K-4 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM Level 4 Line K: Install High-Voltage Systems LEARNING GUIDE K-4 USE HIGH-VOLTAGE TEST EQUIPMENT
Foreword The Industry Training Authority (ITA) is pleased to release this major update of learning resources to support the delivery of the BC Electrician Apprenticeship Program. It was made possible by the dedicated efforts of the Electrical Articulation Committee of BC (EAC). The EAC is a working group of electrical instructors from institutions across the province and is one of the key stakeholder groups that supports and strengthens industry training in BC. It was the driving force behind the update of the Electrician Apprenticeship Program Learning Guides, supplying the specialized expertise required to incorporate technological, procedural and industry-driven changes. The EAC plays an important role in the province s post-secondary public institutions. As discipline specialists the committee s members share information and engage in discussions of curriculum matters, particularly those affecting student mobility. ITA would also like to acknowledge the Construction Industry Training Organization (CITO) which provides direction for improving industry training in the construction sector. CITO is responsible for organizing industry and instructor representatives within BC to consult and provide changes related to the BC Construction Electrician Training Program. We are grateful to EAC for their contributions to the ongoing development of BC Construction Electrician Training Program Learning Guides (materials whose ownership and copyright are maintained by the Province of British Columbia through ITA). Industry Training Authority January 2011 Disclaimer The materials in these Learning Guides are for use by students and instructional staff and have been compiled from sources believed to be reliable and to represent best current opinions on these subjects. These manuals are intended to serve as a starting point for good practices and may not specify all minimum legal standards. No warranty, guarantee or representation is made by the British Columbia Electrical Articulation Committee, the British Columbia Industry Training Authority or the Queen s Printer of British Columbia as to the accuracy or sufficiency of the information contained in these publications. These manuals are intended to provide basic guidelines for electrical trade practices. Do not assume, therefore, that all necessary warnings and safety precautionary measures are contained in this module and that other or additional measures may not be required.
Acknowledgements and Copyright Copyright 2011, 2014 Industry Training Authority All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or digital, without written permission from Industry Training Authority (ITA). Reproducing passages from this publication by photographic, electrostatic, mechanical, or digital means without permission is an infringement of copyright law. The issuing/publishing body is: Crown Publications, Queen s Printer, Ministry of Citizens Services Battery-powered megger tester, Megohmeter pushbutton functions, and Battery-powered megger display reproduced with permission, Fluke Corporation. The Industry Training Authority of British Columbia would like to acknowledge the Electrical Articulation Committee and Open School BC, the Ministry of Education, as well as the following individuals and organizations for their contributions in updating the Electrician Apprenticeship Program Learning Guides: Electrical Articulation Committee (EAC) Curriculum Subcommittee Peter Poeschek (Thompson Rivers University) Ken Holland (Camosun College) Alain Lavoie (College of New Caledonia) Don Gillingham (North Island University) Jim Gamble (Okanagan College) John Todrick (University of the Fraser Valley) Ted Simmons (British Columbia Institute of Technology) Members of the Curriculum Subcommittee have assumed roles as writers, reviewers, and subject matter experts throughout the development and revision of materials for the Electrician Apprenticeship Program. Open School BC Open School BC provided project management and design expertise in updating the Electrician Apprenticeship Program print materials: Adrian Hill, Project Manager Eleanor Liddy, Director/Supervisor Beverly Carstensen, Dennis Evans, Laurie Lozoway, Production Technician (print layout, graphics) Christine Ramkeesoon, Graphics Media Coordinator Keith Learmonth, Editor Margaret Kernaghan, Graphic Artist Publishing Services, Queen s Printer Sherry Brown, Director of QP Publishing Services Intellectual Property Program Ilona Ugro, Copyright Officer, Ministry of Citizens Services, Province of British Columbia To order copies of any of the Electrician Apprenticeship Program Learning Guide, please contact us: Crown Publications, Queen s Printer PO Box 9452 Stn Prov Govt 563 Superior Street 2nd Flr Victoria, BC V8W 9V7 Phone: 250-387-6409 Toll Free: 1-800-663-6105 Fax: 250-387-1120 Email: crownpub@gov.bc.ca Website: www.crownpub.bc.ca Version 1 Corrected, January 2017 Revised, April 2014 New, October 2012
LEVEL 4, LEARNING GUIDE K-4: USE HIGH-VOLTAGE TEST EQUIPMENT Learning Objectives............................................... 7 Learning Task 1: Describe characteristics of cable insulation..................... 9 Self-Test 1......................................... 12 Learning Task 2: Describe the use of a megger for high-voltage insulation testing........ 15 Self-Test 2......................................... 26 Learning Task 3: Describe field testing methods for high voltage cables............. 27 Self-Test 3......................................... 37 Answer Key.................................................. 41 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 5
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Learning Objectives K-4 Learning Objectives The learner will be able to describe characteristics of cable insulation The learner will be able to describe the use of a megger for insulation testing of highvoltage circuits The learner will be able to describe non-destructive testing of cables and equipment. Activities Read and study the topics of Learning Guide K-4: Use High-Voltage Test Equipment. Complete Self-Tests 1 through 3. Check your answers with the Answer Key provided at the end of this Learning Guide. Resources All resources are provided in this Learning Guide. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 7
BC Trades Modules www.bctradesmodules.ca We want your feedback! Please go the BC Trades Modules website to enter comments about specific section(s) that require correction or modification. All submissions will be reviewed and considered for inclusion in the next revision. SAFETY ADVISORY Be advised that references to the Workers Compensation Board of British Columbia safety regulations contained within these materials do not/may not reflect the most recent Occupational Health and Safety Regulation. The current Standards and Regulation in BC can be obtained at the following website: http://www.worksafebc.com. Please note that it is always the responsibility of any person using these materials to inform him/herself about the Occupational Health and Safety Regulation pertaining to his/her area of work. Industry Training Authority January 2011 8 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4
Learning Task 1: Describe characteristics of cable insulation The characteristics of insulation (dielectrics) found in high-voltage (HV) systems were discussed earlier. This Learning Task elaborates on these properties as they relate to HV testing of cable and equipment insulation. Purpose and properties of insulation The purpose of insulation is to prevent current flow between an energized line and ground (ground fault), or between two lines (short-circuit fault). A perfect insulator has infinite resistance and all absorption phenomena and dielectric losses are absent. A perfect vacuum is the only known perfect insulator. Electrical insulation materials should exhibit a high insulation resistance to withstand leakage currents, a high dielectric strength to withstand electrical stress and good heat conducting properties to maintain insulation stability. Resistance The insulation resistance of a cable is defined as the resistance (in megohms) to the flow of current offered by the insulation when a direct voltage is applied. It is determined by applying the direct voltage and measuring the resulting current, which flows through the insulation. This current is referred to as insulation current or leakage current. A perfect vacuum is the only known perfect insulator with infinite resistance. Cable insulation has a very high insulation resistance but when a direct voltage is impressed, a small leakage current will always be present, flowing along and through the insulation to ground. Typically, this current flow is so small it does not cause problems, but a variety of problems can occur if the cable insulation is allowed to deteriorate, creating resistive paths, lowering the insulation resistance and increasing the leakage current. Insulation resistance decreases with temperature rise because insulators have a negative temperature coefficient of resistance. There are several insulation resistance tests that may be conducted with a megger prior to installation of a cable. These will be discussed in Learning Task 2. Capacitance Because insulation lies between energized conductors or between an energized conductor and ground, it essentially forms the dielectric of a capacitor. In the case of an HV cable, the two plates are the energized conductor and the insulation shield, which is grounded. In the case of a motor or transformer, the plates are the energized windings and the frame. In this way, insulation takes on the features and terms that apply to capacitor dielectrics. When you apply a DC voltage to a capacitor, it is like compressing air in a tank or tensioning a spring. This pent-up energy in the capacitor has the potential for some serious consequences. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 9
Learning Task 1 K-4 Dielectric power factor As a result of the capacitance of the insulation when an AC current flows through insulation, the current leads the voltage by nearly 90, which is the phase-angle between the voltage and current. The cosine of this angle is the power factor (PF). Because the dielectric phase angle is close to 90 for most insulating materials, the PF is close to 0. If the insulation is sub-standard or damaged, this angle decreases. Because the faults cause resistive paths to appear in the insulation, the in-phase current component increases and causes an increase in the power loss in the insulation. Dielectric absorption loss Dielectric absorption is a phenomenon that occurs in insulation in which positive and negative charges separate and accumulate at certain regions in the dielectric under the influence of electric stress. In effect, there is a displacement of charges in the insulation. The separation and movement of charge is the dielectric absorption current. Initially, when the voltage is applied this current is at its highest, and then it decreases with time. Like a capacitor, this appears as stored energy in the insulation material, after the application of a DC voltage. The energy stored in the polarized dielectric is referred to as the dielectric absorption phenomenon. When the voltage source is removed, the displaced charges return to their normal state. But this may take time. If it took a long time to move the charges from their normal position, it will also take a long time to return them to their normal position. People in the business of insulation testing must be alert to this potentially hazardous dielectric absorption phenomenon Dielectric loss When the insulation in a cable is subjected to an AC voltage, the electrons in the atoms are strained and dielectric absorption occurs. This stress reverses direction, as the polarity of the AC voltage reverses. As the voltage reverses 120 times a second in a 60 Hz system, the direction of the charge in the dielectric reverses 120 times a second as well. The separation and movement of charge is the dielectric absorption current. This current through the insulation resistance of the dielectric causes an increase in temperature. The resulting heat transfer represents a power loss which is directly proportional to the frequency, and which increases exponentially with voltage. Dielectric loss is measured in watts and is a measurement of the energy dissipated through and over the insulation surface. Dielectric loss increases with temperature, moisture and corona. Insulation deterioration Current may leak from a conductor through the insulation to ground, or it may track across the insulation surface to ground. This leakage or creepage current should normally be extremely small, in the region of microamperes or nanoamperes, and is not usually a problem. But if the insulation is poor, or if there are surface contaminants, then larger currents may flow and the insulation s temperature will increase, causing thermal activity that could eventually lead to its failure. Insulation can also deteriorate over a period of time due to temperature change, electric stress, vibration, chemical changes, and moisture. The major factors of insulation deterioration are manufacturing defects, treeing and partial discharge. 10 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4
Learning Task 1 K-4 Manufacturing defects Voids, inclusions and contaminants in the insulation, and protrusions from the semicon are examples of manufacturing defects that will cause the insulation to deteriorate rapidly under the influence of electric stress. Voids are micro bubbles in the insulation, while inclusions are foreign matter trapped in the insulation. Protrusions are sharp points extending into the insulation from the semicon. These are all forms of cable manufacturing defects. All cause intense local electric fields, which may lead to partial discharge at the site or rapid growth of water or electrical trees near the defect. Water and electric trees Treeing is a name commonly given to the tree-like erosion propagated by electrical discharges in a cable insulation or covering. There are two types of treeing commonly found in serviceaged, high voltage cable insulation: water treeing and electrical treeing. Water treeing Water treeing is the result of deterioration within the dielectric that may occur at electrical stress points, such as protrusions caused by imperfect conductor strands, inclusions, or voids in extruded cross-linked polyethylene insulation or ethylene propylene insulation material when in the presence of moisture and subjected to electrical stress. Bowtie trees Leakage trees Vented trees Figure 1 Various configurations of water trees Now do Self-Test 1 and check your answers. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 11
Learning Task 1 K-4 Self-Test 1 1. What is the primary function of insulation? 2. What factors can cause insulation to deteriorate over time? 3. Is dielectric loss mainly due to AC, DC, or both AC and DC? 4. What is the name given to the phenomenon whereby a charge can reappear on a conductor even after that conductor has been grounded? 5. List the four factors that lead to water trees in cable insulation. 6. Which causes partial discharge: electrical trees or water trees? 12 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4
Learning Task 1 K-4 7. List three factors that may cause partial discharge within a dielectric. Go to the Answer Key at the end of the Learning Guide to check your answers. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 13
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Learning Task 2: Describe the use of a megger for high voltage insulation testing A megger (megohmmeter) is an instrument used to measure very high resistance (measured in millions of ohms), the type of resistance you would expect to find in good insulation. A megger insulation tester is essentially a high-resistance range ohmmeter with a built-in DC generator. The megger s generator, which can be hand-cranked, battery or line-operated, develops a high DC voltage that causes several small currents through and over the surfaces of the insulation being tested. The total current is measured by the ohmmeter, which has an analog indicating scale, digital readout or both. 1 3 2 1 2 3 ( ) Negative Terminal ( + ) Positive Terminal Safety Shutter Figure 1 Battery-powered megger tester. Reproduced with Permission, Fluke Corporation There are several types of insulation resistance tests that are performed with a megger prior to termination to identify the insulation resistance of high voltage cables and equipment. Insulation resistance testing of cable differs from the testing of apparatus windings, mainly CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 15
Learning Task 2 K-4 because of the capacitance of the cable as discussed in Learning Task 1. If the cable is long, the capacitance will be high, which takes a longer time to charge resulting in a longer required test time. Tests should be made between each conductor, between each conductor and ground with other conductors grounded, and between each conductor and ground with other conductors connected to the guard circuit but not grounded. Meggers may be hand-cranked, battery-powered, or motor-driven units depending upon the equipment to be tested. 1 2 3 4 5 6 7 1 2 3 4 5 6 7 Figure 2 Megohmeter pushbutton functions. Reproduced with Permission, Fluke Corporation Battery powered meggers have many advantages over the hand-cranked generators, some of which are indicated in the chart in Figure 2. Advantages such as the ability to store tests results 16 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4
Learning Task 2 K-4 in memory and scroll through them at a later time, incrementally step the test voltage at precise levels, as well as providing timed tests over a lengthy period are desirable features not available from a hand-cranked unit. The downside of course is the requirement for batteries. Hand-crank meggers require two hands to operate, and are generally heavier and bulkier than their battery powered counterparts.. 4 5 6 7 3 2 8 9 1 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 Figure 3 Battery-powered megger display. Reproduced with Permission, Fluke Corporation CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 17
Learning Task 2 K-4 Guard terminal Most insulation resistance tests are conducted using only the (+) and ( ) output connections. The guard terminal (G) is connected internally at the same potential as the negative ( ) terminal but it is not in the measurement circuit path, and is normally only used in high resistance applications. For most low voltage insulation resistance tests, the tester would connect the positive (+) and negative ( ) test leads to the corresponding inputs on the megger. The test leads would then be connected to the cable under test and ground. The guard terminal is left unconnected. When measuring very high insulation resistances, such as in long cables or cables for medium or high voltage applications, incorporating the guard terminal will give more accurate readings. The guard terminal is at the same potential as the negative ( ) terminal, and can be used to prevent surface leakage or other unwanted leakage currents from being included in the test measurements, which would otherwise affect the accuracy of the insulation resistance measurement. If parallel leakage paths exist, a guard connection will eliminate those from the measurement, and give a more precise reading of the leakage between the remaining elements. Figure 4 illustrates the megger leads connected to improve the accuracy of the reading by shunting the surface leakage current away from the measuring device. The guard provides a shunt circuit that diverts surface leakage current around the measurement circuit path. Copper wire shield to positive terminal (+) Dielectric to guard terminal Conductor to negative terminal ( ) A Megger Figure 4 Megger connections with guard terminal Safe test practices Safety is everyone s responsibility, but your safety ultimately is in your hands. When working with insulation resistance test equipment, it is essential that you develop and practice safe work practices that give you maximum protection. Connected equipment (switches, relays, bus, transformers, etc.) may have lower insulation resistance values than the cable, or may not be capable of withstanding the test voltage applied by the megohmeter and should be disconnected before any testing is performed. Always read and follow the manufacturer s recommendations. Never connect the insulation tester to energized conductors or energized equipment. 18 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4
Learning Task 2 K-4 Never exceed the recommended test voltage. Ground all conductors, except the one to be tested. Connect cable shield to ground; ground any adjacent equipment. Ensure adequate clearance of the conductor/terminals to be tested from ground to prevent flashover. Corona-proof conductor/terminal ends of cable by sufficiently taping them. If cable is terminated, cover termination with polyethylene bucket or bag. Erect safety barriers between the test area and surroundings. Keep people out of the area in which the test is to be performed and out of the areas at the other end of energized cables. Remember that remote parts of the system may be energized during the test. Discharge cable capacitance, for a period of time equal to four or five times the duration of the test, both before and after the test. The longer a cable is, the more capacitance there will be, and the longer this discharge time needs to be. Leakage current from conductor ends to ground, flowing over wet or dirty wire and cable ends, will cause low insulation resistance (IR) readings. Tests should be performed in a dry working area when possible. Cable ends should always be cleaned and dried by wiping them with a lint free cloth that has been moistened with an appropriate solvent. Avoid using solvents with 1,1,1 trichloroethane or other hydrocarbons that may damage the cable insulation. Don t use an insulation tester in a hazardous or explosive environment. Always use gloves and a hot stick to handle the cable and its connections. A test should be performed as soon as the cable end is clean and dry. Minimum insulation resistance The Insulated Power Cable Engineers Association (IPCEA) provides the formula to determine minimum insulation resistance per unit length values. R = K log 10 D/d R = IR Value in MΩs per 1000 feet (305 meters) of cable. K = Insulation material constant for XLPE Low Voltage (rated 0-2kV) High Voltage (rated > 2kV) 10 000 20 000 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 19
Learning Task 2 K-4 D = Outside diameter of conductor insulation for single conductor wire and cable (D = d + 2c + 2b diameter of single conductor cable) d Diameter of conductor c Thickness of conductor insulation b Thickness of jacket insulation AWG 14 12 10 8 6 Insulation Thickness inches.047.063.078.094.109.125.141.156.172.188.203.219.234.392.334.283.239.470.405.348.296.225.537.467.404.347.267.594.520.453.392.305.645.568.498.432.340.691.611.538.470.373.732.651.575.505.403.770.686.609.537.431.804.720.641.566.453.836.751.670.594.483.866.779.698.621.506.894.806.723.645.529.921.832.748.669.550 5 4 3 2 1 1/0.206.187.171.155.139.126.245.224.204.186.168.152.281.257.236.215.195.177.314.289.265.243.220.201.346.318.293.269.244.223.373.345.318.293.267.244.401.371.343.316.288.264.426.395.366.338.309.284.450.418.388.359.328.302.463.440.409.379.347.320.495.460.429.398.365.337.515.480.448.416.382.354 Figure 5 Table of values of Log 10 x D/d for different AWG sizes Test voltage A megger is rated in volts, and puts out direct current from a built-in DC generator. The megger shown in Figure 3 has voltage settings of 250 volts, 500 volts, 1000 volts, 2500 volts, 5000 volts and 10 000 volts. Most modern meggers have several voltage options. Lower price batterypowered meggers often have megger voltages of 500 volts and 1000 volts. Insulation should be tested at a specified voltage. When AC voltage is used, test voltage (AC) = (2X name plate voltage) + 1000. When DC voltage is used, test voltage (DC) = (2X name plate voltage). Always refer to the manufacturer s literature for recommended test voltages of equipment insulation. Test currents The amount of current depends on the amount of voltage applied, the system s capacitance, the total resistance, and the temperature of the material. For a fixed voltage, the greater the measured test current is, the lower the value of insulation resistance. The total resistance is the sum of the internal resistance of the core conductor which will be very small in comparison, and 20 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4
Learning Task 2 K-4 the insulation resistance which, if good, will be in megaohms. When the megger applies the test voltage to the cable under test, the current should start at a relatively high value and should then drop off quickly, becoming steady at a low value after a period of time. Conversely, the insulation resistance will start out a low to medium value and should increase over time before levelling out and becoming stable. The fact that this current levels off and becomes constant is as important as test information as is the magnitude of the total leakage. This current characteristic of insulation is due to the capacitive fact that the total current is not one single current, but is actually the sum of several individual currents due to the capacitive nature of the dielectric. These individual components of the total current and their action with time are illustrated below in Figure 6: 0 Insulation resistance Figure 6 Insulation current components Conduction or leakage current (I Conduction ) This current has two components: surface leakage current(which tracks along the surface of the insulation) as well as the conduction path through the insulation. The surface leakage component introduces errors into the measurement and may be shunted from the measurement path with use of the guard terminal. Removing the surface leakage component becomes critical at higher insulation resistance levels to avoid invalid test results. It is only the current that follows the conduction path through the insulation that we want to measure. Capacitive charging leakage current (I Capacitive ) Due to the capacitive nature of the insulation, leakage charging current flows through the conductor insulation. This current lasts only for a few seconds as the DC voltage is applied CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 21
Learning Task 2 K-4 and drops off after the insulation has been charged to its full test voltage. In lowcapacitance equipment, the capacitive current is higher than conductive leakage current, but usually disappears fairly rapidly. Because of this, it is important to let the reading stabilize and become constant before recording the test current value. On the other hand, when testing high capacitance equipment such as long shielded power cables, the capacitive current can last for an extensive period of time. This will affect the type of insulation resistance testing method that is selected for the test. Absorption current (I Absorption ) Absorption current is caused by the polarization of molecules within dielectric material caused by the application of a direct voltage. In low-capacitance equipment, the current is high for the first few seconds and decreases slowly to nearly zero. When dealing with high capacitance equipment or wet and contaminated insulation, the absorption current will remain high for a long time. The longer the time it takes for the absorption current to subside during the test, the longer the time the cable must be grounded after the test is complete. Because the meter can only measure the total current and not the individual current components, the leakage current alone must be measured after the capacitive current and the absorption current have died down. In other words, you can measure true leakage current only when the meter reading becomes steady. Time-dependent currents cause problems when you are testing the insulation on equipment with high capacitance. They are not significant on low capacitance equipment. As a result, the testing methods used for high and low capacitance equipment are different. It is essential, therefore, that you know the approximate capacitance of the equipment you are testing. There are tables available from electrical equipment manufacturers, and some insulation resistance testers have a capacitance measuring feature. High-voltage bus systems, switchgear, and electric cords are examples of low capacitance equipment. If you are testing low capacitance equipment, the time-dependent capacitive current and absorption current will probably decrease to zero very quickly. In this case, you can measure the true insulation resistance with a simple insulation test because the meter reading will become steady almost immediately. Large generators, long lengths of cable, large motors, and similar large apparatus with complex insulation systems are examples of high capacitance equipment. With these kinds of equipment the capacitive current will last a long time, and the absorption current may continue for much longer. With high capacitive equipment you will not be able to get a steady meter reading or use a simple insulation resistance test such as a proof test or spot reading test. Instead, a test that establishes a trend between readings, such as the polarization index test or the step voltage test is required. As an example, a 15kV cable in a medium length and tested at a temperature of 20 C will typically have leakage currents approximating those below for the different dielectric types: Cross-linked polyethylene (XLPE) cable has the lowest leakage current usually less than 10 microamperes, (1500 megohms). 22 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4
Learning Task 2 K-4 Ethylene Propylene Rubber (EPR) and natural rubber cables usually have leakage currents of less than 20 microamperes, (750 megohms). Oil-impregnated lead-covered (PILC) cables usually have leakage currents of less than 50 microamperes, (300 megohms). Cable splices and terminators will increase the leakage current, because they provide additional leakage paths in parallel with the insulation under test. Oil-filled high-voltage equipment has widely varying leakage currents depending on the volume and the quality of the oil and solid materials that are used. Insulation resistance test types There are five basic test types used in insulation resistance testing, the proof test which is a withstand type of test, as well as the spot reading test, the polarization index test, the stepped voltage test and the Dielectric Absorption test which are diagnostic type tests. Proof test Proof tests are performed to insure proper installation and integrity of conductors prior to termination. The proof test is a simple, quick test that gives the instantaneous condition of insulation. It provides no diagnostic data and the test voltages used are much higher than the voltages used in predictive maintenance tests. The proof test is called a pass/fail because the installation is declared acceptable if the insulation resistance reaches a certain value and no breakdown occurs during testing. It tests cable systems for damage or defects occurring in the cable during the installation process, and should be done prior to termination. A proof test can be performed on equipment of any capacitance. It is performed with a single voltage, usually between 500 and 5000 volts, applied for 60 seconds. It is common to stress the insulation above normal working voltages in order to detect subtle weaknesses in the insulation. As a rule, for new equipment the test should be done at about 160% to 180% of the manufacturer s factory test voltage. For cables, test using a voltage about twice the cable s rated voltage plus 1000 volts. Rated voltage is the maximum amount of voltage that the conductor can be exposed to for a prolonged amount of time, and is usually printed on the conductor. Spot-reading/short-time test During the spot-reading or short-time test, the test voltage is applied and slowly increased for 60 seconds to the appropriate test voltage, or until the reading becomes steady. In order to reach a stable insulation reading within a minute, the test usually can only be performed on low-capacitance equipment. When testing good equipment, you should notice a steady decrease in the total current and an increase in insulation resistance due to the decrease in capacitive and absorption currents. If the insulation resistance is below an acceptable value, leave the megger on with the voltage applied for longer than 60 seconds to heat the cable and remove moisture. If that does not result in an increased value of insulation resistance, clean the insulation thoroughly and apply gentle heat to dry it out before retesting. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 23
Learning Task 2 K-4 Polarization index test To overcome the problem of capacitive and absorption time-dependent currents, a polarization index test is often used. The polarization index (PI) is the ratio of two readings: one is taken after one minute and the other is taken after ten minutes. The trend indicated by this ratio will allow you to judge the rate at which the capacitance and absorption currents are disappearing. There are several advantages to a polarization index test. You do not have to wait for a steady meter reading, because you are looking for a trend rather than a single reading. Nor do you do have to adjust the measurement to allow for variations in the temperature of the tested component as you must when you perform a simple insulation resistance test. Moisture and oil ingress may create resistive paths within the insulation, causing an increase in leakage current. This leads to a distinctive flattening effect on the shape of the resultant PI curve and a lower PI index ratio. With good insulation, the insulation resistance will start low and get higher as the capacitive leakage current and absorption current get smaller. Results are obtained by dividing the ten minute test value by the one minute test value. The index ratio will vary with different types of insulation. A low polarization index usually indicates problems with the insulation. Shielded power cables and rotating machine windings with a capacity greater than 0.02 microfarads typically have polarization index values of 1.5 or higher. In general, the higher the polarization index for large capacitance equipment, the better the insulation. Surface conductive dirt and surface moisture films are a frequent cause of low polarization index readings. Clean and dry the surfaces of the insulation, inspect the equipment for conductive surface tracking and then retest. If the polarization index is still low, this suggests an internal problem with the insulation. Probably there is moisture in the bulk of the insulation. In rare cases, there may be some other kind of deterioration. Step voltage test The step voltage test involves taking resistance measurements at various voltage settings. In this test, you apply each test step voltage for 60 seconds and graph the recorded insulation resistance values. Since good insulation is resistive, an increase in test voltage will lead to an increase in current with a result that the resistance remains constant. Any deviation from this could signify defective insulation. By applying increasing voltages in steps, the insulation is exposed to increased electrical stress that can reveal information about flaws in the insulation such as pinholes, physical damage, or brittleness. As the voltage rises we reach a point where ionization and partial discharge can take place within cracks or voids, resulting in an increase in current, and therefore a reduction in the insulation resistance. Good insulation should withstand an increase in electrical stress with only a small variation in the insulation resistance measured at each step. At higher voltage levels, deteriorated, cracked or contaminated insulations which 24 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4
Learning Task 2 K-4 cause a decreased insulation resistance will experience an increased current flow. The step voltage test is independent of insulation material, equipment capacitance, and temperature effect. Because it takes a longer time to run, it may only be performed after an insulation spot test or short time test has been inconclusive. Dielectric-absorption/time-resistance test The Dielectric-absorption/time-resistance test is independent of equipment size, capacitance and temperature. It is based on the relative magnitudes of leakage and absorption currents in clean, dry insulation compared with moist or contaminated insulation. With good insulation, leakage current is relatively small and resistance rises continually as current decreases from the effects of charging and dielectric absorption. Deteriorated insulation will pass relatively large amounts of leakage current at a constant rate for the applied voltage, which tend to mask the charging and absorption effects. The test voltage is applied over a 10 minute period, with the data recorded every 10 seconds for the first minute and then every minute thereafter. The interpretation of the slope of the plotted graph will determine the condition of the insulation. Graphing the resistance reading at time intervals from initiation of the test yields a smooth rising curve for good insulation, but a flat graph for deteriorated equipment. A continuous increase in graphed resistance indicates good insulation. A flat or downward curve indicates cracked or contaminated insulation. Figure 7 Megger-testing of an insulator bushing Now do Self-Test 2 and check your answers. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 25
Learning Task 2 K-4 Self Test 2 1. All meggers are safe because they operate at less than 750 volts. a. True b. False 2. An insulation tester must never be connected to a/an circuit. 3. What is the function of a guard lead on test instruments? Go to the Answer Key at the end of the Learning Guide to check your answers. 26 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4
Learning Task 3: Describe field testing methods for high voltage cables Under optimal environmental and operating conditions, high voltage cables with XLPE or EPPR insulation are designed by cable manufacturers to provide a specified life of anywhere from 20 to 30 years of continued operational service. However, many of the high voltage cables operating in commercial, electric utility, and industrial locations are exposed to a variety of environmental and operational stressors. These stressors include elevated temperature, high UV radiation, high humidity, water submersion, and exposure to dust, dirt and corrosive contaminants. Electromechanical forces resulting from momentary surges of short circuit current through a high voltage cable can potentially cause mechanical damage to the cable jacket, insulation material and cable conductors. High-voltage stress from lightning strikes or power system transient overvoltages can also weaken the dielectric strength of cable insulation. Over time, these stressors may cause aging and areas of insulation stress points that will result in a gradual deterioration of the cable insulation and jacket materials. Cable failure causes The major causes of cable failure are manufacturing defects, installation damage and defects, mechanical damage, operational damage and age-related deterioration. Manufacturing defects Voids, inclusions and contaminants in the insulation and protrusions from the semicon are examples of manufacturing defects that will cause the insulation to deteriorate rapidly under the influence of electric stress. Voids are micro bubbles in the insulation, while inclusions are foreign matter trapped in the insulation. Protrusions are sharp points extending into the insulation from the semicon. These are all forms of cable manufacturing defects. All cause intense local electric fields, which may lead to partial discharge at the site or rapid growth of water or electrical trees near the defect. Modern cables are machine manufactured very accurately in a tightly controlled factory environment with super clean, water tree resistant insulation, superior easily stripped semicons. As a result, manufacturing defects are rare. The manually assembled components of terminations and splices will generally have a considerably higher failure rate because of the difficulty of ensuring quality control and adherence to manufacturers instructions during the field assembly/installation process. Consequently, splices and terminations are more likely sources of defects leading to cable system failures. Installation damage and defects High voltage cable designs are quite complex and many different materials and constructions have been used over the years. A typical cable design includes: The core conductor (copper or aluminum) A semiconducting strand shield The dielectric, most often XLPE A semiconducting insulation shield A metallic shield, usually lead, copper tape or copper wires CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 27
Learning Task 3 K-4 The splices and terminations are equally complicated and technically demanding and must be specifically designed to make a watertight, electrically and mechanically compatible connection to the cable. Failures typically result from partial discharge activity that initiates at component defects. These failures include improper installation of stress relief devices, causing air gaps between surfaces; incorrect overlap of the semicon to insulation interfaces, or accidental nicks or cuts or abrasion of the exposed dielectric, and failure to correctly follow manufacturers instructions. Partial discharge can also occur on termination surfaces where dirt or contamination builds up. Continued partial discharge in such situations can lead to tracking, surface erosion, carbon deposits and eventual flashover and failure. Cable installation damage caused by tight bends exceeding the allowed bending radius of the cable, or excessive pulling tension can create cracks, gaps, separations or other voids in the cable structure. Such voids provide sites for partial discharge initiation. Mechanical damage Mechanical damage may occur during handling in the factory, shipment, handling at the supplier s warehouse or on the job site, Damage may also occur during installation, by accidental excavation or other physical means such as natural movement of the earth due to settling. Such damage can involve cuts, scrapes, excessive crushing, and possibly the intrusion of water into the strands of the core conductor. Cuts, scrapes, and crushing may be enough to prevent the cable from being successfully energized, and may also result in damage to the jacket, the insulation shield, or outer portion of the insulation on XLPE or EPR cables. Once in service, damage to the insulation shield or insulation is likely to produce partial discharge and lead to failure. Damage to the jacket may allow water to enter the space between the jacket and the insulation shield promoting the growth of water trees within the dielectric. Operational damage Operational damage can occur when the cable system is exposed to a severe load duty cycle, overloads, or short-circuit currents. A deep duty cycle with a shorter time base can result in linear expansion and contraction of the cable and can cause abrasion in EPR or XLPE cables; this may also affect stresses, splices and terminations because they may be subjected to significant mechanical forces. Such forces may cause components within the splice or termination to shift, resulting in voids or gaps at insulating and semiconducting interfaces, which could lead to partial discharge. Overloads and short-circuit currents can have similar or even greater effects. They can produce high temperatures that lead to deformation of cable materials and result in gaps or voids. In severe cases, the deformation may allow movement of the conductor within dielectric, leaving voids. Gaps and voids produce partial discharge. If no gaps or voids are created but the insulation/conductor geometry is changed by overheating, the damage may not lead to partial discharges, but may reduce the dielectric strength of the insulation. Age-related damage Age-related deterioration of high voltage cable can result in loss of adhesion between the insulation and semicon strand screen, leading to gaps or voids and partial discharge. A common form of age-related deterioration of XLPE cable is water treeing, resulting from moisture permeation of the insulation in the presence of electrical stress. Water trees may take many years to grow sufficiently large and dense to significantly reduce the dielectric strength of the insulation. 28 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4
Learning Task 3 K-4 Cable splices are exposed to the same environmental and operational stresses as the cables onto which they are installed, which can also cause aging degradation of the polymer insulating materials used in the cable splice. In addition, the individual components that make up a cable splice such as insulating tape, fillers, sleeves, insulating compounds, and compression connectors are also susceptible to aging degradation. A poorly installed or designed splice can result in multiple points of entry for moisture into the interior of the splice not only around the splice materials where they interface with the surface of the cable insulation, but also through the damaged or cracked outer jacket or insulation. As a result it is essential that high-voltage cables be tested at different points in their life cycle to ensure their continued serviceability. IEEE Standard 400-2001, IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems The IEEE Standard 400-2001, IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems is a recommended rather than mandatory standard. It provides an overview of the various field test methods available for testing cables from 5 kv to 500 kv, with a description of the proper procedure for each method. The guide is entirely voluntary and is intended as an aide to assist in the selection of a test method that is appropriate for the requirements. The standard divides field cable testing into three areas and defines them as follows: 1. Installation test: The installation test is conducted after the cable is installed but before any accessories or joints/splices and terminations are installed. This test is intended to detect any shipping, storing, and installation damage that may have occurred to the cable. 2. Acceptance test: The acceptance test is a field test performed after the installation of all cable and accessories including terminations and joints, but before the cables system is placed in normal service. This test is intended to detect installation damage and defects or errors in the installation of the system components. 3. Maintenance test: Maintenance tests are performed during the operating life of the cable system. Their purpose is to assess the condition of the cable system, and to check it for deterioration so that suitable maintenance procedures can be initiated. The recommended cable field tests have been divided into two categories, Type 1 and Type 2: Type 1 field tests are intended to detect defective parts and allow for replacement or repair. Tests are performed after installation or repair and are evaluated on a pass/fail, go/ no go basis. Type 2 field tests determine the health of cable system insulation or locate defects that may become failures in time. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 29
Learning Task 3 K-4 Type 1 (withstand) field tests Referred to as withstand tests, these tests are usually achieved by application of moderately increased voltages across the insulation for a prescribed duration. If the overvoltage doesn t cause the cable to fail, the insulation condition is considered adequate and passes. Such a test overlooks the possibility of smaller issues that may develop over time as a result of electric stress, and which may cause cable failure. All Type 1 tests require the cable to be de-energized, disconnected, and tested with a special voltage source. DC hi-potential testing This method relies on a source of high DC voltage for the testing of cables and other types of electrical equipment. DC hipot testing may still be done for acceptance and installation tests, but the routine periodic DC hipot maintenance testing of service-aged cable is no longer a common practice. The main usefulness of DC high-voltage testing is to detect conducting particles left on the creepage surface during splicing or termination. Advantages DC hipot test equipment is inexpensive, portable and simple to use in comparison to AC testing methods. Input power supply requirements are low. It is effective when the fault is triggered by conduction or by thermal action. It is effective on interface problems at joints and terminations and surface problems at terminations. It s the least costly of any of the testing methodologies. Disadvantages It may not detect certain types of defects, such as clean voids and cuts. The IEEE Standard indicates studies have shown that even massive insulation defects in extruded dielectric insulation cannot be detected with DC at the recommended voltage levels. It does not replicate the bi-directional stress caused by power frequency AC voltage. It may cause undesirable space charge accumulation, especially where cable insulation interfaces with termination or splice accessories. It may cause premature failure in service aged cables more than five years old. The test cannot be performed while the system is energized. AC high-potential testing This method uses high voltage AC for the testing of cables and other types of electrical equipment at a higher magnitude than the rated voltage of the tested equipment. In the past, a bulky and expensive test generator was required when a cable system was stressed above normal operating levels due to the capacitive nature of cable insulation. 30 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4