Partial discharges in a cavity at variable applied frequency part 2: measurements and modeling

The reliability of electrical assets is greatly influenced by the quality of their insulations. Key power installations such as power cables are manufactured with polymer-based materials as part of their insulation system. However, accelerated ageing of equipment insulations due to manifestation of defect(s), and partial discharges (PDs) can offset the operation of these systems or even lead to breakdowns. In this study, a non-deterministic model to simulate the phenomenon of repetitive discharges in a spherical air-filled cavity within a practical power cable has been investigated. In addition, the work contributes to the understanding of PD behaviour and field distribution under different ageing conditions considering changes in cavity surface conductivity. First, a section of the practical XLPE cable containing the cavity is developed in 2D using COMSOL software, and a finite element analysis (FEA) of the electric field distribution within the cable insulation is performed. The magnitude of the cavity local field, that is enough to ignite a PD, is investigated. Alongside the COMSOL model, the activity of sustained internal PD is simulated in MATLAB by introducing a random sample generating factor and adjusting the model’s parameters to obtain something close to the practical results. Furthermore, the impact of continuous PD in the power cable under different cavity dimensions and surface conductivity is likewise investigated, and a phase resolved PD (PRPD) pattern is established. The result shows that the magnitude and number of PDs per cycle increase as the cavity size and its surface conductivity increase. Finally, when the cavity surface conductivity rises, the amplitude of the electric field generated by the surface charge distribution and the number of PDs per cycle approach their maximum values.

This paper examines the influence of cavity size and location in the insulation paper on the Partial Discharge (PD) activities through Finite Element Method (FEM). The model consisted of a conductor wrapped with insulation paper. Two different locations of the spherical cavities were introduced in this study, namely Location 1 (L1) and Location 2 (L2), located at the center and left corner of the insulation paper. The model introduced two different sizes of cavities with diameters of 0.5 mm and 0.8 mm. An AC voltage source of 17 kV, 50 Hz, was applied at the conductor while the bottom of the insulation paper was grounded. The real and apparent PDs were obtained by integrating the current flowing through the cavity and ground electrode with the respective surface area. The simulation was carried out for 100 cycles. The resultant model was used to study the PD occurrence, magnitude, and Phase Resolved Partial Discharge (PRPD) within the insulation paper. It is found that the large cavity size produces a lower number of PD occurrences per cycle than the small cavity size. The large cavity size produces a higher charge magnitude as compared with the small cavity size. The PD occurrence per cycle and charge magnitude are higher for the cavity location at L1 compared to L2. The PRPD yields the same pattern for cavity location at L1 and L2, whereby the differences are only on the charge magnitude and PD occurrence per cycle.

We estimate the attenuation around Cajon Pass, southern California, for frequencies above 10 Hz, and we find total Q to exhibit only weak frequency dependence, ranging from ∼800 at 10 Hz to ∼1500 at 100 Hz. The intrinsic attenuation is approximately twice the level of the scattering attenuation. Measurements are made using earthquake seismograms recorded at 0–3 km depth using the multiple lapse time window method. The results are not dependent on receiver depth and are consistent with previous estimates of Q made from direct waves recorded at 2.5 km depth. Our Q values are therefore thought to be representative of the seismogenic crust, and the technique used is uncontaminated by the highly attenuating near surface, at least above ∼10 Hz. We also calculate Q from surface data between 1 and 10 Hz and find a clear change in the frequency dependence of both intrinsic and scattering attenuation at ∼10 Hz. Q exhibits strong frequency dependence below 10 Hz ( ∝ƒ1.8), consistent with previous studies in active tectonic regions, and only weak frequency dependence at higher frequencies ( ∝ƒ0.34). This change in behavior renders it unwise to extrapolate Q measurements outside the frequency range from which they were derived, for example, in earthquake‐source studies. Possible factors responsible for the apparent change in frequency dependence of Q are considered. Further work is required, however, to resolve the causes of this change.

In the usual methodology for soil investigation, boreholes locations are selected in site, machinery extract, with due care, soil samples (or specimens) and tested, and then soil strength parameters for site are calculated. The soil report may contain some of the field tests, the SPT for instance, as well. The procedure is to pass this information to the structural engineer who, in turn, may design the structure. In soil investigation principle, the small soil specimen extracted from the soil strata have complete representation of the soil body underneath the footing. In soil mechanics theory the soil is well known to be nonhomogeneous and nonisotropic. This, eventually, means that soil properties and strength parameters change not due to location only but due to change of direction as well. This situation imposes the fact that the principle of representing the whole body of soil with a small sample is practically incorrect. The principle is assumed correct only as much as the soil is more and more isotropic and homogeneous. Therefore, it is in reality not uncommon for cavities to be present in soils body due to many reasons. If those cavities are detected before footing construction, then it is assumed that there is no serious problem. On the other hand, if it is not detected, the situation imposes a serious problem to the footing and structure depending on size and location of cavity.
 This study deals with cavity presence directly underneath a single separated footing of proposed multi-storey building. Location and size of cavity are changed and a finite element (FE) analysis is run for each individual case. The subgrade soil is assumed medium-dense sand with a modulus of subgrade reaction 35000 kN/cu.m. Other than cavity location, the subgrade is assumed to be homogeneous and isotropic, i.e. have same material and strength properties. Suitable graphs are used to illustrate the stresses in footing. The study, however, take into account the shear stresses in 3D only (a shear-care study) in concrete. The foundation concrete is assumed to have low compressive strength such as 20 MPa. Other properties of concrete are assumed program’s default.

Markram and Tsodyks, by showing that the elevated synaptic efficacy observed with single-pulse long-term potentiation (LTP) measurements disappears with higher-frequency test pulses, have critically challenged the conventional assumption that LTP reflects a general gain increase. This observed change in frequency dependence during synaptic potentiation is called redistribution of synaptic efficacy (RSE). RSE is here seen as the local realization of a global design principle in a neural network for pattern coding. The underlying computational model posits an adaptive threshold rather than a multiplicative weight as the elementary unit of long-term memory. A distributed instar learning law allows thresholds to increase only monotonically, but adaptation has a bidirectional effect on the model postsynaptic potential. At each synapse, threshold increases implement pattern selectivity via a frequency-dependent signal component, while a complementary frequency-independent component nonspecifically strengthens the path. This synaptic balance produces changes in frequency dependence that are robustly similar to those observed by Markram and Tsodyks. The network design therefore suggests a functional purpose for RSE, which, by helping to bound total memory change, supports a distributed coding scheme that is stable with fast as well as slow learning. Multiplicative weights have served as a cornerstone for models of physiological data and neural systems for decades. Although the model discussed here does not implement detailed physiology of synaptic transmission, its new learning laws operate in a network architecture that suggests how recently discovered synaptic computations such as RSE may help produce new network capabilities such as learning that is fast, stable, and distributed.

We explore high-order soliton evolution in a composite all-normal dispersion photonic crystal fiber with frequency-dependent doping nonlinearity that can vary from positive to negative and vanish at a specific wavelength, named the zero-nonlinearity wavelength (ZNW). The change in frequency dependence of doping nonlinearity dramatically adjusts the position of the ZNW and the nonlinear strength acting on the supercontinuum spectral component. As the negative (positive) frequency dependence decreases (increases), one of two generated fundamental solitons via the combined action of negative nonlinearity and normal dispersion first has a gradually boosting soliton self-frequency shift (SSFS), and then a more suppressed SSFS, but is finally limited within an increasingly small spectral range while the other has a gradually decreasing SSFS toward the ZNW and radiates a broader and stronger phase-matched dispersive wave (DW) in the nonsolitonic region. Note that the collision between two fundamental solitons plays a significant role in regulating their temporal and spectral shift as well as energy redistribution. Furthermore, the numeric sign of the frequency dependence of doping nonlinearity provides another potential tool in controlling the spectral shift direction of fundamental solitons and DWs.

The pH‐dependent chitosan/PEG membrane was prepared by a surface‐mediated process with glucose as a source. Swelling study shows that the glucose‐mediated membrane is more stable in a neutral environment than an acidic one and the stability in all soaking environments generally increases with increasing glucose concentration because of an increase in Schiff's reaction product as demonstrated by a mediated degree analysis. DMA analysis shows that as the glucose concentration increases, the rubber plateau of the glucose‐mediated membrane is prolonged and keeps a higher storage modulus, and the tangent δ peak shifts to a high temperature as the temperature increases. In an isothermal mode, as the glucose concentration increases to 10 wt %, the storage modulus shows a small change in frequency dependence, indicating that the mediating effect is obviously affected by glucose addition. It can also be found that surface‐mediated membrane has a higher complex viscosity than chitosan/PEG membrane at high frequencies. Thus, a new way was found to use glucose to produce stable biomaterial. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 809–820, 2004

When detecting the presence of partial discharge (PD) activity in the insulation system in high-voltage equipment, the excitation voltages at variable frequency have been widely used instead of power-frequency (50/60 Hz) sinusoidal voltage in order to reduce the charging power. This work reviews the relevant research on PD activity at very low frequency (VLF) method, including sinusoidal or cosine-rectangular voltage shape, and damped AC (DAC) method. Based on the research history and development status, some major PD characteristics, such as PD inception voltage (PDIV), PD amplitude, PD charge, PD phase-resolved pattern, and several hot issues, such as surface charge decay and statistical time lag, have been discussed. Moreover, the advantages, disadvantages, and applied conditions of two reviewed methods has been summarized. Finally, the prospects have been made on the main development trends of this research field in the future.

For condition monitoring of the power cable insulation system, different diagnosis procedures are available. The measurement of partial discharge (PD) activity is still an issue of great importance. This paper deals with the effect of the temperature gradient and different voltage waveforms/ frequencies on PD phenomena within voids of power cables insulation. The results of PD measurements and computer simulations are underlined. The current study explains the behaviour of PD considering the effect of different testing conditions. The PD tests were performed considering AC, DC Step and Oscillating Voltage (OVW) voltage waveforms at 50 Hz, Very Low Frequency (0.1 Hz/ VLF) and 1 kHz using the Step-up Voltage Test Method. During PD tests, Partial Discharge Inception Voltage (PDIV) was recorded after measurements developed using polyethylene specimens with artificial cavities embedded. The specimens were realized using three thermally-soldered layers of cross linked polyethylene/ low density polyethylene (XLPE-LDPE-XLPE) and in each specimen the central layer has a circular hole punched of around 400 µm. The computer simulations were developed using Comsol Multiphysics 4.2, Heat Transfer/ Electromagnetic Heating/ Joule Heating. The aim of computer simulations was to fit the experimental results. According to the measured values, PDIV is frequency dependent, as the frequency increases the PD are incepted at higher voltage levels. According to computer simulations, the cavity embedded within specimen is a heat source and this could be an explanation of the frequency dependency behavior of PDIV.

Experimental ac partial discharge (PD) patterns are presented for a disc-void (area greater than length) with metal surfaces, one metal and one insulating surface, and with both surfaces insulating. These patterns indicate that the portion of the surface over which the charge is deposited (discharge area) plays an important role in producing the fluctuations in discharge magnitude commonly observed for such voids. A simulation model for PD patterns in voids is presented, which expressly includes the effects of the charge distribution left on the void surface by consecutive PDs. This model defines two factors that control the PD propagation. These are: a minimum field required to maintain the discharge within an existing surface path Ein and a minimum peripheral field required to extend the discharge path Ep. There is also one other factor, the occurrence probability that controls the incidence of the PD. It is shown that the model gives a variation in PD magnitude throughout the active region of phase without any stochastic factors, i.e. when only variations in the discharge areas are allowed for, but that in order to reproduce the typical (turtle-like) PD patterns observed for the disc-void with insulating interfaces the occurrence probability and surface conductivity have to be included. The current model is compared to previous models and the relationship of the model factors to the change of PD patterns with ageing is briefly discussed.

Current loading of wind farm distribution network is very different from that observed for traditional underground cable systems. Variations in wind power results in a significant number of current load cycles during service. The current can increase from zero to maximum rated load in a short time. This can cause high temperature gradients in the cable systems inducing significant thermo-mechanical forces during heating and cooling, and thermal oxidation at high load. Several premature service failures of cable joints in wind farm distribution networks have occurred, mainly due to the combined action of bad metallic connectors and high load current. Partial discharge measurements are today performed on-site using different frequencies and magnitudes of the test voltage. As the oxidation process causes the conductivity of the bulk and surfaces of the materials involved to strongly increase, this will also likely impact the frequency dependence of the partial discharges. This work includes laboratory high temperature ageing of rubber type heat-shrink MV XLPE cable joints including temperature gradients by application of high load currents. The ageing has been electrically characterised by variable partial discharge measurements in the frequency range of 10 mHz to 100 Hz up to 12 kV (2 U <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> ). The joint material was examined by microscopy examinations after ageing. The results show that the partial discharges are strongly frequency and voltage dependent. For shorter ageing times of the rubber joints, the number of discharges per voltage cycle decrease to zero for frequencies below 10 Hz. At longer ageing times resulting in severe material ageing, the number of discharges per voltage cycle strongly increases by decreasing frequency. In this case the inception voltage at 0.1 Hz was equal or lower than that measured at higher frequencies. It is observed by SEM that especially the stress-control material was strongly degraded by the high temperature conditions.

Green hydrogen, produced through water electrolysis driven by Renewable Energy Sources (RES), is a compelling solution for storing renewable energy. Directly coupling RES with water electrolyzers enhances system efficiency and cost-effectiveness. Alkaline water electrolysis, a mature and cost-effective technology, faces challenges operating under fluctuating RES conditions using AC/DC convertor , which supply rippled DC. A comprehensive understanding of electrolyzer performance in dynamic conditions is crucial for developing durable, efficient systems.In the present experimental study, we extensively explored the performance of alkaline water electrolyzers and the key factors influencing it under dynamic operational conditions. The electrolytic cell was constructed using a zero-gap configuration, comprising a NiCoOx-based anode deposited on a Ni-mesh and an Ln-doped RuOx cathode deposited on a fine mesh of Ni metal. The Zirfon Perl UTP500 membrane served as a separator between the two cell compartments [1]. A 7.0 Molar KOH electrolyte at a room temperature of approximately 30°C was employed. The cell underwent dynamic operational mode through the application of a square wave cell voltage, shown in Fig.1 (a), utilizing the NF BP4620 power supply. Square waves with different frequencies, specifically 1, 100, 1000, and 3000 Hz, were employed. Steady-state operations were conducted while applying various DC voltages, serving as a reference study. The resulting current and the actual cell voltage waveforms were monitored and recorded using a high-resolution oscilloscope (YOKOGAWA DL850E). The flow of H2 gas was measured using a film flow meter (HORIBA STEC, SF series VP-3). The apparent power, real power, and reactive power components were calculated using formulas from a previously published report [2]. Subsequently, the specific energy consumption was determined for each condition.The applied cell voltages were categorized into two groups. In the first category, the maximum cell voltage (Vmax) was set at 2.0 V, a value sufficiently high and suitable for industrial cell operation when applied in a DC mode. Within this category, the minimum cell voltage (Vmin) took various values, namely 1.8, 1.6, and 1.4 V. In the second category, the average cell voltage was maintained at a constant 2.0 V, while the amplitude of the waveform was adjusted to 0.2 and 0.4 V by modifying both Vmax and Vmin. Figure 1(b) depicts the resulting current of the voltage waveform at 100 Hz, illustrating three different Vmin values. The figure reveals that at a high Vmin of 1.8V, the current consistently maintained positive values during the voltage cycle, indicating that the double-layer capacitance remained charged throughout the operational period. However, with Vmin set at 1.6V, a relatively small capacitive current emerged, suggesting partial discharge of the double-layer capacitance. Moreover, at a lower Vmin of 1.4V, the cell voltage became insufficient to drive water electrolysis in the current cell. Consequently, a relatively large negative (capacitive) current manifested during the Vmin period, denoting a deep discharge of the double-layer capacitance.The obtained results revealed a profound influence of the charge and discharge status of the double-layer capacitance on the AWE cell's performance throughout the voltage cycles. Notably, when the double-layer capacitor remained fully charged during the voltage cycles, minimal impact of the frequency was observed on key performance parameters, including H2 flow rate that shown in figure 1 (c), specific energy consumption that shown in figure 1 (d), and power factor. However, under conditions of partial discharge within the cell voltage cycles, a noticeable correlation between the size and frequency of the voltage waveform and the AWE cell's performance emerged. Deep discharge of the double-layer capacitance led to a frequency-dependent reduction in H2 flow rate, attributed to incomplete charge/discharge cycles. Hence, the critical factor influencing the performance of the electrolytic cell during dynamic operation is the condition (charging/discharging) of the double-layer capacitance. These findings offer valuable insights into optimizing the dynamic operation of AWE cells for enhanced efficiency and performance. Acknowledgments: This research was partially supported by New Energy and Industrial Technology Development Organization (NEDO).

The partial discharge (PD) leads to catastrophic failure of the medium voltage (MV) switchgear insulation. Determination of the PD source (defect) location in high voltage (HV) equipment is very important in the maintenance procedures and in isolating the root cause of PD generation. In this paper, the transient earth voltage (TEV) detection method was used to acquire defect-initiated PD signals in a simulated MV switchgear model. An array of four TEV sensors were placed on the surface walls outside an MV switchgear tank to acquire the PD signals generated from the known location(s)/coordinates of sharp needle type defect inside the tank. The time difference of arrival (TDOA) between signals that are captured by the TEV sensors array was critically analyzed. Estimating the TDOA between PD signals generated by PD source at a known location with high accuracy is of great importance for accurate defect localization. The cumulative energy method (CEM) is used to estimate the onset time point of each TEV signal. The estimated TDOA by the cumulative energy method is compared with actual and expected TDOA based on known coordinates of PD source and TEV sensors. Experimental data are used as a basis for determining the TEV method accuracy for PD source localization. Experimental results show the average error of time difference is about 1.34 ns, which is equivalent to the propagation distance of 0.4 m.

Location of a partial discharge (PD) fault inside a transformer can be determined using the Ultrahigh Frequency (UHF) PD measurement technique. Hereby, the electromagnetic (EM) waves that propagate due to the PD are captured using specially designed probes that are inserted into the transformer tank through its oil drain valves. The difference in the arrival time (AT) of these signals is the base of an equation system, where the PD location and the PD inception time are unknown. In this contribution, the influence of erroneous determined ATs on the localization error is investigated using a two-dimensional analogy model. Furthermore, a novel method based on detection of abrupt changes in a certain statistical property of the signal for the AT determination is proposed. A test setup is utilized to evaluate the localization algorithm. Finally, the results are presented and the precision of the proposed method for PD localization using the UHF measurement technique is discussed.

The acoustic emission (AE) method could be used to detect and locate partial discharges (PD) in cast-resin dry-type transformers. However, due to the high sound attenuation in the filled epoxy, the signal is prone to interference from external noises and thus, in practice, there is little possibility of detecting PD. In this study, two techniques were developed to alleviate the shortcomings of the AE method. First, a waveguide is installed on the high-voltage (HV) windings, so that the acoustic signals of PD will propagate to the AE sensors that are installed on both terminals of the waveguide. The location of the winding that has PD can then be detected from the difference in arrival time of the acoustic signals. Test results indicate that the waveguide technique is able to enhance the safety of a measurement system and offers the advantages of easy installation and higher flexibility. Second, a specially designed AE sensor pair is used to distinguish whether acoustic signals are generated by PD inside the HV winding or by the corona outside the transformers. Using these two techniques of waveguide and AE sensor pair not only greatly improves sensitivity but also increases the reliability of the measurement system. Practical test results show that the new techniques can be used to locate precisely the PD in HV windings.