In its investigation of an explosion and loss of propulsion aboard the RMS Queen Mary 2 Britain’s Marine Accident Investigation Branch warns that protection systems for critical equipment must ‘fail safe’, and should be thoroughly tested at regular intervals to prove that all sub-components are functioning correctly. In particular, harmonic filters with current imbalance protection systems should be thoroughly checked by a competent person at the earliest opportunity.
Investigation of the catastrophic failure of a capacitor and explosion in the aft harmonic filter room showed that the protection system for the harmonic filter did not work. As a result the vessel blacked out and was without steering or propulsion for 30 minutes. There were 3823 people on board.
Says MAIB: “there is a need to improve the awareness of the potential risks of high voltage harmonic distortion and arc flash… Awareness of the damaging effects of harmonic distortion needs to improve throughout the marine industry as the risks to equipment caused by harmonic distortion are likely to increase significantly as variable speed AC electric motors become more widely used in ships”.
The vessel had a history of HF capacitor failures, at an average rate of one per year. Although the exact cause of the capacitor failures could not be determined, it was concluded that capacitor degradation was probably caused by a combination of transient high voltage spikes due to frequent switching operations and occasional network overvoltage fluctuations. The capacitor deterioration had not been detected, and because there were no internal fuses or pressure relief devices, it had continued until the capacitor casing failed catastrophically.
The vessel’s alarm logs were found to contain early warnings about the impending failure approximately 36 minutes before the accident. However, as the vessel’s alarm systems regularly logged more than one alarm every minute, this information was not seen and could not be acted upon.
The only protection against catastrophic failure of the capacitors was a current imbalance detection system. It consisted of a current transformer (Figure 2a) which was connected to the capacitor circuit. Under normal conditions, little or no current should have flowed through the transformer. When a capacitor degraded, the current flow across the circuit became unbalanced and induced a current in the transformer’s secondary winding. The system was set to give an alarm when the imbalance reached 400mA and to trip at 800mA.After the accident, the transformer’s windings were found to have failed. There had not been any alarms on this part of the system for several years and it was likely that the imbalance detection system had not worked for some time.