety and equipment reliability concerns all marine classification bodies, without exception, including Lloyds Register (UK), Bureau Veritas (France), American Bureau of Shipping (US), Det Norsk Veritas (Norway), Germanischer Lloyd (Germany) and Polish Register of Shipping impose strict limits on the maximum permissible total harmonic voltage distortion on ships and offshore installations operating under their rules with the risk of declassification should the limits be exceeded and/or insufficient harmonic mitigation installed.
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