Water Tree Degraded XLPE Cables Research Articles

Comparing the Performance of γ-Alumina Nanofiller and Titanium Tetraisopropoxide Catalyst in the Rejuvenation of Water Tree Degraded XLPE Cables

This article evaluates the performance of titanium tetraisopropoxide (TTIP) and γ-alumina nanofiller (Al2O3) in rejuvenation and restoration of degraded XLPE underground medium voltage–power distribution cables. These two materials will be injected into aged cables in the form of two rejuvenation composition groups based on silane coupling agents. The injected and aged specimens were then compared in terms of dielectric loss factor (tanδ) measurement, scanning electron microscopy (SEM) analysis, Fourier transform infrared spectroscopy (FTIR) and energy dispersive x-ray (EDX) analysis. After measurement of dielectric loss factor in 7-day intervals, 1 month after the injection of rejuvenation fluids, it was found that the injected cable specimen dielectric loss tangent in the second group, which contained γ-alumina nanofiller, was lower than the specimens injected with TTIP (first group); even their tanδ was lower than the new unused cables. Microstructures of the rejuvenated and aged specimens were compared by means of SEM analysis and the results indicated that titanium and aluminum nanoparticles filled the water-filled microscopic cavities and modified the polymeric insulation surface. FTIR analysis compared the chemical bonds of the insulation layer in injected and aged cables. EDX analysis also detected the element type and value on the treated insulation specimen surface area. Comparing the four afore-mentioned analysis illustrated that both fluids were efficacious in rejuvenation and rehabilitation of degraded XLPE cable insulations. However, γ-alumina nanofiller performs better than TTIP in rejuvenation of degraded XLPE power cables.

Measurement of residual charge using pulse voltages for water tree degraded xLPE cables diagnosis

The residual charge method is one of the main degradation diagnostic methods for water tree degradation of XLPE cables. In this method, a DC voltage is applied to a water tree-degraded XLPE cable in order to accumulate space charges in water tree parts of an XLPE cable, and an AC voltage is applied to it in order to release deeply trapped space charges, i.e. residual charges. The authors have been developing a new diagnostic method with pulse voltages instead of DC and AC voltages since the method can be applied to the XLPE cables that are connected to gas insulated switchgears. If a diagnostic method using pulse voltages for water tree-degraded XLPE cables is developed, the improvement of the detection sensibility for signals related to water trees is expected because of the averaging effect of signals by the repeated application of pulse voltages in a time that is shorter than conventional method. The diagnostic method with pulse voltages enables us to use more compact and lightweight high voltage devices. In this study, we applied pulse voltages with a pulse width of about 1 ms to an artificial water tree-degraded XLPE cable and a removed XLPE cable with a class of 6.6 kV, and successfully detected current signals related to water tree degradation.

Measurement of residual charge using pulse voltages for water tree degraded xLPE cables diagnosis

The residual charge method is one of the main degradation diagnostic methods for water tree degradation of XLPE cables. In this method, a DC voltage is applied to a water tree-degraded XLPE cable in order to accumulate space charges in water tree parts of an XLPE cable, and an AC voltage is applied to it in order to release deeply trapped space charges, i.e. residual charges. The authors have been developing a new diagnostic method with pulse voltages instead of DC and AC voltages since the method can be applied to the XLPE cables that are connected to gas insulated switchgears. If a diagnostic method using pulse voltages for water tree-degraded XLPE cables is developed, the improvement of the detection sensibility for signals related to water trees is expected because of the averaging effect of signals by the repeated application of pulse voltages in a time that is shorter than conventional method. The diagnostic method with pulse voltages enables us to use more compact and lightweight high voltage devices. In this study, we applied pulse voltages with a pulse width of about 1 ms to an artificial water tree-degraded XLPE cable and a removed XLPE cable with a class of 6.6 kV, and successfully detected current signals related to water tree degradation.

Fundamental Study on Diagnostic Method of Water Tree Degraded XLPE Cables using Residual Charge Detection with Pulse Voltages

One of the degradation diagnostic methods for water tree degradation of XLPE cables is the residual charge method. In this method, a DC voltage is applied to a water tree degraded XLPE cable in order to accumulate space charges in water tree parts of an XLPE cable, and an AC voltage is applied to it in order to release deeply trapped space charges, i.e. residual charges. The authors have been developing a new diagnostic method with pulse voltages instead of DC and AC voltages. In this study, we first applied pulse voltages with a pulse width of about 1ms to an artificial water tree degraded XLPE cable, and found that the residual charge increased with the increase in the degradation degree. Second, we confirmed that residual charges were successfully detected in the removed 6.6kV XLPE cables that had water tree degradation. Finally, we investigated the relationship between the residual charge signal and the AC breakdown voltage of the removed 6.6kV XLPE cables. The results clarified that the removed cables with larger residual charge and longer duration of current signal related to water tree degradation had a lower AC breakdown voltage.

Toward understanding the relationship between insulation recovery and micro structure in water tree degraded XLPE cables

In this paper, to understand the insulation recovery phenomenon of water-tree cables when an applied voltage is removed, the insulation recovery mechanism of the water tree cable is analyzed. A water-needle electrode is used in an accelerated water-tree aging experiment for XLPE (Cross Linked Polyethylene) cables. The dielectric loss factor (tanδ) increases with aging time, but it gradually returns to a lower level once the applied voltage is removed for some time, which means that the insulation recovery of the water tree cables possibly occurs during the process. Depending on the geometric size by micro observations of water-tree slices, a watertree model is constructed for electric field simulation, which includes a series of water-filled micro voids and interconnected channels. According to analysis of the electric field force and the mechanical properties of water tree region, the force generated by the elastic deformation of the molecular chains results in the shrinking of the channels. The water is gradually squeezed out of the channels, which causes the gradual decrease of conductivity and relative dielectric constant in channels. According to the computed results of tanδ at different states of water tree, the reduction of tanδ can be mainly caused by the decrease of conductivity and relative dielectric constant in channels. The time needed for insulation recovery may be determined by the internal electric field in the water tree, the mechanical relaxation time of XLPE, and the diffusion of water.

Application of dielectric response measurement on power cable systems

Water treeing is one of the factors leading to failure of medium voltage XLPE cables in long-term service. Increased moisture content inside oil-paper insulated cable is not desirable. To identify water tree degraded XLPE cables or oil-paper cables with high moisture content, diagnostic tests based on dielectric response (DR) measurement in time and frequency domain are used. Review of individual DR measurement techniques in the time and frequency domains indicates that measurement of one parameter in either domain may not be sufficient to reveal the status of the cable insulation. But a combination of several DR parameters can improve diagnostic results with respect to water trees present in XLPE cables or increased moisture content in oil-paper cables. DR measurement is a very useful tool that reveals average condition of cable systems. However, it is unlikely that DR measurement will detect few, but long water trees. In addition, DR cannot locate the defect or water tree site within the cable system. Combination of DR and partial discharge (PD) measurements can improve diagnostic results with respect to global and local defects. However, it is doubtful whether PD test can identify the presence of water trees inside a cable in a nondestructive manner. Further research is needed for more detailed conclusions regarding the status of a particular insulation and for predicting the remaining life of the insulation system.