## 3 Regulatory Barriers to Implementing an EU-Wide Grid Digital Twin
### Standardization and Interoperability Policies  
A major regulatory challenge in implementing a pan-European grid digital twin lies not in the presence of prohibitive rules, but in the absence of binding mandates for standardization and interoperability [24]. Although the EU has made progress in harmonizing market rules and network codes, the digital aspects of the electricity sector, particularly in relation to ICT interfaces, data formats, and communication protocols, have not advanced at the same pace [25]. This regulatory gap has resulted in a fragmented landscape where individual TSOs, DSOs, and vendors deploy proprietary or incompatible systems, making seamless integration across national borders extremely difficult [5].  
Interoperability is essential for the federation of digital twins to function reliably. Without common data models and standardized interfaces, efforts to share data or link digital systems are often hindered by mismatches in format, semantics, or protocols [5; 12]. Industry studies and surveys consistently identify this fragmentation as a core challenge, with utilities reporting ongoing difficulties in collecting, integrating, and standardizing data from diverse sources [25]. Even when relevant standards exist, such as the Common Information Model (CIM) developed by the IEC and supported by CEN, CENELEC, and ETSI [30], their adoption remains inconsistent and is mostly voluntary [24]. As a result, data often stays confined to siloed, vendor-specific formats that limit scalability and reuse [5].  
Efforts to address these issues are underway, though still insufficient. The EU's Smart Grids Task Force and various industry entities have outlined frameworks and guidelines, while the European Commission's Action Plan emphasizes the importance of interoperable and open digital solutions [1]. Forthcoming initiatives, such as the Implementing Act on interoperability for energy metering data, signal increased regulatory attention [31]. However, these initiatives lack the enforceability and coverage necessary to support a fully interoperable digital twin ecosystem [25]. For example, although smart meter standardization has seen some progress through EU coordination, national variations in specifications and consent mechanisms still impede full interoperability [24].  
The slow pace of regulatory alignment stands in contrast to the rapid development of digital technologies. Although industry associations and working groups such as the joint task force of ENTSO-E and the EU DSO Entity are working toward defining common use cases and roadmaps, progress remains dependent on voluntary adoption [4]. In the absence of regulatory mandates, these initiatives are unlikely to achieve the uniformity needed for plug-and-play integration of digital twins across Europe [24].  
To overcome this structural barrier, EU policymakers must play a more active role. This includes endorsing or mandating specific standards through instruments such as Implementing Acts, incorporating interoperability requirements into grid codes, and aligning funding mechanisms with compliance to open standards. Public funding programs, like those under the Digital Europe Program, can also prioritize projects that demonstrate adherence to common architectures. A consistent regulatory framework that incentivizes modular, vendor-neutral, and interoperable system design is essential for avoiding technological lock-in and ensuring long-term flexibility and competition.  
In summary, while standards for digital twin implementation exist, their regulatory status is often non-binding, inconsistently applied, or narrowly scoped. Bridging this regulatory gap is critical to unlocking the full potential of a pan-European digital twin, ensuring that fragmented legacy systems and proprietary solutions do not obstruct integration, scalability, and innovation.