ELECTRIC POWER SUBSTATIONAn assembly of equipment in an electric power system through which electric energy ispassed for t...
insulation properties of ambient air for insulation to ground. Modern substations       in urban areas are esthetically de...
A link between power systems enabling them to draw on one anothers reserves in time ofneed and to take advantage of energy...
•   Can you use a 120V to 240V step up transformer for an electric range?•   How to determine the right transformer in thr...
2. Actually...NONE. A transformer is a device that steps up, or steps down voltage.       During this process current is a...
As long as you treat the output rating of the step-down transformer as the input rating as astep-up transformer, and dont ...
13. Nothing noticeable. DC power is not transmitted between the coils of a    transformer. There would be no current on th...
Electricity distribution is the penultimate stage in the delivery (before retail) ofelectricity to end users. It is genera...
The losses in a cable are proportional to the square of the current, the length of the cable,and the resistivity of the ma...
and suburban distribution systems continue to use this range of voltages, but most havebeen converted to 7200/12470Y, 7620...
interconnected model is that in the event of a fault or required maintenance a small areaof network can be isolated and th...
materials, and equipment are used among the various utility companies across the U.S.,but the end result is similar. First...
built in 4800 volt delta distribution areas. Step downs are used in areas that have beenupgraded to a 7200/12500Y or 7600/...
•   Distribution Transformers   •   Secondaries, and   •   Services                      Figure 1. Energy flow through a t...
Figure 5. Substation underground distribution                                                                   bus    Fig...
Most industries need 2,400 to 4,160 volts to run heavy machinery and usually their  own substation or substations to reduc...
Residential Customer                                                                       TOP The distribution electricit...
Figure 14. Substation where electricity is conditioned for powering commuter trains  Figure 15. Power runs from the substa...
Illustrated Glossary: SubstationsA substation is a high-voltage electric system facility. It is used to switch generators,...
The specific voltages leaving a step-up transmission substation are determined by the customer needs ofthe utility supplyi...
Figure 4. Step-down transmission substation                                     Figure 5. Step-down power transformerDistr...
Figure 9. Distribution substation               Figure 7. Distribution substationUnderground Distribution Substation      ...
From here the power is distributed to industrial, commercial, and residential customers.Substation Functions              ...
Substation Equipment                                                                                TOPThe major component...
BatteriesBatteries are used in the substation control house as a backup to power the control systemsin case of a power bla...
Capacitor BankCapacitors are used to control the level of the voltage supplied to the customer by reducingor eliminating t...
Figure 1. Circuit switchers                             Figure 2. Circuit switcherControl HouseThe substation control hous...
Figure 2. Substation control house                      Figure 3. Control houseControl PanelsControl panels contain meters...
transmission substations. Two unsynchronized AC transmission systems can be connectedtogether with converter stations.Conv...
Figure 3. Substation line traps              Figure 2. Primary coupling capacitorCurrent TransformersCurrent transformers ...
current transformer                             Figure 3. 400 kV current transformerDisconnect SwitchesDisconnect switches...
Figure 3. Motorized circuit breaker – control box   Figure 4. Motorized circuit breaker - switch detail                   ...
Distribution BusA distribution bus is a steel structure array of switches used to route power out of asubstation.         ...
Grounding ResistorsGrounding Resistors are designed to provide added safety to industrial distribution systemsby limiting ...
Figure 1. Grounding transformer - front view   Figure 2. Grounding transformer - back viewHigh-Voltage Underground CablesH...
Figure 1. High-voltage underground cables            Figure 2. High-voltage underground cablesHigh Voltage FusesHigh volta...
Figure 2. Lightning arrester on distribution pole  Figure 1. Lightning arresters on bus structures                        ...
A manhole is the opening in the underground duct system which houses cables splices and whichcablemen enter to pull in cab...
Figure 2. Recording power meter  Figure 1. An indicating AC amperes meterMicrowaveSubstations commonly use microwave commu...
Figure 2. Microwave power                                                                     transformersOil Circuit Brea...
Figure 1. Potential transformers                Figure 2. Potential transformerPotheadsA type of insulator with a bell or ...
Figure 1. Three conductor potheads on pole      Figure 2. Three conductor pothead                 Figure 3. Potheads on po...
Figure 1. Power-line carrier schematic                                                      Figure 2. Power-line carrier d...
Figure 2. Large power transformers                                                           Figure 4. Step-up transformer...
RelaysA relay is a low-powered device used to activate a high-powered device. Relays are used totrigger circuit breakers a...
Figure 1. SF6 gas power circuit breaker                                                       Figure 2. SF6 gas power circ...
well as transmission and distribution line towers and poles.                       Figure 1. Steel superstructure for circ...
    operate circuit breakers,       operate tap changers on power transformers,       supervise the position and condit...
Figure 2. Suspension insulators               Figure 3. Suspension insulatorsSynchronous CondensersA synchronous condenser...
Transmission buses are steel structure arrays of switches used to route power into asubstation.             Figure 1. Tran...
Figure 1. Vacuum circuit breaker, inside       Figure 2. Vacuum circuit breaker, outsideTransformer VaultA transformer vau...
Figure 1. Pad-mounted transformer for                underground system                                                   ...
Figure 1. 3-phase distribution feeder bay                                                        Figure 3. Distribution fe...
Figure 3. Residential distribution transformer      Figure 1. Air Distribution transformer -                commercial fac...
Figure 1. 3phase distribution feeder bay                                                            Figure 2. Distribution...
layers of redundancy. Through redundancy, power can be shut off in portions of the systemonly, but not the entire system, ...
Figure 6. Non load-break fuse     Figure 3. Substation disconnect switch                                                  ...
Figure 2. Secondaries in a vertical plane   Figure 1. Cabled secondaries                                                  ...
Figure 3. Commercial service                                                    Figure 4. Secondary to underground service...
Figure 4. Single-pole disconnect switch combinedFigure 3. Circuit switchers                                     with a fus...
Figure 7. Group-operated three-pole air break switch
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Electric power substation

  1. 1. ELECTRIC POWER SUBSTATIONAn assembly of equipment in an electric power system through which electric energy ispassed for transmission, transformation, distribution, or switching. Also known assubstation.Electric power substationAn assembly of equipment in an electric power system through which electrical energy ispassed for transmission, distribution, interconnection, transformation, conversion, orswitching. See also Electric power systems.Specifically, substations are used for some or all of the following purposes: connection ofgenerators, transmission or distribution lines, and loads to each other; transformation ofpower from one voltage level to another; interconnection of alternate sources of power;switching for alternate connections and isolation of failed or overloaded lines andequipment; controlling system voltage and power flow; reactive power compensation;suppression of overvoltage; and detection of faults, monitoring, recording of information,power measurements, and remote communications. Minor distribution or transmissionequipment installation is not referred to as a substation.Substations are referred to by the main duty they perform. Broadly speaking, they areclassified as: transmission substations, which are associated with high voltage levels; anddistribution substations, associated with low voltage levels. See also Electric distributionsystems.Substations are also referred to in a variety of other ways: 1. Transformer substations are substations whose equipment includes transformers. 2. Switching substations are substations whose equipment is mainly for various connections and interconnections, and does not include transformers. 3. Customer substations are usually distribution substations on the premises of a larger customer, such as a shopping center, large office or commercial building, or industrial plant. 4. Converter stations are complex substations required for high-voltage direct- current (HVDC) transmission or interconnection of two ac systems which, for a variety of reasons, cannot be connected by an ac connection. The main function of converter stations is the conversion of power from ac to dc and vice versa. The main equipment includes converter valves usually located inside a large hall, transformers, filters, reactors, and capacitors. 5. Most substations are installed as air-insulated substations, implying that the bus- bars and equipment terminations are generally open to the air, and utilize
  2. 2. insulation properties of ambient air for insulation to ground. Modern substations in urban areas are esthetically designed with low profiles and often within walls, or even indoors. 6. Metal-clad substations are also air-insulated, but for low voltage levels; they are housed in metal cabinets and may be indoors or outdoors. 7. Acquiring a substation site in an urban area is very difficult because land is either unavailable or very expensive. Therefore, there has been a trend toward increasing use of gas-insulated substations, which occupy only 5–20% of the space occupied by the air-insulated substations. In gas-insulated substations, all live equipment and bus-bars are housed in grounded metal enclosures, which are sealed and filled with sulfur hexafluoride (SF6) gas, which has excellent insulation properties. 8. For emergency replacement or maintenance of substation transformers, mobile substations are used by some utilities.An appropriate switching arrangement for “connections” of generators, transformers,lines, and other major equipment is basic to any substation design. There are sevenswitching arrangements commonly used: single bus; double bus, single breaker; doublebus, double breaker; main and transfer bus; ring bus; breaker-and-a-half; and breaker-and-a-third. Each breaker is usually accompanied by two disconnect switches, one oneach side, for maintenance purposes. Selecting the switching arrangement involvesconsiderations of cost, reliability, maintenance, and flexibility for expansion.A substation includes a variety of equipment. The principal items are transformers, circuitbreakers, disconnect switches, bus-bars, shunt reactors, shunt capacitors, current andpotential transformers, and control and protection equipment. See also Bus-bar; Circuitbreaker; Electric protective devices; Electric switch; Relay; Transformer; Voltageregulator.Good substation grounding is very important for effective relaying and insulation ofequipment; but the safety of the personnel is the governing criterion in the design ofsubstation grounding. It usually consists of a bare wire grid, laid in the ground; allequipment grounding points, tanks, support structures, fences, shielding wires and poles,and so forth, are securely connected to it. The grounding resistance is reduced enoughthat a fault from high voltage to ground does not create such high potential gradients onthe ground, and from the structures to ground, to present a safety hazard. Good overheadshielding is also essential for outdoor substations, so as to virtually eliminate thepossibility of lightning directly striking the equipment. Shielding is provided by overheadground wires stretched across the substation or tall grounded poles. See also Grounding;Lightning and surge protection.INTERCONNECTION
  3. 3. A link between power systems enabling them to draw on one anothers reserves in time ofneed and to take advantage of energy cost differentials resulting from such factors as loaddiversity, seasonal conditions, time-zone differences, and shared investment in largergenerating units.OVERVOLTAGEA voltage greater than that at which a device or circuit is designed to operate. Alsoknown as overpotential. • What is a substation transformer? • What is the difference between current transformer and voltage transformer? • Why substation is called so? • What is the difference between a power station and a substation? • What is the difference between isolation transformer and step up or step down transformer? • Can you use a step down transformer as step up transformer by reversing the primary voltage as a secondary voltage? • What is pulse transformer? • What is a booster transformer? • What is isolation transformer? • How transformer works? • What is the uses of core transformer? • What is transformer turns ratio? • What happens when DC supply is given to transformer? • How do you identify a liquid transformer? • How did the Transformer franchise begin? • Operation of an isolation transformer? • What is the difference between amplifier and transformer? • How do you measure core loss in a transformer? • What is the input side of a transformer called? • How many bushing do you get on delta to star transformer? • What is the differnce of a two winding transformer and autotransformer? • What is the difference between electronic and magnetic transformer? • What is the name of the company that makes all of the Transformer toys ? • Explain center-tapped transformer? • What is the disadvantage of using dry type transformer? • How do you work out the volt-amp in a transformer? • Will a transformer within a light make a buzzing sound? • Does Megatron have weapons in the 2007 Transformer movie? • How do you calculate the length of the wire conductor in a transformer? • Turns ratio of a single phase transformer? • How do you figure out maximum current of a 240V transformer circuit?
  4. 4. • Can you use a 120V to 240V step up transformer for an electric range?• How to determine the right transformer in three phase system say it should be delta or wye?• Why do a Voltage transformer give me a CLIPPED output signal?• What is the best transformer to purchase to operate a sewing machine purchased in the US in the UK?• How do you calculate transformer turns ratio given primary and secondary winding voltage?• Is it possible to use a 120-240V step up transformer with an electric range when a house is only wired for 120V and if so how?• Can you use the NTSC selectable option on the European television and a step up transformer to make it work in the US?• Do you have parts for a wall furnance Transformer part no AT30 Model s7035st wr62?• Will a 240V water heater operate normally with a 120V 208V service coming off a three phase transformer?• How do you wire a boost transformer for 240v with 32v boostwhat is x1 x4 h1 h4?• If you live in Israel can you use a transformer to step down the 240V power to 120V and then use an American appliance or do you also have a cycle problem and if so can it be cured?• Does varying the resistance in the secondary circuit cause a change in both the input and output currents to keep the power on both sides equal on a transformer?• Can a transformer autobot or decepticon survive falling throught the Earths atmosphere or would the heat and friction instantly kill them?• Could you run your in-house electricity on a 12V system since all your appliances and radios and tvs mostly have a built-in transformer to reduce the current to a much lower voltage?• How do you produce 120V lines from 240V lines without using a transformer?• Is it safe to run a 220volt ac- 12volt dc step down transformer in a 110v outlet- will it transform it to 6v or will it run fine?• If you have a 120V Sony stereo amplifier that you plugged into a wall transformer to step down from 240V in Denmark and it worked fine then zapped can you replace the fried component to accept 240V?• How many turns do you need on a ferrite core transformer with a 1cm by 1cm square center leg at 14 volts and 100Khz switching speed to maintain a Tesla equal to or less than 0.15?• What is the difference between a step up and a step down transformer?1. It is a large encased group of metal plates about the size of a small house with 2 sets of insulated copper wires wrapped around the plates -an input and output - to convert a very high supply line voltage - maybe 44 thousand volts to a lower voltage of 2200 volts that runs down the power line poles in your neighborhood. Look at your streets power poles and you will see a mini substation that converts he 2200 volt to 220 volts and may supply several homes.
  5. 5. 2. Actually...NONE. A transformer is a device that steps up, or steps down voltage. During this process current is also stepped up or down, however, voltage and current are inversely proportional ( meaning an increase in voltage results in a decrease in current and vice versa ) As an example: A step up transformer of 10:1 ratio with 12 volts and 10 ampere of current applied to the primary will have ten times the voltage ( 120 volts ) and ten times less current ( 1 ampere ) at the secondary...and a step down transformer with the same turns ratio with 120 volts and 1 ampere applied to the primary will have 12 volts and ten ampere available at the secondary. The electricity supplied into homes and business uses wires carrying very high voltage and low current over long distances, then uses step down transformers to step down the voltage and step up the current. 3. The noun substation has one meaning - a subsidiary station where electricity is transformed for distribution by a low-network. 4. Power station is where they actually generate the electricity on an industrial scale. Substation is a subsidiary of a power station typically used to step down the voltage for more local use of electricity. 5. Isolation transformer: is a transformer with two separate windings, the primary and the secondary. There is an electrical isolation between the primary and the secondary. There are also transformers with one winding and connections for input and output. If the input is low and the output high you can say STEP Up. If the input is high and the output low you can say STEP DOWN. This transformers can not by used where safety necessary.6. If the transformer has two separate windings, then, YES a step-down can be used as astep-up, and vice-versa.It is more correct to say you are reversing the high and low sides.By definition Primary is the "IN" side and Secondary is the "OUT" side.The Primary could be the high voltage side if it is a step-down,or it could be the low voltage side, if it is a step-up.Design ConsiderationsCare must be taken when reversing the operation of a step-down transformer to insurethat it does not exceed the ratings of the transformer. For example, lets say we have a 12VA step-down transformer that takes 120Vac in and is rated to provide 12V at 1 amp atthe secondary. If we were to reverse it and apply 12Vac to the new primary (the oldsecondary), we would have 120Vac at the new secondary, but to keep within the originalratings it could only be loaded to 0.1A @ 120Vac.
  6. 6. As long as you treat the output rating of the step-down transformer as the input rating as astep-up transformer, and dont try to draw current beyond what would normally havebeen applied to the high voltage primary, you should be fine. Potentially one could pushup the voltage on such a transformer beyond what its stated application specified, whilemaking sure not to exceed the power rating of the transformer and not exceeding thebreakdown voltage of the transformers insulation, for example driving the abovetransformer at 24V to get 0.05A @ 240Vac. It is important to realize though that the ratedinput voltage for a step-down transformer will likely not be a safe input voltage if you useit as a step-up transformer - for example, applying 120V to the above transformer withthe windings reversed would generate 1.2KV! 7. A transformer not intended for power conversion, but for galvanically isolating electrical signals - usually digital, therefore "pulse". 8. Normally used in public address systems, where the audio have to travel long distances and have to drive more than one speaker then a booster transformer is inserted in the audio line, also called audio transformer9. An isolation transformer does not have a direct electrical path from the power inputside to the power output side. The term is also used to define how much electricalisolation exists between the input and output windings. For example when using line-voltage input transformers to power low volatge device handled by humans, a highdegree of isolation is required for safety.Isolated transformers often use separate bobbins for the primary and secondary coilwindings, but usually the windings are just wound on top of each other with insulation inbetween.Non-Isolated transformers are becoming rare. A common example is the "Variac" whichis a non-isolated variable transformer. Usually the term "auto-transformer" is used todescribe non-isolated transformers. They are rarely found in consumer products.10. #1...Wire produces a magnetic field when current is passed through it. If you wrap thewire around something (a core) to make a coil, it concentrates that field. The core isntactually necessary but it helps concentrate the field and make the transformer moreeffecient.#2...If you pass a magnetic field through a wire, it produces electron flow.If you make a coil with 100 wraps and pass current through it, it will produce themagnetic field. If you have another coil close enough to be IN that magnetic field, and ithas 10 turns, you will get about 1/10 the voltage from the second coil that you put intothe first one.It gets a lot more complicated than that with formulas and all kinds of mathematics, butthat is the basics of a "step down" transformer.
  7. 7. 13. Nothing noticeable. DC power is not transmitted between the coils of a transformer. There would be no current on the other side of the transformer, unless the power of the source was constantly modulated.14. An amplifier is what increases a rock bands speaker sound output into a ear splitting experience and small radio signals in a radio until it makes it out to the speaker. A transformer converts AC electricity up or down to a desired level for a required project. Example: A battery charger plugged into a wall socket will transformed to about 18 volts to do the charging15. Core loss is also called “No-Load” loss. To measure the core lose simply you need variable AC supply, Wattmeters, Ampere meters and Voltmeters. The basic principle to perform this measurement is to supply the transformer with its nominal voltage and then record the Watt or Kilo watt values. You will need precision current and voltage transformers to supply your wattmeters, voltmeters and ampere meters. There is a term called “Form Factor” which should be measured in order to identify how sinusoidal is your supply voltage and based on the value of this Form Factor you need to apply relevant corrections to the measured values. For power transformers normally we record no load values between 90% and 110% of rated voltage. Based on the test circuit configuration you may use “2 or 3 wattmeter” arrangements for 3 phase transformers. For further information you can check out my website at www.eonce.com and if you need more information you can simply fill the form in “Contact” page on the website. Hope this helps. B.M. Mirzaei, P.Eng.
  8. 8. Electricity distribution is the penultimate stage in the delivery (before retail) ofelectricity to end users. It is generally considered to include medium-voltage (less than 50kV) power lines, electrical substations and pole-mounted transformers, low-voltage (lessthan 1000 V) distribution wiring and sometimes electricity meters.DescriptionHistoryIn the early days of electricity generation to about 1900, direct current DC generatorswere connected to loads at the same voltage. The generation, transmission and loads hadto be of the same voltage because there was no way of changing DC voltage levels, otherthan inefficient motor-generator sets. Low DC voltages were used (on the order of 100volts) since that was a practical voltage for incandescent lamps, which were then theprimary electrical load. The low voltage also required less insulation to be safelydistributed within buildings.
  9. 9. The losses in a cable are proportional to the square of the current, the length of the cable,and the resistivity of the material, and are inversely proportional to cross-sectional area.Early transmission networks were already using copper, which is one of the besteconomically feasible conductors for this application. To reduce the current and copperrequired for a given quantity of power transmitted would require a higher transmissionvoltage, but no convenient efficient method existed to change the voltage level of DCpower circuits. To keep losses to an economically practical level the Edison DC systemneeded thick cables and local generators. Early DC generating plants needed to be withinabout 1.5 miles of the farthest customer to avoid the need for excessively large andexpensive conductors.Introduction of alternating currentThe adoption of alternating current (AC) for electricity generation following the War ofCurrents dramatically changed the situation. Power transformers, installed at substations,could be used to raise the voltage from the generators and reduce it to supply loads.Increasing the voltage reduced the current in the transmission and distribution lines andhence the size of conductors required and distribution losses incurred. This made it moreeconomical to distribute power over long distances. Generators (such as hydroelectricsites) could be located far from the loads.In North America, early distribution systems used a voltage of 2200 volts corner-grounded delta. Over time, this was gradually increased to 2400 volts. As cities grew,most 2400 volt systems were upgraded to 4160/2400 volt, three-phase systems. Some city
  10. 10. and suburban distribution systems continue to use this range of voltages, but most havebeen converted to 7200/12470Y, 7620/13200Y, 14400/24940Y, and 19920/34500Y.European systems used 3300 volts to ground, in support of the 220/380Y volt powersystems used in those countries. In the UK, urban systems progressed to 6.6 kV and then11 kV (phase to phase), the most common distribution voltage.North American and European power distribution systems also differ in that NorthAmerican systems tend to have a greater number of low-voltage, step-down transformerslocated close to customers premises. For example, in the US a pole-mounted transformerin a suburban setting may supply 1-3 houses, whereas in the UK a typical urban orsuburban low-voltage substation might be rated at 2 MW and supply a wholeneighbourhood. This is because the higher voltage used in Europe (380 V vs 230 V) maybe carried over a greater distance with acceptable power loss. An advantage of the NorthAmerican setup is that failure or maintenance on a single transformer will only affect afew customers. Advantages of the UK setup are that the transformers may be fewer,larger and more efficient, and due to diversity there need be less spare capacity in thetransformers, reducing power wastage. In North American city areas with manycustomers per unit area, network distribution will be used, with multiple transformers andlow-voltage busses interconnected over several city blocks.Rural Electrification systems, in contrast to urban systems, tend to use higher voltagesbecause of the longer distances covered by those distribution lines (see RuralElectrification Administration). 7200, 12470 and 25000 volt distribution is common inthe United States; 11 kV and 33 kV are common in the UK, New Zealand and Australia;11 kV and 22 kV are common in South Africa. Other voltages are occasionally used.In New Zealand, Australia, Saskatchewan, Canada, and South Africa, single wire earthreturn systems (SWER) are used to electrify remote rural areas.While power electronics now allow for conversion between DC voltage levels, AC is stillused in distribution due to the economy, efficiency and reliabilty of transformers. High-voltage DC is used for transmission of large blocks of power over long distances, or forinterconnecting adjacent AC networks, but not for distribution to customers.Distribution network configurationsDistribution networks are typically of two types, radial or interconnected (see SpotNetwork Substations). A radial network leaves the station and passes through the networkarea with no normal connection to any other supply. This is typical of long rural lineswith isolated load areas. An interconnected network is generally found in more urbanareas and will have multiple connections to other points of supply.These points of connection are normally open but allow various configurations by theoperating utility linemen carefully closing and opening switches. The benefit of the
  11. 11. interconnected model is that in the event of a fault or required maintenance a small areaof network can be isolated and the remainder kept on supply.Within these networks there may be a mix of overhead line construction utilizingtraditional utility poles and wires and, increasingly, underground construction with cablesand indoor or cabinet substations. However, underground distribution can cost as much as11 times as much as overhead construction. In part to reduce this cost, undergroundpower lines are sometimes colocated with other utility lines in what are called Commonutility ducts. Distribution feeders emanating from a substation are generally controlled bya circuit breaker or fuse which will open when a fault is detected. Automatic CircuitReclosers may be installed to further segregate the feeder thus minimising the impact offaults.Long feeders experience voltage drop requiring capacitors or voltage regulators to beinstalled, and the phase physical relationship to be interchanged.Characteristics of the supply given to customers are generally mandated by contractbetween the supplier and customer. Deviations from the normal usage pattern usuallyinvoke monthly surcharges. Variables include: • AC or DC - Virtually all public electricity supplies are AC today. Users of large amounts of DC power such as some electric railways, telephone exchanges and industrial processes such as aluminium smelting either operate their own or have adjacent dedicated generating equipment, or use rectifiers to derive DC from the public AC supply • Voltage, including tolerance (usually +10 or -15 percentage) • Frequency, commonly 50 & 60 Hz, 16-2/3 Hz for some railways and, in a few older industrial and mining locations, 25 Hz • Phase configuration (single phase, polyphase including two phase and three phase) • Maximum demand (usually measured as the largest amount of power delivered within a 15 or 30 minute period during a billing period) • Load Factor, expressed as a ratio of average load to peak load over a period of time. Load factor indicates the degree of effective utilization of equipment (and capital investment) of distribution line or system. • Power factor of connected load • Earthing arrangements - TT, TN-S, TN-C-S or TN-C • Maximum prospective short circuit current • Maximum level and frequency of occurrence of transientsSee List of countries with mains power plugs, voltages and frequencies.Modern Distribution SystemsThe modern distribution system begins as the primary circuit leaves the sub-station andends as the secondary service enters the customers meter socket. A variety of methods,
  12. 12. materials, and equipment are used among the various utility companies across the U.S.,but the end result is similar. First, the energy leaves the sub-station in a primary circuit,usually with all three phases.The most common type of primary is known as a wye configuration (so named becauseof the shape of a "Y".) The wye configuration includes 3 phases (represented by the threeouter parts of the "Y") and a neutral (represented by the center of the "Y".) The neutral isgrounded both at the substation and at every power pole. In a typical 12470Y/7200 voltsystem, the pole mount transformers primary winding is rated for 7200 volts and isconnected across one phase of power and the neutral. The primary and secondary (lowvoltage) neutrals are bonded (connected) together to provide a path to blow the primaryfuse if any fault occurs that allows primary voltage to enter the secondary lines. Anexample of this type of fault would be a primary phase falling across the secondary lines.Another example would be some type of fault in the transformer itself.Electric distribution substations transform power from transmission voltage to the lowervoltage used for local distribution to homes and businesses.The other type of primary configuration is known as delta, this method is older and lesscommon. Delta is so named because of the shape of the Greek letter delta, a triangle.Delta has only 3 phases and no neutral. In delta there is only a single voltage, betweentwo phases (phase to phase), while in wye there are two voltages, between two phasesand between a phase and neutral (phase to neutral). Wye primary is safer because if onephase becomes grounded, that is makes connection to the ground through a person, tree,or other object, it should trip out the fused cutout similer to a household circuit breakertripping. In delta, if a phase makes connection to ground it will continue to functionnormally. It takes two or three phases to make connection to ground before the fusedcutouts will open the circuit. The voltage for this configuration is usually 4800 volts.Transformers are sometimes used to step down from 7200 or 7600 volts to 4800 volts orto step up from 4800 volts to 7200 or 7600 volts. When the voltage is stepped up, aneutral is created by bonding one leg of the 7200/7600 side to ground. This is commonlyused to power single phase underground services or whole housing developments that are
  13. 13. built in 4800 volt delta distribution areas. Step downs are used in areas that have beenupgraded to a 7200/12500Y or 7600/13200Y and the power company chooses to leave asection as a 4800 volt setup. Sometimes power companies choose to leave sections of adistribution grid as 4800 volts because this setup is less likely to trip fuses or reclosers inheavily wooded areas where trees come into contact with lines.Economic and PoliticalIn the United States, Electric industry "deregulation" reform, started in the mid-1990s,has led to the creation of electricity markets through the elimination of the former naturalmonopoly of generation, transmission, and distribution. As a consequence, electricity hasbecome more of a commodity. The separation has also led to the development of newterminology to describe the business units, e.g. line company, wires business and networkcompany.Electric Power DistributionA distribution system originates at a distribution substation and includes the lines, poles,transformers and other equipment needed to deliver electric power to the customer atthe required voltages. Customers are classed as: • Industrial Customer • Commercial Customer • Residential Customer • Transportation CustomerDistribution Systems TOPA distribution system consists of all the facilities and equipment connecting a transmissionsystem to the customers equipment.A typical distribution system can consist of: • Substations • Distribution Feeder Circuits • Switches • Protective Equipment • Primary Circuits
  14. 14. • Distribution Transformers • Secondaries, and • Services Figure 1. Energy flow through a typical substationThe following are examples of distribution systems components. Collectively theyconstitute a typical distribution system. These typically deliver voltages as high as 34,000volts (34 kV) and as low as 120 volts. Figure 2. Typical residential service drop Figure 3. Substation pull-off structure
  15. 15. Figure 5. Substation underground distribution bus Figure 4. Substation pull-off structure (connects substation busswork to overhead lines) Figure 6. Distribution primaries and secondaries Figure 7. Distribution underbuild on subtransmission poleAdditional information: • The Linemans and Cablemans Handbook, Shoemaker, T. M., Mack, J. E., Tenth Edition 2002, McGraw-Hill. Industrial Customer TOP
  16. 16. Most industries need 2,400 to 4,160 volts to run heavy machinery and usually their own substation or substations to reduce the voltage from the transmission line to the desired level for distribution throughout the plant area. They usually require 3-phase lines to power 3-phase motors. Figure 8. Industrial facility distribution transformer Additional information: • The Linemans and Cablemans Handbook, Shoemaker, T. M., Mack, J. E., Tenth Edition 2002, McGraw-Hill.Commercial Customer TOPCommercial customers are usually served at distribution voltages, ranging from 14.4 kVto 7.2 kV through a service drop line which leads from a transformer on or near thedistribution pole to the customers end use structure. They may require 3-phase lines topower 3-phase motors. Figure 10. Commercial service drop Figure 9. Distribution transformer to 3-phase service - commercial facility
  17. 17. Residential Customer TOP The distribution electricity is reduced to the end use voltage (120/240 volts single phase) via a pole mounted or pad-mounted transformer. Power is delivered to the residential customer through a service drop line which leads from the distribution pole transformer to the customers structure, for overhead lines, or underground. Figure 11. Residential distribution transformer Figure 12. Pad-mounted residential distribution and service drop transformerTransportation Customer TOPCurrently the only electric transportation systems are light rail and subway systems. A smalldistribution substation reduces the local distribution voltage to the transportation systemrequirements. The overhead lines supply electric power to the transportation system motorsand the return current lines are connected to the train tracks. Figure 13. Public transit train powered by overhead electric lines
  18. 18. Figure 14. Substation where electricity is conditioned for powering commuter trains Figure 15. Power runs from the substation Figure 16. Electric cables carry electricity to underground to the poles where power is power the trains motors delivered to the power lines. The circuit iscompleted through the train tracks, with lines returning to the substation.
  19. 19. Illustrated Glossary: SubstationsA substation is a high-voltage electric system facility. It is used to switch generators, equipment, and circuits orlines in and out of a system. It also is used to change AC voltages from one level to another, and/or changealternating current to direct current or direct current to alternating current. Some substations are small with littlemore than a transformer and associated switches. Others are very large with several transformers and dozens ofswitches and other equipment. There are three aspects to substations: Figure 1. Typical substation • Substation Types: Although, there are generally four types of substations there are substations that are a combination of two or more types.  Step-up Transmission Substation  Step-down Transmission Substation  Distribution Substation  Underground Distribution Substation  Substation Functions  Substation Equipment Step-up Transmission Substation TOP A step-up transmission substation receives electric power from a nearby generating facility and uses a large power transformer to increase the voltage for transmission to distant locations. A transmission bus is used to distribute electric power to one or more transmission lines. There can also be a tap on the incoming power feed from the generation plant to provide electric power to operate equipment in the generation plant. A substation can have circuit breakers that are used to switch generation and transmission circuits in and out of service as needed or for emergencies requiring shut-down of power to a circuit or redirection of power.
  20. 20. The specific voltages leaving a step-up transmission substation are determined by the customer needs ofthe utility supplying power and to the requirements of any connections to regional grids. Typical voltagesare: High voltage (HV) ac: 69 kV, 115 kV, 138 kV, 161 kV, 230 kV Extra-high voltage (EHV) ac: 345 kV, 500 kV, 765 kV Ultra-high voltage (UHV) ac: 1100 kV, 1500 kV Direct-current high voltage (dc HV): ±250 kV, ±400 kV, ±500 kVDirect current voltage is either positive or negative polarity. A DC line has two conductors, so one would bepositive and the other negative. Figure 2. Step-up AC transmission substation Figure 3. Step-up transmission substation to AC transmission linesStep-down Transmission Substation TOPStep-down transmission substations are located at switching points in an electrical grid. They connectdifferent parts of a grid and are a source for subtransmission lines or distribution lines. The step-downsubstation can change the transmission voltage to a subtransmission voltage, usually 69 kV. Thesubtransmission voltage lines can then serve as a source to distribution substations. Sometimes, power istapped from the subtransmission line for use in an industrial facility along the way. Otherwise, the powergoes to a distribution substation.
  21. 21. Figure 4. Step-down transmission substation Figure 5. Step-down power transformerDistribution Substation TOPDistribution substations are located near to the end-users. Distribution substation transformers change thetransmission or subtransmission voltage to lower levels for use by end-users. Typical distribution voltagesvary from 34,500Y/19,920 volts to 4,160Y/2400 volts.34,500Y/19,920 volts is interpreted as a three-phase circuit with a grounded neutral source. This wouldhave three high-voltage conductors or wires and one grounded neutral conductor, a total of four wires. Thevoltage between the three phase conductors or wires would be 34,500 volts and the voltage between onephase conductor and the neutral ground would be 19,920 volts.From here the power is distributed to industrial, commercial, and residential customers. Figure 6. Distribution substation Figure 8. Distribution substation
  22. 22. Figure 9. Distribution substation Figure 7. Distribution substationUnderground Distribution Substation TOP Figure 10. Underground Distribution SubstationUnderground distribution substations are also located near to the end-users. Distribution substationtransformers change the subtransmission voltage to lower levels for use by end-users. Typical distributionvoltages vary from 34,500Y/19,920 volts to 4,160Y/2400 volts.An underground system may consist of these parts: • Conduits • Duct Runs • Manholes • High-Voltage Underground Cables • Transformer Vault • Riser • Transformers
  23. 23. From here the power is distributed to industrial, commercial, and residential customers.Substation Functions TOPSubstations are designed to accomplish the following functions, although not all substations have all thesefunctions: • Change voltage from one level to another • Regulate voltage to compensate for system voltage changes • Switch transmission and distribution circuits into and out of the grid system • Measure electric power qualities flowing in the circuits • Connect communication signals to the circuits • Eliminate lightning and other electrical surges from the system • Connect electric generation plants to the system • Make interconnections between the electric systems of more than one utility • Control reactive kilovolt-amperes supplied to and the flow of reactive kilovolt-amperes in the circuitsAdditional information: • The Linemans and Cablemans Handbook, Shoemaker, T. M., Mack, J. E., Tenth Edition 2002, McGraw-Hill.
  24. 24. Substation Equipment TOPThe major components of a typical substation are:Air Circuit Breaker Distribution Bus PotheadsBatteries Duct Runs Power-line CarrierBus Support Insulators Frequency Changers Power TransformersCapacitor Bank Grounding Resistors RectifiersCircuit Switchers Grounding Transformers RelaysConcrete Foundation High-Voltage Underground Cables SF6 Circuit BreakersConduits High-Voltage Fuses Shunt ReactorsControl House Lightning Arresters Steel SuperstructuresControl Panels Manholes Supervisory ControlControl Wires Metal-clad Switchgear Suspension InsulatorsConverter Stations Meters Synchronous CondensersCoupling Capacitors Microwave Transmission BusCurrent Transformers Oil Circuit Breakers Vacuum Circuit BreakersDisconnect Switches Potential TransformersAdditional information: • The Linemans and Cablemans Handbook, Shoemaker, T. M., Mack, J. E., Tenth Edition 2002, McGraw-Hill. Air Circuit Breakers Air circuit breakers are used to interrupt circuits while current flows through them. Compressed air is used to quench the arc when the connection is broken. Figure 1. Air circuit breaker
  25. 25. BatteriesBatteries are used in the substation control house as a backup to power the control systemsin case of a power blackout. Figure 1. Backup batteries in the control houseBus Support InsulatorsBus support insulators are porcelain or fiberglass insulators that serve to isolate the bus barswitches and other support structures and to prevent leakage current from flowing throughthe structure or to ground. These insulators are similar in function to other insulators used insubstations and transmission poles and towers. Figure 1. Bus support insulators
  26. 26. Capacitor BankCapacitors are used to control the level of the voltage supplied to the customer by reducingor eliminating the voltage drop in the system caused by inductive reactive loads. Figure 1. Capacitor bank, end view Figure 2. Capacitor bank, side viewCircuit SwitchersCircuit switchers provide equipment protection for transformers, lines, cables, and capacitor banks.They also are used to energize and deenergize capacitor banks and other circuits.
  27. 27. Figure 1. Circuit switchers Figure 2. Circuit switcherControl HouseThe substation control house contains switchboard panels, batteries, battery chargers, supervisorycontrol, power-line carrier, meters, and relays. The control house provides all weather protectionand security for the control equipment. It is also called a doghouse. Figure 1. Control house
  28. 28. Figure 2. Substation control house Figure 3. Control houseControl PanelsControl panels contain meters, control switches and recorders located in the control building,also called a doghouse. These are used to control the substation equipment, to send powerfrom one circuit to another or to open or to to shut down circuits when needed. Figure 1. Substation control panel Figure 2. Substation control panel, detailConverter StationsConverter stations are located at the terminals of a DC transmission line. Converter stationscan change alternating current into direct current or change direct current to alternatingcurrent. Sometimes converter stations are located at a generation power plant or at
  29. 29. transmission substations. Two unsynchronized AC transmission systems can be connectedtogether with converter stations.Converter stations are also found in most substations for converting the emergency batteryback-up system to AC power for use in an emergency. Figure 1. Converter station in battery roomCoupling CapacitorsCoupling capacitors are used to transmit communication signals to transmission lines. Someare used to measure the voltage in transmission lines. In signal transmission the couplingcapacitor is part of a power line carrier circuit as shown in the schematic below. A couplingcapacitor is used in this circuit in conjunction with a line trap. Line traps can be installed atthe substation or on a transmission line tower. Figure 1. Power line carrier schematic showing use of coupling capacitors
  30. 30. Figure 3. Substation line traps Figure 2. Primary coupling capacitorCurrent TransformersCurrent transformers can be used to supply information for measuring power flows and theelectrical inputs for the operation of protective relays associated with the transmission anddistribution circuits or for power transformers. These current transformers have the primarywinding connected in series with the conductor carrying the current to be measured orcontrolled. The secondary winding is thus insulated from the high voltage and can then beconnected to low-voltage metering circuits.Current transformers are also used for street lighting circuits. Street lighting requires aconstant current to prevent flickering lights and a current transformer is used to provide thatconstant current. In this case the current transformer utilizes a moving secondary coil tovary the output so that a constant current is obtained. Figure 2. Pole type constant Figure 1. Metering current transformers
  31. 31. current transformer Figure 3. 400 kV current transformerDisconnect SwitchesDisconnect switches or circuit breakers are used to isolate equipment or to redirect current in asubstation. Many different types of disconnect switches are shown below. Figure 1. Disconnect switches on an outgoing Figure 2. Motorized disconnect switch (circuit distribution circuit breaker)
  32. 32. Figure 3. Motorized circuit breaker – control box Figure 4. Motorized circuit breaker - switch detail Figure 5. Substation motorized grounding switches
  33. 33. Distribution BusA distribution bus is a steel structure array of switches used to route power out of asubstation. Figure 2. Distribution bus Figure 1. Distribution busFrequency ChangersA frequency changer is a motor-generator set that changes power of an alternating currentsystem from one frequency to one or more different frequencies, with or without a change inthe number of phases, or in voltage. Sometimes a converter is used to accomplish this. Figure 1. Frequency changers at a transportation substation
  34. 34. Grounding ResistorsGrounding Resistors are designed to provide added safety to industrial distribution systemsby limiting ground fault current to reasonable levels. They are usually connected betweenearth ground and the neutral of power transformers, power generators or artificial neutraltransformers. Their main purpose is to limit the maximum fault current to a value which willnot damage generating, distribution or other associated equipment in the power system, yetallow sufficient flow of fault current to operate protective relays to clear the fault. Figure 1. Grounding resistorGrounding TransformersA grounding transformer is intended primarily to provide a neutral point for groundingpurposes. It may be provided with a delta winding in which resistors or reactors areconnected.
  35. 35. Figure 1. Grounding transformer - front view Figure 2. Grounding transformer - back viewHigh-Voltage Underground CablesHigh-Voltage underground cables are constructed in many different ways, but are usually shieldedcables. They are made with a conductor, conductor-strand shielding, insulation, semi-conductinginsulation shielding, metallic insulation shielding, and a sheath. The sheath can be metallic and maythen serve as the metallic insulation shielding and be covered with a nonmetallic jacket to protectthe sheath. This sheath helps to reduce or eliminate inductive reactance. Such cables are commonlyused in circuits operating at 2400 volts or higher.
  36. 36. Figure 1. High-voltage underground cables Figure 2. High-voltage underground cablesHigh Voltage FusesHigh voltage fuses are used to protect the electrical system in a substation from powertransformer faults. They are switched for maintenance and safety. Figure 1. High voltage fuses in a switch box Figure 2. External switch for high voltage fusesLightning ArrestersLightning arresters are protective devices for limiting surge voltages due to lightning strikes orequipment faults or other events, to prevent damage to equipment and disruption of service. Alsocalled surge arresters.Lightning arresters are installed on many different pieces of equipment such as power poles andtowers, power transformers, circuit breakers, bus structures, and steel superstructures insubstations.
  37. 37. Figure 2. Lightning arrester on distribution pole Figure 1. Lightning arresters on bus structures transformer Figure 3. Lightning arresters Figure 4. Lightning arrester on substation power transformerManholes
  38. 38. A manhole is the opening in the underground duct system which houses cables splices and whichcablemen enter to pull in cable and to make splices and tests. Also called a splicing chamber orcable vault. Figure 1. Manholes Figure 2. Manhole coverMetersVarious types of meters are found in substation control houses. They all are measuringdevices and can be an indicating meter or a recording meter. An indicating meter shows on adial the quantity being measured. A recording meter makes a permanent record of thequantity being measured, usually by tracing a line on a chart or graph. Newer recordingmeters store the information electronically. The photo below left is an indicating amperagemeter. On the right is a recording meter.
  39. 39. Figure 2. Recording power meter Figure 1. An indicating AC amperes meterMicrowaveSubstations commonly use microwave communication equipment for communication withlocal and regional electric power system control centers. This system allows for rapidcommunication and signaling for controlling the routing of power.Electric power for microwave transmission comes from special transformers that reduceincoming transmission voltage to that required for the microwave system. Figure 1. Substation microwave communication tower
  40. 40. Figure 2. Microwave power transformersOil Circuit BreakersOil circuit breakers are used to switch circuits and equipment in and out of a system in asubstation. They are oil filled to provide cooling and to prevent arcing when the switch isactivated. Figure 2. Oil circuit breakers in a distribution circuit Figure 1. Oil circuit breakers in a 41 kV circuitPotential TransformersPotential transformers are required to provide accurate voltages for meters used for billingindustrial customers or utility companies.
  41. 41. Figure 1. Potential transformers Figure 2. Potential transformerPotheadsA type of insulator with a bell or pot-like shape used to connect underground electrical cablesto overhead lines. It serves to separate the bunched-up conductors from one another in thecable to the much wider separation in the overhead line. It also seals the cable end from theweather. Potheads are mounted on a distribution pole and the assembly is called a riserpole.
  42. 42. Figure 1. Three conductor potheads on pole Figure 2. Three conductor pothead Figure 3. Potheads on polePower-line CarrierA power line carrier is communication equipment that operates at radio-frequencies,generally below 600 kilohertz, to transmit information over electric power transmissionlines. A high frequency signal is superimposed on the normal voltage on a power circuit. Thepower line carrier is usually coupled to the power line by means of a coupling capacitor inconjunction with a line trap.A device for producing radio-frequency power for transmission on power lines.
  43. 43. Figure 1. Power-line carrier schematic Figure 2. Power-line carrier device in control housePower TransformersPower transformers raise or lower the voltage as needed to serve the transmission or distributioncircuits. Figure 3. Power Transformer, front view Figure 1. Power transformer, back view
  44. 44. Figure 2. Large power transformers Figure 4. Step-up transformer diagramRectifiersA rectifier is a device used to convert alternating current to direct current. Figure 1. Full wave rectifier circuit diagram Figure 2. Rectifier
  45. 45. RelaysA relay is a low-powered device used to activate a high-powered device. Relays are used totrigger circuit breakers and other switches in substations and transmission and distributionsystems. Figure 1. Substation control panel relays Figure 2. Relay and control panelSF6 Circuit BreakersSF6 circuit breakers operate to switch electric circuits and equipment in and out of thesystem. These circuit breakers are filled with compressed sulfur-hexafluoride gas which actsto open and close the switch contacts. The gas also interrupts the current flow when thecontacts are open.
  46. 46. Figure 1. SF6 gas power circuit breaker Figure 2. SF6 gas power circuit breakerShunt ReactorsShunt reactors are used in an extra high-voltage substation to neutralize inductive reactancein long EHV transmission lines. The photo shows an installation of both an older version anda newer version of the reactor. Figure 1. Shunt reactors in a substationSteel SuperstructuresSteel superstructures are used to support equipment, lines, and switches in substations as
  47. 47. well as transmission and distribution line towers and poles. Figure 1. Steel superstructure for circuit breakers Figure 2. Substation with many steel superstructures for equipment and connection supportsSupervisory ControlSupervisory control refers to equipment that allows for remote control of a substationsfunctions from a system control center or other point of control. Supervisory control can beused to:
  48. 48.  operate circuit breakers,  operate tap changers on power transformers,  supervise the position and condition of equipment, and  telemeter the quantity of energy in a circuit or in substation equipment. Figure 1. Supervisory control room Figure 2. Supervisory control panelSuspension InsulatorsAn insulator type usually made of porcelain that can be stacked in a string and hangs from across arm on a tower or pole and supports the line conductor. Suspension insulators areused for very high voltage systems when it is not practical or safe to use other types ofinsulators. They have an advantage in that one or more of the insulators in a string can bechanged out without replacing the entire string. Figure 1. Suspension insulators
  49. 49. Figure 2. Suspension insulators Figure 3. Suspension insulatorsSynchronous CondensersA synchronous condenser is a synchronous machine running without mechanical load andsupplying or absorbing reactive power to or from a power system. Also called a synchronouscapacitor, synchronous compensator or rotating machinery.In November 1995, the first static synchronous compensator began operating at a TVAsubstation in Knoxville, Tennessee. This compensator can regulate voltage without expensiveexternal capacitors or reactors. Figure 1. Synchronous condenserTransmission Bus
  50. 50. Transmission buses are steel structure arrays of switches used to route power into asubstation. Figure 1. Transmission bus Figure 2. Transmission bus from insideVacuum Circuit BreakersA circuit breaker is a device used to complete, maintain, and interrupt currents flowing in acircuit under normal or faulted conditions. A vacuum circuit breaker utilizes a vacuum toextinguish arcing when the circuit breaker is opened and to act as a dielectric to insulate thecontacts after the arc is interrupted. One type of circuit breaker is called a recloser. Avacuum recloser is designed to interrupt and reclose an AC current circuit automatically, andcan be designed to cycle a set number of times before it must be reset manually.
  51. 51. Figure 1. Vacuum circuit breaker, inside Figure 2. Vacuum circuit breaker, outsideTransformer VaultA transformer vault is a structure or room in which power transformers, network protectors,voltage regulators, circuit breakers, meters, etc. are housed. Figure 1. An underground transformer vaultTransformer - UndergroundAn underground transformer is essentially the same as an aboveground transformer, but isconstructed for the particular needs of underground installation. Vault type, pad-mounted,submersible, and direct-buried transformers are used in underground systems. Pad-mountedtransformers are installed on a concrete pad on the surface near the end-user.
  52. 52. Figure 1. Pad-mounted transformer for underground system Figure 2. Transformer in underground vault DISTRIBUTIONDistribution Feeder CircuitsDistribution feeder circuits are the connections between the output terminals of a distributionsubstation and the input terminals of primary circuits. The distribution feeder circuitconductors leave the substation from a circuit breaker or circuit recloser via undergroundcables, called substation exit cables. The underground cables connect to a nearby overheadprimary circuit outside the substation. This eliminates multiple circuits on the poles adjacentto the substations thereby improving the overall appearance of the substation.Several distribution feeder circuits can leave a substation extending in different directions toserve customers. The underground cables are connected to the primary circuit via a nearbyriser pole.The distribution feeder bay routes power from the substation to the distribution primaryfeeder circuits.In the photo of the distribution main feeder the primary circuit is fed underground to anearby distribution system overhead line. The yellow cables are the primary feeder linesgoing underground.
  53. 53. Figure 1. 3-phase distribution feeder bay Figure 3. Distribution feeder recloser Figure 2. Distribution main feederDistribution TransformersDistribution transformers reduce the voltage of the primary circuit to the voltage required bycustomers. This voltage varies and is usually:  120/240 volts single phase for residential customers,  480Y/277 or 208Y/120 for commercial or light industry customers.Three-phase pad mounted transformers are used with an underground primary circuit and threesingle-phase pole type transformers for overhead service.Network service can be provided for areas with large concentrations of businesses. These areusually transformers installed in an underground vault. Power is then sent via underground cablesto the separate customers.
  54. 54. Figure 3. Residential distribution transformer Figure 1. Air Distribution transformer - commercial facility Figure 2. Industrial facility distribution Figure 4. Pad-mounted residential distribution transformer transformerPrimary CircuitsPrimary circuits are the distribution circuits that carry power from substations to local load areas.They are also called express feeders or distribution main feeders. The distribution feeder bay routespower from the substation to the distribution primary feeder circuits.In the photo of the distribution main feeder the primary circuit is fed underground to a nearbydistribution system overhead line. The yellow cables are the primary feeder lines goingunderground.
  55. 55. Figure 1. 3phase distribution feeder bay Figure 2. Distribution main feeder Figure 3. Overhead primary feeder Figure 4. Distribution primary feeder underbuildProtective EquipmentProtective equipment in a distribution system consists of protective relays, cutout switches,disconnect switches, lightning arresters, and fuses. These work individually or may work inconcert to open circuits whenever a short circuit, lightning strikes or other disruptive eventoccurs.When a circuit breakers opens, the entire distribution circuit is deenergized. Since this candisrupt power to many customers, the distribution system is often designed with many
  56. 56. layers of redundancy. Through redundancy, power can be shut off in portions of the systemonly, but not the entire system, or can be redirected to continue to serve customers. Only inextreme events, or failure of redundant systems, does an entire system becomedeenergized, shutting off power to large numbers of customers.The redundancy consists of the many fuses and fused cutouts throughout the system thatcan disable parts of the system but not the entire system. Lightning arresters also act locallyto drain off electrical energy from a lightning strike so that the larger circuit breakers are notactuated. Figure 1. Substation bus lightning arresters Figure 4. Pole mounted type - lightning arrester Figure 5. Air-break isolator switch Figure 2. Fused cut-out
  57. 57. Figure 6. Non load-break fuse Figure 3. Substation disconnect switch Figure 7. Load-break fuseSecondariesSecondaries are the conductors originating at the low-voltage secondary winding of a distribution transformerfor residential service are three-wire single-phase circuits. They extend along the rear lot lines, alleys, or strecustomers premises. The secondaries can be overhead lines or underground lines.Overhead secondary lines are usually strung below the primary lines and typically in a vertical plane. When sestrung in a vertical plane, they are directly attached to the support pole one above the other. This is in contraprimary lines which are often strung on a cross bar or other attachment in a horizontal or "V" shaped plane.
  58. 58. Figure 2. Secondaries in a vertical plane Figure 1. Cabled secondaries Figure 3. Cabled seconda in a "V" planServicesThe wires extending from the secondaries or distribution transformer to a customers location arecalled a service. A service can be above or below ground. Underground services have a riserconnection at the distribution pole. Commercial and residential services are much the same andcan be either 120 or 220 or both. Figure 2. Service line to residence Figure 1. Distribution system lines and associated equipment
  59. 59. Figure 3. Commercial service Figure 4. Secondary to underground service via a riserSwitchesDistribution systems have switches installed at strategic locations to redirect or cut-off power flowsfor load balancing or sectionalizing. Also, this permits repairing of damaged lines or equipment orupgrading work on the system. The many types of switches include:  circuit-breaker switches  single-pole disconnect switches  three-pole group-operated switches  pad-mounted switchgear Figure 2. Air-break isolator switch Figure 1. Air circuit-breaker switches
  60. 60. Figure 4. Single-pole disconnect switch combinedFigure 3. Circuit switchers with a fuse is called a fused cutout Figure 6. Pad mounted switchgearFigure 5. Circuit breakers
  61. 61. Figure 7. Group-operated three-pole air break switch

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