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Abstract An investigation is made of the influence of an accumulated charge sprinkled on the surface of an epoxy resin plate in corona mode on partial discharge inception and its forms as analyzed by optical imaging at DC voltage. The effects of the polarities of both the deposited hetero‐ and homocharges were investigated. The occurrence of first streamer discharges was detected electrically and observed by optical imaging. The various forms of streamers were acquired by optical imaging and associated with the polarity and intensity of the accumulated surface charges. Surface charge sprinkling on the epoxy specimen surface was quantitatively evaluated by surface charge mapping. In addition, the effect of the subsequent discharges occurring in a heterocharge configuration above the inception voltage was observed. In terms of their directions and lengths, the kinetics of the streamer filaments revealed a strong dependence on both the polarity and the intensity of an accumulated charge. Compared with the reference charge‐free surface, the strongest drop in dielectric strength in the interelectrode space with a sprinkled charge was obtained for the heterocharge configuration. The presented results provide a contribution to the assessment of streamer behaviour in the presence of accumulated surface charges, including its polarity and intensity.

Humidity has been considered as one of the main influencing factors that determine the conduction processes and electric strength of gas-insulated systems. Whereas in the past, various studies focused on the change in the partial discharge inception voltages, breakdown strength of homogeneous and inhomogeneous field arrangements, and insulator flashover voltage, recent studies have investigated the changes in ion currents measured through different gas gaps. In the framework of this contribution, a highly precise humidity control circuit has been developed to analyse the significance of humidity in the range from −25 to −5°C frost-point, which is fully applicable to operating gas-insulated devices. Using sulphur hexafluoride (SF(6)) as the insulation gas at 0.45 MPa, Al(2)O(3)-filled epoxy resin insulators, and technically rough electrodes, the humidity was found to significantly influence the intensity of microdischarges at interfaces. Charge generation from microdischarges at the interfaces substantially increased with increasing humidity. For an electric field of 5 kV/mm that were applicable to the dimensioning of gas-insulated devices, humidity strongly influences the charge provision from technically rough interfaces and potentially contributes to the surface-charge accumulation at insulator surfaces. On the other hand, for low-field conduction phenomena, no increase in the ion currents from natural ionisation or electrophoretic conduction was observed. For the investigated range of parameters, humidity is expected to be highly relevant for the design of gas-insulated devices.

In this paper the behavior of partial discharges (PDs) on epoxy insulation under AC and impulse superimposed voltages are experimentally investigated in SF 6 gas. The lightning impulse (LI) and the switching impulse (SI) voltages with different polarity are superimposed on different phases of the AC voltage. PDs are measured by three sensors, a 50 Ω measurement impedance, an ultra-high frequency sensor and a photomultiplier tube. The max PD amplitude, PD pulse number and PD duration under AC voltage after an impulse voltage are summarized and analyzed. The effect of the impulse superposition phase, the impulse type and the impulse polarity on the characteristics of AC PDs are discussed. Results show that after a positive impulse voltage was applied on the positive semi-wave of AC voltage (0 to 180°), PDs in the negative half cycle of the AC voltage can be remarkably excited. A negative impulse voltage was applied on the negative semi-wave (180 to 360°), AC PDs can be initiated in the positive half cycle of AC voltage. PDs excited by a LI voltage is severe than excited by a SI. PDs excited by a positive impulse is intense than excited by a negative impulse. [ABSTRACT FROM AUTHOR]

Identification of partial discharge (PD) sources in stator insulation is an important and challenging task. Distinct phase‐resolved PD (PRPD) pattern of different PD sources are well established at power frequency (50/60 Hz) and can help to identify each source. In this study, different sources of PD commonly found in stator insulation were created on a 6.6 kV epoxy‐mica stator coil. Each PD source was experimentally investigated at a variable excitation frequency in the range of 50–0.1 Hz. PRPD pattern and PD parameters such as integrated charge per cycle and number of discharge pulses per cycle of each discharge source were compared at different frequencies. The comparison suggests that the parameter of each PD source was dependent on excitation frequency. Some discharge sources have shown a distinct variation in their characteristics with excitation frequency. These distinct characteristics are additional information to the existing ones, which will help to correlate PD characteristics with the source of PD in stator insulation. [ABSTRACT FROM AUTHOR]

In this letter, a test platform capable of generating superimposed voltage with controllable phase is presented. Surface partial discharge(PD) of epoxy insulation was measured and analyzed in SF6. The applied voltage is AC and switching impulse superimposed voltage. Results show that only PD of the negative cycles of the AC voltage is excited or accelerated after impulse. With AC voltage at 90% of partial discharge inception voltage(PDIV), AC PDIV is reduced after imposing the switching impulse. When AC voltage is kept at 110% of PDIV, AC PDs are obviously promoted after impulse. [ABSTRACT FROM AUTHOR]

A review of the literature on nanotechnology for the electrical power sector shows considerable hype in the technology, but the actual usage of nanomaterials in the sector is far below other industries. As an example, the automotive industry has made great strides in using nanomaterials because of their lower weight and higher physical strength. There are several reasons for this, the obvious one being the quantity of nanodielectrics is relatively small compared to nanomaterials in other industries. The second reason is that although there are clear improvements in some dielectric properties, for example resistance to partial discharges, the improvements in other properties are not so significant, or sometimes not even evident. Uniform dispersion of nano-sized inorganic fillers in nanodielectrics during manufacture is critical for repeatable electrical properties improvements, and this has been a problem in some of the reported properties such as breakdown strength. This comes about due to the fact that nanoparticles agglomerate easily because of the high surface energy, and conventional mixing techniques do not break apart the nanoparticle agglomerates. Another problem is the incompatibility of hydrophilic nanoparticles with hydrophobic polymers, such as silicone rubber, which results in poor interfacial interactions. [ABSTRACT FROM PUBLISHER]