Very Fast Transient Overvoltage Research Articles

Research on Nonlinear Modeling for Power Transformer over Wide Frequency Range

This paper proposes a new nonlinear broadband model of the power transformer which is both fit for steady state and transient simulation in power system. The model consists of high-frequency module and low-frequency module in parallel. Based on the black-box method, the high-frequency block is built in the form of π-type equivalent circuit, and the low-frequency block is in the form of conventional gamma-type circuit, in which core saturation characteristic is expressed by a nonlinear inductor. A power transformer is studied in the laboratory, simulations and experiments are carried out in case of power-frequency steady state, excitation currents distortions, lightning overvoltage, and very fast transient overvoltages (VFTO) respectively. The comparisons between the simulated and measured results verify the feasibility and validity of the model.

Characteristic Analysis on Suppressing VFTO in GIS by Ferrite

When disconnecting switches were operated,Very Fast Transient Overvoltage (VFTO) did great harm to power systems and their equipments in Gas Insulated Substations (GIS). Although VFTO can be suppressed by a few of known methods, there were some problems. Low voltage simulation tests show that putting ferrite rings around a disconnecting generatrix can damp traveling wave propagation and suppress VFTO in GIS. This method is economic and practical. We had analyzed ferrite characteristics of suppressing VFTO and got hold of ways of calculating ferrite size in GIS. Investigation shows that ferrite characteristics and sizes have a great effect on suppressing VFTO.

Influence of the Switching Speed of the Disconnector on Very Fast Transient Overvoltage

During the switching of disconnectors in gas-insulated switchgear (GIS), repeated strikes occur in the contact gap and lead to very fast transient overvoltage (VFTO). Severe VFTO may bring damage to transformers and other equipment. The switching speed of the disconnector is one of the important factors which influences VFTO. There are still different opinions on this factor. Very little quantitative study was done before this paper. In this paper, the whole repeated strike process associated with the switching operation of the disconnector was simulated and analyzed. The polarity and the probability characteristics of breakdown voltage were analyzed. The impacts of the switching speed of the disconnector on VFTO were studied by statistical methods. Comparisons were made between simulated and measured results. The simulation result shows that within a certain range of speed, disconnectors with a lower switching speed generate lower trapped charge voltage after opening operation and lower VFTO magnitude during the opening and closing operations.

Development of Full Frequency Bandwidth Measurement of VFTO in UHV GIS

In order to study the characteristics of very fast transient overvoltage (VFTO) in ultra-high voltage (UHV) gas-insulated switchgear (GIS), the authors have developed a full frequency bandwidth measurement system based on a porthole-type capacitive voltage divider. Three kinds of measurement circuits for the measurement system are studied and the influence of cable length on measurement results is analyzed in this paper. The calibration of the measurement system using step response was conducted. The results show that the measurement system of the capacitive voltage divider integrated with an impedance converter has a bandwidth ranging from 0.003 Hz to 300 MHz at least. It can be used to measure the full VFTO during DS operation perfectly. A full-scale UHV DS (disconnect switchgear) is used to establish the test circuit. The measurement system with good anti-electromagnetic interference ability was constructed. Four different measurement locations are chosen on which to install the measurement system. An approximate total of 3000 closing and opening operation tests were conducted. Due to the wide frequency band of the measurement system, it is possible to obtain the full VFTO waveform accurately and conveniently. According to the waveforms, the key parameters of VFTO, such as trapped charge voltage, peak value of VFTO, breakdown characteristic of DS, frequency spectrum of transient waveform, and the overvoltage coefficient could be obtained.

Experimental Research on Suppressing VFTO in GIS by Magnetic Rings

Very-fast transient overvoltage (VFTO) is a serious threat to the insulation of extra-high voltage and ultra-high voltage power-system equipment and should be suppressed. A simpler and more reliable VFTO suppressing method than a shunt resistor is presented in this paper, which uses magnetic rings installed on the center conductor of gas-insulated switchgear (GIS). Two kinds of magnetic materials that can work under high frequency were adopted, one of which is ferrite R2KB and the other is amorphous FJ37. Measurements in a scale model with an air-insulated busbar show that ferrite R2KB rings have a better suppressing effect than amorphous FJ37 when the ring-cluster length is larger than a certain value. The ferrite R2KB rings were applied to 252-kV GIS. The test results show that ferrite R2KB rings can significantly suppress the peak value and the time to peak of VFTO. When the ring-cluster length is 40 cm and the test voltage is 140 kV of power frequency, the overshoot factor and the front steepness of VFTO can be reduced more than 50% and 20%, respectively. The test results are exiting and indicate that using magnetic rings to suppress VFTO is feasible.

Spacer flashover characteristics in SF6 under repetitive nanosecond pulses

Very fast transient overvoltages (VFTO) may result in insulation failures inside or outside gas insulated switchgears (GIS), and most of them are caused by surface flashovers along insulating spacers according to operational experiences. For a good understanding of the discharge process produced by high voltage pulses with a fast rising time, an experimental study on spacer flashover under repetitive nanosecond pulses in high pressure sulphur hexafluoride (SF6) is presented. A solid-state pulse generator is used to generate nanosecond pulses with a rise time of 15 ns and a full width at half maximum of 30~40 ns. The pulse repetition frequency (PRF) varies from single shot to 1 kHz. Brass parallel-plane electrodes and cylinder epoxy resin spacer are used to represent a configuration of uniform electric field and insulator in GIS, respectively. In this experiment, the parameters of amplitude of discharge current, flashover time lag and number of applied pulses to flashover are investigated. And their relationships with gas pressure and PRF are analyzed.

A Reliable Wide-Bandwidth VFTO Sensor Based on Surface-Mounted Devices

A capacitor sensor with a high-voltage electrode inside the GIS tank has the best bandwidth among the sensors developed to measure the very fast transient overvoltage (VFTO). However, the low-voltage arm of the current measurement system is usually a handmade film capacitor which has disadvantages of unassigned capacitance value, temperature effect, and short lifespan. With a novel lower voltage arm based on surface-mounted devices, a wide-bandwidth, stable, and easy maintenance measuring system is proposed in this paper. It is found that the slight inherent inductance in the ground trace of the printed-circuit board can induce unexpected oscillations, which are demonstrated with theoretical analysis and experimental study. With the reforming of the printed-circuit board layout, the oscillation is reduced and 200-MHz high-frequency cutoff frequency is achieved. The environment experiment shows that the temperature effect on the developed sensor is much smaller than that on the conventional sensor. The VFTO generated by disconnector operation in 1100-kV GIS is measured by the developed system and a conventional broadband capacitor sensor. The contrastive results prove that the characteristic of the measuring system can meet the requirement of VFTO measurements. Thousands of measurements on ultra-high voltage GIS indicate that the developed measuring system is stable, and characteristics of the VFTO are gained.

Arcing Model of a Disconnector and its Effect on VFTO

In the computational process of very fast transient over-voltage (VFTO), it is essential to find an accurate model for a gas insulated substation. The arcing model of the disconnector is particularly important. The general arcing model is not able to give a good description of the arc development process. In this paper, based on the physical process of arcing and existing arc models (the exponential time-varying resistance model and the segmental arcing models), a dynamic arcing model is proposed, which is divided into two stages before and after the zero crossing. The dynamic arcing model combines hyperbola time-varying resistance and the Mayr model to describe the dynamic process of arcing. The present paper creates an arc model blockset upon the Matlab/Simulink software platform. Moreover for a specific 1100 kV station, VFTO is simulated in detail based on different arcing models. It is demonstrated that the dynamic arcing model can describe the physical arc process precisely and is useful for improving the accuracy of VFTO simulations.

Nanocrystalline to Suppress VFTO and VFTC for 245kV Gas Insulated Substation

Switching of disconnectors (DS) and various faults in Gas Insulated Substation (GIS) causes number of pre-strikes and re-strikes happens. The short duration voltage collapse, traveling surges in busbar causes Very Fast Transient Over voltages (VFTO) and Very fast Transient Currents (VFTC) in the GIS substation. The main challenge is to reduce VFTO and VFTC amplitudes. Several methods have been proposed and examined in the past, such as damping Resistor and Ferrite rings. The VFTO and VFTC damping solutions utilizing damping Resistor, ferrite rings has been analyzed using most popular and widely used software package Electro Magnetic Transient Programming (EMTP). The simulation of mitigation methods like damping resistor and Ferrite rings shows that a damping effect can be achieved. The damping resistor suffers with the difficulty of inserting inside the busduct and the other one is Ferrite magnetic material rings goes easily into saturation, which complicates the design and reduces its general applicability and robustness. This paper investigates the new techniques to suppress the VFTO and VFTC in a 245kV GIS substation by considering the complete substation model for simulation using EMTP. Rings of a Nanocrystalline alloy placed around the GIS conductor were investigated, developed the equivalent model and simulated using EMTP. Prominent results have been achieved with the Nanocrystalline models.

Modeling of 420 kV GIS Substation and Research on Very Fast Transient Overvoltage (VFTO)

This study conducts simulation of a 420 kV GIS substation in order to study on Very Fast Transient Overvoltage (VFTO) caused by disconnect or Switches operation. All components and its connected equipment in this GIS substation are modeled in EMTP-RV software. The various disconnect or switch operation conditions are considered and the results are presented. Finally, the effect of trapped charge on VFTO magnitude is discussed.

EMTP Simulation of VFTO’s for 420 kV GIS

Very Fast Transient Over voltages(VFTO) are generated due to switching operations and faults in a Gas Insulated Substation(GIS) needs lot of insight to safe guard the equipment and insulation of various systems. VFTOs can also cause malfunction of protection circuits and control circuits. These transients have traveling wave behavior; they travel to the external systems through enclosures, bushings, cable joints etc., and cause damage to the outside equipment. They can lead to secondary break downs in GIS and may give rise to electromagnetic interference. Thus it is important to develop a suitable simulation models for estimation of these over voltages. In order to estimate the over voltages, equivalent electrical network models are developed. This paper focused on 420 kV GIS system and equivalent model of GIS bus duct have been considered for the simulation of VFTOs. GIS equivalent models have been developed for various lengths of bus duct and simulations to estimate VFTOs are carried out with switching operations of Disconnector switch and fault conditions at various locations. The suppression of VFTOs are also simulated by inserting resistance across the disconnector switch. Comparison has been made in terms of voltage magnitudes with and without suppression methods. Significant reduction of voltage magnitudes are observed in this work. For simulation of equivalent GIS models Electro Magnetic Transient Programming (EMTP) is used.

ANALYSIS OF TRANSIENT ENCLOSURE VOLTAGES IN GIS (EMTP SIMULATION STUDIES)

Abstract Transient Enclosure voltage is special case of very fast transient overvoltages which occurs due to disconnect switch operation or earth ground. Transient Enclosure Voltage appears on external of earthed enclosure of Gas Insulated Systems. Despite of proper grounding, this phenomenon indicates presence of high potentials on Gas Insulted System enclosures so the grounding system impedance is thoroughly examined and designed. In this study EMTP Software is used for analysis. Simulation was done by varying the different parameters. Variations of waveforms of the Transient Enclosure Voltage with various parameters have been studied. Index Terms: Transient Enclosure Voltage (TEV), Gas Insulated System (GIS), Very Fast Transient Overvoltages (VFTO) Transient Ground Potential Rise (TGPR), Disconnector Switch ----------------------------------------------------------------------***------------------------------------------------------------------------ 1. INTRODUCTION

Analysis of Magnitude and Rate-of-rise of VFTO in 550 kV GIS using EMTP-RV

Very Fast Transients (VFT) originate mainly from disconnector switching operations in Gas Insulated Substations (GIS). In order to determine the rate-of-rise of Very Fast Transient Overvoltage (VFTO) in a 550 kV GIS, simulations are carried out using EMTP-RV. Each component of the GIS is modeled by distributed line model and lumped model based on equivalent circuits. The various switching conditions according to closing point-on-wave and trapped charge are simulated, and the results are analyzed. Also, the analysis of travelling wave using a lattice diagram is conducted to verify the simulation results.

Experimental Research on Very-Fast Transient Overvoltage in 1100-kV Gas-Insulated Switchgear

Very-fast transient overvoltage (VFTO) generated during switching of the disconnector in gas-insulated switchgear (GIS) may threaten the safety operation of electrical equipment. As the rated voltage of GIS increases, the difference between the level of VFTO and its lightning impulse withstand voltage gradually decreases. Hence, VFTO becomes the main dielectric stress to switchgear optimization and insulation coordination for ultra-high voltage (UHV) systems. To obtain the characteristics of VFTO in UHV GIS, two full-scale 1100-kV GIS test circuits were established in China. Two types of capacitive sensors were developed to measure VFTO generated in two test circuits. The measurement systems based on both types of the sensors were calibrated by a special calibration device in laboratories and onsite. The measurable frequency range of the systems is 0.1 Hz ~ 100 MHz and their division ratios are stable. More than 2000 tests by switching the disconnector in GIS had been carried out in the test circuits, and VFTO waveforms were captured by the measurement systems. From the test results, the waveform properties, the striking number, the predominant frequency, and distribution of the trapped charge voltage for full-voltage waveforms, the typical characteristics and oscillation coefficient of single-strike waveform, the distribution of VFTO magnitude was analyzed. The mitigation of VFTO by a damping resistor in the disconnector was illustrated. The maximum magnitude of VFTO was 2.27 p.u., and it was mitigated to 1.33 p.u. by a damping resistor of 500 Ω. The achieved characteristics of VFTO in the UHV GIS have become the basis of VFTO simulation and insulation coordination for engineering design.

Analysis of very fast transient overvoltages (VFTO) from onsite measurements on 800 kV GIS

In IEC 60071-1 and 62271-102, there is neither standard voltage shape nor standard peak value for very fast transient overvoltages (VFTO). Previous work by other researchers had classified VFTO into two types. A new type was identified in this paper based on an onsite test on a commercially-operated 800 kV GIS. Disconnectors were opened/closed under real operation conditions. Characteristics of the new VFTO type were analyzed, including times of the arc restrikes between the contacts of the disconnectors, trapped charge, damping time constant, magnitude, rise rate, and oscillating frequency. The results of this paper are expected to be of help for the completion of standards on VFTO.

VFTO Measurement System Based on the Embedded Electrode in GIS Spacer

The method using the embedded electrode in a spacer to measure the very fast transient overvoltage (VFTO) is well applicable to the field measurement in a substation. However, the current measurement system has the disadvantage of low high-frequency cutoff frequency (2 MHz), and the problem of fallibility caused by a flashover from the embedded electrode to tank sheath. Based on the fundamental principle of the embedded electrode measurement system, a distributed parameter model has been established in this paper. With theoretical analysis and experimental verification, it is found that the major factors affecting bandwidth include the inherent inductance in external capacitors connected to an embedded electrode, the different port-distribution of external capacitors, and the nonconsistency of external capacitors on different ports. When the capacitors at four ports of an embedded electrode are highly consistent, the partial transient overvoltage of the electrode will be eliminated. In this paper, capacitors with low inductance, high stability, and highly consistency are chosen to be connected on four ports of an embedded electrode, and the consistency of four external capacitors is ensured. Hence, the high-frequency cutoff frequency increases to 10 MHz. The VFTO generated by disconnector operation in 1100-kV gas-insulated switchgear is both measured by the system based on the embedded electrode in a spacer and a broadband measurement system. The contrastive results indicate that the measuring system can meet the needs of VFTO measurement on some conditions.

Mitigation of Very Fast Transient Overvoltages at the More Sensitive Points in Gas-Insulated Substation

Switching operations in a gas insulated substations generate very fast transient over voltages (VFTO) which are dangerous for the transformer and the system insulation because of their short rise time. Under special circumstances the terminal over voltages can arise close to the transformer BIL. The suppression of VFTO is very important in GIS systems. Different techniques are applied to 245 kV GIS model. The simulation used ATP-EMTP package program to evaluate the VFTO at the sensitive points in GIS. The different results are compared to determine the effective mitigation method. The results show that the wave steepness and the amplitudes of VFTO can be reduced to about 1Pu with less oscillation frequency.

Insulation characteristics of 1100 kV GIS under very fast transient overvoltage and lightning impulse

Very fast transient overvoltage (VFTO) in gas insulated switchgear (GIS) is of great concern at the higher rated voltages because the difference between the rated lightning impulse (LI) and the VFTO withstand voltage decreases. In order to clarify VFTO withstand level of 1100 kV GIS, a simulation system was designed for generation of VFTO. The breakdown characteristics of sphere-plane gap and rod-plane gap in SF6 under VFTO and LI were investigated for different gas pressures. Experimental results show that the breakdown voltages in slightly inhomogeneous field gaps under VFTO are higher than that under LI. However, for the rod-plane gaps, the breakdown voltages and voltage-time curves under VFTO fall below that of LI at higher pressure in the case of negative polarity. The polarity effect of rod-plane gaps under LI is reversed at a certain pressure. Based on the analyses, it is concluded that the different breakdown characteristics of SF6 gaps may be attribute to the difference between space charge behaviors during the discharge development under the impulse with different wave front and oscillation tail of VFTO and LI.

SF6 high voltage circuit breaker contact systems under lightning impulse and very fast transient voltage stress

In the past few years gas insulated high voltage circuit breakers have improved considerably, in particular with respect to required drive energy for operation, compactness and reliability. A further reduction in size implies increasing operational field stress inside the breaker and, thus, a reduction in built-in safety margins. If the design comes closer to its physical limits, in particular the dielectric coordination of the contact systems becomes more and more challenging. The aim of the present work is to identify factors that may influence the dielectric coordination of contact systems with reduced coordination margin. The breakdown probability distribution of SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> insulated multi-contact systems was investigated with the focus on two aspects. First of all, the breakdown probabilities under lightning impulse (LI) stress were measured. These indicated that it is not feasible to extrapolate the breakdown probability curves of single contacts to a combined full contact system. Secondly, the findings on the same contact system stressed with very fast transient voltages (VFT) showed a considerable reduction in breakdown voltage and suggested a significant increase in the main contact breakdown probability, compared to the LI tests. The results and interpretation within this work identified the polarity and voltage shape dependent breakdown initiation of different contacts as causes for circuit breaker contact system failure.

Design of Triggering Device for VFTO Measurement System

Very fast transient overvoltage (VFTO) can be generated during the operation of disconnect switch (DS). Especially in ultra high voltage (UHV) gas insulated substation (GIS), VFTO is a threat to the safety of the power system equipment. It is very important to measure VFTO waveforms accurately. VFTO waveforms are comprised of high frequency voltage superimposed on the power frequency voltage in source side of DS and high frequency voltage superimposed on step-wise voltage in load side. How to trigger oscilloscope to acquire data is a big problem in measuring process. According to the features of VFTO waveforms, new triggering device consisting of a high pass filter is designed. The output of measurement sensor is used as the input of the high pass filter, and the output of the high pass filter is used to trigger oscilloscope. When the DS is not operated, there are no signals output from the high pass filter and the oscilloscope is waiting for trigger. After the operation of DS, the high frequency signals will pass the filter to trigger oscilloscope to acquire data. The device can guarantee the oscilloscope to complete a data acquisition properly.