In the past steam turbines were designed to operate on a certain load level. However, plant operators are often forced to operate their steam turbines on various load levels depending on the fluctuating demand of the grid. Due to the influence of renewable energies on the electricity market, these fluctuations significantly increased in the last decade.

One possibility to increase a plant’s flexibility and adapt to these altered conditions is to operate it in low load during periods of reduced electricity demand, instead of shutting the plant down. Doing so, the plant can provide power very quickly, if demanded from the grid and it saves the costs for start-up.

However, like all transient events, the transition to Low Load Operation (LLO) can cause temperature differences in the rotor and thick-walled components resulting in deformations and thermal stresses which increase the lifetime consumption of the turbine or might even damage the turbine. Since LLO was mostly only a secondary criterion in the design of the turbine and installed turbine protection systems (e.g. stress controllers) do not offer sufficient protection in many cases, the possible risks of LLO should be investigated before the implementation of LLO for a plant.

This paper analyses the lifetime consumption and damaging mechanisms at different components of the steam turbine that can be caused by LLO. Therefore, test runs in LLO (down to 15% of nominal load) performed by a hard-coal fired plant are investigated. In a first step the thermal stresses occurring in the rotor during transition to LLO are evaluated. Therefore, the rotor is modeled with Finite-Element-Method and the thermal boundary conditions during transient operation are derived from measurement data recorded during the test runs. The influence of these additional stress cycles on the overall lifetime consumption is analyzed.

In a second step operation data is examined with regard to critical operation regimes (e.g. elevated temperatures at HP and IP outlet caused by windage effects).

The results show that certain components can be exposed to increased lifetime consumption during a start-stop cycle containing LLO phases. Moreover, the analysis of operation data indicates that a close monitoring of steam temperatures and pressures is necessary to avoid operation in critical regimes which might cause damage to turbine components.

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