Electrical variable speed motor control has been utilised on some vessels, both naval and commercial, since the start of 20th Century.
Up until the 1960s most ships generated DC voltage with DC motor speed control facilitated via face plate starters. Other applications including capstans and anchor windlasses sometimes used Ward-Leonard (motor- generator) systems.
The late 1960’s saw the introduction of AC supplies on new builds. With the exception of some specialist vessels (e.g. survey vessels) which still utilised DC supplies, most ships were AC with the use of fixed speed AC motors for general duties. Where variable speed was required for duties such as pump, fans and cargo handling equipment, ‘stepped’ control of AC cage motors was achieved by switching the pole pairs on the multi-pole AC drive motors via contactors. DC motors were however often still used for anchoring handling and mooring winch duties, via first generator electronic SCR controllers or utilising Ward-Leonard systems.
From the late 1980s variable frequency drives (VFDs), popular onshore, started to appear on ships. Today there are naval destroyers with 40MW (2 x 20MW) of PWM VFDs for main propulsion with a number of their naval support vessels having either synchronous or load commutated AC drives for propulsion duties.
From around the late 1990s new cruise vessels were fitted with AC synchronous drives or AC cycloconverters as the propulsion system of choice. Today, VFDs are now commonplace on most classes of vessel for numerous applications including main propulsion, thrusters, pumps, fans, compressors and cranes.
The benefits of electronic variable speed drives are now widely recognised; indeed, a number of new classes of vessels have been designed to specifically to take full advantage of these benefits.
The power conversion process in AC VFDs and DC drive controllers draws harmonic currents from the supply which interact with the system impedances to distort the system voltage. Other adverse effects include ‘line notching’ and ‘voltage spikes’ due to DC SCR drives and ‘common mode voltage’ due to the incorrect installation (from an electromagnetic compatibility (EMC) perspective) of VFDs and their drive motors.
There are a number of power quality issues which, if excessive, can adversely affect the operation and reliability of other electrical equipment connected to the common voltage supply, and in doing so may jeopardise the operational integrity and safety of the vessel and her crew. Marine classification bodies have rules (e.g. ABS, DNVGL, LR) reflect this with strict Uthd limits being placed on harmonic voltage distortion. Recently IACS (UR E24) have in introduced additional harmonic measurement requirements for vessel with electric propulsion.
In the majority of applications VFDs, other AC drives (e.g. synchronous, cycloconverters and load commutated converters) and DC drives will require harmonic mitigation equipment to ensure safe and problem-free operation of the vessel and its electrical equipment. The issue of harmonic voltage distortion and the effects on plant and equipment has to be addressed to ensure that the safety of the ship, the protection of both the crew, and where appropriate, the passengers and the marine environment are to be assured.
Harmonic Solutions Marine are specialist in this area and have a range of appropriate of mitigation solutions available. Please refer to the Mitigation Products section of this website.
Harmonic Solutions Marine can also provide a full range of harmonics, power quality and EMC testing for marine vessels worldwide. Click here for more information.