Kazuhisa Miyajima

Evaluation of relationship between residual charge signal and AC breakdown strength of water-tree degraded 22 to 77 kV classes XLPE cables removed from service using pulsed voltages

The authors have been developing a diagnostic method for water-tree degraded cross-linked polyethylene (XLPE) cables, in which pulsed voltages are used rather than the DC and AC voltages used in the conventional residual charge method. In this study, we first applied the proposed method to the diagnosis of water-tree degraded XLPE cables in classes from 22 to 77 kV, and successfully detected current signals related to water-tree degradation due to residual charges in the XLPE cables removed from service. Next, we investigated the relationship between the residual charge signal and the AC breakdown strength, and our results show that the correlation between the residual charge magnitude and the AC breakdown strength was weak, but the removed cables with a longer duration of the current signal related to water-tree degradation had a lower AC breakdown strength. Also, the ratio of the duration to the rise time of the applied pulsed voltage was strongly correlated with the AC breakdown strength. Therefore, we determined that this ratio can serve as a water-tree degradation index. Thirdly, we demonstrated the reason for this strong correlation using a simplified water tree model. Finally, to obtain greater sensitivity in current signal detection, we developed positive and negative pulse generators and confirmed that an increase in the applied voltage would increase the current signal detection sensitivity.

Study on water tree degradation diagnosis of XLPE cable using charge radar

Water tree is a significant degradation mode of power cables with polymeric insulation. The authors proposed a new diagnosing method for locating water tree degradation. In its procedure, a DC bias voltage is applied for poling the water trees, prior to applying a sharp probing pulse for depolarizing and locating the water trees. However, significant amount of cable lines are in connection with gas insulated stations (GIS). In such a case, applying DC voltage is to be avoided, since it may cause electrification on the insulation in the GIS, which may also induce a reduction in insulation performance. In this report, the period of time for bias voltage application was reduced into as less as 1.5 ms, which is equivalent to the duration of switching surge voltage. As the result response intensity was almost the same as that with 2 s in application time of bias voltage. In addition, theoretical study in terms of pulse response was performed.

Development of external diagnosis for terminals in XLPE cables

To develop diagnostic method for terminals in aged XLPE cable lines, an external diagnosis of deterioration signals relating to partial discharge generated in the aged terminal has been investigated.

Study on short time charge behavior in pulsed residual charge method for water tree diagnostics of XLPE cables

Water tree generated in insulation parts of the XLPE (Cross Linked Polyethylene) cable is concerned as one of the causes of deterioration or breakdown. For the stable supply of electric power, the facilitated and accurate diagnosis technique is important to measure the deterioration of power cable. In our previous study, we proposed a residual charge measurement method using pulse voltage (charge larder method) and suggested that this method makes it possible the degradation position to measure more accurately than conventional methods. In this method, the deterioration signal is detected when the charge trapped at the tip of water tree is depolarized by the short time pulse voltage. However, quantitative evaluations have not been studied yet. In this paper, we investigated the residual charge behavior around the water tree under pulse voltage application and performed the numerical analysis using Maxwell-Wagner model.

Evaluation of relationship between residual charge signal and AC breakdown strength of removed XLPE cables at 22, 66 and 77 kV classes by detecting residual charge due to water tree degradation with pulse voltages

The authors have been developing a new diagnostic method with pulse voltages instead of the DC and AC voltages in the conventional residual charge method. In this study, we applied pulse voltages with the pulse width of about 5 ms to removed XLPE cables at 22, 66 and 77 kV classes, and we succeeded to detect current signals related to water tree degradation. Then, we investigated the relationship between the residual charge signal and the AC breakdown strength of the removed cables. As a result, it was found that the ratio of the duration of the current signal related to water tree degradation to the rise time of the applied pulse voltage had strong correlation with the AC breakdown strength, and it can be a water tree degradation index.

Relationship between residual charge signals and AC breakdown voltages of removed 6.6 kV XLPE cables using residual charge detection method with pulse voltages

The residual charge method is a main diagnostic method for water tree degradation of cross-linked polyethylene (XLPE) cables in Japan. The authors have been developing a new method for diagnosis of water tree degradation of XLPE cables by using pulse voltages for charge accumulation and depolarization. In this study, we first applied pulse voltages with a pulse width of about 1 ms to removed 6.6 kV XLPE cables that had water tree degradation, and confirmed that residual charges were successfully detected in the removed cables. Then, we investigate the relationship between the residual charge signals and the AC breakdown voltages of the removed 6.6 kV 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.