Advanced Partial Discharge Measuring System for Simultaneous Testing of Cable Accessories S. Meijer*, E, Gulski', P.D. Agoris', P.P. Seitz2, T.J.W.H. Hermans3 and L. Lamballais3 'Delft University of Technology, High-voltage Technology and Management PO Box 5031, 2600 GA Delft, The Netherlands 2Seitz Instruments AG, CH-5443 Niederrohrdorf, Switzerland 3Prysmian Cables and Systems B.V., P.O. Box 495, 2600 AL Delft, The Netherlands * E-mail: s.meijergtudelft.nl signals, as is commonly used for on-site/on-line measurements in e.g GIS [I]. The first step in making a measurement is capturing the frequency spectrum. Based on this spectrum, a certain centre frequency which represents PD activity with the highest signal-to-noise ratio is selected, see figure 1. Secondly, a spectrum analyser can also analyse the coupler's signals in the time domain, resulting in similar phase-resolved PD patterns that are obtained with a standardized measuring circuit. Such phase-resolved PD patterns offers the possibility to recognize a certain type of defect and to discriminate between insulation defect and noise. Abstract: In this paper measuring results of partial discharges in power cable accessories are described. For the purpose of measuring simultaneously partial discharge activity in three or more accessories during an acceptance test, a wireless partial discharge detection system was developed. Therefore, each power cable accessory is equipped with either an internal or external sensor to decouple the partial discharge signals. The system uses narrow-band detection and is described in this paper. The computers controlling the measuring units are connected via a wireless network which allows communication between the different units and the main computer situated at the voltage source site. Moreover, effective real time noise-suppression is possible by tuning the system to a noise-free frequency band. Practical examples of this system are shown based on several measurements performed in the field. 50. 00 - o 40.00 a 30.00 - ':5 INTRODUCTION E <J Partial discharges (PD) may occur in power cables as a result of insulation defects. The origin of discharging defects can be related to poor workmanship (new or repaired cables) or to service related insulation degradation effects. In all these case, due to local field enhancement till breakdown, discharge pulses are produced at the defect site. It is known that each of these PD pulses consists of energy frequencies up to hundreds of MHz. < m Ii < m fM 4 f - 1 - : t, A: ) 01i T 10.00 u 0.00 _ f ~ ~ 0 100 F A J X, 4 ~ J W m X Hi ; L L _ - - + Vl. ' 1X 9-4¢ 4X@IV143 ~~~~ 200 300 400 500 Fre quency [M Hz] Fig. 1. Example of a signal-to-noise ratio frequency spectrum as has been measured on-site. Here three frequency ranges show additional activity which could originate from partial discharges. The basic parts of the VHF/UHF measuring setup are shown in figure 2. It contains: A sensor 1) A 30 dB VHF/UHF pre-amplifier 2) A coaxial cable 3) A spectrum analyser (SA) 4) A computer 5) Different types of sensors can be used. In practice, capacitive and inductive sensors are used, which can be either internal or external sensors. Figure 2 shows an internal inductive sensor, installed in the cable. To have better sensitivity, the signals are amplified by a 30 dB pre-amplifier. Using a spectrum analyser, the frequency spectrum is measured, which can contain background noise signals and PD signals. Finally, the Therefore, to achieve a high quality of the installation of power cable accessories, partial discharge detection during the on-site acceptance test is more and more becoming an important issue. The presence of partial discharge activity can be used to assess the condition of the cable accessory and proper action can be taken. To enable simultaneous monitoring of all accessories during such a voltage test, the joints and terminations are equipped with wireless VHF/UHF PD detectors. VHF/UHF PARTIAL DISCHARGE DETECTION As stated before, PD pulses consists of frequencies up to hundreds of MHz. Consequently, a spectrum analyser has been used to capture these high-frequency 1-4244-0189-5/06/$20.00 ©2006 IEEE. 20.00 - 687 Authorized licensed use limited to: Cornell University. Downloaded on April 16,2010 at 14:33:57 UTC from IEEE Xplore. Restrictions apply. o Phase 1 a) 100 -a 1u50 E 0 0 150 Phase 2 a) 100) t -o Fig. 2. VHF/UHF PD detection system as installed in the field consisting of (1) internal inductive sensor, (2) a coaxial cable, (3) a VHF/UHF pre-amplifier, (4) spectrum analyser and (5) PC. 0) 9u E 50 0 data is stored on a PC, which contains software to analyse and further interpret the measured data. 0 150 Phase 3 FIELD MEASUREMENT 1: 127kV CABLE U) 100 Partial discharge measurements were performed during the after-laying test of two 127 kV cable systems. Measuring results as obtained at 130 kV (1.7UO) at the cable terminations of one cable system are shown in figure 4. PD measurements were performed using an external PD sensor around the ground-wire of the cable termination. The test voltage was applied during one hour. -a 0) Cu E 0 aL Phase angle Fig. 4. Examples of phase resolved PD patterns as obtained at the terminations of three phases of a 127 kV cable system. First of all, from the measuring results can be concluded that the noise level was about 10-15 pC in case of all three phases which corresponds quite well to the results of the sensitivity check which was performed before the actual measurements [2]. During the sensitivity check, artificial pulses are injected into the cable termination and the response is measured using the PD detection system. An example is shown in figure 3. From the obtained results can be concluded that the response is different for each measuring frequency. Moreover, the noise level that can be reached at different measuring frequencies is different as well. In the results shown above, the lowest calibration impulse which can be detected is 10 pC for most frequencies, except for 7 MHz (20 pC is still detectable) and 20 MHz (50 pC is detectable). This means that the transfer function for these two frequencies is not so good and should not be used during the measurements. Furthermore, a logarithmic relation between the calibration impulses and the response can be seen. Based on these relations it is possible to estimate the actual noise level and the actual PD level. The presence of a insulation defect as indicated by the PD pattern detected in phase 2 has not yet been confirmed by visual or other inspections. 10000 1000 5; a. 100 a) 10 en 50 FIELD MEASUREMENT 2: 150 kV CABLE -.5 MHz _ 7 The acceptance test of two 2-phase circuits was performed by applying a test voltage of 2.5UO=220 kV during 10 minutes. The cable system was about 800 meters long and simultaneous measurements were performed at the terminations at both sides. The measuring results as were obtained at the far side (i.e. not at the site of the voltage source) of one circuit, in MHz 20 MHz 3 1 10 20 50 100 Calibration Impulse [pC] 200 30 MHz 50 MHz 75 MHz 500 Fig. 3. Response of the VHF/UHF detection system to calibration impulses at different measuring frequencies. 1-4244-0189-5/06/$20.00 ©2006 IEEE. 688 Authorized licensed use limited to: Cornell University. Downloaded on April 16,2010 at 14:33:57 UTC from IEEE Xplore. Restrictions apply. Phase Red CF=3 MHz CF=123 MHz CF=320 MHz Phase Blue Fig. 5. Measuring results as obtained at one side of the cable system at 2.5 Uo. both phases, are shown in figure 5. The following observations can be done. First of all, the thyristor pulses coming from the voltage source are still visible at the far side, though strongly reduced and only at the lower center frequencies of the SA. Secondly, at the lowest center frequencies, corona activity in air is visible. The patterns as were obtained at these 3 MHz and 123 MHz for both phases look remarkably similar, indicating system dependent effects. Therefore, an advanced PD detection system was developed, consisting of three VHF/UHF measuring units as described before and as shown in figure 2 and which were able to communicate to a main computer via a wireless communication network. Having this main computer next to the test engineering controlling the voltage source improves the possibility to act if required. Figure 6 shows the hardware of the wireless system. At 320 MHz, differences can be observed. No disturbances from the thyristors can be seen and, as expected, the corona activity is not clearly visible at this high frequency as well. As a result, it can be concluded that the spectrum analyzer, operating as tunable filter, is a very effective means to reject noise real-time. Nevertheless the patterns as measured for both phases are different as well: in phase red a clear PD pattern is visible, in phase blue, some activity can be seen. However, due to the fact that the inception and extinction voltages are around 2.5 U0, no actions were required. FIELD MEASUREMENT 3: 380 kV CABLE Overview of one side of a 380 kV cable Fig. 6. system. The top inset shows the antenna which is used to make the wireless communication with the main computer and other measuring units possible. The lower inset shows a detail of the internal inductive PD sensor. The sensitivity check as described in [2] was performed. Using a fast impulse generator which was calibrated in the laboratory, artificial PD pulses were Finally the after laying test of two 380 kV XLPE cable circuits is described. A voltage of 1.7Uo was applied during one hour. During this one hour of testing, partial discharge activity had to be monitored for at the accessories: two terminations and one joint. For that purpose, each accessory was equipped with an internal inductive PD sensor. Prior to the measurements, a sensitivity check was performed to determine the performance of each sensor and to obtain an estimation of the relation between the detected uV-level and the PD magnitude [2]. injected into each cable termination. The VHF/UHF measuring unit was tuned at different center frequencies and the response (or transfer function) was measured in [tV's. As a result, 12 cable terminations were investigated in this way. Based on these results, the average transfer function and the spread in the results were available, see figure 7. Moreover, the measurements at each accessory had to be performed simultaneously and of course any change in PD magnitude or PD pattern (if present) had to be reported immediately in order to take the required actions. For that purpose, it would be beneficial to have all data available at the voltage source site. 1-4244-0189-5/06/$20.00 ©2006 IEEE. As can be concluded from figure 7, mainly two frequency ranges show to be of interest: ranging from 60-200 MHz and 320-380 MHz. Although the response in the first frequency range is higher 689 Authorized licensed use limited to: Cornell University. Downloaded on April 16,2010 at 14:33:57 UTC from IEEE Xplore. Restrictions apply. 0.w,^Cl~ 10At;-k.x>w 10Rtqs.l(4-@ visible, which proved the applicability of the system. compared to the second, also the spread in the amplitude is much higher. As a result, only the frequency range between 160-200 MHz is of importance and in this range, about 25 [tV represents 1 pC of PD magnitude. In the second range, a relation of about 7 [tV per pC was found. 80.00 60.00 (D 40.00 E 20.00 < 0.00 0 200 100 300 500 400 No partial discharge activity in the accessories detected during the test. CONCLUSIONS Based on the investigations described in this contribution, the following can be concluded: 1) Using a wireless communication network, measuring data obtained at three different cable accessories could simultaneously be displayed and analyzed. 2) Using a narrow-band VHF/UHF detection system, a noise-level of about 10 pC was reachable. 3) By changing the center frequency of the tunable filter, disturbances coming from thyristor pulses and corona in air can be effectively suppressed. 600 Frequency [MHz] Fig. 7. Average transfer function and its spread of 12 cable terminations. ACKNOWLEDGMENT Examples of measuring results are shown in figure 8. It can be observed that a noise-floor of around 10 pC was reached at all accessories. Large random disturbances were observed, in particular at termination 2. However, the fact that these were just single random disturbances could easily be concluded from analysis of the phase-resolved PD patterns. The measurements shown in figure 7 were performed at a center frequency in the second frequency range. During the rise of the test voltage the measuring units were tuned at other frequencies as well, to ensure that no incepting PD source was missed. Especially at the lower center frequencies, pulses originating from corona at the terminations and originating from the thyristors were Termination 1 70 60 2S -4 E 30 3- REFERENCES [1] o 0 Joint 70 70 60 50 -c- c 40 Termination 2 60 -4 50 -S -- 40 0410 - 20 30 40 30 = co co E 10~~~~~~~~~~~~~I0 50 1 E 2 020-- -20 1.01 0 60 10 30 20 40 50 0 60 - X T X 1 co: 30 E 20 10 0 60 a50 U430 - (D 70 60 50 )3 --- --E E 20 30 40 50 60 L - .=40 30 20 Time [minutes] 70 0, - Time [minutes] Time [minutes] 70 () 60 a. 50 B.F. Hampton, R.J. Meats, Diagnostic measurements at UHF in gas insulated substations, IEE Proceedings, Vol. 135, Pt. C, No. 2, pp. 137-144, 1988. S. Meijer, P.D. Agoris, E. Gulski, P.P. Seitz, T. Hermans, L. Lamballais, Simultaneous Condition Assessment of Accessories of Power Cables using a Wireless VHF/UHF PD Detection System, Conference Proceedings CMD2006, Changwon, Korea, 2006. [2] 20 0)2010 a. E The authors acknowledge TenneT TSO B.V. for their support during the measurements. 40 40 were - 20 - I 10 0 60 120 180 240 Phase-angle 300 360 1-h 0 k 60 - 120 180 240 Phase-angle 300 360 0 60 120 180 240 Phase-angle 300 360 Fig. 8. Measuring results simultaneously obtained at the three cable accessories during the acceptance test of one cable: the PD magnitude versus time for one hour testing and examples of phase-resolved patterns obtained during several cycles of the test. 1-4244-0189-5/06/$20.00 C2006 IEEE. 690 Authorized licensed use limited to: Cornell University. Downloaded on April 16,2010 at 14:33:57 UTC from IEEE Xplore. Restrictions apply.