## 1 Introduction
In reference [9] they propose an agent framework that has been assessed through a lab testbed for interoperability purposes. Intelligent physical agents, the IEC 61850 and data distribution service standards have been used. Interoperability for phasors is studied in reference [10], where a security gateway is also examined for synchrophasor applications in terms of interoperability and integration. In reference [11] interoperability for sensors is addressed. The need for the smart grid sensors to be interoperable is highlighted and sensor interface standards are described. They also perform interoperability tests on smart grid sensors. Interoperability for sensors is also examined in reference [12], where a testbed is suggested for testing interoperability of smart sensors. For this purpose, they perform tests of commercial sensors. Data and protocol interoperability are discussed in reference [13]; a data distribution service is examined for this purpose, whereas a data model is used for configuration of TCP/IP and UDP/IP. The need for interoperable Neighbourhood Access Network NAN communication standards is addressed in reference [14]. The problem of interoperability is also discussed in reference [15], where data and communication issues are examined. They also study the case of publish-subscribe communication techniques as a way to improve communication and resolve interoperability. Communication interoperability is also the topic of reference [16], where the communication network requirements are examined for smart grid interoperability. From the above, it is clear that interoperability is an important issue to be addressed for the smart grid, and interoperability testing is very important. The way to achieve Smart Grid system interoperability is through detailed system specification, through use of standards, and through testing. In order to demonstrate that two implementations are interoperable, several steps should be taken, such as: the definition of functionalities, the selection of standards, and creation of profiles and of course the testing which includes both the conformance and the interoperability testing. Therefore a systematic methodology should be applied to reach interoperability. The Smart Grid Interoperability Laboratory (SGILab) at the Joint Research Centre (JRC) of the European Commission proposes in this document a unified approach towards a European framework for developing interoperability testing specifications, as shown in Figure 2[17]. A successful development and deployment of the future smart grid requires a better understanding of how components interoperate and how the proposed standards ensure interoperability among those components. Towards this objective a methodology serves as a systematic way to evaluate the interoperability of different subsystems or electric grid components and methodically verify the ability of a given equipment under test to communicate effectively with other components. The use of a methodology provides a systematic means to analyse any interoperation flaw against business and user requirements. Further analysis could then be utilised to assess the impact of any inconsistency and propose potential solutions. Figure 2: Joint Research Centre (JRC) proposed methodology flowchart [17]. We further apply the proposed methodology to a use case with a focus on the first two steps (as explained in Section 2) in order to give a practical example of its application and to show the benefits of a structured procedure of interoperability testing. The Use Case used for this purpose is the \"Load Management Based on Demand Response (DR)/Demand Side Management (DSM)\", which we have developed for the scope of applying our proposed methodology. We chose this use case, since Flexibility in Demand is a topic of key scientific interest, which aims at adjusting and/or shifting load consumption in order to avoid peaks in demand and it is considered vital for grid stability.