High-power medium-voltage DC-DC converters : design, control and demonstration

• Mittelspannungs-Gleichspannungswandler hoher Leistung : Auslegung, Regelung und Demonstration

Soltau, Nils; de Doncker, Rik W. (Thesis advisor); Monti, Antonello (Thesis advisor)

1. Auflage. - Aachen (2017)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2017

Abstract

In direct-current (dc) networks, dc-dc converters are used to step up or step down dc voltages and are therefore considered as the equivalent of the transformer in the classical alternating-current (ac) system. This dissertation is dedicated to the implementation of a medium-voltage high-power dc-dc converter. After modeling the converter, a fast current controller is demonstrated that sets a reference current within one third of a switching period. Moreover, a balancing control scheme is developed which increases the utilization of the converter. To increase the converter efficiency, an auxiliary circuit is designed for a medium-voltage high-power application. The prototype demonstrates a compact design and an amortization in less than one year for a wind application. The medium-frequency transformer is a key component in the dc-dc converter. The high power density and the requirements on the stray inductance make its design challenging. Therefore, especially the core losses under the influence of power electronics and the design of the stray inductance are analyzed. Moreover, a 2.2 MVA transformer operated at 1 kHz is constructed. It demonstrates a ten-fold power density compared to a 50 Hz transformer. Finally, the dissertation presents the construction and the setup of a high-power medium-voltage demonstrator of the converter. The demonstrator is rated for 5 kV dc-link voltage and a power of 5 MW. From the commissioning, valuable experience on an industrial construction of the converter are concluded, as well as future research topics.