Partial Discharge Behavior Research Articles

Partial Discharge Behaviour of a Protrusion in Gas-Insulated Systems under DC Voltage Stress

High reliability, independence from environmental conditions, and the compact design of gas-insulated systems will lead to a wide application in future high voltage direct current (HVDC) transmission systems. Reliable operation of these assets can be ensured by applying meaningful and robust partial discharge diagnosis during development tests, acceptance tests, or operation. Therefore, the discharge behavior must be well understood. This paper aims to contribute to this understanding by investigating the partial discharge behavior of a distorted weakly inhomogeneous electrode arrangement in sulfur hexafluoride (SF6) and synthetic air under high DC voltage stress. In order to get a better understanding, the partial discharge current is measured under the variation of the insulation gas pressure, the gas type, the electric field strength, and the voltage polarity. Derived from this, a classification of the different discharge types is performed. As a result, four different discharge types can be categorized depending on the experimental parameters: discharge impulses, discharge impulses with superimposed pulseless discharges, discharge impulses with superimposed pulseless discharges, and subsequent smaller discharges and pulseless discharges. Concluding suggestions for partial discharge measurements under DC voltage stress are given: recommendations for the necessary measurement time, the applied voltage and polarity, and useful measurement techniques.

Numerical study on partial discharge in a dry air cavity with a two-dimensional fluid model considering practical reactions

Partial discharge (PD) is an important factor leading to insulation degradation of electrical equipment. Understanding the PD mechanism is of great significance for maintaining the safety of power systems. Compared with the previously reported PD models, this work develops a well-validated plasma fluid model considering more practical particles and reactions in order to explain the PD behaviour from the physicochemical point of view. The simulation results reveal that, under the power frequency voltage (50 Hz), the PD in a small dry air cavity operates at the weak Townsend mode. Besides, since the electric field nearby the perpendicular wall is significantly distorted by the void geometry, the local intense discharge presents a radial migration. Such a migration route of the local intense discharge can enlarge the discharge area per radial distance and is believed to be capable of inducing the oscillation of the current pulses. Furthermore, the main reaction pathways of ions production/destruction are elaborately analyzed, as well as the contribution of different ions to the surface charge accumulation. Finally, the generation law of the main oxidative product of the dry air discharge-O3 is obtained. It is revealed that the O3 concentration elevates with a fixed increment during each voltage half-cycle in the steady-state, and such an increment basically increases linearly as the voltage amplitude rises. The corresponding discussions in this work would be helpful both to achieve a better understanding of plasma dynamics in PD and to predict the PD intensity or position in the practical application scenarios.

Modelling of supply voltage frequency effect on partial discharge repetition rate and charge amplitude from AC to DC at room temperature

This paper has the purpose to derive an analytical model (continuum model) able to describe the behavior of partial discharge (PD) repetition rate and amplitude, occurring in a cavity embedded in polymeric insulation, as a function of the frequency of the supply voltage, going from AC power supply frequency, 50–60 Hz, to DC. In the range between DC and 50–60 Hz focus is made on data coming from tests under AC sinewave with very low frequency (VLF) such as 0.1 Hz and 0.01 Hz, which are commonly used for cable testing. It is shown that the proposed continuum model can provide reasonably good fit to the experimental results obtained in the range DC to 60 Hz, regarding PD repetition rate and amplitude. To reach such result, the equivalent circuit is modified from that commonly used and made by fully-capacitive or resistive components, in order to take into account the change of polarization mechanisms which, depending on dielectric material, may play a non-negligible role to establish the repetition rate from low frequency to DC power supply. In addition, the residual voltage after a PD event has to vary with frequency to reach good fitting. Also, it is shown that PD amplitude under DC and VLF can be lower than under AC 50–60 Hz due to the delay time of the firing electron, thus experimental PD amplitude varies with frequency depending on material and defect typology and location.

Comparative Characterisation of CNS/Epoxy and BN/Epoxy Nanodielectrics Using Electrical Tree PD Measurements and Atomic Force Microscopy

This paper contributes to the body of knowledge on the efforts to develop nanodielectrics as the next generation of insulation material. The time-to-failure under electrical tree-induced degradation of 1.09-1.35 vol.% hexagonal BN/Epoxy was found to be 3 times longer than in clean epoxy. For 0.31-0.33 vol.% CNS/Epoxy the time-to-failure was 24 times longer than the clean epoxy. The electrical treeing partial discharge behaviour in the BN/Epoxy and CNS/Epoxy showed distinct time-evolution characteristics different from those in the clean epoxy. The improved electrical tree endurance in BN/Epoxy relative to the clean epoxy can be attributed to increased mechanical stiffness. The superiority of the CNS/Epoxy as a nanodielectric is notable. The effect is suggested to be due to the electron affinity properties of the carbon nanospheres at appropriate dispersion levels.

Practical aspects of partial discharge measurement for HVDC cables

High-voltage direct current (HVDC) cables are increasingly being installed to connect new offshore wind farms. Unplanned outages of these connectors can cause high economic impacts. Hence, there are requirements for condition-based maintenance that can improve operational reliability. Partial discharge (PD) is indicative of insulation defects. PD monitoring for AC cables is well established, but before applying the technique to HVDC cable connections, it is important to characterise PD behaviour under DC conditions and the attenuation in HVDC cables. This paper investigates PD activity under non-ideal DC stress, PD signal attenuation in HVDC cables, and electromagnetic noise in converter stations. Under the voltage of superimposed DC and harmonics, PD pulses tend to synchronise with the phase of harmonics. Therefore, synchronised recording of PD pulses can produce phase resolved patterns as an additional tool for insulation diagnostics. Modelling of attenuation in a HVDC transmission cable indicates that a detection bandwidth of tens of kHz to a few MHz may improve detection sensitivity when measuring PD current pulses over very long cable runs is carried out through sensors such as high frequency current transformers (HFCTs) installed at cable ends. Additionally, the RF spectrum measured in a converter station cable hall did not include any switching-related signals, demonstrating the viability of RF sensors based PD monitoring for the HV components associated with the cable connections.

Partial Discharge Behavior and Accelerated Aging Upon Repetitive DC Cable Energization and Voltage Supply Polarity Inversion

DC cables are supposed to withstand a number of operations involving supply-voltage peak-value time variations, such as energization, switch off and polarity inversion. It is known that voltage-supply time variations cause electric field transients in cable insulation that can last for a long time before steady-state conditions are achieved, and during this time the electric field distribution is not only driven by electrical conductivity, as in steady state, but also by permittivity, as in AC. This paper shows that highly energetic degradation phenomena, that is, partial discharges, PD, can occur during voltage transients that a DC cable has to withstand during life. This results in a cumulative damage which can cause premature insulation breakdown, at times well shorter than those expected based on a cable design which is PD free under steady-state conditions. Quantification of the damage caused by PD, based on aging and residual life models, is proposed in the paper, and it is shown that damage can be significant enough to pose a threat for cable life when operation transient rate is e.g., a few per day, and that the faster the slew rate of the supply voltage variation, the highest the cumulative damage due to PD.

Partial discharges activated by impulses and superimposed voltages in a high voltage cable model

In practice, High-voltage (HV) cables are occasionally exposed to impulse and superimposed transient conditions, which may initiate partial discharges (PD) temporarily. Whether such PDs persist under AC voltage after the transient conditions have vanished, is at focus in the research described in this paper. Since for cross-linked polyethylene (XLPE) cables the accessories are weak links in the HV cable insulation system, we investigated the PD behavior of an artificial joint defect in a HV cable model under impulse and superimposed voltages. By applying a dedicated PD measuring system it was found that, the impulse and superimposed voltages can initiate PD in the artificial defect, which under local electrical field conditions can persist for some time. The different parameters of the applied voltages have different effects on the PD behavior.

Surface charging characteristics of GIL model spacers under DC stress in C4F7N/CO2 gas mixture

The C 4 F 7 N/CO 2 gas mixture has attracted much attention due to its excellent insulation property and eco-friendly character. In order to evaluate the feasibility of its application in DC GIL, this paper focuses on the surface charging characteristics of spacers under DC stress in the C 4 F 7 N/CO 2 mixture. First, the DC insulation strength was tested under uniform field. The results show that the critical dielectric strength of pure C 4 F 7 N is more than twice that of SF 6 and the dielectric strength of 15%∼20% C 4 F 7 N/CO 2 mixture is equivalent to that of SF 6 . Next, the surface charge measurement of spacers was performed in C 4 F 7 N/CO 2 mixtures, which reveals that the surface charge density decreases slightly with the increase of C 4 F 7 N content. However, the surface charging level in 25% C 4 F 7 N/CO 2 mixture is still much higher than that in SF 6 . It is found that most of the surface charge stems from partial discharges (PDs) near the flange. The PD measurement shows that in SF 6 , there are more PDs with low magnitude, but much less PDs with high magnitude compared to that in the 15%∼25% C 4 F 7 N/CO 2 mixtures. It is the different PD behaviors between C 4 F 7 N/CO 2 mixtures and SF 6 that lead to their different surface charging characteristics.

Investigation of electrical tree growth characteristics in XLPE nanocomposites

Failure of underground cables due to high degradation-rate phenomena, as partial discharges followed by electrical treeing, is one of the major causes of cable outages. Breakdowns occur mostly in correspondence of joints and terminations, where the electric stress can exceed the design electric field due to bad workmanship or manufacturing. Researches are being carried out to improve the resistance of insulation materials to such harmful electrical-aging phenomena. The use of nanocomposite cross linked polyethylene, XLPE, is investigated in this paper. Experiments of electrical tree growth on silica-nanostructured XLPE specimens, during which the time behaviour of partial discharges is recorded, are presented and discussed. Data are processed through clustering analysis, showing that addition of silica nano fillers with 3 and 5 %wt concentration improves noticeably PD endurance and breakdown time of XLPE material.

Partial discharge behavior of protrusion on high voltage conductor in GIS/GIL under high voltage direct current: Comparison of SF6 and SF6 alternative gases

Recent studies have demonstrated that fluoronitrile (FN) Novec™ 4710 and fluoroketone (FK) Novec™ 5110 show higher dielectric strength than SF 6 . These gases can be mixed with a buffer gas such as CO 2 and technical air to have suitable dielectric properties for high voltage insulation applications. With the rapid growth of HVDC networks, these gases may replace SF 6 in future HVDC apparatus such as GIS/GIL. This paper focuses on the study of the partial discharge (PD) characteristics of SF 6 and SF6 alternative gases with a protrusion defect on the high voltage conductor with DC voltage. The results show that, for calculated equivalent SF 6 dielectric strength under homogeneous electric field, a difference in terms of partial discharge inception voltage was noted for gas mixtures compared to SF 6 . This difference may be due to the influence of gas pressure under highly inhomogeneous electric field. FN-CO 2 and FK-Air mixtures showed the highest PD amplitude under positive polarity while it was higher under negative polarity for SF 6 . Moreover, PD repetition frequency is higher for SF 6 investigated alternative gases. The results can help for the design of SF 6 free HVDC GIS partial discharge monitoring system.

Investigation on partial discharge activities in cross-linked polyethene power cable using finite element analysis

The nationwide usage of cross-linked polyethylene (XLPE) for medium to high voltage distribution networks are practically common due to its excellent electrical, thermal and mechanical properties and widely installed through existing network of cable line in Malaysia. However, cable exposures to harsh climates coupled with inadvertent damage throughout installation or transportation are influencing the presence of voids inside the insulation leading to the initiation of partial discharges (PD) in the cable line. Therefore, this study is important to investigate the activities of PD due to the manifestation of voids in XLPE cable and how it affects the physical, electrical and mechanical characteristics of the cable. Analysis has been performed using Finite Element Analysis (FEA) tool to simulate the PD activities in a 2D model of a three (3) core-XLPE insulated armoured sheathed cable (500 mm2, 11 kV) with several placements of voids. The varied placement and radius of voids is very crucial in order to achieve comprehensive analysis. From the obtained result, it has been established that closest the void to the core yielded higher electric field potential. Additionally, it is verified from the simulation that the larger the size of void, the higher the electric field potential consequently increasing the current density inside the void. This simulation and analysis is quite important to provide better insight pertaining to the behaviour of PD in correspond to the presence of voids which will accelerate ageing failure in insulation framework of the XLPE cable.

Numerical Simulation of Temperature and Pressure Changes due to Partial Discharges in Spherical Cavities Within Solid Dielectrics at Different Ageing Conditions

Partial Discharges (PD) behavior during ageing of the insulation systems exhibits variations that depend on changes in gas filling characteristics and surface condition. In this article, numerical simulations of temperature and pressure behavior in an air-filled spherical cavity within a homogenous solid dielectric material due to PD activity are presented. An Analytical-Finite Element Analysis simulation approach was implemented in MATLAB and results exhibit reasonable agreement with experimental measurements reported by other authors. Simulation results allow concluding that pressure changes are directly related to variations in the PD behavior. In addition, affectations to cavity surface due to temperature increments can be discarded.

Partial discharge behaviour due to irregular‐shaped copper particles in transformer oil with a different moisture content of pressboard barrier under uniform field

This study presents the investigations on Partial Discharge (PD) behaviour of irregular-shaped copper particle in oil with pressboard barrier placed in two positions under uniform field with different moisture contents. Frequency Domain Spectroscopy (FDS) was used to estimate percentage moisture in pressboard. Recovery Voltage Measurement was used to confirm obtained FDS results. PD characteristics like magnitude, number, discharge power, rise time and duration time of PD pulse were analysed. Electric field due to particle in oil with moisture content pressboard barrier was studied by simulations. Phase Resolved PD (PRPD) pattern for different moisture pressboard observed to have similar patterns with variation in PD inception voltage (PDIV), magnitude and number of discharges. It was observed that PD characteristics as a function of voltage, increases nearly quadratic rate with respect to percentage moisture in pressboard. Simulation results complimented experimentally measured PDIV values for particle in oil with moisture in pressboard. Filtering and conditioning of oil is mandatory practice in a new transformer before going to any high voltage (HV) applications. PRPD pattern and PD characteristics data obtained in this study has great practical significance to assess the condition of transformer insulation systems before proceeding for HV application to avoid any potential failure.

Comparison of the PD characteristics of epoxy resin under exponential decay pulse and sinusoidal voltages

The saturable reactor insulation epoxy resin is subjected to exponential decay pulse voltage under normal operating conditions. It is of great importance to study the partial discharge (PD) mechanism of epoxy resin under exponential decay pulse and sinusoidal voltages for optimal insulation design. Thus, the difference in the PD behaviours under the exponential decay pulse and sinusoidal voltages is revealed in this study for the first time, and the PD characteristics of the two types of voltages are compared and analysed at 25 and 110°C. Compared to the results for sinusoidal voltage, a higher amplitude, lower repetitive PD inception voltage peak (RPDIV) and fewer PDs are obtained under an exponential decay pulse voltage. Additionally, for the same type of voltage, the RPDIV is lower, the PD magnitude is higher and the number of PDs is smaller at 110°C than those at 25°C. The experimental results can provide a new reference for the insulation redesign of a saturable reactor under an exponential decay pulse voltage.

Surface Discharge Characteristics of Different Solid–Liquid Insulation Materials in Power Transformers

The surface discharge along the interface of solid–liquid insulation is one of the main causes of the deterioration of the life of a power transformer. This experimental study investigates the effects of different solid and liquid insulation materials on the surface partial discharge (PD) characteristics. The liquid insulation materials utilized are mineral oil and synthetic ester, whereas the solid insulation materials are cellulose board and Nomex board. The surface PD characteristics in various combinations of solid and liquid insulation materials are investigated using PD inception voltages (PDIV), apparent charges (pC), and phase-resolved PD (PRPD) patterns, and the comparative analysis is presented. Moreover, topography and composition of samples during surface discharges phenomena have been discussed using high-resolution images and chemical composition obtained using the scanning electron microscope (SEM) and energy dispersive spectroscopy (EDAX). The findings of this article will assist in understanding the behavior of surface PDs under various conditions.

Numerical modeling of partial discharges in a solid dielectric-bounded cavity: A review

Partial discharge (PD) taking place in a solid dielectric-bounded cavity involves physical processes such as free electron supply, discharge development and surface charge decaying, which bring about memory effects and become the main reasons for stochastic behavior of PDs. This paper reviews numerical modeling of cavity PD in the past 30 years. In the first place, physical processes relevant to PD activity are summarized, and modeling methods for discharge development are classified. Then some differences of PD modeling at AC and DC voltages are distinguished. Subsequently, reproducing methods from simulations to experiments are introduced, as well as their comparison under different conditions and with the emphasis on voltage frequency and PD aging. At last, some problems about current simulation models are discussed, and our suggestions for future work are proposed.

Current pulse characteristic analysis of typical void and point partial discharge based on PIC‐MCC method

The partial discharge (PD) in a void of epoxy solid insulation is a typical kind of PD behaviour, its occurrences may lead to complete breakdown of the insulation system. The microscopic process of particle movement is simulated here by using particle in cell-Monte–Carlo collision (PIC-MCC) method, which possesses great significance for researching typical discharge. The simulation results of void discharge and point discharge show detailed motion process and the Townsend discharge phenomenon. The PD current pulse waveform possesses similar trends of steep rising edge and gentle falling edge compared with Gaussian function. The behaviour of discharge events is influenced by many factors, such as electric field strength, void thickness, and polarity effect. Thus, the characteristics of PD are discussed and compared by adjusting these simulation parameters to find out internal connection and performance differences.

The effects of superimposed impulse transients on partial discharge in XLPE cable joint

In practice, cross-linked polyethylene (XLPE) power cables can be subjected to alternating voltage with superimposed impulse transients. Such impulse transients may initiate partial discharges (PD) in insulation defects even below AC inception voltage. An initiated PD may persist under AC, which will cause insulation degradation. This paper investigates the PD behavior in MV XLPE cable accessories under impulse transients. Different scenarios of PD behavior are measured, described and analyzed. Based on the results, the effects of impulse transients on PD are summarized.

Partial Discharge in Nanofluid Insulation Material with Conductive and Semiconductive Nanoparticles.

This study provides a thorough investigation of partial discharge (PD) activities in nanofluid insulation material consisting of different types of nanoparticles, which are conductive and semiconductive when subjected to high voltage stress is presented. Nanofluids have become a topic of interest because they can be an alternative to liquid insulation in electrical apparatus due to their promising dielectric strength and cooling ability. However, during in-service operation, PDs can occur between conductors in the insulation system. Therefore, this study presents the behavior of PDs within nanofluid dielectric materials consisting of conductive and semiconductive nanoparticles. The results show that there is an improvement in the PD resistance and a reduction in the tan delta of nanofluids at power frequency after the incorporation of conductive or semiconductive nanoparticles in the nanofluid oil. However, the most suitable concentration of conductive and semiconductive nanoparticles in the base fluid was found to be, respectively, 0.01 g/L and 1.0 g/L at PD inception and PD steady-state conditions. The clustering of nanoparticles in a nanofluid suspension due to PD activities is also discussed in this study.

Influence of surface charge decay on cavity PD frequency at DC voltage

In this study, two types of samples, i.e. a disc-shaped cavity embedded within solid dielectrics and the other on an electrode surface, were used to investigate partial discharge (PD) behaviours at DC voltage. It has been found that the discharge frequency of the sample with one metallic surface was much larger under a positive voltage than under a negative one, whereas it was not sensitive to voltage polarities for the sample with two insulating surfaces. In order to explain experimental results, a simulation model based on continuity equations was established to obtain PD sequences at DC voltage, in which surface charge decay and discharge time lag were taken into account. Simulation results showed that positive surface charges dissipated faster than the negative, and the discharge time lag kept almost unchanged at the same voltage amplitude. The discharge time interval consisted of field recovery time and discharge time lag, and the former was determined by surface charge decay rate. In terms of these, the effect of surface charge decay on discharge frequency was discussed. The research is helpful to clarify PD mechanism at DC voltage.