Intelligent Protection System for Smart Grid

The authors propose a concept for protection relay systems for operation within a Smart Grid and describe the results of a prototype development. This paper introduces each of the system characteristics that should be considered for protection operation within Smart Grid, and the evaluation methods that were applied under both normal and faulted conditions. The results of a field trial in the 66kV system operated by Shikoku Electric Power CO., INC indicate that the proposed system has the capability of evaluating power system characteristics in detail on a real-time basis and contributing toward supervision, coordination check and optimisation of setting values. Moreover, it has been proven that an agent system can be applied under practical conditions. CONCEPTS OF PROTECTION RELAY SYSTEM FOR SMART GRID In recent years, trends toward electricity liberalisation and the increased importance of environmental issues have led to diversification of power system configurations and characteristics, through the installation of distributed power sources based on photovoltaic and wind power generation technologies. Under these circumstances, there is concern regarding the effect on the reliability of power networks with respect to aspects such as overload, frequency variation, and harmonic content. Against this background, there has been increased focus on the concept of Smart Grid, which has the capability of collecting wide-area power system information on a real-time basis from intelligent power grids using Information and Communication Technology (ICT). Although the setting values of protection relays in power systems are currently decided based on the power system conditions assumed in advance, instances of unnecessary or missed operations might occur in future due to unexpected power system phenomena or operations. In this paper, the concepts of protection relay systems for Smart Grid are suggested as follows: (a) Protection relay operation suitable for actual power system characteristics Evaluation of relay setting values can be performed by means of supervising the operating and non-operating margins of each relay characteristic quantitatively and detecting variations of power system characteristics precisely on a real-time basis utilising analogue input data under normal and faulted conditions. This approach leads to benefits such as modifications to the setting values suitable for actual power system characteristics, coordination check for each protection relay element based on wide-area data, and critical point detection for system operations. (b) Intelligent collection of device data Each protection relay device is provided with the aforementioned function and carries out first-stage evaluation, before transmitting only the data necessary for analysis. In addition, an agent system is applied, which has the capability of collecting protection relay data intelligently and evaluating data from wide-area protection relays without having any effect on protection functions. This approach can realise updating of applications executed in protection relays without needing to take equipment out of service. EVALUATION OF POWER SYSTEM CHARACTERISTICS AND RELAY SETTING VALUES Based on the concepts mentioned above, the authors have designed and developed a prototype protection relay system. Firstly, each of the system characteristics that should be considered under Smart Grid and their respective evaluation criteria will be described. Next, the evaluation methods for both normal and faulted conditions based on examples of system impedances will be introduced. System characteristics and respective evaluation criteria A key factor in the prototype system is the question of how to capture changes in system characteristics in terms of evaluating the margin and the coordination of relay settings relative to the change in system characteristics. Rather than detecting abnormal conditions directly from the change in system characteristics, the prototype system uses evaluation criteria related to each system characteristic within each protection relay unit, as shown in Table 1. On detection of abnormal conditions, the related disturbance record data can be sent to the information management servers, and a detailed evaluation can be executed with locus data on the characteristic plane being calculated in the server, and finally the results can be displayed on the client PC. The final confirmation of a change of system characteristics and modification of setting values would be made by operators of this system. In some cases the change in system characteristics can be captured based only on the power system quantities under normal conditions, and in other C I R E D 21st International Conference on Electricity Distribution Frankfurt, 6-9 June 2011