CO 2 Cleaning and Its Effectiveness in Cleaning Energized Medium-Voltage Equipment

Transcription

1 CO 2 Cleaning and Its Effectiveness in Cleaning Energized Medium-Voltage Equipment Abstract This paper provides an overview of how CO 2 cleaning works on energized electrical equipment. It also presents the procedures S&C Electric Company used to test the effectiveness of CO 2 cleaning in reducing leakage currents due to contamination. Carbon Dioxide (CO 2 ) Cleaning CO 2 cleaning is a proven, practical method for removing contaminants from the insulating surfaces of energized electrical equipment rated at up to 34.5 kv. Contaminants such as dust, salt spray, dirt, and ash can significantly reduce the dielectric strength of insulating materials. These contaminants can cause increased levels of leakage current to pass over dielectric materials, leading to electrical breakdown due to surface tracking. Such conditions can result in flashovers, which often damage electrical equipment and cause unplanned service outages, as well as safety concerns. CO 2 cleaning has been shown to reduce leakage current by more than 96%. The CO 2 cleaning process uses solid CO 2 (dry ice shavings or pellets), projected through an insulated wand at a specific nozzle pressure. These highdensity dry ice particles (approximately the size of salt granules) are rapidly accelerated using compressed air through a dry air blast line, which requires a compressor that can deliver 150 pounds per square inch (PSI) of pressure at a flow rate of 350 cubic feet per minute. When the CO 2 particles strike a contaminated equipment surface, the impact removes the surface contaminants without damaging insulating or underlying materials. How CO 2 Cleaning Works On impact with the equipment surface being cleaned, the CO 2 undergoes sublimation, transitioning from the solid phase to the gaseous phase, without passing through an intermediate liquid phase. The cleaning process works in three different ways: CO 2 cleaning system. Scan this QR code for a video showing how CO2 Cleaning Works. Thermal-shock effect: The sudden temperature difference between the CO 2 particles ( F) and the insulating surface weakens the bond that holds the contaminant intact, causing it to break apart. Thermal-kinetic effect: During sublimation, the CO 2 gas expands, lifting the remaining contaminants from the insulating surface. Kinetic energy: The accelerated CO 2 blows contaminants off of insulating surfaces. White Paper

2 CO 2 cleaning is non-toxic and does not produce secondary waste; only the lifted contaminants fall from equipment surfaces. CO 2 is also non-conductive, which allows cleaning of many types of energized electrical equipment (rated at up to 34.5 kv), including overhead and underground distribution switching equipment; metal-enclosed switchgear for indoor and outdoor distribution; distribution transformers; substation power transformers, circuit breakers, and buses; and other equipment prone to contamination. Unlike other cleaning methods, such as water blasting and cleaning with solvents, CO 2 cleaning does not require equipment de-energization and grounding; it allows electrical equipment to be cleaned without a planned outage. S&C s CO 2 Cleaning Services As part of an overall asset management program, CO 2 Cleaning Services are intended for routine maintenance of electrical equipment. CO 2 Cleaning Services are particularly beneficial for equipment exposed to harsh conditions in coastal and desert regions, high-humidity areas, and other contamination-prone environments. With routine CO 2 cleaning, costly equipment flashovers can be significantly reduced in any environment, and equipment life can be extended. In addition, the supply of CO 2 is recycled from other industry processes, making it an environmentally friendly option. CO 2 cleaning is the only method that allows cleaning of energized equipment without generating secondary waste, saving the time, costs, and labor associated with planned outages. With reduced set-up and take-down times, CO 2 cleaning enables the cleaning of more equipment in one day than other methods. Working with utilities, S&C estimated the substantial cost savings CO 2 Cleaning Services can provide over current methods. Because CO 2 cleaning does not require an outage, it eliminates the need for technicians to de-energize and re-energize equipment and prevents utility revenue losses due to outages, which can cost on average $1,000 per hour for each piece of equipment taken out of service. When all costs are considered, S&C s CO 2 Cleaning Services can provide utilities with a typical cost savings of more than 60% over their current cleaning method. If there is no current cleaning method used to maintain equipment, this can cause tracking to develop and could eventually lead to a flash-over in the equipment causing an unexpected outage. Before performing CO 2 cleaning, S&C conducts a complete equipment safety inspection, establishes a clearly defined safety perimeter and signage, and provides immediate support if equipment is determined unsafe to clean or requires repair. S&C provides detailed equipment condition inspection reports, which frequently meet routine maintenance mandates in some states. S&C s team of extensively trained Power Systems Solutions field technicians wear appropriate personal protective equipment (PPE) and adhere to S&C s safety procedures while cleaning energized electrical equipment. S&C uses proven safety protocols for CO 2 cleaning. 2

3 How S&C Tested CO 2 Cleaning on Electrical Equipment S&C has investigated the use of CO 2 cleaning on energized electrical equipment at its onsite Advanced Technology Center and at utility sites across the U.S. in order to provide a reliable, efficient, and economical service. S&C s testing includes two comprehensive sets of CO 2 cleaning tests conducted at S&C on heavily contaminated pad-mounted gear. TEST SET 1: Testing the Safety and Efficacy of CO 2 Cleaning on Energized Gear In 2010, S&C evaluated whether CO 2 cleaning could be performed safely and effectively on the two compartments of an energized S&C PMH 9 Pad-Mounted Gear unit, which was heavily contaminated. A core test objective was to determine whether any appreciable leakage current would occur during the CO 2 cleaning process that could be sufficient in magnitude to cause a flashover in the equipment. Taken out of service for use as a test sample, the pad-mounted unit selected for the test was originally installed by a utility in 1986 at an airport, where contaminant levels were high. Multiple rounds of tests were conducted to determine whether CO 2 cleaning of the energized gear could be performed without producing a fault, assess whether CO 2 cleaning is an effective means of cleaning contaminated switchgear, establish precisely how the CO 2 cleaning process should be conducted, and determine the effects of inadvertent CO 2 cleaning of non-insulating gear components (switch contacts, arc compressors, silencers, etc.). S&C used an effectively grounded three-phase test source to energize the gear at 14.4 kv during CO 2 cleaning. Phase-to-phase, phase-to-ground, spray wand to ground, and cabinet to ground leakage currents were monitored and recorded for the entire time the gear was energized in order to determine the presence of any leakage current during the CO 2 cleaning process. S&C s test setup for CO 2 cleaning of energized PMH 9 Pad-Mounted Gear. 3

4 Test Set 1 Results CO 2 Cleaning of Contaminated Insulators in Compartment 2 Before CO 2 cleaning, all of the insulators inside compartment 2 of the PMH 9 Pad-Mounted Gear unit were heavily contaminated. Compartment 2 Mini-Rupter Switch: Heavy contamination on insulators. Rotating strut and lower insulator on B-phase before CO 2 cleaning. CO 2 cleaning of A-phase Mini-Rupter Switch rotating strut inside energized gear. 4

5 While conducting CO 2 cleaning on the insulators inside the energized gear, S&C measured the leakage current levels. S&C s analysis of the leakage current measurements during CO 2 cleaning indicated a significant number of shortduration discharges (SDDs), which varied in magnitude, with a typical duration of 0.5 milliseconds (ms). Because the SDD durations were so brief, S&C determined the running root mean square (RMS) value of the leakage current on a half-cycle by half-cycle basis in order to measure the effective leakage current over that half cycle. Chart 1 and Chart 2 plot the relative frequency of the SDD equivalent RMS discharge level based on the leakage current magnitude. Chart 1. Data curve during CO 2 cleaning of Mini-Rupter Switch. Chart 2. Bottom of data curve magnified. Charts 1 and 2 show that the percentage of time the leakage current exceeds the abscissa decreases dramatically with increasing leakage current. The leakage current never exceeded 34 milliamperes (ma). 5

6 CO 2 Cleaning of Contaminated Insulators in Compartment 1 Before CO 2 cleaning, all of the insulators in compartment 1 of the PMH 9 Pad-Mounted Gear unit were heavily contaminated. While conducting CO 2 cleaning on the insulators inside the energized gear, S&C measured the leakage current levels. As with the cleaning test on compartment 2, S&C s analysis of the leakage current measurements during CO 2 cleaning indicated a significant number of SDDs, which varied in magnitude, with a typical duration of 0.5 ms. Chart 3 and Chart 4 plot the relative frequency of the SDD equivalent RMS discharge level based on the leakage current magnitude. Chart 3. Data curve during CO 2 cleaning of Mini-Rupter Switch. 6 Chart 4. Bottom of data curve magnified. Charts 3 and 4 show that the percentage of time the leakage current exceeds the abscissa decreases dramatically with increasing leakage current. The leakage current was significantly less than that measured during compartment 2 cleaning, never exceeding 8 ma.

7 Inspection of Compartments 1 and 2 After CO 2 Cleaning Compartments 1 and 2 were thoroughly inspected after the gear was de-energized. During and after each test, S&C made the following key observations: CO 2 cleaning tests were performed without a flashover on energized gear. CO 2 cleaning was very effective in removing contamination from the front of the insulator surfaces (with no flying debris). No apparent damage was sustained by any gear components due to CO 2 cleaning. Discharge duration and SDDs decreased dramatically with increasing leakage current. The CO 2 cleaning nozzle was not effective in cleaning the back of the rotating strut or the backs of the support insulators; the nozzle design required additional development. Additional Non-Energized Tests Additional tests were conducted on the gear when it was de-energized. Two tests determined the effect of inadvertent cleaning of non-insulating components (switch contacts, arc compressors, silencers, etc.). One test involved coating the switch contacts with grease and then cleaning the contacts with CO 2. As anticipated, the grease was unaffected by CO 2 cleaning of the contacts. Another test involved CO 2 cleaning of the arc compressor (front and side) in compartment 2 to determine if the device would sustain any damage from cleaning. Disassembly and inspection of the arc compressor revealed no internal or external damage as a result of CO 2 cleaning. Mini-Rupter Switch in compartment 2 after CO 2 cleaning. Test 1 Summary S&C s tests, which evaluated the feasibility of CO 2 cleaning on energized equipment and the effectiveness of CO 2 cleaning on the insulators and rotating struts inside pad-mounted gear, demonstrated that CO 2 cleaning is an extremely effective method for cleaning exposed insulating surfaces. After performing the energized tests, analysis of leakage current measurements indicated that the occurrence of SDDs decreased significantly as the leakage current increased (SDDs were non existent above 34 ma). S&C s analysis of the leakage current demonstrated that CO 2 cleaning of energized pad-mounted gear can be performed effectively, provided that personnel are properly trained and equipped with the appropriate PPE (full flash suit, safety glasses, rubber gloves, insulating mat, proper shoes, etc.). Inadvertent CO 2 cleaning of non-insulating gear components (switch contacts, arc compressors, silencers, etc.) revealed that no damage was sustained by these components and that grease was not removed from switch contacts as a result of CO 2 cleaning. A number of different nozzle designs were tested to determine their effectiveness in cleaning the backsides of the insulating surfaces. The results strongly indicated that a nozzle design, which would effectively clean entire insulator surfaces (including the backsides), could be developed. 7

8 TEST SET 2: Testing the Effectiveness of CO 2 Cleaning in Reducing Leakage Currents in Contaminated Gear In 2012, S&C evaluated the effectiveness of the CO 2 cleaning process for reducing leakage current measurements on contaminated S&C PMH 10 Pad-Mounted Gear and S&C PMH 9 Pad-Mounted Gear units. To simulate typical contamination and evaluate the effectiveness of CO 2 cleaning, all insulators and rotating struts (S&C Mini-Rupter Switches) inside both pad-mounted gear units were artificially and heavily contaminated using hair spray as the bonding agent and diatomaceous earth as the contamination medium. First, each insulator and each rotating strut was coated with hair spray. Diatomaceous earth was then blown onto the insulators, heavily coating both the front and back sides of all insulating surfaces with the contaminant. To simulate field conditions, both heavily contaminated PMH 10 and PMH 9 Pad-Mounted Gear units were later subjected to a dry test and a dew test, where dew was allowed to form on the insulators. During these tests, the insulators were energized at a three-phase test voltage of approximately 15 kv, and the leakage current on each phase and the leakage current of the pad-mounted gear enclosure-to-ground were measured. The insulators inside both padmounted gear units were then cleaned with CO 2, and the tests were repeated. Dew on the insulators was created by placing each pad-mounted gear unit into S&C s environmental test chamber (during separate tests) and cooling it overnight to 12 C. After each unit s temperature reached equilibrium, the chamber temperature was quickly elevated to C while raining on the unit for 15 minutes to create an extremely humid environment, which resulted in heavy dew formation on all insulator surfaces. Mini-Rupter Switch rotating strut with high levels of contaminants applied. Typical dew formation on heavily contaminated insulator during dew tests. An effectively grounded three-phase test source was used to energize the gear at 15 kv. The test source was connected to each phase of one of the Mini-Rupter Switches, and each fuse or switch was closed, allowing all insulators inside the unit to be energized. 8

9 Test Set 2 Results Leakage Currents on PMH 10 and PMH 9 Pad-Mounted Gear Before CO 2 Cleaning The contaminated PMH 10 Pad-Mounted Gear unit and the contaminated PMH 9 Pad-Mounted Gear unit were energized at approximately 15 kv; the leakage currents were then measured when each unit was cold (dry test) and after dew had formed on the insulators (dew test). For both the PMH 10 and PMH 9 Pad-Mounted Gear units, the dry and dew tests were repeated to verify consistency of results. A five-minute interval was allowed between dew tests to enable new dew to form on the insulator surfaces. For the PMH 9 unit, initial leakage currents for the last test were, in some phases, more than double those observed during previous tests. The higher initial leakage current likely resulted from increased dew on the insulator causing additional wetting and flow of contaminants on insulator surfaces. The final dew test leakage currents were closer to each other, particularly for energized tests of 56 seconds or longer. Leakage Currents on PMH 10 and PMH 9 Pad-Mounted Gear After CO 2 Cleaning After testing and removing the contaminated PMH 10 Pad-Mounted Gear unit and the contaminated PMH 9 Pad-Mounted Gear unit from the environmental chamber (during separate tests), each unit was cleaned using S&C s CO 2 cleaning process. A black rag was then used to wipe the back side of each insulator to determine how effective CO 2 cleaning was for hard-to-reach areas of the insulators. Very little residue was found on any of the insulating surfaces in the PMH 10 Pad-Mounted Gear unit, as shown by the negligible amount of residue left on the black rags. Virtually no residue was found on any of the insulating surfaces in the PMH 9 Pad-Mounted Gear unit. CO 2 cleaning of back side of rotating strut (with special curved nozzle) in PMH 10 unit. Black rag test shows minute amount of contamination left on back sides of insulators. 9

10 The PMH 10 Pad-Mounted Gear unit and PMH 9 Pad-Mounted Gear unit were then moved back into the environmental chamber (at separate times) so that post-cleaning dew tests could be performed. As previously described, each unit was cooled overnight to 12 C and then subjected to high-humidity conditions so that a heavy dew could form on the insulating surfaces. Each unit was energized at a three-phase test voltage of 15 kv; the leakage currents were then measured when each unit was cold (dry) and after dew formed on the insulators (dew). For both the PMH-10 and PMH-9 Pad-Mounted Gear units, the dew tests were repeated to verify consistency of results. Again, a five-minute interval was allowed between dew tests to enable new dew to form on the insulator surfaces. These results show that the leakage current under dew conditions is essentially equivalent to that under dry conditions. The black rag test results and the significant reduction in leakage current demonstrated that CO 2 cleaning was extremely effective in removing contamination from all insulating surfaces in the PMH-10 and PMH-9 Pad-Mounted Gear units. Heavy dew formation on fuse support insulator inside PMH-9 unit. Tables 1 and 2 show the leakage currents measured during tests on the energized PMH-10 Pad-Mounted Gear unit before and after CO 2 cleaning: Table 1. Leakage currents on PMH-10 gear before CO 2 cleaning. Leakage Measurement Leakage Current (ma) Initial Final A-Phase B-Phase C-Phase A-Phase B-Phase C-Phase Type Test Duration Seconds Dry 120 Dew 176 Table 2. Leakage currents on PMH-10 gear after CO 2 cleaning. Leakage Measurement Leakage Current (ma) Initial Final A-Phase B-Phase C-Phase A-Phase B-Phase C-Phase Type Test Duration Seconds Dry 58.2 Dew

11 Tables 3 and 4 show the leakage currents measured during tests on the energized PMH-9 Pad-Mounted Gear unit before and after CO 2 cleaning: Table 3. Leakage currents on PMH-9 gear before CO 2 cleaning. Leakage Measurement Leakage Current (ma) Initial Final A-Phase B-Phase C-Phase A-Phase B-Phase C-Phase Type Test Duration Seconds Dry 8.04 Dew 56.6 Table 4. Leakage currents on PMH-9 gear after CO 2 cleaning. Leakage Measurement Leakage Current (ma) Initial Final A-Phase B-Phase C-Phase A-Phase B-Phase C-Phase Type Test Duration Seconds Dry 55.7 Dew 58.1 Test 2 Summary S&C s tests, which evaluated the effectiveness of CO 2 cleaning on the insulators and rotating struts inside pad-mounted gear, revealed several key results. The black rag wiping tests identified only a minute amount of contamination remaining on the back sides of insulating surfaces after the PMH-10 and PMH-9 Pad-Mounted Gear units were cleaned. These tests also confirmed that a specially curved nozzle was extremely effective in cleaning the back sides of the insulating surfaces. The dew tests confirmed that CO 2 cleaning is an extremely effective method of cleaning the exposed portions of the insulating surfaces. Chart 5 shows the significant reduction in leakage current achieved by performing CO 2 cleaning on the pad-mounted gear units. Leakage Current Milliamperes Maximum Leakage Current Measured on Pad-Mounted Gear Units Energized at 15 kv PMH-9 Leakage Current PMH-10 Leakage Current Before C0 2 Cleaning After C0 2 Cleaning Chart 5. CO 2 cleaning can reduce leakage current by more than 96%. 11

12 Conclusion Over several years of comprehensive testing, S&C has proven that the CO 2 cleaning process can be conducted effectively on energized electrical equipment (rated at up to 34.5 kv) by trained, properly equipped personnel, without damaging insulating surfaces or underlying materials. S&C s extensive tests on contaminated gear definitively show that CO 2 cleaning can reduce leakage current by more than 96%. After CO 2 cleaning is completed, the equipment will continue to perform safely at its rated capacity and is better protected from flashovers, thereby helping to prevent damage and unplanned service outages while extending equipment life. For more information or a quote for S&C s CO 2 Cleaning Services please visit our website at sandc.com/co 2 paper or us at CO 2 cleaning@sandc.com Testing services and this test report are provided subject to S&C s Standard Terms and Conditions for Services, Price Sheet 181. No interpretations of this test report should be made beyond the description of the tests performed on the specific equipment tested and the results of the tests as presented. This test report may not be reproduced or published in any form, except as a complete copy in its entirety, without the express written permission of S&C Electric Company, 6601 North Ridge Boulevard, Chicago, Illinois 60626, U.S.A. Phone: (773) , Power Systems Solutions Fax: (773) S&C Electric Company The information contained in this document is proprietary, and is not to be used, reproduced, copied, or distributed without the express written consent of S&C Electric Company. Printed in U.S.A. August 26, 2013 sandc.com twitter.com@sandc_us twitter.com@sandc_uk 180-T104