Analysis of the lightning performance of power distribution networks in rural and urban areas Alberto Borghetti Dept. of Electrical, Electronic and Information Engineering University of Bologna Bologna, Italy

Number of induced-voltage flashovers versus distribution-line insulation level single-conductor, 10 m high, overhead line

The paper deals with the lightning performance of medium voltage (MV) overhead power distribution networks mainly composed by overhead lines and of the connected MV/low voltage (LV) transformers, taking into account: This paper is based on results obtained in the framework of a research project carried out in collaboration with the colleagues of the University of Bologna – Carlo Alberto Nucci, Fabio Napolitano, and Fabio Tossani – and the colleagues of the Federal University of Itajubá (Brazil) and AES Sul (Brazil) – Gustavo Paiva Lopes and Manuel Luis Barreira Martinez (Federal University of Itajubá), Donorvan R. Fagundes, Gilnei J. G. Dos Santos, and Juliana Izabel Lara Uchôa (AES Sul). Outline of the presentation –both indirect and direct strikes to the line conductors; –the AC voltage at the utility frequency; –the flashovers of the insulators; –the shielding of nearby buildings. The results make reference to a single multiconductor line and to a real feeder with complex topology

Monte Carlo method A large number n tot of lightning events is randomly generated. Each event is characterized by four parameters: lightning current amplitude I p time to peak t f stroke location with coordinates x and y. The events are assumed to follow the Cigré log-normal probability distributions for negative first strokes, with a correlation coefficient between t f and I p equal to A is the striking area N g is the annual ground flash density (1 flash/km 2 /yr) For a component at a specific location Mean time between failures

Calculation of the overvoltages due by indirect lightning events LEMP calculation Calculation of the induced voltages The LIOV code calculates: LEMP Coupling The EMTP : calculates the boundary conditions makes available a large library of power components Structure of the feeder LIOV - EMTP Analytical formulation (Napolitano, TEMC, 2011) Link between LIOV and EMTP [Nucci et al., Proc. ICLP ‘94; Borghetti et al, J. Electrostatics ‘04, Napolitano et al. Proc. ICLP‘08 ] - TL MODEL (lightning return stroke current model) - Cooray-Rubinstein approximated formula for the horizontal electric field

Calculation of the overvoltages due by direct lightning events The direct strikes are represented by current sources connected to the pole closest to the randomly-generated stroke location coordinates The insulators flashovers is represented by means of ideal switches that close according to the disruptive effect criterion (Darveniza and Vlastos, 1988), i.e. D(t) given by the following integral becomes larger than a predefined value DE. where v(t) is the voltage at the pole insulator, V 0 is the minimum voltage to be exceeded before any breakdown process can start, k is a dimensionless factor, and t 0 is the time at which |v(t)| becomes greater than V 0 The effect of soil ionization at the grounded poles is accounted by using the Weck’s approximation

Single multiconductor overhead line rated voltage = 13.8 kV height of the line = 9.3 m diameter of conductors = 1 cm span between subsequent poles = 35 m ground conductivity σ g = S/m 15 kV class surge arresters grounding resistance poles not equipped by surge arresters: 400 Ω poles equipped by surge arresters: 10 Ω Typical configuration

The compact design is characterized by a close upper unenergized wire that has the main function of sustaining periodical spacers of the phase conductors. insulation class: 15 kV dimensions in mm The upper wire is assumed located at 9.3 m above ground As the unenergized wire of the compact configuration is periodically grounded. Aim  analysis of its effectiveness in reducing the overvoltages due to indirect lightning  Comparison between the lightning performance of the compact line and the one calculated for a conventional line configuration. The insulation between the upper wire and covered conductors is around 215 kV and it reduces to 95 kV in case of damaged insulation (Napolitano et al. TIEEJ, 2013) Compact configuration

2 km long line matched at both terminations. A 6×4 km 2 rectangular area around the line. Due to the symmetry of the geometry, the considered stroke locations are distributed in a quarter of the area around the line. We have chosen a minimum insulation level equal to 80 kV for the compact configuration and equal to 100 kV for the traditional line configuration one. Adopted configuration Indirect lightning performance of the conventional line for different values of the soil conductivity σ

Indirect lightning performance of the compact line with the resistance of upper wire groundings equal to 50 Ω, for different values of soil conductivity σ and of spacing d between adjacent groundings Indirect lightning performance of the compact line with spacing between adjacent wire groundings equal to 50 m, for different values of soil conductivity σ and of grounding resistance R. Without surge arresters

With surge arresters The upper wire is grounded also at the poles where surge arresters are connected. The upper wire ungrounded. Effectiveness of surge arresters located at different spacing s along the line.

MTBF values of a MV/LV transformer connected to the middle pole of the line with typical configuration SA transformer The Monte Carlo analysis has been performed for two different CFO of the line insulators, namely 100 kV and 165 kV. Parameters of the DE model: k=1, V 0 =90 kV and DE=60.9 kV µs, for CFO equal to 100 kV k=1, V 0 =132 kV and DE=255 kV µs, for CFO equal to 165 kV The distance between subsequent poles is 50 m. The number of generated events in the Monte Carlo procedure is Direct events n d are 1,208. Indirect events n d are The considered rated voltage at the utility frequency is 13.8 kV.

A DE model is adopted for assessing the transformers faults, with parameters: V 0 = kV, k = 1 and DE = 40.9 kV µs, which predict breakdown at 3 µs and 8 µs in case of standard lightning waveform with crest value equal to 121 kV and 110 kV, respectively. The transformer grounding resistance is assumed equal to 10 Ω. without SA with SA Insulators CFO d = 100 md = 200 md = 400 m directind.bothdirectind.bothdirectind.bothdirectind.both 100 kV inf kV inf MTBF (in years) of the transformer for different distances between consecutive SAs and CFO of the insulators. The results are obtained by considering direct and indirect lightning events separately or both together.

Real distribution network (Napolitano et al., ICLP, 2014) Three-phase distribution 13.8 kV feeder located in Brazil. Total length of almost 13.9 km. The span between subsequent poles is 40 m. The total number of MV/LV transformers is 80: 55 utility transformers; 25 private customers. The withstand voltages (WV) of the transformers is assumed to be 125 kV. The ground conductivity σ g is assumed equal to 10 ‑ 3 S/m. Monte Carlo simulation: The total number of Monte Carlo events is n tot = , of which are direct strikes to line conductors. The events are generated uniformly over a striking area A=19 km 2 with borders at least 1 km far from the network.

The topology of the network is extracted from the geographic information system Calculation procedure A procedure automatically discretize each line to meet the FDTD requirements The LIOV – EMTP-rv model of the network is built A statistically relevant number of events are generated and simulated by the LIOV – EMTP (Monte Carlo method) Assessment of the expected MTBF of each MV/LV transformer

Calculated event A(I pA, t fA, dist A ) is classified as not dangerous Criteria to be satisfied to discard an event B -I pB < I pA, -t fB > t fA -dist B > dist A Heuristic technique I sa,A < 100 A The Electrogeometric model is used in order to differentiate direct and indirect strokes 13.8 kV rated voltage Indirect: Direct: σ g =0.001 S/m σ g =0.01 S/m Calculated I sa >100 A Discarded Time saved67.37%86.29%

Environmental shielding

from “IEEE guide for improving the lightning performance of electric power overhead distribution lines,” IEEE Std (Revision IEEE Std ), pp. 1–73, S f = 0 (no shielding) S f = 0.92 (same height of the line on both sides) S f = 1 (complete shielding). For the cases with S f greater than 0, the events with a randomly- generated stroke location at a distance d i ≤ d are repositioned at a distance equal to d. Shielding factor: The effects on the lightning performance due the direct events that do not hit the line thanks to the environmental shielding are accounted by the calculation of the induced voltages caused by the associated LEMP, by neglecting the effect of the multiple reflections along the struck objects and the shielding of the LEMP due to the presence of metallic parts in the nearby objects. Nearby objects assumed placed at distance d = 10 m from the overhead lines on both sides.

Configuration with SAs installed at the MV terminals of all the private transformers and at the terminals of 16 out 55 utility transformers.

Influence of different environmental shielding factors S f on the number of unprotected transformers with MTBF value lower than abscissa calculated for withstand voltage equal to 110 kV and σ g = 1 mS/m.

Influence of the different withstand voltage values of the transformers on the number of unprotected transformers with MTBF value lower than abscissa calculated for S f = 1 and σ g = 1 mS/m.

Influence of different environmental shielding factors S f on the number of unprotected transformers with MTBF value lower than abscissa calculated for withstand voltage equal to 110 kV and σ g = 10 mS/m.

Influence of the different withstand voltage values of the transformers on the number of unprotected transformers with MTBF value lower than abscissa calculated S f = 1 and σ g = 10 mS/m.

Conclusions The influence of the location and characteristics of SAs on the MTBF values of the MV/LV transformers is analysed by means of a Monte Carlo approach taking into account both direct and indirect lightning strikes to the line conductors, the bus voltage at the utility frequency, the flashovers occurrence in the line insulators, and the shielding of nearby buildings The compact configuration has some advantages with respect to the conventional one for the protection against indirect lightning events. These advantages are tangible if the upper wire – whose main function is of sustaining the spacers of the phase conductors – is periodically grounded. For the representation of insulator flashovers with the DE method and the operation of SA, the calculation should include the AC voltage. As expected, insulator flashovers and the operation of nearby SAs provides some protection to transformers not equipped with SA. The relevant MTBF values are also influenced by the adopted model for the representation of the transformer failure.

For the assessment of the flashover rate of components (e.g., transformers) connected to the line, the effects of direct strikes should be taken into account also for elevated shielding factors, in particular in parts of the network characterized by the presence of several laterals. The possibility to defer the installation of SAs in some transformers is strongly dependent on the local topology of the network and on the S f value. The presented methodology appears useful for the selection of the most appropriate strategies for the installation of SAs in order to achieve the desired lightning performance at affordable costs. The voltage stress due to indirect lightning to which power transformers may be exposed is highly dependent on the topological configuration of the distribution network.