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Industrial power supply systems
Thank you for posting a review! We value your input. Share your review so everyone else can enjoy it too. Your review was sent successfully and is now waiting for our team to publish it. Reviews 0. All power systems have one or more sources of power. For some power systems, the source of power is external to the system but for others, it is part of the system itself—it is these internal power sources that are discussed in the remainder of this section. Direct current power can be supplied by batteries , fuel cells or photovoltaic cells. Alternating current power is typically supplied by a rotor that spins in a magnetic field in a device known as a turbo generator.
There have been a wide range of techniques used to spin a turbine's rotor, from steam heated using fossil fuel including coal, gas and oil or nuclear energy , falling water hydroelectric power and wind wind power. The speed at which the rotor spins in combination with the number of generator poles determines the frequency of the alternating current produced by the generator.
All generators on a single synchronous system, for example, the national grid , rotate at sub-multiples of the same speed and so generate electric current at the same frequency. If the load on the system increases, the generators will require more torque to spin at that speed and, in a typical power station, more steam must be supplied to the turbines driving them.
Thus the steam used and the fuel expended are directly dependent on the quantity of electrical energy supplied. An exception exists for generators incorporating power electronics such as gearless wind turbines or linked to a grid through an asynchronous tie such as a HVDC link — these can operate at frequencies independent of the power system frequency.
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Depending on how the poles are fed, alternating current generators can produce a variable number of phases of power. A higher number of phases leads to more efficient power system operation but also increases the infrastructure requirements of the system. However, there are other considerations. These range from the obvious: How much power should the generator be able to supply? What is an acceptable length of time for starting the generator some generators can take hours to start?
Is the availability of the power source acceptable some renewables are only available when the sun is shining or the wind is blowing? To the more technical: How should the generator start some turbines act like a motor to bring themselves up to speed in which case they need an appropriate starting circuit? What is the mechanical speed of operation for the turbine and consequently what are the number of poles required? What type of generator is suitable synchronous or asynchronous and what type of rotor squirrel-cage rotor, wound rotor, salient pole rotor or cylindrical rotor?
Power systems deliver energy to loads that perform a function. These loads range from household appliances to industrial machinery. Most loads expect a certain voltage and, for alternating current devices, a certain frequency and number of phases. An exception exists for larger centralized air conditioning systems as in some countries these are now typically three-phase because this allows them to operate more efficiently.
All electrical appliances also have a wattage rating, which specifies the amount of power the device consumes. At any one time, the net amount of power consumed by the loads on a power system must equal the net amount of power produced by the supplies less the power lost in transmission. Making sure that the voltage, frequency and amount of power supplied to the loads is in line with expectations is one of the great challenges of power system engineering.
However it is not the only challenge, in addition to the power used by a load to do useful work termed real power many alternating current devices also use an additional amount of power because they cause the alternating voltage and alternating current to become slightly out-of-sync termed reactive power. The reactive power like the real power must balance that is the reactive power produced on a system must equal the reactive power consumed and can be supplied from the generators, however it is often more economical to supply such power from capacitors see "Capacitors and reactors" below for more details.
A final consideration with loads is to do with power quality. In addition to sustained overvoltages and undervoltages voltage regulation issues as well as sustained deviations from the system frequency frequency regulation issues , power system loads can be adversely affected by a range of temporal issues. These include voltage sags, dips and swells, transient overvoltages, flicker, high-frequency noise, phase imbalance and poor power factor. For DC supply, the ideal is the voltage not varying from a prescribed level. Power quality issues can be especially important when it comes to specialist industrial machinery or hospital equipment.
Conductors carry power from the generators to the load. In a grid , conductors may be classified as belonging to the transmission system , which carries large amounts of power at high voltages typically more than 69 kV from the generating centres to the load centres, or the distribution system , which feeds smaller amounts of power at lower voltages typically less than 69 kV from the load centres to nearby homes and industry. Choice of conductors is based on considerations such as cost, transmission losses and other desirable characteristics of the metal like tensile strength.
Copper , with lower resistivity than aluminum , was the conductor of choice for most power systems. However, aluminum has a lower cost for the same current carrying capacity and is the primary metal used for transmission line conductors. Overhead line conductors may be reinforced with steel or aluminium alloys.
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Conductors in exterior power systems may be placed overhead or underground. Overhead conductors are usually air insulated and supported on porcelain, glass or polymer insulators. Cables used for underground transmission or building wiring are insulated with cross-linked polyethylene or other flexible insulation. Large conductors are stranded for ease of handling; small conductors used for building wiring are often solid, especially in light commercial or residential construction.
Conductors are typically rated for the maximum current that they can carry at a given temperature rise over ambient conditions. As current flow increases through a conductor it heats up. For insulated conductors, the rating is determined by the insulation.
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The majority of the load in a typical AC power system is inductive; the current lags behind the voltage. Since the voltage and current are out-of-phase, this leads to the emergence of an "imaginary" form of power known as reactive power. Reactive power does no measurable work but is transmitted back and forth between the reactive power source and load every cycle.
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This reactive power can be provided by the generators themselves, through the adjustment of generator excitation, but it is often cheaper to provide it through capacitors, hence capacitors are often placed near inductive loads to reduce current demand on the power system i. Power factor correction may be applied at a central substation, through the use of so-called "synchronous condensers" synchronous machines which act as condensers which are variable in VAR value, through the adjustment of machine excitation or adjacent to large loads, through the use of so-called "static condensers" condensers which are fixed in VAR value.
Reactors consume reactive power and are used to regulate voltage on long transmission lines. In light load conditions, where the loading on transmission lines is well below the surge impedance loading , the efficiency of the power system may actually be improved by switching in reactors. Reactors installed in series in a power system also limit rushes of current flow, small reactors are therefore almost always installed in series with capacitors to limit the current rush associated with switching in a capacitor.
Series reactors can also be used to limit fault currents. Capacitors and reactors are switched by circuit breakers, which results in moderately large steps in reactive power. A solution comes in the form of static VAR compensators and static synchronous compensators. Briefly, static VAR compensators work by switching in capacitors using thyristors as opposed to circuit breakers allowing capacitors to be switched-in and switched-out within a single cycle. This provides a far more refined response than circuit breaker switched capacitors. Static synchronous compensators take a step further by achieving reactive power adjustments using only power electronics.
Power electronics are semiconductor based devices that are able to switch quantities of power ranging from a few hundred watts to several hundred megawatts. Despite their relatively simple function, their speed of operation typically in the order of nanoseconds  means they are capable of a wide range of tasks that would be difficult or impossible with conventional technology. The classic function of power electronics is rectification , or the conversion of AC-to-DC power, power electronics are therefore found in almost every digital device that is supplied from an AC source either as an adapter that plugs into the wall see photo in Basics of Electric Power section or as component internal to the device.
HVDC is used because it proves to be more economical than similar high voltage AC systems for very long distances hundreds to thousands of kilometres. HVDC is also desirable for interconnects because it allows frequency independence thus improving system stability. Power electronics are also essential for any power source that is required to produce an AC output but that by its nature produces a DC output.
They are therefore used by many photovoltaic installations both industrial and residential.
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Power electronics also feature in a wide range of more exotic uses. They are at the heart of all modern electric and hybrid vehicles—where they are used for both motor control and as part of the brushless DC motor.
Power electronics are also found in practically all modern petrol-powered vehicles, this is because the power provided by the car's batteries alone is insufficient to provide ignition, air-conditioning, internal lighting, radio and dashboard displays for the life of the car. So the batteries must be recharged while driving using DC power from the engine—a feat that is typically accomplished using power electronics. Whereas conventional technology would be unsuitable for a modern electric car, commutators can and have been used in petrol-powered cars, the switch to alternators in combination with power electronics has occurred because of the improved durability of brushless machinery.
Some electric railway systems also use DC power and thus make use of power electronics to feed grid power to the locomotives and often for speed control of the locomotive's motor. In the middle twentieth century, rectifier locomotives were popular, these used power electronics to convert AC power from the railway network for use by a DC motor.
The use of power electronics to assist with the motor control and with starter circuits cannot be overestimated and, in addition to rectification, is responsible for power electronics appearing in a wide range of industrial machinery. Power electronics even appear in modern residential air conditioners. Power electronics are also at the heart of the variable speed wind turbine. Industrial power generation refers to applications where customers are seeking to reduce utility expenses by generating the needed electricity and heat for their operation locally. Equally important to remote installations is the requirement that the power source be highly reliable as a loss of power to certain processes such as aluminum smelting can cause enormous disruption and expense.
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