Design of a Flyback-based Input Series Converter as a Wide Input Auxiliary Power Supply for a MMC HV Generator

Abstract

The world is currently undergoing an unprecedented energy transition from fossil fuel-based energy sources to renewable energy sources. Additionally, advancements in power electronics technology have made High Voltage DC (HVDC) the most cost-effective method for long-distance electrical power transmission over both sea and land. These developments have led to an increased presence of Power Electronic Interfaces (PEIs) in the electricity grid. The insulation of High Voltage (HV) equipment in the electrical power grid is subjected to various electrical stresses due to the solid-state switching of PEIs. This can weaken the dielectric material of the HV equipment and therefore reduce the reliability of the electrical power system. Conventional test sources cannot replicate complex waveforms. Therefore, it is necessary to develop an Arbitrary Waveform Generator (AWG) capable of testing HV equipment with complex waveform shapes. A Modular Multilevel Converter (MMC) is a promising topology for this purpose, where each submodule (SM) requires auxiliary power to supply the Gate Drive Units (GDUs) and Distributed Control Units (DCUs). The objective of this thesis is to design the Auxiliary Power Supply (APS).

This thesis proposes using a flyback-based Input Series Converter as an APS. The proposed converter offers advantages such as integrated active Input Voltage Sharing (IVS), a scalable design, wide-input operation, and a minimal component count. The proposed converter topology is compared with other topologies discussed in the literature through SPICE simulations. Furthermore, various synchronous switch driving methodologies are studied and compared, given the necessity of series-connected switches with the currently available devices on the market.

A critical aspect of the design of a flyback-based Input Series Converter is the multi-winding transformer; hence, this thesis places special emphasis on the electrical and mechanical design of the flyback transformer. The transformer provides HV insulation between the SM capacitor and the output of the APS, as well as active IVS for balancing the voltage across the input stage capacitors and the MOSFETs. A prototype of the multi-winding transformer has been developed and used to assemble an open-loop converter. The converter's performance is tested with an input voltage range of 300V–3600V, and the design is further refined with an output diode snubber circuit.

A scalable, compact, and reliable topology has been identified for an HV-wide input APS for an MMC-based AWG. The topology provides suitable HV isolation between the SM capacitor and the APS output, as well as between the gate pulse generator circuit and the APS power stage. However, the transformer winding design led to a mismatch in leakage inductance between the different converter primary side stages. This resulted in a slight discrepancy in the voltage shared between the series-connected MOSFETs during the converter's turn-off stage. Future work should focus on employing a winding design that ensures equal coupling of all primary windings to the secondary winding in the flyback transformer. Additionally, it is recommended to use a coupled inductor-based passive LC snubber rather than an RCD snubber across the primary windings to introduce an active IVS process during the turn-off stage of the converter as well.