Nanosecond pulse discharge plasma has many prospects in industrial applications, and high voltage repetitive nanosecond pulse generators with compact design and light weight have become one of the key issues limiting its development in some applications. This paper presents a high voltage series-connected SiC MOSFETs module which can be served as the main switch in a repetitive high voltage nanosecond pulse generator. This kind of series-connected MOSFETs module with only single external gate driver requiring very few components is very suitable for compact assembly. By analyzing the working principle, three topologies of series-connected MOSFETs module are proposed. The switching behaviors of the three different topologies with four SiC MOSFETs series-connected are compared experimentally. The variation of switching characteristics of series-connection SiC MOSFETs module with different numbers of devices are investigated. The layout is also optimized to shorten pulse front time and improve output pulse quality. Furthermore, a 10kV SiC MOSFETs module with a turn-on transition time~10ns is developed. The double pulse test result demonstrates excellent switching performances. Finally, a compact and high voltage pulse generator composed of three 10kV SiC MOSFETs module is tailored, with a typical rise time ~40ns and peak voltage of ~30kV.

The compact design of the cast resin high-frequency transformers (HFTs) allows the insulation of fine structures withstand severe high-frequency square pulse voltage stress and thermal stress. The partial discharge (PD) behaviors in the presence of thermal-induced defects are investigated. The phase-resolved PD (PRPD) pattern showed that the majority of PDs occurs on the rising and falling edges of the square pulse voltage. The PD inception voltage (PDIV), PD magnitude, and PD repetition rate increase with frequency at very low frequency and decrease with frequency at high frequency. Long charge injection time at very low frequency and the heat effect at high frequency cause the diminution of these PD parameters. The PD and breakdown characteristics are examined under progressive temperature conditions, indicating an early PD degradation and electrical failure phenomenon below the glass transition temperature ( ${T}_{g}$ ). Microscopic observation revealed the formation of delamination gap defects due to the inconsistency of thermal expansion between the cast insulation and metal, which considered the cause of PDs. The decohesion model is employed to describe the structural evolution and the formation of delamination gap with varying temperatures. The model demonstrates an early damage mechanism for interfacial cohesion condition at medium temperatures and temperatures close to ${T}_{g}$ . Additional experiment enables the inference that the release of local interfacial stress under progressive temperature may lead to the formation of damaged interfaces or microcracks, which can trigger occasional PD or instantaneous filamentary breakdown.

It is an important way by detecting SF6 decomposition products to evaluate the insulation condition of SF6 insulated power equipment. In this paper, N3&Ni doped 555-777graphene (N3&Ni-graphene) is proposed as a new gas sensor to detect the characteristic components of SF6 decomposition products: H2S, SO2, SOF2 and SO2F2. In order to get insights into the gas sensing mechanism, the geometry structures, electron structures and conductivity are analyzed by simulation method based on density functional theory. The calculation results show that N3&Ni dopant can improve the conductivity and surface activity of 555-777graphene. Specifically, H2S and SO2 absorbing on N3&Ni-graphene surface will increase the conductivity obviously, while the other two kinds of gas molecules will decrease the conductivity. The sensitivity of H2S is greater than SO2 according to the changed value of energy gap. It is also indicated that N3&Ni-graphene can absorb the above four characteristic components stably with high adsorption energies, complex orbital hybridizations and multi-level charge transfer. This work will provide some theoretical guidance to develop a novel gas sensor to evaluate the insulation condition of SF6 insulated equipment.

The failure of polymeric solid insulation under transient voltages is closely associated with space charge. In this study, DC and AC field dependent conduction characteristics of epoxy resin were investigated using the space charge limited current and high voltage broadband dielectric spectrum methods. The breakdown voltages (BDVs) were examined over a wide range of DC voltage ramp rates. The results reveal a threshold effect of AC conduction loss below 600 Hz with respect to the electric field. By analyzing charge migration laws, a mobility distribution was deduced, with an upper limit of 10−9m2V−1s−1, which is much higher than the measured DC mobility of 10−14m2V−1s−1. A multi-mobility bipolar charge transport model was proposed by introducing the obtained mobility distribution. Compared to the single-mobility model, high-mobility carriers induce larger unblocked space charge currents for high dv/dt voltages, leading to lower values of BDV, which is consistent with experiment results. As the voltage ramp rate decreases, both simulation and experiment demonstrate higher BDVs due to the field shielding effect of homocharges.

The voltage stress on high-frequency transformers (HFTs) features higher levels of repetition frequency, operating voltage, voltage d V /d t and grid voltage superposition, which challenges the epoxy insulation of HFTs exposed to partial discharges (PDs). Wide ranges of voltage parameters (frequency, voltage, rising edge and DC bias) and environmental stresses (temperature, humidity, gas, pressure) are applied to the sphere-gap-insulation-plate structure for PD aging analysis. New patterns are found in this study. For example, the shape of the rising edge affects the lifetime, and the influences of humidity and DC bias are non-monotonic. Surface charge decay and voltage d V /d t have key impacts on the discharge pattern. Early failures with pulse frequencies of 50 kHz and 100 kHz are attributed to the interaction of humidity and electric field. The erosion rate is lower at oxygen-free ambient, while higher with CO ₂ gas. PD parameters (PD number, PD amplitude, PD delay, PD ignition voltage and PD frequency) and phase-resolved PD (PRPD) patterns are measured with ultra-high frequency (UHF) method and discussed for all aging conditions. Cumulative PD amplitude and cumulative PD energy have good correlation to experimental lifetime values. However, due to the transition of PD regime, the fit model has less explainability on the influence of voltage, humidity, gas and pressure on the lifetime, which is rarely revealed before.

Power electronic conversion systems introduce different types of voltage stresses on high-voltage epoxy cast insulation in medium-frequency transformers (MFTs), which challenges the reliability of MFTs. The endurance of epoxy insulation at different voltages (AC, DC and pulse) and environmental (thermal and humidity) stresses was studied using encapsulated electrodes in the absence of partial discharge. To assess aging processes and select aging state indicators, various tests including AC breakdown strength (BDE), broadband dielectric spectroscopy, small-angle x-ray scattering (SAXS), Fourier-transform infrared spectroscopy and differential scanning calorimetry (DSC) were first conducted on aged flat samples. The increase of mesoscopic free volume from the SAXS result and local densification (physical aging) of epoxy network from the DSC result were both found. BDE is proved to be sensitive to various aging conditions and thus determined as a test method for encapsulated samples. It is found that the physical aging effect under thermal stress at early aging stages can cause a reversible increase in BDE compared to non-aged samples. This effect can be erased by coupled medium-low electrical stress during aging. In contrast, a sufficiently high electric field will deepen the physical aging extent. Occasional sample failures were observed simultaneously within this process. According to the SAXS and DSC results on flat samples, the failure of encapsulated samples is possibly attributed to the chain fracture around increased mesoscopic free volume during electron bombardment and microcracks generated by interfacial stress release during physical aging. At longer aging periods, the volumetric absorption and diffusion of chemical reactants (oxygen and water) in epoxy networks at high temperatures determines the eventual decrease in BDE, which can be accelerated by the electric field. Ages with different voltage types were gauged by the reduction of BDE compared to pure environmental stress. Bipolar pulses bring heavier aging effects than ACs at the same RMS value. However, the DC component has opposite effects on the BDE with and without thermal stress.