Search

This paper focuses on the inception of partial discharges within the insulation system of electrical actuators used for the More Electrical Aircraft (MEA). Since these machines should operate in the absence of Partial Discharges (PDs), the dependence of the PD Inception Voltage (PDIV) on voltage impulses typical of wide bandgap (SiC) devices is investigated at both 1 bar, reduced pressures close to those typical of aircraft cruising altitudes (150 mbar – 200 mbar) or lower (down to 5 mbar). Propagation issues are not dealt with here as results were obtained working on insulation models consisting of couples of wires twisted together (twisted pairs), thus knowing exactly the potential differences between all points of the insulation model. The results show that the rise times and the switching frequencies associated with wide bandgap devices have little impact on the PDIV. A model able to predict the PDIV of the turn/turn insulation of random wound motors (the most vulnerable part of the insulation) at different pressures is proposed. The model is also able to deal with temperature changes, with limitations that depend on the type on insulation systems.

The most effective method for insulation assessment in electrical power apparatus is Partial Discharge (PD) detection. During measurements the interference from background environment hampers the PD signal and reduces its measurement accuracy. This paper discuss on the implementation of a Hankel matrix based Fast Singular Value Decomposition (H-FSVD) technique for removing noise from the PD signals. The data is first represented into Hankel Matrix (HM) structure, with appropriate sampling then using Lanczo process the Hankel matrix size is reduced and through Singular Spectral Analysis thresholds are fixed for noise detection and removal. This algorithm has been examined on simulated as well as PD signals measured on two different laboratory environments from transformers with real and simulated noise.The experiment is part of a series of experiments to detect PD patterns related to realistic transformer defects. The denoising performance of H-FSVD is compared with the wavelet based denoising methods, Empirical Mode Decomposition method and normal SVD. Numerical results show that H-FSVD efficiently removes the noise with less computation time, even for large size data.

We have investigated the long term stability of nanofluids exposed to strong divergent fields. The nanofluids were made of conductive nanoparticles (magnetite) dissolved in a base transformer mineral oil. The results show that, due to dielectrophoretic forces, nanoparticles aggregation takes place under DC fields with modalities associated with the polarity of the applied voltage. Under AC fields, aggregation is still possible, but the fields needed are above streamer inception voltage. Comsol simulations and simplified analytical models are provided to investigate the nanoparticle concentration dynamics. The models confirm the role of dielectrophoretic forces as a driver for nanoparticle aggregation.

This paper is focused on the reliability evaluation of an electric vehicle (EV) motor based on hairpin technology. Besides bearings (not discussed here), the weak point of an electric motor is its insulating system. For inverter-fed motors, the inception of partial discharges might lead to failure in a matter of days, and thus, deserves particular attention. Qualification and lifetime evaluation of inverter-fed machines are described in IEC 60034-18-41, which specifies accelerated aging procedures and considers partial discharge inception as the end-of-life criterion. This standard was used as a reference for this paper. The only exception is that insulation models were subjected to the mechanical stress profile reported in the ISO 16750-3-2012 standard since an automotive motor is subject to significant vibrations during its operation. From the tests performed, we observed that, with the hairpin technology, turn-to-turn is the weakest link in the insulation system. All the insulation models were partial discharge (PD) free from the beginning of the tests to breakdown and the root cause for breakdown was, in all cases, traced back to cracks on the surface of the insulation. This suggests that, depending on insulating enamel thickness and conductor geometry, some insulation systems are intrinsically PD free by design, despite the effect of aging. Considering the severe vibration profiles typical of EVs, and the principal breakdown mechanism (cracking of the insulation), mechanical stress coupled with thermal stress appears as the main aging driver. Therefore, this paper spotlights the lack of proper standards for the qualification of automotive electric motors and hints at the possibility that IEC 60034-18-41 considers dealing with motors that might be intrinsically partial discharge free even after long-term exposure to operational stresses.

This paper aims at presenting the designing, prototyping and testing results of a permanent magnet assisted synchronous reluctance machine, suited for A/B-segment electric vehicles. The machine is designed to avoid the use of rare-earths materials in the magnets, compensating the loss of performance from adopting ferrite magnets with a novel hairpin winding for the stator and a lightweight modular design for the rotor. Beyond the motor itself, the paper presents the design of the full drive, with an integrated power-electronics and an air-cooled housing. The simulation results show that the drive provides a maximum torque performance of 133 Nm at 3,600 rpm and a maximum power of 52.9 kW at 4,300 rpm, with peak efficiency above 96% at 4,000 ± 500 rpm and 50 ± 20 Nm, decreasing to 93–94% by including the inverter. These performances are validated with Hardware-in-the-Loop measurements on the prototype, despite small deviations from the operation of the control algorithm, and from the slightly degraded material performance. The proposed drive is finally evaluated based on its machine constant of mechanical power and torque density values, bringing to an improvement of respectively +45% and +25% compared to the 2016 benchmark, thus resulting in the best-in-class ferrite-based PMaSYRM.

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.

At the interface of Power Electronics and High Voltage Engineering, this work combines theoretical considerations and experimental investigations to assess the endurance of polymeric insulation exposed to inverter-type medium-voltage stress, which is characterized by amplitudes in the kilovolt range, switching frequencies of several (tens of) kilohertz and slew rates of several kilovolts per microsecond.

This paper reviews results of partial discharge measurements and accelerated life tests performed on models of random and form wound insulation systems subjected to impulse voltage waveforms. The goal is to highlight how the features of the supply voltage affect the partial discharge activity and how this, eventually, reflects on the endurance of the insulation system. The most salient results are that, for both types of insulation systems, short rise times trigger PD of large magnitude, which are able to Erode the insulation system faster than the partial discharges incepted using waveforms with longer rise times. In particular, failure times estimated by tests under sinusoidal voltages can be overly optimistic in assessing the long-term behavior of the insulation system subjected to impulse voltage waveforms. Frequency also is very important, as it controls the interplay between physical and chemical degradation rates. As a result, above partial discharge inception voltage the frequency acceleration rule does not hold, and can lead therefore to excessively optimistic results.

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.

Measuring PDIV with 50 Hz sinusoidal waveforms to find what happens using wide bandgap devices having slew rates of x 100 V/ns can give misleading results. Since PDIV tests using wide bandgap devices offers contradicting results, a probabilistic model is proposed to figure the probability density function of PDIV results. This is an intermediate step developing a statistical procedure enabling a more correct estimation of the actual PDIV.

A brief summary of the insulation issues associated with the use of power electronics in electrical drives is presented here. The key points that remain open and the perspectives regarding the introduction of wide band-gap devices in aircraft and electrical vehicles actuators are discussed, showing how the benefits brought about by these devices might be offset by insulation issues.

Dealing with electrical rotating machines fed by voltage converters, understanding the effect of impulsive voltage parameters on partial discharge features is of great importance to select appropriate impulsive voltage to perform partial discharge inception voltage measurements. Similarly, the dependence of insulation endurance on voltage parameters might help to develop qualification tests providing more accurate evaluation of insulation performance. The effect of voltage duty cycle on partial discharge characteristics and endurance of inverter-fed motor insulation were investigated through a large number of tests performed on single-contact crossed enameled wires under bipolar repetitive impulse wave voltages. The partial discharge pattern, magnitude, repetition rate show that short impulsive voltage duration (i.e. small duty cycles), can induce asymmetric PD patterns and reduce the probability of PD inception at the falling flanks of impulsive voltages. Moreover, the endurance tests indicate that the duty cycle of the repetitive square wave voltage can affect the insulation endurance significantly. Repetitive square wave voltages with small duty cycles tend to overestimate endurance. Accordingly, when performing partial discharge inception voltage and endurance tests on insulation of inverter-fed motors, the influence of duty cycle should be carefully considered.

To ensure insulation system reliability of rotating machines driven by power electronics, accelerated tests under repetitive impulsive voltages should be performed on turn-to-turn insulation according to the standards provided by the International Electrotechnical Commission. The parameters of repetitive impulsive voltage can significantly influence insulation lifetime, and thus, voltage parameters should be carefully controlled during the accelerated tests. To increase endurance test efficiency when testing many insulation specimens in parallel, the rise time of impulsive voltage should be controlled at certain constant values after the breakdown of one or more specimens to obtain objective endurance test results. This study provides a description of a novel square wave voltage generator used for the insulation evaluation of rotating machines driven by power electronics. A single-channel generator with variable rise times used for one specimen and a multi-channel generator with a constant rise time used to test five specimens in parallel are presented based on a push-pull solid-state switch. In the multi-channel generator, the RC parameter of the high-voltage circuit can be adjusted automatically by resorting to controlling units and a circuit structure after the breakdown of one or more samples when performing endurance tests on parallel specimens to keep the rise time of impulsive voltage at constant values. The multi-channel generator can significantly increase the endurance evaluation efficiency of the insulation systems of rotating machines fed by power electronics.

The state of a transformer depends on many parameters, which should be taken into account to determine its actual condition. In recent years what is known as health indices of transformers has been defined, which is a practical tool to assess the overall status of these electrical devices. Data from field inspection, laboratory tests, and observations during operation of the transformer are combined. All of these result in an index that provides fairly objective and quantitative information as to the condition of the transformer [1]. The research leading to these results has received funding from multiple sources over years, but we would specifically like to acknowledge the support received in the later stages from the Spanish Plan Estatal de I+D under the grant agreement DPI2013-43897-P.

The research on nanofluids has shown important improvements of the electrical performance, but so far the attention has only been focused on their use in transformers. The target of this work is to understand if nanofluids can be applied as insulators for power electronics modules, where the major stresses are caused by square waves or PWM voltages. Breakdown events in non-uniform field conditions and different voltage configurations have been deeply investigated revealing the effects of space charge mechanisms. While for DC fields the nanofluids contribution is positive, a completely different conclusion can be evinced from sinusoidal and square waves, where the situation even worsens at higher frequencies. Schlieren images of breakdown events seem to confirm the postulated hypotheses.

According to the IEC 60034-18-41, the reliability of inverter-fed motor electrical insulation must be assessed through qualification tests with stress levels based on empirical assumptions. For purely organic (Type 1) electrical insulation systems, the inception of partial discharges within the insulation systems is the end-life criterion. The aim of this paper is to start a work in the direction of proposing a framework for designing from scratch motor insulation systems with partial discharge inception voltage adequate for inverter applications. To achieve this purpose, inception voltage estimates obtained using streamer inception criteria were compared with measurements performed in the lab. The results show that, for a large class of magnet wires, conservative estimates can be obtained.

Repetitive square wave voltages should be used to perform partial discharge inception voltage and endurance (lifetime) tests on insulation systems of rotating machines fed by power electronics in qualification and type tests according to new published International Electrotechnical Commission (IEC). To obtain objective test results, the influence of waveform parameters on partial discharge (PD) and endurance results should be carefully considered and thus accordingly, the appropriate waveform selected. In our previous research, we have concluded that the rise time of the repetitive square wave voltages can influence the PD statistics in both time/frequency domain and the lifetime of magnet wires. However, all the results were obtained under symmetrical repetitive square wave voltages having same rise times and fall times. In fact, many researchers and manufacturers have been using impulsive voltages having asymmetric waveforms with fast rise times (ns level) while much slower fall times (μs level). This study aims to investigate the effects of this type of asymmetric waveform on PD statistics and endurance tests results. A large number of PD and endurance tests were performed on single-contact crossed wires made up of enameled wires used in inverter-fed motors. Experimental results show that repetitive square wave voltages with fast rise time and slow fall time give rise to asymmetrical phase resolved PD patterns, i.e. they give rise to PD events with high magnitude when the voltage slew rate is large, while PDs with much lower magnitude can be observed in correspondence of the flanks with low slew rate. Moreover, endurance results indicate the asymmetric repetitive square wave voltages will, probability, lead to much longer endurance compare with that obtained at symmetrical voltages.

A recent investigation on the DC stability of nanofluids has revealed some interesting phenomena regarding the polarization current; some unexpected slow peaks appeared after the voltage reached a precise value. In this work we have deepened and analyzed the possible mechanisms which could cause such experimental results. While in low concentration nanofluids a charge trapping mechanism seems to be the prevailing one, at higher concentration, the reduced nanoparticles distances and the lower energy barrier between two consecutive nanoparticles is able to activate the tunneling charge transfer among them, thus increasing their apparent mobility.

HVDC links with extruded cables are being widely installed in power grids. Thus, methodologies for condition evaluation for HVDC extruded cable systems are required. Partial discharge (PD) measurement is widely adopted in cable commissioning tests under AC, while there is no availability of methodologies to test PD in cables under DC. Therefore, PD measurement techniques for artificially-defected cable models under DC stress are investigated in this paper, and obtained PD characteristics are compared with those under AC stress. It is concluded that the PD test under DC voltage could have a potential for condition evaluation/monitoring, once various problems related to their measurement on field would be solved.

The influence of voltage duty cycle on partial discharge (PD) characteristics and endurance of inverter-fed motor insulation were investigated through tests performed on single-contact crossed enameled wires under bipolar repetitive square wave voltages with different duty cycles. The PD patterns, amplitude, repetition rate and lifetime obtained show that short duty cycles, i.e., lower than 50 µs, at 10 kHz frequency, can induce asymmetric PD patterns and reduce the probability of PD inception at falling flanks of impulsive voltages. Moreover, the endurance tests indicate that the duty cycle of the repetitive square wave voltage can affect the insulation endurance significantly. Accordingly, when performing PD inception voltage and endurance tests on insulation of inverter-fed motors, the influence of duty cycle on PD statistics and endurance should be carefully considered.

The concept of More Electric Aircraft (MEA) aims to address the demand for efficiency, reliability and maintainability of today aerospace industry by means of an in-depth electrification of the currently hydraulic, mechanical and pneumatic on-board actuators. The high power density of electric actuators shall decrease substantially the aircraft weight as well as its fuel consumption and environmental impact. However, safety and reliability are primary drivers of this sector. This paper will review the main stress factors affecting the lifespan of insulation materials used in aerospace actuators, concentrating in particular to the influence of the low atmospheric pressure at high attitude and to the effect of wide band-gap power devices' short rise times on voltage stress in windings.

To study the influence of temperature on partial discharge (PD) statistics and endurance of inverter-fed motor insulation, experiments were carried out focusing on enameled wires tested under bipolar repetitive square wave voltages at different temperatures. The PD pattern, delay time, amplitude and life were obtaiaed according to a large amount of test results. Experimental results show that the influence of temperature on PD features depends significantly on the voltage frequency. At low frequency, higher temperature will give rise to more PD events with longer delay time and higher magnitude. At higher frequencies the behavior is significantly different, and the influence of temperature on PD features can be neglected. The endurance test results performed at 10 kHz indicate that, at ambient temperature, PD would be the dominant factor causing insulation breakdown, while at high temperature, the thermal aging plays an important role on the breakdown process and time.

To ensure insulation system reliability of rotating machines driven by power electronics, accelerated tests under repetitive impulsive voltages should be performed on turn-to-turn insulation according to technical specifications provided by the International Electrotechnical Commission. Due to the fact that the voltage parameters of repetitive impulsive voltage can influence the insulation lifetime significantly, the voltage parameters should be carefully controlled during the accelerated tests. In order to increase the endurance test efficiency when testing many insulation specimens in parallel, impulsive voltage rise time should be controlled at some constant values after one or more specimen breakdown to obtain objective endurance tests results. This paper provides a description of a novel square wave voltage generator used for insulation evaluation of rotating machines driven by power electronics. A single channel generator with variable rise time used for one specimen and multi-channels generator with constant rise time used to test five specimen in parallel are presented. In the multi-channel generator, resorting to controlling units and circuit structure, the RC parameter in high voltage circuit can be adjusted automatically when performing endurance on parallel specimens to keep the rise time of the impulsive voltage at constant values. The multi-channel generator can largely increase the endurance evaluation efficiency for insulation systems of rotating machines fed by power electronics.

The design of the insulation for inverter-fed induction motors is chiefly empirical. In general, a commercial conductor is selected based on experience. Qualification of motor insulation is carried out according to the IEC Std. 60034-18-41. However, qualification might be time consuming. Therefore, an approach based on a preliminary screening of alternative conductors would be useful. Goal of this screening would be to determine which geometries ensure that partial discharges are not incepted under operation. Therefore, we investigate the feasibility of using streamer inception criteria in combination with FEM modelling to determine PDIV. Emphasis is given to the possibility of extrapolating results at different altitudes, to deal with aircraft actuators. PDIV measurement were carried out on twisted pairs. Pressures were in the range from 100 kPa to 20kPa, i.e. that typical of aircraft. Frequencies from 50Hz to 10 kHz were investigated. The results were compared to those provided by streamer inception criteria. A slight dependence on supply frequency was observed, although it could not be explained in terms of the models employed (based on the electrical field lines and the ionization characteristics of air). This dependence seems to be related to phenomena not explicitly considered in the models (e.g., secondary emission from the cathode, although covered by a dielectric, partial discharges occurring below detection sensitivity threshold). Streamer inception criteria failed to predict correctly these phenomena, especially al low pressures, and need further research work to become applicable.

Different types of Partial Discharge (PD) patterns have been proposed in the past to synthesize the information provided by thousand PD-pulse signals recorded during a measurement session. The efficiency in defect identification of each pattern type has been singularly verified but less work has been done to compare their performances. For this purpose, four types of PD patters have been evaluated in the identification of PD occurring in micro-voids distributed within the ground-wall insulation of new bars and coils for rotating machines. This choice is motivated by the rapid changes in PD activity during the transition from initial to steady-state conditions of the discharges within the voids. It was found that, despite some shape modifications, the Phase Resolved PD pattern provides the most effective identification among the four patterns considered in this investigation.

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.

Partial discharge inception voltage (PDIV)and /or repetitive PDIV (RPDIV) tests must be performed in order to design and qualify Type I insulation system of electrical rotating machines driven by voltage converters. RPDIV is measured under voltage surges. Compared to PD detections at sinusoidal voltages (PDIV), these measurements are more complex, since the spectrum of the disturbance produced by the impulse generators might overlap with that of PD pulses. Therefore, the PD detection system design, the selection of impulsive voltage parameters and PD detection procedure should be carefully reviewed to obtain consistent RPDIV values. We have reported that rise time and frequency of repetitive impulsive voltage can influence PD features significantly, both in the time and frequency domain. Thus, the voltage impulse parameters should be carefully selected to improve RPDIV measurements. In this study, the influence of square wave voltage duty cycle on PD statistical features is studied resorting to crossed enameled wires. Based on a large number of PD tests performed under bipolar voltages of different duty cycles, results show that short duty cycles can induce asymmetry in the PD patterns and reduce the probability of incepting PD activities. Moreover, impulse voltage with low duty cycle gives rise to more than one PD at the voltage falling flanks of each cycle, very close to the peak of the disturbance impulses. This phenomenon could hide acquisition of PD pulses, causing not accurate RPDIV estimates when performing qualification and type types on inverter-fed rotating machines according to IEC standards. Therefore, impulsive voltage waveforms having duration time larger than 100 μs should be selected to increase the probability of incepting and detecting PD when performing RPDIV measurements.

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.

High voltage equipment designed for DC applications is stressed differently from ac equipment. Most notably, the load conditions of the equipment play a major role in determining the electric field distribution in insulation under dc, due to the temperature dependence of conductivity. Additionally, polarization and space charge phenomena impose a time dependent behavior on the stress distribution. This has major consequences for the way in which degradation of the insulation under dc stress takes place. The nature of the stresses under dc may lead to accelerated degradation processes which could result in early breakdown of the insulation. This paper provides an overview of the stress conditions specific for dc application and highlights via which routes they may lead to failure. The stress distributions in converter transformers and dc cable systems will serve as typical examples.

In this work, we compare the dielectric withstand performance of nanofluids prepared using Shell Diala D as a base fluid, and magnetite, graphene oxide and silicone dioxide as nanoadditives. The ac withstand capability of the nanofluids was investigated as well as partial discharge inception voltage, partial discharge and repetition rate under ac, dc positive and dc negative voltages. The results indicate that, for all nanofluids, nanoparticle concentrations around 0.2 g/l enhance dielectric withstand properties under quasi uniform fields. Under divergent fields, partial discharge characteristics are improved under ac conditions. Under dc conditions silica nanofluid performs better than mineral oil, but the other two nanofluids do not perform well. In this work, we compare the dielectric withstand performance of nanofluids prepared using Shell Diala D as a base fluid, and magnetite, graphene oxide and silicone dioxide as nanoadditives. The ac withstand capability of the nanofluids was investigated as well as partial discharge inception voltage, partial discharge and repetition rate under ac, dc positive and dc negative voltages. The results indicate that, for all nanofluids, nanoparticle concentrations around 0.2 g/l enhance dielectric withstand properties under quasi uniform fields. Under divergent fields, partial discharge characteristics are improved under ac conditions. Under dc conditions silica nanofluid performs better than mineral oil, but the other two nanofluids do not perform well.

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.