THERMAL PERFORMANCE IMPROVEMENT IN MULTIMEGAWATT POWER CONVERTER SERVING TO ASYNCHRONOUS HYDROGENERATOR

Abstract

Pumped storage power plant (PSPP) is a mature, cost-effective, bulk and oldest form of grid level energy storage technology with a unit capacity of more than 250 MW. PSPP’s are classified into: (i) fixed speed generating unit by employing salient pole synchronous machine (SM), and (ii) variable speed unit equipped with doubly-fed induction machine (DFIM). Fixed speed units suffer from major drawbacks such as inadequate power generation at wide water head variation, torque perturbation and drop in efficiency at part load operation. To overcome these drawbacks, variable speed PSPP are installed at the sites having wide water head variation. In India, the first variable speed PSPP having 4 units of 250 MW DFIM with the capacity of 1000 MW is under construction at the Tehri Dam of Uttarakhand state. In large-rated DFIM, parallel connected power converter is opted in rotor excitation to handle the high amplitude of rotor current. Any failure in the power converters results in excitation failure and it affects the plant operation. Notably, as of now, power converter redundancy is not yet implemented in any of the commissioned variable speed PSPP. In order to overcome the redundancy challenges in variable speed PSPP, reliability of power converter needs to be improved. Initially, comprehensive literature survey is carried out in power converter reliability and studied the factors affecting the life reduction of power semiconductor devices. From the literature, it is found that increase in the amplitude of temperature fluctuation and the mean junction temperature level leads to lifetime reduction of semiconductor devices. It is found that 55% of failure is mainly due to temperature factor. Moreover, thermal performance on multi- MW parallel-connected converters serving to large rated DFIM unit (e.g., 250 MW) in varying water head conditions is not yet studied in detail. For this reason, thermal stress analysis for the power converters serving to variable speed PSPP under water head variations is performed in this research.