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Made available in DSpace on 2020-12-12T00:43:06Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-09-01 Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) This study proposes an alternative method to calculate the transient responses of multi-conductor overhead transmission lines based on ABCD representation, recursive convolutions and numerical method for rational approximation in the frequency-dependent curves. In the classical universal line model, the characteristic admittance matrix Y-c(omega) and propagation matrix H(omega) are represented by rational functions. In H(omega), previous and correct identification of the modal travelling times is required. However, these modal travelling times are estimated numerically, which do not offer good accuracy in some cases. This fact has a considerable effect on the rational representation of H(omega) itself and consequently on the transient responses. In the alternative method, a previous estimation of the modal travel times is unnecessary because when using the ABCD representation for each propagation mode, a direct relation exists between the currents and tensions of the receiving and sending ends. These relationships are fitted by smooth rational functions, and then, using recursive convolution methods, the authors obtain the explicit equations of the currents and voltages to calculate the transient responses. The results demonstrate that the alternative method is accurate when compared with the frequency-domain solution of the transmission line transformed to the time domain using the numeric Laplace transform and with the Bergeron method. Sao Paulo State Univ UNESP, Ilha Solteira, Brazil Sao Paulo State Univ UNESP, Ilha Solteira, Brazil FAPESP: 2014/17051-0 FAPESP: 2017/08486-0

An approach is demonstrated to visualise overhead line failure rates and estimated wind power output during extreme wind events on transmission networks. Reanalysis data is combined with network data and line failure models to illustrate spatially resolved line failure probability with data corrected for asset altitude and exposure. Wind output is estimated using a corrected power curve to account for high speed shutdown with wind speed corrected for altitude. Case studies demonstrate these methods' application on representations of real networks of different scales. The proposed methods allow users to determine at-risk regions of overhead line networks and to estimate the impact on wind power output. Such techniques could equally be applied to forecasted weather conditions to aid in resilience planning. The methods are shown to be particularly sensitive to the weather data used, especially when modelling risk on overhead lines, but are still shown to be useful as an indicative representation of system risk. The techniques also provide a more robust method of representing weather-related failure rates on lines considerate of the altitude, voltage level, and their varying exposure to weather conditions than current techniques typically provide, which can be used to usefully represent failure probability of lines during storms.

This chapter presents the coupling of lightning electromagnetic fields to overhead and underground lines based on the transmission line approximation. The chapter starts with an introductory section in which the advantages and the drawbacks of three approaches, namely the quasi-static approach, the transmission line approach, and the antenna approach, are presented and discussed. The conditions of validity of the application of the transmission line approximation and its suitability for the case of lightning generated fields are also presented in that section.

Through analysing the infrared temperature measurement data obtained from composite insulators for overhead lines in actual operation state and atmospheric environment, detection requirements of infrared accurate temperature-measuring technology of composite insulators for overhead lines were discussed, such as the detector, detection environment, and position. According to the local detection, two kinds of insulators' local heating, caused by insulation deterioration and damp contamination on the surface, were researched. A judgment method was proposed, and overheat of the electric equipment, which was caused by voltage effect, should be judged from both the contrast of similar equipment and the thermal image characteristic. This method can greatly reduce the insulators overheat error rate, improve the detection efficiency of composite insulators, and enhance the accuracy of composite insulators' insulation defects judgment.

The fast DC line protection technology is one of the main challenges to high-voltage direct current (HVDC) grids. Based on the Zhangbei HVDC grid project, this paper proposes a non-unit overhead transmission line protection scheme for bipolar voltage-source converter-based HVDC grids equipped with DC circuit breakers. The voltage gradient is used to detect faults rapidly and start the protection algorithm, and the voltage derivative criterion is adopted to discriminate the internal faults from the external ones. Two kinds of faulty pole identification methods are proposed based on the propagation characteristics between the line- and zero-mode voltages, ensuring the selectivity of the protection. The Zhangbei HVDC grid model is built in power systems computer-aided design/electromagnetic transients including DC and the performance of the proposed protection scheme is verified by simulation results of various fault conditions.

The ±500 kV half-bridge modular multilevel converter-high-voltage DC (MMC-HVDC) grid project with overhead transmission line is under construction in China at present. For MMC-HVDC grid, only one electrical grounding point is allowed and thus, the metal return lines on the same towers with positive and negative polar lines are necessary. However, current studies mainly focused on the control methods of MMC-HVDC substation and few studies have been done on the transient overvoltage of overhead transmission line for the MMC-HVDC project. This study analysed the main transient overvoltage types of metal return lines of ±500 kV half-bridge MMC-HVDC grid which transmit electric power by overhead lines and clear the grounding fault by DC breakers. Based on a ±500 kV MMC-HVDC grid project, the simulation models were built using PSCAD and the possible transient overvoltage types were studied. The results showed that the most serious overvoltage was caused by the DC breaker clearing the grounding fault. Then the insulation coordination design for metal return lines was carried out. The recommended arcing horn clearances at different altitude were given. The research results provided support for the insulation design of the transmission line.

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.

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.

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.

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.

Recently, there has been significant interest in developing the modular multilevel converter (MMC) based DC grids in the power industry. However, the immature DC circuit breaker (DCCB) technology hampers the development of DC grids in overhead lines transmission field. The full controlled power electronic devices based hybrid DCCB has been proposed and researched substantially. But the hybrid type DCCB suffers from the drawback of high cost and low reliability, leading to high capital cost for DC grids that require numerous DCCBs. In this paper, a mechanical type DCCB, consisting of a mechanical switch, a circuit and an oscillation circuit, is proposed. Then, a four-terminal bipolar MMC-HVDC system implementing the mechanical DCCBs is studied. Extensive simulations have been conducted to analyse the transient performance under different fault conditions. The simulation results verify the technical feasibility of this mechanical DCCB in DC grid.

When a DC line fault happens, the converter will be blocked instantly. This leads to an interruption of power transmission and influences the stability of DC and AC systems. To avoid the blocking of converters, this paper presents a novel protection scheme for VSC-HVDC system, which consists fault isolation and auto-reclosing of DC circuit breaker (DCCB). First, this paper coordinates the protective actions of converters and DCCB. The DC line fault is cut immediately by DCCB while the blocking signal of converter is isolated during the fault. Second, since the faults of long overhead transmission DC lines are mostly transient, an auto-reclosing strategy is developed. The emphasise of this part, which includes fault discrimination and time sequences, between DCCB and converter is elaborated, and the value of reclosing delay is derived by analysing the charging and discharging characteristics of capacitor paralleled with converter. Finally, the protection scheme is evaluated by overcurrent, overvoltage, and recovery time of DC system, and simulation results of a point-to-point VSC-HVDC transmission system proves its validity.

This work studies the application of short-time Fourier transform (STFT) to extract current harmonics of various fault types to design a protection method for meshed multi-terminal voltage source converter-high-voltage direct current (VSC-HVDC-MTDC) grids. The frequency spectrum of fault current harmonics is used to detect internal and external faults, the faulty pole and fault type. The method does not need the implementation of series inductance on the sides of the DC lines and it is also effective for lines combining overhead lines and cables. The window function and hop size play the primary role in STFT, which are investigated in a sensitivity analysis. A modified version of CIGRE meshed DC grid system is simulated in PSCAD in order to show the robustness of the method, and further signal processing analysis is done in MATLAB. The results show the accurate detection and discrimination between faulty sections using the proposed harmonic-based method and how important the STFT parameters are in order to have a robust protection algorithm.

The current standards related to live working provide no determined path for equal potential working, in which only safety distance is referred, so that the works are often executed relying on the line maintenance experience. As high-voltage and strong space electrical field intensity exist in 1000 kV ultra-high voltage transmission lines, the real-time safe distance measurement is difficult to be conducted and the exhaustive full-scale experiments are difficult to accomplish due to the varied true tower models and lines. Simulation calculation is aroused for improving the efficiency of safety distance calculation and safety path planning during the process of importing and exporting the potential according to the different kind of tower and the actual line situation. Here, with the cup type iron tower and hanging basket mode, the electrical field intensity in and outside of screening clothes at different part of body while climbing tower and during equal-potential process are calculated, and then the safety distance, the combination gap, and the optimal path planning are carried out. The simulation results demonstrate that electrical field intensity meet safety requirements using the proposed planning path, which enriches the theoretical basis for the safety operation of ultra-high voltage live working.

With the rapid development of China's power grid, it is becoming increasingly common that the multi-circuit overhead transmission lines cross over each other. It is important to design the lines' configuration reasonably and evaluate whether the power-frequency electric field meets the national standard. In order to analyse the electric field distribution in the maintenance area of the 220 kV tower with an overhead 1000 kV UHV AC double circuit transmission line, a three-dimensional finite element model of the crossing tower and transmission line was established. The influences of the crossing angle, span height and offset distance on the electric field distribution in the maintenance area of the lower tower and the line worker body surface electric field were analysed. The results show that the maximum electric field strength of the tower will increase 28.7% when the cross-angle decreases from 90° to 30°, and the impact of span height on the electric field distribution is much smaller at 90° than that at 30°. At 60°, when the crossing height is 16 m, the electric field strength on the worker's body surface is much less than the limiting value.

Several kinds of widely used detection technology of wire clamp on overhead lines were introduced. Although infrared temperature measurement technology can detect the overheating defects of the clamp, it is difficult to ensure the effectiveness of on-site testing due to the environmental factors, the accuracy of testing instruments, and the experience of test personnel. X-ray inspection technology can accurately measure the exact size of the steel core without damaging the wire, and find the problem of poor crimping of the clamp. According to the field test, the typical clamp defect spectrum of X-ray inspection was listed in this study. It could effectively improve the hydraulic quality detection efficiency of the strain clamp in field work. A method was proposed, to judge the inside corrosion condition of the strain clamp, and to evaluate the current conduction capability, by detecting the current on the clamp and the wire.

The Journal of Engineering : JoE 2019(16), 2318-2323 (2019). doi:10.1049/joe.2018.8628, Published by IET Digital Library, Stevenage

The ±1100 kV Zhundong–Wannan ultra high-voltage direct current (UHVDC) transmission project is under construction in China, which has the highest operating voltage in the world. The length of the UHVDC overhead transmission line is greater than 3000 km. The electromagnetic transient process caused by grounding faults might have significantly impact on the overvoltage level of DC transmission line, which is crucial for the external insulation configuration of the overhead transmission line. This paper presents a detailed electromagnetic transient simulation model for the UHVDC transmission system using an open-loop extinction angle control principle. The simulation results demonstrate that the single pole grounding fault into a bipolar system causes the most serious damage. The maximum overvoltage is 1.628 p.u. and the 2% statistic overvoltage is 1.635 p.u., while the reference value is 1122 kV. The influence of DC filters for line overvoltage is also analysed here. Furthermore, results show that adding a no gap line arrester at the midpoint of transmission line can suppress line overvoltage, while the maximum overvoltage reduces by 0.138 p.u. The results obtained provides a reference for the optimisation of the external insulation configuration.

As the load of the power system continues to increase, the capacity of switchgear climbs, resulting in that the overheating problems becoming more serious. In order to realise the monitoring of the temperature of the switchgear, the authors need to conduct a thorough analysis of the temperature rise principle of the switchgear firstly. For the purpose of getting the detailed temperature distribution in the switchgear, a series of temperature rise tests were carried out based on the KYN28A-12(Z) overhead line switchgear, and the data were analysed and compared in detail to summarise the factors that affect the temperature distribution in the switchgear.

This paper presents a technical/economic comparison between remedial measures aimed at improving the lightning performance of an existing Italian three-phase 150-kV overhead line. The line is characterized by a very high back-flashover rate (BFOR), due to large grounding resistance values. Two countermeasures are proposed: grounding system improvement with additional vertical rods and line metal oxide surge arrester (MOSA) installation on one or all phases. A Monte Carlo ATP-EMTP procedure developed by the authors, which takes into account both the tower grounding nonlinear transient response due to soil ionization and MOSA nonlinear response, has been applied to evaluate and compare the effectiveness of the proposed countermeasures. The installation of MOSA on all phases is technically the best option, but it is relatively expensive. Tower grounding improvement and MOSA installation on the lower phase yield very similar BFORs: the economic comparison strongly depends on tower's accessibility and soil nature.

Transients from lightning strikes can enter underground cables at overhead line to power cable transitions. Possible overvoltages on these surges at cross-bonding connections of the cable screens are of major concern. A model is developed for modelling overvoltages from transient signal propagation through a combined cross-bonding cable and box. This model is applied to the first Dutch 400 kV cable connection. Such model incorporates model parameters whose values depend on design details of the cross-bonding box. The values for these model parameters are extracted from the measured transmission and reflection signal on steep pulses injected into the actual cross-bonding box configuration. The model combines transmission line description for the cross-bonding cables with mainly inductive behaviour of the cross-bonding box. The obtained results are verified by measurements. The model is applied to investigate overvoltages induced at the cross-bonding cable and box on 1.2/50 µs impulse voltage injection representing a lightning impulse voltage applied to the core conductor of the cross-bonding cable. Furthermore, the effectiveness of surge arresters to reduce overvoltages at the cross-bonding cable screen is demonstrated by simulations with this model in PSCAD.

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.

Meshed HV subtransmission networks are subject to undesirable parallel (unscheduled) active power flows from the EHV system. The phenomenon is worsened by conversion of HV overhead lines to underground cables (UGCs), which increases parallel flows through the HV network because of the attendant reduction in HV line reactance; the effect increase with network extension and are compounded by UGC charging power. The study deals with the assessment of the maximum undergrounding degree for a given subtransmission network. An analytical formulation of maximum HV line extension length as a function of EHV power flow and undergrounding degree is given; optimum UGC shunt compensation requirements are evaluated. Application to a typical 150 kV-50 Hz network is presented, showing a very good agreement with power flow simulation. In particular, the case study evidences that complete undergrounding of the HV network is possible if line length is under 50 km; the proposed shunt compensation effectively raises this limit to 70 km.

Extreme weather conditions threaten Finnish electricity distribution networks that are mostly built as overhead lines. This paper presents selected methods and technologies that can be taken to improve the networks' resilience during different phases of a disruptive event.

In this paper, we first extend the Sunde logarithmic approximation for the single-wire line ground impedance to the case of a multiconductor line. The new approximate forms are compared to the general expressions which involve integrals over an infinitely long interval and an excellent agreement is found. The inverse Fourier transform of the ground impedance presents singularities which complicate the numerical solution of the transmission line equations. The order of the singularity is reduced by 1, and a careful numerical treatment is then employed to derive an equivalent and numerically more appropriate form of coupling equations in which there is no longer a singular term. Finally, finite-difference time-domain (FDTD) solutions of the coupling equations are presented and the theory is applied to calculate lightning-induced voltages on a multiconductor line. The lightning-induced voltages are calculated for the case of lossless/lossy, single-conductor/multiconductor lines and the effect of ground losses and the presence of other conductors on the magnitude and shape of induced voltages are illustrated.