Arrester

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Arrester

  1. 1. Abstract—Distribution substations feed power to the actual consumers through distributors and service lines. The main equipments are generators and transformers. To protected these equipments and for stability purpose, over-voltages and over currents protection are important to consider. Lightning is one of the most serious causes of over-voltage. If the power equipments especially at outdoor substation are not protected, the over-voltage will cause burning of insulation. Lightning arrester can protect the damages of equipments. This paper describes the arrester type, lightning terminal and earthing plan of Dagon East substation in Myanmar. DynaVar station class and intermediated arrester (Vrated = 72kV and I charge (max) = 10kA) are used in this substation. Most of substation equipments are designed to match with the insulation coordination. If the insulation equipments are higher, the cost is also high. So, to relax this, the lightning arrester must be put in front of the protected equipments and protected zone. For this purposes, this paper specially indicates the safety and saving cost of equipments for over- voltage protection in distribution substation. Keywords—Lightning arrester, Earthing plan, DynaVar station, Intermediated arrester. I. INTRODUCTION UBSTATION design involves more than installing apparatus, protective devices and equipment. The significant momentary investment and required reliable continuous operation of the facility requires detailed attention to preventing surges from entering the substation facility. The effects of disturbances with limiting in a power system, which if allow to persist, may damage plant and interrupt the supply of electrical energy. Lightning is one of the most serious causes of over voltage. If the power equipment especially at outdoor substation is not protected the over-voltage will cause burning of insulation. Thus it results into complete shutdown of the power and the loss may run into cores of kyat. Electrical equipment can be damage due to over-voltage such as switching surge over-voltage, Lightning surge over-voltage, transient recovery voltage and power frequency temporary over-voltage in transmission line and receiving end of substation. It is important to protect power equipment against them wherever possible, consistent with sound economic. Lightning Arrester can protect the damages of electrical equipments. So, Lightning Arrester needed to install in the terminal end of the transmission line, substation, high voltage Miss NayKyiHtwe is a student of Mandalay Technological University. (e-mail :naykyihtwe08@ gmail.com). transformers and low voltage transformer. The analysis of electromagnetic transient is depended on operating voltage, lengths of the lines and contactor configuration. So, it can be chosen correctly the technical specifications of the apparatus of Lightning Arrester base on the amounts of receiving over- voltage. II. PHYSICAL PHENOMENON OF LIGHTNING Lightning is a huge spark caused by the electrical discharge taking place between the clouds, within the same cloud and between the clouds and the earth. The turmoil that is apparent inside a thundercloud is most impressive to the viewer. It shape changes continually, and one notes especially the development of the towering ‘thunderhead’. It is very easy to imagine the fierce updrafts with the cloud and the downdrafts near it surface which are matters of practical experience for aviators. It is generally accepted that the updraft is responsible for charge separation within the cloud, like some gigantic electrostatic generator, which leads to the creation of electric fields within and around the cloud and ultimately to the electric breakdown that is called lightning. III. WAVE SHAPE OF LIGHTNING STROKE The lightning stroke current rises to crest value very quickly and then starts decaying at a low rate as illustrated in Figure.1. The generalized wave shape can be characterized as: 1. Crest or peak value and it have been observed that the maximum value of this current is 400 kA. 2. The wave front line varies from 1 t 10 sec. 3. The time at which the stroke current reduces to 50 percent value of that crest value and it has been estimated that the time varies from 10 to 100 sec. Figure.1 Generalized wave shape of lightning stroke Analysis and Design Selection of Lightning Arrester for Distribution Substation Nay Kyi Htwe S World Academy of Science, Engineering and Technology 48 2008 174
  2. 2. IV. STATIC CHARGING OF THE CONDUCTOR DUE TO A CHARG CLOUD Suppose that the cloud is positively charged, then the line will be charged to a negative potential by the electrostatic induction. This negative charge will be present right under the cloud and the portions of the line away from this point will be charge positively as illustrated in Figure.2. Figure.2 Static charging of the line due to a cloud The charge on the line will not flow since it is a bound charge. The positive charge on the far ends of the line will however leak to the earth slowly through insulators metallic parts etc, thus leaving only the negative charge on the line. Due to a direct discharge occurring between this cloud and another passing by cloud the charge on the cloud is neutralized then the charge on the line is no more a bound charge and is free to travel in both directions in the form of traveling waves. V. OVERVOLTAGE DUE TO LIGHTNING STROKE In case of direct strokes, a line having a surge impedance of Zs and the discharge current be Id, then the over-voltage due to a direct stroke is sdd ZIV ×= (1) When the traveling waves flow in one direction, the equation is true. However, when they travel in both directions, the current is halved and the over-voltage is 2 ZI V sd d × = (2) When the lightning stroke is on the earth wire or top of a tower, the over-voltage is t i ccdd d d lZIV +×= (3) Where Z c is the impedance of the earth conductor and l c is the inductance of the line conductor. VI. INTERACTION BETWEEN LIGHTNING AND THE POWER SYSTEM When lightning strikes a power line, a current is injected into the power system. This is very useful concept. This current will give rise to depend upon its wave shape and the impedances through which it flows. If a tower is struck, the impedance of the tower will be of concern. The voltage drop down the tower will appear across the line insulation. If this is excessive, flashover of the insulation will occur and a fault will be placed on the system. The current comings into a tower have been postulated by lightning stroke from a cloud, and then disappear into the ground. A useful concept is to think of the cloud and earth as forming a vast capacitor which is being discharge by the stroke. The return circuit would be completed by displacement current in the electric field. This is suggested by Figure.3. Stroke Earth cloud Figure.3 Lightning stroke from cloud to earth discharges a vast Capacitor VII. LIGHTNING ARRESTER Lightning arresters are the most effective means of protecting an electrical apparatus against traveling voltage waves caused by lightning and switching. Lightning arresters are connected across and apparatus to provide a Low- resistance path to ground, thus limiting the transient voltages below the Basic Impulse Level of the apparatus. There are four different classes of arrester. 1. Station 2. Intermediate 3. Distribution, and 4. Secondary The functions of a lightning arrester are 1. To act like an open circuit during the normal operation of the system i.e., to hold off the system voltage, 2. To limit the transient voltage to a safe level with the minimum delay and fitter, and 3. To bring the system back to its normal operation mode as soon as the transient voltage is suppressed, i.e., to interrupt the power-follow current and to reseal itself. The normal operation or operational mode includes the system under faulted condition. Under several types of system faults, such as the single line-to-ground faults, the voltage to ground across the unfaulted phases will rise above the normal voltage level. The arrester must not go into conduction during this fault condition. It should also be able to interrupt the power-follow current and reseal itself under system fault conditions when the power-frequency voltage across it rises. VIII. SELECTION OF LIGHTNING ARRESTER The lightning arresters are designated by the crest magnitude of the discharge current having 10×20 second wave shape which the arrester can safely pass without damage. The lightning arresters are designated as 8, 10, 20 KA. They World Academy of Science, Engineering and Technology 48 2008 175
  3. 3. can safely discharge these current crests. As the arrester is a protective device, if is a general impression that if should be rated for most severe conditions of discharge currents says 20 A. The discharge current from the arrester varies from a few hundred amperes to kilo-amperes and sometimes if is even 20 KA. Maximum discharge voltage and discharge factor for the arrester is defined the maximum value of voltage which appears across the arrester terminals at the time of discharging if rated current determines its impulse level of protection. The discharge factor if is, DF= arrestertheofvalue)(RMSvoltageRated arrestertheofvalue)(crestvoltagedischarge (5) In the early designs of arresters, this discharge factor was quite high (about 5.6) but due to the advent of better material and Modern research it has been reduced varying to 2.4 to 3.0. The above ratio for arresters manufactured by different firms varies from 1.5 to 1.7, so, the average value may be taken as 1.6 E is the rated arrester voltage KV (R.M.S) and LS is the minimum impulse insulation level in kV (crest valve) its value after allowing 10% as tolerance factor and 25% as margin factor can be obtained. LS= E 0.8 21.61.11.25 ××× =3.88E (6) In case Extra High Voltage system LP in kV, LP = 2.3 × power frequency withstand voltage in kV (RMS) = 2.3 × EL (7) For 75% arrester, LP = 2.37 EL (8) For 80% arrester, LP=2.53EL (9) IX. EARTHING SYSTEM The frame of every generator, stationary motor, and so far as particable, portable motor, and metallic parts of all transformer and regulating and controlling apparatus connected with supply shall be earthed by the owner by separate and distinct connection with earth. Every conductor used on earthing shall be of stranded as solid copper or suitable copper alloy, and shall be protected wherever liable to mechanical damage and also, where necessary, against corrosion, particular attention being given in these respects to the earthing leads at its point of connection with the earth electrode. The coefficient of earthing is below 80 percent. On four wire distribution systems, with solidly ground transformer neutral at every voltage level, coefficient of earthing is generally less than 80%. On high voltage transmission systems the coefficient of earthing does not exceed 75%. In resistance of Common Types of Earth-electrode, there are three types. 1. Plates 2. Pipes and rods 3. Strip or conductor electrodes X. INTERMEDIATE CLASS DYNAVAR ARRESTER Application is based upon the maximum continuous operation voltage, line to neutral, at the arrester location. For grounded neutral systems, this is computed as maximum system voltage divided by √3. For historical comparison, the maximum continuous operating voltage is 81% of the conventional 71% arrester installed on an affectively grounded neutral system. Figure.4 Section view of typical unit A. Lightning Arrester Design At Dagon East substation, DynaVar station class and intermediate surge arresters are used. The system voltage is 66 kV and maximum continuous operation voltage is 48 kV rms. The duty cycle rating is 60 kV rms and maximum discharge current is 10 kA. 1. Maximum 0.5µs discharge voltage = 163.5 kV 2. Maximum switching surge protective level=116.4 kV 3. Maximum discharge voltage using an 8/20=148.8 kV Current wave-kV The maximum discharge voltage for a10kA impulse current wave produces a voltage wave cresting in 0.5µs. B. Structure of Lightning Arrester In Figure.5, PVN 48.0kV maximum continuous operation voltage arrester is shown. It has 0.56 diameter holes, clamp type terminal, arrester name plate. Its internal diameter is 9.3 diameters. Figure.5 PVN 48.0 kV maximum continuous operation voltage Arrester 9.3 1.7 Clamp Type Terminals Suitable for Use with CU DR AL Conductor 0.25 to 0.81 Dia 0.56 Dia 3 Holes at 120˚ On 10.0 Dia BC 1.750.56 Diameter 4 Holes Arrester Nameplate 32. 3 World Academy of Science, Engineering and Technology 48 2008 176
  4. 4. C. Specification of Lightning Arrester for Incoming Side Incoming side of Dagon East substation, the specifications of lightning arrester are as follow. Figure.6 One Line Diagram of East Dagon 30MVA Substation System nominal voltage = 66kV Rated normal Voltage = 66 × 1.1 = 72.6kV Continuous Operating Voltage (kV) rms = 48.0kV Normal Discharge Current (8 /20µs) kA = 10kA 1/50 Impulse Spark over Voltage = 163.5kV Frequency (Hz) = 50Hz Type = outdoor D. Specifications of Lightning Arrester for Outgoing Side The followings are the specifications of lightning arrester for outgoing side of Dagon East substation. System nominal voltage = 33kV Rated normal Voltage = 33 × 1.1 = 36kV Continuous Operating Voltage (kV) rms = 24kV Normal Discharge Current (8 /20µs) (kA) = 10kA Frequency (Hz) = 50Hz Type = outdoor XI. LIGHTNING EARTHING In earthing system, lightning arresters with PVC coated wire and cable lug are used. 1. 66kV Lighting Arrester(70mm2 PVC Coated Wire) = 50' 2. 33kV Lighting Arrester(70mm2 PVC Coated Wire) = 100' 3. 70mm2 Cable Lug = 2 No Figure.7 shows the earthing plan of Dagon East substation. XII. DETAILED DESIGN DATA For incoming side and outgoing side, the discharge voltage, insulation level, minimum impulse insulation level and power frequency withstand voltage base on 100% arrester are as shown in Table.1. Ground voltage peak value and switching surge withstand voltage are also shown in Table.1. Table.1. Detailed Design Data Sheet System Voltage 66kV for Incoming Side 33kV for Outgoing Side Rated Voltage (kV) 72.6 36 Discharge Voltage on 100% Arrester (kV) 188.76 93.6 Insulation Level for 100% Arrester (kV) 284.78 164.4 Minimum Impulse Insulation Level on 100%(kV) 732.389 363.168 Power Frequency withstand Voltage on 100% (kV) 151.8 75.9 Ground Voltage Peak Value(kV) 59.28 29.63 Require Switching Surge Withstand Voltage (kV) 215.6 107.8 XIII.CONCLUSION In this paper, the basis theory of lightning, lightning shielding and design of lightning arrester are presented. DynaVar station class and intermediate arresters are used. The type of arrester is outdoor type. The rated voltages of arresters are 72kV and 36kV, the maximum discharge current is 10kA and MCOV are 48kVrms and 24kVrms. The lightning arrester in this paper is provided for overvoltage protection in distribution substation. This paper will help and give the electrical knowledge of the protection system in distribution substation which coach to the technicians, the professional engineers, the students who facing the overvoltage condition and protection coordination of distribution substation. ACKNOWLEDGMENT Firstly, the author would like to express her indebtedness and gratitude to her beloved parents, for their kindness, support, understanding during the whole course of this work and encouragement to attain ambition without any trouble. The author is indebted to all her teachers who give her knowledge from M.T.U and Y.T.U in Myanmar. REFERENCES [1] Allan Greenwood 1923 “Electrical Transients in Power System” Second Edition, John Willey & Sons, Inc [2] Anderson 1987 “Transmission Line Reference Book”, Second Edition Substations Committee [3] ANSI/ IEEE 1989 ” IEEE Standard for Gapped Silicon-Carbide Surge Arresters for AC Power Circuit” [4] Franked- Graham 1970 “New Electric Library” Vol-2 [5] IEEE Std 998- 1996 “Guide for Direct Lightning Stroke Shielding of Substation”, IEEE Working Group D5, [6] U Tin Swe “Power System Analysis Part 3” World Academy of Science, Engineering and Technology 48 2008 177
  5. 5. Figure.7 Dagon East Sub-station Earthing Plan 66kV TR: 30 MVA R = 2 66kV LA 66kV Switch Yard 33kV LA 33kV TR: 10 MVA 100kVA Transformer Switch Gear Panel Earth Ring System 10MVA×R Neutral Earth 100VA Neutral Earth Lightning Arrester Lightning Arrester Body Earth LA Earth LA Earth DS Earth R = 1.8 2 Pole DS LA PT CT CB CB CT PT DS CB CT PT DS 10´ 10´ 10´ 10´ 10´ 10´ 10´ 10´ 10´ 10´ 10´10´ 10´10´ 10´ R = 2 Lightning Earth R = 2 R = 2 Lightning Earth 30MVA Body Earth 30MVA× R World Academy of Science, Engineering and Technology 48 2008 178

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