Switching from time-based to condition-based maintenance of modular frequency converters

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19 March 2020

Not the age, but the actual aging should be leading when replacing modular frequency converters, says Bakker Sliedrecht, adding action to the word. The first offshore crane ship is already being successfully maintained in this way.

Vessels with electrically driven motors, but also cranes, terminals, pumps and rollers, use modular frequency converters to control their speed and power. Each frequency converter contains a capacitor bank, the capacitor bank that is subject to aging. In the current situation, the manufacturer's prescribed replacement period, e.g. 9 years, is adhered to. This is called time-based maintenance. The replacement period is based on the most severe conditions imaginable, not on the actual aging of the capacitors. As a rule, therefore, maintenance is carried out at an early stage, which makes it more advantageous to postpone it with the associated downtime of operation to a more suitable period. The capacitors of each frequency converter then has to be replaced, which takes 7 to 8 hours of work per module, and an average ship contains dozens of modules on board. 

Measuring makes maintenance predictable

Bakker Sliedrecht therefore has started condition monitoring of modular frequency converters. The capacitor bank of one module per cabinet is measured for inspection. By doing this periodically, it becomes possible to determine the progression of aging, and to plan a replacement moment based on the actual condition of the capacitors. Bakker Sliedrecht has experience data at their disposal that shows how the ageing curve proceeds. By projecting this on the measured data, the ageing over time can be predicted. Prolonged downtime of the frequency converters also causes the ageing of the capacitor bank, which is not taken into account in time-based maintenance schedules. Bakker Sliedrecht is familiar with the parameters of this thus making maintenance predictable. 
The client can exchange the module to be measured for a spare module. Bakker Sliedrecht measures and keeps track of obsolescence, after which the module can be replaced at a later date by the customer.

Replacing when necessary and suitable

The advantages of condition-based maintenance over time-based maintenance are clear. Actual aging is taken into account, so replacement in most cases is delayed in comparison to the estimate of time-based maintenance. As a result, condition-based maintenance can save money and postpone maintenance. Replacement can also be scheduled at a more convenient time. For an offshore crane vessel, this form of condition-based maintenance of frequency converters has already been applied satisfactorily.

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