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Basic Electrical Generation and Distribution

This is a document for everyday use of electricity in a household. Many circuits are a mixture of electrical,mechanical, and electronic components, which interact in different ways to produce strange and useful effects.Topics include commercially generated AC as well as AC generated from inverters for alternative power use (suchas off-the-grid homes, cabins or recreational vehicles.) Electricity has become an integral part of life and difficult toimagine to be without it.

Distribution and Domestic Power Supply

Unlike DC, AC can be easily stepped up to a higher voltage. Because of Ohm's law (Voltage equals Current timesResistance), electrical energy losses are dependent on current flow, not on energy flow. By using transformers, thevoltage of the power can be stepped up to a high voltage so that the power may be distributed over long distancesat low currents and hence low losses due to the resistance of the conductor. The voltage can then be stepped downagain so that it is safe for domestic supply.

Three-phase electrical generation is very common and is a more efficient use of conductors as the ampere-rating ofeach conductor is not exceeded in transporting power from generation through transmission and distribution toutilization end. Three-phase electricity distribution is common only in industrial premises and many industrial electricmotors are designed for it. Three current waveforms are produced that are 120 degrees out of phase with eachother. At the load end of the circuit the return legs of the three phase circuits can be coupled together at the neutralpoint, where the three currents sum to zero when it is supplied to a balanced load. This means that the currents canbe carried using only three cables, rather than the six that would otherwise be needed. Three phase power is a kindof polyphase system.

In many situations only a single phase is needed to supply street lights or residential consumers. When distributingthree-phase electric power, a fourth or neutral cable is run in the street distribution to provide a complete circuit toeach house. Different houses in the street are placed on different phases of the supply so that the load is balanced,or spread evenly, across the three phases when a lot of consumers are connected. Thus the supply cable to eachhouse can consist of a live and neutral conductor with possibly an earthed armoured sheath. In North America, themost common technique is to use a transformer to convert one phase to a center-tapped 220V winding; theconnection to the consumer is typically two 110-volt power lines out of phase with each other, a neutral wire, and aground wire which also acts as the physical support wire. In India there is a recent trend of providing a HighVoltage line up to the residence & then stepping it down to domestic power on premises to avoid pilferage of theEnergy. Although this method has certain advantages, there are obvious potential dangers associated with it.

The use of "split phase" power, two 110-volt power lines out of phase with each other, as described above, allowshigh-powered appliances to be run on 220V, thus decreasing the amount of current required per phase, whileallowing the rest of the residence to be wired for the safer 110V. For example, a clothes dryer may need 3500Wof power, which translates to a circuit rating of 40A at 110V. If the dryer can instead be run off 220V, the servicerequired is only 20A. Granted, you would then need two 20A circuit breakers, one for each half of the powerdistribution, and you would need to provide two hot lines, one neutral, and a ground in the distribution wiring, butthat is offset by the higher costs of distributing the higher current. Additionally, houses are generally wired so that

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the two phases are drawn down about equally; connecting the high-power appliances such as clothes dryers,kitchen ranges, and built-in space heaters across both phases helps to assure that the load will remain balancedacross the two phases.

For safety, a third wire is often connected between the individual electrical appliances in the house and the mainelectric switchboard or fusebox. The third wire is known in Britain and most other English-speaking countries as theearth wire, whereas in North America it is the ground wire. At the main switchboard the earth wire is connectedto the neutral wire and also connected to an earth stake or other convenient earthing point (to Americans, thegrounding point) such as a water pipe. In the event of a fault, the earth wire can carry enough current to blow afuse and isolate the faulty circuit. The earth connection also means that the surrounding building is at the samevoltage as the neutral point. The most common form of electrical shock occurs when a person accidentally forms acircuit between a live conductor and ground. A residual-current circuit breaker (also called a Ground FaultInterrupter, GFI, or Ground Fault Circuit Interrupter, GFCI) is designed to detect such a problem and break thecircuit before electric shock causes death. As many parts of the neutral system are connected to the earth,balancing currents, known as earth currents, may flow between the generator and the consumer and other parts ofthe system, which are also earthed, to keep the neutral voltage at a safe level. This system of earthing the neutralpoint to balance the current flows for safety reasons is known as a multiple earth neutral system.

Overcurrent protection

In households circuit breakers or fuses are used to switch off the supply of electricity very quickly if the current istoo large, for example a limit of 15 Amperes in a 115 Volt circuit.

In distribution systems automatic protection systems are used for that purpose. There may be two stages:

A very fast disconnection if the problem causing the overcurrent is nearby, andA time-delayed backup operation if the overcurrent originates outside the local area.

Unfortunately in some cases that can have a cascading effect, because a switching-off of one circuit can lead toovercurrent(s) in adjacent circuits that will switch off later. "Blackouts" can be the result.

There is also the problem of the source of power generation getting disconnected from the load, causing bigproblems with the balance between the amount of power needed and the amount of power available in many

parts of, or even in the whole system.

The amount of time it will take to restore that balance depends on the kind of generation (from coal, oil, or nuclear),and after a "blackout" it may take many hours to restore that balance.

Single phase electric power

The generation of AC electric power is commonly three phase, in which the waveforms of three supply conductorsare offset from one another by 120°. The design of the power generators has three sets of coils placed 120 degreesapart rotating in a magnetic field. This creates three separate sine waves of electricity that are displaced from eachother in time by 120 degrees of rotation (1/3 of a circle). Standard frequencies of rotation are either 50 Hertz(cycles per second) in Europe or 60 Hertz in North America. The voltage across any pair of these threeconductors, or between a single conductor and ground (in a grounded system) is what is known as "single phase"electric power. Single phase power is what is commonly available to residential and light-commercial consumers inmost distribution power grids. In North America, the single phase that is supplied is developed across a transformer

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coil at the utility pole (for aerial drop) or transformer pad (for underground) distribution. This single coil is centertapped and the tap is grounded to develop two waveforms that are 180 degrees out of phase with each other with1/2 the voltage. This then creates a 120/240 volt system that is delivered to the customer. The voltage from eitherside of the coil to the center tap (ground) is 120 volts whereas the voltage between the two conductors on eitherend of the coil develops the full voltage of 240 volts.

Inverters and Battery Based AC

An inverter is a circuit for converting direct current to alternating current. An inverter can have one or twoswitched-mode power supplies (SMPS).

Early inverters consisted of an oscillator driving a transistor, that is used to interrupt the incoming direct current tocreate a square wave. This is then fed through a transformer to smooth the square wave into a sine wave and toproduce the required output voltage.

More efficient inverters use various tricks to try to get a reasonable sine wave at the transformer input rather thanrelying on the transformer to smooth it. Capacitors can be used to smooth the flow of current into and out of thetransistor. Also, it is possible to produce a more sinusoidal wave by having split-rail direct current inputs at twovoltages, or positive and negative inputs with a central ground. By connecting the transformer input terminals insequence between the positive rail and ground, the positive rail and the negative rail, the ground rail and the negativerail, then both to the ground rail, a stepped sinusoid is generated at the transformer input and the current drain onthe direct current supply is less choppy.

Modified Sine Wave inverters convert the (usually 12 V DC) battery voltage to high frequency (20 kHz) AC, sothat a small transformer can be used. This is then stepped up to a higher voltage (say 160 V) AC. This output isconverted to DC at the same voltage, and then inverted again to a quasi sine wave output (about 120 V RMS).Another disadvantage of the modified sine wave inverters is that the output voltage depends on the battery voltage.

It is quite expensive to obtain a good sine wave from an inverter. The quoted accuracy (harmonic distortion) formost is less than 60%, and this will have an effect on the appliances connected to the output of the inverter. Thisleads to noise in lot of appliances and damages electric motors, as they run significantly hotter. High end inverters (>$2,000) produce waveforms which are closer to a mathematical sine wave than those produced by the utility.

Batteries

Most home systems use conventional lead acid batteries for storage. They are cheap, and are deep cycle batteries,i.e., they can be discharged completely and charged again many times. You cannot use automobile batteries ininverters, as they are only used to provide a large starting current, and are not meant to be discharged completely.The lead acid batteries have the disadvantage that they have to be replenished with distilled water every fewmonths, and if it dries out, it cannot be repaired. However, they can provide the large surge currents which arerequired by many loads (such as induction motors) which may be connected to the system.

Switched Mode Power Supply

A switched-mode power supply, or SMPS or switching regulator, is an electronic power supply circuit thatattempts to produce a smoothed, constant-voltage, output from a varying input voltage.

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Switched-mode power supplies may be designed to convert from alternating current or direct current, or both.They generally output direct current, although an inverter is technically a switched-mode power supply.

Switched-mode power supplies operate by using an inverter to convert the input direct current supply to alternatingcurrent, usually at around 20 kHz. If the input is alternating current but at a lower frequency (such as 50 Hz or 60Hz line power) then an inverter is still used to bump the frequency up.

This high frequency means that the output transformer of the inverter will operate more efficiently than if it were runat 50 Hz or 60 Hz, due to hysteresis in the transformer core, and the transformer will not need to be as large orheavy. This high-frequency output is then fed through a rectifier to produce the output direct current.

Regulation is achieved through feedback. The output voltage is compared to a reference voltage and the result usedto alter the switching frequency or duty cycle of the inverter oscillator, which affects its output voltage.

Switched-mode PSUs in domestic products such as personal computers often have universal inputs, meaning thatthey can accept power from most mains supplies throughout the world, with frequencies from 50 Hz to 60 Hz andvoltages from 100 V to 240 V.

Unlike most other appliances, switched mode power supplies tend to be constant power devices, drawing morecurrent as the line voltage reduces. This may cause stability problems in some situations such as emergencygenerator systems.

Also, maximum current draw occurs at the peaks of the waveform cycle. This means that basic switched modepower supplies tend to produce more harmonics and have a worse power factor than other types of appliances.However, higher-quality switched-mode power supplies with power-factor correction (PFC) are available, whichare designed to present close to a resistive load to the mains.

The term power factor with respect to switched-mode supplies is misleading as it doesn't have much to do withleading or lagging voltage, but the way in which it loads the circuit (i.e. only at certain points in the cycle).

There are several types of switched-mode power supplies, classified according to the circuit topology.

1. buck regulator (single inductor; output voltage < input voltage)2. boost regulator (single inductor; output voltage > input voltage)

3. buckboost regulator (single inductor; output voltage can be more or less than the input voltage)

4. flyback regulator (uses output transformer; allows multiple outputs and input-to-output isolation)

5. forward regulator (uses output transformer; allows multiple outputs and input-to-output isolation)6. Cuk converter (uses a capacitor for energy storage; produces negative voltage for positive input)

Major Classes of Appliances

Single-Phase AC motors

The most common single-phase motor is the shaded-pole synchronous motor, which is most commonly used indevices requiring lower torque such as electric fans, microwave ovens and other small household appliances.

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Another common single-phase AC motor is the induction motor, commonly used in major appliances such aswashing machines and clothes dryers. These motors can generally provide greater starting torque by using a specialstartup winding in conjunction with a starting capacitor and a centrifugal switch. When starting, the capacitor andspecial winding are temporarily connected to the power source and provide starting torque. Once the motorreaches speed, the centrifugal switch disconnects the capacitor and startup winding.

Shaded-pole synchronous motor

Shaded-pole synchronous motors are a class of AC motor that uses single phase electric power to convert electricpower to mechanical energy. They work by using a squirrel-cage rotor and a split stator that has copper shortingrings placed on it so as to shade a portion of the stator's magnetic field enough to provide starting torque.

The number of poles in an induction motor is an important factor in its interaction with non sine wave input. As arule of thumb, motors with larger number of poles are more sensitive to harmonic distortion.

Incandescent Lamps

Early applications of lighting was using lamps which used a heated filament to provide light. The filament was madeof tungsten and was placed inside a near vacuum glass enclosure. While it was cheap, it produced a lot of heat, sothat it was inefficient too. Note that the incandescent bulb is a purely resistive load (power factor 1).

Inrush Current

The incandescent bulb is designed to operate at high temperatures. At normal operating temperatures, a tungstenfilament has a resistance nearly 20 times its room-temperature resistance. So when a bulb is turned on, it draws acurrent nearly 20 times the normal current until it warms up. This current surge is called the inrush current, whichlasts for 30-100 milliseconds. Again, something different from the "dumb load" point of view. Thus, 5 100 W bulbsin parallel, which would consume just 500 W in normal circumstances, will have a inrush load of more than 10000W. More importantly, a huge current flows, and it is important that all components on the line can carry the current.For larger lamps, a small current flows to keep it at a reasonable temperature, called the "keep alive".

Evaporation

Another factor often overlooked in lamps is the resistance vs. time values. For an incandescent lamp, the power isproportional to the area. The tungsten slowly evaporates as the bulb ages, so that the power (and hence the light)produced by the lamp drops. Further, the light drops at about 5 times the rate of the power drop, so that the lampbecomes very inefficient with age.

After running for 75% of its rated life, an incandescent lamp must produce more than 93% of its initial light output inorder to pass the standard test described in IEC Publication 60064.

Voltage and Efficiency

The efficiency of an incandescent lamp is measured in terms of the amount of light produced per watt of powerconsumed. As the temperature of the lamp decreases, the light output per watt decreases. Thus, at a lower voltage(brownout), the efficiency of the lamp is very low.

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saying that the efficiency is "very low" is purely subjective - "very low" compared to what?

The tungsten filament normal operating temperature is selected to minimize the net cost of running lighting fixtures,balancing efficiency and lifetime. Hotter filament temperatures cost more because they wear out the filament fasterand require more frequent replacements. Colder filament temperatures cost more because they require moreelectrical power for a given amount of visible light.

The luminous efficiency of any black-body radiator increases with temperature up to 6300 °C (6600 K or 11,500°F). Tungsten melts at 3695 K (6192°F), where it, like any black-body radiator, would theoretically have aluminous efficiency of 52 lumens per watt.

A 50-hour-life projection bulb is designed to operate at 50 °C (90 °F) below that melting point, where it mayachieve up to 22 lumens/watt.

A 1000 hour lifespan general service bulb typically operates at 2000 K to 3300 K (about 3100-5400°F),

achieving 10 to 17 lumens/watt[citation needed]. As you increase the voltage V of an incandescent light bulb, theincandescent bulb puts out more light -- proportional to the fourth power of V -- but the life of the incandescent

bulb is then decreased by the eighth power of V.[1]

Fluorescent Lamps

The tungsten lamp has been replaced in most applications by fluorescent lamps. Fluorescent lamps have a powerfactor close to 0.25. Fluorescent lamps typically rate about 40 W, and they provide much more (about 5 times)light compared to an incandescent lamp of the same wattage. they also give out less heat.

Passive Control

Early fluorescent lamps used a ballast (also called a choke coil), which was essentially an inductor to control thecurrent in the lamp. Also, the lamp was started by using a starter, which is essentially a neon thermistor which heatsup and closes a circuit. With the choke coil in series with it, the lamp has a relatively small voltage drop across it sothat the starter doesn't close again. As the starter is in parallel with the lamp, the same starter can be used to startseveral lamps. One particularly annoying aspect of the electromagnetic ballast is the 60 Hz flicker produced. Whileit does not bother most people, some find it extremely irritating. Also, the electromagnetic ballast increases powerconsumption by about 25% when on utility power.

Active (Electronic) Control

Modern lamps use electronic circuits to control the current, so that both the starter and the choke coil areredundant, and they behave much better on both inverter based and utility power. Many electronic ballasts willboost the frequency to something in the range of 20 kHz, so that there is no flicker problem.

CRT Based Appliances

The other major source of power consumption are CRTs (Cathode Ray Tubes) like computer monitors andtelevisions.

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Computer Towers

The towers of a modern computer draw their power from a SMPS, which has been detailed below. The mostpopular computers today (running P4s and 3D cards) consume several hundred watts of power.

Other Electronic Loads

Other electronic items in a household draw their power from the mains using a wall wart. The steady state powerconsumed by each component is pretty low, and in many cases (likeprinters, scanners etc.), they don't workcontinuously.

Control Elements

Control elements are the switches, dimmers, and regulators which are connected to the circuit. They are, by theirvery nature non linear elements and their behavior is quite complicated, and not quite well represented by theirsimple schematic symbols.

Light Dimmers

Light dimmers work by cutting off parts of the input sine wave. While this works for resistive loads, even here it hasside effects.

Energy Meters

Most households are on the grid, i.e. their electricity comes from a utility, which installs an energy meter on thepremises. The meter is then read either manually or by phone line connection to the utility offices.

The utility wants your power factor to be as close to 1 as possible, and businesses are penalized if they cannotachieve a target set by the utility, as the transmission losses are nearly the same for both active and reactive powerconsumed. For home users no such rule exists, and it is interesting to see the changes in the power consumptionpatterns now that most of the home electricity use is not lighting, and even the lighting is by fluorescent lamps whichare not resistive in nature. The utility only charges the home uses for active power, so that a low power factor is notan issue from an economic perspective, and transmission losses within the household are negligible.

Mechanical Energy Meters

Mechanical energy meters are discussed in high school physics books as applications of Lenz's law, viz., thegeneration of eddy currents which oppose the change that caused it. The number of revolutions of a metal discbetween the poles of an electromagnet represents the amount of energy consumed. They are more accuratelydescribed as electro-mechanical meters, as they use mechanical components like a spinning disc to measure theenergy consumed.

Electronic Energy Meters

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Electronic meters work by measuring the current flowing through the resistors in it at any time. The unit ofmeasurement in the meter is the number of pulses, which is the smallest unit of energy measured by the meter. Thepulses are calibrated in terms of kilowatt-hours of electricity, typically 3200 pulses per unit. Apart from thenumbered wheel display found in mechanical meters, the energy consumed is also noted inside chips in the meter,so tampering can be detected.

Lightning

Lightning is a very major cause for concern for a home user. Lightning consists of an immense current source whichdischarges itself through anything it can find. Proper lightning control and defense is very tricky, and impropermethods can increase the risk to man and machine.

A simple lightning arrestor consists of a choke which is in series with the loads. A spark gap which is grounded runsparallel. When lightning strikes, the pulse is almost a square wave, and the choke acts as a large resistance. At thesame time, the large voltage generated causes the air to break down across the spark gap, and it acts as a short,

See also

Power Electronics

Energy and Power: Production, Distribution, and Society

Open Circuits: SMPS (http://opencircuits.com/SMPS) has more information on switching regulators

References

1. ↑ Bob Pease. "What's All This LED Power Stuff, Anyhow?"

(http://electronicdesign.com/article/components/what-s-all-this-led-power-stuff-anyhow-18278.aspx). 2008.

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