Discharges In Transformer Oil Research Articles

Research on image acquisition and contour extraction algorithm of partial discharge in oil

Abstract Partial discharge in oil paper insulation is an important reason for insulation breakdown. The study of partial discharge in transformer oil is of great significance for ensuring the reliability of transformers. In this paper, a comprehensive observation platform of partial discharge with needle plate electrode and oil-paper insulation is built, and the original shape image set of partial discharge is collected and established. Through multi-channel convolution processing of the true color image, gray level equivalent distribution of 3D section, segmentation stretching of threshold interval and binary neighborhood search, a general technical method suitable for processing a series of low brightness discharge images with background interference is finally obtained.

Molybdenum Disulphide Coated Fluorescent Fiber for Detection of Corona Discharges in Transformer Oil

Early detection of corona discharge plays a critical role in avoiding catastrophic failure of transformer insulation. In this study, we explore the use of two-dimensional material coated fluorescent fiber for early detection of corona discharge. Specifically, Molybdenum disulphide (MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) material is dip coated over a commercial grade fluorescent since the bandgap of the coated MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> material corresponds to the visible and ultraviolet (UV) spectral regions. Corona inception voltage detection adopting MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> coated fluorescent fiber exhibits detection efficiency with the percentage difference of 32% in comparison to the UHF technique. Also, fiber coating enhances the energy content of the discharge signal by the difference of 150% with respect to uncoated fluorescent fiber. The phase resolved partial discharge studies reveal that the discharge is observed at the peak of the applied AC voltage.

Simulation of positive streamer discharges in transformer oil

To reveal the impact of formation and development of the streamer discharge on the dielectric liquid formed between pin-plate electrodes, a numerical model of transformer oil discharge in the electrode system is built which is based on the continuity equations coupled with Poisson’s equation. The influence of applied impulse voltage parameters such as rise time and voltage magnitude on the formation and development of the streamer discharge is evaluated in this model. In addition, the characteristic of the streamer discharge such as streamer velocity, electric field, and radius of streamer head have been investigated. Modeling results reveal that the higher impulse voltage amplitude form streamer discharges with longer paths, thicker columns, higher velocity, and greater radius. In addition, we find that the radius of the streamer head is greatly affected by the percentage of the predetermined electric field tube at the head, and slightly affected when streamer length increased. Modeling results also showed that the rise times had a clear effect on the radius streamer discharge and the distribution of electric fields.

Comparative Analysis of Partial Discharge Detection Features Using a UHF Antenna and Conventional HFCT Sensor

This article presents a design of ultra-high frequency (UHF), ultra-wide band (UWB) antenna used for partial discharge (PD) detection on high voltage and medium voltage power system equipment. The proposed UHF antenna has a working frequency band of 1.2GHz-4.5GHz, covering a total bandwidth of 3.3GHz with a return loss less than -10dB in the entire antenna’s operating frequency. The computer simulation technology (CST) Microwave Studio software was used to design, simulate and optimize the proposed antenna. Upon simulation and optimization process, the antenna prototype was fabricated on the FR-4 substrate of 1.6 mm thickness and dielectric permittivity of 4.4. This antenna has a compact size of 100mm x100mm.The radiating patch and the ground plane of this antenna are made of annealed copper whose thickness is 0.035mm. The simulations and measurement results for the proposed antenna are in a good agreement, and the return loss of this antenna is less than -10dB with voltage standing wave ratio, VSWR, < 2 within the frequency range of interest. The proposed antenna performance in PD sensing is compared with a commercial high-frequency current transformer, HFCT. To validate the sensitivity performance of the designed antenna, experimental PD measurements were carried out by using an epoxy slab inserted between two parallel plates electrode model, in order to generate surface discharge on the insulator, and using a needle-plate electrode configuration to generate corona discharge in transformer oil. Based on PD measurement results, it was shown that the designed antenna has a high sensitivity, which make it a suitable candidate for UHF partial discharge monitoring on high voltage and medium voltage power assets.

Systems Theory: Simulation Analysis of Space Charge Generation Mechanisms in Transformer Oil under High Electric Field

Simulation analysis plays an important role in Systems Theory nowadays. In order to reveal the mechanism of space charge injection and generation in the process of liquid dielectric breakdown, the discharge of transformer oil between needle-plane electrodes under high electric field is studied. Based on the 2-D hydrodynamic model and Poisson equation of electric field, the system simulation model of oil discharge by different charge generation mechanism is established, based on which the inception and propagation process of discharge is simulated by COMSOL. By system simulation, the temporal and spatial distribution of the electric field intensity, space charge density, electric potential and temperature of transformer oil is obtained and deeply analyzed. The simulation results prove that the space charge generated by metal field emission and ionic disassociation are neither of the major factor for streamer formation in transformer oil, while the Zener molecular ionization and impact ionization are the major factors. Our research improves the understanding of the inception, propagation and breakdown process for discharge in transformer oil, and also the ionization mechanism in the liquid dielectric

Fluid simulation model and analysis of partial discharge characteristics of tip defects

The tip discharge is modeled and the micro discharge characteristics are studied. The hydrodynamic model of tip discharge in transformer oil is established. The development process of streamer discharge under pulse voltage is analyzed. The distribution of space charge density and space electric field under different pulse voltage amplitude and rising edge is obtained. The numerical results show that: with the increase of applied voltage amplitude, the average velocity of streamer development changes more significantly with the same discharge spacing; the steeper the rising edge pulse is, the faster the average discharge development speed is and the larger the discharge radius is. At the same time, according to the change law of streamer morphology development under AC voltage, the discharge frequency is less, the amplitude is higher, and the phase distribution is narrow when the applied voltage is positive half cycle. When the applied voltage is negative half cycle, the discharge times are more and the phase distribution is wide, and the tip discharge has obvious polarity effect.

Detection and Localization of Partial Discharge in Transformer Oil and Winding using UHF Method

Transformers are important part in power transmission. One of the serious faults that is seen in transformers is electrical insulation failure. Insulation failure is usually initiated by partial discharges (PD). Due to high voltage stress, the fluid insulation in electrical system or a small solid portion can get dielectric breakdown, this is known as partial discharge and this does not create a space between the conductors. The accurate location of PD is an effective method for assessing the existing electrical insulation condition and preventing future accidents. PD is detected by many methods, but for onsite detection of PD, one of the efficient methods is sensing the electromagnetic waves radiated by the discharge source with a frequency range that can go up to 3GHz. In this technique Ultra High Frequency (UHF) sensors are mounted on the transformer tank to measure these waves. This provides a suitable solution for on-site detection of insulation failure in power transformers.

Feasibility Study on the Audible Noise Measurement of Partial Discharge in Transformer Oil

Partial discharge fault detection based audible noise is a kind of new method. This paper set up a needle-plate discharge system, and investigates the feasibility of measuring the audible noise of partial discharge in transformer oil. A needle-plate discharge system is built, and audio noise of partial discharge inside and outside of transformer oil is measured by a microphone and a specially designed hydrophone. According to signal processing, it’s found that the characteristics of internal and external noise have good consistency on time domain wave, spectrum distribution, and time-frequency distribution, except that the former is much larger than the latter in amplitude. The experiment results show that it’s feasible to measure the noise of partial discharge in the transformer oil and have significant advantage in signal to noise ratio.

Effect of magnetic nanoparticles on partial discharges in transformer oil

We report on an experimental study of partial discharges (PD) in transformer oil with four different volume concentrations of superparamagnetic iron oxide nanoparticles (SPION) ranging from 0 to 0.0004 vol%. To investigate the PD, a specific high-voltage setup was designed. Thanks to the small amount of SPION in the oil (transparency), the PD events in the form of visible corona and streamer discharges were visualized by a CCD camera. Furthermore, the PD inception and extinction voltage, apparent charge and quantities derived from the PD recurrence, as pulse repetition rate and phase resolved patterns, were analyzed in dependence on SPION concentration. It was found that the PD inception voltage increases quasi linearly with increasing SPION concentration but the extinction voltage is practically unaffected. The pulse repetition rate decreases as a result of the increasing SPION concentration. The decreased PD activity is clearly concluded also from the reducing shape and growth of the visual corona discharge patterns. To interpret the observed suppression of PD in transformer oil-based magnetic nanofluids, the electric field modification and injected charge trapping by the dispersed nanoparticles is considered. Moreover, a possible effect of the local magnetic fields around the SPION on the space charge migration is taken into account, too.

Circular Array of Ultrasonic Sensor based DOA estimation: Location of multiple Partial Discharge in Transformer oil

Location of Partial Discharge (PD) in transformers is very important, and many methodologies that have been carried forward in previous decades have a restriction theoretically, specially, in power transformers they cannot locate multiple PDs. Based on array signal processing a novel multiple partial discharges localization is presented in this paper. Ultrasonic signal emission occurs when the partial discharge occurs inside the transformer oil tank. Using ultrasonic sensor arrays located on the wall of the transformer tank, emitted ultrasonic signals is detected. Generally a single partial discharge will not occur in transformer there may be multiple partial discharge occur at different location. For detection of multiple partial discharges, we have used circular array of ultrasonic sensors (CAUS) in our proposed method. Primarily, by using Translation Invariant Wavelet Transform the partial discharge signals are de noised. To change the wide-band signal received by CAUS to the narrowband one Total Least Square algorithm is utilized. After this, the number of PD signals is estimated and to find the directions of arrival (DOA) of the signal sparse representation of array covariance vector technique is utilized. Lastly, on the basis of estimation of DOA position of PD source is detected with the help of optimization algorithm in order to find the exact coordinates of the PD source.

Efficient method for location and detection of partial discharge in transformer oil by DOA estimation of circular array of ultrasonic sensors

The electrical insulation failures in oil transformers are mainly occurs due to the inappropriate placing of Partial Discharge (PD) sources. In order to eliminate the insulation defects and also to locate the PD sources in an appropriate location, a new approach called circular array of ultrasonic sensors (CAUS) with various analysis is proposed. At first de-noise the PD signal from the CAUS using the fast independent component analysis (Fast ICA) algorithm. Secondly, the wide band signal from CAUS is converted into narrow band signal by using the total least square algorithm (TLS). Third, parse representation of array covariance vector (SRACV) technique is utilized to separate DOA (Direction of Arrival) in three directions from PD to CAUS. Finally, the PD sources are placed in an appropriate location by using the pitch and azimuth angles of those three DOAs and the exact coordination of three planes are calculated by using the particle swarm optimization algorithm. The simulation result demonstrates the effectiveness of proposed approach in terms of PD location in transformer oil.

Influence of microbubbles motion state on partial discharge in transformer oil

To study the influence of microbubbles motion state on partial discharge (PD) in transformer oil, an experimental platform of suspended microbubbles in transformer oil is established. Deformations of suspended microbubbles in mobile and stationary states are observed. The Ultra-high frequency (UHF) and pulsed current methods are applied to detect the PD of mobile and stationary microbubbles. In addition, the timefrequency characteristic of different PD is analyzed, and an x-U-N map is constructed. A deformation parameter (c) of microbubble is defined, and the electric field distribution is analyzed with the change of c. The results show that oil flow has a significant influence on the PD of microbubbles. When the motion state of microbubble changes from stationary to mobile, we find that the stretched shape of microbubble changes from horizontal ellipsoid to vertical one, the PD inception voltage increases, and the electric field strength inside microbubble decreases. Therefore, mobile microbubbles are harder to lead to PD than stationary microbubbles, signifying that increasing the oil flow helps to enhance the dielectric strength of oil.

Surface Modification of Nanoparticle and Its Charging Dynamics During Streamer Discharge in Transformer Oil

Surface Modification of Nanoparticle and Its Charging Dynamics During Streamer Discharge in Transformer Oil

Effects of electrode size and solid barrier orientation on streamer discharge in transformer oil

Geometrical effects of electrodes and solid barriers immersed in transformer oil are investigated on positive streamer initiation, propagation, and transformation to surface flashover using a 2-D axisymmetric model. Electrode radii of curvature in the range of 20 μm to 6.35 mm are selected such that only positive streamers form. Modeling results indicate that the positive electrode size directly determines the streamer initiation voltage, while breakdown voltage and delay are mainly determined by the grounded electrode size. Specifically, sharper positive electrodes require lower voltages to initiate positive streamers and sharper grounded electrodes result in lower delays and higher breakdown voltages. Incorporating perpendicular and parallel orientations of solid barrier interfaces in the electrohydrodynamic model shows that polarization forces from the barrier dielectrics on charge carrying streamers are proportional to the permittivity difference between transformer oil and the solid dielectric. If the barrier permittivity is greater than the oil permittivity, the solid dielectric applies an attractive force on the volume charge that turns the streamer into a surface flashover expanding on the barrier surface. On the other hand, a low permittivity pressboard interface repels the approaching streamer. Theoretical analysis of electric field propagation and charge distribution along the streamers, surface flashovers, and through the interfacial surfaces is presented. Barrier dielectric relative permittivities of 1.1 and 4.4 have been studied while the oil relative permittivity is assumed 2.2.

Location of partial discharge in transformer oil using circular array of ultrasonic sensors

Partial discharge (PD) in transformer oil is one of the major causes of electrical insulation failures in oil transformers. An accurate location of PD provides important information required for detection and elimination of insulation defects. To improve accuracy of PD location, a new location method is proposed in this paper based on using circular array of ultrasonic sensors (CAUS) and implemented as follows. First, the fast independent component analysis (FastICA) algorithm is used to de-noise PD signal obtained by CAUS. Second, the total least square (TLS) algorithm is used to transform the wide-band signal obtained by CAUS to the narrowband one. Third, the mode excitation algorithm and the fast direction of arrival (FastDOA) algorithm are combined together to find three different directions of arrival (DOA) of the signal from PD to CAUS. Next, using the azimuth and pitch angles of these three DOAs, the PD location is determined in the center of a sphere whose surface tangents to these three DOAs in three different planes. Finally, the improved genetic algorithm is used to obtain accurate coordinates of the PD source. An experimental system of PD-location has been developed, which consists of (i) CAUS made of 8 sensors evenly placed along a circle, and a single sensor placed in the center of the circle, (ii) classic three-capacitance PD model, (iii) 16-channel synchronous data collector, (iv) oil tank, and (v) computer. The preliminary experimental results show that the accuracy of PD location after denoising is better than that before de-noising.

Experimental Study of Partial Discharge UHF Detection in Transformer Oil

This paper studies the characteristic frequency of partial discharge in transformer oil [, and with the spectrum characteristics of partial discharge in air was measured by the comparison of uhf signal inside the metal tank and the attenuation characteristics and metal wire in the air on the radiation characteristics of uhf signals. The results show that the frequency spectrum of the partial discharge in transformer oil is higher than the frequency spectrum of the partial discharge in air. Ultra high frequency signal have the oscillation characteristics in the metal box body propagation. The transformer has the certain ability of outgoing line radiation of partial discharge signal inside.

Simulational study on streamer discharge in transformer oil under positive nanosecond pulse voltage

In this paper, we investigate the streamer discharge process in transformer oil under positive nanosecond pulse voltage through developing a two-dimensional axially symmetric fluid model and simulating the physics of discharge inception and propagation. The streamer discharge profile and distributions of electric field and space charge density are obtained under different conditions such as the amplitude of applied voltage, rise time and gap distance. Simulation results show that space charges enhance the front field of streamer head, which is conducive to the longer propagation of discharge channel, therefore "ionization wave" is formed. The magnitude and rise time of applied voltage have evident influences on the average speed of streamer propagation. It can be observed that the higher the applied impulse voltage, the faster the streamer propagates, and the steeper the rise time of applied impulse, when streamer arrives at the same position, the larger the discharging radius will be and the smaller maximal electric field will be. The cases of different gap distances indicate that longer gap distance corresponds to a faster average speed of streamer. It is considered that field-dependent molecular ionization predominates the charge generation mechanism of streamer discharge process in transformer oil, and space charge effect contributes to further developing ionization until the whole gap eventually breakdowns. The study is dedicated to the better understanding of the process from inception to breakdown of discharging in transformer oil, as well as ionization mechanism in liquid dielectric.

NIR and optical fiber sensor for gases detection produced by transformation oil degradation

In this paper, an innovative optical fiber sensor and near-infrared (NIR) based on reflexion mode were adapted to follow in situ ageing of transformer oil under operation in order to avoid any risk of explosion due to the hydrogen formation. Optical fiber sensor is based on the variation of oil refractive index due to the presence of gases resulting from partial discharges and that may lead to failure of transformer in the long term. We demonstrate that with the use of NIR method we can identify disturbed chemical groups associated with each chemical species created (CH, C C, Si OH, OH, etc.) and detect the formation of methane, ethylene, propane and butane due to partial discharges in transformer oil.

Study on dielectric surface discharge in transformer oil under high voltage nanosecond pulse

Electrical equipments are always subjected to different operating conditions and voltage waves which cause electrical failure on the surface and in the bulk of dielectrics. In this paper, experiments have been carried out to study main factors that influence the characteristic of dielectric surface discharge in transformer oil. Experimental study on flashover of nylon 6 and polymethyl methacrylate (PMMA) under applied voltage ranged from 50 to 250 kV shows that the flashover voltage U is influenced by the insulation structure, rise velocity and pulse width of the applied voltage. Flashover electrical field value E in pulse with rise time less than 10 ns is higher than that in DC and AC voltages. When angle between electrode and the sample is about 45°, nylon 6 and PMMA show the best flashover performance. From these results, the influence of the secondary electron emission and space charges on the electrical breakdown of dielectrics has been stressed.

Influence of insulating barrier on the creepage discharge in transformer oil

The propagation aspects of a creepage discharge in transformer oil are observed simultaneously and in detail under a positive impulse voltage condition, by means of a high-speed Schlieren optical system, an LED current measurement system and a charge measurement system using a capacitor. In the present paper, the influence of the material type and the thickness of insulating barriers on the propagation of the impulse creepage discharge are discussed for a point to plane electrode geometry. In addition, the influence of a barrier surface to the electric field direction on the propagation of the impulse creepage discharge is discussed for a parallel plane electrode gap with a protruding point.