The derate tab (figure 99) can be used to limit a turbine’s maximum power with increasing temperature. This is a strategy implemented by some manufacturers in order to avoid losses associated with high temperature shutdown hysteresis. The derating strategy can be varied with air density or elevation. Air densities should increase from left to right. The derating strategy interacts with the regular energy capture by limiting the maximum energy that can be generated in a calculation step.
The example shown above shows both low and high temperature derating. The low temperature derating only varies with temperature; going from full power at -30C down to 1800 kW at -45C. The high temperature derating behavior varies with elevation. For elevations below 1000m, the turbine goes from generating full Power at +30C down to 1800 kW at +45C. For elevations between 1000m and 1500m, it goes from full power at +27C down to 1800 kW at +40C.