Abstract:
Partial discharge testing of in-situ power cable accessories is an emerging technology. Many diagnostic systems are being developed, both off-line and on-line that are providing much data. These data need interpretation and understanding as the technology emerges. This paper will address some of the commercially available diagnostic methods and cover the field experiences with these diagnostics. Both PILC and polymeric insulated cables are included in the discussion.
Introduction
The age and size of most utility underground installed plant is increasing and the overall failure trend will continue to grow unless a directed maintenance program is implemented. For example, Colorado Springs Utilities has 62% of their distribution network underground, with an average age of failed cable of 21.6 years, that were first installed 33 years ago (1).
Utilities need a predictive maintenance tool, which will allow proactive maintenance to occur before unplanned customer outages. By using a diagnostic it is hoped to accomplish termination or splice replacement before failure and to provide scheduled cable system maintenance. Additional budget information will be gathered to reduce costs and to target replacement money (2,3,4).
Both off-line and on-line diagnostics are commercially available to determine the condition of cable and cable accessory insulation. Two major types of insulation degradation occur in cable systems. One is an average or overall condition caused by chemical aging and/or water treeing. The diagnostics used for this type of aging includes dissipation factor (loss angle), harmonic analysis, return voltage, isothermal relaxation current, dielectric response or dc leakage current. A summary of these methods is provided in the table.
The second type of degradation is discrete or incremental condition assessment, that utilizes dissipation factor measurements or partial discharge (PD) level measurements. This paper will focus on PD diagnostics specifically applied to the detection of degradation in cable accessories. No matter which type of diagnostic is used, it should be applied in a non-destruction manner, so that the diagnostic itself does not reduce cable or accessory life.
Cable accessories are treated quite differently from cables. For example, the accessory design is not always properly tested, they are man-made in the field, so workmanship is a concern, and they are not properly tested after installation. Most cable accessory materials are more resistant to partial discharge activity than the cable and will withstand PD and treeing activity longer than the adjacent cable insulation. However, there are likely to be more defects in a cable accessory than in a cable, so PD detection is more applicable to cable accessory assessment (3).
Partial Discharge
Partial discharges occur at voids (or cavities) in electrical insulation or at interfaces between materials, in cable accessories, for example. Microsparks occur in these voids that emit broadband radiation from 50 kHz to >500 MHz, with pulse rise times of 1.0 ns. The location of the discharge remains the same, but the magnitude and number of the pulses can vary considerably with time, voltage, temperature, load and humidity, making okdetection and location difficult. In addition, attenuation along the cable length, particularly at the higher frequencies, and background noise in the field can compound the methodology. Multiple discharge sites, cable branches, contamination and the splice material type can add to the complexity of interpretation. In spite of all these concerns, PD detection on installed power cables is an emerging technology showing considerable promise (6,7,8,910,11,12,13).
The energy inherent in a partial discharge leads to damage of the material surface surrounding the void or cavity. Erosion of the surface occurs and electrical trees initiate and grow in the body of the insulating material. The process becomes self-perpetuating until the electrical tree bridges the insulation and complete breakdown occurs. Partial discharge accompanies the whole process (14).
Material erosion rates due to PD activity are very material dependent. Some rubber materials can tolerate PD activity with minimum erosion, while other polymeric materials have minimum tolerance of PD activity making degradation assessessments unreliable. Also the PD magnitude is dependent upon many variables, including the shape of the offending void, a parameter that can change with time.
Unfortunately the time-to-failure predictions for a cable system based upon PD magnitude is not possible. It has been found that growth of electrical trees can be quite rapid during low PD levels and large PD levels can be quite harmless. Cable life estimation will depend upon the ability to distinguish between harmful and harmless PD (6).
Detection and location of PD activity in in-situ underground power cable accessories can be performed both "off-line", with the cable out of service, or "on-line" without the need for switching or reduced system contingency. For off-line detection, a portable, alternative voltage supply is necessary.
Off-Line PD , Diagnostic Techniques
Off-line diagnostic techniques require the de-energization of the cable to be tested and in some cases, complete removal from the underground distribution system. Before any diagnostic test begins it is usually necessary to remove any residual space charge remaining in the cable under test, as this may influence the test results. This is carried out by either direct grounding of the cable conductor, or for longer cable lengths, a graduated resistive grounding stick can be utilized. The latter device limits internal switching surges that can further damage cable insulation.
At the conclusion of the diagnostic test program, it is further necessary to remove any charge left on the cable by the test procedure. This will prevent any damage to the cable insulation from polarity reversals upon re-energization.
Off-line testing has the advantage that the test voltage can be controlled closely, and if necessary, can be extended to voltages above the normal operating voltage. However, caution should be exercised if overvoltages are used.
The following are examples of off-line diagnostics.
60 Hz Test: The advantage of 60 Hz diagnostic testing is that the test results obtained relate to the cable operating conditions, as the frequency is exactly the same. By using a resonant test set, the test set size is substantially reduced. Further more, the test voltage and frequency can be controlled and overvoltage testing can lead to location of problem areas that normally would not be seen at the rated voltage (15). It must be emphasized, however, that caution must be exercised during any overvoltage test due to the potential of damaging the insulation in other weak locations.
Partial discharge patterns at 60 Hz can provide a great deal of information on the type severity and location of the degradation area, but may change with load and or temperature. It is not easy to assess PD severity, but a good proportion of serious defects can be found with PD analysis.
VLF (0.1 Hz) Test: VLF testing at 0.1 Hz, and lower frequencies for longer cable lengths, has been used successfully to measure dissipation factor and PD levels in cables and their accessories, is less damaging to the insulation than dc testing and can locate potential failure sites more reliably that a dc hipot test. VLF has the advantage that the energy requirements are 1/600 for 60 Hz voltages and 1/500 for 50 Hz voltages, so the test sets can be much smaller than for normal operating frequency test sets. Portability is of utmost importance for field testing. VLF PD detection has been successfully employed to find major defects in cable accessories (3).
Combined AC and VLF Diagnostic: Complex Discharge Analysis (CDA) combines the meaningfulness of an ac test with the small power demand of a VLF test. The cable under test is charged slowly for 10 seconds and when the recommended peak voltage is reached the cable is discharged with a periodic oscillation of about 10 ms, that is, about 50-60 Hz. PD activity is recorded during the discharge cycle. For statistical significance the charge/discharge cycle is repeated 10 times. The PD pulse repetition rate and magnitude is determined during the discharge period and any defects located.
The Oscillating Wave Test System (OWTS): The Oscillating Wave Test System (16,17) was developed as a non-destructive, afterlaying test, standardized by the IEC. Further development has lead to a diagnostic suitable for off-line applications in noisy environments. The method has been used on XLPE cables with a maximum length of 7348 feet and it is suggested that the time for each test is the same as a dc hipot test.
The OWTS produces ac voltages in the range of 20 to 1000 Hz depending upon the cable length and can be used for PD pattern analysis provided there are no high disturbances present. The method is also used for dissipation factor measurements.
On-Line Diagnostics: On-line diagnostics have the advantage that the cable is not switched out of service and is still energized as for the normal operation. No system contingency problems will arise and the potential of damage due to inappropriate switching is removed. The disadvantage is that the test voltage cannot be controlled and the opportunity for overvoltage testing is not available.
Ultrasound: Ultrasonic detection of PD in cable accessories can be readily used to pinpoint a suspected problem if the cable accessory is not direct buried, or is at least physically accessible. Ultrasonic detectors are lightweight and very portable.
Partial discharge within a cable accessory produces a broad range of sound that can be detected with ultrasonic translators. The high frequency ultrasonic components are extremely short wave in nature, fairly directional and easy to isolate from background noise allowing easy location. Parabolic reflectors or concentrators can be used to detect emissions at a distance (18).
Partial Discharge: Direct, capacitive or inductive couplers (sensors) are installed on or near the cable accessory to locate and measure PD in cable splices and terminations while the cable is on-line. Some diagnostics require that the sensors be installed during initial installation of the cable system while others can be retrofitted. On-line diagnostics can be used for data trending and analysis such as Pulse Phase Analysis. Trends can be evaluated so that the influence of load, voltage, temperature and humidity can be taken into account in the quest for an understandable assessment and improved data interpretation.
One PD detection method (19) can be used to detect PD components up to 300 MHz near the cable accessory.
Pattern recognition combined with Neural Networks is being used in another diagnostic to improve PD phenomena and hence cable accessory diagnostics (20). The system can recognize each detected pulse as PD or noise and dramatically improve data analysis.
Utility Experience
Utility experience with cable accessory PD field testing has been quite variable. A summary of several experiences follows.
Colorado Springs Utilities (CSU) removed many of the splices with an indicated high partial discharge and dissected each splice to determine the cause of the high PD (1,21,22). Most problems were linked to workmanship. In addition, some samples were sent to a laboratory for further investigation, but the high PD could not be repeated, nor could any related defects be found (23). CSU also found that PD attenuation occurred in locations where the concentric neutral was missing. They concluded that considerable cost savings could be realized with one diagnostic test.
After a series of in-situ PD tests, Oklahoma Gas and Electric concluded that it was difficult to take the correct action when the accuracy of the applied voltage was limited, the PD types were difficult to assess and the accuracy of PD location needed some improvement. Repeatability was quite good, however, but the effectiveness was very dependent upon the test personnel (2).
PECO found that there was an inability to distinguish discharge sites that lead to failure from those that do not lead to failure. Dissection of three high discharge splices did not locate any evidence of an incipient failure (24).
Xcel Energy linemen expressed concern about splices removed from service because the splices appeared to be in good condition (25). The question was posed, “Was replacement really necessary?” Partial discharge testing of the splices gave only limited information about the condition of splices, so this utility is proceeding with caution.
To maximize information gained TXU Electric and Gas went to overvoltage testing at three times the rated voltage (3Uo). It is interesting to note that one termination failed after testing that indicated a low PD level. This utility suggested that 60 Hz testing may be used successfully to determine the condition of cable and accessories, but that the cable is more susceptible to failure than the accessories (26).
North East Utilities were concerned about further damage to the cable caused by the diagnostic, what effect does cable length have on the diagnostic and the advantages/disadvantages of off-line versus on-line diagnostics. They will continue to use the ultrasonic detector (27).
PD Monitoring of Rotating Machines
Partial discharge monitoring of rotating machines has been used for several decades to evaluate the insulation condition in the stator windings of motors and generators (28). Most users prefer to use an on-line diagnostic, as it is more effective to diagnose the winding condition with continuous monitoring. The presence of partial discharges in a rotating machine stator winding can be a symptom of ongoing deterioration of the insulation in the machine.
The technology has advanced sufficiently to allow the identification of specific types of partial discharge activity and so assess the general condition of the insulation as well as the most prominent aging mechanisms.
Data interpretation by pulse peak magnitude, pulse polarity, quantity (repetition rate) and phase location with respect to the line-to-ground voltage is utilized. Three partial discharge plots are created that include Pulse Height Analysis, Pulse Phase Analysis and a Trend plot.
It has been found that operating conditions, such as operating voltage, load, temperature and humidity have a significant impact on partial discharge behaviour. Thus to determine if winding insulation has deteriorated with time and to establish a reliable trend curve it is necessary to accurately record the operating parameters along with the on-line PD data. If the relationship between these parameters is unknown, then it is not possible to assess whether a change in PD is due to an operating change or to insulation deterioration.
Conclusion
Partial discharge detection and location in cable accessories is still undergoing development. Interpretation is difficult due to the number of variables involved.
Future diagnostics should;
References
[1] “The Evolving Application of Partial Discharge Diagnostic Testing at Colorado Springs Utilities”, Kraig Bader, IEEE, ICC Minutes,
Fall 2000, Page 75.
[2] “Partial Discharge Testing Experience at Oklahoma Gas and Electric”, Dale T. Metzinger, IEEE, ICC Minutes Fall 2000, Page 66.
[3] “Non-destructive Diagnostic Testing of Distribution Cable Accessories”, Willem Boone, IEEE, ICC Minutes, Spring 1999, Page 129.
[4] “Partial Discharge Site Location Experience at Colorado Springs Utilities”, Kraig Bader, IEEE, ICC Minutes Spring 1998, Appendix 10J.
[5] “Test Methods to Determine Condition of Insulation”, H. Oetjen, IEEE, ICC Minutes,
Fall 1999, Page 99.
[6] “Fundamentals of Partial Discharge in the Context of Field Cable Testing”, by Steve Bogs and John Densley, IEEE ICC Minutes,
Fall 2000, Page 186.
[7] “Partial Discharge Characteristics of Interfaces in Extruded Cable Systems – Influence of Contaminants”, J. Densley,
J.M. Braun and Z. Nadolay, IEEE,
ICC Minutes, Spring 1998, Appendix 5-I.
[8] “Partial Discharge XXII. High Frequency Attenuation in Shielded Dielectric Power Cable and Implications thereof for PD Location”,
S. Boggs, A. Pathak and P. Walker, IEEE, Electrical Insulation Magazine,
January-February 1996, pp 9-16.
[9] “Fundamentals of Partial Discharge in the Context of Field Cable Testing”, S. Boggs and J, Densley, IEEE Electrical Insulation Magazine, September-October 2000, pp 13-18
[10] “Partial Discharge VII, PD Testing of solid Dielectric Cable”, J.C. Chan, P. Duffy, L.J. Hiivala, J. Wasik, IEEE, Electrical Insulation Magazine, September-October 1991,
pp 9-16, 19-20.
[11] “Partial Discharge VI, Commercial Discharge Testing”, J.P. Steiner, IEEE, Electrical Insulation Magazine, January–February 1991, pp 20-33.
[12] “Application of Advanced Diagnostics for Condition Based Maintenance of MV Power Cables”, F.J. Wester, E. Gulski et al, IEEE Conference Record of ISEI, 2000, pp 115-118.
[13] “Estimation of Medium Voltage Parameters for PD-Detection”, R. Villefrance, J.T. Holboll, M. Hendriksen, Conference Record of the ISEI 1998, pp 109-112.
[14] “Small Partial Discharges and their Role in Insulation”, M.G. Danikas, IEEE DEIS Transactions 1997, Vol. 4, pp 863-867.
[15] “Discussion of Current PD Testing Topics in the Light of Recent Field Experience”,
M. S. Mashikian, IEEE, ICC Minutes,
Fall 1999, Page 225.
[16] “The Use of Oscillating Wave Test Systems with PD Site Location, Field Experiences in the US and Canada”, Bill Larzelere, IEEE,
ICC Minutes, Fall 2000, Page 130.
[17] “Advanced Partial Discharge Diagnostic of MV Power Cable System using Oscillating Wave Test System”, E. Gulski, F.J. Wester, et al,
IEEE, Electrical Insulation Magazine,
March-April 2000, pp17-25.
[18] “Detection of Partial Discharges in Cable Splices and Terminations”, M. Goodman, IEEE, ICC Minutes, Appendix 10-F.
[19] “On-line Partial Discharge Detection in Cables”, N.H. Ahmed, N.N. Srinivas, IEEE DEIS Transactions, 1998, Vol. 5, pp 181-188.
[20] “An Application of Neural Network for Detection of Partial Discharge in Power Cables”, IEEJ Technical Report, EC-95-34, 1995.
[21] “Results Analysis of Partial Discharge Testing in Colorado Springs”, Kraig Bader, IEEE, ICC Minutes, Spring 1999, Page 117.
[22] “Further PD Testing on Distribution Cables and Accessories”, Kraig Bader and Lyn Kimsey, IEEE, ICC Minutes, Fall 1998, Page 179.
[23] “Partial Testing on Field Splices”,
Wolfgang Haverkamp, IEEE, ICC Minutes,
Fall 1998, Page 192.
[24] “PECO Experience with Partial Discharge Testing”, Stan Heyer, IEEE, ICC Minutes,
Fall 2000, Page 37.
[25] “60 Hz Testing Cables and Accessories”,
J.M. Braun, IEEE, ICC Minutes,
Fall 2000, Page 40.
[26] “60 Hz PD Testing of Terminations and Splices at TXU”, Richie Harp, IEEE, ICC Minutes Fall 1999, Page 192.
[27] “Partial Discharge Testing – Utility Perspective”, Rachel Mosier, IEEE,
ICC Minutes, Fall 2000, Page 56.
[28] “Experience with Continuous Partial Discharge Monitoring of Stator Windings”,
M. Fenger, G. Stone, et al, Proceedings of EIC/EMCW Conference, Cincinnati, OH, October 15-19, 2001, Page 417.
Partial discharge testing of in-situ power cable accessories is an emerging technology. Many diagnostic systems are being developed, both off-line and on-line that are providing much data. These data need interpretation and understanding as the technology emerges. This paper will address some of the commercially available diagnostic methods and cover the field experiences with these diagnostics. Both PILC and polymeric insulated cables are included in the discussion.
Introduction
The age and size of most utility underground installed plant is increasing and the overall failure trend will continue to grow unless a directed maintenance program is implemented. For example, Colorado Springs Utilities has 62% of their distribution network underground, with an average age of failed cable of 21.6 years, that were first installed 33 years ago (1).
Utilities need a predictive maintenance tool, which will allow proactive maintenance to occur before unplanned customer outages. By using a diagnostic it is hoped to accomplish termination or splice replacement before failure and to provide scheduled cable system maintenance. Additional budget information will be gathered to reduce costs and to target replacement money (2,3,4).
Both off-line and on-line diagnostics are commercially available to determine the condition of cable and cable accessory insulation. Two major types of insulation degradation occur in cable systems. One is an average or overall condition caused by chemical aging and/or water treeing. The diagnostics used for this type of aging includes dissipation factor (loss angle), harmonic analysis, return voltage, isothermal relaxation current, dielectric response or dc leakage current. A summary of these methods is provided in the table.
The second type of degradation is discrete or incremental condition assessment, that utilizes dissipation factor measurements or partial discharge (PD) level measurements. This paper will focus on PD diagnostics specifically applied to the detection of degradation in cable accessories. No matter which type of diagnostic is used, it should be applied in a non-destruction manner, so that the diagnostic itself does not reduce cable or accessory life.
Cable accessories are treated quite differently from cables. For example, the accessory design is not always properly tested, they are man-made in the field, so workmanship is a concern, and they are not properly tested after installation. Most cable accessory materials are more resistant to partial discharge activity than the cable and will withstand PD and treeing activity longer than the adjacent cable insulation. However, there are likely to be more defects in a cable accessory than in a cable, so PD detection is more applicable to cable accessory assessment (3).
Partial Discharge
Partial discharges occur at voids (or cavities) in electrical insulation or at interfaces between materials, in cable accessories, for example. Microsparks occur in these voids that emit broadband radiation from 50 kHz to >500 MHz, with pulse rise times of 1.0 ns. The location of the discharge remains the same, but the magnitude and number of the pulses can vary considerably with time, voltage, temperature, load and humidity, making okdetection and location difficult. In addition, attenuation along the cable length, particularly at the higher frequencies, and background noise in the field can compound the methodology. Multiple discharge sites, cable branches, contamination and the splice material type can add to the complexity of interpretation. In spite of all these concerns, PD detection on installed power cables is an emerging technology showing considerable promise (6,7,8,910,11,12,13).
The energy inherent in a partial discharge leads to damage of the material surface surrounding the void or cavity. Erosion of the surface occurs and electrical trees initiate and grow in the body of the insulating material. The process becomes self-perpetuating until the electrical tree bridges the insulation and complete breakdown occurs. Partial discharge accompanies the whole process (14).
Material erosion rates due to PD activity are very material dependent. Some rubber materials can tolerate PD activity with minimum erosion, while other polymeric materials have minimum tolerance of PD activity making degradation assessessments unreliable. Also the PD magnitude is dependent upon many variables, including the shape of the offending void, a parameter that can change with time.
Unfortunately the time-to-failure predictions for a cable system based upon PD magnitude is not possible. It has been found that growth of electrical trees can be quite rapid during low PD levels and large PD levels can be quite harmless. Cable life estimation will depend upon the ability to distinguish between harmful and harmless PD (6).
Detection and location of PD activity in in-situ underground power cable accessories can be performed both "off-line", with the cable out of service, or "on-line" without the need for switching or reduced system contingency. For off-line detection, a portable, alternative voltage supply is necessary.
SUMMARY TABLE OF DIAGNOSTIC TECHNIQUES | ||||
| Destructive | Non-Destructive Off-line | Non-Destructive, On-Line | ||
| Dissection and Microscopic Examination | Electrical Tests | Methods to Detect Singular Faults | Integral Measurement Methods | Integral & Singular Measurement Methods |
| Contaminants, Voids and Water Trees | 60 Hz step tests, acbd tests, radial "power factor" | PD at 60 Hz Kinectrics | Dissipation Factor at 0.1 Hz BAUR, Kinectrics | Leakage Current Sumitomo |
| Chemical Evaluation FTIR, DSC, DMA, OIT, DP, PIXE | DC hipot Resonant Cct 0.1 Hz Sine Oscill.Wave Impulse | PD at 0.1 Hz KEMA | Dielectric Spectroscopy ABB | DC Component Fujikura |
| Mechanical Evaluation Tensile, Elongation, Burst test | DIACS | PD Location System (<2U0) (IMCORP) | LIpATEST Powertech | Harmonic Current Sumitomo & NRC |
| CDA & OWTS PD Lemke & Univ. Delft | Isothermal Relaxation Current SINTEF | PD, Power KEMA, DTE, Sumitomo & Eaton | ||
| Return Voltage Hagenuk & Univ. of Siegen | ||||
Off-Line PD , Diagnostic Techniques
Off-line diagnostic techniques require the de-energization of the cable to be tested and in some cases, complete removal from the underground distribution system. Before any diagnostic test begins it is usually necessary to remove any residual space charge remaining in the cable under test, as this may influence the test results. This is carried out by either direct grounding of the cable conductor, or for longer cable lengths, a graduated resistive grounding stick can be utilized. The latter device limits internal switching surges that can further damage cable insulation.
At the conclusion of the diagnostic test program, it is further necessary to remove any charge left on the cable by the test procedure. This will prevent any damage to the cable insulation from polarity reversals upon re-energization.
Off-line testing has the advantage that the test voltage can be controlled closely, and if necessary, can be extended to voltages above the normal operating voltage. However, caution should be exercised if overvoltages are used.
The following are examples of off-line diagnostics.
60 Hz Test: The advantage of 60 Hz diagnostic testing is that the test results obtained relate to the cable operating conditions, as the frequency is exactly the same. By using a resonant test set, the test set size is substantially reduced. Further more, the test voltage and frequency can be controlled and overvoltage testing can lead to location of problem areas that normally would not be seen at the rated voltage (15). It must be emphasized, however, that caution must be exercised during any overvoltage test due to the potential of damaging the insulation in other weak locations.
Partial discharge patterns at 60 Hz can provide a great deal of information on the type severity and location of the degradation area, but may change with load and or temperature. It is not easy to assess PD severity, but a good proportion of serious defects can be found with PD analysis.
VLF (0.1 Hz) Test: VLF testing at 0.1 Hz, and lower frequencies for longer cable lengths, has been used successfully to measure dissipation factor and PD levels in cables and their accessories, is less damaging to the insulation than dc testing and can locate potential failure sites more reliably that a dc hipot test. VLF has the advantage that the energy requirements are 1/600 for 60 Hz voltages and 1/500 for 50 Hz voltages, so the test sets can be much smaller than for normal operating frequency test sets. Portability is of utmost importance for field testing. VLF PD detection has been successfully employed to find major defects in cable accessories (3).
Combined AC and VLF Diagnostic: Complex Discharge Analysis (CDA) combines the meaningfulness of an ac test with the small power demand of a VLF test. The cable under test is charged slowly for 10 seconds and when the recommended peak voltage is reached the cable is discharged with a periodic oscillation of about 10 ms, that is, about 50-60 Hz. PD activity is recorded during the discharge cycle. For statistical significance the charge/discharge cycle is repeated 10 times. The PD pulse repetition rate and magnitude is determined during the discharge period and any defects located.
The Oscillating Wave Test System (OWTS): The Oscillating Wave Test System (16,17) was developed as a non-destructive, afterlaying test, standardized by the IEC. Further development has lead to a diagnostic suitable for off-line applications in noisy environments. The method has been used on XLPE cables with a maximum length of 7348 feet and it is suggested that the time for each test is the same as a dc hipot test.
The OWTS produces ac voltages in the range of 20 to 1000 Hz depending upon the cable length and can be used for PD pattern analysis provided there are no high disturbances present. The method is also used for dissipation factor measurements.
On-Line Diagnostics: On-line diagnostics have the advantage that the cable is not switched out of service and is still energized as for the normal operation. No system contingency problems will arise and the potential of damage due to inappropriate switching is removed. The disadvantage is that the test voltage cannot be controlled and the opportunity for overvoltage testing is not available.
Ultrasound: Ultrasonic detection of PD in cable accessories can be readily used to pinpoint a suspected problem if the cable accessory is not direct buried, or is at least physically accessible. Ultrasonic detectors are lightweight and very portable.
Partial discharge within a cable accessory produces a broad range of sound that can be detected with ultrasonic translators. The high frequency ultrasonic components are extremely short wave in nature, fairly directional and easy to isolate from background noise allowing easy location. Parabolic reflectors or concentrators can be used to detect emissions at a distance (18).
Partial Discharge: Direct, capacitive or inductive couplers (sensors) are installed on or near the cable accessory to locate and measure PD in cable splices and terminations while the cable is on-line. Some diagnostics require that the sensors be installed during initial installation of the cable system while others can be retrofitted. On-line diagnostics can be used for data trending and analysis such as Pulse Phase Analysis. Trends can be evaluated so that the influence of load, voltage, temperature and humidity can be taken into account in the quest for an understandable assessment and improved data interpretation.
One PD detection method (19) can be used to detect PD components up to 300 MHz near the cable accessory.
Pattern recognition combined with Neural Networks is being used in another diagnostic to improve PD phenomena and hence cable accessory diagnostics (20). The system can recognize each detected pulse as PD or noise and dramatically improve data analysis.
Utility Experience
Utility experience with cable accessory PD field testing has been quite variable. A summary of several experiences follows.
Colorado Springs Utilities (CSU) removed many of the splices with an indicated high partial discharge and dissected each splice to determine the cause of the high PD (1,21,22). Most problems were linked to workmanship. In addition, some samples were sent to a laboratory for further investigation, but the high PD could not be repeated, nor could any related defects be found (23). CSU also found that PD attenuation occurred in locations where the concentric neutral was missing. They concluded that considerable cost savings could be realized with one diagnostic test.
After a series of in-situ PD tests, Oklahoma Gas and Electric concluded that it was difficult to take the correct action when the accuracy of the applied voltage was limited, the PD types were difficult to assess and the accuracy of PD location needed some improvement. Repeatability was quite good, however, but the effectiveness was very dependent upon the test personnel (2).
PECO found that there was an inability to distinguish discharge sites that lead to failure from those that do not lead to failure. Dissection of three high discharge splices did not locate any evidence of an incipient failure (24).
Xcel Energy linemen expressed concern about splices removed from service because the splices appeared to be in good condition (25). The question was posed, “Was replacement really necessary?” Partial discharge testing of the splices gave only limited information about the condition of splices, so this utility is proceeding with caution.
To maximize information gained TXU Electric and Gas went to overvoltage testing at three times the rated voltage (3Uo). It is interesting to note that one termination failed after testing that indicated a low PD level. This utility suggested that 60 Hz testing may be used successfully to determine the condition of cable and accessories, but that the cable is more susceptible to failure than the accessories (26).
North East Utilities were concerned about further damage to the cable caused by the diagnostic, what effect does cable length have on the diagnostic and the advantages/disadvantages of off-line versus on-line diagnostics. They will continue to use the ultrasonic detector (27).
PD Monitoring of Rotating Machines
Partial discharge monitoring of rotating machines has been used for several decades to evaluate the insulation condition in the stator windings of motors and generators (28). Most users prefer to use an on-line diagnostic, as it is more effective to diagnose the winding condition with continuous monitoring. The presence of partial discharges in a rotating machine stator winding can be a symptom of ongoing deterioration of the insulation in the machine.
The technology has advanced sufficiently to allow the identification of specific types of partial discharge activity and so assess the general condition of the insulation as well as the most prominent aging mechanisms.
Data interpretation by pulse peak magnitude, pulse polarity, quantity (repetition rate) and phase location with respect to the line-to-ground voltage is utilized. Three partial discharge plots are created that include Pulse Height Analysis, Pulse Phase Analysis and a Trend plot.
It has been found that operating conditions, such as operating voltage, load, temperature and humidity have a significant impact on partial discharge behaviour. Thus to determine if winding insulation has deteriorated with time and to establish a reliable trend curve it is necessary to accurately record the operating parameters along with the on-line PD data. If the relationship between these parameters is unknown, then it is not possible to assess whether a change in PD is due to an operating change or to insulation deterioration.
Conclusion
Partial discharge detection and location in cable accessories is still undergoing development. Interpretation is difficult due to the number of variables involved.
Future diagnostics should;
- Look at PD activity over time or Trend Analysis that includes operational parameters, such as voltage, load, temperature and humidity,
- Distinguish between PD types, and especially between harmful and harmless PD,
- Data interpretation improvement is necessary.
References
[1] “The Evolving Application of Partial Discharge Diagnostic Testing at Colorado Springs Utilities”, Kraig Bader, IEEE, ICC Minutes,
Fall 2000, Page 75.
[2] “Partial Discharge Testing Experience at Oklahoma Gas and Electric”, Dale T. Metzinger, IEEE, ICC Minutes Fall 2000, Page 66.
[3] “Non-destructive Diagnostic Testing of Distribution Cable Accessories”, Willem Boone, IEEE, ICC Minutes, Spring 1999, Page 129.
[4] “Partial Discharge Site Location Experience at Colorado Springs Utilities”, Kraig Bader, IEEE, ICC Minutes Spring 1998, Appendix 10J.
[5] “Test Methods to Determine Condition of Insulation”, H. Oetjen, IEEE, ICC Minutes,
Fall 1999, Page 99.
[6] “Fundamentals of Partial Discharge in the Context of Field Cable Testing”, by Steve Bogs and John Densley, IEEE ICC Minutes,
Fall 2000, Page 186.
[7] “Partial Discharge Characteristics of Interfaces in Extruded Cable Systems – Influence of Contaminants”, J. Densley,
J.M. Braun and Z. Nadolay, IEEE,
ICC Minutes, Spring 1998, Appendix 5-I.
[8] “Partial Discharge XXII. High Frequency Attenuation in Shielded Dielectric Power Cable and Implications thereof for PD Location”,
S. Boggs, A. Pathak and P. Walker, IEEE, Electrical Insulation Magazine,
January-February 1996, pp 9-16.
[9] “Fundamentals of Partial Discharge in the Context of Field Cable Testing”, S. Boggs and J, Densley, IEEE Electrical Insulation Magazine, September-October 2000, pp 13-18
[10] “Partial Discharge VII, PD Testing of solid Dielectric Cable”, J.C. Chan, P. Duffy, L.J. Hiivala, J. Wasik, IEEE, Electrical Insulation Magazine, September-October 1991,
pp 9-16, 19-20.
[11] “Partial Discharge VI, Commercial Discharge Testing”, J.P. Steiner, IEEE, Electrical Insulation Magazine, January–February 1991, pp 20-33.
[12] “Application of Advanced Diagnostics for Condition Based Maintenance of MV Power Cables”, F.J. Wester, E. Gulski et al, IEEE Conference Record of ISEI, 2000, pp 115-118.
[13] “Estimation of Medium Voltage Parameters for PD-Detection”, R. Villefrance, J.T. Holboll, M. Hendriksen, Conference Record of the ISEI 1998, pp 109-112.
[14] “Small Partial Discharges and their Role in Insulation”, M.G. Danikas, IEEE DEIS Transactions 1997, Vol. 4, pp 863-867.
[15] “Discussion of Current PD Testing Topics in the Light of Recent Field Experience”,
M. S. Mashikian, IEEE, ICC Minutes,
Fall 1999, Page 225.
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