1. business and industrial
2. energy
3. electricity

Lightning: detection and protection
ETH, Zürich
Oct. 14th, 2011
Impact of lightning
on the reliability
of future power systems
Prof. Mario Paolone
DESL ‐ Distributed Electrical Systems laboratory
École Polytechnique Fédérale de Lausanne
Outline
 Introduction
 Lightning performance of distribution networks
 Lightning impact on HVDC overhead transmission lines
 Conclusions
Outline
 Introduction
 Lightning performance of distribution networks
 Lightning impact on HVDC overhead transmission lines
 Conclusions
Introduction
The mission of modern and future power systems is to supply electric energy satisfying conflicting requirements:
reliability and security of supply;
Economy/rational use of energy
environmental protection
Massive introduction of renewables at various voltage levels.
Introduction
Renewable Electricity Generating Capacity Worldwide
Source: U.S. Dept. of Energy, Renewable Energy Data Book, August 2010 Introduction
Renewable Electricity Generating Capacity Worldwide
 Global renewable electricity installations (excluding hydropower) have more than tripled from 2000–2009.
 Wind and solar energy are the fastest growing renewable energy technologies worldwide. Wind and solar PV generation grew by a factor of more than 14 between 2000 and 2009.
 In 2009, Germany led the world in cumulative solar PV installed capacity. The United States leads the world in wind, geothermal, biomass, and CSP installed capacity.
Source: U.S. Dept. of Energy, Renewable Energy Data Book, August 2010 Introduction
Remark: mismatch between renewables location
and demand
Wind speed (annual avg m/s)
2010 Population density (prs/km2)
Introduction
Remark: mismatch between renewables location
and demand
Daily solar irradiation (annual avg Wh/m2)
2010 Population density (prs/km2)
Introduction
Impact of renewables on transmission networks:
increase of transmission capacity over long distances
Example: required number of lines in parallel to transmit ≈6 GW
EHV and UHV AC transmission lines
+ straightforward integration
+ reliability
+ investments
- stability
- voltage control
- complexity in power flow control
HVDC
+ power flows control
+ transfer capacity
+ stability
- reliability on long term
Introduction
Impact of renewables on transmission networks: HVDC
USA future AC‐EHV
and HVDC installations
Europe future
HVDC installations
Source: J. A. Fleeman*, P.E., R. Gutman, P.E., M. Heyeck, M. Bahrman, B. Normark, “EHV AC and HVDC Transmission Working Together to Integrate Renewable Power”, Cigré ‐
Integration of Wide‐Scale Renewable Resources into the Power Delivery System, Calgary, Canada. 29 ‐ 31 July 2009
Introduction
Impact of embedded generation on distribution networks
the Italian example
Primary substation minimum power
Major issues
Voltage control
Secure network operation after transients subsequent to the loss of major dispersed generation and subsequent reconnection
Protections and short circuit levels
Detection and operation in islanding conditions
16 % of the Italian primary substations experience power flow inversions to the sub‐
transmission network (courtesy of ENEL, Italy)
Introduction
Remarks
Transmission and distribution networks reliability is
a crucial element for the integration of renewables
Revamping of topics related to insulation coordination of both transmission and distribution lines
Example – Cigré SC4, System technical performances
Lightning protection and insulation coordination, their modeling and analysis with changing technologies
(wind turbines, UHV lines, active distribution networks).
Outline
 Introduction
 Lightning performance of distribution networks
 Lightning impact on HVDC overhead transmission lines
 Conclusions
Lightning performance of distribution networks
Impact of lightning on the power quality of distribution networks
Sags / month / bus
14
12
10
8
6
4
2
0
2
4
6
Lightning Flash Density (flash / km2 / year)
Source: E.W. Gunther and H. Metha, ‘A survey of distribution system power quality’, IEEE TPWD, 10‐1, 1995
8
Lightning performance of distribution networks
Remark: the different geometry and insulation characteristics of transmission
and distribution overhead lines  direct or indirect lightning events differently
concern the two line types:
 direct lightning major concern for transmission lines
 htl>>hdl
 indirect lightning major concern for distribution lines
 CFOtl>>CFOdl
Overhead distribution lines
hdl
htl
Overhead transmission lines
Lightning performance of distribution networks
The evaluation of the lightning performance of overhead distribution lines, available “standards’:
IEEE Std. Guide 1410;
Joint Cigré‐Cired WG C4.4.02;
Inherent complexity of distribution systems:
number of lines (main feeder with laterals) presence of power components (transformers, surge arresters, groundings, etc.)
is well far from the straight line configuration generally adopted in the literature and in the above ‘standards’. Distribution systems
insulation coordination 
Evaluation of the number of
annual flashovers due to
indirect lightning that a
distribution overhead line
may experience, as a
function of insulation level
and line construction
design.
Number of induced voltages with magnitude
exceeding the value in abscissa/100km/yr
Number of Flashovers
Lightning performance of distribution networks
Note: a so called ‘incidence model’ is needed
To distinghish between direct and indirect
LIGHTNING PERFORMANCE
OF A DISTRIBUTION LINE
Voltage [kV]
CFO [kV]
Lightning performance of distribution networks
What was available within IEEE Std. 1410-2004 ?
Rusck simplified formula
U max


1
Z I h
1 v
 0 0 1 
2
y 
2c
1  v

1  
2  c

Too simple: not adequate in many cases!






v return stroke velocity
Z 0  1 / 4 0 /  o  30
Assumptions: a. single‐conductor b. infinitely long lines above a
c. perfectly cond. ground
d. step current waveshape

h
Zswc
U'
 1  sw 
U
h Zsw  2Rg
Lightning performance of distribution networks
What is available within the new IEEE Std. 1410-2010 ?
i(0,t)
RSC
i(z,t)
Lightning return stroke current model
i(z,t)
LEMP
E, B
Lightning ElectroMagnetic Pulse Model
E, B
EMC
ElectroMagnetic Coupling Model
U(x,t)
I(x,t)
Lightning performance of distribution networks
What is available within the new IEEE Std. 1410-2010 ?
Single conductor line
Source: A. Borghetti, C. A. Nucci, M. Paolone, “An improved procedure for the assessment of overhead line indirect lightning
performance and its comparison with the IEEE Std. 1410 method”, IEEE Trans. on Power Delivery, pp. 684-692, January 2007.
Lightning performance of distribution networks
Influence of groundings – spacing
Single conductor line
plus grounded conductor
Source: A. Borghetti, C. A. Nucci, M. Paolone, “An improved procedure for the assessment of overhead line indirect lightning
performance and its comparison with the IEEE Std. 1410 method”, IEEE Trans. on Power Delivery, pp. 684-692, January 2007.
50 m
370 m
Zc
SW
gr. point
Ideal ground, Rg=0 
Stroke
location
SW
gr. point
500 m
SW
gr. point
Zc
Lightning performance of distribution networks
Influence of groundings – ideal/lossy ground
Single conductor line
plus grounded conductor
2.2
0.52
Comparison between phase-to-ground and phase-to-grounded-wire flashover rate
Comparison between phase-to-ground and phase-to-grounded-wire flashover rate
curves calculated for different ground conductivity σg and grounding resistance Rg.
curves calculated for different ground conductivity σg and grounding resistance Rg.
(Shielding wire grounded each 200 m. A linear model is assumed for the grounding impedance of
(Shielding wire grounded each 200
A linear
model is wire
assumed
for the grounding impedance of
the m.
neutral
or shielding
)
the neutral or shielding wire )
Lightning performance of distribution networks
Influence of surge arresters
σg =1 mS/m
Single conductor line
Lightning performance of distribution networks
Influence of network topology
5090
5088
5086
5084
1.3 m
b
5082
2347
2349
2351
2353
Coordinate Gauss-Boaga x [km]
2355
c
10 m
5080
2345
a
10.8 m
Coordinate Gauss-Boaga y [km]
5092
CFO =125 kV
Lightning performance of distribution networks
fl.num:30260#4 del 30/8/2007 11:23:36.407205945 -48.4 kA
Influence of network topology
CESI‐SIRF event n. 30260‐4
Aug. 30, 2007
Ip= ‐48.4 kA
Primary 132/20 kV
substation ' Ponterosso'
5091
5090
Coordinate Gauss-Boaga y (km)
Experimental observations
5092
Venus
5089
Maglio
5088
5087
5086
5085
Torrate
5084
5083
5082
5081
2345 2346 2347 2348 2349 2350 2351 2352 2353 2354 2355 2356
Coordinate Gauss-Boaga x (km)
Lightning performance of distribution networks
Influence of network topology
Annual number of events exceeding the value in
abscissa
1.0000
straight line
H-shaped network (type 1)
H-shaped network (type 2)
T-shaped network
0.1000
0.0100
0.0010
0.0001
50
100
150
Voltage [kV]
200
250
Outline
 Introduction
 Lightning performance of distribution networks
 Lightning impact on HVDC overhead transmission lines
 Conclusions
Lightning impact on HVDC overhead transmission lines
HVDC typical configuration
Lightning impact on HVDC overhead transmission lines
The designer of a power system needs to know the flashover rate of an overhead power line for a selected insulation level to meet the reliability criteria set for the system.
The lightning flashover rate (lightning performance of the line) is the sum of:
direct strikes flashover rate;
nearby strikes flashover rate (disregarded in view of TL CFO);
flashover rate from failures of protective equipment.
Only first strokes of negative downward flashes are generally taken into account in lightning performance studies.
To predict the lightning performance one needs the knowledge of:
the lightning activity (the ground flash density Ng (fl/km2/yr));
the exposure to lightning;
lightning consequences.
Lightning impact on HVDC overhead transmission lines
Exposure (i.e. lightning incidence)
The models applied to calculate lightning incidence on transmission lines are based on consideration of the physical processes involved during the final stages of progression of a charged downward lightning leader (usually assumed negative) in its approach to the earth, or toward structures such as a line or transmission tower.
These models, based on downward leader approach, could be divided into:
conventional models  electrogeometric model;
more recent models leader progression model (LPM).
Lightning impact on HVDC overhead transmission lines
Conventional electrogeometric model: basic concept
Single conductor overhead line of a given height h:
 rc: striking distances to a phase conductor;
direct stroke
 rg: striking distances to ground;
 dl: lateral attractive distance of the line
rc  Arc  I b
rg  Arg  I
nearby stroke
r g  k  rc
b
rc
rc
rg
A
b
k
Armstrong and
Whitehead
6.7
0.8
0.9
IEEE WG
10
0.65
0.55
h
dl
rg
d l  r  rg  h 
2
c
2
Lightning impact on HVDC overhead transmission lines
Advanced models: leader progression model
Sequential solution of the Poisson’s equation:
   ( 0 V  P)  
Electric potential iso‐surfaces associated to a downward leader corresponding to a peak current of 20 kA. Downward leader at 360 m from the ground. Inception conditions for the formation of the upward leader from the earthed structure have not yet been reached.
Lightning impact on HVDC overhead transmission lines
Lightning leader approaching ground: downward motion unperturbed unless critical field conditions develop  juncture with the nearby tower, called final jump.
For each electric field streamline connecting the two leaders it has determined whether the electric field exceeds the value of 500 kV/m along the overall streamline length.
Peak current of 20 kA located at 23 m from the 30 m high earthed
structure: electric field iso‐surfaces and streamlines.
Lightning impact on HVDC overhead transmission lines
Shielding failure: basic concept
For a specific value of stroke current, arcs of radii rc are drawn from the phase conductors and from the shield wires with the horizontal line at a distance rg from the earth. A shielding failure is a stroke that terminates on a phase conductor, in spite of the presence of overhead ground wires. The flash collection rate is:
Dc
Dg

N s  2 N g L [ Dg ( I )  Dc ( I )] f ( I )dI

I min
Integrating only the exposure area of the phase conductors, we obtain the shielding failure rate (SFR)
rc
I max
SFR  2 N g L  Dc f I dI
rc
I min
Where:
 Imin is the minimum lightning current (2 or 3 kA);
 Imax is the maximum current at and above which
no stroke will terminate on the phase conductor.
rg
Lightning impact on HVDC overhead transmission lines
Shielding failure flashover rate: SFFOR
If the voltage E is set to the CFO, negative polarity, then the critical current Ic, at and above which flashover occurs.
The initial condition of the HVDC could be taken into account into the estimation of Ic.
Therefore SFFOR is:
SFFOR  2N g L
I max

Ic
Dc f  I  dI
Lightning impact on HVDC overhead transmission lines
Shielding failure on HVDC: experimental observations
Source: Hengxin He, Junjia He, Zhang, D., Li Ding, Zhenglong Jiang, Cheng Wang, Huisheng Ye, “Experimental Study on Lighting Shielding
Performance of ±500 kV HVDC Transmission Lines”, 2009 Asia-Pacific Power and Energy Engineering Conference (APPEEC 2009).
Lightning impact on HVDC overhead transmission lines
Shielding failure on HVDC: experimental observations
Source: Hengxin He, Junjia He, Zhang, D., Li Ding, Zhenglong Jiang, Cheng Wang, Huisheng Ye, “Experimental Study on Lighting Shielding
Performance of ±500 kV HVDC Transmission Lines”, 2009 Asia-Pacific Power and Energy Engineering Conference (APPEEC 2009).
Lightning impact on HVDC overhead transmission lines
Shielding failure on HVDC: experimental observations
Remark: it seems that the applied DC voltage on the polar conductor, as well as its
polarity, plays a role into the upward streamer inception criterion and, therefore, into the
attachment process.
Source: Hengxin He, Junjia He, Zhang, D., Li Ding, Zhenglong Jiang, Cheng Wang, Huisheng Ye, “Experimental Study on Lighting Shielding
Performance of ±500 kV HVDC Transmission Lines”, 2009 Asia-Pacific Power and Energy Engineering Conference (APPEEC 2009).
Lightning impact on HVDC overhead transmission lines
Back flashover: basic concepts
When lightning strikes the tower (or the OHGWs), the current on the tower and ground impedances causes the rise of the tower voltage. A small fraction of the tower and shield wires voltage is induced in the phase conductors due to the electromagnetic coupling, nevertheless the tower and shield wires voltage becomes greater then phase conductors voltage. If the voltage exceeds the line CFO, flashover occurs called back flash or back flashover and the corresponding minimum lightning current that produces such a flashover is called critical current.
The term “back” refers to the fact that the highest voltage is on a part of the power system normally at ground potential.
Lightning impact on HVDC overhead transmission lines
Estimation of the backflash rate
Use of the Electromagnetic Transient Program
The calculation of the critical current Ic by means of EMTP‐like programs allows to take in account:
 waveshape of the current source;
 flashover criteria in the form of volt‐time characteristics;
 transmission line models including all line conductors (em‐coupling);
 representation of the soil ionization;
 frequency dependent grounding models;
 surge arresters;
 representation of all the power system components;
For HVDC:
For HVDC:

conductors potential (pre‐lightning DC voltage status);
conductors potential (pre‐lightning DC voltage status);

frequency dependency of input impedance of HVDC power electronics.
frequency dependency of input impedance of HVDC power electronics.
Lightning impact on HVDC overhead transmission lines
Calculation of the backflashover rate (BFR)
The BFR is the probability of exceeding the critical current multiplied by the number of flashes to the line NL. However, since the crest voltage and the CFO are both functions of the time‐to‐crest tf of the lightning current, the critical current previously determined is variable. Therefore, the BFR considering all the possible time‐to‐crest values is:

BFR  0.6N L  
0 Ic
I


fl
f   f t f dIdt f 

 tf 
 100km  yr 
 
Where f(I/tf) is the conditional probability density function of the stroke current given the time‐to‐crest and f(tf) is the probability function of the time‐to‐crest.
Note: in order to obtain the BFR for strokes to the tower and stroke and to the spans, the BFR obtained for strokes to the tower is multiplied by a coefficient, equal to 0.6 [Hileman, 1999]
Outline
 Introduction
 Lightning performance of distribution networks
 Lightning impact on HVDC overhead transmission lines
 Conclusions
Conclusions
 Renewables integration is increasing the need of electrical network reliability  revamping of the studies on insulation coordination of T&D systems.
 Distribution: advanced models integrated into international standard for the evaluation of lightning performance taking into account realistic network configurations.
 Transmission with HVDC: inherent characteristics influence the lightning performance  need of more research on
 attachment process and relevant analytical formulation;
 high frequency models for line terminations.