Evaluation and Improvement of Lightning 
Performance of Pole-Mounted Transformers and 
Medium Voltage (MV) Distribution Lines 
This Thesis was submitted to 
the Department of Electrical Engineering 
University of Moratuwa- Sri Lanka 
In partial fulfillment 
of the requirement for the 
Degree of Master of Engineering 
By 
D A J Nanayakkara 
Thesis Supervisor Professor J R Lucas 
December 2001 
Unnversity of M o r a t u w a 
7 4 4 8 1 
ABSTRACT 
This dissertation presents the means to evaluate lightning performance of pole-mounted 
distribution transformers and Medium Voltage (MV) distribution lines. The main intention is to 
provide a general bench-mark procedure for analysis of lightning performances of pole-mounted 
transformers and MV distribution lines and thereby to ensure proper installation and reduction 
of the high lightning failure rate existing in Sri Lanka. 
In installation of HV surge arresters, the separation length between the arrester and the high 
voltage bushing is of paramount importance. Thus the calculation of the separation lengths and 
failure rates are first discussed and the drawbacks of the present method of installation are 
presented. The required separation depends on several factors such as tower configuration, 
system voltage, tower surge impedance, the length of the earth wire of the arrester etc. Effect of 
ground flash and residual voltage of arresters impart a considerable contribution to the 
transformer protection. An alternative approach for this is also presented. The role of the earth 
resistance and its contribution to High Voltage and Low Voltage failures due to lightning are 
discussed\ indicating typical modes of failure. The determination of a safe value for the earth 
resistance is also presented 
Flashover due to lightning induced surge voltages is of paramount importance in evaluating 
reliability and power quality of Medium Voltage distribution lines. First, analysis has been done 
on flashovers due to direct lightning. Next a comprehensive analysis has been done to develop a 
method to evaluate lightning performance against lightning induced flashes. Accuracy of the 
algorithms to calculate lightning induced line flashes highly depends on parameters such as 
wave front time of return stroke current, magnitude of the return stroke current and its 
probability distribution and the speed of the return stroke current. Having selected the most 
suitable values for these parameters, a technique has been developed to determine line flas hover 
rates due to induced lightning. Next, cumulative effect of both direct and indirect lightning surges 
has been discussed. Analysis has been done to study dependence of line flas hover rates on line 
insulation levels,isokeraunic level, line height, wave front time, corona damping, shielding due 
to near by objects etc. Variation of line flas hover rates with above parameters has been 
discussed. Theoretical results have been compared with practical data as a measure to check 
adequacy of the developed analysis technique. 
The report finally presents simplified preventive measures that can be implemented in Sri Lanka. 
Acknowledgements 
I \\ish to express my gratitude and sincere thanks to those who help me out through out the Master of 
Engineering Degree programme: Profc~~or J R Luca~ for the invaluable guidance and advice given 
during the research work; Mr l' D Jaya\\ardana for providing me the opportunity to engage in M~ng 
degree course with out hesitation; Distribution planning branch of Ceylon Electricit} Board for 
providing some valuable information: and linall} my family members for providing me the time and 
courage to complete the dissertation. 
I 
... 
II 
CONTENTS 
Ackno~ ledgemcnt 
Abstract 
List of symbols 
1. INTRODUCTION 
.I 
2. LIGHTNING PERFORMANCE OF POLE MOUNTED DISTRIBUTION 
TRANSFORMERS 
2.1 Direct Lightning Surges 
2.2 Protection Against Direct Strokes 
2.3 Effect of Lightning Induced Surges on Transfonner 
Performance 
2.4 Protection With Arresters I laving I .ow Re~idual Voltages 
2.5 Effect of Earth Resistance 
2.6 Typical Application in Sri Lankan System 
3. LIGHTNING PERFORMANCE OF MEDI UM VOLTAGE 
DISTRIBUTION LINES 
3.1 Evaluation of Lightning Performance of Medium Voltage 
Distribution Lines. 
3.2 Overoltages due to Direct Lightning 
3.3 Overvoltages due to Indirect Lightning Surges 
3.4 Determination ofTotal Flashover rate of Medium 
Voltage Overhead Lines 
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3.5 Comparison Of Results With Real Flash Over Data of 
MY Lines 
3.6 Improvement To Lightning Performance of MY Lines 
4. FUTURE WORK 
S. CO~CLUSION 
6. APPENDIX 
I 
7. REFERENCES 
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iv 
LIST OF SYMBOLS 
(3 : Reflection coefficient at the transformer 
BIL : Basic insulation level <" V) 
d : Horizontal span between outennost conductors (m) 
de/dt : Rate of rise of surge voltage(" V / f.l.S) 
E, :Peak surge voltage at transformer<" V) 
GfD : Ground flash density (strokes/krn '/year) 
H : Line height above ground (m) 
lAM : Maximum current through the arrester (kA) 
10 : Lightning stroke current, peak value (kA) 
10 , : Lightning stroke current, peak value. v .. hich exceeds slope SA at{, at a distance x away 
from A (kA) 
K :corona-damping constant (f.ls/kVm). 
I : Separation between arrester and transformer (m) 
MY : Medium voltage 
Nr : Number of strokes per km2 per year 
Nr :Number of lightning surges arriving at A with a slope higher than SA per year. 
P1o : Probability of lightning current exceeding Io. 
... 
R31 : Impulse resistance of arrester earth rod. (0). 
SA (de/dt) rate of rise of surge voltage at recci\ ing end (kV /f.ls) 
Sr : Shielding factor due to nearb) objects 
S., : (de/dt) rate of rise at point of stri"e <" V /f.ls) 
tr : Wave front time (f.1S) 
t. : Duration between two lightning surges \.\.hich cause damage to the transformer (yr) 
U1 : Line insulation level (k V) 
Up : Arrester residual voltage (kV) 
v :Velocity of wave propagation (m/~ts} 
Ypk :Peak value of surge voltage (kV) 
YRa :Voltage developed across earth resistance of arrester earth rod. (kV) 
X : Distance in which a surge with an infinite slope will decay to slope SA alA (m) 
Z :Surge impedance of line (Q) 
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