Youping Tu

In high-voltage pulse energy storage systems, coupled electrical-thermal stress from high-repetition-rate pulses during initial discharge induces self-healing effects in capacitors that may ultimately trigger insulation failure. In this work, self-healing characteristics of metallized biaxially oriented polypropylene (BOPP) film under high-repetition-rate pulsed voltage based on the actual capacitor electrode structure was investigated, while concurrently elucidating their mechanism through electro-thermal coupling analysis. The results indicate that below 6 kHz repetition-rate, the electrode region exhibits the most pronounced degradation, with metal layer ablation forming interconnected dendritic patterns. In contrast, above 20 kHz frequency, the electrode damage area remains nearly invariant as frequency increases, and the self-healing points are predominantly isolated, exhibiting high circularity and low fractal dimension. Simultaneously, Fourier Transform Infra-Red (FTIR) scanning results of carbonization reaction induced by Joule heating above 20 kHz further confirm that the self-healing mechanism is governed by a repetition-rate dependent transition from metal layer ablation to localized dielectric breakdown. Bridging the voltage thermal coupling and self-healing characteristics, this work provides understanding of failure mechanism of dielectric film under high dv/dt and high-repetition-rate pulsed voltage.

Charge accumulation at the gas-solid interface of DC (direct current) spacers distorts electric fields, which severely hampers the development of DC gas-insulated equipment. The charge on the gas phase critically influences the distribution characteristics of the interfacial charge and electric field. This work investigates the influence of the average surface roughness Ra of conductors on the surface potential distribution of spacers, as well as the effects of temperature gradients and voltage polarity on charge accumulation. The results show that the surface potential on the spacers does not increase monotonically with Ra, but instead reaches a relative minimum at Ra=3.2 μm. The temperature gradient exacerbates the nonhomogeneous distribution of the surface potential on the spacers, resulting in the formation of regions with heteropolar potentials.The influence of protrusion morphology on conductor micro-discharge current was investigated. It was found that the electric shielding effect dominates at low aspect ratios, whereas an optimal morphology minimizing discharge current exists at high aspect ratios. This research provides references for designing conductor surfaces in DC gas-insulated equipment where gas-solid interface charging effects must be considered.

The DC-link capacitor in the converter valve is a critical component within flexible DC transmission systems. The biaxially oriented polypropylene (BOPP) base film in capacitors operates under DC voltage superimposed with AC voltages of varying frequency and amplitude. However, the dielectric breakdown strength of BOPP base films is currently evaluated exclusively with DC ramp voltage, which does not correlate well with the actual service conditions. In this paper, a multi-layer aluminum foil electrode system is designed to evaluate the breakdown performance of the 5.8 μm BOPP base film used in converter valve capacitors under AC-DC superimposed voltage. The effects of AC frequency, AC amplitude, and the superposition of multiple harmonics on breakdown voltage are systematically investigated. Results show that as the AC component amplitude increases, the sample's breakdown voltage first decreases and then increases, with the maximum reduction of 10.5% occurring at 200–300 V compared to the DC breakdown voltage. As the AC frequency increases, the sample's breakdown voltage gradually decreases. At 200 V AC amplitude, increasing the frequency from 50 Hz to 1000 Hz causes the largest drop in breakdown voltage—9.3%. Superimposing multiple AC components has the same effect on breakdown voltage as adding a main single AC component. This paper proposes an AC-DC superimposed breakdown test method, which employs a DC voltage ramped at 100 V/s superimposed with a 200 V, 50 Hz AC component. The research results presented in this paper can provide a reference for the design optimization of the DC-link capacitor in the converter valve.

Self-healing performance constitutes a critical determinant for ensuring reliability of metallized film capacitors (MFCs). By utilizing capacitor component to investigate the internal self-healing distribution and mechanisms under actual operating conditions (3.92 kV DC and 70°C) enable the location of faults and evade inaccuracy of multi-physics decoupling during film-level experiment. In this work, self-healing distribution was investigated, while concurrently elucidating the mechanism combined with microstructure analysis. The results indicate that within the two internally series-connected capacitances, the upper capacitance demonstrates 126.9% max higher quantity and 38.1% max larger area of self-healing, with this asymmetric distribution exhibiting strong spatial correlation with metallized film location. The Scanning Electron Microscope (SEM) and Energy Dispersive X-ray (EDX) spectroscopy results indicate that self-healing points in inner layer area exhibit high circularity and low fractal dimension, along with a decrease in Zn elemental content. The increased internal pressure (8-16 MPa) is the primary cause of reduced self-healing by inhibiting plasma expansion and increasing the energy threshold, while the pressure-induced 2.1% increase in film crystallinity acts as a secondary factor that contributes to altered self-healing distribution. This work elucidates the self-healing distribution mechanisms within MFC and offers critical guidance for the structural optimization and reliability design of MFCs.

Surface micro-defects on glass-fiber-reinforced polymer (GFRP) insulating rods in gas-insulated switchgear (GIS) can initiate incipient partial discharges (PDs) long before visible degradation. A multimodal framework integrating ultra-high-frequency (UHF) PD sensing and optical single-photon counting is developed to detect incipient PD under mechanical fatigue. Hollow GFRP rods containing representative micro-defects (scratch, subsurface bubble, micro-crack) are mechanically cycled for 10,000 operation-equivalent cycles to emulate breaker actuation. After each 1,000-cycle block, specimens are energized with AC high voltage in SF₆ while phase-resolved UHF and photon-count PD signals are synchronously recorded. Photon counting identifies PD inception one fatigue block earlier (ΔN ≈ 1,000 operation-equivalent cycles) than UHF for all defected specimens, while both channels remain at baseline for the pristine rod until ≈7,000 cycles. Under typical GIS duty rates, this lead corresponds to several to tens of service years. Phase-resolved PD patterns reveal distinct defect-specific behaviors: scratches show the earliest PD onset and later exhibit discharges on both voltage polarities; bubbles develop dominant negative halfcycle activity under advanced fatigue; and micro-cracks confine PD activity to a narrow phase band near 270°. The combined UHF–optical method provides early warning of insulation degradation and enables quantitative defect severity ranking, supporting practical online condition monitoring of GIS components. An IEC 60270–compatible phase-resolved pulse-current reference is also derived from the coupling capacitor to quantify apparent charge (pC) in the weak-discharge regime.

In gas‐insulated high‐voltage direct current (HVDC) devices, micro partial discharges (PD) caused by micro‐protrusions or surface roughness on conductors or enclosures may not cause immediate insulation system failure due to their weak discharge intensity. However, the continuous charging of carriers generated by the discharges to the insulator is one of the potential triggers of surface flashover of insulators. This work investigates the micro PD characteristics in SF ₆ at 0.1–0.7 MPa, which is below the lower limit of conventional detection methods, via a highly sensitive micro‐discharge measurement platform. The experimental results show that the micro PD exhibits a strong polarity effect. With increasing pressure, the mean amplitude and mean repetition rates exhibited an extremum at 0.3 MPa. As the applied voltage increased, the mean amplitude shifted from negative polarity dominance over positive polarity to positive polarity dominance over negative polarity. Time‐resolved PD (TRPD) and pulse sequence analysis (PSA) patterns indicate that the micro PD is characterised by clustered bursts and amplitude stratification. Furthermore, the negative polarity micro PD is more stable than the positive polarity counterpart. The results of this work can provide references to guide the optimized design of gas‐insulated HVDC devices.

Metallized polypropylene film capacitors (MPPFCs) are an important structure in superconducting magnetic energy storage (SMES) systems. This article systematically investigates the correlation mechanisms between self-healing parameters in metallized polypropylene films (MPPFs) under different environmental conditions. Results show that elevated temperature at $65~^{\circ }$ C enhances plasma activity, increasing self-healing energy to 6.5 mJ, three times higher than $25~^{\circ }$ C, while thermal expansion mismatch between metallized layer and polypropylene layer at $85~^{\circ }$ C induces activity in self-healing process. Pressure, however, reduces the self-healing energy from 0.2 to 10 MPa and changes the situation of the polypropylene layer’s surface, causing damage to the hydrophobic property in the polypropylene layer. The data of arc initiation time and arc extinction time in the self-healing waveform exhibit a two-stage development, representing two stages of the self-healing process under different conditions. Finally, a self-healing simulation model was developed and validated against experimental data, successfully replicating the non monotonic trend in the self-healing area, which is an initial increase followed by a reduction with rising temperature.