Second biannual progress report

WP 1: Inventory


TUD-EPP: Analysis of the implications of system architecture choice inside the NSTG project. There is a lot of research on the benefits of both modularity and integrality but the evaluation of which is better and when it needs to be applied is often subjective, qualitative, or speculative. TUD-EPP Report 8 contains the analysis of how can the NSTG system grow with time by applying the tools utilized in modular architecture systems. If one analyses the complexity involved in the development of a system such as the NSTG, it is immediate that an integrated architecture is evidently not the most appropriate for development and construction of the system. Modifications to features or components are likely to occur regularly during the growing phases and there should be little, if any, redesign of the whole system. One possible solution for this inconvenient is to change from an integrated system development architecture to a more modular one. Usually, in the timeline of the design of a modular complex system, the architecture is the first element to be defined. Next are the interfaces and control structures between the system's modules. The last element to emerge usually is the comparative tests of individual modules performance.


WP2: Techno-Economic Evaluation


ECN: An inventory has been made of the wind farm locations and sizes by the different countries in the North Sea (ECN-E–10-072). The total amount of wind power is about 56 GW. Based on these locations and the location of HV lines and substations on land, a development plan of 56 GW wind power has been made. In this plan the interconnection of the wind farms and the transnational connections have been included. The size of the required connections has been determined (wind farms to land) or estimated (all other interconnections). The result has been compared to the Northe Sea offshore network proposed by EWEA. This report will be the starting point to build the steady state models for the different stages of the North Sea Transnational Grid.


WP3: Multi-Terminal Converter


 TUD-EPP: A model of a three-terminal DC network composed of VSC-HVDC transmission schemes was build and simulated for Matlab Simulink. The most important hardware inside the VSC were included in the simulation as well as the complete control structure necessary to operate the converters inside multi-terminal DC networks (MTDC). A description of the control structure of VSC-HVDC transmission systems together with the deduction of the maximum obtainable closed-loop bandwidths for the inner current controller and the respective outer controllers was prepared (TUD-EPP Report 7). For the voltage-source converter (VSC) it is important to understand that the obtainable system performance provided by control actions will be in strict relationship with the converter switching frequency. Only after understanding the constraints set by the system physical parameters can the control parameters be establish through the analysis of the system's transfer functions. An important conclusion from the performed study concerns the fact that in a multi-terminal DC network it will be most likely impossible to operate the system with just one station controlling the DC voltage and there are mainly two reasons for that, viz.: limited power rating of the VSC station controlling the DC voltage and the possibility of AC faults in that station.


 ECN: A dynamic model of a wind farm with variable speed turbines (permanent magnet generators with full converter) connected to a Voltage Source Converter HVDC connection to shore has been implemented. This model is intended to simulate the effect of transients in the offshore DC grid, for instance caused by AC grid faults and by the DC grid control actions. This model still has to be developed further and will in a later stage of the project be integrated with the multi-terminal DC grid. This model will also provide input for the optimization stage (WP5).


TUD-EPS: Wind power dispatching methods for multi-terminal HVDC networks have been investigated. One specific way to control power flows in dc systems is the voltage-margin method. The particular benefit of this control method is its robustness: by applying a voltage margin between the onshore terminals, control hunting is prevented. Other benefits include insensitivity to single converter failure and communication delays. The following control strategies were explored: fixed power exchange, priority power sharing and proportional power sharing (the latter realized via supervisory dispatch). An important recommendation was that a high degree of flexibility is introduced when using this method by adjusting the steady-state characteristics accordingly as the offshore dc network is extended.


WP6: Grid


TUD-EPS: The efforts to include multi-terminal VSC-HVdc systems into stability simulations have been continued by focusing on so-called hybrid simulation methods. Occurrences in dc grids may cause severe oscillations in the connected ac systems which may affect transient stability. These most notably include protective behavior of individual VSCs as well as that of the entire HVdc scheme. It is therefore critically important to include these phenomena into grid integration studies that focus on dynamics, which are usually performed by stability-type time-domain simulations. As these rely on a simplified representation of the network and its connected devices, dc systems are hard to include in existing simulation tools. A more detailed simulation approach must be employed, such as the electromagnetic transients (EMT) type simulation. With this method, the  ability to simulate large networks is however limited and detailed information on connected equipment is often not easily available. A solution is to combine both simulation types by modeling the ac system by stability simulations and to include converters and dc systems by EMT-type simulation. At some points and/or areas in the network these simulations are interfaced with each other. The interfacing methods to be used determine the performance (speed and accuracy ) of the entire hybrid simulation. This is the main focus of this line of research. The present state is that the simulation test environment has been finished and will be described in future contributions (currently abstracts accepted for PSCC and EWEC 2011). The  VSC controllers developed by TUD-EPS in WP3 can be conveniently included in this method. The developed hybrid simulation method constitutes the basis for assessing the dynamic interaction between a transnational offshore grid and the future onshore power system.  


In this respect, TenneT has committed to provide a network model of the Dutch (and Western European) power system, which includes foreseen network reinforcements and accurate generator representation via dynamic models. Together with one to two Master students we will include future offshore wind scenarios and offshore network topologies, most notably those provided by WP2, into the model provided by TenneT. The network model is built using a software package called PSS/e. This part of WP6.2 is closely related to the work being done in WP6.1, which will provide information on the commitment of conventional power plants as well as the actual wind power plant outputs. The resulting load flows can provide the initial conditions for the time-domain simulations. We intend to recruit an additional researcher starting in Feb. 2011 to focus mainly on WP6.1. During the course of the M.Sc. project (February 2011 – November 2011), both the load flow and dynamic simulation studies will start.


WP7: Market analysis


ECN: Following the suggestion of the Advisory Group, a draft work plan for WP7 has been composed (Costs, benefits, regulations and policy aspects related to NSTG. F. Nieuwenhout). After discussion in the project team, this plan will be discussed with (part of) the Advisoty Group.


WP9: Dissemination of Results: TUD-EPS: Arjen van der Meer has attended the 9th International Workshop on Large-Scale Integration of Wind Power in Power Systems and Transmission Networks for Offshore Wind Power Plants, in Quebec City, Canada, Oct. 2010 and presented two papers:

“Offshore Transnational Grids in Europe: The North Sea Transnational Grid Research Project in Relation to Other Research Initiatives” and “Control of Multi-Terminal VSC-HVDC for Wind Power Integration Using the Voltage-Margin Method”. He also presented at the Power and Energy Society General Meeting 2010 in Minneapolis, USA with the contribution “Combined Stability and Electro-Magnetic Transients Simulation of Offshore Wind Power Connected through Multi-Terminal VSC-HVDC”.


WP10: Coordination

ECN: The first Advisory Group meeting was organised at Schiphol on 16 June 2010.