Diagnostic testing of thermally aged medium voltage XLPE cable joints

Insulation degradation mechanism and identification of defect types for cross-linked polyethylene (XLPE) cable and joint have not been cleared yet. Measurement of partial discharge (PD) is one of the most important things of testing the high voltage cable. In this paper, we investigated PD characteristics measured for practical XLPE cable joint by the so-called foil electrode method, in which aluminum foils are rapped onto the cable sheath at insulated cable joint to form coupling capacitance. We measured aging time dependence of /spl phi/-q-n patterns and PD statistic parameters when an artificial defect (a needle void) was placed In XLPE/EPR (ethylene propylene rubber) interface. As a result, phase-resolved PD patterns (i.e., /spl phi/-q-n patterns) were found to depend upon the defect type varying with aging time. Moreover, we investigated mechanism of degradation process leading to breakdown by comparing experimental results of XLPE cable joint and simulated XLPE cable joint (model sample).

At the present, Thailand has turned the attention to Underground Cable (UC) systems and trends to use it more to safety, stability and reliability of electrical system. However, one problem that can affect to this electrical system is the Partial Discharge (PD) problem. Most of PD problems are caused by incomplete installation, maintenance reason and insulation degradation on the weakest part, especially cable joint. This paper proposes a method of analysis to locate the PD source in UC systems with Cross Bonding (CB) focus on cable joints. This work has carried out in laboratory on CB system model using 50ohm R58 coaxial cable instead of underground cable. The PD detection technique according to IEEE 400.3 standard with a RFCT sensor to refer the appropriate signal. This work uses a PD diagnostic device that can filter noise from the PD groups and separate groups of PD for greater analysis accuracy. To determine which cable joint is PD source and to be able to fix it on the spot. The PD signal analysis are performed by using time of arrival method considered together with the characteristics and the polarity of the PD pulse.

Partial discharges (PD) detection is an effective method to evaluate the insulation condition of power cable joint. However, optimization of the partial discharge sensor was hard to be achieved because of the complex nature of cable joint. In this paper, the waveform and frequency characteristics of the partial discharge pulse in power cable were studied with experiment firstly. Then, the partial discharge pulses were numerical modeled to be used as excitation in cable joint simulation model. Furthermore, an electromagnetic simulation method of PD propagation in cable joint with considering the frequency dependent dielectric parameter was proposed. Afterwards, the quantitative relation between the sensor sensitivity and the sensor structure was discussed based on the developed pulse model and cable joint model. Calculations and measurements indicated the numerical sensor design can be achieved with the proposed pulse numerical model and cable joint simulation method.

Defect in a cable joint is one of the main reasons of a power system being faulty. A defect can cause the electric field in the cable joint to become higher than the material surrounding the defect. This may result in partial discharge (PD) activity to occur within the defect site. When PD is repeating at the defect site, the material surrounding the defect will be affected, where PD may extend its path in the material. This may result in a breakdown at cable joint and consequently causes breakdown on the whole system. Therefore, the purpose of this research is to study the electric field distribution in 132 kV one piece premolded cable joint with and without the presence of defects. A better understanding of the electric field distribution at a cable joint can be attained by modelling the cable joint structure using finite element analysis (FEA) method. Through modelling and simulation results obtained in this work, an understanding of the electric field distribution in defects within cable joint structure can be enhanced.

Cable joints and terminations play a vital role in providing dependable electrical connection, mechanical support and physical safeguard. These provide electrical stress control to shielded power cables. Despite their usefulness, they suffer from partial discharges (PD) because of enhanced voltage stress, moisture ingress and poor workmanship (during installation). Therefore, it is necessary to undertake on-line PD detection to determine their state. Some of these techniques are capacitive coupler (CC); acoustic emission (AE); and high frequency current transducer (HFCT). This article presents a literature review of these techniques based on the cost, availability, and applicability. The comparative analysis is also provided on location of their sensors, quantification and detection ability. The CC technique involves the quantification of the coupler sensor input measured in mV/pC by realizing the time of flight between two sensors for which the results are used to estimate location of the PD. Meanwhile, the AE technique has an advantage of high immunity to electrical noise, with a caveat that acoustic signals are highly attenuated within the cable joints. Additionally, the combination of the acoustic sensors and PD electrical couplers can be used to discriminate PDs from electrical noise. The HFCT has two methods – with demonising (HFCT-WiD) and without demonising (HFCT-WoD). The HFCT-WiD technique can significantly reduce the sensor’s detection sensitivity due to its high value of noise to signal ratio (NSR). Comparatively, the HFCT has the best results of quantification and detection ability for PDs among all three techniques investigated. However, in places where electrical noise is severe, PD activities may be detected effectively with AE technique. Further work is needed to statistically map these methods and establish their correlation with experimental data.

Monitoring partial discharge (PD) activities in the joint of cross-linked polyethylene (XLPE) cables has attracted much attention due to its extensive application and high fault rate. In this paper, we designed a wide-band sensor with simple configuration, which was embedded in the outer semi-conductive layer of the cable. Based on the analysis of circuit and antenna models, the structural parameters of sensor were determined, and it was found that the effective bandwidth reached up to 500 MHz. In addition, the sensor had a quick response to the pulse with a rising time longer than 2 ns by testing its square response, as well as a linear input-output characteristic and a sensitivity of 3 pC. At last, the sensor was applied to insulation defect type identification in the cable joint. As for three common defects, both the single PD pulse and PD patterns showed distinct. These indicated that the sensor had a potential application prospect in the PD online detection for cable joint.

XLPE cable joint defects measurement method based on point cloud remapping

Since the introduction of partial discharge (PD) detection, the majority of conventional electrical measurement of PD has been performed with detectors operating in narrow bandwidths from 10 kHz to 1 MHz for several decades. In the past decade, advances in electronic instrumentation have permitted the measurement of PD with bandwidths up to 1 GHz. Since PD pulses are fundamentally events with duration as low as a few nanoseconds which have a very wide frequency spectrum, comparing with narrow band PD detection, PD measurement with such ultra-wideband (UWB) detection methods can provide more information on the deterioration of stator winding insulation subjected to combined stresses. A UWB PD measuring technique with bandwidths from 20 kHz to 1 GHz is presented, and some waveforms of UWB PD signals of stator bar specimens in different aging periods both in time domain and frequency domain are given in this paper. It has been found that the ultra high frequency components contained in PD pulse increase and the rise-time of the pulse decrease gradually with aging time. It would be suggested that the ultra high components of PD pulse may be used to characterize the aging degree of stator winding insulation.

The measurement of PD(Partial Discharge) plays a major role in the assessment of the cable system's condition. The PD test applied to the power cable system is generally performed in short-term. However, since PD has physical characteristics of irregularly repeating occurrence and extinction, there is a possibility that even if a PD test is applied to a defective cable system, it cannot be detected. Therefore, the long-term PD test is necessary. In this work, on-load PD measurement was performed at AC 22.9kV power cable terminations. PD was measured on the high frequency band using inductive sensor attached on termination, and monitored for 14days. The result shows that PD was measured at one of the measurement points, and it was observed that occurrence and extinction were repeated irregularly. To verify the PD test result, X-ray test was taken and internal defect was found. After those non-destructive inspections, dismantlement of termination was performed and the exact coincided with the result of PD test. Based on this result, this PD measurement and analysis could be effective solution to assess condition of cable termination.

Partial discharge (PD) based condition monitoring has been widely applied to power cables. However, difficulties in interpretation of measurement results (location and criticality) remain to be tackled. This paper aims to develop further knowledge in PD signal propagation in power cables and attenuation by the PD monitoring system devices to address the localization and criticality issues. As on-line or in-service PD monitoring sensors commonly comprise of a high frequency current transformer (HFCT) and a high-pass filter, the characteristics of detected PD pulses depend on the attenuation of the cable, the HFCT used and the filter applied. Simulation of pulse propagation in a cable and PD monitoring system are performed, based on analyses in the frequency domain using the concept of transfer functions. Results have been verified by laboratory experiments and using on-site PD measurements. The knowledge gained from the research on the change in pulse characteristics propagating in a cable and through a PD detection system can be very useful to PD denoising and for development of a PD localization technique.

A partial discharge (PD) locating system, which can automatically distinguish PD from noise and locate the PD site in power cables, has been fabricated successfully and verified in an after-laying test of a Gas Insulated transmission Line (GIL). The system provides sufficient detection accuracy to meet the requirements of the practical measurement on site. This paper presents the PD locating principle and the system configuration. A new concept of recognizing PD based on the relative distribution between PD magnitude, PD site location, and measuring time is described. The results of applying the system in the on-site test for GIL are also reported.

The fundamental partial discharge characteristics of an oil-impregnated insulation system with an oil gap were investigated in this study, in addition to the effects of the oil gap conditions such as size, depth, location, and volume on the partial discharge inception electric field (PDIEF). We initially assumed the oil gap to be a defect of oil-filled (OF) cable systems. A breakdown occurred at the oil gap in all test samples. We then selected one of the oil gap conditions that gave the lowest PDIEF, and we investigated its time transition of partial discharge characteristics under a continuous alternating current (AC) voltage application. As a result, partial discharges continued with the AC voltage that was lower than that of the partial discharge inception, and the test sample broke down after approximately 16 days. Partial discharges occurred near the timing of the zero crossing of the applied voltage. Their characteristics such as charge magnitude and number did not increase, and partial discharges with larger magnitudes began at the timing closer to the peaks of the AC voltage.

Epoxy/paper insulated materials, with the merits of great insulation performance, excellent mechanical strength, are widely used in tubular busbars playing an important role in safe and steady operation of national grid. In this work, the partial discharge (PD) in epoxy/paper insulated tubular busbars with two types of artificial defects and different aging time were measured. The aging voltage imposed on the samples was 18 kV, and the aging temperature was 428 K. PD of samples at varied aging stage were measured. Sample with defect of cavity and metallic impurity (sample B) showed larger apparent charge magnitude, compared with the sample with defect of cavity (sample A). For example, the measured largest apparent charges of sample B was 8337 pC, much larger than the value of 2266 pC measured in sample A when aged for 700h. In addition, the value of PD inception voltage of sample A was fluctuated around the 16 kV, while PD inception voltage of sample B varied from 10 kV to 8.4 kV. And we analyzed the results in the article. To figure out the influence of defects on the PD activity, MATLAB Simulink tools were used to simulate the PD of samples with different defects. The results showed that the defect of cavity and metallic impurity discharged more severely than the defect of cavity. Moreover, finite element analysis (FEA) method was applied to obtain the electrical field distribution, which revealed that the defect of cavity and metallic impurity caused local electrical field distortion whose maximal value was 1.8 times of the defect of cavity.

For condition monitoring of the power cable insulation system, different diagnosis procedures are available. The measurement of partial discharge (PD) activity is still an issue of great importance. This paper deals with the effect of the temperature gradient and different voltage waveforms/ frequencies on PD phenomena within voids of power cables insulation. The results of PD measurements and computer simulations are underlined. The current study explains the behaviour of PD considering the effect of different testing conditions. The PD tests were performed considering AC, DC Step and Oscillating Voltage (OVW) voltage waveforms at 50 Hz, Very Low Frequency (0.1 Hz/ VLF) and 1 kHz using the Step-up Voltage Test Method. During PD tests, Partial Discharge Inception Voltage (PDIV) was recorded after measurements developed using polyethylene specimens with artificial cavities embedded. The specimens were realized using three thermally-soldered layers of cross linked polyethylene/ low density polyethylene (XLPE-LDPE-XLPE) and in each specimen the central layer has a circular hole punched of around 400 µm. The computer simulations were developed using Comsol Multiphysics 4.2, Heat Transfer/ Electromagnetic Heating/ Joule Heating. The aim of computer simulations was to fit the experimental results. According to the measured values, PDIV is frequency dependent, as the frequency increases the PD are incepted at higher voltage levels. According to computer simulations, the cavity embedded within specimen is a heat source and this could be an explanation of the frequency dependency behavior of PDIV.

Aim: This article proposes a method based on oscillation wave partial discharge detection for locating power cable faults, with the aim of quickly identifying the insulation aging status and faults in power cables. Methodology: This method introduces the principles and experimental devices used in oscillation wave partial discharge experiments. It also compares and analyzes three power cables with significant differences in insulation aging levels and insulation defects at cable joints and terminals. We established the relationship between the degree of insulation aging and the partial discharge parameters of the cable, including discharge starting voltage and discharge quantity. We applied the partial discharge localization method to analyze the location of cable insulation aging faults. Results: The experiments demonstrated that, in comparison to cables A and C, the insulation strength of cable B was significantly worse, even exceeding three orders of magnitude. Conclusion: The tan δ value for each phase of Cable B was two to three orders of magnitude larger than that of Cable A and Cable C. Both experimental results were in agreement, highlighting the effectiveness and applicability of the partial discharge experiment with damping oscillating wave voltage.