Why 10 GW is a qualitatively different engineering problem

A 10 GW offshore wind farm transmission system is not an incremental scaling of existing HVDC installations — it is a threshold at which four distinct engineering domains simultaneously reach the limits of current industrial practice. Patent analysis across approximately 25 records spanning EP, US, WO, DE, and CN jurisdictions, filed between 2007 and 2025, maps the precise technical barriers: offshore converter station design, internal collection grid architecture, system startup under HVDC-isolated conditions, and submarine cable corridor planning.

The dominant technical approaches identified in the dataset fall into four clusters: offshore converter topology design (particularly the choice between Modular Multilevel Converters, diode rectifier bridges, or hybrid combinations); internal collection grid architecture (specifically the transition from medium-voltage AC to medium-voltage DC); system startup and black-start capability under HVDC-islanded conditions; and offshore substation physical integration and submarine cable corridor planning. Each cluster presents engineering difficulty that compounds at 10 GW in ways not encountered at smaller installations.

Offshore converter topology: the central cost and complexity driver

The selection and sizing of the offshore converter station is the most fundamental engineering challenge at 10 GW scale. For HVDC transmission, the offshore end must convert the aggregated AC generation into high-voltage DC suitable for long-distance submarine cable transmission. The conventional approach relies on Voltage Source Converters based on MMC architecture at the onshore inverter station, but the offshore rectification stage is actively contested in recent patent filings.

A major cost-reduction strategy explored in recent patents involves replacing full MMC offshore converters with simpler diode-rectifier-based topologies. As described in Zhejiang University’s 2022 patent on high-frequency uncontrolled rectification DC transmission, the offshore converter can be a three-phase six-pulse uncontrolled rectifier bridge operating at elevated frequencies between 100 Hz and 400 Hz, with the onshore converter remaining a full MMC. This approach simplifies offshore equipment and reduces capital expenditure, but introduces harmonics and requires wind turbines with full-scale power converters based on permanent magnet synchronous generators — a constraint that limits turbine procurement flexibility at 10 GW scale.

“Hybrid diode-MMC topologies can reduce offshore equipment cost but constrain turbine technology selection and introduce complex control interactions — a trade-off that becomes acute when coordinating multiple converter assemblies across a 10 GW plant.”

A more nuanced topology is proposed by China Three Gorges Corporation in their 2025 patent on series-parallel converter circuits, which combines three groups of diode valves in series with a single offshore auxiliary MMC connected in parallel with one of the diode valve groups. This hybrid reduces converter volume and cost while preserving some controllability. However, coordinating the series-parallel interactions across a 10 GW plant — where multiple such converter assemblies must operate in synchrony — multiplies the control and protection engineering burden significantly.

Physical integration of the offshore HVDC substation is an equally serious structural challenge. Aibel AS’s 2025 HVDC Solution patent introduces a modular approach using two self-sufficient, mirrored HVDC pole modules mounted side-by-side on a single jacket substructure. For a 10 GW system requiring multiple such platforms — or one very large one — the logistical complexity of marine installation, weight distribution on the jacket, and inter-module electrical isolation presents a construction challenge that is qualitatively different from anything previously built. According to IRENA, offshore wind is central to meeting global energy transition targets, yet substation installation at this scale remains an unsolved logistical problem.

Internal MVDC collection grids and the module reliability challenge

At 10 GW scale, the internal collection network connecting hundreds of individual wind turbines to the offshore substation becomes a critical bottleneck. Traditional medium-voltage AC collection grids face limitations in cable losses and reactive power compensation over the distances involved in a 10 GW array, which may span tens of kilometres.

A DC-based internal collection approach is described in the DC Connection Scheme for Windfarm with Internal MVDC Collection Grid, filed by LI, JUN in 2013 across US and WO jurisdictions. In this architecture, each wind turbine contains a generator-rectifier subsystem whose DC output feeds a bipolar MVDC collection network. At the offshore substation, multiple DC-DC converter modules connect the MVDC bus to the HVDC transmission system, with module outputs serially connected to step up voltage to HVDC levels. The same scheme is covered in parallel filings through ABB Research Ltd. (EP, 2015), indicating that this topology was actively pursued by major industrial actors for large-scale deployment. Standards bodies including IEC are actively developing frameworks for MVDC systems, reflecting growing industrial readiness.

The challenge of this approach at 10 GW scale is the sheer number of DC-DC converter modules required, each of which must be individually reliable in a harsh offshore environment. Series-stacking of converter module outputs to achieve HVDC voltage levels means that a single module failure can disrupt an entire pole string. The need for bypass switching, fault isolation, and module redundancy across potentially thousands of units in a 10 GW installation represents a systems engineering challenge with no current industrial precedent.

Black-start capability and grid fault resilience at scale

The absence of a synchronous voltage reference on the offshore AC bus during startup or following a grid disconnection is one of the most technically demanding challenges unique to HVDC-connected offshore wind farms. This “black-start” problem is severely amplified at 10 GW scale because there is no practical way to energize the offshore collection grid from the onshore grid through a DC link without specialized auxiliary infrastructure.

China Three Gorges Corporation addresses this directly in their Self-Starting Offshore Wind DC Transmission System (EP, 2025), which incorporates an offshore half-bridge flexible DC converter valve alongside diode valves and a diesel generator-converter group connected to the offshore AC bus. The diesel generator provides the initial voltage reference required to energize the offshore MMC and start wind turbines sequentially. For a 10 GW installation with potentially multiple isolated offshore AC collection segments, replicating this auxiliary startup infrastructure at each segment adds capital cost, weight, and maintenance complexity offshore.

A parallel approach from China Three Gorges (EP, 2025) uses an offshore auxiliary flexible DC converter valve connected in parallel with a main diode valve, with bypass switches allowing the auxiliary converter to establish the offshore AC voltage before transitioning to the main power path. This reduces dependence on diesel generation but requires precise coordination of bypass switch timing and converter current sharing.

Vestas Wind Systems addresses the broader grid fault resilience problem in their patent on Providing Auxiliary Power Using Offshore Wind Turbines (US, 2024). When the HVDC link to shore is lost, a substation backup generator creates a weak grid to sustain auxiliary systems in a pilot turbine; subsequent turbines switch to auxiliary control to sustain substation and inter-turbine auxiliary power. At 10 GW, the energy required for auxiliary systems across hundreds of turbines and multiple substations is non-trivial, and the control architecture for staged turbine re-engagement must be designed to avoid cascading inrush currents on the offshore AC buses. As noted by IEC and reported in research published by IEEE, black-start and auxiliary power resilience are increasingly recognised as system-level design requirements rather than afterthoughts.

RWE Offshore Wind GmbH (DE, 2024) discloses architectures in which individual turbine strings are switchable between two independent high-voltage grid connections, reducing the consequence of a single HVDC cable or substation failure — a critical reliability requirement at 10 GW output where a single fault could otherwise cause a nationally significant generation loss.

Submarine cable corridor planning as a marine governance challenge

At 10 GW scale, submarine cable corridor planning ceases to be a purely electrical engineering problem and becomes a marine governance and spatial planning discipline. Multiple wind farms at different shore distances may share a common transmission corridor, creating mixed AC and DC submarine cable arrangements where the precise calculation of required seabed corridor widths is essential to minimise marine territory usage.

The Shanghai Survey and Design Research Institute’s 2024 patent on deep-sea offshore wind power AC/DC hybrid transmission corridor sea area calculation explicitly identifies that when transmission distances exceed 70 km, flexible DC (VSC-HVDC) becomes economically superior to AC transmission. At 10 GW scale, this threshold is routinely crossed, and the resulting mixed AC/DC hybrid corridors serving multiple wind farms must be precisely engineered. The calculation must account for cable spacing, burial depth, water depth, and cable type per segment — each of which varies along the route.

Furthermore, for very deep-water or far-offshore installations, as demonstrated in the Electrical Power Transmission System patent (Appleford, WO 2007), remote power conversion facilities more than 100 km from the host station must incorporate local DC-AC inversion and transformer stages. This design requirement becomes exponentially more complex when the remote station itself must also aggregate 10 GW of variable generation. Regulatory frameworks from bodies including IMO and national maritime authorities govern the cable burial and corridor exclusion zone requirements that ultimately constrain what is technically feasible.

Who is leading the patent landscape — and what it signals

The patent dataset reveals a convergence between Chinese state-owned enterprises and Western industrial conglomerates on similar high-voltage, large-scale DC transmission solutions — but from different technological starting points, with different near-term project drivers.

China Three Gorges Corporation is the most active recent filer, with three EP patents in 2025 alone, all focused on startup control and hybrid converter topologies for offshore HVDC. Their filings consistently address the real operational gap of self-starting HVDC-connected wind farms, suggesting active project development at scale rather than exploratory research.

ABB Research Ltd. / ABB Technology AG holds multiple active patents on MVDC collection grid schemes (EP 2014, EP 2015) and meshed HVDC network power flow control (KR, 2014), reflecting a systems-level approach to multi-terminal and large-scale DC grid design that anticipates multi-farm, multi-connection architectures.

Zhejiang University contributes three active patents on high-frequency uncontrolled rectifier topologies (EP 2021, EP 2025, US 2022), representing an academic-led push to radically simplify offshore converter hardware — an approach that trades controllability for capital cost reduction.

Vestas Wind Systems A/S holds two US patents on auxiliary power and black-start capability, indicating that turbine-level contribution to system resilience is increasingly expected at the wind turbine OEM level rather than being solely a substation design problem. This signals a structural shift in how system responsibility is allocated across the supply chain.

Aibel AS (WO, 2025) and RWE Offshore Wind GmbH (DE, 2024) represent the offshore construction and project developer perspective, addressing modular substation assembly and multi-grid-connection topologies respectively. Their participation confirms that the engineering challenges are being addressed simultaneously at the equipment, system, and project levels. As documented by WIPO, the internationalisation of patent filings in offshore energy infrastructure reflects the genuinely global nature of the supply chain and the regulatory jurisdictions involved.

“Both Chinese state-owned enterprises and Western industrial conglomerates are converging on similar high-voltage, large-scale DC transmission solutions — but from different technological starting points, suggesting parallel rather than collaborative development paths.”

The overall pattern in the dataset suggests that no single organisation has yet solved all four engineering challenges simultaneously. China Three Gorges leads on startup and converter topology; ABB leads on collection grid architecture and multi-terminal grid control; Zhejiang University leads on hardware simplification; and Vestas leads on turbine-level resilience. At 10 GW scale, integrating these contributions into a coherent system design remains the overarching engineering challenge — and the primary gap in the current patent landscape.