Lightning Outage System using Two-Point Method

Lightning Outage System using Two-Point Method

Riza Alfita

Faculty of Engineering, Trunojoyo University of Madura Bangkalan, Indonesia

Aeri Rachmad

Faculty of Engineering, Trunojoyo University of Madura Bangkalan, Indonesia

Rosida Vivin Nahari

Faculty of Engineering, Trunojoyo University of Madura Bangkalan, Indonesia

Abstract – The main role of an electric power system is to provide and distribute electrical energy reliably and continously to its electrical loads. Nowadays, power plants are interconnected. This interconnection provides a backup system, that is, if there are disruptions in a power plant or transmission lines, the electric power supply can still be maintained. Lightning outage in transmission lines is a disturbance caused by lightning strikes at phase conductors of a 500 kV Extra High Voltage (EHV) Transmission Lines. Since the continuity of electricity supply is important, an application software that able to examine the reliability level of distribution lines- which its results can be later compared with the real disturbances- is needed. The two- point method is one of the methods that can be used in the optimazation process of lightning outage calculations. Research results show that the R-Phase is the most sensitive one toward lightning strikes due to its position at the uppermost of the tower of EHV lines, with the number of strikes of 0.34/23,093km/year.

Keywords – Lighting outage, two-point method, phase.

1. INTRODUCTION

   The demand of electrical energy in Indonesia is increasing, along with the increasing population and economic growth. To be highly-realiable, the electrical power system should be supported by a reliable protection system whichprotect the system safely and efficiently. Nowadays, power plants are interconnected. This interconnection provides a backup system, that is, if there are disruptions in a power plant or transmission lines, the electric power supply can still be maintained.

   Although the system has been well planned, in fact there have been total blackouts which occurred in several cities or countries.For instance, in Indonesia on April 13, 1997; in New Zealand on February 28, 1998; and in London on August 23, 2003. The incident in 1997 reoccured in Indonesia on August 18, 2005 which there was a total blackout for 3 hours in most of the island of Java, resulting in 42 trips of Electric Railway (KRL) in JABOTABEK canceled, 15 flights at Soekarno-Hatta delayed and a lot of traffic jams [1].

   The main role of an electric power system is to provide and distribute electrical energy reliably and

   continously to its electrical loads. In general, the reliability of a power system can be defined as the ability of a system to provide a sufficient power supply with satisfactory quality. The reliability of an electric power systemis determined by adequacy assessment and security assessment.It means that the reliability of electric power system is determined by the system ability to supply enough electrical energy to customers,that fulfill certain requirements, satisfactorilyas well as its ability to keep functioning under un-anticipated disturbances, such as short circuit or loss of system elements[1];[3];[4].

   One of the drawbacks of the electric power system in Indonesia is the existence of voltage instability and overloads which in turn affect other systems. Therefore, we need a software that can predict the lightning outage due to lightning strikes on 500 kV EHV transmission lines (SUTET).

2. LIGHTNING OUTAGE

   Lightning outage is a disturbance caused by lightning strikes on transmission lines, either directly or indirectly (induction strikes), which cause disruption tothe continuity of electrical power distribution from the corresponding transmission line.

   On the transmission line, lightning is an unplanned disruption of electrical power distribution. The unit of the disturbance or so-called the number of lightning outagesis expressed in unit of outage/100 km / year

   Types of Lightning Outages

   Outages caused by lightning strikes are divided into two groups [5,6]:

   1. Outage due to direct strikes, which consists of:

      • Lightning outage on the ground wire

      • Lightning outage on the phase conductor

   2. Outage due to indirect lightning (induction strikes).

   On the extra high voltage transmission line, outages due to indirect lightning (induction strikes) can be ignored since its probability is very small. For calculation purposes,

   disturbances due to lightning on the ground wire can be further classified into three types, depending on the place where the lightning struck, i.e.:

   1. Lightning disturbances on the transmission tower, which is a disturbance caused by lightning strikes on ground wire close enough to the tower

   2. Lightning disturbance on the quarter distance of the

      The transmission line can form electric shadows on the ground below it. Lightning usually head to the ground within the shadow the transmission line instead. Contrary, lightnings outside the shadow area do not hit the transmission line. Figure 1 shows the shadow width of a transmission line using 2-wire ground approach.

      The average height of wires in Figure1is :

      tower

   3. Lightning disturbance on the mid-rangedistance of the tower

   H = Y –

   where :

   2 . Average Sag (2)

   3

3. TWO-POINT METHOD

   If there are lightning strikes on a transmission tower, then there will be overvoltages propagating along the

   H = average height of conductors above ground (m) Y = conductor height above ground (m)

   The width of electric shadow proposed by whitehead is as follows :

   g

   W = b + 4. h 1,09 (3)

   tower. Due to weak insulation of the insulator, sparks to the phase conductors occur, resulting in short circuit to the ground. This disturbance is called backflashover.

   If the lightning does not hit ground wires or transmission tower but directly hit phase conductors, there will be a very high overvoltage on the striking point which propagate along the the phase conductor and reach the

   where :

   W = width of electric shadow(m)

   b = distance between ground wires(m) hg = average height of ground wires (m)

   Then, the shadow area of a 100 km transmission line can be expressed as:

   g

   A = 100 . (b + 4. h 1,09) . 10-3 km (4)

   insulator. If the insulator cannot withstand the resulting overvoltage, sparks occur, causing a short circuit to the ground.

   1. Iso-keraunic Level (IKL)

      Isokeraunic Level (IKL) describes theaverage

      Or

      A = 0,1 . (b+4 . hg 1,09) km2 /100 km lines length. (5) Using equantions 1 and 5, the number of strikes to the transmission line can be calculated as :

      g

      NL = 0,012 . IKL. (b + 4. h 1,09) (6)

      number of thunderstorm per year in an area. The value of IKL determines the number of lightning strikes to the ground and transmission lines.

      IKL data of Indonesia can be obtained from the Indonesian Agency for Meteorology and Geophysics.

   2. Lightning Strikes to the Ground

      The number of lightning strikes to the ground in an area is proportional to the corresponding areas IKL, which can be expressed as

      N = 0,12 . IKL (1)

      where :

      N = number of lightning strikes to the groundper km2/year

      IKL = number of thunderstorm per year

   3. Lightning Strikes to the Transmission Lines

      where :

      NL = number of strikes to the transmission line per 100 km per ear

      IKL = number of thunder storm per year hg = average height of ground wires (m) b = distance between ground wires (m)

   4. Lightning Strikes to the Ground Wires and Tower

      The above equation indicates the number of lightning strikes to all parts of transmission lines, including to the tower and phase conductors, either at the quarter or half distance from the tower.

      As discussed in the previous sections, 60% of the total strikes hit the tower while the rest is negligible. This is because the transmission tower is higher than the ground wire in the mid-distance between the towers. Hence, the transmission towers attract lightnings stronger. The number of lightning strikes to the tower can be calculated as follows:

      G b

      Ground

      NT = 0,6 (NL – Ns) (7)

      where:

      NT	=	number of strikes to the tower per 100 km per year
      NL	=	number of strikes to the transmission line per 100 km per year
      NS	=	number of outagedue to protection failure per 100 km per year

      R R

      h h

      b

      g

      2.h 1,09

      2.h 1,09

   5. Probability of Lightning Current

      A lightning usually consists of several strikes. Approximately 55% of lightnings consist of more than one strike, with the average is three.

      g

      W

      Figure 1. Width of electric shadow Source [4]

      Based on the research results of R. Anderson, the probability of lightning current peak valueis obtained using:

      P =

      1

      1

      I 2,6

      (8)

      Eo = gradient threshold of corona where its sheath cannot re-emerge (kV/m)

      The effective radius of a bundled conductor, by taking into

      where :

      31

      account the corona, is:

      Rc = R + Rek (11)

      P = Probability of Lightning Current I = Lightning Current (kA).

      where :

      Rc = radius of the bundled conductor corona sheath (m) R = radius of the corona sheath (m)

      Rek = radius of the single equivalent conductor (m)

      3.8. Calculation of Surge Impedance

      The self surge impedance of a single conductor, by taking account corona, is:

      Znn = 60

      ln 2.h ln 2.h

      (12)

      where :

      Rek Rc

      Znn	=	self surge impedance of the single conductor n (ohm)
      H	=	average height of conductors above ground (m)
      Rek	=	radius of single conductor (m)
      Rc	=	radius of the corona sheath of the bundled conductor (m)

      Figure 2. Accumulative probability of lightning current

   6. Converting The Radius of Bundled Conductor into Those of Single Equivalent Conductor

      Mutual surge impedance of two ground wires is:

      12

      Z = 60 . ln b12

      a

      (13)

      Extra high voltage transmission line usesbundled conductors. To simplify calculations, those bundled

      where :

      12

      conductors are reduced to a single equivalent conductor using the equation:

      Z12 = mutual surge impedance of ground wire 1 and ground wire 2 (ohm)

      Rek = N r11 . r12 .

      where :

      r13……r1n

      (9)

      b12 = distance from ground wire 1 to ground wire 2 (m) a12 = distance between ground wire 1 and ground wire 2

      (m)

      Rek R11 R1n	= = =	radius of the single equivalent conductor (m) radius of sub-conductor 1 (m) distance between sub-conductor 1 to sub-conduktor
      N	=	n (m) number of conductors

      1 2

      a12

      a2

      Kawat Ground a1

      Fasa T

   7. The Effective Radius of Ground Wire and Phase Conductor

   If there are high voltages on the ground wire and the phase conductor, then the effect of the arising corona should be taken into account. The diameter of the corona sheath of the ground wire can be more than one meter and will affect the induced voltage in the phase conductor. The radius of the corona sheath of a single conductor can be calculated using the following equation:

   2 . YG

   b12

   a2'

   Bidang Referensi a1'

   Fasa T

   Kawat Bayangan

   where :

   R ln

   2. h V R Eo

   (10)

   1' 2'

   Figure 3. Cross section of a transmission line

   Figure 3. depicts the cross section of a transmission line, along

   R = radius of corona sheath

   h = average height of conductors above ground (m) V = voltage across the conductors (kV)

   with its wires and shadows.

   On the other hand, the equivalent surge impedance of two non-identical ground wires can be expressed as:

   Zs =

   where :

   Z11 Z 22 Z12 4

   (14)

   The lightning propagation time from the top to the arm of the tower follows the equation:

   Zs = equivalent surge impedance of two ground wires (ohm)

   Z11 = self surge impedance of ground wire 1 (ohm)

   pn = where :

   Yn . 0,85 300

   (18)

   Z12

   = mutual surge impedance of ground wire 1 and 2 (ohm)

   = lightning propagation time from the top to the nth

   pn

   arm ( s )

   Z22 = self surge impedance of ground wire 2 (ohm)

   To calculate the surge impedance of a tower, a model approach from Sargent and Darveniza with adjustments

   Yn = height of the nth arm (m)

   The lightning propagation time to the nearest tower is :

   Average of span distance

   according to the towers shape, is used.

   Taking into account the shape of the tower, the tower surge impedance is calculated using the following equation:

   s

   where :

   =

   300 x 0,9

   (19)

   2 ( p r 2 )

   r

   ZT = 30 ln

   2

   where

   (15)

   s = lightning propagation time to the nearest tower ( s )

   1. Reaction of The Transmission Tower To Lightning Strikes

      To get the induced voltage of a phase conductor, the

      ZT = tower surge impedance (ohm) h = tower height(ohm)

      r = tower base radius (m)

      3.9 Determining the Coupling Factor of A Phase Conductor

      Lightning strike currents flowing in the ground wire will induce voltages, called the coupling voltages, to each phase conductor. The ratio of theinduced voltage of each phase to the tower peak-voltage is called the coupling factor. The coupling factor between the ground wire and the phase conductor can be calculated using two ground wires approach, as follows: [4]

      voltage at the top of the tower should be calculated first. After each surge voltage of insulators have been calculated, these voltages will be used for futher calculation.

      The discussion in this section use a simplified method based on the following conditions :

      1. Among many strikes contained in a lightning, only one is used in the calculation. It is assumed that the the strike waveform reaches the peak value at2 s

      2. Calculations are only carried out at two time points, i.e. at t = 2 s and t = 6 s .

      Sparks occured at time greater 6 s (t > 6 s ) is

      Z n1 Z n 2

      considered rare because of the volt-time curvature on this

      Kn =

      where :

      Z11

      Z12

      (16)

      area is flat.

   2. Calculation of Voltages at The Top of The Tower

   Kn = coupling factor

   Zn1 = mutual surge impedance of phase-n conductor and ground wire 1 (ohm)

   Zn2 = mutual surge impedance of phase-n conductor and

   If at t0 = 2 s there is no reflection of lightning current wave from a nearby transmission tower, then the voltage at the top of the tower that is struck by lightning is as/p>

   follows:

   ground wire 2 (ohm)

   Z11 = self surge impedance of ground wire 1 (ohm)

   (VT)2 =

   ZI

   Z 1

   T

   .I

   (20)

   Z12 = mutual surge impedance of ground wire 1 and 2 (ohm)

   where :

   1

   1

   1. Lightning Propagation Time on the Tower

      The lightning propagation time on a tower includes two propagations, i.e.the lightning propagation time from the top to the base of the tower; and the propagation time from the top to the arm of the tower.

      The lightning propagation time from the top to the base of the tower follows the equation:

      (VT)2 = voltage at the top of the tower at t = 2 s (kV) ZT = intrinsic impedance (ohm)

      Z = tower wave impedance (ohm)

      = damping ratio

      T = lightning propagation time from the top to the base of the tower ( s )

      = h s

      (17)

      I = lightning current of 1 pu (1kA)

      T

      where :

      300

      Intrinsic impedance is a parallel combination of the tower surge impedance ZTand the ground wires surge impedance

      T

      = lightning propagation time from the top to the base of the tower ( s )

      h = tower height (m)

      Zs/2, i.e. :

      ZI =

      where :

      Zs .ZT Zs 2.ZT

      (21)

      ZI = intrinsic impedance (ohm)

      Zs = equivalent surge impedance of two ground wires (ohm)

      ZT = tower surge impedance (ohm)

      The wave impedance of the tower is calculated using the following equation:

4. RESEACRH METHODS

   The purpose of developing this lightning outage software is to facilitate the detection of the electrical power distribution continuity.The reliability of the transmission line depends on the means of distribution and equipment used. The means of distribution is arranged based on the time needed to

   T

   Z = Zs2 .Z

   Z R

   . T

   (22)

   revive the power supply after aninteruption due to lightning strikes.

   (Z 2.Z

   Z R

   dimana :

   S

   T T

   The data used in this research is based on the data

   of 500 kV EHV lines in Gresik West Surabaya area, which

   Z = tower wave impedance (ohm)

   ZT = tower surge impedance (ohm)

   R = average tower foot resistance (ohm)

   Damping factor is obtained from:

   2.ZT ZS ZT R

   includes :

   1. The average sag of phase conductors and ground wires.

   2. The average tower resistance

   3. The length of hanging insulators

   4. Diameter kawat fasa dan tanah

   5. The diameter of phase conductors and ground wires.

      =

      .

      (23)

   6. The length of the lines

   2.ZT ZS ZT R

   where :

   Start

   = damping factor

   Problem Definition

   ZT = tower surge impedance (ohm)

   Study of Literature

   Zs = equivalent surge impedance of two ground wires (ohm)

   Analysis of System Requirements

   R = average tower foot resistance (ohm)

   Coding

   If there are reflections of the current wave fromanearby tower, then there are reflected voltages at the top of the tower at 2

   Coding Standardization

   Continuous Integration

   s which can be expressed as

   4. Ks . (V ) 2 1 2.(V ) 2

   Coding Testing

   (VT)2 =

   T 2 . T 2 . (1 S )

   (24)

   Zs Zg

   where :

   (VT)2 = reflected voltage from the nearest tower at 2

   s (kV)

   System Design

   System Implementation Using 2-point Method

   (VT)2

   = tower peak voltage at 2 s (kV) A

   Ks = goal damping factor (0.85)

   System Testing

   s = lightning propagation time to the nearest tower ( s )

   Bug ?

   Yes

   Zg = surge impedance of the ground wire (ohm)

   3.13 Calculation of Tower Arm Voltage

   To obtain arm voltage, the voltage of the transmission tower,

   i.e. the voltage of the tower foot resistances, should be calculated first. The refraction factor is obtained by:

   No

   End

   Figure 4. General Research Flowchart

   R = where :

   2.R ZT R

   (25)

   R = refraction factor of the tower foot resistances.

   ZT = tower surge impedance (ohm)

   R = average tower foot resistance (ohm)

   A

   Determine the Required Data :

   – IKL of Area Passed by Lines

   – Average Goal Distances

   – Tower Coordinates And Conductor Radius

   Start

   Type of insulator used in the EHV lines is made of ceramic. The number and type of disc insulators in one string is determined by the level of insulation required and the level of local pollution in the corresponding area. The conductor used on the 500 kV line of Gresik – West Surabaya is theAluminum Conductor Steel Reinforced (ACSR) with the average sag of each tower is 11.97046 meters.

   Determine the Insulation Surge Voltage of Each Phase & Determine the Reflected Voltage from Nearest Tower

   Determine The Radius of the Single Equivalent Phase Conductor & Determine the Radius of Sheath Corona of The Single Conductor

   Determine the Voltage of Tower Foot Resistances & Determine the Phase Arm Voltage

   Determine the Average Height of Phase Conductors & Determine the Radius of Sheath Corona of the Single Equivalent Phase Conductor

   Determine the Refraction Factor of the Tower Foot Resistance & Determine the Voltage at the Top Of the Tower

   Determine the Number of Strikes to the Transmission Line (NL) & Determine Insulation Spark Voltage (V1)6

   Determine the Wave Impedance & Determine the Dumping Factor

   1

   Determine the Number of Strikes

   to the Ground (N) & Determine the Average Height of Ground Wires (hg)

   Determine the Number of Outage due to one-side Protection Failures & Determine Total Outages Due to Protection Failure

   Calculate Total Outages EHV Transmission Lines (SUTET) & Calculate the Reliability Level

   Determine the Maximum Lightning Current Protection Failure Occur & Determine the Probability of Exceeding Minimum Lightning Current

   Calculate the Number of Strikes to Tower of Each Phase & Calculate the Number of Strikes that Cause Total Backflashover

   Determine the Width of Unprotected Area &Determine the Maximum Distance of Lightning Strike

   Calculate the Time Share Where Phase is Dominant & Calculate the Probability to Exceed the Average Current During the Nth Phase is Dominant

   Determine the X-Coordinate of Ground Wire for Effective Protection & Determine the Width of Unprotected Area

   Calculate Each Phase Insulator Critical Lightning Current in the Presence of Power Frequency

   Determine the Minimum Lightning Current Protection Failures Occur & Determine the Minimum Distance of Lightning Strikes

   Determine The Insulator Surge Voltage & Determine the Critical Lightning Current of Each Phase

   1

   End

   Figure 5. Flowchart of Two-Point Method

   The data of nature required is the number of thunderstorm per year (IKL) for area under the 500 kV EVH Transmission Lines of Gresik West Surabaya. The transmission data is based on those of steel tower of AA type with dual configuration (i.e. with parallel phase arrangement). The ground wire, which protects phase conductors from lightning strikes, uses galvanized steels with diameters of 16 mm2 and 12,6 mm2 while the average sag is 10.10964 meters.

   Figure 6. Flowchart of Two-Point Method (cont..)

5. RESULTS AND DISCUSSION

   Two-points method yields values which are close to the real disturbances since this method takes into account the existing parameters, i.e.: tower foot resistance, the shape f the tower, and the magnitude of lightning current.

   Figure 7. Software display of sheath corona radius calculation of a single equivalent phase conductor

   Figure 8. Software display of outage calculation due to line protection failures

6. CONCLUSION

Based on the results of this research, some conclusions can be inferred as follows:

1. The reliability level of two-point method yields the number of outage of 0.341/23.093 km/year.

2. In planning the protection system against lightning strikes, the lightning strike density should be reviewed in advance to determine the quality of protectionthat will be installed. The greater the lightning strike density in an area then the failure of the transmission line protection will be greater.

The realiability of research results are expected can be improved by increasing the number of insulator strings.

REFERENCES

1. Alvarado, Fernando. dan Oren, Shmuel.(2008). Transmission System Operation and Interconnection. University of Wisconsin, Madison, Wisconsin.

2. Ditjen Listrik dan Pemanfaatan Energi Departemen Energi dan Sumber Daya Mineral. (2010). Aturan Jaringan Jawa-Madura-Bali. Jakarta.

3. Hutauruk, T.S. (2009). Gelombang Berjalan dan Proteksi Surja.

   Jakarta : Erlangga

4. Karnoto, Tarsiah S. Hardiono, Agung Warsito, (2003),Sosialisasi dan Evaluasi diri Konservasi Energi Universitas Diponegoro tahun 2003. Badan Penerbit Universitas Diponegoro.

5. Kim, Hyungchul. (2010). Evaluation of Power System Security and Development of Transmission Price Method. A Dissertation, Texas A&M University.

6. Marsudi, Djiteng. (2005). Operasi Sistem Tenaga Listrik. Balai Penerbit dan Humas ISTN, Jakarta.

Figure 9. Software display of the calculation of mutual surge impedance of phase inductors and ground wires.

Figure 10. Software display of the calculation of phase insulator-critical lightning current