2012 International Conference on Lightning Protection (ICLP), Vienna, Austria

Protection Systems against Lightning-Originated Overvoltages in Resonant Grounded Power

Distribution Systems

A. Borghetti1, F. Napolitano1, C.A. Nucci1, M.L.B. Martinez2, G.P. Lopes2, J.I.L. Uchoa3 1 University of Bologna, Italy. E-mail: {alberto.borghetti;fabio.napolitano;carloalberto.nucci}@unibo.it

2 Federal University of Itajuba, Brazil. E-mail: martinez@lat-efei.org.br 3 AES SuI, Brazil. E-mail: juliana.uchoa@aes.com

Abstract- A joint research project between AES SuI, the Federal

University of ltajuba and the University of Bologna aims at

defining the required protections against lightning-originated

overvoltages in a resonant grounded distribution system in order

to achieve the adequate lightning performance level with

minimum cost. The paper aims at illustrating the effectiveness of

the presence of a surge arrester or an arcing-horn gap and its

protective zone by means of simulations obtained with a

computer code for the calculation of lightning induced voltages

that takes also into account the steady-state voltage. The results

of a statistical analysis for a single multiconductor line are

eventually presented and discussed.

Lightning protection; power distribution networks; induced

overvoltages.

1. INTRODUCTION

The main aspect of the AES SuI research project consists in the adoption of a neutral resonant grounding device (e.g. [1]) as an alternative standard neutral ground treatment in medium voltage distribution networks previously operated with solid grounded neutral. The neutral resonant grounding allows the system operation also in presence of a permanent phase-to­ground fault without the opening of the three-pole circuit breakers at the substation. In these conditions, as a consequence of the increase of the voltage in the sound conductors, the rated voltage of all surge arresters installed in the feeder must be increased with respect to the one chosen for the surge arresters installed with solid grounded neutral. As the region where the distribution networks are located is characterized by high annual average values of cloud to ground flashes density [2], the above motivates the development and implementation of a procedure aimed at minimizing the number of required surge arresters.

The paper proposes a procedure for the reassessment of the insulation coordination and lightning protection of the medium voltage feeders composed mainly by overhead lines. It is focused in particular on the analysis of the effectiveness of a surge arresters or an arcing-horn gap along the feeder and on the evaluation of its protective zone (e.g. [3 ]-[7]) for the case of a line exposed to the lightning electromagnetic pulse (LEMP) of an indirect stroke. The analysis is carried out by

This work is supported by AES Sui in the framework of a research agreement with Fondazione Alma Mater and Fundacao Apoio ao Ensino Pesquisa e Extensao (F APEPE) de Itajuba.

means of a statistical simulation procedure presented in [8],[9] suitably adapted in order to take adequately into account the steady-state phase voltages. The effects of the presence of the steady state voltage, i.e. the conductor voltage at the utility frequency due to the line energization, are often neglected in the studies on the subject (e.g. [10]-[14]).

The influence of the steady state voltage on the number of dangerous overvoltages without the presence of surge arrester has been shown in [15].

This paper describes the procedure able to take into account the presence of the steady state voltage in the LTOV-EMTP code [16],[17] which implements the LEMP-to-line coupling model proposed by Agrawal et al. [18] and an analytical formulation presented in [19] for the fast and accurate calculation of the lightning induced voltages. The LlOV­EMTP code is used in the already mentioned statistical procedure based on the Monte Carlo method for the case of a typical Brazilian overhead distribution line equipped with surge arresters or horn gaps.

The structure of the paper is the following. Section IT describes the type of feeder taken into account in the analysis. Section TIT describes the procedure in order to represent the steady state voltage in the model of a line terminated with its surge impedance. Section IV shows the voltage waveforms for some test cases. Section V presents the results of the statistical analysis.

II. DESCRIPTION OF THE DISTRIBUTION NETWORK AND

TYPE OF ANALYSIS

The analysis refers to a MV network composed by overhead lines. The protection devices are surge arresters and/or arcing­horn gaps connected to all the three conductors or to one conductor only.

For the analysis, the pole configuration shown in Fig. 1 and the following data are assumed: rated voltage = 23.2 kV; voltage class of the insulation level = 25 kV; residual voltage of the surge arresters = 80 k V; operating voltage of the horn gaps = 125 kV; ground conductivity equal to 1 mS/m or 10 mS/m.

The aim of the statistical calculation is to provide the graph of the expected number of annual voltages exceeding the value

978-1-4673-1897-6/12/$31.00 mO l2 IEEE

in abscissa for the all system and for specific locations along the line. In this paper, the procedure is applied to the case of a 2.5 km long line matched at both ends. The procedure could be also applied to more complex configurations by using the procedure presented in [20].

Figure 1. Pole of the line.

TIT. CALCULATION OF INDUCED VOLTAGE ON ENERGIZED

LINES

The technique adopted to link the LlOV (Lightning Induced Overvoltage) code [21 ],[22] with the Electromagnetic Transient Program (EMTP) have been presented in [23],[17]. Other approaches have been also proposed in the literature (e.g. [24]-[26]). In [27] an improvement of the technique is presented that exploits the structure of the recently developed EMTP-rv simulation environment [28].

In this paper, the method is adapted in order to perform induced voltage calculations along overhead line taking into account the voltage at the utility frequency.

As the time horizon usually adopted for induced voltage calculations is of the order of some tens of microseconds, the 50 or 60 Hz voltage is assumed constant and, for simplicity, uniform along the line.

In the induced voltage analysis, the overhead line of finite length is often assumed to be terminated on its surge impedance at one or both ends. This simplifies the analysis of the results as the effects of the reflections of the travelling surge waves at the line terminations are avoided.

For the case of a line with a single conductor, Fig. 2 illustrates the circuit that links the Agrawal et al model of the illuminated line implemented in the LlOV code with EMTP for the left termination of the line (represented by function [0). The meaning of the symbols reported in the figure is the following:

Z is the surge impedance of the line (assumed to be frequency independent);

Co and C� are the Bergeron equivalent generators

[29] ( Ck""" and C;""" at the right termination);

v"o and io are the total voltage and line current,

respectively, at the left termination (V"km" and ikmox at

the right termination);

VI, V2, . . . , Vkm,,-I and ii, i2, . . . , ikm,,-I are the so-called

scattered voltages and line currents, respectively, at nodes 1, 2, ... , kma[ I of the second-order finite­difference time-domain spatial grid used to solve the equations of the Agrawal et al. model.

C; and G;"x at the two line terminations at time step n are

Gn _ n-I -z ·n-I n o -VI II + ve,o (1)

Gn = vn-I + Z t-I + vn (2) klTJJ.L\. knuL\. -1 knn" -1 e ,klTJJ.L\. where Ve,i is the incident, or exciting, voltage at node i, i.e.

h

Ve,i = -f E;,i dz (3) o

with h the conductor height and E'. the vertical component of Z,'

the exciting electric field,

G�n and G;:", at time step n are

GIn n Z

'n n o = V"O + 10 -Ve,o

C'n = vn - Z t - vn kmax l,kmax kmax e ,kmax •

(4)

(5)

C; , C:'n" ' c�n and C;::" are calculated by the LlOV code

and defined in the EMTP-rv simulation environment by means of a specific dynamic link library (DLL).

Figure 2, Interface between a L10V-line and the EMTP-RV at its left termination, by means of a Bergeron line,

In case of unenergized lines, a matched line is represented by defining [0 equal to the surge impedance Z of the line (or a set of branches of coupled impedances in case of

multiconductor lines) in the EMTP circuit. Total voltage v"o is equal to the half value of sources Co.

In order to preserve the possibility to represent a matched line at one or both ends also when stationary voltage V is taken into account, the line termination is kept open in the

EMTP circuit. Sources Co and/or Gkmox are calculated as the

half of the value given by (1) and (2) plus an offset corresponding to V:

Cn = �( n -I _ Z ·n-I + n )+V o

2 VI II ve,o (6)

en = .!..(V;-II+Zi;-II+V�k )+V. (7) k,"", 2 "H" - m", - C, m.x Sources e� and/or e�m", are always null so to simulate the

absence of voltage wave reflected inward the line at the matched termination.

The linking method presented In [23] and [17] includes a

time delay in the calculation of C; and Cn or c�n and km", C'n corresponding to the propagation time along the km' Bergeron lines. Such a delay is avoided in the technique presented in [27], where the Bergeron lines are included in the illuminated line. In this approach, (1) and (2) becomes

en _ n-I Z ·n-I n fu (En En-I) -v - I +V -- + o I I e,O 2

x,O x,1 (8)

C�H" = V;H��-I + Z i;H��-1 + V;,km" + � (E;,�,�,,_I + E;,k",,,)' (9)

In order to represent also the stationary voltage and the matched terminations, as in (6) and (7), Co and/or Gkmax are calculated as the half of the value given by (8) and (9) plus an offset corresponding to V. The current value at the matched terminations are calculated in the LTOV code by

.n e; 1 =-­o Z (10)

en = k"H"

Z

IV. TEST RESULTS OF INDUCED VOLTAGE WAVEFORMS

(11)

Tn order to illustrate the effect of the stationary voltage on the calculation of the induced voltages along the line with the presence of surge arresters, this section presents the results obtained for the 3-phase 2.5 km-long overhead line described in section II for two different stroke locations and number of surge arresters.

For illustrative purposes, the results of this section have been obtained by assuming a surge arrester characteristic with a 30 kV residual voltage that is lower than that usually adopted (i.e. 80 kV).

The assumed lightning current has peak amplitude equal to 12 kA and a maximum time derivative of 40 kA/IlS. The current wave shape is represented by the sum of two Heidler functions [30] with the same parameters adopted in [31], i.e.: 101 = 10.7 kA, Til = 0.25 Ils, T2J = 2.5 Ils, nl = 2, for the first function, 102 = 6.5 kA, TI2 = 2.1 Ils, T22 = 230 Ils, n2 = 2, for the second function. The ground conductivity is equal to 10 mS/m.

100

50

�

-50

-100 -1500

stroke location

A

• ,� "

point of surge observation arresters

stroke location

B

-1000 -500 0 500 1000 1500 (m)

Figure 3, Stroke locations,

140 r----------------------,

120

100

� 80 ID 2 60 �

40

20

-without surge arresters

- -with three surge arresters

-with one surge arrester (protected conductor)

_ . with one surge arrester (unprotected conductor)

o u-_�_�_� __ �_�_� __ �� o

time (�s)

Figure 4, Lightning induced voltages on an un energized line calculated for stroke location A: in absence of protections, with three surge arresters, and

with one surge arrester.

t40

120

100

80 � i 60

g 40

20

-20

a) time (�s)

140

120

100

80 � -;;; 60 g> g 40

20 -----

-20 0

b) time (�s)

140

120

100

80

:[ 60 ID 0> ;g 40 g

20

c) time (�s)

-phase a

-phase b ---phase c

-phase a

-phase b - - phase c

-phase a

-phase b - - phase c

Figure 5, Lightning induced voltages on an energized line calculated for stroke location A: a) in absence of protections; b) with three surge arresters; c)

with one surge arrester.

Fig. 3 shows the two considered stroke locations. Both are 40-m far from the nearest point of the line: stroke location A is equidistant from the line terminations, stroke location B is near to the right termination. The calculation is repeated with surge arresters located in the middle of the line and without

surge arresters. The following figures show, for each case, the voltage waveforms at an observation points of the line located 50 m far from the line center on the left side.

Fig. 4 and Fig. 5 show the induced voltages at observation point A in three different conditions: in absence of protections, with three surge arresters, each installed on a different phase, and with one surge arrester installed on the central conductor of the line (phase b). In Fig. 4, the line is assumed to be not energized, whilst Fig. 5 the three phases are characterized by stationary voltages equal to Va=16.4 kV, Vh=-16.4 kV, Vc=O, respectively.

The same calculations have been repeated also for stroke location B. Taking into account the stationary voltage, Fig. 6 compares the results obtained by assuming the terminations open or matched. As expected, reflections due to the open terminations disappear when the proposed technique for the representation of matched terminations is applied.

100 - - open end

80 -matched end

60

� 40 v phase a � 20 rn � phase c

phase b ·20

·40

a) 1ime (�s)

40 - - open end

30 -matched end

20 phase a

� 10 v phase c � rn �

·10 phase b

·20

·30

b) 1ime (�s)

100 - - open end

80 -matched end

60

� 40 v g' 20 �

phase a

phase c

phase b ·20

·40 o C) 1ime (�s)

Figure 6. Lightning induced voltages on an energized line calculated for stroke location B: a) in absence of protections; b) with three surge arresters; c)

with one surge arrester.

v. STATISTICAL ANALYSIS

As mentioned in the Introduction, the adopted statistical analysis is based on the application of the Monte Carlo method and of the LTOV-EMTP code. This section presents the results obtained for the case of the 3-phase 2.5 km long line matched at both end with the characteristics described in Section IT.

Fig. 7 shows the top view of the line and the area that surrounds the line is assumed wide enough to include all the lightning events that could produce induced voltages larger than the minimum value considered in the analysis.

As the configuration is symmetric, only the area at one of the two sides of the line is shown. The figure also shows the stroke locations of the indirect events. The perspective stroke locations (i.e. the stroke locations in the absence of the line) of the considered 10000 lightning events are assumed to be uniformly distributed over the entire area. The low density band near the line is due to the 616 direct strokes to the line conductors. The direct strokes are inferred by applying the electro-geometrical model (EGM) adopted in [32].

For the statistical analysis, ground flash density Ng is assumed equal to 1 flash/year/km2 and each flash is composed by a single stroke. Lightning current parameters (i.e. peak amplitudes and time to peak values) follow the log-normal distributions recommended in [33] for the first strokes of negative downward flashes.

1200

1000

800

600 I

400

200

0

·200 ·1000 ·500 500 1000 1500 (m)

Figure 7. Top view of the line and the stroke locations of the indirect events considered in the Monte Carlo simulations.

The stationary voltages of the three phase of the energized line are assumed to be symmetrical and uniformly distributed.

For the case of an energized line without protections, Fig. 8 shows the graph of the annual number of lightning events that cause a voltage greater than the value in abscissa in at least a point of the line. The maximum values of the voltage amplitudes in all the points along the line are taken into account. All the direct events produce an overvoltage larger than the maximum value. The calculation is repeated for two different ground conductivity, namely (5=1 mS/m and (5=10 mS/m.

10 ,-_

-_

-_

--_

-_

-_

-_

--_

-_

-_

--_

-_

-_

-_

--_

-_

-_

-_

--_

-_

-_-,-

-----

d�

ir-ec

-t-------- '

-indirect a = 1 mS/m

--- indirect a = 10 mS/m , .... -... :::-.... -:: ... - - - - - - - - - - - - - - - - - T - ----------

� � � � �-��-��������-� � � = � � � t � � � � = = � � � � � .......... I

- .... -.... I -1 .... - ............. ---------------------- � -----:��-�: ----------------------+---------------------------------+---------------------------------1---------------------------------T-----------

0.01 �--------------------------�------------� 50 100 150 200

voltage (kV)

Figure 8. Annual number of voltages having amplitudes greater than the abscissa for the energized line without protections.

Assuming a ground conductivity value equal to ()=I mS/m, the calculation is repeated including a protective device at the line center. We consider 7 different observation points located at the line center and at a distance x equal to 50 m, 100 m, 150 m, 200 m, 250 m, and 300 m from it, respectively.

The calculation is repeated for two types of surge arresters, the usual one with residual voltage equal to 80 kV and the one characterized by the lower residual voltage of 30 kV already adopted in section VI for illustrative purposes.

Fig. 9 shows the results obtained for the case of three surge arresters with residual voltage equal to 30 kV, each connected to a different line conductor, whilst Fig. 10 shows the results obtained for a single surge arrester for both the protected conductor and the other two conductors. Fig. 11 and Fig. 12 show the corresponding results obtained for the case of surge arresters with residual voltage equal to 80 kV.

10 �-�-�-�-�-�-�-�-�-�-�-=-�-�-�-=-�-�-�-�-�-�- - - - unprotected ==========�========== --e-x = 50 m --+-x=100m --e-x= 150 m -x=200m -x = 250 m

___ ....

�-,.....�_-:::. ____ : ___________ T _ �� =

_3�

O � ___ _ --=���?���;��i�������������������

I ........ __ .... I

"--'�� ..... -- - - - - - - -�-=--f-=----�-- - ------

0.01 L-______ --".-__________ =--________ ----'I..-________ � 50 100 150 200

voltage (kV)

Figure 9. Annual number of voltages having amplitudes greater than the abscissa for the energized line with a three surge arrester with residual voltage

equal to 30 kV connected to the three line conductors in the middle point. (cr=1 mS/m)

10 ,--------------,-------------- --- unprotected --------------------- -I ---e---x=50m ---------- �---------- .

--+-x= 100 m ----------�---------- . --------------------- - -x= 150 m

I -x=200m ---------- �---------- . ---x = 250 m ---x = 300 m - - --'::�-""---':"'::::-_ - - -1- - ---------------------

- - - - - = =��:��-��;-��-��-; ; ; ; ; ; ; ; ; ; ; ; ; ; ---

0.01 50 100 150 200

voltage (kV) a)

b)

10 �-�-�-�-=-�-�-�-=-�-�-�-�-�-�-�-�-�-�-�-�-�--�-�-�-�un�pr�ot�ec�te�d--� ==========�========== -x=O

-x=50m -x= 100 m -a-x= 150 m -x=200m

- - ---�--':-=�-�������-�-�-�- � � � � = � � ��:�;���� � � =

__________ �_I ____ � ___ � � __ - --:C�b---O - � � � ��-� �

---------- �-------------------------------------------

0.01 50 100 150 200

voltage (kV)

Figure 10. Annual number of voltages having amplitudes greater than the abscissa for the energized line with a single surge arrester with residual

voltage equal to 30 kV located in the middle point of the line: a) protected conductor. b) unprotected conductor. (cr=1 mS/m)

10 �-�-�-�-�-�-�-=-�-�-�-=-�-�-�-�-�-�-�-�-�-�- --- unprotected ---------- �---------- --x=O ---------- �---------- --x=50m

----------�---------- - -x=100m I -x= 150 m

-x=200m 1 ---x=250m

-��-::-�_�-����-�- �_� � � � � � �

� ==

==

���3�Ot== == ==

== 0.01 L-________ ---l-____________ -".-________ ---"'---____ -----'

50 100 150 200 voltage (kV)

Figure II. Annual number of voltages having amplitudes greater than the abscissa for the energized line with a three surge arrester with residual voltage

equal to 80 kV connected to the three line conductors in the middle point. (cr=1 mS/m)

Fig. 13 shows the results calculated for the case of three hom gaps (each assumed in series with an inductance equal to 20 mH), whilst Fig. 14 shows the results calculated assuming that only one arcing hom is connected to one of the conductors at the line center.

a)

� .-2 � ro .�m > 00 � "ii 00.0 c '" w e > ro w£

10 ,------------,------- - - - unprotected

0.01 50

--------------------- -I -x=O ---------- �---------- -

---x = 50 m ----------�---------- - -x=100m

I -x= 150 m ---------- �---------- --x=200m ---x = 250 m

-=--"'-�.::-.... - __ 1_ - - - - - - - - - - ---x= 300m ---���;;��:=================

100 150 200 voltage (kV)

10 �-�-�-�-�-�-�-�-�-�-�-=-�-�-�-=-�-�-�-�-�-�-�-�-�-�un�p=m=te�cte�d--� -x=Q -x=50m -x=100m -B-X = 150 m -x=200m ---x = 250 m ---x = 300 m ------------

-------- -.... �-:;;:-=-�-= = = = = = = = � = = = = = = = = = = = -,

I

-'::-.l�-_cc - -------

� � 0.1 ---------------------- + --.0 ro �- - - - - - - - - - - - - - - - - - - - - - - i - ----------------------------------------� ______________________ 1 __________ _

I

ro 0.01 "--------------------------'

50 100 150 200 voltage (kV)

b) Figure 12. Annual number of voltages having amplitudes greater than the abscissa for the energized line with a single surge arrester with residual

voltage equal to 80 kV located in the middle point of the line: a) protected conductor, b) unprotected conductor. (cr=1 mS/m)

! <i E ro .�m � .� 00.0 C ro w e > ro w£ '0 � CD �

10 ,-------,------� ---------------------I ----------�----------

----------�----------I ----------�----------

1 � 0.1

i ro

- - - unprotected -x=O ---x=50m -x= 100 m -e-x= 150 m -x=200m -+-x= 250 m -+-x= 300 m

0.01 '----------------I-------+-----� 50 100 150 200

voltage (kV)

Figure 13. Annual number of voltages having amplitudes greater than the abscissa for the energized line with a three horn gaps connected to the three

line conductors in the middle point. (cr=1 mS/m)

In order to complete the evaluation of the lightning performance, the calculated graphs that show the frequency of occurrence of the overvoltages need to be compared with the surge withstand capabilities of the line insulators and of the connected components, with particular reference to the distribution transformers.

a)

b)

w � <i E '" .� m > 00 � "2 00.0 C ro w e > '" w.c --

10 ,-------------,-------------�------------� - - - unprotected -x=O --e-x = 50 m -x=100m --e-x= 150 m -x=200m --+-x = 250 m --+-x = 300 m

� � 0.1 .0 ", �-1 ro

� .� � ro .�rn > 00 � .

�

00.0 C ro w e > ro W£

I ----------�------

0.01 "-__________ --1-_____ +---___ -----' 50 100 150 200

voltage (kV)

10 �-�-�-�-=-�-�-�-�-�-�-�-�-�-�-�-�-�-�-=-�-�·--�-�-�-�un�pr�ot�ect�e�d--� -x=Q -x=50m -x=100m --e-x=150m -x=200m --+-x=250m --+-x= 300 m -------------------------

� � 0.1 -----------------�----

��-��-� �----�----

.0 ro �-1 '"

0.01 50 100 150 200

voltage (kV)

Figure 14. Annual number of voltages having amplitudes greater than the abscissa for the energized line with a single horn gap located in the middle

point of the line: a) protected conductor, b) unprotected conductor. (cr=1 mS/m)

VI, CONCLUSIONS

The characteristic and number per phase of surge arresters need to be adequately selected in order to take advantage of the relevant protection zone. The issue is particularly interesting for the case of lightning-induced voltages, as nearby lightning can induce dangerous voltages at both sides of a protection device,

The results here presented are obtained by Monte Carlo simulations carried out by using a tool for the calculations of the lightning induced overvoltages in distribution networks (LlOV-EMTP-RV code) properly modified in order to take into account the stationary voltage of an energized line,

The obtained results are obtained by assuming certain values of grounding conductivity and annual flash density and are valid for the assumed overhead line configuration, e.g. the absence of laterals and of nearby buildings (typical in urban area). However, the proposed procedure appears to be able to adequately take into account the main factors that influence the lightning performance of the line, The obtained results provide the preliminary basis to select the minimum spacing and number (per phase) of protective devices in order to achieve the requested lightning performance leveL

[I]

[2]

[3]

[4]

[5]

[6]

[7]

[8]

[9]

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