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IEC standard for the design and qualification of the insulation system of inverter-fed motors controlled by power electronics refers to the measurement of partial discharge inception voltage (PDIV) or repetitive partial discharge inception voltage (RPDIV). This is, however, not a simple testing procedure when carried out under repetitive impulse voltage. Indeed, impulse voltages cause strong switch-on/off disturbance, which make much more difficult to distinguish PD signals from noise. To overcome this problem, the effects of impulse rise time on PD pulse signals should be investigated in order to design appropriate PD sensors with high signal-to-noise ratio and to choose suitable digital signal processing methods after the high-speed analog to digital conversion. This work investigates the influence of repetitive impulsive rise time on PD statistics focusing on experiments performed on crossed enameled wires under square wave impulsive voltages with different rise times, from 50 ns (the minimum) to sinusoidal. The dependence of PD magnitudes and voltages (firing voltages), as well as PD pulse energy distribution maps (in the time/frequency domain), on square wave voltage rise time, are analyzed. It is shown that the rise times of the square wave voltage can have significant influence on PD statistics both in time and frequency domains; impulse voltages with shorter rise time are likely to induce PD events with relatively higher energy in 1.0 to 1.5 GHz domain, which will be beneficial for increasing signal-to-noise ratio when using ultra-high frequency antenna; PD repetition rate decreases with rise times.

We have compared the DC conduction of a nanofluid based on mineral oil and magnetite nanoparticles with that of the base mineral oil using a point/plane electrode configuration. The results show that the nanofluid has higher conduction currents and that transition from ohmic to tunneling-controlled conduction occurs at higher fields. This latter evidence might explain why nanofluids improve partial discharge inception voltage under AC conditions. After some tests we observed that the nanofluid is not stable under DC: the nanoparticles are separated from the base fluid by coulombic forces and dielectrophoresis. We have compared the DC conduction of a nanofluid based on mineral oil and magnetite nanoparticles with that of the base mineral oil using a point/plane electrode configuration. The results show that the nanofluid has higher conduction currents and that transition from ohmic to tunneling-controlled conduction occurs at higher fields. This latter evidence might explain why nanofluids improve partial discharge inception voltage under AC conditions. After some tests we observed that the nanofluid is not stable under DC: the nanoparticles are separated from the base fluid by coulombic forces and dielectrophoresis.

Endurance tests under repetitive impulse voltages should be performed on inverter-fed motor insulation to evaluate its insulation performance according to the technical specification (TS) released by the International Electrical Commission (IEC), i.e. IEC TS 60034-18-42. To simplify testing procedures in the application, the TS allows life tests to be performed under sinusoidal waveform, as it assumes that sinusoidal and impulsive voltages will provide comparable results if they have same peak-to-peak voltage and frequency. However, we have found that voltage rise time can have significant effect on PD features in both time and frequency domains, so that if PD are deemed the cause of accelerated aging, rise time must be taken into account and sinusoidal waveform may be not the best choice for accelerated aging procedures which resemble the real stress experienced in service by insulation systems of rotating machines controlled by Pulse Width Modulation (PWM). This study aims at summarize the effect of impulsive voltage rise time on insulation life based on endurance tests on crossed enameled wires used for type I motors and turn/turn insulation for Type II motors. Repetitive voltages of 6 rise times (i.e. 50 ns, 200 ns, 1 µs, 16 µs, 200 µs and 500 µs, sinusoidal voltage with same peak-to-peak voltage and frequency) and 3 rise times (i.e. 150 ns, 2 µs and sinusoidal voltage with same peak-to-peak voltage and frequency) are used in the endurance tests on Type I and Type II motor insulation, respectively. Statistical analysis indicate that, due to the different PD features caused by the impulsive voltage rise time, impulsive voltage rise time can have significant effect on insulation lifetime. Accordingly, when performing endurance tests on high-voltage inverter-fed motor insulation, the rise time influence on final life of inverter-fed motor insulation should be carefully considered.

Power transformers are one of the critical components in power system; traditionally mineral oil is used in transformers for the purpose of dielectrics as well as cooling. In this work, an investigation has been carried out to analyze the performance of Ferrofluids (nano fluids) along with mineral oil for various concentrations. Ferrofluids concentration of 0.1g/liter, 0.2g/liter and 0.5g/liter has been prepared with moisture content less than 5 ppm. Partial Discharge Inception Voltage (PDIV), discharge characteristics of test samples for AC, DC positive and negative voltage is analyzed. Moreover charges produced for each Partial Discharge (PD) have been evaluated. The results illustrate that Ferrofluids with lower concentration shows enhanced PDIV and charge accumulation characteristics than other concentrations as well as mineral oil.

The effect on partial discharge activity of the application of hybrid organic-inorganic nanostructured coating to XLPE insulation is investigated in this paper. Specimens having an embedded flat cavity were manufactured and coated with two different silica-based nanocoating obtained by sol-gel process. Aging tests under ac voltage were performed and partial discharge activity taking place in the cavity was monitored and compared to that occurring in uncoated specimens. Results show that coated materials are affected by less intense partial discharge activity, in terms of amplitude and repetition rate, than pure XLPE. In particular, partial discharge activity decreases more quickly with aging time with respect to uncoated materials. This fact could be associated with the good impermeability to oxygen shown by the hybrid nanocoating here used. Oxygen, in fact, is a driving factor for partial discharges and for polymer degradation caused by partial discharges.

Permanent Partial Discharge Monitoring (PDM) systems were installed in several generators in South America from 2009 to 2011. The installation fell within a long term project envisaging the installation of PDM systems in several generators placed in different power plants. All PDM units send automatically data and alarms to a Central Unit (CU) placed in the utility headquarter. Through the use of Time Frequency (TF) separation tools it is possible to reject noise and disturbances, as well as to separate on line multiple PD sources and tune proper alarm logics for each type of detected PD phenomenon without being affected by the significant on-line noise and disturbances. In addition, different criteria can be used to evaluate the trend of PD phenomena depending on PD source nature, i.e., if they are relevant to microvoids, slot, stress grading, bar to bar, delamination or corona. Furthermore multiple PD activities occurring in the same phase can be dealt separately avoiding false alarms coming from the sum of the statistical parameters relevant of each PD source.

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.

The electric properties of hybrid organic-inorganic nanostructured coated XLPE are investigated in this paper with the purpose to understand if this class of nanostructured insulating material could have applications in the field of high voltage electrical insulation. Space charge measurements and aging tests under partial discharge activity were performed on different kinds of nanocoated specimens and the results compared to the base XLPE. The paper shows that space charge accumulation can be reduced significantly by some coatings which could increase the injection barrier. Moreover, surface erosion caused by partial discharges can be slowed down even by a thin coating of about 1 µm, which creates a ‘barrier effect’ to partial discharge propagation in the insulation bulk.

Electrical utilities, stretched between requests for higher quality and lower costs are operating their assets at or close to their ratings, very often beyond their design life and with minimum maintenance. Keeping grid reliability at reasonable levels in these conditions is indeed a technical challenge, that can be won getting the most comprehensive information about assets that are of critical importance for the grid. This information, provided by global monitoring systems, once suitably processed, should be able to highlight which assets are in critical conditions and need actions to restore their reliability. Furthermore, it should be able to identify particular conditions of the network causing accelerated degradation problems as, e.g., harmonic resonances and voltage dips. Eventually, this information should be available in SCADA centres to allow, e.g., load flow and voltage regulation to be optimized, so as to minimize the stress applied to apparatus in critical conditions. These concepts fall into the Smart Grid approach, as it is elucidated first. Then, examples of global monitoring tool application are presented and discussed.

Partial discharges (PD) in HVDC equipment are not the predominant degradation factor. However, they can be a failure precursor (particularly when electrical trees are formed), so that monitoring PD could be a tool to infer imminent failures. In order to monitor PD on site, one major problem at DC voltage is the absence of tools able to distinguish noise from PD signals. In this paper, corona, surface and internal discharges measured under DC voltage are extracted from noise and interference resorting to a new strategy of noise rejection at DC voltage.

The research work reported in this paper has the aim to contribute in the development of a new physical aging model for damage inception and growth from micro-cavities in polymeric insulating materials subjected to partial discharges (PD) under AC voltage. Aging tests were performed on polyethylene (PE) layered specimens with cylindrical micro-cavities. The PD activity taking place inside the micro-cavities was monitored during the aging process. PD patterns, discharge repetition rates and discharge amplitudes were recorded. Common features can be extracted in the changing trend of PD repetition rate and discharge amplitude. Furthermore, damage growth rate, as well as degradation severity, are inferred through simple statistics.

Partial Discharge (PD) off- line and on-line measurements are required to prevent early failures on low voltage induction motors fed by inverter drives. Conventional methods use commercial Digital Scopes to visualize simultaneously the waveform of the applied impulsive voltage and the acquired PD pulses, as well as appropriate solutions to attenuate power electronic voltage impulses. A new approach is proposed in this paper, relying upon a Phase-Resolved PD (PRPD) pattern, which constitutes a simple and efficient tool for PD activity detection in adjustable speed drives. Off-line PD measurements were done on low voltage induction motors fed by a voltage impulse generator and detecting PD Inception Voltage (PDIV) on each phase. Then, PD on line measurements were carried out using a commercial PWM inverter and showing interesting correlation between off-line and on-line tests. All the measurements were performed using antenna sensors, achieving excellent sensitivity.

This research aims at establishing criteria for the automatic identification of defects in paper/oil insulation systems. Statistical analysis of partial discharge (PD) data collected in a reference database (built up from tests on artificial defects: internal PD, surface PD, corona and discharges due to bubbles in oil) is carried out to highlight the best identification markers. The development of an automatic identification system based on fuzzy logic is presented. Eventually, applications of identification are discussed.

In this paper the effect of partial discharge induced degradation on epoxy-resin specimens filled by silica nanoparticles is discussed. In particular, the experimental results show that nanostructured materials exhibit longer failure times with respect to base epoxy resin and increasing nanofiller concentration the resistance to partial discharge activity improves significantly. Degradation evolution is investigated through continuous monitoring of partial discharges and periodic physical surface profile measurements. It is shown that the former provides useful quantities, e.g. inception phase, to be associated with the extent of specimen surface erosion. This allows PD endurance of nanostructured materials to be evaluated by means of on-line measurements which can provide further information also on PD erosion phenomenology (e.g., surface erosion or pit formation/growth).

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.

Partial discharge (PD) measurement is a fundamental tool to evaluate the state of apparatuses of medium voltage (MV) power systems. An alternative to traditional PD sensors (e.g. current transformers, capacitive couplers) for online measurements can be constituted partly by electromagnetic field sensors, capable to pick up the signals generated by PD occurring in the insulation system and traveling in the surrounding media. In this paper, the results of on-line PD measurements, performed on two PILC MV power lines by means of a UWB PD detector and a narrow band log periodic antenna sensor, are presented and commented. The sensor was chosen in order obtain the maximum sensitivity on the basis of the frequency content of the input signal (i.e. electromagnetic pulse generated by PD) and to give an optimized output signal according to the PD detector characteristics (such as input impedance and bandwidth). The results of the on-field tests show that the system composed by the PD detector and the antenna sensor is able to detect PD occurring in cable accessories. Rejection of noise and of external disturbances is also possible by means of time-frequency mapping of recorded pulses. The correct identification of the PD phenomenon and the consequent risk assessment can thus be performed. The used antenna sensor is directional and allows localized investigation on each component of the system to be performed and information about the PD source location to be gathered.

Drug development and personalized therapy require accurate and frequent monitoring of the metabolic response by living tissues to treatments. In case of high risk side effects, e.g. therapies with interfering anti-cancer molecule cocktails, direct monitoring of the patient’s drugs metabolism is essential as the metabolic pathways efficacy is highly variable on a patient-by- patient basis. Currently, there are no fully mature point-of-care bio-sensing systems for drugs metabolism monitoring directly in blood. The aim of the paper is to investigate solutions to develop point-of-care systems for drugs monitoring in personalized therapy. P450 enzymes are the considered probe molecules as they are key proteins directly involved in drugs metabolism of humans. Sensitivity improvement is ensured by means of enzyme integration onto electrodes structured with carbon nanotubes. Component Off-The-Shelf (COTS) based circuits design is investigated toward bio-chip development. Results show that the proposed circuitry is suitable for the aim and confirm that nanotubes are detection enhancers.

This paper presents a method to determine travel time, attenuation and frequency characteristics of partial discharge signals travelling along a power cable. The method is based on the Advanced Transient Program. Laboratory tests performed on 13 kV EPR insulated cable are used to validate simulation results. The model developed here will be used in forthcoming papers to assess partial discharge detection sensitivity as a function of cable length and detector bandwidth. Here, some preliminary results are presented.

Results of investigations having the purpose to select meaningful diagnostic indexes for the identification of different defects of insulation systems of AC rotating machines are reported in this paper. Different discharge sources were created artificially on six Roebel bars arranged in a mock-up of eight stator slots. These bars have been firstly tested one by one, then connected in series to simulate different locations of PD sources. Standard quantities and parameters, derived from PD-pulse height and phase distribution analysis, were compared in order to verify their ability to identify the defects in the various set-up conditions (bars alone or connected together, changing couplers and instrument bandwidth). It was found that parameters related to the shape of PD patterns remain almost constant and their suitable processing allows identification of the different PD sources.

This paper presents a new technique to perform Partial Discharge (PD) monitoring on high voltage apparatus that are critical to transmission systems, focusing on high voltage cable systems. The hardware used for PD monitoring is innovative since it can provide information concerning PD pulse waveforms, allowing, in particular, disturbance and noise rejection. The PD system is also endowed with an identification tool that, at a first level, is able to discriminate between internal, surface and corona PD. This identification tool can be used to improve monitoring effectiveness, since the warning system can be trained, as an example, to weight alarm severity against PD identification, providing more emphasis to internal activities.

This work presents a new PD (partial discharge) measuring system for inverter-fed motors based on a bridge circuit topology. The bridge circuit was adopted in the measuring system for picking up PD signal and suppressing interference. The experimental results show that it is possible to detect and record PD notwithstanding the large interference due to inverter commutations if measuring frequencies are selected adequately

Long-term and short-term electrical properties of impregnated paper and polymeric insulating materials employed in oil-filled HVDC cables, that is, paper and paper-polypropylene-paper laminates, are investigated in this paper. Specimens were subjected to electric strength tests, life tests and space charge measurements, at different electrical field and temperatures. The results indicate that PPPL is by far superior to paper in terms of time-to-breakdown. As regards, space charge, paper accumulates more charge than PPPL but, generally, it is able to expel it at a slightly faster rate.

The effect of space charge accumulated in insulation cavities on partial discharge activity is dealt with in the paper. Partial discharges were measured on non-polarized and DC-polarized XLPE specimens. In order to characterize space charge distribution, pulsed-electroacoustic (PEA) space charge measurements were also performed on polarized and PD-aged specimens. The results show that space charge can affect, although in some complex way, partial discharge phenomenology. Moreover, space charge measurements on stressed specimens can help in the inference of degradation mechanisms (e.g., trap formation at the cavity surface) associated to partial discharge activity.

Partial discharge (pd)-based risk assessment in electrical insulation systems becomes effective only if the nature of the defect generating pd can be ascertained. In oil/paper insulation systems this is a fundamental item since e.g. harmfulness of pd generated by gas bubbles is much smaller than that of pd occurring between paper layers. For such reasons, partial discharge analysis of oil/paper insulated systems often lacks a clear correlation between cause (i.e., defect type) and effect (partial discharge recording). Results relevant to partial discharge measurements performed on lab-scale objects having different electrode configuration are presented in the paper with the purpose to spotlight characteristic features aimed at identifying the basic nature of partial discharge-generating defects in oil/paper insulated systems.

The application of a novel partial discharge measurement system to the identification of defects in insulation systems of electrical apparatus is presented in this paper. It records the whole waveshape of a large number of partial discharge pulses and, then, splits the measured dataset into subgroups on the basis of the pulse waveshape. When different waveshapes can be attributed to different partial discharge sources or, possibly, noise, artificial intelligence techniques enable the system to reject noise and apply identification/risk-assessment routines to homogeneous patterns, that is, to phase-resolved partial discharge patterns relevant to a single partial discharge source.

This paper proposes a new model which provides the life estimation of polymeric insulation subjected to dc stress, with or without voltage reversals, on the basis of accelerated life tests carried out at constant dc stress and measurements of space charges. Accordingly, the performance of a new material under dc electrical stress can be characterised by short-term tests, so that, for example, the expected life of a cable can be evaluated once the rate of voltage reversals expected during its life has been guessed. The model is applied to results of tests carried out on polyethylene materials.

Nowadays the increase of polluting loads in electrical networks affects supply voltage sinusoidality more significantly than in the past. Voltage distortion can decrease reliability of electrical devices, that is, increase their failure rate. This paper deals with the effect on aging acceleration due to harmonics for a simple insulation system, i.e. low-voltage self-healing capacitors. The most stressing features of the nonsinusoidal voltage and current waveforms (peak value, rms value, slope, etc.), as well as temperature, are investigated in order to single out the extent of their effect on life reduction. For this purpose, life tests under temperature and voltage distortion were carried out considering different kinds of harmonics, summed to fundamental one, able to vary peak and slope of the voltage waveform. The tests were performed both at room and at high temperatures. The results, statistically processed through Design of Experiment (DOE) techniques, show that the effect of temperature on ageing is significant, but the most important effect is that relevant to peak voltage. A life model involving all the main stressing factors (peak voltage, rms value and slope, as well as temperature) is therefore proposed.

Buffer traps can induce “false” surface-trap signatures in AlGaN-GaN HEMTs, namely the same type of current-mode DLTS peaks and pulse responses that are generally attributed to surface traps. Device simulations are adopted to clarify the underlying physics. Being aware of the above phenomenon is important for both reliability testing and device optimization, as it can lead to erroneous identification of the degradation mechanism, thus resulting in inappropriate correction actions on the technological process.

This paper deals with power factor compensation system design by probabilistic criteria. The compensator is considered as an assembly consisting of switchable units, thus assuming that a single unit is not sufficient to ensure satisfactory power factor correction and/or compliance with harmonic-limiting standards for all the operating conditions of the load. The expected cost of the compensator (including energy costs, utility penalties and installation mortgage) is selected as the objective function for optimum design. An analytical expression of the objective function, based on measurement results statistics, is obtained.