Proceedings of the 13th IFAC Conference on Proceedings of Devices the 13thand IFACEmbedded Conference on Programmable Systems Proceedings of the IFAC Conference on Proceedings of the 13th 13thand IFACEmbedded Conference on Programmable Devices Systems Available online at www.sciencedirect.com May 13-15, 2015. Cracow, Poland Programmable Devices and Embedded Systems Programmable Devices and Embedded Systems May 13-15, 2015. Cracow, Poland May May 13-15, 13-15, 2015. 2015. Cracow, Cracow, Poland Poland
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Time Time Time Time
Synchronization in Power-line Synchronization in Power-line Synchronization in Synchronization in Power-line Power-line Communication Communication Communication Communication
∗∗∗ Stastny ∗∗ Lesek Franek ∗∗ ∗∗ Miroslav Jirgl ∗∗∗ Stastny ∗ Lesek ∗∗ Miroslav ∗∗∗ ∗∗∗∗Franek † Jirgl ∗ ∗∗ Stastny Lesek Franek Miroslav Jirgl Stefan Zdenek Bradac ∗∗∗∗Franek † Jirgl ∗∗∗ StastnyMisik Lesek Miroslav Stefan Misik ∗∗∗∗ Zdenek Bradac † ∗∗∗∗ Zdenek Bradac † Stefan Misik Stefan Misik Zdenek Bradac ∗ ∗ Brno University of Technology, Technicka 12, Brno, 616 00 Czech ∗ Brno University of Technology, Technicka 12, Brno, 616 00 Czech ∗ Brno University of Technicka Republic (e-mail:
[email protected]) Brno University of Technology, Technology, Technicka 12, 12, Brno, Brno, 616 616 00 00 Czech Czech Republic (e-mail:
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[email protected]) University of Technology, Technicka 12, Brno, 616 ∗∗ Brno Republic Republic (e-mail:
[email protected])00 Czech ∗∗ Brno University of Technology, Technicka 12, Brno, 616 00 Czech ∗∗ Brno University of Technicka Republic (e-mail:
[email protected]) Brno University of Technology, Technology, Technicka 12, 12, Brno, Brno, 616 616 00 00 Czech Czech Republic (e-mail:
[email protected]) ∗∗∗ Republic (e-mail:
[email protected]) Brno University of Technology, Technicka 12, Brno, 616 00 Czech ∗∗∗ Republic (e-mail:
[email protected]) ∗∗∗ Brno University of Technology, Technicka 12, Brno, 616 00 Czech ∗∗∗ Brno University of Technicka Republic (e-mail:
[email protected]) Brno University of Technology, Technology, Technicka 12, 12, Brno, Brno, 616 616 00 00 Czech Czech Republic
[email protected]) ∗∗∗∗ Republic (e-mail: (e-mail:
[email protected]) of Technology, Technicka 12, Brno, 616 00 Czech ∗∗∗∗ Brno University Republic (e-mail:
[email protected]) ∗∗∗∗ Brno University of Technology, Technicka 12, Brno, 616 00 Czech ∗∗∗∗ Brno University of Technicka Republic (e-mail:
[email protected]) Brno University of Technology, Technology, Technicka 12, 12, Brno, Brno, 616 616 00 00 Czech Czech Republic (e-mail:
[email protected]) † Republic (e-mail:
[email protected]) Brno University of Technology, Technicka 12, Brno, 616 00 Czech † Republic (e-mail:
[email protected]) † Brno University of Technology, Technicka 12, Brno, 616 00 Czech † Brno University of Technicka Republic (e-mail:
[email protected]) Brno University of Technology, Technology, Technicka 12, 12, Brno, Brno, 616 616 00 00 Czech Czech Republic (e-mail:
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[email protected]) Abstract: This paper proposes possible way how to realise time synchronization for Smart Abstract: This paper proposes possible way how to realise time synchronization for Smart Abstract: proposes possible how realise time for Smart Grid devicesThis overpaper power-line communication methods 1Abstract: This paper proposes possible way way (PLC). how to to Time realisesynchronization time synchronization synchronization for like Smart Grid devices over power-line communication (PLC). Time synchronization methods like 1Grid devices over power-line communication (PLC). Time synchronization methods like 1PPS, IRIG-B, GPS time or IEEE1588 are well known and used generally in Smart Grids, Grid devices over power-line communication (PLC). Time synchronization methods like 1PPS, IRIG-B, GPS time or IEEE1588 are well known and used generally in Smart Grids, PPS, IRIG-B, GPS time or IEEE1588 are well known and used generally in Smart Grids, but not suitable for most devices with PLC. PLC channel is considered as difficult due to PPS, IRIG-B, GPS time or IEEE1588 are well known and used generally in Smart Grids, but not suitable for most devices with PLC. PLC channel is considered as difficult due to but not suitable most devices PLC considered as due to strong interferences, slow communication speeds andchannel variableis topology. Proposed solution for but notinterferences, suitable for for slow mostcommunication devices with with PLC. PLC. PLC channel istopology. considered as difficult difficult due for to strong speeds and variable Proposed solution strong interferences, slow communication speeds and variable topology. Proposed solution for low-voltage segment uses significant event speeds on line,and zero-crossing for synchronization purpose.for strong interferences, slow communication variable topology. Proposed solution low-voltage segment uses significant event on line, zero-crossing for synchronization purpose. low-voltage segment uses event on zero-crossing for synchronization purpose. low-voltage uses significant significant on line, line, zero-crossing forElsevier synchronization purpose. © 2015, IFACsegment (International Federation ofevent Automatic Control) Hosting by Ltd. All rights reserved. Keywords: power-line communication, PLC, time synchronization, zero-crossing, Smart Grids Keywords: power-line communication, PLC, time synchronization, zero-crossing, Smart Keywords: power-line power-line communication, communication, PLC, PLC, time time synchronization, synchronization, zero-crossing, zero-crossing, Smart Smart Grids Grids Keywords: Grids 2. TIME SYNCHRONIZATION 1. INTRODUCTION 2. TIME TIME SYNCHRONIZATION 1. INTRODUCTION INTRODUCTION 2. 1. 2. TIME SYNCHRONIZATION SYNCHRONIZATION 1. INTRODUCTION As mentioned before, Smart Grids are nice example of As mentioned before, Smart Grids Grids are are nice example of of mentioned before, Smart nice example distributed system. They consist ofare autonomous nodes As mentioned before,They Smart Gridsof nice example of Smart Grids concept is evolving every day. With growing As distributed system. consist autonomous nodes Smart Grids Grids concept concept is is evolving evolving every every day. day. With With growing growing distributed system. They consist autonomous that communicate with each other.of Each of these nodes Smart distributed system. They consist of autonomous nodes areas and growing requirements, Smart Grids becoming Smart Grids conceptrequirements, is evolving every day. Withbecoming growing that that communicate communicate with with each each other. other. Each Each of of these these nodes nodes areas and and growing Smart Grids has its own local time, whichother. is generally from areas requirements, Grids becoming communicate with each Each ofderived these nodes nicer example of large distributedSmart system, where every that areas and growing growing requirements, Smart Grids becoming has its own local time, which is generally derived from nicer example of large distributed system, where every has its own local time, which is generally derived from an internal clock oscillator. However, this oscillator is nicer example of large distributed system, where every has internal its own clock local time, whichHowever, is generally derived from deviceexample has some intelligence. Smart system, metering, as aevery part an nicer of intelligence. large distributed where oscillator. this oscillator is device has some Smart metering, as a part an internal clock oscillator. However, this oscillator is produced with certain tolerance and is also temperature device has some intelligence. Smart metering, as a part an internal clock oscillator. However, this oscillator is of Smart Grids, is also evolving, because to control somedevice hasGrids, someisintelligence. Smart metering, as asomepart produced with with certain certain tolerance tolerance and temperature of Smart Smart also evolving, evolving, because to control control and is is also also temperature dependent.with To conclude, synchronization process is not a of Grids, someproduced certain tolerance and is also temperature thing, you haveis toalso have some because kind of to feedback (mea- produced of Smart Grids, is also evolving, because to control somedependent. To conclude, synchronization process is not not aa thing, you have to have some kind of feedback (meadependent. To conclude, synchronization process is one-time event and that is why time synchronization hasa thing, you have to some (meaTo conclude, synchronization process is not surement). With two-way communication it is possible to dependent. thing, you With have two-way to have have communication some kind kind of of feedback feedback (meaone-time event and that is why time synchronization has surement). it is possible to one-time event and that is why time synchronization has two main goals: surement). With two-way communication it is possible to one-time event and that is why time synchronization has do not justWith remote readouts of measured itdata, but also surement). two-way communication is possible to two two main main goals: goals: do not just remote readouts of measured data, but also do not just remote readouts of measured data, but also two main goals: remotely set parameters and control meters or actuators. do not just readouts of measured but also • Establishment of the common time-base across disremotely set remote parameters and control metersdata, or • Establishment Establishment of of the the common time-base time-base across disdisremotely set parameters and control meters or actuators. actuators. • remotely set parameters and control meters or actuators. tributed systems. • Establishment of the common common time-base across across disDetailed information from meters can be used to make retributed systems. systems. Detailed information from meters can be used to make retributed • tributed Maintain nodes of a distributed system in synchrony. Detailed from meters can be reviews for information areas and find quickly black consumptions, faults Detailed from meters canconsumptions, be used used to to make make reMaintainsystems. nodes of of a distributed system system in synchrony. synchrony. views for for information areas and and find find quickly black faults ••• Maintain nodes views areas quickly black consumptions, faults Maintain nodes of aa distributed distributed system in in synchrony. or other losses. With remote control of meters/valves, views for areas and find quickly black consumptions, faults or other losses. With remote control of meters/valves, 2.1 Reasons for time synchronization or other losses. With of debtors can be disconnected orcontrol some kind of social pro- 2.1 or othercan losses. With remote remoteor control of meters/valves, meters/valves, Reasons for time time synchronization debtors be disconnected disconnected some kind kind of social social propro- 2.1 Reasons debtors can be or some of gram can bebeactivated. Two-way communication allows for time synchronization synchronization debtors canbe disconnected or some kind of socialallows pro- 2.1 Reasons for gram can activated. Two-way communication gram can be activated. Two-way communication allows regulation of production and consumption based on real Time synchronization is used as a source of accurate time gram can be activated. Two-way communication allows regulation of of production production and and consumption consumption based based on on real real Time Time synchronization synchronization is is used used as as aa source source of of accurate accurate time time regulation values fromofend-users and so avoid overloads and spike in Time information that is needed to make the system consistent. regulation production consumption on real synchronization is used as a source of accurate time values from end-users end-users andand so avoid avoid overloadsbased and spike spike in information information that is is needed needed to make the system consistent. values from and so overloads and in that to make the system consistent. distribution networks. and Especially in overloads emergencyand situations, Time synchronization is also part ofthe a systems, which are values from end-users so avoid spike in information that is needed to part makeof system consistent. distribution networks. Especially in emergency situations, Time synchronization is also a systems, which are distribution networks. in situations, synchronization part aa systems, controlled disconnection of same consumers can bring back Time not normally consider is as also real-time systems, but which where are acdistribution networks. Especially Especially in emergency emergency situations, Time synchronization is also part of of systems, which are controlled disconnection of same consumers can bring back not normally consider as real-time systems, but where accontrolled disconnection of same consumers can bring back not normally consider as real-time systems, but where acoverall stability and avoid collapse of entire network. More curate time information is necessary for their functioning. controlled disconnection same consumers can bringMore back not normally consider asisreal-time systems, where acoverall stability stability and avoid avoidofcollapse of curate time information information necessary for their theirbut functioning. overall and collapse of entire entire network. network. More More curate time is for functioning. information in Zhou et al. collapse (2010). of These include systems that use for example: file systems, overall stability and avoid entire network. curate time information is necessary necessary for their file functioning. information in Zhou et al. (2010). These include systems that use for example: systems, information in Zhou et al. (2010). These include systems use for example: file systems, database systems, cash that transactions, stock, cryptographic information in Zhou et al. (2010). These include systems that use for example: file systems, database systems, systems, cash cash transactions, transactions, stock, stock, cryptographic But to gain all benefits and possibilities of Smart Grids, database cryptographic operationssystems, and planning. Time synchronization in automaBut to to gain gain all all benefits benefits and and possibilities of Smart Grids, Grids, database cash transactions, stock, cryptographic But possibilities of Smart operations and planning. Time synchronization in automathey need, what almost every distributed system needs, But to gainwhat all benefits and possibilities ofsystem Smart Grids, operations and planning. Time synchronization in automation is used to measure time, control and planning events, they need, almost every distributed needs, operations planning.time, Timecontrol synchronization in automathey need, what almost system tion is is used usedand to measure and planning events, cooperation of whole gridevery - all distributed devices in this grid.needs, This tion they need, what almost every distributed system needs, to measure time, control and planning events, coordination and synchronization of parallel processes, cooperation of whole grid all devices in this grid. This tion is used toand measure time, controlofand planning events, cooperation of whole whole grid -- all all devices in -this this This coordination synchronization parallel processes, can be achieved by common sense of time timegrid. synchrocooperation of grid devices in This coordination of system tuningand and synchronization postmortem analysis in case processes, of failure. can be be achieved achieved by common common sense of time time - time timegrid. synchrocoordination and synchronization of parallel parallel processes, can by sense of synchrosystem tuning and postmortem analysis in case of failure. nization between devices. Because many devices, especially can be achieved common sensemany of time - timeespecially synchro- system tuning and postmortem analysis in case of failure. nization betweenby devices. Because devices, system tuning andcan postmortem in case ways, of failure. nization between devices. Because devices, especially electric meters are equipped with many power-line communicaTime information be used inanalysis many different but nization between devices. Because many devices, especially Time electric meters are equipped with power-line communicainformation can can be be used used in in many many different different ways, ways, but but electric meters are equipped with power-line communicaTime information tion, thismeters paper are is focused on possible time synchronization in general, it is possible toused summarize the reasonsways, for time electric equipped with power-line communicaTime information can be in many different but tion, this this paper paper is is focused focused on on possible possible time synchronization synchronization in general, general, it it is possible possible to to summarize summarize the the reasons reasons for time tion, for time solution for power-line communication. synchronization in two main categories: tion, thisfor paper is focused on possible time time synchronization in in general, it is is possible to summarize the reasons for time solution power-line communication. synchronization in two main categories: solution for power-line communication. synchronization solution for power-line communication. synchronization in in two two main main categories: categories:
Ladislav Ladislav Ladislav Ladislav
Copyright © 2015, IFAC IFAC 2015 (International Federation of Automatic Control) 83 Hosting by Elsevier Ltd. All rights reserved. 2405-8963 © Copyright © IFAC 2015 83 Copyright IFAC 2015 83 Peer review© of International Federation of Automatic Copyright ©under IFAC responsibility 2015 83 Control. 10.1016/j.ifacol.2015.07.012
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• Timestamps- when an event occurs • Planning- in which order actions will be done
a wide range of methods, which vary at the maximum achievable accuracy, computational cost, demands on the communication channel and other parameters. The choice of the appropriate synchronization method must be subordinated to the purpose and options for its deployment. The following section describes the methods and their basic features, which found their application in the area of Smart Grids, but non of them is directly suitable for devices like electric meters, that are hidden in buildings and only communication is done through PLC.
But before using any time information, it is necessary to think about quality of this information. This can be done by analysing quality and type of synchronization method. 2.2 Quality of synchronization The quality of synchronization can be seen from two different angles - as a type of synchronization and its quality parameters.
3.1 1-PPS
1) Type of synchronization There are 2 basic types of synchronization:
One of the oldest methods of clock synchronization is 1PPS - 1 pulse per second. This signal is characterized by pulses whose width is less than one second (typically 100 ms) and sharp ascending and descending edges, with pulses repeating exactly one second. Example of 1-PPS signal can be found in Fig. 2. Although it is a simple method that does not carry absolute time information, it is especially popular for the local clock synchronization. Due to its simplicity of implementation, it is possible to find this method in various frequency standards, radio beacons, GPS receivers and other types of precision oscillators.
• Local (internal) - the maximum difference of time between any pair of nodes • Global (external) - the maximum difference between the reference time and any other time of nodes Local synchronization is used in application, where the key requirement is, that the nodes are synchronized with each other mutually. Therefore, it is not necessary, that this time somehow reflects real time (such as UTC), it is sufficient just to be the same at all nodes. On the other hand, in the case of global synchronization, the condition of correspondence time information of the nodes with some external (reference) time, which may be mentioned, for example, UTC or other ”human” time system. 2) Quality parameters The second way of expressing the quality of synchronization is known as ”precision and accuracy” or how stable and accurate results we can get. This can be explained best by Fig. 1.
Fig. 2. Example of 1-PPS signal, from Vig (2004) Main features 1-PPS, based on Ingram et al. (2012b) + + -
Simple Smallest jitter of mentioned methods Does not provide absolute time information Does not compensate propagation time of signal Requires cable / additional channel
Nowadays, it is usual to find this method in combination with a precise time obtained from the GPS. GPS receiver provides information about the absolute time using serial interface. By principle, serial communication can achieve only limited accuracy. Therefore, high-quality GPS receivers are equipped with dedicated output pin for 1-PPS signal, which edges specify starts of seconds with minimal jitter. This allows the GPS receiver to provide absolute and also very accurate time information. In the case of using cheaper GPS receivers, that usually have larger jitter on PPS-output, the jitter can be reduced by different techniques as discussed in the Zhou et al. (2010). Reference Ingram et al. (2012b), which also deals with sync methods in the substation automation, it is also stated, that thanks to minimal jitter of this method, it is possible to obtain a standard deviation less than 2 ns using 1000 m optical cable.
Fig. 1. Quality of time synchronization, from Vig (2004) Every time synchronization deals with 3 basic challenges that also affect the assessment of the overall quality of synchronization: • Precision and accuracy • Reliability- fault models, the number of tolerable errors, ... • Efficiency- the number of nodes, the number of required messages, ...
3.2 IRIG-B
3. COMMON SYNCHRONIZATION METHODS
IRIG or Inter Range Instrumentation Group standardized various time formats. One of the latest is just IRIG standard 200-04. This standard defines the time codes A, B, D, E, G and H, which mainly differ in time frame lengthrange from 0.1 up to 1 hour. These codes standardized
The time or clock synchronization can be achieved by various methods. These methods have evolved over time as well as requirements have evolved. Therefore, there is 84
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serial form of date and time and so allow time synchronization to devices from different manufacturers.
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3.4 Packed-based methods Packed-based methods are mainly known from ethernet. They use fast connection speeds and packet exchange to establish common time base derived from master clock. One of the most famous is IEEE 1588 standard. The IEEE 1588 standard, also known as PTP, exists in two versions: an older one from 2002, and a newer one released in 2008. The latter variant is also known as the IEEE 1588-2008 or PTPv2, defined in IEEE (2008) . This method is suitable for systems requiring high accuracy of the synchronized time with an error of less than 1 us. It is nevertheless also possible, as listed in Ingram et al. (2012a), to achieve an error of less than 100 ns with proper implementation of PTPv2. The IEEE 1588 uses hardware timestamping to obtain the precise time of the received/transmitted packet and the subsequent calculation of the time offset and delay on line. The basic principle of the IEEE 1588 synchronization is presented in Fig. 3.
The most common time codes of this standard include the IRIG-B, which may be used at logic level (unmodulated) or as amplitude modulated (AM) signal of 1 kHz carrier wave. Each bit is represented by 10 ms period starting with high level. In case of bit 0, it is 2 ms of high level, for bit1, it is 5 ms and the P bit is 8 ms. P bit is used as a separator of individual items within a single frame. This time code contains the seconds, minutes, hours and number of days of the year, all in BCD format. It also contains a number of seconds and control bits, but these are in binary form. However, synchronization accuracy of this method is heavily dependent on its implementation in devices. Reference Ingram et al. (2012b) compares 1-PPS, IRIG-B and PTP and during tests it used two IRIG-B master devices and various jitters were observed. This reference also states, that the standard deviation is around 120-times greater for IRIG-B as for the 1-PPS signal, using the same transmission medium.
The use of the IEEE 1588 standard within Smart Grids was promoted by the Power profile, which is declared in the standard profile IEEE C37.238-11. The main purpose of this profile is to define the methods and parameters of the applied time transfer from a UTC-synchronized device (e.g., by GPS) to devices that use this time for their functionality (synchrophasor measurements; event logging; protection; ...), explained more in Symmetricom (2013).
IRIG-B also found its application in the area of Smart Grids, mentioned in Ingram et al. (2012b), Aweya and Al Sindi (2013). In practice, we can often found solution with IRIG-B master device equipped with a GPS receiver to obtain a reference time. Time is then distributed to individual devices using IRIG-B standard, which is supported by many manufacturers for substation automation. Its disadvantage is inability to provide information about the origin of the time. This is required by IEC 61850 used in the distribution net. 3.3 GNSS (GPS) GNSS or Global Navigation Satellite System, as the name suggests, it is a system of satellites with global coverage, which provides information about location and precise time. Two most famous GNSS include American GPS and Russian GLONASS. Time accuracy achieved via GPS is better than 100 ns, last results are even better than 10 ns, as stated in Aweya and Al Sindi (2013). This accuracy is more than enough for majority of devices and even sufficient for industrial use, including Smart Grid synchrophasors in Carta et al. (2008). GPS is preferred and used as source of precise time in substations and devices for electricity distribution. For these devices there is no pressure on their prices, so they are usually equipped with GPS receiver to ensure precise synchronization of time.
Fig. 3. The basic principle of the IEEE, from NI (2013) 4. POWER-LINE COMMUNICATION Power-line communication, for short PLC, is communication based on transfering data over power-lines. These lines are primary used to supply customer with electric power, but can be also used to communicate with devices connected to these lines. This technology is deployed in wide area, from home automation to broadcasting internet connection. PLC connection is mainly used only in one layer of grid, i.e. within one building, but some can penetrate to higher layers of distribution grids. PLC signal is usually blocked by transformer, therefore to cover bigger areas it is necessary to combine PLC with other technologies. Usually, it is combined with GPRS, where GPRS gateway to internet is installed in transformer. Another typical use-case is PLC communication over high-voltage lines, used to monitor and control substation automation. More information in Hyun and Lee (2008).
GPS might seem like perfect solution for any need for time synchronization, but the reality is different. GPS is suitable for installations, where accurate time is absolutely critical for proper operation. In these cases, higher prices caused by GPS receiver and necessity of placement antenna to see the sky, are acceptable. Antenna placement is often problematic for devices located in buildings, basements, or even underground. Solution for this situation is to use appropriately positioned GPS receiver to obtain precise time and then distribute this information by different synchronization method based on wired connection (IRIG-B, NTP, PTP, ...). For example, in the case of substation automation, it is often the IRIG-B signal, which is distributed to individual devices. 85
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While this may not seem at first look, PLC communication is similar to wireless communications in many ways. The only difference is in communication channel/medium metal vs. air. Whether PLC or radio signal, both must operate in relatively inhospitable, crowded channel, in which occurs a lot of interferences and attenuations.
hundreds of Mb/s. Such high speeds allow providing connection to the Internet including audiovisual content. On the other hand, these high frequencies can be used only for small distances, often only within a single building or even less. Suitability of broadband PLC itself within smart grids is questionable, since such small distances can only be used in densely populated areas. But on the other hand, the combination of smart metering and broadband internet access for households with home automation can be helpful in large buildings with several apartments.
4.1 PLC classes PLC communication can be categorized, as any other communication, according to different parameters- frequency band, modulation, transmission speed, distance and others. The most appropriate seems to be frequency band, because together with physical laws, it predetermines the properties of the individual communication channels. PLC communication can be divided into three categories according to frequency band. Description of classes based on Galli et al. (2011).
5. STATE OF ART Recent development in PLC communications starts to deal with need for time synchronization method via PLC channel, but standard to define protocol with native support of synchronization is still missing. This issue was started to be dealt with some scientific publications, like Dong et al. (2011), Ferrari et al. (2013) or Gallina et al. (2012). The basis for the implementation of any packet-oriented synchronization mechanism is estimation of packet arrival time or other synchronization event. Papers Ferrari et al. (2013) or Luo et al. (2008) choose PRIME as default standard because they use its frame synchronization based on preamble in form of chirp signal. Papers discuss utilization of timestamps derived from this signal and possible increase of accuracy. Reference Feng et al. (2007) use CEBus standard instead, but this standard also use chirp signal as synchronization event. It declares achieved accuracy around 10ms. This accuracy is insufficient for more sophisticated measurements.
4.2 Ultra Narrow Band These communications using frequency bands from 0.3 to 3 kHz or sometimes from 30 to 300 Hz. Low frequencies allow signal transmission distances up to 150 km and penetration of the signal through a transformer, but this is paid by low bit rates around 100 b/s. A typical representative of this category is Ripple Carrier Signalling which operates from 125 to 2000 Hz. More interesting is a system called TWACS (Two-Way Automatic Communications System), which uses voltage and current distortion at the time of zero-crossing. This system is popular in the United States, in places with lower population density, where despite the low bit rates can be collected relatively large area.
Generally, time synchronization via PLC communication is considered as difficult and the main reasons are stated in Georg Gaderer (2004):
4.3 Narrowband Frequency from 3 to 500 kHz are used for the communication. Higher frequencies allow higher transmission speeds in range ones to tens of kb/s with single carrier frequency. Unlike the ultra narrow band PLC, these frequencies penetrate through the transformer reluctantly. Problem may also occur in low-voltage systems, which contain many branches or impedance differences. In this case, it is necessary to install equipment to enhance or repeat signal.
• Slow communication speed - in compare to ethernet, communication speeds are much slower, because most common PLC communications are narrow band. Slow speed causes higher latencies and uncertainty of communication and also of possible packet-based synchronization method. • Difficult environment - PLC communication channel is typical with strong interferences. Together with low transmission power allowed by regulations, reflections on interface of two different impedances and variable topology of the grid, utilization of this channel is difficult. • Signal processing - strong interferences require high quality algorithms to process signals, that are commonly hidden in noise. These algorithms generally bring another delay to communication path, but that can be compensated because it tends to be deterministic. • Variable topology - This variability can be on different level. Most of the changes are done on low-voltage segment of the grid. Different appliances are connected and disconnected to the grid every moment. Another level of variability is when whole subgrids are connected/disconnected to another grid, this is knows as island mode of the grid.
In Europe, communication in this area is regulated CENELEC standard EN 50065 which divides the frequency range from 3 to 148.5 kHz into four ranges according to the method of use. The existence of many standardized and also proprietary protocols proves suitability of narrowband PLC for communication in smart grid area. The most important standards of PLC communication include: • • • • • •
PRIME G3 S-FSK (Spread frequency shift keying) LonWorks KNX CEBus
4.4 Broadband Broadband PLC communication uses a frequency range from 1.8 to 250 MHz, which reaches speeds of ones to 86
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6. TIME SYNCHRONIZATION OVER PLC FOR LOW-VOLTAGE DEVICES
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6.1 Identification of synchronization period First phase is about getting information, that will identify one specific synchronization period. For this task is sufficient only local synchronization between devices in one low-voltage segment. Dataconcentrator (master device),that is located close to transformer, must somehow specify to electric meters (slaves) one period, that will be used in second phase.
For wide area measurement systems it is necessary to have measurement on different places synchronous. Synchronization can be done by 2 approaches: • Start measurement right after synchronization event • Time synchronization and planning of measurement to specific time
One of the following methods is assumed:
According to nature of PLC communication, first approach is not suitable, because starting command may not be delivered on first try. Second approach is far more sophisticated, when time synchronization is used to set precise time for device and measurement is planned to specific time. With planning measurement to the future, we get enough time to synchronize time and safely deliver command to all devices.
• Packet-oriented approach - similar to the NTP/PTP protocol, similar to reference Feng et al. (2007) using a chirp signal. • Form of very narrowband PLC - synchronization signal transmitted during zero crossing. 6.2 Detection of zero crossing
As was stated before, synchronization over PLC is not simple for different reasons. Slow communication speed is one of them. Majority of referenced papers does not state archived accuracy of synchronization, only in reference Feng et al. (2007) is state accuracy around 10 ms after filtration. This is insufficient - for SCADA realization is 1 ms considered as satisfactory, or for synchrophasor measurement is needed around ± 30 µs. Therefore, new synchronization method is proposed for PLC channel, which accuracy should ± 30 µs or better.
In second phase, all devices will focus on synchronization period selected in first phase and try to detect zero crossing event as accurate as possible. This will restart internal high-accurate counter. Master device, that is equipped with GPS receiver, is able to timestamp this event with accurate time for GPS. After this phase, all devices are synchronized locally, but not globally according to some reference time (i.e., UTC). The key of accuracy for this method is derived from accuracy of zero crossing detection. Based on consultation with employees of company ModemTec s.r.o., which is engaged in production of PLC communication modules and smart meters, with proper filtration, sampling and analyse of signal should be possible to achieve accuracy about 10 µs. But this professional estimation must be verified.
Proposed method is based on assumption, that only one period of voltage is presented on one low-voltage segment of the grid. According to consultation with employees of distributor company, length of one low-voltage segment (from supplying transformer to the farthest consumer) is almost exclusively less then 2 km, because of voltage drop limitation. This assumption was verified by calculating λ for distribution grid operating at frequency f = 50Hz. Because wave propagation speed in cooper conductor is approximately 0.6 times slower than the speed of propagation in vacuum, λ can be stated as: k∗v 0.6 ∗ 3 ∗ 108 λ= = = 3600km (1) f 50
6.3 Assignment of precise time In third phase, master device would distribute timestamp, that was acquired from GPS in the moment of synchronization zero crossing. Slave devices add this timestamp to current value of precise counter present in every device. This value is cumulated from the moment of zero crossing detected by slave. After this, every slave has globally synchronized time information and it is able to operate in wide area measurements and other associated activities typical for distributed system.
It is clear from calculation, that only one period can be present on whole low-voltage segment in the moment. However, due to the length of the segment, there is a shift of the zero crossing at 2 km distance for: l 2 . ∆t = ∗ T = ∗ 0, 02 = 11µs (2) λ 3600 But this shift is deterministic and therefore can be easily compensated. During device installation to the grid, only temporary connected GPS receiver can provide extremely accurate time for calibration of device. Calibration would be able to calculate this shift and compensate it. After this, zero crossing event would be at ’same’ moment for connected device and for master device, that will later provide accurate time from GPS.
7. CONCLUSION This paper discusses time synchronization issue in Smart Grid area with focus on power-line communication (PLC). Time synchronization is essential for any distributed system and also for Smart Grid, where precise cooperation of devices lead to more stable and effective grid. Common synchronization methods in Smart Grids were introduced and were explained difficulties and importance of time synchronization over PLC, that is still at beginnings. Finally was proposed possible way of synchronization over PLC, that utilizes significant event like zero-crossing to provide synchronization event for connected devices.
Proposed synchronization method consists from 3 phases: (1) Identification of synchronization period (2) Detection of zero crossing (3) Assignment of precise time 87
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Future work will focus on high-precise zero-crossing detection methods and its utilization in proposed concept of time synchronization over PLC channel. ACKNOWLEDGEMENTS The research was financially supported by Brno University of Technology and the European Regional Development Fund under project No. CZ.1.05/2.1.00/01.0014. The above-mentioned funds and institutions facilitated efficient performance of the presented research and associated tasks. This work was supported also by the project TA02010864 - Research and development of motorized ventilation for the human protection against chemical agents, dust and biological agents, project TA03020907 - REVYT - Recuperation of the lift loss energy for the lift idle consumption and project TA04021653 - Automatic Lift Inspection granted by Technology Agency of the Czech Republic (TA R). Part of the work was supported by project FR-TI4/642 - MISE - Employment of Modern Intelligent MEMS Sensors for Buildings Automation and Security granted by Ministry of Industry and Trade of Czech Republic (MPO). Part of the work was carried out with the support of core facilities of CEITEC Central European Institute of Technology under CEITEC open access project, ID number LM2011020, funded by the Ministry of Education, Youth and Sports of the Czech Republic under the activity Projects of major infrastructures for research, development and innovations. Part of this paper was made possible by grant No. FEKTS-14-2429 - ”The research of new control methods, measurement procedures and intelligent instruments in automation”, and the related financial assistance was provided from the internal science fund of Brno University of Technology. REFERENCES Aweya, J. and Al Sindi, N. (2013). Role of time synchronization in power system automation and smart grids. In Industrial Technology (ICIT), 2013 IEEE International Conference on, 1392–1397. doi: 10.1109/ICIT.2013.6505875. Carta, A., Locci, N., Muscas, C., and Sulis, S. (2008). A flexible gps-based system for synchronized phasor measurement in electric distribution networks. Instrumentation and Measurement, IEEE Transactions on, 57(11), 2450–2456. doi:10.1109/TIM.2008.924930. Dong, L., Baohui, Z., Dongwen, N., Bo, Z.Q., and Klimek, A. (2011). Design and implement of timing synchronizatin algorithm for ofdm plc system in low voltage powerline networks. In Universities’ Power Engineering Conference (UPEC), Proceedings of 2011 46th International, 1–4. Feng, P., Wu, G., Li, G., and Bai, Y. (2007). Study of a new time transfer method of low voltage power line. In Frequency Control Symposium, 2007 Joint with the 21st European Frequency and Time Forum. IEEE International, 823–826. doi:10.1109/FREQ.2007.4319191. Ferrari, P., Flammini, A., Rinaldi, S., Rizzi, M., and Sisinni, E. (2013). Time of arrival estimation in power line communication systems for home smart 88
