Partial discharge (PD) measurements provide valuable information for assessing the condition of high voltage (HV) insulation systems, contributing to their quality assurance. Different PD measuring techniques have been developed in the last years specially designed to perform on-line measurements. Non-conventional PD methods operating in high frequency bands are usually used when this type of tests are carried out. In PD measurements the signal acquisition, the subsequent signal processing and the capability to obtain an accurate diagnosis are conditioned by the selection of a suitable detection technique and by the implementation of effective signal processing tools. This paper proposes an optimized electromagnetic detection method based on the combined use of wideband PD sensors for measurements performed in the HF and UHF frequency ranges, together with the implementation of powerful processing tools. The effectiveness of the measuring techniques proposed is demonstrated through an example, where several PD sources are measured simultaneously in a HV installation consisting of a cable system connected by a plug-in terminal to a gas insulated substation (GIS) compartment.

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Application of HFCT and UHF Sensors in On-Line Partial Discharge Measurements for Insulation Diagnosis of High Voltage Equipment

Power utilities are struggling to reduce power failure incidents in substations and their components to operate more reliably and economically [1]. Many power failures are produced directly or indirectly because of the insulation system of utility components [2], [3]. The selection of the insulation should ensure power plant operational continuity along with completely resolving or significantly limiting the actual power system's failures [4]. Gas insulated substations (GIS) have the best insulation performance which ensures achieving minimum failure incidents, although at high installation cost. The most common insulating gas used in GIS is Sulfur hexafluoride (SF 6 ) gas, which is widely used as an effective electrical insulation as well as an arc-quenching medium [5]. Basic GIS and gas insulated transmission lines (GITL or GIL) consist of a conductor supported by solid insulators inside an enclosure filled with SF 6 gas or its mixture [6].

Partial discharge detection and diagnosis in gas insulated switchgear: State of the art

SF6 is widely used in electrical equipment as an insulating gas. In the presence of an electric arc, partial discharge (PD) or spark, SF6 dissociation products (such as SF2, SF3 and SF4) react with the unavoidable gas impurities (such as water vapor and oxygen), electrodes and surrounding solid insulation materials, forming several toxic and corrosive byproducts. The main stable decomposition products are SO2F, SO2F2 and SOF2, which have been confirmed experimentally to have a direct relationship with discharge faults, and are thus expected to be useful in the fault diagnosis of power equipment. Various studies have been performed of the main SF6 decomposition species and their concentrations under different types of faults. However, most of the experiments focused on the qualitative analysis of the relationship between the stable products and discharge faults. Although some theoretical research on the formation of main SF6 derivatives have been carried out using chemical kinetics models, the basic data (chemical reactions and their rate constants) adopted in the model are inaccurate and incomplete. The complex chemical reactions of SF6 with the impurities are ignored in most cases. The rate constants of some reactions obtained at ambient temperature or in a narrow temperature range are adopted in the models over a far greater range, for example up to 12 000 K, due to the difficulty in the experimental measurement and theoretical estimation of rate coefficients, particularly at high temperatures. Therefore, improved theoretical models require not only the consideration of additional SF6 decomposition reactions in the presence of impurities but also on improved values of rate constants. This paper is devoted to determining the rate constants of the chemical reactions relating to the main byproducts of SF6 decomposition in SF6 gas-insulated power equipment: SO2F, SOF2 and SO2F2. Quantum chemistry calculations with density functional theory, conventional transition state theory and Wigner’s tunneling effect correction are employed to estimate the rate constants of four important chemical reactions: F + SO2F → SO2F2, F2 + SO2 → SO2F2, SO2F + SF5 → SF6 + SO2 and SOF3 + SF3 → SF4 + SOF2. The results are derived for a large temperature range, from 300 to 12 000 K, and finally fitted by a three-parameter Arrhenius equation. This work lays a basis for the further study of the SF6 decomposition mechanism by means of chemical kinetics modelling. Journal of Physics D: Applied Physics Calculated rate constants of the chemical reactions involving the main byproducts SO2F, SOF2, SO2F2 of SF6 decomposition in power equipment

Calculated rate constants of the chemical reactions involving the main byproducts SO2F, SOF2, SO2F2 of SF6 decomposition in power equipment

Condition monitoring of an operating apparatus is essential for lifespan assessment and maintenance planning in a power system. Electrical insulation is a critical aspect to be monitored, since it is susceptible to failure under high electrical stress. To avoid unexpected breakdowns, the level of partial discharge (PD) activity should be continuously monitored because PD occurrence can accelerate the aging process of insulation in high voltage equipment and result in catastrophic failure if the associated defects are not treated at an early stage. For on-site PD detection, the ultra-high frequency (UHF) method was employed in the field and showed its effectiveness as a detection technique. The main advantage of the UHF method is its immunity to external electromagnetic interference with a high signal-to-noise ratio, which is necessary for on-site monitoring. Considering the detection process, sensors play a critical role in capturing signals from PD sources and transmitting them onto the measurement system. In this paper, UHF sensors applied in PD detection were comprehensively reviewed. In particular, for power transformers, the effects of the physical structure on UHF signals and practical applications of UHF sensors including PD localization techniques were discussed. The aim of this review was to present state-of-the-art UHF sensors in PD detection and facilitate future improvements in the UHF method.

Application of UHF Sensors in Power System Equipment for Partial Discharge Detection: A Review

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.

Hilbert fractal antenna for UHF detection of partial discharges in transformers

A partial discharge (PD) sensor based on two-arm equiangular spiral antenna is designed and optimized in this research, and key parameters which mainly influence the performance of sensor are optimized. Through simulation and contrastive study, the key parameters' values are confirmed, which improves the sensor's performance to a great extent. To match the impedance of the sensor and the coaxial transmission line, a kind of micro strip impedance transformer which is of an exponentially gradual type is designed, the parameters of which are defined through simulation. Two improvement schemes for the designed transformer are also proposed to make an easy package and installation for the sensor and the transformer. Then, the physical model is made and tested. The results of simulation and test show good agreement, and verify that the sensors designed can meet the GIS internal partial discharge monitoring requirements well in bandwidth, performance and size, etc. The designed impedance transformer (IT) achieves impedance matching within the sensor and transmission line in ultra wideband range, and also enhances the sensitivity of the sensor.

Development simulation and experiment study on UHF Partial Discharge Sensor in GIS

Using ultra-high-frequency (UHF) method in practical partial discharge (PD) detection can be affected by the positioning and placement of the UHF sensor. This in turn can affect the PD diagnosis. To ensure optimal performance of the sensor and understand the propagation process of electromagnetic (EM) wave, there is a need to fully analyze how the sensor?s placement affects the output signal and the attenuation of EM wave in various positions and directions. As the previous researches are mainly concentrating on the radial component of the UHF signal, the propagation of the signal components in axial and radial directions and that perpendicular to the radial direction of the GIS tank are investigated in detail in this paper. Firstly, the attenuation of UHF signals at different radial positions of a GIS model is analyzed using the finite difference time domain (FDTD) method. Then, secondly, the coupled signals in the three directions are calculated respectively. By comparing the signal received for different directions and circumferential angles, the peak to peak value (Vpp) and cumulative energy of the coupled voltage in each case are considered both in the time and frequency domain. As well, the attenuation characteristics and rules are summarized, based on which a new method for circumferential and axial location is proposed. The investigation on the propagation and the detection mechanism of EM wave in GIS provides a significant understanding of the application of UHF sensor and actual PD detection.

Investigation on the placement effect of UHF sensor and propagation characteristics of PD-induced electromagnetic wave in GIS based on FDTD method

To clarify the propagation process and improve the utilization of ultrahigh frequency (UHF) technique in partial discharge diagnosis, the propagation characteristics of electromagnetic wave in different directions in an L-shaped gas insulated switchgear tank are profoundly investigated in this paper. According to the property of electric field distribution and attenuation curves of the UHF signal, the influence of L-shaped structure is analyzed.

The Influence of L-Shaped Structure on Partial Discharge Radiated Electromagnetic Wave Propagation in GIS

Diagnosis method for mechanical faults of circuit breaker

For detecting partial discharge (PD) phenomenon in gas insulated switchgear (GIS), ultra-high-frequency (UHF) method, which detects electromagnetic wave (EM wave) in UHF band with good immunity to noise, has been widely employed to avoid insulation accidents. In order to deeply understand the propagation process of EM wave and improve the utilization of the UHF method, it is necessary to investigate the attenuation characteristics of UHF signal from broadband PD sensor. In this research, by using the self-designed planar equiangular spiral antenna, the output UHF signal is detected and analyzed experimentally in a 252 kV GIS with L-shaped structure (LS) and disconnecting switch (DS) tank. When DS is closed and is open, the output signal is measured under two applied voltage levels individually in the test. And the results are mainly investigated in detail by time, statistics and frequency analysis. The propagation rules of the peak to peak value (Vpp) and transmission rate of the UHF signal are summarized. This research provides some referential significance towards the rational use of UHF method and better implementation of PD detection.

Tianhui Li

Papers

Development simulation and experiment study on UHF Partial Discharge Sensor in GIS

Investigation on the placement effect of UHF sensor and propagation characteristics of PD-induced electromagnetic wave in GIS based on FDTD method

The Influence of L-Shaped Structure on Partial Discharge Radiated Electromagnetic Wave Propagation in GIS

Diagnosis method for mechanical faults of circuit breaker

Experimental research on partial discharge radiated UHF signal attenuation characteristics in GIS