IEEE 1588 synchronization in distributed measurement systems for electric power networks

Modern electric power systems can be considered as the consequence of the continuous technological evolution, often pushed by economical, political and social requirements. As an example, the main transformations in electric distribution systems arise from the diffusion of “Distributed Generation” (DG), i.e. small production plants, often supplied through renewable energy sources, whose presence has significant implications on both energy management (since “active networks” are needed to take into account bidirectional energy flows by means of innovative devices) and protection systems (since adaptive protections can be used to automatically reconfigure the network in the case of fault occurrence). In general, in both transmission and distribution networks, monitoring, control and protection tasks are usually performed by Intelligent Electronic Devices (IEDs), which can be, by their nature, connected to each other by suitable communication links. A famous example of this approach is represented by the series of Standard IEC 61850 (Communication Networks and Systems in Substations). These standards are related to networks and communication systems within the substation, but are used as a reference in all those circumstances in which an electrical system is managed through the use of IEDs communicating with each other (as in the case of active distribution networks). In this way, control and protection schemes practically become algorithms, whose correct behavior is determined firstly by the availability of data measured in strategic points of the network. The critical role of the above mentioned applications, which clearly emerges from their implications on safety, as well as from economical considerations, makes it of fundamental importance the evaluation of correctness and trustworthiness of the information on which such actions are based. Many of these applications implemented for control and protection purposes in electric power networks require the acquisition of information by Wide Area Monitoring System (WAMS) from strategic points of the system and need that the acquired data have an extremely accurate common time reference. Generally, amplitudes and phases of the positive sequence voltages are the quantities to be estimated in the network nodes. Because of the extension of power networks, suitable measurement devices should be used to ensure proper synchronization between the collected data. Thus, the key components of WAMSs are represented by Phasor Measurement Units (PMUs) designed to measure synchronized phasors (synchrophasors). Typical synchronization specifications for synchrophasors measurement are in the order of few microseconds. Such a tight synchronization requirements lead to the need of highly accurate clock settings, such as the ones bases on satellite systems. Currently, the Global Positioning System (GPS) is the only system to provide a time reference with sufficient availability and accuracy for most distributed monitoring and control applications in power systems. As an alternative, in situations where many devices are located in a geographically limited sub-area (e.g. a substation) of the system and are connected to each other by suitable communication networks (as described by the series of standard IEC 61850), it could be advantageous to distribute the time reference of a high accuracy clock to the devices through suitable network synchronization protocols. Between them, the PTP (Precision Time Protocol) defined in the Standard IEEE 1588 offers the best accuracy performance. It is worth mentioning that the Standard IEC 61850-9-2 practically indicates Ethernet as a preferred communication solution, thus offering an optimal support to implement 1588 synchronization in electric power plants. In this context, it should be recalled that the IEEE 1588 profile for power system applications (project PC37.238) is being developed under IEEE Power System Relaying Committee (PSRC) and Power System Substation Committee (PSSC). The scope covers all power system applications, including Synchrophasors. The group works in close coordination with TC57 WG10, which plans to adopt the PTP profile in the next revision of IEC 61850. In the first part of this thesis, the state of the art regarding power system evolution, IEEE Standard on synchrophasor measurements and synchronization system is presented. In particular, the problems related to the evolution of the power system along with some possible advantages due to the implementation of Phasor Measurement Units in Wide Area Monitoring Systems are introduced. After a general description of the architecture of a distributed measurement system based on PMUs, the new synchrophasors standard is analysed, highlighting the differences with previous versions, the requirements for the measurement of synchrophasors and the definition of synchrophasor under steady-state and dynamic conditions. Moreover, a summary of the possible synchronization solutions is introduced. For each solution, advantages and disadvantages are highlighted. In particular, satellite system and network based protocol are analysed in detail. In the second part of the thesis, a synchronization solution able to exploit the worldwide availability of the GPS and the possibility to disseminate the synchronization signal with high accuracy by means of the network synchronization protocol IEEE 1588 is proposed. This solution is used for the synchronization of PMUs. The objective of this work is to analyse the possibility to synchronize PMUs via PTP and to study the impact that such a synchronization solution has on the performance of measurement systems under both steady-state and anomalous operating conditions, as well as its effects on the applications that make use of their data. Two different versions of the PTP are used: the first one uses hardware-assisted time-stamp mechanism whereas the second one uses software-only time-stamp mechanism. Two experimental systems are characterized in detail with an accurate description of all the used hardware and software components, and their synchronization performances under different operative conditions are analysed. Finally, among all the sources which may contribute to the uncertainty introduced by PMUs, the last part of this thesis analyses the impact of the phasor estimation models on the accuracy of these devices, with particular attention to algorithms proposed in literature for the estimation of dynamic phasors and studies their performances under several different conditions.