Proceedings of the Nordic Insulation Symposium

On Field Monitoring Design

Tord Bengtsson — 2022-07-05

Transforming a diagnostic technique into a monitoring product imposes a large number of challenges, both known and unknown. This paper will discuss a number of the known technical challenges that has been encountered in our history, like considerations on system properties, sensors, platform choice and algorithm design. To detect the unknown challenges as early as possible, field prototypes and pilots are needed and considerations on those are discussed.
The paper ends by discussing some aspects of monitoring applications that we have designed in the past in light of the presented challenges.

Partial Discharge Behavior of Epoxy-Mica Insulation System under Superimposed AC and DC Voltage Stress

Thomas Linde — 2022-07-05

Recent technology advances in wind energy conversion systems include a new type of permanent magnet synchronous generator with a segmented stator that allows lightweight concepts with high efficiency and superior controllability. Each stator segment is driven by a separate power converter. The various cascaded power converters can be connected in series and in parallel, which ultimately leads to the superposition of a DC and an AC voltage stress in the insulation of the stator coils. The resulting DC-AC mixed voltage is an entirely new electrical stress for the epoxy-mica insulation system that is typically utilized as main insulation of rotating electrical machines. In order to resemble this stress, typical dielectric tests such as partial discharge (PD) measurements and long-term aging of model insulation systems have to be be carried out under DC-AC mixed voltage stress. The mentioned investigations are presented in this contribution. The results indicate that the AC voltage is dominant with regard to the PD inception and influences the PD parameters such as the apparent charge and the repetition rate substantially when compared to the DC voltage proportion. The accelerated aging tests with significantly higher AC-DC mixed voltage stress than is expectable under operating conditions revealed no measurable PD deterioration of the investigated insulation systems.

Possibilities of Conventional PD Measurements with Non-Sinusoidal Waveforms for Electric Vehicles

Maurizio Zajadatz — 2022-07-05

Electrical traction machines in electric vehicles are normally fed by converters with DC link voltages up to 800 V. The resulting voltage pulses place particular stress on the insulating system of the drivetrain. In order to be able to investigate insulating material samples, e.g. twisted pair enameled wire, with voltages of different shapes and high frequency, a shielded experimental
setup for the investigation of partial discharges (PD) at low voltages and high frequencies is presented. A medium frequency transformer with a frequency range up to 2500 Hz is used for this purpose, fed by a linear power amplifier on the primary side. The amplifier has a slew rate of 52 V/μs and is capable of sourcing various voltage waveforms such as sinusiodal, triangular
or rectangular, with a maximum frequency of 30 kHz. Electrical PD measuring methods according to IEC 60270 as well as acoustic and optical measurement methods are applied for PD diagnosis. The experimental setup is intended to demonstrate the possibilities and limitations of conventional PD diagnostics for nonsinusoidal voltages. Focus is placed on the application of filters, the damping behavior of the step-up transformer and the rise times of the voltages.

On-site Measurement of Complex Waveforms on Transmission Network Components

Peter Wouters — 2022-07-05

Superimposed voltage waveforms may result in electrical stresses in dielectric materials not anticipated for. Non-intrusive ethodologies to observe these events, both capacitively and inductively, are discussed. This paper presents two examples where complex waveforms arise from energizing and de-energizing inductive loads. Case I: A compensation coil connected through overhead lines showed high overvoltage levels when being energized by means of a SF6 switchgear. Case II: A compensation coil connected via a power cable with a vacuum switchgear experienced high overvoltage levels from recurrent restrikes during its de-energization.

Non-contact Voltage Measurement Technique for On-Line Monitoring of Transient Overvoltages

Durga Pawan Mahidhar Gorla — 2022-07-05

The electric power system is continuously growing in its size, complexity and required reliability. There is an increased risk due to transient overvoltages which can deteriorate the insulation for different power system components. There is a need for monitoring of transient voltages to better monitor the effects due to them on the insulation of the components. It can be achieved using a non-contact voltage measurement technique. The study elucidates the theory and practical application of the non-contact voltage measurements. The measurement of voltage is achieved by utilizing the stray parasitic capacitance between the high voltage conductor and the ground. A metal plate is used as a sensor to detect the voltage, which indirectly acts as a capacitance divider for voltage measurement. The current study uses operational amplifier based differential-integrator circuit topology in order to accurately measure the voltage over a wide bandwidth of 20 Hz - 1 MHz. The measurement technique is used for measurement of three phase voltages and a methodology is proposed for it. The sensor system is also tested in an online test scenario in a substation for monitoring the shunt reactor switching transients.

Partial Discharge Evolution under Half-sine Voltage Excitation

Gan FU — 2022-07-05

Partial discharges (PD) analysis is the common technique for detection and identification of dielectric defects. Certain PD characteristics can be an early sign of degradation. For further understanding, combined voltages can enhance the analysis of PD characteristics in some specific cases. This paper presents an experimental study on the PD appearance under repetitive negative half-sine voltage excitation, as such a combination of AC and DC voltage. Adopting the time-resolved PD pattern technique, PD signals originated from dielectric barrier corona discharge (DBCD) source are recorded in time domain by a DL750 Scope Corder. Preliminary filtered results indicated the strong relationship between surface charge accumulation and back discharge, a time delay movement of back discharge can be clearly observed on the pattern during ‘relaxation time period’ with various insulation materials.

Review of Water Treeing in Polymeric Insulated Cables

Amar Abideen — 2022-07-05

Since discovering the water treeing phenomenon (WT) in polymeric cables in the early 1970s, water treeing has been presumed to be the main cable degradation mechanism for polymeric cables and has been extensively researched up to the late 90s. Historically, different theories were proposed to describe this phenomenon's mechanism. The two most prominent theories link the initiation and growth of WTs to (i) Environmental stress cracking (ESC) and (ii) Stress-induced chemical degradation (SIED). Additionally, different experimental investigations were conducted to highlight the correlation between different operating conditions and the initiation and growth of WTs. Despite that it has been challenging to determine or agree upon a particular water treeing mechanism, it is generally accepted that there are main factors that can influence the inception and growth of water trees and can possibly steer the water tree growth mechanism to adhere ESC or SIED theories. This paper presents a comprehensive review of different selected papers and technical reports between 1969-2020 on the topic of water treeing in polymeric cables with an emphasis on the development of standing theories and operation factors that influence the initiation and growth of WTs.

Electric Field Modelling and Simulation of the High-voltage High-frequency Transformer Insulation Applied for Power Electronics Based on Circuit-field Coupling Analysis

Zhaoxin Wang — 2022-07-05

This paper established a transient electric field model of a high-voltage high-frequency (HVHF) transformer based on circuit-field coupling analysis. As the key component of power electronic transformers (PETs), the operating conditions of the HVHF transformer are determined by the converter circuit containing it. To find out the electrical stress withstood by the solid insulation, it is essential to model the transient electric field while considering the circuit model of the converter. The circuit model of the converter and the 3-D transient electric field model based on the finite element method (FEM) are built in ANSYS. The voltage and current waveforms on both primary and secondary sides of the HVHF transformer obtained from the converter model are coupled to the transient electric model as the excitations. Based on the established model, a case study of a 60 kHz HVHF transformer used for the dual active bridge (DAB) converter is investigated, and the electric field characteristics of the insulation are analyzed. The electric field distribution inside and on the surfaces of the insulation is present, and magnitude values of the highest electrical stress inside and on the surfaces of the insulation are identified. The theoretical verification is carried out based on the grid convergence index (GCI) method and it verifies the accuracy of the proposed model. This model could be used to calculate the overvoltage capacity of the HVHF transformers in various power electronics applications and it is an essential step in building the digital twin model of the HVHF transformer.

Longitudinal Breakdown Strength of Wet-mate Solid-Solid Interfaces at VLF and 50 Hz AC voltages

Erling Ildstad — 2022-07-05

The 50 Hz AC breakdown strength of dry interfaces is known to strongly depend upon the mechanical properties, contact pressure, roughness of the surfaces, and the type of lubricant used at the interface. This paper aims to experimentally examine how these factors affect the longitudinal AC breakdown strength of interfaces assembled in water, so-called wet interfaces. The main aim is to obtain data relevant to the design of power equipment operating at very low frequency (VLF) or DC voltages.
Experiments were conducted using identical specimens made from 4mm thick plaques of PMMA and plane sections cut from XLPE cable insulation. The findings were discussed with respect to expected dimensions of interface voids and contact regions, considering tribology-based contact theory, including the impact of surface roughness, modulus of elasticity, and applied mechanical interface pressure.

The longitudinal 50 Hz AC breakdown strength values of wet samples were typically as low as 80 % of samples assembled in the air under dry conditions. In addition, the results verified previous findings that the AC breakdown strength strongly increases with reduced surface roughness, stiffness, and increased interface pressure. The breakdown values obtained during VLF breakdown testing were found to be 2 – 3 times higher than in the case of testing at 50 Hz AC voltages.

Hybrid Method for Numerical Implementation of Segmented Power Cable Conductors in Finite-element Based Ampacity Calculation

Henrik Strand — 2022-07-05

This paper addresses challenges with modelling of segmented power cable conductors using finite element analysis (FEA) for ampacity calculation. Segmented conductors improve current distribution by minimizing skin and proximity effects, thus reducing conductor losses. 2D FEA simulation offers high flexibility and accuracy beyond IEC 60287 for complex laying geometries, but the modelling of losses in segmented constructions using FEA has proven difficult due to the big difference in wire size and twisting pitch, requiring great amounts of computational power. In this paper a hybrid method is proposed, in which the IEC 60287 empirical formulae for segmented conductors are included in a 2D FEA model. The proposed method shows a good correspondence to IEC standard calculations, with deviations in conductor AC resistance of less than 1 %.

Pre-Breakdown Phenomena in Technical Air with and without C5-Fluoroketone for a Rod-Plane Gap

Fanny Skirbekk — 2022-07-05

Technical air with a few %vol C5-fluoroketone (C5-FK) is one of several alternatives to the potent greenhouse gas SF6 in medium voltage (MV) gas insulated switchgear. MV products using this gas mixture are already commercially available, but there is still a lack of knowledge about this gas mixture especially regarding pre-breakdown phenomena. In this study, positive and negative streamers are studied in technical air with 7.5 %vol C5-FK at 1.0, 1.3, and 1.5 bar (absolute pressure). Similar tests were done in technical air (80% N2, 20% O2) for comparison with the C5-FK results. The streamers emerged from a grounded needle electrode placed 53mm below a planar electrode. The electrode setup was stressed by a non-standard lightning impulse voltage with voltage peaks varying from 52 to 129 kV. A photomultiplier tube (PMT) was used to register the streamers, and an iCCD camera was used to image the streamers. The pre-breakdown phenomena in C5-FK+air were found to be shorter, less than 1/5 of the length, and to branch less than in pure technical air. The streamer branches in both gas mixtures became wider and less clear when the pressure was decreased. Leader-like channels were observed for C5-FK+air at 1.5 bar (both polarities) and at 1.3 bar (negative streamers).

AC Breakdown Voltage of 50-Year-Old Service Aged Hydro Power Generator Stator Bars

Torstein Aakre — 2022-07-05

Randomly selected generator bars from a 50-year-old Norwegian hydro power generator were examined in this work. Typical non-destructive tests, as partial discharge and dissipation factor measurements were initially performed at both 50 Hz and 0.1 Hz. Then, the AC breakdown strength of the epoxy/mica/glass fibre reinforced bar insulation was measured by gradually increasing the voltage in steps of 5 kV with duration of 1 min until breakdown occurred. The field graded terminations were soaked in transformer oil during breakdown testing to prevent external surface flashovers. The AC breakdown voltage ranged between 60 kV and 75 kV, which corresponds to 8-10-fold the service voltage of 7.4 kV. No significant difference in breakdown voltage was observed between bars being located close to the high voltage or neutral terminal during service. This indicates minor degradation caused by the AC service stress, even after 50 years in service. No correlations were found between the measured breakdown voltage and the diagnostic partial discharge activity and dissipation factor measurements.

Advances in Interpreting On-Line Partial Discharge Test Results on Stator Windings

Mladen Sasic — 2022-07-05

On-line partial discharge (PD) testing has been used for decades to help maintenance personnel detect rotating machine stator winding insulation problems at an early stage and thus optimally plan any required maintenance. Specifically, the test can find loose, overheated, and contaminated windings, well before these problems lead to failure. The test has also found use in determining the effectiveness of any repair work.
The widespread application of on-line PD testing has enabled the accumulation over the past 20 years of a database with, currently, approximately 700,000 test results. This proposed contribution presents the basics of on-line data collection, including the implementation of noise rejection algorithms and possible methods in improvements in interpreting test results. One case study describing application of on-line partial discharge testing on a hydro generator will be presented.

Use of Data-Driven Approaches for Defect Classification in Stator Winding Insulation

Emre Kantar — 2022-07-05

Partial discharges (PD) in the high voltage insulation systems are both a symptom and cause of terminal and impending failures. The use of data-driven methods based on PD measurements will enable predictive strategies to replace traditional maintenance strategies. This paper employs machine learningbased classification models to identify and characterize
PD signals originating from lab-made artificial defects in epoxy-mica material samples. Three different PD sources were studied: surface discharges in air, corona discharges, and discharges caused by internal cavities/delaminations. To generate high-quality datasets for the training, validation, and testing of classification models, Phase-Resolved PD (PRPD) data for each test object was obtained at room temperature under 50 Hz AC excitation at 10 % above the PD inception voltage (PDIV) of each sample. Relevant statistical and deterministic features were extracted for each observation and were labeled based on the defect type (supervised learning). Finally, the trained and validated ML models were used to identify PD sources in the service-aged stator winding insulation. Support vector machines (SVM), ensemble, and k-nearest neighbor (kNN) algorithms achieved significantly high accuracy (≥ 95 %) of defect identification.

Comparison of Methods to Detect Thermomechanical Ageing of the Insulation System for Rotating High-Voltage Machines

Lena Elspass — 2022-07-05

Increased dynamic operation of long rotating high-voltage machines as well as elevated operating temperatures lead to intensified thermomechanical stress in the insulation system of global vacuum-pressureimpregnated machines. Meanwhile, the requirements regarding reliability of the machine and the electric insulation system remain high. Consequences of thermomechanical stress include delaminations and abrasion. To satisfy the high standards of longevity, reliable diagnosis of thermomechanical ageing is essential to allow manufacturers to develop and improve countermeasures. This work identifies diagnostic tools, which investigate the effects of thermomechanical ageing on model replicates of machine insulation systems. The longitudinal thermal expansion of the conductor during dynamic operation is replicated by applying mechanical force to the conductor of specimens, thus inducing mechanical stress in the insulation system. Recurring measurements of partial discharges, dielectric losses and capacitance are evaluated regarding their sensitivity in detecting resulting ageing phenomena. The study reveals that partial discharge measurements detect preliminary damages before insulation rupture caused by mechanical
stress occurs. Knowledge of these capabilites enables future-oriented development of insulation systems for dynamically-operated long rotating machines.

Multi-stress Cyclic Testing of Roebel Stator Bars for Hydropower

Espen Eberg — 2022-07-05

A multi-stress test rig was built to investigate the effect of many start-stops (load cycling) on hydropower generator insulation. To emulate real-life start-stops, stator bars were subjected to accelerated temperature cycling by circulating a 3.5 kA current and then cooling down with fans while high voltage (service voltage) was applied simultaneously to the insulation. A total of 250 load cycles were applied. Partial discharges (PD) were recorded on-line during load cycling, and after every 50 cycles, off-line dielectric loss and PD measurements were conducted. There was an apparent increase in dielectric losses after they had been subjected to a total of 250 load cycles. During load cycling, the PD level generally decreased with temperature, but there was a temporary increase in the PD activity during temperature rise and fall. This trend can be explained by different thermal expansion in copper and insulating material (epoxymica), suggesting that a stator bar that is load-cycled will be exposed to substantially higher PD levels than a stator bar under a uniform load and temperature, leading to further deterioration of the insulation quality.

Selective PD Measurements on DC Cable Joints Using a HFCT-Balanced Circuit Arrangement

Bernhard Schober — 2022-07-05

Due to the many advantages of medium-voltage (MV) and high-voltage (HV) DC cable transmission lines and the possibility of converting AC transmission to DC transmission, this technology is becoming increasingly relevant. One of the most important diagnosis tools used to investigate the condition of the insulation of highvoltage cables is to apply partial discharge (PD) measurements. Cable joints and terminations are of particular importance, as these represent weak spots due to human interaction. Especially under DC voltage, it is of crucial importance to determine whether the detected PD pulses occur inside or outside the cable accessories.
For this purpose, a PD test method for cable joints using two high-frequency current transformers (HFCT) was optimized with modern electronic devices, including an instrumentation amplifier and a modern PD measuring instrument. Two HFCT with opposite winding directions were placed on the outer semi-conductive layer on both sides of a cable joint. The signals were fed into an instrumentation amplifier to separate the PD pulses originating from inside and outside the cable joint. This method automatically suppresses PD pulses occurring outside the cable joint and was tested on a 12/20 kV AC cable joint under DC voltage and with thermal and electrical stress.

MVAC XLPE Cables for MVDC – DC Conductivity of Plaque Samples During Temperature Changes

Patrik Ratheiser — 2022-07-05

The rising demand for electric energy poses increasing challenges for the distribution grid. To overcome these challenges, direct current (DC) transmission will play a dominant role in the medium voltage (MV) level. On the one hand, MV alternating current (AC) cable systems can be converted into MVDC cable systems and, on the other hand, new MVDC cable systems can be implemented into the existing grid.
However, MVDC cables with extruded insulation show special phenomena (e.g. space charges). Due to the interest in characteristics of this insulation this paper provides information about the DC conductivity of XLPE plaque samples during steady thermal and electric stress and during temperature changes. The plaque samples are extracted from 12/20 kV MVAC XLPE cables. The DC conductivities at steady state and during temperature changes are presented in a temperature range from 25 °C – 90 °C and an electrical stress up to 30 kV/mm.

Efficient Sensitivity Calculation for Insulation Systems in HVDC Cable Joints

M. Greta Ruppert — 2022-07-05

Many high voltage direct current cable systems include field grading materials with can be mixed such that the properties can be tailored to the particular configuration. This comes with a drastic increase in the number of design parameters to be considered in the nonlinear transient electrothermal finite element model.

This paper introduces the adjoint variable method to efficiently calculate the sensitivities of the quantities of interest with respect to a large set of design parameters.

Breakdown Voltage of Polymeric HVDC Insulation at DC Stress and Superimposed Lightning Impulse Voltages

Erling Ildstad — 2022-07-05

Space charge formation makes it difficult to predict the resulting electric field distribution within HVDC cable insulation.
The main purpose of the work presented here has been to experimentally study how the lightning impulse breakdown strength of thin (0.1 mm thick) insulation foils of polypropylene (PP) and Polyethylene Terephthalate (PET) are affected by DC prestress, polarity of the impulse voltage and the temperature.
Tests were performed using parallel plate electrodes and prior to DC prestressing the lightning impulse breakdown strengths were found to be approximately 17.5 kV for samples of both PP and PET. Prior to superimposing lightning impulse breakdown voltages, a DC voltage of 8 kV was applied for 60 seconds. The effect of DC prestressing was to significantly increase the impulse breakdown strength during aiding and reduce the breakdown strength during opposing impulse polarity. This strongly support the assumption of homo-charge formation close to the metal electrodes during DC stress.
The results showed higher and more temperature dependent charge accumulation in samples of PET. It is indicated that at 22 °C the homo-charge formed during DC stressing approximately reduced the electric field at the electrodes from 100 to 48 kV/mm in the PP samples and from 80 to 22 kV/mm in the PET samples, respectively

Lifetime of Oil-Impregnated Paper under Pulse Stress at Different Frequencies

Philip Mathew — 2022-07-05

With the increasing penetration of power electronic interfaces in the power grid, insulation materials will begin to experience stresses at higher frequencies than the conventional 50 Hz AC. This article studies the lifetime curves of oil-impregnated paper (OIP) under pulsed stresses and compares them at 10 kHz and 50 kHz. A pulse modulator is constructed consisting of a rectified DC supply feeding an H-bridge pulse driver connected to a 4:200 pulse transformer. The modulator is used to apply medium voltage pulse waveforms with rise-times of T_r ≈ 1.8 µs across single-layer OIP samples. The results clearly show that an increase in pulse frequency significantly accelerates insulation ageing. However, it is also observed that below a certain threshold of field strength, the slope of the lifeline decreases dramatically thereby indicating decelerated ageing. Possible reasons for this phenomenon are also discussed in this article.

Improvement and uncertainty evaluation of a 1 MV lightning impulse measurement system

Lauri Aaltonen — 2022-07-05

This paper focuses on development of an existing 1 MV lightning impulse (LI) measuring system as part of European project for i.a. LI calibration capabilities development. After carefully characterization of the original LI divider a new low voltage arm was designed and built with e.g. improved shielding and also RC circuits for compensating the detected drift in the step response. With the compensation step response was successfully corrected resulting in clear improvements in LI time parameter measurements. However, persistent high frequency noise was present in the step response complicating further fine-tuning of the response. The noise could not much be improved at least partly due to non-optimal damping resistor structure, which is difficult to improve afterwards. After the improvements an extensive uncertainty evaluation was carried out for the original system as well as for two versions of the improved system.

The Usage of High Voltage Amplifiers to Set up Reference Calibrators for Combined and Impulse Voltages up to 1 kV

Mohamed Agazar — 2022-07-05

An approach to set up reference calibrators to generate combined voltages, using voltage amplifiers, has been demonstrated. Four high voltage amplifiers were tested and studied. Measurements and experiments have shown that this approach could be used to set up calibrators for combined voltages, at least up to 1 kV. The advantage is the flexibility to generate separate or combined wave shapes (combination of AC, DC, distorted shapes, double exponential impulses such as lightening and switching impulses). This method could be beneficial to the fields where impulses, combined or not with DC or AC voltages, need a reference calibrator to calibrate digitizers. This method, relatively cheaper and simple to develop, reaches high metrological performance. For example, for lightning impulses up to 900 V, the uncertainties are lower than 0.2 % for the peak voltage, 1 % for the front time and 0.5 % for the time to half value . The traceability to the international system of units can be ensured by characterizing the gain using a step response, followed by the convolution technique.

Universal Measuring Unit for High Voltage Measurements

A Khamlichi — 2022-07-05

Traditionally, high voltage (HV) laboratories have required different measuring instruments to measure alternating voltages 50/60 Hz (AC), using peak voltmeters, lightning impulses, through peak voltage meters or oscilloscopes and the apparent charge, by means of partial discharge (PD) measuring instruments. All of them with very different technical requirements stablished in applicable hardware and software standards (IEC 61083 series [1], IEC 60060 series [2, 3] and IEC 60270 [4]). However, the technological evolution of measuring instruments has allowed these measurements could be carried out through highperformance digital recorders. This article describes a new universal measuring instrument, developed by the LCOE for high voltage quantities, known as UMU (Universal Measuring Unit), for the five typical measurements to be carried out in any high voltage laboratory: AC 50/60 Hz, DC, Lightning impulse 1.2/50 μs (LI), Switching impulse 250/2500 μs (SI), partial discharge measurements, Radio Interference Voltage (RIV) measurements, transmitted overvoltage in secondary winding of voltage and current measuring transformers.
A new software package has been also developed in the framework of 19NRM07 HV-com2 project [5].

Optical Partial Discharge Detection in Insulating Substrates

Ivan Semenov — 2022-07-05

Photomultiplier tubes (PMT) in combination with flexible liquid light guides and lenses have been tested for partial discharge (PD) detection. The coincidence technique was used to eliminate the random noise of PMTs. Results of PD measurements in two different insulation systems, namely a needle-plane arrangement in air and a ceramic substrate in silicone liquid, are presented and discussed.

Design and Verification of a Calculable Composite Voltage Calibrator

Jussi Havunen — 2022-07-05

Components used in the power grid need to be tested to verify they can withstand specified voltage stresses. Composite voltage test according to IEC 60060-1:2010 is a test method where impulse voltage is applied simultaneously with AC or DC voltage to the same terminal of the test object. To provide traceability to instruments used for composite voltage measurements, low-voltage calibrator based on combining existing calibrators was developed. The developed composite voltage calibrator can generate lightning and switching impulses together with DC and AC. The calibrator design is based on a series connection of a DC/AC calibrator and a calculable impulse voltage calibrator. Maximum impulse voltage peak is 330 V whereas the DC/AC voltage can be within 1000 V. Reference parameters for composite waveshape are calculated based on the applied voltage, charging voltage of the impulse calibrator, internal impedance of the impulse calibrator, and the input impedance of the device under calibration. The main uncertainty components of this composite voltage calibrator are the uncertainties related to the separate calibrators and the uncertainty of the load impedance needed for the parameter calculation.

Streamer Propagation along Profiled Insulator Surfaces under Positive Impulse Voltages

Hans Kristian Hygen Meyer — 2022-07-05

Controlling discharge growth on insulator surfaces is important in high voltage gaseous insulation systems. In this study, the effect of small-scale surface profiles on streamer discharge propagation is examined experimentally. The experimental test objects were 5x72x150 mm polycarbonate plates with and without machined surface profiles. One test object had a surface with 0.5 mm deep semi-circular corrugations, while the other profile had 0.5 mm deep rectangular corrugations. The semi-circular profile increased the surface area with 20 %, while the rectangular profile increased the area with 110 %. A plain surface was also examined as a reference. Positive impulse voltages were applied to a 1 mm thick disk electrode placed 2 mm above the insulator. The insulator was placed in a grounded aluminium casing. The streamer development was imaged with a light-sensitive high-speed camera. Surface charges left on the surface after the impulse were examined using an electrostatic probe and simulations of saturation charge. The rectangular surface profile reduced the streamer range significantly, which suggests an effect of added surface area. Imaging indicated that the wavelike surface streamers follow the profiles closely. Surface potential measurements showed a saddle-shaped distributions, with values in line with saturation charge computations.

Health Index Analysis of Transmission System Components

Soumya Thakur — 2022-07-05

Optimizing maintenance strategies for power transmission components is crucial to prevent failure in the system while maintaining and enhancing the overall economic efficiency. Condition-based maintenance can fulfill this and in the present paper health index-based efficient condition assessment is applied as an essential input to this type of maintenance. A method for the calculation of the health index of primary transmission system components is presented and discussed. Condition-based assessment is utilized for developing a calculation method for evaluating the health index at different levels in the system. The health index of each component is calculated by using the Weighting and Scoring Method. Several factors affecting the condition of different equipment are considered in the algorithm including environmental effects, mechanical stress, and accessibility issues. The special focus is on assigning a numeric value to the health of individual sub-components of the equipment to plan the condition-based predictive maintenance of power system components. The data indicate that in contrast to considering the health index of the entire system, making decisions considering the individual componet health indices can result in a more reliable operation.

Characterization of Ion Transport Properties in Synthetic Ester Oil by Polarization Current and Dielectric Spectroscopy

Andrea Cremasco — 2022-07-05

The accurate estimation of the charge carrier mobility in synthetic ester oil-based insulation systems is required for the reliable prediction of the electric field induced by dc voltages. Some discordance is observed in the literature, therefore, this quantity is investigated by two alternative methods. First, the mobility is estimated in a material sample by measuring the polarization current and the ion time-of- ight during a voltage polarity reversal test. Next, the mobility is derived from the frequency-dependent loss tangent measured by dielectric spectroscopy. The values obtained by the two methods are consistent, in the range of ≈ 2-4·10^-10 [m2/Vs] at 303 K.

Diffusion Behavior of Greases in Interfaces in Organic Insulation Materials

Lucas Höfer — 2022-07-05

The diffusion of greases in organic insulation materials, which is the core parameter of the electrical and mechanical long-term behavior of the interface, is studied. The diffusion of grease depends both on the solid organic insulation material and on the grease. The highest diffusion can be observed by the combination of a silicone paste and silicone rubber. However, for silicone rubber there are low and no diffusion lubricants existing.
The diffusion in EPDM, EP and XLPE is commonly much lower. In EP and XLPE, no type-dependency of the grease can be observed. With the knowledge of the diffusion behavior, the desired interface properties can be achieved for the whole lifespan of the apparatus.

DC-biased dielectric measurements using an existing frequency-domain spectroscopy (FDS) instrument and series battery

Nathaniel Taylor — 2022-07-05

We present a simple method for adding DC bias to frequency-domain spectroscopy (FDS) measurements by inserting a battery in series in the ‘high’ lead to the device under test. An instrument designed for FDS measurements with pure AC can be used in this way without modification, even for DC voltages above the instrument’s AC range. Issues and limitations of the method are discussed, along with some alternative methods. Experimental results from FDS measurements on well defined linear objects are compared with/without the DC bias, to check that the measurement is not disturbed by the DC source. The only detected difference was the expected effect of including the battery impedance in series with the specimen in the measurement. This effect was negligible for typical lab-specimen capacitances, around and below power frequency. The nature of battery impedance is further described, as multiple small batteries in series can strongly affect the results for large specimens and at high frequencies.

Characterization of Silicone Elastomers for Refractive Field Grading under Sinusoidal Voltages

Jun Ting Loh — 2022-07-05

This contribution compares three different numerical approaches to characterize a nonlinear field dependent
permittivity when stressed with a sinusoidal voltage. Silicone elastomers with refractive field grading properties serve as test samples. Their dielectric properties such as the DC-conductivity and the complex permittivity are determined experimentally and applied to compute the numerical models. Principally, all three models apply the J-E relationship to quantify the nonlinear permittivity, different numerical methods are however implemented to solve the models. The numerical approaches are compared with experimental results and analysed. Based on the results, the most suitable numerical method to determine the nonlinear permittivity could be determined. This model serves as a fundamental approach for sinusoidal voltages and can be further adapted for non-sinusoidal voltages in the future.

Novel Silicone Resin Binder and Compound for Thermally Highly Stressed Applications

Jens Lambrecht — 2022-07-05

This paper deals with a new silicone resin compound that can be cured by addition curing and by peroxide curing, respectively. Curing and post-curing behavior has been investigated and optimized. Numerous trials towards the optimization of the binder-filler combinations have been performed. The resulting material POWERSIL® Resin 710 has been investigated with respect to its mechanical and thermal behavior. The new material exhibits heat class R and can be processed by press molding, pressure gelation, and injection molding.