ABSTRACT This study presents a piecewise analytical transient behavioral model for hybrid relays, aiming to describe the multistage current evolution during switching under combined resistive–inductive loads. By introducing a time‐sliced modeling framework based on the dominant conduction mechanism in each stage, the transient process is decomposed into semiconductor conduction, arc‐dominated conduction, and metal oxide varistor (MOV) energy absorption phases. Compared with conventional piecewise models mainly developed for single‐mechanism semiconductor devices, the proposed approach extends the analytical transient modeling methodology to hybrid switching devices involving multiple coupled physical mechanisms. An experimentally derived dynamic current model of MOV is incorporated to improve the prediction of transient characteristics during the commutation process. The influence of key parameters, including arc voltage and stray inductance, on current commutation characteristics is quantitatively evaluated through sensitivity analysis. The proposed model is validated by experimental tests and simulation calculations on hybrid relays in typical low‐voltage DC systems, demonstrating good agreement between measured and calculated results, as well as the effectiveness of arc suppression during switching.

Solid-state circuit breakers (SSCBs) enable rapid and reliable isolation of short-circuit faults in photovoltaic (PV) storage and charging DC microgrids. However, the main switch typically experiences high current and voltage stress during fault isolation, thus challenging the safe operation of the SSCB. Hence, a novel SSCB topology based on a normally-on silicon carbide (SiC) junction field-effect transistor (JFET) is proposed, which can achieve fully zero-current turn-off for both the main and auxiliary switches. During a short-circuit fault, an LC resonant circuit injects a reverse pulse current into the SiC JFET main switch to achieve zero-current turn-off, while a metal-oxide varistor (MOV) branch absorbs the fault current flowing through the SiC metal-oxide-semiconductor field-effect transistor (MOSFET) auxiliary switch, thereby facilitating the zero-current turn-off of the SiC MOSFET. Subsequently, the operating characteristics and design considerations for key components, including the inductor, capacitor, and MOV, are analyzed. Finally, a 375 V/63 A SSCB prototype is constructed and validated. Experimental results confirm that the proposed topology achieves zero-current turn-off for all the switches when interrupting a 260 A fault current, thereby significantly enhancing the interrupting performance and energy efficiency of the SSCB in PV storage and charging DC microgrids.

The series capacitor compensation technology can improve the transmission capacity of long-distance transmission lines and the stability of the power grid. However, it faces limitations due to quick failure of the mainstream capacitor protection device, the metal oxide varistor, after multiple actions. To improve the conduction performance and service life of the fast protection device for series compensation capacitors, this paper proposes a long-spacing air gap switch (LAGS) based on double capillary discharge triggered plasma jet and focuses on the influence of microcavity structure parameters on trigger characteristics of the LAGS. The results show that the conduction performance of LAGS is optimal under the structure of a 20-25mm long secondary cavity, a 4 or 5mm long primary cavity, and a 10mm thick ground electrode. Under the optimized parameters, the switch can be conducted stably for about 110 times at a 1.4kV trigger voltage. Due to the recoil effect of the secondary cavity on the primary cavity, the primary cavity is severely eroded, which lowers the density and velocity of initial plasma, making it hard to penetrate the gap. By transforming the middle electrode into a 2-0.6mm contracted shape, the recoil effect can be mitigated without reducing trigger energy. Therefore, at a 1.4kV trigger voltage, the service life of the gap switch can be extended to 220 times with a less than 500 μs conduction delay. The research results indicate that a 10cm long LAGS is expected to meet the sub-millisecond and reusable conduction requirements of the series compensation protection system.

This article introduces an extensive directional relaying algorithm designed for series-compensated transmission lines. The proposed algorithm relies on observing the locus of the calculated impedance of the change of the positive-sequence circuit. Both the voltage and current signals in positive sequence circuit are observed to acquire the impedance perceived by the relay. The fault direction is identified by the quadrant in which the locus of the obtained impedance settles. The proposed algorithm is distinguished by its high performance under varying prefault power factor and active power flow direction. It is tested for a range of fault types, varying fault resistance values, diverse fault locations, and different compensation ratios. All tests have been conducted while considering the nonlinear behaviour of the shunt metal oxide varistor (MOV) with the series-compensation capacitor. Furthermore, the proposed scheme is validated with a T-connected multi-terminal Egyptian transmission system. It is extensively tested and compared with other relevant schemes in the literature.Supplementary InformationThe online version contains supplementary material available at 10.1038/s41598-025-33564-9.

Hypergeometric behavior of metal oxide varistors in DC circuit breakers

This research addresses real faults in photovoltaic (PV) systems in Cuba, as these faults result in damage to all converters, whether in operation or not, when atmospheric discharge strikes nearby. This phenomenon occurs in PV systems connected to the grid through a power transformer with an ungrounded connection on the converter side. The research examines the phenomenon of impulsive overvoltages originating from the grounding system of PV installations after lightning. A simplified model was created using MATLAB/Simulink, encompassing PV system devices with converters, surge protection devices (SPDs), metal oxide varistors (MOVs), a grounding system, and an impulsive overvoltage source simulating a lightning strike. The simulation results suggest that these overvoltages have a detrimental effect on the coordination of the SPDs and MOVs inside the inverter, which in turn damages the inverter and other devices in the PV system. This research is pioneering in its approach to addressing this phenomenon in PV systems, characterized by this study as back flash overvoltage (BFO). BFO impacts the devices connected to direct current (DC) and alternating current (AC) branches. The model demonstrates that modifying the output transformer connection on the low-voltage side (the converter side) to a grounded star connection significantly reduces the occurrence of damage to the inverter and other components of the PV system. The BFO has not been adequately addressed in international standards pertaining to safety in PV installations. The paper concludes with a series of recommendations pertaining to the implementation of the proposed solution.

This work presents an interdisciplinary experimental study on the impact of gamma radiation on the performance and reliability of surge protective components, focusing on both switching-type and limiting-type technologies. Gas discharge tubes, metal-oxide varistors, and transient voltage suppressor diodes were experimentally evaluated before and after prolonged exposure (~740 to ~2370 hours) to gamma radiation from Cesium-137 and Barium-133 sources, having activities of ~5.7 MBq and ~4.1 MBq, respectively. The novelty of the study lies in the evaluation of (i) switching-type components performance under standardized 8/20 μs impulse current waveforms and (ii) limiting-type components voltage-current characteristic over the 30–1000 Hz range up to ~1 A. The investigation considers both varying dose rates and prolonged exposure durations applied to the devices under test. The resulting data were systematically examined to assess radiation-induced variations in electrical characteristics, providing insight into the radiation resilience of surge protective components and supporting their deployment in radiation-prone environments, including aerospace, nuclear, medical, and defense applications.

Series-connected insulated gate bipolar transistors (IGBTs) are employed to support higher voltage levels in high power conversion systems. However, dynamic voltage imbalance remains a critical challenge, adversely affecting device reliability and overall system performance. Conventional hybrid balancing strategies that combine active control with passive clamping circuits exhibit significant limitations under extreme switching transients or fault conditions, primarily due to their inability to absorb overvoltage-induced energy. This results in excessive energy dissipation, increased thermal stress, and insufficient protection, ultimately limiting long-term performance. To enhances fault tolerance and energy balance in high-voltage series-connected IGBTs, this paper proposes a fault-resilient voltage balancing strategy that integrates adaptive gate delay feedback control with a Modular Gapped Metal-Oxide Varistor (MG-MOV) circuit for bottom-line protection. The MG-MOV activates only during critical overvoltage events to clamp voltage spikes and absorb surplus energy, offloading stress from the IGBT. A complete analysis of the proposed strategy is presented, including parameter design and lifetime evaluation. Experimental results show that the MG-MOV approach reduces transient turn off losses by 52.97% and total turn-off energy by 32.56% compared to active clamping. Multi-cycle voltage balancing test results further validate its long-term effectiveness and compatibility with flexible gate control strategies.

Electromechanical relays continue to play a vital role in modern control and protection systems, yet they are inherently prone to electromagnetic interference (EMI) and electromagnetic compatibility (EMC) issues caused by contact bounce and arcing phenomena. This paper presents a detailed simulation-based analysis of EMI and transient behavior for various relay contact materials, including Silver, Copper, Gold, Tungsten, and Silver Tin Oxide (AgSnO?). The proposed MATLAB model integrates both bounce and arc effects as a single overlapping event, providing a realistic representation of dynamic contact behavior during switching. Multiple suppression techniques, such as RC snubbers, metal oxide varistors (MOV), flyback diodes, and hybrid RC–MOV configurations, are evaluated across a broad frequency range to determine their effectiveness in reducing radiated and conducted emissions. The results demonstrate that AgSnO?, when paired with a combined RC–MOV suppression network, yields the lowest EMI energy and the most stable transient response. This study establishes a quantitative foundation for selecting optimal contact materials and suppression strategies to achieve improved EMC performance in electromechanical systems.

Abstract Microstructure homogeneity and grain size reduction are needed to ensure the current uniformity and reliability required in metal oxide varistors for DC circuit breakers. Cold sintering (CS)–a low‐temperature digestive liquid and high‐pressure densification process–has previously shown reduced grain growth, but it is not clear how the lower processing temperatures will impact the bismuth intergranular phases that induce current nonlinearity. This study investigates the impact of CS on the microstructure and nonlinearity of bismuth added to ZnO varistors. ZnO powders with 0–1 mol% Bi 2 O 3 are either CS at 300°C and 300 MPa or high temperature sintered (HTS) at 1100°C. CS samples reach >97% of the theoretical density. The 1 mol% Bi 2 O 3 CS samples have limited grain growth with a final average grain size of 120 ± 85 nm, while the HTS sample shows over 1.991% increase to a grain size of 2.99 ± 1.17 µm. No Bi‐rich intergranular phase is observed in the CS samples, but bismuth clearly increases resistivity with increasing bismuth concentration, with a higher voltage onset for nonlinearity and an increase in nonlinearity with increasing bismuth. This work demonstrates the ability to use cold sintering to create dense ZnO ceramics with reduced grain growth and controlled nonlinearity.

Solid-state circuit breaker (SSCB) has been identified as a potential game-changing technology for dc distribution systems. However, fewer SSCB research and applications have been reported in ac due to direct competition with mechanical CB (MCB), which has a proven record as ac circuit protection device. The proposed three-phase SSCB integrates the application-specific functions of soft starters, contactors, CBs, and thermal relays commonly present in motor control centers (MCCs). This article primarily discusses the current protection, soft start, and contactor functions of the proposed SSCB. Key design considerations for a 380-VAC/63-A three-phase SSCB are introduced, covering semiconductor device, metal oxide varistor (MOV), snubber, and heatsink selections, as well as MOV lifetime analysis. The current fault protection of the SSCB is verified at 200 A. Furthermore, novel soft start and soft turn-off strategies are proposed. The turn-off time of both strategies is determined by a grid voltage phase-locked loop (PLL) angle instead of current sensors. Finally, both simulation and experimental results using open- and closed-loop soft start strategies validate a significant reduction in motor inrush current. In addition, experimental results confirm that employing the novel soft turn-off strategy extends the lifetime of the SSCB when used as a contactor, as it does not activate MOVs. By combining the contactor, CB, and soft starter functions into one, SSCB can offer competitive advantages in MCC applications.

The dynamic models of the metal-oxide varistors (MOV) have been developed successively and they show good performances in the prediction of the residual voltage. However, energy absorption, one of the most important electrical characteristics of MOV, is also worth studying, because it is a non-negligible factor to evaluate the accuracy of the model. This paper proposes a dynamic nonlinear model for MOV considering the residual voltage and the energy absorption under wide-range transients with the rise time ranging from 20 ns to 30 μs. During modeling, the characteristics of the propose model including the capacitance, the inductance and the nonlinear resistance are studied first. Then the electrical circuit is developed in ATP-EMTP. The performance of the proposed model is analyzed and compared with the IEEE model. The results indicate that the proposed model is accurate and reliable in calculating both the residual voltage and the energy, and the maximum relative errors of the residual voltage and the energy are 5.74% and 5.29%, respectively.

Varistors are passive, two-terminal solid state semiconductor devices that are used to provide over-voltage or over-current protection to electrical and electronic circuits. Metal-oxide varistors (MOVs) are typically made up of a durable ceramic material, which protects them against sudden surges of high current or energy. However, there are some drawbacks of MOVs, such as having a short life span and a large dynamic resistance. NAMICS has developed a solvent-less epoxy paste that has shown non-linear properties and sustained reliability, very similar to that of an MOV. With NAMICS’s proprietary blend of epoxy and other additives, we were able to achieve low-voltage varistor values (< 16 V) with minimal leakage current, comparable to commercially available MOVs being sold today. Printed varistor epoxies have a chance to revolutionize the semiconductor industry by offering miniaturization of components and a solder-less, one-step curing process, all while maintaining the same properties that are offered by MOVs.

High-altitude electromagnetic pulse (HEMP) can be divided into three parts: 1) early-time HEMP (E1); 2) intermediate-time HEMP (E2); and 3) late-time HEMP (E3). Most studies focus on E1 alone, as E2 and E3 pose lower peak value, especially E2, which is considered less severe than lightning. However, E2 coexists with E1 and affects victim equipment simultaneously, which may result in different effects than E1 or E2 alone. As the most widely used surge protection device, the behavior of metal-oxide varistors (MOVs) under the consecutive E1 and E2 should be investigated and compared with those under E1 or E2 alone. Based on a pulse current injection platform, the voltage, current, susceptibility, and nonuniformity behavior of two types of MOVs under the separate E1, E2, and consecutive E1 and E2 disturbances are investigated and compared. The peak residual voltages of the MOV samples under consecutive E1 and E2 disturbances reach 15 kV, which is the same as that under the E1 disturbance and much higher than that under the E2 disturbance (1.5 kV). The MOV samples are more sensitive to the consecutive E1 and E2 disturbance, particularly those with a higher nonuniformity. Finally, the response process is discussed based on the microstructural nonuniformity of MOVs.

Lightning represents the primary threat to power grids, with approximately 80% of natural occurrences being multi-lightning strikes, which have been extensively studied and confirmed to pose even more severe hazards. Hybrid DC circuit breakers (DCCBs) exhibit promising application prospects due to their superior interruption performance. During multi-lightning strikes, lightning intrusive waves pose a greater threat to the insulation of equipment such as hybrid DC fuses, fast mechanical switches, and insulated gate bipolar transistors. Therefore, this paper establishes a simulation model using the Real-Time Digital Simulator based on the model and parameters of the DCCB in the ± 500 kV Zhangbei HVDC project in China. It analyzes the lightning intrusive waves on the DCCB and investigates the energy absorption of metal oxide varistors (MOVs) in the energy-consuming branch of the DCCB under multi-lightning strikes, as well as the overvoltage levels of various branches of the DCCB under such conditions. The research results indicate that the energy-consuming branch has a certain tolerance to multi-lightning strikes. However, if the MOV collapses under multi-lightning strikes, it can lead to severe overvoltage issues. Therefore, the design and selection of DCCBs should fully consider the impact of multi-lightning strikes.

Automotive advancement mainly focuses on Personalization, Automation, Connectivity and Electrification (PACE) which includes advancement in wireless and telecommunication domain. Different mobile wireless-charging technologies are being developed in recent automotive trends. This enables the smartphone to start charging simply by placing on the charging pad. The heart of the technology is its Printed Circuit Board (PCB) which contains multiple power-electronics to meet the customer demands. Epoxy moulded chip type Metal Oxide Varistor (MOV) is a widely used in automotive sector. In present study, an in-service failed molded MOV of a wireless charger was received for failure investigation. As per failure investigation, the MOV failed due to puncture type failure which is confirmed by presence of puncture type hole in the ceramic, smooth fracture surface and Ag precipitates in the ceramic matrix. The failure can be prevented by selecting MOVs as per threshold acceptable current limit and load dump factor of the application. Keywords: Failure Analysis, Metal Oxide Varistor, Wireless Charger, Automotive Vehicle, MOV, Materials, SEM/EDS

Metal-Oxide Varistor (MOV) is a key component in electrical protection systems, safeguarding devices from excessive voltage surges. MOV exhibits non-linear behavior, acting as an insulator at normal voltages and a conductor at high voltages, absorbing excess energy to protect connected devices. This study employs the Finite-Difference Time-Domain (FDTD) method to analyze the electro-thermal characteristics of MOV, including the voltage-current relationship, resistivity changes with temperature, and temperature distribution within MOV. The FDTD method models the distribution of electric fields, magnetic fields, and temperature within MOV, which is modeled as small rectangular elements with resistivity dependent on the local electric field and temperature. Temperature distribution is calculated using the heat transfer equation, with resistivity determined based on experimental measurement data. Visualizations include graphs of the electric field and resistivity relationship at various temperatures and temperature distribution maps. Simulation results show that the Gaussian impulse current wave generates significant voltage surges and uneven temperature distribution within MOV. Above 600 K, the material's resistivity significantly decreases, allowing larger currents to flow through MOV. Temperature distribution in the form of heat maps identifies hotspots that may cause local degradation of MOV. These findings provide crucial insights for the design and analysis of overcurrent protection in electrical devices, ensuring the effectiveness of MOV in protecting devices from excessive voltage surges.

ABSTRACT The application of cryogenic and superconducting technologies could revolutionise aircraft electric propulsion systems by substantially increasing their power density and energy efficiency. However, designing and selecting components that can operate at cryogenic temperatures present significant challenges. Fault protection in the closely coupled onboard DC power networks of electric aircraft is particularly difficult due to the rapid rise and high magnitude of fault currents. A hybrid DC circuit breaker integrated with a current limiter is a promising solution for fault protection. In this paper, a high‐power 3.3 kV/1.5 kA press‐pack IGBT has been chosen and investigated as the main breaker of the proposed hybrid DC circuit breaker, with metal oxide varistors (MOVs) to clamp the voltage during current interruption. Modifications to the press‐pack IGBT and MOV are introduced to make them compatible for cryogenic operations. The junction temperature rise of the press‐pack IGBT during current interruption is then simulated. High‐current interruption tests are conducted at both room temperature and cryogenic temperatures. The experimental results demonstrate that the 3.3 kV/1.5 kA press‐pack IGBT can interrupt currents exceeding 5.1 kA when immersed in a liquid nitrogen bath. Additionally, the voltage across the DC circuit breaker is clamped below 1 kV using cryogenically compatible MOVs.

The dc Solid-State Circuit Breaker (SSCB) is commonly used to protect sensitive equipment and enhance system stability by quickly interrupting fault currents, thereby increasing overall safety. The double Metal Oxide Varistors (MOVs) and the single capacitors are the snubber circuit topologies represented by MOV-MOV-C that have been successfully demonstrated through design and experimentation, effectively suppressing high-frequency oscillations caused by the capacitance of the MOV and blocking low-frequency harmonics introduced by the RC snubber. However, previous research highlights critical areas for improvement, particularly in reducing surge voltage across the main breaker switch, minimizing power loss in on-state normal operation, and discussing the overall design cost in low-scale applications. This paper analyzes and compares power loss between different types of semiconductor switches to optimize and minimize it. The surge voltage also improves from the traditional MOV-MOV-C into a proposed enhancement incorporating a new MOV featuring a lower dc-rated and maximum clamping voltage aimed at optimizing surge voltage protection, represented by a Triple-MOV-C snubber circuit. A cost model for critical components was also developed to discuss the total design based on a low-scale dc SSCB application of both snubber circuits and validated through the graph. Finally, the validation of power loss and surge voltage improvements was conducted through small-scale experiments.

Abstract The active clamping circuit can effectively suppress overvoltage and is often used for the series protection of IGBTs. However, when active clamping is in operation, it will increase the turn-off time of the IGBT. If it remains in an unbalanced voltage state for a prolonged period, this can lead to a significant increase in IGBT losses, particularly in high-voltage applications. This paper first analyzes the issue of turn-off losses under overvoltage conditions through theoretical derivation and then proposes a novel voltage-limiting circuit—Metal Oxide Varistor (MOV)—capable of absorbing overvoltage energy and suitable for series HV-IGBTs. Finally, the superiority of the voltage balancing circuit is demonstrated.