Powertech Labs Inc th Ave., Surrey, B.C. Canada V3W 7R7 POWERTECH LABS INC.

Transcription

1 POWERTECH LABS INC. ELECTRICAL ASSESSMENT OF ULTRA HIGH PERFORMANCE FIBRE REINFORCED CEMENTITIOUS COMPOSITE CROSS ARMS FOR 230 KV WOOD POLE TRANSMISSION LINES Project December 2009 January 2009 Prepared for: Lafarge Canada Inc. ABSTRACT: Electrical tests were done on ultra high performance fibre reinforced cementitious composite cross arms for 230 kv wood pole transmission lines. The electrical properties are very comparable to conventional wood cross arms. The one area of concern is the damage to the material during the lightning impulse tests. It was recommended the impact of this damage on the mechanical properties of the material be investigated. Subsequent to the Powertech tests structural tests were carried out by the University of Calgary on the damaged cross arm. A copy of the University of Calgary report is included as Appendix A. Tested by: Prepared by: R.G. Pollock Senior Technologist, High Voltage Lab A.J. Vandermaar, P. Eng. Manager, High Voltage Lab Project No: Page 1 of 24

2 TABLE OF CONTENTS 1.0 SUMMARY BACKGROUND PROJECT OBJECTIVES TEST PROCEDURE Test Setup AC Voltage Partial Discharge Tests AC Voltage Flashover Tests Lightning Impulse Voltage Flashover Tests Dielectric Strength Tests Tracking Wheel Test TEST RESULTS Test Date AC Voltage Partial Discharge Tests AC Voltage Flashover Tests Lightning Impulse Voltage Flashover Tests Dielectric Strength Tests AC Voltage Dielectric Strength Tests Lightning Impulse Voltage Dielectric Strength Tests Tracking Wheel Test DISCUSSION OF RESULTS CONCLUSIONS...22 APPENDIX A University of Calgary Report Project No: Page 2 of 24

3 1.0 SUMMARY Electrical tests were done on 9 m long ultra high performance fibre reinforced cementitious composite cross arms for 230 kv wood pole transmission lines. The cross arms were simultaneously tested in a cross brace configuration. The partial discharge on the test assembly did not exceed 20 pc at 120 % of rated maximum operating voltage (169.7 kv) with and without the hardware (metal parts) grounded. AC flashovers to the surface and across the surface caused no visible damage to the reinforced cementitious composite material. Lightning impulse flashovers across the surface also caused no visible damage to the material. Lightning impulse flashovers to the surface did cause spalling, punctures through the material and visible cracks of unknown depth. It is recommended that the effect of the potential damage that could be done by AC fault currents or lightning impulse on the mechanical strength of the reinforced cementitious composite material be investigated. Subsequent to the Powertech tests structural tests were carried out by the University of Calgary on the damaged cross arm. A copy of the University of Calgary report is included as Appendix A. The power-frequency and the lightning impulse dielectric strength of the reinforced cementitious composite material exceeded the strength of wood samples. The samples passed the tracking and erosion test on the tracking wheel with no damage to the reinforced cementitious composite material. The electrical properties are very comparable to conventional wood cross arms. For structures with un-bonded hardware the slight conductivity of the material is a good feature. This means that under normal operating conditions there will be good voltage grading between the insulators and the transmission line structure. 2.0 BACKGROUND It is proposed to use ultra high performance fibre reinforced cementitious composite cross arms manufactured by Lafarge Canada Inc., on 230 kv Transmission Lines as a replacement for wooden cross arms and potentially to also replace cross braces. The fibre reinforced cementitious cross arms contain no metal. Since the cross arms are exposed to an electric field the effect of the electric field should be considered. The electric field could cause partial discharge within the cross arms. Over an extended period in-service this can cause external or internal damage and eventual failure of the cross arms. There is also concern as to what the impact of AC flashovers and lightning strikes would have on the cross arms. If this material were used as cross braces the same concerns would apply. Project No: Page 3 of 24

4 To study these effects tests were conducted on the fibre reinforced cementitious composite cross arms and on material samples. Figure 1 shows a manufacturer drawing and Figure 2 and 3 show the cross arms tested. Figure 1 Ultra high performance fibre reinforced cementitious composite cross arms Project No: Page 4 of 24

5 Figure 2 Ultra high performance fibre reinforced cementitious composite cross arms Figure 3 Ultra high performance fibre reinforced cementitious composite cross arms Project No: Page 5 of 24

6 3.0 PROJECT OBJECTIVES The objectives of this project were to: Determine if there is damaging internal partial discharge (PD) on the fibre reinforced cementitious concrete members under operating conditions. Evaluate the effect of AC flashovers on the cross arms by initiating low energy flashovers to the cross arms and across the cross arms. Evaluate the effect of lightning impulses applied to the cross arms and across the surface of the cross arms. Evaluate the ability of the ultra high performance fibre reinforced cementitious composite cross arms/cross braces to withstand long term electrical stress over the surface. Evaluate the dielectric strength of the fibre reinforced cementitious members compared to wooden members. 4.1 Test Setup 4.0 TEST PROCEDURE Two 9 m long ultra high performance fibre reinforced cementitious composite cross arms were mounted back to back on 14 x 4 x 38 timbers to simulate the mounting arrangement on a 230 kv wood pole structure. A second pair of ultra high performance fibre reinforced cementitious composite cross arms were used as cross braces on the simulated structure. A 230 kv insulator was suspended from the center of the cross arms to simulate the centre phase of a 230 kv circuit. A conductor was suspended from this insulator at right angles to the cross arms. The complete assembly was suspended from insulators in the High Voltage Laboratory. The test setup is shown in Figure 4 and Figure 5. Project No: Page 6 of 24

7 Figure 4 Test Setup Project No: Page 7 of 24

8 4.2 AC Voltage Partial Discharge Tests Figure 5 Test Setup The conductor was energized to expose the cross arms to the electric field they would experience in-service. The partial discharge (PD) was measured as a function of the applied voltage. The partial discharge tests were done in three configurations as follows: with all metal hardware floating (not electrically bonded to ground or electrically bonded together), with the lower metal hardware (bolts) on the cross braces electrically connected to ground, with all metal parts (cross brace mounting hardware, cross arm mounting hardware and insulator ground end hardware) electrically connected to ground. Project No: Page 8 of 24

9 The cross arms and cross braces were examined with an image intensifier during the tests for visible external discharge and with an acoustic ultrasonic detector for audible partial discharge. This was done to eliminate external corona as the source of any measured partial discharge. 4.3 AC Voltage Flashover Tests A minimum of twenty AC flashovers were initiated from the energized AC conductor to the cross arms to obtain data on the effect of AC flashovers on the cross arms. The test was done at voltages up to 350 kv. A minimum of twenty AC flashovers were also initiated lengthwise over the surface of one of the cross arm over a distance of 30 cm. For the surface flashover two strips of copper tape were applied to the surface of the crossarms at the 30 cm spacing. One copper strip was energized and the other strip was grounded. Note: The energy available from the AC test source is relatively low so it is expected that the flashovers would cause minimal damage. 4.4 Lightning Impulse Voltage Flashover Tests The previously energized AC conductor was connected to the lightning impulse generator and twenty lightning flashovers (1.1/48 µs waveform) were initiated to the cross arms to obtain data on the effect of lightning on the cross arms. The test was done at voltages up to 1.6 MV. Twenty lightning impulse flashovers were also initiated lengthwise over the surface of one of the cross arms over a distance of 30 cm. This was done over the same surface as used for the AC surface flashovers. 4.5 Dielectric Strength Tests The power-frequency (AC) and lighting impulse material dielectric strength was determined for the ultra high performance fibre reinforced cementitious composite cross arms material. For comparison normal Douglas Fir used for wood pole transmission line structures were also tested. Insulation samples were tested by placing 1 inch diameter electrodes on opposite sides of the test sample. For the AC tests the test voltage was applied to the electrodes and ramped up to sample breakdown. For the lighting impulse test standard 1.2/50 µs waveforms were applied to the test electrodes starting at a voltage below the expected breakdown voltage. The applied impulse voltage was increased in steps up to sample breakdown. 4.6 Tracking Wheel Test Samples of the ultra high performance fibre reinforced cementitious composite cross arms material were tested in accordance with CEA Purchasing Specification LWIWG- Project No: Page 9 of 24

10 03(96), Clause 5.4, tracking and erosion test, with the exception that only the post power frequency test was done i.e. no steep front test. Four 28.5 cm x 9.5 cm x 12 mm samples with 1.9 cm holes drilled in the samples at a 22.9 cm center to center spacing were mounted on the tracking wheel. On one of the samples the cut ends of the sample cut hole edges were sealed with epoxy. This was done to evaluate if sealing cut edges would have any impact on tracking performance. The other three samples were tested as received. Electrodes were attached so the was a distance of 20 cm between the ground and high voltage electrode the samples. The four tested samples were subjected to the aging process on a tracking wheel for 330 hours using Test Method 1. The minimum applied test voltage was 8.1 kv rms (leakage distance of 230 mm and minimum stress of 35 V/mm). The salinity of the water was 0.22 grams/litre. After every four days of testing there was a rest period of 24 hours, when the samples were not sprayed. The total test duration, including the rest periods was 402 hours. Test evaluation: no tracking, no erosion to the core and no coating puncture may occur. 5.0 TEST RESULTS 5.1 Test Date 18 December 2009 to 20 January AC Voltage Partial Discharge Tests Table 1 gives the partial discharge results with all metal hardware floating (not electrically bonded to ground or electrically bonded together). Table 1 Partial Discharge Test with all Metal Hardware Floating Voltage (kv) Partial Discharge (pc) Comments 100 < <1 Rated operating voltage <1 Rated maximum operating voltage x rated operating voltage x rated maximum operating voltage x rated operating voltage x rated maximum operating voltage 236 >1000 inception voltage Extinction voltage Project No: Page 10 of 24

11 Table 2 Partial Discharge Test with the Lower Bolts) on the Cross Braces Electrically Connected to Ground Voltage (kv) Partial Discharge (pc) Comments 100 < Rated operating voltage Rated maximum operating voltage x rated operating voltage x rated maximum operating voltage x rated operating voltage x rated maximum operating voltage 240 >1000 inception voltage extinction voltage Table 3 gives the partial discharge test results with all metal parts (cross brace mounting hardware cross arm mounting hardware and insulator ground end hardware) electrically connected to ground. Table 3 Partial Discharge Test with all Metal Parts Grounded Voltage (kv) Partial Discharge (pc) Comments 100 < Rated operating voltage Rated maximum operating voltage x rated operating voltage x rated maximum operating voltage x rated operating voltage x rated maximum operating voltage 239 >1000 inception voltage extinction voltage The cross arms were examined with an image intensifier for visible external discharge in all the test configurations. No external discharge was observed. 5.3 AC Voltage Flashover Tests The flashover to the surface and across the surface had no visible effect. Figures 6 shows the gap that was used for the flashovers across the surface and figure 7 shows surface flashover. Project No: Page 11 of 24

12 Figure 6 Surface Flashovers Gap Figure 7 Surface Flashover Project No: Page 12 of 24

13 5.4 Lightning Impulse Voltage Flashover Tests The flashovers to the surface caused spalling, punctures and visible cracks on the cross arm web. Figure 8 shows a flashover to the surface. Figures 9 and 10 show the damage caused by the flashovers. The flashovers over the surface had no visible effect. Figure 8 Lightning Impulse Flashover to the Surface Project No: Page 13 of 24

14 Figure 9 Lightning Impulse Flashovers Damage to the Web Figure 10 Lightning Impulse Flashovers Damage to the Web Project No: Page 14 of 24

15 5.5 Dielectric Strength Tests AC Voltage Dielectric Strength Tests The results of the AC dielectric strength tests are given in Tables 5 and 6. Sample No. Table 5 Fibre Reinforced Cementitious Samples Puncture Voltage (kv) Thickness (mm) Dielectric Strength (V/mm) Comments 874-A Smooth surface 874-A Smooth surface 874-A Smooth surface with some roughness 874-A Smooth surface 874-A Smooth surface 874-A Smooth surface with some roughness 874-A Smooth surface 874-A Smooth surface 874-A Rough surface 874-A Rough surface 874-A Smooth surface 874-A Smooth surface Average Table 6 Wood Samples Sample No. Puncture Voltage (kv) Thickness (mm) Dielectric Strength (V/mm) A A A A B B Comments see Note below Old cross arm material tested parallel to the wood grain Old cross arm material tested parallel to the wood grain Old cross arm material tested across the wood grain Old cross arm material tested across the wood grain Old cross arm material tested parallel to the wood grain Old cross arm material tested across the wood grain Project No: Page 15 of 24

16 Sample No. Puncture Voltage (kv) Thickness (mm) Dielectric Strength (V/mm) C Comments see Note below Old cross arm material tested parallel to the wood grain C Old cross arm material tested across the wood grain Average Note: The measured puncture voltage on the wood samples was the maximum voltage reached by the test source before the increasing load current caused the source voltage to collapse Lightning Impulse Voltage Dielectric Strength Tests The results of the lightning impulse dielectric strength tests are given in Tables 7 and 8. Sample No. Table 7 Fibre Reinforced Cementitious Samples Puncture Voltage (kv) Thickness (mm) Dielectric Strength (V/mm) Comments 874-A Smooth surface 874-A Smooth surface Smooth surface with 874-A3 > some roughness 874-A Smooth surface 874-A Smooth surface Smooth surface with 874-A some roughness 874-A7 > Smooth surface 874-A8 > Smooth surface 874-A Rough surface 874-A10 > Rough surface 874-A11 > Smooth surface 874-A Smooth surface Average Project No: Page 16 of 24

17 Sample No. Puncture Voltage (kv) Thickness (mm) Table 8 Wood Samples Dielectric Strength (V/mm) A A A A B B C Comments Old cross arm material tested parallel to the wood grain Old cross arm material tested parallel to the wood grain Old cross arm material tested across the wood grain Old cross arm material tested across the wood grain Old cross arm material tested parallel to the wood grain Old cross arm material tested across the wood grain Old cross arm material tested parallel to the wood grain C Old cross arm material tested across the wood grain Average Tracking Wheel Test The ultra high performance fibre reinforced cementitious composite cross arms material passed the tracking and erosion test with no tracking, no erosion to the core and no coating puncture of the cross arm material. Figures show the samples at the end of the tracking test. Sample B4 (with sealed edges), Figures17 & 18, had carbonization around one of the mounting holes. This damage was on the epoxy used to seal the edges. There was no damage to the tested material. It was observed during the aging on the tracking wheel that the there was considerable heating of the samples. It was found the fibre reinforced cementitious composite material has a slight conductivity. The resistance over a 2.1 m distance was measured to be 6.3 megohm. For comparison a sample of wood was also tested and it measured 5 megohm over a 2.6 m distance. Normally there is a post tracking wheel steep front impulse test, power-frequency flashover test and power-frequency voltage heating test done on the samples. The steep front impulse test and the flashover test is done to determine if there has been water ingress between the fibreglass core and the polymer housing interface on insulators. This does not apply in this case as there is no interface. The power-frequency heating test is done to determine if there has been water absorption into the material. If there is heating of the sample it indicates water absorption. Since the material heated up during the aging test, due to its slight conductivity, the power-frequency heating test would serve no purpose and was not done. Lafarge Canada stated that does not absorb any moisture. Project No: Page 17 of 24

18 Figure 11 Sample 1 Front after Tracking and Erosion Test Figure 12 Sample 1 Back after Tracking and Erosion Test Project No: Page 18 of 24

19 Figure 13 Sample 2 Front after Tracking and Erosion Test Figure 14 Sample 2 Back after Tracking and Erosion Test Project No: Page 19 of 24

20 Figure 15 Sample 3 Front after Tracking and Erosion Test Figure 16 Sample 3 Back after Tracking and Erosion Test Project No: Page 20 of 24

21 Figure 17 Sample 4 Front (with sealed edges) after Tracking and Erosion Test Figure 18 Sample 4 Back (with sealed edges) after Tracking and Erosion Test Project No: Page 21 of 24

22 6.0 DISCUSSION OF RESULTS There is no specification for acceptable partial discharge in the cross arms/cross braces but based on normal requirements for other equipment we would recommend that the internal partial discharge (extinction voltage) should be below 20 pc at 120 % of rated maximum operating voltage (1.2 x kv =169.7 kv). The test structure met this requirement in all test configurations. The AC flashovers to the surface and across the surface caused no visible damage to the fibre reinforced cementitious composite material. The lightning impulse flashovers across the surface also caused no visible damage to the material. The lightning impulse flashovers to the surface did cause spalling, punctures through the cross arm web material and visible cracks on the web of unknown depth. It is recommended that the effect of the potential damage that could be done by AC fault currents or lightning impulse on the mechanical strength of the fibre reinforced cementitious composite material be investigated. The power-frequency and the lightning impulse dielectric strength of the fibre reinforced cementitious composite material exceeded the strength of wood samples. The samples passed the tracking and erosion test on the tracking wheel with no damage to the fibre reinforced cementitious composite material. 7.0 CONCLUSIONS The ultra high performance fibre reinforced cementitious composite cross arms electrical properties are very comparable to conventional wood cross arms. For structures with unbonded hardware the slight conductivity of the material is a good feature. It means that under normal operating conditions there will be good voltage grading between the insulators and the transmission line structure. The one area of concern is the damage to the material during the lightning impulse tests. It is recommended the impact of this damage on the mechanical properties of the material be investigated. Subsequent to the Powertech tests structural tests were carried out by the University of Calgary on the damaged cross arm. A copy of the University of Calgary report is included as Appendix A. Project No: Page 22 of 24

23 APPENDIX A University of Calgary Report Project No: Page 23 of 24

24 Project No: Page 24 of 24

25 Structural Testing of Fibreglass Reinforced Cementitious Composite Cross Arm after being Exposed to Electrical Lightning Impulses Prepared by: Dr. Raafat El-Hacha, PEng University of Calgary Department of Civil Engineering Prepared for: Lafarge Canada Inc. April 2010

26 Executive Summary The objective of this test is to asses the structural performance and mechanical properties of a cross arm with surface damage created by electrical lightning impulses. A mechanical test was performed on this cross arm in order to asses its structural integrity. The cross arm that was damaged during Powertech s lightning impulse test is shown bellow. Upon visual inspection, the impact caused by the lightning impulse seemed to be on the web s surface and was not larger than 25 mm 2 (1 inch 2 ). The depth of these damage spots did not exceed 3 mm (1/8 of an inch) on both sides of the web. The surface damage seemed to be localized along 500 mm of the cross arm. There was no other apparent damage anywhere on the cross arm. 500 mm Figure 1: Damage due to lightning impulse Page 1 of 8

27 In order to investigate the structural integrity and the mechanical properties of this damaged cross arm, it was decided that a flexural test on the damaged section of the cross arm should be performed with bending about the x-axis. For this reason, the 9,000 mm long cross arm was cut into 3 specimens, each 2550 mm long, labelled specimens S x#1-6.6, S x#2-6.6, and S x# Specimen S x#2-6.6 had the damage area and was cut in a way to have the 500 mm long damaged section located at the mid span for it to be loaded under a 3 point bending test and have the web exposed to maximum stresses.. All specimens were tested in a simply supported configuration under three point bending, with the point load applied at midspan, the ends supports were roller and hinge. The width of the end supports was 100mm. The span centre-to-center between the supports is 2350mm. The shear span (distance between the support reaction and the midspan point load) is 1175mm. Therefore, the shear-span-todepth ratio a/d= 6.6, where d is the effective depth measured from the compression fibre of teh cross arm to the centroid of the bottom tension reinforcement taken equal to 178mm. Figure 2 shows the test-set-up. The specimens were instrumented at midspan using Linear Strain Conversion transducers to measure the vertical deflection and any potential of the occurrence of lateral displacement. The cross arms were loaded on the top flange such that the point load is only applied over the width of the web. Figure 2: Test Set-up for flexural test Figure 3 shows the load versus midspan vertical deflection of the three cross arms. From these curves, it can be concluded that the overall flexural behaviour and capacity of all specimens are similar. The average ultimate load was 31 kn. Page 2 of 8

28 Cross Arm Strong Axis Shear Span-to-Depth Ratio=6.6 Specimens #1, 2 &3 35 Load (kn) S x#1-6.6 S x#2-6.6 (damaged with lightning impulse) S x# Mid-span Deflection (mm) a/h=5.9 (h=200mm) a/d=6.6 (d=178mm) S x#1-6.6: P max = 32.4 kn M max = kn m S x#2-6.6: P max = 30.8kN M max = kn m S x#3-6.6: P max = 30.2 kn M max = 15.1 kn m Figure 3: Load versus midspan deflection of the three specimens The mode of failure of the three specimens was similar and can be described as shear failure followed by longitudinal splitting cracks starting at both ends of the diagonal crack, one (top longitudinal crack) towards the point load and the other (bottom longitudinal crack) towards the support. Figure 4 shows the failure of specimens S x# Figure 5, shows the failure mode of specimen S x# Moreover, it can be seen that the cracks and damaged areas on the cross arm s web caused by the lightning impulses did not promote or attract further cracking or further damage when subjected to the midspan point load from the 3 point bending tests. Moreover, specimens S x#1-6.6, S x#2-6.6, and S x#3-6.6 had comparable results to similar specimens that were cut from undamaged fibreglass reinforced cementitious cross arms and previously tested. The bending test results of these undamaged specimens with the same length reached an average ultimate load of 38 kn with shear as the dominate failure mode accompanied with longitudinal splitting at the bottom flange towards the support. It can be concluded from this test, that the damage caused by the electrical lightning impulse test did not affect the structural performance and mechanical properties of this fibreglass reinforced cementitious composite cross arm. Page 3 of 8

29 S x#1-6.6 S x#1-6.6 Figure 4(a): Cross arm S x#1-6.6 before flexural testing to failure S x#1-6.6 Figure 4(b): Close up view of the left shear-span before flexural testing to failure Page 4 of 8

30 S x#1-6.6 S x#1-6.6 Figure 4(c): Cross arm S x#1-6.6 after flexural testing to failure S x#1-6.6 Figure 4(d): Close up view of the left shear-span after flexural testing to failure Page 5 of 8

31 Figure 5 (a): Cross arm S x#2-6.6: one side of entire length after flexural testing to failure Figure 5 (b): Cross arm S x#2-6.6: close up of the damaged area caused by the lightning impulses (white spots) after flexural testing to failure Figure 5 (c): Cross arm S x#2-6.6: close up of the shear crack (in the right shear span) after flexural testing to failure Page 6 of 8

32 Figure 5 (d): Cross arm S x#2-6.6: close up of left shear span after flexural testing to failure Figure 5 (e): Cross arm S x#2-6.6: other side of entire length after flexural testing to failure Figure 5 (f): Cross arm S x#2-6.6: close up of the damaged area by the lightning impulses (white spots) after flexural testing to failure Figure 5 (g): Cross arm S x#2-6.6: close up of the shear crack after flexural testing to failure Figure 5 (h): Cross arm S x#2-6.6: close up of the cracks on the other shear span after flexural testing to failure Page 7 of 8

33 Figure 5 (i): Cross arm S x#2-6.6: close up of the damaged area by the lightning impulses (white spots) after flexural testing to failure Page 8 of 8