International Journal of Science, Engineering and Technology Research (IJSETR)

Volume 1, Issue 1, July 2012

[(]

Analysis of Transmission Line Protection

System against from Lightning Effects

Phyo Phyo wai

Abstract— In this paper the authors analyze the lightning behavior of extra- high voltage transmission lines. The transmission line is usually built along mountain terrain or open space area, so it is often struck by lightning. This paper aims to analysis of transmission line protection system against from lightning effects. The shield wire method can be used as a useful tool in the design of transmission line, aiding in a more effective protection of them against lightning strokes.

Index Terms— Lightning, Stroke Effect, High voltage,

Transmission line.

I. INTRODUCTION

The lightning is a huge spark caused by the electrical discharge taking place between the clouds, within the cloud, and between the cloud and the earth. When lightning makes a direct hit on a transmission line, it deposits a large electrical charge, producing and enormous overvoltage between the line and ground.

Lightning cannot be prevented but it is current can be conducted to a grounding system without side flashes where it is harmless dissipated. Lightning cannot be prevented but it is current can be conducted to a grounding system without side flashes where it is harmless dissipated.

Lightning is one of the most serious causes of overvoltage. Every year, millions of dollars are lost by the devastating effects of lightning to buildings, electrical power systems, and facilities with sensitive electronic equipment. Lightning is a leading cause of outages on power unity transmission and distribution systems.

Lightning discharges can be classified into three main types. They are:

(1) Intra cloud discharge,

(2) Cloud-to-cloud discharge and

(3) Cloud-to-ground discharge

Figure1. Types of Lightning

II. Typical Lightning Flash

A typical lightning flash consists of a stepped leader that progress toward the ground at a velocity that can exceed 50 m/μs. When sufficient potential difference between the cloud and the ground exists, arcs move from the ground to the leader column, completing the ionized column from cloud to ground. A fast and bright return stroke then moves upward along the leader column at about one-third the speed of light. Peak currents from such a lightning flash may exceed 100 kA, with a total charge as high as 100 coulombs. Although averages are difficult to assess where lightning is concerned, a characteristics flash exhibits a 2 μs rise time, and a 10 µs to 40 μs decay to a 50 percent level. The peak current will average 18 kA for the first impulse, and about half that for the second and third impulses. Three to four strokes per flash are common.

A lightning flash is a constant-current source. Once ionization occurs, the air becomes a conductive plasma reaching 60000°F, and becomes luminous. The resistance of an object struck by lightning is small consequence except for the power dissipation on that object, which is equivalent to I2R. 50 percent of all strikes will have a first discharge of at least 18 kA, 10 percent will exceed 60 kA, and only 1 percent will exceed 120 kA.

An idealized lightning flash is shown in Figure 2. The stepped leader initiates the first return stoke in a negative cloud-to-ground flash by propagating downward in a series of discrete steps, as shown. The breakdown process sets the stage for a negative charge to be lowered to the ground. A fully developed leader lowers 10 coulombs or more of negative cloud charge to near the ground within a few tens of milliseconds. The average return leader current is measures from 100 A to 1 kA. During tip toward earth, the stepped leader branches in a downward direction, producing the characteristic lightning discharge.

Figure 2. Mechanics of a lightning flash

The electrical potential difference between the bottom of the negatively charged leader channel and the earth can exhibit a magnitude in excess of 100 MV. As the leader tip nears ground level, the electric field at sharp objects on the ground increases until the breakdown strength of the atmosphere is exceeded. At that point, one or more upward-moving discharges are initiated, and the attachments process the previously ionized and charged leader path. This process will repeat if sufficient potential exists after the initial stroke. The time between successive strokes in a flash is usually several tens of milliseconds.

III.  TYPE OF LIGHTNING STROKES

The lightning stroke is classified as into two types. They are

direct stroke and indirect stroke

(1) Direct Stroke

Lightning direct stroke can be defined as the lightning stroke that directly hit any part of the electrical network. In most cases in power distribution lines, the insulation flashover is occurred although the return stroke current is small due to the high generated overvoltage which is very high comparing with the overhead distribution lines insulation level. For example, when the strok e current is 10 kA, the generated overvoltage can be 2000 kV. Also, direct strokes cause faults in the high voltage lines due to the overvoltage and therefore flashover across the insulator strings.

(2) Indirect Stroke

Indirect lightning stroke can be defined as the lightning stroke that does not directly hit any part of the electrical network; however, the induced overvoltage is generated and travelled over the network. This type of strokes is responsible for many of lightning outages of low insulation lines. Most of the flashes hitting near to the line produce overvoltage less than 300 kV.

Although the indirect strokes generate induced overvoltage with small amplitude comparing with generated overvoltage by direct stroke, they are frequently occurred affecting on the performance of overhead distribution lines. In the medium voltage distribution lines, indirect stroke is the main source of recorded faults.

IV.  LIGHTNING STROKE EFFECTS ON

TRANSMISSION LINE

Lightning surges entering a power system through direct strokes are the primary concern in planning surge protection.

These strokes may hit phase conductors directly, or they may strike the overhead ground wires or masts that shield the conductors. There are six effects of the lightning on transmission line.

(1) Flashover

(2) Shielding

(3) Induced Surges

(4) Surge Voltages

(5) Back Flashover

(6) Travelling Wave

The negative charges at the bottom of the cloud induce charges of opposite polarity on the transmission line. These are held in place in capacitances between the cloud and the line and earth, until the cloud discharges due to a lightning stroke. Figure 3 shows various type strokes on transmission line.

Figure 3. Geometry of lightning leader stroke and transmission line

In the first discharge path (1), which is from the leader core of the lightning stroke to the earth, the capacitance between the leader and earth is discharged promptly, and the capacitances from the leader head to the earth wire and the phase conductor are discharge ultimately by travelling wave action, so that a voltage is developed across the insulator string. This is known as the induced voltage due to a lightning stroke to nearby ground.

The second discharge path (2) is between the lightning head and the earth conductor. It discharges the capacitance between these two. The resulting travelling wave comes down the tower and, acting through its effective impedance, raises the potential of the tower top to a point where the difference in voltage across the insulation is sufficient to cause flashover from the tower back to the conductor. This is the so-called back-flashover mode.

The third mode of discharge (3) is between the leader core and the phase conductor. This discharges the capacitance between these two and injects the main discharge current into the phase conductor, so developing a surge-impedance voltage across the insulator string. At relatively low current, the insulation strength is exceeded and the discharge path is completed to earth via the tower. This is the shielding failure or direct stroke to the phase conductor.

V. CLASSIFICATION OF VOLTAGE LEVEL ON

TRANSMISSION LINE

In the transmission line, the voltage level is classified as

Follows:

- low voltage : < 1 kV

- Medium voltage : > 1 kV and ≤ 72.5 kV

- High voltage : > 72.5 kV and ≤ 242 kV

- Extra-high voltage : > 242 kV and < 1000 kV

- Ultra-high voltage : ≥ 1000 kV

VI.  OVERVOLTAGE ON TRANSMISSION LINE

The overvoltage causes many destructive in power system. Thus, there are three categories of overvoltage in the power system.

(1) Temporary overvoltage

(2) Switching overvoltage

(3) Lightning overvoltage

A temporary overvoltage is an oscillatory phase-to-ground

or phase-to-phase overvoltage which usually originates from faults, sudden change of load, open conductors and etc.

A switching overvoltage is an overvoltage at a given location due to a switching operation or fault.

Figure.4 General Scheme of Lightning Overvoltages in Power Systems

Lightning is a naturally occurring phenomenon where in clouds get charged to several thousand kilovolts, and a discharge (stroke) can occur to high ground objects, or even to the ground.

VII.  ANALYSIS OF LIGHNING STROKE EFFECTS ON

230KV TWIN BUNDLE DOUBLE CIRCUIT TRANSMISSION LINE

There are six calculations to analysis the lightning stroke effects on the high voltage transmission line should be studied.

(1) the number of lightning strokes incidence

(2) the number of flashes

(3) the minimum and maximum strike distance

(4) the effective shielding angle

(5) the shielding failure rate

(6) the total back flashover

(7) the total failures

Table .1 Details of Transmission Tower

Cond: No. / Power factor / Phase Coordinates / Cond:
radius / Bundle spacing / VPh-to-Ph
(kV) / Phase angle
α˚
X (m) / Y (m)
1 / Shield / -2.6 / 39.5 / 0.38 / - / 0 / -
2 / Shield / 2.6 / 39.5 / 0.38 / - / 0 / -
3 / A / -4 / 36 / 1.2635 / 40 / 230 / 0
4 / B / -4.5 / 30 / 1.2635 / 40 / 230 / -120
5 / C / -5 / 24 / 1.2635 / 40 / 230 / 120
6 / C׳ / 4 / 36 / 1.2635 / 40 / 230 / 120
7 / B׳ / 4.5 / 30 / 1.2635 / 40 / 230 / -120
8 / A׳ / 5 / 24 / 1.2635 / 40 / 230 / 0

Table.2 Specification of 230kV Transmission line

Specifications / Symbol / Value
Insulator length in meter / W / 2.5
Tower height in meter / hg / 39.5
Spacing between shield wires in meter / b / 5.2
Span distance in meter / Sd / 375
Distance from tower top to phase A & C-prime / upper / 3.5
Distance from tower top to phase B & B-prime / middle / 9.5
Distance from tower top to phase C & A-prime / lower / 15.5
Keraunic level in thunder days per year / T / 30
Sag in meter / sag / 7
Corona Gradient, E0 in kV/m / E0 / 1500
Coefficient Beta of Strike Distance 1 for HV / β / 1
Select grounding resistance / R / 15

The number of the lightning stroke incidence is calculated

by the following equation.

N = 0.12T (1)

The number of flashes NL is calculated by using the

equation (2).

NL= 0.012 T (b+ 4h1.09) (2)

The minimum strike distance is calculated by the

following equation.

S = 10 I 0.65 (3)

Figure .5 Critical Stroke Current of each Phase for a full 360˚

Table. 3 Output Results

Specifications / Unit / Value
Minimum strike distance / m / 38.355
Maximum strike distance / m / 52.629
Minimum current / kA / 7.910
Maximum current / kA / 12.974
Probability of minimum current / % / 0.972
Probability of maximum current / % / 0.906
Total shielding failure / times / 0.068
Total flash / times / 70.918
Critical stroke current at 2microsec.(ph A) / kA / 255.871
Critical stroke current at 2microsec.(ph B) / kA / 187.810
Critical stroke current at 2microsec.(ph C) / kA / 166.964
Critical stroke current at 6microsec.(ph A) / kA / 402.155
Critical stroke current at 6microsec.(ph B) / kA / 250.817
Critical stroke current at 6microsec.(ph C) / kA / 200.947
Dominated average current (phase A prime) / kA / 160.716
Dominated average current(phase C) / kA / 153.950
Dominated average current (phase B B prime) / kA / 167.431
Probability of current A_pr will be exceed / % / 1.367
Probability of current C will be exceed / % / 1.526
Probability of current B B pr will be exceed / % / 1.231
Total back-flash over / times / 0.677
Total failures / times / 0.744
Effective shield angle / degree / 7.909

VIII. Conclusion

Lightning is a capricious, random and unpredictable event. Lightning effects can be direct or indirect action. Once lightning enters a power system, the surge current is unlikely to cause any damage. That can destroy lines, towers, substations and generating stations. In protecting power systems against lightning, surge voltages and currents must be considered.

The best way to protect the tower and line is by introducing the overhead earth wire or shield wires. Therefore, this paper is intended to analyze 230kV twin bundle double circuit transmission line about the effect of the lightning performance by using the MATLAB program.

Acknowledgement

The author wishes to express her deepest gratitude to her teachers, Department of Electrical Power Engineering, Mandalay Technological University. Similar thanks to all for their instructions and willingness to share their ideas throughout all those years of study.

REFERENCES

[1] Anonymous, 2008, “Record of Department of Electric System Planning”, Myanma Electric Power Enterprise (MEPE).

[2] Anderson, J.G, 1975, “Transmission Line Reference Book 345 kV and Above”, 2nd ed. Palo Alto, California Electric Power Research Institute.

[3] Hileman, A. R. 1999. “Insulation Coordination for Power Systems”, New York, U.S.A.: Marcel Dekker, Inc.

[4] Anderson, R.B and Eriksson, A.J, 1979, “Lightning Parameters for Engineering Applications”.

[5] Lucas, J. R, 2001, “Lightning Phenomena for High voltage Engineering”.

[6] Nagrath, I.J and Kothari, D.P, (1994), “Power System Engineering”, New Delhi: Rajkamal Electric Press.

[7] Pritindra.C.: “Overvoltages Caused by Direct Lightning Strokes”, CRC Press LLC., (2001).

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All Rights Reserved © 2012 IJSETR

[(]Manuscript received Oct 15, 2011.

Phyo Phyo Wai, Department of Electrical Power Engineering, Mandalay Technological University, (e-mail:).

Mandalay , Myanmar, Phone/ Mobile No 09-43159407