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When a single-phase-to-ground fault occurs in a power system, a secondary arc will normally be generated at the fault position after the fault phase has been interrupted. A detailed investigation of multiple-reignition features of secondary arcs on extra-high-voltage/ultra-high-voltage transmission lines is, therefore, very important. In this paper, an experimental setup for generating a secondary arc across a transmission-line insulator string is presented, and both the discharge waveforms and images are synchronously recorded. According to the properties of the discharge waveforms, the discharge process is divided into two stages and each stage is analyzed. The evolution of the volt–ampere characteristic and the resistance–current curve of the secondary arc is found to exhibit hysteresis during the discharge process. Multiple-reignition images of the secondary arcs are converted into binary images using image threshold processing. The arc length and breakdown voltage are discussed in association with the discharge images. Morphological changes in the secondary arc during a single discharge process are analyzed in terms of self-magnetic compression theory, and the extinction mechanism of the arc discharge is discussed on the basis of two theoretical approaches. [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]