Ensuring performance and reliability in multi-node grids: the importance of converter interoperability

As electric grids evolve toward increasingly converter-dominated systems, the ability of power electronic devices to operate not only effectively as standalone devices, but also cohesively, has become a key requirement. This is particularly the case in applications where multiple converters operate in close proximity or need to share responsibility for system control. 

In multi-terminal DC (MTDC) networks, for example, HVDC converters must coordinate to regulate DC voltage, manage power flow, and maintain stability across geographically dispersed nodes. Similarly, in modern electric rail systems that use traction feeding concepts based on static frequency converters (SFCs), the converters must be synchronized to manage highly dynamic and unbalanced loads across catenary sections.  

In both cases, interoperability – defined as the ability of systems or equipment from one or multiple vendors to communicate, coordinate, and operate together effectively through defined interfaces and functionalities – is critical. Poorly coordinated control strategies and interfaces between vendors can lead to oscillations, degraded power quality, or system instability. Conversely, well-designed control schemes enable resilient, flexible, and scalable systems capable of accommodating diverse operating conditions and future expansion. 

Traditionally, HVDC transmission has been deployed in point-to-point configurations, with converter stations installed at each end of the link to regulate power flow and ensure stable operation. In recent years, however, increasing integration of renewable energy sources, along with the need for cross-border electricity exchange, has driven interest in multi-node, networked HVDC systems.  

MTDC extends the HVDC paradigm by interconnecting three or more converter stations to form radial or meshed networks capable of routing power dynamically across multiple nodes. This provides significant advantages when it comes to large-scale renewable energy integration. Offshore wind farms, in particular, can benefit from a shared transmission backbone, as they are often developed in clusters and located far from load centers. 

MTDC systems facilitate interconnection of multiple wind farms and onshore grids, enabling power to be distributed more efficiently and reducing curtailment during periods of excess generation. The flexibility also allows operators to balance variability across geographically dispersed renewable installations, smoothing fluctuations in output. 

Improved resiliency is another advantage of MTDC systems. In contrast to traditional point-to-point HVDC links, DC networks can reroute power in real time in response to disturbances, congestion, or shifting demand patterns. This capability supports more robust system operation, including improved fault tolerance. It also creates more opportunities to provide inertia and frequency support services to AC grids dominated by inverter-based resources.  

Specifically in Europe, the broader vision underpinning MTDC development is the realization of large-scale DC grids. Interconnected networks will allow countries to share renewable resources, optimize generation across time zones, and enhance energy security—ultimately creating the foundation for a more integrated and decarbonized energy system.  

In a multi-terminal HVDC network, multiple converters must be synchronized to regulate voltage, control power flows, and maintain system stability. Unlike point-to-point links, where control strategies are relatively straightforward, MTDC requires sophisticated schemes, as interactions between converters can lead to oscillations or instability if not properly managed.

Protection and fault management are especially important. DC faults propagate much faster than AC faults due to the low impedance of DC lines. The absence of natural current zero crossings makes interruption inherently more difficult. Developing reliable and fast-acting DC circuit breakers has been a central focus of industry innovation, but these technologies are still maturing. Ensuring selective fault isolation in an MTDC grid remains a critical area of research and development among grid technology providers.    

Operation in multi-vendor environments is an equally pressing issue. Historically, HVDC projects have been delivered by a single supplier, resulting in proprietary control and protection systems. In a multi-terminal network involving equipment from different manufacturers, integration is more complex and requires careful consideration of interfaces and protocols. This challenge extends beyond technical compatibility to include procurement practices and long-term operational considerations. 

The InterOPERA¹ project has been established to address precisely these challenges. As a major European research and innovation initiative, InterOPERA focuses on enabling interoperability in multi-vendor HVDC systems. Its work encompasses the development of standardized functional specifications, control frameworks, and validation methodologies, as well as the creation of real-time demonstrators to test concepts in practice.

By aligning stakeholders across industry, academia, and transmission system operators, InterOPERA aims to lay the groundwork for the first fully interoperable MTDC grids in Europe within the next decade. 

Hitachi Energy has played a leading role in the development of HVDC grids over several decades and has been instrumental in advancing voltage source converter (VSC) technology, which forms the foundation of modern MTDC systems.  

VSC converters, like the HVDC Light®, which is part of Hitachi Energy’s Grid-enSure® portfolio, allows for independent control of active and reactive power, making it well suited for multi-terminal applications and weak grid connections, such as offshore wind integration. 

Within the context of MTDC development, Hitachi Energy has focused on several critical areas, including the design of hybrid HVDC circuit breakers, switching stations, and DC grid controllers. The company has also played a leading role in the InterOPERA project by contributing hardware, software, and system integration expertise to real-time testing platforms and helping to validate interoperability concepts. 

In addition to ongoing research and development activities, Hitachi Energy has experience designing and delivering real-world multi-terminal HVDC systems.  

One recent example is the Caithness–Moray–Shetland (CMS) HVDC link project in the United Kingdom, which is the first implementation of a regional DC grid in Europe. The project interconnects multiple converter stations across mainland Scotland and the Shetland Islands. 

For the project, Hitachi Energy provided a solution based on three HVDC Light® converter stations. The system is implemented as a symmetric monopole rated at ±320 kV DC, enabling efficient and flexible power transmission between mainland Scotland and the Shetland Islands while supporting the integration of remote renewable generation. 

The CMS project highlights Hitachi Energy’s expertise in addressing the practical challenges of MTDC deployment, including coordinated control across multiple terminals, system protection strategies, and reliable operation under varying grid conditions. By successfully delivering the system, Hitachi Energy has established itself as a leader in MTDC, providing a validated reference for future deployments. It also reinforces the company’s role as a developer of next-generation transmission networks and demonstrates that multi-terminal HVDC is not only technically feasible, but already being realized at scale. 

Modern traction rail networks are another area where converter interoperability is critical.  

Rail power systems have traditionally relied on multi-zone feeding concepts based on conventional transformer-only substations, where catenary sections are electrically isolated. Increasingly, however, as rail operators look to maximize energy efficiency and minimize impact on the grid at the point of connection, SFC-based feeding concepts are being adopted.   

In many cases, multiple SFCs (often supplied by different manufacturers) are required to operate in parallel at different points across the rail network. This places a greater emphasis on harmonized control strategies, standardized interfaces, and demonstrated multi-vendor compatibility to ensure coordinated performance (i.e., with respect to phase angle synchronization and load sharing). 

The ability of SFCs to integrate with legacy infrastructure, including conventional traction substations and/or rotary converters is also important.  

Many rail operators continue to rely on a hybrid mix of technologies, either due to phased modernization programs or to take advantage of the long service life of existing assets. SFCs must be able to operate alongside these systems without introducing disturbances or requiring extensive modifications. This demands careful consideration of harmonic performance and dynamic response, particularly during disturbances, such as faults, load steps, or switching events. 

Achieving deterministic converter behavior requires extensive engineering on the part of the OEM, including detailed system modeling, hardware testing, and rigorous validation of control algorithms across a wide range of scenarios. 

Hitachi Energy brings extensive, field-proven experience in deploying its rail SFCs within complex, multi-vendor rail networks and alongside legacy traction power technologies. The company has successfully delivered solutions based on the PCS6000 and SFC Light that integrate seamlessly with existing infrastructure, while maintaining stable and predictable system performance. 

One recent implementation of Hitachi Energy’s SFC technology was in the Czech Republic. Czech railway infrastructure manager, Správa železnic, is currently undergoing a project to modernize and unify the country’s railway power supply network, which consists of two different traction systems: a 3k DC supply in the north and a 25kV 50Hz AC supply in the south.  

Conversion of the first 3k DC catenary line to 25kV AC was completed in 2022 on a 50-km section of double-track. The track is part of the country’s main railway connecting Vienna, Austria to Warsaw, Poland.  

For the project, Hitachi Energy supplied three PCS6000 SFCs to create a multi-source power feeding arrangement. The SFCs decouple the railway grid from the feeding utility grid, which eliminates power quality issues and ensures full grid code compliance.  

In addition, the SFCs control power flow and output voltage on the traction side, so catenary sections fed by a different substation no longer need to be isolated. Besides saving on costs, this allows the current to circulate between different sections and for regenerative braking energy to be re-used over a larger catenary distance. 

With multiple substations able to share the peak load, maximum power drawn from each grid connection point is reduced, resulting in a more energy efficient system.  

The SFCs also create the opportunity for Správa železnic to potentially support the utility grid with ancillary services in the future. 

As power grids incorporate more power electronic technologies, converter interoperability is becoming an important enabler of reliability, flexibility, and scalability.  

Hitachi Energy is uniquely capable of addressing the technical challenges associated with developing complex, multi-node systems. The breadth of the company’s Grid-enSure® portfolio, coupled with extensive real-world experience provides a critical advantage in anticipating and mitigating interoperability challenges before they arise in large-scale deployments. 

Within HVDC, Hitachi Energy is actively shaping the evolution toward fully meshed MTDC networks. Through the InterOpera project, the company is also helping to define the frameworks necessary for true multi-vendor interoperability—aiming to reduce integration risks, foster a more competitive ecosystem, and ultimately lay the foundation for scalable, future-proof grids. 

For more information on HVDC and SFC solutions, visit the Grid-enSure® webpage.