Abstract: Cross-bonded cable models are compared against their versions with normal bonding (no bonding) for propagation characteristics, frequency, and time-domain responses. Contrary to no-bonding cables, coaxial modes cannot propagate in cross-bonded cables. Instead, intercore modes propagate, being always accompanied by intersheath modes. The latter property cannot be observed if a cross-bonded cable is modeled with its homogeneous equivalent. It is shown that sheath cross-bonding induces intersheath modes circulating between grounding points and impacts on the cable response at resonant frequencies. Alternate models for cross-bonded cables, including both homogeneous and nonhomogeneous equivalents, are compared based on frequency- and time-domain responses, including a field test on a 225 kV 64 km cable with 17 major sections.

Abstract: This study covers research and development efforts to develop, design and test a novel very low frequency cosine-rectangular voltage generator (CRVG) specialised not only for a withstand test, but also a partial discharge (PD) test of standard medium voltage (MV) cross-linked polyethylene power cables. The system schematic and operating principle of the proposed system are presented and described. The cosinusoidal polarity reversing process is realised through L–C series resonance. A charging and discharging switch is developed by means of an insulated gate bipolar transistor series-connected semiconductor switch. A laboratory prototype is assembled and tested. Maximum output voltage of the proposed CRVG is measured as 20 kV, which is in accordance with the expected acceptance test voltage level given in IEEE standard 400.2. Finally, the proposed system is applied to PD detection of a 300 m cable circuit with a built-in artificial defect, and the PD locating results are presented, proving the system's feasibility.

Abstract: Tangent delta (TD) measurements using very low frequency (VLF) sinusoidal waveform in aged cable system have been proved sensitive to water tree degradation and insulation aging. Whereas, the application of VLF in acceptance tests on new cable systems of medium voltage is still on research and development. This paper presents an experimental study of the capability of the main VLF testing methods for newly installed 10kV XLPE cable systems in acceptance tests. Compared with oscillation wave excitation, the different waveforms including sinusoidal (Sinus) and cosine-rectangular (CR) of VLF voltage were employed to stress the nine samples of artificial defects of different types. Various characteristics of TD measurement and partial discharge (PD) measurement were analyzed. The results showed that VLF-Sinus voltage has a distinctive ability of detection for a wet or lossy accessory through TD or PD measurements. It is recommended to employ a combination of the monitored VLF-Sinus withstand with TD measurement and the PD measurement under damped alternative current (DAC) or VLF-CR as field diagnostic tools in acceptance test for newly installed cables.

Abstract: System reliability and improving System Average Interruption Duration Index (SAIDI) and System Average Interruption Frequency Index (SAIFI) performance are of utmost importance for utility companies today. The first step to systematically achieve a reliable distribution network which contains a high percentage of Medium Voltage underground cables is Very Low Frequency (VLF) testing. VLF testing on cable networks identifies critical defects during a scheduled test when the circuit is de-energized intentionally. At the same time the n-l principle ensures continued supply to customers. The next step beyond VLF testing is diagnostics testing, namely offline Partial Discharge (PD) analysis in the field. There are various technologies available, but real-world comparisons have shown significant differences in performance between the different types of excitation voltages. This paper will illustrate those differences and give recommendations for best testing practices.

Abstract: Transient enclosure voltage (TEV) induces a relative potential difference between grounding conductors and then couples to secondary cables and may cause malfunction and even damage to secondary equipment. This study investigates the coupling mechanism of TEV to secondary cables from an experimental point of view and presents a cable model suitable for calculation. Experiments are carried out on test platforms established in a laboratory. The experimental results indicate that the magnitudes of port voltages are roughly 1.5‰ of TEV magnitudes, and the waveform of the start port voltage has the same profile but in antiphase with that of the end port voltage. Moreover, only common-mode voltage with no differential-mode voltage exists for the twin core cable. Based on these results, an equivalent circuit for a secondary cable is proposed. This circuit can generate similar waveforms as those measured in the experiment. The experimental and theoretical results presented in this paper should prove helpful for evaluating the interference severity of secondary cables resulting from TEV for the electromagnetic protection of secondary ports.