Dynamic analysis of a system with RPT
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The purpose of this master thesis is to establish and validate a simulation modelof the transition from pump to turbine mode of operation for hydropower systemswith reversible pump-turbines (RPTs). The model builds on the one-dimensionalturbine model presented by Nielsen [Nielsen, 1990. "Transient characteristics of high head Francis turbines". NTH], but uses pump mode of operation as a basis.Simulations have been carried out in MATLAB, and compared with measurementsfrom the RPT rig at the Waterpower Laboratory at NTNU. Simulation of transition from pump mode to turbine mode was conducted by cuttingthe torque from the motor that drives the RPT in pump mode. This reducesthe rotational speed of the runner and it starts rotating in the other direction dueto the hydraulic torque from the water, and ends up at runaway speed in turbinemode. The results from the simulations and measurements are fairly similar in pumpmode, but very different in turbine mode. At the end point of the simulations therotational speed is only 1=3 of the speed at the end point of the measurements.There are several reasons for this difference, but insufficient accuracy of the modelof the pump-turbine characteristics in turbine mode plays an important role. Themodel does not take into account that there are two different heads for zero flowdepending on whether the RPT is going from pump to pump brake or from turbineto reverse pump mode. The simulation results are also influenced by the assumptionof neglecting elasticity and the switch of causality between pump and turbine mode.Torque is the controlling parameter in pump mode, while this is the role of thehydraulic head in turbine mode. All these factors are part of the explanation ofwhy the simulations do not follow the same trajectory as measurements in theH-Q-diagram.