Earthing System Lightning

Earthing System Lightning 1.1Introduction Earthing system electrodes are used to divert high currents to the earth. Therefore a proper design of an earthing system is required to dissipate high currents magnitude to the earth safely regardless the fault type. Lightning and other surges subject the electrical power system to high magnitude and fast rise-time transient currents and voltages requires dissipation to earth in controlled methods. In order to minimise damage to the electrical equipments and safety measures have to be taken to protect the human beings. On high voltage transmission and distribution systems, lightning protection and insulation co-ordination schemes are employed to protect power system equipment from damage. The main factor that determines the effectiveness of these schemes is the soil resistivity properties of the earth. In this thesis, the performance of wind turbine earthing systems subjected to power frequency and impulses is investigated by considering both their high frequency and transient behaviour. 1.2 Earthing System Functions Earthing systems are designed primarily for power frequency earth fault conditions. However, certain plant within substations such as surge arresters will provide a path to earth for transient currents and in such cases the standards recommend the installation of a high frequency earth electrode, usually an earth rod [1.1]. High frequency earth electrode means that the earth rod is to dissipate to earth all the high frequency components of the transient. In practice, all the parts of the earthing system are interconnected and may play a role in the dissipation of both power frequency faults and surges. In the case of wind turbine there will earthing termination system especially designed for lightning protection. . 1.3 Components of earthing systems Generally, substation earthing grids consist of a system of conductors buried in the ground occupying an area related to the dimensions of the substation. Additional components may include the metallic sheaths of cables and earth wires of tower lines and their associated tower footings. These extended earth systems emanate from the substation and are bonded solidly to the earth grid. The performance of these components is difficult to predict because soil has a non-homogeneous resistivity ranging from 10 to 10,000 m [1.2]. 1.4 Wind turbine earthing system IEC 61400-24 IEC technical Report of wind turbine generation lightning protection [1.3] states that each turbine must be equipped with its own earth termination system, even if it connected to a general wind farm earthing system. The report [1.3] described the basic requirements for lightning protection of the windfarm. Earthing systems should be designed in accordance with ICE 61024-1 standard of lighting protection of structures [1.4]. This standard recommends two types of earth electrode systems for wind turbines and are referred to as Type A and Type B arrangements. 1.4.1Type A arrangement: This type employs of horizontal or vertical electrodes connected to each down conductor in the case of wind turbine the down conductor would be the wind turbine tower. 1.4.2Type B arrangement: This type uses a ring earth electrode external to the structure soil. A wind turbine normally uses the Type B arrangement with a ring earth electrode placed around the foundation reinforced [1.3] as can be seen in Figure 1. in the standard the ring earth requires to be in contact with the soil for at least 80% of its total length of a foundation earth electrode. The ring electrode should be buried to a depth of at least 0.5m. However, Additional vertical and horizontal earth electrodes may be used in combination with the ring electrode. The radius of the ring electrode will therefore normally be defined by the radius of the turbine foundation and extra conductors may have to be installed as stated below. Figure 1. Typical wind turbine earthing arrangement Foundation reinforced bar normally bonded to lightning protection earth Top view Wind turbine tower 2m vertical electrodes Transformer If the recommended 10W resistance by [1.4] is not achieved then an additional horizontal or f vertical electrodes may be used in combination with the ring electrode. Also, in the standard it states that no more than 50m of the horizontal electrode can be attached to the wind turbine, may be a low resistance value achieved, but could have high transient impedance due to high series impedance of the conductor. 1.5 Soil resistivity Conduction properties of soil are important, particularly its specific resistivity. The soil resistivity is one of the main factors determining the resistance of any earth electrode. Most soils and rocks are poor conductors of electricity when dry. The exceptions to this are certain mineral bodies. However when soils contain water, the resistivity drops, and they may then be considered as moderate conductors, although they are very poor when compared with metals. For example, pure copper resistivity is 1.6cm whilst a quite normal value for soil would be 10,000 cm. The resistivity is determined by the following factors: a) type of soil b) chemical composition of the soil c) concentration of salts dissolved in pore water d) overall moisture content e) temperature f) grain size and the distribution of grain size [1.2]. 1.6Tolerable voltage During a ground fault on a transmission line, a number of towers near the fault are likely to acquire high potential. Potential gradients are also set up in the ground surface and these may present a hazard to humans and livestock. These hazard voltages are generally referred to as touch and step potentials, which are defined [1.5] as follows: a) The touch potential is the difference between the earth potential rise (EPR) and the surface potential at the point where a person is standing, while at the same time having his hand in contact with a ground structure [1.5]. b) The step potential is the difference in surface potential experienced by a person bridging a distance of 1m with his feet without contacting any other grounding object [1.5]. There are limits placed on the allowable EPR of an earthing system as stated in International Telecommunication Union ITU-T [1.6]. These limits are as follows. 650 V for sites fed from high reliability lines where faults are rare and cleared quickly (200 ms maximum). 430 V for sites fed from lines having standard protection. The limits of touch and step potentials are related to the current that can be withstood by a human body before there is a serious risk of fatality [1.5, 1.7]. A magnitude in the order of 50mA is sufficient to cause ventricular fibrillation, which will normally result in death [1.8-1.9]. 1.7 Standard Lightning Impulse The lightning impulse is characterised by three parameters, the peak current magnitude, the time to peak current and time to half peak current which is the time required for the current impulse to decay to half of its peak magnitude. However, standard lightning impulse shapes are generally described by the peak current or voltage and the time to the peak T1 and the time to the half peak T2 and usually written as T1/T2 in microseconds defined as the rise-time. e.g. a 10kA, 5/20ms. Figure 1.2 Lightning Impulse Lightning impulses are usually described by their peak current (or voltage) and by T1 and T2 written as T1/T2 in microseconds. e.g. a 10kA, 5/20ms impulse. 1.7 Standards Applicable to Earthing Systems There are UK and international standards provide guidelines to different earthing systems configurations design. These standards are concerned with power frequency earthing systems design and give limited guidance when earthing system subjected to transient conditions. These standards are: Engineering Association Technical Specification 41-24 (EA TS 41-24), Guidelines for the Design, Testing and Maintenance of Main Earthing Systems in Substations (UK) [1.1]. This standard recognises that equipment such as surge arresters and CVTs are more likely to pass high frequency current due to the low impedance they present to steep fronted surges. This standard suggests that unless a low impedance earth connection is provided, the effectiveness of a surge arrester may be compromised. It is therefore recommended that the connection from the equipment to earth should be as short, and as free from changes in direction, as is practicable. Another guideline in the same standard states that the effectiveness of a surge arrester can be improved by placing a high frequency earth electrode in its immediate vicinity. The standard does not specify any quantitative limits such as over all earthing impedance limit. ANSI/IEEE standard 80 [1.5] Guide for safety in AC substation grounding widely used throughout the world. This standard does not provide detailed guidance for designing for earthing systems subjected to transient conditions. However, it considers that an earthing system that is designed to be safe for power frequency faults should also be safe for transient faults. This is based on the assumption that the human body can withstand higher currents for very short duration. It also considers the safety issues and defines the touch and step potential and provides equation which can be calculated. CENLEC HD 637 SI, Power Installation Exceeding 1kV ac or 1.5kV dc [1.10]. The standard provides measures to reduce the amount of interference created when surges are dissipated to earth. These measures include reducing the inductance of the current path by significantly meshed the earthing system. Moreover, the connection to the earthing system should be as short as possible. BS 6651 [3] Protection of structures against lightning the standard recommends that the earthing system designed for lightning protection should have an earth resistance of less than 10W. The same requirement appears in BS 61400-24:2002 Wind turbine generator system __ Lightning protection with some details regarding earthing system arrangements as mentioned previously. 1.9 Aims of the research Wind turbines become the largest growing electrical energy in the world. Wind farms, however, have particular requirements for earthing compared with conventional electrical installations. They are often very extensive, stretching over several kilometres and subject to frequent lightning strikes due to the height of the structures and their location on high rocky terrain with associated high earth resistivity. Consequence of these developments, there is an increase in fault current associated with windfarms and an increase in the probability of a lightning strike a wind turbine. Therefore, the protection of human beings and livestock becomes more important due to the trend towards large scale high capacity wind farms. The aims and the objectives of this work therefore can be summarised by the following: To review the methods that representing the wind turbine earthing system and their limitations To develop models that produce more accurate results. To use computer simulation to overcome problems in modelling of the wind turbines earthing system To carry out computer simulations in order to investigate the importance of the wind turbine steel tower in modelling of wind turbine earthing system. Design mitigation techniques for fast surge injections. To carry out soil resistivity using a 2-D technique in order to obtain a soil model to be used for Wind Turbine earthing simulations. To carry out dc, ac and impulse measurement of resistance/ impedance of an isolated wind turbine. To carry out dc, ac and impulse measurement of resistance/ impedance of the wind farm interconnected at the location of wind turbine used in point above. To carry out measurement of safety voltages (step and touch voltages) around an isolated wind turbine. To carry out measurement of safety voltages (step and touch voltages) around the same wind turbine when its earthing system is bonded / connected to the wind farm earthing network. To obtain details of the design and construction of the test wind turbine (s) for CDEGS simulation purposes. To draw conclusions and suggest recommendations for a better estimation of Wind farm earthing systems and related safety precautions. 1.10 References: [1.1] Technical Specification 41-24, Guidelines for the Design, Testing and Maintenance of Main Earthing Systems in Substations, Electricity Association, 1992. [1.2] Tagg, G. F.: Earth Resistance. Gorge Newnes Limited. (London), 1964. [1.3] IEC Technical Report, Wind Turbine Generation System -24: Lightning Protection, TR61400-24, 2002. [1.4] IEC International Stand Protection of Strictures against Lightning IEC61024 -1998. [1.5] IEEE guide for safety in AC substation grounding, ANS/IEEE standard 80, 2000. [1.6] ITU-T (International Telecommunication Union): Calculating Induced Voltage and Currents in Practical Cases. Volume II, 1999 [1.7] International Electrotechnical Commission IEC Report 479-1: Effects of current passing through the human body, IEC, 1984. [1.8] Electricity Association: Engineering Recommendation S.34: A guide for assessing the rise of earth potential at substation sites. Engineering Management Conference, May 1986. [1.9] IEEE: IEEE Std 81.2-1991, IEEE Guide for measurement of Impedance and Safety Characteristics of Large, Extended of Interconnected Grounding Systems, The Institute of Electrical and Electronic Engineers, New York, 1991. [1.10] HD 637S1, Power Installations Exceeding 1kV AC, European Committee for Electrotechnical Standardisation (CENELEC) 1999. [1.11] BS 6651, Code of Practice for Protection of Structures Against Lightning, British Standards Institution, 1992.

Effects of the Vietnam War

Abstract While it takes societies to start wars, war changes societies. Whether it be the loss of life, wealth or influence, war determines what a society becomes. This paper will look at some of the effects on U. S. society following the war in Viet Nam. The Effects of the Vietnam War on American Society The Conflict in Southeast Asia had lasting effects on the United States society. It was sold to the American people as necessary to stop the spread of Communism and quickly become one of the longest and costliest wars in U. S. history.Looking at the war strictly from the point-of-view of society, the after effects of the war were staggering. There was weakened public faith in the government and a change in the public image of the American soldier. The loss of national pride was deep. America had effectively lost a war to what Secretary of State Henry Kissinger had called â€Å"a fourth-rate power. † The U. S. was no longer looked at as an invincible giant. Because President J ohnson wanted to fund the war without raising taxes, the government deficit skyrocketed. Additionally, troops returning home could not find jobs in the private sector.These factors together caused double digit inflation and unemployment rates. The people felt that the government was not taking care of them like it used to. These factors led to a change in the country's role around the world. Congress was worried that the world was viewing the U. S. as the planet's policeman, ready to jump in anytime another country seemed ready to fall victim to Communism. In 1973, the mostly Democratic Congress enacted the War Powers Resolution, which limited the president's powers. The Commander-in-Chief could only send U. S. troops into combat for less than ninety days without congressional approval.Besides the lack of jobs, returning soldiers were not given parades and hailed by the people as conquering heroes as they were following previous conflicts. Instead, they were viewed with disdain, bot h for losing the war and for the vicious actions taken by a few soldiers. The hero gave way to the villain, the soldier became the baby-killer. The American sensibilities had changed as well. While soldiers were fighting and dying in Viet Nam, the United States had a few other conflicts within its own borders. Race riots and anti-war protests were shown side by side with battlefield reports on the nightly news.National Guard intervention and the shooting of students at Kent State and Jackson State universities did little to gain support for the government. According to Sen. Frank Church, Viet Nam â€Å"has already stretched the generation gap so wide that it threatens to pull the country apart. † (â€Å"Vietnam War Quotations†, para. 13) Whether the outcome of the war was beneficial or detrimental depends entirely on your point of view. On one hand, the more liberal attitude of government towards foreign intervention, the redefining of the right to protest and the skep ticism toward the government can be looked at as positive effects of an unpopular war.On the other hand, the image of the country in the eyes of the world, the loss of traditional values and the blow to the economy can be viewed as negative. No matter what personal stance one has on the Viet Nam Conflict, there is one truth; The War in Southeast Asia definitely changed the United States as a world power, as a country and as a people. References Frey-Wouters, E. , & Laufer, R. S. (1986). The Vietnam Generation's Views of the Combatants. In Legacy of a war: the American soldier in Vietnam. (pp. 108-109). M. E. Sharpe. Sitikoff, H. (n. d. ). The Postwar Impact of Vietnam.Universtiy of Illinois. Retrieved February 18, 2012, from http://www. english. illinois. edu/maps/vietnam/postwar. htm The Vietnam War: An Overview. (n. d. ). The Wars for Viet Nam: 1945 to 1975. Retrieved February 18, 2012, from http://vietnam. vassar. edu/overview/index. html Vietnam War and the American Economy. (n. d. ). History Central. Retrieved February 18, 2012, from http://www. historycentral. com/sixty/Economics/Vietnam. html Vietnam War Quotations. (n. d. ). VietnamWar. net. Retrieved February 18, 2012, from http://www. vietnamwar. net/quotations/quotations. htm

The Negative Effects Of Internet Addiction - 1545 Words

In the society we live in today, the internet has become a dominant force in our lives. The ones who are exposed the most to it are the ones who are in the most danger. In particular, teenagers are the ones in harm’s way mainly due to the fact that they spend the most time on the internet whether it be for academics or for leisure activities. However, when do we know when internet usage is too much? The effect of the increase of social media and technology has affected our teenagers is more than we know and the effects are only going to get worse as technology becomes a bigger focal point in our lives. The effects of social media on teenagers range from internet dependence and addiction, privacy information being leaked to other problems†¦show more content†¦It’s no secret that over the past few years, our society has experienced numerous technological advancements in various industries including the internet. 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What Is Surface Tension Definition and Experiments

The Romantic And Classic Ages Of European Music - 1015 Words

Music is widely considered to be the least rational of the arts, and thus the one least openly associated with fundamentals of the outside world; all the same, musicians of considerable significance have been both consciously and subconsciously molded by the environment in which they reside and as such they may interpret and bring the world to life in the language of music. The Romantic and Classic ages of European music can be simply put as the period of time between the deaths of Ludwig Van Beethoven in 1827, and that of Gustav Mahler in 1911. Despite this convenient partition, history has never been one to divide itself so precisely. There were certainly early makings of Romanticism well before 1827, and after 1911. So a better time†¦show more content†¦One of the emerging voices of the era was a French composer named Jean-Jacques Rousseau, who, along with his followers, expressed the idea that both progress and science were taking mankind away from their natural roots, and nature had all of the makings of a simpler life. Vivaldi, in the Four Seasons, and Beethoven, in the Pastoral Symphony, also depict nature or natural phenomena as a part of everyday life; depicting it sometimes pleasant, while other times annoying, and even, at times, rather terrifying. In this romantic mentality, nature was viewed as a mysterious and independent, yet awe inspiring force. This new found connection with nature provided a foundation of inspiration, as well as beauty and truth; and is one of the major themes of Romanticism. The form in which Romantic music first visibly manifested itself was through hymn, both voice and piano; these pieces were almost always quite simple, sounding most closely to folk music, and were usually referred to by the name Lied. Further, these songs were usually strictly Strophic, which simply means that they divided into verses which are repeated without variation until the entire song has been sung. Mozart and Beethoven, are two of more famous Composers, who wrote many such Lieder (plural of Lied); which were often lovely works, but in no way represented major

Vietnam’s Inflation Free Essays

Vietnam’s government is set to raise electricity prices by 15% on Tuesday, at a time when the population is already struggling with a soaring cost of living. Last week the Vietnamese government also raised the retail prices of oil products by as much as 24%. In February, inflation hit a two-year high of 12. We will write a custom essay sample on Vietnam’s Inflation or any similar topic only for you Order Now 31%. Analysts say despite the government’s measures to tighten monetary policy, price pressures are likely to continue. â€Å"The increases in energy, electricity and petroleum indicate that we are going to see inflation get a little worse despite the shift in government policy,† said Christian de Guzman of Moody’s Investor Group. Overheating For years the Vietnamese government has kept a loose interest rate policy and subsidized lending in order to boost growth. The government expects the economy to expand as much as 7. 5%, up from 6. 8% in 2010. But the cost of that rapid pace is that the economy has started to show signs of overheating. Credit rating agencies cut the country’s sovereign-credit rating last year. Inflation fears have also caused a sell-off in Vietnamese markets. Vietnam’s benchmark stock index has slid 6. 7% in the past year. The Vietnamese government was focused on growth at all costs,† said Mr de Guzman. â€Å"By the middle of last year they had already reached their inflation target but then they continued to pursue other macroeconomic policy targets like credit growth and gross domestic product growth,† he said. Fighting inflation But since the beginning of this year, the government seems to have shifted its policies towards stabilizing prices. Last week the Vie tnamese government announced a set of measures to curb inflation. Electricity prices have been raised in an effort to reduce budget spending The central bank recently raised the cost of borrowing. It increased the benchmark refinance rate by 2 percentage points from 9% to 11%. The government has also vowed to reduce government debt. To that effect, it cut the budget-deficit target to less than 5% of gross domestic product, from 5. 3%. Reducing government spending on subsidies for fuel and electricity are also part of that plan. â€Å"In order to stave off inflation, they want to cut back on subsidies. It does alleviate some of the pressure on the budget,† said Mr de Guzman. Dong devaluation The other major strain on the Vietnamese economy is the currency. The central bank devalued the dong against the US dollar by 8. 5%. It is the latest in a series of devaluations the government has implemented to reduce the risk of a shortfall in foreign currency reserves. However, that will lead to higher import costs, which in turn, could again increase inflationary pressures. How to cite Vietnam’s Inflation, Papers

okay.doc Essay Example For Students

okay.doc Essay Okay, on to CLSA. Heres a base case on why CLSA/e Equity Access nixedSybase ASE 11.9 for Microsoft SQL Server 2000 on their research database. Problem to solve:=================Provide a mechanism that will enable secure electronic contribution andstorage of information related to equity research data, as well as, amechanism to enable secure electronic distribution of the information storedin a repository or data warehouse. Factors affecting the implementationincluded:* Providing access to the mechanisms via intranet and internet connectivity* Ensuring the contribution / storage / notification process can beperformed in ; 5seconds once the information has been submitted forconfirmation* Ensuring performance (i.e., user response time) as it relates to thedistribution of information can be performed in ; 5 seconds once requested* The contribution function must support no less than 1000 simultaneoususers located in 15 countries. * The distribution mechanism must support not less than 2,000,000simultaneous users located in 57 countries. * Rapid startup time (3 months for a prototype 9 months for production)Those were my requirements --The issues:=================Our parent firm was already using a home-grown research database forcontribution and distribution of 400 GB of research information. It wassuggested that we copy their setup and code to meet our rapid startuprequirement. Our parent firm had implemented a Sybase ASE 11.9 database running onhigh-end HP Unix servers (hp superdome with 256GB memory with a 2000GB (2TB)mirrored storage array)engineered for large-scale databases. This high-endarchitecture is ideal for the specific demands of online transactionprocessing workloads and high availability (never goes down). Unfortunately,the whole setup is very expensive and very centralized (Hong Kong). In orderto meet their performance criteria, they resorted to distributing all orparts of the database across multiple servers in different time zones. Thiswas very expensive solution to maintain because of the database licensing,hardware, communication costs associated with distributing the data, andpersonnel to support the distributed databases (3 databases x 55 tables ineach database x 4 countries). We were starting with 6 countries and over 100,000 simultaneous users. Theparent solution cost would have an initial capital expenditure of $4,200,000and a run rate of $46,000 per month (to start). Our budget to start was somewhere around $125,000 (not including personnel). -This is how I make a living ==============================Using a copy of Microsoft SQL Server 2000, I prototyped the database andsupporting structures (indexes, stored procs, tables, etc) in two weeksusing our parent firm and another successful internet-based researchsupplier as templates for our base service. The license for the software was$900, and the server was already being used for other non-production work. SQL Server license was free because we already had a development license. I set SQL (ANSI SQL-92), JAVA (J2EE), Javascript, XML, and HTLM as thebaseline for our standards for development. Notepad was the developmenttool. I picked up a free evaluation copy of ColdFusion 5.0 for themiddleware components and downloaded JDK 1.3 from Sun.com for the Javadevelopment environment. The majority of the database activity revolves around a master transactiontable for collecting and distributing data. Seperate reference tables (in1-to-many relationships with the transaction tables) are used for useraccess, markets, countries, clients, contributors, names, address, etc, inorder to minimize table maintenance and complicated screen inputs. Command-line batch jobs using SQL BULK INSERT we created to transfer databetween database servers. In six weeks we were ready to test the systems performance on a CompaqProliant 8500 class machine with 8 CPUs, 16GB memory, with external storagefor 200GB. We had