## 4 Technological Challenges and Integration Issues
### Interoperability and Integration Complexity  
One of the most fundamental technological challenges in building a pan-European grid digital twin is achieving true interoperability across a diverse landscape of systems and data sources [5]. Europe's electricity grid is a federated \"system of systems\" operated by dozens of TSOs and hundreds of DSOs, each historically developing their own SCADA, EMS/DMS, GIS, and data management tools, often based on proprietary formats and differing conventions [25]. These legacy systems were not designed to interoperate, especially not at the scale and granularity required by a continent-wide, real-time digital twin [24]. Integrating these disparate systems into a unified digital representation requires addressing heterogeneity at multiple levels: syntactic, which involves data formats and communication protocols; semantic, which involves terminology and ontologies; and pragmatic, which involves business processes and operational contexts [5]. For example, the substation identifier used by one operator may not align with another's naming scheme, and their telemetry protocols may differ, using IEC 60870-5-104, OPC UA, or vendor-specific interfaces. As a result, data fusion becomes a time-consuming and error-prone task, with grid operators frequently reporting mismatches in data formats as well as incomplete or inconsistent datasets [24]. A joint survey by ENTSO-E and the EU DSO Entity confirms that system operators across Europe consistently identify interoperability and data integration as major bottlenecks [4; 12]. Standards such as the IEC Common Information Model (CIM) provide a shared data schema for power system components and have been adopted in initiatives such as ENTSO-E's CGMES-based grid model exchanges [25]. However, implementation remains inconsistent. Transmission system operators have made more progress than distribution system operators, and many DSOs have not yet adopted CIM extensions or aligned their internal models with the standard [24]. In addition, CIM does not cover all layers of integration. Real-time data flows, for example, require further standardization in telemetry protocols and API interfaces [5]. Without harmonized models and communication standards, there is a risk that each local digital twin will remain an isolated silo, which would undermine the broader vision of a federated and interoperable system [4]. The challenge is further intensified by the need for continuous and bidirectional data exchange. A digital twin is not a static database but a dynamic system that must both ingest and produce data in real time [20]. Achieving this requires robust interface specifications, event-driven architectures such as those using MQTT or Kafka, and potentially message buses that support publish and subscribe communication among different grid actors [5; 38]. Many utilities still depend on batch data exchanges or manual imports, which are not capable of providing the temporal precision required for accurate and timely operation of a digital twin [24]. Integration must also reach beyond the power system. A high-fidelity digital twin will need to incorporate data from multiple domains, including weather forecasts, electricity market conditions, mobility patterns such as electric vehicle charging, and sector-coupling data from systems such as gas networks and heating infrastructure [5]. Each of these domains operates with its own data conventions and formats, which necessitates the use of advanced translation layers and semantic mediation [20]. The European Commission'sDigitalization of Energy Action Plan has emphasized the importance of cross-sector interoperability and has encouraged the alignment of data spaces across energy, mobility, and finance to support broader system integration [1]. Solutions must bring together both technology and governance. On the technological side, middleware platforms, semantic data layers, distributed data lakes, and standardized APIs can help bridge differences between systems [5].