Switching Impulse Research Articles

Improved Electrogeometric Model Suitable for EHV and UHV Transmission Lines Developed through Breakdown Testing for Long Air Gaps

As a type of engineering model used for solving lightning-shielding problems, the electrogeometric model has been widely used for assessing lightning-shielding failure in high-voltage and extra-high-voltage transmission lines. However, with the increase of voltage levels in ultra-high-voltage lines and the growth of the air gap, the results previously yielded by the electrogeometric model are no longer consistent with those observed. Therefore, this study corrected the equation for the relationship between lightning current and striking distance, by using data on a negative polarity 50% discharge voltage from the rod-rod gap −20/2500 μs switching impulse test wherein an air gap of up to 10 m was first tested. Subsequently, the return stroke velocity distribution observed by Idone was newly applied. In addition, the correction factor for the ground was corrected in this study. Thus, an improved electrogeometric model for application in EHV and UHV transmission lines is proposed in this paper. By employing the improved electrogeometric model to calculate the frequency of lightning strikes of 500 and 1000 kV transmission lines in Japan and a 1000 kV transmission line in China, we found that the results calculated using the improved electrogeometric model were closer to the actual observations in Japan than was the original electrogeometric model. The improved electrogeometric model provides a more reliable foundation for lightning-shielding design of extra-high-voltage and ultra-high-voltage transmission lines.

Electrostatic model to understand the influence of salt deposit in gfrp material during lightning discharges

A methodical experimentation and mathematical modeling is carried out to study the influence of salt deposit on wind turbine blade material (Glass Fiber Reinforced Plastics/GFRP) during lightning discharges. Electrical discharge measurements combined with optical emission spectroscopy technique and electrostatics is utilized to understand the dynamics of surface discharge and the level of damage on GFRP material. COMSOL simulation studies shows that the electric field intensity on the surface of the polluted GFRP gets enhanced when compared to that of virgin GFRP, irrespective of the applied voltage profile and polarity. Flashover voltage, discharge current and optical emission spectroscopy measurements indicates that the deterioration of GFRP material is severe when there is an adhesion of salt deposit. It is also observed that the damage induced on GFRP material is severe for winter lightning (switching impulse voltage of 250/2500 μs) compared to that of summer lightning (lightning impulse voltage of 1.2/50 μs) due to a longer front and tail period of the pulse.

Study of surge arrester model under influence of various current pulses

The model of surge arrester, which consists of two nonlinear resistors, connected with each other via two inductive and a resistive element is studied. To determine the model parameters are only required by the data given in the catalogs of surge arresters manufacturers. Until now, in the simulation of surge arresters insufficiently investigated the effect of currents typical for lightning surges. It is shown that the model of surge arrester can be investigated by comparing the results of computer simulation with the data given in the catalogs of surge arresters. The study will use the following data, specific both switching and lightning surges: the residual voltage at switching impulse current, the residual voltage at lightning impulse current, as well as the residual voltage at steep current pulse. Residual voltages corresponding to the 250-2000 A standard switching current pulses, 1.5-20.0 kA standard lightning current pulses and 10.0 kA steep current pulses were defined using Micro-Cap model of surge arrester. Residual voltages obtained in the simulation were compared with the corresponding values given in catalogs of surge arresters. It is shown that the smallest error takes place under the influence of lightning current pulses on the model of surge arrester. The model of surge arrester in Micro-Cap program allows high accuracy determination of the residual voltage on the surge arrester during the flow of discharge pulses, which are typical for lightning surges. The model also allows to calculate the transients during the flow of the lightning currents, which are composed with several successive pulses, which can pose a significant danger to the surge arresters. The results of the research can specify the parameters of substation protection against different overvoltages and, thus, reduce economic losses due to possible faults.

Partial discharge test under standard oscillating impulses on a gas-insulated bus with artificial metal particle defects on the insulator surface

In this work, PD diagnosis was performed under standard oscillating lightning impulse (OLI), oscillating switching impulse (OSI) and alternating current (AC) voltages on a 110-kV GIS bus model with artificial particles on the insulator surface. The results demonstrated that the difference between the PD inception voltage (PDIV) and breakdown/flashover voltage (BDV) under AC voltages was smaller than that under the oscillating impulse voltages (OIVs), especially as gas pressure improved. Under OIVs, most PDs occurred at the rising slopes of the oscillating periods, while few reverse polarities were detected around the troughs. PD sequences excited by particles located at different positions presented their own different features in terms of PD magnitude and PD number. The average magnitudes of the PDs decreased as the absolute gas pressure increased from 0.05 to 0.4 MPa. Basically, the PDs at the positive point and those under the OIVs had the highest values of magnitude compared to those in other conditions. These results prove that the defects of particles on the insulator surface are more sensitive to OIVs than to AC voltage.

Partial discharge characteristics of uniform gap in oil-impregnated paper insulation under switching impulse voltage

In comparison with the AC and DC working voltages, the effect of switching impulse voltage on insulation is very different because of its fast-rising rate and large amplitude. This paper mainly focuses on the characteristics of PD in oil-impregnated paper insulation under switching impulse voltage. First, a robust experimental platform was set up including the impulse power source, test samples with uniform gas and oil gap, and the signal acquisition and processing system. PD characteristics in the air and oil gap are analyzed and compared involving the time-domain waveforms, the variation tendency of the average amplitude and numbers of PD pulses, the occurrence time and the instantaneous value of impulse corresponding to the first PD. Based on the dielectric barrier discharge (filament) theory, a parallel microcircuit model was established to explain the similarities and dissimilarities, and to evaluate the gap discharge intensity. The occurrence and development of PD under switching impulse voltage of typical oil-impregnated paper defect were studied systematically with simulation, experiments, and modeling.

Partial discharges in void defect of gas insulated switchgear insulator under standard aperiodic and oscillating switching impulses

This study focuses on partial discharges (PDs) in the gaseous void in an insulator under aperiodic switching impulse (SI) and oscillating SI (OSI). The PD measurements are carried out using a 110-kV GIS model with an artificial known void defect in a basintype insulator. Experiments show that the PD sequences under SI and OSI are composed of the main discharge, which occurs at the rising edges or the peaks of the applied impulse, and the polarity reverse discharge, which occurs at the falling edges or troughs. For a new sample, the 50% PD inception electric field (PDIE50) under different stress types obviously decreases with increasing void radius, and their values follow the pattern PDIE50, OSI > PDIE50, SI > PDIE50, AC. For an aged sample, the difference between the experimental PDIEs and the streamer inception value becomes close. The relationships between the time lag and the applied voltage can be calculated by the V–t curves, which roughly describe the distribution of the time lag under impulses. The experiment indicates that OSI can more easily excite PD compared with SI. Repeated tests show that the magnitudes and numbers of the main and reverse PDs gradually change as the test is repeated.

A New Switching Impulse Generator Based on Transformer Boosting and Insulated Gate Bipolar Transistor Trigger Control

To make the switching impulse (SI) generator more compact, portable and feasible in field tests, a new approach based on transformer boosting was developed. To address problems such as triggering synchronization and electromagnetic interference involved with the traditional spark gap, an insulated gate bipolar transistor (IGBT) module with drive circuit was employed as the impulse trigger. An optimization design for the component parameters of the primary winding side of the transformer was realized by numerical calculation and error correction. Experiment showed that the waveform parameters of SI and oscillating switching impulse (OSI) voltages generated by the new generator were consistent with the numerical calculation and the error correction. The generator was finally built on a removable high voltage transformer with small size. Thus the volume of the generator is significantly reduced. Experiments showed that the waveform parameters of SI and OSI voltages generated by the new generator were basically consistent with the numerical calculation and the error correction.

Partial Discharge Measurement under an Oscillating Switching Impulse: A Potential Supplement to the Conventional Insulation Examination in the Field

Partial discharge (PD) detection under oscillating switching impulse (OSI) voltage was performed on three types of insulation defects, including a protrusion on a conductor, a particle on an insulator surface, and a void in an insulator, which are three kinds of the common potential insulation hazards in gas insulated power apparatus. Experiment indicated that the PD sequences under OSI were composed of various combinations of the single pulse, the multiple pulses, and the reverse polarity pulse. The difference between the PD inception voltage (PDIV) and the breakdown voltage (BDV) under OSI voltage was greater than that under alternating current (AC) voltage in some cases, which can provide a more sufficient margin below the BDV for PD diagnosis. The OSI voltage also showed a better performance for exciting PDs with detectable magnitudes from small-scale defects, of which the AC voltage was incapable under our test conditions. The different PD activities with different interfaces under an impulse and a slowly varying voltage were speculated to be associated with the gradient of the background electric field and the space-charge mobility.

Electrical and optical measurements investigation of the pre‐breakdown processes in natural ester oil under different impulse voltage waveforms

A study of pre-breakdown processes in natural ester oil is realised in the present study. A point-to-plane electrode configuration was used and the oil was subjected to lightning and switching impulse (SI) voltage stress. The inter-electrode gap distance was set to 1 and 2 cm long. Hence, electrical and optical measurements, concerning breakdown voltage, gap current, and light emission were acquired. Experimental data shows significant differences between the pre-breakdown processes taking place under the application of lightning 1.2/50 μs and switching 250/2500 μs impulse voltage. Ionisation/excitation activity, prior to breakdown, clearly occurs mainly under the application of SI voltage. This activity is mirrored on the optical emission signal while gap current measurements prior to breakdown were negligible in all cases. Furthermore, secondary-reverse streamers are detected during the SI voltage.

Effect of branching on spikes of positive leader current

Positive leader current spikes were observed in long air gap discharge. However, there has been no explanation on this phenomenon so far, and some leader advancement models can only produce the current without spikes. Therefore, it is necessary to figure out this phenomenon in order to deepen the understanding of leader discharge. In this paper, experiments of leader development were carried out in a 10 m rod-plane gap under positive switching impulse. The discharge current and high speed photographs of discharge process were recorded synchronously. Then the high speed photographs during the period when the leader current fluctuated were carefully analyzed. The experimental results showed that the leader current spikes were with leader branching. Finally, the effect of branching on the spikes of positive current was explained qualitatively and a hypothesis on leader branching was proposed.

Failure analysis and maintenance of a surge capacitor on the neutral bus in a ±500 kV HVDC converter station

This study analyses a surge capacitor failure on the neutral bus in a ±500 kV high voltage direct current (HVDC) converter station. Both overvoltage and insulation of the surge capacitor are investigated through simulation, practical record data analysis, and experimentation. A highly accurate HVDC model is established on the basis of the topological structure and parameters of the HVDC project. Using this model, the failure process is simulated by combining neutral bus overvoltage record analysis and voltage divider performance test. Results indicate that the 246 kV uncharacteristically high overvoltage on the neutral bus is a measurement error caused by voltage divider fault. From the field test, difference equation of the voltage divider is derived to reconstruct the actual overvoltage, the amplitude of which is 73 kV. The amplitude is significantly lower than that of the switching impulse withstand level of the surge capacitor, which should not cause capacitor failure. Therefore, the primary cause of capacitor failure is insulation damage rather than overvoltage. Finally, maintenance measures are proposed and applied in the station.

Influence of voltage waveforms on partial discharge characteristics of protrusion defect in GIS

The CIGRE WG D1.03 has indicated the breakdown voltage of GIS with protrusion defect under standard lightning impulse (LI) voltage is significantly smaller than that under standard switching impulse (SI) or AC voltage, and LI withstand test is more efficient to find protrusion defect. The question is that whether the partial discharge (PD) inception voltage of protrusion defect under LI is also lower than that under standard switching impulse or AC voltage. In order to understand the relations between voltage waveforms and protrusion defect, three kinds of protrusion with different curvature radius are designed, namely, 0.025 mm curvature radius, 0.25 mm curvature radius, and 0.5 mm curvature radius, and the PD characteristics of protrusion defects are measured under four types of voltage waveforms (standard lightning impulse, standard switching impulse, oscillating switching impulse (OSI), AC voltage), and the influence of voltage waveforms on PD characteristics of protrusion is analyzed. The results indicate that, for protrusion with 0.025 mm curvature radius, the steeper the impulse voltage front is, the lower PD inception voltage of protrusion defect is, and PD under LI has higher detection sensitivity to protrusion compared with AC voltage and SI. While for protrusions with 0.25 mm curvature radius and 0.5 mm curvature radius, the steeper the impulse voltage front, the higher the PD inception voltage of protrusion defect is, and PD under LI has lower detection sensitivity of protrusion with 0.25 mm and 0.5 mm curvature radius compared to AC voltage. As PD exacerbated, the shape changes occur in the q−φ scatter plot, N−φ histogram under oscillating switching impulse voltage, and q−1 scatter plot and N−1 histogram under non-oscillating impulse voltage, which can be used as important criterion in assessing the severity levels of the partial discharge triggered by protrusion under impulse voltage waveforms.

Statistical characteristics of breakdowns in long air gaps at negative switching impulses

Discharge characteristics in long air gaps follow certain statistical regularities. Statistical methods and finite element simulation were both employed to deeply study discharge characteristics. The statistical results indicate that the height of grounding rod has a slight influence on the 50% breakdown voltage of rod-rod air gaps and that the breakdown time is longer than the wave front time when 20/2,500 μs switching impulse is applied and shorter than the wave front time when 80/2,500 μs switching impulse is applied. Furthermore it can be shown that the distribution of the breakdown voltages in 10 m rod-plane gap follow the normal distribution and that the V-t characteristics of rod-rod gaps is lower than that of rod-plane gaps. Calculation results indicate that the non-uniform coefficient of the electric field increases with increasing gap distance and that the ratio of gap distance to the non-uniform coefficient also increases with the gap distance. The work described in this paper can be used for the insulation coordination of UHV transmission lines.

Research on Overvoltage for XLPE Cable in a Modular Multilevel Converter HVDC Transmission System

Due to the rapid development of the voltage-source converter (VSC), the demands for HVDC cables have increased significantly. Although more than ten VSC-HVDC projects are under construction and many more are in operations globally, the testing criteria for HVDC XLPE cables are mainly referenced to test guideline TB 496 recommended by CIGRE. Due to the large stray capacitance of the VSC-HVDC cables, the switching impulse overvoltage stress of the cables is the main electrical parameter concerned, a unified overvoltage factor of 2.1 p.u. being generally accepted. However, the switching impulse overvoltage stress of a VSC-HVDC cable system depends on not only the VSC topology but also the cable parameters and the volt-ampere characteristics of the dc cable surge arresters used. This paper studies the switching impulse overvoltage stresses for the dc cable system of a symmetrical monopole VSC-HVDC transmission system using the modular multilevel converter technology. The studies were performed according to the key technical parameters for a typical 1000-MW/320-kV VSC-HVDC transmission scheme, the XLPE dc cable links being assumed. The results show that a switching impulse overvoltage level of 2.3 p.u. should be applied during the type test of the overvoltage capability of the HVDC cables.

Rogowski coil with a non-inverting integrator used for impulse current measurement in high-voltage tests

This paper presents design and construction of an impulse current measuring system based on a Rogowski coil with a non-inverting integrator used for impulse current measurement in high-voltage tests. For convenience in current measurement in the high-voltage tests, the developed Rogowski coil is specifically designed as a clamp on device. For verification, the developed current measuring system was tested in comparison with a non-inductive shunt resistor and the same Rogowski coil with previous developed integrators. The various standard impulse currents, i.e. the steep impulse current (1/20μs), the high impulse current (4/10μs), the lightning impulse current (8/20μs), the switching impulse current (30/80μs), and the long-duration impulse currents with duration time of 1−3ms were generated for comparative measurement in a high-voltage laboratory. From the experimental results, the developed Rogowski coil with the proposed non-inverting integrator has fairly high accuracy in wide frequency bandwidth covered the frequency bandwidth of the standard impulse currents in high-voltage tests. It can be concluded that the developed current measuring system has a promising characteristic being appropriate for measurement of impulse currents in high-voltage tests.

Effect of superimposed impulses on AC partial discharge characteristics of oil-impregnated paper

The effect of HV impulses on surface and cavity partial discharges (PD) of oil-impregnated paper (OIP) is investigated in this work, by subjecting specimens to impulse voltages superimposed on a continuous AC voltage. The superposition of transient and AC voltage is relevant to insulation systems in power-system equipment, which is exposed to transients such as lightning and switching impulses during its operation. For several different PD-generating defects in oil-impregnated paper, the PD behaviors before and after the impulse are compared. The influence of moisture content is studied by using OIP with moisture content <0.5%, 3.0% and 5.5%. The results show a distinctive behavior of PD due to the impulse for each defect and insulation condition, either by change in the PD rate and magnitude or by causing PD inception or extinction. Recordings of the voltage signals and associated PD measurements during and after the incidence of a high voltage transient are of potential use for condition assessment of the insulation of power transformers and their bushings, as a form of online diagnostics.

Dielectric response measurement by impulse stimulus on AC: Measurement considerations, and laboratory testing on a bushing

Dielectric response (DR) measurement is commonly used for condition assessment of insulation systems of power components. A method for measurement of dielectric response using the stimulus of natural transients such as lightning and switching impulses was proposed in earlier work. Its desirable features include the ability to make measurements online over a range of frequencies, without requiring a voltage source. This article presents a laboratory demonstration of the method on a 150 kV service-aged transformer bushing, where the stimulus was a standard lightning impulse voltage superimposed on an AC voltage. Several aspects of the measurement and data processing that affect the results are studied experimentally and numerically. The results are compared with low-voltage frequency domain spectroscopy (FDS). Reasonable accuracy for monitoring changes in DR can be achieved by suitable choice of measurement circuit and data acquisition. The study suggests an approach for online monitoring of dielectric properties of power transformer bushings, and diagnostics of defects that affect the high frequency region of DR, such as moisture content.

Analysis of Higher Order System with Impulse Exciting Functions in Z-Domain

This paper deals with mathematical modelling of impulse waveforms and impulse switching functions used in electrical engineering. Impulse switching functions are later investigated using direct and inverse z-transformation. The results make possible to present those functions as infinite series expressed in pure numerical, exponential or trigonometric forms. The main advantage of used approach is the possibility to calculate investigated variables directly in any instant of time; dynamic state can be solved with the step of sequences (T/6, T/12) that means very fast. Theoretically derived waveforms are compared with simulation worked-out results as well as results of circuit emulator LT spice which are given in the paper.

Partial discharge measurement and analysis at standard oscillating switching and lightning impulses on a GIS with artificial protrusion defects

Partial discharges (PDs) induced by artificial fixed protrusions inside an SF6 gasinsulated switchgear (GIS) test object under oscillating switching and lightning impulse voltages (OIVs) were investigated experimentally. The features of PD current and light pulses (sequences) under impulses were analyzed. The characteristics of PD behaviors under oscillating lightning impulse (OLI), oscillating switching impulse (OSI) and 50 Hz AC voltages were studied according to their statistical distributions. The experimental data were quantitatively analyzed to compare the effectiveness of AC and OIV voltages in exciting the PDs in terms of PD inception and PD activity. Most PDs occurred successively around the peaks of the oscillation period of OIVs and periodical PD sequences were formed. Compared to positive point PDs, negatives occurred more frequently, but with smaller magnitudes. The positive OSI was found to have relative high efficiency in exciting PDs from fixed protrusion comparing to other types of voltage investigated. The occurrence frequency of PD under negative OLI was one or more orders of magnitude greater than that under AC voltage. The average PD magnitudes under positive OIVs were much greater than the others. Both the average PD magnitude and the occurrence frequency were greater at 0.2 MPa, than at 0.4 MPa. This indicates that the AC PD test is not very sensitive to local enhanced field defects, such as fixed protrusions on conductors, because of the limited PD measurement sensitivity and the corona stabilization under AC voltage.

Study on voltage-number characteristics of transformer insulation under transformer invading non-standard lightning impulses

According to the IEC Standard, the insulation performances of power apparatus under lightning impulse voltages are evaluated by the withstand test of standard lightning impulse voltage, with the wave front/tail time of 1.2/50 μs. However, the standard lightning impulse voltage originally obtained from the transmission lines or triggering lightning towers, is quite different from the invading lightning impulse voltage in the power transformers. Hence, in this work, an attempt has been made to obtain the waveform characteristics of invading lightning impulses in power transformers using an overvoltage online monitoring system. Energy Method was then proposed to convert the recorded oscillatory waveform into double-exponential waveform defined by IEC Standard. And the statistical waveform parameters of recorded invading lightning impulses, namely, the non-standard lightning impulses, were determined based on the Energy Method. Current insulation assessment methods, for example, the V-t characteristics, are far from satisfactory to meet the demand of insulation design for the power apparatus, whereas the impulse breakdown voltage-number of voltage applications characteristics (V-N characteristics) could well evaluate the long term performance of insulation system. Therefore, we experimentally identified the V-N characteristics of oil paper insulation under impulse voltage with different waveforms, including non-standard lightning impulse voltage, and then mathematically analyzed the obtained data. At last, the influence of wave front/tail time on the V-N characteristics of oil paper insulation was experimentally studied. The test results indicate that the insulation performances of oil paper insulation under repeated nonstandard lightning impulse voltages and under standard switching impulse voltages were better than that under repeated standard lightning impulse voltages. And the capacity to withstand repeated voltage became increasingly stronger with the increase in wave front time or the decrease in wave tail time. The breakdown voltage, however, was only related to the wave front time of applied impulse voltage.