Highly Flexible Indirect Modular Multilevel Converter for High Power Pumped Hydro Storage Plants

Abstract

As fossil-fuel-and nuclear-based generating facilities are being phased-out in favor of intermittent renewable energy sources, the need for large scale, highly flexible balancing storage is imminent. By the generating capacity of readily available units, enormous potential lays in retrofitting of large pumped hydro storage plants to variable speed operation, increasing their flexibility to meet future power system demands. Indirect Modular Multilevel Converter is a highly scalable and inherently redundant topology that meets the power and voltage ratings of large pumped hydro generating units. Variable DC link converter operation, offering common-mode-free machine-side operation has been evaluated, considering constraints in retrofit of old machines, not designed for converter-fed operation. Performance evaluation over the entire operating range of the plant is performed. Grid support dynamics has been assessed. Finally, application-specific challenges in the design stage are pointed out.

Distributed Real-Time Model of the M3C for HIL Systems Using Small-Scale Simulators

“…10 can be defined. The main objective to reduce the number of state space matrices has successfully been achieved as with the splitting, this number was reduced to 3 × 2 6 .…”

“…The high power requirements of PHSP constitute a challenge for Variable Speed Drive (VSD) due to the converter requirements, making conventional converters unsuited for such applications. However, the rise of the Modular Multilevel Converter (MMC) technology [5] allows, due to its modularity which relies on the series connection of cells, each with its own capacitor, to be scaled to the power and voltage requirements of PHSP [6], while current scaling can be achieved as explained in [7], [8]. Additionally the elimination of the connected transformer as well as the This paper was submitted on the 25 th of August 2021.…”

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

“…CM voltage-free low frequency operation of a machine-side MMC is enabled through variable DC link voltage operation [34], [23]. In the VSD application, only positive DC voltage values are of interest, defined henceforth by the factor k DC ∈ (0, 1] such that V DC = k DC V DC,n , as briefly analyzed hereafter.…”

“…CM-voltage-free operation can be performed by varying the DC link voltage with output machine frequency, in the range (0, V DC,n ], V DC,n denoting rated DC voltage value [22], [23]. However, HB-based MMC Active Front-End (AFE) operation is limited to a narrow band around V DC,n , owing to fixed grid-side AC voltage amplitude.…”

“…Operation at variable DC link voltage has also been studied specifically with VSD application in mind. An extreme case utilizing a FB MMC as AFE is presented in [22], [23], enabling [−V DC,n , V DC,n ] operating range while doubling AFE stage losses. In [34], constant DC voltage is chopped by a DC link switch, to achieve desired average V DC , while in [35] further SM capacitance reduction through decreased SM average voltage operation has been studied.…”