Partial Discharge Signal Propagation Research Articles

Comparing the New Improved RLC and CMTL Models for Measuring Partial Discharge in Transformer Winding

Nowadays, one of the most well-known methods for accurate simulation of the fast-transient phenomenon as well as the study and location of partial discharge (PD) in transformer windings is the method based on winding modeling. In this regard, the circular multi-conductor transmission line (CMTL) model, which is an improved model of the MTL model, has been proposed as the finest solution since it is acceptable in an extensive frequency range and it can investigate the effect of winding rotation by considering the radius and angle variables. However, one of the fundamental problems of using the CMTL model is that a set of large equations, known as complete admittance matrices, must be solved for all winding turns, which typically contain thousands of turns. To overcome this problem, we presented an improved RLC model, which has the same accuracy as the CMTL model in terms of frequency validity range and less computational complexity. In this model, the variables of length and curvature of the turns are also considered in the formulations. Although this method is theoretically accurate, its reliability method must also be determined in practice and how the PD signal propagation must be carefully considered due to the problems in high-frequency modeling of the winding. Numerical analyses and laboratory measurements resulting from the use of PD pulses on a 20 kV winding of a distribution transformer, as well as a 63 kV, 30 MV winding, revealed that a valid frequency range is extendable up to 5 MHz for the proposed improved model without a significant increase in computational complexity.

3D Model of partial discharge in defects with different sizes and positions in power cable for distribution and transmission networks

The knowledge of partial discharge (PD) phenomena inside electrical insulation of power cables is an important issue for assessing the insulation condition and its degradation state, obtaining information about the likelihood of failure. PDs cause signals to propagate along the cable, as noise phenomena, and contribute to the insulation degradation, culminating in a disruptive fault with the interruption of power supply. Therefore, PDs are considered the best ‘early warning’ indicators of insulation degradation and their modelling, with the development of on-line PDs location methods, are important topics to increase the networks’ electricity security. In this study, a three-dimensional (3D) model of PD events using the CST STUDIO® Dassault software is proposed. PDs inside air inclusions in epoxy-resin are analysed with different shapes, positions and sizes of the defect. The electric-field distribution is evaluated with the conduction current inside the void and the apparent charges induced on the electrode. The effectiveness of the model is validated by comparing the simulation results with other published experimental results. Finally, a description of a 3D–1D hybrid model useful to describe the propagation of PD signals in power networks is given.

Localization of Partial Discharge in Electrical Transformer Considering Multimedia Refraction and Diffraction

Partial discharge (PD) is a widely adopted method for the internal insulation detection of electrical transformers. The impact of the refraction and diffraction of the ultrasonic signal in the oil, winding, and core in the transformer makes the localization of the PD source inside the transformer a nontrivial task. The refraction and diffraction factors can introduce additional complexity to the localization equations that can hardly be solved. This article proposes an algorithmic solution based on the time difference of arrival algorithm and semidefinite relaxation convex optimization to improve the PD source localization accuracy. The refraction and diffraction error, measurement error, and their relationship in the process of PD signal propagation are fully considered. The proposed solution is extensively assessed through simulation, testbed, and field experiments, and the results confirm that the proposed solution outperforms the Chan and particle swarm optimization algorithm with the localization error of about 0.1 m.

Partial Discharge localization on cross-bonded cable systems

When a Partial Discharge (PD) signal, which travels through a cross-bonded (CB) power cable system, reaches a CB joint, the signal not only continues its way on the defect phase but it also enters and propagates in the other faultless phases. For analyzing the effect of CB joints on PD signal propagation, an experimental model of CB cable systems is designed. The experimental setup uses 1km length of a PE insulated cable. The setup models one major section of real sectionalized CB high voltage three-phase cable systems. Transient signals are applied to the setup to model PD in different possible locations and the signals are detected in different locations. It is concluded how the faulty phase can be identified according to the results obtained from the CB and the non-CB joints. Single-phase PD measurements are also carried out, and the results are compared with those of PD in the CB cable system. It is suggested to use summation of the PD signals acquired from three high frequency current transformers installed on three screen conductors of three phases at each joint for PD localization and charge estimation. Finally, a monitoring algorithm is proposed for PD localization and defected phase identification.

Propagation characters of PD UHF signal in a prefabricated joint of XLPE cable

Investigating the propagation characters of UHF partial discharge (PD) signals in cable joint is helpful to the application of UHF method in XLPE PD monitoring. In this paper, a propagation model about UHF signal in a prefabricated joint is constructed. Based on it, it effects of some factors, i.e. longitudinal distance of detection location, lateral location of detection, detecting angle, pulse width, pulse amplitude, the size of metal shield and the thickness of semiconductive layer, on the PD signal propagation are studied. It is found that the signal amplitude decays faster in the joint than that in the body, and the signal intensity is much stronger inside the metal shield than that outside. The maximum amplitude of electromagnetic wave radiated by the PD source appears in the radial direction along the radius of the cross-section. The short duration of discharge pulse contributes to the appearance of high-order modes, while the response signal is proportional to the PD source. The metal shield and semiconductive layer increase the signal attenuation. These results provide insight into some design parameters of UHF antenna and its installation location.

Partial discharge signal propagation in medium voltage branched cable feeder

Rising global and regional electricity use is accelerating the need to upgrade networks. The adoption of sustainable ways of energy generation (renewables energy resources) is the top priority of today’s grid, and these resources are predominantly embedded within the distribution networks that are mostly connected by medium voltage (MV) cables. Driven by urbanization trends, negative land value impacts, public safety, environmental aesthetics, and network reliability, the resistance to overhead lines in distribution networks is gradually increasing in many countries. Either choosing the proactive path considering the operational superiority of underground cables compared with overhead lines or following the ongoing legislative policies, the use of cables has been increasing rapidly over the past 30 years. This trend is likely to accelerate [1].

New high frequency multi-conductor transmission line detailed model of transformer winding for PD study

The first step of partial discharge (PD) detection is to obtain a model through which the winding behavior is monitored precisely in high frequencies. The multi-conductor transmission line (MTL) model is one of the best models conducting the PD studies. It is worth to say that one of the main problems of the MTL model is that it considers the windings in a parallel way as well as equal length. Solving the mentioned problem can decrease the simulation error relating to the PD detection. This paper presents an accurate approach in order to solve the problem. This modified MTL model uses the circular effect of windings. Considering the windings circular is one of the important parameters which should be evaluated with respect to PD signal propagation. This method has been applied on a 20 kV transformer winding. The simulation and experimental results show the powerful performance of circular multi-conductor transmission line (CMTL) model in comparison with the MTL model.

Effect of ground return path on partial discharge signal propagation along single-core and three-core power cables

Summary High frequency signal components from partial discharge in an underground power cable propagate according to available modes provided by the cable structure. Which mode or combination of modes is detected depends on type and installation of sensors. Without the ground return impedance, the earth screen voltage is assumed to be equal to the ground potential and all current will flow through phase conductor(s) and earth screen. However the soil may contribute to the transmission line propagation modes and affect the detected partial discharge signals. This paper investigates the effect of the ground return path on partial discharge detection at cable ends. Existing theory and modeling (modal analysis, ground impedance) are customized for the analysis. Transmission line parameters measurements approve the modelings. Time domain signal propagation simulation shows that the ground mode has a minor contribution to the signal propagation. The implication for partial discharge detection is discussed. Copyright © 2016 John Wiley & Sons, Ltd.

Partial discharge pulse propagation in power cable and partial discharge monitoring system

Partial discharge (PD) based condition monitoring has been widely applied to power cables. However, difficulties in interpretation of measurement results (location and criticality) remain to be tackled. This paper aims to develop further knowledge in PD signal propagation in power cables and attenuation by the PD monitoring system devices to address the localization and criticality issues. As on-line or in-service PD monitoring sensors commonly comprise of a high frequency current transformer (HFCT) and a high-pass filter, the characteristics of detected PD pulses depend on the attenuation of the cable, the HFCT used and the filter applied. Simulation of pulse propagation in a cable and PD monitoring system are performed, based on analyses in the frequency domain using the concept of transfer functions. Results have been verified by laboratory experiments and using on-site PD measurements. The knowledge gained from the research on the change in pulse characteristics propagating in a cable and through a PD detection system can be very useful to PD denoising and for development of a PD localization technique.

An analysis of the influence of microclimate on partial discharge activity in emissions of electric machines in the conditions of industrial operation – own research

Abstract The article presents the results of diagnostic measurements of partial discharge signal propagation from the winding insulation in electrical machinery, which were performed using an on-line method. This paper describes the results of experiments and the acquired experience in the monitoring of winding insulation in high power and high voltage electrical machines which are important in industrial production processes. The authors show the measurement techniques employed in their research. Representative measurement results are presented along with their analysis. The authors use an SKF monitoring systems to measure: vibrations, temperature, and humidity, as major factors affecting partial discharge activity in the from winding insulation of electrical machines.

Partial discharge criterion in AC test of oil-immersed transformer and gas-filled transformer in terms of harmful partial discharge level and signal transmission rate

The soundness of a power transformer under an operating voltage is evaluated in partial discharge (PD) test of long-duration induced ac voltage test. The acceptance criterion for this PD test is 500 pC according to IEC standard; however, few basic data backing this criterion are available. To establish a clear criterion for this PD test, the authors initially conducted a study on the harmful PD level of materials themselves constituting the insulation of an oil-immersed transformer and a gas-filled transformer. This PD level was evaluated based on the rate of decline in the lightning impulse breakdown voltage using the insulating materials themselves and the element models simulating the insulating structure which had been exposed to a PD. Consequently, it emerged that insulations of both types of transformers were degraded if exposed to a PD of 7,000 pC to 10,000 pC. With the safety factor for this PD value taken into account considering the long-term operation and the structural difference of an actual transformer, 5,000 pC was deemed as the harmful PD level at the PD occurrence position. Subsequently, using a winding model of a transformer, PD signal propagation characteristics inside the winding were investigated through actual measurement and analysis. As a result, it was found that the PD having occurred inside the winding is measured as the signal significantly damped depending on the position of occurrence. The transmission rate was 2.2% for an oil-immersed transformer and 2.8% for a gas-filled transformer in the respective lowest cases. What should be controlled in the PD test of an actual transformer is the value of the harmful PD level at the PD occurrence position multiplied by the transmission rate at the PD detection position. Therefore, the conclusion was reached that the acceptance criterion in the test should be set to 5,000 pC × 2.2% = 110 pC or less for an oil-immersed transformer and 5,000 pC × 2.8% = 140 pC or less for a gas-filled transformer, respectively.

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.

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

The soundness of a 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, a study was conducted on PD signal propagation characteristics for an oil-immersed transformer. This involved a winding model of a three- winding transformer, or the structure of an actual transformer being produced to investigate the PD signal propagation characteristics relative to the occurrence position, measurement position, and front time of the PD as parameters. Subsequently, since a preceding study on a gas-filled transformer confirmed that an equivalent circuit of the winding could be used for analytical evaluation, an analysis model was also constructed for a three-winding model of the oil-immersed transformer in the present study to investigate the propagation characteristics. Consequently, it emerged that an analysis using an equivalent circuit could also effectively simulate the propagation characteristics, waveform shape, and transmission rate for an oil- immersed transformer. As for the transmission rate, it was found that the signal was detected as significantly damped to a level of several percent depending on the occurrence and measurement positions of the PD, as was also the case for a gas-filled transformer.

Partial discharge signal propagation characteristics inside the winding of gas-filled power transformer - study using the equivalent circuit of the winding model

The soundness of a gas-filled 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, the authors initially conducted a study on the harmful PD level of materials themselves used in a gas-filled transformer. In the actual PD test, since the PD having occurred inside the transformer is measured from the outside, the measured signal is damped due to various influential factors. Since this damping must be taken into consideration to establish control criteria, a preceding study investigated the propagation characteristics of PD signals through actual measurement using transformer winding models. As a result, it was found that the PD having occurred inside the winding is detected as the signal significantly damped depending on the position of occurrence. This paper studies whether PD signal propagation characteristics can be analytically evaluated using the equivalent circuit of the transformer winding. This analytical evaluation, if feasible, can be applied to various winding structures and will make it possible to evaluate the propagation characteristics in further detail as well as the validity of the actual measurement results. The study was conducted on the interleaved and continuous disk windings to make a comparison between the analysis and measurement results of the voltage waveform and the propagation pattern at various locations inside the winding with the position of occurrence and measurement position of the PD as parameters. Consequently, it emerged that the analysis using the equivalent circuit could adequately simulate the propagation pattern and waveform shape of the PD signal. A detailed study using this equivalent circuit also clarified the fact that the PD signal propagation waveform was determined by the winding constant and the terminal conditions. Furthermore, the analytical and measurement results of the transmission rate were almost identical, whereby it was analytically confirmed that the transmission rate significantly decreased depending on the position of occurrence of the PD. It also emerged that, since the frequency characteristics of the PD measurement device may influence the transmission rate, the PD control criteria must be studied taking such influence into consideration.

Partial discharge signal propagation characteristics inside the winding of gas-filled power transformer - experimental study using winding models in the air

To establish clear control criteria for a partial discharge (PD) test of long duration ac withstand voltage test of a gas-filled power transformer, the authors have conducted a study on the harmful PD level of materials themselves using the insulating material- and structural-models of an actual gas-filled power transformer. In the PD test on an actual transformer, since the PD having occurred inside the transformer is measured from the outside, the measured signal is damped due to various influential factors, such as the position of occurrence, measurement position, and internal structure of the equipment. In particular, in the event of the PD between sections or turns, which might actually occur, significant damping of the signal is anticipated in certain cases due to the influence of the position of occurrence and the internal structure of the equipment, which is a propagation path. Therefore, this damping must be taken into consideration to establish control criteria for the PD. In this paper, to understand the characteristics of this damping, such as its magnitude, two types of winding, interleaved disk and continuous disk, were produced as transformer models to investigate the propagation characteristics of PD signals with the position of occurrence, measurement position, and wavefront duration of the PD as parameters. Consequently, it emerged that, where the PD occurs between sections or turns, the transmission rate at the measurement position was generally low because the PD signal circulates within the PD occurrence parts and does not come outside. Assuming measurement at the bushing test terminal, the lowest transmission rate was 2.8% and 2.0% between sections and turns, respectively. A study was also conducted on the occurrence of the PD against ground, despite the low anticipated potential of actual occurrence, and the lowest transmission rate was 4.0%. Accordingly, the PD having occurred inside the winding of the transformer can be detected with a magnitude of 2% or more; however, from a reverse perspective, the signal is detected as one significantly damped to a level of about 1/50.

Semiconducting layer as an attractive PD detection sensor of XLPE cables

Online monitoring of high voltage (HV) cross linked polyethylene (XLPE) cables is a major requirement and interest of utilities for reliable operation of underground cables. Although XLPE cables have a relatively reliable insulation system, however since its insulating material is less resistant to partial discharges (PD), the failure risk increases significantly after occurrence of any partial discharge. Therefore sensitive sensors and a reliable detection system are required for an effective cable life management. Due to attenuation effects of semiconducting layers of XLPE cables on high frequency components of partial discharge signal, the detection process is easily distorted with application of conventional sensors such as high frequency current transformers (CTs). In this paper a new sensitive internal sensor of partial discharge signal, the semiconducting layer, is introduced which is available normally in the H.V. XLPE cables and access can be easily provided at the terminals and also at some specific distances from the terminals, i.e. every one hundred meters, if the manufacturer was asked for. Two 11 kV, XLPE cable setups are employed for study of high frequency signals and partial discharge signals propagation in the XLPE cable and its semiconducting layers. The J Marti's frequency dependent cable model is employed for simulation of partial discharge signal propagation in the core conductor and semiconducting layers of single phase XLPE cables

Propagation of Partial Discharge Signals in Stator Windings of Turbine Generators

We studied the propagation of partial discharge pulses in a stator winding by means of measurements on a dismantled 35-MW generator and found that a stator winding acts as a transmission line. Therefore, a partial-discharge (PD) signal manifests itself at the generator terminals after a transit time that depends on the location of the discharge. Due to capacitive and inductive couplings in the end-winding region, a second propagation mode is present for higher frequencies. This "fast mode" manifests itself at the terminals without appreciable time delay. The capacitive and inductive couplings also cause crosstalk between the phases. As a consequence, a signal measured in one phase does not necessarily originate from a discharge in that phase. The amplitudes of the fast mode and, to a lesser extent, the slow mode (or transmission-line mode) are heavily damped when the discharge occurs further away from the measuring terminal. The consequence is that only discharges close to the measuring terminal can be observed. The propagation of a PD signal is not only influenced by the construction of the generator but also by its external connections.

PD Signal Propagation Characteristics in GIS and Its Location System by Frequency Components Comparison

Propagation characteristics of partial discharge (PD) signal in gas-insulated switchgear (GIS) were investigated in a laboratory and at a 300 kV underground substation. The laboratory test indicates that signal damping can be attributed to two major mechanisms: reflection due to irregularity of characteristic impedance such as spacers and the mode shift from TEM to TE or TM. Tests at the substation show that the signal damping phenomena are independent of propagation direction and the mode shift gives rise to a large signal damping. The major higher mode seems to be TE11. Amplitudes of observed waveforms using narrow-band filters that agree well with the corresponding frequency components observed with a spectrum analyzer show irregular or random behavior depending upon their observation point. This indicates that narrow-band filters do not seem to be suitable for a PD location system by a frequency component comparison, and the reliability of the system using broadband filters instead is discussed.