Deliverable 1.3: Synthesis of available studies on offshore meshed HVDC grids

Several previous studies have already addressed the development of offshore meshed HVDC grids and the associated technical challenges and regulatory and financial barriers. The PROMOTioN “Progress on Meshed HVDC Offshore Transmission Networks” project builds on those studies, in order to address a number of the remaining barriers for the implementation of offshore grids including: high cost of connection for wind resources, security and robustness of the grid, international regulations and access to financial resources.

The aims of this deliverable (D1.3), which forms part of work package 1 (WP1), are the synthetization of existing studies relevant for the project, and the justification of PROMOTioN scope of work in respect to current barriers and gaps. Therefore, this report intends to provide a state-of-the-art picture of the offshore meshed HVDC grids.

A fundamental ingredient to unlock the full potential of Europe’s offshore resources in the North Seas (North Sea, Irish Sea, English Channel, Skagerrak Strait and Kattegat Bay) is the demonstration of the cost effectiveness of a solution to harness these resources. Numerous roadmaps have been proposed in the past decade for the development of an offshore meshed grid in the North Seas.

Exact geographical scopes, methodologies and assumptions can differ strongly from one study to another. Moreover, very different levels of details are used to model power systems: from macro-levels, modelling only transfer capacities between hubs, to node-breaker models. Nevertheless, some trends emerge from the analysis of these past roadmaps.

Firstly, it is unlikely that the final solution consists of a single large interconnected offshore grid: roadmaps usually come up with several offshore grids not connected together by DC branches. Secondly, complex offshore topologies (i.e. radial multi-terminal and meshed grids) appear to be cost-efficient only for scenarios considering both a high offshore wind generating capacity and numerous offshore hubs to collect this energy (geographical spreading).

Otherwise, purely radial configurations stay the most economical way to collect wind energy. It must also be noted that offshore mixed AC/DC grids can be relevant from an economic point of view in some cases. Finally, the economical advantage of complex offshore topologies such as radial multi-terminal and meshed grids can only be demonstrated when the overall grid structure is optimized.

The development of HVDC grids based on VSC and Diode Rectifier Units (DRU) converter technologies introduces new operation and control challenges to maintain the stability of the offshore DC grid, the offshore AC grids connecting wind power plants, and the onshore AC grid. A first issue is the steady-state control of the DC grid to maintain voltages across the grid in an acceptable range, but several kinds of dynamic instabilities could occur as well.

For example, the massive integration of converters based on power electronics in the grid introduces new possible interactions between components and the literature shows that this can lead to new resonance phenomena between the converters, either within the DC grid, or through the AC grid. Moreover, developping an offshore HVDC grid in the North Seas will not be done in one step: it is expected that the offshore grid will be developed over several decades, following the development of offshore wind generation.

Consequently, several technologies will be integrated, from different manufacturers. The interoperability of converters in such offshore grids remains an open question. The connection of OWFs to the main onshore gridthrough HVDC systems also leads to small islanded offshore AC grids dominated by cables and power electronics (i.e. there is no synchronous generator to provide an inherently stabilising source of inertia).

If wind turbines and HVDC converters are not properly operated, instabilities could occur, as has already been observed in the BorWin1 HVDC system connecting an offshore wind farm to the shore in Germany. In this case, unexpected harmonics issues occurred, leading to the outage of the HVDC system when converters entered into resonance with the offshore AC grid natural frequencies.

The introduction of the DRU leads to new phenomena and the appropriate control of converters and wind power plants in that context is still unknown, in particular when the connection of wind turbines from different vendors must be enabled. Similarly to AC grids, faults can occur on transmission elements in a DC grid.

Consequently, DC grid protection systems must be able to detect and isolate faults and minimize their negative impacts. In a point-to-point HVDC system, it is usually sufficient to open circuit breakers on the AC side to isolate the fault. The problem is much more difficult for complex DC grid topologies such as radial multi-terminal and meshed grids, because, depending on the protection philosophy, there is the need for selective fault detection and clearing.

Moreover, a speed requirement is expected for the fault detection and identification in these complex DC grid topologies, such that the faulty element can be disconnected before the current increases beyond the acceptable limits of the system. Numerous HVDC grid protection schemes have been proposed by academia and industry, but they remain at the theoretical level with no practical implementation having been made so far.

The literature shows that there is no final consensus on the protection schemes that will be the most suitable for practical implementation. Indeed, no single basic protection principle fulfills all the requirements needed for meshed HVDC grids. Protection systems must thus combine several basic protection principles to perform as required.

Another challenge to isolate faults is the development of adequate DC circuit breakers (DCCBs) which must interrupt the fault current quickly, due to the high rate of rise of fault current in DC grids (which is limited only by the resistive part of cables). Additionally, a sufficient means of generating a current zero or an appropriate counter voltage is also required in DCCBs because there is no natural zero crossing of fault current to extinguish the arc, as exists in conventional AC circuit breakers,.

If adequate technological concepts already exist for DCCBs (e.g. resonant DCCB, solid-state DCCB, hybrid DCCB) no practical installation exists at this moment. A main barrier for the deployment of DCCBs is the lack of detailed data on the behavior of DCCBs and their interaction with their electric environment.

In particular, it is difficult to compare devices from the different manufacturers because they have varying characteristics and they were not compared on the basis of standardized methodologies. Finally, a number of regulatory and financial barriers are also hampering a large scale deployment of meshed HVDC grids.

A vast amount of studies have been done regarding the regulation of financing of transnational infrastructures such as a meshed offshore grid. However, some topics are not fully covered. For example, the amount of literature on the financing mechanisms for such an infrastructure is still limited. Moreover, there is no unanimity on the way to alleviate regulatory and financial barriers: several reports propose different and sometimes conflicting recommandations to attain transnational infrastructure development.

Thus, even if progress has been made, there is not yet a definite international regulatory and financial framework for an offshore grid in the North Seas.