A Novel On-line Inspection System for Transmission Lines Using Optical Ground Wires Jinrui Tang*, Xianggen Yin, Zhe Zhang State Key Laboratory ofAdvanced Electromagnetic Engineering and Technology (Huazhong University of Science and Technology), Wuhan 430074, Hubei, China Email: tangjinrui@hotmail.com Abstract. In order to obtain the desired visible images, the infrared images and the meteorological data for the inspection, a novel on-line inspection system for transmission lines is designed and proposed in this paper. The system consists of tower terminal units, Ethernet passive optical networks and master stations. Due to the huge monitoring data sampled by the tower terminal units at each tower, Ethernet passive optical networks are used to transmit the monitoring data in real-time. The fibers in the optical fiber composite overhead ground wires are used as the trunk fibers in the communication systems. They extend from the optical line terminal to the passive optical splitters located in the splice trays at each tower. Then the optical drop fibers are used to connect the tower terminal units and the splitters. A single-trunk-fiber network scheme, multiple-trunk-fiber network scheme and cascaded-OLT network scheme are proposed for the communication systems. The effectiveness and practicability of the proposed inspection system were verified by simulation results and the field data. Keywords: EPON, inspection, OPGW, transmission line Sistem za sproten nadzor vodnikov na daljnovodih z uporabo optičnega omrežja V članku je predstavljen komunikacijski sistem za sproten nadzor vodnikov na daljnovodih. Sistem sestoji iz terminalne enote, optičnega omrežja ethernet in nadzorne postaje. Zaradi velike količine podatkov, katere zajemamo na vsakem daljnovodnem stolpu, smo za prenos podatkov v resničnem času uporabili omrežje ethernet. Podatki se po zajemu prenesejo do terminalne enote, kjer se izvede predobdelava zajetih podatkov, nato pa se prenesejo prek optičnega omrežja v nadzorno postajo. V prispevku smo predstavili tehnike za prenos velikih količin podatkov po optičnem komunikacijskem sistemu. Učinkovitost in uporabnost predlaganega pristopa smo preverili s simulacijami in testi v resničnem okolju. 1 Introduction With the development of the power systems, the total length and coverage area of the high-voltage transmission lines has increased significantly. For the transmission lines and their attaching equipments to work safely, inspections are usually required. The main inspection method is the manual inspection on schedule. Its disadvantages are high working intensity and high danger. Meanwhile, the manual inspection will be very hard to perform in a complicated geographical environment, such as mountain areas and aboriginal forests, or in severe weather conditions. Recently, the helicopter and robot inspections for transmission lines have been presented and used in routine inspections [1-3]. But it is difficult for the helicopters to inspect in a complicated geographical environment or severe weather. More researches are needed to make the robot inspections practical, such as obstacle crossing and obstacle recognition. Meanwhile, the routine inspections cannot monitor the equipments in real-time and may cause overwork or absentation. So a novel inspection method is urgently needed to replace the routine manual inspections. In order to monitor the equipment in real-time, the condition monitoring systems have been introduced into the electric power systems, including electric magnitudes monitoring [4], mechanical magnitudes monitoring [5], environmental monitoring, etc [6]. These systems are generally focused on one particular aspect and cannot obtain the overall operating parameters of transmission lines. Meanwhile, the general packet radio service (GPRS) or the code division multiple access (CDMA) are usually used as the communication systems in these monitoring systems, so the desired visible and the infrared images sampled at each tower cannot be transmitted in real time. Because these systems cannot meet the inspection requirements, the utilization ratio of these systems is low. To overcome these drawbacks, a novel on-line inspection system for transmission lines is designed and proposed in this paper. It consists of tower terminal Received 15 January 2014 Accepted 26 February 2014 units, optic-fiber communication systems and master stations. The tower terminal units are designed to obtain the desired data for inspections and are installed at each tower along the transmission lines. The optic-fiber communication systems are used to transmit the monitoring data, including the visible and the infrared images and meteorological data. Meanwhile, a practical fusion splicing technique of the optical fiber composite overhead ground wire (OPGW) is applied in the proposed on-line inspection system. 2 Structure of the proposed on-line INSPECTION SYSTEM Fig. 1 shows the structure of the proposed on-line inspection system, including the tower terminal units, optic-fiber communication systems and master stations. The tower terminal unit consists of measuring units, terminal-host unit, power unit and optical network unit (ONU). The measuring units are used to sample the desired visible and infrared images and the meteorological data for the inspection. The terminalhost unit is used to collect the monitoring data, preprocess the data, upload the data and receive the commands given by the master station to control the devices. The appropriate structure of the inspection system should be selected according to the field situation. Fig. 2 shows a two-layer structure, which includes the tower terminal units and the master station. Fig. 3 shows a three-layer structure, which includes the tower terminal units, the vice-master stations and the master station. Tower terminal unit ! | ^Hf^ ^Terminalhost^| ONU ~| I | Power unit -^ Tower • < terminal unit ! | —►Terminal hort~^| ONU ~ I | Power unit -^ Figure 1. Structure of the proposed on-line inspection system Figure 2. Two-layer structure of the inspection system Master station Figure 3. Three-layer structure of the inspection system For the three-layer structure shown in Fig. 3, the monitoring data is transmitted to the vice-master stations firstly. Then the data reception, data storage, data analyses, data query and early warning are realized in the vice-master stations. After processing, the data is transmitted to the master station via electric power private telecommunication networks. The master station can query the storage data in the vice-master stations and analyze the accidents. Table 1. The monitored objects in the transmission line inspections Monitoring types Monitored objects Transmission line corridor Tower fundations and protection facilities Tower and bracing wires Visible images Conductors and ground wires monitoring Insulators, insulator cross-arms and fittings Lightning protection facilities Vibration dampers and other subsidiary _facilities_ Infrared images Composite insulators monitoring_splices_ The functions of the system depend on the inspection contents adopted by electric utilities, mainly including visible-image monitoring, infrared-image monitoring and microclimate monitoring. The monitored objects are listed in Table 1 according to the inspection contents. 3 OPTIC-FIBER communication systems 3.1 Communication mode The monitoring data sampled by the tower terminal units installed at each tower contains the visible and the infrared images, etc. So a high-bandwidth communication system should be designed to transmit the monitoring data in real-time. The bandwidth required by one tower terminal unit is given in Table 2. Table 2. The bandwidth required by one tower terminal unit Monitoring types Bandwidth/(bps) Visible images monitoring Infrared images monitoring microclimates monitoring 1.2M, two cameras 0.6M, one camera 0.014M Table 2 shows that the visible and the infrared images dominate the monitoring data. The bandwidth required by one tower terminal unit is about 2M bps. The communication modes used in the electric power systems have Ethernet passive optical networks (EPONs), industrial Ethernets, GPRS/CDMA and wireless communication mode. In this paper, EPONs are chosen to transmit the huge data because of their low-cost point-to-multipoint optical infrastructure with low-cost high-bandwidth Ethernet. 3.2 Fusion-splicing technique of OPGW The monitoring data sampled at each tower should be uploaded to the master station. Firstly, the sampled data is transmitted to the terminal-host unit. After preprocessing, the data is transmitted to the optical network units (ONU) installed in the tower terminal unit. Then ONU is connected to the passive optical splitter (POS) through the drop fiber. The splices are carried out at each tower as shown in Fig. 4. Splice tray Tower terminal ' unit Figure 4. Fusion splicing of OPGW Based on the fusion splicing technique of OPGW, the monitoring data can be transmitted to EPONs at each tower. 3.3 Network schemes for EPON EPON is a passive optical network (PON) that carries all data encapsulated in THE Ethernet frames. All transmissions in PON are performed between an optical line terminal (OLT) and ONUs. In the proposed inspection system, OLT resides in the electric substation connecting the optical access network to the electric- power private telecommunication networks. ONU is located in the tower terminal unit installed at each tower. An appropriate network scheme for EPON should be designed based on the transmission route and the idle fibers in OPGW. The designed schemes should guarantee that the optical total loss between OLT and each ONU is less than the optical power budget shown in (1). P = ZX + Z H + Z R + Z F + G ^ pr (1) where, P represents the optical total loss between OLT and ONU. PT is the optical power budget determined according to the fiber optic communications equipment. Xi is the transmission loss of the optical fiber in the ith section. Hi is the loss of the optical fiber connectors in the ith section. Ri is the splitting loss in the ith section. Fi is the insertion loss of the POS in the /'th section. And G represents the optical power margin loss. 3.3.1 Single-trunk- fiber network scheme As shown in Fig. 5, only a single fiber in the OPGW is used to transmit the monitoring data in the single-trunk-fiber network scheme. In this scheme, cascaded optical splitters are used and the loss of the optical power can be calculated by n 2 2n n-\ Pn = ZX + ZH + ZR + (£F + Fzn) + G (2) i=\ i=\ i=\ i=\ n \ 2n+\ n Pn =Z X- +Z H + Z R +Z F, + G (3) i=\ i=\ i=\ i=\ where, Pzn represents the optical power loss between OLT and ONU installed at the nth tower. Pgn represents the optical power loss in the trunk fiber between OLT and the nth tower. Fgi is the insertion loss of POS along the trunk fiber at the nth tower. Fzi is the insertion loss of POS along the branch fiber at the nth tower. The splitting ratios of the splitters should be chosen so as to guarantee that the network can provide as many ONUs as possible to access. Then the following splitting ratios should be chosen in the sequence of 95:5, 90:10, 80:20, etc. 3.3.2 Multiple-trunk- fiber network scheme As shown in Fig. 6, the multiple-trunk-fiber scheme uses a multiple trunk fiber in OPGW to transmit the data. Firstly, ONUs access the first trunk fiber. When the actual loss between OLT and the last ONU is larger than the optical power budget, ONUs stop accessing the first trunk fiber and access the second fiber, and so on. 3.3.3 Cascaded-OLT network scheme As shown in Fig. 7, two fibers are used and OLTs are installed along the transmission lines. This scheme can be used in the inspection system for the ultra long transmission lines. OLT^ ONU ONU ONU ONU ONU ONU Figure 5. Single-trunk- fiber network scheme Figure 6. Multiple-trunk- fiber network scheme Figure 7. Cascaded-OLT network scheme 4 Case study In the case study: 1) the optical power budget is set of 30 dB; 2) the tranmission loss of the fiber is set of 0.35 dB/km when the wavelength is 1310 nm; 3) the loss of the active optic-fiber connector is set of 0.5 dB per connector; 4) the splitting loss is set of 0.1 dB per joint; 5) the POS insertion loss is given in Table 3; 6) the optical power margin is given in Table 4.; 7) the tower span is set of 300 m. For the single-trunk-fiber network scheme taken as an example, the calculation results of the optical power along the network are given in Table 5. Namely, twenty-four tower terminal units can access EPON. Table 3. The insertion loss of the passive optical splitter The insertion loss/(dB) Splitting Ratio Output port along the trunk Output port along the branch 95:5 0.45 15.2 90:10 0.6 11.3 80:20 1.2 7.9 70:30 1.9 6.0 60:40 2.7 4.7 50:50 3.6 3.6 Table 4. The optical power margin Distance from OLT to ONU/(km) Typical value/(dB) <5 1 <10 2 >10 3 Table 5. Calculation results of the optical power along th single-trunk-fiber network shown in Fig. 5 Tower number The loss from The loss along the Splitting OLT to ONU at trunk from OLT to ratio at the the ¿th tower/(dB) the 2th tower/(dB) i