josef.kindersberger@tum.de

Solid/Gaseous Insulation Systems for Compact HVDC Solutions

A. WINTER, J. KINDERSBERGER

TU Muenchen Germany

M. TENZER, V. HINRICHSEN

TU Darmstadt Germany

L. ZAVATTONI, O. LESAINT

Grenoble University and CNRS France

M. MUHR

TU Graz Austria

D. IMAMOVIC

Siemens AG, Erlangen Germany

SUMMARY When energizing an electrical insulation system with a direct voltage the initial field distribution, being a capacitive field distribution free of space charges like under normal ac voltage stress, turns into a stationary resistive field distribution. Both, the stationary field distribution and the duration needed for the transition from the capacitive to the resistive field distribution are governed by the bulk and the surface conductivity of the insulating materials involved. Any change of the field distribution compared to the capacitive field is associated with the accumulation of space charges in the bulk material or surface charges on the interface between different insulating materials. Particularly for gaseous dielectrics a simulation model is presented, which takes into account the generation, recombination and motion of charge carriers. The field calculations using this model are verified by measurements of the surface potential on cylindrical epoxy resin insulators under low electric field stress. By means of the simulation model, the charging of conical insulators under high electric field stress and the influence of field-induced electron emission from the cathode is investigated. The influence of temperature and field strength on the surface and bulk conductivity of epoxy resin material is shown experimentally. The influence of temperature-dependent bulk conductivity on the resistive field distribution is shown using simulations. Current measurements in gas under high field conditions indicate the existence of a source of electric charges besides natural ionization. In order to achieve a faster transition from the initial capacitive field distribution to the stationary resistive field distribution (e.g. when an HVDC system is energized, or when a polarity reversal takes place) and in order to provide a faster decay of surface charge carriers, an increase of the insulators’ surface and bulk conductivity is recommended. For experimental investigations, the surface and bulk conductivity of conventional epoxy resin insulators were adjusted by functional fillers providing a well-defined conductivity of the compound. Aiming for eco-friendly solutions, the electric direct voltage strength of nitrogen, oxygen, and gas mixtures has been determined. Further, the influence of argon in such mixtures has been investigated. With respect to possible application in dc insulation systems the electric strength of three typical electrode arrangements with uniform, slightly and strongly non-uniform electric field has been determined experimentally and relevant values compared to the electric strength in SF6 are given. KEYWORDS HVDC, insulators, surface charge accumulation, functional fillers, insulating gases

21, rue d’Artois, F-75008 PARIS D1_102_2014 CIGRE 2014 http : //www.cigre.org

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1. Introduction

For the transmission of large electric power over long distances, high voltage direct current (HVDC) transmission is an important technology. Overhead lines, underground cables or gas insulated lines (GIL) are the different options for transmission lines. Gas insulated underground transmission systems can cover many important aspects of future HVDC transmission systems, such as high transmission power capacity, low electrical losses, high operational reliability (auto-recloser function), enormous space reduction, high public acceptance, etc. The stationary distribution of the electric field in an electrical insulation system significantly differs for alternating and direct voltage stress. After energizing a system with a direct voltage the initial capacitive field distribution, which is governed by the permittivities of the involved dielectrics, changes towards a stationary resistive distribution depending on the bulk and surface conductivities of the insulating materials. Therefore, the choice of insulating materials with suitable electrical properties is essential for the design of gas-solid insulation systems. Since a considerable amount of insulating gas is required for gas insulated systems over large distances, the replacement of SF6 might be desirable regarding its high global warming potential. The dielectric strength under direct voltage is a crucial factor for alternative insulating gases or gas mixtures. This paper summarizes results of investigations concerning the field distribution and charge accumulation in gas/solid insulation systems, the use of solid materials with adjusted electrical properties and the dielectric strength of insulating gases and gas mixtures under direct voltage stress. 2. Electric field distribution under direct voltage stress

The accumulation of charges in the stationary resistive case depends on the geometry of the insulation system and the conductivity of insulating materials. Bulk and surface conductivity of polymeric materials are highly affected by temperature, field strength and the humidity of the bulk material and the surrounding gas, respectively [2.01]. As long as other sources of electrical charges like partial discharges, field emission from the cathode or charged particles can be neglected, the electric conductivity of insulating gases like air or SF6 is based on charge carriers generated by natural ionization, either of cosmic and terrestrial origin. Due to a constant generation rate, the conductivity of a gas is assumed to be field-dependent [2.02]. A physical gas model is developed that takes into account generation, recombination and motion of electrical charges in a gas and is implemented in the FEM-program COMSOL Multiphysics®. The underlying equation system and the discontinuous Galerkin-method, which is used to stabilize the numerical calculation of the transient field simulations, are explained in detail in [2.03] and [2.04]. Investigations of the electric field distribution in gas-solid insulation systems are carried out on cylindrical epoxy resin insulators between a high voltage electrode and a ground electrode inside of a grounded screen in ambient air under low electric field stress (Figure 2.1a). A positive direct voltage U = 15 kV is applied to the high voltage electrode at the time t = 0. The surface potential is measured using a contact-free electrostatic voltmeter. The insulators are made from different epoxy resin materials, i. e. EP1 and EP2. Some insulators are modified by using a coating of low electric conductivity. Simulations are performed on a rotational symmetric model of the experimental setup.

2.1. Influence of bulk conductivity

Simulations are performed using an applied direct voltage U = 15 kV, relative permittivity of the solid insulating material εr = 5, negligible surface conductivity κS = 0, ion pair generation rate ∂nIP/∂t = 10 IP/(cm3s) [2.05, 2.06], mobility of positive and negative ions b+ = 1.36 cm2/(Vs) and b- = 1.87 cm2/(Vs), respectively [2.07] and recombination rate kr = 1.4×10-6 cm3/s [2.08]. The initial capacitive and the stationary resistive distribution of the surface potential along the insulator for values of the bulk conductivity of the solid insulating material 1.00×10-20 S/m ≤ κV ≤ 3.33×10-16 S/m are shown in Figure 2.1b. After application of the direct voltage, charges accumulate on the insulator in correlation to the distribution of the normal component of the electric field strength En. For κV < 2.00×10-17 S/m, charge transport through the gas dominates the accumulation of charges on the insulator surface. Mostly negative charges accumulate on the dielectric interface, leading to a

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decreasing surface potential along the insulator. For κV > 2.00×10-17 S/m, mostly positive charges accumulate on the insulator, leading to an increasing surface potential. For κV < 1.00×10-20 S/m and κV > 3.33×10-16 S/m, the potential distribution does not change anymore, because the electric field distribution is altered in a way that the field lines run predominantly in the gas and insulator volume, respectively. The duration of the field transition decreases with increasing bulk conductivity as a consequence of an increased charge transport.

2.2. Influence of surface conductivity

For the investigation of the influence of surface conductivity, a bulk conductivity of the solid insulating material κV > 3.33×10-18 S/m is assumed. The initial capacitive and the stationary resistive distribution of the surface potential along the insulator for values of the surface conductivity 1.00×10-17 S ≤ κS ≤ 1.00×10-20 S are shown in Figure 2.1c. Note that the surface conductivity κS, quoted in S, corresponds to the inverse of the surface resistivity, quoted in Ω, while in practice this is sometimes referred to as Ω per square. For κS ≤ 1.00×10-20 S, charge transport through the gas volume dominates the stationary resistive distribution and mostly negative charges accumulate on the insulator. For κS ≥ 1.00×10-17 S, charges accumulate in correlation to the tangential component of the electric field strength Et due to a locally differing surface current as a consequence of an inhomogeneous distribution of Et at the moment of voltage application. In the stationary resistive state, an almost homogeneous distribution of Et is achieved, i.e. the surface potential shows an almost linear distribution along the insulator height. The duration of the field transition decreases with increasing surface conductivity as a consequence of an increased charge transport along the insulator.

hI

rI

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t = 03,33e-16_____2e-173,33e-181.00E-20

z

r

hI

2.00×10-17 S/m

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3.33×10-16 S/m

3.33×10-18 S/m

t = 0 h

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t = 0

1e-17--------

1e-20

z

r

hI

1.00·10-20 S

1.00·10-17 S

t = 0 h

a) b) c) Figure 2.1: a) Experimental setup ; b) Simulated stationary resistive surface potential for different values of bulk conductivity κV while κS = 0; c) Simulated stationary resistive surface potential for

different values of surface conductivity κS while κV = 3.33×10-18 S/m; U = 15 kV; εr = 5; ∂nIP/∂t = 10 IP/(cm3s); b+ = 1.36 cm2/(Vs); b- = 1.87 cm2/(Vs); kr = 1.4×10-6 cm3/s

2.3. Measured potential distribution on cylindrical insulators

Measurements of the surface potential are performed on cylindrical insulators under atmospheric air at temperature ϑ = 20 °C and relative humidity RH ≤ 2%. Before the measurements, the insulators are dried for at least 48 h in an oven at ϑ = 100 °C and cooled down at ϑ = 20 °C and RH ≤ 2% for at least 24 h. After application of a direct voltage U = 15 kV, the surface potential is measured periodically. For uncoated inulators, the measurement results are corrected for the influence of the probe, which is calculated using three-dimensional field calculations [2.03]. Values of permittivity, bulk conductivity and surface conductivity of the epoxy resin materials and the layer conductivity, which can be achieved by applying a coating of well defined thickness and of low electric conductivity, are taken from measurements on material samples [2.03, 2.04]. Measurements on insulators made of EP1 match simulation results with κV,EP1 = 3.33×10-19 S/m and κS,EP1 = 1×10-20 S in the initial capacitive state and during the capacitive-resistive field transition (Figure 2.2a). The measurement is stopped after one year, although the stationary resistive state is not reached up to then. Simulation results indicate that the field transition would be finished after approx. 40000 h, i.e. about four and a half years. EP2 has a considerably higher bulk conductivity than EP1. For κV,EP1 = 5.00×10-16 S/m and κS,EP1 = 7×10-19 S, the measured potential distribution matches closely the simulated one in the initial capacitive state (Figure 2.2b). The stationary resistive state is reached after approx. 170 h. However,

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the measured resistive potential distribution slightly differs from the simulated distribution, presumably due to inhomogeneous electrical properties of the solid material. In acordance with simulations using κS,coated = 9.1×10-11 S, an almost linear potential distribution can be measured on insulators, which are coated with a material of low electric conductivity (Figure 2.2c). Since the evaluation of the surface potential on an insulator takes about 2.5 min due to the limited velocity of the measurement system, only the stationary resistive potential distribution can be measured. Simulations indicate that the capacitive-resistive field transition is finished after approx. 0.3 s.

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S: t = 0S: t = 2000 hS: t = 8000 hS: t = 25000 hM: t = 0M: t = 2000 hM: t = 8000 h

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hI

t = 0 ht = 2000 ht = 8000 ht = 60000 ht = 0 ht = 2000 ht = 8000 h

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r

hI

M: t ≈ 2...150 s

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S: t = 0.001 s

a) b) c) Figure 2.2: Measured (M) and simulated (S) surface potential on different cylindrical insulators:

a) uncoated EP1; b) uncoated EP2; c) coated EP1; Simulation (S): U = 15 kV; εr = 5; ∂nIP/∂t = 7 IP/(cm3s); κV,EP1 = 3.33×10-19 S/m;

κV,EP2 = 5.00×10-16 S/m; κS,EP1 = 1×10-20 S; κS,EP2 = 7×10-19 S; κS,coated = 9.1×10-11 S; b+ = 1.36 cm2/(Vs); b- = 1.87 cm2/(Vs); kr = 1.4×10-6 cm3/s

Measurement (M): Mean values around the insulator’s circumference; U = 15 kV; = 20 °C; RH ≤ 2%

2.4. Simulated surface charge accumulation on conical insulators

The physical gas model for the conductivity is used to calculate the transient electric field in a gas insulated electric insulation system consisting of a conical epoxy resin insulator in a coaxial electrode arrangement (Figure 2.3a). Below, the concave side is refered to as interface 1, the convex side as interface 2. The gas is SF6 at gas pressure p = 0.5 MPa. A negative direct voltage U = -500 kV is applied to the inner conductor. Insulators made of EP1 and EP2 are investigated using values of bulk conductivity κV,EP1 = 9.53×10-17 S/m and κV,EP2 = 5.80×10-16 S/m respectively, which are calculated from current measurements at different temperatures [2.04], and negligible surface conductivity κS,EP1 = κS,EP2 = 0. Ion pair generation rate is assumed as ∂nIP/∂t = 30 IP/(cm3s) [2.06], ion mobility as b+ = b- = 0.048 cm2/(Vs) [2.06, 2.09], the diffusion and recombination coefficients are calculated with the Langevin- and the Einstein-equation [2.03].

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1) EP1, EP22) EP22) EP13) EP1

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1) EP1, EP22) EP22) EP13) EP1

nC

/cm

2

z

rri

2

ra

a) b) c) Figure 2.3: a) Conical insulator in coaxial electrode arrangement ; b) Simulated stationary resistive

surface charge density on interface 1; c) Simulated stationary resistive surface charge density on interface 2 of a conical insulator made of either EP1 or EP2 for different cases:

1) without field emission (FE); 2) with FE and βFN = 175; 3) with FE and βFN = 200; U = -500 kV; εr = 5; κV,EP1 = 9.53×10-17 S/m; κV,EP2 = 5.80×10-16 S/m; κS,EP1 = κS,EP2 = 0; ∂nIP/∂t = 30 IP/(cm3s);

b+ = b- = 0.048 cm2/(Vs); kr = 1.74×10-7 cm3/s; αFN = 7.5×10-8; λFN = 5.0025; WW = 2.86 eV

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Without other sources of electrical charges except natural ionization, charge accumulation is dominated by ion transport through the solid material as a consequence of decreasing gas conductivity with increasing electric field strength [2.03]. Mostly positive charges accumulate on interface 1 (Figure 2.3b) and negative charges on interface 2 (Figure 2.3c) for EP1 as well as for EP2. To investigate the influence of field emission from the cathode on charge accumulations, the Fowler-Nordheim-equation was implemented as Neumann-boundary condition on the inner conductor [2.04, 2.10]. Average values from literature are assumed for the ratio of emitting area to the whole electrode area αFN = 7.5×10-8 [2.10], the correction factor λFN = 5.0025 [2.10] and the work function of aluminum WW = 2.86 eV [2.11]. Simulations are carried out for different values of the factor βFN representing the field enhancement due to electrode roughness. Less positive charges accumulate on interface 1 due to an increased bulk conductivity of the gas as a consequence of an increased density of electrical charges. The influence of field emission increases with decreasing bulk conductivity of the solid material and increasing electrode roughness, i.e. emission of electrons from the cathode. For βFN = 200 and EP1 mostly negative charges accumulate on interface 1, i.e. charge transport through the gas volume is the dominating mechanism. The accumulation of negative charges on interface 2 is basically not influenced by field emission as a consequence of the insulator geometry. 3. Conduction phenomena in solid and gaseous insulating materials

3.1. Bulk and surface conductivities of solids

Conductivities of alumina-filled epoxy samples are derived from experimental measurements of conduction currents (Figure 3.1a), either through the volume (voltage applied to electrode 1, electrode 2 grounded), or along the surface (voltage applied to electrode 2, electrode 1 grounded). The electrode system is placed in a test enclosure containing SF6 (0.6 MPa). Samples are first dried under vacuum at = 90 °C, and a controlled amount of water (mass fraction Wc, measured by weighing) can be added by diffusion in a climatic chamber. After an initial de-polarization to remove residual charges (one day in short-circuit conditions at = 80 °C), voltage is applied until a stabilized current is reached, which may occur after a long time up to several days. The resolution of current measurements is 10 fA. Bulk conductivity κV (Figure 3.1b) strongly increases when either temperature or water content Wc are increased. The large increase of κV is certainly due to an enhanced ionic conduction [3.02]. Measurements of κV versus electric field strength show an ohmic behaviour within the measurement range (up to 2 kV/mm) at temperatures up to = 80 °C.

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(kV

/mm

)

A B

20mm

A

B

Central conductor (100kV)

ϑ= 20 °C

Δϑ = 60 °C

a) b) c) Figure 3.1: a) Electrode system for the measurement of surface or volume currents on material

samples (distance e between electrodes 2 and 3: 2 mm); b) Bulk conductivity κV vs. temperature at 0.5 kV/mm under dry conditions and Wc = 0.2% [3.01];

c) Calculated electric field strength from A to B for dry (full line) and Wc = 0.2% (dashed line) samples for uniform temperature ( = 20 °C) and with a temperature gradient ( = 60 °C) [3.01]

Numerical field strength calculations (FEM software COMSOL Multiphysics®) are carried out on a conical GIS insulator of simplified shape, considering ohmic behaviour of materials. Figure 3.1c shows results obtained with either a dry material or with Wc = 0.2% at a uniform temperature ( = 20 °C) and with a temperature difference between the central conductor ( = 80 °C) and the enclosure ( = 20 °C). The temperature distribution is first calculated considering heat conduction

3

1 2 e

pA

5

through gas and insulator. Local values of κV() obtained from Figure 3.1b are then implemented and used for current and field strength calculations. The temperature difference induces a considerable change compared to a uniform temperature. The field distribution is nearly reversed since hot regions become more conductive. The region of maximum field strength moves from the central conductor side towards the grounded enclosure side. For a uniform temperature, an identical field strength distribution is calculated for dry and Wc = 0.2% materials (curves are perfectly superposed), whereas with a temperature gradient a slight difference exists. The surface conductivity κS versus average tangential electric field strength, i. e. applied voltage U over gap distance e (Figure 3.1a), and temperature is measured in SF6 at 0.6 MPa (Figure 3.2a). In contrast to bulk conductivity, surface conductivity shows a large non-linearity versus electric field strength. For the sake of numerical handling, the surface conductivity is simulated by considering a layer with arbitrary thickness d, and with a bulk conductivity κVL, different from κV. Then, values of κVL are adjusted in order to obtain a good agreement between calculated and measured currents (Figure 3.2b). For realistic layer thicknesses (< 400 µm), adjusted values of κVL, significantly higher compared to the bulk conductivity κV, fairly well correspond to a constant surface conductivity (κS = κVLd), evidencing that the measured current in the set-up mainly flows along the surface. The high values of κVL indicate that the current densities in the insulator surface are higher than in the bulk. In the geometry of Figure 3.1c, the implementation of surface layers induces only minor differences on the total current flowing through the insulator, but a larger impact might exist in other geometries.

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κv = 7.5x10-17 S/mU=1 kVϑ = 40 °C

kV/mm

High Voltage electrode

Measurement electrode

2mm

a) b) Figure 3.2: a) Surface conductivity κS in SF6 versus tangential electric field strength and temperature

(pressure: 0.6 MPa, SF6 dew point: -27°C @ 0.1 MPa); b) Adjusted bulk conductivity κVL of the surface layer providing a good agreement between measured and calculated currents versus layer

thickness [3.03]

3.2. “Dark” current through gases at high electric field strength

To estimate the influence of charges coming from the gas on the charging of insulators in GIS, currents through gases at high electric field strength are measured, using a coaxial electrode system with a guard electrode [3.04]. Experiments are performed in SF6 and air, varying water content of the gas (in mg/l) and roughness Ra of the high voltage electrode. Figure 3.3a shows a significant influence of Ra in SF6 for both polarities. Unexpected large current densities are measured (up to 0.1 nA/cm² at 10 kV/mm), considerably higher than the values predicted if only natural ionisation occurs. Figure 3.3b shows the large influence of the gas water content on the measured current. Higher currents are measured at positive polarity, indicating that field emission is not the only mechanism to be considered. These measurements point to the fact that the water content of SF6 (usually determined by the dew point) probably constitutes an important parameter for the charging of insulators in GIS structures. Further experiments are still necessary to fully ascertain this fact.

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Positive Polarity_Ra=2.26µm

Positive Polarity_Ra=0.57µm

Negative Polarity_Ra=2.26µm

Negative Polarity_Ra=0.57µm

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rent

(A)

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Air_9.26mg/l

SF6_2.85mg/l

Air_0.925mg/l

Air_0.035mg/l

a) b)

Figure 3.3: a) Current density on the high voltage electrode versus electric field strength and surface roughness in SF6 (coaxial brass electrodes); b) Current versus voltage with positive polarity in air

with different water contents (1 MPa) and SF6 (0.8 MPa) [3.04]

4. Application of functionally filled insulating materials with adjustable conductivity

In state of the art epoxy resin based ac insulation materials, different kinds of fillers, e.g. quartz flour or alumina, are applied, mainly for improving their mechanical properties. When refractive or resistive field grading shall be achieved in addition, carbon black is often used. However, the electrical properties of carbon black filled polymers strongly depend on the filling degree and on temperature. They exhibit a steep percolation curve in the region of the percolation threshold, making it thus extremely difficult to exactly achieve specified and reproducible electrical characteristics [4.01]. A promising solution for the development of high-field dc insulators is to use functional fillers of well-defined intrinsic electrical conductivity, filled into the polymer matrix distinctly above the percolation threshold. They should as far as possible be insensitive to the filling degree in the epoxy resin matrix and should provide defined, long-term stable properties of the electric field and temperature dependence of the insulator's overall conductivity. The aspired goals for the resulting insulators are improved electric field distribution, reduction of maximum electric field stress, minimization of electric field inversion effects under the impact of a non-uniform temperature distribution (e.g. due to a heated conductor in a GIS) and fast decay of charges from the insulator surfaces.

4.1. Introduction to antimony doped tin oxide particles

Antimony doped tin oxide (ATO) is well known in applications for transparent conductive layers, i.e. flat TV screens or antistatic floor grounds. A recently developed type of Minatec®- particles are flake shaped mica particles, covered with a nanoscaled semiconducting ATO layer, hereinafter referred to as MFF (mica functional filler). By using multiple layers and by a combination of different dopants, particularly antimony and titanium dioxide, a very fine structure with a large number of potential barriers is obtained (Figure 4.1a). Mixed into acrylate-based films above the percolation threshold, a specific sheet resistance corresponding to a layer conductivity of κS = 10-13 S (ϑ = 35 °C, E = 0.1 kV/mm) and a nonlinearity exponent α = 3 are measured on the compound material [4.02, 4.03]. The mechanism of conduction is based on potential barriers at the grain boundaries of the semiconducting layer, comparable to zinc oxide (micro)varistors [4.04]. The nonlinear field dependent conductivity κV(E) is an intrinsic property of the particles. When they are mixed into a polymeric material at a filling degree distinctly above the percolation threshold one gets an insulation material with reproducible conductivity кV [4.05].

4.2. Measurements and theoretical investigations on functionally filled epoxy resin specimens

MFF particles are applied as filler in an epoxy resin matrix; the filling degree is approximately 39 % of weight. Cylindrical specimens are manufactured and electrically characterized. Bulk conductivity is measured on small discs, which are cut from each specimen and contacted by silver conductive lacquer as electrodes according to IEC 60093 [4.06]. On the same specimens, surface conductivity is evaluated. The measured curves show that the functionally-filled epoxy resin has a nonlinear, field and temperature dependent electric conductivity κV(E,ϑ). The electric bulk conductivity κV is in the range

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of (10-13…10-11) S/m (Figure 4.1b), which is an appropriate range for compact dc GIS insulators in order to achieve a desired field distribution but low losses at the same time. The increase of conductivity with temperature is only moderate, e.g. by a factor of 20 from ϑ = 30 °C to ϑ = 100 °C at E = 4 kV/mm. This result of temperature dependence is in good agreement to the one measured in thin films [4.03]. Due to the anisotropy of the particles, the surface conductivity is considerably higher (cf. Figure 4.1c), which can be helpful for fast surface charge decay. However, if this effect is unwanted the high surface conductivity can be eliminated by machining the surface-near region of the specimen. This has been verified by measurements. Long term investigations with polarity reversals have shown the dc stability of this material as well [4.07].

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100 °C 60 °C 30 °C

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Surface, 60 °C Surface, 100 °C Bulk, 100 °C

a) b) c) Figure 4.1: a) SEM micrograph of MFF particles with visible nanostructure; b) measured bulk

conductivity; c) measured surface U/h/-I curve of the filled epoxy resin specimens

4.3. Field grading with functionally filled nonlinear conductive insulating materials

In order to evaluate the effect of the nonlinear conductivity of a compact dc GIS insulator on the electric field distribution, FEM field simulations are performed taking the field and the temperature dependence of the material’s conductivity into account. The conductivity κV of a simplified 2D axisymmetric disc-type insulator geometry is modelled with nonlinear temperature and field dependence, κV = f(E,ϑ). The MFF-filled insulator is modelled with the measured bulk conductivity and temperature dependence acc. to Figure 4.1. For the conventional material, a conductivity of κV(ϑ)30 °C = 1×10-13 S/m with a typical low temperature dependence of epoxy resin is assumed. The conductor temperature is set to ϑcond = 80 °C and that of the enclosure to ϑencl = 50 °C. The simulation results indicate a more homogeneous field distribution for the MFF insulator (Figure 4.2). The maximum electric field stress is reduced by both the temperature dependence and the additional electric field dependence of the MFF material κV(E,ϑ). The tangential electric field Etan along the insulator’s surface, normalized to the maximum Emax, is significantly reduced for the MFF material, compared to the conventional material (Figure 4.2c).

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Conventional 30 °C / 30 °C MFF-filled 30 °C / 30 °C Conventional 80 °C / 50 °C MFF-filled 80 °C / 50 °C

a) b) c) Figure 4.2: Simulation results of the stationary, temperature dependent electric field distribution on a

disc-type GIS insulator at U = 500 kV, gas conductivity κV,gas = 10-18 S/m. The potential lines and normalized E-field are plotted. a) Insulator with conductivity κV(ϑ)30 °C = 10-13 S/m; b) MFF-filled

insulator with bulk conductivity κV(E,ϑ); conductivity also temperature dependent in both cases; c) Normalized tangential electric field distribution Etan at the insulator’s surface for the two cases “hot”

(ϑcond = 80 °C/ϑencl = 50 °C) and “cold” (ϑcond = ϑencl = 30 °C)

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5. Dielectric strength of insulating gases under direct voltage stress

Today’s standard insulating gas mixture SF6/N2 is used for insulation purposes in GIL systems. However, even with the reduced total amount of the SF6 (gas mixture) and the application of the welding technology (virtually no leakage), there is an ongoing discussion on SF6 alternatives due to its high global warming potential. Therefore, many investigations were performed to substitute SF6 with more eco-friendly insulating gases and gas mixtures. Alternative gases like carbon-nitrogen mixtures, fluorite-carbon combinations but also pure nitrogen and nitrogen-oxygen mixtures have been tested. In the following, results on the dielectric strength of nitrogen, nitrogen-oxygen and nitrogen-oxygen with argon in electrode arrangements with different field distributions under direct voltage stress are reported.

5.1. Dielectric strength of different insulating gases

The dielectric strength of different gases (N2, N2/Ar, N2/O2, N2/O2/Ar, SF6, N2/SF6) is tested in terms of 50% breakdown voltages ud50, using three electrode arrangements with different degrees of uniformity, i. e. a uniform sphere-to-sphere arrangement (sphere diameters 75 mm), a slightly non-uniform sphere-to-plate arrangement (sphere diameter 15 mm) and a highly non-uniform needle-to-plate arrangement (needle tip radius 0.4 mm). All electrode arrangements have a gap distance of 5 mm and are installed in a gas insulated test vessel. Positive and negative direct voltage of up to 150 kV is applied to the test setup. The statistical evaluation is assured by over 30 000 breakdown experiments. The 50% breakdown voltages of an electrode arrangement with different gases are shown in Figure 5.1a (negative direct voltage) and Figure 5.1b (positive direct voltage) for a gas pressure of 0.8 MPa in a uniform field configuration as well as in Figure 5.2a (negative direct voltage) and Figure 5.2b (positive direct voltage) in a non-uniform field configuration, respectively.

a) b) Figure 5.1: 50% breakdown voltage of a sphere-to-sphere arrangement under a) negative and

b) positive direct voltage for different gases at a pressure of 0.8 MPa: ( 1) N2-100%, (2) N2-50%/Ar-50%, (3) N2-95%/Ar-5%, (4) N2-70%/O2-30%, (5) N2-80%/O2-20%, (6) N2-90%/O2-10%, (7) N2-70%/O2-20%/Ar-10%, (8) N2-75%/O2-20%/Ar-5%, (9) N2-80%/O2-10%/Ar-10%, (10) N2-80%/O2-15%/Ar-5%, (11) N2-85%/O2-10%/Ar-5%, (12) SF6-100%(0.3 MPa), (13) N2-80%/SF6-20%(0.5 MPa)

1 2 3 4 5 6 7 8 9 10 11 12 13 1 2 3 4 5 6 7 8 9 10 11 12 13

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a) b) Figure 5.2: 50% breakdown voltage of a needle-to-plate arrangement under a) negative and

b) positive direct voltage for different gases at a pressure of 0.8 MPa: ( 1) N2-100%, (2) N2-70%/O2-30%, (3) N2-80%/O2-20%, (4) N2-90%/O2-10%, (5) N2-70%/O2-20%/Ar-10%, (6) N2-75%/O2-20%/Ar-5%, (7) N2-80%/O2-10%/Ar-10%, (8) N2-85%/O2-10%/Ar-5%, (9) SF6-100%(0.3 MPa) (10) N2-80%/SF6-20%(0.5 MPa)

5.2. Influence of electric field distribution

The electric field distribution (uniform, slightly non-uniform and highly non-uniform) has a significant influence on the dielectric strength of insulating gases for positive and negative polarity. Depending on the results it is possible to characterize the different dielectric behaviour of the gases. For comparison, Table 5.1 shows the 50% breakdown voltages of electrode arrangements with different degrees of uniformity and the investigated insulating gases in relation to the breakdown voltage of the relevant arrangement with SF6.

Table 5.1: 50% breakdown voltages of different electrode arrangements with the investigated insulating gas-mixtures related to the breakdown voltage of the relevant electrode arrangement

insulated with SF6 at 0.8 MPa uniform field

(η = 0.97) slightly non-uniform

field (η = 0.67) highly non-uniform

field (η = 0.17) positive negative positive negative positive negative

N2 (100%)

0.34 0.35 0.34 0.36 0.21 0.26

N2/O2 (80/20%)

0.35 0.33 0.38 0.40 0.32 0.31

N2/O2/Ar (75/20/5%)

0.38 0.38 0.36 0.37 0.33 0.32

The results of the investigation in the strongly non-uniform field (needle-to-plate electrode arrangement) show that the breakdown voltage for the positive polarity of the needle is always lower than that for the negative polarity. According to the literature this polarity effect is influenced by polarity dependence of the generation rate and the nature of the space charges in the vicinity of the needle electrode [5.01] and has been validated by experimental studies for strongly electro-negative gases (SF6), gas mixtures (N2/SF6) as well as for neutral gases (N2) [5.02, 5.03]. These investigations have shown that the effect is not that distinctive for air like in gas mixtures and that addition of a certain amount of inert gas may affect the breakdown voltage positively.

1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10

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5.3. Discussion

In this study the focus is on an investigation of the dielectric strength of tertiary gases, which are mixed by addition of various amounts of argon to the N2/O2 gas mixture. Only for a specific gas mixture of N2/O2/Ar (75/20/5%) a measurable improvement of the 50% breakdown voltages is observed. As possible cause, the formation of neutral, excited and charged molecules is considered, which are generated under the influence of gas discharges and/or the presence of an electric field. With their properties, such as formation of negative ions, the relevant electron affinity, ion and molecule mobility and larger number of energy levels, they could lead to the higher electric breakdown field strength. In the literature the formation of positive ionic excimers [5.04] and argon oxygen cluster ions ArnO

+* [5.05] has been reported and their influence on the gas discharge was proven. Further, in a large number of theoretical and experimental studies, eco-friendly gas mixtures were also analysed with regard to the negative differential mobility and negative differential conductivity (NDC). NDC gas mixtures were discussed as medium for diffuse discharge switches [5.06]. In [5.07] the NDC for argon/oxygen mixtures was found. That means that with regard to the insulating gases research, a gas-mixture is needed, whose conductivity possesses a minimum in the range of the critical reduced field strength. 6. Conclusion Using a simulation model, which takes into account generation, recombination and motion of electric charges in gas, it was shown that the stationary resistive field distribution and the transition time of the capacitive-resistive field transmission in gas-solid insulation systems depend on the ratio of the bulk and surface conductivity of the solid material and the field-dependent bulk conductivity of the insulating gas. Adjustable (and possibly non-linear electric field dependent) bulk and/or surface conductivity of a solid material represents a way to control the resistive electric field. Assuming natural ionization as exclusive generation mechanism for electric charges in a gas, bulk conduction through the solid material dominates the charge transport in gas/solid insulation systems under high electric field stress. However, if an additional source of charges, e.g. field emission from the cathode, is considered, charge transport through the gas might play an important role in the accumulation of charges on dielectric interfaces. The investigations have shown that the existence of a temperature gradient must be considered in simulations since it induces large variations of the electric field distribution. Current densities through gases under high electric field stress of up to 0.1 nA/cm2 suggest additional sources of electric charges besides natural ionization. Measurements have shown the large influence of electrode surface state and water content of a gas on the measurable current density. These phenomena should be implemented in simulations. The manufactured functionally MFF-filled material improves the electric field distribution compared to conventional insulating materials of adjusted conductivity κV(ϑ). It has so far shown robust electrical material properties in short and long term investigations, also under polarity reversal conditions. A more compact geometry of dc compact gas insulated systems may thus be achieved. However, the material can still be optimized. A controlled decrease of conductivity κV by one or two orders of magnitude and a higher nonlinearity exponent α may further help to optimise the dimensions of such systems. Finally, it can be concluded that air like gas mixtures can be used for high direct voltage insulation and that the addition of a very low amount of argon to a nitrogen/oxygen gas mixture leads to improved dielectric performance. However, the dimensions of equipment insulated with these gas-mixtures would increase significantly in comparison to that insulated with SF6, and the gas pressure would have to be higher, resulting in a non-economic solution. Further investigations of eco-friendly insulating gases with the consideration of technical and economic issues are required.

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