# Toward a Practical Digital Twin Platform Tailored to the Requirements of Industrial Energy Systems
## 4 Discussion
### Alignment with the Requirements on Industrial Energy Systems
\begin{table}
\begin{tabular}{l l l l} \hline \hline \multirow{2}{*}{**ID**} & **Requirement (Abbreviation)** & **SD-DT** & **DT Platform for IES** \\ \hline R1 & Real-time observation & Physical entity & SCADA/PLC system \\ R2 & Reaction on physical & Service dimension & Various orchestrated services + \\ R3 & Physical entity actuation & Physical entity & SCADA/PLC system \\ R4 & Modularity & Connection & Microservice architecture + \\ R5 & Decision capability & Service dimension & Service orchestration \\ R6 & Scalability & Service dimension & Microservice architecture + \\ R7 & Optimization capability & Service dimension & containerization \\ R8 & User interaction & Service dimension & HMI + workflow engine \\ R9 & Robust modelling & Virtual entity & Model management \\ R10 & Prediction capability & Virtual entity & Simulation and \\  & & Prediction services \\ R11 & Model versatility & Virtual entity & Model management + \\  & & Service encapsulation \\ R12 & Standardized foundation & Data dimension & Alignment with RAMI 4.0 \\  & & based on existing ontologies \\ R13 & Heterogeneous & Data dimension & Federated knowledge graph \\  & data processing & & Federated knowledge graph + \\ R14 & Semantic & Data dimension & Federated knowledge graph + \\  & interoperability & & message broker \\ R15 & Secure operation & Physical entity & SCADA/PLC system \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Alignment of the identified requirements (see Table 3) supported by implementation aspects of the proposed DT platform. The requirements are listed along with their given IDs, an abbreviation of their description in Table 3, and structured via localization within the five dimensions (5D-DT) of the DT platform for IES. Our implementation of the data dimension (see Section 3.4) with a federated knowledge graph facilitates heterogeneous data processing (R13) and provides semantic interoperability (R14). It holds information about plant equipment, topology, instrumentation, etc. in a machine-readable and standardized format and integrates runtime data via OBDA. The implementation with the RDF framework provides means to semantically access data via SPARQL endpoints. A wrapper service, which translates MQTT requests to SPARQL queries, enables information exchange within the DT. Standardization technologies and metadata (R12) are applied throughout our DT platform, e.g., by the use of established protocols such as MQTT and standards such as FMI for virtual model description. However, these requirements are especially crucial in the data dimension. Therein, domain-specific standardized ontologies are a crucial methodical foundation, fulfilling this requirement. Existing IES ontologies, as introduced in Section 3.4, can be (semi-)automatically merged to fit the demands of specific DT services, exploiting their highly reusable terminological components. AAS or ISO frameworks, as introduced in Section 2.5.2, can be conveniently incorporated in the data dimension of the platform to ensure conformance in case these are established as recognized standards. The knowledge graph in the data dimension of the DT platform provides a single access point to acquire and integrate external information sources, e.g., metadata, further aiding the scalability of the DT (R6). Furthermore, the whole DT platform builds on the GDTA [63] and RAMI 4.0, hence aiding the re-use of existing frameworks and technologies. Security requirements (R15) are only inherently covered in our DT platform. We argue that cyber-security mechanisms should be contained in the applied fundamental technologies and further investigated in the pertinent literature. As outlined in Section 3.2, plant safety measures and critical high-performance control loops should be implemented on the PLC/SCADA system. However, specialized services could be integrated to automatically reconfigure and compile these control mechanisms on the PLC if, for example, the plant topology changes.