Multi-megawatt three-phase dual-active bridge dC-dC converter : extending soft-switching operating range with auxiliary-resonant commutated poles and compensating transformer saturation effects
Aachen / E.ON Energy Research Center, RWTH Aachen University (2019) [Book, Dissertation / PhD Thesis]
Page(s): 1 Online-Ressource (x, 182 Seiten) : Illustrationen, Diagramme
In this dissertation a high-power dc-dc converter, called Dual-Active Bridge (DAB), is investigated at a nominal operating voltage of 5 kV. As part of the research campus Flexible Elektrische Netze (Flexible Electrical Networks - FEN), a three-phase DAB test bench has been built in the test hall of the Institute for Power Generation and Storage System (PGS) at the E.ON Energy Research Center (ERC). This test bench is scheduled to be connected to a medium-voltage direct-current (MVDC) microgrid connecting several multi-megawatt test benches at the RWTH Aachen Campus Melaten. In many applications, converters operate primarily in partial-load conditions at a lower efficiency. To improve the efficiency and realize full soft switching, a modified design of the auxiliary-resonant commutated-pole (ARCP) was implemented. The new design requires only simple control and thus improves the reliability under auxiliary- resonant commutated operation. The concept is based on a non-symmetrical resonant circuit using a dc-link separated into three capacitors. Conventional ARCP systems require additional expensive high-speed boost-current control units. With the improved setup, those costs are eliminated. Losses and dead-time effects can be compensated using the new system. The technique was simulated and fully embedded in hardware. Three single-phase medium-frequency transformers are used to link the input and output converters. During commissioning, high unwanted voltage harmonics appeared, impressively proving the presence of dc-magnetizing currents. The saturation leads to high core losses, high voltage stress at the windings and increased magnetostriction of the transformer core, which can result in undesired acoustic emissions. Multi-megawatt converters with medium-frequency transformers can suffer from unwanted dc-currents causing core saturation. The cause of this saturation phenomenon is analyzed and identified in this thesis. A novel control method for avoiding transformer saturation in high-power three-phase DABs was developed, simulated and embedded in the hardware setup. In this approach, the star-point voltage of the transformer is evaluated to determine the dc components of the magnetizing currents; therefore, the need is eliminated for expensive high-precision current sensors in the ac-link, as proposed in previous studies to compensate the transformer saturation. All power electronic components are integrated in electric cabinets in a unique compact setup with a power density of 0.6 kW/dm3.
de Doncker, Rik W.
Mantooth, H. Alan