If you read the report it's been suggested that a battery would have prevented the failure when switching in a new battery charger.
The significant contributors to the Unit C4 incident have been identified, and aresummarised below in order of importance:
1) Interlocking design preventing battery being connected during switching operation
The interlocking design prevents two batteries being connected in parallel during switching operations, thereforelimiting the reliability of the critical DC supply system to that of the AC supply system.Had a charged battery been able to be connected through all switching operations, the incident would likely havebeen prevented.
2) Unintended tripping due to arc flap protection
Loss of the AC supply due to arc flap protection on loss of DC supply was not an expected occurrence and is notthe intent of the protection design. This resulted in the loss of turbine oil pumps and prevented the battery chargerrecovering the DC voltage.Had the AC supply remained in service, the turbine oil and sealing oil pumps would have remained in service and it islikely that the Unit C4 220V battery charger would have recovered the 220V DC, enabling the unit to trip on reversepower in a matter of seconds.Arc flap protection should only operate when the arc flaps operate and should be able to ride through short termdeviations in DC supply voltage.
3) Automatic Changeover Switch did not operate
The Automatic Changeover Switch did not automatically change over to the available Station Main 220V Switchboardupon the loss of the Unit C4 Main Switchboard supply. This prevented the ‘Y’ Protection system from tripping thegenerator and likely ending the generator motoring.
4) Failure of battery charger to maintain voltageUpon opening of the Station Interconnector,
The battery charger did not instantly maintain the DC voltage within therequired operating range. Ensuring a battery is always connected reduces the criticality of the battery charger response time and is thepreferred method to address this.
