Partial Discharge Pulse Propagation Research Articles

Wide-Frequency Finite Element Based Model for Partial Discharge Pulse Propagation in Stator Winding of Generators

Condition monitoring of generators is one of the main keys to reduce outages and keep the system stable during disturbances. The partial discharge (PD) test is among the most reliable tools for diagnosing generator insulation defects in the early stages. However, various studies show that it is impossible to correlate the measurement results with the apparent charge of the occurred PD in the generator winding. Therefore, no specific threshold can be set for the harmful PD activities in the generator stator windings. Modeling the stator winding for PD pulse propagation helps to interpret the PD test results properly. In this regard, this paper presents a finite element method (FEM)-based model with acceptable accuracy in a wide frequency range. Practical laboratory tests performed on stator bars are used to validate the model. The results and findings of the proposed model are fully matched with practical tests. Also, the proposed model covers the features obtained for PD pulse propagation in previous experimental studies. The proposed model can be employed to investigate the PD pulse propagations at both low and high frequencies, which is an important tool to analyze the results of a PD test correctly.

Analytical investigations on discrimination of partial discharge defective cable termination among healthy ones with joint sheath‐ground connection

Industrial complexes suffer from crosstalk of Partial Discharge (PD) defects in cable terminations connected to the same medium voltage busbars. This phenomenon happens because the sheath-ground connections of all cables of one switchboard are first fastened together and then connected to ground, which leads to PD pulses being induced from the source termination to others. In this paper, based on mathematical investigations on PD pulse propagation and by introducing an energy-based index, a simple rule-based procedure is proposed for discrimination between defective terminations and healthy ones connected to common busbars. The proposed procedure has been verified by simulations even when there exists noise pollution or multiple defects of different strengths.

Modelling of partial discharge pulse propagation in transformer windings using multi conductor transmission line modelling scheme

Power transformers are considered as an essential component in the electric power systems. They are placed in services in different circumstances like electrical, environmental and mechanical stresses, and through the progress of operation it can affect with various threats and hazards. Transformer failure should be diagnosed earlier in order to avoid its unsafe and unstable operation. The main reason for failure of insulation in power transformer is due to partial discharges. The partial discharge activity occurring in the transformer have to be identified earlier, afore it progresses into a complete discharge, otherwise it results in the maintenance of the device and causes loss of economic cost to the utilities. In this paper, multi conductor transmission line theory is used for the modelling of transformer windings and the partial discharge pulse propagation through the winding is simulated in the MATLAB Simulink. The effect of partial discharge on the windings are also discussed.

Modelling and experimental validation of EM wave propagation due to PDs in XLPE cables

Monitoring of electromagnetic (EM) waves due to partial discharge (PD) is a promising tool for condition monitoring of cables. This study presents the development of a modelling scheme for simulation of EM wave propagation due to PDs occurring at cavities in the cross-linked polyethylene (XLPE) insulation. Simulation models for generation and propagation of PD pulses through the cable geometry are developed. A finite element method (FEM)-based model is developed to simulate the PD current along cylindrical voids in the cable insulation. The initial fields due to the PD currents in the voids are simulated by an FEM model. The propagated EM fields inside and outside the cable surface due to PD's occurring inside the voids are analysed by a finite-difference time-domain model. The analysis of propagated EM fields due to a Gaussian-shaped PD pulse identifies the dominant components of propagated electric and magnetic fields along the cable geometry. Furthermore, the propagated EM fields at the cable sheath due to different void dimensions are simulated and experimental validation of the propagated magnetic field is conducted with measurement by an high-frequency current transformer sensor installed around an earth wire from the sheath of an 11 kV XLPE cable.

Validity of circuit‐based models for air‐cored helical windings at high‐frequency regimes

Helical coils in different forms find applications in various areas of electrical engineering. While power engineers use close-pitched coils in inductors, reactors and transformer windings, communication engineers use large-pitched helical coils as antennas. In classical power engineering, coils are represented as lumped inductors for power frequency applications and switch-over to distributed circuit models while dealing with switching and lightning transients. Even for very fast rising excitations such as very fast transient overvoltages, chopped lightning waves and propagation of partial discharge pulses, multi-conductor transmission-line-based models have been employed, which appears to be an over-simplification. It would be very useful if an upper-frequency limit for the circuit-based models is quantified, which requires an extensive solution of the electromagnetic fields. After realising the inherent late-time instability of marching-on-in-time methods, a relatively new method called the marching-on-in-degree-based scheme is adopted for the solution of the associated electric field integral equation. A deeper insight is first obtained by analysing the helical antennas. Subsequently, based on the extensive simulation results for the single-layer helical coils, an empirical relation relating the upper-frequency bound for the circuit-based modelling has been deduced.

A Review of Online Partial Discharge Measurement of Large Generators

Online partial discharge (PD) measurements have long been used as an effective means to assess the condition of the stator windings of large generators. An increase in the use of PD online measurement systems during the last decade is evident. Improvements in the detection capabilities are partly the reason for the increased popularity. Another reason has been the development of digital signal processing techniques. In addition, rapid progress is being made in automated single PD source classification. However, there are still some factors hindering wider application of the system, such as the complex PD mechanism and PD pulse propagation in stator windings, the presence of detrimental noise and disturbances on-site, and multiple PD sources occurring simultaneously. To avoid repetition of past work and to provide an overview for fresh researchers in this area, this paper presents a comprehensive survey of the state-of-the-art knowledge on PD mechanism, PD pulse propagation in stator windings, PD signal detection methods and signal processing techniques. Areas for further research are also presented.

Analysis of the partial discharge pulse propagation in the stator winding of a synchronous machine

In this paper, a study of the propagation of partial discharge pulses in the stator winding of a synchronous machine is presented. This study was performed by injecting artificial partial discharge pulses in the stator winding of a 2.4 kV/210 kVA salient-pole synchronous machine. It was found that the propagation of the partial-discharge PD signals is presented in two modes: the slow mode, in which the coil acts as a transmission line; and the fast mode, in which the pulse propagates through the overhangs. The fast mode is due to capacitive and inductive couplings in the overhangs. In the slow mode, a PD signal manifests itself at the generator terminals after a transit time that depends on the distance traveled by the pulse. A strong positive linear correlation was observed between the arrival time in the slow mode and the length traveled by the pulse in this mode. The DC ohmic resistance between the injection point and the measurement point was used to represent the distance traveled by the pulse in the slow mode. The capacitive and inductive couplings also cause a crosstalk between the phases. As a consequence, a PD signal can be measured at different phases to the origin phase. In the slow mode, the signal will suffer attenuation in amplitude due to the dispersion and the absorption of energy while it propagates in the slow mode.

Partial Discharge Pulse Propagation and Attenuation in Power Cable Insulation: Simulation in Finite Element Method

Travelling wave method is commonly used for partial discharge (PD) location since the past. However, for a long cable, the attenuation of PD pulse propagation could be significant. This is due to more energy loss during the propagation of pulse, which can affect the sensitivity of the PD source location within cable insulation. In this work, PD pulse propagation along cable insulation from the defect site was modelled using finite element analysis (FEA). The model was used to simulate the electric field distribution in the cable insulation due to the PD pulse propagation and to calculate the attenuation constant. The effect of cable length and PD source location in the cable insulation on the attenuation constant was also studied. Through this work, a better understanding of PD pulse propagation and attenuation constant using the FEA model geometry that has been developed can be attained.

On-line PD detection and localization in cross-bonded HV cable systems

This paper addresses the detection and localization of partial discharge (PD) in crossbonded (CB) high voltage (HV) cables. A great deal has been published in recent years on PD based cable insulation condition monitoring, diagnostics and localization in medium voltage (MV) and high voltage (HV) cables. The topic of pulse propagation and PD source localization in CB HV cable systems has yet to be significantly investigated. The main challenge to PD monitoring of CB HV cables is as a result of the interconnectedness of the sheaths of the three single phase cables. The cross-bonding of the sheaths makes it difficult to localize which of the three phases a PD signal has emanated from. Co-axial cables are used to connect cable sheaths to cable link boxes, for ease of installation and protection against moisture. A second challenge is, therefore, the coupling effect when a PD pulse propagates in HV cable joints and the co-axial cables, making PD detection and localization more complex. The paper presents experimental investigations into PD pulse coupling between the cable center conductor and the sheath and the behavior of PD pulse propagation in CB HV cables. It proposes a model to describe PD pulse propagation in a CB HV cable system to allow monitoring and localization, and also presents the knowledge rules required for PD localization in CB HV cable systems.

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.

Partial Discharge Pulse Propagation in Shielded Power Cable and Implications for Detection Sensitivity

Boggs and Stone (1982) defined the fundamental limits to the electrical detection of corona and partial discharge (PD), i.e., wideband detection of a PD-induced pulse in the presence of thermal noise. This paper treated the effect of frequency-dependent attenuation in shielded power cable in that context. However, most of the plots in that paper were the result of numerical computations. In the same year, Stone and Boggs set out a theory for high-frequency attenuation of shielded power cable. They showed good agreement between attenuation predicted from measured material properties and measured, high-frequency attenuation of shielded power cable. Since 1982, measurements of high-frequency cable attenuation have been reported by a number of authors for a variety of cables. In addition, software tools have become available that facilitate an analytic solution for the parameters of interest. This article summarizes the theory for PD propagation in shielded power cable for both symmetric (Gaussian) and asymmetric PD-pulse waveforms, based on the assumption that the attenuation constant (dB/m or Nepers/m) of the cable is proportional to frequency. This appears to be the most complete possible analytic exposition of PD attenuation in shielded-power cable, which has obvious applications to field PD measurements of such cable.

Partial Discharge Pulse Propagation in Shielded Power Cable and Implications for Detection Sensitivity

Partial Discharge Pulse Propagation in Shielded Power Cable and Implications for Detection Sensitivity

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.

The application of matched filters to PD detection and localization

In practice, partial discharge (PD) signals are corrupted mainly by narrowband noise caused by radio communications. Matched filters are very well suited for PD detection in the presence of such noise, since they exploit spectral regions with high SNR to maximize the SNR of a measured PD signal. In addition to its noise suppressing capabilities, a matched filter provides a means to estimate accurately PD pulse magnitude and arrival time. Consequently, PD detection can benefit greatly from the application of matched filters. However, good knowledge of the signal waveform is required. In many cases, such information can be obtained by measurement or modeling of the PD pulse propagation.

Simulation of transformer PD pulse propagation and monitoring for a 500 kV substation

This paper discusses the development of a simulation model to study the propagation of partial discharge (PD) pulses of transformers in a 500 kV substation. The results will be used to design an on-line PD monitoring system for transformers. The impedance matrix of a 500 kV, single-phase transformer is computed based on geometry data and its external coupling network (often referred to as coupling impedance) is calculated with electromagnetic transients program (EMTP). It was found that the frequency characteristic of the coupling network is complex and that the traditional lumped coupling capacitance representation is only feasible at <60 kHz. The entire substation network was included to calculate the PD propagation of the transformer. PD responses are obtained at the tank grounding and 500 kV bushing tap grounding with a simulated PD pulse injected at the 500 kV terminal and 220 kV terminal of the transformer, respectively. Simulation shows that, using the resonant frequency of the substation network as monitoring frequency, one effectively can increase the monitoring sensitivity. The off-line and on-line PD calibration can also be made based on the developed simulation model.

Investigation of PD pulse propagation characteristics in GIS

Concentrating on the TEM mode, the damping characteristics of partial discharge (PD) pulses propagating through each gas-insulated switchgear (GIS) component were investigated. As a result, it was discovered that frequency components above 500 MHz were remarkably damped by spacers, T-shape branches, and L-bends. Quantitative studies were undertaken on the frequency dependency of these damping characteristics. Then the equivalent circuits for analyzing the propagation characteristic of PD pulses were proposed. Furthermore, the validity of concentrating on the TEM mode was denoted by tests at a substation in service.

On-site measurement for the development of on-line partial discharge monitoring system in GIS

This paper deals with the results of field tests carried out to develop an online sensor for monitoring partial discharges generated in the GIS. It covers noise distribution in the GIS tank, the propagation of partial discharge pulses, and its mechanisms, while presenting applications of partial discharge online monitoring sensors. >

Partial-Discharge Measurement on High-Voltage Power Transformers

Many kinds of partial discharge detectors are available, and many kinds of windings are adopted for EHV transformers, where propagation of partial discharge pulses is characterized mainly by series capacitance. The acoustic method is predominantly used for partial discharge location, because of the present scarcity of investigations on the electrical location method.