Ingeniería e Investigación ISSN: 0120-5609 [email protected] Universidad Nacional de Colombia Colombia

Gómez, S.; Buitrago, M.P.; Roldán, F.A. Portable High Voltage Impulse Generator Ingeniería e Investigación, vol. 31, núm. 2, octubre, 2011, pp. 159-164 Universidad Nacional de Colombia Bogotá, Colombia

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Portable High Voltage Impulse Generator

Generador Portátil de Impulsos de Tensión.

S. Gómez 1, M.P. Buitrago 2, F.A. Roldán 3

Abstract — This paper presents a portable high voltage impulse 1. INTRODUCTION generator which was designed and built with insulation up to 20 Dielectricstrength testsof materialsused aselectrical kV. This design was based on previous work in which simulation software for standard waves was developed. Commercial insulatorsare part of widely used andinternationally accepted components and low-cost components were used in this work; qualitytests or trials and they are subject to rulesor however, these particular elements are not generally used for standardsestablished bycorresponding institutions,such asthe high voltage applications. The impulse generators used in AmericanSociety forTestingof Materials(ASTM) and the industry and laboratories are usually expensive; they are built to InternationalElectrotechnicalCommission(IEC). withstand extra high voltage and they are big, making them An insulationcoordination study must be done toensurethat impossible to transport. The proposed generator is portable, thereby allowing tests to be made on devices that cannot be high voltagematerialstoleratedifferent overvoltage throughout moved from their location. The results obtained with the their life. These techniquesare used to selectthe dielectric proposed impulse generator were satisfactory in terms of time strengthor insulationlevel for high voltage materials which and waveforms compared to other commercial impulse mustbe able to support normalised voltages havingdifferent generators and the standard impulse wave simulator. waveforms (the most common types are lightning and switching). Keywords — Electrical insulation, voltage impulse generator, Some authors, (ASTM,2004;IEC, 2001), have stated that insulation coordination, power disruption, standardised waves, standardised wave simulator. impulse voltage generatorscapable ofproviding impulsewaveslarge enough tocause apowerdisruptionin the Resumen —En este trabajo se presenta un generador portátil proof element are neededfordielectric strengthtesting.The de impulsos de tensión, diseñado y construido con un aislamiento tested material’s electrical parameters,such ascapacitance,can hasta para 20 kV. El diseño fue basado en un trabajo previo en el affectmagnitude and the waveformappliedby the generator. cual se desarrolla un software de simulación implementado Such capacitance should thus be taken into account when exclusivamente para ondas de impulso normalizadas. Los measuring,adjusting andmonitoring thevoltagewaveform. componentes empleados fueron en su totalidad de bajo presupuesto, comerciales y algunos generalmente no son usados An impulse generator was designed in (Lora,2008)where en alta tensión. Con el generador de impulsos se obtuvieron most ofthe projectcomponentswere imported,expensive, not resultados satisfactorios en cuanto a tiempos y formas de onda, verycommercial and built for very specific applications, comparados con otros generadores de impulsos comerciales y el thisbeingthe greatest disadvantage(high implementation costs). simulador de ondas de impulso normalizadas. Los generadores A simulation and numerical optimisation tool was de impulso utilizados en la industria y laboratorios eléctricos son developedin (Carmano et al) which used a minimum squares normalmente de gran tamaño, costosos y fabricados para soportar trabajos en extra alta tensión, ocupando demasiado variant to compare mathematical model output against the espacio e imposibilitando su transporte. De ahí la importancia de output system. This tool calculated electrical circuit values este proyecto, pues siendo portátil facilita realizar pruebas en during impulse trials for elements which could be handled. It elementos que no se puedan desplazar de su ubicación. was stated that the optimisation model would be better as soon as the amount of difficult to obtain experimental data became Palabras Claves: Aislamiento eléctrico, Coordinación de expanded. aislamiento, Disrupción eléctrica, Generador de impulsos de tensión, Ondas Normalizadas, Simulador de Ondas de Impulso. Another article (Electrical Testing Group) has shown how a voltage impulse generatoris typically used in techniques forfindingfaults inelectricaltransmission and distribution systemsin high and mediumvoltage,calledhigh power reflectometry. It was concluded thatan impulse 1Works is with the Department of Electrical, Electronical and Computational Engineering, National University of Colombia, Manizales.(e- generatorallows testing mail: [email protected]). transformerstoobtaindatarepresentation, associated capacitance 2Is with the Department of Electrical, Electronical and Computational and fault detection regarding transformer insulation. Engineering, National University of Colombia, Manizales.(email: To complement the aforementionedwork, a voltage impulse [email protected]) 3Is with the Department of Electrical, Electronical and Computational wave simulator wasdeveloped,based on wave normalisation Engineering, National University of Colombia, Manizales. (e-mail: using agraph technique ornomogramstudied in (Aguet and [email protected]). Ianoz, 1990) and previously usedinthe proposedsimulation by

159 PORTABLE HIGH VOLTAGE IMPULSE GENERATOR

(Idarraga and Roldán, 2005) ,where it was onlynecessary to set thecomponents to be simulatedwithout obtainingpreliminaryexperimental datatoconduct an impulsewave analysis . A portable generatorwas thus designedfrom simula tion results,considering the field applicationnoted above ; a portable impulse generator was then constructed giving normalised voltage waves for lightning and switching types, using low -cost implementation components. Because of the small scale design, th ere were limitations on the voltage generator supply as the generator only delivered up to 20kV impulse voltage waves .

2. THEORETICAL BACKGROUND Voltage im pulse generatorsproducewaves which can be classified asimpulselightning and impulseswitching,with 1.2 - 250 sstandard front time and 50 -2,500 s for tail time (IEC Standard 60060 -1, 1989). Fig. 2. RLC Circuits . Rs1,Rs2,Rs: Front , Rp: Tail Resistor, Cg: Discharge , Cc: Charge capacitor, L:

These kinds of circuit give an impulse wave as output (such as that in Figure3) resulting fro msubtracting twoexponential functions (Aguet and Ianoz, 1990) .

Fig. 1. Lightning Impulse

A. Time measurements for a lightning wave Fronttime T for a lightning impulse is 1.67 times time 1 interval T (Figure 1, (IEEE Standard 4, 1995) ) between the Fig. 3. Characteristic Impulse Voltage instants when an impulse is 30% and 90% of peak value .Tail time T2for a lightning impulse is the time interval between virtual origin T and the instant on the tail when the voltage Equation (1) describes this kind of impulse: o has decreased to half (50%) peak value.Standardtolerances for front and tail timeare 30 % and 20%, respectively (IEEE , (1) Standard 4, 1995; Kuffel andZaengl, 1970) . ∆ where, and are time constants depending on circuit components (Aguet and Ianoz, 1990). B. Time measurement for a switching wave

Front time Tcr ,ismeasuredby reaching peak voltage , while D. Normalising the wave equation tail timeThis measuredwhenmaximum voltagedrops to50%.Standard front and tail timetolerances are 20 % and According to (Aguet and Ianoz, 1990) , im pulse wave(2) is 60% , respectively (IEEE Standard 4, 1995;Kuffel and Zaengl, used fornormalisation: 1970) . (2) C. Impulse generator The generalised schemes for a single stage withcapacitive , Such simplificationis associated with a graph resistiveand inductive components are used to generatea callednomogram orabacus (shownin Figure4) (Aguet and standard impul se wave , as shown in Figure 2. Ianoz, 1990) .This graph relatesthe determinant factor

160 REVISTA INGENIERÍA E INVESTIGACIÓN Vol. 31 Suplemento No. 2 (SICEL 2011) , OCTUBRE DE 2011(159-164) GÓMEZ, BUITRAGO, ROLD ÁN. ofvoltage impulse shape α,and the determinant coefficient of Table 2. F ormulas for the components time θ. Circuit X(1) Rsi( Ω) Rp( Ω)

1 1 1 1 √1 1 √1 2 1 1 1 √1 1 √1 3 1 2 1 1 1 √1 4 2 √1 4 ------ 5 --- Ω 3. EXPERIMENTAL FRAMEWORK A. Simulat or The standardisedvoltage im pulsesimulator shownin Figure5was designed using the Matlab platformguide . This softwareallows theuser to obtain thewaveform forthe type of selected circuitfromfive possible optionsby determiningfr ont andtail times forlightningorswitching . The component values can also be obtained the typeof impulse,theselected circuitand capacitor values.

Fig. 4.Nomogram o rAbacus

Expressions forT 2/T 1, T 2/θorT h/θandT h/T cr are derivedfromthe nomogram curvesusedfor component andtime calculations. Theseequations simplify the characteristic component calculation for an im pulse generatorfrom thetype of knownvoltagewave or voltage impulse waveregardingthe components being used . E. Characteristic coefficients Characteristic coefficients α, θ and η are determined for each type of circuit according tothe equations shownin Table1, which were obtained from (Ague t and Ianoz, 1990) .

Table 1. F ormulas for the characteristic coefficients

Circuit Θ(s) η(1) α(1)

1 1 1 2 2 1 1 1 2 3 1 2 1 1 2 4 1 1 1 2 5 --- 1 2 Fig.5. Graphical interface of the simulator The equations presented inTable2, which were obtained from (Aguet and Ianoz, 1990) , can be used for calculatingthe The procedurecan be summarised bythe scheme presented componentsbased onthe se lection of thekind of scheme . inFigure 6 .

REVISTA INGENIERÍA E INVESTIGACIÓN Vol. 31 Suplemento No. 2 (SICEL 2011) , OCTUBRE DE 20 11(159-164) 161 PORTABLE HIGH VOLTAGE IMPULSE GENERATOR

Fig. 7.Elements of the Portable Voltage Impulse Generator Fig.6.Flowchartof the simulator Table 4. design and construction The simulator wasused to testcommercial generatordatabases and selectappropriate valuesfor space Charge Capacitor Discharge Capacitor Impulse Cg(µF) Cc(µF) requirements,constructioncosts and electrical insulation. It was Type Individual Amount of Individual Amount of then decided to buildthe elements using the values describedin Value (µF) Capacitors Value (µF) Capacitors Table3. Lightning & 0.56 21 0.047 39 Table 3. Nominal Values of Generator Components Switching 0.025 µF 0.0012 µF Impulse Circuit Capacitors type Type Cg(µF) Cc(µF) Rs1 (Ω) Rs2 (Ω) Rp(Ω) C. Resistors Lightning 2 0.025 0.0012 ---- 350 2,400 The resistors weremade fromtraditionalelectronic carbon Switching 2 0.025 0.0012 ---- 46000 120,000 resistors connected in seriesto withstand the required stress. The resistors were isolated from each other by using rigid B. Capacitors polyurethane foamand encapsulating themin acrylic, thereby obtaininggreaterdielectric strength. Table5 showsthe values Theproposedcapacitorshad to withstand 20kVvoltage and forthe resistors used;the resistance configuration for lightning theirsmallcapacitanceswere notcommercially available.For is presented in Figure7 (c). eachcondenserit was necessary toassemble aseries ofcapacitors, insulated from each otherbyrigid polyurethane Table 5. Resistors design and construction foamand encapsulatedin acrylic, thereby Front Resistor Tail Resistor obtaininggreaterdielectric strength.Thebuildingmodelsare Impulse Rs2 (Ω) Rp(Ω) shownin Figure 7 (a)and 7 (b). Type Individual Amount of Individual Amount of Value (Ω) Resistors Value (Ω) Resistors Table4summarisesthe technical characteristics andrequired 20 18 200 12 Lightning amounts of elements usedto build thecapacitors for the 360 Ω 2400 Ω portable voltage impulse generator. 4.7k 10 10k 12 Switching 47k Ω 120k Ω

162 REVISTA INGENIERÍA E INVESTIGACIÓN Vol. 31 Suplemento No. 2 (SICEL 2011), OCTUBRE DE 2011(159-164) GÓMEZ, BUITRAGO, ROLD ÁN.

D. Sphere Gap The sphere gap is used as voltage in voltage im pulse generators, asin IEEE Standard 4, 1995; Bedoya, 2004) . Due to the impulse generator’s designed voltage , the spheregapwas proposedforuniform fielddistribution , using horizontalarrangementand supportedon an acrylic structure. The switch could thus be calibratedto the generator’s maximum possible voltage. The spheres had 30mm diameterand maximum 8 mm distance; they were made o faluminiumand designed to withstand a maximum 20kV voltage. The sphere gap is shown in Figure 7d . E. Powersupply The power supply was formed by a 120/7,000Velevatortransformer followed by a Schenkel voltage doubler circuit, as proposedin (Aguet and Ianoz, 199 0) , to achieve maximum 15kV voltage . The circuit was built using two 0.07 F/8000Vcapacitorsand tworectifier diodeshaving 7,000Vpeak inversevoltage.

4. RESULTS AND DISCUSSION The portable voltage impulse generator was tested in the laboratory to confirm thatt he results conformed to established standards andwere withinthe tolerances setby them. Simulations were made to test the generator’s performance. Table6 (a ) shows the data obtained from laboratory testing foralightning impulse usingthe portable voltage imp ulse generator. The data obtained for acommercial im pulse generator(having the same resistor andcapacitor values) and thev alues calculated by the im pulse wave simulator are also presented .

Fig.8. Waves from Portable Generator Table 6. a). impulse lightning r esults Time [Front/Tail] (µs) Peak Voltage Lightning and switching impulses were within established Portable Commercial Wave Simulator (kV) standards when error rate associated with the portable vo ltage Generador Generator impulse generator was within such range (Table 7). 1,184/43.2 1.172/43.1 1.25/45.36 10-20 b). impulse typeswitching results Table7. Tolerances Time [Front/Tail] (µs) Impulse Type Admisible Error Portable Generator Peak Voltage Portable Commercial Wave Simulator (kV) [Front/ Tail] (%) [Front/ Tail] (%) Generador Generator Lightning 30/20 1. 33/13 .6 224/2140 ---- 256.4/2,360 10-20 Switching 20/60 10.4/14. 4

The lightning impulse registered by the oscilloscope as The resultsshowed thatthe errorscalcul ated for lightning described by (IEC Standard 60060 -1, 1989; IEEE Standard 4, and switching impulses came withinthe percentage limits set 1995) is presented in Figure 8 (a). bythe aforementioned regulations.

Table 6 (b) presents the measured switching impulse The final version of the portable voltage impulse generator values from a portable voltage impulse generator compared components is presented in Figure 9. Tuning tests were to those supplied by the impulse wave simulator. The performed with a bell -type i nsulator and the results showed switching impulserecorded by theoscill oscopeas that the generator outputs came within the range of tolerances describedby (IEC Standard 60060 -1, 1989 ;IEEE Standard 4, mentioned above. 1995) is presentedin Figure 8 (b).

REVISTA INGENIERÍA E INVESTIGACIÓN Vol. 31 Suplemento No. 2 (SICEL 2011) , OCTUBRE DE 20 11(159-164) 163 PORTABLE HIGH VOLTAGE IMPULSE GENERATOR

6. ACKNOWLEDGEMENTS The authors are grateful to the Universidad Nacional de Colombia inManizales,especiallythe High Voltageresearch groupfor their supportin carrying out laboratorypractice. This work was supported in part by the National University of Colombia, Manizales.

7. REFERENCES Aguet, M. and Ianoz, M. Haute Tensión, TraitéD’Electricité 2éme Édition, París, 1990. ASTM Standard Test Method for Dielectric Breakdown Voltage and Dielectric Strength of Solid Electrical Insulating Materials Using Impulse Waves, Standard ASTM International D3426 -1997, Reapproved 2004. Bedoya, D. R.; " Diseño y construcción de un espinterómetro para alta tensión – impulso tipo rayo," Engi neeringThesis, Department of Electrical, and ElectroncialEngineering, Univ. Nacional de Colombia, Sede Manizales, 2004. Carmona, F.; Jiménez, J. E. and Vázquez, F. "Modelado y simulación del circuito generador de impulsos para el ensayo en transformadore s,” Department of informatic and numericalanalisys, Córdoba, Campus Rabanales, TR -14071. ElectricalTestingGroup, " Reflectometría de alta energía mediante el uso del Fig.9. Portable High Voltage Impulse Generator generador de impulsos," Inducor Ingeniería S.A., Buenos Aires, [en línea]. Available on ht tp://www.inducor.com.ar/investigacion.html 5. CONCLUSIONS Idarraga, C. and Roldán, F.: “Simulador de Ondas de Choque”, presentado This paper has presented the design and construction of a en VII Congreso Latinoamericano y IV Iberoamericano en Alta Tensión y portable voltage impuls e generator. The proposed Aislamiento Eléctrico, Panamá, 2005. generator’s performance was compared to that of IEC Electric Strength of Insulat ing Materials_ Test Methods Part 3: Additional Requirements for 1.2/50µs Impulse Test, IEC Standard 60243 - commercial generators and the established standards for 3, Jul. 2001. such instruments. The results came within the ranges established by the standards and the generator could thus be IEC Standard 60060 -1, Nov. 1989; IEC High -voltage test techniques. Part 1: General definitions and test requirements, regarded as being valid. The generator satisfied the main objective and needs IEEE Standard 4 -1995, Aug. 1995;IEEE Techniques for High - Voltage Testing proposed in this work due to the low cost of its implementation and its comfortable size for use and Kuffel, E. and Zaengl, W.; High Voltage Engineering. Ed. Londres: Pergamon Press, 1970. transport. Future work will be aimed at expanding in sulation Lora, A.J. " Diseño de un generador de impulsos de alta tensión basado en las normas ASTM -D3426 e IEC -60243-3 para ensayos de rigidez dieléctrica components and power supply le vel to encompass jobs de materiales poliméricos sólidos,” Engineering Thesis, Science and inhighervoltage rangesand diversify thenumber of Engineering Faculty, Univ. Católica Perú, 2008. componentsto be tested.

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