22-06-2010, 09:40 PM
Lightning Protection using LFA-M arrester presentation.ppt (Size: 1.27 MB / Downloads: 321)
Lightning Protection using LFA-M arrester
PRESENTED BY:
RAJLAXMI SAHA
7TH SEM,ELECTRICAL,0501211500
CONTENTS
INTRODUCTION
PRINCIPLE
NEED OF LFA-M ARRESTER
LFA-M DESIGN
FLASHOVER CHARACTERISTICS
TYPES OF LIGHTING STROKES
PROTECTION USING LFA-M
TESTS FOR LFA-M ARRESTER
FUNCTIONING
APPLICATION
BENEFITS OF LFA-M ARRESTER
FUTURE EXPANSION
CONCLUSION
INTRODUCTION
A new simple,effective & inexpensive method for
Lightning protection of medium voltage overhead
Distribution line is using a long flashover arrester
It designated as LFA-M which comprisres three
Three flashover modules using the discharge effect
The total arrester stressing voltage is applied
Simultaneously to all the three modules so that voltage time
Charactertics of the arresters are improved and In order to prevent
Arc fusion of insulated conductors on distribution lines due to
lightning flashover it is recommand to install an arrester at every
insulator against both induced overvoltages &direct lightning
Strokes.
PRINCIPLE
When a lightning surge gets to an insulator
May flashover depending on the overvoltage
Value & insulation level of the line .
PAF (power acr flashover) depends on
Many parameters such as:
>nominal voltage of line Vnom
>length of flashover path L
>moment at which lightning stroke occurred ,lightning curent magnitude,line parameters
E=Vph/L
Vph =Vnom/1.732,phase voltage kv
L=length of flashover ,m
PAF decrease with decrease in E analysis of available data on spark
Over discharge transition to PAF conclude that E=7 to 10 kv/m probablity of
PAF is pratically zero .Flashover length is greater for lines with wooden structures rather
Then steel or concrete support. So this analysis presented above it is clear that it is
Possible to improve the protection against lightning by increasing the length of lightning
Flashover path. LFAâ„¢s length may be several times greater than that of an insulator .
Due to special inner structure the LFA impulse flashover voltage is lower than that of the insulator &
When subjected to lightning over voltage the LFA will flashover before insulator.
In russia for lightning over voltage & conductor burn protection of 10kv overhead lines.
NEED OF LFA-M ARRESTER
Distribution power lines are most common. According to information provided
by the Lenenergo (St Petersburg Power Administration), the total length of 6-
10 kV overhead lines in service in the Russian Federation exceeds 1200
thousand kilometers, 698 thousand km of which (including 450 km
in the Lenenergo grid alone) must be replaced or uprated. Power supply
reliability is very much a function of the reliability performance of 6- 10 kV
overhead lines. For a number of reasons the reliability performance of 6- 10
kV overhead lines remains low. Because a majority o loads are supplied by 6-
35 kV lines whose reliability is below that of higher voltage lines distribution
Grids account for a large percentage of power supply interruptions, both in
terms of incidence and duration. This holds true both for Russia and other
countries. Forexample, 11 to 33 kV overhead lines in Japan account for 88%
of total supply interruptions and 77% of total line outages for lines of
all voltage ratings [1]; 2 to 33 kV lines in UK are responsible for 77% of supply
interruptions [2].In Russia some 30 to 50% of line outages stem from mechanic
causes (falling towers, conductors broken by wind or ice, vandalism etc.),
while some 50 to 70% are due to electrical causes including:
¢ insulation flashovers and power arc onset at lightning overvoltages
¢ insulation flashovers at switching and quasistationary overvoltages
¢ operating voltage insulation flashovers due to pollution and wetting
¢ lightning impulse puncture of insulators
¢ insulator failures due to power arc
¢ conductor burn down due to power arc
¢ other electrical causes.
A low reliability level of 6-10 kV lines results in high undersupply penalties, as well
as in a considerable increase of maintenance costs. Among objective causes of a
poor reliability Performance of 6-10 kV lines one should point out a fairly low
impulse strength of line insulation. For lines with reinforced concrete poles it is
around 130-150 kV. Thus each lightning stroke on or near such a line of a
conventional design that results in a lightning overvoltage in excess of 150 kV
causes a flashover. There is also a heavy probability of a steady power arc,
which brings about quasi-stationary overvoltages and equipment damage and
necessitates disconnection of the line and thus load shedding. Unless special
lightning protection steps are taken to overhead lines with covered conductors, a
lightning overvoltage leads first to a flashover of a line insulator and next to a
breakdown of the solid conductor insulation. With a high probability such a
lightning flashover brings about a power frequency arc which keeps burning at
the insulation breakdown point until the line is disconnected.The arc can easily
burn the insulating covering and, with heavy fault currents, melt the conductor [3].
A subjective reason is that utilities resigned themselves to an "inherently low"
lightning performance of 6-10 kV lines. The above brief analysis shows that it
highly imperative to enhance the reliability performance of 6-10 kV lines. Their
Operational reliability can be improved many times over through ruling out or
reducing radically the percentage of outages due to electrical causes. To achieve
this, it is necessary to assure a high lightning performance of such lines and to
make the line insulation less vulnerable to other electrical stresses.
LFA-M DESIGN & OPERATION
An LFA-M arrester consists of two cable like pieces
With a resistive core[3].There are also intermediate
Ring electrodes on its surface. Cable pieces are
Arranged so as to form three flashover modules 1,2,3
As shown in figure .The resistive core of the upper
Piece whose resistance is R applies the high potential
V to the surface of the lower piece at its middle
Similarly the resistive core of the lower piece of the
Same resistance R applies the low potential 0 to the
Surface of the upper piece also at its center. In this way the
Total voltage V is applied to each flashover modules at the
Same moment & all three modules are assumed conditions for
Simultaneous initiation of creeping discharge developing into single
Long flashover channel.
FLASHOVER CHARACTERISTICS
LFA-Insulation Tube
LFA-IT prototype for 10 kV distribution lines was built in accordance with Fig. .The diameter of the conductor was 9mm and the tube wall thickness was 8mm .The flashover length of the line insulator was 17 cm (without sparkover horns) and the flashover length of the LFA-IT prototype was l=75cm. The air gap S varied from 0 to 17 cm. The LFA was tested by 1.2/50 ms lightning impulses of positive and negative polarity. Test impulse was applied to the conductor while the pole was grounded (Please note that after the gap is sparked over gap, the polarity of metal tube clamp which is placed in the middle of the insulated tube becomes opposite to that of the conductor). Test results are
Table 1. LFA-IT lightning impulse flashover voltages
Sparkover gap S, cm 50% flashover voltages, U50% , kV
polarity
positive negative
0(the insulator is shunted) 112 170
5 150 175
17(without sparkover horns) 310 215
POWER ARC FOLLOW (PAF)
The test circuit (see Figure 4) consisted of three impulse current generators. Lightning ImpulseGenerator (LIG) provided lightning impulses with crest voltages up to 220 kV and current amplitudes up to 15 kA, front time from 1 to 5 µs and duration (time to half value) from 40 to 60 µs. Lightning impulses were applied to the tested object via
Figure 4: Power Arc Follow Test Circuit
LIG = Lightning Impulse Generator;
K1, K1= Keys;
DC1, DC2 = Direct Current Sources;
GAP1, GAP2 = Air Gaps;
IG = Triggering Impulse Generator;
LFA = Tested LFA Sample;
C1=C2=0.1 µF; C3=300 µF; C4=1000 µF; L1= L2=
L3= 1 mH, L4= 100 µH, R1=100 Ohm, R4=100 Ohm,
R2 and R3 varied from 0 to 120 Ohm.
protective GAP1. As the protective gap the LFA sample with a 1 m flashover length was used.
Two generators with discharge capacitors C3 and C4 were used to simulate the positive and negative half-periods of the operating voltage, respectively. The second generator was applied after a time delay of 3 to 5 ms using triggering Impulse Generator (IG). Capacitors C3 and C4 were charged from dc current sources DC1 and DC2, respectively. The simulated operating voltage was applied to the LFA through a distribution line model which consisted of capacitors C1 and C2, inductances L1, L2 and L3 and a 350-m long cable, simulating an overhead distribution line of approximately 6 km in length. After application of the lightning impulse to the LFA sample, capacitor C3 discharged through the line model forming a positive half-cycle of the operating voltage. After 3-5 ms, GAP2 was triggered and a negative operating voltage half-cycle was applied from circuit C4,L4 and R4. The length of the tested LFAs varied from 0.2 to 2 m, the
resistances R2 and R3 varied from 0 to 120 Ohms and the charging voltage U of capacitors C3 and C4 varied from 0 to 9 kV. During testing, U was increased in 0.5 kV steps. The highest charging voltage U at which there was no Power Arc Follow was determined as the critical voltage, Ucr. Thus the mean critical gradient was defined as Ecr=Ucr/l, where l = flashover length along the LFA. For measuring the short-circuit current Is.c., the LFA was replaced by a short copper wire. Test results including typical voltage and current
oscillograms are presented in [6]. They can be approximated by a formula:
Ecr=100(42-3.4U)/Is.c. (1)
where: Ecr = [kV/m]
U = [kV] in the range from 0 to 9 kV It can be seen from equation (1) that Ecr depends on the short circuit current Isc and the operating voltage, U. The higher are the short circuit current Is.c and the voltage U,
the lower is the critical gradient Ecr.Is.c = [A] in the range from 0 to 1000A; all parameters are peak values.
TYPES OF LIGHTNING STROKES
There are two main ways in which the lightning
May strike the power system.
>Direct stroke
>Indirect stroke
Direct stroke:
In direct stroke the lightning discharge is directly
From the cloud to the overhead line. From the
Line, current path may be over the insulators down to
The pole to the ground. Over voltages set up due to
Stroke may be large enough to flashover this path directly to
Ground.
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 & line exceed the breakdown value of air, the lightning discharge occurs
Between the cloud & the line.
Stroke B:
The lightning discharge on the overhead line as the result of stroke A between the clouds. There are three clouds
P,Q&R having positive , negative &positive charge respectively .Charge on the cloud Q is bound by cloud R .If the clouds P shift too
Nearer to cloud Q, then lightning discharge will occur between then R charges & both these cloud disappear quickly. The result is
That the charge on the cloud are suddenly become free & it then discharges rapidly to earth, ignoring the tall object.
Indirect stroke:
The results from electrostatically induced charges
On the conductors due to the presence of
Charged cloud. If a positively charged cloud is
Above the line & induce 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 & the portion of the
Line away from it will be positively charged. The
Induced positively charge leaks slowly to earth. When
The cloud discharges to earth or to another cloud,
Negatively charge on the wire is isolated as it cannot flow quickly
To earth over the insulator. The result is that negative charge
Rushes along the line is both direction in the form of traveling wave.
Majority of the surges is a transmission line are caused by indirect lightning
Stroke.
PROTECTION USING LFA-M
It follows from sequence of event that direct
Lightning stroke causes flashover of all the
insulators on the pole. Therefore in order to protect
The line against the direct lightning stroke LFA should be
Mounted on the pole in parallel with each line insulator .
A delta arrangement of conductors maximizes direct lightning stroke on
The top which acts as a shielding wire for the bottom spheres .LFA mounted on
The top phase must flash over before the top phase insulator. It is stressed by fairly step
Over voltage impulses associated with direct lightning strokes on conductor. Therefore this arrester
Should be relatives short .After atop phase LFA flashes over, lightning current will flow, through pole
To the ground. Thus voltage on the cross area increases at much slower rate then it does on the
Lightning struck conductor before the flashover of the top phase LFA. On the other hand the potential
Of the adjacent phase also increases due to electromagnetic coupling between conductors but at
much slower rate then that applied to the top phase insulator consequently, an outer phase arrester
operates under much easier. Coordination condition then a top phase arrester with
One or both outer phase arrester activated, a two or three phase lightning flashover is Initiated.
TESTS FOR LFA-M ARRESTER
We perform two tests for LFA-M arrester
-Quenching tests for LFA-M
-Dielectric tests
Quenching tests for LFA-M:
->The quenching tests for LFA-M was performed in NIIVAâ„¢S lab st.petersburg Russia.
->The power frequency which was 50Hz & resistance R which was short circuited .
->Test results are summarized in table.
Table II: Quenching Test Results for LFA-M, R = 0 _, Ishortcircuit
= 4000 A, V50Hz = 8.7 kVrms. Ifoll = Following
current.
Vg (kVdc) Tests Ifoll (Acr) Result
200 5 1084 All quenching
240 2 1196 All quenching
248 1 1215 Quenching
248 1 1400 No quenching
->There are two possibility of current quenching :
# when current impulse finishes (further quenching at front)
#when power frequency passes zero (further quenching at zero)
->LFA-M successfully quenches power follow current for a circuit with short circuit current of 4kv & for power generator voltage of 8.7 kvrms.
->Therefore for lower grid voltage of 7.5 kvrms it is expected that LFA-M quench high value of power flow currents
Dielectric tests: (using insulators)
-Lightning impulse tests
The determination of the impulse withstand
Voltage was made using the up & down test
Method with 30 impulses at both polarities in
The following conditions:
With out LFA under dry condition
With LFA under dry condition
With LFA under wet condition
LFA “M was connected to the conductor directly,
Without air gap. During test all flashover occurred
At LFA while the insulators withstood applied impulses. The obtained flashover voltages of all types of insulators.
-Radio interference voltage tests
RIV tests were also carried out in the arrangement with LFA & insulators. The criterion consider RIV max limit =200microV,1MHZ under 8.8KVrms.Test in following condition.
Without LFA under dry condition
With LFA-M under dry condition
-Power frequency tests
The determination of 60HZ disruptive discharge voltages was made of 34KVrms it withstand voltage level for 15KV distribution lines applied to LFA-M.
Test in following condition.
It made under dry &wet condition during 1min
No discharge occurred for all test arrangement & conditions
FUNCTIONING
> It protects conductors from burn outs
> It protects lines insulation against lightning over voltages
> It protects overhead lines & mounted equipments against lightning outages & damage
> It protects power grids against arc faults.
APPLICATION GUIDLINES
>Protection against induced over voltages:
To eliminate high short circuit currents associated with two-or three-phase lightning flashovers to ground, LFA-Ls are recommended to be installed one arrester per pole with phase interlacing figure With such an arrangement, a flashover to ground results in a circuit comprising two phases, two arresters and two grounding resistors that limit the fault current and ease arc quenching. The higher are the values of the grounding resistance, the more effective is LFA-L operation.
>Protection against direct lightning strokes:
A direct lightning stroke causes flashover of all the insulators on the affected pole. Therefore, in order to protect the line against a direct lightning stroke, LFA-Ms should be mounted on the pole in parallel with each line insulator figure. Phase-to-phase faults on a pole can give rise to follow-up current on the order of 10 kA or more. In order to quench such currents, flashover length of the LFA-M 13.8 kV should be 1.7 m, i.e. much higher than that of LFA--L(0.9 m) which intended to protect overhead lines against induced over voltages.
BENEFITS OF LFA-M ARRESTER
->The LFA is that current passes outside the apparatus ,flowing along arrester surface. Therefore the arrester cannot be destroyed by excessive current at direct lightning stroke
->It is the simple & reliable construction.
->It can efficiently protect distribution lines from induced over voltages flashover.
->It is inexpensive & no need of maintance.
FUTURE EXPANSION
LFA-M described here consists of three flashover Modules. We can increase the flashover modules .If number of flashover modules increase by Increasing the cable pieces this LFA-M can be Used for lightning protection of very high voltageLines. When the modules increases the total Arrester stressing is distributed these modules also Then it can withstand very high over Voltage.
CONCLUSION
Long Flashover Arresters were developed to protect overhead distribution lines against lightning over voltages
and conductor-burn and have been successfully used in Russia 10 kV lines. In order to check their dielectric and
operating performances considering their possible application in 13.8 kV overhead lines, laboratory tests
were carried out under high voltage and impulse current conditions. Based on the technical information and test
results presented here, the following conclusions can be drawn:
a. Long Flashover Arresters of Loop (LFA-L) and
Modular (LFA-M) types had a good performance
in the dielectric, lightning impulse, radio
interference, power frequency and power arc
quenching tests.
b. LFA-Ls can protect overhead 13.8 kV lines
against induced over voltages. LFA-Ls are
recommended to be installed one arrester per
pole with phase interlacing.
c. LFA-Ms can protect overhead 13.8 kV lines
against over voltages of direct lightning strokes.
LFA-Ms should be mounted on the pole in
parallel with each line insulator.