Insulator String Flashover Modeling with the aid of an ATPDraw Object

Zacharias G. Datsios1, Pantelis N. Mikropoulos2 and Thomas E. Tsovilis3,High Voltage Laboratory, School of Electrical & Computer Engineering, Faculty of Engineering,

Aristotle University of Thessaloniki, Thessaloniki 541 24, Greece 1zdatsios@auth.gr, 2pnm@eng.auth.gr, 3tsovilis@ee.auth.gr

Abstract- Accurate modeling of line insulation flashover is very important in evaluating the fast-front surges arising at overhead transmission lines and impinging on the connected substations. A new ATPDraw object, called ISF, has been developed with the aid of which insulator string flashover is modeled on the basis of several flashover models from literature. The ISF object was employed in ATP-EMTP simulations of a 150 kV GIS substation. The computed backflashover surges impinging on the substation vary considerably among the insulator string flashover models employed in simulations. The ISF object is a useful tool within the ATP-EMTP environment for insulation co-ordination studies. The effects of insulator string flashover modeling on backflashover surges arising at overhead transmission lines and impinging on the connected substations can be easily quantified.

Index Terms-- ATP-EMTP, ATPDraw, GIS substation, insulation coordination, insulator string flashover, lightning surges, overhead transmission lines.

I. INTRODUCTION

Lightning is a major cause of transmission system outages resulting in power interruptions, thus, also, in economic losses. Overvoltages due to lightning strokes to shield wires or phase conductors of transmission lines may exceed the line insulation strength resulting in flashover. Modeling of transmission line insulator string flashover is very important for insulation coordination studies of overhead transmission lines and of the connected substations. It allows for the accurate computation of the fast-front surges arising at transmission lines and impinging on substations due to either backflashover or shielding failure of the lines. According to [1], flashover of transmission line insulator strings can be modeled by using either volt-time curves [2], [3] or the integration method [4]-[6] or by employing leader development models [1], [7]-[10].

In this study, a new ATPDraw [11] object is presented with the aid of which the flashover of a transmission line insulator string can be represented on the basis of several insulator string flashover models reported in literature. The new object, called ISF, has been developed by using MODELS language [12], [13]. The ISF object has been applied to simulate the flashover of a typical insulator string of the 150 kV lines of the Hellenic transmission system and to evaluate the computed backflashover surges impinging on a connected GIS substation with respect to the flashover model adopted; a preliminary account of this has been given in [14]. The

computed overvoltages vary notably among insulator string flashover models employed in simulations; this variation has been easily quantified with the aid of the new ATPDraw object.

II. INSULATOR STRING FLASHOVER MODELS

According to [15], transmission line insulator strings are represented in simulations by voltage-dependent flashover switches in parallel with capacitors which simulate the coupling effects of conductors to the tower. Line insulator string flashover can be modeled by employing either volt-time curves [2], [3] or the integration method [4]-[6] or by employing leader development models [1], [7]-[10].

Based on the volt-time characteristics of line insulation, flashover occurs when the potential difference across the insulator strings becomes equal to or higher than the flashover strength under standard lightning impulse voltages, VFO (kV); the latter is estimated as a function of insulator string length as [2], [3]

0.75400 710 cFOV t D (1) where D (m) is the insulator string length and tc (s) is the time to flashover.

According to the integration method [4]-[6], flashover process of line insulation begins after a minimum voltage, U0,has been exceeded and depends on the magnitude of the applied voltage and on the time during which the voltage is applied. The general expression of the integration method is

0

0

t k

tDE U t U dt (2)

where DE (kVks) is the disruptive effect of the surge, U(t)(kV) is the applied voltage, U0 (kV) is the minimum voltage, t0 (s) is the instant when the applied voltage exceeds U0, t (s) is the elapsed time after the lightning stroke and k is a constant. Flashover occurs when the integral becomes equal to or higher than the critical disruptive effect, DE*, which is a characteristic of the insulation configuration under study. The critical disruptive effect DE*, the minimum voltage U0 and the constant k are determined based on experimental results under standard impulses so as to provide the best fit between the experimental and computed voltage-time characteristics.

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Leader development models [1], [7]-[10] are based on the physical breakdown process of long air gaps which consists of three phases: corona inception, streamer propagation and leader propagation. Thus, the time to flashover, tc, is determined by the expression

c i s lt t t t (3) where ti is the corona inception time commonly assumed equal to zero, ts is the streamer propagation time and tl is the leader propagation time.

When the average gradient in the gap becomes equal to a critical value, E0, the streamers are considered to bridge the gap and the streamer propagation phase is completed. Subsequently, the leader starts propagating and flashover occurs when the leader bridges the gap. However, if the average electric field strength in the unbridged part of the gap becomes lower than the critical value E0 the leader stops developing and flashover does not occur. Several expressions, derived from experimental investigations, have been proposed yielding leader velocity as a function of the applied voltage and length of the unbridged part of the gap. Such expressions referring to insulator strings [1], [8] and rod-rod gaps [7], [9], [10], are given in Table I.

III. ISF OBJECT

A new ATPDraw [11] object, called Insulator String Flashover (ISF), has been developed by using MODELS language [12], [13] within the ATP-EMTP [16] environment. The ISF object implements the insulator string flashover models detailed above by incorporating a MODEL that controls a TACS Type 13 switch.

Fig. 1 shows the ATPDraw dialog box of the ISF object; the user enters the input data, namely values for the insulator string/gap length (D), the constants K1 and K2 used in Shindo and Suzuki leader development model [7], the constant K3 used in CIGRE leader development model [1],

the critical electric field strength E0 used in leader development models of CIGRE [1], Pigini et al. [8] and Motoyama [9], [10] and finally the critical disruptive effect DE, minimum voltage U0 and constant K used in the integration method [4]-[6]. The user also selects the insulator string flashover model to be used in simulations, by assigning a value to the parameter model selection MS, ranging from 1 to 6 as provided, together with the description of the input parameters, in the help viewer window of the object. Values for the input parameters required to use the leader development models (Table I) can be estimated from Appendix A.

The ISF object was applied to simulate the flashover of a typical insulator string of the 150 kV lines of the Hellenic transmission system with standard lightning impulse withstand voltage level of 750 kV and length of 1.86 m.

Fig. 1. ATPDraw input dialog box of the developed ISF object.

TABLE I LEADER DEVELOPMENT MODELS

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Fig. 2 shows the volt-time curves under standard lightning impulse voltages obtained by employing several insulator string flashover models. It is obvious that the voltage-time characteristic of the insulator string varies considerably among flashover models. This, influencing the flashover voltage and time, may affect considerably the computed backflashover surges arising at the overhead transmission lines, thus also impinging on the connected substations. The latter is demonstrated in what follows.

Fig. 2. Volt-time characteristics of a 150 kV insulator string under standard lightning impulse voltages (1.2/50 s) according to several insulator string flashover models; standard lightning impulse withstand voltage level 750 kV, insulator string length 1.86 m.

IV. APPLICATION OF THE ISF OBJECT FOR THE EVALUATION OF BACKFLASHOVER SURGES IMPINGING ON SUBSTATIONS

The ISF object was employed in ATP-EMTP [16] simulations for the evaluation of the backflashover surges impinging on a 150 kV GIS substation, with configuration shown in Fig. 3. Simulations were performed for the following worst case scenario: negative lightning is assumed to strike to the top of the first tower (Fig. 4) close to the substation, at the time instant of positive power-frequency voltage peak of the upper phase of the overhead transmission line.

Lightning stroke was represented by a current source producing a current with an amplitude of 200 kA and a waveshape 8/77.5 s with front upwardly concave and maximum steepness calculated according to [1]. The last section of the incoming overhead transmission line, 1.75 km in length, was represented by a sequence of J.Marti frequency-dependent models, considering the line span (350 m) and tower geometry (Fig. 4). Towers were modeled as vertical lossless single-phase frequency-independent distributed parameter lines with a surge impedance of 167 calculated according to [3], [17]. Towers were terminated by a constant grounding resistance of 10 [18]. Transmission line insulator strings, with standard lightning impulse withstand voltage level of 750 kV and length of 1.86 m, were represented by the ISF object. The underground XLPE power cables were represented by the Bergeron model with

Fig. 3. Schematic diagram of the evaluated 150 kV GIS substation.

Fig. 4. Tower of typical 150 kV double circuit overhead lines of the Hellenic transmission system and lightning stroke location considered in simulations.

parameters calculated at 500 kHz. Surge arresters were represented by the Pinceti and Giannettoni frequency-dependent model [19] as shown in Fig. 5, with parameters calculated based on the surge arrester characteristics given in Table II. GIS bays were represented as lossless stub lines with a surge impedance of 75 [15]. The step-up transformer was represented by a capacitance pi-circuit together with a BCTRAN model. Cable connections and the surge arrester lead lengths shorter than 3 m were modeled by a lumped parameter inductance of 1 H/m [15].

Finally, simulations were performed with and without surge arresters operating at the line-cable junction so as to evaluate the protection offered against impinging surges with respect to the basic insulation level, BIL, of the GIS system (750 kV), considering also a safety factor of 1.15 [20].

Fig. 5. Frequency-dependent surge arrester model [19]; parameters calculated based on the surge arrester characteristics given in Table II.

TABLE IISURGE ARRESTER CHARACTERISTICS

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The overvoltages across the 150 kV transmission line insulator string for several insulator string flashover models are shown in Fig. 6; there is a pronounced effect of model selection on both flashover time and voltage of line insulation. Fig. 7(a) shows the computed overvoltages arising at the 150 kV GIS entrance, associated with the insulator string flashover models used in the ISF object, without surge arresters operating at the line-cable junction. The computed overvoltage varies notably in terms of both peak and waveshape, among flashover models. However, it is obvious from Fig. 7(b) that this is less pronounced when surge arresters are operating at the line-cable junction.

The computed peak overvoltages arising at the entrance of the 150 kV GIS substation vary up to 17% among insulator string flashover models, especially when surge arresters are not installed at the substation entrance (Fig. 8). However, all models agree that surge arresters should be installed at the substation entrance so as to reduce the arising overvoltages to values lower than BIL/1.15. These findings are in accordance with common practice [21], and consistent to IEC [20] suggestion for the installation of surge arresters at the line-cable junction when substations are connected through underground cables to the transmission system.

Fig. 6. Overvoltage across 150 kV transmission line insulator string.

Fig. 7. Overvoltage at the entrance of the 150 kV GIS substation due to backflashover of the incoming line; dashed line depicts the safety margin of BIL/1.15, (a) and (b) without and with surge arresters operating at the line-cable junction, respectively.

Fig. 8. Computed peak overvoltages at the entrance of the 150 kV GIS substation due to backflashover of the incoming line, with and without surge arresters operating at the line-cable junction; dashed line depicts the safety margin of BIL/1.15.

V. CONCLUSIONS

A new ATPDraw object, called ISF, has been developed by using MODELS language. The ISF object represents the flashover of overhead transmission line insulator strings by implementing several flashover models reported in literature. The ISF object enables the easy quantification of the effects of insulator string flashover models on the fast-front surges arising at overhead transmission lines and impinging on the connected substations.

The ISF object was employed in ATP-EMTP simulations of a 150 kV GIS substation. The computed backflashover surges impinging on the substation vary significantly in terms of both amplitude and waveshape among the selected insulator string flashover models; such variation, which also depends on substation layout, may affect the selection of the required protection measures and standard insulation level of the substation equipment.

The ISF object is a useful tool within the ATP-EMTP environment for utilities in assessing the fast-front surges arising at overhead transmission lines and impinging on the connected substations, as well as in selecting the necessary protection measures. It can also be used for educational purposes and is available at http://www.eng.auth.gr/hvl/.

APPENDIX A

The input parameters required for the leader development models implemented in the ISF ATPDraw object are given as follows:

I. Shindo and Suzuki [7] TABLE 1

CONSTANTS k1 AND k2 [7]

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II. CIGRE [1] TABLE 2

CONSTANT k3 AND PARAMETER E0; ADAPTED FROM [1]

III. Pigini et al. [8] TABLE 3

GAP FACTORS; ADAPTED FROM [8]

Fig. A. Rod-plane configuration. Average gradient at U50 as a function of the gap clearance; adapted from [8].

Fig. B. Other configurations. Average gradient in the gap at U50 as a function of the gap factor. Gradients given in p.u. of the values relevant to rod-plane with the same gap length; adapted from [8].

IV. Motoyama [9] E0 = 750 kV/m.

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[3] IEEE Working Group, A Simplified method for estimating lightning performance of transmission lines, IEEE Trans. Power App. Syst.,vol. PAS-104, no. 4, pp. 919-932, Apr. 1985.

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[7] T. Shindo, and T. Suzuki, A new calculation method of breakdown voltage-time characteristics of long air gaps, IEEE Trans. Power App. Syst., vol. PAS-104, no. 6, pp. 1556-1563, Jun. 1985.

[8] A. Pigini, G. Rizzi, E. Garbagnati, A. Porrino, G. Baldo, and G. Pesavento, Performance of large air gaps under lightning overvoltages: Experimental study and analysis of accuracy of predetermination methods, IEEE Trans. Power Del., vol. 4, no. 2, pp 1379-1392, Apr. 1989.

[9] H. Motoyama, Experimental study and analysis of breakdown characteristics of long air gaps with short tail lightning impulse, IEEE Trans. Power Del., vol. 11, no. 2, pp 972-979, Apr. 1996.

[10] T. Mozumi, Y. Baba, M. Ishii, N. Nagaoka, and A. Ametani, Numerical electromagnetic field analysis of archorn voltages during back-flashover on a 500-kV twin-circuit line, IEEE Trans. Power Del.,vol. 18, no. 1, pp. 207-213, Jan. 2003.

[11] L. Prikler and H. K. Hidalen, ATPDRAW version 3.5 Users Manual, SINTEF Energy Research, Norway, 2002.

[12] L. Dub, MODELS in ATP, language manual, Feb. 1996. [13] L. Dub, Users guide to MODELS in ATP, Apr. 1996. [14] P. N. Mikropoulos, T. E. Tsovilis, Z. G. Datsios, and N. C. Mavrikakis,

Effects of simulation models of overhead transmission line basic components on backflashover surges impinging on GIS substations, in Proc. 45th UPEC, Cardiff, Wales, 2010, paper no. 72.

[15] IEEE Task Force, Modeling guidelines for fast front transients, IEEE Trans. Power Del., vol. 11, no. 1, pp. 493506, Jan. 1996.

[16] Canadian-American EMTP Users Group, ATP Rule Book, 1997. [17] M. A. Sargent and M. Darveniza, Tower surge impedance, IEEE

Trans. Power App. Syst., vol. PAS-88, no. 5, pp. 680-687, May 1969. [18] A. Ametani and T. Kawamura, A method of a lightning surge analysis

recommended in Japan using EMTP, IEEE Trans. Power Del., vol. 20, no. 2, pp. 867-875, Apr. 2005.

[19] P. Pinceti and M. Giannettoni, A simplified model for zinc oxide surge arresters, IEEE Trans. Power Del., vol. 14, no. 2, pp. 393-398, Apr. 1999.

[20] IEC 60071-2, Insulation co-ordination Part 2: Application guide, 1996.

[21] Joint Working Group 33/23.12, Insulation co-ordination of GIS: Return of experience, on site tests and diagnostic techniques, Electra,no. 176, pp. 67-97, Feb. 1998.

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