Surface charge accumulation is the potential criminal for surface insulation failure on spacers under direct current (DC) voltages. The existence of thermal gradient posing the difficulty of surface charge regulation. This study proposes a temperature‐dependent adaptive conductivity coating technique for surppressing surface charge accumulation under electro‐thermal coupling field. A two‐dimensional axis‐symmetrical simulation model regrading surface charge computation is established. The effects of thermal gradients and coating conductivity on surface charge and electric field distribution are investigated. The results show that the thermal gradient increases the bulk conductive current, therefore aggravating surface charge accumulation. The effects of the coating condcutivity on surface charge and electric field contains three stages. The lower coating conductivity leads to aggravated homo‐polarity charge accumulation. By increasing the coating conductivity, the surface charge and electric field are significantly suppressed at the obtained optimal conductivity, where the bulk and surface conductive current reach a balance stage. Continuously increasing the coating conductivity results in aggravated hetero‐polarity charges. Besides, the increase of thermal gradient to an appropriate extent contributes to the further suppression of surface charge on coated spacers. It is hoped that this study could provide some references for designing highly reliable DC GIL under electro‐thermal coupling field.

In recent years, the performance of polluted suspension insulators operating in coastal foggy regions has attracted attention because of the occurrence of flashover. Surface wetting by salt fog accretion increases the surface conductivity and degrades the flashover performance of insulators. Normally, the effect of conductivity under fog condition and pre-contamination on the flashover of polluted insulators are analyzed separately. However, this does not reveal the flashover behavior in salt fog, which requires further research. This paper describes salt fog experiments with an AC test voltage conducted using polluted suspension insulators in a 110-kV power system. The electrical strength performance of sample insulators with salt fog treatments is analyzed, and the effects of salt deposit density (SDD) and the conductivity of fog water (CFW) on the flashover of three types of suspension insulators are studied. The concept of additional SDD is proposed to quantify analyze the effects of salt fog. The results show that there is less dependence on CFW at higher pollution levels than at lower pollution levels. That is, the effect of CFW on U50 can be neglected for SDD>0.15 mg/cm2, whereas CFW has a strong effect on U50 for SDD<0.15 mg/cm2. CFW has a greater influence on the critical flashover voltage of ceramic insulators than on that of silicone rubber insulator. A model for calculating the critical flashover voltage of insulators is proposed by considering the combined effect of CFW and pre-polluting SDD.

Surface charge accumulation subjected to direct current (DC) voltages poses hazardous effects on the surface insulation performance of the involved insulators. It is of great significance to investigate surface charge regulation methods applicable to industrialized spacers. In this study, we propose a graded conductivity coating method, with excellent adhesion strength consisting of TiO ₂ /epoxy composites with different TiO ₂ contents, realizing a gradually decreased conductivity distribution on the spacer surface from high voltage (HV) to grounded (GND) electrodes. Surface charging behavior and flashover performance under DC voltages on this kind of spacer is obtained. A simulation model for surface charge computation is constructed for the sake of better understanding the surface charging mechanism on spacers with graded conductivity coating. The results indicate that the randomly distributed surface charges are obviously suppressed by this coating manner and the ring-shaped homo-charges adjacent to HV and GND electrode dominate surface charge patterns. The main cause lies in the fact that surface charge dominant mechanism changes from the bulk conductivity model to the surface conductivity model, where surface leakage current becomes the primary sources of surface charges, with the increase in coating conductivity. DC flashover performance is enhanced as well due to the improvement of surface charge accumulation and electric field distribution. This study supplies an important reference for designing DC gas insulated transmission line spacers of high reliability as well as inspires novel ideas in surface charge regulation.

Capacitor voltage transformers (CVTs) are widely used in high-voltage power systems. Their main problem is poor stability in measurement errors. The existing approach of utilizing standard voltage transformers for regular offline error calibration is insufficient in real-time and cannot meet the requirements of intelligent substations for online operations monitoring of key equipment. Therein, this paper proposes a data-driven self-diagnosis method based on metering errors of operational CVTs. This method synchronously collects the secondary signals of three-phase CVT outputs. Firstly, under the rigid constraints of three-phase symmetrical power system operations, the residual subspace parameters that characterize the measurement errors are extracted using principal component analysis (PCA). Then, bilateral threshold detection is combined with a multi-dimensional comprehensive assessment to determine the boundary conditions of the abnormal overall error state of the three-phase CVT and its deterioration direction. Finally, the coded multi-dimensional detection results are input into the learning vector quantized (LVQ) classifier to rapidly diagnose CVT measurement error deterioration. The experimental results show that the proposed method effectively identifies error fluctuations at 0.02% with a detection accuracy for the CVT amplitude error overshoot reaching 0.1%. The method can accurately locate the faulty phases and direction of error deterioration in real-time, realizing the demand for self-diagnosing for CVT metrological errors at the 0.2-level.

The poor long-term stability of measurement error is the main problem in the operation of an electronic voltage transformer (EVT). Prevalent methods of error detection involve calibrating it regularly with a standard voltage transformer, but this is not conducive to the timely detection of the degradation in the error of the EVT. In light of this, this article proposes a method to self-detect error based on principal component analysis-wavelet packet decomposition (PCA-WPD). A standard of comparison for error detection is first established based on the characteristics of three-phase symmetrical operation. PCA is then applied to the measurement data of the three-phase EVTs, and their error is mapped to the online process monitoring of squared prediction error (SPE) statistics. Furthermore, the SPE statistics are decomposed by a three-level wavelet packet to eliminate the influence of the time-varying characteristics of node imbalance on the results of detection. The results of experiments show that the proposed method can detect the measurement error with an accuracy of 0.2 class without a standard.

The gas-solid interface electric field distortion is an essential factor leading to surface insulation failure. This study proposes the conductivity and permittivity-graded coating layer (ε/γ-layer) method, aiming at regulating surface charge and electric field distribution on GIL spacers under DC/ DC superimposed impulse voltages. The iteration optimization algorithm is developed to obtain the optimal conductivity and permittivity distribution of the ε/γ-layer. The surface charge and electric field behavior on GIL spacers with the ε/γ-layer are investigated through the numerical simulation approach. The polarity of the accumulated surface charges reverses on both the convex and concave sides as the conductivity of the ε/γ-layer increases under DC voltages, indicating the dominant mechanism changing from the bulk conductivity model to the surface conductivity model. The electric field distribution is also improved with the ε/γ-layer at the optimal permittivity and conductivity distribution. Compared with the raw spacer, the maximum electric field strength with ε/γ-layer is reduced by about 31.97% under DC, 20.56% under DC superimposed positive impulse (DC+Po.Im), 42.68% under DC superimposed negative impulse (DC+Ne.Im) on the convex surface, about 64.22% under DC, 21.37% under DC+Po.Im, 36.46% under DC+Ne.Im on the concave surface, correspondingly. It is hoped that the results of this study could spark off novel ideas for the optimal design of DC GIL spacers.