* Brugg Kabel AG, Klosterzelgstr. 28, CH-5201 Brugg, E-mail: weissenberg.werner@brugg.com

ON-SITE PD DETECTION AT CROSS-BONDING LINKS OF HV CABLES W. WEISSENBERG*

BRUGG KABEL AG

(Switzerland)

F. FARID

GIZA CABLES CO.(Egypt)

R. PLATH

IPH BERLIN (Germany)

K. RETHMEIER, W. KALKNER

TU BERLIN (Germany)

On-site PD measurements on HV cables have to concentrate on the cable accessories because there is a remaining risk for assembling faults on site. PD sensors with an appropriate coupling behavior to accessory-internal PD give sensitivities of a few pC or even better. Unfortunately, two main reasons prevent the general use of PD sensors in cable accessories. First of all, the costs for PD sensors have to be balanced with the costs of the accessories, importance of the cable link, consequential costs for outages etc. This is the reason why PD sensors were mainly used for EHV cable systems. The second reason is limited accessibility: the PD sensor cable at the accessory has to be connected to a PD detection unit. Accessibility is much more difficult for direct buried cable systems than for cable terminations and for tunnel-laid cable systems: the sensor cable must pass the ground and end up in a box on the surface to provide access. This solution causes additional costs and new problems like sealing the sensor cable against humidity, capability to withstand sheath testing etc. By looking for alternative access to PD signals from cable joints of long cable systems, a very simple solution proved suitable: detecting PD at the cross-bonding links. To investigate the high frequency propagation of PD pulses in cross-bonding links, computer simulations and laboratory measurements were done. On-site PD measurements at cross-bonding links of a 220-kV-XLPE cable system showed unexpected high sensitivity. Using HF transformers for PD coupling and appropriate signal processing led to PD sensitivities of a few pC or even better. The use of cross-bonding links for on-site PD measurements is suited for direct buried systems, has no impact on the cable system, needs no PD sensors in the cable joints, offers a low-cost solution and so opens a wider range of applications in cable testing. Keywords: High Voltages Cables – Cross-Bonding Links – Partial Discharges – Unconventional Decoupling – On Site 1. INTRODUCTION After installation testing of HV and EHV cable systems is actually carried out by AC voltage tests according to IEC 60840 and IEC 62067, respectively. These standards offer alternatives for the AC

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Session 2004B1-106

test procedure: besides testing with test voltage levels higher than rated voltage, it is also possible to test with Uo/24h. Due to the fact, that the most part of possible defects in cable systems will cause partial discharges (PD) in accessories after installation under AC stress, combined AC resonance testing and sensitive PD measurements will result in best test efficiency [1]. AC test systems provide free adjustable test voltage levels, so sensitive PD measurements would discover defects at lowest possible voltage level, resulting in minimized risk for breakdown during testing. Nevertheless, a Uo/24 h-testing benefits from sensitive PD measurements, too. In practice, sensitive PD-measurements on long HV cable lengths can only be realized if either PD sensors have been installed inside the accessories (as in many new installed EHV cable systems) or sensitive PD decoupling is possible at joints or in cross-bonding links, if existing. The latter possibility would be of advantage for technical and economical reasons for both, new installed cable systems and especially cable systems already in service. 2. PD MEASUREMENTS ON LONG HV/EHV CABLE SYSTEMS HV cable lengths are already routine tested at the manufacturer, including high-sensitive PD measurements in screened laboratories. The components of prefabricated accessories are also pre-tested, but there is a remaining risk for assembling faults on site. Consequently, on-site PD measurements have to concentrate on the cable accessories. 2.1 PD Detection at the Cable End To achieve high sensitivity for on-site PD measurements on long cable lengths, conventional PD detection at the cable end is clearly not the best way. The sensitivity for PD detection at the cable end depends strongly on the length of the cable system and will give acceptable results only for cable lengths up to 1 or max. 2 km. This method is very sensitive to external disturbances, but not to PD from defect sites at long distances. The damping behavior of the cable is mainly determined by the thickness and conductivity of the semicon layers. A strong attenuation can be found at conductivities of 1...10 S/m, which is in the range of typical values of semicon layers of high voltage XLPE-insulated cables. Increasing semicon thickness leads to higher attenuation [2]. 2.2 PD Detection with Sensors inside Cable Accessories In order to get maximum PD sensitivity, PD sensors installed inside joints and terminations are recommended. HF or UHF PD sensors inside the accessories take full advantage from cable attenuation. Noise from the end of the cable (just like PD signals) is increasingly attenuated with propagation along the cable length. Therefore, sensor PD sensitivity on site increases with longer distance from the end. Small PD signals from one joint will usually not interfere with PD from the next accessories, either because cable attenuation pushes small PD signals below noise level (UHF PD detection) or because propagation time measurements clearly separates PD sources. PD sensors with an appropriate coupling behavior to accessory-internal PD will give best performance with sensitivities of a few pC or even better [3]. Experience with PD sensors was made especially on EHV cable systems, e.g. [4].

Unfortunately, two main reasons prevent the general use of PD sensors in cable accessories. First of all, the costs for PD sensors have to be balanced with the costs of the accessories, importance of the cable link, consequential costs for outages etc. This is the reason why PD sensors were mainly used for EHV cable systems. The second reason is limited accessibility: the PD sensor cable at the accessory has to be connected to a PD detection unit. Accessibility is very easy to achieve for cable terminations as well as for tunnel-laid cable systems. However, for direct buried cable systems it is difficult: the sensor cable must pass the ground and end up in a box on the surface to provide access. This solution causes additional costs and eventually new problems like sealing the sensor cable against humidity, capability to withstand sheath testing etc. 2.3 PD Detection at Cross-Bonding (CB) Links of HV/EHV Cables By looking for alternative access to PD signals from cable joints of long cable systems, a very simple solution proved suitable: decoupling PD at the cross-bonding link-boxes. Long HV and EHV cable systems are usually designed as cross-bonding systems to minimize screen losses and to limit voltage rise. Using cross-bonding links for on-site PD measurements is suited for direct buried systems, has no impact on the cable system, needs no PD sensors integrated in the cable joints, offers a low-cost solution and so opens a wider range of applications [5], [6] in cable testing after installation as well as on service-aged or repaired cable systems. 3. PD PROPAGATION ON CROSS-BONDING CABLE SYSTEMS To analyze the possibility of decoupling PD-pulses in cross-bonding links computer simulations were done. In addition, for laboratory measurements, different test set-ups were developed to investigate the high frequency propagation of PD-Pulses in cross-bonding links.

Fig. 1. PD propagation in CB links (schematic) Fig. 2: Injection of calibration impulses in L1, detection at CB links

In a first basic approach a LV test set-up using 50 Ω coaxial measuring cables was built up. The CB-joints were realized by ohmic resistors to simulate the discontinuity of the wave impedance for propagating impulses. The interrupted cable sheath has been cross-bonded with the corresponding sheath of the other phases in a separate cross-bonding box. In the cross-bonding wires of the joint, the PD-signals injected by a calibrator in one phase of the cable model could be detected and allocated to its origin (Fig. 2). To improve the validity of the LV model, especially respecting the attenuation of the semicon layers, the 50 Ω cables were substituted by MV cables with laminated sheath. As a result the cross talking effects of the cables laid parallel on the ground were minimized. The main measuring results concerning the propagation of PD impulses could be confirmed. Finally, a HV test set-up (Fig. 3) was constructed using similar components as applied in a 220-kV-cable project in Cairo. Especially the CB joints and the coaxial cross-bonding wires were structurally identical to achieve the most possible consistency with the expected on-site conditions. To enforce PD signals, one joint was mounted with an artificial PD defect. Also a new multi-channel PD-measuring

device mpd520 was integrated for calibration matters [4]. A PD measurement was performed successfully. However, the exact calibration was highly frequency dependent and mainly influenced by the set-up (Fig. 4). According to this it was decided to repeat the calibration for the Cairo cable project on site.

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measured at the cross-bonding box in the lab 4. ON-SITE INVESTIGATIONS Experiences with decoupling PD at different types of joints (transition, insulating, cross-bonding) without integrated PD sensors were gained from measurements on XLPE- as well as oil-paper-insulated HV and EHV cable systems. 4.1 Transition Joints and Insulating Joints Transition joints are used to extend existing oil-paper-insulated cable systems with new XLPE-insulated cables. PD impulse attenuation of oil-filled cables is several times higher than that of XLPE cables, so PD testing from the (XLPE) cable end can be very difficult, even for shorter (XLPE) cable lengths. For sheath corrosion testing, transition joints usually provide external screen connections, which are very useful for PD detection. Furthermore, PD signal propagation through transition joints causes not any crosstalk effects, like in cross-bonding links, making evaluation easier. Fig. 5 shows the frequency response of an insulating joint, which is very similar to the signal transmission behavior of the transition joint.

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of a 400-kV- insulating joints of a XLPE-insulated cable

In the analyzed frequency range there is only a minor deviation in the decoupling attenuation from 100 kHz up to 10 MHz for both cases. Therefore differences in the technical realization of the cross-bonding connection (respectively the capacities between the screens) are of minor influence on the results for measurement frequencies up to about 10 MHz. 4.2 400 kV Cross-Bonding Joints in Joint Chambers Investigations of PD impulse propagation in cross-bonding links of an oil-paper insulated double 400-kV-cable system (Fig. 6) confirmed the suitability of this PD detection method. Inductive sensors (HF transformers) were directly installed on the CB links of two three-phase joints groups and synchronized by fiber optic cable. The distance in between one substation and the first joint group as well as between the first and second joint group was approximately 450 m. Between the second joint group and the far cable end the distance was approx. 9 km.

Fig. 6. PD calibration at CB links of

400-kV-joints Fig. 7 External PD, decoupled at CB links of 400-kV-joints of

two cable systems (about 450m distance to power plant) At 4 MHz centre frequency, the attenuation was 25 dB/km. Fig. 7 shows the PD finger prints of the first joint groups (M18, close to substation) of both three-phase cable systems (no. 907 and 908) obtained at on-line operation (both sides connected, load condition). The similarity of the PD fingerprints and the detected PD levels were caused by similar PD sources. By comparison of PD propagation time and impulse attenuation depending on the switching state (single or both sides connected) it was possible to clearly identify the nearby substation as the source of detected PD. When the far-distance substation energized the cable system, only noise of a few pC was detected due to strong attenuation of any external interference. 4.3 220 kV Cross-Bonding Joints On-site online PD measurements at the cross-bonding boxes of a sensor-less 220-kV-XLPE cable system showed high sensitivity, confirming preliminary investigations. Using HF transformers for PD decoupling and appropriate signal processing led to PD sensitivities of a few pC. The HF transformers were installed only temporarily during the on-site measurements and afterwards removed for further use. Fig. 8 shows the installed HF transformer inside a cross-bonding box. For safety reasons, the cable system was switched off for installing and removing the HF transformers. Due to transient overvoltages during switching operation, the PD measurement has to operate potential free. Battery power supply, communication and synchronization via fiber optic cable as well as 10-kV-spacer for CB-to-ground insulation provided safe operation of the PD measurement system.

Fig. 8: CB Box with mounted HF transformer (inductive PD sensor)

Fig. 9: On-site measurement set-up 220-kV-XLPE-insulated system

To minimize network interference, one substation was disconnected from the cable system under test, leaving the second system still in service. Corona protection spheres were mounted on the top of the outdoor terminations to avoid PD on the bushing bolts. 2000-pC-calibration impulses for re-checking the frequency behavior were injected at the outdoor terminations. Figure 10 shows the test set-up for this case. The result of re-checking frequency dependency on damping is shown in fig. 11, respectively.

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behavior on site Fig. 11: PD attenuation and phase-to-phase cross-

talk relations vs. frequency (test set-up fig. 10) Results from figure 11 (on site) delivered high PD sensitivity, which was not expected from the results of the test set-up in the high-voltage laboratory (see Fig. 4). The results of phase-to-phase cross-talk led to the choice of 4 MHz mid-frequency for PD measurements to have approximately equal sensitivities for all three joints within one joint group. For higher measurement frequencies, under- or over-estimation of PD levels of different phases is probable. Lower measurement frequencies would cause the disadvantage of lower cable damping, resulting in higher external interference from both ends of the cable link. The results of the on-site PD measurements are shown in figure 12. By the use of the synchronized multi-terminal measurement system it is possible to clearly distinguish between external disturbances and possible PD from the accessories. Due to that signal separation it was proved that all joints showed no PD higher than the base noise level of a few pC.

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Fig. 12: Summary of on-line PD measurement results

220-kV-cable system Cairo 5. CONCLUSIONS As could be shown in laboratory and on-site investigations, sensitive PD-decoupling is possible at insulating joints and cross-bonding links, respectively, even without integrated field-sensors. Using HF transformers at the cross-bonding links for PD decoupling and appropriate signal processing led to PD sensitivities of a few pC. The cross-bonding link-box is usually accessible also in case of direct-buried cable systems. The use of inductive sensors is possible without any interference and therefore has no impact to the cable system. 6. REFERENCES [1] W. Hauschild, P. Coors, W. Schufft, R. Plath, U. Herrmann, K. Polster: The Technique of AC

On-Site Testing of HV Cables by Frequency-Tuned Resonant Test Systems, Cigré Session 2002, Paris, France, 2002, paper 33-304

[2] R. Heinrich, K. Rethmeier, W. Kalkner, R. Plath, W. Weißenberg: Sensitive PD detection on high voltage XLPE cable lines using field coupling sensors. Jicable, Versailles, 2003, paper C.8.2.6.

[3] Boone et al.: Partial Discharge Detection in Installed HV Extruded Cable Systems. Technical Report 182, CIGRE, April 2001. Cigré Working Group 21.16

[4] J. Kaumanns, E. Plieth, R. Plath: On-site AC Testing and PD Measurement of 345 kV/2500 mm XLPE Cable Systems for Bulk Power Transmission. Jicable, Versailles, 2003, paper A.8.4.

[5] C. Min, K. Urano, H. Ueno, Y. Yoshino: Investigations of partial discharge measuring method via coaxial bonding wire for direct-buried power cable. T. IEE Japan, 121-B, No. 1, 2001

[6] C. Min, K. Urano, H. Ueno, Y. Yoshino: Verification of partial discharge measuring method via coaxial bonding wire for direct-buried power cable. T. IEE Japan, 122-B, No. 4, 2002.