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• The packaging insulation material is exposed to extreme operating conditions with rapid voltage rise rate. • The voltage rise time appeared to be the most important parameter influencing the early failure of insulating materials. • A strong local vibration of the surrounding molecules to produce phonons in addition to the normal thermal vibration. • With assistance of phonons, a portion of charges is excited to high-energy states, thereby leading to insulation deterioration. Due to the extreme operating conditions of high voltage rising rate, silicone gel materials used in the encapsulation insulation of power electronic devices are prone to insulation failure. In this paper, we proposed a mechanism of insulation degradation under the ultra-fast pulsed electric field by considering the synergistic effect of molecular vibrations and charge behaviors. With the quantum dynamic simulation, it is found that the motion of space charge will induce a strong local vibration of the surrounding molecules to produce phonons in addition to the normal thermal vibration. The steeper the edge time, the higher the amplitude of vibration. With assistance of phonons, a portion of charge is excited to a high-energy state, resulting in an increased probability of induced charge trapping and detrapping. This image well microcosmically confirms the phenomenon of molecular vibration and intensified electric tree formation under ultra-fast pulsed electric fields. [ABSTRACT FROM AUTHOR]

• The bipolar charge transport and hydrodynamic drift-diffusion models were used. • The PD waveforms under the assessed conditions were unimodal. • Pressboard surface charge can form a shield layer that hinders streamer evolution. • The streamers have voltage-dependent shapes. • The time relationships between PD pulse and streamer evolution were found. A better understanding of the complicated characteristics and interrelationships between the charge distributions and partial discharge (PD) pulse waveforms occurring in oil-pressboard insulation can be used to characterise the PD mechanism as a basis for the optimisation of the insulation structure to minimise PD activity. These characteristics were studied by using a non-inductive resistor to measure the PD pulse waveforms produced in oil-pressboard insulation by a needle-plate at positive DC voltage and through additional simulations based on bipolar charge transport and hydrodynamic drift-diffusion models. It was found that the PD pulse waveforms are unimodal, and with increased applied voltage, their rising and falling edges and equivalent times decrease, and their apparent charge amplitudes and equivalent frequencies increase. These parameters have a saturation state at high voltages (>45 kV) and the oil streamers have voltage-dependent shapes and evolution velocities (v s); the higher the voltages, the higher the values of v s , and the wider are the streamer bodies. Accumulated charge can form a shield layer that hinders streamer evolution. Analysis of the PD mechanism in terms of the relationships between charge distribution and PD pulse waveform reveal that the rising and falling edges of a PD pulse correspond to streamer propagation in the oil gap and charge accumulation at the pressboard surface (i.e., the charging process in the pressboard), respectively. The peak value of the PD pulse waveform is shown to be related to the conduction current in the coupling loop as reflected by the current density at the streamer head. [ABSTRACT FROM AUTHOR]

Highlights • First, based on the general form of Maxwell's equations, the field source is transformed into an equivalent form with a constraint source, and a full-current PEEC model with a surface charge component element is established. • Second, a fast and accurate calculation of the inductance parameters of SCSR components involving multiple integrals is carried out using the geometric mean distance and complex component division. • Third, the equivalent structure and circuit equations for the SCSR internal windings are given, and the variation of winding resistance and inductance parameters with VFTO frequency is analyzed. Abstract Stepped Controlled Shunt Reactors (SCSRs) have advantages of flexible control and reliable compensation and are likely to become widely used in Ultra-High-Voltage/Extra-High-Voltage (UHV/EHV) transmission systems. Very fast transients that occur in such systems will generate a Very Fast Transient Overvoltage (VFTO) in the SCSR. This may cause damage to the SCSR and lead to system losses. In this paper, the electric field strength and magnetic induction intensity are taken as the basic physical variables when these transients occur. Maxwell's equations are used to separate the constraint sources, such as the conduction current, magnetization current, and polarization current, from the field source. Then, an all-current Partial Element Equivalent Circuit (PEEC) model for these component elements that considers the surface charge is established. Fast and accurate calculations of the inductance parameters, which involve multiple integrals, are performed using geometrical mean distances and a complex component segmentation method. The equivalent structures and circuit equations of the SCSR windings are presented, and the voltage distribution of the SCSR windings under VFTOs of different frequencies is obtained. The results show that, the voltage waveform in the SCSR windings more rapidly attenuates as the VFTO frequency increases. The peak value of the oscillating voltage in the high-voltage winding of the SCSR is higher than that in the low-voltage winding. [ABSTRACT FROM AUTHOR]