Elsevier

Ad Hoc Networks

Volume 50, 1 November 2016, Pages 41-57
Ad Hoc Networks

Time synchronization over heterogeneous network for smart grid application: Design and characterization of a real case

https://doi.org/10.1016/j.adhoc.2016.04.001Get rights and content

Distributed monitoring and control systems are mandatory for the efficient management of the distribution grid, due to the growing presence of renewable energy sources. Time synchronization among devices deployed in distribution grid plays a key role, since a common time reference is essential to correlate power quality and high sampling frequency measure, and generally coordinate any distributed actions. The design and the deployment of a time synchronization system over an existing heterogeneous network is investigated in the paper. The Network Time Protocol (NTP) has been chosen due to its backward compatibility, the good security level, and the intrinsically robust hierarchy. Several NTP synchronization architectures have been proposed and then applied to a real (operating) distribution grid, both at Medium Voltage (MV) and Low Voltage (LV). The experimental validation was carried out during several weeks on the live system. The results in the MV grid highlight the trade-off existing between synchronization accuracy and availability: the synchronization accuracy over MV Broadband Power Line (BPL) links decreases from 20 ms to 50 ms when a more reliable architecture, from the point of view of time synchronization, is used. On the other hand, experimental results show as end-nodes connected on LV grid by means of BPL have a mean offset typically below 50 ms, and a synchronization accuracy on the order of 150 ms. These results are due to the limitation of the synchronization protocol deployed in the LV end-nodes and to scalability issues of power line communication networks when the number of nodes increases.

Introduction

The modern power grid relies on distributed automation architectures, which can be implemented only by means of a communication infrastructure connecting transformer substations; the union of the automation and communication infrastructures as well as of the power grid is called Smart Grid. Recently, automation of the distribution grid is becoming essential since many Distributed Energy Resources (DER) are often present in the system. Such a requirement forces extensive deployment of network interconnections between substations. The use of a single technology for the communication infrastructure over large regions is not generally feasible from the economical point of view [1]. For this reason, this infrastructure for the automation of the distribution grid could be formed as a heterogeneous network, in which several communication technologies are used together [2], [3], [4].

In any Smart Grid, the distribution of a common reference time is required to merge information, correlate power quality measure, and generally coordinate any actions. Clearly, each automation application may require different synchronization accuracy: the IEC 61850 standard [5] gives some guidance proposing a classification. The time synchronization requirements can be satisfied using GPS-based or protocol-based time synchronization. The latter uses the communication infrastructure also for the distribution of the time reference. Among the most common solutions, it should be mentioned the Network Time Protocol (NTP) [6] and IEEE 1588 [7]. NTP is able to distribute time reference with accuracy about hundreds of microseconds in a Local Area Network (LAN) [8], [9]. On the other side, IEEE 1588 is able to provide, in a LAN, a time synchronization below one microsecond [10], thanks to the hardware-based implementation. Nevertheless, other considerations, in addition to performance, may influence the design of a time synchronization system for a distributed monitoring system over Smart Grid. Among them, the most important are availability, backward compatibility and security.

The aim of the work presented in following paper is to present practical criteria that should guide the designer of a time synchronization system for Smart Grid with heterogeneous communications infrastructure. In particular, the following topics will be discussed:

  • Tradeoff between synchronization accuracy and cost of infrastructure.

  • Effect of synchronization architecture (single layer versus multi-layer) on synchronization accuracy.

  • Distribution of time synchronization down to the end user level in the Low Voltage grid.

  • Classification of the designed system with respect to the performance classes required by typical Smart Grid applications.

The design guidelines resulting from the investigation will be adopted in a real test case: a time-synchronized Smart Grid over a heterogeneous communications infrastructure deployed to control a part of the distribution grid of the City of Brescia (North of Italy). A comprehensive, on-the-field, experimental, characterization of the synchronization performance of the designed system is carried out during normal grid operations.

Section snippets

Time synchronization requirements in Smart Grid

Several activities are performed in a Smart Grid, each of them taking part in different physical places. Some of the scheduling and control actions nowadays performed at the transmission level (such as generation scheduling) could be, in the next future, performed also at the distribution grid level. However, the perfect coordination of these activities requires a time synchronization mechanism, even more important, since the great number of elements forming the distribution grid. The

Design criteria

Actually, when decisions must be taken on real working systems many other aspects have to be considered in addition to performance, as already introduced by the authors in [17] and further investigated in [18]. In real distribution grids, the following constrains are very important when considering the time synchronization protocol to be adopted:

  • High availability: The power utilities must guarantee the delivery of power with a high quality of service and reduced downtimes. The mechanism

The communication network: heterogeneous approach

Commonly, the deployment of brand new, high performance, network infrastructure is limited to a small number among the most important substations (typically primary substations); the reason is in the high cost and the long time for cabling technologies like the Fiber Optic (FO) link. The communication infrastructure can be extended to the other substations by using other technologies with lower installation costs, like wireless (e.g., IEEE802.11), narrowband Power Line Communication (PLC) [33],

The validation methodology

As usual, in NTP the time information exchange is based on a client-server mode: the NTP client sends periodic requests for time information to the NTP server (sending time tn1). When the NTP server receives at tn2 the time request, it creates a response at tn3 and immediately sends it. Last, the NTP client receives the packet at tn4. The client, using the four previous timestamps, which are contained in the messages, estimates the round trip delay (δNTP), using (1), and the time offset of the

Conclusions

The increasing presence of distributed energy resources on MV and LV level is pushing DSOs to extend the monitoring, the control and the protection functions all along the distribution grid. This goal can be achieved only by deploying a high performance network infrastructure that reaches primary and secondary substations. Mainly for economic reasons, this infrastructure cannot be implemented in a short/middle time by using a single technology. For these reasons, the first Smart Grid pilot

Acknowledgements

This research activity has been partially funded by research grant MIUR SCN00416, “Brescia Smart Living: Integrated Energy and Services for the enhancement of the welfare”, whose support the authors gratefully acknowledge and by EU's seventh framework funding program FP7 (INTEGRIS project ICTEnergy-2009 under grant 247938).

Stefano Rinaldi was born in Seriate, Italy, in 1982. He received the M.S. (cum laude) degree in electronic engineering and the Ph.D. degree in electronic instrumentation from the University of Brescia, Brescia, Italy, in 2006 and 2010, respectively. He is currently an assistant professor with the Department of Information Engineering, University of Brescia. His current research interests include industrial real-time Ethernet network, communication in smart grids, wireless sensor network and

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    Stefano Rinaldi was born in Seriate, Italy, in 1982. He received the M.S. (cum laude) degree in electronic engineering and the Ph.D. degree in electronic instrumentation from the University of Brescia, Brescia, Italy, in 2006 and 2010, respectively. He is currently an assistant professor with the Department of Information Engineering, University of Brescia. His current research interests include industrial real-time Ethernet network, communication in smart grids, wireless sensor network and smart sensors, time synchronization methods, field-programmable gate array system-on-a-chip design, and Linux-embedded software development. Dr. Rinaldi is member of the IEEE 1588 WG.

    Davide Della Giustina received his M.S. and Ph.D. in physics from Università degli Studi di Milano, Italy in 2007 and 2010, respectively. He is currently working as project manager for a distribution utility, in research and innovation projects about the Smart Grid. Dr. Della Giustina is a member of IEC Working Group 10, Technical Committee 57.

    Paolo Ferrari was born in Italy, in 1974. In 1999 he graduated M. Sc. with honors in Electronic Engineering at the University of Brescia, Italy, where, in 2003, he received the Ph.D. degree in Electronic Instrumentation. He is employed as associate professor with the Department of Information Engineering, University of Brescia. His main research activities are signal conditioning and processing for embedded measurement instrumentation, smart sensors, sensor networking, smart grids, Real-time Ethernet and fieldbus applications. He is the author of more than 90 international papers. He is member of IEC SC65C MT9, IEC TC65C WG10 and CENELEC/IEC TC65X IRWC.

    Alessandra Flammini graduated with honors with a Laurea degree in Physics at the University of Rome in 1985. From 1985 to 1995, she was involved with industrial research and development on digital drive control. She is currently with the University of Brescia, Italy, where she was a Researcher from 1995 to 2002 and has been an associate professor since 2002. She is the responsible of the electronics laboratory and since 2004 she realized the National Competence Centre of PNI (Profibus Network Italia) for PROFIBUS and PROFINET. Her main research activity includes: electronic instrumentation; digital processing of sensor signals; smart sensors; wired and wireless sensor networks with a special attention to synchronization. She has authored and co-authored more than 150 international papers. She is a IEEE senior member since 2010. In 2009 she was the conference co-chair of ISPCS2009. She is the Conference Co-Chair of SAS2012.

    Emiliano Sisinni was born in Lauria, Potenza, Italy, in 1975. He received the Laurea degree in electronics engineering and the Ph.D. degree in electronic instrumentation from the University of Brescia, Brescia, Italy, in 2000 and 2004, respectively. Currently, he is as associate professor with the Department of Information Engineering, University of Brescia. In the past, he focused on numerical signal analysis, with particular interest in DSP-based instrumentation. His current research interests include smart sensors industrial communications, wireless sensor networking and software defined radio for cognitive radio with a focus on industrial applications. Dr. Sisinni is a member of International Electrotechnical Commission Technical Committee 65C WG 16 and WG17.

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