Improved start-up strategy for a doubly fed induction machine fed large rated variable speed pumped storage unit in pumping mode operation

The starting time of a large-rated variable speed pumped storage unit (PSU) operating in pumping mode is crucial in the power balancing scenario in a modern power system as it defines the transition period from generation to pumping modes and vice versa, which determines the stability of the power system. Typically, doubly fed induction machines (DFIM) are preferred in large-rated variable speed PSUs (> 100 MW) as they ensure variable speed operation through partial-rated power converters. The start of DFIM in pumping mode is challenging, and it is ensured through rotor side power converters. DFIM is typically started by short-circuiting the stator windings and supplying power to the rotor windings via power converters. The conventional starting method leads to more starting time and power consumption due to the saturation of the iron core in the earlier speed range because of the higher magnetizing inductance seen from the rotor side. This paper proposed an improved starting method in which the machine's magnetization current is supplied via stator windings and acceleration current is supplied via rotor windings. The proposed method is based on the field-oriented vector control method, and it significantly reduces the starting time and power consumption compared to the conventional starting method. The proposed method is validated through a 306 MVA DFIM-fed variable speed PSU, and results show that the improved starting method reduces the starting time and power consumption by 27.5 and 37.2%, respectively, as compared to the conventional starting method. This reduced starting time is preferred by the grid authorities as it improves the stability of the power system. Furthermore, experimental results are validated in the laboratory with a scaled-down unit of 7.5 kW DFIM.