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

Fast-Convergence Microsecond-Accurate Clock Discipline Algorithm for Hardware Implementation

Discrete microprocessor-based equipment is a typical synchronization system on the market which implements the most critical features of the synchronization protocols in hardware and the synchronization algorithms in software. In this paper, a new clock discipline algorithm for hardware implementation is presented, allowing for full hardware implementation of synchronization systems. Measurements on field-programmable gate array prototypes show a fast convergence time (below 10 s) and a high accuracy (1 μs) for typical configuration parameters.Ministerio de Educación y Cultura HIPER TEC2007-61802/MI

A Gossip Algorithm based Clock Synchronization Scheme for Smart Grid Applications

The uprising interest in multi-agent based networked system, and the numerous number of applications in the distributed control of the smart grid leads us to address the problem of time synchronization in the smart grid. Utility companies look for new packet based time synchronization solutions with Global Positioning System (GPS) level accuracies beyond traditional packet methods such as Network Time Proto- col (NTP). However GPS based solutions have poor reception in indoor environments and dense urban canyons as well as GPS antenna installation might be costly. Some smart grid nodes such as Phasor Measurement Units (PMUs), fault detection, Wide Area Measurement Systems (WAMS) etc., requires synchronous accuracy as low as 1 ms. On the other hand, 1 sec accuracy is acceptable in management information domain. Acknowledging this, in this study, we introduce gossip algorithm based clock synchronization method among network entities from the decision control and communication point of view. Our method synchronizes clock within dense network with a bandwidth limited environment. Our technique has been tested in different kinds of network topologies- complete, star and random geometric network and demonstrated satisfactory performance

Algorithms for the synchrophasor measurement in steady-state and dynamic conditions

Phasor measurement units (PMUs) are becoming one of the key issues of power network monitoring. They have to be able to perform accurate estimations of current and voltage signals either under steady-state or dynamic conditions. The first part of this PhD thesis analyses the impact of the phasor models on the estimation accuracy, focuses on algorithms proposed in the literature for the estimation of phasors and studies their performance under several different conditions. On the basis of the results of this analysis, in the second part of this thesis an innovative approach to improve the performance of synchrophasor estimation is presented. The method proposes a modified version of the synchrophasor estimation algorithm which uses the non-orthogonal transform defined as Taylor-Fourier Transform (TFT) and which is based on a Weighted Least Squares (WLS) estimation of the parameters of a second order Taylor model of the phasor. The aim of the proposed enhancements is to improve the performance of the algorithm in presence of fast transient events and to achieve a Phasor Measurement Unit that is simultaneously compliant with both M and P compliance classes, suggested by the synchrophasor standard IEEE C37.118.1. In particular, while the TFT based adaptive algorithm is used for synchrophasor estimation, frequency and Rate of Change of Frequency (ROCOF) are estimated using the higher derivatives outputs of the adaptive TFT. Frequency estimation feedback is used to tune the algorithm and achieve better performance in off-nominal conditions. The proposed approaches are validated by means of simulations in all the static and dynamic conditions defined in the standard. In the last chapter, the algorithm proposed above is used in a novel architecture, compliant to IEC 61850, for a distributed IED-based PMU, to be used in electrical substations. In particular, a measurement architecture based on process bus and sampled values synchronized with IEEE 1588-2008 is proposed, so that voltage and current signals are acquired by a Merging Unit device, while the PMU signal processing is performed on a IED (Intelligent Electronic Device), in compliance with IEEE C37.118.1-2011

Algorithms for the synchrophasor measurement in steady-state and dynamic conditions

Phasor measurement units (PMUs) are becoming one of the key issues of power network monitoring. They have to be able to perform accurate estimations of current and voltage signals either under steady-state or dynamic conditions. The first part of this PhD thesis analyses the impact of the phasor models on the estimation accuracy, focuses on algorithms proposed in the literature for the estimation of phasors and studies their performance under several different conditions. On the basis of the results of this analysis, in the second part of this thesis an innovative approach to improve the performance of synchrophasor estimation is presented. The method proposes a modified version of the synchrophasor estimation algorithm which uses the non-orthogonal transform defined as Taylor-Fourier Transform (TFT) and which is based on a Weighted Least Squares (WLS) estimation of the parameters of a second order Taylor model of the phasor. The aim of the proposed enhancements is to improve the performance of the algorithm in presence of fast transient events and to achieve a Phasor Measurement Unit that is simultaneously compliant with both M and P compliance classes, suggested by the synchrophasor standard IEEE C37.118.1. In particular, while the TFT based adaptive algorithm is used for synchrophasor estimation, frequency and Rate of Change of Frequency (ROCOF) are estimated using the higher derivatives outputs of the adaptive TFT. Frequency estimation feedback is used to tune the algorithm and achieve better performance in off-nominal conditions. The proposed approaches are validated by means of simulations in all the static and dynamic conditions defined in the standard. In the last chapter, the algorithm proposed above is used in a novel architecture, compliant to IEC 61850, for a distributed IED-based PMU, to be used in electrical substations. In particular, a measurement architecture based on process bus and sampled values synchronized with IEEE 1588-2008 is proposed, so that voltage and current signals are acquired by a Merging Unit device, while the PMU signal processing is performed on a IED (Intelligent Electronic Device), in compliance with IEEE C37.118.1-2011

Development of a Power Quality Based Digital Energy Meter Educational Platform

Phasor Measurement Units (PMUs) are being used extensively for electrical grid monitoring and control. However, their cost prohibits further adoption on the distribution grid and easy access for educational purposes. This paper proposes that simple and fundamental functions of a PMU can be achieved using an energy metering IC and integrated into smart electricity meters, providing a lower cost and more widely available method of monitoring and control of distribution grids, and presents a proof-of-concept platform with aforementioned functionality. The described platform's construction and flexibility emphasizes its educational capabilities PMUs and electricity meters as well

Synchrophasor Technology: PMU Applications in Smart Grids

Abstract: Synchrophasor technology was first introduced in 1980s. Since then, many electric utilities such as American Electric Power (AEP), Bonneville Power Administration (BPA), Southern California Edison (SCE), New York Power Authority (NYPA), as well as universities such as Virginia Polytechnic Institute (VPI), have been conducting R&D to explore and advance the application of synchrophasor technology. The R&D pace has accelerated recently, mainly due to active participation and leadership provided by US Department Of Energy (DOE) and North American Electric Reliability Corporation (NERC). These R&D efforts have shown that the synchrophasor technology has promise to greatly enhance the transmission planning, design & operations functions. This paper provides a brief description of synchrophasor technology and summarizes potential synchrophasor technology applications that can contribute towards the development of a smarter transmission grid

PMU’s behavior with flicker-generating voltage fluctuations: an experimental analysis

Phasor measurement units (PMUs), which are the key components of a synchrophasor-based wide area monitoring system (WAMS), were historically conceived for transmission networks. The current trend to extend the benefits of the synchrophasor technology to distribution networks requires the PMU to also provide trustworthy information in the presence of signals that can occur in a typical distribution grid, including the presence of severe power quality (PQ) issues. In this framework, this paper experimentally investigates the performance of PMUs in the presence of one of the most important PQ phenomena, namely the presence of voltage fluctuations that generate the disturbance commonly known as flicker. The experimental tests are based on an ad-hoc high-accuracy measurement setup, where the devices under test are considered as “black boxes” to be characterized in the presence of the relevant signals. Two simple indices are introduced for the comparison among the different tested PMUs. The results of the investigation highlight possible critical situations in the interpretation of the measured values and provide a support for both the design of a new generation of PMUs and the possible development of an updated synchrophasor standard targeted to distribution systems