A numerical model is developed in Simulink to simulate transient alkaline electrolyzer operation. The model is built to simulate a large scale hydrogen plant in Rjukan, Norway. The model is based on physical relations where available, some plant-specific data is modeled using empirical equations. The model describes the anodic and cathodic compartments using electrochemical equations, lumped heat, and a perfectly mixed mass model. The balance of plant is modeled using the same heat and mass model. The model can predict the potential, temperature, efficiency, temperature, and mass flows and is validated using 30 days of measured data of the large scale plant. Transient current input, such as sudden current ramp up from 0 to 100% load, is used to simulate dynamic operation. From these simulations flexibility limitations of the electrolyzer are determined. The results suggest that the liquid levels in the stack andgas-liquid separator and temperature are limiting flexibility. The liquid levels form the biggest limitation of dynamic operation due to potential flooding or draining of the gas-liquid separators.