4th Progress report

WP 1: Inventory
All: In this work package an inventory of the relevant technology for the different work packages has been made. This is an ongoing process and will be reported in the individual work package reports.

WP2: Techno-Economic Evaluation

ECN: As first step in the Techno-Economic Evaluation a relatively simple business case has been investigated, as was suggested during the first Advisory Group Meeting in 2010. This case compares two main options for the "Cobra cable" between the Netherlands and Denmark. The next figure shows the first option. The case showed that results for the transnational connection are especially sensitive to the relative power of this connection with respect to the wind power connections. The results have been presented at the second Advisory Group Meeting in August 2011.


Figure 1: First option of the Cobra case.

Secondly, the first phases of the NSTG and the Separate Wind and Trade Scenario have been built and analysed in EeFarm.



WP3: Multi-Terminal Converter

ECN: A coupling has been made of a dynamic model of a small MTDC system with four VSCs and a wind farm with PM generators and full-rated VSC converters.  The setup of the MTDC model is modular so that the model can be split up in the real-time simulator used for validation in WP4. Other features developed are automated initialization and post-processing, which makes it easy to modify or extend the model. The first simulations of a congestion of the produced power in the DC-grid showed proper control of the MTDC converters using a simple power control scheme. Several different power control schemes should be developed and tested for more complex MTDC networks.


Next step is to investigate different control options for wind farms connected to MTDC grids. Already a selection of suitable combinations for converter and wind farm technologies has been made for controller development. Also the model will be linearized so that it can be applied for designing MTDC controllers.


TUD-EPP: The dynamic model of the multi-terminal HVDC (MTDC) transmission system composed only of voltage-source converters was used in order to compare four different methods for controlling the DC voltage inside MTDC networks. The compared methods are the most found in the corresponding literature, i.e.: droop control, ratio control, priority control and voltage margin control. The developed work is the foundation for the Master Thesis work of Sílvio Fragoso Rodriguez, titled “Dynamic Modeling and Control of VSC-based Multi-terminal DC Networks.” Additionally, the MTDC grid model was updated to be directly obtained from its state-space representation which simplifies the models’ implementation in simulation software packages.


For large DC grids the best control strategy will be the one with good dynamic behaviour, high flexibility and expandability, and low communication requirements. From all the methods analysed, none has shown all the necessary requirements combined. In order to ensure a secure and reliable DC grid operation in future large offshore multi-terminal networks it seems necessary to distribute the DC voltage control responsibility to more than one converter terminal. Thus, as a conclusion from the work a possible solution could be to combine different DC voltage control strategies.


Additionally to the dynamic models, a DC load-flow program was developed. The idea behind the load-flow analysis is to show that a configuration where all the HVDC stations onshore function as DC voltage regulators is superior to the case where only one or a couple of HVDC stations are left with the task of balancing the power within the network.


The DC load-flow analysis was performed for a possible NSTG layout involving the countries with the highest expected offshore installed capacity. It was shown that a control strategy where more than one node is controlling the DC voltage inside the MTDC network is superior when compared to a strategy in which only one node is given that task, regarding N-1 contingencies and the overall losses in the transmission system.


WP4: Multi-Terminal Converter testing


TUD-EPP: For the testing phase, the EPP group has acquired two small three-phase AC/DC converters from Belgium company Triphase. A third converter will be added to the setup in a later stage. Initially the converters will be connected in back-to-back in order to test the operation and control strategies developed in the previous task (WP3) and acquire stand-alone measurements for converter model validation. Later, the small scale multi-terminal converter will be connected to the OPAL-RT simulator at JRC in Petten for final testing including the ECN wind farms models of WP3.


The VSC-HVDC dynamic model developed in WP3 was updated to include the protection measurements necessary to safeguard the real small three-phase AC/DC converters. That includes the development of a current limit strategy, to protect the small-scale converter IGBTs against overcurrent and the inclusion of the VSC capability chart in the control schemes. The inclusion of the  capability chart in the controls guaranties that the VSC voltage magnitude (modulation index) is limited, which protects the converter from surpassing its rated power (overloading).


WP5: Optimization.


TUD-EPP: This work package is delayed due to problems regarding the visum of the optimization tool expert dr. Kumar. TUD is working on a solution, either by having the work done in India or by organising assistance from the Mathematics Dept. at TUD.


WP6: Grid


TUD-EPS: Work package 6.1 focuses on integrated ac/dc security analysis. This method can be applied to planning as well as operation of power systems. In the ac part of the grid, the n-1 contingency analysis will be performed with classical linearised load flow techniques, whereas the dc grid will be analyzed with a load flow method particularly suited for dc networks. The analysis adopts a round-the-year approach by combining market simulations with the security analysis, in order to capture the various uncertainties in load, wind and solar profiles. This means that the security analysis is repeated for every hour of the year. Before performing the security analysis on a detailed grid model, a sensitivity analysis with respect to capacities and topologies of the offshore grid will be done by using the market simulation tool. The results of this study enable accurate identification of bottlenecks in the system and decision making with respect to their corresponding mitigation options. The completed work on this subpackage was in the last 6 months devoted to data preparation (realistic wind and solar power time series), and market and network model building.


Work Package 6.2: During the past 6 months, the part of the research that considers inclusion of dc networks into stability simulations was directed towards a generic implementation of the combined EMT-stability simulation method in Matlab. The method is adequate for three-phase short circuits, which have the most severe impact on first-swing stability (period immediately after a fault). Besides, it was shown that the speed gain of the combined simulation with respect to full-EMT simulation was approximately 40% in the case of a small network. For networks in which the ac system is large compared to the dc system, this ratio is expected to improve in favour of the proposed combined simulation method. It was also shown that the coupling location (between the EMT and the stability simulation) does matter. The simulation is more accurate in case the interface location is at the point of common coupling of the VSC terminal. Whether this also holds for larger networks is still under investigation.


The commercial package identified for possible use for this WP (PSS/e) does not inherently offer a unified method to include dc structures. A Master student has been conducting work on this topic and results are promising: dc grids can now be included with arbitrary topologies and corresponding dynamics in the ac stability simulation. Time-step sizes must however be decreased, leading to longer simulation times. PSS/e does not lend itself easily to implementing a combined simulation method similar to the prototype developed in Matlab. Possible future research directions include the investigation of a more sophisticated way to include dc systems into PSS/e by inner integration loops or reducing the network size by dynamic equivalencing.


Grid model: In co-operation with TenneT, TUD-EPS is working with a UCTE grid model, which contains a load flow layer as well as a layer for dynamic simulations. The security analysis performed in WP6.1 is however being conducted with a less detailed network model, in order to keep the computation times reasonable for the round-the-year approach. It is vital that both models used in WP6.1 and 6.2 have the same starting conditions (i.e. scenarios, generation portfolios, unit commitment and dispatch). The next period will focus on coupling the detailed UCTE grid model with the market simulation output of WP6.1, and picking interesting cases for dynamic studies.  Another Master student has recently begun his thesis on the topic of identifying extreme cases resulting from wind power injections into the grid.


WP7: Costs, benefits, regulatory and policy aspects


ECN: The cost-benefit analyis will combine the offshore grid costs of WP2 and the trade benefits of NSTG for the three main scenario’s. The effect on electricity prices (level and volatility) will be investigated by using the COMPETES electricity market model. IRENE-40 RES scenario 2030 quantities of installed offshore wind will be used as COMPETES input. This activity is expected to start at the end of 2011.


WP8: IEA Annex 25 collaboration

TU Delft and ECN will contribute to Task 25 activities:,

1. North Sea Transnational Grid (NSTG) project (TUDelft and ECN):

Ø     determine the best solution (modular, flexible, most cost effective) for a high capacity transnational offshore grid, connecting future far and large wind farms in the North Sea.

Ø     develop and test a multi-terminal converter control strategy

Ø     determine the effects of the offshore grid on the national AC grids: planning and operating rules to ensure security, regulate power exchange correctly, and avoid congestion, as well as the  effect of the offshore grid on the stability of national grid

Ø     costs, benefits, and policy regulations related to the realisation of the North Sea grid will be investigated.


2. Identifying the options for designing an offshore electricity grid and the legal instruments to create such a grid (TUDelft)

Ø     legal, technological and market considerations which coastal states, the EU and national legislators and policy makers should take into account when planning and weighing the grid design options. The research takes the North Sea area as an example. Four different options will be examined, varying from a national option, to a bilateral option, a bilateral-multilateral option, and finally a multilateral option.

Ø     assessment tool is developed, which will deliver quantitative results for each of the four technical options. These results will be used to adjust the legal framework, thus ensuring an adequate development of a (trans)national offshore grid.


3. Grid code requirements and possibilities for ancillary service provision by HVDC-connected wind power plants (including inertia emulation and oscillation damping), one national and one EU-funded  project proposal has been submitted.


The IEA Annex 25 meeting in Lisbon in September 2011 was attended.

WP9: Dissemination of Results:


See section 6 for the publications during the period of this progress report.


WP10: Coordination and administration


The second Advisory Group meeting was organised at Schiphol airport on August 29 2011.