Very Fast Transient Overvoltage Research Articles

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.

The Study of VFTO Distribution in the Insulation System of IOCT Used in Traction Network

The FDS (Frequency-domain Dielectric Spectroscopy) of oil-immersed insulation paper, and semi-conductive paper with different moisture content, has been measured. The data measured are fitted as a function of frequency and moisture content using the amendatory Cole–Cole model utilizing the least square technique. Then, the broadband MTL model of the insulation system of IOCT (Inverted-type Oil-immersed Current Transformer) is established considering the capacitive electrodes thin layer, and the distribution parameters consider the moisture and frequency dependence. A new method for VFTO (Very Fast Transient Overvoltage) distribution calculation of insulation systems is proposed.

Research on Conducted Disturbance to Secondary Cable Caused by Disconnector Switching Operation

The disconnector switching operation in GIS not only generates very fast transient overvoltage (VFTO) in primary equipment, but also couples to the secondary system, which affects normal operation of the secondary equipment. In this study, aiming at the conducted disturbance caused by the disconnector switching operation of the 1,000-kV UHV GIS test circuit on the secondary cable, a broadband equivalent circuit model of the potential transformer and the grounding grid is proposed based on the vector fitting method and the impedance synthesis method, and the accuracy of the model is tested. On the basis of this model, the conducted disturbance voltage of the secondary cable core is obtained by combining the measured typical disturbance source waveform. The research results of the influencing factors of conducted disturbance show that the amplitude of the disturbance voltage generated by the capacitive conduction is higher than that generated by the resistive conduction, but the main frequency of the resistive conducted disturbance voltage is higher. The amplitude of the conducted disturbance voltage will decrease with the increase of the length of the cable and the length of the grounding wire. The single-ended grounding of the secondary cable shield at the GIS side will cause serious disturbance voltage. The research results of this study will be beneficial to the protection of secondary cable electromagnetic disturbance in the intelligent substation and have reference significance for the implementation of secondary equipment protection measures in the intelligent substation.

Research on Improved Arc Model of Isolating Switch and VFTO Characteristics

The simulation of very Fast Transient overvoltage (VFTO) requires an accurate isolating switch arc model. Based on gas discharge theory, the trend of electron number in the gap is analyzed by particle continuity equation simulation. Based on this, the traditional time-domain arc resistance model is improved, and combined with the characteristics of time-domain arc resistance model, dynamic arc model and normal distribution, the improved arc model is established, which conforms to the physical law of arc discharge process of isolating switch. The simulation model of simulation experiment platform was established, and the simulation waveform obtained by the improved arc model, exponential resistance arc model and other arc models was compared with the experimental waveform to prove the accuracy of improved arc model.

Research on On-Line Detection Method of Transformer Winding Deformation Based on VFTO.

At present, the detection of transformer winding deformation faults is carried out in an offline state, which requires the transformer to cooperate with the implementation of planned power outages, or it takes place after the sudden failure of the transformer when it is out of operation. It is difficult to obtain the status information of the windings online in time. Since the transformer will suffer very fast transient overvoltage (VFTO) impact during operation, combined with the principle of the frequency response method, an online detection method of transformer winding deformation based on VFTO is proposed. In order to study the frequency response characteristics of transformer winding under the impact of VFTO, the generation process of VFTO is simulated by simulation software, and the equivalent circuit model of transformer winding before and after deformation is established. The VFTO signal is injected into the transformer circuit model as an excitation source, and the changes of resonant frequencies of frequency response curve under different deformation types and different deformation degrees of winding are analyzed. The simulation results show that the frequency response curves of different winding deformation types are different. Different deformation degrees are simulated by increasing the radial capacitance by 4%, 13%, and 23%, series inductance by 2%, 4%, and 6%, and longitudinal capacitance by 3%, 6%, and 9%, and the change of resonance frequencies can comprehensively reflect the deformation information of winding. At the same time, the tests of different deformation types and deformation degrees of the simulated winding are carried out. The results show that with the deepening of the change degree of the simulated fault inductance value, the frequency response curve shifts to the low-frequency direction, confirming the feasibility of the online detection method of transformer winding deformation based on VFTO.

Filtering for the Interference Signal Caused by Grounding Potential Difference in High-Voltage Disconnector

The disconnector operation in the high-voltage substations forms a very fast transient overvoltage (VFTO), which leads to a sudden rise in local grounding potential. Due to double grounding of the secondary cable, the current transformer signal is interfered by the grounding grid potential difference. To study the mechanism of the interference signal, a simulation model based on the electro-magnetic transient program is built and the coupling path of the grounding grid potential difference is analyzed. According to the characteristics of interference signal, an adaptive algorithm is put forward and it mainly consists of three parts, including starting element, decision algorithm, and filtering algorithm. By selecting the appropriate parameters of wavelet transform, the filtering algorithm can discompose and reconstruct signal to remove the interference signal. The simulation results demonstrate that the filtering algorithm can effectively eliminate the interference signal caused by the grounding grid potential difference, with a 91% reduction of the root mean square error and a 135% improvement of the signal-to-noise ratio. Moreover, the accuracy of the measured data is ensured and the reliability of the relay protection is improved.

Numerical simulation and analytical calculation of the GIS transient enclosure voltage interaction with grounding system

Gas-insulated substations (GISs) are widely used in power systems due to high reliability, convenient maintenance, and small area occupation. Transient enclosure voltage (TEV) is a consequent effect of very fast transient overvoltage (VFTO) during the switching operations in GISs. It can lead to insulation degradation, damage to the secondary equipment connected with the shell of GIS, and harm to the working staff. This paper presents a correlation between a lumped-circuit model based on transmission line theory (TLT) for purpose of the simulation and an analytical approach based on the scattering matrix for the evaluation of TEVs. Furthermore, a frequency-dependent model is proposed to quantify the transient grounding impedance (TGIM) against the waves with high-frequency content such as TEVs. The interaction between the TEV and the grounding system has been investigated analytically, and experimentally based on the field measurements in other efforts. The comparisons indicate that the modified wide-band TGIM results in an improvement in comparison with the other works to track both the front and tail of a measured voltage-wave within acceptable accuracy. Furthermore, the role of the ground strap as a damping structure using a modified model on TEV profile has been scrutinized regarding the grounding system model and the rising rate of TEVs. It is indicated that considering frequency-dependent TGIM could lead to a 25% variation in the estimation of amplitude of TEVs.

Investigating the Effect of Several Model Configurations on the Transient Response of Gas-Insulated Substation during Fault Events Using an Electromagnetic Field Theory Approach

Assessment of very fast transient overvoltage (VFTO) requires good knowledge of the behavior of gas-insulated substation when subjected to very high frequencies. The international standards and guidelines generically present only recommendations regarding the VFTO suppression without a technical and mathematical background. Therefore, to provide an accurate image regarding the critical locations across a gas-insulated substation (GIS) from a transient response point of view, a suitable modeling technique has to be identified and developed for the substation. The paper aimed to provide an accurate assessment of the GIS holistic transient response through an electromagnetic field theory (EMF) approach. This modeling technique has always been a difficult task when it came to gas-insulated substations. However, recent studies have shown that through suitable Computer-aided design models, representing the GIS metallic ensemble, accurate results can be obtained. The paper investigated several simplifications of the computational domain considering different gas-insulated substation configurations in order to identify a suitable modeling approach without any unnecessary computational effort. The analysis was performed by adopting the partial equivalent element circuit (PEEC) approach embedded into XGSLab software package. Obtained results could provide useful hints for grounding grid designers regarding the proper development and implementation of transient ground potential rise (TGPR) mitigation techniques across a gas-insulated substation.

Analysis of VFTO during the failure of a 550-kV gas-insulated substation

This paper presents an analysis of a very fast transient overvoltage (VFTO) experienced by a 550-kV gas-insulated substation (GIS) that caused the failure of a disconnector switch (DS) and circuit breaker (CB), after manual closure of the DS. Simulations were performed using an electromagnetic transient program with GIS component modeling for high-frequency analysis and multi-spark electric-arc modeling. The simulation results were verified by comparison with field test measurements and showed an acceptably accurate waveform and amplitude prediction for VFTO on the 550-kV GIS under study, allowing for improved failure analysis. The simulation results using equivalent electrical circuit models indicated that the failures occurred at a voltage below the lightning impulse withstand voltage (LIWV) with a high quantity of sparks. After the analysis, it was possible to conclude that the manual closure of the DS generated excessive sparks, causing significant damage to the fixed and mobile contacts and abnormal stress in the insulation of SF6 gas in the inhomogeneous electric field region of the CB, impacting on the reduction of the dielectric strength of SF6 gas, thereby producing a high-intensity arc between the shield and the enclosure.

Study on fluid model of disconnector arc applied in very fast transient overvoltage

The essence of the mathematical model of disconnectors' multiple reignition arc is the repeated breakdowns of SF6. In this study, the kinetic properties of microparticles are studies based on the streamer discharge theory, and a fluid arc mathematical model applied in centimetre clearance is established. Ultra-High Voltage (UHV) disconnector manufactured by Pingao Electric Co. Ltd of Henan Province is selected as the object and the resistance change process of contact clearance multiple reignition arc is simulated by finite difference method-finite element method-flux corrected transport self-programming in MATLAB based on the chamber structure and operating characteristics. UHV gas-insulated switchgear very fast transient overvoltage (VFTO) test platform is built to gain the characteristics of VFTO waveforms generated by the disconnector's operation. Compared with simulation results, the test data shows that the fluid arc mathematical model can scientifically explain the dynamic development process of microparticles in the contact clearance and the calculation results are consistent with the test data. In addition, the arc fluid mathematical model combines the structure and operating characteristics of the switch with the development process of microparticles, which can better guide the design of the switch, and provide a theoretical basis for the suppression of VFTO with the disconnector itself.

Design and Verification of VFTO Differentiating-Integrating Measurement Equivalent Circuit

Operation of isolating switches and circuit breakers in fully enclosed gas insulated substations (GIS) produces very fast transient overvoltage (VFTO). According to IEC61321-1, VFTO contains four frequency components with frequencies ranging from 0.1 MHz to 100 MHz. Therefore, the VFTO measurement system needs to have good transient response and steady state response and puts high requirements on the measurement frequency band. As a measurement method that does not need to modified GIS, the differentiating-integrating measurement system has the characteristics of high frequency bandwidth and high voltage divider ratio. In this paper, the theoretical derivation and numerical calculation of the equivalent circuit of differentiating-integrating measurement are carried out, and the simulation of time domain, frequency domain and synthetic VFTO waveform as excitation source was constructed, and its performance is fully verified, which proves that this system can meet the needs of VFTO measurement.

Research and Experiments on an External Miniaturized VFTO Measurement System.

Disconnect switch and circuit breakers operations in gas insulated switchgear (GIS) systems may produce very fast transient overvoltage (VFTO). Detecting VFTO is the first step for researchers to reduce the damage to other equipment of the substation caused by VFTO. Most of the existing sensors used for VFTO are generally bulky, complex to install, and require modification of the GIS structure. In this paper, a miniaturized measurement system that uses capacitive voltage divider and differentiating–integrating circuit is proposed. A special sensor structure and optimized differentiating–integrating circuit components arrangement were designed to increase the bandwidth of the measurement system. The frequency-domain, time-domain and voltage divide calibration experiment was performed, and a comparison experiment with an internal VFTO sensor was conducted. The measurement system was applied in the 500 kV GIS substation, and the VFTO measurement under specific conditions was carried out. The measured time domain and frequency domain waveforms conformed to the definition of standard VFTO according to IEC 60,071. It was found that the proposed measurement system meets VFTO measurement requirements and can be applied to actual VFTO measurements.

Numerical Analysis of a Spiral Tube Damping Busbar to Suppress VFTO in 1000 kV GIS

Gas insulated substations (GISs) are broadly used for transmission and distribution in electric power networks. Very fast transient overvoltage (VFTO) caused by SF6 discharge during switching operations in a GIS may threaten the insulation of electrical equipment. In this paper, a novel VFTO suppression method with great prospects in engineering, called the spiral tube damping busbar, is proposed. The suppressing mechanism of the new method is analyzed. The structure and the design characteristics of the damping busbar are introduced as well. Moreover, a calculation method for the self-inductance of the damping busbar at high frequency is presented. According to the structural characteristics of the damping busbar, the inductance effect on suppressing VFTO is analyzed. A further improvement in damping VFTO is investigated by designing a spiral litz coil connected in series with the busbar, which increases the damping effect. The simulation results show that the improved damping busbar has a significant suppressing effect on the amplitude and the frequency of VFTO.

Effect of oil impregnated paper thickness and impulse waveshapes on voltage–number characteristics

Inter-turn insulation plays a major role in the dielectric integrity of any transformer. Though the inter-turn insulation thickness is designed based on standard lightning impulse (LI) voltage distribution, the insulation fails due to various types of overvoltages from switching impulse (SI) to very fast transient overvoltages (VFTO). Moreover, the insulation gets degraded due to many factors such as service ageing, exposure to repeated applications of overvoltages due to various reasons, etc. The effect of the repeated impulses is characterized using the voltage–number (V–N) characteristics recommended by CIGRE Working group C4.302. Based on the design and rating of the transformer, the thickness of the inter-turn insulation is normally changed. To quantify and compare the impulse breakdown strength and degradation due to repeated impulses under SI, LI and VFTO, different thicknesses of oil impregnated paper, ranging from 0.25 to 0.75 mm are considered. Experiments are carried out to obtain the V–N characteristics for all the cases. V–N characteristics are mathematically modeled by enhancing the existing exponential model to incorporate the thickness of insulation and are then validated experimentally. Based on the model parameters, the effect of the various waveshapes and the degradation due to repeated impulses are quantified and compared with the standard LI.

Comparison between suppressing approaches of very fast transient over voltages in gas insulated substation

Very Fast Transient Over-Voltage (VFTO), which is a unique electromagnetic transient phenomenon generated by SF6 gas discharge in disconnector gap in Gas Insulated-Substation (GIS), could bring damage to power devices, especially to power transformers. The peak magnitude and high-frequency oscillations of VFTO cause reduction of dielectric of gas insulation, high inter-turn voltages and internal resonance in the transformer windings. Due to all of these disadvantages related to VFTO, VFTO becomes a criterion to determine the insulation level of the GIS, and VFTO damping is rising as an essential issue to be solved. However, the researchers concern on finding the optimum technique for suppressing VFTO, this paper aims to discuss the most feasible methods for mitigation of the overvoltage magnitude of VFTO and determine the most efficient approach for VFTOs attenuation by presenting the advantages and disadvantages of each approach. In this review paper, some suppression methods of very fast transients in GIS are presented, including adopting a resistor-fitted disconnector, ferrite rings, surge arresters, and high-frequency resonator. The results show that the peak value of VFTOs can be decreased to a great level by employing the proposed damping methods. Finally, the best approach for the suppression of VFTOs is chosen based on deep discussion for each method with its features.

The Configuration Effect of SCSR on VFTO and Its Inhibitory Effect Analysis

The very fast transient overvoltage (VFTO) is changed when stepped controllable shunt reactor (SCSR) is installed in substation, this paper simulates and analyzes the closing sequence of the disconnector (DS) and voltage transformer (PT) installation location’s influence on VFTO with Dommel transmission line, and establishes the VFTO suppression equivalent impedance model in frequency domain, and analyzes the effect of various measures to suppress VFTO time-frequency amplitude. The results show that the VFTO dominant frequency and the corresponding amplitude can be reduced by reasonable choice of DS closing sequence and PT installation site. Compared to the closing resistor, ferrite has advantages in suppressing VFTO. The peak of VFTO increases first then decreases with the increase of the length of the ferrite.

Application of PEEC Model in Calculation of Transformer High Frequency Voltage

Overvoltage will occur on transformer windings when high frequency waves such as lightning strike and Very fast transient overvoltage(VFTO) intrude into transformer windings. The model and parameters is the basis of calculating transient overvoltage in transformer windings. In this paper, partial element equivalent circuit (PEEC) is used to calculate High Frequency Voltage on transformer windings. the inner grid of conductor through which conductive current passes and the surface grid of conductor with charge distribution is used to transform into the inductance unit and capacitance unit of PEEC respectively. The mutual capacitance between different windings is solved by PEEC. The change regulation of resistance coefficient of transformer windings under different frequency waveforms is analyzed, and the internal voltage of transformer under different frequency applied voltage is solved. The results show that the self-inductance of winding decreases slightly with the increase of applied voltage frequency, and basically remains unchanged in high frequency band. With the increase of frequency, the voltage waveform on winding turns decreases rapidly.

Simulation and analysis of a gas insulated switchgear explosion accident caused by a failure of high‐voltage circuit breaker in a thermal power plant

This study analyses a gas insulated switchgear explosion accident caused by a failure of the circuit breaker (CB) in a thermal power plant, which resulted in the explosion of the CB and the burning of the potential transformer (PT). Through X-ray detection and disassemble examination of the damaged CB, it is found that the A-phase moveable contact was not opened completely. After analysing the voltage waveform obtained from the phasor measurement unit recorder system, it is speculated that this failure is caused by the process of repeated arc strike. For verification and further analysis, the fault process is simulated in the EMTP; the arc is properly modelled by an extended KEMA's equation-based model which is proposed in this study. The simulation results match well with the actual situation; the contact gap would be repeated breakdown because its breakdown voltage is reduced, and the very fast transient overvoltage (VFTO) would be generated in these repeated strike processes. Due to the steep front and high amplitude of VFTO, the insulation of the PT would be damaged and led to the occurrence of PT short-circuit fault. The current would increase rapidly and causes the CB to explode subsequently.

The influence of VFTO on SCSR windings with an equivalent PEEC model

Stepped Controlled Shunt Reactors (SCSRs) have advantages of flexible control and reliable compensation and are likely to become widely used in Ultra-High-Voltage/Extra-High-Voltage (UHV/EHV) transmission systems. Very fast transients that occur in such systems will generate a Very Fast Transient Overvoltage (VFTO) in the SCSR. This may cause damage to the SCSR and lead to system losses. In this paper, the electric field strength and magnetic induction intensity are taken as the basic physical variables when these transients occur. Maxwell’s equations are used to separate the constraint sources, such as the conduction current, magnetization current, and polarization current, from the field source. Then, an all-current Partial Element Equivalent Circuit (PEEC) model for these component elements that considers the surface charge is established. Fast and accurate calculations of the inductance parameters, which involve multiple integrals, are performed using geometrical mean distances and a complex component segmentation method. The equivalent structures and circuit equations of the SCSR windings are presented, and the voltage distribution of the SCSR windings under VFTOs of different frequencies is obtained. The results show that, the voltage waveform in the SCSR windings more rapidly attenuates as the VFTO frequency increases. The peak value of the oscillating voltage in the high-voltage winding of the SCSR is higher than that in the low-voltage winding.