Hybrid Modular Multilevel Converter Design and Control for Variable Speed Pumped Hydro Storage Plants

Abstract

Hybrid modular multilevel converter control and design for pumped hydro storage plants is presented, addressing application-specific shortcoming of conventional back-to-back half-bridge modular multilevel converter: high common-mode-voltage machine stress. Common-mode-voltage-free operation in the entire or partial frequency range is enabled by variable DC link voltage control, through introduction of minimal full-bridge submodule share in hybrid active front-end converter stage. The developed generalized converter design approach for arbitrary DC link voltage range operation and additional internal energy balancing control layers enable down-to-zero DC voltage control. The results are verified through highfidelity switched-model simulations of 6 kV converter, with 10/6 ratio of full-bridge to half-bridge submodules in active front-end. The analyzed hybrid active front-end stage benefits from lower converter losses for equal machine operation flexibility compared to full-bridge design, while trade-off between gridside power factor range and full-bridge submodule share is offered within the design stage. Compared to the state-of-the-art, zero-to-rated DC link voltage operation is possible at lower full-bridge submodule share (62 % against 75 %), at the penalty of reduced grid-side power factor. Alternatively, operation at higher fullbridge submodule share (62 % against 50 % existing solution) enables grid-side reactive power support over wide speed range, without branch current overload.INDEX TERMS Hydroelectric power generation, modular multilevel converter, retrofit.

Research on Simulation and Power Regulation Control Strategy of Fengning Variable-Speed Pumped Storage Unit

“…where s is the unit slip. Formula (7) shows that the total electromagnetic power of the unit closely aligns with that of the stator end. Consequently, the electromagnetic power of the unit can be managed by controlling the rotor current through the converter, or the electromagnetic torque of the unit can be controlled, enabling further manipulation of the unit's speed.…”

Calculation method of external fault short-circuit current for variable-speed pumped storage units based on time-domain dynamic circuit

“…A VSPSU is essentially a large, doubly-fed induction motor, and its characteristics are similar to those of doubly-fed wind turbines, but the excitation system of VSPSUs is more complicated. Referring to VSPSPPs that have been put into construction (e.g., 300MW VSPSPP, Fengning (China) and 250 MW VSPSPP, Tehri (India)), VSPSUs often use multiple sets of parallel neutral-point clamped three-level back-to-back converters for excitation (Desingu et al, 2019;Basić et al, 2021). In addition, in order to protect the converter and the unit, the excitation system of VSPSUs is also equipped with jumper protection, which is similar to the crowbar protection of the doubly-fed wind turbine, and the jumper will act when serious external short-circuit faults occur.…”

World's First Fully Rated Direct ac/ac MMC for Variable-Speed Pumped-Storage Hydropower Plants

“…Until recently, CFSMs were facing challenges due to inadequate converter terminal voltage (compared to typical machine terminal voltages), requiring dv/dt filters and transformers on the machine side, making the pump start difficult due to transformer saturation, as in the world's first CFSM PSHP Grimsel 2, which was commissioned in 2013 and with a three-level VSC SFC rated 100 MVA [13] or multilevel monolithic VSCs [14], which may still require dv/dt filters, series-connected semiconductors, potential additional passive components for mitigating circulating currents between parallel converters and higher losses compared to MMCs. Indirect MMC has only been considered [15], [16], [17], but so far to our best knowledge none has been commissioned. With a direct ac/ac MMC, the SM is directly connected to a SFC with a very low harmonic content, which leads to the following simplified single-line diagram (cf.…”