Stijn Cole’s research while affiliated with KU Leuven and other places

This paper discusses the extension of electromechanical stability models of voltage source converter high voltage direct current (VSC HVDC) to multi-terminal (MTDC) systems. The paper introduces a control model with a cascaded DC voltage control at every converter that allows a two-terminal VSC HVDC system to cope with converter outages. When extended to an MTDC system, the model naturally evolves into a master-slave set-up with converters taking over the DC voltage control in case the DC voltage controlling converter fails. It is shown that the model can be used to include a voltage droop control to share the power imbalance after a contingency in the DC system amongst the converters in the system. Finally, the paper discusses two possible model reductions, in line with the assumptions made in transient stability modeling. The control algorithms and VSC HVDC systems have been implemented using both MatDyn, an open source MATLAB transient stability program, as well as the commercial power system simulation package EUROSTAG.

Phasor based power system simulation software was originally conceived to study electromechanical interactions between rotating machinery and the network. Today, fast power electronic equipment is also commonly represented in phasor based power system software, although their dynamic behavior may violate some assumptions that lie at the basis of the phasor approximation. Modeling fast acting devices in phasor based power system software presents a number of challenges: due to the small time constants involved, time efficient simulation is a challenge. Furthermore, adverse interactions can occur between detailed models and state-of-the-art solution methods. The paper presents a robust implementation method on three levels: robustness of the tool, of the interface, and of the model itself, that leads to robust, accurate and efficient simulation of power systems with fast acting devices.

Until now most HVDC schemes have been point to point connections. A few multi-terminal schemes have been built with one extra terminal. But there have been many discussions of using HVDC for more advanced grids. The TB investigates the technical and economic feasibility to build such HVDC grids. The first question to answer is if HVDC grids offer any advantage over many point to point HVDC connections inside an AC grid. Another important question is if it will be possible to build HVDC breakers that are necessary to make the grid reliable. One more question is if one can make protections and control to the grid. These and many other challenging questions are studied in the Brochure.

In many papers, in dealing with bifurcation analysis of power systems, the power system is modelled as an ordinary differential equation system, in order to conveniently use the theory and numerical tools developed for bifurcation analysis of ordinary differential equation systems. We explain in this work how state‐of‐the‐art numerical techniques for bifurcation analysis, not well known in the power system community, can be extended to the more conventional power system model, that is a differential algebraic equation system, including power system specifics such as limits. Bordered matrices are extensively used, notably for the detection and exact localisation of singular points, as well as their continuation in two‐parameter continuation. The methods are tested on a simple power system with grid‐connected voltage source converter. Copyright © 2011 John Wiley & Sons, Ltd.

In this paper, a steady-state multi-terminal voltage source converter high voltage direct current (VSC MTDC) model is introduced. The proposed approach is extended to include multiple AC and DC grids with arbitrary topologies. The DC grids can thereby interconnect arbitrary buses in one or more non-synchronized AC systems. The converter equations are derived in their most general format and correctly define all set-points with respect to the system bus instead of the converter or filter bus, which is often done to simplify calculations. The paper introduces a mathematical model to include the converter limits and discusses how the equations change when a transformerless operation is considered or when the converter filter is omitted. An AC/VSC MTDC power flow is implemented using MATPOWER to show the validity of the generalized power flow model.

In this paper, we present a new Matlab-based toolbox for power system analysis, called MatDyn. It is open-source software, and available for everyone to download. Its design philosophy is based on the well-known open-source Matlab toolbox MATPOWER, but its focus is transient stability analysis and time-domain simulation of power systems, instead of steady-state calculations. MatDyn's philosophy, design criteria, program structure, and implementation are discussed in detail. A trade-off is achieved between the flexibility of the program and readability of the code. MatDyn retains overall flexibility by, for instance, allowing user defined models, and custom integration methods. The software is validated by comparing its results with those obtained by the commercial grade power system analysis package, PSS/E. Despite the fact that MatDyn is fairly new, it has already been extensively used in research and education. This paper reports interesting results obtained with MatDyn in recent research that would be hard to obtain using commercial software.

Europe has a clear objective of obtaining a large share of wind power in the overall energy mix. A significant part of the installed wind power capacity will come from offshore wind farms. As an alternative to connecting every wind farm to the onshore grid separately, the 'supergrid' concept has been proposed. A supergrid would allow international trade and balancing, and can accommodate renewable energy sources, such as concentrated solar power and offshore wind energy. In this paper, we give an overview of technical challenges associated to meshed multi-terminal direct current supergrids, and present alternatives and a roadmap that could expedite the development of a European supergrid.

The European transmission grid is facing an investment challenge. There is a strong call for more transmission capacity. At the same time, the investment climate is fierce and troubled by public opposition, a complex regulatory framework, etc. Many transmission capacity expansion projects are delayed or canceled. In this paper different technology options suitable for increasing transmission capacity are discussed. The aim is to provide policy-makers with information on technologies without going too much into technical details. The focus is on opportunities and limitations to implement various technological alternatives in practice, including technical solutions that go beyond constructing new connection lines. The criteria used in this technology assessment are based on the obstacles reported in the European Priority Interconnection Plan. This ensures a realistic approach based on problems encountered in real projects. Although AC overhead lines (OHL) will remain the standard solution for grid expansion, it is argued that different technology options can overcome many obstacles that OHL face. Additionally, it is illustrated that the higher investment costs for some solutions can be offset with an increased benefit, e.g. by accomplishing investments with smaller delays due to fewer obstacles encountered.