It is well known to many individuals who high voltage electricity transmission network signifies the backbone of the complete regional power scheme in the country. The main reason for this research is studying the most typical common failures in the HV transmitting lines and understanding the real reasons for these failures in the transmitting network of a private electric powered company in the Sultanate of Oman known as OETC Oman Electricity Transmission Company. The business of that company is providing, developing and regulating the Electrical Transmitting System in the Sultanate of Oman. The data offered and information provided hereafter will be considered extremely private and therefore the assignment information is supposed only for the use of this project and shall not be sent out to any other get together without permission from the initial source.
In the beginning, the assignment first of all provides a brief explanation about the business and its role of electricity syndication. Then, it further analyzes different kinds of equipment failures that are came across with HV network operation and reported by an area company. It sums up on general findings, results, summary and the recommendation regarding future maintenance.
Oman Electricity Transmitting Company OETC manages, organizes and maintains the majority HV transmitting system where electrical power is transmitted through 220kV and 132kV transmitting lines to insert centers in Muscat Governorate and the parts of Dakhliyah, Batinah, Dhahirah and Sharqiyah. It dispatches power from all Centrally Dispatched Technology Stations managed by the next companies: (1).
1. Ghubrah Electric power & Desalination Station
2. Rusail Power Station
3. Wadi Jizzi Power Station
4. Manah Electricity Station
5. Al Kamil Vitality Station
6. Barka Electric power & Desalination Station
7. Sohar Vitality Station
1. Muscat Electricity Circulation Company (MEDC) - for Muscat area
2. Mazoon Electricity Company (MZEC) - for South Batinah, Dakhliyah and Sharqiyah regions
3. Majan Electricity Company (MJEC) - for North Batinah and Dhahirah regions
Fundamentals of Power Era and Transmission
After the electricity leaves a power era (1), the voltage is increased at a "step-up" grid substation (2). Eventually, the energy moves along a transmitting line to the area where the electric power is necessary (3). Once there, the voltage is reduced, or "stepped-down, " at another major substation (4), and a distribution power series (5) provides the electricity until it extends to a home or business (8). (1)
Fig -1 ELECTRICAL ENERGY System (1)
The main the different parts of the HV high voltage power transmitting are; the over head towers, conductors, insulators, lightning arrestors, CVT & CT and cable connection sealing ends. It has been well recognized since the starting of electric power generation that over head transmission lines (OHL) have displayed the most important element for the electric power transmission and circulation. The over brain transmission range generally dedicated for high voltage range, as the buried type (underground cords) are generally found in lower voltage range for the distribution goal. However, in Oman, both systems are used in various applications depending on cost, development conditions and topography constraints. OETC has prepared, designed and erected overhead electric power lines for various voltage levels in many parts of the sultanate of Oman.
Line voltage Selection
According to IEC 60038 there are standard voltage amounts used for the energy transmission and syndication.
-Low voltage range from 220v up to at least one 1 kV AC
Medium voltage range from 1 kV to 36 kV AC
High voltage range from 52 kV to 765 kV AC) and higher
Generally the Low-voltage transmitting and distribution networks serve households and other small company consumers. Networks on the medium-voltage level usually supply larger properties and settlements, commercial plant life and other large consumers; the energy supply capacity is typically significantly less than 10 MVA per circuit. The high voltage ranges up to 145 kV are usually used for sub-transmission of the energy regionally, and also supply the medium-voltage electric network.
This level is generally selected to support the medium-voltage level even when electric power is leaner than 10 MVA. Moreover, some of high-voltage transmission lines are also used to transmit the electric power from medium size electricity plant life, like hydro vitality plants on water streams, channel or rivers, and provide energy for large-scale products, such as sizeable power plants or metal factories. The bandwidth of electro-mechanical transported vitality corresponds to the broad range of usage, but it almost never exceeds 100 MVA per circuit, while the surge impedance fill is 35 MVA (about).
In most European countries, the high voltage lines of 245 kV were greatly found in interconnection of power supply systems and this prior to the 420 kV level was taken to this purpose. Nowadays, the usage of 245 kV lines is reduced somewhat due to the option of the 420 kV transmission network. The 420 kV level represents the highest operation voltage used for AC transmission in Central Europe. It typically interconnects the energy resource systems and transmits the energy over long distances. Some 420 kV lines hook up the countrywide grids of the individual European countries enabling interconnected network procedure (UCTE = Union for the Co-ordination of Transmission of Electricity) throughout European countries. Large power vegetation such as nuclear channels feed straight into the 420 kV network. The thermal capacity of the 420 kV circuits may reach 2, 000 MVA, with a surge impedance load of around 600 MVA and a transmission capacity up to 1 1, 200 MVA. [SIEMENS Power Executive Guide 2009]11
Selection of conductors and globe wires
Electric conductors are the main important part in the over head power lines network and they must be selected carefully for the electric transmitting lines because this will ensure economical and reliable transmitting and contribute directly to the total series costs. Therefore, to accomplish better monetary solution, aluminum and its alloys have been used as performing materials for vitality lines because of the favorable price, the reduced weight and the need of certain least cross-sections. On the other hand, aluminum is an extremely corrosive metal. But when a thick oxide part is created it can stop further corrosion. Therefore, up to certain level, lightweight aluminum conductors are well-suited for areas in which corrosion can be an concern, such as humid weather in areas located near coastal zone. Generally, there are a variety of different designs in use for metal conductors. As an advantage, All-aluminum conductors (AAC) have the highest conductivity for a given cross-section; however, they possess relatively low mechanical strength, which limits their assembly to brief spans and low tensile makes.
To boost the mechanical strength, light weight aluminum wires are made of mixing with other alloys like aluminum-magnesium-silicon alloys. In this manner, the strength can be increased about twice that of real aluminum. But practically, all-aluminum and metal alloy conductors have exhibited some susceptibility to vibrations. To resolve this problem, element conductors with a material core, so-called light weight aluminum conductor, steel-reinforced (ACSR), can avoid this drawback. The percentage between aluminum and steel amounts from 4. 3:1 to 11:1. An aluminum-to-steel proportion of 6. 0 or 7. 7 has an inexpensive solution. Conductors with a ratio of 4. 3 should be utilized for lines installed in regions with heavy wind flow and ice lots. Conductors with a ratio higher than 7. 7 provide higher conductivity. But because of lower conductor strength, the sags are bigger, which requires higher towers. Experience shows that ACSR conductors, exactly like aluminum and aluminium alloy conductors, supply the most economical solution and give you a life span higher than 40 years. Conductors are selected according to electric powered, thermal, mechanised and economical aspects. The electric amount of resistance as a result of the executing material and its cross-section is the main feature affecting the voltage drop and the energy losses across the series and, therefore, the transmission costs. The cross-section has to be selected so the permissible temperatures will never be exceeded during normal procedure as well as under short-circuit condition. With increasing cross-section, the brand costs increase, while the costs for deficits decrease. With regards to the length of the series and the power to be transmitted, a cross-section can be driven that results in the cheapest transmission costs. The heat balance of ohmic loss and solar radiation against convection and radiation establishes the conductor temperature. A present density of 0. 5 to at least one 1. 0 A/mm2 predicated on the aluminum cross-section has shown to be a cost-effective solution in most cases. [SIEMENS Power Anatomist Guide 2009] (9)
The stand below shows the characteristics of AC over head lines (data make reference to a one circuit of any double-circuit range)
Table -1 attribute of AC over head lines (9)
High-voltage ends up with correspondingly high-voltage gradients at the conductor's surface, and in corona-related results such as visible discharges, radio disturbance, audible noises and energy deficits. When selecting the conductors, the AC voltage gradient has to be limited to prices between 15 and 17 kV/cm. Since the
sound of the audible sound of DC lines is mainly brought on at the positive pole which sound is different from those of AC lines, the subjective feeling differs as well. Therefore, the maximum surface voltage gradient of DC lines is higher than the gradient for AC lines. A maximum value of 25 kV/cm is recommended. The brand voltage and the conductor diameter are one of the primary factors that infl uence the top voltage gradient. To keep this gradient below the limit value, the conductor can be divided into subconductors. This results within an similar conductor diameter that is bigger than the diameter of a single conductor with the same cross-section. This aspect is important for lines with voltages of 245 kV and above. Therefore, so-called bundle conductors are mainly followed for extra-high-voltage lines. Desk 2. 5-2 shows typical conductor configurations for AC lines. From a mechanical perspective, the conductors need to be designed for each day conditions and for maximum tons exerted on the conductor by wind and ice. As the rough figure, a day to day stress of approximately 20 % of the conductor graded tensile stress can be used, producing a limited risk of conductor damage. The maximum working tensile stress should be limited to around 40 % of the scored tensile stress. Earth wires, also called shield wire or earth cable, can protect a range against direct lightning hits and improve system behavior in case of short-circuits; therefore, lines with single-phase voltages of 110 kV and above are usually outfitted with earth wiring. Earth wires manufactured from ACSR conductors with a sufficiently high aluminum cross-section satisfy both requirements. Since the start of the 1990s, more and more earth cables for extra-high-voltage over head electric power lines have been performed as optical earth wires (OPGW). This sort of earth wire combines the functions just detailed for the normal earth line with the additional service for large data transfer capacity via optical fi bers that are integrated into the OPGW. Such data transfer is essential for the communication between two converter channels within an HVDC interconnection or for distant controlling of ability stations. The OPGW in such a case becomes the major communication link within the interconnection. OPGW are mainly designed in a single or more layers of aluminum alloy and/or aluminum-clad metallic wire connections. One-layer designs are used in areas with low keraunic levels (small amount of possible lightning strikes per season) and small short-circuit levels. [SIEMENS Vitality Executive Guide 2009](9)
Selection of insulators
Usually, insulators in the overhead line are subject to electrical and mechanical stresses, because they have to isolate the conductors form potential to earth and must definitely provide physical aids. Therefore, Insulators must manage to withstanding these stresses under all conditions came across in a transmitting line.
The steady-state functioning power-frequency voltage (highest procedure voltage of the system)
Temporary over voltages took place at specific ability frequency
Switching and lightning over voltages
Electrical insulators are incredibly critical and important component in the electric power systems such as circulation & transmitting lines. Recently, the electronic insulators which is made of ceramic and glass materials. However in 1963, a polymeric insulator were developed and its advancements in design and creation in the modern years have managed to get attractive to utilities. polymeric insulator consists of a fibreglass core rod included in weather-sheds or skirts of polymer such as silicon rubber, prepared with metallic end fittings. It is also called amalgamated insulators, which means made of at least two insulating parts - a primary and housing equipped with end fittings. Polymeric insulators have many advantages above the ceramic and wine glass insulators such nearly as good performance in contaminated environment, light-weight, easy handling, maintenance free, and considerably low priced etc. Because of these properties it is gaining popularity worldwide and upgrading the conventional ceramic and cup insulators in many countries. Therefore, our research shall concentrate the light on the silicon rubber insulator and the main advantages can be achieved by using such kind of electrical power insulators.
The following is a comparison showing the various factors between ceramic and composite insulators.
Resistance to flashovers in Polluted atmosphere.
Resistance to puncture
(School: B insulators)
Resistance to Cracking and Erosion in Polluted atmosphere.
Contamination & Pollution
Performance not affected
Unique Hydrophobicity persona.
Self cleaning property
Due to Glaze and inclination of sheds.
Due to Hydrophobicity recovery characteristic.
Needs maintenance like cleaning, cleansing, greasing.
No maintenance is required
10% to 35% of Ceramic Insulator
Resistance to damage and Vandalism
Breakable in Vandalism vulnerable areas
Artificial Pollution Test
Power Arc Test
Table -1 contrast different facets between ceramic and composite insulators (10).
Cap-and-pin insulators (fig. 2) are constructed of porcelain or pre-stressed goblet. The individual models are linked by fixtures of malleable cast flat iron or forged flat iron. The insulating body are not puncture-proof, which is the reason for a relatively high number of insulator failures.
In Central Europe, long-rod insulators made from aluminous porcelain (fig. 3) are most frequently implemented. These insulators are puncture-proof. Failures under procedure are extremely unusual. Long-rod insulators show superior behavior, especially in polluted areas. Because porcelain is a brittle material, porcelain long-rod insulators should be guarded from bending lots by suitable fixtures.
Composite insulators are the third major kind of insulator for over head power line applications (fig. 4). This insulator type provides superior performance and trustworthiness, specifically because of improvements over the last 20 years, and has been in service for more than 30 years.
Fig -2 Cap and pin (disc insulator) (9)
Fig -3 Long-rod insulator with clevis cover (9)
Fig -4 A glass fibre reinforced amalgamated insulator with ball and socket accessories (Lapp insulator) (9)
The composite insulator is constructed of a glass dietary fiber reinforced epoxy pole. The glass fibres applied are ECR glass fibres that are repellent to brittle fracture (ECR = electric grade corrosion resilient glass materials). In order to avoid brittle fracture, the glass fiber fishing rod must additionally be sealed very carefully and durably against water. That is done by software of silicone rubber. Nowadays, high temperature vulcanized (HTV) silicon is used.
Sealing the a glass fiber rod
Molding into insulator sheds to determine the mandatory insulation
Light weight, less size and less damages
Shorter string period compared to cap-and-pin - and porcelain long-rod - insulator strings
Up to 765 kV AC and 600 kV DC, only one product of insulator (sensible length is merely limited by the power of the development collection) is required
High mechanical strength
High performance in polluted areas, predicated on the hydrophobicity (drinking water repellency) of the silicon rubber
Silicone plastic offers exceptional hydrophobicity over the future; almost every other polymeric housing material will loose this property over time
Silicone rubber can recover its hydrophobicity after having a temporary loss of it
The silicone rubber insulator is able to make pollution levels on its surface water-repellent, too (hydrophobicity transfer)
Low surface conductivity, even with a polluted surface and incredibly low leakage currents, even under wetted conditions.
Insulator string sets
Suspension insulator pieces take the conductor weight, including additional loads such as snow and breeze, and are arranged pretty much vertically. A couple of I-shaped (fig. 5a) and V-shaped models in use. Anxiety insulator pieces (fig. 5b, fig. 5c) terminate the conductors and are organized in the direction of the conductors. They can be packed by the conductor tensile make and also have to be graded accordingly. Multiple solitary, double, triple or even more sets manage the mechanised loadings and the design requirements.
Fig -5a; I-shaped suspension insulator place for 245 kV (11)
T Fig -5b&c; Increase tension insulator set for 245 kV (Elevation, Top & Plan, lower part) (9)
The general electrical design of insulation is ruled by the voltages to be withstood and the pollution to that your insulation is subjected. The specifications IEC 60071-1 and IEC 60071-2 as well as the technological report IEC 60815, which provides four pollution classes, give assistance for the design of the insulation. Because IEC 60815 is applicable to AC lines, it should be noted that the creepage distances recommended are based on the phase-to-phase AC voltage (UL-L). When transferring these creepage distances advised by IEC 60815 to a DC series, it ought to be mentioned that the DC voltage is a pole-to-earth value (UL-E). Therefore, these creepage distances have to be multiplied by the factor 3. Furthermore, it ought to be noted that the AC voltage value identifies a mean value, while the DC voltage is related to a top value, which requires a further multiplication with factor 2. Insulators under DC voltage procedure are subjected to more unfavorable conditions than they are really under AC, anticipated to an increased collection of surface contamination caused by the constant unidirectional electric field. Therefore, a DC pollution factor has to be applied. Desk shown with number 5a shows specific creepage distances for different insulator materials under AC and DC request, and is based on industry experience published by power supply companies in South Africa and China. The results shown were validated by an experienced insulator producer in Germany. The modification factors shown are valid for porcelain insulators only. When taking amalgamated insulators under consideration, an additional reduction factor of 0. 75 can be employed. The worth for a DC system must be observed as a guideline only, that must definitely be verified on the case-by-case basis for new HVDC projects.
To handle switching and lightning overvoltages, the insulator pieces need to be designed with value to insulation coordination relating to IEC 60071-1 and IEC 60071-2. These design aspects determine the difference between the earthed fi ttings and the live part. However, for HVDC software, switching impulse levels
are of minor important because circuit-breaker functions from AC lines do not arise on DC back-to-back lines. Such lines are manipulated via their valve control systems. In order to coordinate the insulation in an effective way, it is strongly recommended to apply and use the same SIL and BIL as is employed for the equivalent AC insulation (dependant on the arcing distance). [SIEMENS Power Engineering Guide 2009](9)
Selection and design of supports
Together with the brand voltage, the amount of circuits (AC) or poles (DC) and kind of conductors, the construction of the circuits poles decides the design of overhead power lines.
Additionally, lightning protection by earth wires, the surfaces and the available space at the tower sites need to be considered. In densely populated areas like Central European countries, the width of right-of-way and the space for the tower sites are limited. In the case of extra-high-voltages, the conductor construction affects the electro-mechanical characteristics, the electrical and magnetic field and the transmission capacity of the range. Very often there are contradicting requirements, such as a tower height only possible and a thin right-of-way, which can only be met by compromises. The lowest clearance of the conductors is determined by the voltage and the conductor sag. In ice-prone areas, conductors shouldn't be arranged vertically, to avoid conductor clashing after snow shedding.
For low-voltage and medium-voltage lines, horizontal conductor configurations prevail; these configurations feature lines post insulators as well as suspension system insulators. Poles made of wood, concrete or steel are preferred. Fig. 6 shows some typical collection configurations. Earth cables are omitted at this voltage level.
For high-voltage and extra-high-voltage power lines, a large variety of configurations can be found that depend on the amount of circuits (AC) or poles (DC) and on local conditions. Because of the not a lot of right-of-way, more or less all high voltage AC lines in Central European countries comprise at least two circuits.
Electric field requirements
For DC lines, two basic outlines (monopole and bipole), with versions is highly recommended. Fig. 7i-l show samples for HVDC collection configurations that are valid for all those voltage levels. The plans of insulators depend on the use of a support within the collection. Suspension system towers support the conductors in straight-line parts and at small perspectives. This tower type offers the most affordable costs; special attention should therefore be paid to applying this tower type normally as you possibly can. Angle towers have to transport the conductor tensile makes at angle details of the series. The strain insulator sets completely transfer high forces from the conductors to the supports. Finally, dead-end towers are being used at the terminations of any transmission lines. They carry the total conductor tensile causes at risk aspect (even under unbalanced load condition, e. g. , when conductors of 1 tower aspect are cracked) and a lower life expectancy tension in to the substations (slack span).
Fig. 6 Configuration of Medium voltage supports
Various launching conditions given in the particular national and international requirements have to be met when making towers. The climatic conditions, the earthquake requirements and other local environmental factors will be the next determining factors for the tower design. When making the support, a number of conditions have to be considered. High breeze and ice loads cause the utmost forces to act on suspension towers. In ice-prone areas, unbalanced conductor tensile makes can bring about torsional loading. On top of that, special loading conditions are followed for the purpose of failing containment, that is, to limit the magnitude of damage. Finally, provisions have to be made for engineering and maintenance. Depending on voltage level and the operating pushes of the over head brand, differing designs and materials are followed. Poles made of wood, cement or steel are very often used for low voltage and medium-voltage lines. Towers with lattice steel design, however, prevail at voltage levels of 110 kV and above (fig. 7). Guyed lattice metallic structures are used in some elements of the entire world for high-voltage AC and DC lines. Such design requires a relatively fl at topography and a secure environment where there is no danger from vandalism and theft. Guyed lattice metal structures give you a substantive amount of cost benefits with respect to tower weight and basis quantities. However, a wider right-of-way has to be considered.
Foundations for the supports
Usually, overhead ability line works with are installed on concrete foundations. The foundations have to sustain the entire weight of the tower and really should be designed relative to the neighborhood or international standard suitable for the particular projct.
Fig. 7;(a-h) Tower configurations for AC high-voltage lines. (i-l) Tower configurations for DC high-voltage lines
Total weight caused by tower
Location and Dirt conditions
Accessibility to the series route
Availability of machinery
Constraints of this country and the site
Concrete blocks or concrete piers are used for poles that exert bending moments on the foundation. For towers with four hip and legs, a foundation is provided for each individual lower leg. Pad and chimney and concrete stop foundations require good bearing garden soil conditions without groundwater. Influenced or augured piles and piers are followed for low-bearing dirt, for sites with bearing soil at a greater depth and then for high groundwater level. In case of groundwater, the dirt conditions must permit pile driving. Concrete slabs can be utilized for good bearing soil, when subsoil and groundwater level prohibit pad and chimney foundations as well as piles.
Fig. 8; Foundations for four-legged towers
Route selection and tower spotting
Selection of route and planning represent increasingly difficult jobs, because the right-of-way for transmission lines is limited and many aspects and interests have to be considered. Way selection and authorization rely upon the statutory conditions and procedures prevailing in the united states of the job. Course selection nowadays requires initial desktop studies with a variety of path alternatives, environmental impact studies, community communication hearings and acceptance approval from the neighborhood authorities. [SIEMENS Electricity Executive Guide 2009](9)
The books and journals referred are complete in references. The methodology has been chosen after studying different literatures. The societal loss calculation have been taken from the paper Vitality String Management Audit Service Concentration Professional Engineering Services/ www. powerchainmanagement. com. The effective of electric powered systems is crucial to the success of businesses and facilities. The electronic problems facing businesses today could seem overwhelming, especially understanding that important elements of electrical power systems are susceptible to failure. It could be costly and difficult to design something that predicts failure and minimizes dangers of dangerous dangers such as arc flash and from the graph which shows how does it cost in enough time of losing the power supply.
Fig -2 cost raven (2)
To approximate cost of transmitting losses. The loss calculations derive from an peak insert current for a line. (7)
EC (Energy Cost) = 3 x R x I 2 x 8760 x LF x AIC x LIF,
DC (Demand Cost) = 3 x R x I 2 x IDC x LIF
EC = energy cost, $ / yr
DC = demand cost, $ / yr
R = conductor amount of resistance (ohms/phase/mile) X brand length (kilometers)
I = peak load current on the line (amperes)
8760 = hours / year
LF = damage factor (average reduction / peak reduction)
AIC = average incremental energy cost for the entire year ($ / kWh)
LIF = damage increase factor (1 + PU system deficits reflecting
IDC = incremental demand cost ($ / kW-yr)
Run to failure
condition founded monitoring
on series monitoring
Hot line maintenance
The cost marriage between materials established exclusively on purchase prices, the life cycle economics at all the factors and provides consideration to enough time value of money predicated on a "present value" analysis. The methodology of using present value, life pattern costs is often considered the fairest method of comparison since it considers and properly weighs about all the material factors. This life routine cost study offers consideration to the next:(6)
Material costs and availability
Projected service life
Inspection costs / Inspection frequency
Maintenance costs / Maintenance frequency
For the goal of present value computations, a 4% inflation rate and a 10% discount rate are assumed. The formula used for processing the present value (PV) of a single expenditure is given below. The present value for multiple project expenditures is a simple summation of all specific PVs. (6)
PV = (Cost) x (IF) (DF)
PV = Present Value
Cost = Today's Value
IF = Inflation Factor [ 1 + inflation rate ]
DF = Discount Factor [ 1 + discount rate ]
N = Time No [ no. of years from present ]
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