Very Fast Transient Overvoltage Research Articles

Regional Evaluation Study of VFTO Interference to Secondary Side Cables Based on Cloud Model and MARCOS.

With the advent of the data era, most power secondary side equipment tends to be digitized. The power system needs more accurate numerical results to further improve its operating efficiency. Therefore, it is important to study the electromagnetic interferences of very fast transient overvoltage (VFTO) generated by gas-insulated switchgear (GIS). To protect the secondary side cable from interferences, the secondary side cable is wrapped with an outer shield and the shield is grounded. When the interference of VFTO comes, it will couple the interference current and interference voltage on the shield of the cable. By grounding, the interference is greatly discharged. However, due to the grounding resistance, there will be a potential difference between the grounding points at the two ends of the shield of the cable. This causes a corresponding interference current to flow through the shield, which will affect the transmission of signals inside the cable. In the actual substation, the resistivity of the soil, the ambient temperature and humidity of the area, and so on will have impacts on the grounding resistance. In addition, the irregularity of the cable arrangement and the time of the use of the cable will have impacts on the signal transmission of the cable. Based on the abovementioned issues, this article proposed a comprehensive assessment method based on the combination of the cloud model and measurement of alternatives and ranking according to compromise solution (MARCOS). The method brings the cloud model into MARCOS by the algorithm of the contribution of the cloud droplets. It overcomes the difficulty of cloud model quantification. By comparing the results of the proposed method with the actual conditions at the substation and the results of the common MARCOS assessment method, the validity of the method is verified, and a reference scheme is provided for substation optimization.

Design Optimization of Ferrite Rings for VFTO Mitigation

Very fast transient overvoltage (VFTO)-related problems are raising growing concerns in China with the development of ultra-high voltage. The practice of fitting ferrite rings on gas-insulated switchgear busbars for VFTO mitigation has long been proposed but few investigations on its' design optimization were conducted. Based on theoretical analysis of the simplified model, the optimal damping condition of ferrite rings is proposed, which is conducive to the type selection. The number of rings to be employed to ensure visible suppressing effects is given through simulations and the influence of the saturation is discussed. The impact on power frequency is analyzed to be negligible. Finally, comparisons are made between theoretical conclusions and existing experiment results, which approve the optimal damping condition and agree in the number of rings to be employed.

Nonlinear modeling of magnetic materials for circuit simulations

Magnetic materials in the form of magnetic rings are widely used in power engineering products. In many cases, they operate in high frequency and in nonlinear conditions, e.g., as damping elements in electrical power substations equipped with Gas-Insulated Switchgear (GIS) where they provide suppression of Very Fast Transient Overvoltages (VFTOs). To model phenomena in GIS with magnetic rings it is required to have realistic time-dependent models of magnetic materials operating in a wide frequency range and nonlinear conditions. Nowadays, this has become even more relevant due to the actual trend in the industry to create digital twins of physical devices. Models composed of high-precise discrete lumped nonlinear elements are in demand in circuit simulators like SPICE. This work introduces a method based on classical algorithms that find equivalent lumped models of magnetic cores based on frequency-dependent measurements of impedance under DC-bias current. The model is specifically designed to have smooth behavior in the current domain and thanks to that to improve numerical stability in the time domain simulations.

Analysis of the Very Fast Transient Overvoltage in a Nuclear Plant 500kV GIL-GIS System

The simulation models of the 500kV gas-insulated transmission lines (GIL) and gas-insulated substation (GIS) systems in a realistic nuclear plant are presented in the paper. The Very Fast Transient Overvoltage (VFTO) which is generated during switching operations, is analyzed. Some affecting factors on the level of the VFTO are investigated. These factors are the location of the power source, the entrance capacitance of the potential transformer (PT), and the capacitive voltage transformer (CVT). At last, some suggestions on overvoltage calculations or calculations of electromagnetic interference sources are proposed. The conclusions can be used in insulation designing and analyzing the electromagnetic interference on the secondary system in nuclear plants and substations.

Dispersion characteristics of switching transients from large‐scale experiments in a 1100 kV gas insulated switchgear circuit

AbstractThe switching operation of gas insulated switchgear (GIS) disconnector will produce multiple random spark discharges between the gas gaps of the disconnector. Each spark discharge can be affected by various random factors, such as trapped charge on the load side, the initial operating phase, gap distance and operating speed of the disconnector, leading to dispersion characteristics. At present, some studies have mentioned this dispersion, but its impact is still unclear due to the lack of experimental data, resulting in a deviation between the simulation results and the measured results. In this study, thousands of switching operations were carried out based on the 1100 kV full‐scale GIS circuit in Wuhan ultra‐high voltage alternating current Test Base. The maximal information coefficient analysis, linear regression fitting and other statistical methods were used to discuss the dispersion characteristics of very fast transient overvoltage (VFTO) and transient enclosure voltage (TEV) from large‐scale experimental data. The results show that the dispersion of VFTO and TEV in amplitude aspects both increase gradually as the breakdown voltage increases, with the maximum difference of five times of TEV amplitude at normalised breakdown voltage. In terms of frequency characteristics, the dispersion of different frequency components in TEV is always greater than that of VFTO, especially for the high‐frequency components. Moreover, by comparing the frequency characteristics of TEV at different positions, we notice that the dispersion is almost independent of the spatial position but only determined by the randomness of spark discharges. These discoveries reveal the importance of the dispersion in switching transients and make up for the lack of theoretical understanding of the correlation among them.

The Statistical Characteristics Analysis for Overvoltage of Elevated Transmission Line under High-Altitude Electromagnetic Pulse Based on Rosenblatt Transformation and Polynomial Chaos Expansion

A High-Altitude Electromagnetic Pulse (HEMP) could induce very fast transient overvoltage (VFTO) with nanosecond level rise time and mega-volt amplitude, which severely threatens the electrical devices connected to the elevated transmission line. An elevated transmission line with different locations may suffer different levels of HEMP threat since the dip angle could influence the polarization of the HEMP wave. The combination of Rosenblatt Transformation and Polynomial Chaos Expansion (R-PCE) is introduced in this paper. With this method, the efficiency of calculating the overvoltage of an elevated transmission line under HEMP is improved, speeding up 24.75 times. The influence of different factors (dip angle, elevated height, and earth conductivity) on the overvoltage of elevated transmission lines applied in power systems is calculated and analyzed. The numerical result shows with 99.9% confidence that the overvoltage would be over 3.7 MV of amplitude and 6.7 × 1014 V/s of voltage derivative, which is much more rigorous than a lighting pulse. We also find that elevated transmission lines may have a larger HEMP threat in a small dip angle area. The corresponding data are shown at the end of the paper, which could be useful for relative researchers in pulse injection experiments and reliable evaluation.

Research on Interferences of VFTO to Irregular Distributions of Secondary Side Cables

In the digital era, the secondary equipment of power system is becoming more and more sensitive to electromagnetic interference. Gas insulated switchgear (GIS) in substations will generate very fast transient overvoltage (VFTO) after switching action. The VFTO will generate transient electromagnetic fields and cause interferences to the secondary side cables. In recent years, the anti-interference measures for cable protection have been greatly improved. However, in the current cable trenches of some substations, there are still many irregular cable arrangements. It may have some influences on the electromagnetic interference to the cable generated by VFTO. This paper expresses a method for finding the exact interference areas of the cable and standardizing the wiring standard of the cable. Firstly, the relationship between the cable position variation and parameters is analyzed. Then, simulation experiments based on speculation are conducted to observe the effect of irregular cable arrangement on port voltage, and computer software simulation is performed for comparison. After that, an irregular cable is simulated and an interval type Ⅱ fuzzy neural network is used to predict the parameters of the shielding layer of the cable at each irregularly arranged point. Finally, the voltage and current at each point of the cable shield are calculated through the finite-difference time-domain (FDTD) algorithm. In this manner, the actual position of the higher interference voltage can be determined, which can provide a reference standard for cable arrangement.

Calculation and analysis the dynamic breakdown characteristic of SF6 discharge caused by GIS disconnector operation

In order to accurately calculate the very fast transient overvoltage(VFTO), it is very important and necessary to carry out the study on breakdown characteristic of SF6 gas discharge and get the expression of critical breakdown voltage during GIS disconnector operation. Therefore, a study of dynamic breakdown characteristic of SF6 discharge caused by GIS disconnector operation is conducted. For gas discharge modeling, considering the micro discharge process will lead to the problems of complex modeling and difficult solution. A new method which can avoid the problems of complex modeling and solution is proposed in this paper. Researches have revealed that there is a certain relationship between the critical electric field breakdown strength and the gas density. Thus, the solution of dielectric strength is transformed into the solution of gas density. Then a 3D multiple physical field coupling model of disconnector operation to solve the gas density is established. The dynamic meshing technology is applied to solve the problem of dynamic description contact moving. And then linear regression fitting method is used to obtain the expression of dynamic breakdown voltage of SF6 gas discharge. After checking the quality of fitting function by index F, P and R2, the SF6 gas breakdown voltage expression with the characteristic of dynamic variation under different operation conditions are obtained successfully.

Study of the behavior of low voltage ZnO varistors against very fast transient overvoltages (VFTO)

• Complete VFTO(s) generation and measurement system based on (MPC) and (TLT) technologies. • Five commercial ZnO varistors of the same reference were used in the characterization. • The influence of the applied overvoltage front time on the capacitive effect of the varistor is confirmed. To study the behavior of low voltage ZnO varistors against Very Fast Transient Overvoltage (VFTO), the authors have developed a complete VFTO(s) generation and measurement system based on magnetic pulse compression (MPC) and Transmission Line Transformer (TLT). This system has an output voltage of 24 kV, rise time of 32 ns, and a bandwidth of 10 MHz. The measurement system comprises a damped resistive voltage divider with an impedance of 1000 MΩ and a maximum bandwidth of 10 MHz. The experiment was performed with five commercial zinc oxide varistors with a nominal voltage of 150 V, each one was subjected to VFTO-type voltage pulses and, by Analysis of Variance (ANOVA), a single varistor was selected to perform the case study. This work presents an experimental methodology to observe the response of low voltage ZnO varistors against VFTO. This response is the delay of the residual voltage to the applied discharge current and indicates the poor performance of these devices against VFTOs. Our methodological proposal consists of a series of laboratory tests that will allow manufacturers and consumers to take corrective measures in case of malfunction, damage of the varistor material, or permanent degradation of the device.

Development and Experimental Research of VFTO Measuring Sensor

Very fast transient overvoltage (VFTO) generated by an operating disconnector is one of the main reasons for electromagnetic disturbance in gas-insulated switchgear (GIS) substations. Generally, the amplitude of VFTO can be used as one of the references for the insulation design of GIS primary electric power equipment, so it is necessary to obtain its accurate amplitude. In this study, a new VFTO measuring sensor is developed and its measurement performance is demonstrated through hundreds of operations by a disconnector in a 220 kV GIS test circuit. The validation shows that the low cut-off frequency of the new VFTO measuring sensor has been greatly expanded to 0.01 mHz, which is improved by about 50% compared with the old sensor. The measurement accuracy of amplitude of VFTO micro-pulse improves greatly by about 80% compared with the old one. Thus, the new VFTO measuring sensor can fully meet the measurement needs of trapped charge voltage, power frequency voltage, and high-frequency transient voltage in VFTO waveform. It can be used to provide more accurate data support for insulation design of GIS primary power electric equipment in extra-high voltage (EHV) and ultra-high voltage (UHV) GIS substations.

A low‐cost and high‐performance self‐trigger method for high‐voltage transient measurement

AbstractStable triggering of acquisition devices is critical to accurately measure electromagnetic transients, especially for very fast transient overvoltages (VFTOs). A highly stable and low‐cos t self‐trigger method for transient measurement is proposed. An integrated low‐voltage arm module was developed to synchronously realise the triggering and VFTO measurement. The trigger branch was developed using a high‐pass filter. This component obtained the high‐frequency component of the VFTO to trigger the oscilloscope. The trigger branch was integrated with the measurement branch through an impedance converter. An attenuator was implemented before the impedance converter to avoid overvoltage damage to the chip. The circuit parameters of the attenuator were optimised to alleviate the disturbance of the measurement branch. The calibration results demonstrated that the ±1 dB bandwidth of the measurement branch in this system ranged from 11.8 Hz to 105 MHz. The low cutoff frequency of the trigger branch was 710 Hz. The VFTO waveforms generated by the operation of a disconnector in a real 252 kV gas insulated switchgear were measured using the proposed system. The test results verified that the self‐trigger measurement system can provide stable trigger signals and accurately measure the VFTO waveforms.

Implementation of Radiation and Soil Losses in VFTO Simulation by Frequency-Domain TL Model

Transmission line (TL) models are widely used within the simulation of the very fast transient over-voltages (VFTO) for gas-insulated substations (GISs); however, due to quasi-TEM approximation and model complexity, energy losses related to bushing radiation and frequency-dependent soil parameters are either not available or simply neglected, limiting the simulation accuracy of TL models. In this study, a new frequency-domain TL model is proposed, enabling the model to consider the influences of both radiation and soil losses in a VFTO simulation. Derived from the radiation power of a line antenna model, the radiation resistance of a “lossy” TL is put forward as an equivalent of the radiation loss for GIS bushing. For the soil loss, the Sunde approximation formula and the L-S soil dispersion expression are involved, and the parameters of the overhead busbar are also analyzed. The model gets validation by the measured VFTO generated at a full-scale 1100 kV GIS testing circuit. By considering radiation loss to suppress high-frequency response and soil loss to improve low-frequency response accuracy, the calculated and measured waveforms have the best consistency, which may greatly help assess the risk of insulation breakdown and the level of very fast transient disturbance in GISs.

A Test Method for Response Behavior of Metal-Oxide Arrester Subjected to Transient Electromagnetic Disturbances

Metal-oxide arresters (MOA) are widely used to protect equipment against transient electromagnetic disturbances (TED) like very fast transient overvoltage (VFTO), high-altitude electromagnetic pulse (HEMP), lightning pulse, <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al.</i> Researches on the performance of MOA under microsecond level rise-time TED are abundant, such as lightning pulse. However, when MOA is subjected to TED of rise-time at nanosecond level, the voltage/current responses of MOA are not fully studied due to the lack of a standardized and reliable test setup. In this paper, a test setup is proposed for acquiring the response behavior of MOA under nanosecond level pulse. The setup is comprised of a pulse generator, a two-branch transmission line system and a high voltage power divider. The generator can provide stable double exponential pulse with tunable rise-time ranging from 5 ns to 100 ns. The divider ensures an even distribution of voltage/current surge wave along the two-branch line. Through the numerical analysis, both impinging and residual surge waveforms could be retrieved from measurements. The proposed method has been applied to investigate response behavior of MOA subjected to nanosecond level rise-time pulses.

Very fast transient overvoltage calculation and evaluation for 500‐kV gas insulated substation power substation with double circuit and long gas insulated substation busbar

The operation of disconnectors in gas insulated substation (GIS) or power plant will produce very fast transient overvoltage (VFTO), which is considered to have serious harm to transformer longitudinal insulation and other high-voltage equipment insulation. VFTO may also lead to secondary circuit failure. In this paper, the holistic modelling of 500 kV GIS step-up substation equipment with a double circuit and long GIS busbar in a coastal power plant is established using Electromagnetic Transient Program (EMTP). The authors calculated the VFTO waveform that may be generated in different operations of the GIS substation. The authors analyze the amplitude and frequency characteristics of the VFTO waveform at lightning arresters and the entrance of transformers and find that the amplitude of VFTO at the entrance of #1 MT is less than its impulse withstand voltage. In some cases, the VFTO waveform at the entrance of #1 MT has high-frequency content with fast attenuation. The VFTO waveform generated when operating the disconnector close to the arrester will cause the arrester to act many times and count frequently in a short time.

Research on VFTO Simulation Analysis of 1000 kV GIS Test Circuit Considering Dynamic Arcing Model

In the research process of very fast transient overvoltage (VFTO), the accuracy of the switching arc model largely determines the calculation results of VFTO. How to more accurately simulate the arc striking and extinguishing process of the arc during the action of the disconnector is the key issue in the study of the arc model. In this article, the ATP-EMTP electromagnetic transient program was used to simulate arc damping with a series connection of a fixed-value resistance and a time-varying inductance, and the built-in MODELS module was used for programming to simulate the reignition and extinguishing process of the arc based on the gaseous dielectric theory and the energy balance theory. The model was applied to a 1000 kV gas insulated substation (GIS) test circuit. The relationship between the arc damping and arc voltage and current was analyzed, and the arc reignition law during opening and closing is studied. The arcing law obtained is compared with the experimental results of the existing literature, and the accuracy of the simulation model is verified. Simultaneously, the influence of the opening speed of the disconnector on VFTO was analyzed by simulation. The results show that when the opening speed is less than 0.7 m/s, the maximum value of VFTO in the test circuit increases with the increase in opening speed. After the VFTO amplitude reaches the maximum and continue to increase the opening speed, the VFTO amplitude decreases. This law is consistent with the experimental results of the existing literature, which proves that the arc model built in this article has a certain engineering application value.

Study on the characteristics of the potential rise of shell under VFTO in the GIS pumped storage power station

Compared with the traditional GIS station, complex wiring and operation mode of pumped-storage plant diversity, working condition of frequent transformation, makes the plant to produce very fast transient overvoltage (VFTO) probability increase greatly. VFTO will form a transient coupled to the GIS shell casing transient earth voltage (TEV), and then to station personnel safety and the reliability of the secondary equipment poses a great threat. But at the moment because of the limitation of testing technology such as understanding of the academic study of TEV is lack of system. Based on a typical 500 kV gis pumped-storage power station, this paper established a system equivalent model of GIS, network frame and calculates the amplitude, frequency spectrum characteristic of the TEV and influence on VFTO characteristics, and analyzes the fault location, power supply mode, branch bus, the influence of vertical grounding electrodes on the TEV characteristics, For the characteristics of the TEV rules and parameters. Research on TEV characteristics of GIS of pumped storage power station has important reference value for further perfecting relevant theories.

Study on Very Fast Transient Overvoltage Characteristics of Gas-Insulated Switchgear Substation Based on Hybrid Reactive Power Compensation

In recent years, gas-insulated switchgear (GIS) substations have become widely used in power systems. Due to their compact structure, extremely high oscillation frequencies are prone to extremely high oscillation transient overvoltages or very fast transient overvoltages (VFTOs) when the isolating switch or circuit breaker is operated. The characteristics of these special fast transient processes in GIS and the effect of hybrid reactive power compensation (HRPC) on these fast transient processes were studied. A simulation of the GIS model was established, and based on the transient model of HRPC and the theory of the transmission line, the effects of VFTO produced by HRPC switching operation and the parameters of HRPC on VFTO were studied and the differences in the characteristics of VFTOs in substations with or without HRPC. Based on the conclusions of our study on the influence of HRPC on VFTO, the inhibitory effects of a ferrite magnet ring and an inhibitor busbar on VFTO were verified.

Research on VFTO Identification of GIS Based on Wavelet Transform and Singular Value Decomposition

The accurate identification of Very Fast Transient Overvoltage (VFTO) is the key of overvoltage control in modern smart grids. In order to accurately identify VFTO generated by the operation of a disconnector in Gas Insulated Substation (GIS), a VFTO identification method based on Wavelet Transform (WT) and Singular Value Decomposition (SVD) is proposed. The simulation model of VFTO is established in ATP-EMTP software first, and then wavelet decomposition is used in MATLAB software for VFTO simulation of the waveform from the ATP-EMTP software. Then, the feature matrix is composed of the coefficients of each frequency layer of the wavelet. The SVD is used to decompose the feature matrix, and finally the characteristic parameters of the VFTO are obtained. The simulation results in Matlab software indicate that the characteristic parameters of VFTO have an obvious difference compared with those of power frequency AC voltage, especially in the load-side, which verifies the effectiveness of the VFTO identification method based on WT and SVD proposed in this paper.

Frequency domain algorithm in the whole process simulation from VFTO to its conducted disturbance via voltage transformer in GIS substations

Simulation of very fast transient over-voltage (VFTO) and its conducted disturbance via voltage transformer (VT) is significant in guiding the insulation design and electromagnetic compatibility of gas-insulated switchgear (GIS) substations. However, the commonly used time-domain algorithms are difficult to deal with the complex frequency-dependent parameters in the VFTO simulation, and might introduce instability of conducted disturbance due to an active circuit synthesis of the VT in some cases. In this paper, a frequency-domain algorithm is proposed, which calculates the VFTO in the frequency domain and then directly uses the VFTO frequency spectrum and the VT transfer function to calculate the conducted disturbance without circuit synthesis. The proposed algorithm has been applied in the simulation of VFTO and its disturbance in the 1000 kV Wuhu substation. The calculated and measured VFTO waveforms are in good agreement, verifying the accuracy of the algorithm. The transfer functions of the 1000 kV GIS VT are obtained by the impulse test method, and the conducted disturbances are also calculated with their severity and suppression method analyzed. The proposed frequency-domain algorithm covers the whole calculation process from VFTO to its conducted disturbance and has good engineering application potential in the simulation, with better convenience and stability.

Interference to the Secondary Cable Caused by a Very Fast Transient Overvoltage in a Gas-Insulated Switchgear Substation

When a very fast transient overvoltage (VFTO) propagates in a high-voltage bus, the capacitive voltage transformer (CVT) causes considerable interference on its secondary side, which will affect the normal operation of the secondary equipment. In this article, a CVT equivalent calculation circuit model was established, and the disturbance voltage of the secondary cable was deduced and calculated. Using electromagnetic transient simulation software, a simulation model of the secondary cable disturbance caused by the VFTO in a 500 kV gas-insulated switchgear (GIS) substation was established, the disturbance voltage at the end of the secondary cable was simulated and analyzed, and the effects of the length of the secondary cable and the grounding mode of the shield on the disturbance were studied. Combined anti-interference measures are also suggested, which use a spiral tube damping busbar, grounding at both ends of the shield using multistrand conductors and installing filter capacitors, all of which greatly reduce the amplitude and harmonic components of the disturbance voltage of the secondary cable.