AC Partial Discharge Research Articles

Breakdown and Partial discharge characteristics of Mineral oil‐based nanofluids

In this study, the AC breakdown (BD) voltages and partial discharge (PD) activities of mineral-oil (MO)-based nanofluids were investigated under controlled conditions. The aim of this research is to improve the dielectric properties of MO by amalgamation with various types of nanoparticles, i.e. semiconducting and dielectric nanoparticles. Two types of nanoparticles, i.e. titanium dioxide and barium titanate with mean diameters of <;100 nm were used in the nanofluid samples prepared for the experiments. The AC BD voltages of the nanofluids were investigated according to IEC 60156. In addition, PD characteristics of the MO-based nanofluids were examined including PD inception voltage (PDIV) characteristics and PD activity with a needle-plane electrode configuration. The test circuit was set up in accordance with IEC 60270 and the experiment was performed in conformity with IEC 61294 at room temperature. The test results showed that the AC BD voltages of the modified liquids were clearly influenced by the type of nanoparticles added, with the AC BD voltages of the MO with nanoparticles being slightly higher than those of the unmodified MO used for comparison. Furthermore, the nanoparticles also demonstrated a propensity to increase the PDIV of the nanofluids compared with the PDIV of the MO.

氷雪中で起きる局部放電の紫外線画像による電流の大きさの推定

In a transmission line passing through a snow region, snow inevitably accretes on insulators more or less and partial discharge might occur on insulator strings covered with snow. Since partial discharge occurring at the surface of polymeric insulator may deteriorate the insulating property of it, it is necessary to find the magnitude of discharge current as well as the location or occurrence frequency of partial discharge to grasp the degree of degradation. The paper shows the method for estimating the magnitude of the discharge current from the images taken by an ultra violet camera. To simulate partial discharge occurring at an air gap in capped snow over an insulator, ac partial discharge was formed between a pair of ice electrodes or snow walls. By increasing applied voltage to the ice or snow electrodes, discharge current was increased. The luminous area in UV image was increased with the magnitude of discharge current below 10mA. The magnitude of discharge current flowing through an air gap formed in accreted snow can be estimated from the luminous area in UV image.

Effects of DC prestressing on partial discharge in oil-impregnated pressboard insulation

The AC (50 Hz) partial discharge (PD) characteristics of oil-impregnated pressboard (OIP) insulation are severely influenced by the DC voltage component in the converter transformer. Therefore, this study investigates the effect of different DC prestressing voltage (DCPV) amplitudes, prestressing time, and repetitions times of the applied AC voltage on the OIP insulation AC PD characteristics. Meanwhile, the space charge characteristics of distribution and dissipation are measured by pulsed electro-acoustic method. The experimental results show that the PD inception voltage (PDIV) and PD extinction voltage (PDEV) after DCPV were lower than that under AC voltage without DCPV. The PDIV decrease with higher DCPV at prestressing time from 30 to 60 min. But it increase with higher DCPV at prestressing time from 90 to 120 min. However, PDIV initially increase then decrease with the increasing repetition times of the applied AC voltage. The space charge distribution in 1 mm OIP insulation takes 60 min to reach a balanced condition. The residual space charge is relatively higher at a longer prestressing voltage time and higher DC voltage when the DC voltage was turn off. The PDIV and PDEV after DCPV is improved by Joule heating with longer prestressing time and higher DCPV. The space charge had negative effect on them.

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.

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.

絶縁体/空気ギャップ複合絶縁系における直流および交流部分放電開始電圧に及ぼす表面帯電の影響

The electrical insulating materials in motors are exposed to higher electric field in improving their performance such as high outputs and/or miniaturizations. Especially, the motor windings have the small gap at which partial discharge (PD) may occur under the high electric field and it can lead to dielectric degradation and breakdown. The actual power equipment is used under various environmental conditions. In actual case, PD may easily occur due to charge on the surface of the insulating parts. Therefore, it is important to clarify the partial discharge inception voltage (PDIV) characteristic at the small gap of the motor windings. In this study, we investigated the influence of surface charge on DC or AC partial discharge inception voltage in the insulation-air gap composite system.

Investigation on insulation material morphological structure of 110 and 220 kV XLPE retired cables for reusing

Most of the 110 and 220 kV cables are in service till the end of life, but part of them would be out of service by refurbishment. This research is focused on evaluating the cable insulation and determining whether the cable can be reused. All the cables used in this research satisfactorily passed the AC voltage and partial discharge tests. Then the fourier transform infrared spectroscopy, thermogravimetry analysis, differential scanning calorimetry, dynamic mechanical analysis, and tensile machine were used to analyze the insulation morphological structure and performance of 20 retired 110 and 220 kV crosslinked polyethylene cables obtained from the Beijing and Zhuhai districts. The results show that the structure parameters which can represent the physical change and the irreversible chemical change of cables have obvious time dependence characteristic. DSC result shows that crystallinity, melting range, and crystallization rate increase slightly with service years. DMA experiment illustrates that the temperature of mechanical loss peak, Tγ, shifts to a higher temperature while the mechanical loss increases slightly and temperature dependency of storage modulus increases with service years. The electrical treeing experiment utilized cable operated for 17 years, which is the longest operation year for the samples. Compared with new cable, the cable operated for 17 years has a slight decrease in tree initiation time and an obvious increase in tree propagation time. These experiment data show that the insulation material morphological structure is mainly stable, and the test results are also a contribution to the lifetime expectancy evaluation of the 110 and 220 kV XLPE cable.

Suitability of Palm Based Oil as Dielectric Insulating Fluid in Transformers

Mineral oil has been widely used as dielectric insulating fluid in transformers due to its excellent performance in-service. However, there are few issues with mineral oil such as it has poor biodegradability and could contaminate the environment if a spillage occurs. With the increasing tight regulation on safety and environment, alternative fluids for mineral oil are currently being investigated and among the suitable candidate is the vegetable oil. There are different types of vegetable oils and one of them is the palm based oil. At the moment, extensive research works are carried out to examine its feasibility to be applied in transformers. This paper will review the previous research works that were carried out to examine the suitability of palm based oil as dielectric insulating fluid in transformers. The physical and chemical properties of palm based oil are studied based on viscosity, acidity, oxidation stability and flash point. Next, the electrical characteristics of palm based oil are examined based on AC breakdown voltage, relative permittivity, dissipation factor and partial discharge.

Measurement and simulation of partial discharge in oil-paper insulation under the combined AC–DC voltage

Depending on unique operation characteristics for voltage applied on valve side winding of the converter transformers, it is extraordinarily significant to study the partial discharge (PD) behavior with oil-paper insulation under combined AC–DC voltage. Therefore, this paper investigated PD inception characteristics by pulse current methods with needle-plate electrode system under combined AC–DC voltage. Furthermore, 3D electric field distributions versus combined AC–DC voltage in different ratios were calculated by Finite Element Analysis (FEA). An experimental conclusion was drew that AC partial discharge inception voltage (PDIV) in pure oil would decrease linearly with the DC component increasing but the inception voltage with oil-paper insulation appeared to be independent of DC voltage and dependent of AC voltage. And 3D electric field distribution deduced from simulation provided a supplementary proof on the experimental results. Moreover, high speed photography was used to capture emitted light produced by discharge, estimate streamer velocity (1.8 km/s) and record streamer initiation and propagation process in oil gap. Previous studies have shown that the prebreakdown phenomena involving the generation and propagation of vapor channels through the oil could be divided into a three-stage process.

고압전동기 고정자 권선의 온도변화에 따른 절연특성 분석

A variety of diagnostic tests are widely applied in the field in industry to evaluate the condition of high voltage (HV) motor stator insulation. In this paper, the influence of temperature on the stator insulation diagnostic tests such as the insulation resistance, AC current, dissipation factor, and partial discharge measurements are studied and reported. The tests are performed with the HV motor stator winding temperature set between to in intervals. It is shown that the AC current, dissipation factor, and partial discharge magnitude steadily increase with temperature, which suggests that temperature must be taken into account in the interpretation of the test results.

Transient Earth Voltage Measurement in PD Detection of Artificial Defect Models in $\hbox{SF}_{6}$

This paper mainly discussed the utilization of transient Earth voltage measurement (TEVM) in partial discharge (PD) detection of gas insulation switchgear. Four typical artificial defect models in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> such as spikes fixed on high voltage (HV) conductor, suspended metals, voids in dielectric, and metal particles on the surface of dielectric had been used as test objects in ac PD experiments with the aid of the synchronized PD signals sampled from a transient Earth voltage sensor, photomultiplier, Rogowski monitor, and PDCheck system. The measuring results of TEVM results, which included the phase distribution of PD, the relative amplitude, and the pulse number of different voltage thresholds, showed good consistency with three other methods and had better sensitivity of detection in spark or corona PD patterns. For further study, the frequency distributions of the three PD patterns were analyzed and discussed for understanding the different measurement sensitivities showed in different typical defects in SF <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> . In addition, TEVM was applied to detect PD under oscillating impulse voltage. As an example, the needle-plane electrode with short gap was adopted under 613.2-kHz oscillating impulse, which showed the effectiveness in this kind of detection.

6.6 kV 전동기 고정자 권선의 절연진단과 절연파괴 특성

To assess the condition of stator insulation, nondestructive and overpotential tests were performed on four high voltage motors. The stator windings under these tests have nominal ratings of 6.6 kV. After completing nondestructive tests, the AC overvoltage applied to the stator windings was gradually increasing until insulation failure in order to obtain the breakdown voltage. No. 1, No. 2, No. 3 and No. 4 of 6.6 kV motors failed near rated voltage of 18.4 kV, 19.8 kV, 19.7 kV and 21.7 kV, respectively. The breakdown voltage of four motors was higher that expected for good quality coils(14.2 kV) in 6.6 kV motors. Almost all of failures were located in a line-end coil at the exit from the core slot. The breakdown voltages and the types of defects showed strong relation to the stator insulation tests such as in the case of AC current, dissipation factor(<TEX>$tan{\delta}$</TEX>) and partial discharge magnitude.

Gas by-products of CF3I under AC partial discharge

This paper presents the decomposition by-products of trifluro-iodo-methane and their relative proportions in the gas phase under the occurrence of partial discharge. The experiment was performed in the presence of water vapor from 250 to 400 ppm under a non-uniform electric field configuration. The experimental results reveal that the by-products of C2F6, C2F4, C2F5I with the amount of 1300, 200, and 55 (CH3I) ppm, respectively, were produced for a cumulative charge of 161 mC. Other by-products, such as C3F8, CHF3, C3F6 CH3I were obtained at less than 30 ppm C2F6 was the dominant gas by-product of trifluro-iodo-methane suffering partial discharge.

High Voltage Dielectric Characteristics of Epoxy Nano-Composites in Liquid Nitrogen for Superconducting Equipment

Cryogenic dielectric insulation skills play a significant role in the development of superconducting electric equipment for transmission and distribution electric network. Nowadays, newly developed nano-composites have shown enhanced electrical, thermal, and mechanical properties of polymer insulation materials. And the application of nano-composites in high voltage power systems could be implemented in the near future. Among the various nano-composites, epoxy nano-composites have been paid much attention as a new insulating material for high voltage insulation. In this paper, we presented experimental results of epoxy nanocomposites in liquid nitrogen and determined the possible applications of nano-composites as insulating material for superconducting equipment. In order to determine their dielectric breakdown properties in liquid nitrogen, various kinds of epoxy based nano-composites have been made by mixing SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , TiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> fillers, respectively. AC withstand voltage test and partial discharge (PD) inception voltage tests have been performed to verify the insulation breakdown characteristics of nano-composites in cryogenic environment. Consequently, it was deduced that the breakdown strength of epoxy nano-composites have been improved compared to epoxy with micro-fillers in cryogenic environment. And epoxy nano-composites immersed in liquid nitrogen showed a similar breakdown voltage characteristic when compared with that of insulating oil. In addition, the effects of nano-composites have been varied according to the concentration of nano-fillers and their content of fillers.

Partial discharge-induced degradation characteristics of insulating structure constituting oil-immersed power transformers

The soundness of an oil-immersed power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration ac withstand voltage test. The acceptance criterion for this PD test is 500 pC according to IEC standard; however, only insufficient basic data backing this criterion are available. In addition, the harmful level of the insulation performance has not necessarily been perceived in such a manner as the present one, for example, only the insulation breakdown, instead of the insulation performance degradation, has been assumed to be harmful. In light of the consideration presently paid, it is necessary to determine the PD level that causes the insulation performance to be degraded in order to confirm the soundness by the test using an actual transformer. As the initial step of the study for this purpose, the previous paper investigated the PD and the insulation damage level using oil-impregnated press-boards (PBs) used in the transformer, at which PD is more likely to occur when ac voltage is applied than at any other parts, and evaluated the harmful PD level for insulating materials themselves. To investigate the harmful PD level for the actual transformer, this paper studied the harmful PD level that decreases the residual lightning impulse (LI) withstand voltage using the main insulation model, which is the insulating structure constituting the oil-immersed power transformer. As a result, black discoloration occurred on the surface of the PB and the residual LI withstand voltage also tended to decline when exposed to ac PD of 20,000 pC or more. On the other hand, when it had been exposed to ac PD of 10,000 pC, no visible damage was observed and there was no drop in the residual LI withstand voltage either. Consequently, the harmful ac PD level for the oil-immersed transformer can be determined at between 10,000 pC and 20,000 pC. This result coincided with that for insulating materials themselves, hence it was revealed that the harmful PD level be set to 10,000 pC on a conservative side.

High Voltage Testing of a 5-meter Prototype Triaxial HTS Cable

High voltage tests were performed on a 5-m long prototype triaxial HTS cable (supplied by Ultera) at ORNL in preparation for installation of a 200-m HTS cable of the same design at the AEP utility substation in Columbus, Ohio. The triaxial design comprises three concentric phases and shield around a common former with the phase to phase dielectric at cryogenic temperature. Advantages of this design include increased current density, a reduced amount of HTS tape needed, and reduced heat load. The phase to phase voltage will be 13.2 kVrms (7.6 kVrms to ground). Preliminary testing was done on half-scale and full-scale terminations which successfully passed AC withstand, partial discharge, and impulse tests. High voltage tests conducted on the 5-m cable with the cable straight and after bending 90 degrees were ac withstand to 39 kVrms, partial discharge inception, and a minimum of 10 positive and 10 negative lightning waveform impulses at 110 kV. Phase to phase insulation was tested by applying high voltage to each phase one at a time with all the other phases grounded. Partial discharge data will be presented. The 5-m prototype triaxial HTS cable passed all the HV tests performed, with a PD inception voltage significantly above the required voltage.

Electrical Insulation Design and Experimental Results of a High-Temperature Superconducting Cable

A 22.9 kV/50 MVA class high-temperature superconducting (HTS) power cable system was developed in Korea. For the optimization of electrical insulation design for HTS cable, it is necessary to investigate the ac and impulse breakdown and partial discharge inception stress of the liquid nitrogen (LN2)/ laminated polypropylene paper (LPP) composite insulation system. These results were used to insulation design of the model cable for a 22.9 kV class HTS power cable and the model cable was manufactured. The insulation test of the manufactured model cable was evaluated in various conditions and was satisfied standard technical specification in Korea. Based on these experimental data, the single-phase and three-phase HTS cable of a prototype were manufactured and verified.

Assessment of Insulation Degradation in Rotating Machines

To assess the degradation of stator winding insulation in high voltage motor, a total of forty-eight coils were manufactured. These coils are classified into five groups and there are twelve strands in each coil. Four coil groups are artificial defects such as shorted turn of strand, internal separation between conductor and insulation, large void within insulation and removal of semi-conductive tape. The other coil group is normal. Before ac breakdown test was performed, nondestructive test that included ac current, dissipation factor and partial discharge (PD) was done with five coil groups. This paper carried out by comparing the results between nondestructive test and ac breakdown test. When the values of ac current increase rate, tip-up dissipation factor and PD magnitude are large, the insulation is occurred the deterioration. AC breakdown voltage is also low in degraded insulation.

Impulse and ac partial discharge inception and breakdown characteristics of LN2/polypropylene laminated paper composite insulation system

Partial discharge (PD) inception and breakdown (BD) characteristics of liquid nitrogen (LN2)/polypropylene (PP) laminated paper composite insulation system for high temperature superconducting (HTS) cables are presented. In this paper, we measured PD inception characteristics under lightning impulse and AC voltage applications by electrical, optical and acoustic methods. The impulse PD inception characteristics were compared with those under AC voltage application, and the PD generation mechanisms are discussed. Experimental results reveal that PD detection sensitivity under lightning impulse voltage application is as small as 6 pC. PD inception at the lower stress was verified to be generated not only in the butt gaps but also in micro gaps between PP laminated paper layers under both lightning impulse and AC voltage applications. After PD inception, PD activity could be developed in the butt gaps at the higher stress, finally leading to BD

The contribution of discharge area variation to partial discharge patterns in disc-voids

Experimental ac partial discharge (PD) patterns are presented for a disc-void (area greater than length) with metal surfaces, one metal and one insulating surface, and with both surfaces insulating. These patterns indicate that the portion of the surface over which the charge is deposited (discharge area) plays an important role in producing the fluctuations in discharge magnitude commonly observed for such voids. A simulation model for PD patterns in voids is presented, which expressly includes the effects of the charge distribution left on the void surface by consecutive PDs. This model defines two factors that control the PD propagation. These are: a minimum field required to maintain the discharge within an existing surface path Ein and a minimum peripheral field required to extend the discharge path Ep. There is also one other factor, the occurrence probability that controls the incidence of the PD. It is shown that the model gives a variation in PD magnitude throughout the active region of phase without any stochastic factors, i.e. when only variations in the discharge areas are allowed for, but that in order to reproduce the typical (turtle-like) PD patterns observed for the disc-void with insulating interfaces the occurrence probability and surface conductivity have to be included. The current model is compared to previous models and the relationship of the model factors to the change of PD patterns with ageing is briefly discussed.