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Abstract A bipolar short‐circuit transient fault identification method for overhead transmission lines in modular multi‐level converter‐based high voltage direct current (MMC‐HVDC) grid based on the hybrid HVDC breaker is proposed. The circuit breakers on both sides of the positive and negative overhead transmission lines will trip after a bipolar short‐circuit fault in the MMC‐HVDC grid. In order to identify whether the fault is a transient fault, the isolating switch of the DC breaker on one line side is reclosed first, and the voltage change on this line side in the case of transient and permanent faults is analysed. By comparing the electrical characteristics of the voltage on the line side under transient and permanent faults, a transient fault identification method is proposed, and the calculation method of the setting value of the criterion is obtained. Finally, by building the MMC‐HVDC system model for simulation in power systems computer aided design, the accuracy and effectiveness of the proposed method for transient fault identification are verified.

Abstract This paper presents an analysis of 64 lightning‐induced voltage waveforms recorded in an experimental system implemented in the campus of the University of São Paulo, Brazil. The voltages were measured on a 10 m high, 2.7 km long un‐energized overhead line consisting of two conductors installed on 6 m crossarms. One of the conductors had surge arresters, while the other one was left unprotected. Strictly speaking, all the recorded voltages on the unprotected conductor were bipolar, but in many cases only one semi‐cycle could be considered important for practical purposes. Different criteria are discussed for the determination of the waveshape parameters, which may differ significantly from those of the standard lightning impulse voltage (1.2/50 μs). Based on the proposed criterion for classifying the induced voltages, unipolar waveforms accounted for 67% of the total and, except for one case, all of them had positive polarity. The median values of the front time and time‐to‐half‐value were, respectively, 5.2 μs and 15.8 μs. Bipolar waveforms were 33% of the total and in approximately 62% of the cases the first semi‐cycle was of positive polarity. In 91% of the cases the voltage peak occurred either in the first or the second semi‐cycle.

Based on a ±10 kV three-terminal MMC-MVDC (medium voltage DC distribution system based on modular multilevel converter) system, in this study, the authors mainly study on the influencing factors and protection schemes of the lightning intruding overvoltage (including direct lightning and back-flashover lightning). Firstly, a lightning intruding overvoltage simulation model of the MMC-MVDC distribution system using overhead lines is established based on the PSCAD/EMTDC program. The influence of steady-state operating voltage and the number of converter stations on the simulation results are simulated and analysed. Meanwhile, the proper simulation modelling method of the lightning intruding overvoltage for multi-terminal MMC-MVDC system is proposed. Secondly, the lightning intruding overvoltage is simulated when the line is struck by the lightning stroke current that occurs once in 20 years and the different lightning protection schemes are proposed. Finally, based on the lightning impulse withstand voltage, lightning trip-out rate and the cost of the protection schemes, the lightning protection schemes in different scenarios are proposed. The simulation results show that if the cost of the system is prioritised, the protection scheme of increasing the number of parallel stacks of the DL arresters can be selected, and if the power supply reliability of the system is prioritised, the protection scheme of installing the overhead ground wire as well as reducing grounding resistance can be selected.

With an impetuous growth of DC links and future grids that will be often hybrid, providing AC and DC supply to nearby or the same customer, an important element is often neglected, that is, electrical insulation. There are no doubts that overhead lines will step back compared to insulated cables, for known reasons of environmental impact, right of way and reliability, especially in an urban or harsh environment. On the other hand, while AC insulation systems have been investigated and used for decades, there is no analogue experience for DC polymeric insulation, nor for insulation systems subjected to power electronics supply. This study aims at enhancing the attention on this topic, focusing on insulation ageing mechanisms and models of relevance for hybrid AC/DC assets and grids. Ageing and life models for AC, DC and power electronics supply are discussed and compared to experimental data, focusing on partial discharge and space charge. It is shown how these two phenomena affect lifetime for different AC, DC and transient operating conditions. The main drivers for insulation design and material selection are discussed, showing that insulation reliability can be achieved only if operating stresses, ageing mechanisms and life models are known and accounted for properly by insulation designers.

Flashover of ice-covered insulators seriously affects the safe operation of transmission lines. It is necessary to study the electric field distribution for flashover analysis of ice-covered insulators. The electric field of ice-covered insulators was mostly calculated by a two-dimensional (2D) axisymmetric model that cannot be well corresponded to actual icing situations. In this study, a 3D electric field simulation model of ice-covered suspension insulators under moderate icing condition with applied DC voltage was established. Two characteristic parameters, E(av) and E(max) were proposed to measure electric field distortion degree. Based on the simulation results, the effects of water film conductivity, icicle length, icicle deviation angle, and icicle distribution on E(av) and E(max) were studied. The results showed that icicles have a significant influence on the electric field distribution of insulators. E(av) increases with the increase of icicle length and decreases with the increase of icicle deviation angle. E(max) increases with the increase of the icicle length, icicle spacing and adjacent icicle length difference. When the icicle deviation angle is 45°, E(max) is the largest. Lastly, the possible flashover paths were analysed based on the simulation results, which would provide a theoretical basis for building a flashover model of ice-covered insulators.

Here, using the probabilistic evaluation based on the Monte Carlo method, back-flashover rate and shielding failure flashover rate of 230 kV overhead transmission lines in the western regions of Iran are evaluated. To such an aim, first, the number of thunderstorm days per year is collected from the reported weather information in order to determine the ground flash density. Then, using MRU-200 equipment, the tower-footing resistance of several towers is measured. Matlab® software is used in order to produce lightning surges considering its probabilistic nature and randomly distribution on the ground to evaluate striking distance based on the geometric model. Then, calculated parameters are transferred to EMTP-RV software by establishing a link to perform the transient simulation and report the results for modelled 230 kV transmission line. Finally, considering IEEE-1243 standard, it is shown that due to high ground flash density, using 230 kV tower with one shield wire is not sufficient to protect the line against lightning phenomena.

Condition monitoring of outdoor insulation systems is crucial to the integrity of distribution and transmission overhead lines and substations. The objective of this study is to use a commercial acoustic sensor along with artificial neural network (ANN), to classify different typical types of discharges in outdoor insulation systems. First, ANN was used to distinguish between five common electrical discharges that were generated under controlled conditions. Next, this approach was extended to include outdoor ceramic insulators. Three types of defects were tested under laboratory conditions, i.e. a crack in the ceramic disc, surface pollution discharge, and corona near the insulator surface. Both a single disc, and three discs connected in an insulator string were tested with respect to these defects. For both controlled samples and full insulators, a recognition rate of more than 85% was achieved.

This study sets out to discuss the challenges that need to be addressed effectively to facilitate the adoption of long distance high-voltage and ultra-high-voltage direct current (UHVDC) power transmission cables. As in the alternating current case, the desire to minimise the losses has involved the use of increasingly high transmission voltages, which presents particular problems where UHVDC cables are concerned. Specifically, this study presents the case for a novel, integrated approach to the problem, which links three distinct aspects of the problem, namely: (i) cable ratings and network operability; (ii) cable design and constituent insulating material requirements; and (iii) potential strategies for the design of novel insulation materials, where the required properties are actively ‘designed in’. The study concludes that the impact of these factors acting in concert on network operation/performance requires a more integrated approach to the problem than has traditionally been the case. A number of complementary strategies for the development of novel material technologies and UHVDC cable designs are required that can meet the service performance and ultimate service lifetimes expected for new network applications.

When a transmission line is subjected to adverse weather conditions, rain drops or ice can be formed on its surface, leading to discharge phenomena known as corona. The corona discharge increases corona loss, leakage conductance, and capacitance, the latter two effects facilitating harmonic conditions and power quality degradation. This study investigates the electrical characteristics of an energised conductor under rain, rime, and glaze ice conditions. The experiments were carried out in a climate room using a corona cage. It was found that the leakage conductance of a corona-cage configuration was at its highest under glaze ice condition and at its lowest under rime condition for the same background surface electric field. With the presence of icicles, the change in capacitance under glaze ice accretion was complex. From the experimental results, it can be concluded that the corona-cage configuration is mainly characterised by its capacitive nature before the corona onset. When the corona discharge occurs, the corona cage exhibits a resistive–capacitive nature. With the increase of applied voltage, the configuration becomes more resistive.