Control of Reactive Power for Stabilized Junction Temperature in Power Electronic Devices Serving to a 250-MW Asynchronous Hydrogenerating Unit

In order to enable a variable-speed operation in existing synchronous-machine-based fixed-speed pumped-storage power plant (PSPP), full-scale power converters are required. But it is not economical in case of large-size PSPP. Furthermore, full-scale power converters lead to increase the complexity of civil construction in case of underground powerhouses. Therefore, a doubly fed induction machine (DFIM) with a slip power rated power converter is an acceptable option for the storage dams with wide variation in water head (for e.g., Tehri Dam, India). In view of the reliability of converters used in such projects, thermal stress across the power semiconductor devices is a major cause for the failure of power converters. It is reported that 55% of failures in electric drives are due to thermal cycling or temperature rise in semiconductor devices. In consideration of this issue, this paper analyses active-thermal-control-based reactive power circulation in a five-channel parallel-connected back-to-back power converter fed 250-MW DFIM system (data collected from a 4 × 250 MW PSPP under construction at Tehri dam, India). The allowable limit of reactive power range is calculated in consideration of an operational strategy to be adopted, and the reactive power among the converters is circulated using the vector control technique. It is concluded that because of reactive power circulation among parallel-connected converters, temperature fluctuation is effectively stabilized. PLECS Blockset in MATLAB/Simulink software is used for this analysis. Experimental validation is carried out through a 2.2-kW scale-down DFIM laboratory prototype.