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To explore the surface charge behavior of polyimide films under corona discharge, the surface potential at both high-voltage (HV) stage and short-circuit (SC) stage is measured by a noninvasive method. DC HVs with different magnitudes, polarities, and durations are applied to a needle electrode to generate corona discharge. The surface potential without corona discharge exhibits a homopolar pulse at the HV stage and a heteropolar pulse at the SC stage because of interfacial polarization and depolarization. This article proposes a three-region model (including corona region, homopolar-charge region, and sample surface region) to explain the surface potential evolution with corona discharge. Surface potential under corona discharge shows a large fluctuation followed by decay with oscillation during the HV stage. The combined effect of interfacial depolarization and surface charge leakage explains the “backtracking” phenomenon at the SC stage. From the comparison under different polarities, it is found that negative corona produces more charge deposition than the positive case, and the leakage rate of negative charge is faster than that of positive charge. Moreover, both a lower applied voltage and a longer duration lead to a smaller amount of residual surface charge and a more obvious “backtracking” phenomenon. It is expected that this article will help to understand the surface charge behavior under dc corona in future studies. [ABSTRACT FROM AUTHOR]

The growth and partial discharge (PD) characteristics of electrical tree in crosslinked polyethylene (XLPE) needle-plate electrode samples are investigated under constant dc voltage at different temperatures. The results reveal that under the experimental conditions, the electrical trees are all branch trees, and the discharge intensity fluctuates with time. The increase in temperature promotes the growth of electrical tree as well as the intensity of PD. Five texture features of PD ${q}$ - ${t}$ histogram are extracted and used as the elements to evaluate the insulation status of XLPE needle-plate electrode under dc voltage based on the gray correlation degree. The area ratio and lateral tree ratio of electrical tree is extracted based on the gray histogram. The lateral tree ratio increases with the increase in temperature, which is closely related to the intensity of PD. Gray correlation theory is used to evaluate the status of XLPE insulation. The results reveal that the evaluated insulation statuses based on the gray correlation degree are the same as those based on experience, which indicates that the proposed diagnosis method of XLPE insulation status based on the gray correlation degree with ${q}$ - ${t}$ histogram texture features as elements has a high accuracy. [ABSTRACT FROM AUTHOR]

Space charge problems have been widely acknowledged for solid insulation such as polyethylene for high-voltage dc cable insulation. However, the effect of space charge on the reliability of high-voltage power module packaging insulation has not been systematically investigated. In this article, a simultaneous measurement of space-charge and isothermal relaxation current (IRC) is performed on silicone packaging insulation. An iterative algorithm based on the nonnegative least-square method is introduced to estimate multilevel trap energy distribution in silicone insulation. It is found that homocharges are accumulated around the electrodes, and more charges are accumulated with the increase in applied electric field and temperature. The local electrical field is distorted, and the threshold of the external applied electric field for charge accumulation decreases with increasing temperature. The trapped charges tend to be trapped in deep traps as the temperature increases. Furthermore, to investigate the charge dynamics in the complex power module package structure, a 2-D space-charge simulation model based on the combination of the bipolar charge transport model and finite-element method (FEM) is proposed with the parameters derived from the simultaneous measurement. According to the simulation results, the maximum electric field is located at the triple-junction position of the high-voltage power module package. With the accumulation of homocharges and the increase in temperature, the electric field decreases at the triple-point position but increases between the injected positive and negative charges. Space-charge behavior affects the local electric field of power module packaging insulation and should be thoroughly investigated in the future. [ABSTRACT FROM AUTHOR]

To explore dc flashover characteristics and evaluate the dc flashover resistance of epoxy resin, an online surface potential measurement is proposed and verified in this article. Rising positive and negative dc high voltages (HVs) with a step of 0.5 kV per 5 min are applied to epoxy resin until the intensive flashover occurs, and the surface potential at the triple junction point is measured during flashover. It is found that the surface potential profile appears as the recurrence of flashover units during intensive flashover. A three-point peak-finding algorithm with threshold judgment is proposed to determine the interval between adjacent flashover units. Both appropriate surface potential difference threshold (SPDT) and flashover interval (FI) are used to evaluate the severity of flashover (i.e., the opposite of flashover resistance). It is interesting to find positive and negative flashover has different surface potential evolution characteristics. Positive flashover exhibits larger fluctuation, higher SPDT, and shorter FI, which may result from severe positive charge accumulation before every single flashover due to the low surface mobility of the positive charge. The results manifest positive flashover is more serious than negative flashover for the investigated epoxy resin. [ABSTRACT FROM AUTHOR]

Charge transport in high-voltage direct-current (HVDC) cable joint with complicated structure has not been profoundly investigated. In this study, a two-dimensional electrothermal coupled charge transport model combining finite element method (FEM) and bipolar charge transport algorithm is proposed, and the effects of temperature gradient and stress cone tilt angle on charge transport and electric field distribution in a 320 kV HVDC cable joint is investigated by this model. In the simulation, the interfacial area between the cross-linked polyethylene (XLPE) cable insulation and the ethylene-propylene-diene monomer (EPDM) joint insulation is specially modeled with different trap parameters from XLPE and EPDM. To obtain the simulation parameters, simultaneous measurements of space charge and relaxation current are performed on XLPE/EPDM double-layer samples, XLPE single-layer samples, and EPDM single-layer samples. It is found that with the increase of temperature gradient, more positive charges are injected and migrate from the inner side of XLPE insulation to the interface, thus, the charges accumulated at the interface change from negative to positive under large temperature gradient. At the same time, the position of the maximum electric field changes from the inner side of XLPE insulation to the root area of the stress cone at the interface. With the increase of the stress cone tilt angle, both the positive charges in XLPE insulation and the negative charges near the stress cone become less, and the overall maximum electric field first decreases and then increases under 20°C temperature gradient. [ABSTRACT FROM AUTHOR]