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The direct-current (dc) fault-clearance capability of the modular multilevel converter (MMC) is a key issue for overhead line applications of voltage source converter-based high-voltage dc (VSC-HVDC) transmission systems. An MMC integrated with a dc circuit breaker (IDCB-MMC) is proposed to provide a cost-effective DC fault clearance and isolation solution. A mechanical switch (MS) and a load commutation switch are connected in series as the main branch of the dc circuit breaker. DC fault currents can be forced to transfer to converter- and line-side energy absorbing branches when submodule bypass protection and dc circuit breaker interrupting operations are coordinated. Therefore, the bulky and costly solid-state branch that uses numerous semiconductor devices and surge arresters can be eliminated. The dc fault clearance and isolation capability of the IDCB-MMC were verified through the simulation results for one pole of a 1000 MW/±320 kV VSC-HVDC system. Compared with an MMC that uses half-bridge SMs, the IDCB-MMC only requires few additional capacitors and switching devices and a simple MS, thereby resulting in several advantages, such as low cost, footprint requirements, and additional conduction losses.

This paper presents a comprehensive analysis and a novel solution for the discharge problem on the half-bridge submodules’ (HBSMs’) capacitors in the hybrid modular multilevel converter (MMC)-based high-voltage direct current transmission systems under a long-term (e.g., several minutes or longer) pole-to-ground fault condition. The mechanism of this issue is analyzed, and then a solution based on fundamental-frequency reactive circulating current (FFRCC) injection is proposed. The influence of FFRCC injection is analyzed in terms of the fluctuated arm capacitor energy and the current stress of semiconductors. Moreover, an enhanced arm voltage modulation method based on HBSMs and full-bridge submodules coordinate control is proposed to accelerate the capacitor voltage balance of HBSMs. Finally, the performance of the proposed control strategy is verified by the simulation results.

This paper presents an enhanced control for HVDC systems employing hybrid modular multilevel converters (MMCs) during asymmetrical dc voltage faults. The characteristics of pole-to-ground (PTG) fault under different system grounding schemes are analyzed. Then, the principle of enhanced control strategy to deal with the PTG fault is presented for a hybrid-MMC-based HVDC system with symmetrical monopolar configuration. A new control variable is proposed on the arm voltages to distribute the power transmitted from the ac side between the upper and lower arms. By adopting the enhanced control, the overvoltage and the fault current can be eliminated when a PTG fault occurs. Furthermore, the hybrid-MMC-based HVDC system can operate continuously with adjustable active power and controllable reactive power by controlling the upper and lower arms of the converter asymmetrically during a fault period. Finally, the feasibility of the proposed control scheme is verified by simulation results in PSCAD/EMTDC.

In order to improve the shielding effect of the transmission line, the accurate line system design is significant. The electrogeometric model and leader progression model are two typical and most widely used numerical analysis models to analyze the shielding failure performance. However, both models call for more experimental evidence for accurate application. This paper investigates the characteristics of an upward leader emerging from 110-, 220-, 500-, and 1000-kV single-phase conductors through simulated experiments, and the conductor parameters and heights adopted are similar to the actual transmission lines in operation. The ac voltage generator is connected to the conductor to apply an operating voltage to the conductor. The influence of bundle number and conductor height on the upward leader is also discussed in detail. The obtained characteristics of an upward leader will be helpful to build appropriate numerical analysis models for the shielding failure of the transmission line.

Lightning is a substantial cause that leads to serious lightning failures, which has a significant influence on the normal operation of the entire power grid. This paper mainly focused on several regions of China with serious lightning activity, presenting the lightning performance of power grids by statistics obtained from long-term monitoring data, and intended to indicate some general characteristics. The lightning tripout rate, the shielding failure tripout ratio, the occurrence frequency corresponding tripout failures of different phases of single-circuit or double-circuit transmission lines, and the multicircuit simultaneous tripout failures of multicircuit transmission lines were analyzed. These statistical results presented a panorama of the lightning protection effect of transmission lines in China. The statistical data in this paper are informative to guide the future research or engineering practice of lightning protection of transmission lines, and will be helpful to evaluate the analysis methods of lightning performance of transmission lines.

The space charge produced by the glow corona from tall objects is known to inhibit the initiation of upward leaders. However, in contrast to consideration of the glow corona at the surface of conductors caused by thunderclouds, the inhibitive effect of ion flow on the initiation of upward leaders from conductors due to dc operation voltage has been relatively neglected in previous studies of the leader progression model. In this paper, the glow corona at the surface of a conductor due to the thundercloud and the operation voltage have been analyzed quantitatively and compared with each other. It is found that the influence of the operation voltage on the distribution of ion flow is the largest in the area near the conductor. Since the operation voltage has a greater influence on the inception of the upward leader than the thundercloud, it should therefore also be considered in the leader progression model. A new simplified model is hence proposed to account for the distribution of ion flow under thundercloud conditions. In addition, a simulation experiment is designed to study the influence of ion flow on the initiation of the upward leader from a 6-bundled conductor, and the results are used to verify the validity of the simplified model. Finally, the model is applied to assess the influence of ion flow on the inception of upward leaders under different lightning currents. The calculation results show that the ion flow causes the upward leader to incept later, and reveal that the influence of the thundercloud on the inception of the upward leader is small, although it influences the distribution of the ion flow significantly.

It is necessary to restrain the electric field near some civic facilities in the earth, such as gas pipes, communication cables, and so on, if a grounding device is buried nearby. This paper proposed an approach to restrain the electric field by building an underground insulating wall between the grounding device and the facilities. The leakage current from the grounding device can be forced to bypass the wall to reduce the electric field and the ground potential rise behind the wall. The electric-field distribution was simulated by using a numerical approach, coupling method of moments with a boundary-element method. The effectiveness of the proposed approach and the accuracy of the numerical simulation were verified by the field test on a real built insulating wall. The effects of the insulating wall on the electric field, the ground potential rise, and the step voltage nearby were discussed.

An understanding of the characteristics of upward leaders from transmission lines is crucial to the analysis of shielding failure. Since data on these characteristics are difficult to capture under natural circumstances, experiments are needed. In this paper, 7-m plane/rod to conductor and plane/rod to ground wire air gap have been used for studying the characteristics of an upward leader from ground wire and conductor, and the differences in the upward leader's initial characteristics between these two were compared. The experimental results show that the streamer and upward leader from the ground wire were incepted much earlier, but the difference in their velocity is small compared with that from the conductor. These conclusions may suggest that the length of the upward leader from the ground wire is longer than that from the conductor, and this condition may lead to the ground wire being struck by lightning more often compared to the conductor when no operational voltage is applied.

The sizes of ultra-high-voltage (UHV) towers and substations are related to the air clearances which are usually determined by switching overvoltage. Based on electromagnetic transient simulation of the first 1000-kV UHV ac transmission line in China, this paper extracted the effective wave information in accordance with the concept of the active part of the waveshape from calculated main types of switching overvoltage. Moreover, based on test results of switching impulse flashover voltages, a practical and accurate method to calculate the risk of failure was presented, which considered the variable characteristics of the insulation strength of the transmission line under different switching overvoltage waveshapes. Evaluation of the results focused on the comparison between two new methods and the traditional simplified method considering the effect of the front time.

The standard waveform of switching impulse is important for insulation tests of air gaps as well as external insulation and inner insulation of power apparatus in the power system, which should be consistent with the waveforms in actual power systems. Different definitions of waveform parameters of standard switching impulse are discussed in this paper, and a definition based on breakdown characteristics of air gaps is suggested to obtain the front times. Waveform parameters of switching overvoltages in two 1000-kV ultra-high voltage systems in China are studied and their statistical regularities under different operation conditions are analyzed. Based on the breakdown characteristics of air gaps and the concept of equivalent insulation performance, the typical front times in the system are obtained by a weighted average method.

The evaluation of the lightning shielding failure of a transmission line is very important in power transmission system design, and the operation voltage has rarely been considered before. However, when the operation voltage reaches 800 kV and the tower height is near 100 m, the influence of operation voltage may not be ignored. In this paper, an experimental system consisting of an air gap between a plate/rod and a conductor was established, where the dc voltage could be applied to the conductor and was accompanied by a negative impulse voltage on the plate/rod electrode. An upward leader up to 1.54 m in length was observed, and the leader velocity and charge density were obtained in a 11-m-long air gap, whereas the voltage on the conductor was 800-kV dc. The influence of dc operation voltages on the upward leader was studied. The experimental results showed that the upward leader will emerge much earlier and may progress longer with dc operation voltages. That is, the lightning attachment to the conductor may occur more easily when the voltage of the conductor is opposite in polarity to that of the downward lightning than when the conductor has no operation voltage.

The determination of the maximum ground fault current, which has a close relation to the maximum hazardous step-and-touch voltage, is essential for the safety of a substation. Seasonal frozen soil changes the grounding resistances of the substation grounding grids and the tower footing devices, which seasonally influences the maximum grid current. This paper first introduces the method to calculate the fault current-division factor, and then discusses the influence of the seasonal frozen soil layer on it by using the proposed method. Based on the cases of two substations connected by the transmission lines and without neutrals or other connection paths, the influence of several factors on the fault current-division factor, including the thickness of seasonal frozen soil, and the number of transmission lines with overhead ground wire connected to the substation grounding grid, is analyzed. It is reasonable to consider the influence of the seasonal frozen soil on the fault current-division factor in grounding grid design. According to the analysis, adding vertical grounding rods to the grounding grid and the tower footing grounding device can almost eliminate this seasonal influence.

When the HVDC system works in monopole ground return mode, dc current flows through the neutral of ac power transformer with the neutral grounded, which will enhance its vibration and audible noise. In this paper, dc bias experiments on ac power transformers carried out on 500-kV power transformers both in the transformer factory and the substation are described; the experimental results show how the vibration and audible noise of the transformer change under the dc bias current or windings voltage. The frequency characteristics of the vibration and audible noise of the dc-biased transformer are discussed under the conditions with and without loads. The direct connections between the vibration peak value and the dc bias current, and between the sound pressure and the dc bias current are revealed. Long-term online monitoring shows that the vibration of the transformer changes seriously even if there is not a dc system operating in monopole earth return mode, so when we analyze the influence of dc bias on the vibration and audible noise of the transformer, other influential factors should be expelled.

Closing resistors for breakers are used to suppress switching overvoltages, but their shortcomings make it a long-standing goal to eliminate closing resistors. In this paper, different arresters are evaluated as alternative measures to eliminate closing resistors in 1000-kV ultra-high voltage (UHV) systems. Two kinds of arresters with lower residual voltage are introduced to suppress switching overvoltages. The performance of these arresters under different operation conditions is also analyzed to ensure the safety of arresters. It is shown that closing resistors in UHV systems can be eliminated after applying proper arresters and adopting certain distributing measures.

The insulation level of power apparatus is mainly determined by the amplitudes and waveforms of system overvoltages. The substation is handled as a lumped equivalent model, and the detailed substation structure is not taken into account in present switching transient calculations. In this study, the influence of considering a substation structure on the switching transient calculation of a 1000-kV ultra-high-voltage system is analyzed, and the factors affecting this calculation are obtained. Generally, considering the substation structure will not affect the calculation of switching overvoltage amplitudes, but it will generate a damped oscillation with a very sharp wavefront and very high frequencies upon the familiar switching transient waveform. Attention should be paid to the insulation withstand level of air gaps and power apparatus to this kind of complex wave of a short-front rapid impulse superimposed on a long-front slow impulse.

Sharing the same corridor of the HVAC and HVDC transmission lines can efficiently increase the power transfer capacity of the limited corridor and will be erected in East China. An accurate and efficient numerical analysis method for analyzing the ion-flow field of the ac-dc hybrid lines is helpful for engineering design. In this paper, an upwind finite volume-based algorithm for the solution of the ac-dc hybrid ion-flow field is proposed, which can take the influence of the space charges generated by corona into account. The electric field is solved by the charge simulation method and the finite-element method, while the advection and recombination effects of the space charges are solved by a time-dependent finite volume method. The reduced-scale ac-dc hybrid models are analyzed and the calculating results by the proposed approach in this paper have good agreements with the experimental ones in the literature and in this paper. Finally, the ground-level electric field and ion current density of the hybrid UHVAC and UHVDC transmission lines in parallel are analyzed, providing a reference for the engineering design.

How to control overvoltages in ultra-high voltage (UHV) systems is critical to ensure their safe operations, and the switching overvoltage level is an essential aspect of insulation coordination. The deep suppression of switching overvoltage is thus a key issue of current ultra-high voltage (UHV) systems. Presently, the low residual voltage surge arresters and the arresters with controllable residual voltage are being developed. These new arresters could be an effective way to provide deep suppression of switching overvoltages in UHV systems. Simulations show that the low residual voltage arresters will dramatically reduce overvoltage levels of various types of switching overvoltages. Much lower overvoltage levels can be obtained by distributing arresters along the line. As a result of using these low residual voltage arresters, present overvoltage protection methodologies and insulation coordination rules will be changed; overvoltage withstand requirements for UHV power apparatus will be greatly reduced, and the closing resistors for breakers can be eliminated.

The transient characteristics of groundings are decided by the impulse breakdown characteristics of soil, thus, the influence of soil ionization neighboring the grounding electrodes should be considered in the accurate lightning protection analysis. The breakdown delay is an important parameter of soil impulse characteristics, which are closely related to the soil properties. In this paper, the impulse breakdown delay of soils under varied temperatures and moistures were tested and the impacts of the amplitude and polarity of the applied impulse, the temperature and moisture of the tested soil samples on the breakdown delay were analyzed. A concept of critical breakdown delay of soil was proposed. From our tests, the soil breakdown under impulse is possibly an electrical but not a thermal process. According to the electrical breakdown mechanism of soil and the impulse breakdown mechanism of gas, the main conclusions were explained.

Photoemission replenishment criterion (PRC) is widely used to evaluate the onset conditions for negative corona discharges in atmospheric air. The relationship between the PRC and the air density is built up in this paper, which is validated for a wide range of electrode radii and low air densities. The total effective region responsible for photoemission is taken into account and a new geometric parameter that characterizes the photon absorption in the discharge volume is proposed, although the modification insignificantly affects the results. The numerical results agree well with Peek's experimental data for negative corona discharges in air, which are also consistent with Peek's formula for coaxial cylinders and Whitehead's formula for negative corona onset.

Deutsch assumption greatly simplified the calculation of the ion-flow field under HVDC transmission lines. However, its applicability remains to be proved for the 800-kV HVDC projects in contemplation in China. The ground-level electric field and ion current density are calculated with an upwind differential-based 2-D method and a Deutsch assumption based on the 1-D method to analyze the errors of the ion-flow field caused by Deutsch assumption. A dc bipolar line laboratory model is established to verify the methods. The parameters indicating the corona discharge intensity and the field distortion degree are introduced to evaluate the errors caused by Deutsch assumption. Then, the applicability of the Deutsch assumption is discussed on the 800-kV HVDC test line established in China. The relationship of the corona discharge intensity, the field distortion degree, and the errors caused by Deutsch assumption are analyzed. Using the corona discharge intensity to evaluate the errors caused by Deutsch assumption seems to be reasonable.

The insulation of secondary system in substation, including secondary cables and devices, have to endure the high ac power-frequency voltage during the short-circuit fault. The ac withstand characteristics of the secondary cables and devices are experimentally investigated and discussed in this paper. The insulation breakdown mechanism of relays can be classified into three modes: air gap breakdown, creeping discharge, and solid insulation breakdown, that of the microcomputer protection device is solid state component damage. The ac power-frequency breakdown voltages of the microcomputer protection device, the small-size electromagnetic and solid-state relays all are as low as about 2 kV. The statistic method to obtain the withstand voltage of long communication cable from experimental data of short cables is proposed. The minimum 3-s withstand voltages of secondary system are 6.5 kV and 2 kV, respectively, this should be considered in substation design. What's more, the volt-second characteristics of the power-frequency withstand voltage of the secondary cables and devices are analyzed and compared in detail, which would be important in the selection of secondary cables and devices. At last, the method how to guarantee high ac breakdown voltage of electromagnetic and solid-state relays and microcomputer protection device is suggested.

Live working is important for the continuity of electricity supply in power system. Experiment results of 50% switching impulse sparkover voltages (U50) for different live working entering routes on a real tower are presented. The potential and electric-field distribution around the 750 kV straight tower window with a line worker inside, was analyzed via an approach combing charge simulation method and indirect boundary element method. The comparison demonstrates the close relationship between electric-field distribution and U50. The sparkover voltage and the maximum electric-field strength change synchronously for a specific structure. Generally speaking, the higher the electric filed strength is, the lower the sparkover voltage will be. Thus, the experiment procedure for deciding the best entering route can be simplified according to the electric-field distribution calculation.

If surge protection devices (SPDs) are installed without consideration of the concept of lightning protection zones, the equipment to be protected might be damaged despite the correct energy coordination of SPDs. This damage is induced by the reflection phenomena on the cable connecting an external SPD and the load protected. These reflection phenomena depend on the characteristics of the output of the external SPD, the input of the loads, and the cables between the load and the external SPD. Therefore, the SPD has an effective protection distance under the condition of the specific load and the specific voltage protection level of SPD. In this paper, the equipment with different load characteristics and SPDs with different voltage protection levels are analyzed combinatorially under the operation of the surge combination wave. The influence of voltage protection level on the effective protection distance is discussed. Moreover, the effective protection distance of the SPD with the same parameter in a different voltage protection level is also analyzed in detail. The analyzed results show that the effective protection distance would be very long if the impedance of equipment is close to or smaller than the characteristic impedance of the connecting cable.

This paper presents the study on the potential distribution of the ac 1000-kV ultra-high voltage metal-oxide arrester (MOA). Based on the idea of field-circuit combination, the 3-D finite-element method (FEM) and circuit-analytic method are applied to analyze and improve the potential distribution of the 1000-kV MOA. The 3-D FEM is used to obtain the 1000-kV MOA's potential distribution and stray capacitances to build the equivalent circuit of the MOA. With the equivalent circuit, the method to improve the potential distribution of the MOA by adding parallel capacitors is analyzed. A scheme to obtain the recommended values of parallel capacitors is achieved to improve the potential distribution coefficient within plusmn 5%. In addition, the influences of the grading ring and mounting height of the MOA on the potential distribution are also discussed. The potential distribution of the developed 1000-kV surge arrester was tested by the photoelectricity measurement method, which is close to the analysis results.

The line surge arrester can effectively improve the lightning protection performances of transmission lines by eliminating the insulator flashover. If the line arrester is very close to the insulator string in parallel, the lightning discharging would take place in the gap between the upper discharging ring of the line arrester and the bottom grading ring of the insulator string, there is no discharging in the series gap of the line arrester. Sometimes, the insulator string would have a flashover, the line surge arrester cannot provide protection for the insulator string. This so-called ldquotransverse dischargerdquo phenomenon is caused by the proximity effect of the discharging ring of the line arrester. This phenomenon is explained in this paper, and the transverse discharge phenomenon is simulated in high voltage laboratory. The demanded minimum distances between the 110-kV and 220-kV line arrester and insulator string protected are proposed from experimental results.

The impulse characteristics of frozen soil are the fundament for the lightning protection design of the overhead transmission lines in frozen soil environments. In this paper, the conductivity and lightning impulse breakdown characteristics of the frozen soil were experimentally studied, and those key factors influencing the impulse breakdown characteristics of the frozen soils such as temperature, water content and salinity were analyzed systematically. Comparing with the normal soil, some important conclusions on the impulse breakdown characteristics of the frozen soil were proposed. The electric breakdown process accompanied by a thermal thaw process was proposed to explain the impulse breakdown mechanism of the frozen soil.

Covered conductors have been widely used on overhead distribution lines, and are very easily broken by short-circuit arcs caused by direct lightning strokes or lightning induced overvoltages. The analysis and comparison of all measures and devices to prevent breakage of covered conductors applied in different countries are presented. All of these methods are classified into three categories according to their operation principles, and according to their functions, these methods are summarized into the arc-blocked method and the arc-guided method.

The line surge arrester with series gap can effectively improve the lightning protection performance of the transmission line and guarantee its safe operation. This paper discussed the design of two different series gap structures, and their influence on the line surge arresters. Experimentally, the line surge arresters with separated gaps were found to have higher lightning impulse discharging voltage versus time characteristics than the other arresters with fixed gaps. It has contributed to the effect of the composite insulator which fixes the two discharging electrodes. As a result, the length of the separated gap can be designed to be shorter than that of the fixed gap.

When dc current is injected into the earth through the grounding electrodes of an HVDC system, transformers in an ac system may be under dc bias if the dc currents flowing through the transformers are large enough. In this paper, a circuit model is presented to calculate the dc current distribution in an ac power system caused by the ground return current of a dc system. A complex image method is used to calculate the ground potential rise of each substation in multilayer earth. An indirect boundary element method is used to take into account the effects of mountains, rivers, and sea. The circuit equations are used to take into account the effects of the transmission lines. By using the method, the dc current distribution and the effectiveness of restraining measure in an ac power system are analyzed. The method and the results are not only useful for analyzing the influence of the ground return current from an HVDC ground electrode on ac power systems, but also useful for analyzing the effectiveness of the restraining measure.

When lightning strikes a substation or a transmission tower near the substation, high current generated by the stroke would flow into the grounding grid of the substation and dissipate in the soil. The transient characteristic of the grounding grid is very important to the safe and reliable operation of the power system. A circuit model with a lumped time-variable parameter is proposed to calculate the transient characteristics of a grounding grid under lightning current. The ionization phenomena around the ground conductors in the soil are simulated by means of time-variable parameters under appropriate conditions. The influences of different parameters on the transient characteristics of the grounding grid subjected to lightning impulse current are analyzed and the effective usage area of the grounding grid under lightning is discussed.

As the need for electric power grows continuously, transmission systems with more delivery capability, such as HVDC lines, are also needed. However, one of the biggest obstacles to construct a new transmission system is the obtaining of sufficient right-of-way, due mainly to the economical and ecological reasons. As a consequence, ac and dc transmission lines are to be built in parallel sharing the same right-of-way. In this paper, the electromagnetic interference in a dc line, due to the capacitive and inductive coupling from a parallel ac line in close proximity, is studied in detail. A multiconductor transmission line model is elaborated in the electromagnetic transient program to calculate the induced voltage from an ac line upon a dc line. A method based on Fourier series theory, to obtain the direct current flowing into the converting transformer, is presented. The generated interferences upon the dc line are calculated and analyzed through an example regarding steady state and fault conditions. Finally, the influence of various parameters upon such interferences is discussed. The paper provides an insight on how to control the generated electromagnetic interference in the dc line on account of the induced voltage from the neighboring ac line.

The low-resistivity material (LRM) is widely used to decrease the power-frequency grounding resistances of grounding devices for transmission lines in the regions with high soil resistivity. The study about the influence of the LRM on impulse performances of grounding devices cannot be found in literature. Experimental results of lightning impulse properties of grounding devices covered with LRM were presented in this paper. The influences of impulse current, geometrical dimensions of grounding devices, and soil resistivity on impulse grounding resistance and impulse coefficient of grounding devices with the LRM coverings are still the same with those without the LRM coverings. The impulse grounding resistance decreases from about 25% to 45% when the LRM covering is used. The fitting formulae to calculate the reducing ratio of impulse grounding resistance of different grounding devices are provided. On the other hand, the influence of LRM on the effective impulse length of grounding electrodes is discussed.

In order to measure the grounding impedance of a grounding system more precisely and easily, two key problems of the measurement system: the location of the voltage electrode and the elimination of the noise and interference must be conquered. The new theoretical analysis and measurement system of the grounding impedance based on short-current lead wire and a swept frequency ac source is proposed. The compensating position of the voltage electrode to obtain the real grounding impedance is evaluated by analyzing the soil structure, the grounding system under test, and the position of the current electrode. This new system measures the ground impedance at several frequencies other than power frequency, then interpolates them to acquire the ground impedance at the power frequency. The mutual inductance between the potential and current wires is analyzed and eliminated. The industrial computer-based grounding impedance measurement system is developed and the analysis software is programmed. A field test was performed and compared with the traditional method in the substation and the novel system worked well.

In this paper, the electric-field distribution around the composite insulators and the heads of transmission towers is calculated by a model coupling the indirect boundary-element method to the moment method. The heterogeneous media, such as the composite insulators and the filament metal structures such as the transmission towers and the transmission lines, are considered altogether. The method can take into account not only the permittivity but also the resistivity of the media by using the complex resistivity in the frequency domain. The electric-field distribution around a three-phase 330-kV transmission tower with composite insulators was analyzed. The electric fields on the surfaces of the grading rings and the phase subconductors and at the ends of the composite insulators are analyzed. Both the method and the results are useful for the design of high-voltage transmission lines.

The 500-kV polymeric ZnO surge arresters for compact transmission line against lightning overvoltage were developed and have been put into operation, which consists of arrester unit and series gap. The design of the arrester unit, including structure design, selection of ZnO varistor, parameter determination, polymeric housing design, and mechanical structure design was discussed. The design of the series gap and the determination of the series gap length were presented. The calculated results state the line surge arrester can highly improve the lightning withstand level of the compact transmission line, and the line surge arrester can withstand the effects of lightning.

How to precisely measure the grounding impedance of large-scale substations is a fundamental guarantee for the safe operation of power systems. If the ground wires were connected to the grounding grid, these ground wires would shunt a part of the injected measuring current and allow the measured grounding impedance to be smaller than the actual impedance of the grounding system. Based on an actual 500-kV grounding system, this paper discusses how to analyze the influence of the overhead ground wire on the measured grounding impedance and how to obtain the real grounding impedance of the grounding system. A numerical analysis model considering the grounding system, the transmission lines, towers, and their grounding devices, based on the multilayer soil model analyzed from field test data of apparent soil resistivity, was constructed to analyze the influence of ground wires and tower grounding resistances on grounding impedance, and the general effects of different parameters, such as tower impedance, grounding impedance, and circuits of transmission lines, were analyzed. The real grounding impedance of the substation with connected overhead ground wires was validated, and some useful results have been obtained, based on the field-measured values and numerical analysis.

Grounding system is very important to maintain the safe and reliable operation of power network and ensure the safety of power apparatus and operators. A new technique to decrease grounding resistance of substation is presented in this paper, which makes use of deep ground well to decrease the grounding resistance. The ground well is formed by metal tube with water percolation apertures in the soil with groundwater, which has the ability to gather the groundwater and wet the surrounding soil. The principle of deep ground well to decrease grounding resistance is to decrease the resistivity of the soil region surrounding the grounding well by leading the groundwater moving to the deep ground well under pressure, and consequently decrease the grounding resistance of the deep ground well. This novel method was applied in a substation grounding engineering, and good effect was achieved.

The shielding failure is the occurrence of a lightning stroke that bypasses the overhead ground wires and terminates on the phase conductors. The correct design to improve the shielding effect of transmission line to lightning is one of the key problems of transmission line design. A numerical analysis model for shielding failure of transmission line based on electromagnetic field theory is proposed in this paper. The electrical field of the system including lightning leader, transmission line and ground is solved, the range of lightning current striking the phase conductor is obtained, and the shielding failure probability of transmission line is calculated. The lightning leader model and the critical breakdown electrical field intensity of long gap are discussed. The analyzed results by numerical analysis model are compared with those by the electrogeometric model.

In a high soil resistivity area, counterpoise wires are applied to decrease the grounding resistance of tower grounding devices. If the conductor of counterpoise wire is very long, although the power frequency grounding resistance of the tower grounding device is decreased, the lightning protection performance of the transmission line is still not good. The influences of the length of grounding electrodes on the lightning transient characteristic were analyzed. The dynamic and nonlinear effect of soil ionization around the grounding electrode was considered in the analysis model of transient characteristics for the grounding electrodes under lightning impulse. The counterpoise wire has an effective length when lightning passes through it. When the length of a grounding electrode exceeds the effective length, the grounding conductor will not be utilized effectively. The simulating experiments were performed to analyze influences of the length of the counterpoise wire on the impulse characteristics. The formulae to calculate the impulse effective lengths of counterpoise wires were proposed. The model proposed in the paper has been validated by comparing the numerical results with experimental tests.

Frozen soil leads to the change of the soil model. This would affect the safety of the grounding system. The design of the grounding system considering the seasonal frozen soil should be based on the full investigation of the actual maximum depth of the frozen soil and the actual layered soil models. The final design scheme of the grounding system should be determined synthetically from two phases-first, the grounding system is designed in the normal soil model and its safety is checked in the frozen soil model, second, it is designed in the frozen soil model and its safety is checked in the normal soil model. The influences of the frozen soils on the optimal design of grounding systems in homogeneous and double-layer soil models were systematically analyzed, and the respective design methodologies of the grounding systems were proposed. The influence of vertical electrodes on the design of the grounding system considering the seasonal frozen soil was discussed. The analyzed results state that the safety of grounding system can be sufficiently improved by vertical grounding electrodes added to the horizontal grounding grid.

Analyzing the nonuniformity of electrical parameters of ZnO varistors is useful to select and coordinate the ZnO varistors in parallel operation and improve the protection capability of surge-protection device (SPD). The nonuniformity of 1-mA dc voltage and impulse residual voltage of commercial low-voltage ZnO varistors were estimated by statistic analysis in this paper. The Minimum Gobble Distribution was used to describe the distribution of 1-mA dc voltage and impulse residual voltage, ZnO varistors were verified conforming to the Minimum Gobble Distribution by the Kolmogorov-Smirnov test. The relationship between the impulse residual voltage and 1-mA dc voltage has a chaos phenomenon, and they have a close relative degree by t-test, the impulse residual voltage can be estimated by voltage ratio and 1-mA dc voltage. The nonuniform phenomena of electrical parameters of ZnO varistors were simulated by Voronoi network. The current overload of ZnO varistor with reduced residual voltage is serious, in order to keep safe operation of SPDs, when we design a SPD with ZnO varistors in parallel, a safe margin coefficient of discharge withstand current should be considered according to the actual relative standard deviation of surge residual voltages of used ZnO varistors.

Installing surge protection devices for a low-voltage power supply is important to ensure the survival of electric or electronic devices and systems. Sometimes one group of surge protective devices (SPDs) installed on the distribution board is expected to protect all equipment supplied power by this distribution board. This equipment is connected to the distribution board by cables with different lengths, the influence of the cable length between the SPD and protected equipment on protective effects of SPD was discussed. The reflection and oscillation phenomena of the transient voltage caused by the connecting cables were analyzed, which would affect the protective effects of SPDs. These phenomena depend on the characteristics of the SPDs, the properties of the protected loads, and the length of the connecting cable. Five different types of loads and three different cable lengths have been simulated by electromagnetic transient analysis software, and their respective transient voltages across the SPD and the load were discussed in detail. The effective protection distances of SPD to equipment were analyzed for different types of loads.

How to precisely measure the grounding resistance of substations is the fundamental factor to guarantee the safe operation of power systems. The placement rules of test electrodes for the grounding resistance measurement of grounding system in vertical-layered soil were analyzed by the numerical method in this paper. The actual demanded location of the test potential electrode respective to the true grounding resistance of grounding system in different measuring directions changed (in a wide range) when the grounding system was arranged in different soil models. The test error of 0.618 method to measure grounding resistance sometimes is very high. The test route that the test circuit is in parallel with the vertical boundary is recommended for measuring the grounding resistance of grounding system in the vertical-layered soil area. The scientific and suitable measuring method of grounding resistance is based on analyzing the actual soil model and the grounding system structure to obtain the suitable test route and choose the correct compensated point location of the potential electrode.

The nonuniformity of electrical characteristics of grain boundaries in ZnO varistors was systematically analyzed. The high nonuniformity exists in barrier voltages and nonlinearity coefficients in different grain boundaries. The barrier voltages have normal distributions, only a few grain boundaries were electrically active, and the grain boundaries can be simply classified into good, bad, and ohmic ones according to the electrical characteristics of grain boundaries. The average barrier voltage is equal to 3.3 V by the direct method, but it is only 2.3 V by the indirect method. There is a high difference between the barrier voltages by direct and indirect measurement methods. These few good grain boundaries are responsible for the good varistor effect, and control the leakage current of ZnO varistor at low values of applied voltage. The Al/sub 2/O/sub 3/ dopants affect the electrical characteristics of grain boundaries by changing the electron status in grain boundary and intragrain.

Reducing grounding resistance of a transmission tower grounding device is an important method to improve performance of transmission lines against lightning. Principles of impulse characteristics of grounding devices were discussed. Impulse experiments using grounding device models were systematically performed to analyze the influence of different factors on the impulse characteristics of grounding devices. During the experiments, soil resistivity was changed in the range of 100 to 5103 /spl Omega//spl middot/m. Effects of different parameters on impulse grounding resistance and impulse coefficients of different transmission tower grounding devices were discussed. Formulae to calculate impulse coefficients and power frequency grounding resistance of different grounding devices were obtained.

The low or high resistivity soil layer formed in raining or freezing season affects the safety of the grounding system, and leads to changes of grounding resistance of the grounding system, step and touch voltages on the ground surface. This paper systematically discusses the seasonal influence on the safety of grounding system by numerical analysis method. If the thickness of the low-resistivity soil layer formed in raining season is smaller than the burial depth of the grounding system, the touch voltage would increase. On the other hand, the raining season leads the limit of touch voltage is smaller than the actual touch voltage, even if a surface granite layer is used. The influences of raining on the safety of grounding system must be comprehensively considered. When the high resistivity soil layer is formed in freezing season, the grounding resistance, step and touch voltages would increase with the thickness or resistivity of the freezing soil layer. When the thickness of high resistivity soil layer exceeds the burial depth of the grounding system, the grounding resistance, step and touch voltages would increase sharply, and the touch voltage would exceed the limit of touch voltage decided by a surface granite layer. In order to ensure the safety of the grounding system in freezing season, the grounding system must be designed carefully, and special measures should be used, long vertical grounding electrodes added to the grounding system can effectively attenuate the seasonal influence, and improve the safety of the grounding system.

A hybrid 2-D/3-D numerical method is introduced to analyze the ion-flow field around the metal board house near a bipolar high-voltage direct current (HVDC) transmission line. First, a 2-D method is used to obtain the ion-flow field without the house. Then, let the 2-D results at a distance from the house be the boundary conditions of the 3-D calculation. Finally, a simple 3-D method is used to analyze the field in the vicinity of the house.