Condition-based maintenance for modular frequency converters


Bakker Sliedrecht introduced condition monitoring of modular frequency converters earlier this year after successful experience aboard an offshore crane vessel.Bakker Sliedrecht introduced condition monitoring of modular frequency converters earlier this year after successful experience aboard an offshore crane vessel.



The main criterion for the replacement of modular frequency converters should be the actual condition of the components, rather than their age. Bakker Sliedrecht began to offer condition-based monitoring after successfully maintaining an offshore crane ship this way.

Vessels with electrically driven motors use modular frequency converters to control their speed and power. Each frequency converter contains a capacitor bank, which is subject to aging. Until now, the replacement period for capacitor banks has followed manufacturers’ guidelines, (typically 9 years). This time-based maintenance approach is based on the most severe conditions imaginable, rather than the actual condition 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 have to be replaced. As the replacement of each module can take  7 to 8 hours of labour, and an average ship contains dozens of modules on board, this is a considerable task. 

Measuring makes maintenance predictable
Bakker Sliedrecht began to conduct detailed monitoring of modular frequency converters in order to optimise maintenance. By measuring the condition of the capacitor bank of one module per cabinet periodically, it becomes possible to determine the progression of aging, and to schedule a replacement based on the actual condition of the capacitors. Bakker Sliedrecht has experience data at its disposal that shows how the ageing curve proceeds. By comparing the measured data with the ageing curve, it is possible to predict how the condition of the modular frequency converters will age over time. An additional variable is the impact of prolonged frequency converter downtime on the capacitor bank itself, which is not taken into account in time-based maintenance schedules. 

The process of monitoring 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
Bakker Sliedrecht notes that switching to a condition-based maintenance approach over a time-based one offers clear advantages. As actual condition rather than age is taken into account, in most cases replacements can be delayed compared with time-based maintenance regimes, with reduced maintenance requirements. Replacements can also be scheduled at a more convenient time.

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