A modular multilevel converter (MMC) is one of the promising voltage source converter topologies in the field of high-voltage direct current (HVDC) transmission. MMCs are vulnerable to the direct current (DC) short-circuit faults due to the high discharge current stress. This study first introduced the simplified equivalent circuits of MMCs under both normal and DC fault operation conditions. Then the MMC fault spread loops and fault energy distribution principles are presented. Moreover, a detailed comparative study of the existing DC fault handling schemes, which are classified as the rigid blocking and flexible blocking methods are carried out. It provides a technical reference for further study on the MMC DC fault protection schemes for the HVDC transmission system.

Development of power electronics transformers (PETs) has made dc distribution and hybrid ac/dc systems a competitive solution for future network expansion. This study focuses on the functions of a multiport PET interfacing medium-voltage and low-voltage networks. A comprehensive power flow control scheme based on a globalised multiport transmission model of the PET and droop control is derived. This allows for a coordinated energy exchange between the ports of the PET, which enables autonomous operation. Simulations of the proposed approach in a hybrid ac/dc distribution network with different levels of distributed generation and loads verify the effectiveness of the method.

DC–DC modular multilevel converter (DC–DC MMC) is an attractive candidate for high-voltage DC (HVDC) interconnections since it can provide the required voltage matching, galvanic isolation and flexible power control abilities. The semiconductor loss of such a DC–DC MMC is a major concern for both system evaluation and parameter design. However, it cannot be measured directly since it is out of the precision range of the high-voltage measuring equipment, and thus, mathematical analysis is considered as a feasible alternative. This paper proposes an accurate off-line loss calculation method for DC–DC MMC modulated by fundamental frequency modulation. Based on the characteristics of the modulation, the switching moments along with the instantaneous voltage and current can be calculated exactly, and the conduction loss and switching loss of each submodule can be expressed in a mathematical way. The calculation results are compared with the simulation results from a comprehensive switched model and show good accuracy performance within a relative error of  ± 2% under different operating conditions.

Reducing the converter loss, sub-module voltage ripple and handling DC short-circuit faults in high-voltage direct current systems are crucial for the economical and reliable operation of modular multilevel converters (MMCs). This study proposes a coordinated control method of power loss reduction and capacitor voltage balancing for AC voltage boosted full-bridge sub-module (FBSM)-MMC by controlling the second harmonic component in the circulating current. The relationship between converter loss and second harmonic circulating current (SHCC) is firstly derived for AC voltage boosted FBSM-MMC. From the analysis, it can be seen that the converter loss and ripple can be optimised by injecting the SHCC into the arm current when compared with SHCC elimination. Simulation results of an AC voltage boosted FBSM-MMC with typical parameters using PSCAD/EMTDC confirm the feasibility and validity of the proposed loss and ripple reduction method.

Based on completely mastering ±800 kV transmission technologies, the first ±1100 kV direct current (DC) transmission demonstration project is being constructed in China. Combining theoretical analysis and a large number of experiments, the margin of switching overvoltage in converter stations, the configuration scheme and performance of lightning arresters, the shielding angles of ground lines under different geographical conditions, and the maximum air gap of lines have been determined. The switching impulse flashover characteristics of equipotential sphere and typical rod-plane air gaps are also provided. The external insulation is designed differently for light, medium, and heavy pollution areas. For the ±1100 kV project, if the 8 × 1250 mm² conductors are applied, the pole gap is 26 m and the height of the line is 25 m, all the electromagnetic parameters will meet the requirement of international electromagnetic standards. The key design points of ±1100 kV converter transformers, smoothing reactors, converter valves, and wall bushings are researched and the 75 mH smoothing reactors, ±1100 kV/5000 A converter valves, and ±1100 kV/5523 A wall bushings are successfully made. The optimised hierarchical connection modes and coordination control measure of the ±1100 kV project are studied and the results can provide sufficient technical support for the demonstration project.

Thermal depolymerisation induced tracking and erosion of polymeric insulators is one of the key insulation failure modes and this process adversely affects the reliability of power delivery networks. This study reports the tracking, erosion and thermal distribution of micron-AlN and micron-AlN + nano-SiO₂ co-filled silicone rubber composites. A tracking-erosion model is presented to explain how the co-filled set of particles directly affects such mechanisms. Aluminium nitride (AlN: 5–10 μm) and silica (SiO₂: 20 nm) particles were procured for fabricating test samples. The inclined plane test according to IEC 60587 was carried out using tracking voltage method 2 with an initial applied voltage of 3 kV and a ramping rate of 0.25 kV/h over the duration of 240 min. Measurement results show co-filled composites exhibit significantly lower physical tracking and erosion as compared to micron-AlN filled composites. Thermal accumulation and average leakage current in co-filled composites are found noticeably lower than micron-filled counterparts. Moreover, the increased surface area of the combined co-filled particles in the composites provides better scattering and reduce secondary electron collision. This may impede the release of high energy causing thermal degradation.

This study attempts to elucidate whether the addition of micro and/or nano-silica (SiO₂) particles can enhance the resistance of pure polydimethylsiloxane against synergistic effects of UV, temperature and high-voltage stress. Four types of composites (U-SR, M-SR, MN-SR and N-SR) are fabricated by adding micro and/or nano-silica particles and then subjected to multi-stress degradation in a test chamber. Results show that there is an apparent surface discoloration in the form of yellowish pale tint and a significant resistance to hydrophobicity reduction is offered by N-SR and MN-SR followed by M-SR and U-SR. Scanning electron microscopy and surface roughness findings proclaimed that N-SR and MN-SR offer excellent resistance against filler exposure and an increase in surface roughness. There is a minor reduction in absorbance level of Si (CH₃)₂ and Si–O–Si functional groups of composites but interestingly, hydrophilic hydroxyl group absorbance level is found higher in the U-SR comparatively. Furthermore, dielectric response measurements indicate considerable sensitivity to weathering with N-SR and MN-SR giving the lowest dielectric loss. Results indicate that the addition of nano-silica to pure and micro-silica filled SR can enhance its UV weathering resistance considerably by building an effective UV shielding layer.

Accurate faults diagnosis in power transformers is important for utilities to schedule maintenance and minimises the operation cost. Dissolved gas analysis (DGA) is one of the proven and widely accepted tools for incipient fault diagnosis in power transformers. To improve the accuracy and solve the cases that cannot be classified using Rogers’ Ratios, IEC ratios and Duval triangles methods, a novel DGA technique based on Parzen window estimation have been presented in this study. The model uses the concentrations of five combustible hydrocarbon gases: methane, ethane, ethylene, acetylene and hydrogen to compute the probability of transformers fault categories. Performance of the proposed method has been evaluated against different conventional techniques and artificial intelligence-based approaches such as support vector machines, artificial neural networks, rough sets analysis and extreme learning machines for the same set of transformers. A comparison with other soft computing approaches shows that the proposed method is reliable and effective for incipient fault diagnosis in power transformers.

Interleaving of the high-voltage winding turns is a widely-accepted method for restraining the surge voltage stresses in extra high voltage (EHV) transformers. Reduction of voltage stresses under external surge is attributed by the increase in the self-capacitance of the interleaved winding disks. However, the apparent benefit of interleaving may be counterpoised by a significant decrease of natural frequency of windings, which in turn, may force a winding to resonate under external surges. Winding resonance is particularly noticeable when uniform interleaving is adopted for the whole winding, uniform interleaving attributes a unique natural frequency to all interleaved coils and consequently, the collective resonance of the winding disks is plausible. Uniform interleaving in large EHV transformers therefore leads to complications and the windings reportedly exhibit an unexpectedly poor surge voltage response because of such internal resonances. This study describes a novel concept termed as ‘graded interleaving’ for EHV transformer windings to overcome resonance and to achieve an improved response against surge voltage. The performance of the proposed winding was tested on a 132/33 kV two-winding transformer with a tap changer using EMTP(ATP) simulations with encouraging results. Manufacturing and real-time implementation feasibility of the winding were verified with transformer manufacturers.

Results of experimental study of nanosecond diffuse discharge in SF₆ and gas mixtures of SF₆ with H₂, D₂ and C₂H₆ are presented. The aim of this work is to study parameters of discharge between two extended electrodes with a small radius of curvature in SF₆ and SF₆ with additives. It was shown that diffuse discharge can be formed in SF₆ at elevated pressure between blade electrodes with length of 30 cm. It was also confirmed that in a sharply non-uniform electric field a beam of runaway electrons is generated and that the gap breakdown occurs due to ionisation waves which begin on electrodes with small radius of curvature. Laser action in the infrared spectral region was obtained in SF₆–H₂ (D₂, C₂H₆) mixtures. The laser output up to 110 mJ was easily achieved which corresponds to ultimate intrinsic efficiency (with respect to deposited energy) of 10%.