Explosion at a turbine-generator of a power plant

The DC power system was serving as back-up power and was based on a battery storage system. It had been taken off-line to be replaced with a more modern system. After its replacement, at the moment of switching it on, it caused a zeroing of the voltage and the loss also of the AC powers, which was providing power station control.
The control room lost any information on the operation of the station, which could not be disconnected from the grid or shut down safely. The turbine generator spun, and without power to the oil pumps and to the oil sealing the hydrogen coolant, the cooling power disappeared. Eventually turbine and generator failed mechanically igniting the escaping hydrogen.
No-one was injured. An extensive blackout was the main consequence.

The INITIATING CAUSE was a loss of power controlling the power plant, due to the tripping of circuit breakers. This induced a loss of control on the operations, the running out of control of the turbine-generator unit and eventually the spilling of the oil and hydrogen used as coolants.
The ROOT CAUSE was severe shortcoming in the design of the interconnection and interlocking of both the AC and DC systems and a lack of redundancy. Moreover, a failure to implement effective process safety practices at the plant was reported as result of the investigation.

turbine,generator

During the previous 18 months, an upgrade program had been initiated to replace the battery chargers on the two coal-fired units C3 and C, and C3 battery chargers were already replaced and successfully brought back into service. TO do the same for C4, the battery and battery charger had been disconnected from the system and the battery charger had been replaced. During this time, the battery of the backup system of C4 could not provide DC power, and the system was configured to receive power from common station, via a switch, called a Station Interconnector. The Unit C4 incident occurred during the re-connection of the new Unit C4 220V battery charger.

DESCRIPTION OF THE PLANT
The power station comprised of two power plants, B and C, each with two generating units. All four units were operated from a common control room. C power plant was coal fired, where coal is grinded to powder and burned it in a boiler, producing steam sent to a turbine. The steam rotates the turbine at 3,000 revolutions per minute, driving a generator to convert the mechanical energy into electricity.

There were no fatalities or injuries, but turbine-generator unit was destroyed and the whole state power grid was destabilised. The explosion initiated a cascading failure of nine major generator units across the state, which caused almost half a million customers to lose power.
At three years from the event, the affected unit was still not back in operation.

This is an incident in which a hydrogen system (the coolant system of the generator) plaid a role only at the end of the sequence of events. It certainly contributed to the severity of the final damage, but the main damage to the turbine-generator unit was caused by their loss of control.

The investigation of the causes was diced in two: the only available report, arrived three years after the incident, regards the technical dimension. This report is available on the site of the power plant owner, and delves in very complex technical explanations, which confirm the management statement that “the explosion was the result of the simultaneous failure of key electrical equipment and system backups in a series of complex events
that could not have been anticipated”.

A second report focussed on the role of the organisation. This is not publicly available, and only online news summarises it, stating “failure to implement effective process safety practices that would have increased the likelihood of identifying and managing the risks associated with replacing the new Unit C4 battery charger and bringing it into service”.
Due to the lack of availability of this second report, it is impossible to attempt an independent lesson learnt on the management responsibilities. Nevertheless, considering the extensive blackout experienced by half a million of customers, it seems correct to conclude that a simple maintenance activity should never had such consequences.

The power plant management decided several design changes to prevent the occurrence of similar incidents, not only at the affected power plant but also to other similar plants.
System redundancies were installed, such extra switchboard, and the interlock design was reviewed and improved.
Moreover, the workers pool was increased, a safety team and an engineering team were created to focus on standards and quality assurance.
Finally, maintenance backlog system and incident reporting system were improved, aiming at a better oversight and management of changes.