In practice, High-voltage (HV) cables are occasionally exposed to impulse and superimposed transient conditions, which may initiate partial discharges (PD) temporarily. Whether such PDs persist under AC voltage after the transient conditions have vanished, is at focus in the research described in this paper. Since for cross-linked polyethylene (XLPE) cables the accessories are weak links in the HV cable insulation system, we investigated the PD behavior of an artificial joint defect in a HV cable model under impulse and superimposed voltages. By applying a dedicated PD measuring system it was found that, the impulse and superimposed voltages can initiate PD in the artificial defect, which under local electrical field conditions can persist for some time. The different parameters of the applied voltages have different effects on the PD behavior.@en

The problem of impaired data sets refers to data sets containing a vast majority of unwanted signals than signals of interest. With increased interest in partial discharge (PD) testing with arbitrary waveforms and transients, these kind of data sets are becoming more and more common. Traditional clustering techniques cannot be applied due to big differences in spatial densities of the existing clusters in the data set. This paper contributes a simple yet efficient technique to recognize PD signals from noise and other disturbances. The signal recognition features are based on two specific areas extracted from the cumulative energy signal (CE) of each recorded waveform. These areas weigh up the extent to which the recorded signals have a pulse-like shape. A third feature, defined as a shape factor, extracts additional metrics from the CE signal that serves the purpose of accounting for the factors affecting the computation of the proposed recognition features and threshold for data size reduction. These three CE-based features are used to create a graph from which a real PD can be spotted in large impaired data sets. The performance of this technique is tested using PD measurements from superimposed impulse tests on a 150 kV cable system.@en

Occasionally sporadic defects in high voltage cable insulation may escape from detection by commissioning or maintenance tests. Usually these tiny defects will not initiate significant partial discharges or cause breakdown in the cable insulation system under normal AC operating condition and normal transient situations. However, nowadays more often complex transient’s behaviour occurs in power systems. For example, in a mixed line and cable system, superimposed transients with large overvoltage can occur due to switching operations. Such transients may have unexpected influences on the partial discharge behaviour and degradation of the cable insulation. This thesis aims to investigate which potential effects such transients have on the insulation condition of the HV cable system, in particular in one of its weakest links, the cable joint.@en

This paper presents a new wavelet analysis approach in partial discharges cable joint measurements in noisy environments. The proposed technique uses the Cross Wavelet Transform (XWT) to separate PD signals from noise and external disturbances in partial discharges measurements in cable joints using two opposite polarity sensors. The partial discharge measurements were performed during impulse and superimposed voltages, leading to a huge amount of noise and pulse shaped external disturbances. The XWT foundations, the experimental setup and the XWT methodology proposed are presented together with the results of the recognition of PD originated in the cable joint. In the experiments, 51,898 signals were acquired, in which 733 were PD signals from the joint and 51,165 corresponded to noise or external disturbances. The XWT performance was studied, finding that 97% of the PD signals were correctly separated by the technique proposed. The results demonstrate the effectivity of the XWT in separating PD signals from noise and external disturbances in this particular measuring system configuration.@en

A partial discharge (PD) measuring system has been deployed in order to identify and measure PD in a high voltage (HV) cable joint under impulse and superimposed voltages under laboratory conditions. The challenge is to enable the detection of PD during the impulse conditions. The method of measurement has been investigated by introducing an artificial defect in the cable joint in a controlled way to create conditions for partial discharges to occur. Next the HV cable system is subjected to AC, impulse and superimposed voltage. Two high frequency current transformers (HFCT) installed at both ends of the cable joint were used to identify PD from the cable joint and to separate PD from disturbance. Transient voltage suppressors and spark gaps are applied to protect the measuring equipment. Band pass filters with selected characteristics are applied to suppress transient disturbances and increase the chance to detect PD during the impulse. PD signals are separated from transient disturbances during data post processing and by means of pulse polarity analysis. The developed system enables the detection of so-called main and reverse discharges respectively occurring during the rise and tail time of the superimposed impulse. The measurement results obtained show the effectiveness of the presented PD measuring system for investigating the effects of voltage transients on a HV cable system in laboratory conditions.

@en

In practice, cross-linked polyethylene (XLPE) power cables can be subjected to alternating voltage with superimposed impulse transients. Such impulse transients may initiate partial discharges (PD) in insulation defects even below AC inception voltage. An initiated PD may persist under AC, which will cause insulation degradation. This paper investigates the PD behavior in MV XLPE cable accessories under impulse transients. Different scenarios of PD behavior are measured, described and analyzed. Based on the results, the effects of impulse transients on PD are summarized.@en

This paper investigates the partial discharges (PD) at artificial defects in a cross-linked polyethylene (XLPE) insulated cable joint under superimposed voltage. The experiments are conducted on a 16-meter long 150 kV commercial XLPE cable, together with a cable joint and two terminations. Defects are fabricated on purpose in the cable joint. The cable system was subjected to a 50 Hz AC voltage, being in between PD extinction and inception values, superimposed with a lightning impulse voltage. Partial discharges are measured by two HFCT sensors at the two ends of the cable joint. The measurement results show that, the impulse voltage could trigger partial discharges. The partial discharge occurrence is mainly influenced by the time period during which the applied voltage is higher than PDIV.@en

This paper investigates the triggering and development of partial discharges at artificial defects in the cross linked polyethylene (XLPE) insulated cable with accessories under superimposed voltage. The experiments are conducted on a 4-meter long 6/10 kV commercial XLPE cable sample, which is assembled with a cable joint and terminations. Defects are fabricated on purpose in the cable accessories. The cable sample was subjected to the superimposed voltage of 50 Hz AC with impulse voltage. Partial discharge activities are measured by a HFCT sensor and analyzed by PDflex. The measurement results show that, the impulse voltage could trigger partial discharges, which might be kept sustained by the AC voltage. The partial discharge activities are influenced by the AC voltage level, which is determined by the PDIV and PDEV.@en

The worldwide demand for electricity, which is steadily increasing, leads to a continuous need for developing and extending the electrical transmission networks. However, the installation of new overhead lines (OHL) faces many challenges due to societal and environmental reasons. One solution that gains widespread public support is the installation of EHV AC XLPE underground cables (UGC). Although this development is quite encouraging from a societal perspective, new challenges might arise, mainly from a technical perspective. This is due to different electrical characteristics of underground cables compared to OHL as well as long repair time in the case of failure in cable systems. These aspects highlight the necessity of investigating power system technical performance issues related to application of EHV cables to prevent any unwanted condition in partially cabled grids. This paper, among various system operation issues, investigates optimum shunt compensation sizing, reliability analysis, and condition monitoring of EHV cable systems. Reactive power compensation by means of shunt reactors should be allocated for long cables to consume their reactive power surplus. It is crucial to have sufficient size of shunt compensation because both undercompensation and overcompensation can lead to undesirable system operation like overvoltage and zero-missing phenomenon. In this work, the shunt compensation sizing of a 80 km double-circuit connection in the Dutch transmission system consisting of series connected OHL and cable sections is studied. The sizing is performed for two load-flow scenarios and based on four sizing criteria and different cable lengths in the case study. Moreover, the most decisive sizing criterion in each case is determined too. The next studied issue is the reliability analysis of partially cabled EHV grids. The additional components of UGC (joints and terminations) together with a significantly larger repair time (compared to OHL), reduce the reliability of the whole system. In this work, a reliability assessment approach is developed in order to examine how the installation of EHV UGC in transmission networks impacts the overall reliability level. A contingency analysis regarding failures of 380 kV OHL and UGC is performed through state enumeration. Reliability indicators are calculated by performing a dc load-flow calculation and applying remedial actions (if necessary) to relieve overloads. Varying cable length is installed in three connections in the Dutch network, the probability of load curtailment is calculated and the main factors that influence the probability of overload with EHV UGC are explored. The third studied aspect is the condition monitoring of cable systems. An advanced real time Condition Monitoring System (CMS) was installed in the Dutch 380 kV grid and its goal is to monitor the status of the cable connection and its impact on the 380 kV electricity transmission network. In this paper, a first comparison between measurement and simulation results, in both time and frequency domain, is presented. In this way a first validation of the simulation models is performed and the reasons of possible deviations are discussed. @en