Protection of HVDC grids using DC Hub

Abstract

Nowadays there is a movement towards the creation of HVDC grids. The massive integration of renewable energy sources, the operation of the current grid close to its limits and the need to facilitate the power trade are some of the reasons driving the research on HVDC grids. Towards HVDC grids, there are still some obstacles and technical limitations to be confronted. To address these problems, the idea of a DC hub is introduced. A DC hub is a multiport DC-DC converter, which serves as an actively controlled DC node. The basic parts of the DC hub are the ports, where AC/DC converters are used and the intermediate AC link. Until now there is no standardization of HVDC grids and thus, if different projects need to be interconnected a DC-DC conversion is necessary. A DC hub can offer solutions in the connection of HVDC lines with different voltage levels and different configurations. Moreover, it can facilitate the power exchange between any two HVDC lines connected in the DC hub. This thesis deals with the development of a DC hub model and its study under transient phenomena. More specifically, the ability of the DC hub to aid in the protection of HVDC grids is examined.
The first step to address this problem is to propose the most suitable multiport DC-DC converter topology. A literature review over different DC-DC converter topologies took place. The advantages, disadvantages and the ability to upgrade into a multiport DC-DC converter are examined. The most promising is the front-to-front topology where either a transformer or an LCL filter is used. Although the LCL DC hub offers small footprint and fault ride-through capability, its efficiency depends on the level of active power flow and its expandability is limited. In the current thesis, the front-to-front topology with an intermediate AC transformer is studied.
Before studying the DC fault response, the basic attributes of the DC hub must be acquired. A three-port DC-DC converter is designed where the state of the art modular multilevel converter is used in every port. The model operation is validated through step reference changes in the AC voltage, DC voltage, and active power. Moreover, the control role distribution is suggested, where the port with the higher power rating operates in AC voltage control mode. Being one of the main attributes, the connection/disconnection of a line without disturbing the operation of the DC grid is presented. For a smooth transition, the suggested steps are presented. The next step is to study the natural DC fault response of the DC hub. In the case of pole-to-ground faults, a design methodology is presented in order to acquire fault ride-through capability. Finally, the pole-to-pole fault contingency study leads to useful conclusions. A design methodology of adding large phase reactors between the converter and the transformer is suggested in order to limit the large fault currents.
The main contributions of this thesis are the following. First, a DC hub average model, using three modular multilevel converters in front-to-front configuration is provided. Moreover, the DC hub response to line connection/ disconnection is studied and steps are suggested for a smooth transition. Finally, after studying the natural DC fault response of the DC hub, the design requirements are provided in order to acquire fault ride-through.