Aixuan Zhao

Study of dielectric response characteristics for thermal aging of XLPE cable insulation

In urban power grids, the proportion of cross-linked polyethylene (XLPE) power cable continues to increase. Judging the insulation condition of XLPE cable precisely is of great significance. In this paper, a number of cable samples are thermally accelerated aged at different temperatures and during various periods. Polarization/depolarization current (PDC) and frequency-domain dielectric spectroscopy (FDS) tests are performed on these samples. Based on the results of PDC and FDS, combining with Debye model and dielectric theory, we analyzed their dielectric response characteristics and acquired some characteristic parameters representing the aging degree of XLPE insulation. From the aspect of PDC measurement, at a certain temperature, with the aging time increases, the attenuation time of depolarization current rises correspondingly and becomes slower to reach the steady state. Whats more, an aging factors A could be calculated which rises with the aging time. As for the FDS test, with the aging time increases at one temperature, the curve of dissipation factor tanδ moves upward; curves of complex permittivity real component have a tendency of moving right. The results are alike when observing samples at different aging temperatures. With the increase of aging time and aging temperature, the pyrolysis reaction and oxidizing reaction will become more drastic. As a result, there will be more boundary between sphaero-crystal and amorphous region. Some polar oxides could also be created. These changes can produce more interfacial polarization and space charges which could make the dielectric relaxation phenomenon more obvious.

Development of time and Frequency Domain dielectric response testing device for power apparatus insulation

Dielectric response(DR) measurement has been deployed as a non-destructive and potential diagnostic method for moisture or aging condition of power system apparatuses. In order to promote the application of DR method, development of precise, easy-to-use and cost-effective testing devices is an urgent issue, in which accurate acquisition of the insulation current is critical. In this paper, a DR testing device which combine time-domain Polarization and Depolarization Current(PDC) and Frequency-Domain Spectroscopy(FDS), which are two major approaches for DR measurement, is developed. Firstly, the principle of PDC and FDS is introduced. Then the technical specification, hardware and software structure are demonstrated. In addition, using high input impedance PCI data acquisition card, phase lag error of the current measurement module is calibrated by measuring the voltage signal at both the input and the output ends of current measurement module. Finally, PDC and FDS test waveform of XLPE cable and bushing sample are demonstrated, and the effectiveness of this device is proved.

Insulation status diagnosis of XLPE cable based on polarization and depolarization current (PDC)

Cross-linked polyethylene (XLPE) insulation cable is widely used in distribution power system, so it is important to reasonably estimate cable insulation status. Cable insulation is mainly affected by high temperature and water tree. Therefore, in this paper, we use PDC method to evaluate cable insulation, which is non-destructive due to its low voltage. In order to find out the relationship between PDC curve and aging status, we put 5 single-core cable samples in oven and set aging temperature as 14°C. Then, we divide these samples into different aging time (new, 10 days, 20 days, 30 days, 40 days) and measure the PDC curve. The result shows that with the increase of aging time, the PDC curve decreases quicker and the absolute value of steady current becomes higher. PDC will be a very practical way to judge cable insulation in the future.

Research on aging parameters of XLPE cable based on isothermal relaxation current

Dielectric response is closely related to the aging status of XLPE cables, and the isothermal relaxation current (IRC) has been applied to find out the change trend of cable insulation as a non-destructive method. In this paper, 4 groups of XLPE cable samples (16 cables in total) are artificially acceleratedly aged with different time interval at 120, 140 and 160°C, respectively, and their IRC curves are measured by a micro-current device. The results show that with the increasing aging time, IRC curves decrease slower and the absolute values of steady current become larger. In order to quantify the aging characteristics of XLPE cables, the trap parameters calculation theory is employed to work out the peak trap level of different polarization types. The experiment results show that the peak trap level of bulk polarization (type 1) remains stable and is around 0.76eV which is considered to be independent of aging degree. As the aging time increases, the peak trap level of amorphous crystalline relaxation (type 2), which is from 0.785eV to 0.82eV, increases at low temperature (120°C) due to the breaking of molecular chain and decreases at high temperature (more than 140°C) due to the over cross-linking. The peak trap level of impurities interface relaxation (type 3) is highly influenced by aging temperature, which is most sensitive to the aging degree of power cable. It is considered that IRC can be used as a practical way to judge cable insulation in the future.Dielectric response is closely related to the aging status of XLPE cables, and the isothermal relaxation current (IRC) has been applied to find out the change trend of cable insulation as a non-destructive method. In this paper, 4 groups of XLPE cable samples (16 cables in total) are artificially acceleratedly aged with different time interval at 120, 140 and 160°C, respectively, and their IRC curves are measured by a micro-current device. The results show that with the increasing aging time, IRC curves decrease slower and the absolute values of steady current become larger. In order to quantify the aging characteristics of XLPE cables, the trap parameters calculation theory is employed to work out the peak trap level of different polarization types. The experiment results show that the peak trap level of bulk polarization (type 1) remains stable and is around 0.76eV which is considered to be independent of aging degree. As the aging time increases, the peak trap level of amorphous crystalline relaxation ...

Research on conversion of polarization/depolarization current and frequency domain spectroscopy for XLPE cable

Cross-linked polyethylene (XLPE) insulation cable is the main type of cable in use now. Its main insulation is aging with the increasing of service years. Two methods are used to diagnose the condition of the cable insulation-polarization/depolarization current (PDC) and frequency domain spectroscopy (FDS). The PDC is a time domain method, while the FDS is frequency domain. In this paper, we put forward a method to get the dissipation factor curve of FDS under low frequency by carrying out the PDC experiment. This method is based on three R-C series branches extended Debye model. In this model each branch stands for a type of polarization process. Firstly, The measurement of cable samples aged for different days is carried out to get the PDC and dissipation factor curves. Then, the PDC curves are fitted with a third-order exponential decay function, and the parameters of the extended Debye model are calculated. Comparing the calculated dissipation factor curves with the measured, we find that they agree very well with each other under low frequency range. By this way, we can reduce the measuring time.

Research on the defect development of cable accessories under AC voltage

As cable insulation failure usually started with PD activity, a PD detection system based on pulse current was built to study the PD characteristics of typical defects in XLPE cable. To facilitate the test, the oil cup test terminal was used. Considering the installation procedure, three different kinds of insulation defects in cable accessory samples were made, which were knife incision defect, metal floating potential defect and misplaced stress cone defect. Under long time power frequency voltage of U», 1.7U0, 2.5U0 and 3.5U0 (U0=8.7kV), this paper proposed a variety of methods to characterize partial discharge of these samples by PRPD spectrum, δ t spectrum and PSA spectrum. Results showed the δ t spectrum presented a clear distribution of the peak and the sub-peak. The distribution regularity of partition probability in PSA pattern before the breakdown was found that the probability in the area 1.1 as from 50% to 60%, and the probability in the area 3.1 was from 30% to 40%.

Cable Insulation Diagnosis Equipment Based on Polarization/Depolarization Current

Traditional cable insulation status diagnosis method is comprised of withstand voltage experiment. However, withstand voltage experiment injures the insulation part. Insulation resistance and tanδ provide the limited information because of single frequency test. However polarization and depolarization(PDC) can be used for determining the ageing extent of the cables. Existing equipments have poor anti-jamming capability, Aiming at the problems, we have developed cable insulation diagnosis equipment based on (PDC) process in dielectric insulation. This equipment can collect the low current which can be used to analyze cable insulation status by applying high voltage excitation. Due to possible device noise and environment electromagnetic interference, low current measurement encounters is usually the most difficult issue in device development. To solve this, first of all, the equipment using the modular components avoid self-interference. Then, to achieve good polarization/depolarization current measurement, software Butterworth filter is employed to deal with the noise in measured current waveforms. In addition, the equipment is designed with floating potential in order to have the ability to measure the current without disconnecting grounding wires of the device under test. Calibration of the equipment is also conducted with the usage of a precise RC network. In this paper, the equipment with high accuracy is verified, and tests in the field, which get good results. It can be used more widely due to small volume and reliable operation.

The characteristics of switching impulses in cable OHL mixed network

In recent years, in order to improve the efficiency of urban power supply, automatic reclosing operation is gradually applied in the XLPE cable-OHL mixed line. Operation experience shows that the grounding fault occurred in the overhead line, in most cases, is instantaneous and the success rate of reclosing is high. The grounding fault occurred in the cable line is almost permanent and the success of reclosing is quite low. In this paper, a typical mixed line of urban power network was built in PSCAD software. The amplitude-frequency characteristics of overvoltage, caused by single-phase grounding fault of cable segment and overhead line segment, were studied. And the characteristic parameters of two kinds of overvoltage were extracted. According to these parameters, the fault location could be determined and the basis of reclosing strategy was provided.