D. Van Hertem

Phase shifting transformers: principles and applications

The purpose of this paper is to give a short overview of existing technologies regarding phase shifting transformers (PSTs). A classification is made based on the symmetrical or asymmetrical and on the direct or indirect character of the PST. As a case-study, the PSTs in Meeden, The Netherlands are studied more profoundly. Furthermore, a model is developed on a real-time digital simulator (RTDS) in order to demonstrate the capabilities of the PST

Transmission System Topology Optimization for Large-Scale Offshore Wind Integration

A method to determine the optimal transmission system topology for multiple offshore wind farms is introduced using a rule-based generic algorithm. The methodology is implemented as a software tool determining the best economical and technically feasible offshore transmission solution. The optimization algorithm takes radial, ring shaped, and meshed transmission topologies into consideration. Besides the investments for the offshore grid also new connections or reinforcements in the existing onshore grid are proposed by the algorithm. The developed tool delivers a set of transmission system topologies ranked after their life cycle system costs with their corresponding transmission voltage and technology (HVAC or HVDC), the necessary transmission equipment, and its rating. The optimization results are based on publicly available data. The achieved results are compared with other studies and validated. The developed tool can be used in the planning phase as a guidance for offshore developers with investment decisions and for long-term offshore grid planning, e.g., towards an offshore supergrid.

Grid Impact of Voltage Control and Reactive Power Support by Wind Turbines Equipped With Direct-Drive Synchronous Machines

Summary form only given. This paper proposes voltage control and reactive power support with direct-drive synchronous machines in wind turbines during normal operation and transient events. The wind turbines are equipped with voltage and reactive power control for normal operation and undervoltage ride-through modes for voltage dips. The wind turbine contributions are evaluated both for theoretical test cases as well as for two specific locations in the Belgian grid. Simulations reveal that the preferred mode for voltage support during a voltage dip depends on the grid characteristics short-circuit power and X-R ratio. Additionally, the angle stability of both induction motor load and nearby synchronous generators can be improved by adding reactive power support by the wind turbines. From the Belgian case study, it is concluded that voltage control by wind power plants is preferred, especially in remote areas where the additional control becomes necessary to maintain operation in between voltage limits. In case of voltage dips, a co-ordinated control of wind power plants in the area lowers the voltage reduction.

Choosing the Correct Mitigation Method Against Voltage Dips and Interruptions: A Customer-Based Approach

Voltage dips and interruptions may cause major economic damage, not only can they create considerable loss of production (manufacturing) or data (ICT), there is also loss of market, the loss of client trust, comfort, etc. that incite energy consumers to implement certain forms of protection for their systems. However, the selection of the most cost-effective mitigation method is difficult because of the wide range in available protection devices and the mostly unknown and variable interruption cost. In this paper, the theoretical selection method is compared with the currently used selection methods in industry, showing a discrepancy between theory and practice. Using an alternative selection method, an optimal mitigation method can be found. This paper describes this method and its use by means of a practical example. The case examined is the protection of a 250 kVA installation, sensitive to voltage dips (ICT load). The proposed method makes a cost-benefit analysis of several proposed mitigation solutions resulting in an overview giving the optimal solution for a certain interruption cost interval. It enables to clearly interpret the investment costs and to compare completely different mitigation methods, only using data available to an industrial customer

Effect of Demand Response on transformer lifetime expectation

Demand Response is seen as important to support the integration of renewable energies into the grid. In Flanders, a residential Demand Response setup is realized in the Linear pilot. The aim is to assess the potential benefits and ways of technical realization of residential Demand Response. In this paper, household devices, like washing machines, are used to offer a flexible load. These are also devices which are used in the pilot. The effect of using flexible loads on the lifetime of a low-voltage transformer is assessed. An IEEE transformer model is used to calculate the lifetime. To calculate the effect of Demand Response, aging is first calculated based on the load of a group of customers and then based on their load being optimized by Demand Response. In this paper, devices are scheduled based on the transformer temperature. The temperature is optimized by using a simulation model based on a mixed integer quadratic programming (MIQP) scheduler. To assess the effect of Demand Response on the transformer lifetime, aging for the improved load curve is compared with aging for the initial load curve. To demonstrate the impact, realistic data for household load curves and the usage of household devices are employed. Results for this input data show reductions in aging of up to 75 % for transformers operating at rated load. The setup will be used to calculate a benchmark for the setup in the Linear pilot, which will use an on-line scheduler. It will be also used to determine potential outcome of a business case.

Technical developments for the future transmission grid

Recent technical and political developments require investments in the transmission grid. Up to recently, the only solutions for grid reinforcement were transmission lines and underground cables. Today much more technologies are or become available. The rising environmental concerns and the difficulties of obtaining right-of-way show that an assessment of these technologies on a mere techno-economical basis is no longer sufficient. This paper investigates overhead lines, new conductor types, underground cables, conventional ones as well as gas insulated and superconducting, FACTS and HVDC on a technical, economical, political, social and environmental basis. It can serve as aid in selecting the most appropriate technology for a given case

Power flow controlling devices: an overview of their working principles and their application range

The ongoing liberalization process causes an increase in international power flows, putting substantial strain on the transmission grid. This is especially a problem when unidentified power flows or loop flows are involved. At this moment, several congestion problems arise in the European grid. Controlling power flows is one way to extend the utilization of the current grid, without investments in new transmission lines, being problematic due to political, environmental and social considerations. In this respect, power flow controlling devices are rapidly gaining interest of utilities and transmission system operators (TSO). New devices are installed, and it is expected that their number will rise significantly in the near future. This paper provides an overview of the current available power flow controlling devices and their applications. Both electromagnetic transformer based solutions as the fairly recent power electronics developments are covered. Special attention is given to HVDC, both line commutated and voltage source converter based, as they are able to fully control the power flow. In a final part, the use of multiple controllable devices in a meshed grid is treated with special emphasis on the need for global control. As a conclusion different technologies are compared

Border-Flow Control by means of Phase Shifting Transformers

In this paper, a control scheme is demonstrated that regulates multiple phase shifting transformers (PSTs) to equally load the interconnectors of a border. A crucial step in the development of the control scheme is the derivation of phase shifter distribution factors (PSDFs), which indicate the influence of a PST on the active power flow on a certain line. Based on these PSDFs, the Linear Least Squares (LLS) method is used to calculate the optimal PST settings. The degree to which an even repartition can be obtained, depends on the number of PSTs in relation to the number of interconnectors. As a case- study, simulations are performed involving the Dutch and Belgian interconnectors.

Building a new overlay grid for Europe

The strong growth and the expected further increase in renewable energy generation in Europe requires a fundamental upgrade of the transmission system. A potential option is to realize these upgrades at a higher voltage by constructing a new overlay grid or supergrid. Such a supergrid is likely to be built using VSC HVDC technology, especially when considering that a significant part will be offshore. The paper will first discuss how such a grid should be conceived. Aspects that will be covered are the potential topology of such a grid and the balance between mesh size and the connection to the AC system. It will be discussed how such a new overlay grid with multiple controllers interacts with the existing system. The second aspect that will be covered is the stepwise development of such a system. A new overlay grid will not be conceived in a single phase, but link by link. The differences between “green field” and “brown field” approach are highlighted and shown using an example.

Technology and Topology Optimization for Multizonal Transmission Systems

This paper presents a method to optimize equipment investments in multizonal transmission systems, considering spatial properties of the areas of focus. Together with a probabilistic technique for assessing nodal injection capability, the method in the paper completes the methodology of a long term transmission system planning tool. Transmission topology, line route and technology are optimized through iterative application of linear integer programming and Dijkstras shortest path algorithm. By optimizing the transmission route, the spatial properties of the area of focus are taken into account, which in turn can significantly influence the installation costs of transmission equipment. The optimization method considers both AC and DC technology and takes the N-1 security criterion into account.