Partial Discharge Propagation Research Articles

Study of Localization of Partial Discharges in Oil-filled Transformers using Acoustic Emission Signals

The measurement of partial discharges (PD) in power transformers is crucial for fault detection and maintenance scheduling. In this paper, the relationship between detection intensity, type of PD source and propagation distance is investigated using an acoustic emission (AE) sensor. The AE wave intensity by the corona discharges are relatively strong. Creeping discharges were next, followed by PD in bubbles. Furthermore, two methods for calculating time difference of arrival (TDOA) in locate calculations, energy reference and generalized cross-correlation (GCC), were experimentally compared. The results showed that the energy reference method is suitable when sensors can be placed around the tank, while the GCC method is suitable when sensors are concentrated in specific parts of the tank. This finding may contribute to improving the accuracy of maintenance diagnostics.

An improved technique for quantifying PD activity in cross-linked polyethylene (XLPE) power cables

The presence of air-filled cavities within the insulation of active power cables affects the electric field distribution in the insulation bulk and increases the electrical stress inside and around the cavities. This causes accelerated ageing of the materials and creates regions for partial discharge (PD) development that may subsequently result in cable failure. In this paper, an improved PD quantifying technique applied for insulated power cables is first presented. Its effectiveness is demonstrated through an in-depth study of electric field distribution across a defective cross-linked polyethylene (XLPE) power cable with practical dimensions for HV applications and by analysing the PD behaviour under different ageing conditions. In realizing the aforementioned, the cable model with different arrangements, in series and parallel, of two discharging air-filled cavities was implemented in COMSOL software. The simulated cable fields are analysed for different separations and positions of the cavities before and after PD activity. The cavity fields are extracted in MATLAB and utilized for PD pattern characterization under three ageing conditions. Through the PD patterns and statistical quantities measured from simulated PD behaviour, the severity of PD in the cable model was quantified over the different ageing periods. It was found that the series configuration of the cavities was the most harmful configuration due to its high-field magnitude and greater tendency to initiate a PD propagation. Furthermore, the PD behaviour changes during ageing and the highest PD activity, greater PD magnitude and rate, were recorded at the most advanced ageing stage. Thus, indicating that PD characteristics can be used for inferring the ageing stage.

Propagation and localisation of partial discharge in transformer bushing based on ultra‐high frequency technique

Propagation and localisation of partial discharge in transformer bushing based on ultra‐high frequency technique

Optical Measurement of Partial Discharge Propagation Phenomena including Leader Transition in SF<sub>6</sub> Gas

Optical Measurement of Partial Discharge Propagation Phenomena including Leader Transition in SF<sub>6</sub> Gas

Simulations of partial discharges in a chain of gas cavities at AC voltage

The stochastic model of partial discharges (PDs) in chains of gas caverns in condensed dielectrics is improved. The equations for electric field potential are solved in a region filled by dielectric containing gas caverns. The opportunity of a “relay-race” propagation of PDs in a linear chain of gas caverns under constant and ramp voltage was shown in previous studies. This mechanism is possible if the interval between caverns is relatively small. In this case, the wave of PDs can travel along the chain. This paper is devoted to modelling “relay-race” waves in a zigzag chain of gas-filled caverns under AC voltage.

Flexible Sensor Array Based on Transient Earth Voltage for Online Partial Discharge Monitoring of Cable Termination.

Cable termination is a weak point in an underground cable system. The transient earth voltage (TEV) method is an effective and nonintrusive method for estimating the insulation condition of cable termination. However, the practical application of TEV detection is mainly focused on switchgears, generators, and transformers with a flat and conductive shell. A flexible sensor array based on the TEV method is presented for online partial discharge (OLPD) monitoring of the cable termination. Each sensing element is designed with a dual-capacitor structure made of flexible polymer material to obtain better and more stable sensitivity. Based on the electromagnetic (EM) wave propagation theory, the partial discharge (PD) propagation model in the cable termination is built to analyze and verify the rationality and validity of the sensor unit. Some influencing factors are discussed regarding the response characteristics of sensors. Finally, the performance of the sensor array is verified by simulations and experiments. Besides, an OLPD monitoring system is introduced. The monitoring system is composed of the on-site monitoring device and the remote monitoring host. The two parts of the system exchange the data through wireless networks using a wireless communication module. The experiment results show that the monitoring device could supply the PD condition monitoring demand for cable termination.

Elaboration and Performance Analysis of a Bio-Based Insulating Liquid from Castor Oil for Power Transformers

The research for new mineral oil substitutes focuses on vegetable oils known for their biodegradability and low toxicity. This paper focuses on the development and analysis of physicochemical and dielectric properties of a bio-based insulating liquid from castor oil. Castor oil is an inedible tropical resource. It has a good annual oil yield and is widely available in developing countries. Cold pressing of castor seeds is the most appropriate non-chemical process for extracting oil. A refining process was used to remove metal and chemical residues. Refined Castor Oil was filtered and degassed in order to minimize the dissolved gases, solid particles and moisture. A transesterification operation was performed to significantly reduce viscosity oil. Finally, obtained Castor Oil Methyl Esters (COME) are finally distilled in a rotary evaporator under vacuum to remove traces of water and methanol. Physicochemical properties as visual examination, relative density, kinematic viscosity, and acidity were measured in accord-ance with ASTM D6871. AC Breakdown voltage was performed according to IEC 60156, and had been analyzed using Weibull distribution. Processed Castor Oil (PCO) has low viscosity than certified transformer vegetable oils (BIOTEMP, FR3) and high Dielectric Strength (74.67 kV/2.5 mm). Partial Discharges characteristics including the Partial Discharge Inception Voltage and the Partial Discharge Propagation Voltage were also investigated according to the recommendations of IEC 61294. PCO has satisfactory properties for their use as an insulating oil for transformer.

"Relay-race" mechanism of partial discharges in a long chain of cavities for stochastic nature of process

A stochastic model of partial discharges (PDs) inside a linear chain of gas cavities in condensed dielectrics is developed. The equations for electric field potential and electric charge transfer are solved together for dielectric with these inclusions. Computer simulations show the possibility of a "relay-race" mechanism of propagation of partial discharges in this chain of gas cavities even if the stochastic nature of partial discharge is taken into account. This mechanism can be realized if the distances between cavities are small enough and the dependence of probability function of partial discharges r(E) on electric field strength is rather sharp. In this case, the wave of partial discharges can propagate along the chain of cavities. The PD waves can be initiated in the first cavity as well as in the last cavity in the chain. Occasionally, two PD waves can arise from the both edges of the chain. The sequence of partial discharges in the inclusions has a completely stochastic character for a weak mutual influence of cavities on each other.

A simple approach of partial discharge source location in high voltage cables

This article presents a new approach of partial discharges (PD) source location in power cables. The main advantage of this approach is the detection of the propagation direction of the PD. The method provides a pattern named by the authors PD+D, which, in contrast to the well–known phase-resolved partial discharge (PRPD) pattern, graphically shows the direction of PD propagation and uses the polarity of a sensor array on top of the magnitudes of the positive and negative parts of each PD pulse to detect the direction of propagation, is described in detail. The proposed technique is useful for the evaluation and diagnosis of power cables, since it can identify incipient faults. The attained results that demonstrate the value of this methodology and their scope and limitations are discussed.

"Relay-race" mechanism of propagation of partial discharges in condensed dielectrics at linearly increasing voltage

A stochastic model of partial discharges (PDs) inside gas cavities in condensed dielectrics is developed. The equations for electric field potential and electric charge transfer are solved together for dielectric with gas voids. Computer simulations show the possibility of a "relay-race" mechanism of propagation of partial discharges in a linear chain of gas cavities. This mechanism can be realized if the spacing between cavities is relatively small and the dependence of probability function of partial discharges on electric field strength r(E) is relatively sharp. In this case, the wave of partial discharges propagates along the chain of cavities. The PD waves can be initiated in the first cavity or in the last cavity in the chain. Occasionally, two PD waves can be observed together from the both ends of the chain. The sequence of partial discharges in the inclusions has a completely stochastic character at a relatively weak dependence r(E) or if the mutual influence of cavities on each other is weak.

Simulation of waves of partial discharges in a chain of gas inclusions located in condensed dielectrics

A stochastic model of partial discharges inside gas inclusions in condensed dielectrics was developed. The possibility of a "relay-race" wave propagation mechanism of partial discharges in a linear chain of gas inclusions is shown. The lattice Boltzmann method is successfully implemented for three-dimensional computer simulations of flows of dielectric fluid with bubbles. Growth and elongation of bubbles in a liquid dielectric under the action of a strong electric field are simulated. The physical model of propagation of partial discharges along a chain of gas bubbles in a liquid is formulated.

Modeling dispersion of partial discharges due to propagation velocity variation in power cables

Existing models for partial discharge (PD) propagation based on a single attenuation constant are unable to explain how each frequency component travels with a different propagation velocity. This paper proposes a new model based on a complex propagation term whose real component does not depend on the frequency (f), and whose imaginary part is modeled with a second order polynomial in f. The proposed model explains how the PD is attenuated, delayed, and dispersed due to the fact that each frequency component is differently delayed.A closed-form expression is proposed for the PD peak value and width, and a method to derive the model parameters from a reference model existing in the bibliography. Simulation results show that the peak value and width of the propagated PD pulse are similar to those obtained with the proposed model. Additionally, the proposed model provides the velocity of each PD frequency component, which is crucial to get an accurate estimation of the PD source location.The parameters of the proposed model have been estimated using a vector network analyzer for a XLPE cable. These results have been compared to the measurement obtained in a medium voltage test bench where intentionally induced PDs have been captured and processed, confirming the results of attenuation, delay and dispersion predicted by the proposed model.

Partial discharges triggered by metal-particle on insulator surface under standard oscillating impulses in SF<sub>6</sub> gas

The characteristics of partial discharge (PD) triggered by a metal particle on an insulator’s surface were studied experimentally under standard oscillating lightning impulse (OLI) and standard oscillating switching impulse (OSI) conditions. Light emission detection was carried out to assist in PD measurement. Experiments showed that the largest number of successive PDs occurred during the rise time of each oscillation period of applied impulses and that a small number of reversed-polarity PDs occurred during the fall time. The composition of the PD sequence was determined by the position of the particle on the insulator’s surface. Specifically, when a needle particle detached from the plate electrodes, the PD sequences consisted of both negative point PDs and positive point PDs. Two types of negative discharge, three types of positive discharge, and reversed-polarity discharge were differentiated by their current and light pulse waveforms. In these experiments, detectable partial discharge inception voltages (PDIVs) of the three particle positions were investigated at gas pressures from 0.05 to 0.35 MPa. The light emission inception voltages (LEIVs) were lower than PDIVs under OLI, OSI, and AC voltages. These inception voltages of different particle positions could be sorted as follows: LEIV < PDIV+ (d = 30 mm) < PDIV- (d = 0 mm) < PDIV+/- (d = 15 mm). We demonstrated that the OLI voltages could excite PDs from defects at lower equivalent voltage levels than for OSI and a 50 Hz AC voltage, and the PDs under OLI were more active in terms of occurrence frequency than those under OSI and AC voltages. A simple model was proposed to explain PD inception and propagation under oscillating impulses. The rapid change of background electric field and the formation and accumulation of charges on the insulator’s surface were considered the main causes of the greater efficiencies of OLI and OSI at exciting PDs compared with the AC voltage.

Numerical sensor design for partial discharge detection on power cable joint

Partial discharges (PD) detection is an effective method to evaluate the insulation condition of power cable joint. However, optimization of the partial discharge sensor was hard to be achieved because of the complex nature of cable joint. In this paper, the waveform and frequency characteristics of the partial discharge pulse in power cable were studied with experiment firstly. Then, the partial discharge pulses were numerical modeled to be used as excitation in cable joint simulation model. Furthermore, an electromagnetic simulation method of PD propagation in cable joint with considering the frequency dependent dielectric parameter was proposed. Afterwards, the quantitative relation between the sensor sensitivity and the sensor structure was discussed based on the developed pulse model and cable joint model. Calculations and measurements indicated the numerical sensor design can be achieved with the proposed pulse numerical model and cable joint simulation method.

Parameter estimation technique for the semi-conducting layers in single-core XLPE cable

Partial discharge (PD) detection and location is of great significance for the power cable insulation condition monitoring, where the high-frequency (HF) model is needed to analyze PD propagation. Considering the influences of conductor skin effects and semi-conducting layers (SCL), this paper presents the equivalent Γ-section lumped parameter model for single-core cross-linked polyethylene (XLPE) insulation cables. The longitudinal parameters per-unit-length in the model are obtained using available geometric data by vector fitting method. Unfortunately, the dielectric characteristics of SCL in XLPE cables, especially for an installed one, are usually unknown and hard to measure. To address this issue, based on the simulated annealing algorithm, an optimization approach for estimating the transversal parameters in the Γ-circuit model is developed. The proposed technique is then carried out on the XLPE cable with simulated and real SCL, respectively. Furthermore, the estimation results of the real case are validated on a 603 m cable with kinds of HF pulses responses.

Simulation of Partial Discharge Propagation Characteristics in XLPE Cables and Joints

Accurate Partial discharge (PD) detection and location is of great significance for the power cable insulation condition monitoring. To achieve this, an adequate cable model to analyze pulse propagation is needed. Considering the complex structure of the cable, the equivalent circuit model is no longer applicable. In this paper a numerical electromagnetic method, FDTD, is applied. To analysis wave propagation characteristics in time domain, a well-established cable model is constructed. By simulating the electromagnetic (EM) field propagation inside the cable and the cable joint, the components of the electric fields excited by the PD source in different directions are calculated in order to evaluate the influence of the cable joint in practice, the calculated PD waveforms in the cable are compared with those of the cable joint model in FDTD.

Propagation of Cavities caused by Surface Discharges in Gel-filling for Semiconductors

Silicone gel is widely used to encapsulate power electronic circuits. It is known that the weakness of this insulation system is the triple point which is composed of metallization, gel and substrate. Partial discharges (PDs) occurs at this triple point and propagates between gel and the substrate. The propagation of PDs gives rise to electrical degradation, which may lead to eventual failure of the insulation. Despite the fact, surface discharges in gel has not yet been studied enough. Therefore we have obtained images synchronized with the applied voltage. It showed that the degradation occurs when the applied voltage is positive.

Time Domain simulation of PD Propagation in XLPE Cables Considering Frequency Dependent Parameters

Partial discharge (PD) detection and location is of great significance for the power cable insulation condition monitoring, where analysis of the PD propagation is needed. This paper presents the single-core model of cross-linked polyethylene (XLPE) cable considering the influence of skin effect and semi-conducting screens. To analysis the propagation of PD pulse in time domain, the model firstly is rational approximated using vector fitting method. Based on the fitting results, the π section lumped parameters model is then proposed. Examples on two cables with different permittivity approximating forms of semiconducting screens are presented.

Partial discharge signal propagation characteristics inside the winding of oil-immersed power transformer using the equivalent circuit of winding model in the oil

The technical 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. To establish clear control criteria for this PD test, it is necessary to study PD propagation characteristics inside the transformer windings. In this paper, an analysis model was constructed using constants in the oil for a 500 kV-class transformer to analytically obtain PD signal propagation characteristics. For this analysis model, a circuit model for high frequencies up to several MHz was used in order to obtain the characteristics of highfrequency PD propagating inside the winding in detail. Using this model, PD propagation characteristics were investigated in terms of the PD occurrence position, measurement position, and front time as parameters. As a result, it emerged that the propagation pattern was determined by the winding constants as viewed from the PD injection position and the terminal conditions. When the charge amount was the same, the steeper the PD signal front time, the higher the peak voltage value was likely to be because the waveform became more oscillatory. Conversely, assuming an actual test using the ERA device (PD detector developed by Electrical Research Association), it was found that the front time had little impact on the charge amount measured because the oscillatory high-frequency components were removed. As with the previous studies, it emerged that the PD signal transmission rate was lowest when a PD occurred between sections and the signal was detected as one significantly damped to a level of several percent depending on the conditions.

Study of partial discharge radiated electromagnetic wave propagation characteristics in an actual 154 kV model GIS

A method to detect partial discharge (PD) is considered effective for gas insulated switchgear (GIS) insulation diagnostics. In this paper, for a 154 kV model GIS, the influence of the enclosure diameter on PD propagation characteristics was initially investigated using model GIS by varying the enclosure size. Secondly, an experiment was conducted for metallic particles placed in different locations as a source of PD (particle adhering to the center conductor or free metallic particle). Thirdly, an actual PD experiment was conducted in an Lshaped form to study the influence of GIS shape on PD-radiated electromagnetic (EM) waves. In addition, in order to study the experimental results in detail, a simulation using the FD-TD method was conducted for comparison with these experimental results. Following the experiment, the larger the enclosure diameter, the greater the measured amplitude of PD having occurred around the center conductor. In addition, it emerged that, where the particle was close to the center conductor, EM waves propagating due to PD were less likely to be damped compared with the case where particles were placed on the bottom of the enclosure. Furthermore, following the investigation of the propagation characteristics in the L-shaped form, it emerged that, while a high frequency component of frequency exceeding that of the TEM wave component was reflected at the L-shaped part, the TEM wave component passed through almost unaffected by the latter. The propagation of the TEM mode component could be also found via FD-TD analysis.