The invention relates to a method of operating a hydraulic turbine or a pump-turbine (referred to as a hydraulic machine).
A hydraulic power plant, which generally generates electric power and pumps up the stored water, is frequently used to improve the power factor of the electric power system by operating a motor-generator as a synchronous condenser. In the synchronous condenser operation mode, the water level in the draft tube is moved down by using compressed air to let the runner rotate in air for the purpose of reducing the rotation loss of the runner. In a water turbine generator, the power output may readily be adjusted by merely opening or closing the guide vane, unlike a steam turbine generator. For this reason, the water turbine generator is used for adjusting the load of an electric power system. When the water turbine generator is running in the synchronous condenser operation mode in the above-mentioned manner, that is to say, by moving down the water level in the draft tube by compressed air to decrease the input, in a hydraulic power station, if a load condition of the power system suddenly changes, the synchronous condenser operation mode of the generator must be changed to a power generation mode. How to effect such an operation mode change is the important and pressing problem to immediately be solved in this field.
There has been used a mode of changing the synchronous condenser operation mode to the power generation mode in the hydraulic power station including a pumped storage power station.
In the operation mode changing method from condensor to generating the air in the draft tube and the runner chamber is firstly exhausted and then water is filled into them. Then, when the pressure within the runner chamber exceeds a predetermined value, the guide vane is opened to allow supply of necessary electric power to the electric power system. In the synchronous condenser operation mode, the runner rotates in the direction of the turbine rotation and the generator is coupled with the electric power system. Therefore, the power may be supplied to the electric power system by merely exhausting air within the runner chamber and then opening the guide vanes.
In a recent power station equipped with small sized and high speed machines, when the synchronous condenser operation mode is changed over to the power generation mode, the hydraulic machine sometimes enters a motoring region in the term of the machine characteristic because of its hydrodynamic characteristic, with the result that it does not act as a generator to generate electric power but as a motor to consume electric power. The consumed electric power by the machine temporarily reaches the approximately 50% of the maximum output. In such a situation, the operation of the hydraulic machine becomes unstable with excessive vibration and noise.