Thermal Loading of Multi-Megawatt Medium-Voltage Power Converters Serving to Variable Speed Large Pumped Storage Units

In recent era, large rated doubly fed induction machines (DFIM) are being preferred for variable speed pumped storage power plant (PSPP), due to its high round trip efficiency and high power discharge. To operate the large rated DFIM under variable speed operation, parallel connected back-to-back (BTB) power converters are employed in rotor excitation circuit. The semiconductor devices associated with the rotor excitation circuit are considered as the most vulnerable components in large hydrogenerating systems due to its thermomechanical fatigue stress. Especially, the major failure in the BTB converters are influenced by the thermal stress across the power semiconductor devices. In order to increase the BTB converter reliability, thermal stress on power semiconductor devices must be within the acceptable limits. With respect to this prime aim, this paper implements an active thermal control based on discontinuous pulse width modulation (DPWM) technique for multi-megawatt BTB power converters serving to a variable speed PSPP (4×250 MW DFIM, to be commissioned in Tehri dam, India). The implemented active thermal controller regulates the temperature fluctuation across power semiconductor devices with respect to varying speed (active power) conditions. The efficacy of the designed active thermal control is implemented and analyzed for real-time 250 MW DFIM with 5-channel 25 MW power converters in Matlab/PLECS environment.