ACCEPTANCE AND MAINTENANCE TESTING FOR MEDIUMVOLTAGE ELECTRICAL POWER CABLES: EXPLORING TECHNOLOGY DEVELOPMENTS OVER THE PAST 20 YEARS Thomas Sandri Shermco Industries ABSTRACT Over the years there have been several methods and/or philosophies regarding the acceptance and maintenance field testing of medium voltage underground electrical power cable. Time honored methods such as direct current (DC) High Potential, Insulation Resistance (IR), and Dielectric Absorption (DA) testing have been employed for many years by utilities and industrial facilities for both field acceptance testing and periodic maintenance evaluation of medium voltage insulated power cables. Insulation Resistance provides a simple test for identifying gross defects in the cable and accessories, but offers little to no diagnostic value. A test like dielectric absorption is useful because it can be performed on even the longest of cables, and yields a self-contained evaluation based on relative readings rather than absolute values. In the early 1990s the benefits of using DC high potential testing for maintenance purposes of extruded dielectric cables has been questioned and written out of most industry guides. Since the days of high potential DC, other testing methods have been developed that may provide a better indication of the integrity of cables, splices, and terminations. These methods include Very Low Frequency (VLF) high potential testing, Tan Delta, and both Online and Offline Partial Discharge testing. So where do we stand today? What are the modern techniques and what value do they offer? Do we treat Acceptance testing the same as Maintenance testing? What is the difference? This paper will explore developments in testing and evaluation of medium voltage cable seen over the last 20 years. The intended application of each technique along with the advantages and limitations of the technique will be reviewed providing the knowledge necessary to develop an effective cable testing program.
INTRODUCTION As time progresses and a cable system ages, the system’s bulk dielectric strength degrades. During this aging process artifacts such as water trees, delamination, voids, and shield corrosion raise the local stress placed on the cable during normal operation. The exact way in which the strength of a device degrades will depend upon many factors such as voltage, thermal stresses, maintenance practices, system age, cable system technology and materials, and environment. For years, high-voltage direct current (DC) testing has been the traditionally accepted method for judging the serviceability of medium-voltage cables. DC high-potential tests have worked well for conducting dielectric strength and condition assessment tests on paper insulated lead covered (PILC) cable. When cable materials began to change and plastic insulated cables were first introduced in the 1960’s, little was known as to the overall aging characteristics of the new materials and therefore DC continued to be the preferred method of testing. As time moved on plastic insulated cables became more abundant and as they aged they began showing effects of service age. DC continued to be the dominate test, but concerns began to grow over the effectiveness of this test. In the early 1990’s reports started to surface indicating that DC high potential testing could be the blame for latent damage experienced by extruded medium voltage cable insulation.
© 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved
As insulation materials continue to change and improve and as reliability demands grow testing methods have been developed that may provide a better indication of the integrity of cables, splices, and terminations. To use these methods effectively the operator must understand the mechanisms of aging and of failure in cable systems. It is important to have an understanding of testing techniques and the ability to diagnose test results. It is equally important for trained personnel to be thoroughly familiar with the fundamentals of power cable design, operation and maintenance. This paper will review the evolution of testing methods and philosophies over the past 20 years. The intended application of each technique along with the advantages and limitations of the technique will be reviewed providing the knowledge necessary to develop an effective cable testing program.
SIX BASIC LAYERS OF MEDIUM VOLTAGE CABLE CONSTRUCTION In a typical medium voltage cable, copper or aluminum wires (either stranded or solid) are used as the conductors. These conductors are covered with an extruded polymeric stress-control layer made of semiconductive compounds, often referred to as the conductor shield. The insulation layer immediately surrounds and is fully-bonded with the conductor shield. An insulation shield encases the insulation and in some cases may be composed of the same semi-conductive material as the conductor shield. The copper neutral wires or tape are wound around the insulation shield and are usually covered with a thermoplastic polyethylene jacket for mechanical protection from the external environment and to reduce moisture intrusion into the cable, all of which can cause the premature cable failure.
Layers of Medium Voltage Cable Figure 1 Conductor Strand Types Various conductor strand types are commonly used in cable construction. The various types provide advantages in certain applications by either reducing the diameter of the cable or by lowering total AC resistance for a given cross-sectional area of conducting material. Examples include:
Concentric Strand A concentric stranded conductor consists of a central wire or core surrounded by one or more layers of helically laid wires. Each layer after the first has six more wires than the preceding layer. Except in compact stranding, each layer is usually applied in a direction opposite to that of the layer under it.
Concentric Strand Figure 2
© 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 2 of 23
Sector Conductor A sector conductor is a stranded conductor whose cross-section is approximately the shape of a sector of a circle. A multiple conductor insulated cable with sector conductors has a smaller diameter than the corresponding cable with round conductors.
Sector Conductor Figure 3 Segmental Conductor A segmental conductor is a round, stranded conductor composed of three or four sectors slightly insulated from one another. This construction has the advantage of lower AC resistance due to increased surface area and skin effect.
Segmental Conductor Figure 4 Compact Strand A compact stranded conductor is a round or sector conductor having all layers stranded in the same direction and rolled to a predetermined ideal shape. The finished conductor is smooth on the surface and contains practically no interstices or air spaces. This results in a smaller diameter.
Compact Strand Figure 5 Conductor Shield The conductors of the cable are covered with an extruded polymeric stress-control layer made of semiconductive compounds, often referred to as the conductor shield. The conductor shield shields the cable insulation from any air surrounding the conductor strands. This is very important since air gaps will lead to ionization and partial discharge activity which will prematurely fail the insulation. © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 3 of 23
Conductor Shield Figure 6 Insulation Materials Comparing the dielectric losses of various insulation types we can see that polyethylene (PE) and crosslinked polyethylene (XLPE) offer the lowest dielectric losses. Paper/Oil (PILC) has low to medium dielectric losses and ethylene propylene rubber (EPR) offers the highest dielectric losses. This comparison of materials also shows that PE and XLPE have sensitivity to water contamination (treeing) while EPR offers relatively low sensitivity to water contamination. An understanding of the insulation material plays a key factor in the testing of cable and analysis of test results. When testing hybrid or mixed circuits, the insulation with the higher dielectric loss may mask the true condition of the cable section with lower dielectric losses.
Table 1 Insulation Materials Insulation Shield Material PE
XLPE
EPR
Paper / Oil
Advantages Lowest dielectric losses High initial dielectric strength Low dielectric losses Improved material properties at high temperatures Does not melt but thermal expansion occurs Increased flexibility Reduced thermal expansion (relative to XLPE) Low sensitivity to water treeing Low – medium dielectric losses Not harmed by high potential DC testing Known history of reliability
Disadvantages Highly sensitive to water treeing Material breaks down at high temperatures Medium sensitivity to water treeing (although some XLPE polymers are water tree resistant)
Medium – high dielectric losses Requires inorganic filler/additive
High weight High cost Requires hydraulic pressure / pumps for insulating fluid Difficult to repair Degrades with moisture
© 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 4 of 23
Insulation Shield The insulation shield encases the insulation and in some cases may be composed of the same semiconductive material as the conductor shield. It serves a similar purpose as the conductor shield and shields the insulation from air that might cause ionization and partial discharge activity.
Insulation Shield Figure 7 The purposes of the insulation shield are to: Obtain symmetrical radial stress distribution within the insulation. Eliminate tangential and longitudinal stresses on the surface of the insulation. Exclude from the dielectric field those materials such as braids, tapes, and fillers that are not intended as insulation. Protect the cables from induced or direct over-voltages. Shields do this by making the surge impedance uniform along the length of the cable and by helping to attenuate surge potentials.
Cable Shielding Medium and high-voltage power cables, in circuits over 2000 volts, usually have a shield layer of copper or aluminum tape or conducting polymer. If an unshielded insulated cable is in contact with earth or a grounded object, the electrostatic field around the conductor will be concentrated at the contact point, resulting in corona discharge, and eventual destruction of the insulation. Leakage current and capacitive current through the insulation presents a danger of electrical shock. The grounded shield equalizes electrical stress around the conductor and diverts any leakage current to ground. Stress relief cones should be applied at the shield ends, especially for cables operating at more than 2 kV to earth. There are several different types of shields that are commonly used for medium voltage cable applications. These shielding styles include: Tape Shielded (also called ribbon shielded): Tape Tape Shielded Cable shields over ethylene Figure 8 propylene rubber (EPR) insulation have been a favored power cable construction for years. The way the tape is wrapped can deliver significant reliability and performance benefits. The overlap of the tape windings is a key design feature in helical-tape construction. The Insulated Cable Engineers Association (ICEA) recommends a © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 5 of 23
minimum tape overlap of 10 percent; however, extra overlap delivers increased short-circuit capacity and better mechanical reliability. Caution should be taken during installation of this style cable. If a tapeshielded cable is bent too sharply or pulled around bends with too much tension, the tape windings may separate and the tape can buckle when the cable straightens out. The buckling can damage the underlying insulation shield. In a conduit, the damage is invisible, and even in a cable tray the cable jacket may conceal the condition. What’s worse, most electrical proof tests performed in the field will not reveal this kind of damage. Wire Shielded (also called concentric neutral): This cable offers the same construction as the tapeshielded cable except for the different metallic shield layer. It is often considered interchangeable with the tape-shielded cable and is very common in utility applications. When concentric neutral cables are specified, the concentric neutrals must be manufactured in accordance with the Insulated Cable Engineers Association (ICEA) standards. These wires must meet ASTM B3 for uncoated wires or B33 for coated wires. These wires are applied directly over the nonmetallic insulation shield with a lay of not less than six or more than ten times the diameter over the concentric wires. UniShield®, a registered trademark of BICC Cables: Note the “Uni” in the name, which refers to the outer three layers being combined into a single layer (insulation shield, which also functions as a jacket, with metallic drain wires imbedded into the jacket to form a single functional layer).
Wire Shielded Cable Figure 9
UniShield Cable Figure 10
PILC, paper-insulated, leadcovered cable: The paper insulation is impregnated with oil which must be kept contained within the cable by use of a lead jacket.
CABLE AGING A power cable fails when local electrical stresses are greater than the local dielectric strength of the dielectric material(s). Reliability and the rate of failure of the whole cabling system depend on the Paper-insulated Lead-covered Cable difference between the local Figure 11 stress at any given point in the system and the local strength at that point. A failure of the dielectric results in an electrical puncture or flashover at the location of the degraded dielectric. This flashover can occur between two dielectric surfaces, such as the cable insulation and joint insulation or it can occur as an external flashover at the © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 6 of 23
cable terminations. A cable failure can occur as a result of the normally applied 50/60 Hz voltage or during a transient voltage such as lightning or switching surges. As time progresses and the cable system ages, the bulk dielectric strength degrades. The main aging factor of extruded dielectric cable is electrical although under abnormal situations thermal aging can be significant. The electrical aging mechanisms; partial discharge, electrical treeing, water treeing, and charge injection occur at contaminants, defects, protrusions and voids and thus tend to be localized. Looking at specific mechanisms, the excessive electrical stress or bulk deterioration of the insulation can occur as a result of:
Manufacturing Imperfections: Increases the local stress leading to either early failure or higher rates of aging. o Voids o Contaminants in insulations o Poor application of shield material o Protrusions on the shields o Poor application of jackets Poor Workmanship: Increases the local stress leading to either early failure or higher rates of aging. o Cuts o Contamination o Missing applied components or connections o Misalignment of accessories Aggressive Environment: Reduces the dielectric strength. The impact can be local if the environmental influence is local. o Chemical attack o Transformer oil leaks o Floods o Petrochemical spills o Neutral corrosion Wet Environment: Reduces the dielectric strength and increase the local stress. o Bowtie trees o Vented water trees o High rates of corrosion o Can reduce dielectric properties Overheating: Reduces the dielectric strength. The impact can be restricted to short lengths (local) if the adverse thermal environment is localized. o Excessive conductor current for a given environment and operating condition (global) o Proximity to other cable circuits for short distances (local) Mechanical: Reduces the dielectric strength. The impact can be restricted to short lengths if the mechanical stress is localized. o Damage during transportation. This is typically localized. o Excessive pulling tensions or sidewall bearing pressures. This can be either localized or global. o Damage from dig-ins. Typically localized. Water Ingress: Reduces the dielectric strength and increase the stress in the area surrounding the moisture. o Normal migration through polymeric materials o Breaks in seals or metallic sheaths
Water Trees Water trees, sometimes called electrochemical trees, have basic characteristics different than electrical trees. Electrical trees are characterized by the occurrence of partial discharge and require high electric © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 7 of 23
stress to initiate and then rapidly lead to catastrophic dielectric failure. Water trees can be initiated at much lower dielectric stress and grow very slowly. These trees are associated with no measurable partial discharge and can completely bridge the insulation from conductor to shield without dielectric breakdown. Although breakdown does not occur, the dielectric strength is greatly reduced. This is particularly true with regards to the direct current (DC) breakdown value. Water tree degradation is a major problem for medium voltage extruded dielectric cables, particularly service aged XLPE cables. It is perhaps the worst degradation process of the polymeric insulation that contributes to the failure of the cable. Water tree are formed and grow in the presence of moisture, impurities or contamination and the electric field over time. There are generally two types of water trees, namely Bow-Tie Trees and Vented Trees.
Types of Water Trees Figure 12 Bow-Tie trees are water trees that grow from the insulation outwards towards the surfaces of the insulation. These trees grow in the direction of the electric field in both directions, towards the two electrodes. Bow-Tie trees have faster initial growth rate as compared to vented trees. Bow-Tie trees are however, not capable to growing to large sizes and usually do not grow to a size significant enough to cause failure of the insulation. Vented Tree are water trees that grow from the surface of the polymer inwards into the insulation system. These trees will also grow in the direction of the electric field. Vented trees have a lower initial growth rate as compared to bow-tie trees; however, vented trees are capable of growing right through the entire insulation thickness.
Electrical Trees The cumulative effect of partial discharges within solid dielectrics is the formation of numerous, branching partially conducting discharge channels, a process called electrical treeing. Repetitive discharge events cause irreversible mechanical and chemical deterioration of the insulating material. Damage is caused by the energy dissipated by high energy electrons or ions, ultraviolet light from the discharges, ozone attacking the void walls, and cracking as the chemical breakdown processes liberate gases at high pressure. The chemical transformation of the dielectric also tends to increase the electrical conductivity of the dielectric material surrounding the voids. This increases the electrical stress in the unaffected gap region, accelerating the breakdown process.
© 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 8 of 23
CABLE TESTING OPTIONS Over the years there have been several methods and/or philosophies regarding the testing of underground electrical power cable in the field. The Insulated Conductor Committee of the IEEE Power & Energy Society has divided these methods or philosophies into two fundamental categories, Type 1 Field Tests and Type 2 Field Tests. Type 1 tests are intended to detect defects in the insulation of the cable system in order to improve the service reliability after the defective part is removed and appropriate repairs are performed. These tests are usually achieved by application of moderately increased voltages across the insulation for a prescribed duration of time. Such tests are categorized as pass/fail. Tests include:
Insulation Resistance (Under-voltage) DC Hipot (IEEE 400.1 - GUIDE FOR FIELD TESTING OF LAMINATED DIELECTRIC, SHIELDED POWER CABLE SYSTEMS RATED 5 KV AND ABOVE WITH HIGH DIRECT CURRENT VOLTAGE) VLF Hipot (IEEE 400.2 - GUIDE FOR FIELD TESTING OF SHIELDED POWER CABLE SYSTEMS USING VERY LOW FREQUENCY (VLF)) High Potential Power Frequency (Typically this is considered a factory test and not a field test.)
Type 2 tests are intended to provide indications that the insulation system has deteriorated, hence, are termed “diagnostic” tests. These tests include:
Dissipation Factor (Tan Delta) Testing (IEEE 400.2 - GUIDE FOR FIELD TESTING OF SHIELDED POWER CABLE SYSTEMS USING VERY LOW FREQUENCY (VLF)) Partial Discharge (IEEE 400.3 - GUIDE FOR PARTIAL DISCHARGE TESTING OF SHIELDED POWER CABLE SYSTEMS IN A FIELD ENVIRONMENT) Isothermal Relaxation Current Test, and Return Voltage
The IEEE further categorizes cable testing into three areas: 1. Installation tests: These are field tests that are conducted after cable installation is complete, but before splicing or terminating occurs. The test is intended to detect shipping, storage, or installation damage. 2. Acceptance tests: These are field tests made after the cable system installation, splicing and terminations are completed, but before the cable system is placed in normal service. The tests are intended to further detect installation damage and to show any gross defects or errors in installation of the various system components. 3. Maintenance tests: Field tests made during the operating life of a cable system. They are intended to detect deterioration of the system and to check the serviceability so that suitable maintenance procedures can be initiated. The International Electrical Testing Association (NETA) addresses field testing of medium and high voltage cables in their “Standard for Acceptance Testing Specifications for Electrical Power Equipment and Systems” and “Standard for Maintenance Testing Specifications for Electrical Power Equipment and Systems” books.
UNDER VOLTAGE TESTING PERFORMED WITH DIRECT CURRENT (DC) Under voltage test performed with direct current (DC) are typically performed with a test set referred to as a megohmmeter. Since these tests use voltages under the rating of the insulation being tested, the test is considered to be a non-destructive test and does not produce any of the ill effects associated with high voltage DC testing that we will discuss later in this paper. The traditional insulation resistance test is the simplest way to gain an overall indication of the condition of the insulation. Although the Insulation © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 9 of 23
Resistance (IR) test can be applied as a simple Type 1, or go/no-go test, it can also be used to give more extensive diagnostic information. Type 2 or diagnostic insulation tests electrically stimulate the insulation and measure the response of the insulation to that stimulus. Dependent on that response, we can draw some conclusions about the condition or heath of the insulation. A megohmmeter is a portable instrument that provides a direct reading of insulation resistance in ohms, meg-ohms, gig-ohms, or tera-ohms regardless of the test voltage selected. For good insulation, the resistance usually reads in the meg-ohm or higher range. An insulation resistance tester is essentially a high range resistance meter or ohmmeter, with a built-in DC generator to produce the test voltage. The most common voltages applied for non-destructive insulation tests on medium voltage cables are 2.5 and 5.0 kV DC. The test set’s internal DC generator, which can be hand-cranked, battery- or line-operated, develops a high DC voltage that causes several small currents to flow through and over surfaces of the insulation being tested. The total current is measured by the ohmmeter, which has an analog indicating scale, digital readout or both. The test current in the body of the cable’s insulation can be split into three components; the capacitance charging current, the absorption (or polarization) current and the conduction or leakage current. The capacitive current is initially large, but goes to zero as the test piece is charged. The polarization current is caused by charges in the insulation material moving under the effect of the electric field or by molecular di-poles lining themselves up with the applied field (orientation Polarization). It is greatly affected by moisture or contamination in the insulation, as the water molecule has additional orientation polarization. This process takes much longer than the capacitive charging. Finally we have steady leakage current through the insulation, which is usually represented by a very high resistor in parallel with the capacitance of the insulation.
Spot Test
Components of Test Current Figure 13
The spot reading test is the simplest of all insulation tests; the test voltage is applied for a short, but specific period of time (typically 1 to 2 minutes as usually any capacitive charging current will have decayed by this time) and a reading is then taken. On longer cables offering increased capacitance the time period for the capacitive charging current may be significantly increased. The reading can then be compared to the minimum installation specifications. Spot readings can be performed as part of an inspection or as part of troubleshooting and can be used as a simple good/bad indicator. Spot test readings can also be trended against previously obtained values, however, insulation resistance is highly temperature dependent, and thus the results should be corrected to a standard temperature. If the insulation resistance reading taken was high and if the reading increased or remained steady during the test, the insulation is considered to be in good condition. As the capacitance current and absorption current decreases, the insulation resistance increases. If the insulation resistance reading decreased during the test, the insulation of the cable is probably wet or otherwise in bad condition. If the final value of resistance is low (or the current is high), the insulation of the cable is in poor condition.
Time Resistance Test (Polarization Index / Dielectric Absoption) A valuable property of insulation is that it “charges” during the course by the megohmmeter causes re-alignment of the insulating material orient themselves with the field. This movement of charge constitutes indicator is based on two opposing factors: the current dies away
of a test. The polar DCield applied on the molecular level, as dipoles a current. Its value as a diagnostic as the structure reaches its final
© 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 10 of 23
orientation, while “leakage” promoted by deterioration passes a comparatively large, constant current. The net result is that, with “good” insulation, leakage current is relatively small, and resistance rises dramatically as charging goes to completion. This changing resistance provides diagnostic information related to the degree of degradation of insulation. Deteriorated insulation will pass relatively large amounts of leakage current at a constant rate for the applied voltage. This will “flood out” the charging effect and will show little to no change in resistance value. Time-resistance test methods take advantage of this charging effect. 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 insulation. The ultimate simplification of this technique is represented by the Polarization Index (PI) and Dielectric Absorption (DA) tests, which requires only two readings and a simple division. When performing the PI test the one-minute reading is divided Polarization Index Test into the ten-minute reading to provide a ratio. In DA the Figure 14 time values are typically 30 seconds and 60 seconds. Obviously, a low ratio indicates little change, hence poor insulation, while a high ratio indicates the opposite. Spot readings alone can be difficult to work with, as they may range from enormous values in new cables down to a few meg-ohms just before the cable is removed from service. A test like the PI is particularly useful because it can be performed on even the longest of cables, and yields a self-contained evaluation based on relative readings rather than absolute values.
Discharge Based Insulation Tests There have emerged a range of techniques that look at the response of the insulation during its discharge. These tests all target the polarization behavior of the insulation, because as mentioned earlier in this paper, this property is sensitive to moisture in the insulation. Since all three components of current are present during the charging phase of an insulation test, the determination of polarization or absorption current is hampered by the presence of the capacitive and leakage currents. The discharge phase of the test, however, can more rapidly remove these effects, giving the possibility of interpreting the degree of polarization of the insulation and relating this to moisture and other polarization effects.
Isothermal Relaxation Current Test (Irc Test) This test has been developed for testing of service aged medium voltage cables and grew as a response to problems associated with DC high potential testing of plastic cables. The early installed base of these cables from the 1960’s and early 1980’s are particularly problematic. The IRC test uses a 1kV test voltage for 30 minutes to polarize the dielectric. The polymer polarization traps charge at specific discrete energy levels and during the discharge process these energy levels give rise to different time constants in the discharge current. The major use of the effect in the IRC test is to look for the time constant associated with water trees in degraded cross-linked polyethylene (XLPE) cable material. The ‘relaxation current’ occurring after the capacitance has been discharged is digitized for processing in PC based software. The software processing is based on a modelling technique, which converts the current into Charge and plots this Charge against Time. The total charge plot is then treated as a composite of standard shapes whose time constants are ‘fitted’ to the composite curve by iteration. Aging of the cable is identified by
© 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 11 of 23
the relative values of the time constants. The test was initially developed using artificially aged cable and has now been applied to operational XLPE cables.
HIGH POTENTIAL TESTING PERFORMED WITH DIRECT CURRENT (DC) For years, high-voltage DC testing has been the traditionally accepted method for judging the serviceability of medium-voltage cables. Direct current tests have worked well for conducting high potential and condition assessment tests on paper insulated lead covered (PILC) cable. When plastic insulated cables were first introduced in the 1960’s DC continued to be the preferred method. As time moved on plastic insulated cables became more abundant and began showing effects of service age. Direct current continued to be the dominate test, but concerns began to grow over the effectiveness of this test. In the early 1990’s reports started to surface indicating that DC high potential testing could be the blame for latent damage experienced by extruded medium voltage cable insulation (cross linked polyethylene and ethylene propylene rubber). After receiving these reports the Electrical Power Research Institute (EPRI) funded a study relating to cross linked polyethylene (XLPE) and ethylene propylene rubber (EPR) cables. This study (EPRI reports TR-101245) yielded the following conclusions regarding XLPE cable:
DC high potential testing of field-aged cable reduces its life. DC high potential testing of field-aged cable generally increases water tree growth. DC high potential testing before energizing new medium voltage cable doesn’t cause any reduction in cable life.
To better understand the conclusions of the EPRI study, it is beneficial to once again review the aging characteristics of extruded dielectric cables and the process of “water treeing.” Water trees begin to form when a cable is exposed to water and normal operating voltage over an extended period of time. Electrical forces acting on water molecules, electrophoresis, at a microscopic point within the insulation, increases the separation between polymer units. These water droplets become oriented into a chain-like channel. The result is a sharp electrode, producing highly localized stresses. Once treeing is initiated, an electrical stress exists from the base of the tree channel to its extremity. When high levels of DC voltages are impressed on solid extruded polymeric materials their good electrical insulation properties become degraded. For example, trapped or low mobility electrically charged species within the bulk can give rise to space charge, resulting in localized electric stress enhancement. This can cause further concentration of charge and accelerate electrical ageing. The water tree part contains more ionic impurities than the sound part. It is therefore likely that, in XLPE cable under DC voltage stress, the space charges are formed in the water tree region and its vicinity promoting an increase in water tree growth and reduced service life. In 1996, a few years after the EPRI reports, the insulated conductor industry determined that DC high potential testing of the plastic (XLPE) insulation systems, either in the cable factory as a routine production test or after installation as a high voltage proof test, were determined to be detrimental to the life of the insulation and therefore discontinued recommending DC testing for plastic (XLPE) insulation systems. In 2001, a new “IEEE Guide for Field Testing and Evaluation of the Insulation of Shielded Power Cable Systems”, IEEE Std 400™-2001 was released. The new standard includes statements such as, “testing of cables that have been aged in a wet environment (specifically XLPE) with DC at the currently recommended DC voltages may cause the cable to fail after they are returned to service. These failures would not have occurred at that point in time if the cables had remained in service and had not been tested with DC.” This standard also indicates other testing has shown that “even massive insulation defects in extruded dielectric insulation cannot be detected with DC at the recommended voltage levels.” Current versions of most industry recommendations no longer include DC high potential testing of extruded cables (XLPE and EPR) as a maintenance test. Of those that still do, all have reduced the © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 12 of 23
recommended test duration from 15 minutes to only 5 minutes. None endorses DC high potential testing as a factory test for extruded cables, but most documents continue to include DC high potential testing as an acceptance test on newly installed extruded cable. These industry recommendations and guides also no longer endorse DC high potential testing as a maintenance test for extruded cables that have been in service for more than five years. Laminated insulation systems are not subject to the same aging characteristics as XLPE and EPR. Therefore, DC testing is more accepted for acceptance and maintenance testing of paper insulated lead covered cable (PILC). Keep in mind, however, the test has limitations. The DC leakage can be affected by external factors such as heat, humidity, and water level, if unshielded and in ducts or conduits. It can also be affected by internal heating if the cable under test had recently been heavily loaded. These factors make comparisons of periodic data obtained under different test conditions very difficult. In practice, the shape of the leakage curve, assuming constant voltage, is more important than either the absolute leakage current of a “go-no-go” withstand test result.
HIGH POTENTIAL TESTING PERFORMED AT POWER FREQUENCY As the name implies, this test method is based on using alternating current (ac) at the operating frequency (50/60 Hz) of the system as the test source. This method has the advantage of stressing the insulation comparably to normal operating conditions. Power frequency high potential testing is most commonly conducted by cable manufacturers. The withstand testing determines the dielectric strength of the insulation system and identifies gross defects. The test looks at the bulk of the insulation and renders a pass or fail conclusion. A further advantage of power frequency testing is that it allows partial discharge and dissipation factor (tan δ) testing for diagnostic purposes. It should be noted that power frequency testing methods can be categorized as withstand or diagnostic. In withstand testing, insulation defects are caused to break down (fault) at the time of testing. Faults are repaired or removed, and the insulation is retested until it passes the withstand test. The withstand test is considered a destructive test. Diagnostic testing allows the identification of the relative condition of degradation of a cable system and establishes, by comparison with figures of merit, if a cable system can or cannot continue operation. Diagnostic testing is considered nondestructive. As a field test power frequency testing suffers a series disadvantage since at increased voltage levels the test sets require heavy, bulky and expensive transformers. Field portable AC high potential test sets of the power frequency variety are used worldwide for field testing vacuum bottles, switchgear, reclosures, circuit breakers, etc. Field portable units will typically offer ratings of 3 to 7 kVA and will be portable and cost effective for the applications mentioned. The reason for large transformers when referring to cable testing has to do with the capacitance of the load being tested. Capacitive reactance (Xc) changes as a function of frequency as seen in formula: Xc =
1 2πfC
Therefore, if we are testing a 15 kV rated cable of approximately 10,000 feet the capacitance would be around 1uF. Based on the formula the capacitive reactance at 60 Hz would be 2654 ohms: 2654 Ω =
1 6.28 (60 × 1E−6 )
To apply the IEEE recommended 22 kV test voltage, it would require a power supply rated for 8.3 amps, or 183 kVA: 8.3 amps =
22,000 (Volts) 2,654 (Ohms, Xc)
183 kVA = 22,000 (Volts) × 8.3 (Amps) © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 13 of 23
The physical size and weight of a transformer capable of this rating is obviously not practical as a “portable” field unit. The size of the necessary equipment can be substantially reduced by using the principle of resonance. If the effective capacitance of the cable is resonated with an inductor, the multiplying effect of the resonant circuit (its Q factor, presently between 50 and 120) will allow the use of a smaller test source. In the ideal case of a perfect resonance, the test source will only be required to supply energy to balance the true resistive loss in the inductor and cable system. One of two resonant circuits is normally used, either series or parallel. Even with the application of resonance, these power frequency high voltage supplies can be quite large and heavy, requiring dedicated test vehicles to transport to and from field job sites.
HIGH POTENTIAL TESTING PERFORMED AT VERY LOW FREQUENCY (VLF) Similar to power frequency test methods; if used alone, VLF testing is considered a Type 1, go-no-go test, but can be used with auxiliary equipment for the purpose of Type 2, diagnostic testing. Auxiliary equipment can be partial discharge and/or dissipation factor (tan δ) couplers and measurement equipment. The main advantage of testing at very low frequency is that it significantly reduces the size, weight and cost of the required power supply and thus offers greater attraction for field testing of medium voltage cables. If the test frequency were dropped to 0.1 Hz, the capacitive reactance, as calculated earlier, becomes 1.6 megohms. The same 22 kV would now only draw 14 mA or 0.303 kVA. Therefore, the size, weight and portability of the power supply become very convenient for field use. VLF power supplies can be constructed as either a cosine-pulse (rectangular) waveform or sinusoidal waveform output.
Cosine-Pulse Waveform VLF The cosine-pulse waveform version is constructed using a DC test set that acts as the high-voltage source. A DC to AC converter then changes the DC voltage to the VLF AC test signal. The converter consists of a high-voltage inductor (choke) and a rotating rectifier that changes the polarity of the cable system being tested every 5 seconds. This generates a 0.1-Hz bipolar wave. A resonance circuit, consisting of a high-voltage inductor and a capacitor in parallel with the cable capacitance, assures sinusoidal polarity changes in the power frequency range. The use of a resonance circuit to change cable voltage polarity preserves the energy stored in the cable system. Only leakage losses have to be supplied to the cable system during the negative half of the cycle. The intent of the VLF cosine-pule waveform test is to generates a 0.1-Hz bipolar pulse wave that changes polarity sinusoidally. Sinusoidal transitions in the power frequency range will then initiate a partial discharge at an insulation defect, which the 0.1-Hz pulse wave develops into a breakdown channel. During the test period, typically within minutes, a defect can be detected and forced to break through. After the defect breaks through and faults, it can then be located with standard, readily available cable fault locating equipment. Identified faults can then be repaired during the scheduled outage. When a cable system passes the VLF test, it can be returned to service. The recommended testing time periods range from 15 to 60 minutes. Advantages and disadvantages of the cosine-pulse waveform include: Advantages The sinusoidal transitions in the power frequency range can initiate a partial discharge at an insulation defect. The 0.1Hz pulse wave can then develop the defect into a breakthrough channel faulting the defect during the scheduled outage.
Disadvantages When testing cables with extensive water tree damage or ionization of the insulation, VLF testing alone is often not conclusive.
© 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 14 of 23
Due to continuous polarity changes, space charges cannot develop and therefore no new defects will be initiated during the testing process
Cables can be tested with an AC voltage up to three times the normal phase-toground voltage with a device comparable in size, weight, and power requirements to a DC test set.
Due to the layout of cosine-rectangular test voltage generation, the waveform is dependent on the cable length being tested. A DC offset or bias may be possible.
Sinusoidal Waveform VLF The VLF test set generates sinusoidally changing waves that are less than 1 Hz (0.1 Hz, 0.05 Hz or 0.02 Hz). When the local field strength at a cable defect exceeds the dielectric strength of the insulation, partial discharge starts. The local field strength is a function of applied test voltage, defect geometry, and space charge. After initiation of partial discharge, the partial discharge channels develop into breakthrough channels within the applied test period. When a defect is forced to break through, it can then be located with standard, fault locating equipment during the scheduled outage. VLF testing guides usually recommended a test time duration of 60 minutes or less. Advantages and disadvantages of the sinusoidal waveform include: Advantages Cables are tested with an AC voltage up to three times the normal phase to ground voltage. After initiation of a partial discharge, a breakthrough channel at a cable defect develops very rapidly and can be located during a scheduled outage. Due to continuous polarity changes, space charges cannot develop and therefore no new defects will be initiated during the testing process Cables can be tested with an AC voltage up to three times the normal phase-toground voltage with a device comparable in size, weight, and power requirements to a DC test set. Due to the sinusoidal regulated waveform and to the highest electrical tree growth rate as compared to the cosine-rectangular waveform, electrical trees will be initiated at a defect within minutes. The test voltage level and waveform is defined as RMS voltage and is completely independent of the cable length.
Disadvantages When testing cables with extensive water tree damage or ionization of the insulation, VLF testing alone is often not conclusive.
The total charging energy of the cable has to be supplied and dissipated by the test in every electrical period. This limits the size of the cable system that can be tested.
Test Duration and Voltage Recommendations for VLF According to the IEEE 400.2 standard for acceptance testing cables ranging from 5 kV - 35 kV … cables which are tested between 2 Vo and 3 Vo (phase to ground); 68% of the failures occurred within 12 minutes, 89% within 30 minutes, 95% after 45 minutes and 100% after 1 hour. These results were found using both types of power supply waveforms, the sinusoidal and cosine-rectangular with the most commonly used being the sinusoidal waveform. © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 15 of 23
Suggested maintenance testing is generally 15 minutes in length slightly above or below the phase to phase voltage using the sinusoidal wave. The reason for this is that the test is performed at 0.707 rms of the peak value while testing using the cosine-rectangular waveform the test is performed at a slightly higher voltage assuming the rms is equal to the peak value.
DISSIPATION FACTOR (TAN Δ) TESTING Dissipation factor, also called Tan δ or Loss Angle, testing, is a diagnostic method of testing cables to determine the quality of the cable insulation. This is done to try to predict the remaining life expectancy and in order to prioritize cable replacement and/or injection. It is also useful for determining what other tests may be worthwhile. Such as VLF Withstand or Partial Discharge. If the insulation of a cable is free from defects, like water trees, electrical trees, moisture and air pockets, etc., the cable approaches the properties of a perfect capacitor. It is very similar to a parallel plate capacitor with the conductor and the neutral being the two plates separated by the insulation material. In a perfect capacitor, the voltage and current are phase shifted 90 degrees and the current through the insulation is capacitive. If there are impurities in the insulation, like those mentioned above, the
Ideal Model for Insulation Figure 16 resistance of the insulation decreases, resulting in an increase in resistive current through the insulation. It is no longer a perfect capacitor. The current and voltage will no longer be shifted 90 degrees. The shift will be something less than 90 degrees. The extent to which the phase shift is less than 90 degrees is indicative of the level of insulation contamination, hence quality/reliability. This “Loss Angle” is measured and analyzed.
The tangent of the angle delta is measured. This will indicate the level of resistance in the insulation. By measuring IRp/ICp we can determine the quality of the cable insulation. In perfect cable insulation, the angle would be Typical Model for Insulation nearly zero. An increasing angle generally Figure 15 indicates an increase in the resistive current through the insulation, meaning contamination. Keep in mind, however, that different insulation materials have higher or lower dielectric losses, therefore, the angle or tan δ value may be higher for some insulating materials due to their associated dielectric losses. We need to keep in mind that the goal of tan δ testing is to provide a quality/reliability indication of an insulating system. In a cabling system the terminations and splices are part of that system. If the insulation of that system and all accessories are free from defects, like water trees, electrical trees, moisture and air pockets, etc., the cabling system approaches the properties of a perfect capacitor as stated earlier. It should also be noted that in the event the cable circuit has transitions between multiple insulation types, for example XLPE transitioned to EPR, the Tan δ value will be influenced by the © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 16 of 23
component with the higher dielectric losses. In this example, EPR has a higher dielectric loss and the circuit would need to be evaluated based on the EPR component. This obviously hampers the effectiveness of the test since contamination growing in the XLPE may be masked by the higher dielectric losses of the EPR. When analyzing Tan δ results we look at the stability of the Tan δ reading at a given test voltage, the absolute value of Tan δ and the gradient of change in the Tan δ value as test voltage is increased.
Table 2 Typical Ranges for Standard Deviations, TD Gradient and TD Values for Various Insulation Types (G = Good / M = Monitor / A = Action Required) CABLE TYPE
TD Std. Dev. (%)
TD Gradient (E-3) (1.5Vo – 0.5Vo)
TD Value (E-3)
G
M
A
G
M
A
G
M
A
XLPE
<0.02
<0.04
>0.04
<0.6
<2
>2
<2
2 to 4
>4
Black EPR / Butyl
<0.02
<0.04
>0.04
<3
3 to 10
>10
<12
12 50
to
>50
Pink /Red EPR
<0.02
<0.04
>0.04
<3
3 to 8
>8
<15
15 30
to
>30
Brown EPR
<0.02
<0.04
>0.04
<5
5 to 15
>15
<50
50 60
to
>60
It is important to note that Tan δ does not “locate” defects in cabling systems, it simply gives an indication of the insulation quality between point A and B in the system. Tan δ test results may justify conducting other analysis such as time domain reflectometry (TDR) or offline Partial Discharge analysis and mapping to uncover possible locations of contamination and ionization. Unfortunately, there is not an extensive library of test values for all types of cables and accessories. Testing is typically done on a comparative basis. Keep in mind that the purpose of the test is to grade cables tested on a scale from high quality to low. The point in the testing is to help an asset owner to prioritize cable replacement or injection … comparative testing will show which cables are worse than others. While it is beneficial to have previous tests to compare to for trending purpose, it is not absolutely necessary. The very first test on a cable will render valuable information. If the cable’s insulation is in good condition, the Tan δ will change little as the applied voltage is increased. The capacitance and loss will be similar with 0.5 Vo or 1.5 Vo applied to the cable. If there is contamination changing the capacitive/resistive nature of the insulation, then the Tan δ will be higher at higher test voltages. A Tan δ measurement system consists of a high voltage divider and measurement circuit. The high voltage divider measures the voltage and current input to the cable, sends this information to the controller, which analyzes the voltage and current waveforms and calculates the Tan δ number. A voltage supply is required to energize the cable under test to the desired test voltage(s). Although power frequency can be used, and is used in factory testing, VLF is typically the chosen power supply for © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 17 of 23
field application. As mentioned earlier, to test a cable with 60 Hz power requires a very high power supply. When testing in the field it is not practical, or in many locations possible, to test a cable of several thousand feet with a 60 Hz supply. A typical VLF frequency of 0.1 Hz takes 600 times less power to test the same cable compared to 60 Hz and therefore provides a significant size, weight and cost advantage in field testing. Secondly, the magnitude of the tan delta numbers increase as the frequency decreases, making measurement easier. As the below equation shows, the lower the frequency (f), the higher the Tan δ number. Tan δ = (Tan δ is measured in radians)
IR 1 = IC 2πfCR
PARTIAL DISCHARGE A Partial Discharge (PD) is an electrical discharge or spark that bridges a small portion of the insulation between two conducting electrodes. PD activity can occur at any point in the insulation system, where the electric field strength exceeds the breakdown strength of that portion of the insulating material. When partial discharge is initiated, high frequency transient current pulses will appear and persist for nanoseconds to a microsecond, then disappear and reappear repeatedly as the voltage sine wave goes through the zero crossing. The PD happens near the peak voltage both positive and negative. As mentioned, the PD burst creates a current pulse that can be measured in different frequency bands based on the test configuration. The severity of the PD is measured by measuring the burst interval between the end of a burst and the beginning of the next burst. As the insulation breakdown worsens, the burst interval will shorten due to the breakdown happening at lower voltages. This burst interval will continue to shorten until a critical point is reached. At this point the discharge is very close to the zero crossing and will fail with a full blown discharge and major failure. PD activity usually begins within voids, cracks, or inclusions within a solid dielectric. Since these activities are limited to only a portion of the insulation, the discharges only partially bridge the distance between electrodes. PD can also occur along the boundary between different insulating materials. Partial discharges within an organic or polymer insulating material are usually initiated within gas-filled voids within the dielectric [Figure 18]. Because the dielectric constant of the void is considerably less than the surrounding dielectric, the electric field across the void is significantly higher than that across an equivalent distance of dielectric. When the applied 50/60Hz increases sinusoidally, the apparent electric stress across the void increases until it reaches the equivalent breakdown voltage in the void. If the voltage stress across the void is increased above the corona inception voltage (CIV) for the gas within the void, PD activity will start within that void.
PD Activity Relative to AC Cycle Figure 17
PD within Solid Insulating Systems Figure 18
© 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 18 of 23
Once the breakdown occurs, the voltage across the gap collapses to a voltage level sufficient to sustain the discharge. No further detectable discharges will occur until the gap voltage has reversed in polarity and another over voltage condition established. Thus, for each pulse position there will be a detectable PD occurring twice in an AC cycle. However, the occurrence, magnitude, and pattern in a void are a complex phenomenon depending on the size, shape, internal gas pressure, and nature of the void surface and will deviate from cycle to cycle due to past space charge trapping. Simplified models of voids in insulating systems have been described as consisting of capacitance only. When we review the progressive failure mode of these voids we can also see semiconducting films inside the voids. These films can also consist of carbonization of organic insulation material within the void due to the arching damage caused by partial discharge. Therefore the model of the partial discharge void is similar to that of the insulation medium itself and can be represented as a capacitance and resistance in parallel. Actual failure modes have indicated a drop in partial discharge intensity shortly prior to complete failure in solid dielectrics. This occurs when the internal arcing had carbonized to the point where the resistive component of the partial discharge void model was low enough to prevent a build-up of voltage across the void. This newly formed low resistive component would allow higher current to flow and additional heating and resultant insulation degradation. The partial discharge void model, including the resistive component correlates to the actual failure mode of a partial discharge void, where the resistive component passes more leakage current as the partial discharges increase with time.
PD Characteristics If both sides of the void have similar insulation materials then the charge distribution will be equal during the positive and negative cycles. In theory, there will be two observable PD pulses in each AC cycle of equal magnitude and opposite polarity per void within the bulk of the insulation. These pulses clump at the classic positions for phase-to-ground dependent pulses, that is, negative pulses at 45 degrees and the positive pulses at 225 degrees with reference to the 50/60Hz phase-to-ground voltage.
Polarity Predominance Figure 19
Service aged cables may develop delamination of the conductor shield resulting in a void near the copper conductor. A void bounded by the copper conductor and insulation, exhibits a different phenomenon than those within the bulk of the insulation. Though the basic breakdown mechanisms are the same, because the electrodes are of dissimilar materials, polarity predominance occurs. The mobility of the positive ions on the insulation surface is much lower than the negative ions on the conductor surface. The result is a predominance of negative ions migrating through the gap to the positive insulation surface. In this case, there will usually be an observable predominance of negative PD pulses clumped at 45 degrees during the positive AC cycle.
Cables that develop delamination of the insulation shield result in a void near the metallic shield. As with those voids near the copper conductors, these discharges occur between electrodes made of different materials. Here, the immobile positive charges on the insulation and mobile negative charges on the © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 19 of 23
grounded metallic electrode lead to pulses occurring during the negative AC cycle. Because the metallic electrode is grounded, the observable PD pulses will be predominantly positive clumped at 225 degrees.
Partial Discharge Inception and Extinction Voltage For partial discharge to occur, a sufficient voltage must be applied to the system under test to meet the minimum voltage required to start partial discharge activity. This is known as the partial discharge inception voltage (PDIV). Once the PDIV has been reached, voltage may be lowered and PD will remain present at the lower voltages until finally they extinguish at what is referred to as the partial discharge extinction voltage (PDEV). The PDEV is therefore less than the PDIV. If the PDEV voltage level is lower than the system operating voltage (phase-to-ground) this implies that an over voltage surge on the insulating system could initiate PD, and then even when the system voltage returns to normal, the PD activity may continue. Partial discharge activity that can continue at operating voltage is therefore more likely to result in an insulation failure than PD that extinguishes above normal operating voltage. Provided that PD activity occurs at the operating voltage level it can be detected and/or measured through online detection methods and therefore testing for partial discharge activity can be performed either online at operating voltage levels or offline at test voltage levels.
PARTIAL DISCHARGE TESTING Online PD testing has the obvious advantage in that it does not require disconnecting or an outage. The main disadvantage when testing cables under operation is that the test is only performed at the operating voltage level and cannot be adjusted. Obviously if the applied voltage is fixed and cannot be changed the PDIV and PDEV voltages cannot be determined and therefore, in comparison to offline testing, where voltages can be adjusted to simulate transients or other over voltage conditions, a lower percentage of defects in the cable’s insulating system can be detected through online methods.
Power Frequency and Alternative Test Voltage Sources As stated earlier; for partial discharge to occur, a sufficient voltage must be applied to the system under test to meet the minimum voltage required to start partial discharge activity. When testing cables the online testing approach uses the system voltage of a constant fixed magnitude. In an offline approach a temporary voltage source will be required. Considerations for an offline voltage source should include:
The applied voltage should cause partial discharges in the insulating system under test that have characteristics close, if not identical, to those that occur when the insulating system is in service. The temporary voltage source should cause no appreciable damage to the insulating system during the time required to perform the measurements. The temporary voltage source should have a variable voltage output so that PDIV and PDEV tests can be performed. The size and weight of the equipment required to produce the voltage levels required for testing various assets needs to be considered. Is the equipment to be used in a fixed location or used in a field application?
Voltage sources that are used for commercially available field partial discharge measurement systems will fall into the general categories of power frequency and alternative voltage sources such as Very Low Frequency (VLF).
VLF Test Voltage Depending on the type of defect, sinusoidal VLF voltage sources, usually 0.1 Hz, for extruded dielectric systems may require a higher test voltage to generate the same partial discharge level compared with tests performed with power-frequency voltages. For example, the conductivity of the surface of a cavity that has been exposed to PD increases, which allows any charges deposited on the surface by PD to leak away and thus lowers the electric field in the cavity. As more charge can leak away between polarity © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 20 of 23
reversals at VLF than at power frequency, the PDIV at sinusoidal VLF will be larger than that at power frequency. If there has been no previous PD activity to increase the conductivity of the cavity surface, the PDIV at sinusoidal VLF and power frequency will be similar [IEEE Std. 400.3]. A VLF cosine-pulse waveform generates a 0.1 Hz bipolar pulse wave that changes polarity sinusoidally. Since the sinusoidal transitions are in the power frequency range the PDIV measurement will be comparable to power frequency. The VLF cosine-pulse voltage work according to the principle of 50/60 Hz Slope Technology. This is particularly important for PD diagnosis, since reliable evaluation of the measured results requires a direct comparability with the power frequency. Partial discharge characteristics change in the case of large frequency differences, making reliable evaluation to power frequency impossible. The 50/60 Hz Slope Technology ensures comparability for both voltage wave shapes. Figure 20 shows a typical example of how PD measurement is carried out during the slope of the applied voltage. The steepness of the VLF cosine-pulse slopes in comparison to the 0.1 Hz sine wave can be clearly seen. It is precisely this rise in voltage which is so important for the PD inception voltage. Therefore, the 0.1 Hz sine wave test voltage cannot be directly compared to the 50/60 Hz power frequency and critical partial discharge defects are therefore not always reliably detected.
VLF Sine Wave versus VLF Cosine-pulse Figure 20
Damped AC Voltage (DAC) Another approach to reduce the size and weight of the test voltage supply from that of a conventional power frequency supply is the Damped AC Voltage (DAC) technique. For the purpose of partial discharge analysis, the cable under test is charged to the pre-selected peak value by a direct current high voltage source within a couple of seconds and afterwards shorted with an electronic switch via a resonance coil. Thus a sinusoidal oscillating AC voltage with low damping is created. The frequency is fixed in a range from 50 Hz to several 100 Hz, depending on the capacitance of the test object. Since the frequency of the test voltage is close to nominal service conditions, all measured PD activities can be effectively evaluated and compared to that of power frequency. Due to the decaying amplitude of the test voltage, the partial discharge extinction voltage can be easily determined.
Noise Influence
Damped AC Voltage Figure 21
Another consideration that needs to be reviewed when comparing online and offline measurements is noise effect on the measurement. In the offline approach the detection equipment can be calibrated at the time of the test by injecting a known PD pulse level into the specimen under test. This is not possible in the online approach. Further,
cables can be isolated during offline testing. © 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 21 of 23
When detecting PD activity using online methods further analysis will typically be necessary to ensure that the suspect activity was not caused by external noise or activity generated upstream or downstream of the cable section under investigation. It will also be advantageous to locate the source of the partial discharge activity and to quantify and assess the severity of the problem. This can be accomplished by measuring and analyzing activity over time to detect deterioration and to raise an alarm or call-to-action if PD activity reaches a critical level. As an example, if PD activity that is intermittent or possibly influenced by environmental conditions (changing temperature, humidity, or electrical noise) is found, temporary installed multi-sensor systems that automatically monitor your plant can be utilized.
Locating Partial Discharge in Cable Systems When PD activity occurs, high frequency current pulses are created and will propagate along the cable. These high frequency current pulses will propagate in both directions along the cable and can used to locate the PD activity.
Time Domain Reflectometry Technique for PD Location Figure 22
© 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 22 of 23
When partial discharge occurs in a cable system, two pulses of similar size and characteristics propagate away from the partial discharge site towards the terminations, Figure 22. Depending on the cable insulation type, shield construction, and other factors, the speed in which the pulses travel is relatively consistent. For instance, partial discharge pulses travel at a speed of approximately 468 feet per microsecond (142.65 meters / u-sec.) in XLPE insulation.
Effects of Shield Type In most industrial facilities, a large portion of the cables are of the tape shield design where overlapping layers of copper tape are wrapped around the insulation shield of the cable. In addition a widespread use of EPR insulation is found in industrial plants. In contrast, the electrical utility distribution sector tends to use concentric neutral wires instead of tape design for the ground shields and XLPE for the insulation. The concentric neutral wires hold certain advantage as the cable matures and begins to show signs of service age. As tape shielded cable ages and corrosion of the copper tape occurs, particularly at the overlap of the consecutive tape layers, the cable starts to attenuate high frequencies. Even with slight corrosion of the overlapping layers, the cable shield starts to behave as a coil or inductor to high frequencies. The net effect of this attenuation is that a PD detection system connected to a cable may not always detect the high frequency PD pulse as it would be so attenuated by the time it reaches the PD monitoring equipment. The further away the detection equipment is from the active PD site, the more severe this limitation. This limits the effectiveness of partial discharge testing on tape shielded cable.
SUMMARY As we had seen there are numerous insulation tests to assist in assessing the quality and condition of a cable’s insulation. The pass/fail tests provide the means of identifying gross defects while the diagnostic tests provide us with an understanding of the severity of degradation or the extent of contamination in the insulation. Technology and philosophies toward testing have advanced over the past twenty years and push toward predictive maintenance solutions. A key element in predictive maintenance is monitoring the trend of diagnostic test results. Not all tests are appropriate to all circumstances and neither can any single test give you the complete answer. Each type of test serves as a window looking into the condition of the cable and by putting together a number of different tests you build a more complete picture.
© 2015 Doble Engineering Company – 82nd International Conference of Doble Clients All Rights Reserved 23 of 23
