Basic Electrical Generation and Distribution
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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 (such as off-the-grid homes, cabins or recreational vehicles.) Electricity has become an integral part of life and difficult to imagine to be without it.
Distribution and Domestic Power Supply
Alternating Current is used for electric power distribution because it can easily be transformed to a higher or lower voltage. Electrical energy losses are dependent on current flow. By using transformers, the voltage can be stepped up so that the same amount of power may be distributed over long distances at lower currents and hence lower losses due to the resistance of the conductors. The voltage can also be stepped down again so it is safe for domestic supply.
Three-phase electrical generation and transmission is common and is an efficient use of conductors as the current-rating of each conductor can be fully utilized in transporting power from generation through transmission and distribution to final use. Three-phase electricity is supplied only in industrial premises and many industrial electric motors are designed for it. Three voltage waveforms are generated that are 120 degrees out of phase with each other. At the load end of the circuit the return legs of the three phase circuits can be coupled together at a 'neutral point', where the three currents sum to zero if supplied to a balanced load. This means that all the current can be carried using only three cables, rather than the six that would otherwise be needed. Three phase power is a type of polyphase system.
In most situations only a single phase is needed to supply street lights or residential consumers. When distributing three-phase electric power, a fourth or neutral cable is run in the street distribution to provide one complete circuit to each 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 consumers are connected. Thus the supply cable to each house can consist of a live and neutral conductor with possibly an earthed armoured sheath. In North America, the most common technique is to use a transformer to convert one distribution phase to a center-tapped 'split-phase' 240 V winding; the connection to the consumer is typically two 120-volt power lines out of phase with each other, and a grounded 'neutral' wire, which also acts as the physical support wire. In India there is a recent trend of providing a High Voltage line up to the residence & then stepping it down to domestic power on premises to avoid pilferage of the Energy. Although this method has certain advantages, there are obvious potential dangers associated with it.
The use of "split phase" power, two 120-volt power lines out of phase with each other, as described above, allows high-powered appliances to be run on 240 V, thus decreasing the amount of current required per phase, while allowing the rest of the residence to be wired for the safer 120 V. For example, a clothes dryer may need 3600 W of power, which translates to a circuit rating of 30 A at 120 V. If the dryer can instead be run on 240 V, the service required is only 15 A. Granted, you would then need two 15 A circuit breakers, one for each side of the circuit, and you would need to provide two 'hot' lines, one neutral, and a ground in the distribution wiring, but that is offset by the lower cost of the wires for the lower current. Houses are generally wired so that the two phases are loaded about equally; connecting the high-power appliances such as clothes dryers, kitchen ranges, and built-in space heaters across both phases helps to ensure that the loads will remain balanced across the two phases.
For safety, a third wire is often connected between the individual electrical appliances in the house and the main electric switchboard or fusebox. The third wire is known in Britain and most other English-speaking countries as the earth wire, whereas in North America it is the ground wire. At the main switchboard the earth wire is connected to the neutral wire and also connected to an earth stake or other convenient earthing point (to Americans, the grounding point) such as a water pipe. In the event of a fault, the earth wire can carry enough current to blow a fuse and isolate the faulty circuit. The earth connection also means that the surrounding building is at the same voltage as the neutral point. The most common form of electrical shock occurs when a person accidentally forms a circuit between a live conductor and ground. A residual-current circuit breaker (also called a Ground Fault Interrupter, GFI, or Ground Fault Circuit Interrupter, GFCI) is designed to detect such a problem and break the circuit 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 distribution transformer and the consumer and other parts of the system, which are also earthed, this acts to keep the neutral voltage at a safe level. This system of earthing the neutral points to balance the current flows for safety reasons is known as a multiple earth neutral system.
In households circuit breakers or fuses are used to switch off the supply of electricity quickly if the current is too large, for example there is a limit of 15 amperes in a normal 115/120 volt circuit.
- In distribution systems automatic protection is used for the same purpose. There may be two stages:
- A very fast disconnection if the problem causing the overcurrent condition is nearby, and
- A time-delayed backup operation if the overcurrent originates outside the local area.
Unfortunately in some cases this 'protection' can have a cascading effect, because the switching-off of one circuit can lead to an overload of adjacent circuits that may switch off later. "Blackouts" can be the result if further failures occur.
- There is also the problem of a power source thereby becoming disconnected from its load, causing disruption to generation and altering the balance between the amount of power needed and the amount of power available in many parts of, or the entire system.
The amount of time taken to restore generation and reestablish that balance depends on the type of generation (thermal, hydroelectric. nuclear or other) available, - after a "blackout" it can take many hours to restore the system.
Single phase electric power
The generation of AC electric power is commonly three phase, in which the waveforms of three supply conductors are offset from one another by 120°. The design of the power generators has three sets of coils placed 120 degrees apart rotating in a magnetic field. This creates three separate sine waves of electricity that are displaced from each other 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 three conductors, 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 in most distribution power grids. In North America, the single phase that is supplied is developed across a transformer coil at the utility pole (for aerial drop) or transformer pad (for underground) distribution. This single coil is center tapped and the tap is grounded. This creates a 120/240 volt system that is delivered to the customer. The voltage from either side of the coil to the center tap (ground) is 120 volts whereas the voltage between the two conductors on either end 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 two switched-mode power supplies (SMPS).
Early inverters consisted of an oscillator driving a transistor as an on/off switch, that is used to interrupt the incoming direct current to create a square wave. This is then fed through a transformer to smooth the square wave into a sine wave and to produce the required output voltage.
More efficient inverters use various methods to produce an approximate sine wave at the transformer input rather than relying on the transformer to smooth it. Capacitors can be used to smooth the flow of current into and out of the transformer. It is also possible to produce a more sinusoidal wave by having split-rail direct current inputs at two voltages, (positive and negative inputs with a central ground). By connecting the transformer input terminals in a timed sequence between the positive rail and ground, the positive rail and the negative rail, the ground rail and the negative rail, then both to the ground rail, a 'stepped sinusoid' is generated at the transformer input and the current drain on the direct current supply is less variable.
Modified Sine Wave inverters convert the (usually 12 V DC) battery voltage to high frequency (20 kHz) AC, so that a smaller transformer can be used for stepping up to a higher voltage (say 160 V) AC. This output is converted to DC at the same voltage, and then inverted again to a quasi sine wave output (about 120 V RMS). A disadvantage of the modified sine wave inverters is that the output voltage depends on the battery voltage.
It is quite difficult to obtain a good sine wave from an inverter. The quoted accuracy (harmonic distortion) for most is less than 60%, and will have an effect on the appliances connected to the output of the inverter. This might mean noisy operation in some appliances and/or damaged electric motors, because they will run less efficiently and could overheat. High end inverters (> $2,000) produce waveforms which are closer to the sine wave produced by a utility.
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 in inverters, 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 few months, and if it dries out, it cannot be repaired. However, they can provide the large surge currents which are required 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 that attempts to produce a smoothed, constant-voltage, output from a varying input voltage.
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 alternating current, usually at around 20 kHz. If the input is alternating current but at a lower frequency (such as 50 Hz or 60 Hz 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 run at 50 Hz or 60 Hz, due to hysteresis in the transformer core, and the transformer will not need to be as large or heavy. 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 used to 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 that they can accept power from most mains supplies throughout the world, with frequencies from 50 Hz to 60 Hz and voltages from 100 V to 240 V.
Unlike most other appliances, switched mode power supplies tend to be constant power devices, drawing more current as the line voltage reduces. This may cause stability problems in some situations such as emergency generator systems.
Also, maximum current draw occurs at the peaks of the waveform cycle. This means that basic switched mode power 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, which are 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 with leading 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.
- buck regulator (single inductor; output voltage < input voltage)
- boost regulator (single inductor; output voltage > input voltage)
- buckboost regulator (single inductor; output voltage can be more or less than the input voltage)
- flyback regulator (uses output transformer; allows multiple outputs and input-to-output isolation)
- forward regulator (uses output transformer; allows multiple outputs and input-to-output isolation)
- 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 in devices requiring lower torque such as electric fans, microwave ovens and other small household appliances.
Another common single-phase AC motor is the induction motor, commonly used in major appliances such as washing machines and clothes dryers. These motors can generally provide greater starting torque by using a special startup winding in conjunction with a starting capacitor and a centrifugal switch. When starting, the capacitor and special winding are temporarily connected to the power source and provide starting torque. Once the motor reaches 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 electric power to mechanical energy. They work by using a squirrel-cage rotor and a split stator that has copper shorting rings 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 a rule of thumb, motors with larger number of poles are more sensitive to harmonic distortion.
Early applications of lighting was using lamps which used a heated filament to provide light. The filament was made of tungsten and was placed inside a near vacuum glass enclosure. While it was cheap, it produced a lot of heat, so that it was inefficient too. Note that the incandescent bulb is a purely resistive load (power factor 1).
The incandescent bulb is designed to operate at high temperatures. At normal operating temperatures, a tungsten filament has a resistance nearly 20 times its room-temperature resistance. So when a bulb is turned on, it draws a current nearly 20 times the normal current until it warms up. This current surge is called the inrush current, which lasts for 30-100 milliseconds. Again, something different from the "dumb load" point of view. Thus, 5 100 W bulbs in parallel, which would consume just 500 W in normal circumstances, will have a inrush load of more than 10000 W. 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".
Another factor often overlooked in lamps is the resistance vs. time values. For an incandescent lamp, the power is proportional 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 lamp becomes 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 in order 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 power consumed. 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.
- 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 faster and require more frequent replacements. Colder filament temperatures cost more because they require more electrical 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 a luminous 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 may achieve 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. As you increase the voltage V of an incandescent light bulb, the incandescent 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.
The tungsten lamp has been replaced in most applications by fluorescent lamps. Fluorescent lamps have a power factor 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.
Early fluorescent lamps used a ballast (also called a choke coil), which was essentially an inductor to control the current in the lamp. Also, the lamp was started by using a starter, which is essentially a neon thermistor which heats up and closes a circuit. With the choke coil in series with it, the lamp has a relatively small voltage drop across it so that the starter doesn't close again. As the starter is in parallel with the lamp, the same starter can be used to start several lamps. One particularly annoying aspect of the electromagnetic ballast is the 60 Hz flicker produced. While it does not bother most people, some find it extremely irritating. Also, the electromagnetic ballast increases power consumption 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 are redundant, and they behave much better on both inverter based and utility power. Many electronic ballasts will boost 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 and televisions.
The towers of a modern computer draw their power from a SMPS, which has been detailed below. The most popular 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 power consumed by each component is pretty low, and in many cases (like printers, scanners etc.), they don't work continuously.
Control elements are the switches, dimmers, and regulators which are connected to the circuit. They are, by their very nature non linear elements and their behavior is quite complicated, and not quite well represented by their simple schematic symbols.
Light dimmers work by cutting off parts of the input sine wave. While this works for resistive loads, even here it has side effects.
Most households are on the grid, i.e. their electricity comes from a utility, which installs an energy meter on the premises. 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 cannot achieve a target set by the utility, as the transmission losses are nearly the same for both active and reactive power consumed. For home users no such rule exists, and it is interesting to see the changes in the power consumption patterns now that most of the home electricity use is not lighting, and even the lighting is by fluorescent lamps which are not resistive in nature. The utility only charges the home uses for active power, so that a low power factor is not an 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., the generation of eddy currents which oppose the change that caused it. The number of revolutions of a metal disc between the poles of an electromagnet represents the amount of energy consumed. They are more accurately described as electro-mechanical meters, as they use mechanical components like a spinning disc to measure the energy consumed.
Electronic Energy Meters
Electronic meters work by measuring the current flowing through the resistors in it at any time. The unit of measurement in the meter is the number of pulses, which is the smallest unit of energy measured by the meter. The pulses are calibrated in terms of kilowatt-hours of electricity, typically 3200 pulses per unit. Apart from the numbered wheel display found in mechanical meters, the energy consumed is also noted inside chips in the meter, so tampering can be detected.
Lightning is a very major cause for concern for a home user. Lightning consists of an immense current source which discharges itself through anything it can find. Proper lightning control and defense is very tricky, and improper methods 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 runs parallel. When lightning strikes, the pulse is almost a square wave, and the choke acts as a large resistance. At the same time, the large voltage generated causes the air to break down across the spark gap, and it acts as a short,
- Power Electronics
- Energy and Power: Production, Distribution, and Society
- Open Circuits: SMPS has more information on switching regulators
- Bob Pease. "What's All This LED Power Stuff, Anyhow?". 2008.