* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Insulation condition assessment of medium voltage power
Survey
Document related concepts
Variable-frequency drive wikipedia , lookup
Chirp spectrum wikipedia , lookup
Stray voltage wikipedia , lookup
Immunity-aware programming wikipedia , lookup
Spectrum analyzer wikipedia , lookup
Voltage optimisation wikipedia , lookup
History of electric power transmission wikipedia , lookup
Opto-isolator wikipedia , lookup
Buck converter wikipedia , lookup
Switched-mode power supply wikipedia , lookup
Alternating current wikipedia , lookup
Telecommunications engineering wikipedia , lookup
Mains electricity wikipedia , lookup
Transcript
Insulation condition assessment of medium voltage power cables using on-site PD detection and analysis techniques Edward Gulski, Bjorn R. Hamerling, Frank J. Wester, Johan J. Smit, Edwin Groot(1), Peter Schikarski(2) Delft University of Technology, High Voltage Laboratory, The Netherlands (1) InfraCore, NUON, Alkmaar, The Netherlands (2) Haefely Test AG, Tettex Instruments Division, Dietikon, Switzerland SUMMARY In the last ten years, to determine the insulation condition of the distribution power cable grid, several on-site diagnostic measuring methods have been developed. As a result, on the basis of on-site testing and measurements of partial discharges, important information is generated about discharging insulation defects, see figure 1. In particular, applying off-line the PD diagnosis at oscillating voltage waves has shown that PD knowledge rules can be defined to support condition-based maintenance, see figure 2. Figure 1. PD mapping made using OWTS® method for a 840m long 10kV PILC power cable The concentration of discharges at the 120m location originates from a oil-filed splice. To obtain maximum efficiency in applying advanced on-site diagnosis to a large number of different cable sections, a certain pre-selection is necessary. This pre-selection could be done in different ways, e.g. using failure history, position in the network or on the basis of an on-line diagnosis, see figure 3. In particular, to support the off-line diagnosis by prioritising particular cable sections on the one hand, and after an on-site inspection to trend the discharge process in a suspicious cable section the on-line PD diagnostics is becoming more and more interesting. Unfortunately, in contrast to off-line PD diagnosis, the sensitivity of on-line PD detection as well as the interpretation of measuring results is strongly dependent on local external disturbances and operation conditions of the cable section. Finally, based on systematic laboratory and filed investigations between off-line and on-line PD diagnosis transferability rules have to be developed supporting field application of both techniques. To contribute to this investigation, this paper will firstly describe the measurement method of the online PD diagnostic, secondly it will present laboratory measurement results and thirdly it handles the first application of the diagnostic in practice and compares the results with the OWTS measuring method. Finally there will be drawn some conclusions with respect to the applicability of the on-line PD discharge detection diagnostic. Figure 2. OWTS® PD system during off-line diagnosis of a 10 kV PILC cable section; location Alkmaar, The Netherlands. spectrum analyser 10 kV PILC cable terminations and the HFCT sensor Figure 3. VHF PD detection system during online PD detection of a 10 kV PILC cable section. The HFCT sensor is temporally installed in the earth circuit of the 10 kV PILC cable termination; location Purmerend, The Netherlands. Insulation condition assessment of medium voltage power cables using on-site PD detection and analysis techniques Edward Gulski, Bjorn R. Hamerling, Frank J. Wester, Johan J. Smit, Edwin Groot(1), Peter Schikarski(2) Delft University of Technology, High Voltage Laboratory, The Netherlands (1) InfraCore, NUON, Alkmaar, The Netherlands (2) Haefely Test AG, Tettex Instruments Division, Dietikon, Switzerland ABSTRACT some conclusions with respect to the applicability of the on-line PD discharge detection diagnostic. In this contribution fundamental aspects of on-site partial discharge detection on distribution power cables as a tool for insulation condition assessment is discussed. In particular, based on laboratory and field tests the combination of off-line and on-line PD diagnostics is presented. HV Supply Inductor Cable under test HV Switch 1. INTRODUCTION Computer unit In the last ten years, to determine the insulation condition of the distribution power cable grid, several on-site diagnostic measuring methods have been developed [1-2]. As a result, on the basis of on-site testing and measurements of partial discharges, important information is generated about discharging insulation defects. In particular, applying off-line the PD diagnosis at oscillating voltage waves has shown that PD knowledge rules can be defined to support condition-based maintenance [3-5]. To obtain maximum efficiency in applying advanced on-site diagnosis to a large number of different cable sections, a certain pre-selection is necessary. This pre-selection could be done in different ways, e.g. using failure history, position in the network or on the basis of an on-line diagnosis [6]. In particular, to support the off-line diagnosis by prioritising particular cable sections on the one hand, and after an on-site inspection to trend the discharge process in a suspicious cable section the on-line PD diagnostics is becoming more and more interesting. Unfortunately, in contrast to offline PD diagnosis, the sensitivity of on-line PD detection as well as the interpretation of measuring results is strongly dependent on local external disturbances and operation conditions of the cable section. Finally, based on systematic laboratory and filed investigations between off-line and online PD diagnosis transferability rules have to be developed supporting field application of both techniques. To contribute to this investigation, this paper will firstly describe the measurement method of the online PD diagnostic, secondly it will present laboratory measurement results and thirdly it handles the first application of the diagnostic in practice and compares the results with the OWTS measuring method. Finally there will be drawn HV + PD detection V A t q t A Figure 1. Principals of off-line PD diagnosis of distribution power cables using oscillating wave test system OWTS® : Upper photo: 36 kV HV Supply, PD analyzer (30kg), Lower photo: inductor, HV Switch, HV+PD detection (65kg). Spectrumanalyser & laptop To termination HFCT High voltage Earthshield power cable Figure 2. Principals of on-line VHF PD analysis of distribution power cables using 50 MHz CT, a Spectrum Analyzer controlled by a computer. 2. PD MEASURING SYSTEMS 2.1. Oscillating Wave Test System OWTS® (off-line) With this method, see figure 1, the cable sample is charged with a DC power supply over a period of just a few seconds to the usual service voltage [25]. Then a specially designed solid state switch connects an air-core inductor to the cable sample Figure 3. Measurement results from a 10kV 2870 m long paper/oil power cable, installed in 1960: a) PD pattern at inception voltage; b) PD pattern at 2*U0. The air core inductor has a low losses factor design, so that the resonant frequency lies close to the range of power frequency of the service voltage: 50Hz to 1 kHz. Due to the fact that the insulation of power cables usually has a relative low dissipation factor, the Q of the resonant circuit remains high depending upon the cable: 30 to more than 100. As a result, a slowly decaying oscillating waveform (decay time of 0.3 to 1 second) of test voltage is applied to energise the cable sample. During tens of power frequency cycles the PD signals are initiated in a way similar to 50 (60) Hz inception conditions, see figure 3. Since the oscillating frequency represents the AC conditions of the power line frequency, the measurement bandwidth of the PD circuit has been chosen in accordance with IEC 60270 recommendations. For the purpose of location by travelling waves, the bandwidth is increased up to several MHz. In combination with a 100 MHz digitiser and depending upon the cable type, e.g. XLPE or paper-oil, the detection and location of PD pulses remains sensitive for cable lengths of few kilometres, see figure 4. PD spectrum Noise spectrum 3-core PILC; 945m; 1991 Cable A (PD activity upto U0) 8000 PD magnitude [pC] 7000 6000 5000 4000 3000 2000 1000 0 0 190 380 570 760 950 cable length [m] Figure 4. PD phase-resolved pattern and the PD mapping of discharge activity in the PILC cable insulation due to mechanical damage at 85m location. in a closure time of <1µs. Now series of voltagecycles starts to oscillate with the resonant frequency of the circuit: f =1/( 2π * √L*C ) , where L represents the fixed inductance of 0.8 H of the air core and C represents the capacitance of the cable sample. PD activity Figure 5. Laboratory results of VHF PD measurement of discharge activity in a XLPE cable termination: artificial defect. The PD signals, which are ignited during one or more oscillating waves and are detected by the system, can be processed for two purposes. Firstly, each of the PD pulses can be analysed for reflections using travelling wave analysis (figure 3). Statistical evaluation of PD signals obtained after several oscillating waves can be used to evaluate the location of discharge sites in the power cable (figure 4). ms finally resulting in a phase-resolved PD pattern, known from off-line PD detection and recognition. 2.2. VHF PD Analyzer (on-line) In contrast to off-line PD diagnosis, where the cable sample is energized by an external power supply by applying an on-line diagnosis the cable sample is energized by the power line frequency of the network. To detect PD pulses and to suppress the external disturbances present in the network a VHF PD detection system can be used [7]. In particular, a high frequency split-core current transformer (HFCT) connected to the earth of the cable section has been used. This 5 MHz HFCT is connected through a coax-cable to the spectrum analyzer (HP-8590L). To control the SA and to process the data, the SA was connected via a serial data-cable to a computer, see figure 2. Using spectrum analysis [8] the distinction between external disturbances and cable section internal PD have been obtained, see figures 5-6. In particular, using difference spectrum and by tuning the SA to frequency ranges where the presence of disturbances was minimal, typical phase-resolved PD pattern could be measured and analyzed for the presence of internal discharges, see figures (5-7). For this purpose, the SA which Figure 7. Laboratory results of a PD phase resolved PD pattern measured at a PD free cable section. coil (HFCT) termination high voltage cable splice spectrumanalyser Test transformer IEC 60270 coupling capacitor 1nF PD spectrum Noise spectrum Figure 8. The measuring set-up as used in the laboratory for VHF PD detection system. Using 5m and 3m long XLPE cable samples, different artificial defects (typical examples of bad workmanship) have been investigated. 3. LABORATORY MEASUREMENT RESULTS PD activity Figure 6. Laboratory results of VHF PD measurement of discharge activity in a XLPE cable splice: artificial defect. has to been set to the ‘zero-span’ mode on a certain center frequency (CF), triggers to the 50 Hz voltage wave of the cable and makes sweeps of 20 The PD activity of two different cable samples was measured in the way as described above (figure 8): 1. a bad installed field grading in a splice 2. interruption between the semi-conductive layer and the field grading of the termination. Using IEC60270 PD detection at test voltage < 10kV the presence of discharges was confirmed. Using the VHF PD detection system the discharge activity has been also confirmed. In figure 5-6 the frequency spectra as well as the phase-resolved PD patterns of the PD activity are shown. It follows from these results that: 1. the PD-activity only appeared in the frequency spectrum range from about 10 kHz to 10 MHz 2. the best signal to noise ration was gained when the SA resolution bandwidth was set to 100 kHz. To be sure that the increase (in the range of about 1 MHz to 5 MHz) in the frequency spectrum is caused by internal PD activity, the phase-resolved PD pattern has been used. As a result, cable internal PD-activity shows stable pattern occurring at several frequencies, see figures 5-6. To check the sensitivity of the VHF PD detection a 10 kV XLPE cable containing discharges was stressed at different voltage levels. The PD-level was detected by means of an IEC 60270 PDmeasurement system. As a result, using VHF PD detection a sensitivity starting at 200pC…400pC has been obtained. These values of 200 to 400 pC is obtained by doing several measurements and calculating the mean. Figure 10. OWTS PD system during off-line diagnosis of a 10 kV PILC cable section; location Alkmaar, The Netherlands. spectrum analyser For of-line measurements the set-up as shown in figure 10 has been used. For this purpose the cable was disconnected from the network and the OWTS unit consisting HV coil was connected to the cable section. The results of testing several cable sections using both measuring methods confirm that when the online VHF PD detection system did not detect any PD activity also using off-line OWTS method no PD activity has been observed. On the contrary, the on-line presence of PD phaseresolved patterns could also be confirmed by the off-line OWTS method. As an example, the results of an 849m long PILC 10kV cable section consisting discharges in a oil-filled joint at 120m location are shown. 1. On-line using VHF PD detection system; for measuring results example see figure 11 2. Off-line using OWTS® 25 analyzer; for measuring results example see figure 12 It follows from these results that the presence of a particular discharge site, e.g. oil-filled joint produced discharges can be detected using both methods. In particular, using on-lie detection rough indication about discharging site is provided whereas using off-line detection the location as well as the severity of the discharges can be determined. 10 kV PILC cable terminations and the HFCT sensor Figure 9. VHF PD detection system during online PD detection of a 10 kV PILC cable section. The HFCT sensor is temporally installed in the earth circuit of the 10 kV PILC cable termination; location Purmerend, The Netherlands. 4. PRACTICAL MEASUREMENT RESULTS For the on-line PD VHF field measurements detection circuit as shown of figure 2 has been used. In figure 9 the on-lie connection of the PD detection system to the cable section is shown. In particular, the location of the HFCT sensor was of importance to have sensitive PD coupling path. Figure 11. VHF PD on-line detection measuring results obtained on an 840m long 10kV PILC cable showing discharges in a oil-filed joint, see also figure 12. (a) (b) As a result, combining the less sensitive on-line PD detection and high sensitive off-line diagnosis e.g. OWTS can be of benefit by insulation condition assessment of distribution power cable networks. In particular, in combination with failure history and the position in the network, the on-line PD detection could be useful by prioritising particular cable sections for an off-line in-depth analysis. To complete this work, more systematic investigation is necessary resulting in the following knowledge. 1. Transferability rules for both methods e.g. voltage and momentary power load dependence of discharging sites, and their online and off-line detection ability. 2. On-line detection criticality limits for applying off-line diagnosis. 3. Based on 1) and 2) PD off-line diagnosis knowledge rules for condition assessment of cable insulation. 6. REFERENCES Figure 12. OWTS off-line detection measuring results obtained on an 840m long 10kV PILC cable, see also figure 11: a) PD phase-resolved pattern obtained at 10kV b) PD mapping showing PD concentration of 7.000 pC at the 120m location (oil-filled splice). 5. CONCLUSIONS In this study two diagnostic methods for distribution power cables in service have been investigated. An on-line PD detection has been studied and compared with an off-line PD diagnostic method. Based on this study with regard to on-line PD detection the following can be concluded: 1. To detect on-line PD activity in distribution power cable networks, the VHF detection technique through the earth circuit of the power cable is possible. 2. To suppress disturbances and noises present during on-line PD measurements spectrum analysis can be used. 3. To detect PD signals of discharging defects the phase-resolved patterns can be used. 4. The sensitivity of an on-line VHF PD detection circuit is in the range of few hundreds of pC’s. Comparing both methods provides the following conclusions: 1. Using both methods no PD have been detected in discharge free cable sections. 2. Using both methods discharges could be detected in cables containing discharging sites: e.g. bad workmanship in XLPE accessories, aging effect in PILC accessories. [1] R. Plath, W. Kalkner, I. Krage, Vergleich von Diagnosesystemen zur Beurteilung des Alterungszustandes PE/VPE-isolierter Mittelspannungskabel, Elektrizitätswirtschaft, Jg. 96 (1997) [2] E. Gulski, J.J. Smit, P.N. Seitz, J.C. Smit, PD Measurements On-site using Oscillating Wave Test System, IEEE International Symposium on EI, Washington DC, USA, June 7 - 10, 1998 [3] E. Gulski, F.J. Wester, J.J. Smit, P.N. Seitz, M. Turner, Advanced PD Diagnosis of MV Power Cables using Oscillating Wave Test System, IEEE EI Magazine March/April 2000, Vol. 16 No.2 [4] E. Gulski, F.J. Wester, P. Schikarski, Insulation Condition Assessment of MV Power cables Using PD Detection And Analysis At Oscillating Voltages, paper 2.3 at ERA HV Plant Life Extension Conference, Brussels, 23-24 November, 2000 [5] F.J. Wester, E. Gulski, J.J. Smit, Condition Based Maintenance of MV Power Cable Systems on Basis of Advanced PD diagnostics, paper in this proceedings [6] C. Walton, Regulatory and Business Drivers For Reducing Underground Cable Failures, paper 3.6 at ERA HV Plant Life Extension Conference, Brussels, 23-24 November, 2000 [7] J.P. Zondervan, E. Gulski, J.J. Smit, T. Grun, M. Turner, A New Multipurpose Partial Discharge Analyzer For On-site and On-line Diagnosis of HV Components, 11th ISH, London, 23-27 August, 1999 [8] S. Meijer, E. Gulski, J.J. Smit, R. Brooks, Digital Analysis of Multiple Faults in GIS, 1999 IEEE Iner. Symp. On EI, Arlington, USA, June 7-10, 1998