Britain’s Marine Accident Investigation Branch, MAIB, has issued an urgent Safety Digest which recommends checks on vessels with electric propulsion which have large capacitors in harmonic filters and has appealed for information regrading similar events elsewhere. The bulletin follows the catastrophic failure of a capacitor and explosion in an 11kV harmonic filter on board the passenger cruise vessel RMS Queen Mary 2.
At 0426 (UTC+1) on 23 September 2010, the cruise liner RMS Queen Mary 2 was approaching Barcelona when one of 12 capacitors in a harmonic filter1 failed (Figure 1), accompanied by a loud explosion. The explosion resulted in extensive damage to the surrounding electric panels and caused the vessel to black out. There were no navigational hazards nearby, main power was restored at 0455 and the ship was able to get back underway at 0523.
Preliminary findings of the investigation carried out by the manufacturer of the capacitor indicate that the capacitor had deteriorated gradually. The monitoring device based on current unbalance detection did not give any indication of the fault developing. The dielectric oil appeared to have vaporised due to internal short circuiting, causing internal pressure to build up and resulting in the oil leaking out and being sprayed onto the high voltage bus bars. The oil vapour is likely to have provided a conducting path between the live phases of the 11kV terminals, resulting in a major arc flash event.
Catastrophic failure of large capacitors has been experienced in land-based high voltage installations. One or more of the following can cause such failures:
• Excessive harmonic currents and voltages produced by electronic power converters during the voltage rectification process in variable speed electric motor drives.
• Transient voltage spikes and ‘line notching’ produced in thyristor power converters.
• Radiated heat from resistors and electromagnetically induced heating from inductor coils within passive harmonic filters.
• High ambient temperatures or blocked ventilation.
• Degradation due to ageing.
A full investigation is currently underway. In the mentime MAIB recommends that operators of vessels with electric propulsion which have large capacitors in harmonic filters, should urgently:
• Inspect the capacitors and check for any physical distortion, unusual smells (indicating dielectric fluid leakage) and signs of external overheating.
• Ensure that the cooling and ventilation systems are functioning correctly.
• Calibrate and function test any fault detection or condition monitoring system fitted.
• Ensure that if there is a history of capacitors in the equipment needing to be replaced, the underlying causes are understood.
• Carry out a thorough check for the cleanliness of all exposed conductors as well as for external signs of chaffing damage on high voltage cables.
Owners and operators of vessels or offshore platforms who have experienced catastrophic failures or bulging of capacitors in harmonic filters are requested to inform the MAIB by e-mail (email@example.com) using the title ‘Capacitor failures’ and include the name of the vessel or platform, the system manufacturer, and the date and place of installation.
Says MAIB: “This information is for internal use only and will be treated in the strictest confidence”.