S.1.2 Protection analysis of the long transmission lines in amazon region from lightning climatic patterns

13/03/2014
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S.1.2 Protection analysis of the long transmission lines in  amazon region from lightning climatic patterns

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Protection analysis of the long transmission lines in amazon region from lightning climatic patterns Laure Madeleine Dentel / Federal University of Pará Electrical Engineering UFPA Belém, Brazil lauredentel@ufpa.br/ldentel.lab@gmail.com Brígida Ramati Pereira da Rocha / Sistema de Proteção da Amazônia e Federal University of Pará Electrical Engineering SIPAM/UFPA Belém, Brazil brigida.rocha@sipam.gov.br Abstract— Lightning is one of the biggest risk factors for the Electricity Sector, especially in the Amazon, a region of the world identified with high lightning densities and peak currents. In this context, the Sferics Timing And Ranging NETwork (STARNET), which is presently the only free and continuous lightning detection system covering the whole Amazon Region was chosen to generate climatic products required for the optimization of protection systems and of lightning monitoring systems for transmission lines. The data were regularized over the region in order to produce reliable lightning densities maps. However, we also found that there are Amazon regions which always have positive lightning density anomalies like the region of Belém and Manaus, and the state of Tocantins. Finally, the consequences of these lightning patterns were analyzed for the protection of the long transmission lines presently installed in amazon region. Keywords— Lightning statistics; System protection. I. INTRODUCTION The Brazilian electrical sector is connecting the Amazon region to the National Interconnected System by expanding the transmission lines over thousands of kilometers covered by dense forests, crossed by large rivers which cause seasonal flooding. To meet this technical challenge, the transmission line towers have to reach heights between 79 m and 295 m. In 2013, the installation of a 1500 km transmission line was completed, connecting the Manaus city (state of Amazonas) to the hydroelectric plant of Tucurui (state of Pará). These transmission lines play a strategic role for the energy distribution across the Brazilian country, as well as their protection to ensure their operation. Lightning is one of the biggest risk factors for the electrical sector, especially in the Amazon, a region of the world identified with high lightning densities and peak currents. Indeed, in the eastern Amazon, the analysis of ground base lightning measurements with the SIPAM´s Lightning Detection Network (SIPAM-LDN) show in this region some high Cloud- to-Ground (CG) lightning density values (of up to 11 events/km2 /year) and strong peak currents values (7% was determined to be between 100 and 250 kA) [1]. To design lightning protection systems, the lightning parameters, such as estimating lightning incidence, calculated from lightning density and peak current distribution, need to be extended to the entire Brazilian Amazon and different structure heights. In this study, the electrogeometrical model (EGM) described by [2] was used to estimate lightning incidence for different structures (100 km of the transmission line and transmission line towers) with different heights (79, 150, 185 and 295 m). The Sferics Timing And Ranging NETwork (STARNET), which is presently the only free and continuous lightning detection system covering the whole Amazon Region was chosen to generate the strokes density map. Such a strokes density map was standardized as a function of the number and location of active stations, to compensate their increasing number, to correct their intermittent service and balance the detection level of their different combination [3]. The ground flash density and peak current distribution were extracted from the network inter-comparisons study between the SIPAM-LDN (the only high resolution system which partially covered the amazon region) and the STARNET network [4]. I. LIGHTNING DETECTION NETWORK DESCRIPTION A. The STARNET lightning detection network The STARNET is a Very Low Frequency (VLF) Lightning Detection Network, based on radio antennas. Each STARNET station measures continuously the vertical electric field pulses emitted by a lightning discharge in the frequency range from 7 to 15 kHz, which constitute the Sferics waveforms [5]. The stations are synchronized with a microsecond GPS and the Sferics waveforms are used to compute their Arrival Time Difference (ATD) that is evaluated by a time cross-correlation between 2 sensor stations [6]. The STARNET was covering the entire South America with 5 stations in 2008, 8 stations in 2010 and 11 stations in 2012. The database provides the time and position of a lightning stroke at ground level. B. The SIPAM lightning detection network The SIPAM lightning detection network consists of 12 LPATS-IV sensors, which were operating in the eastern Amazon region (in the States of Pará, Maranhão, Tocantins and Mato Grosso, in Brazil) during 5 years, between May 2004 and May 2009 [1]. The system is based on the VLF/LF (Very Low Frequency/Low Frequency) return strokes waveform signals discrimination, using microsecond GPS synchronization and a Time-of-Arrival (TOA) [7]. The lightning database of the SIPAM-LDN provides time and position of the lightning flashes at ground level, type of flash (CG and intra-cloud [IC] lightning), polarity, and peak current estimation. II. METHODS First, the installed and future transmission lines in the Brazilian amazon region were drawn on the standardized lightning density map from STARNET network in order to identify the critical areas for the electrical system protection. The lightning density map was calculated with 4 years of STARNET database from 2008 to 2011 and standardized as a function of active station combination [3]. Next, the lightning incidence on the transmission line and tower was estimated with the electrogeometrical model (EGM). According to [2], the lightning incidence (Ir in [flash/year]) can be estimated by integrating on peak current (Ip in [kA], from 0 to Ipmax), the product of the ground flash density (Ng in [flash/m2 /year]), the resultant capture surface (S(Ip) in [m2 ]), and the distribution of lightning peak currents f(Ip) : ∫ ( ) ( ) (1) The resultant capture surface can be estimated from the striking distance (rs(Ip) in [m]), defined as the distance from the tip of the descending leader to the object to be struck at the instant when an upward connecting leader is initiated from this object [2]. The striking-distance can be calculated from different expressions. According to [2], the most popular expression, included in many lightning protection standards, is: (2) However, this expression does not take into account the height (H in [m]) of the structure unlike the expression used by [8]: (3) The Figure 1 presents one comparison between these two expressions where the striking-distance was plotted as a function of peak current. It was observed that the expressions (3) taking into account the height of the structure presented higher values than the other expression (2). For a peak current value of 20 kA, the striking-distance estimation was: 62 m for the expression (2); 123 m for H=79 m, 187 m for H=150 m, 215 m for H=185 m and 294 m for H=295 m in the expression (3). For a peak current value of 100 kA, the striking-distance estimation reached a values of 910 m for H=295 m in the expression (3). Fig. 1. Striking-distance as a function of peak current. The SIPAM-LDN is the only high resolution system which partially covers the amazon region. Thus, the distribution of lightning peak currents recorded by the SIPAM-LDN in the best and uniform detection efficiency area (a 130 km diameter circle centered on 4 ° S, 48 ° W) is used as the reference for the Brazilian Amazon [4]. In Figure 2, the cumulative distribution of SIPAM-LDN peak currents is compared to the classical cumulative distributions of peak currents adopted by IEEE [2].For the IEEE distribution, 24% of peak currents is below 20 kA and 4.5 % of peak currents exceeds 100 kA, whereas for SIPAM-LDN 73% is below 20 kA, and 1% exceeds 100 kA. As the detection efficiency is high, much lower peak current values were detected. Fig. 2. Cumulative distribution of these peak currents. The product of the ground flash density (Ng in [flash/m2 /year]) and the distribution of lightning peak currents f(Ip) can be calculated from the standardized stroke density from the STARNET network and the Relative Detection Efficiency (RDE) of CG flash of the STARNET network as a function of peak currents. The RDE calculated during the network inter-comparisons study between the SIPAM-LDN and the STARNET [4] is presented Figure 3. It was observed that the CG flashes with peak currents values below 20 kA have a RDE < 12%, while this kind of flashes represent 73% of the detected CG flashes by the SIPAM-LDN. The CG flashes with peak currents above 40 kA have a RDE between 30% and 50%. Moreover in this study, the STARNET network was able to detect about 9.7% of CG flash detected by the SIPAM-LDN. Next The product of the ground flash density, Ng in [flash/m2 /year], and the distribution of lightning peak currents f(Ip) can be calculated by: ( ) ( ) (4) where NgSTARNET is the standardized stroke density from the STARNET network in [stroke/m2 /year]. Fig. 3. Relative detection efficiency (RDE) for CG lightning flashes of the STARNET as a function of the peak currents. III. RESULTS A. Critical areas The Figure 4 presents the actual and future transmission lines installed in the Brazilian amazon region drawn on the standardized lightning density from the STARNET network. The results show that lightning activity is stronger in the region of Belém (state of Pará), in the region of Manaus (state of Amazonas), in the state of Tocantins and the southern of the state of Rondônia. These areas are more critical for the electrical systems protection. Fig. 4. Installed (solid line) and future (dotted line) transmission lines in the Brazilian amazon region on standardized stroke density from the STARNET network in [stroke/m2 /year]. B. Lightning incidence The lightning incidence on 100 km of transmission line and tower was estimated as a function of standardized stroke density from the STARNET network for different expressions of the striking-distance, respectively Figure 5a and Figure 5b. For example, in the Manaus region, the standardized stroke density registered by the STARNET network reach around 9 stroke/km2 /year which leads to a lightning incidence of 100 flash/year for 100 km of transmission line and 0.2 flash/year on a tower for the striking-distance expression (2). Taking into account the height of the structure, the lightning incidence was estimated between 200 and 480 flash/year on 100 km of transmission line and between 0.8 and 4.6 on a tower for the striking-distance expression (3). Fig. 5. Lightning incidence as a function of standardized stroke density from the STARNET network for different expressions of the striking- distance: a) on 100 km of transmission line, b) on tower. ACKNOWLEDGMENT This work was partially funded by CNPq (The Brazilian "National Council for Technological and Scientific Development"). The authors wish to thank the SIPAM (The Amazonian Protection System Agency) and the USP- IAG (The University of São Paulo) for providing access to the data used for this research. REFERENCES [1] A. C. Almeida, B. R. Rocha, J. R. Souza, J. A .Sá and J. A. Pissolato Filho, “Cloud-to-ground lightning observations over the eastern Amazon Region,” Atmospheric Research, vol. 117, pp. 86-90, 2012. [2] V. A. Rakov, “Lightning Discharge and Fundamentals of Lightning Protection,” Journal of Lightning Research, vol. 4, pp. 3-11, 2012. [3] L. M. Dentel, “Modelagem de sistemas de detecção de descargas atmosféricas na Amazônia”, PhD Tese, Federal University of Pará, Belém, Brazil, 2013. [4] L. M. D. Dentel, B. R. P. Rocha, J. R. S. Souza, “Evaluation of STARNET lightning detection performance in the Amazon region,” International Journal of Remote Sensing, vol. 35, No. 1, pp. 115-126, 2014. [5] C. A. Morales, J. R. Neves e E. M. Anselmo, “Sferics Timing and Ranging Network – STARNET: Evaluation over South America,” in XIV International Conference on Atmospheric Electricity, Rio de Janeiro, Brazil, 2011. [6] A. Lee, “An experimental study of the remote location of lightning flashes using a VLF arrival time difference technique,” Quart. J. R. Met. Soc., vol. 112, pp. 203-229, 1986. [7] E. A Lewis, R. B. Harvey, and J. E. Rasmussen, “Hyperbolic direction finding with sferics of transatlantic origin,” Journal of Geophysical Research vol. 65, n. 7, pp. 1879–1905, 1960. [8] A. J. Eriksson, “The incidence of lightning strikes to power lines,” IEEE Trans. on Power Delivery, vol. 2, n. 3, p. 859-870, 1987. b) a)