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The development of more electrical transportation systems requires rotating machines having higher efficiency and specific power. To date, this is achieved by feeding them with rising supply voltages with higher frequencies. This generalized strategy is introducing several challenges in the correct design of the insulation that is necessary for a reliable system. A major issue to be dealt with are partial discharges, an aging mechanism that can bring insulation to failure in short times. A solution often proposed by manufacturers involves the use of the so-called corona resistant materials, which supposedly can withstand partial discharge activity. This paper investigates the possibility of using corona resistant magnet wires for electrical machines operating at reduced pressures, such as the actuators for primary control surfaces in the more electric aircraft. The results show that corona resistant insulations are characterized by an improved behavior at ground level but are not a viable option at reduced pressures.

Current standards base the evaluation of partial discharge (PD) harmfulness on their amplitude and repetition rate. This work shows how, on the contrary, the non conventional environmental conditions introduced by aerospace applications (i.e. low-pressure atmosphere, higher frequency supplies) lead respectively to lower or unchanged PD apparent charges, which are however characterized by different mean energies, hence destructive potential, in a counter intuitive manner.

Polyimide (PI) is an important insulation material due to its high thermal, chemical and electrical resistance along with good mechanical strength [1]. It has been widely used in equipment such as high voltage motors, wind turbine and aerospace craft [2].

Partial discharge testing of complete, low-voltage induction machines and dry-type convertor transformers for the More Electric Transportation is discussed. The current standard (i.e. IEC 60034-18-41, 60317-0-1 and 61378-1) offers very few answers to the new challenges posed to the insulating systems reliability of these increasingly electrified electrical machines. Based on tests performed on different kind of specimens and the literature, the most promising assessment technique to ensure a reliable insulation is based on optical sensors. In addition, this work seeks to give some precise, but simple, indications on how to take into account the effect of altitude and high frequency in the qualification of an insulation system for More Electric Transportation and the limits of the different standards involved are highlighted.

The reliability of low-voltage inverter-fed motors is highly dependent on the inception of partial discharges. The effect of impulsive waveform parameters must be investigated to predict the repetitive partial discharge inception voltage (RPDIV) and define test procedures that can indicate properly the behavior in service of the insulation system. This paper focuses on the RPDIV of magnet wires using twisted pairs subjected to repetitive unipolar impulsive voltage waveforms. The effects of supply frequency (5 to 200 kHz) at pressure levels that are typical for aircraft (20 to 100 kPa) is examined. Results show that RPDIV steadily decreases with frequency up to 100 kHz where it reaches a plateau. This behavior is explained as an effect of the oscillations that inevitably exist in the applied voltage waveform. Therefore, a conservative estimate of the RPDIV could be achieved by raising the supply frequency well above the operation frequency. In the experiments, the RPDIV is decreasing linearly with pressure. If this behavior could be confirmed for other insulation systems, the design of systems working at pressures typical of aircraft would result relatively easy.

Type II insulation systems, typical of MV motors or generators, were aged under sequential thermal cycling and voltage, and diagnostic quantities related to partial discharge activity recorded, such as partial discharge inception voltage (PDIV). Accelerated aging was carried out until breakdown statistics was reached. Sinusoidal and impulsive voltage waveforms were used, with the purpose of comparing breakdown times and PD phenomenology changing supply voltage waveform and rise time. The results undoubtedly indicate that the shape of the waveform, particularly the rise time, can affect dramatically both PD phenomenology and life.

EO has proved to be a viable substitute for MO. Its dielectric performance is always comparable with, if not superior to that of MO, at least when new, dried, and highly purified. The smaller permittivity mismatch between pressboard and EO, compared to that between pressboard and MO, leads to a more uniform field distribution at the oil/pressboard interface. We have observed that EO has higher PDIVs than MO for corona and creepage discharges. However, our understanding of the oil/pressboard interface is incomplete. There is a lack of standardization in PDIV measurements, particularly in the case of creepage discharges. Because industry is striving to provide new insulating oils for use at increased transmission voltages, improving electrical testing techniques and the understanding of the oil/pressboard interface is important.

To address the reliability of rotating machine insulation, the International Electrotechnical Commission implemented new standards and technical specifications that are relevant to low-voltage and medium-voltage inverter-fed motors. Insulation evaluation under such power supply relies upon partial discharge (PD) measurements and accelerated life tests, which should be conducted (at least for the turn-to-turn insulation) under impulsive voltage waveform. The effect of impulsive voltage parameters on PD inception voltage and, in particular, endurance tests must be investigated to design test procedures that can indicate insulation quality and its behavior in service. This paper examines the effect of repetitive impulsive voltage waveform parameters on PD and the endurance (life) of organic insulation for rotating machine wires based on experiments performed on single-contact crossed pairs. Results show that the features of repetitive voltage impulse affect PD time and frequency characteristics, PD phase distribution, and endurance. Consequently, evaluation of insulation subjected to power electronics waveforms should take into account the appropriate choice of impulsive voltage parameter to fit to the type of PD phenomenology occurring in service conditions.

Partial discharges (PD) are very familiar in the electrical insulation world. A phenomenon able to reveal or cause incipient failure, as well as need of maintenance, of an electrical apparatus must play a fundamental role in the development of electrical asset diagnosis and design.

According to International Electrotechnical Commission (IEC) technical specification (TS) 60034-18-42, to ensure the insulation system reliability of inverter-fed form wound motors, endurance tests should be performed on formettes to compare new insulation systems with proven ones. IEC TS 60034-18-42 makes reference to sinusoidal testing, as it assumes that sinusoidal and square voltage waveforms will provide comparable breakdown times if they have the same frequency and peak-to-peak voltage values. Given this premises, it becomes crucial to assess this assumption and, in case, provide the limit values for square voltage waveform parameters (i.e., rise time) that can provide results that are comparable with those obtained using sinusoidal waveforms. Therefore, the effects of square voltages waveform parameters on life tests are discussed in this paper. The tests have been performed on magnet wires, as their insulation is similar to turn insulation of form-wound machines. Partial discharge (PD) data were recorded during the tests resorting to a ultra-high frequency system and are used to explain the influence of square voltage waveform on life test results. Based on the experimental results obtained in this study, when performing endurance tests on inverter-fed motor insulation systems, square voltages waveforms having appropriate parameters should be selected to get objective endurance test results.

This paper focuses on the influence of repetitive square wave voltage rise time on partial discharge pulse characteristics in the ultra-high frequency range. Paerial Discharge (PD) tests on single-point contact crossed pairs were carried out using repetitive square wave voltages with a wide range of rise times, from 200 ns to 400 μs, including also 50 Hz sinusoidal voltage waveforms. Experimental results show that rise time has a significant influence on magnitude and frequency spectrum of PD pulses. Voltages of shorter rise times generate larger PD pulses with a predominance of high frequency components in the energy spectrum. Interpretations are presented for this phenomenon.

Ester oils are making their way into the transformer market, but still some doubts exist about their ability to be successful fluids for high and extra-high voltage transformers. In this paper, we compare the PDIV of two commercially available fluids: a natural ester and mineral oil. Two different setups were considered: Point to plane in oil and point to plane on a pressboard sheet immersed in oil. Fluids and pressboard sheets were fully conditioned prior testing. Results suggest that ester oils perform slightly better with simple point-to-plane electrodes, independently of the gap length. When testing oil/plane electrode configurations with pressboard, ester oils perform much better at short gaps. This behavior has been explained considering the hydrophilicity of ester oils, which helps in further drying the pressboard prior testing. Therefore, ester oils could prove superior to mineral oils in suppressing creeping discharges.

The implementation of underground distribution lines has grown significantly over the last decades due to the rapid increase of electric energy demands. But, the lifetime distribution of lines is strongly affected by the single or multiple stresses that could develop in the insulation system of the power cables. In this paper, the influence of applied voltage waveform on partial discharge activity and lifetime distribution of insulation material for power cables was investigated. In this sense, the Inverse Power Law-Weibull model was applied in order to develop a life model of the PD induced degradation processes under different voltage waveforms occurring in micro-cavities from the power cables insulation system. For the insulation design it is of great importance to assess the insulation threshold by using the voltage tests. As result, the step-up test method was used.

Download Citation Email Print Request Permissions Save to Project Smartening the grid is becoming imperative to deal with several issues as, e.g. competitive energy markets and the impact of renewable energy sources. Condition monitoring techniques and smart asset management are a fundamental part of this framework. Indeed, techniques that are able to evaluate apparatus condition and residual life/maintenance time can allow failure risk to be managed in transmission and distribution grids, thus enabling optimum economic exploitation of the involved assets. This paper deals with technological improvements that have led recently to raise expectations about the use of partial discharge monitoring systems as a tool to avoid failures of electrical apparatus insulation systems. General considerations on the requirements that monitoring techniques should fulfil to be able to be part of a smart grid are presented, along with a working case where an improved partial discharge monitoring system was able to highlight insulation faults and suggest an appropriate course of action for maintenance.

Identifying partial discharge (PD) phenomena in power transformers is the key to achieve proper maintenance actions. In order to tag a recorded PD pattern obtained from either on site testing or monitoring as, e.g., internal, surface or corona discharges, a diagnostic database has to be built first. This database would contain patterns and relevant statistic parameters recorded in the lab under controlled conditions. One of the limits of this approach is that several factors may influence the PD pattern or the partial discharge inception voltage and, thus, identification procedures. In this paper, focus will be on the effects of both temperature and oil flow velocity on surface PD in a pressboard layer at five different oil speeds and with static oil. Statistical analysis is presented to compare the PDIV values

This paper discusses the role of high voltage direct current (HVDC) voltage waveforms on partial discharge (PD) activity in paper/oil insulation systems. A variety of voltages were simulated by the PSIM software and synthesized using a high voltage amplifier. Kraft paper sheets were used to create defects incepting PD. The PD signals were coupled by a high frequency ferrite core current transformer. PD quantities including PD inception voltage (PDIV), pattern, amplitude distribution and repetition rate were recorded. The results show that the PDIV under notched voltage becomes higher increasing firing and overlap angles. PD patterns change with the HVDC voltage waveforms showing that large PD likely occurs at the rising front of the notches. PD amplitude and repetition rate are both directly proportional to notch depth. It is expected that such results can improve the design of UHV converter transformers.

With the aim of developing defect-based life models (i.e. life models in which breakdown is explicitly associated with a partial discharge-induced damage growth from a defect beyond a critical level), ageing tests were carried out in the lab on XLPE sandwich specimens containing artificial cavities. PD activity taking place in the cavities was continuously monitored during ageing. The behavior of discharge repetition rate and amplitude were extracted from the monitoring records. The modification of the cavity surface at various ageing times was investigated using microscope observation. Brownish solid byproducts, having oxygen containing moieties, were found covering uniformly the entire area of the cavity surface. After the byproduct was wiped off, degradation pits were observed. The variation of PD repetition rate and amplitude, as well as the relevant PD patterns, could be related to the modification of the cavity surface.

This article is the second of a series of three examining on-line partial discharge monitoring of Type I motors and highlighting critical aspects of IEC technical specification 60034-18-41. Motors passing the off-line type test can still experience premature failure when driven by variable-speed drives.

The increased diffusion of power-electronic drives in electrical plants and networks is worsening the quality of energy supply, particularly voltage-waveform distortion. The harmonic content of voltage and current has been shown to contribute to accelerated degradation of insulation systems and, thus, to premature failure of electrical components. This paper focuses on life performance of insulation systems (capacitors and induction motors) supplied with nonsinusoidal voltage. The results of life tests under sinusoidal and distorted voltage waveforms are presented and discussed on the basis of the Voltage Endurance Coefficient of the life lines, considering various reference factors, such as voltage peak, peak-to-peak and rms. In particular, the distorted waveforms are obtained considering either low-frequency harmonics (typically present on supply ac lines) or high-frequency harmonics (affecting insulation systems of power-electronics controlled loads). It is shown that the main acceleration factor should be associated to the increase of voltage peak and supply-waveform frequency, while waveshape and rms value can play a minor role.

The dependence of partial discharge inception voltage (PDIV) and repetitive partial discharge inception voltage (RPDIV) on some voltage impulse parameters (impulse width, rise time and repetition frequency) is investigated in this paper focusing on crossed pair specimens made of polyimide-amide enameled wires. The results are explained in terms of partial discharge physics and provide basis to improve evaluation during both type and quality tests of random wound motors.

Partial discharge (PD) measurements performed online can provide important information regarding insulation system reliability. However, PD detectors suitable for online measurements (or monitoring) should be endowed at least with tools that are able to deal with noise and multiple PD sources. PD source identification tools that can exploit data streams from other sensors are, moreover, very important in order to provide an accurate failure risk assessment. This paper gives specifications about PD detection systems suitable for monitoring purposes, describes a detector that fits these requirements and provide examples of processing and analysis of PD data detected on in-service apparatus.

The application to power transformers of a novel system for partial discharge detection and analysis is presented in this paper. The system exploits the diversity of partial discharge pulses to separate phenomena relevant to disturbance and/or multiple sources. By this way, measurement data can be treated in a way that allows accurate diagnostic results to be obtained. Besides signal processing tools, this system is also endowed with automatic identification routines that have been specialized for paper/oil insulated systems. Working cases are presented to show the effectiveness of the proposed system.

Applications of a new methodology, aimed at the identification of defects occurring in insulation systems of HV apparatus and based on partial discharge (PD) mea- surements, are presented in this paper. This methodology relies upon the digital acquisition of a large amount of PD pulses and separates the acquired pulses into homogeneous sub-classes. Signal processing tools recognize the presence of noise among the different classes. Identification of basic PD source typologies (i.e., inter- nal, corona and surface discharges) is then achieved, resorting to fuzzy algorithms. The proposed procedure is applied to measurements performed on different HV apparatus, such as cables, transformers and rotating machines. The purpose of this paper is to show that the identification process is robust, regarding the measuring circuit, and flexible, so that it can constitute an advanced tool for condition based maintenance, guiding maintenance experts in making decisions on the condition of the insul

A new system aimed at performing partial discharge measurements and condition assessment on electrical apparatus is presented. It is able to separate the conventional phase resolved partial discharge pattern into a series of sub-patterns, each of them containing partial discharge data generated by a single kind of defect, which makes easier identification of the source generating partial discharges. Application of the system to partial discharge data recorded on an hydrogenerator is reported to support its effectiveness.

This paper describes a new algorithm for the automatic identification of the type of partial discharge sources in insulation systems of rotating machine stator windings. The proposed algorithm is based on fuzzy rules that associate each partial discharge phenomenon to a fuzzy space class trough a tree path selection. The output is a simple frame where the defect type and a membership are evidenced. Applications of the algorithm to analyze data collected both on field and in laboratory are reported and discussed to show its validity and robustness.

Partial discharge (PD) measurements constitute one of the most promising tools for electrical insulation diagnosis. This paper describes how a procedure based on PD measurements can provide early detection of electrical trees in polymeric insulation systems. Such an application relies upon a new methodology, which provides enhanced tools for the identification of PD generating defects. Tree inference is carried out step wise. Acquired signals are primarily separated according to their waveform, thus achieving data sets related to a specific PD typology. Then, fuzzy algorithms are applied to PD height and phase derived quantities belonging to these homogeneous datasets, in order to assign a membership degree to specific output categories. If the data set is relevant to an internal defect, a further analysis is performed in order to establish whether or not this defect is a treed region. The algorithm described in this paper was developed resorting to tests performed on artificial test s

This paper presents results of tests having the purpose to derive indexes, from PD measurements, which can enable the recognition of PD generated from different sources, such as internal voids, surface and corona discharges. Moreover, recognition of the location and shape of cavities can be achieved. PD measurements are performed by a digital instrument which allows PD-pulse amplitude, phase and shape to be recorded. Such results can be employed to improve recognition efficiency of PD-based diagnostic tools used for on field applications.

Results of investigations performed in order to improve the current diagnostic techniques used for the evaluation of insulation systems of HV apparatus are presented in this paper. Improvements come from the development of a new measuring system which allows the digital acquisition of Partial Discharge (PD) signals and a new separation method, based on a Fuzzy Classifier, for the analysis of the PD-pulse shape signals. The identification of the classes, relevant to different PD phenomena, is then performed by means of PD-pulse height and phase analysis. The proposed approach is supported by the analysis of PD data obtained from insulation systems of stator bars with artificially-reproduced defects.

Results of investigations performed in order to improve the current diagnostic techniques used for the evaluation of insulation systems of AC rotating machine stator windings are presented. Improvements come from the development of a new measuring system and the stochastic analysis of partial discharge (PD) pulse signals. The measuring system allows the digital acquisition of PD-pulse signals, operates at a sampling rate high enough to avoid frequency aliasing and provides an amount of PD-pulses which enables PD stochastic analysis. In addition to the traditional PD-pulse height and phase analysis, PD-pulse shape processing is also available. The first results of PD measurements performed on different types of complete stator bars, with and without defects, are reported to show the system performance.