CITY OF GLASGOW COLLEGE
Electrical Power Systems Assessment Report (DN3W 34) Tariq M Tariq M ir ir
2011
HNC Elect ri ri cal Eng al Eng in in eerin eerin g g
Contents:
Page: 3.0
Electrical Power Systems
4.0
Types of Distribution Systems
5.1
The Earthing Arrangements
5.2
Earthing Standards
5.3
Ground Rise Potential
5.4
International Standards
6.0
TN-Networks Schematic of Primary Substation
8.0
Energy Sources
8.1
Wind Power
8.2
Coal
8.3
Nuclear
8.4
Natural Gas
9.0
Load Matching
9.1
Maximum Demand (Peak Load)
9.2
Demand Factor
9.3
Diversity Factor
10.0
The Reasons and Applications of Energy Monitoring
10.1
Power Quality
10.2
Problems Harmonics Create
11.0
What a Filter is and How it Operates
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3. Electrical Power Systems
Electricity is one of the most convenient and friendly forms of energy available today for our day to day needs. Electrical power is used for domestic lighting, heating and motive power for driving various types of loads in industries etc. Electrical power & energy is produced (or converted from one form of energy to electrical energy) at power stations. These stations are located at various places in our country and most of them are far and remote from consumer loads. The sets of equipments installed, from sources up to the consumer loads, performing the processes of generation, transmission/transformation and distribution of electrical energy, is known as an electrical power systems. This means that the gen erated energ y has to be transferre d from the sources up to the consumer loads commonly through large distances. All of the generating stations are interconnected to each other through transmission lines. Most generating plants (especially if they are hydroelectric) are located in remote places with respect to the load centre. To deliver this generated energy to the load centres, a transmission system is required. The transmission system should be able to carry this energy reliably, and with a minimum loss, at a virtually stable voltage and frequency. A transmission system can be broken down into three sub systems.
Primary Transmission System (generating points to bulk power receiving points)
Sub Transmission System (bulk power receiving points to area substations)
Distribution System (area substations to distribution substations)
We know the generated electrical energy is transmitted over long distances to reach the load centres. Generator voltages are in the range of 11 to 33 kV; higher generator voltages are difficult to achieve owing to insulation problems in the narrow confines of the generator stator. Long distance transmission cannot be done at generator voltage levels (11-33 kV) because of the huge material requirement and the associated high Copper Loss (sometimes we call it I2R loss). Therefore, the voltage is first stepped up at the generating point using transformers, depending upon the power system and the amount of power that has to be transmitted through transmission lines. Then this power flows through the high voltage transmission lines to the load centres. Transmission voltages worldwide range from 110 to 1000 kV. One reason for using higher transmission voltages is to improve transmission efficiency. Basically, transmission of a given amount of power (at a specified power factor) requi res a fixed product of voltage and li ne current. Thus, the higher the voltage, the lower is the current required. Lower line currents cause lower resistive losses (I2R) in the line. For example the present UK Transmission System consists of a network of 400 kV, 275 kV and 132 kV transmission lines feeding several 400/132 kV and 132/33 kV bulk power receiving stations. These receiving stations are also known as grid substations. The voltage of a 3-phase line is the voltage between any two wires. At these receiving points, the voltage is stepped down to 33 kV (or 11 kV in a few cases) and fed to the Sub Transmission System for shorter transmission runs. For example, The Sri Lanka sub transmission system comprises a 33 kV network, but there are a few 11 kV sub transmission lines, mainly in urban and suburban areas. Thereafter, the voltage is further reduced to 440 V by means of distribution transformers at distribution substations located in the residential and commercial areas for distribution purposes.
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4. Types of distribution systems Imagine that several consumer loads (f ed through distribution substations) are connected to a single source (main grid substation) located in one area. The simplest method is to connect each consumer load to the grid substation through feeder lines. Such a network will need a large number of feeder lines to be installed between the grid substation and the consumer loads and therefore is not recommended. Instead of connecting a single consumer to a dedicated feeder, it is recommended to connect a group of consumers to each of these fee der lines thus minimizing the overall distributio n cost. However, care should be taken to avoid violation of any technical constraints such as over loading of distribution lines, voltage drops etc. Since all the lines are radially emanating from the source in this case, this type of distribution system is known as a radial main system. The radial main distribution system is the cheapest because it requires the least amount of conductors and simple line protection methods compared to the other systems available for power distribution. If one of these radial lines goes out of service the consumer (or group of consumers), connected to the feeder line will not get any electrical power. This is the main disadvantage of radial systems. Mostly, these systems are used in rural areas. If we can find a way to connect the consumers and the source in a ring, then the consumers will receive supply from both sides and even if a portion of the line is on forced outage, the system still receives supply from either side. This type of distribution system is called a ring main system. Normally, ring main systems receive supply from multiple sources. Usually this type of system is used or recommended in areas where the higher reliability for the consumers is a requirement. As of today, most of the distribution systems are interconnected to each other, in which the ring main systems have additional interconnections between nodes.
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5.1 The Earthing Arrangements The two primary functions of a safe earthing system are:
To ensure that a person who is in the vicinity of earthed facilities during a fault is not exposed to the possibility of a fatal electric shock To provide a low impedance path to earth for currents occuring under normal and fault conditions.
5.2 Earthing Standards There are a variety of national and international standards available, which provide empirical formulae for the calculation of earthing design, parameters and shock potential safety limits. There is some variation in formulae between different standards. Three standards, which are widely referred to are:
BS 7354 - 1990: Code of Practice for design of High-Voltage Open-Terminal Stations. IEEE Std 20-2000: IEEE Guide for Safety in AC Substations Grounding. Electricity Association Technical Specification 41-24: Guidelines for the Design, Installation, Testing and Maintenance of Main Earthing Systems in Substations.
5.3 Ground Rise Potential (GPR) The substation earth grid is used as an electrical connection to earth at zero potential reference. This connection, however, is not ideal due to the resistivity of the soil within which the earth grid is buried. Burying typical earth fault conditins, the flow of current via the grid to earth will therefore result in the grid rising in potential relative to remote earth to which other system neutrals are connected. This produced potential gradients within and around the substation and ground area. This is defined as ground potential rise of QPR. GPR of a substation under earth fault conditions must be limited so that step and touch potential limits are not exceeded, and is controlled by keeping the earthing grid resistance as low as possible.
5.4 The international standard for earthing classification is TN TT and IT where: T > Direct connection of a point with earth (Latin: terra); I > No point is connected with earth (isolation), except perhaps via a high impedance. The second letter indicates the connection between earth and the electrical device being supplied: T > Direct connection of a point with earth N > Direct connection to neutral at the origin of installation, which is connected to the earth
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The conductor that connects the exposed metallic parts of the consumer is called protective earth ( PE ). The conductor that connects to the star point in a three-phase system, or that carries the return current in a single-phase system, is called neutral ( N ). Three variants of TN systems are distinguished:
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TN-S: separate protective earth (PE) and neutral (N) conductors from transformer to consuming device, which are not connected together at any point after the building distribution point.
TN-C: combined PE and N conductor all the way from the transformer to the consuming device.
Schematic f a Pr i mary Substati n 7
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TN-C-S earthing system: combined PEN conductor from transformer to building distribution point, but separate PE and N conductors in fixed indoor wiring and flexible power cords.
in-f eeds and three f eeders):
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8. Energy sources 8.1 Wind Power Wind power is the world¶s fastest growing electricity generation technology. Wind is a renewable resource because it is inexhaustible. They use large spinning blades to capture the kinetic energy of the moving wind and this is transferred to rotors that produce electricity. The best wind farm sites today are nearly competitive with the conventional natural gas fired plants. Regions where average wind speed exceeds 12 miles per hour are currently the best sites. Wind power is the lowest cost renewable energy. The costs of wind power are expected to fall and may rank the cheapest energy source of all by 2020. Wind often skips the high voltage national grid and connects to the system locally. This is know as embedded generation.
Coal, gas and nuclear connect the power they generate to the HV national grid 8.2 Coal Coal is the cheapest fossil fuel for making electricity. Coal is burned to produce steam. The steam propels turbine blades at high speed. A generator is mounted at one end of the turbine shaft and consists of carefully wound wire coils. Electricity is generated when these are rapidly rotated in a strong magnetic field. The electricity generated is transformed into the higher voltages (up to 400,000 volts) used for economic, efficient transmission via power line grids. When it nears the point of consumption, such as our homes, the electricity is transformed down to the safer 240 voltage system used in the domestic market.
8.3 Nuclear A nuclear reactor produces and controls the release of energy from splitting the atoms of elements such as uranium and plutonium. In a nuclear power reactor the energy released from continuous fission of the atoms in the fuel as heat is used to make steam. The steam is used to drive turbines which produce electricity ± but without the combustion of fossil fuels and resultant greenhouse gas emissions. Most reactors need to be shut down for refuelling so this is a downside of nuclear. Another danger of nuclear is if the radiation escapes from the protective reactor this would be very damaging to the environment.
8.4 Natural Gas Gas has become a very popular fuel for the generation of electricity. In the 1970s and 80s most electricity was generated by burning coal or nuclear powered plants. Due to economic, environmental and technological changes, natural gas has become the fuel of choice for new power plants. Natural gas fired electricity generation is expected to increase dramatically over the next 20 years.
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9.0 Load matching This is a technique of electric circuit design in which one of the components provides power ot another, and the output circuit of the first component has the same impedance as the input circuit of the second component. Maximum power transfer is achieved when the impedances in both circuits are exactly the same. Load matching is important wherever power needs to be transmitted efficiently, as in the design of power lines, transfo rmers and signal processing de vices such as au dio and computer circuits.
9.1 Maximum Demand (peak load) The transmission system provides for base load and peak load with safety and fault tolerance margins. The peak load times vary by region largely due to the industrial mix. In very hot and very cold climates air conditioning and heating loads have an effect on the overall load. They are typically highest in the late afternoon in the hottest part of the year and in mid-mornings and mid-evenings in the coldest. This makes the power requirements vary by season and the time of the day. Distribution systems designs always take the base load into consideration. The transmission system usually does not have a large buffering capability to match loads with the generation. Thus generation has to be kept matched to the loads to prevent overloading failures of the generation equipment. Multiple sources and loads can be connected to the transmission system and they must be controlled to provide transfer of power. In centralised power generation , only the local control of generation is necessary and it involves synchronization of the generation units to prevent large transients and overload condition.
9.2 Demand Factor Demand factor is the ration of the maximum demand of a system or part of a system to the total connected load on the system, or part of the system under consideration. Demand factor is always less than one.
9.3 Diversity Factor Diversity factor is the ration of the sum of individual maximum demands of the various subdivisions of a system, or part of a system, to the maximum demand of the whole system or part of the system, under consideration. Diversity factor is usually more than one. The sum of the connected loads supplied by a feeder-circuit can be multiplied by the demand f actor to determine the load used to size the components of the system. The sumof the maximum demand loads for two or more feeders is divided by the diversity factor for the feeders to derive the maximum demand load.
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10.1 The Reasons and Application of Energy Monitoring Benefits:
Facilitate the management of energy usage in buildings, facilities or the high voltage infrastructure of the grid. Trending and monitoring energy consumption Automatic and consistent reaction to events allowing the frequency of supply to be monitored Provide a means to gather and view information quickly and provide enough supply online to meet demand at all times A comfortable and safe environment for the high voltage transmission network at the lowest possible cost.
The thermal stress on large power transformers can be monitored in the national grid. Monitoring loads and stress on these transformers at any given time is crucial to the service of the national grid.
10.2 Power Quality Harmonics are currents or voltages with frequencies that are integer multiples of the fundamental power frequency being 50Hz. The second harmonic is at 100Hz, the third at 150 Hz and so on. In rd modern test equipment harmonics can be measures up to the 63 harmonic. When harmonics e xist electrical systems and transformers become mechanically resonant to the magnetic fields in the distribution systems. Harmonics are caused by and are the by-product of modern electronic equipment. There are two types of non-linear loads: single-phase and three-phase. Single phase are prevalent in modern homes and office buildings. Three phase loads are widespread in factories and industrial plants and high voltage transmission systems operating in the national grid.
10.3 Problems harmonics create
Large load currents in the neutral wires of a 3 phase system. Since only the phase wires are protected by circuit breakers and fuses this can result in a potential fire hazard. Overheating transformers which shortens their live. High Current and Voltage Distortion High neutral to ground voltage Poor power factor conditions that result in monthly utility penalty fees for major users (factories, manufacturing and industrial) with a power factor less than 0.9 Resonance that produces over-current surges. This is equivalent to continuous audio feedback through a PA system. This results in destroyed capacitors and their fuses and damaged surge suppressors which will cause electrical system shutdown. False tripping of branch circuit breakers
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11.0 What a filter is and how it operates Harmonics can be treated by cancellation and filtering. A harmonic filter consists of a capacitor bank and an inductor coil. The filter is designed or tuned to the predetermined non-linear load and to filter a predetermined harmonic frequency range. Usually this frequency range only accounts for one harmonic frequency. Harmonic cancellation is performed with ha rmonic cancelling tra nsformers also known a s phaseshifting transformers. This transformer has patented built in electromagnetic technology designed to rd st remove high neutral current and the most harmful harmonics from the 3 through 21 .
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References:
Distribution networks http://hubpages.com/hub/Dist ribution -Networks Grid Operation http://uk.answers.yahoo.com/question/index?qid=20100311112902AAis0A1 The National Grid and Power http://www.nationalgrid.com/uk/Electricity/MajorProjects/faqs.htm Geomagnetically induced currents in the Scottish Power Grid http://www.geomag.bgs.ac.uk/documents/estec_gic_171201.pdf Wind Turbines http://makelectricalsupply.com/Documents/Lesson%202%20 %20Electrical%20supply%20systems.pdf Electrical Supplies http://makelectricalsupply.com/Documents/Lesson%202%20 %20Electrical%20supply%20systems.pdf
Introduction to Low Voltage Power Systems http://makelectricalsupply.com/Documents/Lesson%202%20 %20Electrical%20supply%20systems.pdf
The Effects of Harmonics http://literature.rockwellautomation.com/idc/groups/literature/documents/wp/mvb wp011_-en-p.pdf Electrical Supply Substation http://en.wikipedia.org/wiki/Electrical_substation
Load Matching http://en.wikipedia.org/wiki/Impedance_matching
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Earthing Arrangements http://en.wikipedia.org/wiki/Earthing_system Substation Design
http://www.scribd.com/doc/25059713/Calculation -of-the-Fault-Level-Contribution-Of Ring Mains http://www.dh.gov.uk/prod_consum_dh/groups/dh_digitalassets/@dh/@en/document s/digitalasset/dh_4119451.pdf The UK Grid http://en.wikipedia.org/wiki/Electrical_grid Load Demand in UK http://www.nationalgrid.com/uk/Electricity/Data/Realtime/Demand/demand24.htm
http://www.nfpa.org/assets/files//PDF/necdigest/CodeIssues072704.pdf http://www.scribd.com/doc/8411593/Electrical -Power-Supply-and-Distribution Wind Power http://www.bwea.com/onshore/index.html Load Matching http://en.wikipedia.org/wiki/Imped ance_matching http://www.iea -shc.org/publications/downloads/Task40a Load_Matching_and_Grid_Interaction_of_ Net_Zero_Energy_Buildings.pdf http://www.thefreedictionary.com/impedance+matching
impedance matching Impedance Matching http://www.maxim-ic.com/app-notes/index.mvp/id/742 Max demand, Load factor, Diversity factor
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http://cr4.globalspec.com/thread/28459/Max -demand-Load-factor-Diversity-factor http://www.nfpa.org/assets/files//PDF/necdigest/CodeIssues072704.pdf http://www.jec.co.uk/userfiles/files/Tariff%20Leaflets/Maximum_kW.pdf Energy Monitoring http://www.onsetcomp.com/products/energy_logging_systems Harmonics http://en.wikipedia.org/wiki/Harmonics_(electrical_power ) http://www.cpccorp.com/harmonic.htm http://www.cpccorp.com/harmonic.htm#How can we treat harmonics?
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