Feasibility analysis of reduced-scale air breakdown tests in high voltage laboratories combined with the use of scaled test cages

1590 J. Hernández-Guiteras et al.: Feasibility Analysis of Reduced-scale Air Breakdown Tests in High Voltage Laboratories

1070-9878/13/$25.00 © 2013 IEEE

Feasibility Analysis of Reduced-scale Air Breakdown Tests in High Voltage Laboratories Combined

with the Use of Scaled Test Cages

Joan Hernández-Guiteras, Jordi-Roger Riba Universitat Politècnica de Catalunya Electrical Engineering Department

Rambla Sant Nebridi 22, 08222 Terrassa, Spain

Pau Casals-Torrens and Ricard Bosch Universitat Politècnica de Catalunya Electrical Engineering Department

Avda. Diagonal 647, 08028 Barcelona, Spain

ABSTRACT Breakdown and corona extra- and ultra-high voltage tests are expensive, time-consuming and require large and extremely costly high-voltage halls, of which there are very few worldwide. Some of these limiting requirements may be minimized by using test cages and by applying reduced-scale tests. In this work the feasibility of performing breakdown reduced-scale tests combined with the use of test cages is analyzed. For this purpose a three-dimensional finite elements based methodology is introduced to determine the geometric dimensions of the test cage in order to adjust the test voltage according to the allowable output voltage of a conventional high-voltage laboratory. To validate the findings of this work, breakdown tests of a full-scale 500 kVRMS substation connector and a 1:3 reduced-scale model using test cages are conducted in a high-voltage laboratory, which maximum allowable output voltage is of 125 kVRMS. Results from this work show that by using appropriately designed test cages and scaled versions of the test samples the results may be scaled with high accuracy.

Index Terms — Air breakdown, high-voltage, test cage, finite element method, simulation.

1 INTRODUCTION

FOR over three decades, test cages have been used as an appropriate, fast, space-saving, repeatable and economical method to study corona performance of transmission lines under ac excitation [1, 2]. Test cages allow obtaining high electric field strengths by applying much lower voltage levels than those required in actual operating conditions [3]. By applying a lower voltage to the test sample, the cage allows reproducing the electric field distribution around the test object similarly to that resulting in an actual test [4]. Cage tests have been often used to reduce the scale of the experimental setup and to predict corona effects, including corona losses, audible noise and radio interference [5]. However, under monopolar or bipolar dc excitation some corrections must be applied depending on the measured parameters [6]. Cage tests allow assessing several test objects geometries without the time delays and high costs involved in full-scale tests [7]. It is recognized that empirical

surface electric fields strengths obtained from cage tests under laboratory conditions may be used to reproduce actual transmission line conditions [1, 2]. A test cage is usually a metallic grounded structure which surrounds the test object [4]. To simplify construction and reduce costs, large cages usually have rectangular cross-sections whereas small cages tend to be circular [7]. Electric flux lines generated by circular cross-sections cages tend to be more radially-directed near the test object surface [7]. To minimize space requirements, large cages often use guard rings. When not used, the cage length must be at least three times its diameter [7].

Breakdown tests are being applied in insulating materials such as laminated paper, or different types of rubbers among others. These insulating materials are widely applied in high voltage power devices such as power cables, transformers or insulator strings among others. Since premature degradation of insulation systems due to streamer activity may reduce the availability and performance as well as shorten the lifetime of these expensive devices, the assessment of breakdown properties of insulation systems becomes essential [8]. Insulations tests are often carried Manuscript received on 15 February 2013, in final form 21 June 2013.

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20, No. 5; October 2013 1591

out in small test specimens under laboratory conditions. Breakdown mechanisms present a complex nature, so they are still being investigated because the physicochemical processes involved are difficult to formulate and simulate accurately. Therefore, to perform realistic breakdown tests it is necessary to reproduce in the laboratory operating conditions similar to those found in the real environment. Breakdown tests carried out under realistic operating conditions are also appealing since they allow obtaining the weakest points of the analyzed test objects or electrodes regarding the electrical stress. Since extra- and ultra-high voltage apparatus require very expensive laboratory facilities [9], reduced-scale breakdown tests are an attractive and economical option.

Reduced-scale tests and comparisons with full-scale experiments are often applied in research projects. In particular they have been widely applied to analyze the electric field distribution and the corona activity associated with dc and ac power lines [10-14]. According to [12], when analyzing the electric field distribution, ion current density and corona activity of single phase ac and monopolar dc lines, a great similarity between full-scale and reduced-scale models is obtained. Depending on the size of the objects to be tested, full-scale tests may be very expensive, time-consuming and occasionally may present a limited scope [10]. However, reduced-scale models have several advantages including the possibility of minimizing costs and testing time, models may be studied and adjusted systematically and geometric parameters may be tested over a wide range among others [10]. Reduced-scale modeling is based on the fact that an energized electrode generates an electric field whose amplitude and distribution depends on the geometries of the electrode and the surrounding ground potential objects. In consequence the reduced-scale test model must represent the full-scale object in a realistic manner in order to infer the results in regard the electric field distribution.

It is often difficult to perform full-scale tests to some high-voltage devices because of the difficulty for measuring some required parameters, the very high voltages involved and the size of both the tested objects and the laboratory required [5]. Laboratory-scale tests of electrical breakdown and spark formation have also been applied to study the physical nature of lighting [15]. More recently, Hu et al. [16] applied FEM simulations to calculate the electric field distribution around reduced-scale models and full-scale bundle conductors. The equivalence between reduced-scale models and full-scale bundle conductors was obtained from the information provided by FEM simulations.

Additionally, in [17] the average current ion density and the ground-level electric field were obtained by means of reduced-scale ac-dc hybrid lines and good agreement was attained when compared with experimental results found in the technical literature.

This work is devoted to analyze the feasibility of performing reduced-scale breakdown tests of extra high-voltage substation connectors combined with the use of scaled test cages. Similarly as done in [16], three-dimensional finite elements method (3D-FEM) simulations are applied to calculate the electric field

distribution around the reduced- and the full-scale connector when using test cages with appropriate dimensions. From the 3D-FEM results, the breakdown voltage and the electric field strength on the peak stress point and along the critical field line are calculated by applying the method proposed in [18], which is based on the Pedersen criterion [19]. From the simulation results a methodology to determine the appropriate test cage dimensions is presented. The usefulness of this method is validated by applying breakdown tests to a full-scale and a 1:3 reduced-scale substation connector. These tests are performed by using inexpensive tailor-made cylindrical test cages which have been calculated according to the substation connectors dimensions and the maximum available output voltage of the high-voltage laboratory in which the tests are conducted. The methodology presented in this work allows testing the connector under limit conditions, even harsher than those found under operating conditions in real environments, thus allowing settling a safety margin. These breakdown tests are useful for design engineers since they allow testing the connector and to find out the most vulnerable points of its geometry when the connector is pushed to its limits regarding the electrical stress. The use of scaled models allows reproducing these tests in a fast and inexpensive manner, since the laboratory apparatus required to perform these tests (mainly the generator, measurement system and accessory tools) are much less costly that those found in specialized extra- and ultra-high voltage laboratories and the space and clearances required are minimized, these being severe limiting factors to carry out factory tests. In addition, the manufacture of metallic cylindrical test cages is simple and economical, thus simplifying the application and requirements of the factory tests.

The paper is organized as follows. Section 2 describes the substation connectors and the test cages analyzed, whereas Section 3 gives details about the 3D-FEM method combined with the Pedersen criterion, which are applied to calculate, for a given substation connector, both the cage dimensions and the breakdown voltage according to the available output voltage of the laboratory. Section 4 describes the experimental setup and Section 5 details the experimental tests carried out to verify the suitability of the proposed methodology. Finally, the conclusions are settled in Section 6.

2 THE SUBSTATION CONNECTORS AND THE TEST CAGES ANALYZED

The analyzed Z12SZ2LS full-scale substation connector is from SBI catalogue and connects two aluminum substation bus bars of 120 mm diameter aluminum each. It was designed to operate at 500 kVRMS (line-to-line) and is made of A356 cast aluminum alloy.

As explained, a 1:3 reduced-scale model of this connector was especially built for this purpose. Figure 1 and Table 1 show the main geometric dimensions of both versions of the Z12SZ2LS substation connector.

Figure 2 shows both the reduced-scale (1:3) and the full-scale (1:1) connectors and their respective bus bars, which also preserve the same geometric ratios, i.e. 1:3 and 1:1.


Figure 1. Three-dimensional view of the full-scale Z25SZ2LS substation connector analyzed in this work.

Table 1. Main geometrical dimensions of the full-scale and reduced-scale connectors.

A (mm) B (mm) C (mm) Full-scale (1:1) 204 231 120 Reduced-scale (1:3) 68 77 40

The substation connectors analyzed in this work were tested in

the high-voltage laboratory of the Universitat Politècnica de Catalunya (Barcelona, Spain), which has a maximum ac output voltage of 125 kVRMS. In order to test them under these constrains, test cages with suitable geometric dimensions are required. For this purpose, 3D-FEM simulations jointly with a revised version of the Pedersen criterion, which is found in [18], are performed. The theoretical bases of such calculations are detailed in Section 3. As stated in [18], the accuracy of this method is usually better than 6.5%. Therefore, the geometrical dimensions of the cage for which a pre-settled voltage level (when testing the analyzed substation connector) leads to the breakdown condition are calculated. By means of this procedure, the full-scale cage is calculated, whereas the reduced-scale one is obtained by rescaling this latter by a scale factor 1:3. However, simulation results need to be validated by means of experimental tests, which are reported in Section 5.

Figure 2. The reduced-scale and full-scale connectors and bus bars dealt with in this work.

3 THE 3D-FEM METHOD COMBINED WITH

THE PEDERSEN CRITERION The 3D geometric model of the substation connector was

carefully prepared and simplified in order to limit its complexity, while trying not to influence its accuracy. The 3D model was developed in the commercial COMSOL® FEM package using the electrostatic module (which allows solving both DC and AC stationary problems). Therefore, the Poisson’s equation was solved in the air gap between the connector and the test cage,

0

2

V (1)

where V is the electric potential, ε0 is the vacuum electric permittivity, and ρ is the charge density, which was settled to zero since air is considered free of space charge. Therefore, equation (1) leads to Laplace’s equation. Next, the electric field in each point of the domain was calculated as in

VE (2) The material parameters used in the 3D-FEM calculations are

shown in Table 2.

Table 2. Materials parameters used in the 3D-FEM simulations.

Material ASTM

code Relative

permittivity Resistivity

(Ω·m) Substation connector Aluminum A356 1 4·10-8 Bus bars Aluminum 6101B 1 3·10-8 Test cage Iron A536 1 6·10-7 Air Vacuum 1 6·1015

The boundary conditions for the electric potential are schematically shown in Figure 3. The aluminum bus bars and the substation connector are connected to the voltage source whereas the test cage is grounded.

Figure 3. Schematic view of the substation connector analyzed in this work and the cylindrical cage. a) Simulated setup. b) 3D-mesh of the connector and bus bars.

In order to minimize the material requirements and produce

radially-directed electric flux lines near the test object surface, the test cages are supposed to have circular cross-section. Additionally, to minimize their cost, the cages are made of commercially available iron pipes. To avoid contour effects, the cage length must be at least three times the longitudinal dimension of the test object. In addition, the cage length must be at least three times its diameter [7].

Results obtained from 3D-FEM simulations combined with those provided by the Pedersen criterion explained in the following paragraphs are used to select the test cage


dimensions in order not to exceed the laboratory output voltage capacity.

It is recognized that in the case of uniform gaps, a spark breakdown may occur between the two energized electrodes without any preliminary discharges. Nevertheless, when dealing with non-uniform gaps, the breakdown is always preceded by localized corona discharges [20].

Following the Pedersen hypothesis [19], in [18] a FEM-based model combined with the semi-empirical Pederson criterion is developed to predict the breakdown or corona onset condition in non-uniform gaps. Based on this methodology, the points of the electrode surface where breakdown may occur, if any, are located for a given applied electric potential. This methodology is based on the following sequential steps:

1.- By means of FEM simulations the electric field distribution in all points of the analyzed domain is calculated.

2.- The peak stress point of the analyzed electrode surface is located. Next, the critical field line is plotted. This is the field line that starts in the peak stress point on the electrode surface.

3.- It should be computed the value of (the effective

first ionization coefficient) in all points of the critical field line. These values are tabulated in [21] for atmospheric air as a function of the electric field strength. Note that α is the first ionization coefficient (the most probable number of ionizing collisions for an electron per unit length of path) and η is the attachment coefficient. The values of and are obtained from [22] and [23], respectively, and are a function of the electric field strength. Next, the values of the function that explains the development of streamers in air when dealing with non-uniform fields

z

uniformnon 'dz)'z()(lnzg0z)(

(3) are computed, where z’ is a dummy variable. For a detailed mathematical background of this equation the reader is addressed to [18, 19].

4.- In the case of a uniform field gap in atmospheric air, equation (3) results in

z)(lnzguniform )( (4)

5.- The functions guniform(z) and gnon-uniform(z) are calculated and compared along all points of the critical field line. If any point accomplishes that gnon-uniform(z) ≥ guniform(z), it means that in this point the breakdown condition may occur. This is the Pedersen’s criterion.

Following the above-described procedure, different diameters of the cylindrical cage were checked in order that the breakdown condition was attained for a voltage level less than 125 kVRMS, the maximum allowable output of the high-voltage laboratory dealt with. After some simulations, in the case of the full-scale Z12SZ2LS substation connector, a 450 mm diameter test cage was selected. Figure 4 summarizes the iterative process carried out for sizing the full-scale cage. The dimensions of the reduced-scale cage are obtained by dividing

those of the full-scale cage by a suitable factor, which is 3 for the case under analysis.

)(z

'd)'()(0

zzzz

Figure 4. Procedure carried out for sizing the full-scale test cage.

Figure 5 shows the electric field distribution on the full-

scale Z25SZ2LS connector surface jointly with the critical field line. This is the field line that starts on the peak stress point of the connector surface. The 3D-FEM simulations consider that the connector and the bus bars are placed concentrically with the cylindrical iron cage of 450 mm diameter, as shown in Figure 3. Next, from these results, the five-steps-procedure detailed in the former paragraphs is applied to determine whether or not the breakdown condition is attained in any point placed between the substation connector and the inner surface of the cylindrical cage.

By applying the five steps of the above-mentioned methodology to the full-scale substation connector when using a 450 mm diameter cylindrical cage, the breakdown condition is achieved for an applied stationary voltage of 107.8 kVRMS. Note that as explained, the breakdown condition is attained when gnon-

uniform(z) given by (3) (which corresponds to the connector gap) intersects the guniform(z) function given by (4) (which corresponds to the plane-to-plane air gap). For the analyzed geometry, this condition is reached in the critical field line point given by z ≈ 3.5 mm. Therefore, in the case of being the results scalable, the breakdown voltage for the reduced-scale connector should be about one third of that corresponding to the full-scale connector (107.8 kVRMS / 3 = 35.9 kVRMS) when using a 150 mm diameter cylindrical cage. However, these assumptions must be validated through experiments to carefully verify the feasibility of the proposed methodology.


a)

Critical field line (z, p.u)

0.0 0.2 0.4 0.6 0.8 1.0

Ele

ctric

fie

ld (

kV/m

m)

0

1

2

3

4

5

6

Ele

ctric

po

tent

ial (

kV)

0

20

40

60

80

100

120

Electric potential

Electric field

b)

Figure 5. Full-scale connector with the bus bars when energized at 107.8 kVRMS. FEM simulations assuming a cylindrical cage of 450 mm diameter which is concentric with the connector and the bus bar symmetry axis. a) Electric field distribution and critical field line starting at the peak stress point of the connector surface. b) Electric potential and electric field strength along the critical field line. The origin, i.e. z = 0, is taken on the peak stress point of the connector surface.

Figure 6 shows the comparative results between the calculation of guniform(z) and gnon-uniform(z) when the full-scale version of the Z12SZ2LS substation connector is energized at an stationary voltage of 107.8 kVRMS. Note that this voltage is compatible with the maximum output voltage of the laboratory dealt with, i.e. 125 kVRMS.

Therefore, from the above-mentioned simulation results it is proposed to carry out two sets of tests in the high-laboratory, one for each scale. In accordance with the explanations above, each connector must be tested with its own scaled cage, whose dimensions are shown in Table 3.

All dimensions of both cages are related by the same scale factor (1:3). Additionally, this scale factor is applied to the dimensions of the two scaled versions of the analyzed connector and their respective bus bars dimensions.

Critical field line (z, mm)0 5 10 15 20

g(z)

func

tion

5

10

15

20

25

guniform(z)

gnon-uniform(z)

zbreakdown

Figure 6. Full-scale connector. Simulation results obtained by means of the combined action of 3D-FEM and the Pedersen-based method when using a cylindrical cage of 450 mm diameter. Comparative results of guniform(z) and gnon_uniform(z) when applying a stationary voltage of 107.8 kVRMS. Note that z is the distance to the connector surface measured along the critical field line and zbreakdown = 3.5 mm is the critical avalanche length measured from the connector’s surface.

Table 3. Geometrical dimensions of the full-scale and reduced-scale cages. Diameter (mm) Length (m) Thickness (mm)

Full-scale (1:1) 450 3 3 Reduced-scale (1:3) 150 1 1

4 EXPERIMENTAL SETUP In this section the experimental setup used to validate the

methodology detailed in Section 3 is described. As explained, a full-scale Z25SZ2LS connector and a 1:3 reduced-scale model were tested in the high-voltage laboratory of the Universitat Politècnica de Catalunya (Barcelona, Spain). Their respective bus bars and cylindrical cages also maintain the same scale factor, as detailed in Section 3.

Figure 7 shows a diagram of the single-phase experimental setup used to obtain the ac high-voltage output. It consists of a 0-220 VRMS, 25 kVA, 50 Hz variable voltage autotransformer which is connected to a 220-440 VRMS, 25 kVA insulating transformer. The output of this transformer is connected to the 440-300000 VRMS, 75 kVA high-voltage transformer. The output current of this transformer is limited by means of a 300 k resistance. A 127000/63.5 VRMS voltage instrument transformer is used to adapt the test voltage to the requirements of the voltmeter used to measure the voltage. The autotransformer output voltage is mechanically limited to 91.7 VRMS in order to not exceed the 125 kVRMS high-voltage output measurable by the instrument voltage transformer.

Figure 7. Experimental setup.

As explained in Section 3, the calculated breakdown voltages according to the methodology proposed in this work are 107.8 kVRMS and 35.9 kVRMS for the full-scale and the reduced-scale test objects, respectively. These values are compatible with the


maximum allowable output voltage of the high-voltage laboratory dealt with, which is 125 kVRMS.

5 EXPERIMENTAL RESULTS In this section experimental breakdown tests are carried out

using both the full-scale and reduced-scale test objects (connector and bus bars) and their respective test cages, whose dimensions are described in Table 3. For this purpose the substation connector and the two bus bars are placed inside the cylindrical test cage. Both the longitudinal axis of the connector and the bus bars and the symmetry axis of the cage are aligned to ensure a regular disposition of the test object inside the cage. Next, the laboratory high-voltage output is connected to the test object whereas the cylindrical iron cage is grounded. This arrangement is depicted in Figure 8.

Figure 8. a) Experimental test setup with both test cages. b) Photograph of the interior of the test cage showing the bus bar, the substation connector and the inner surface of the test cage.

To perform the breakdown tests, the output voltage applied to the test object is regularly and smoothly increased from zero until spark breakdown of air. Next, the setup is disconnected and a new test is performed. The experimental tests were performed with the laboratory in partial dark. This process was performed four times under the same local atmospheric conditions and the average values of the breakdown voltage were calculated, as shown in Tables 4 and 5. Breakdown voltages presented in Tables 4 and 5 have been corrected to standard atmospheric conditions (20 ºC, 101.3 kPa and 11 g/m3 humidity).

Table 4. Full-scale breakdown test results using a 450 mm diameter cage. Test Breakdown voltage (line-to-ground, kVRMS)*

1 100.1 2 101.9 3 102.0 4 101.2

Average value 101.3 *The tests were conducted at local atmospheric conditions (18 ºC, 1017.2 hPa, and 72% relative humidity).

Table 5. Reduced-scale breakdown test results using a 150 mm diameter cage. Test Breakdown voltage (line-to-ground, kVRMS)*

1 33.8 2 33.9 3 33.6 4 35.2

Average value 34.1 *The tests were conducted at local atmospheric conditions (18 ºC, 1017.2 hPa, and 72% relative humidity).

Table 6 shows the comparative results of both full-scale and reduced-scale tests. From these results it can be concluded that these types of tests are feasible since the results differ in a scale factor of 2.97 which is very close to its theoretical value of 3.

Table 6. Experimental breakdown tests results. Experimental

Full-scale (1:1) breakdown voltage (kV) 101.3 Reduced-scale (1:3) breakdown voltage (kV) 34.1 Experimental breakdown voltage ratio 101.3/34.1 = 2.97 Theoretical scale factor 3

To determine the accuracy of the methodology presented in

this work (3D-FEM combined with the Pedersen criterion), the results of the experimental tests are compared with those obtained by means of the simulations presented in Section 3, as detailed in Table 7.

Table 7. Comparative results between simulations and experimental breakdown tests.

Experimental Simulation Error Full-scale (1:1) 101.3 107.8 6.4 % Reduced-scale (1:3) 34.1 35.9 5.3 %

The results presented in Table 7 show differences between experimental and simulation results of 6.4 % and 5.3 % for the full-scale and reduced-scale tests, respectively. These results prove that the simulation methodology analyzed in this work may be a valuable tool for designing reduced-scale tests and their associate cages.

Figure 9 shows the trace of the spark breakdown in the full-scale connector.

Figure 9. Photograph of the tested full-scale connector showing the trace produced by the spark breakdown.

6 CONCLUSION Electrical equipment designed to operate at extra- and ultra-

high voltages required to be tested in very large and costly high-voltage laboratories, of which there are very few worldwide. Test objects designed to operate under these special voltages are very large, and require the use of cranes and other special equipment. Hence, these tests are time consuming and very expensive.

To minimize these requirements, the combined use of cage tests and reduced-scale test samples has been investigated in this work. The dimensions of the test cages have been designed in such a way that the expected breakdown voltage is adapted to the maximum output voltage level of a usual high-voltage laboratory. To calculate the test cage dimensions, a 3D-FEM methodology combined with the Pedersen criterion has been applied.


To validate the usefulness of the methodology proposed in this work, the electrical behavior of a 500 kVRMS substation connector has been tested in a high-voltage laboratory which maximum output voltage is 125 kVRMS. For this purpose two types of tests have been performed. The first one deals with the full-scale connector and the second with a 1:3 reduced-scale model of the connector. Both breakdown tests have been performed by using cylindrical test cages, which maintain the same 1:3 geometric scale factor. Experimental results show the feasibility and usefulness of the proposed methodology because the ratio between the experimental breakdown voltage of the full-scale and reduced-scale tests is 2.97, whereas the applied scale factor is 3.

Therefore, it is concluded that by the combined use of reduced-scale tests and the appropriate design of test cages as suggested in this work, it is feasible to perform factory breakdown tests in a fast and cost-effective manner. These tests allow locating the most vulnerable points of the analyzed connectors regarding the electrical stress while providing assistance during the design and optimization process of the connectors before performing normalized full-scale tests.

ACKNOWLEDGMENT The authors wish to acknowledge the financial support

received for carrying out this work from CDTI (Centro para el Desarrollo Técnico e Industrial of the Ministerio de Ciencia e Innovación de España) under the MEGAVOLT research project and from the KIC InnoEnergy under the CIPOWER research project.

REFERENCES [1] R.D. Dallaire, P.S. Maruvada and N. Rivest, “HVDC monopolar and

bipolar cage studies on the corona performance of conductor bundles”, IEEE Trans. Power Appar. Syst., Vol. 103, pp.84-91, 1984.

[2] M. J. Lekganyane, N. M. Ijumba and A. C. Britten, “A comparative study of space charge effects on corona current using an indoor corona cage and a monopolar test line”, IEEE PES Power Africa Conf. Exposition, Johannesburg, South Africa, pp. 1-6, 2007.

[3] F.-C. Lü, S.-H. You, Y.-P. Liu, Q.-F. Wan, and Z.-B. Zhao, “AC conductors’ corona-loss calculation and analysis in corona cage”, IEEE Trans. Power Delivery, Vol. 27, No. 2, pp. 877-885, 2012.

[4] Y. Nakano and Y. Sunaga, "Availability of Corona Cage for predicting Audible Noise generated from HVDC Transmission Line," IEEE Trans. Power Delivery, Vol. 4, No. 2, , pp. 1422-1431, 1989.

[5] X. Bian, D. Yu, X. Meng, M. MacAlpine, L. Wang, Z. Guan, W. Yao, and S. Zhao, “Corona-generated space charge effects on electric field distribution for an indoor corona cage and a monopolar test line”, IEEE Trans. Dielectr. Electr. Insul., Vol. 18, No. 5, pp.1767-1778, 2011.

[6] N.M. Ijumba, M.J. Lekganyane and A.C. Britten, “Comparative Studies of DC Corona Losses in a Corona Cage and a Point-Plane Gap”, pp. 1-7, 8th IEEE Africon Conf., Windhoek, Namibia, pp. 1-7, 2007.

[7] R.G. Urban, H.C. Reader and J.P. Holtzhausen, “Small corona cage for wideband HVAC radio noise studies rationale and critical design”, IEEE Trans. Power. Delivery, Vol. 23, No. 2, pp. 1150 -1157, 2008.

[8] A. C. Garolera, J. Holboell, M. Henriksen, “Breakdown and Tracking Properties of Rubber Materials for Wind Turbine Blades”, IEEE Int’l. Sympos. Electr. Insul. (ISEI), pp. 516-519, 2012.

[9] J.-K. Seong, W. Choi, W.-J. Shin, J.-S. Hwang, and B.-W. Lee, “Experimental and Analytical Study on DC Breakdown Characteristics of Butt Gap Condition in LN2/PPLP Composite System”, IEEE Trans. Appl. Supercond., Vol. 23, No. 3, pp. 5401604, 2013.

[10] S. A. Sebo. D. G. Kasten, T. Zhao, L. E. Zaffanella, B. A. Clairmont, and S. Zelingher, “Development of reduced-scale line modeling for the

study of hybrid corona”, IEEE Conf. Electr. Insul. Dielectr. Phenomena, pp. 538–543, 1993.

[11] D. G. Kasten, S. A. Sebo, T. Zhao, L. E. Zaffanella, B. A. Clairmont, “Corona tests on reduced-scale two-conductor hybrid lines”, IEEE Conf. Electr. Insul. Dielectr. Phenomena, pp. 624-629, 1993.

[12] T. Zhao, J. Illan, J. M. Cohol, R. D. Hinton, S. A. Sebo, and D. G. Kasten, “Design, Construction and Utilization of a New Reduced-Scale Model for the Study of Hybrid (AC and DC) Line Corona”, IEEE Conf. Electr. Insul. Dielectr. Phenomena, pp. 239-245, 1993.

[13] L. A. Metwally, “Investigation of Corona Phenomenon for A Reduced-Scale Model Of Hybrid AC And DC Lines”, IEEE Conf. Electr. Insul. Dielectr. Phenomena, San Francisco, California, USA, pp. 610-613, 1996.

[14] Z. M. Al-Hamouz, M. Abdel-Salam, and A. M. Al-Shehri, “Inception Voltage of Corona in Bipolar Ionized Fields Effect on Corona Power Loss”, IEEE Trans. Ind. Appl., Vol. 34, No. 1, pp. 57-65, 1998.

[15] R. J. Van Brunt, T. L. Nelson, and K. L. Striklett, “Early streamer Emission Lightning Protection Systems: An Overview,” IEEE Electr. Insul. Mag., Vol. 16, No. 1, pp. 5-24, 2000.

[16] Q. Hu, L. Shu, X. Jiang, C. Sun, S. Zhang, and Y. Shang, “Effects of air pressure and humidity on the corona onset voltage of bundle conductors”, IET Gener. Transm. Distrib., Vol. 5, No. 6, pp. 621–629, 2011.

[17] H. Yin, J. He, B. Zhang, and R. Zeng, “Finite Volume-Based Approach for the Hybrid Ion-Flow Field of UHVAC and UHVDC Transmission Lines in Parallel”, IEEE Trans. Power Delivery, Vol. 26, No. 4, pp. 2809-2820, 2011.

[18] J. Hernández-Guiteras, J.-R. Riba, P. Casals-Torrens, “Determination of the Corona Inception Voltage in an Extra High Voltage Substation Connector”, IEEE Trans. Dielectr. Electr. Insul., Vol. 20, No. 1, pp. 82-88, 2013.

[19] A. Pedersen, “Calculation of spark breakdown or corona starting voltages in nonuniform fields,” IEEE Trans. Power Appar. Syst., Vol. 86, No. 2, pp. 200-206, 1967.

[20] H. Anis, and K. D. Srivastava, “Pre-breakdown discharges in highly non-uniform fields in relation to gas-insulated systems,“ IEEE Trans. Electr. Insul., Vol. 17 No. 2, pp. 131-142, 1982.

[21] J. P. Donohoe, “Physical characteristics of criteria governing the computation of air gap breakdown voltages for slightly divergent fields,” IEEE Trans. Dielectr. Electr Insul, Vol. 5 No. 4, pp. 485-492, 1998.

[22] D. W. V. Planck, "Calculation of initial breakdown voltages in air," Trans. American Inst. of Electr. Engineers, Vol. 60, No. 3, pp. 99-104, 1941.

[23] D. B. Phillips, R. G. Olsen, and P. D. Pedrow, "Corona onset as a design optimization criterion for high voltage hardware", IEEE Trans. Dielectr. Electr. Insul., Vol. 7, No. 6, pp. 744-751, 2000.

Joan Hernández-Guiteras (S’13) was born in Barcelona, Spain in 1986. He received the M.S. degree in Industrial Engineering from the UPC, Universitat Politècnica de Catalunya (Spain) in 2010. The same year, he became a member of the Electrical Engineering Department and a member of MCIA Research group as Ph.D. degree student. He is part of MCIA research activities concerning high voltage energy transmission systems and energy efficiency. His interests are atmospheric air partial discharges,

high voltage testing and computer coupled simulations in high voltage devices using FEM tools and CAD design.

Jordi-Roger Riba Ruiz (M’09) was born in Igualada, Spain in 1966. He received the M.S. degree in Physics and the Ph.D. degree from the Universitat de Barcelona, Barcelona, Spain, in 1990 and 2000, respectively. In 1992, he joined the Escola d’Enginyeria d’Igualada, Universitat Politècnica de Catalunya (Spain), as a full-time Lecturer and the Electric Engineering Department in 2001. He belongs to the Motion and Industrial Control Group (MCIA). His research interests include partial discharges, high

voltage engineering, electromagnetic devices modeling and electric machines.


Pau Casals-Torrens received the Electrical Engineering degree from the University of Orient (Venezuela) in 1984, the postgraduate in electrical power systems from the Central University of Venezuela in 1989, and the Master and Ph.D. Degrees from the Universitat Politècnica de Catalunya (Spain) in 2004 and 2007, respectively. He was engineer and head of the Quality and Electrical Testing Department at the BICC-Cable Company (Spain). Actually he is a Professor in the Electrical Engineering Department of the Universitat Politècnica de Catalunya, Barcelona,

Spain. He combines his research on on-line tests and measures in high voltage equipment with projects in utility companies.

Ricard Bosch Tous received the M.S. degree in electrical engineering and the Ph.D. degree from the Universitat Politècnica de Catalunya, Barcelona, Spain in 1982 and 1987, respectively. From 1970 to 1982 he worked at the automobile factory of SEAT as maintenance manager and from 1982 to 1983 at Nassarre foundry. He joined the Electrical Engineering department of the Universitat Politècnica de Catalunya in 1984 where he currently is an

Associate Professor. His research interests include high-performance electrical machines, axial flux motors, electrical generation, electrical applications for naval propulsion, partial discharges, and high-voltage applications.

Documents

Feasibility analysis of reduced-scale air breakdown tests in high voltage laboratories combined with the use of scaled test cages