Influence of Field Grading in Setup for Electric Breakdown Testing of Polyethylene Films

High dielectric strength of solid electrical insulation materials for high voltage applications is essential for high reliability and long-term performance.The IEC 60243 and ASTM D149 both describe very similar test methods to determine the short-term electric strength of solid insulation materials. A test sample, usually thin plaque, is placed in an electrode system with surrounding insulating oil. The voltage is then steadily increased until an electric breakdown occurs. Despite the relatively simple test setup, testing materials with high electric strength can be difficult. The breakdown channel is often located outside the active testing area of the electrodes and found at the electrode edge at the triple point between the electrode, test object, and surrounding oil.In this study, we have investigated different possibilities to control the electrical field enhancement in the vicinity of the electrode edges using silicone rubber, field grading silicone rubber, and a high permittivity oil. The testing was performed with semi-spherical electrodes and electrodes as recommended in IEC 60243 on polyethylene films. Electrodes covered with the field grading rubber increased the short-term breakdown strength compared to standard testing without modification. The high permittivity oil and silicone rubber seemed to have limited effect on the breakdown strength.

Performance of UV and IR Sensors for Inspections of Power Equipment

Electric power infrastructure, such as transmission lines or substations, is usually routinely inspected to assess its condition. The vast majority of typical defects in power transmission equipment manifests itself either through corona phenomena or through thermal effects. Therefore, an IR camera and a solar blind UV camera are sufficient for the detection of most defects in power transmission equipment. In the past, many network operators have relied mostly on manual inspections. In recent years, however, manned as well as unmanned aerial inspection methods, which are significantly more time effective, have become increasingly affordable and are therefore gaining in popularity rapidly.To obtain meaningful measurement results, many factors must be taken into account, which can even be difficult with conventional, static measurements. In the case of highly dynamic measurement practices (airborne or vehicle based), the combination of velocity and distance presents further challenges.This contribution is focused on the detection performance of UV and IR sensors under dynamic conditions. For this purpose, experiments were carried out with a typical IR and UV/corona camera at various distances to artificial defects. Additionally, a method for the automatic evaluation of UV und IR data based on machine learning is presented.

Silicone Gel Compounds for Non-Linear Stress Grading Applications

The paper deals with a field-grading insulation usingsilicone gel as main material/binder. Additives, which providethe nonlinearity of the compounds, consist of a support materialwith metal oxide coating. The nonlinear electrical propertieswere adjusted by the addition of different dopants. In thisinvestigation the fundamental properties of new non-linearconductivematerials (NLCM) were examined. One aim of theinvestigation is to attribute the macroscopic electrical behaviorto the respective doping. For a typical compound, fieldcalculations for a cable joint model are presented and comparedto a conventional design without NLCM.

Comparison of Low and High Frequency Partial Discharge Measurements on Stator Windings

Partial discharge (PD) testing has been used for over 60 years primarily as a method to assess condition of the stator winding insulation in motors and generators rated 6 kV and above. More recently it has also been used by some machine manufacturers as a means of assuring the quality of the insulation on single winding elements (coils and bars). Although both on-line and off-line tests mainly use a high voltage capacitor to detect the PD, the PD measuring systems in use work either in the low frequency (LF) regime (less than about 1 MHz) or in the very high frequency (VHF) (30-300 MHz) range. By reference to several international standards, published work as well as some experiments described in this contribution, the advantages and disadvantages of the two approaches are compared. Based on this work, it is now clear that off-line PD tests should be done in the LF range. For on-line tests, either method may be used, but use of the VHF method has become more widespread with machine end users, since the owners themselves can perform and interpret the results with a relatively low risk of false indications.

Cable Capacity Enhancement by Utilizing Trapezoidal Voltage Waveform

To avoid installation of expensive new underground cable connections in locations where peak load exceeds existing cable capacity, it is advantageous to transfer more power through a cable than its nominal power capacity, without endangering its reliability. Dynamic current rating of cables is a typical approach that is used to exceed the nominal cable capacity for a short time period. In this paper a new method for cable capacity enhancement based on dynamic voltage rating is introduced. The method can be applied if multilevel converters are installed at both ends of cable as will become more commonplace in the future when an inverter rich power system is realized. In this study the influence of trapezoidal voltage waveform on the electric field distribution inside cable insulation is investigated. The results shows that by using trapezoidal waveform it is possible to achieve a more homogeneous field distribution inside the cable insulation. This enables better utilization of the insulation system which translates into higher continuous power transfer capacity.

Effective surface functionalization of fumed silica with NBR telechelic oligomer for high-voltage polyolefin-based dielectric composites

In this study an effective surface functionalization of fumed nano-silica for dielectric insulation composites is developed. A three step modification procedure was applied: 1. Silanization of the silica surface with (3-glycidoxypropyl)trimethoxysilane (GOPTMS); 2. Grafting of an acrylonitrile-butadiene telechelic oligomer terminated with two carboxylic acid groups (o-NBR); 3. Hydrogenation of the unsaturated polybutadiene segments for better compatibilization with the saturated polyolefin matrices and for higher dielectric performance.The deposition of the coupling system was monitored by thermogravimetric analysis (TGA) revealing 6.4 wt. % of GOPTMS and 13.6 wt. % of o-NBR bond to the silica, resulting in an exceptionally high total deposition (20.0 wt. %). The o-NBR grafting was analyzed qualitatively by Fourier transform infrared spectroscopy (FTIR) showing successful deposition of o-NBR and effective hydrogenation.The silica modified according to this procedure is a promising functional nano-filler for high-voltage insulation composites allowing improved dispersion and distribution in polyolefine matrices and therefore enhancing their electrical properties.

Time-Dependent Electric Field Distribution in Layered Paper-Oil Insulation

Layered paper–oil insulation is used in several types of HVDC equipment. In order to better understand breakdown mechanisms and optimize the design, it is important to understand the electric field distribution in the insulation. In the present work, a test object with such insulation has been modeled as a series connection of oil and impregnated paper. The permittivity, conductivity, and the dielectric response function has been measured for impregnated paper and oil separately and used as parameters in a dielectric response model for the layered insulation system. A system of differential equations has been established describing the voltages across each material, i.e. across each layer of the test object. These equations have been solved considering a DC step voltage across the whole test object. Based on this, the time-dependent electric field in each material as well as the time-dependent polarization current density in the test object have been calculated. The calculated polarization current density was found to agree well with the measured polarization current density of the test object. This indicates that application of dielectric response theory gives a good estimate of the time-dependent electric field distribution in layered insulation systems. The results show that 90 % of the change from initial values to steady-state values for the electric fields has occurred within the first 35 minutes after the voltage step. This applies to the electric fields in both of the materials of the examined test object at a temperature of 323 K.

An Electrode Setup for Non-contact Dielectric Response Measurement

The dielectric response measurement is a widely used technique for characterizing dielectric materials. However, the contact problems between samples and electrodes existing in the use of conventional electrode setup limit the accuracy of the measurement. This paper studies an electrode arrangement that avoids direct contacts of the sample with the electrodes on both sides. The edge effect of this arrangement is calculated by the FEM model. The equations to derive the complex permittivity are presented. The measurement instrument is described and the influence of the small voltage between its measure and guard terminals is analyzed. The error sensitivities of the non-contact and contact methods are compared. The results show that this electrode arrangement can be used to perform non-contact measurements, the edge effect of it is not significant, and the guard voltage is not an obstacle to obtain results with high accuracy. Overall, the non-contact electrode arrangement combined with the IDAX 300 can potentially improve the accuracy of dielectric response measurements although the non-contact methods can increase the sensitivities to errors.

Influence of Standard Lightning Impulse Front Time Tolerances on the Flashover Voltage of Suspension Insulators

For high voltage impulse testing, a standard lightning impulse is defined in IEEE Std. 4 and IEC 60060-1 as a double exponential waveform having a front time T1 = 1.2 μs ± 30% and time to half-value T2 = 50 μs ± 20%. It has been noticed that for a given specimen, it is possible to successfully pass a flashover test at one end of the T1 tolerance range while failing the same test at the opposite end of the tolerance spectrum. Consequently, a systematic approach was adopted to investigate this observation. Up-and-down tests were performed to define the disruptive discharge voltage (critical flashover voltage CFO, U50) for 1, 5, 10, and 15 unit glass insulator strings standard lightning impulses using the minimum acceptance front time value (T1 ≈ 0.84 μs). Tests were repeated using the maximum tolerance value (T1 ≈ 1.56 μs) to investigate the degree of divergence in the flashover value. Particular attention is given to the steepness (voltage-time characteristics) of the applied impulse to consider if tolerance criteria amendment is merited in a future standard revision. As the steepness impact is more renowned in non-uniform geometries, field homogeneity as a function of string length is also incorporated into the analysis.

Surface Hydrophobicity in Medium Voltage Terminations

The surface hydrophobicity in medium voltage terminations is studied in this paper to see if the performance of aged heat shrink terminations could be improved. Terminations which are used inside of unheated outdoor enclosures can often be subjected to leakage currents and surface discharges along the insulating surface if the environmental conditions inside the enclosure become harsh. The discharge phenomena on the insulating surface is generally not critical but continuous electrical activity on the surface could eventually lead to a flashover or breakdown as the materials age. The effect of surface hydrophobicity was studied by comparing the performance of heat shrink terminations over a longer time in operation. Improvements were done to aged heat shrink indoor terminations which already had signs of discharge occurrence on the termination surfaces. The results indicate that the performance of these aged terminations improved simply by increasing the surface hydrophobicity of the original installation. The improvement can be done by applying a silicone coating on the termination surface. This solution offers a convenient method for improving the performance of aged heat shrink terminations used in unheated outdoor enclosures.