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9Experimental Investigation on Live Working for 1000kV AC Transmission LineYi Hu, Linong Wang, Guiwei Shao, Kai Liu, Yuanfang, Wen, Kangzhen WenFinancial support should be acknowledged here. Example: This work was supported in part by the U.S. Department of Commerce under Grant BS123.The paper title should be in uppercase and lowercase letters, not all uppercase.The name and affiliation (including city and country) of each author must appear on the paper. Full names of authors are preferred in the author line, but are not required. Initials are used in the affiliation footnotes (see below). Put a space between authors initials. Do not use all uppercase for authors surnames. Examples of affiliation footnotes:J. W. Hagge is with Nebraska Public Power, District Hastings, NE 68902 USA (e-mail: ).L. L. Grigsby is with the Department of Electrical Engineering, Auburn University, Auburn, AL 36849 USA (e-mail: ).Abstract-The overvoltage level of ultra high voltage transmission line with 1000KV altering current was analyzed, and the minimum working distance and minimum combinational gap under different overvoltage level as well as the minimum effective insulating length of insulating tools were determined by various experiments. The screen clothing which could be applied to live work for 1000kV transmission line was developed. The surface and inner electrical-field intensities of screen clothing in different work positions on the pole were measured. The safe guarding measures under different operation mode were revised. The outcomes of these investigations provide technical reference to engineering design and live line work for 1000kV transmission line.KEY WORDS：1000kV AC transmission line；Live working；Working distance；Combinational gap；Insulating tool；Safe guarding；Screen clothingI. IntroductionChina is about to carry out the demonstration project of power transmission of extra-high-voltage 1000kV AC. In order to provide parameters for the design of circuitry pole as well as the technical basis and the regulations standard of the live working and the overhauling maintenance, we need to make the experimental research on the live working of the 1000kV AC transmission line. It is the first time for China to develop the research on the live working under the condition of 1000kV AC transmission line 2-4. On the basis of each tentative data and the research results from the live working of 500kV and 750kV transmission line5-8, this thesis has researched the work distance, the combinational gap, the work way, the work tool and the safe guarding apparatus in the live working of 1000kV AC transmission line. These research results can provide the technical basis for the engineering design as well as the live working of 1000kV AC transmission line.II. The experimental research on the work distance and the combinational gapA. The operating overvoltage and risk rate in the live workingWhen considering the operating overvoltage in the live working, the line overvoltage of closing circuit without load needs not to be considered. In addition, according to the stipulation Electrical industry Trouble-free service Regulations 9, when carrying out the live working in the system whose neutral point is effectively earthed you should stop using the automatically reclose switch. Therefore, the limit of overvoltage and the probability density of overvoltage in the system of the single-phase earth with the divided three-phase is the governing factor in choosing The minimum safe distance, the combinational gap, the shorexperiment insulation length of a work tool and so on in the live working. After the computation, in 2008, for the Chinese first extra-high voltage power transmission line SanXi - south JingZhou, the limits of overvoltage at the situation of the single-phase earth with the divided three-phase are obtained. The limits are 1.72p.u.（circuitry）and 1.55p.u (the side of bus bar) without gate resistance. The limit are 1.66p.u（circuitry） and 1.53p.u(the side of bus bar) with gate resistance（700）. Figure 1, 2 lists the overvoltage density when the single-phase earth with the divided three-phase happens to transmission Line. Fig.1 probability density in the situation of the single-phase earth with the divided three-phase (without gate resistance)Fig.2 probability density in the situation of the single-phase earth with the divided three-phase (with gate resistance)The risk of the live working may obtain by the formula,where is the probability density function of the amplitude U of operating overvoltage and is shown in Fig. 1 and 2; is the probability distribution function of breakdown of air gap which bear the amplitude U of the operating overvoltage and is expressed as follow:where is 50% discharge voltage (kV) of air gap under the impact of operation overvoltage; is the standard deviation (kV) of the discharge voltage of air gap under the impact of operation overvoltage.B. Experimenting arrangementThe experiment is carried out in the outdoor extra-high-voltage experimenting ground of Wuhan High Voltage Research Institute of State Grid. The area of experimenting ground is 120 445 sq. meters and the length of transmission line imposed extra-high voltage is 200 meters.During the experiment, according to the constructional feature of typical iron tower (cathead tower) used in the design of 1000kV AC transmission line, the equipotential aisles and the working positions are selected and there the working distance and the combinational gap are experimented. Fig. 3 depicts the concrete experimenting arrangement. In Fig. 3, S1 is the minimum distance between human body and earthy electric potential, S2 is the minimum distance between human body and high electric potential, and position 14, 6, and 7 are the locations of the live working. Position 1 and 3 are the locations of the live working where anthropomorphic dummy contacts the wire close. Position 2 and 4 are the locations of the live working where anthropomorphic dummy contacts the tower close. Position 6 and 7, located in the middle between high electric potential and earthy electric potential, is one of the point of the equipotential aisle along which worker enters working area.During the experiment in the working distance and combinational gap of the serial insulators of 1000kV AC line, the anthropomorphic dummy wears the shield shaped into clothing and squats in the different positions of the serial insulators by the way of cross three and short four shown in Fig. 4 ( position 5 and 8 respectively express where personnel is at high electric potential and at intermediate electric potential). In Fig. 4, S is the distance between anthropomorphic dummy that is at high electric potential and earth, B is the width of anthropomorphic dummys body and takes the value of 0.5 meter, and S1 and S2 is the same as that in Fig. 3.The anthropomorphic dummy used for experiment is made from the aluminum alloy, has the same configuration as actual human, and has freely curving four limbs so as to adjust the posture.The high of anthropomorphic dummy that stands and sits are respectively 1.8 meter and 1.45 meter. The width of anthropomorphic dummys body is 0.5 meter. The computation indicates that the time to the peak of the overvoltage from the transmission line between Nanyang and SanXi to earth, is at least 3000s, which is far more than the time to the peak of standard operating overvoltage (250s).Combining the results from the calculation of the time to the peak of operating overvoltage with the waveform of the operating overvoltage resulted from switching impulse used to experiment the external insulation under extra-high voltage, 720/4000 s operating overvoltage, whose waveform is the same as that used to experiment external insulation under extra-high voltage, is applied to the experiment of the live working 10. All tentative data, according to the standard GB/T16927.1-1997, are revised to the standard meteorological condition. Fig. 3 arrangement of experimenting in working distance and combinational gap Fig. 4 arrangement of experimenting in working distance and combinational gap in the serial insulatorsC. The minimum safe distanceA great deal of research indicate that the U50 (50% discharge voltage) between human located at wire (high electric potential) and tower is less than that between human located at tower (earthy electric potential) and wire 11-12. Thus, for the research on minimumwor safe distance, U50 between human located at wire (high electric potential) and tower is experimented and shown in Fig. 5. Where, d is the distance between human located at wire and the simulation tower whose position can change.Fig. 5 Discharge characteristic at the different work positions (equipotential positions) On the basis of the fact that 1100kV is the maximum service voltage in power system, and 1.72p.u. (without gate resistance) and 1.66p.u.（with gate resistance）respectively express the maximum overvoltage, according to the curve shown in Fig. 5, and by calculation, the acceptable minimum gap distance (satisfied with the acceptable maximum risk rate) from human located at side-phase (position 1), middle-phase (position 3), or Serial insulators(position 5) to tower, is obtained, respectively. The acceptable minimum gap distance rounded to one decimal, and the corresponding and risk rate are all shown in Table 1 and 2. In the last column of Table 1 and 2 is the minimum safe distances, considering the sphere of human body in live working (0.5metre). TABLE IThe minimum safe distance (without gate resistance)TABLE The minimum safe distance (with gate resistance)D. The minimum combinational gapThe combinational gap is Sc (Sc=S1S2). S1 and S2 are shown in Fig.3 and 4. When leaving the wire, the human will be at the position between high electric potential and earthy electric potential such as position 6 ,7 and 8 shown in Fig.3 and 4. Hence, Sc is divided into two sections by the position where the human locates. According to a lot of experiments on tower which have been carried out, we have known 12-14: U50 imposed on the combinational gap is changeable along with the position of the human in the combinational gap, and there is a minimum U50 corresponding to a position of the human. Therefore, the experiment in minimum combinational gap divides into the following two steps: (1) After fixing Sc (Sc=S1+S2), altering the position of human in the combinational gap, and respectively on the combinational gap putting operating overvoltage and measuring U50, we, on the combinational gap, can obtain the minimum U50 and the position of human synchronously. This position is named as the minimum discharge position. Here, S1 is the minimum distance from the human to the tower, S2 is the minimum distance between the human and the simulation wire, and Sc is the sum of S1 and S2.(2) After hanging the human on the minimum discharge position and fix the position of the human, altering the distance between the tower body and the anthropomorphic dummy (S1), carrying on the discharge experiment of operation impact, we obtain the corresponding 50% discharge voltage by experiment; Again, according to its discharge curve, through the computation of risk rate, we find the minimum combinational gap (Sc).Fig. 6 the discharge curve of different positions (the high of a serial insulator is 170mm) For one of three curves in Fig. 6, U50 alters along with the distance between the wire and the human in Sc. For middle-phase or side-phase in Fig. 3, when the anthropomorphic dummy(located at position 6 or 7) is apart from high electric potential for approximately 0.4 metre, U50 imposed on the combinational gap is minimum. For serial insulators in Fig. 4, when the anthropomorphic dummy(located at position 8) is apart from (high electric potential) for 2 serial insulators ( 0.34 metre ), U50 imposed on the combinational gap is minimum.On the basis of the above experimenting results, putting the anthropomorphic dummy on the minimum discharging position, altering S1 and seeking ，according to the process and method which are applied to seek the minimum safe distance, we find the minimum combinational gap, as shown in Table 3 and 4. In the table listed the minimum safe combinational gap when considering the sphere of human body in live working (0.5metre).TABLE The minimum combinational gap (without divided resistance)TABLE The minimum safe distance (with divided resistance)III. The experimental investigation of the minimum effective insulating length of insulating tools As the proximity effect of the tower window frame, the discharge-voltage of insulation tools in the mid-phase is always lower than that in the side-phase, therefore, in order to determine the effective length of insulating tools and the security of operations, taking the actual operating mode in the experimental into account, two insulating sleeve and four insulating rope are parallel mounted in the mid-phase to determine the minimum effective insulating length of the supporting tools. In the course of the effective length insulation test for the operational tools, a robot is employed to hold one end of the operating arm, and the another end of the operating arm is shorted with the high electric potential (conducting wire). The specific layout of the experiment shown in figure 7 Fig. 7 The layout of insulating tools test By changing the length of operational tools, the 50% impulse discharge-voltage of the operational tools could be attained by the experiments. The curve of the corresponding which changes following the effective length of operate tools is shown in Figure 8. Fig. 8 The discharge characteristic of operation toolsWhen the highest system working voltage is 1100kV and the highest over-voltage is 1.72p.u. (without breaking resistance) or 1.66p.u. (with breaking resistance), according to the operation impulse discharge characteristic curve as shown in fig.8, the minimum effective length of the operation tools could be obtained, as is shown in Figure 5. The minimum effective insulating length is determined by the risk-discharge rate and the safe margin of 0.5m.TABLE The minimum effective insulating lengthIV. The Safeguard investigation of the live line workerThe purpose of this experiment is to test the main technical parameters of the shield cloth which used in the live line work at 1000kV. When the system is operating at the highest working voltage, the strength of electric field of outside and inside surface of the shield clothes is measured. The arc is validated and the impulse current is measured when the electric potential comes equal. Whether the shield clothes could be used in 1000kV line live work is determined by the experiment and the corresponding security measures is presented.A. The safe guarding equipmentThe shield clothes used in 1000kV line live work is an important the safe guarding equipment. The shield clothes could shield electric field and bypass current, and form a equipotential shield face on the surface of the operators who is in the high electric field, thus, protects human body against the impact of high electric field 15-16. It should be well shield, with low resistance, and suitable carrying capacity, and some ability of the flame retardant. It also should have the performance of resistance against sweat corrosion, washing, and electric spark 17.In allusion to the high voltage and electric-field intensity of the 1000kV line live work shield clothes when used, according to the experience of operation and testing on our homemade shield clothes for 500kV line live work and the test result of the 750kA line live work shield clothes, the cloth shield rate which is larger than 60dB is chosen to make the 1000kV line live work shield clothes. To reduce the exposed area, the pattern is designed as coalescing the caps, frock and trousers together, and adopting the meshy shield masks formed by conductive materials and fire retardant fiber.B. The material and resistance test of shield clothesThe material and the resistance testing of the 1000kV line live work shield clothes was proceeding in the Wuhan High Voltage Research Institute shield clothing test chamber, and the test equipments there meet standards of GB6568.2-2000, “Test procedure of shield clothes for live working” . The result of resistance test and material of the shield clothes are shown in Tab.6.TABLE The material of the 1000kV line live work shield cloth and the performance test of the finished productsThe performance test of materials of 1000kV shield clothes shows that the shield materials completely conforms to the stipulation GB6568.1-2000 and IEC60895-2003, Which needs to be pointed out is national standard GB6568.1-2000 is required to the voltage level of 500kV and below, and national standard IEC60895-2003 is to the voltage level of 500kV and below, however, at present there are no national standard for the voltage level of 1000kV. From the results of testing, the technical performance of the materials of 1000kV shield clothes were significantly higher than those in the above two national standard.C. The electric field shield test of the shield clothes The electric field shield test inside and outside of the 1000kV shield clothes was proceed in the Wuhan high voltage research institute outdoor test field, the strength of electric field of the human body in the condition of equipotential work and the earth electric potential is measured separately. As shown in Tab.7 is part of the measured results of the strength of the human body