Abstract:
Since a long time, it is known that hydraulic machines can work with an increased efficiency at partial load when the speed can be adapted to this partial load. In a standard configuration when the hydraulic machine is coupled to the electric grid this speed variation is not possible at all. So for variable speed generator operation some modifications have to be made in hydro power plant. Speed governor may not be used to control frequency of isolated plant. Depending upon the type of generator whether it is synchronous or induction the suitable topology of converter need to be selected to convert variable voltage or frequency output into the constant voltage or frequency output. The variable speed concept of electrical machines is the optimum solution in the case of large variation in the turbine and pumping heads enables load control in pumping mode, results in the best overall efficiency and improves the stability in the power system. The reason for these advantages comes from the ability to run the group at the optimum speed (in a given speed range) of the hydraulic machine for all hydraulic conditions. The control systems proposed for a variable speed generator becomes nowadays more and more complicated. When the load is suddenly decreased by a step reduction in conductance, the current does not change instantly; however, the voltage across the load suddenly increases because of the reduction in conductance (or increase in resistance). This causes the output power to suddenly increase initially. With a rate determined by the inductance, the current decreases exponentially until a new steady value is reached establishing the new steady output power. The responses of current, voltage and power are very similar to those of velocity, head and power for a step reduction in gate position. Speed governor performs speed control functions for satisfactory operation of power plant. The main part of the dissertation is dedicated to the behavior of this variable speed generator system under varying head and discharge conditions in respect of grid stability and grid transients.
