UHF and current pulse measurements of partial discharge activity in mineral oil
09 Mar 2006-Vol. 153, Iss: 2, pp 47-54
TL;DR: In this article, the relationship between radiated ultra-high frequency (UHF) signals and electric charge in a partial discharge (PD) current pulse for PD sources in oil insulation was explored.
Abstract: Investigations are reported into a range of defects causing partial discharge (PD) activity in high-voltage oil-insulated equipment. The aim is to explore the relationship between radiated ultra-high frequency (UHF) signals and electric charge in a PD current pulse for PD sources in oil insulation. Experiments are performed under high-voltage AC conditions using electrode configurations that included a sharp protrusion, a bad contact, and a free metallic particle. A broadband current transformer is used to measure the PD current pulses with the UHF signals being detected using a pair of sensors mounted inside a metal tank surrounding the PD sources. Current pulses and UHF signals were recorded simultaneously using an oscilloscope with a bandwidth of 3 GHz. The level of correlation between the energy of the radiated UHF signals and the charge content of PD pulses is shown to vary depending on the defect type. Polarity-dependent UHF spectral content and ratio of UHF signal energies measured at the two sensors are shown to exhibit characteristics dependent on the discharge behaviour. These results and the analysis procedures introduced will contribute to the development of effective UHF monitoring systems for oil-insulated high-voltage plant.
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TL;DR: In this article, three different methods of acquiring the time differences of arrival (TDOA) from the PD waveforms are discussed, including the first peak of the signal as the arrival instant, or from the cross-correlation of the PD signal waveforms, or by applying the similarity function to the plots of PD signals cumulative energy.
Abstract: The location of a partial discharge (PD) source inside a transformer can be determined from the time differences of arrival (TDOA) between signals that are captured by an array of UHF sensors. The TDOA can be acquired from the received PD waveforms. In this paper, three different methods of acquiring the TDOA from the PD waveforms are discussed. The time difference can be calculated either by taking the first peak of the signal as the arrival instant, or from the cross-correlation of the PD waveforms, or by applying the similarity function to the plots of the PD signals cumulative energy. Computation algorithms for determining the TDOA automatically are introduced so that possible bias from human interpretation is avoided. The presence of noise and its effect on the accuracy of the PD localization will also be presented. Experimental results show the first-peak method has higher accuracy than the two other methods. The application of signal denoising further improves the localization accuracy.
133 citations
Cites background from "UHF and current pulse measurements ..."
...In some cases, the PD pulse rise time can be considerable, up to 17 ns for bad contacts in oil [13] and hence a higher location error....
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TL;DR: In this article, the relationship between the IEC 60270 pC and Radio Frequency (RF) signal energy is analyzed for a given test configuration, and the correlation between the two techniques produces characteristic patterns specific to each defect type.
Abstract: Partial discharge (PD) measurement is an established condition monitoring technique used to facilitate the detection of incipient faults in high voltage electrical insulation systems such as gas-insulated switchgear and power transformers. By simultaneously recording partial discharge using both the IEC60270 'apparent charge' measurement technique and the more recent Radio Frequency method, which measures the energy radiated from the discharge, it is thought that more information can be gained about the nature of the PD source. Combined measurement has been carried out on various common PD source topologies recreated under laboratory conditions using PD test cells in a range of insulating media including SF6, oil and epoxy resin. Test cell geometries include floating electrodes, free particles and protrusions. By plotting apparent charge against the energy of the RF signal for a large number of PD pulses, the relationship between the two techniques can be quantified for a given test configuration. It has been found that the correlation between the two techniques produces characteristic patterns specific to each defect type. It is envisioned that combined RF/IEC measurements will contribute to a more widespread acceptance of the RF technique in terms of its ability to quantify PD severity by relating the RF energy to the more widely accepted IEC60270 pC level.
108 citations
Cites background from "UHF and current pulse measurements ..."
...A distinctly quadratic trend can be observed, indicating a fairly consistent pulse shape [19]....
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TL;DR: In this paper, a knowledge-based decision support system is proposed to provide engineers with a comprehensive diagnosis of the defects responsible for partial discharge activity detected in oil-filled power transformers.
Abstract: The abstraction of meaningful diagnostic information from raw condition monitoring data in domains where diagnostic expertise and knowledge is limited and constantly evolving presents a significant research challenge. Expert diagnosis and location of partial discharges in high voltage electrical plant is one such domain. This paper describes the functionality of a knowledge-based decision support system capable of providing engineers with a comprehensive diagnosis of the defects responsible for partial discharge activity detected in oil-filled power transformers. Plant data captured from partial discharge (PD) sensors can be processed to generate phase-resolved partial discharge (PRPD) patterns. This paper proposes a means of abstracting the salient features characterizing the observed PRPD patterns. Captured knowledge describing the visual interpretation of these patterns can be applied for defect diagnosis and location. The knowledge-based PRPD pattern interpretation system can support on-line plant condition assessment and defect diagnosis by presenting a comprehensive diagnosis of PD activity detected and classification of the defect source. The paper also discusses how the system justifies its diagnosis of the PD activity to offer the expert greater confidence in the result, a feature generally absent in 'black-box' pattern recognition techniques. The incremental approach exhibited by the system reflects that of a PD expert's visual interpretation of the PRPD pattern. The paper describes how this functional system design has evolved from the approach taken by PD experts to the visual interpretation of PRPD patterns.
105 citations
Cites background or methods from "UHF and current pulse measurements ..."
...Partial discharge data captured using UHF sensor technology was analyzed by University of Strathclyde researchers in an attempt to understand the physical process underpinning dielectric breakdown within large oil-filled power transformers and the PD behavior arising from particular defect geometries [7][8]....
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...Limited knowledge of the nature of partial discharges generated in the oil insulation of power plant with complex geometry, such as oil-filled power transformers, prompted some empirical research within the University of Strathclyde [7]....
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...The range of defect classes for which the nature of the discharge behaviour was observed in the programme of research reported in [7] include:...
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TL;DR: The experimental results show that the proposed Hilbert fractal antenna is suitable and effective for UHF online monitoring of PDs in transformers.
Abstract: Partial discharge (PD) online monitoring is an effective tool of inspecting insulation defects and identifying potential faults in power transformers Ultra-high-frequency (UHF) approaches have caught increasing attention recently and been considered as a promising technology for online monitoring PD signals The size of a UHF sensor for PD online monitoring of transformer is a critical factor for practical installation inside transformer This communication presents a compact fourth order UHF Hilbert fractal antenna with desired performance and suitable size for easy installation Actual PD experiments were carried out for four typical artificial insulation defect models while the antenna was used for PD measurements The experimental results show that the proposed Hilbert fractal antenna is suitable and effective for UHF online monitoring of PDs in transformers
74 citations
Cites methods from "UHF and current pulse measurements ..."
...An inverted cone antenna was presented in [4] to...
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TL;DR: In this paper, a compact multi-band UHF Hilbert fractal antenna with wide frequency band and suitable size for easy installation was proposed for partial discharge (PD) detection in a transformer.
Abstract: Electromagnetic waves generated due to partial discharge (PD) can be captured by ultra-high-frequency (UHF) antennas. The size and frequency band of an UHF antenna for PD online monitoring are critical factors for practical installation inside a transformer. This paper presents a compact multi-band UHF Hilbert fractal antenna with wide frequency band and suitable size for easy installation. Design criteria of a PD antenna were proposed and the Hilbert fractal antenna was selected as an appropriate candidate for PD detection. Principle of Hilbert fractal antenna was discussed and a fourth order Hilbert fractal antenna was optimized for detecting PD signals. Actual PD experiments have been carried out for two typically artificial insulation defect models while the antenna was used for the PD measurement. More simulation studies and actual PD experiments have been done to further examine the effects of electromagnetic wave refraction and reflection by transformer components. The results show that the proposed UHF antenna is qualified for PD online monitoring.
70 citations
Cites methods from "UHF and current pulse measurements ..."
...A Dipole antenna model and its waveform characteristics were presented in [8] and an inverted cone antenna was given in [9] to measure PD signals in transformer insulation oil....
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References
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TL;DR: In this article, the authors present a more user-friendly graphical form of the IEC TC10 databases of DGA results corresponding to faults identified by visual inspection of faulty transformers in service.
Abstract: IEC Publication 60599 provides a coded list of faults detectable by dissolved gas analysis (DGA): PD = partial discharges; D1 = discharges of low energy; D2 = discharges of high energy; T1 = thermal faults of temperature 700/spl deg/C. The IEC TC10 databases of DGA results corresponding to faults identified by visual inspection of faulty transformers in service have been presented in a previous paper (see IEEE Elec. Insulation Mag., vol. 17, no. 2, p.31-41, 2001). The present paper reviews these DGA results in a more user-friendly graphical form. It also reviews the DGA results of laboratory models attempting to simulate these faults, as published in the scientific literature or technical reports. The specific case of on-load tap changers (OLTC) is reviewed much more extensively, and separately, since DGA interpretation in this case must take into account the large background of residual gases resulting from the normal current-breaking operation of the OLTC. Particular attention is also given to DGA results related to PDs and low-temperature thermal faults.
531 citations
TL;DR: In this article, the propagation velocity matrix (PVM) and propagation time matrix (PTM) were introduced to detect partial discharge (PD) in power transformers, which can be used to index a database of physical descriptions of the transformer that could provide additional information to assist with diagnosis.
Abstract: Determining whether power transformers are suffering from internal arcing or dangerous levels of partial discharge (PD) is important because failure without warning can result in damage to neighboring equipment, customer dissatisfaction, disruption to economic activity, and the imposition of regulatory fines. This paper provides an overview of the excitation of UHF signals by PD inside transformers. The use of externally mounted sensors has been outlined, and the structure of a dielectric window that can be constructed on an inspection hatch has been outlined. Attenuation of UHF signals propagating inside a transformer tank has been shown to be relatively low. A new approach to locating PD sources in three dimensions has been presented, based on using a numerical model of the transformer materials, which defines electromagnetic propagation velocities on a mesh of 5-cm sub-cells. The concepts of the propagation-velocity matrix (PVM) and the propagation-time matrix (PTM) were thereby introduced. Once the PD source has been located to a specific region of the transformer, the PVM might be used to index a database of physical descriptions of the transformer that could provide additional information to assist with diagnosis.
390 citations
TL;DR: In this article, the fundamental theory of the UHF method for detecting partial discharge (PD) in gas insulated substations (GIS) is presented, and the effects of position, size, current amplitude, and pulse shape of the PD source on the signal can be predicted using this theory.
Abstract: The fundamental theory of the UHF method for detecting partial discharge (PD) in gas insulated substations (GIS) is presented. The effects of position, size, current amplitude and pulse shape of the PD source on the UHF signal can be predicted using this theory. Excitation of propagating electromagnetic waves by a PD current pulse within the coaxial waveguides formed by GIS components is explained by making use of dyadic Green's functions for the electric fields of propagating modes. Experiments with a coaxial test chamber are used to verify the theoretical predictions, and comparisons are made between measured and simulated UHF signals. Some implications for the UHF measurement of PD are discussed, together with positioning and sensitivity requirements for UHF couplers. A scheme is proposed for standardizing PD measurements made using the UHF technique.
330 citations
TL;DR: In this article, a defect such as a free metallic particle or stress-raising edge on an electrode could lead to breakdown in a gas insulated substation (GIS), but may be detected by the partial discharges it generates.
Abstract: A defect such as a free metallic particle or stress-raising edge on an electrode could lead to breakdown in a gas insulated substation (GIS), but may be detected by the partial discharges it generates. Of the various detection means available, most success has been achieved with either acoustic transducers to detect sound waves from the discharge, or electrical couplers to pick up the ultra-high-frequency (UHF) signals generated inside the GIS chambers. All new GIS in the UK are fitted with internal couplers for UHF monitoring, and two examples of continuous monitoring schemes on 400 kV GIS at nuclear power stations are described. Further developments, which are already well advanced, include specifying and measuring the coupler characteristics, making use of expert systems to interpret the UHF discharge data, and extending the monitoring system to include the performance of circuit breakers and other equipment. >
193 citations
TL;DR: In this paper, the UHF technique is applied increasingly to monitor partial discharges (PD) in power transformers and has been shown to be much more sensitive than acoustic techniques when the signal path passes through solid insulation.
Abstract: Because of the large capital value of power transformers and their critical role in the electricity network, there is an increasing need for non-intrusive diagnostic and monitoring tools to assess their internal condition. The UHF technique is being applied increasingly to monitor partial discharges (PD) in power transformers and has been shown to be much more sensitive than acoustic techniques when the signal path passes through solid insulation. Field trials were performed on five power transformers ranging in size from 18 to 1000 MVA and in voltage from 25 to 400 kV. The results of PD tests on power transformers provide sufficient evidence to justify making provision for UHF sensors on new transformers to facilitate their monitoring when required during the service lifetime.
183 citations