Search Results (26)

With the increase in voltage levels, higher requirements are imposed on the energy-dissipation capability of high-voltage direct current (HVDC) networks. Existing energy-dissipation schemes cannot satisfy the demands of future HVDC systems. In this paper, a composite energy-dissipation branch circuit based on liquid metal, zinc oxide varistors, and damping resistors is proposed for HVDC circuit breakers. First, the self-constricting arc initiation mechanism and energy-dissipation characteristics of gallium–indium–tin liquid metal are studied. The results show that the energy-dissipation process exhibits an obvious stage-wise characteristic. Subsequently, an energy-dissipation topology incorporating liquid metal elements is established. A simulation model for the liquid-metal module is developed using the Mayr arc theory, and the conductance evolution during arc initiation is simulated. The model is combined with a hybrid HVDC circuit breaker model for analysis. Finally, a composite energy-dissipation branch circuit is constructed. The energy allocation among different components and the corresponding power density are evaluated. In the case of connecting three liquid-metal components in series, the energy density reached 0.248 kJ/cm³, representing a 22.2% increase compared to the original. The results support the coordinated application of liquid-metal modules and conventional absorption units in HVDC circuit breakers. Full article

Wind turbines are persistently threatened by both lightning overvoltage and switching overvoltage due to their ultra-high structure, dense power electronics, and harsh operational environments, which severely endanger the safe and stable operation of the units. This paper systematically reviews the generation mechanism, type characteristics, and hazards of overvoltages in wind turbines. An internal and collaborative overvoltage protection system based on lightning protection zones (LPZs) is described. Focusing on three core protective devices—metal oxide varistors (MOVs), gas discharge tubes (GDTs), and Transient Voltage Suppressors (TVSs)—the research progress in material modification, structural optimisation, and performance evolution laws is explored. Additionally, the development of series-parallel topological collaborative design for multiple devices and active-triggered intelligent protection technologies is analysed. It is highlighted that current wind turbine overvoltage protection still faces bottlenecks in standard applicability, device operating condition adaptability, and system-level collaborative design. Future research should focus on the application of a wide bandgap and nanomaterials, the improvement of test standards tailored for actual operating conditions, and the construction of multi-physics coupling simulation and active intelligent early warning protection systems, so as to provide theoretical and technical support for high-reliability overvoltage protection of large-capacity and offshore wind turbines. Full article

Accident records from the actual operation of metal oxide arresters (MOAs) indicate that even MOA products that have passed standard tests still suffer from frequent damage. This phenomenon may be related to the fact that the current standards for MOA testing do not cover multiple stroke conditions. To investigate the damage mechanism of MOA under the effect of multiple strokes, this study conducts continuous current impulse tests on MOA and simultaneously performs finite element simulation analysis. A comparative analysis of the test and simulation results shows the following: The continuous impulse discharge process of multiple strokes causes instantaneous heat accumulation in the varistors, leading to a sudden temperature rise and inducing significant non-uniform thermal stress in the varistors; Under the condition of consistent total impulse energy, multiple strokes are more likely to cause damage to MOA varistors. Moreover, the higher the amplitude of the lightning current in multiple strokes, and the shorter the impulse interval, the higher the risk and degree of thermal damage to the MOA varistors; By analyzing the rate of change in the maximum thermal stress of the varistors, the significant effective range of the superposition effect of multiple strokes under different impulse intervals can be obtained. Full article

Metal-oxide surge arresters (MOSAs) are critical devices for overvoltage protection in power systems, and the aging characteristics of their zinc oxide (ZnO) varistors under impulse and power-frequency voltages exhibit significant differences. However, traditional methods for monitoring the aging state of surge arresters suffer from limitations such as insufficient sensitivity and vulnerability to harmonic interference. Therefore, this study conducted accelerated aging experiments on ZnO varistor samples under negative-polarity impulse currents. Key parameters were measured, including the DC reference voltage, the DC leakage current, nonlinear coefficients, and the full current under harmonic-containing power-frequency voltage at a voltage ratio of 0.6. The resistive component was accurately extracted from the full current using a separation method based on the Levenberg–Marquardt (LM) optimization algorithm. Spectral analysis was then performed on both the full current and the extracted resistive current components. The experimental results demonstrate a significant polarity effect in the aging of ZnO varistors under unipolar impulse currents. The LM optimization algorithm enables precise extraction of the resistive current component from the full current. Furthermore, compared to the fundamental and third harmonic components, the amplitude of the DC component within the resistive current exhibits the highest sensitivity to aging, establishing it as a viable aging criterion. Full article

DC circuit breakers (DCCBs) are the key equipment to rapidly interrupt the fault current in high-voltage DC power grids and ensure the safe operation of the system. However, most DCCBs do not take current-limiting measures and rely solely on current-limiting reactors in the system to limit the rate of current rise during the interruption process. The extensive use of fully controlled power electronic devices in circuit breakers (CBs) results in high costs. To address the issues above, this paper proposes a DCCB topology with a current-limiting function based on thyristors and diodes, which can reduce the cost of CB while ensuring reliable interruption. The impact of various parameters on CB performance is analyzed using numerical calculations to optimize the parameters. Then, a simulation model of a 500 kV/16 kA DCCB is built in PSCAD/EMTDC, and the performance of the proposed CB topology is compared with the other CB topologies. By comparison, the proposed DCCB topology can reliably isolate fault currents and reduce the amplitude of fault currents and the cost of CBs. Significantly, the energy absorbed by the metal oxide varistor (MOV) during the interruption process decreases by 64.2%, reducing the cost and volume of the MOV. Finally, the feasibility of the CB is further verified in the ±500 kV 4-terminal high-voltage DC power grid simulation model. The results show that the proposed DCCB topology can limit the fault current rise rate, interrupt and isolate the fault reliably, and reduce the cost. Full article

Under the premise of using the solid-phase method to pre-sinter Bi₂O₃ and Sb₂O₃ into BiSbO₄ as a substitute for equal amounts of Bi₂O₃ and Sb₂O₃ in the formula, the effects of co-doping with In(NO₃)₃, Al(NO₃)₃, and Y(NO₃)₃ on the microstructure and electrical properties of ZnO varistors were studied. The experimental results show that with an increase in In³⁺-doped molar concentration, the leakage current of the ZnO varistor shows a rapid decrease and then a slow increase trend. However, the nonlinear coefficient is the opposite of it. With the combined effect of the rare earth element Y³⁺, the average grain size is significantly reduced, which leads to an increase in the voltage gradient. At the same time, a certain amount of doped In³⁺ and Al³⁺ is dissolved into the grains, resulting in a decrease in grain resistance and thus a low level of residual voltage. The varistor with 0.6 mol% In³⁺, 0.1 mol% Al³⁺, and 0.9 mol% Y³⁺ doping ratios exhibits excellent overall performance. The nonlinear coefficient is 62.2, with the leakage current being 1.46 µA/cm² and the voltage gradient being 558 V/mm, and the residual voltage ratio is 1.73. The prepared co-doped ZnO varistors will provide better protection for metal oxide surge arresters. Full article

Varistors processed from mixtures of certain metal oxides (as additives to the main component, zinc oxide, ZnO), called MOVs, represent the devices most used for overvoltage protection and are integrated into the construction of high-performance surge arresters. The manufacturing process of these powerful electronic devices is crucial for their electronic performance. For manufacturing temperature-related studies, we used two seven-varistor experimental series: one based on two added oxides and the other on five ones. The main goal of these series was to identify the suitable sintering temperature in the case of each chemical composition from the point of view of assessing the most important electric/electronic behavioral parameters. A simple study considering mass losses after the sintering process was carried out in order to provide a brief reference for the manufacturing engineers. Before performing these studies, each varistor was sintered at a different temperature. In order to draw a general set of conclusions about the impact of the sintering pressure on the main electrical and electronic performances, a second activity involved producing two additional smaller varistors series with similar chemical compositions (two main oxides and five main oxides as additives) all processed at two different sintering pressures 4900 N/cm² and 9800 N/cm². The electrical/electronic parameters considered for the assessment are the main current–voltage characteristics, the non-linearity logarithmic coefficient, and the normal operational leakage current. All electrical/electronic behavioral tests were performed according to the IEC standards and regulations for both types of varistor devices (seven different temperatures and two pressure values). We concluded that a sintering temperature of 1300 °C and a pressure of 4900 N/cm² are optimal for both types of varistors (with two and five additives). Full article

The discovery of the transient-surge-withstanding capability of electrochemical dual-layer capacitors (EDLCs) led to the development of a unique, commercially beneficial circuit topology known as a supercapacitor transient suppressor (STS). Despite its low component count, the new design consists of a transient-absorbing magnetic core which takes the form of a coupled inductor placed between the AC-main- and load-side varistors. With an introduction to the structural features of metal oxide varistors (MOVs), gas tubes, thyristors, and EDLCs, this research presents a frequency (S)-domain analysis of an STS circuit to accurately model the surge propagation through its coupled inductor. Transient energy distribution trends among STS components are estimated in this paper, with an emphasis on peak energies absorbed and dissipated by the various inductive, capacitive, and resistive circuit elements. Moreover, this study reveals STS transient-mode test waveforms validated by a standard lightning surge simulator with supporting simulation plots based on LTSpice numerical techniques. Both experimental and simulation results are consistent, with the analytical findings showing 90% of the peak transient propagating through the primary coil, whereas only 10% is shared into the secondary coil of the coupled inductor. In addition, it is proven that the two STS MOVs dissipate over 50% of the transient energy for a standard 6 kV/3 kA combinational surge, while the magnetic core absorbs over 20% of the energy. All test procedures conducted during this research adhere to IEEE C62.41/IEC 61000-4-5 standards. Full article

While traditional AC mechanical circuit breakers can protect AC circuits, many other DC power distribution technologies, such as DC microgrids (MGs), yield superior disruption performance, e.g., faster and more reliable switching speeds. However, novel DC circuit breaker (DCCB) designs are challenging due to the need to quickly break high currents within milliseconds, caused by the high fault current rise in DC grids compared to AC grids. In DC grids, the circuit breaker must not provide any current crossing and must absorb surges, since the arc is not naturally extinguished by the system. Additionally, the DC breaker must mitigate the magnetic energy stored in the system inductance and withstand residual overvoltages after current interruption. These challenges require a fundamentally different topology for DCCBs, which are typically made using solid-state semiconductor technology, metal oxide varistors (MOVs), and ultra-fast switches. This study aims to provide a comprehensive review of the development, design, and performance of DCCBs and an analysis of internal topology, the energy absorption path, and subcircuits in solid-state (SS)-based DCCBs. The research explores various novel designs that introduce different structures for an energy dissipation solution. The classification of these designs is based on the fundamental principles of surge mitigation and a detailed analysis of the techniques employed in DCCBs. In addition, our framework offers an advantageous reference point for the future evolution of SS circuit breakers in numerous developing power delivery systems. Full article

This paper introduces a novel approach to fault localization in transmission lines with series compensation using nonlinear optimization. The objective function minimization provides information on the fault distance and the current at which the protective varistor starts conducting. Terminal source data, the maximum current capacity of the capacitor, and the operating curve of the metal oxide varistor protection element are not required. The proposed method successfully met the predefined objectives, with a maximum error of 1.13% in simulated scenarios. In validating the method using real cases from a Brazilian energy concessionaire, the errors were below 0.5%. Full article

This article reviews the progress in developing ZnO-V₂O₅-based metal oxide varistors (MOVs) using powder metallurgy (PM) techniques. The aim is to create new, advanced ceramic materials for MOVs with comparable or superior functional properties to ZnO-Bi₂O₃ varistors using fewer dopants. The survey emphasizes the importance of a homogeneous microstructure and desirable varistor properties, such as high nonlinearity (α), low leakage current density (J_L), high energy absorption capability, reduced power loss, and stability for reliable MOVs. This study investigates the effect of V₂O₅ and MO additives on the microstructure, electrical and dielectric properties, and aging behavior of ZnO-based varistors. The findings show that MOVs with 0.25–2 mol.% V₂O₅ and MO additives sintered in air over 800 °C contain a primary phase of ZnO with a hexagonal wurtzite structure and several secondary phases that impact the MOV performance. The MO additives, such as Bi₂O₃, In₂O₃, Sb₂O₃, transition element oxides, and rare earth oxides, act as ZnO grain growth inhibitors and enhance the density, microstructure homogeneity, and nonlinearity. Refinement of the microstructure of MOVs and consolidation under appropriate PM conditions improve their electrical properties (J_L ≤ 0.2 mA/cm², α of 22–153) and stability. The review recommends further developing and investigating large-sized MOVs from the ZnO-V₂O₅ systems using these techniques. Full article

In this study, commercially low-voltage MOVs are exposed to switching surges to analyse and model the relationship between the number of surges and the MOV grain barrier height response. Repeated slow-front overvoltage transients are used to degrade the protective qualities of metal oxide surge arrester devices, affecting their reliability and stability. A total of 360 MOVs with similar specifications from three different manufacturers are degraded under switching surges at a constant temperature of 60 °C. The reference voltage and C-V characteristics of MOVs are measured before and after the degradation process to analyse the MOVs’ conditions. Grain barrier heights are determined from the C-V characteristics curve. An F-statistical analysis is then applied to analyse the effects of number of surges on the grain barrier height. The T-test is used to assess the statistical difference between the tested groups. Linear regression analysis is then applied to model the relationship between the number of surges and MOV grain barrier height. The results obtained show that the number of surges has a significant impact on grain barrier height. MOV grain barrier height is found to decrease as the number of surges applied increases. Regression models obtained for the tested MOV groups across all three manufacturers agree and indicate that the reduction in grain barrier height results from an increased number of surges. Regression coefficients of a developed model indicate that for one surge applied, the MOV grain barrier height decreases by 0.024, 0.055, and 0.033 eV/cm for manufacturers X, Y, and Z, respectively. Therefore, there is a linear relationship between grain barrier height and the number of applied switching surges. Full article

This paper proposes a systematic and deterministic method for metal-oxide varistor (MOV) surge arrester selection based on the comprehensive analysis in line-commutated converter (LCC)-based high-voltage direct current (HVDC) transmission systems. For the MOV surge arrester, this paper investigates several significant impacts on the transient overvoltage (TOV) phenomena, which is affected by practical factors such as an operating point of the LCC-HVDC system, synchronous machine operating status of the power system, AC passive filter trip, and communication delay in a special protection system (SPS). In order to determine an appropriate rating of surge arrester, especially for TOV, this paper considers a pattern, magnitude, and duration of TOV based on various fault scenarios in an electrical power system with an LCC-HVDC system. A screening study method with 60 Hz and RMS-based balance system is conducted for examining a wide range of fault scenarios, and then for the specific test cases that need a detailed analysis, electro-magnetic transient (EMT)-based analysis models are developed with an approvable boundary setting method through the equivalent network translation tool. A detailed EMT study is subsequent based on the distinguished cases; as a result, the exact number of metal-oxide resistor stacks could be obtained through the detailed TOV study according to this procedure. The efficacy of the selection method from the proposed procedure based on the comprehensive analysis are verified on a specific power system with a 1.5 GW DC ± 500 kV symmetric monopole LCC-HVDC transmission system. Full article

Deep decarbonization is the goal of modern power systems, so it is inevitable that large-scale wind farms will be integrated into systems. This also gives rise to many problems, which have been studied in detail in the literature. However, these studies basically have two deficiencies. One is to assume that traditional generator units are fully loaded, and the other is not to compare the differences in the impact of different types of wind farm. This paper discusses these two points in detail. Taking a series-compensated transmission system as the research object, and assuming that the wind farm only replaces part of the power of the traditional generator unit in the transition period of energy conversion, the difference between the torsional vibration behaviors of the traditional unit caused by adopting different types of wind farm is discussed. The results of the study show that the impact of integration of the type 3 wind farm is dominated by the induction generator effect of doubly fed induction generator units. The penetration rate as low as 19% could cause instability. However, the paralleled metal-oxide varistor can effectively improve the stability. For the type 4 wind farm, the dominant factor turns out to be the de-rating operations of the steam turbine generator unit. The allowable penetration rate depends on the turbine damping. When the turbine damping is sufficient, the penetration rate can be as high as 87.5%. In conclusion, in order to integrate wind farms into a series compensated transmission system, one should not only focus on the compensation factor to avoid the sub-synchronous torsional vibrations, but also pay attention to the types of wind farm as well as the penetration rate. The findings can be used as the decision-making basis for the integration of wind farms during the energy transition period. Full article

This article presents an original experimental method applied to assess the stability limits of a given Metal Oxide Varistor (MOV), with cylindrical symmetry (cylinder or disk shape), as a direct relation between the ambient temperature and the service rated voltage, in the permanent operational regime. As the crossing current of a certain varistor is heavily influenced by its physical temperature, we must find an empirical relationship between these two parameters for a symmetrical configuration. Each ambient temperature can increase the temperature reference and any higher voltage will also produce an increased current, causing a uncontrolled runaway heating process, in an avalanche model. We tried to eliminate any references to technical dimensions or device values, focusing on material parameters. In the case of a symmetric MOV, we will consider the load coefficient, which is the ratio between the service DC voltage and the nominal DC opening voltage. By using experimental measurements and a simple mathematical model, we will establish the relation between the critical load coefficient and the ambient temperature in the case of symmetrical MOVs. This procedure could be applied to the design of more performant and safe surge arrester devices using existing MOVs, for all voltage levels and symmetrical configurations. Full article