At high intermittent renewable energy share, plants are forced to operate more flexible beyond their original design intent. Many plants are older than 30 years with only limited residual lifetime. Decreasing energy prices and capacity factors will further enforce older plants to more transient operation with steeper gradients. Steam Turbine (ST) protections systems on site often are not designed for such flexible operation and therefore do not properly supervise the resulting impact on lifetime consumption.
Therefore precise lifetime management concepts are required to increase plant reliability and flexibility, mitigate risks for new implemented operation modes like faster start-ups and extended turn down.
The prerequisite for properly managing a plants lifetime is the accurate knowledge of the current state of consumed lifetime, which is evaluated in a lifetime assessment (LTA). This includes a standardized process for the calculation of creep and fatigue damage. For this long-term field operation data is used as input. The current LTA procedure typically limits the analysis to transient operation during start-up and shut-down. Other types of transient operation such as improved turn-down also need to be considered in order to take all relevant operation modes into account.
Therefore dedicated, new advanced methods are required for the precise estimation of the lifetime impact due to new operation modes with higher requirements compared to an assessment based on known current procedure. These methods will allow optimizing asset lifetime. In order to maintain power plant competitiveness, operation and maintenance cost must be reduced despite the demand for improved turn-down and more frequent start-up procedures.
This paper presents results of ST lifetime studies combining long-term operation data (including improved turn-down operation), FEM calculations and simplified and advanced constitutive material models.
In a first investigation, the rotor is modelled with FEM and thermal boundary conditions during isolated transient operation are derived from measurement data combined with generically elaborated new turn-down profiles.
The impact of the start-stop (enclosing cycle) and turndown cycle (sub-cycle) on the lifetime consumption is evaluated. For this, sub-cycles are considered in combination with the enclosing cycle and as isolated cycle.
In a second investigation, the above described calculation model is fed with 2 years of continuous operation data. Postprocessing is performed with common rain-flow methods and compared to an accumulative start-stop cycle assessment.
Based on the input of the calculation with the advanced constitutive material model further fracture mechanic investigations are performed.
The paper closes with an outlook on opportunities arising from digitalization of power generation equipment.