Digital Fault-Tolerant Infrastructure for the Control of Electrical Networks

An Interdisciplinary Research Project

The energy supply of the future is characterized by the increasing share of renewable energies and the replacement of fossil fuels. The volatility of many renewable energy sources (wind, solar) contradicts to the increasing demand due to electromobility, hydrogen production and many more. To ensure security of supply in the medium and long term, our energy supply must be redefined for these new requirements.

To address this challenge the project DEFINE has been set up within dtec.bw – Digitalization and Technology Research Center of the Bundeswehr. The project is dedicated to the development of fault-tolerant energy supply systems through the use of digitally controllable MVDC networks. A uniform solution approach is being pursued and, for the first time, joint optimization is being targeted from the system to the component level. The research fields include secure IT systems and control structures as well as power electronic hardware and building structures for converter stations. In addition to the focus on efficiency, the security of electrical power supply is the central topic of the project. It is addressed in two ways: The aim is both to ensure uninterrupted power supply in the event of grid faults, such as short circuits, and to protect against cyber attacks on this critical supply infrastructure.

 

© University of the Federal Armed Forces Munich, Prof. Dr. T. Brückner - istock.com/ASKA

 

The concepts and components to be developed will be designed as well as tested for the pilot scenario of an urban medium-voltage DC grid. The technological core questions will be addressed on three levels,

1. the system or grid level:
   • How to make the utility grid secure and stable?
2. the station level:
   • How to best integrate a converter station into an (urban) environment?
3. the module and component level:
   
   • How to reduce losses and cost and increase reliability?

 

Six institutes of both universities of the German Armed Forces (Munich and Hamburg) are participating in the project, the overall project lead is in the hands of Prof. T. Brückner. At the Laboratory of High Power Electronics Systems, within the project, a novel MMC submodule based on space- and loss-saving technologies is developed and the resulting system behavior and system design for an urban converter station are investigated.

 

System / grid level

In order to obtain universally transferable results regarding controllability and protection, all investigations are carried out on a generic, meshed example grid (Fig. 1). The proposed grid structure consists of three key components: 1. MMC with full-bridge functionality for coupling the DC network with AC networks, 2. passive nodes with disconnectors and grounding switches for each connected line, 3. active nodes containing power flow controllers (PFC) in addition to the components mentioned above. For complete current control in the three meshes of the grid, three active nodes with PFC are required, which in this example are located next to the MMC.

Fig. 1: General structure of a meshed MVDC grid

 

It can be shown that an uninterrupted DC power flow during single pole-to-ground faults can be enabled by smart fault handling using MMC and PFC, only [1].

Since the dynamics of the DC grid are controlled by its power electronic components, their energies must also be controlled. In the developed DC grid simulation model, the MMC and PFC energies are therefore also represented with high temporal resolution. Fault events or changed requirements of the connected AC grids force the DC grid to be operated in a new state. For such transients between operating points, an innovative method was developed and validated by simulations [2].

The proposed MVDC grid structure is designed to couple and support existing AC grids, mainly. In later steps, medium-size renewable power plants or storage systems could be directly integrated into the grid, as well as links to other DC voltage levels (LVDC, HVDC) to form part of a universal DC grid.

 

More Information about DEFINE (in German) can be found at the dtec.bw-Webpage.

 

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