| Transmission & distribution
Aquila Lite: pioneering HVDC multi-vendor interoperability
The Aquila Lite project represents a significant advancement in the field of high voltage direct current (HVDC) transmission, particularly in achieving multi-vendor interoperability. This article, contributed by SSEN Transmission and the National HVDC Centre, UK, delves into the technical challenges faced and innovative approaches developed as part of the project, highlighting its global context and unique differentiators
To manage the integration of large amounts of offshore wind power connecting to the north of Scotland, new HVDC reinforcement is required to deliver the generated power to demand centres in the south of England. The Aquila Lite project (shown schematically in Figure 1) proposes a DC-switching-station (DCSS) based multi-terminal HVDC topology to facilitate power transfer, requiring fewer bipolar converter stations onshore than traditional point-to-point systems. This innovative approach aims to de-risk the supply chain by ensuring interoperability and reliability through a multi-vendor scheme.
HVDC interoperability: enhancing system efficiency and reliability HVDC interoperability is the capability of one manufacturer’s HVDC system to work seamlessly with systems from other manufacturers. This eliminates the requirement for AC conversion, increases supply chain efficiency, and enhances future system operation. Without interoperability, relying on a single supplier for large parts of the developing offshore network could disproportionately expose future network operations to outage risks.
Challenges of achieving HVDC multi-vendor interoperability One of the primary challenges addressed in the Aquila Lite project is achieving interoperability among different HVDC vendors. By proving
interoperability, the project aims to enhance competition, reliability, and reduce risks associated with single-vendor systems. The project involves collaboration with four principal HVDC manufacturers: GE Vernova (GEV); Mitsubishi Electric (MELCO); Hitachi Energy; and Siemens Energy. Ensuring seamless operation between these vendors’ systems requires a vendor-agnostic control architecture and rigorous testing protocols.
Vendor-agnostic control architecture
The dynamics of vendor-agnostic onshore voltage source converters (VSCs) have been defined as individual blocks with a minimum number of interfaces for control inputs and electrical outputs. This approach manages the intermittency of communications and the condition of “N-1” pole ends. The inputs are updated by upper-level controls to manage three baseline functions: power dispatch; DC voltage bias regulation; and neutral current control. This architecture supports a range of operational scenarios, including the interconnection of rigid and full bipolar sections and subsequent unbalanced operations.
Specifying HVDC converters for guaranteed DC-side small-signal stability
To ensure small-signal stability, an indexing approach has been developed to scope out the eligibility of operating points with mathematical
AC system SPITAL
AC–DC converter DC switching station (DCSS)
Multi-vendor, multi- terminal opportunities for connection
proof. A frequency-domain approach has been patented to specify individual VSCs, generating specifications as envelopes for DC impedance in the frequency domain (Figure 2). This method is assisted by time-domain simulations at the single- converter stage to guarantee small-signal stability.
Demonstration of multi-vendor HVDC grid
The Aquila Lite project has developed and demonstrated multi-vendor multi-terminal (MVMT) interoperability in a lab environment. In March 2025, the operation of a two- vendor, four-terminal, five-node ±525 kV HVDC grid was demonstrated in the real-time digital simulator (RTDS) environment at the Institution of Engineering and Technology (IET) AC/DC conference. See Figure 3. Bipolar terminals in the simulation, supplied by GEV and MELCO through distinctive black-box approaches, were interconnected with HVDC Centre models. The demonstration included a range of operations such as energisation, power ramps, soft switching of control modes, and loss/restoration of pole ends.
Having already successfully tested interoperability with two vendors, it is crucial to include a third vendor in the testing phase to showcase further multi-vendor multi-terminal interoperability. This third model testing is important to demonstrate the seamless integration of HVDC systems from multiple manufacturers. Discussions are now underway to enable third model testing, which will further validate the robustness and scalability of the interoperability framework.
Some key elements of Aquila Lite can be summarised as follows:
PETERHEAD
and cost and
SCOTLAND
Eastern Green Link (EGL) 2: Drax
Eastern Green Link (EGL) 4: South Humber
Figure 1. Schematic of the Aquila Lite project. Source: SSEN Transmission/National HVDC Centre
Aquila Lite control and protection systems. The Aquila Lite project employs advanced control and protection systems to manage the complex interactions between different HVDC vendors. These systems are designed to handle various fault conditions and ensure the stability and reliability of the HVDC network. DC switching station. The DCSS is a critical component of the Aquila Lite project. It allows flexible routing of power between different terminals and helps to optimise the overall efficiency of the HVDC network. The DCSS is equipped with state-of-the-art switching technology to handle high voltage and current levels.
Hybrid earthing scheme. To avoid significant cable costs incurred for a dedicated metallic
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Interconnector (Northconnect) to Norway
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