AR-461: BUILDING SCIENCE
Department of Architecture and Planning
LECTURE NO. 20
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current. Electrical networks that consist only of sources (voltage or current), linear lumped elements (resistors, capacitors, inductors), and linear distributed elements (transmission lines) can be analyzed by algebraic and transform methods to determine DC response, AC response, and transient response. A network that contains active electronic components is known as an electronic circuit. Such networks are generally nonlinear and require more complex design and analysis tools.
DESIGN METHODS FOR ELECTRICAL CIRCUIT:
To design any electrical circuit, either analog or digital, electrical engineers need to be able to predict the voltages and currents at all places within the circuit. Linear circuits, that is, circuits with the same input and output frequency, can be analyzed by hand using complex number theory. Other circuits can only be analyzed with specialized software programs or estimation techniques such as the piecewise-linear model. Circuit simulation software, such as HSPICE, and languages such as VHDL-AMS and verilog-AMS allow engineers to design circuits without the time, cost and risk of error involved in building circuit prototypes.
A number of electrical laws apply to all electrical networks. These include:
Kirchhoff's current law: The sum of all currents entering a node is equal to the sum of all currents leaving the node.
Kirchhoff's voltage law: The directed sum of the electrical potential differences around a loop must be zero.
Ohm's law: The voltage across a resistor is equal to the product of the resistance and the current flowing through it (at constant temperature).
Norton's theorem: Any network of voltage and/or current sources and resistors is electrically equivalent to an ideal current source in parallel with a single resistor.
Thévenin's theorem: Any network of voltage and/or current sources and resistors is electrically equivalent to a single voltage source in series with a single resistor.
Other more complex laws may be needed if the network contains nonlinear or reactive components. Non-linear self-regenerative heterodyning systems can be approximated. Applying these laws results in a set of simultaneous equations that can be solved either algebraically or numerically.
Electrical elements are conceptual abstractions representing idealized electrical components, such as resistors, capacitors, and inductors, used in the analysis of electrical networks. Any electrical network can be analysed as multiple, interconnected electrical elements in a schematic diagram or circuit diagram, each of which affects the voltage in the network or current through the network. These ideal electrical elements represent real, physical electrical or electronic components but they do not exist physically and they are assumed to have ideal properties according to a lumped element model, while components are objects with less than ideal properties, a degree of uncertainty in their values and some degree of nonlinearity, each of which may require a combination of multiple electrical elements in order to approximate its function. Circuit analysis using electric elements is useful for understanding many practical electrical networks using components. By analyzing the way a network is affected by its individual elements it is possible to estimate how a real network will behave.
A simple electric circuit made up of a voltage source and a resistor.
Here, V = iR, according to Ohm's Law.
SINGLE-PHASE ELECTRIC POWER:
In electrical engineering, single-phase electric power refers to the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors. A single-phase supply connected to an alternating current electric motor does not produce a revolving magnetic field; single-phase motors need additional circuits for starting, and such motors are uncommon above 10 or 20 kW in rating. In contrast, in a three-phase system, the currents in each conductor reach their peak instantaneous values sequentially, not simultaneously; in each cycle of the power frequency, first one, then the second, then the third current reaches its maximum value. The waveforms of the three supply conductors are offset from one another in time (delayed in phase) by one-third of their period. Standard frequencies of single-phase power systems are either 50 or 60 Hz. Special single-phase traction power networks may operate at 16.67 Hz or other frequencies to power electric railways.
Single-phase Polemount Stepdown Transformer
Single-phase power distribution is widely used especially in rural areas, where the cost of a three-phase distribution network is high and motor loads are small and uncommon. High power systems, say, hundreds of kVA or larger, are nearly always three phase. The largest supply normally available as single phase varies according to the standards of the electrical utility. In the
a single-phase household supply may be rated 100 A or even 125 A, meaning that there is little need for 3 phase in a domestic or small commercial environment. Much of the rest of Europe has traditionally had much smaller limits on the size of single phase supplies resulting in even houses being supplied with 3 phase (in urban areas with three-phase supply networks). UK
In North America, individual residences and small commercial buildings with services up to about 100 kV·A (417 amperes at 240 volts) will usually have three-wire single-phase distribution, often with only one customer per distribution transformer. In exceptional cases larger single-phase three-wire services can be provided, usually only in remote areas where poly-phase distribution is not available. In rural areas farmers who wish to use three-phase motors may install a phase converter if only a single-phase supply is available. Larger consumers such as large buildings, shopping centers, factories, office blocks, and multiple-unit apartment blocks will have three-phase service. In densely populated areas of cities, network power distribution is used with many customers and many supply transformers connected to provide hundreds or thousands of kV·A, a load concentrated over a few hundred square meters. Three-wire single-phase systems are rarely used in the
where large loads are needed off only two high voltage phases. Single-phase power may be used for electric railways; the largest single-phase generator in the world, at Neckarwestheim Nuclear Power Plant, supplies a railway system on a dedicated traction power network. UK
Typically a third conductor, called ground (or "safety ground") (U.S.) or protective earth (Europe, IEC), is used as a protection against electric shock, and ordinarily only carries significant current when there is a circuit fault. Several different earthing systems are in use.
THREE-PHASE ELECTRIC POWER:
Three-phase electric power is a common method of alternating current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other large loads. A three-phase system is generally more economical than others because it uses less conductor material to transmit electric power than equivalent single-phase or two-phase systems at the same voltage. The three-phase system was introduced and patented by Nikola Tesla in the years from 1887 to 1888.
Three-phase transformer with four wires output for 208Y/120 volt service:
one wire for neutral, others for A, B and C phases
one wire for neutral, others for A, B and C phases
In a three-phase system, three circuit conductors carry three alternating currents (of the same frequency) which reach their instantaneous peak values at different times. Taking one conductor as the reference, the other two currents are delayed in time by one-third and two-thirds of one cycle of the electric current. This delay between phases has the effect of giving constant power transfer over each cycle of the current and also makes it possible to produce a rotating magnetic field in an electric motor.
Three-phase systems may have a neutral wire. A neutral wire allows the three-phase system to use a higher voltage while still supporting lower-voltage single-phase appliances. In high-voltage distribution situations, it is common not to have a neutral wire as the loads can simply be connected between phases (phase-phase connection).
Three-phase has properties that make it very desirable in electric power systems:
The phase currents tend to cancel out one another, summing to zero in the case of a linear balanced load. This makes it possible to eliminate or reduce the size of the neutral conductor; all the phase conductors carry the same current and so can be the same size, for a balanced load.
Power transfer into a linear balanced load is constant, which helps to reduce generator and motor vibrations.
Three-phase systems can produce a magnetic field that rotates in a specified direction, which simplifies the design of electric motors. Three is the lowest phase order to exhibit all of these properties.
Most household loads are single-phase. In
North America and some other countries, three-phase power generally does not enter homes. Even in areas where it does, it is typically split out at the main distribution board and the individual loads are fed from a single phase. Sometimes it is used to power electric stoves and electric clothes dryers. The three phases are typically indicated by colors which vary by country.
THREE-PHASE ELECTRIC POWER TRANSMISSION
GENERATION AND DISTRIBUTION:
At the power station, an electrical generator converts mechanical power into a set of three alternating electric currents, one from each coil (a.k.a. "winding") of the generator. The windings are arranged such that the currents vary sinusoidally at the same frequency but with the peaks and troughs of their wave forms offset to provide three complementary currents with a phase separation of one-third cycle (120° or 2π/3 radians). The generator frequency is typically 50 or 60 Hz, varying by country. (See Mains power systems for more detail.)
Large power generators provide an electric current at a potential which can be a few hundred volts or up to about 30 kV. At the power station, transformers step this voltage up to one suitable for transmission. After numerous further conversions in the transmission and distribution network, the power is finally transformed to the standard utilization voltage for lighting and equipment. Single-phase loads are connected from one phase to neutral or between two phases. Three-phase loads such as larger motors must be connected to all three phases of the supply.
ANIMATION OF THREE-PHASE CURRENT FLOW
CONNECTING SINGLE-PHASE LOADS TO A THREE-PHASE SYSTEM:
Single-phase loads may be connected to a three-phase system in two ways. Either a load may be connected across two of the live conductors, or a load can be connected from a live phase conductor to the neutral conductor. Single-phase loads should be distributed evenly between the phases of the three-phase system for efficient use of the supply transformer and supply conductors. If the line-to-neutral voltage is a standard load voltage, for example 230 volt on a 400 volt three-phase system, single-phase loads can connect to a phase and the neutral. Loads can be distributed over three phases to balance the load. Where the line-to-neutral voltage is not the standard voltage for example 347 volts produced by a 600 V system, single-phase loads are connected through a step-down transformer.
In a symmetrical three-phase system, the system neutral has the same magnitude of voltage to each of the three-phase conductors. The voltage between line conductors (Vl) is √3 times the phase conductor to neutral voltage (Vp). That is: Vl = √3Vp.
In some multiple-unit residential buildings of
North America, three-phase power is supplied to the building but individual units have only single-phase power formed from two of the three supply phases. Lighting and convenience receptacles are connected from either phase conductor to neutral, giving the usual 120 V required by typical North American appliances. In the split-phase system, high-power loads are connected between the opposite "hot" poles, giving a voltage of 240 V. In some cases, they may be connected between phases of a three-phase system, giving a voltage of 208 V. This practice is common enough that 208 V single-phase equipment is readily available in North America. Attempts to use the more common 120/240 V equipment intended for split-phase distribution may result in poor performance since 240 V heating and lighting equipment will only produce 75% of its rating when operated at 208 V. Motors rated at 240 V will draw higher current at 208 V; some motors are dual-labelled for both voltages.
Where three-phase at low voltage is otherwise in use, it may still be split out into single-phase service cables through joints in the supply network or it may be delivered to a master distribution board (breaker panel) at the customer's premises. Connecting an electrical circuit from one phase to the neutral generally supplies the country's standard single phase voltage (120 V AC or 230 V AC) to the circuit.
The currents returning from the customers' premises to the supply transformer all share the neutral wire. If the loads are evenly distributed on all three phases, the sum of the returning currents in the neutral wire is approximately zero. Any unbalanced phase loading on the secondary side of the transformer will use the transformer capacity inefficiently.
If the supply neutral of a three-phase system with line-to-neutral connected loads is broken, the voltage balance on the loads will no longer be maintained. The neutral point will tend to drift toward the most heavily loaded phase, causing undervoltage conditions on that phase only. Correspondingly, the lightly loaded phases may approach the line-to-line voltage, which exceeds the line-to-neutral voltage by a factor of √3, causing overheating and failure of many types of loads.
For example, if several houses are connected through a 240 V transformer, which is connected to one phase of the three-phase system, each house might be affected by the imbalance on the three phase system. If the neutral connection is broken somewhere in the system, all equipment in a house might be damaged due to over-voltage. A similar phenomenon can exist if the house neutral (connected to the center tap of the 240 V pole transformer) is disconnected. This type of failure event can be difficult to troubleshoot if the drifting neutral effect is not understood. With inductive and/or capacitive loads, all phases can suffer damage as the reactive current moves across abnormal paths in the unbalanced system, especially if resonance conditions occur. For this reason, neutral connections are a critical part of a power distribution network and must be made as reliable as any of the phase connections. Where a mixture of single-phase 120-volt lighting and three-phase, 240-volt motors are to be supplied, a system called high-leg delta is used.
ELECTRIC POWER DISTRIBUTION HISTORY:
In the early days of electricity generation, direct current (DC) generators would be connected to loads at the same voltage. The generation, transmission and loads all needed to be of the same voltage because, at the time, there was not a common way of doing DC voltage conversion (other than motor-generator sets which today have became super efficient). The voltages usually had to be fairly low with old generation systems due to the difficulty and danger of distributing high voltages to small loads. The losses in a line transmission cable are proportional to the square of the current, the length of the cable, and the resistive nature of the conductor line wire material, and are inversely proportional to cross-sectional area. Early power transmission networks were already using copper, which is one of the best conductors that is also very economically feasible for this application. To reduce the current while keeping power transmission constant requires increasing the voltage which, as previously mentioned, was, at that time, problematic. This meant in order to keep losses to a reasonable level the (DC)
Edison power transmission system needed thick cables and local power generators.
SIMPLIFIED DIAGRAM OF AC ELECTRICITY DISTRIBUTION
FROM GENERATION STATIONS TO CONSUMERS
FROM GENERATION STATIONS TO CONSUMERS
ALTERNATING CURRENT (AC) BECOMES MOST COMMON STANDARD:
Soon, the adoption of alternating current (AC) for electricity generation dramatically changed the situation. Power transformers, installed at power 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 power transmission and distribution lines. Thus the size of conductors required and distribution losses incurred were also reduced. This made it more economic to distribute power over long distances. The ability to transform to extra-high voltages enabled power generators to be located far from loads with transmission systems to interconnect generating stations and distribution networks.
Though due to power line losses, it is still often valuable to locate the power generators nearby the actual power load.
North America, the early power 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 2400/4160 Y three-phase systems, which also benefited from better surge suppression due to the grounded neutral. Some city and suburban power distribution systems continue to use this range of voltages, but most have been converted to 7200/12470Y.
European systems used higher voltages, generally 3300 volts to ground, in support of the 220/380Y volt power systems used in those countries. In the
, urban power generation and transmission systems progressed to 6.6 kV and then upgraded to 11 kV (phase to phase), the most common power distribution voltage. UK
North American and European power distribution systems also differ in that North American power distribution systems tend to have a greater number of low-voltage step-down transformers located closer to customers' premises. For example, in the
US a pole-mounted transformer in a suburban area may supply only one or a very few houses or small businesses, whereas in the a typical urban or suburban low-voltage substation might be rated at 2MW of power and supply a whole neighborhood. This is because the higher voltage used in UK Europe (230V vs 120V) may be carried over a greater distance without an unacceptable power loss. An advantage of the North American setup is that failure or maintenance on a single power transformer will only affect a few customers. Advantages of the setup are that fewer transformers are required; larger and more efficient transformers are used, and due to diversity there need be less spare capacity in the transformers, reducing power wastage. UK
Rural power electrification systems, in contrast to urban power systems, tend to use higher voltages because of the longer distances covered by those power distribution lines. 7200 volts is commonly used in the
United States; 11kV and 33kV are common in the UK, New Zealand and Australia; 11kV and 22kV are common in . Other voltages are occasionally used in unusual situations or where a local utility simply has engineering practices that differ from the normal practices South Africa
GENERAL LAYOUT OF ELECTRICITY NETWORKS
POWER DISTRIBUTION NETWORK LAYOUT:
Power distribution networks are typically arranged out in one of two types, radial or interconnected. A radial network leaves the station and passes through the network area with no connection to any other supply. This is typical of long rural lines with isolated load areas. An interconnected network is generally found in more urban areas and will have multiple connections to other points of supply.
These points of connection are normally open but allow various configurations by closing and opening switches. The benefit of the interconnected model is that in the event of a fault or required maintenance a small area of network can be isolated and the remainder kept on supply. The only downside to this design occurs when there is a major power outage that causes a domino effect damaging the power supply systems from the whole network leaving more customers without power. There are protections in place to keep this from happening though it still occurs every few years in places where this method of power distribution and transmission is used.
Characteristics of the supply given to customers are generally mandated by law and by contract between the electric power supplier and customer. 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 aluminum smelting either operate their own generating equipment or have equipment to derive DC from the public AC supply).
Electrical wiring in general refers to insulated conductors used to carry electricity, and associated devices. This article describes general aspects of electrical wiring as used to provide power in buildings and structures, commonly referred to as building wiring. This article is intended to describe common features of electrical wiring that may apply worldwide. For information regarding specific national electrical codes, refer to the articles mentioned in the next section. Separate articles cover long-distance electric power transmission and electric power distribution.
Materials for wiring interior electrical systems in buildings vary depending on:
- Intended use and amount of power demand on the circuit
- Type of occupancy and size of the building
- National and local regulations
- Environment in which the wiring must operate
Wiring systems in a single family home or duplex, for example, are simple, with relatively low power requirements, infrequent changes to the building structure and layout, usually with dry, moderate temperature, and non-corrosive environmental conditions. In a light commercial environment, more frequent wiring changes can be expected, large apparatus may be installed, and special conditions of heat or moisture may apply. Heavy industries have more demanding wiring requirements, such as very large currents and higher voltages, frequent changes of equipment layout, corrosive, or wet or explosive atmospheres. In facilities that handle flammable gases or liquids, special rules may govern the installation and wiring of electrical equipment in hazardous areas.
Wires and cables are rated by the circuit voltage, temperature rating, and environmental conditions (moisture, sunlight, oil, chemicals) in which they can be used. A wire or cable has a voltage (to neutral) rating, and a maximum conductor surface temperature rating. The amount of current a cable or wire can safely carry depends on the installation conditions.
 Electrical Network; From: http://en.wikipedia.org/wiki/Electrical_network (Retrieved April 23, 2011)
 Electrical Element; From: http://en.wikipedia.org/wiki/Electrical_element (Retrieved April 23, 2011)
 Three Phase Electric Power; From: http://en.wikipedia.org/wiki/Three-phase_electric_power#Single-phase_loads (Retrieved April 23, 2011)
 William D. Stevenson, Jr. Elements of Power System Analysis Third Edition, McGraw-Hill, New York (1975). ISBN 0070612854. Page 2.
 Single-Phase Electric Power; From: http://en.wikipedia.org/wiki/Single-phase_electric_power (Retrieved April 28, 2011)
 Three-Phase Electric Power; From: http://en.wikipedia.org/wiki/Three-phase_electric_power (Retrieved April 28, 2011)
 History and Development of Electric Power Distribution; From: http://www.3phasepower.org/3phasedistribution.htm (Retrieved April 28, 2011)