The Research Of Time Unified System In Smart Grid

2012 7th International ICST Conference on Communications and Networking in China (CHINACOM) The Research Of Time Unified System In Smart Grid Lv Yao Yunnan Power Grid Corporation Information Center Kunming, P.R.China lvyao723@163.com Wang Jianjuan Luo Yunlong Kunming power supply bureau Kunming, P.R.China 18908718692@189.cn aklylrrr@163.com Abstract According to the intelligent substation and the increasing synchronization requirements of Green Power Grid, it is important to establish a communication support network of time synchronization in the whole power grid. This paper offers a comprehensive introduction and detailed analysis of the basic concept, signal type and the networking technology of time synchronization. According to the application requirements of power system in Yunnan Power Grid and the characteristics of power communication network, the DCLS +NTP time synchronization networking mode based on SDH+IP network, the NTP time synchronization networking mode based on IP network, the PTP time synchronization networking mode based on SDH +IP network and the PTP time synchronization networking mode based on PTN+OTN in future are investigated, they will be the reference of time synchronization network formation in Yunnan Power Grid. Keywords- time synchronization; DCLS; NTP; PTP; Networking mode. I. THE REQUIREMENTS FOR SMART GRID ON THE TIME SYNCHRONIZATION SYSTEM Power Grid is time-related system, the voltage, current, phase angle, power angle changes are based on the timeline waveform. Power Grid on time synchronization requirements reflect the analysis and judgment in power dispatching, power grid failure and real time control areas directly related to electric power production [1]. In the smart grid, the application of digital substation construction and intelligent dispatch techniques will provide better guarantee in power grid operation safe and stability. The time synchronization system is an important part of the digital substation construction, whether it is in the digital substation protection devices, monitoring and control devices, electronic transformer, and intelligent switch cannot be separated from the synchronization information, so the smart grid on time synchronization for more urgent needs[2]. Intelligent dispatch technique adopts dispatching data integration technology to achieve the normal operation of power systems, early warning control, incident identification, failure analysis and processing, dispatching, power grid operation and management intelligent. The High precision time synchronization system is more conducive to the power system fault analysis, fault location, troubleshooting and prediction, adaptive protection, selfhealing control and other functional requirements [3]. In addition, the smart grid raised very high demand to information system time synchronization. On the one hand, the same application system in the Province Corporation and Prefectural-Level city Power Supply Bureau to longitudinal unified; on the other hand, the application system and the electric power production system must also be horizontal unified. Therefore, the time synchronization between application systems in power grid data center is the important constituent in the whole power grid Unified time system. II. OVERVIEW OF TIME SYNCHRONIZATION NETWORK A. Time synchronization Time synchronization is used to transmit the time signal; the information is years, months, days, hours, minutes, seconds. The time deviation between devices on the network and UTC (Coordinated Universal Time) is defined in a sufficiently small range based on time synchronization. From a time source GPS standard time, the time information is through the time distribution link of Bureau/Board sent to the various equipments which need of time synchronization. B. Networking mode Time synchronization network is mainly composed of a time source, a time allocator equipment and transmission link. For the large-scale networks which contains several subnets in the power grid, in order to achieve real time synchronization of the whole network should use master-slave structure to setup time synchronization network. In this structure, the first and foremost task is the classification of the time source, usually in the network, the GPS receiver to provide the reference time source (PTS) as a zero-level of time synchronization network, and one-level time server is connected to GPS receiver which through time signal direct transmission technology, it receives PTS which provided by GPS, and the reference time is transmitted to a connected network element or a two-level time server. Two-level time server only receives a time reference which provides by one-level time server, it completes time allocation and protocol conversion function, provides the network timing service, and transmits time signal to the downstream time server, to realize the time synchronization of master-slave hierarchical structure. Usually the one-level time servers called the primary time server, the two or two levels below time server called the slave time server. 81 978-1-4673-2699-5/12/$31.00 2012 IEEE

C. Timing source GPS (Global Positioning System) in the land, sea and sky, is a full range of real-time three-dimensional navigation and positioning capabilities of satellite navigation and positioning systems. The all-weather, high precision, automation, high efficiency and other significant features, to provide timing and positioning functions in a global scale, anywhere in the world of GPS users to use low-cost GPS receiver to receive the signal from the satellite, and obtain accurate location information, synchronous time scale and standard time through the latitude, longitude, altitude, and time deviation between user clock and GPS master clock standard time. BeiDou (COMPASS) Navigation Satellite System is independent development by China, and independent operation of the global navigation satellite system. The system is designed for the users who need high accuracy, high reliability of satellite navigation and provide positioning, Velocity Measuring, timing, communication service and system integrity information [4]. D. Time signal transmission mode Power system communication network is based on a wired communication network, which is responsible for data information transmission, such as SDH/MSTP, data communication network, the IRIG-B code ( inter range instrumentation group-b), DCLS (DC level shift), NTP ( Network Time Protocol), PTP (Precision Time Protocol) is widely used for wired communication network transmission time synchronization information in the communication industry. IRIG-B: the transmission medium can be twisted pair (1.2 km) and a coaxial cable, the accuracy is 10~100 s; DCLS: a special form of the IRIG-B, is transmitted through the DDN special line of 64 Kbps, no transmission distance limit, the accuracy is 100 ~ 1000μs; NTP: Provides highly accurate, reliable time standard Internet protocol in the network, based on UDP packets. Its accuracy in LAN for10μs~10ms, 100~1000 ms in the Internet; PTP: IEEE 1588(precision clock synchronization protocol for networked measurement and control system) for a precision time protocol, used for standard Ethernet or other using multicast technology of distributed bus system terminal equipment to the sub-μs-level synchronization. Compared with the NTP mode, the PTP has obvious advantages: two-way channel, precision is the ns level, low resource consumption, and can adapt to different access environment. III. YUNNAN POWER GRID TIME SYNCHRONIZATION NETWORKING TECHNOLOGY RESEARCH A. Yunnan Power Grid digital synchronization network status Yunnan Power Grid digital synchronization network is a province synchronization network of China Southern Power Grid. Yunnan Power Grid Corporation Communication Center and Yuxi Power Supply Bureau Communication Center building one-level clock node, set the LPR regional reference clock and standby for each other, as the reference clock of the Yunnan Power Communication Network. Yunnan Power digital synchronization network with master-slave structure, in Dali, Qujing, Honghe, Chuxiong, Puer and Lincang 6 regional Power Supply Bureau communication center building twolevel clock nodes. The three-level clock nodes synchronize two-level clocks in local region; the entire network formed the province synchronization area by the satellite signal as the source. When the satellite signal is not available, synchronization with the terrestrial is linked. Yunnan Power Grid Corporation Communication Center and Yuxi Power Supply Bureau set up a time server, which receives the time reference signal provided by the local LPR, while providing high-precision (1PPS, TOD (IS-95), IRIG-A/B, DCLS, NTP) time output signal. In Dali, Qujing, Honghe, Chuxiong, Puer and Lincang 6 regional Power Supply Bureau communications centers to set up a time server, which receives the time reference signal provided by the regional bureaus, while providing the time signal of NTP, TOD and DCLS. Figure 1. Yunnan Power Grid Synchronization Network Status. 82

Yunnan Power Grid digital synchronization network mainly to provide clock synchronization and frequency reference for a variety of digital communication equipment, to solve the frequency and phase synchronization problem in communication network; the digital synchronization network with time synchronization function, but time synchronization has not yet been solved. B. DCLS+NTP time synchronization networking mode based on SDH+IP network Using the DCLS interfaces of Yunnan Power Grid Corporation synchronous equipment, DCLS time code signal is transmitted to the two-level time server node through the provincial SDH network E1channel, forming a master-slave structure of the time synchronization network; Using the NTP interfaces of Yunnan Power Grid Corporation synchronous equipment, the time signal is transmitted to the application system server of the province company data center through the Yunnan power integrated service data network; two-level node as a time server to provide reference time signal, DCLS signals transmitted to the substation through regional optical transport network, establishing the two-level time synchronization network, substation unified time synchronization server via the expansion unit provides the time application of every systems. At the same time, two-level time server using NTP interface provides time signal to the application system of regional data center through the IP metropolitan area network (MAN). Network structure is shown in figure 2. Using E1 channel of SDH network transmission DCLS signal, subject to the inherent drift of the SDH network, jitter and pointer adjustment, making the accuracy of DCLS time signal decreased, cannot meet the time precision requirements of traveling wave fault location and lightning location [5]. Using DCLS transmission mode will take up a lot of communication resources, and bring certain pressure to the entire transmission network. C. NTP Time Synchronization networking based on IP network Yunnan Power Grid Corporation synchronous equipment receives the time reference signal provided by the local LPR, using the time server NTP interface, the time signal is transmitted to the province company data center application system servers and each of the two-level time servers through the IP network consisted by Yunnan power integrated service data network, forming a master-slave structure of the time synchronization network, the time signal of two-level nodes as a reference time, NTP time signals transmitted to the regional data center application system servers and unified time servers in substation through the regional IP MAN, establishing the two-level time synchronization network, final time synchronization servers in substation via the expansion unit provide time application for the client systems. Network structure is shown in figure 3. In the NTP time synchronization network, time signal transmitted through the IP network. External NTP signal received from the master time server which can be manually or automatically switch between multiple master time servers by selecting the IP; thereby achieving 2 or more redundant reference signal configuration, redundancy is determined by the types of slave time server. This feature of the NTP time synchronization network has greatly improved the reliability of the system. NTP networking technology is mature, suitable for the sites which power system IP network has covered, but due to the inherent properties of the IP network, the synchronization precision is low, cannot meet many requirements of μs and ms precision level in power system [6]. Figure 3. NTP time synchronization networking mode based on IP network. Figure 2. DCLS +NTP time synchronization networking mode based on SDH+IP network. D. PTP Time Synchronization networking based on SDH+IP network PTP is IEEE 1588 (network measurement and control system for precision clock synchronization protocol) 83

specification of a Precision Time Protocol, is slightly different from the NTP which use the software, PTP through the way which hardware and software used in conjunction to send more precise time synchronization information, to achieve synchronization of slave clocks with master clock. PTP protocol not only inherited the NTP Ethernet-based transmission mode, while providing superior to the IRIG-B time synchronization accuracy, and time information will not be affected by too much delay and jitter when signal transmitted in the general network [7]. Power SDH optical transmission communication network in the present and future for a long time will remain the dominant position of power communication, so SDH network can be used to realize the PTP synchronization [8-9]. Time information transmission must take into account delay and jitter effects, SDH mapping delay and jitter caused by pointer adjustments will be eliminated through a reasonable algorithm to E1 signal, and the PTP data package as a business use can be transmitted by SDH. PTP time synchronization network structure is based on the SDH network shown in Figure 4. PTP switch configured in the Yunnan Power Grid Corporation will extend a one-level PTP time signal to the multiple output ports, so that two-level PTP time server in the regional communications center synchronous with one-level time server. At the same time protocol converter will be configured, the output Ethernet frame from switch will be converted to E1signal, and protocol converter will filter E1signal jitter through the compensation algorithm. Using SDH provincial network, open E1channel of Province Company to Power Supply Bureau communication center, this is used for transmission of PTP time synchronization service. Protocol Converter also configured around the Power Supply Bureau Communication Center, E1 signal of the SDH output is converted to the Ethernet frame format as the input of the PTP two-level time server. PTP switch configured around the Power Supply Bureau Communications Center, two-level PTP time signal is extended to the multiple output ports, and provides time synchronization service to regional substations, the switch output Ethernet frame is converted to E1signal by Protocol Converter, using regional SDH network, opened E1 channel of Power Supply Bureau Communication center to the substation for the PTP time synchronization service transmission. In the substation are equipped with one set of protocol converter, the SDH output E1signal is converted to the Ethernet frame format for PTP time server input. One set of time expansion equipment is configured in each substation, PTP synchronization signal from time server to time expansion equipment, time expansion equipment has a wealth of synchronous signal interface, the signal in the form of PTP, NTP, B code assigned to the secondary equipment. current mainstream switches and routers are able to support the PTP protocol, province company PTP time server through Yunnan power integrated service data network for each application system server in province company data center provides service of time synchronization; two-level PTP time servers through regional IP MAN for regional data center application system servers provide the time synchronization service. Figure 4. PTP time synchronization networking mode based on SDH+IP network. E. PTP time synchronization networking based on the next generation transmission network of PTN+OTN Power communications applications are gradually changing from voice communication to data communication. Data communication business has exceeded 80% of the total bandwidth requirements. In a variety of data services, IP protocol occupies a dominant position, some of the traditional business, such as dispatching automation, is being gradually abandoned special Statute of use for many years, instead of using the IP protocol; International Electrotechnical Commission promulgated digital substation communication standard IEC 61850is based on the Ethernet standard, using the standard TCP/IP communication [10]-[11]. IP-based business inevitably require IP-based transmission network, which requires the transmission network from circuit cross core converse to packet switching core, the use of packet switching core to achieve the efficient transmission of the packet service. By the development of smart grid, the proportion of power communication in IP-based business will rise in further. With the development of the power communication, a new generation of IP-based multi-service transmission technology based on packet transmission PTN and IP-based WDM technology OTN will give the power grid a new generation of information and communication platform, so the IP-based time synchronization system need to be provided to the real-time business and the protection business which require high quality, high reliability and low delay. When the synchronization information transmitted in the network using the PTP protocol, even if the synchronous 84

control signal to produce a certain fluctuations, the output time synchronization signal can still meet the accuracy requirements, the accuracy up to μs level, which makes the PTP is particularly suitable for Ethernet-based system, special suitable for the next generation transmission network of PTN+OTN. 1) Technical scheme PTN is a new generation of IP multi-service transmission technology, the PTP time synchronization is one of the most important features.otn Wavelength division multiplexing technology based on IP can also solve the time synchronization problem, OTN is asynchronous mapping network, it does not need to transmit a synchronizing signal, but OTN can transmit synchronization signal which SDH require as its user. OTN opened time synchronization function requires increased clock processing module, support PTP protocol and provides a time delay compensation function, and need to transform the line transmission module to provide the processing functions of the PTP packets. After OTN supports PTP transfer function, a real end-to-end time synchronization transmission network can be built. The core backbone layer OTN obtain time information from mutual backup BITS first. Then Time synchronization information is sent to the various PTN subnets through OTN. At last PTN send time synchronization information to each site. 2) Networking Scheme 110 kv and above voltage class substation of Yunnan Power Grid will be commonly used by the IEC 61850 specification, the provincial OTN transmission network will cover the provincial data center, regional data centers and 500kV substation, regional PTN transmission network will cover the 110kV and above substation, provincial OTN transmission network and regional PTN network will form a unified classification time synchronization network based on the PTP time protocol. In the province company set up a PTP master clock as time center of Yunnan Power Grid, which provide a time reference source; at the same time, a set of PTP master clock as a standby center, which complement each other with the clock of provincial company; PTP clock configured around regional Power Supply Bureau as a boundary clock of the hierarchy time synchronization network, which both as a slave clock of provincial company master clock and standby center master clock, and as a master clock of regional substations; each substation configuration PTP clock, as the slave clock of regional Power Supply Bureau clock, as well as the master clock of automation equipment in substation. OTN transmission network and the PTN transmission network establish a structured time synchronization network between provincial company of Yunnan Power Grid, regional power supply bureau, data center and substation. PTP master clock synchronize with a GPS, set the PTP switch to increase PTP output port. The synchronization system based on PTN+OTN ground link transmission, the network time synchronization accuracy could reach μs level, or even higher, which fully meet the accuracy requirements for power system internal time synchronization signal. Figure 5. PTP time synchronization networking mode based on PTN+OTN. IV. CONCLUSIONS The whole network synchronization is the real synchronization. The time synchronization network based on ground link transmission, mutual standby for air and ground link, which provides time and frequency synchronization of high precision and high reliability for Yunnan Power Grid. This paper discusses the DCLS +NTP time synchronization networking mode based on SDH+IP network, the NTP time synchronization networking mode based on IP network, the PTP time synchronization networking mode based on SDH +IP network and the PTP time synchronization networking mode based on PTN+OTN in future, which can effectively solve the problem of time synchronization signal transport in ground, they will be the reference of time synchronization network formation in Yunnan Power Grid. REFERENCES [1] YU Yuehai, ZHANG Daonong and HU Yonghui, et al., Time Synchronizing System for Power System, Automation of Electric Power Systems. vol.32 No.7, pp.82-86,april 2008. [2] HUA Huang-sheng, WANG Li, The Related Issues about Time Synchronization System in Digital Substations, Telecommunications for Electric Power System. Vol.32 No.219, pp. 28-32, Janurary 2011. [3] HU Zhiguang, WENG Yishan and ZOU Guohui, et al., Network Time Synchronization in Power Dispatching Communication Based on IEEE1588, Water Resources and Power. Vo l. 29 No.4, pp. 174-176, April 2011. 85

[4] Chen Hongqing, Beidou satellite navigation system and the application foundation ofpositioning, navigation & timing (PNT) in China, Journal ofelectronicmeasuramentandinstrument, Vol.24 No.12, pp.1075-1081. [5] HU Chun-yang, JIAO Qun, Feasibility Study on Time System in Power Grid, Telecommunications for Electric Power System, Vol.32 No.219, pp. 1-2, Janurary 2011. [6] Huang Xiaoyun, Discussion on the application of NTP in the time synchronization of power automation devices, Automation of Electric Power Systems,29(15), pp. 93-95, 2005. [7] IEEE 1588. Standard for a Precision Clock Synchronization Protocol fornetworked Measurement and Control. 2008. [8] Liu Weihua, PTP Synchronization System Based on SDH Transmission Terrestrial Links, Shandongelectric power, NO.5, pp. 36-39, 2010. [9] KIHARA M, IMAOKA A and IMAE M, et al., Two-Way Time Transfer Through 2.4 Gbit/s Optical SDH System, IEEE Transactions On Instrumentation and Measurement, 50(3), pp. 709-7152001. [10] IEC 61850-9-1. Communication Networks and System in Substations. 2003. [11] IEC 61850-9-2. Communication Networks and System in Substations. 2003. 86