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Abstract
A long flashover arrester (LFA), which comprises of three flashover modules using the creeping discharge effect, is described in this paper. In this design, the total arrester stressing voltage is applied simultaneously to all of the three modules so that the voltage-time characteristics of the arresters are improved. It assured reliable protection of medium voltage (e.g., 10kv) over head power line against both induced over voltages and direct lightning strokes. A single LFA per support or pole is found to be sufficient to protect an over head line against induced over voltages. An LFA should be arranged in parallel with each insulator in order to protect a line against direct lightning strokes.
INTRODUCTION
A new simple, effective and inexpensive method for lightning protection of medium voltage overhead distribution line is using long flashover arresters (LFA). A new long flashover arrester model has been developed. It is designated as LFA-M. It offers great number of technical and economical advantages. The important feature of this modular long flashover arrester (LFA-M) is that it can be applied for lightning protection of overhead distribution line against both induced overvoltages and direct lightning strokes. The induced over voltages can be counteracted by installing a single arrester on an overhead line support (pole). For the protection of lines against direct lightning strokes, the arresters are connected between the poles and all of the phase conductors in parallel with the insulators.
LIGHTNING
WHAT IS LIGHTNING Lightning is an electrical discharge between cloud and the earth, between clouds or between the charge centers of the same cloud. Lightning is a huge spark and that take place when clouds are charged to at a high potential with respect to earth object (e.g. overhead lines) or neighboring cloud that the dielectric strength of the neighboring medium(air) is destroyed.
TYPES OF LIGHTNING STROKES
There are two main ways in which the lightning may strike the power system . They are
1. Direct stroke
2. Indirect stroke
DIRECT STROKE
In direct stroke, the lightning discharge is directly from the cloud to the an overhead line. From the line, current path may be over the insulators down to the pole to the ground. The over voltage set up due to the stroke may be large enough to flashover this path directly to the ground. The direct stroke can be of two types
1. stroke A
2. stroke B
In stroke A, the lightning discharge is from the cloud to the subject equipment(e.g. overhead lines). The cloud will induce a charge of opposite sign on the tall object. When the potential between the cloud and line exceed the breakdown value of air, the lightning discharge occurs between the cloud and the line.
In stroke B the lightning discharge occurs on the overhead line as the result of stroke A between the clouds. There are three clouds P,Q and R having positive, negative and positive charge respectively. Charge on the cloud Q is bound by cloud R.If the cloud P shift too nearer to cloud Q,Then lightning discharge will occur between them and charges on both these cloud disappear quickly. The result is that charge on cloud R suddenly become free and it then discharges rapidly to earth, ignoring tall object.
INDIRECT STROKE
Indirect stroke result from eletrostatically induced charges on the conductors due to the presence of charged cloud. If a positively charged cloud is above the line and induces a negative charge on the line by electrostatic induction. This negative charge however will be only on that portion on the line right under the cloud and the portion of the line away from it will be positively charged. The induced positive charge leaks slowly to earth. When the cloud discharges to earth or to another cloud, negative charge on the wire is isolated as it can not flow quickly to earth over the insulator. The result is that negative charge rushes along the line is both directions in the form of traveling wave. Majority of the surges in a transmission lines are caused by indirect lightning stroke.
THE LFA PRINCIPLE
When a lightning surge gets to an insulator, the insulator may flashover depending on the overvoltage value and insulation level of the line. Probability of power arc flow (PAF) depends on many parameters: nominal voltage of the line Unom, length of the flashover path L, moment at which lightning stroke occurred, lightning current magnitude, line parameters, etc. It was found that the main factor, which determines the probability of PAF, is the mean gradient of operational voltage along the flashover path.
E = Uph/L
Where Uph = Unom /3 =phase voltage, kV;
L = length of flashover,
The probability of PAF sharply decreases with a decrease in E. An analysis of available data on spark over discharge transition to PAF concluded that for E=7 to 10 kV/m probability of PAF is practically zero. The flashover length, L is greater for lines with wooden structures rather than steel or concrete structures, because wooden Cross-arm increases the flashover path. As a result probability of PAF for wooden structures is sufficiently lower than for steel or concrete supports. From the short analysis presented above, it is clear that it is possible to improve the protection against lightning by increasing the length of lightning flashover path. The suggested LFA accomplishes this principle. The LFA's length may be several times greater than that of an insulator (string, etc.). Due to a special inner structure the LFA impulse flashover voltage is lower than that of the insulator and when subjected to lightning overvoltage the LFA will flashover before the insulator.
DESIGN OF LFA-M
An LFA-M arrester consists of two cables like pieces. Each cable piece has a semi conductive core of resistance R. The cable pieces are arranged so as to form three flashover modules 1,2,3 as shown in figure1.Semiconductive core of upper piece, whose resistance is R ,applies the high potential U to the surface of the lower piece at its middle.Similiarly,the semi conductive core of the lower piece of the same résistance R applies the low potential 0 to the surfaces of the upper piece, also at its center. Therefore the total voltage U is applied to each flashover module at the same moment, and all three modules are assured conditions for simultaneous initiation of creeping discharges developing in to a single long flashover channel.
Tests have been shown that, as the line conductor is stressed by lightning over voltage impulse, flash over channel develop at different rates.Modules1 and 3 flashover first, followed by module 2 ,and thus, forming a rather long flashover channel along the LFA.
Due to long flashover path, a flashover does not give rise to a power arc as the arc extinguishes when the power frequency current crosses zero. This assures uninterrupted power supply of a LFA protected over head line.
FLASHOVER PERFORMANCES
The flashover performance of modular long-flashover arresters(LFA-M) arresters of two different flashover lengths and the voltage-time characteristics of LFA loop arresters, as well as those of the most common Russian insulators ShF 10-G and ShF 20-G with lengths 17 and 23 cm,respectively,were studied. The 50% flashover voltages of these units are 130 and 160 KV when stressed by 1.2/50 lightning impulses of negative polarity. therefore, these units will be referred hereafter as INS 130 and INS 160,respectively.
The voltage-time characteristics of the arresters and insulators can be approximated by the expression
U=a tb
Where, U=flashover voltage in kilovolt.
t=time to crest in microseconds.
a,b are empirical coefficients whose values are given in the table
test object impulse polarity A b
insulator ins130 + 190 -0.352
insulator ins 130 - 185 -0.285
insulator ins160 + 243 -0.407
insulator ins160 - 280 -0.28
LFA-M,L=1m +,- 109 -0.784
LFA-M,L =2 m +,- 173 -1.05
LFA-M,L=0.8m + 159 -0.5
LFA-M,L =0.8m - 107 -1.64