B Improved Performance of OPGW Under Lightning Discharges in Brazilian Regions with a High Keraunic Level

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1 21, rue d'artois, F Paris B2-316 Session 2004 CIGRÉ Improved Performance of OPGW Under Lightning Discharges in Brazilian Regions with a High Keraunic Level by M.G.Alvim (*) A.O.Silva B.S.L.Moreira Furnas-Brazil Furnas-Brazil Cemig-Brazil D.B.Niedu C.F.Loewenthal C.Falqueiro Cepel-Brazil Consult-Brazil Eletrosul-Brazil Abstract The first optical fiber ground wires (OPGW) installed on transmission lines in Brazil crossed areas with a high keraunic level, and were often damaged by lightning discharges. The utility companies, together with the manufacturers of OPGW and the Brazilian Electrical Energy Research Center (CEPEL), decided to develop their own studies, looking for solutions to those specific problems in Brazil, since the technical literature on the effects of atmospheric discharges on OPGW was scarce. This research has led to the creation of a Brazilian Standard on lightning discharge tests for OPGW, as well as its validation, so that OPGW may be used in areas in Brazil with a high keraunic level. Keywords : OPGW - Conventional Ground Wire - Lightning - Continuing Current 1 - INTRODUCTION The first OPGW installed in high keraunic level regions of Brazil were imported from countries where weather conditions differ from those in Brazil, such as the United States, Canada, Germany, Switzerland, Japan and Portugal. The results of the tests according to the technical standards available at that time were not in accordance with the damages observed in the field. In the same way, the information obtained in the available technical literature was not in accordance with the field experience in Brazil. The utility companies, together with the manufacturers of OPGW and the Brazilian Electrical Energy Research Center, decided to develop their own studies, looking for solutions to those specific problems in Brazil. The studies started by simulating in the laboratory, in two separate tests, two components of a lightning discharge current. The first component was a short-duration, high-amplitude current pulse and the second component was a low-amplitude, long-duration, DC current, intended to simulate the continuing current of a lightning discharge. Lightning discharge damage observed in the field was compared with that caused by the tests performed in the laboratory. It was observed that the damage caused by the first component was not (*) Maria das Graças Alvim Furnas Centrais Elétricas S.A. alvimmg@furnas.com.br Address: Rua Real Grandeza 219 sala 406A Rio de Janeiro Brazil Cep.: Phone: (55-21) Fax (55-21)

2 significant due to its low energy. So it was decided to test the OPGW only with the simulation of the continuing current as this current had sufficient energy to produce damage on the cable similar to that caused by lightning discharges. As there were no measurements of continuing current intensity and discharge duration time in Brazil, the parameters of the DC current were determined through the comparison of the damages observed in the laboratory with those occurring in the field. In order to continue the supply of OPGW, foreign manufacturers had to adapt their designs so as to maintain the reliability of these cables at the same level of the conventional ones. 2 MAIN DAMAGES ON OPGW DUE TO LIGHTNING IN BRAZIL The main damages caused by lightning that occurred up to October 2002 in the southern states of Paraná (PR), Santa Catarina (SC) and Rio Grande do Sul (RS) and in the central state of Tocantins (TO) are shown in Table I. Transmission Line Cable Type State Length (km) Installation year/ Occurrence number/ Maximum broken wires 500 kv Serra da Mesa Gurupi (D9) TO /17/8 500 kv Gurupi Miracema (D9) TO /24/ kv F. Iguaçu Ivaiporã III (D8) PR /18/ kv Blumenau Joinvile 1 (*) SC /3/3 230 kv Jorge L. Siderópolis 2 (*) RS /1/7 230 kv Blumenau Palhoça (*) SC /1/3 230 kv Blumenau J.Lacerda 2 (C4) SC/ RG /2/7 230 kv Blumenau Palhoça (C4) SC /1/3 500 kv C. Novos Gravataí (C4) RS /3/6 230 kv Siderópolis Caxias 5 (C4) RS /3/7 230 kv Curitiba Joinville (C4) RS /1/3 230 kv Jorge L. B Siderópolis 1 (C4) RS /1/7 (D8), (D9) and C4 Designations as shown in tables VI and VII (*) This cable was not tested in this research Table I 3 LABORATORY TESTS 3.1. Test Method According to the Brazilian standard [1], the effect of the continuing current is simulated in the laboratory by a DC current. The test circuit is shown in Figure 1. The cable shall be tensioned to an initial value equal to 15% ± 1% of the rated tensile strength (RTS). The steel electrode has 12.7 mm diameter and the gap length between the electrode and the OPGW is 60 mm. The electrode is positioned in a plane normal to the axis of the cable and makes a 45 o angle with the horizontal plane, as shown in Figure 2. 2

3 According to the standard [1], after each discharge the cable shall be submitted to a load equal to its maximum working load in the transmission line, multiplied by a safety factor, to verify the remaining tension of the cable. The standard [1] recommends four classes of tests, as shown in Table II. The DC current and its duration time were defined through the comparison of the damages observed in the laboratory with those occurring in the field on conventional ground wire cables (designations E1 to E4/Table V) and on OPGW (designations A4/Table III, B1 Table/IV, C4 and C15/Table VI and D8 e D9/Table VII) and others. 1. direct current source 6. load cell or dynamometer 2. insulator 7. optical measuring equipment 3. tension device 8. OPGW 4. dead end hardware Figure 1 9. connector 5. electrode 10. current measuring device Figure 1 Figure 2 The discharge electrode shall be connected to the positive terminal of the DC source and the test sample to the negative one. This arrangement, simulating a descending discharge is the only one included in the standard [1]. Considering that the effect of a DC current discharge as the one performed in the tests depends, on its polarity, and on the standard [2] adopted for testing aircraft material in Brazil that specifies that tests be made with the electrode connected to the negative terminal, it was decided to make a new series of tests with both polarities, keeping the other requirements of the Brazilian standard [1]. The test with negative polarity at the electrode simulates an ascending discharge. Figure 3 shows an example of a wave form obtained with the electrode connected to the negative terminal of the DC source. Class A B C D Electric current (A) Time (ms) Electric charge (C) Tolerance (%) ±10 ±10 ±10 ±10 Table II Figure 3 During the tests, some registers of the electric arc were obtained with a high speed camera. These registers have shown that the arcs of different polarities have different characteristics as following: Positive polarity (Figure 4a) In the area between the electrode and the cable, the electric arc looks like a cone whose base faces the cable. This effect is well defined when the OPGW s are composed of galvanized steel wires. In these cables most of the discharges do not cause the breaking of the wires but causes an apparent 3

4 pulverization of the zinc that, among the materials used to coat the wires, is the one with the lowest fusion temperature (420 o C). The highest concentration of the electric arc is at the electrode side. Negative polarity (Figure 4b) In the area between the electrode and the cable, the electric arc looks like a cone whose base faces the electrode. This effect causes the broken wires to look like as if they had been cut with a saw. The highest concentration of the electric arc is at the cable side. The damages occurred in the field on the conventional ground wires composed of galvanized wires are very similar to those occurred during the tests with negative polarity electrode(-). Figure 4a Figure 4b 3.3. Types of cables tested After the development of the Brazilian Standard several cables, listed in tables III to VII, were tested to verify their behavior in laboratory tests. Through these tests lightning resistant OPGW were designed to be installed on transmission lines crossing regions with a high keraunic level. In these tables, the following symbols were adopted: GS: galvanized steel wire OL: outer layer A: aluminum wire IL: inner layer AC: aluminum-clad steel wire RTS: rated tensile strength AA: aluminum alloy wire φ: diameter of the wires Designation Stranding Diameter (mm)/ RTS( dan) Designation Stranding Diameter (mm)/ RTS( dan) A1 7GSφ3.00 mm 10.00/5,948 B1 (*) 10AC φ 3.70 mm 14.80/10,500 A2 7GSφ3.20 mm 10.64/6,875 B2 (**) 12AC φ 2.82 mm 13.89/9,180 A3 6GSφ3.78 mm 11.16/7,910 Table IV Trapezoidal wire/channeled rod A4 6ACφ3.60 mm 10.80/7,144 Table III Stainless steel tubes/single layer Designation Stranding Diameter (mm)/ CTS( dan) E1 (3/8 ) 7GSφ3.05mm 9.52/6,990 E2 (3/8 ) 7ACφ3.05mm 9.52/6,800 E3 (Minorca) OL:12Aφ2.44mm 12.21/5,120 IL:7GSφ2.44mm E4 (Dotterel) OL:12Aφ3.08mm IL:7GSφ3.08mm 15.42/7,870 Table V Conventional ground wires 4

5 Designation Stranding Diameter (mm)/ CTS( dan) Designation Stranding Diameter (mm)/ CTS( dan) 15.00/8,300 IL: 6 GS φ3.00 mm 14.80/12,170 IL I: 8AAφ2.16 mm 14.80/14,770 IL: 8 ACφ2.16 mm 15.20/8,100 IL: 7ACφ2.85 mm 18.00/11,385 IL: 6ACφ3.60 mm OL: 3AAφ3.50 mm 16.4/13,420 D6 (**) +8 ACφ3.50 mm IL: 6ACφ3.10 mm 14.40/11,523 IL: 5AAφ2.60 mm 12.50/4,800 IL: 5ACφ2.50 mm 13.90/6,166 IL: 5ACφ2.90 mm 15.00/7,388 C1 (*) 11GSφ2.94 mm 13.38/9,180 D1 (*) OL:12 AAφ3.00 mm C2 (*) 10GSφ3.34 mm 14.18/10,770 D2 (**) OL:10ACφ3.44 mm C3 (*) 7GSφ3.78 mm 12.56/9,230 D3 (**) OL: 10ACφ3.44 mm C4 (*) 11ACφ 2.5 mm 11.50/6,660 D4 (**) OL: 13AAφ 2.85 mm C5 (*) 7ACφ3.78 mm 12.56/8,940 D5 (**) OL: 12AAφ3.60 mm C6 (*) 9ACφ3.80 mm 15.10/11,600 C7 (*) 8ACφ3.90 mm 14.30/10,870 D7 (***) OL:10GSφ3.30 mm C8 (*) 8ACφ4.10 mm 14.70/11,700 D8 (***) OL: 12AAφ2.50 mm 15.20/8,960 D9 (***) OL: 13AAφ2.60 mm C9 (*) 6ACφ3.85 mm 3AAφ3.85 mm C10 (*) 5ACφ3.91 mm 15.32/8,220 D10 (***) OL:12AAφ3.00 mm 4AAφ3.91 mm IL: 5ACφ3.00 mm C11 (**) 10GSφ3.10 mm 13.40/7,474 Table VII Stainless steel tubes/double layer C12 (**) 9ACφ 3.6 mm 14.40/9,069 C13 (***) 11ACφ3.00 mm 14.00/7,198 C14 (***) 10ACφ3.60 mm 15.70/9,283 C15 (***) 4ACφ3.30 mm /5,000 4AAφ3.30 mm C16 (****) 11GSφ2.67mm 12.40/6,650 C17 (****) 12GSφ3.09mm 15.50/11,351 C18 (****) 13GSφ3.09 mm 16.40/12,300 Table VI Aluminum tubes 3.3 Analysis of the Test Results The following analysis was based on the results of laboratory tests. The tests were performed according to the standard [1], and consisted in applying 3 to 5 discharges on different samples. After each discharge, the sample was submitted to a tension equal to 45% of its rated tensile strength (RTS). In each test the highest number of broken wires was taken into consideration. Figures 5 to 10 show the breakage tendency of wires of different materials. In these figures, the sign + or before the electric charges in coulombs indicates that the discharge was performed with the electrode connected respectively to the positive or negative terminal of the DC source. 5

6 Since the test results with charges up to 150C were considered representative of the damages in the field, it was decided to limit the tests to that charge OPGW with outer layer composed of galvanized steel wires (designations A1 to A3 Table III/C1 to C3, C11 and C16 to C18 TableVI/D7 Table VII) In the tests with positive polarity electrode the breakage of wires (diameter range 2.67 to 3.78mm) did not occur. However, it was observed a removing of the zinc coating in 3 to 5 wires. In the tests with negative polarity electrode, the breakage of wires only occurred during the electric discharge. This type of OPGW has a good behavior under lightning discharges. The number of broken wires and the extension of the zinc coating area removed are requirements to be checked. Therefore, these cables should be evaluated considering applications of 150 C with positive polarity to verify the extension of the zinc coating area removed and applications of 150 C with negative polarity to check the number of broken wires. Considering the high number of wires that lost their zinc coating when a 150C electric charge with positive polarity was applied, it is strongly recommended that the number of wires in the outer layer be no less than 10. For the single layer OPGW with a central metallic tube, the removing of the zinc coating may accelerate the corrosion of the steel wires, and, after some time, the wires may break and cause the twist of the optical unit. Figure 5 Figure OPGW with outer layer composed of aluminum clad steel wires (designations A4/ Table III/B1 to B2 Table IV/ C4 to C8 and C12 to C14/ Table VI/D2 and D3 Table VII) The test results did not show significant differences for either electrode polarities. The number of broken wires decreases with increasing diameter of the wires as shown in Figure 7 and Figure 8 with electric charges of ±50 C, ±100 C and ± 150 C, respectively. 6

7 It was shown that the arrangement of the OPGW affects the number of broken wires. The OPGW composed of 3.70 mm diameter trapezoidal wires (designation B1, Table III) presented a different behavior as can be seen in Figure 8. These cables should be evaluated with 150C and positive or negative polarity to check the number of broken wires. For single layer OPGW laboratory tests and field experience have shown that if the number of broken wires is greater than about one third of the total number of wires, the twist of the optical unit may happen. It is recommended a minimum of 10 wires of diameter equal or higher than 3.4 mm, or a minimum of 9 wires, if the wire diameters are higher than 3.60 mm. Figure 7 Figure OPGW with outer layer composed of aluminum-alloy (6201) wires (designations D1, D4, D5, and D8 to D10/Table VII) and conventional ground wires composed of aluminum (1350) wires (E3 and E4/ Table V) The test results did not show significant differences for either electrode polarities. The number of broken wires decreases with increasing diameter as shown in Figure 9 with to electric charges of ±50C, ±100C and ±150C. The aluminum-alloy used affects the materials that are in contact with the inner part of the cable, as the metallic tube or the internal layer wires. This effect occurs when charges equal or higher than 100 C are applied and seems happen randomly and does not depend on the diameter of the wires used. These cables should be evaluated with a 150C electric charge with positive or negative polarity to check the number of broken wires. On single layer OPGW it is not recommended the use of aluminum-alloy wires as in cable C15, shown in Table VI. In the outer layer of a double layer OPGW it is not recommended the use of the aluminum-alloy wires alone. They may be used in reduced number, mixed with aluminum clad steel wires, if the optical unit is in the center of the cable, as on cable D6, shown in table VII. If this unit is part of the internal layer in cables D1 and D8 to D10, shown in table VII, aluminum-alloy wires should not be used in the outer layer. 7

8 Figure 9 OPGW The conventional ground wires show a better behavior than the OPGW, when wires of the same diameter are used as shown in Figure 10. Figure 10 8

9 4 - CONCLUSIONS The damage on OPGW cables due to DC current arcs, in the laboratory tests according to the Brazilian standard, is similar to the damage caused by lightning discharges on OPGW installed on the transmission lines crossing regions with a high keraunic level in Brazil. OPGW with outer layer composed of galvanized steel wires have shown a different behavior for each polarity of the electrode. OPGW with outer layer composed of aluminum-alloy or aluminum clad steel wires have shown similar results for both polarities. In regions with a high keraunic level in Brazil, the charge of 150C (+) is the most adequate to test OPGW with outer layer composed of aluminum clad steel wires. For OPGW composed of galvanized steel wires the charge of 150C (-) is the most destructive. OPGW with outer layer composed of aluminum alloy wires are commonly used in Europe and Canada; however, these cables are not appropriate for most of the Brazilian regions with high keraunic level, because of the poor field and laboratory performance. 5 - REFERENCES [1]] ASSOCIAÇÃO BRASILEIRA DE NORMAS TÉCNICAS NBR (SET/2000): Optical Ground Wires (OPGW) for overhead transmission lines Determination of the effects of lightning discharge Test Method. [2] MIL.STD.-1757A: Lightning Qualification Test Techniques for Aerospace Vehicles and Hardware. 9