This study describes the research and design of a novel and compact very low frequency cosine-rectangular (VLF-CR) and damped alternating current (DAC) voltage generator that can be used to test the insulation condition of the medium-voltage power cables. Further, a complete system based on a voltage multiplier (VM), which can be cascaded to increase the output voltage, is proposed to simultaneously realise VLF-CR and DAC voltages for the withstand voltage and offline partial discharge (PD) tests, enabling the test system to be lightweight and efficient. The system principle and operating characteristics associated with the generation of two different voltages are analysed in detail according to the new circuit topology of the proposed generator. Furthermore, oscillation suppression and multiple output units are considered to improve the voltage waveform and provide power supply to the VM module, respectively. A laboratory prototype is also developed and tested. Finally, the integrated generator is used for the PD testing on cables with artificial defects, verifying the practicability of the proposed system.

In distribution networks with arc suppression coil grounding, the fault line can be set as the energy leakage channel of the capacitive current of the entire system. During the period from fault occurrence to the whole action process of arc suppression coil, there is abundant fault information in the transient waves. However, the data of this period are usually ignored in previous methods. In this study, a novel fault section location method based on the leakage energy function is proposed. Only the fault phase voltage and fault phase current are used in the proposed method, to avoid the difficulty of the zero-sequence current measurement. Then, the action characteristics of two types of arc suppression devices are analysed. The influence of the capacitor current on the fault phase voltage and current during the entire process of arc suppression coil operation is deduced. The difference between the leakage energy on both sides of the fault section is used as the criterion for fault section location. Finally, the effectiveness of the method is verified by a case study. The simulation results indicate that the proposed method has high sensitivity and high impedance adaptability, that will have more advantages in the actual situations.

The DC conductivity of insulating oil is key to converter transformers, in which AC and DC electrical fields simultaneously exist. Research on the influence of the electric field on oil conductivity can optimise the insulation design of converter transformers. Although numerous studies have been conducted on the conductance mechanism of non-polar liquids, the explicit mechanism and properties of oil conductivity under a high electrical field remain unclear. This study developed an optimised conductance model considering charge carrier elimination, dissociation, and charge injection process. Thereafter, the authors chose three types of oils for converter transformers and measured the variations in oil conductivity and ion mobility under different electric fields with 0–10 kV/mm range. The result indicates that ion mobility will increase under a high electric field owing to the EHD effects. The conductivity of the insulating oil follows a U-shaped pattern with an increasing electric field, thereby generally matching the optimised theoretical model. Therefore, the decrease in conductivity is mainly the result of the elimination process of free ions, whereas the increase is the result of the injection process that occurs on the electrodes.

Ethylene–propylene rubber (EPR) cable is the most commonly used type in high-speed railway. Several flashover accidents take place in cable termination in an extremely cold environment. In this study, EPR cable terminations were tested in a simulated low-temperature system with varied applied voltages. The results showed that the differences in elastic modulus and free space volume between EPR insulation and stress control tube (SCT) in a frigid environment would lead to structural defects between them. Damages on the EPR insulation occurred in the form of flashover point when the ambient temperature declined to −30°C and below, with the growth rate of discharge and resulting extent of damage across the EPR/SCT interface being non-linearly related to ambient temperature. Besides, the partial discharge (PD) initial voltage and PD extinction voltage under low-temperature conditions reduced by >40% compared with those at 20°C. The flashover probability of EPR terminations increased significantly, and the shape characteristics of PD patterns have also changed obviously at low temperatures. It can be verified that under the high-voltage and low-temperature conditions, the EPR/SCT interface is prone to organisational mismatch, which will lead to the PD phenomenon and seriously affect the stable operation of EPR cable terminations.

Transformer oil is widely used in high-voltage transformers as a cooling and insulation medium. Electrical and thermal stresses expose transformer oil to degradation, which may produce dissolved decay products (DDPs) that are partially adsorbed on the insulation paper and accelerate the degradation of oil and paper. An evanescent field D-shaped plastic optical fibre (POF) sensor was fabricated using the side-polishing technique to detect DDPs in transformer oil. A laser light source and power meter were used in the prototype to measure the output power of the POF sensor. The measurement of DDPs was conducted on seven samples of insulating oil collected from the field. The sensor performance was evaluated based on different sensing areas, and the sensor sensitivity was measured. Results show an excellent correlation between the output power and DDPs. A correlation analysis was conducted amongst sensor output power, transformer oil refractive index and area under the absorbance curve. The performance of the sensor demonstrates improved linearity and sensitivity of 97.49% and 26.6779 µW/RIU, respectively. The POF sensor is easily fabricated, low cost and can be used for real-time monitoring of high-voltage transformers.

Electrical insulation of high voltage (HV) power equipment/apparatus plays very important roles in the sound functioning of power systems. Examining the insulation condition through partial discharge (PD) measurements has considerable importance as the presence of PDs in any HV systems stands for a sign of defects and degradations in electrical insulation. Generally, the PD measurements are performed with a supply voltage of normal power frequency (PF-50/60 Hz). As an attractive alternative, the PD measurement with a very low frequency (VLF–1 Hz or lower) applied voltage has emerged as a powerful diagnostic tool. A PD event is a random physical process, it depends mostly on the availability of free electrons that triggers the PD and the presence of space charge usually originated after a discharge. As the discharge characteristics vary with the supply voltage frequency; thus, the existing interpretation knowledge at 50 Hz may or may not be directly applicable to other test frequencies, particularly in the VLF range. Therefore, to provide a general insight, this paper presents various diagnostic aspects of PD measurements, and at the end, various discharge sources (e.g., void, surface, and corona) and their behaviours under varying the test voltage frequencies are discussed in detail.

Voltage endurance coefficient (VEC) is a parameter used in the design of withstand test and insulation thickness of cross-linked polyethylene (XLPE) cables. Thermal ageing is inevitable in operation of cable, but the thermal ageing effect on the VEC is rarely considered. In this study, the trend of VEC of XLPE films under DC voltage during thermal ageing is investigated by 120 and 135°C thermal ageing experiments. The results show that, after 14 days of 120 and 135°C thermal ageing, the VEC decreases from 20.74 to 7.05 and 3.93, respectively. To understand the effect of thermal ageing on the VEC, the VEC is described as a function of the free energy increment which is proportional to the degree of polymerisation (DP) and the electric field. A simulation calculating the free energy increment of several molecular chains is conducted to confirm the proportional relationship between the free energy increment and the DP, the electric field. Based on the function of the VEC, a predicting method for the VEC under thermal ageing is developed. The experimental results of 120 and 135°C thermal ageing are used to confirm the preciseness of the proposed predicting method.

The flashover voltage of high temperature vulcanised (HTV) silicone rubber will be decreased after corona ageing, and it further decreases under the effect of mineral oil. The mechanisms for the phenomena can be explained by considering the role of trap level distribution. In this study, the corona ageing of silicone rubber was conducted and the flashover voltage was tested. The isothermal surface potential decay was tested to explore the internal relationship between trap distribution and flashover voltage. Besides, the surface microtopography and chemical structure of silicone rubber were gained by scanning electron microscopy and Fourier infrared spectroscopy to explain the change of trap distribution. The test results show that the flashover voltage of HTV silicone rubber decreases with AC corona ageing time, the reduction of flashover voltage for an oil-impregnated sample is more serious. The density of shallow electron traps increases at first and then decreases, while the density of shallow hole traps increases with ageing time. The density of deep traps decreases after corona ageing. These trap level distribution characteristics are beneficial to promote the development of flashover, leading to lower flashover voltage. The effect of mineral oil can be regarded as an accelerated ageing effect.

Dielectric response measurement is a widely used technique for characterising dielectric materials in terms of their capacitance and dielectric loss. However, the widely used approach with contact between samples and electrodes can in some cases limit the accuracy of the measurement. The authors introduce an easily realised electrode arrangement for non-contact measurements, which avoids these contact problems. The performance of the electrode arrangement in terms of the edge effect is assessed. The non-contact and contact methods are compared based on error-sensitivity analysis and experimental results. Differences are studied further, with attention to contact pressure. The non-contact method is also compared experimentally with the one-sided non-contact method. Air-reference measurements, comparing the sample to an air-gap for improved calibration, are used for all measurements. The results show that the non-contact method can be an alternative to reduce contact problems between the sample and electrodes, although error sensitivity can be higher when the non-contact method is used. The non-contact method can decrease the influence of the pressure applied to the sample compared to the contact method, and can also reduce the problem of poor contact that can arise from the absence of pressure in the one-sided non-contact method.

Surface tracking and erosion is an irreversible degradation occurring on the insulator surface, and this can ultimately lead to failure of the insulator. Polymeric materials such as silicone rubber have many advantages; including a superior hydrophobic surface. However, polymers are exposed to ageing and degradation resulting from electrical and environmental stresses. The rotating wheel dip test is adopted to conduct a comparative study of surface conduction on the two insulators. A conventional design has been selected and compared with insulators having textured surface. Monitoring of the shed surface and insulator trunk using an IR camera were carried out to assess the temperature distribution along insulator profiles. A spatial analysis was also performed to identify key features of the two designs. Localised surface conductance measurements are proposed in this study. This helps to understand and distinguish the trends of conductance and its distribution on each surface, helping to predict the future surface degradation associated to each design.