ANALYSIS OF DYNAMIC BEHAVIOR OF VARIABLE SPEED PUMP STORAGE UNITS UNDER CONVERTER FAULTS

Abstract

Several possible technologies for electricity storage are developed including high energy batteries, flywheels, superconducting magnetics, compressed air, and Pumped Storage Power Plants (PSPP). Among the mentioned technologies, PSPP is considered as reliable and bulk energy storage system. The PSPP’s that are constructed in the beginning of the 20th century in the European continent were of fixed speed type employing synchronous machine and the same were continually established in Asian and American continents also. The total installed capacity of fixed speed PSPP in India is 4804 MW out of worldwide capacity of 140 GW. However, the fixed speed PSPP suffers from major drawbacks including: (i) inability to generate power over full range of water head, (ii) reduced efficiency during partial generation/pumping modes of operation. The aforementioned drawbacks can be overcome by the transformation of PSPP from fixed speed mode to variable speed mode. In order to enable variable speed operation, synchronous machines of fixed speed PSPP need to be driven by power electronic converters with a rating equivalent to the rating of machine. Such design of high capacity power electronic converter is not economical. Furthermore, these full size converters (> 200 MW) are very challenging in size, cost and site clearance in case of underground power houses. Therefore, variable speed PSPP employing Doubly Fed Induction Machine (DFIM) is an acceptable option for the sites with wide variation in water head since they provide variable speed operation with reduced power converter rating and high dynamic stability. In India, the first variable speed PSPP (with 3-level VSI) having 4 nos. of 250 MW DFIM totaling to a capacity of 1000 MW is under construction at the Tehri dam of Uttarakhand state. In DFIM, rotor side power converters act as excitation system and control the real and reactive powers of the machine based on set points (reference) and feedback signals from various sensors. A comprehensive literature survey is carried out in the area of power converter topology, modulation techniques, parallel converter schemes, circulating current reduction techniques, machine control, grid disturbances, protection of power converter, fault analysis, fault tolerant control and power converter redundancy techniques. In addition, operational challenges for the power converter redundancy and the protection circuit is studied through simulation and experimental tests. Smooth starting/regenerative braking of the DFIM unit is discussed with real and reactive power consumption/delivery. Time required during smooth starting and regenerative ii | P a g e braking of large rated DFIM unit plays an important role in transition from generating to pumping mode and vice versa. Furthermore, it is beneficial for the better management of grid operation and energy balancing. An Energy efficient method for starting of DFIM fed pump turbine is discussed. Variable voltage/frequency applied in rotor side and fixed dc supply provided in stator circuit during starting, saves considerable amount of energy compared to conventional smooth starting. It is observed that 35% of electrical energy shall be conserved in comparison with conventional method during starting. Dynamic behavior of power and control circuit (excitation system) faults of a 250 MW DFIM hydrogenerating unit, to be commissioned in 1000 MW Tehri PSPP, operating at generation, pumping and condenser modes are discussed. In addition, survivability status of power and control circuit faults of DFIM at said modes are assessed based on performance measures. Further, Economic analysis of 1000 MW PSPP under power and control failures are also investigated. The present work also investigates fault tolerant operation of 250 MW DFIM unit at open switch fault in converters to increase the continuity of the unit operation, where power electronic converter redundancy is not available in large rated DFIM unit. Open switch fault is detected through Park’s vector phase currents technique and variation in dc link voltage. An experimental set-up with 2.2 kW DFIM is developed in the laboratory to support the simulation results. Overall, the present research work shall be helpful to the project authorities/policy makers in hydropower engineering during the design stage of their future projects.