Loss of lubrication and seal oil for the 133MW steam turbine and hydrogen-cooled generator unit at this power generation plant resulted in mechanical damage and a hydrogen fire. A turbine rotor out inspection revealed scoring of the thrust and journal bearing rotor surfaces, and blade rubbing. Nondestructive testing will determine if the rotor components are fit for service. Also, the generator requires electrical testing to determine if the hydrogen fire caused damages.
Following a boiler maintenance outage, operators initiated normal plant start-up. With the steam turbine at part load during the start-up sequence, switching from the start-up transformer to the auxiliary transformer was initiated according to standard operating procedures. During switching operations, all AC auxiliary power was lost.
Investigations revealed that during the start-up process the auxiliary transformer breaker had actually not closed, preventing the auxiliary transformer to come under load. This changeover is a manual process with no interlocks to prevent an error. Therefore, when the start-up transformer breaker was opened as part of the start-up procedure, all AC auxiliary power was lost and an automatic trip initiated. Unfortunately, the emergency DC lube oil pump that acts as a last line of defense to supply the unit with lube and hydrogen seal oil in the event of AC power loss, was isolated from the DC electrical supply at that time.
Several factors played a role in this loss:
- The start-up procedure did not have a detailed checklist and sign-off for the critical steps, such as ensuring the DC power supply breakers are in the correct position, conducting a DC pump test prior to start-up, and ensuring the start-up and auxiliary transformer operate in parallel before isolating the start-up transformer.
- Various plant alarms and warning lights were not reliable. This includes a defective annunciator light that indicates a breaker-open position of the auxiliary transformer, and the lack of an annunciator in the control room indicating the position of the breaker between the DC power supply and emergency lube oil pump.
- Inadequate Lock Out/Tag Out procedures allowing the DC breaker to be left in the open position. In addition, critical breakers were not properly tagged and labeled.
The turbine and generator repairs are currently estimated in excess of US$3 million.