Three steam turbine generator units were damaged due to the loss of lubricating oil supply to the bearings as a result of the loss of DC power and subsequent operator actions.
The facility is a 54-MW, base-loaded power-generating station consisting of three steam turbine generator units. Each unit has a rated capacity of 18.0MW and utilizes an individual lube oil skid that is located adjacent to the unit. The lube oil skids are provided with two auxiliary AC power driven pumps and one 120-V DC emergency pump.
The DC power system has two independent DC battery strings for all three units: one provides power to protective relays and DC lube oil pumps and the other provides power to the distributed control system (DCS) located in the control room. There is a relay that monitors the DC voltage to power the emergency and protective devices, but no alarm was provided to the control room in the event of a low voltage.
When DC power was lost, all three steam turbines tripped. As part of the trip sequence, the steam admission valves to the turbines closed, shutting down the steam supply to the turbines. However, because the generator circuit breakers rely on DC power to open, they remained closed, which led to “motoring” of the generators. The auxiliary lube oil pump that was in operation prior to the trip, remained in operation as the AC power supply was not interrupted at this stage.
Recognizing that generator motoring may cause severe damage to the steam turbines, operators tried to open the generator circuit breaker that connects the generator to the generator step up transformer (GSU) but were unable to do so. Operators then proceeded to open the circuit breaker at the utility owned substation, downstream of the GSU. This action stopped the generator motoring and the units started to coast down. However, this action also resulted in a loss of AC power to the entire plant, and hence to the auxiliary lube oil pumps.
With the loss of both DC and AC power supplies, the lube oil supply to the bearings was interrupted. The three turbines came to a stop without any lubrication in about 6 minutes.
The bearing babbitts were severely worn, and the journals on the rotors were scored on all three turbines and generators. Labyrinth and oil seals caused further damage to the rotor surfaces. One steam turbine also sustained rubbing damage in the high pressure section.
The generator exciters showed major electrical energy induced scorching, discoloration, and deformation to pole surfaces. Several loose field winding wedges were identified.
Investigations revealed that that the loss of DC power was caused by a disconnected battery charging system and depleted batteries. The batteries had gradually discharged while continuing to supply DC power to the protection system. The voltage of the DC bus eventually dropped below the holding voltage of the relay that monitors the voltage to power the protection devices. This caused lock-out relays to trip the units.
The disconnection of the chargers was the result of an AC side circuit breaker trip, likely caused by an electrical transient event that occurred two days prior the incident. Although an alarm is provided in the control room to alert the operators of the DC battery charger breaker open position, this was not a recurring alarm requiring acknowledgement for charger out of service. The single alarm that was generated when the AC side circuit breaker tripped open, was not noticed by the operators after the transient event. Also, as no low DC voltage alarm is provided in the control room, the ongoing depletion of the batteries went unnoticed.
This loss demonstrates the importance of properly designed and maintained alarm management systems, including low voltage alarms with readily visible indicators for critical DC bus systems at a continuously monitored location such as control room. Alarm set points should be set at a value that allow operators to take appropriate corrective actions before cascading events are initiated. DC systems in a power plant are vital components to reliable operation. Prime consideration should be given to all aspects related to their reliability such as proper design, operating procedure, maintenance practice and periodical testing.
Operator knowledge and operator preparedness to properly recognize and respond to alarms and upset conditions is of equal importance as demonstrated by this loss. FM Global loss history shows that large losses are often times a combination of failing protection systems, and a failure of operators to recognize and/or adequately respond to the escalating conditions.on.