Yüksek gerilim istasyonlarında topraklama sistemi
Başlık çevirisi mevcut değil.
- Tez No: 75149
- Danışmanlar: YRD. DOÇ. DR. ÖZCAN KALENDERLİ
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Topraklama, Yüksek gerilim sistemleri, Grounding, High voltage systems
- Yıl: 1998
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Elektrik-Elektronik Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Yüksek gerilim istasyonlarında yıldırım boşalmaları, açma-kapama olayları, kısa devreler ve toprak temasları gibi durumlar sonucunda meydana gelebilecek aşın gerilimler çok yüksek potansiyel artışlarına sebep olmakta bu da canlılar ve aygıtlar için tehlikeli durumlar yaratmaktadır. Yüksek gerilim sistemlerinde insanları tehlikeli gerilimlere karşı korumak için çarelerden biri topraklama yapmaktır. Topraklamanın yapılmasındaki amaç, istenmeyen nedenlerden dolayı meydana gelebilecek temas ve adım gerilimlerinin izin verilen sınır değerlerden küçük kalmasını sağlamak ve bu gibi tehlikeli gerilimleri ortadan kaldırmaktır. Topraklama amacıyla kullanılan birçok topraklayıcı çeşidi vardır. Bunlar arasında yüksek gerilim istasyonlarında yaygın kullanıma sahip olan topraklama ağlarının diğer topraklayıcılara göre daha karmaşık bir yapı içermesi, analizlerinde farklı analitik ve sayısal yöntemlerin kullanılmasını zorunlu kılmaktadır. Bu çalışmada Bölüm 2' de topraklama ile ilgili temel kavramlar ele alınmış, Bölüm 3'te topraklama sisteminde kullanılan topraklayıcı çeşitleri anlatılmıştır. Bölüm 5'te yüksek gerilim istasyonlarında tehlikeli koşullarda meydana gelecek olan adım ve temas gerilimleri anlatılarak bu gerilimlerin izin verilen sınır değerlerde kalması için gerekli hesapların kolay bir şekilde sonuçlandırılması için bir bilgisayar programı geliştirilmiş ve bilgisayar programı ile toprak özdkencinin, toplam iletkenin uzunluğunun ve de iletkenin gömülme derinliğinin değiştirilerek adım ve temas gerilimine etkisi gösterilmiştir. Bilgisayar programı Delphi 2.0 ile hazırlanmıştır. Bu programla girilmesi gereken ve hesaplanması gereken değerler iki ayrı ekranda gösterilmiş hesap sonuçlarına da bu ara ekranlarda ulaşılmıştır. Programın ana içeriğine girilerek çözümleme yapma zorluğu giderilmiştir. Böylelikle hızlı ve kolay kullanılabilir bir program tasarlanmıştır. Hesaplama sonucunda elde edilen topraklama direncinin değeri topraklama ölçüm yöntemleriyle denetlenir. Bu amaçla bu çalışmada Potansiyel Düşümü Yöntemi, % 61,8 yöntemi, Arakesit Yöntemi, Eğim Yöntemi gibi topraklama direnci ölçüm yöntemleri de incelenmiştir. V11I
Özet (Çeviri)
SAFETY GROUNDING AT HIGH VOLTAGE SUBSTATIONS SUMMARY In principle, a safe grounding design has two objectives: 1. To provide means to carry electric currents into the earth under normal and fault conditions without exceeding any operating and equipment limits or adversely affecting continuity of service, 2. To assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock. A practical approach to safe grounding concerns and strives for controlling the interaction of two grounding systems: 1. The intentional ground, consisting of ground electrodes buried at some depth below the earth surface, 2. The accidental ground, temporarily established by a person exposed to a potential gradient in the vicinity of a grounded facility. People often assume that any object grounded, however crudely, can be safely touched. This misconception probably contributed to accidents in the past, as a low station ground resistance is not, in itself, a guarantee of safety. There is no simple relation between the resistance of the ground system as a whole and the maximum shock current to which a person might be exposed. Therefore, a station of relatively low ground resistance may be dangerous under some circumstances, while another station with very high resistance may be safe or can be made safe by careful design. For instance, if a substation is supplied from an overhead line with no shield or neutral wire, a low grid resistance is important. A substantial part of the total ground fault current enters the earth causing an often steep rise of the local ground potential; Fig 1 (a). If a shield wire, gas-insulated bus, or underground cable feeder, etc, is used, a part of the fault current returns through this metallic path directly to the source. Since this metallic link provides a low impedance parallel path to the return circuit, the rise of local ground potential is ultimately of lesser magnitude; Fig 1 (b). In either case, the effect of that portion of fault current that enters the earth within the station area should be further analyzed. If the geometry, location of ground electrodes, local soil characteristics, and other factors contribute to an excessive potential gradient at the earth surface, the grounding system may beinadequate despite its capacity to carry the fault current in magnitudes and durations permitted by protective relays. If *- Q m lF = IG 7xw >\\y//\/t/sv\\\v;v\KY/sy\\\YWv^'<w/ (a) Q 'f = 'g + 1. (b) Fig 1 Typical Faulted Substation With and Without Multiple Ground Return Paths During typical ground fault conditions, the flow of current to earth will produce potential gradients within and around a substation. Figure 2 shows this effect for a station with a simple rectangular grounding grid in homogeneous soil. Unless proper precautions are taken in design, the maximum potential gradients along the earth surface may be of sufficient magnitude during ground fault conditions to endanger a person in the area. Moreover, dangerous potential differences may develop between structures or equipment frames that are grounded and the nearby earth. The circumstances that make electric shock accidents possible are: 1. Relatively high fault current to ground in relation to the area of ground system and its resistance to remote earth, 2. Soil resistivity and distribution of ground currents such that high potential gradients may occur at points at the earth surface, 3. Presence of an individual at such a point, time, and position that the body is bridging two points of high potential difference, 4. Absence of sufficient contact resistance or other series resistance to limit current through the body to a safe value under the above circumstances,5. Duration of the fault and body contact, and hence, of the flow of current through a human body for a sufficient time to cause harm at the given current intensity. Fig 2 Equipotential Contours of a Typical Grounding Grid With and Without Ground Rods The relative infrequency of accidents of the type being studied, as compared to accidents of other kinds, is due largely to the low probability of coincidence of all the unfavorable conditions mentioned above. Nevertheless, some fatalities due to gradients have occurred in the past. Therefore, it is the responsibility of the engineer to lower this possibility. Conceptual analysis of a grid system usually starts with inspection of the station layout plan, showing all major equipment and structures. In order to establish the basic ideas and concepts, the following points may serve as guidelines for starting a typical grounding grid design: 1. A continuous conductor loop should surround the perimeter to enclose as much area as practical. This measure helps to avoid high current concentration and hence high gradients both in the grid area and near the projecting cable ends. Enclosing more area also reduces the resistance of the grounding grid, XI2. Within the loop, conductors should be laid in parallel lines and, where practical, along the structures or rows of equipment, to provide for short ground connections, 3. A typical grid system for a substation may include 4/0 bare copper conductors buried 1.3-0.5m below grade, spaced 3-7m apart, in a grid pattern. At cross- connections, the conductors would be securely bonded together. Ground rods may be at the grid corners and at each second junction point along the perimeter. Ground rods may also be installed at major equipment. In multilayer or very resistive soils, it might be useful to use longer rods, 4. This grid system would be extended over the entire substation switchyard and often beyond the fence line. Multiple ground leads or larger sized conductors would be used where high concentrations of current may occur, such as at a neutral-to-ground connection of generators, capacitor banks, or transformers, 5. The ratio of the sides of the mesh usually is from 1:1 to 1:3, unless a precise (computer-aided) analysis warrants more extreme values. Frequent cross- connections have a relatively small effect on lowering the resistance of a grid. Their primary role is to assure adequate control of the surface potentials. The cross-connections are also useful in securing multiple paths for the fault current, minimizing the voltage drop in the grid itself, and providing a certain measure of redundancy in the case of a conductor failure. In areas where the soil resistivity is rather high or the substation space is at a premium, it may not be possible to obtain a low impedance grounding system by spreading the grid electrodes over a large area, as is done in more favorable conditions. Such a situation is typical of many GIS installations, occupying only a fraction of the land area normally used for conventional equipment This often makes the control of surface gradients difficult. Some of the solutions include: 1. Connections) of remote ground grid(s) and adjacent grounding facilities; a combined system utilizing separate installations in buildings, underground vaults, etc. A predominant use of remote ground electrodes requires careful consideration of transferred potentials, surge arrester locations, and other critical points. A significant voltage drop may develop betwen the local and remote grounding facilities. 2. Use of deep-driven ground rods and drilled ground wells, in combination with a chemical treatment of bentonite clays for backfilling. 3. Use of counterpoise wire mats. In exposed areas, it is feasible to combine both an insulating material and fabricated mats made of wire made of wire mesh, expanded metal, or gratings; first to equalize the gradient field near the surface and then to reduce conductance from the surface to the underlying metal structures. A typical counterpoise mesh might consist of copper-clad steel wires of AWG No 6 size, arranged in a 0.6x0.6m grid pattern, installed 0.05- 0.15m below the earth's surface and overlaying the main grounding grid, which is installed in greater depth, usually between 0.3-0.5m. 4. Where feasible, controlled use of other available means to lower the overall resistance of a ground system, such as connecting static wires and neutrals to the ground. Typical is the use of metallic objects on the site that qualify for Consequences of such applications, of course, have to be carefully evaluated. Xll5. Wherever practical, a nearby deposit of low resistivity material of sufficient volume can be used to install an extra (satellite) grid. This satellite grid, when sufficiently connected to the main grid, will lower the overall resistance and, thus, the ground potential rise of the grounding grid. The nearby low resistivity material may be a clay deposit or it may be a part of some large structure, such as the concrete mass of hydroelectric dam. May decrease with new connections is when new transmission lines are added with ground or neutral wires, or both. In general, if no margin for increase in la is included in the original ground system design, the design may become unsafe. Also, subsequent additions will usually be much less convenient and more expensive to install. Allowance for an increase in IG can be made by decreasing the value of system impedance used in the calculations; or simply by multiplying the value of calculated fault current by an appropriate factor, Cp ; Cp> 1. It has been a widely accepted practice to assume the total fault current, IF, between the grid and surrounding earth (that is, ignoring any current division) in an attempt to allow for system growth. While this assumption would be overly pessimistic for present year conditions, it may not exceed the current IG computed considering current division and system growth. If the system growth is taken into account and current division is ignored, the resulting grid will be overdesigned. An estimate of the future system conditions can be obtained by including all system additions forecasted. Caution should be exercised when future changes involve such design changes as disconnection of overhead ground wires coming into the substations. Such changes may have an effect on ground fault currents, resulting in an inadequate grounding system. However, future changes such as additions of incoming overhead ground wires may decrease the current division ratio, resulting in the existing ground system being, in effect, overdesigned. There are two main design goals to be achieved by any substation ground system under normal as well as fault conditions. These are (1) to provide means to dissipate electric currents into the earth without exceeding any operating and equipment limits, and (2) to assure that a person in the vicinity of grounded facilities is not exposed to the danger of critical electric shock. The design procedures are aimed at achieving safety from dangerous step and touch voltages within a substation. That it is possible for transferred potentials to exceed the GPR of the substation during fault conditions. The design procedure is based on assuring safety from dangerous step and touch voltages within, and immediately outside, the substation fenced area. Since the mesh voltage is the worst possible touch voltage inside the substation (excluding transferred potentials), the mesh voltage will be used as the basis of this design procedure. Step voltages are inherently less dangerous than mesh voltages. If, however, safety within the grounded area is achieved with the assistance of a high resistivity surface layer (crushed rock), which does not extend outside the fence, then step voltages may be dangerous. In any event, the computed step voltages xmshould be compared with the permissible step voltage after a grid has been designed that satisfies the touch voltage criterion. For equally spaced ground grids, the mesh voltage will increase along meshes from the center to the corner of the grid. The rate of this increase will depend on the size of the grid, number and location of ground rods, spacing of parallel conductors, diameter and depth of the conductors, and the resistivity profile of the soil. In a computer study of three typical grounding grids in uniform soil resistivity, the data shown in Table were obtained. These grids were all symmetrically shaped square grids with no ground rods and equal parallel conductor spacing. The corner Em was computed at the center of corner mesh. The actual worst case Em occurs slightly off-center (toward the corner of the grid), but is only slightly higher than the Em at the center of the mesh. Tablel Typical Ratio of Corner-to-Center Mesh Voltage As indicated in Table 1, the corner mesh voltage is generally much higher than that in the center mesh. This will be true unless the grid is unsymmetrical (that is, has projections, or is L-shaped, etc), has ground rods located on or near the perimeter, or has extremely nonuniform conductor spacings. Thus, in the simplified equations for the mesh voltage Em only the mesh voltage at the center of the corner mesh is used as the basis of the design procedure. Analysis based on computer programs, may use this approximate corner mesh voltage, the actual corner mesh voltage, or the actual worst-case touch voltage found anywhere within the grounded area as the basis of the design procedure. In either case, the initial criterion for a safe design is on limit the computed mesh or touch voltage to below the tolerable touch voltage. XIV
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