Varistor Research Articles

Metal Oxide Varistor Design Optimization and Main Breaker Branch Switch Control of a Progressively Switched Hybrid DC Circuit Breaker

In this article, metal oxide varistor (MOV) design optimization and switching control in the main circuit breaker (MCB) branch of a progressively switched hybrid dc circuit breaker (DCCB) is presented. A progressively switched hybrid DCCB can achieve faster fault isolation with reduced peak fault current magnitude and transient recovery voltage compared with a regular hybrid DCCB due to its modified switching strategy. Consequently, thermal stress on the semiconductor devices in MCB is significantly reduced. Analytical model of the system dynamics during fault isolation with progressive switching is derived to demonstrate the switching scheme’s effect on the energy-absorbing component, MOV, during turn- <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">off</small> process. Derived analytical model in conjunction with the displacement curve of the fast mechanical switch of the hybrid DCCB is utilized to optimize the components of the main circuit breaker branch to reduce MOV degradation through asymmetric energy dissipation. A model of the circuit breaker is built in PSCAD to validate the performance of the proposed optimization method in a 10-kV/250-A system with four stage progressive switching. Additionally, a low voltage system model at 380 V is developed in PLECS for two stage progressive switching that works as the basis of experimental validation. This includes both lookup table-based MOV model and device thermal model for junction temperature estimation. Experimental results are provided for a 380-V system to demonstrate reduced fault current peak in a progressive switching and near uniform energy absorption in optimally selected MOVs.

An Energy Absorbing Method for Hybrid MMCs to Avoid Full-Bridge Submodule Overvoltage During DC Fault Blocking

The full-bridge submodules (FB-SMs) in hybrid modular multilevel converters need to absorb the enormous energy stored in dc side and arm inductors during dc fault blocking, which may lead to severe overvoltage. An energy absorbing branch (EAB) composed of metal oxide varistor (MOV) and thyristors is proposed in this letter. No extra power loss is produced by the EAB during normal operation. The EAB can absorb a part of the energy and reduce the energy absorbed by FB-SMs suffering from severe overvoltage by clamping the dc voltage. Thus, the maximum FB-SM overvoltage is reduced. Turning off the EAB several milliseconds after the blocking of the converter can accelerate the decaying of the dc fault in the transmission line and reduce the required energy volume of MOV with minimal effect on the maximum FB-SM overvoltage. The proposed EAB shows better technical and economic performance than existing methods. The proposed EAB is validated by simulations and experiments.

Implementing Symmetrical Structure in MOV-RCD Snubber-Based DC Solid-State Circuit Breakers

This article deals with metal-oxide varistor (MOV) and resistor&#x2013;capacitor&#x2013;diode (RCD) snubber-based solid-state circuit breakers (SSCBs). There are two main contributions. First, a design methodology of MOV-RCD snubber parameters is proposed by considering thermal dissipation and snubber charging time for the first time. The thermal profile of the main conduction switches determines nominal current under passive cooling. Based on the nominal current, the fault current magnitude is derived, and a design method of key parameters of an MOV-RCD snubber is proposed. Four snubbers are designed with different fault currents and dc voltages. Second, symmetrical layouts with distributed parallel cells are proposed for high current capability, which enables scalability. Q3D simulations facilitate the evenly distributed stray inductances among paralleled branches. Both low- and medium-voltage SSCBs are implemented, ranging from 500&#x00A0;V&#x002F;300&#x00A0;A to 2&#x00A0;kV&#x002F;1&#x00A0;kA. The power density achieves 37.8&#x00A0;kW&#x002F;L at 500&#x00A0;V&#x002F;1&#x00A0;kA. Experiments demonstrate that the reverse voltage reaches 4.88&#x00A0;kV with an ultrafast interruption speed of 24&#x00A0;&#x03BC;s for 2&#x00A0;kV&#x002F;1.144&#x00A0;kA fault current including the MOV conduction time interval.

A Durability Model for Analysis of Switching Direct Current Surge Degradation of Metal Oxide Varistors

In this study, a durability model for predicting the lifetime of MOV devices used to prevent DC switch damage due to occasional switching surges is proposed and validated. In addition, MOV devices are subjected to induced switching DC surges of a constant amplitude and variable time durations. Each MOV of the 270 selected devices sourced from three different manufacturers with similar size and electrical specifications was subjected to 5000 degrading surges. Three samples of 30 of the selected MOV devices from each manufacturer were degraded by induced switching DC surge durations of 2, 3, and 4 ms in order to reach an undesirable degradation level of 10% change in V1 mA. A statistical analysis of the three MOV manufacturer sample averages of the accumulated conduction charge transfer at 10% change in V1 mA supported the proposed durability model irrespective of the surge charge content variation and MOV material differences. The results show that MOV device durability or resilience may be more accurately modelled by using the surge average accumulated conduction charge transfer of a statistically significant MOV device sample.

A Novel Solid-State Switch Scheme With High Voltage Utilization Efficiency by Using Modular Gapped MOV for DC Breakers

Metal oxide varistors (MOV) is often used to limit the overvoltage generated by the current breaking of dc breaker to protect power electronic device from breakdown. However, while suppressing overvoltage, the MOV also causes the degradation of continuous operating voltage capability. This letter proposes a novel solid-state switch scheme with high voltage utilization efficiency by using modular gapped-MOV (MG-MOV) for dc breakers. The breakable gap is first applied to the protection of power electronic devices. By using the gap to share the voltage in the static-state and be broken down in the dynamic state, the overvoltage and continuous operating voltage of the solid-state switch can be decoupled. Moreover, based on the modular scheme, the instability of the gap discharges and the voltage imbalance of the series devices can be solved well. The working mechanism, voltage-balance and stability issue of the MG-MOV are discussed in detail in this letter, and a 15-kA prototype is developed to verify the feasibility of the novel solid-state switch scheme. The experiment under results show that voltage utilization efficiency can be increased from 44.4% to 72.0% by using MG-MOV.

Three‐phase overvoltage at lightning strike point due to direct triggered lightning to the phase wire of 10 kV power distribution line

This paper reports the observation results of three-phase overvoltage at the lightning strike point of a 1.5 km, 10 kV power distribution line that was struck directly by rocket triggered lightning to its phase wire C. The arrester of the direct struck phase at the lightning strike point operated in all return strokes corresponding to return stroke current from −3.3 kA to −32.9 kA. The waveform of direct struck phase includes an initial spike before negative peak, operation area of metal oxide varistor, and the final attenuation with oscillation. The overvoltage waveforms of the indirect struck phase at the lightning strike point show a small initial negative peak, rapid transition to a large positive peak, and the final negative attenuation with oscillation. The negative peak value of the indirect struck phase at lightning strike point ranged from −4.1 kV to −25.2 kV and that of the direct struck phase was between −78.1 kV and −167.8 kV. The positive peak value of indirect struck phase was between 0.2 kV and 49.8 kV. The negative peak value, zero-crossing time and the peak-to-peak value of voltage all have a good linear relation with the peak value of lightning return stroke current.

Failure analysis of ZnO arresters in the 500kV AC filter of the HVDC/UHVDC power transmission system

The ZnO arresters in 500kV AC filters in many converter stations have abnormal discharge during operation, which accelerated the aging of arresters and even caused arresters out of work. In this paper, through the disassembly experiment of the whole arresters and the test of the electrical characteristic parameters of the varistors, the failure reason and aging characteristics are preliminarily analyzed. Firstly, it is found that there are obvious cracks and surface discharge traces on one individual varistor of the F1 arrester for AC filter; while disassembling the F2 arrester for AC filter, it is found that the appearance of the porcelain sleeve and the internal va ristors of the arrester are intact. Secondly, The electrical characteristic parameters measurement results show that the electrical characteristic parameters of each varistor of F1 arrester for AC filt er are dispersed, as high as 8%; the DC reference volta ge U1mA and leakage current IL at 0.75 times U1mA of each varistor of F2 arrester for AC filter have obvious polarity effect. Therefore, it is preliminarily inferred that the frequent large impulse current is the main reason of the arresters failure, and p arameters dispersion and polarity effect of the varistors in one arrester are critical influencing factors.

A SSSC protection scheme based on thyristor integrate with phase change material

With the development of the economy and society, the power of distribution network increases and the demand for power quality is higher and higher. At the same time, with the rapid development of renewable energy such as photovoltaic power generation, wind power generation and etc, more and more distributed energy systems are connected to medium-voltage networks. Nevertheless the existing distribution networks have limited capacity. Static synchronous series compensator (SSSC) is one of the solutions for these issues. Since SSSC is connected to the power grid in series, it is a great challenge to protect the SSSC especially when a short-circuit fault occurs in the power grid. In this paper, a parallel connection of metal–oxide varistors (MOV) and the thyristors with SSSC is proposed to protect SSSC in case of transmission system failure. In the protection process, MOV only works for about 1ms, providing the time of fault detection and thyristor conduction. The thyristors are turned on to bypass the SSSC, and the short-circuit current flows through the thyristors. Furthermore, it is inevitable that the junction temperature of the thyristors rise sharply and even causes damage to the thyristors, when the surge current passes through the thyristors. In order to restrain the junction temperature of the thyristors from rising too fast and improve the current surge handing capability, this paper proposes to integrate phase change materials (PCMs) into the thyristors. During the current surge, PCMs absorb a large amount of heat for phase change while its temperature remains constant, which can reduce the temperature rise rate of the chip, and ensure that the thyristors work properly. The protection scheme proposed in this paper is simulated in MATLAB/ Simulink and implemented experimentally in the laboratory. The thermal simulation model of thyristors is built in COMSOL. The finite element simulation results show that PCMs can reduce the junction temperature of the thyristors during current surges.

Evaluation Tests of Metal Oxide Varistors for DC Circuit Breakers

With emerging technologies and products in power semiconductors and ultrafast mechanical switches, direct-current circuit breakers (DCCBs) have been developing rapidly. Yet the metal-oxide varistors (MOVs), being the indispensable DCCB components, are less analyzed and tested so far. Generally, MOVs in DCCBs are used to dissipate energy and limit overvoltage surges during switching events. This emerging application of MOVs calls for new design procedures and evaluation tests to ensure MOV performance under non-standard stresses and repetitive energy pulses. In this paper, online-monitored stress endurance tests are performed on MOVs that are employed in a 12 kV DCCB. Most MOVs have endured the DCCB-derived stresses for about 3500 pulses with little variation in their voltage-current characteristics, while only one fails after 1000 pulse tests and gets damaged by a shunt conductive channel. Different from conventional operating points of MOVs in their voltage-current characteristics, the MOVs in DCCB applications are found to work routinely in the temporary overvoltage (TOV) region with a decreasing TOV current over time. As these behaviors of MOVs are scarcely reported in the literature or covered by their datasheets, this study presents one of the first publicly available endurance test data that address the knowledge gaps about energy-dissipating MOVs in switchgear applications.

Fault Current Bypass-Based LVDC Solid-State Circuit Breakers

This letter presents a new low-voltage direct-current fault current bypass-based solid-state circuit breaker (SSCB) using silicon-carbide <sc xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">mosfet</small> s. The proposed SSCB provides the possibility to select the clamping voltage of metal–oxide varistors (MOVs) close to the nominal voltage of the dc system. This reduces voltage overshoots across the main switch and snubber components and extends the maximum allowable dc bus voltage on the SSCB. The MOVs are removed from the power line, and their leakage currents are completely eliminated. The clamping voltage of the MOV and its surge energy rating is considered to optimize the MOV. The <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">dv/dt</i> across the main switchis controlled by an auxiliary capacitor, where a design procedure is presented to optimize its value. Also, the stored inductive energy of the line inductor in dc systems is bypassed using an auxiliary branch and prevented from flowing through the faulty section to enhance the safety. LTspice simulations are presented to show the significance of the proposed SSCB. The experiments of 375 V/170 A/2.4 μs and 600 V/163 A/2.4 μs verify the effectiveness of the proposed design in practice.

Investigation of Limitations in Passive Voltage Clamping-Based Solid-State DC Circuit Breakers

This paper investigates limitations of passive voltage clamping based dc solid-state circuit breakers (SSCBs). There are two major contributions of this paper. First, it presents a unique quantifying study of limitations in metal oxide varistor (MOV) clamping based SSCBs in terms of gate voltage distortions and power shock. Second, it provides a comprehensive quantifying investigation of limitations in MOV and resistor-capacitor-diode (MOV-RCD) clamping based SSCBs regarding the power shock and response speed. For MOV based SSCBs, both gate voltage distortions and power shock during turn-off are extremely high, causing switch degradation/failure. For MOV-RCD clamping based SSCBs, a snubber capacitor is added to reduce d <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">v</i> /d <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> and power shock. However, this capacitor brings two main limitations: a) inevitable high transient power/energy shock at high-current interruption and b) low turn-off speed caused by high-inertia capacitor charging at low-current interruption. Two prototypes are implemented to illustrate limitations: a discrete SiC MOSFET-based 500 V/20 A SSCB, and a SiC MOSFET module-based 800V/100A SSCB. Experiments validate that the transient power shock can reach 256 kW at a 1.2 kA fault current interruption, which affects system reliability. Moreover, the response speed at a 100 A case (7.8 μs) can be 14 times slower than the 1 kA case (0.56 μs) due to the snubber capacitor charging process, which leads to inconsistent control and coordination of SSCBs.

A Breaker Failure Detection Method Based on Nonlinear Parameter Estimation for Modular Hybrid DC Circuit Breakers

Modular hybrid dc circuit breakers (DCCBs) have been widely employed to protect multiterminal high-voltage dc (MT-HVdc) grids. However, breaker failures still pose a challenge to the protection of MT-HVdc grids, especially for a single module failure of breakers. To this end, a breaker failure detection method based on nonlinear parameter estimation is presented. This proposed method consists of three parts. The first is a signal preprocessing method that eliminates the effect of the RC circuit of the hybrid DCCB. The second is an offline optimal parameter acquisition method that has an improved numerical model of the metal–oxide varistor (MOV) and combines the Levenberg–Marquardt (LM) algorithm with the particle swarm optimization (PSO) algorithm. The third is an online breaker voltage error calculation that estimates the breaker voltage based on the improved numerical MOV model to detect the partial failure of the modular hybrid DCCB. The performance of the proposed algorithm is evaluated by both power system computer aided design (PSCAD)/electro-magnetic transient design and control (EMTDC) tests and field tests. The study results prove the effectiveness of the proposed method for the partial failure of the modular hybrid DCCB considering different faulty conditions and the aging of MOVs.

Effect of GIC Neutral Blocking Devices (NBDs) on Power Network Ferroresonance in Malaysia

Geomagnetic disturbance (GMD) arises during space weather and solar activity can result in geomagnetically induced current (GIC) flow in the grounded power transformers in the power network. This GIC may cause half-cycle saturation of transformers and lead to severe damage or blackout. To block the GIC flow into the power transformers, the neutral blocking devices (NBDs) based on capacitor banks are often installed in the neutral ground paths of transformers to mitigate the GIC. However, the high voltage (HV) can build up across these capacitors during ground faults and may cause a ferroresonance phenomenon in the power network. This phenomenon generates high voltages/currents in the transformer windings and results in transformer failure. This work investigates the effect of connected NBDs to the power transformers on the potential power network ferroresonance in Peninsular Malaysia. The complete analysis was carried out using the Power System Computer-Aided Design for Electromagnetic Transients including Direct Current (PSCAD/EMTDC) software. These transformers were selected in the power network due to the sensitivity of their locations to GMD events. The NBD systems were tested under different working conditions. The simulation results found that the metal oxide varistors (MOVs) arresters in NBDs fault protection mode effectively clamped ferroresonance overvoltages below the protection level under faulty conditions. Also, the results showed that GIC protection modes with 1 &#x03C9; and 3180 &#x03BC;F in the mitigation systems had the lowest ferroresonance overvoltages in the neutrals of the transformers under faulty conditions. Based on the results, the recommendations were provided to the local power utility which will help to improve the reliability of the power supply to the consumers.

Transient DC Over-Voltage Protection for ITER PF AC/DC Converter

The International Thermonuclear Experimental Reactor (ITER) poloidal field (PF) AC/DC converters are composed by thyristor-based phase controlled converter modules. As the core component of ITER PF AC/DC converter, the thyristor is very sensitive to over-voltage and damaged in microseconds, therefore, the transient over-voltage protection strategy is desperately essential to ensure the converter safety operation. In this paper, a nanosecond respond and high reliability protection strategy which combined by Metal Oxide Varistor (MOV) and external bypass is proposed to protect the ITER PF AC/DC converter from transient DC over-voltage. The MOV is designed to certify the fast respond in nanosecond. Moreover, a bidirectional BreakOver Diode (BOD) circuit board is designed to activate external bypass to ensure the reliability of the transient DC over-voltage protection strategy. The performance-testing platform is built to study its performance. The experiments on ITER PF AC/DC converter test facility are carried out. According to the experiment results, the external bypass is triggered by BOD board effectively and the load current is transferred to the external bypass in 2 us when BOD suffers from an over-voltage. The effectiveness of the proposed transient DC over-voltage protection strategy is verified.

DISTRIBUTION NETWORK MODERNIZATION IN TNB WITH THE APPLICATION OF LINE LIGHTNING PROTECTION DEVICE ONTO THE 33KV OVERHEAD LINES WITH HIGH SOIL RESISTIVITY IN TENAGA NASIONAL BERHAD - REIMAGINED TNB TOWARDS GRID OF THE FUTURE

This paper focuses on the use of a line lightning protection device (LLPD) for the improvement of the 33kV overhead line lightning performance in TNB Distribution Network. This initiative is part of TNB Distribution Network Modernization Program that seeks for long term and efficient solutions for medium voltage overhead lines (MV OHL) lightning protection issues especially for those MV OHL that have been erected on high soil resistivity areas. The current lightning protection installed on MV OHL is metal oxide varistor (MOV) Line Surge Arrester (LSA) Class II, as per the IEEE 1410 Guide for Improving the Lightning Performance of Electric Power OH Distribution Lines. However, the earthing system high resistance and the high soil resistivity values can affect LSA service life. MOV LSA requires low earthing resistance so that surge current can be discharged and dispersed effectively to ground in improving the line lightning performance and ensuring that MOV LSA operates according to its intended lifetime. The use of LLPD that is based on long arc flashover concept without the use of MOV is a proven solution for improving line lightning performance for poles located in areas with high resistivity soils and high intensity and frequency of lightning activities.

Protection From Indirect Lightning Effects for Power Converters in Avionic Environment: Modeling and Experimental Validation

This article focuses on a protection system from lightning indirect effects applicable to power converters for avionic applications. The case study considered in this article is a dc-dc pulsewidth modulation buck-boost converter protected by a metal oxide varistor and a series inductive blocking element. The converter is investigated when operated in normal operating conditions, under lightning indirect strokes without protection, and under lightning indirect strokes with protection. A numerical model of the system based on a finite-difference time-domain scheme is proposed. Validation of the system is performed through a comparison with experimental measurements and alternative numerical modeling techniques, showing the effectiveness of the blocking system and the accuracy of the proposed model.

Development and Verification of Indirect Lightning-Induced Transient Protection Circuit for Avionics System

In this paper, an indirect lightning-induced transient protection circuit for avionics system is proposed, and its effectiveness is verified. The proposed circuit consists of a metal oxide varistor (MOV), a transient voltage suppression (TVS) diode, and a resistor. Compared with the conventional circuits (MOV or TVS diode), the improved noise suppression of the proposed circuit against indirect lightning strikes are experimentally verified in accordance with radio technical commission for aeronautics (RTCA) DO-160G Sec. 22. The highest attenuation levels of indirect lightning strike WF5A reference voltage and current signals are approximately 91.0% and approximately 98.4% for the input lightning signals, respectively.

A linearized MOV model-based method for fault location on off-terminal series capacitor bank-compensated transmission line using one-end current

This paper proposes a method for fault location on single and double-circuits series capacitor bank-compensated transmission line (SCTL). A common situation for installing the series capacitor bank (SCB) is to place it some point away from the terminal substations. One-end measurement-based fault locators are favorable for many utilities due to simplicity and less cost especially if they are accurate enough. For impedance-based one-end fault locators on SCTL, the current passing in the SCB is unknown for about 50% of faults. This prevents the prediction of the challenging metal oxide varistor (MOV) behavior that is inevitable for a one-end method. Herein, the nonlinear characteristics of the MOV protecting the SCB are simplified by a linearized simple yet precise model. This makes it possible to express the SCB-MOV characteristics as a linearized function of the far-end current for faults upstream the SCB keeping the nonlinear characteristics of the MOV. This eventually enables to estimate the fault current contribution of the far-end. Hence, an analytical fault distance estimation algorithm is derived for both single and parallel transmission lines using the sequence-networks modeling of the system. Beside simplicity and robustness, the proposed method is found to be accurate enough in locating a big number of faults under a variety of conditions.

Adaptive Solid-State Circuit Breaker Without Varistors in VSC-Interfaced DC System

The rapid discharge of the dc-link capacitors during the short-circuit fault would cause the transient current with fast propagation speed in the voltage-source-converter-based dc system. Therefore, it is necessary to limit the dc fault current and lower the requirement of fault-detection speed. However, the traditional fault current limitation method with directly installed inductors in the dc line will prolong the fault-isolation time and result in the system instability during the power flow shifting. In this article, an adaptive solid-state circuit breaker (SSCB) is proposed for the low- and the medium-voltage dc system, which can bypass the installed inductors during the normal state and connect them into the dc line when the fault occurs. Besides, the energy dissipation circuit and the snubber circuit are applied to release the energy stored in the installed inductors and the dc line inductance after tripping. Hence, the fault-isolation speed can be accelerated, and it is not necessary to use the metal-oxide varistors in the proposed SSCB. By comparing with other SSCB schemes with the fault current limitation function, the effective fault current limitation, the instantaneous fault isolation, and the valid fault ride-through of the health part of the proposed SSCB are verified in the OPAL-RT-based simulation and the hardware test.

Solid-State DC Circuit Breakers and Their Comparison in Modular Multilevel Converter Based-HVDC Transmission System

This paper proposes a new and surge-less solid-state direct current (DC) circuit breaker in a high-voltage direct current (HVDC) transmission system to clear the short-circuit fault. The main purpose is the fast interruption and surge-voltage and over-current suppression capability analysis of the breaker during the fault. The breaker is equipped with series insulated-gate bipolar transistor (IGBT) switches to mitigate the stress of high voltage on the switches. Instead of conventional metal oxide varistor (MOV), the resistance–capacitance freewheeling diodes branch is used to bypass the high fault current and repress the over-voltage across the circuit breaker. The topology and different operation modes of the proposed breaker are discussed. In addition, to verify the effectiveness of the proposed circuit breaker, it is compared with two other types of surge-less solid-state DC circuit breakers in terms of surge-voltage and over-current suppression. For this purpose, MATLAB Simulink simulation software is used. The system is designed for the transmission of 20 MW power over a 120 km distance where the voltage of the transmission line is 220 kV. The results show that the fault current is interrupted in a very short time and the surge-voltage and over-current across the proposed breaker are considerably reduced compared to other topologies.