Solid-gas insulation in HVDC gas-insulated system: Measurement, modeling and experimental validation for reliable operation

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1 Solid-gas insulation in HVDC gas-insulated system: Measurement, modeling and experimental validation for reliable operation R. Gremaud 1 *, C. B. Doiron 1, M. Baur 1, P. Simka 1, V. Teppati 1, B. Källstrand 2, K. Johansson 2, M. Hering 3, J. Speck 3, S. Großmann 3, U. Riechert 4, U. Straumann 4 1 ABB Switzerland, Corporate Research, Switzerland 2 ABB AB, Corporate Research, Sweden 3 Institute of Electrical Power Systems and High Voltage Engineering, TU Dresden, Germany 4 ABB Switzerland, High Voltage Products, Switzerland Abstract Robust dimensioning of DC gas-insulated systems requires the knowledge of the electrical field distribution at all times. Starting from a capacitive distribution at voltage switch-on, the field distribution evolves towards a resistive distribution at a pace dictated mainly by the conductivity of the solid epoxy insulation. This article presents an experimental validation scheme for modeled DC electrical field distribution. It is based on (i) measurements of electrical currents in the solid and the gas, (ii) measurement of surface potential distribution of the surface of epoxy insulators, and (iii) determination of flashover voltage of insulators under HVDC stress. To mimic the effect of ohmic losses in the conductor, a temperature gradient is applied across the insulator. DC Field distributions are calculated with a finite element model taking into account the relevant charge transport processes in the gas as well as in the solid. 1. Introduction Interfaces are critical locations in gas-insulated systems: Failure occurs in the gas, but is often mediated by charges accumulated on insulators: surface flashovers generally follow a short path of high electric field. In practice, this often means that the field component normal to the insulating surface is critical for surface charge accumulation, but flashover is mainly triggered by a high tangential field component [1]. The temporal evolution of the DC electric field in the system is mainly determined by conduction in the solid insulation. The radial potential distribution changes with time from capacitive to resistive-controlled. This causes a field redistribution, as the location of highest electric field on the insulator-gas interface moves from near the conductor to the enclosure area between switch-on time until DC steady-state is reached (see Ref. [2]). Figure 1(a), Components of a HVDC gas-insulated system: (1) conductor, (2) enclosure, (3) insulating gas, (4) solid insulator, (5) capacitive screen, (6) insert, (7) current collector, (8) dielectric coating on metallic surfaces. Electrical effects for the design of a HVDC insulator in gas-solid insulation: electronic and/or ionic conduction, polarization, space charge accumulation. In gas insulation: Ion generation by (a-d) and ion drift (e); On interfaces/surfaces (f-j). (b), Design optimization scheme for HVDC insulators. In red: optimization constraints on the electrical volume conductivity σ of the insulator material. Other constraints are in blue, and potential modification of components in orange color. E n and E t : electrical field components normal, respectively tangential to the epoxy insulator-gas surface. *robin.gremaud@ch.abb.com KEYWORDS Charge emission, epoxy insulation, gas insulated system, high voltage direct current (HVDC), surface potential. 133

2 An optimal design of an epoxy insulator with respect to the DC resistive field distribution follows many sometimes contradictory constraints, as represented in the scheme of the Figure 1(b). Note that not all design measures are necessary to achieve a robust design: for the actual DC design, a sufficient reduction of the dielectric stress compared to the AC design was obtained by geometrical optimization and insertion of a current collector. More than 10 DC insulators in realistic arrangement were dielectrically tested by superimposed lightning and switching impulses (unipolar and bipolar) under highload steady-state DC stress (both polarities) [3]. The magnitude and sign of charge accumulation on the insulator depends on the difference between the normal components of electrical current from the gas and from the solid. The resulting electrical field at the solid-gas interface therefore depends on voltage level, nominal current and induced thermal gradient in the insulation, system geometry, and nature and magnitude of gas ion sources [4]. Due to the complexity of the often non-linear [5] physical effects at hand in the gas and solid insulation, it is necessary to validate the multiphysics finite element modeling (FEM) approach used to design DC insulation by a direct monitoring of the electric field [6] or surface potential at the solid-gas interface during the transition from capacitive to resistive field [7, 8]. In this contribution, we present surface potential measurements of an epoxy insulator in a real-scale DC gas-insulator system under high current load. The temporal evolution of the surface potential is modelled by our standard finite-element modeling of gas and solid insulation. The model is calibrated by independent measurements of the leakage current of solid insulation and emission current in the gas of technical metallic surfaces. Finally, the applicability of the same modeling approach is demonstrated on breakdown voltage experiments. This present experimental validation is essential to derive reliable testing procedures, in particular to confirm the DC stress duration needed for withstand verification tests. 2 Theory and electrical modeling Considered are electrical conduction mechanisms in the solid and in the gas; the two parts of the model being connected at the solid-gas interface (Figure 1(a)). Surface and bulk of the solid are assumed to have the same conduction properties. 2.1 Solid: DC conductivity and polarization Considering an insulator contacted on one side to a ground electrode, and on the other side to a high voltage (HV) electrode at potential U, the total current density through any cross-sectional area of the insulator is then: where (1) is the DC conductivity of epoxy and the displacement current. The first term represents the fast capacitive displacement current. At the time scale of experiments (seconds to week), the starting instant polarization ε is well-approximated by the 50 Hz permittivity value. The second term P accounts for all slow polarization processes such as structural processes e.g. dipole reorientations-, interface polarization or inter-phase polarization. Measurements of the leakage current as a function of time on a plate insulator geometry are used to determine the DC conductivity and the slow polarization of the epoxy composite insulation material. (2) The procedure for such measurement is to rise the voltage from 0 to the test voltage U with constant rate du/dt and then keep the voltage constant. The leakage current between HV and ground electrode is monitored continuously. As soon as the applied voltage (U) is constant, the current density becomes (3) An empirical description for the amplitude of the slow time-dependent polarization current may be given by the empirical Curie-von Schweidler relation: (4) 134

3 If the polarization obeys linear relaxation dynamics, the depolarization current (i.e., the current flowing after switching-off the voltage) becomes The DC conductivity can therefore be determined for long measurement times as. 2.2 Gas: ion generation and drift Unlike solids, the current-voltage characteristics of gases is linear only for very low applied fields. Hence, the gas conductivity should rather be described by a carrier density based model, where is a purely local quantity, with carrier density n i, mobility µ i and charge q i for each carrier (ion) type i. Field lines in the gas ending onto the solid insulation surface define a capture volume V capt for ions. This capture volume generally changes with time during surface charging. Ions generated in the capture volume drift along field lines and, if not recombined with counter ions, contribute to surface charging of the solid. Because of the comparatively large mobilities, HV fields in the gas-insulated systems (>> 100 V/m) imply high ionic drift velocities, and consequently separation of ions of opposite polarity is fast enough for recombination in the bulk gas to play a negligible role. A local ionic current can therefore be defined if one takes into consideration the density of gas ion (generation), and ion drift Charge injection and multiplication in SF 6 In SF 6 insulation, accumulated surface charges are associated to positive or negative SF 6 ions which can be produced by three different mechanisms: 1. Ionization by cosmic and earth radiation forming a free electron and a positive SF 6 ion. If the electron is attached by another SF 6 molecule an ion pair (IP) is formed. This process is usually referred to as natural ionization and is characterized by a production rate between 20 and 50 IP/(s cm 3 ), dependent on geographic location and gas pressure. To reflect the fact that volume ion production conserves charge neutrality, our model assumes a symmetric ion generation scheme, and a constant natural production rate of ion pairs (IP). 2. Electron emission from a metallic cathode by thermionic or field emission. If the electron is attached by a SF 6 molecule a negative SF 6 ion is formed. The natural ionization current saturates at a rather low electric field values, <1 V/mm. The emission current grows exponentially with increasing electric field, but starts usually at very high electric fields (~1 MV/mm). For both cases free electrons can again be released from SF6- by collisional detachment. The above mentioned mechanisms are called charge injection from now on. 3. If the applied electric field is sufficiently high to make the electron attachment rate to SF6 molecules lower than the ionization rate, then collisional ionization can lead to an exponential growth of free electrons (and positive SF6 ions). This mechanism is called charge multiplication from now on. In the model, the additional 1 combined effect of charge injection and multiplication near surfaces is taken into account by a current source term on conductor and enclosure with boundary condition Ion drift To describe the non-linear nature of the conduction behavior in the gas, the best approach is to solve the drift-diffusion-reaction equations for the gaseous ions in parallel with the Poisson equation for the electric field [9]. Negative and positive charge carriers (expressed as charge densities ρ and ρ +) in an electric potential ϕ move because of the electric field and diffusion. The equation of continuity reads: (5) with the recombination coefficient R and the elementary charge e. The ionic current densities can be expressed by the driftdiffusion equation 2 : (6) In this equation, is the ion mobility, which for SF 6 can be assumed to be the same for both polarities, 1- Additional to the natural ionization contribution. 2- Neglecting any additional background current distribution, e.g. caused by convection. 135

4 Figure 2 Left: Polarization (I Pol ) at 20 kv source voltage and Depolarization (I Depol ) currents for a cast alumina-filled epoxy sample. The grey dashed line indicates the level of DC current. Temperature: 50 C. I 0 : base current at 0 V source voltage. Right: cross-section of cast sample geometry used for the (de-)polarization measurement. [10] and D the diffusion coefficient. The diffusion coefficient is related to the mobility via Einstein s relation the Boltzmann constant, T 0 = 300 K is the gas temperature. A direct solution of these equations is numerically challenging for complex geometries, so we instead use a model inspired from Ref. [11]. In this model, only the ionic current from the gas impinging the insulator surfaces is solved for, and not the ion concentration in the whole gas volume. In practice, this is done by solving the steadystate drift-diffusion equations(6) together with the source term for natural ionization (7): constant (7) for each ion species in the gaseous domain at every time step, making use of the fact that the time needed for newly created ions to drift to a surface is much shorter than all the other relevant timescales in our problem. 2.3 Charging of gas-solid interface, surface potential Gas ions located within the capture volume accumulate on the insulator surface where they build a surface charge. Similarly, space charge from the solid insulation also tends to accumulate on the other side of the interface. Accumulated charges typically reduce the normal component of the field at the insulator interface at the side from which they reach the interface. Eventually, they can also contribute to conduction in the solid. The charging of the gas-solid interface is modeled by introducing the following boundary current source on the insulator interface: (8) where n g is the normal vector pointing from the surface of the insulator in the gas and the current density from the gas phase. So-called open boundary conditions are applied to the other interfaces (electrodes and grounded tank), i.e. using in regions where the charges flow out of the domain, and otherwise. The development of surface charge on the solid-gas interface ( insulator surface ) is thus determined by the continuity equation: where is the current in the solid. Electric field and surface potential ϕ are then calculated using the Poisson equation: (9) (10) Whether gas or solid charging current dominates on the interface depends locally on the ratio of normal currents. If natural ionization is the only source of gas ions, this ratio is where L is the length of the field line ending on the insulation surface and E n the normal field component in the solid [2, 11]. Therefore, the solid tends to dictate the field distribution for high voltages, unless the insulator geometry creates large capture volume or charge injection and multiplication dominates. This ratio determines also the sign of the surface charge and of the remaining surface voltage when the main voltage is switched off. 3. Experimental measurments of electrical currents in insulation 3.1 Solid insulation: DC conduction Figure 2 shows current curves for polarizationdepolarization experiments of alumina-filled epoxy composite cast into an aluminum electrode system, including guard-ring. This cast block geometry offers several advantage respectively to the previously used contacting system of conductive rubber on epoxy plates [2]: namely i) the same metal/insulation contacting as in the real insulator application ensures realistic charge injection behavior at contact, ii) reduced quantity of void at contact compared lead to lower partial discharge noise, and ii) less influence of short-time temperature fluctuation (increased thermal mass). 136

5 Figure 3 Left: Simplified sketch of the emission current measurement system. The inner conductor (emitter) radius is r in = 7 cm, the outer conductor (collector) radius is r out = 17.8 cm, the length of the collector is l = 2 m, the length of the guards is 30 cm. Right: Realized vessel, connected to the vacuum system composed by three turbopumps. The system is capable of reaching vacuum with pressure < 0.08 Pa and a SF 6 pressure of 0.45 MPa. Figure 4 Left: Emission current at low electric fields, and right, at higher fields, for negative polarity and smooth (rms roughness R a = 3±0.2 mm) emitting electrode both for vacuum and 0.45 MPa SF 6 As seen in Figure 2, in this case an estimation (within 20% error) of the DC current is already achieved after one day ( s) of measurement time, while a steady state necessitates 11 days (10 6 s) of measurement. By repeated polarization/depolarization measurement at all operating temperatures, the Arrhenius behavior of the DC conductivity is determined. The conductivity value varies over three decades from 20 C to the maximum operating temperature of 105 C [4]. As a consequence, an inhomogeneous temperature distribution results in space charges in the solid and field redistribution under DC compared to AC stress. 3.2 Gas insulation Although the physical mechanisms of charge injection and multiplication are well known, the absolute number of charges and the onset-fields can vary in orders of magnitude. The biggest unknown in the system which can also not be described analytically is the electrode surface roughness. Therefore the amount of injected charges for a technical relevant geometry was investigated experimentally (see Ref. [12] for a description of the experimental setup). In order to separate gas dominated effects from electrode-dominated effects the experiments were conducted using both vacuum and pressurized SF 6. Small electric fields were applied (< 2 kv/mm) in order to detect the onset electric fields of charge injection and multiplication. Figure 3 shows a simplified sketch of the measurement system: High voltage (positive and negative polarity, up to 100 kv) is applied to the injecting electrode while another electrode (collector) collects these charges. The collector is connected to a current measurement system. The size of the electrodes, i.e. the volume in between was chosen in order to reach approximately 1-2 pa due to natural ionization. (11) where V is the volume of the gas. With a value of = 37.5 IP/(s cm 3 ) [13], the current due to natural ionization in the given volume (~ m 3 ) is ~2 pa. Although the natural ionization current is dependent on the volume, it is plotted as current density (A/m 2 ) to represent an injected charge. The surface of the inner conductor is ~1 m 2, therefore the absolute values of current and current density approximately coincide. The onset for charge injection can be located in a range between 0.3 and 0.7 kv/mm. This value is much lower than values from literature [14, 15]. Emission currents at higher electric fields are shown in the right panel of Figure 4, in both SF 6 and vacuum, for negative polarity. Figure 4 Left: Emission current at low electric fields, and right, at higher fields, for negative polarity and smooth (rms roughness R a = 3±0.2 μm) emitting electrode both for vacuum and 0.45 MPa SF

6 Figure 5 a) Test set-up for the long-term measurement of surface potential on insulator-gas interfaces. System nominal voltage: U DC = 350 kv DC. Max. load current = 4.6 ka. Potential of non-contact electrostatic probe: 0 < U p < 350 kv DC. b) Angleadjustable surface potential probe for surface scanning of DC insulators. c) Projection of insulator and measured azimuthal angle ϑ (ϑ= 0 : vertical upward direction). 4. Surface potential measurement Surface potential measurement are performed with a modified non-contact electrostatic voltmeter from TREK, Inc, using a miniature electrostatic field chopper probe [16]. A positioning system for the probe allows scanning of two insulator surfaces within a 320 kv DC gas-insulated system (see Figure 5). Tested are DC insulators under 0.45 MPa SF6 at relative humidity RH 8%. Measurements are performed at full load current, resulting in an average 25 C thermal gradient across the insulation 3. To speed up the transition to DC state, the minimum enclosure temperature is lifted up to 50 C. After system thermalization, a voltage U DC = 100 kv is applied on the central conductor and the surface potential of the gas-insulator interface is measured periodically at various azimuthal angles ϑ and radii r until steady state is reached. Figure 6a) shows the temporal evolution of the measured and modeled surface potential change at an average radius r = 120 mm. For modeling of the potential evolution, DC conductivity values of the alumina-filled epoxy insulator material as function of temperature and field are determined independently by polarization experiments on plates and cast blocks 4. The only adjusted parameter in the model is the injection/multiplication current (see Figure 6b)). Effective values used are within the same range as measured in section 3.2 and Ref. [14] for comparable surface roughness (R a, see Ref. [17]). The time-scale of the transition to the resistive state (which depends mainly on the conductivity of the solid insulation) is well-predicted by the model 5. For early times (t < 300 h), the measured potential increases in most of the cases more rapidly than predicted, which is to be expected as fast polarization currents are neglected. The amplitude of the change in surface potential U is however controlled by charges in the gas: more charge injection/multiplication reduce the final potential change, as can be seen in Figure 6c). In this particular case, considering natural ionization only, the expected increase in surface potential would be U=13 kv on the insulator surface, corresponding to a steady-state injection current = 0.35 pa/m 2 into the insulator surface. In contrast, the maximum experimentally observed U(ϑ= 30 )= 8 kv, corresponding to an additional injection current 10 pa/m 2 from the metallic enclosure, corresponding to = 2.2 pa/m 2 onto the insulator surface. These relatively low additional ionic currents reduce the change in radial distribution between the capacitive and the steady state DC electric field (see Figure 6c). 5. Flashover breakdown voltage After proper experimental calibration of the DC simulation model at nominal field values, its applicability should be tested at near-breakdown conditions. For this purpose, a setup for the determination of the flashover voltage of cylindrical epoxy specimens subjected to a thermal 3- Due to natural convection, the thermal distribution depend on the azimuthal angle. A separate measurement of the temperature distribution as function of load current is performed prior to voltage switch-on by a network of thermocouples distributed on the insulator surface. 4- A direct scale-up of the fast polarization current obtained from thin samples (plates/cast blocks) to this real-size component is difficult and thus neglected here. 5 An exception is the upward direction (0 ). In this location the natural convection is considerably higher than anywhere else, and the associated gas velocity field might interfere with the field-driven ion drift. 138

7 Figure 6 a) Measurement and modeling of surface potential change ΔU as a function of time and azimuthal angle (see b)) at an average radius of 120 mm on an alumina-filled epoxy partition insulator. Initial values of the potential are distributed on the y-axis for better readability. Natural ionization rate = 50 IP/cm 3 /s, Epoxy DC conductivity from independent (de-)polarization experiments. b) Effective injection/multiplication current as a function of absolute azimuthal angle used for the modeling, and compared to experimental emission currents from the literature. c) Corresponding modeled surface potential and electric field (gas side) at the insulator surface (10 7 s = 115 d) for various injection currents (ϑ= 90 ). Voltage on conductor U DC = 100 kv, max. load current (4.6 ka) gradient at 0.1 MPa SF 6 pressure was built and operated at TU Dresden [18]. 5.1 Capacitive state: cold and warm Performing voltage rising tests (VRT), the dielectric strength under electrostatic field conditions comparable to AC stress was examined. The tests were executed in cold condition without a temperature gradient and in warm condition with the temperature distribution seen in Figure 7a). The breakdown strength is about 15 % lower in warm than in cold condition, due to the locally seen lower gas density in the vicinity of the heated electrode [19]. The corresponding breakdown voltage V bd,vrt,warm is used for scaling the flashover voltages V bd,dc after long-term stress with V DC for (6, 48, 92) hours. 5.2 Capacitive-resistive transition For suitable DC pre-stress times, the experiments were carried out at a voltage V DC of 45 % of the breakdown voltage of the AC-like voltage rising test: (12) Under cold conditions, the conductivity of epoxy is low and the time of the field transition is in the order of weeks. Hence, no decrease of the breakdown voltage occurred after 48 h or 92 h (see Figure 8). Figure 7 a) Equilibrium temperature distribution b) Calculation of capacitive-resistive transition after DC energizing at voltage V DC with temperature distribution neglecting charge multiplication. (calculation 1 in Figure 8) 139

8 Figure 8 Left: symbols: experimental flashover voltage V bd,dc as a function of the DC stress duration with voltage V DC. Lines: calculation 1: simulation for no-charge emission from gas (solid conduction only, low homogeneous gas conductivity (10-21 S/m) ). Calculation 2: including additional charge injection on insulator surface (see text). Right: Test execution to investigate time-dependence of flashover voltage during field transition to resistive state. VRT: voltage rising test. Under warm conditions, all long-term tests show a decrease of the breakdown voltage in comparison to the values of the voltage rising test and show the effects of the capacitive-resistive transition. After 48h, the insulation strength is on average about 10 % lower than in the VRT. Due to the temperature-dependent conductivity of the insulator material, the highest field strength is increasing and its location is shifted to the cooler part of the insulator into the gap near the high voltage electrode (Figure 7b). 5.3 Comparison with simulation The simulation of the capacitive-resistive transition reproduces the execution of the experimental test (see Figure 8, right): Using i) Curie-von Schweidler approach to simulate the polarization processes, and ii) the independently measured DC conductivity in the epoxy resin, the electric field is calculated for a DC voltage application of various pre-stress durations. Subsequently, a fast voltage rise is implemented in the FEM tool, until reaching the breakdown field strength on the HV electrode. Considering conduction in the gas, simulation was carried out for two cases: 1. Calculation 1: residual homogenous conductivity of S/m in the whole gas volume. This approximation is justified, as the contribution from natural ionization is negligible for this voltage level and geometry. 2. Calculation 2: as calc. 1, with additional field-dependent injection current = 10 pa/m 2 ) on the insulator interface subjected to high field next to the HV electrode. Comparing the calculated values with the measurements (Figure 8a)), agreement is found for both cases for times of DC stress up to 100 h. For longer times, calculation 1 predicts that flashover should happen after twelve days under pure DC stress at V DC. However, two experiment have been carried out at V DC for 14 days, respectively 27 days without breakdown. Subsequent VRT applied show a moderate reduction of the minimum flashover breakdown strength of 0.8 V bd,vrt,warm. This behavior can be explained with calculation 2 by the presence of low injection ionic currents from the neighboring HV electrode. The magnitude of these currents is comparable to those reported in Ref. [14]. 6. Conclusion/outlook This contribution presents a joint experimental/theoretical approach for the determination of electric potential at the gas-solid interface in HVDC gas-insulated systems. The associated model enables prediction of the spatial and temporal evolution of the electric field. The input experimental parameters needed for modeling are the transient and steady-state currentvoltage characteristics of the insulation media, i.e. the temperature-, field- and time-dependent residual DC currents in gas and solid insulation. These are determined by independent measurements on model experimental setups: for the solid, epoxy plates or cast blocks, for the gas, a coaxial electrode geometry. The time scale of the capacitive-resistive surface potential transition is controlled by the DC conductivity of the solid epoxy insulation. The timescale predicted by the independently-calibrated model is in good agreement with the measured temporal evolution of the surface potential. This result is essential to derive reliable testing procedure of DC gas-insulated system. Furthermore, the applicability of the model for the prediction of breakdown voltage as a function of DC stress duration is demonstrated. Further investigations should aim at completing the characterization of charge injection and multiplication in the gas, with e.g. the effects of paint, various surface roughness, and gas humidity content. Effects on conduction in the solid, such as long-term thermal and electrical aging, as well as the scalability of fast polarization currents to real-scale insulation components deserve also further study. 140

9 7. Bibliography 1. Pedersen, A., T. Christen, A. Blaszczyk, and H. Boehme. Streamer inception and propagation models for designing air insulated power devices. in Conference on Electrical Insulation and Dielectric Phenomena Gremaud, R., F. Molitor, C.B. Doiron, T. Christen, U. Riechert, U. Straumann, B. Källstrand, K. Johansson, and O. Hjortstam. Solid Insulation in DC Gas-Insulated Systems. in Cigré Session Paris, France. 3. Riechert, U., U. Straumann, F. Blumenroth, and E. Sperling. Dielectric Testing of Gas/Solid Insulation Systems for HVDC GIS/GIL. in SCD1 Colloquium Trends in Technology, Materials, Testing and Diagnostics Applied to Electric Power Systems Rio de Janeiro, Brazil. 4. Gremaud, R., F. Molitor, C. Doiron, A. Krivda, T. Christen, K. Johansson, N. Lavesson, U. Riechert, and U. Straumann. Solidgas interfaces in DC gas insulated systems. in 4. ETG-Fachtagung Grenzflächen in elektrischen Isoliersystemen Dresden, Germany. 5. Lavesson, N. and C.B. Doiron. 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Capacitive-resistive transition in gas insulated DC systems under the influence of particles on the insulator surface. in The 19th International Symposium on High Voltage Engineering Pilsen, Czech Republic. 19. Hering, M., J. Speck, S. Großmann, and U. Riechert. Investigation of the Temperature Influence on the Breakdown Voltage in Gas Insulated Systems under DC Voltage Stress. in The 18th International Symposium on High Voltage Engineering (ISH) Seoul, Korea. 8. Biographies Robin Gremaud received his diploma in physics from the University of Fribourg, Switzerland, in 2003, and his Ph.D. degree in condensed matter physics from the VU University Amsterdam, Netherlands, in After a postdoctoral position during at the Research Institute for Materials Science EMPA, Switzerland, he joined the Swiss ABB corporate research center in 2010 and is currently Principal Scientist in the Materials, Design and Testing group. His current research addresses the physics of solid and gas insulation under HVDC stress. Charles B. Doiron received his B.Sc and his M.Sc. degrees in physics from the University of Sherbrooke, Canada, in 2002 and 2005, respectively, and his Ph.D. degree in theoretical physics from the University of Basel, Switzerland, in He joined the Swiss corporate research center of ABB in 2009, where he currently leads the Theoretical Physics group. His current research interest include gaseous insulation, alternatives to SF6, gas discharges, as well as high-temperature properties of gases. Matthias Baur received his diploma in physics in 2007 and later his Ph.D. degree in physics in 2012 from ETH Zurich, Switzerland. After a short postdoctoral position in 2013 at the University of Queensland, Australia, he joined the Swiss ABB corporate research center in 2014, where he is now working as a Scientist in the Materials, Design and Testing group. His current research activities focus on the design of power capacitors and the physics of high voltage solid insulation materials. Philipp Simka received the M.Sc. degree from the Department of Electrical Engineering and Information 141

10 Technology, ETH Zurich, Switzerland in 2005 and Ph.D from the Institute for Power Systems and High Voltage Technology, ETH Zurich, Switzerland in Since then he is with the Swiss corporate research center of ABB. His research interests include gaseous insulation under AC and DC, high voltage test techniques and high frequency phenomena associated with high voltage equipment. Valeria Teppati received her M.Sc. degree in Electrical Engineering and Ph.D. degree in Electronic Instrumentation from Politecnico di Torino, Italy, in 1999 and 2003 respectively. From 2003 to 2014 she was Assistant Professor at Politecnico di Torino, her main research interests being microwave measurements of active and multiport devices. From 2012 to 2014 she was Visiting Professor at ETH Zürich developing a 100 GHz active load-pull measurement system. In 2014 she joined the Swiss ABB Corporate Research Center in the gas circuit breakers group. Her current research activities include gas insulation, alternatives to SF6 and gas circuit breaker CFD simulations. Birgitta Källstrand received her BSc degree in meteorology from Uppsala University, Sweden, in 1992 and her PhD degree in meteorology from the same university in She joined the Swedish corporate research center of ABB in Main focus of her research during the years with ABB has been on HVDC cable insulation and processing, and electrical measurement techniques for solid and gaseous insulation materials. She is now team manager for the High Voltage physics team at ABB Corporate Research, in Västerås, Sweden. Kenneth Johansson received a technical college graduate in electronics in He started to work at ASEA Research and Innovation (later ABB Corporate Research) in Västerås in 1986 as a laboratory engineer with measuring techniques for dielectric materials. He develops scientific measuring equipment for dielectric and high voltage studies. Main focus areas have been measurements of very low currents, electric field measurements in gas, fluid and solid insulation materials and high frequency behaviors of power products. He holds the position Senior Engineer. Maria Hering finished her studies in electrical engineering at the Technische Universität Dresden (TUD) in 2011 with the diploma degree (Dipl.-Ing.) and received the Ph.D. degree from the TUD on the topic of gas-insulated DC systems in Maria Hering is Young Member of CIGRÉ and contributes to different Working Groups of the Scientific Committee D1. She is also a member of the German Power Engineering Society, department for Materials, Electrical Insulations and Diagnostics. Maria Hering received the Young Researcher Award of the International Symposium on High Voltage Engineering in 2013 (Seoul) and 2015 (Pilsen). Joachim Speck was born in Dresden, Germany, in He received the Ph.D. degree in electrical engineering from the Technische Universität Dresden, Germany, in He has been with the Institute for High Voltage and High Current Engineering, Technische Universität Dresden since His research interests include field calculation, statistical evaluation, SF6 insulation and breakdown of solid dielectrics. Steffen Großmann was born in Dresden, Germany, in He received the Ph.D. degree in electrical engineering from the Technische Universität Dresden, Germany, in From 1976 to 2003 he was with Starkstrom-Anlagenbau Dresden and RIBE Electrical Fittings Radebeul and Schwabach. Since 2003 he is Professor for High Voltage and High Current Engineering, Technische Universität Dresden. His research interests include high voltage insulation systems, ampacity and heating of electrical power devices and electrical contacts and connections. Uwe Riechert finished the studies in electrical engineering at the Dresden Technical University (TUD) in 1994 and received the Ph.D. degree at the TUD on the topic of polymeric insulated HVDC cables in Since 1999 he is with ABB Switzerland. He conducted several product development projects. In 2013, he became a principal manager. Since 2011 Dr.-Ing. Uwe Riechert is working as project manager for HVDC substations. Uwe Riechert is a member of DKE, CES and different CIGRE and IEC working groups. Ueli Straumann received his diploma in theoretical physics from the University of Zurich, Switzerland, in 2001 and his Ph.D. degree in electrical engineering from ETH Zurich, Zurich, Switzerland, in Since then, he has been a Lecturer of High Voltage Engineering, ETH Zurich. After being Senior Assistant at the High Voltage Laboratory of ETH Zurich between 2007 and 2012, he became Senior and Principal Engineer at the GIS R&D of ABB Switzerland Ltd. 142