DESIGN AND OPERATIONAL CHALLENGES IN VARIABLE SPEED PUMPED STORAGE PLANT

Abstract

Energy storage technologies play significant role in energy systems and are important tool in achieving a low-carbon footprint. These technologies allow for the decoupling of energy supply and demand thereby provide a valuable resource to system operators. Among various energy storage technologies, pumped storage plant has come up as matured energy storage technology available in MW range. The global installed capacity of pumped storage unit has increased around 158 GW. In grid level energy storage system, pumped storage power plant is adopted in view of (i) large energy storage option in MW range, (ii) ability to provide flexibility to the power system, and (iii) load balancing thereby ensuring grid stability and reducing the risk of black-outs when critical disparities occur between supply and demand. Fixed speed synchronous machines that are employed for pumped storage applications have demerits like drop in operating efficiency at partial generation/pumping mode of operation, requirement of separate SFC for starting, no power regulation in pumping mode etc. With the advancement in power electronics, Variable Speed Pumped Storage Hydro Plants (VSPSHP) has emerged as state of art technology in pumped storages. In variable speed technology, adjustment in the speed of turbine throughout the operating range in generation/pumping mode allows to get maximum efficiency, better dynamic response, power regulation during pumping mode etc. Variable speed operation with synchronous machines employing full rated converters above 100 MW is not viable due to converter rating limitations, cost escalation and increased physical sizing of power house. Doubly fed asynchronous machine (DFAM) based VS PSHP surpasses limitations of synchronous machine based VSPSHP and are gaining prominence all over the world. The outline of thesis is identification and analysis of the design and operational challenges associated with variable speed pumped storage plants. Besides, major benefits associated with operation of VS PSHP along with its dynamic performance have been elaborated in comparison to FS PSHP. Enhancement in fault ride through capabilities of DFAM based VS PSHP with implementation of two-stage protection has been examined. In the present work, major benefits offered by variable speed pumped storage unit in generation, pumping, and synchronous condenser mode of operation have been evaluated. Based on analysis for a 250MW VSPSHP (Tehri variable speed PSHP, India with the speed variation of -10.73% to +8.33% and head variation of 90 mtrs.), it’s summed up that variable speed operation provides additional generation of 3.81% per year and additional energy storage of 9.64% per year in comparison to fixed speed PSHP which get translated to 3.85 M$ annually in financial terms. Furthermore, other advantages of VS PSHP like quick voltage and frequency support even in pumping mode; faster ramp rates, faster mode transition, better power fluctuation control under torque perturbation and dynamic performance are highlighted. As part of electro-mechanical design challenges, selection of pump- turbine, designing of components of motor-generator, GCB, Excitation system etc. is outlined. During design & engineering stage of VSPSHP, criteria for various civil structures like surge, water conductor systems, site selection etc are elaborated. Ecological challenges posed by PSHP viz. change in morphology of reservoir, destabilization of reservoir banks, biological impacts on habitat of aquatic flora and fauna etc. have also been put forth. On part of operational challenges, the present research works draws attention towards vitality of power converter redundancy in large rated DFAM and analyse reasons behind its absences. Variable speed drives failure rate of around 90% is associated with faults in power converter and control circuits which includes power devices failure, driver circuit failure, failure of dc link capacitor, sensor failure and controller failure. Despite, there is no full converter redundancy available in any of the operational DFAM based variable speed pumped storage plant in the world having parallel converter system. The present work further studies protection circuits designed for power converters in large rated DFAM based VS PSHP. A two-stage protection circuit using dc-link braking chopper circuit and crowbar protection circuit is evaluated along with its operation strategies. To examine dynamic performance and operation strategy of two stage protection, a 250-MW DFAM hydro-generating unit equipped with a five-channel ii | P a g e 3L-NPC back-to-back power converter is designed and implemented in MATLAB/Simulink environment. Survivability of the drives against various grid disturbances is demonstrated The obtained simulation results confirm the viability and effectiveness of the designed methodology (Two-Stage protection scheme) for variable speed pumped storage unit under marginal grid disturbances. In addition, results obtained from MATLAB/Simulink are verified through a 2-channel 3L-NPC back-to-back power converter fed 2.2kW DFAM unit in laboratory experimentation. To summarise, the present research work emphasise the critical role played by Variable speed pumped storage plants in stability, security and operation of RE-rich grid. Benefits offered by variable speed operation of a reversible pump turbine in generation mode and pumping mode are demonstrated in context of Tehri PSHP along with financial paybacks. Additional benefits of VS PSHP like faster ramp rate, better dynamic performance during grid faulty, rapid mode change, frequency regulation in either mode etc. are highlighted. A comprehensive design and operational challenges associated with VS PSHP are discussed. A two-stage protection strategy against severe grid disturbances has been designed & analysed for large rated DFAM. The present research work shall surely assist project developers, State agencies, regulatory bodies, research scholars etc to have comprehensive insight of a VS PSHP project right from conceptualisation to its operationalization and evaluate Variable Speed options for various PSHP sites on merits basis.