Elements of the AC Power System
The process of generating, distributing, and controlling the large amounts of power required for a municipality or geographic area is highly complex. However, each system, regardless of its complexity, is composed of the same basic elements with the same basic goal: deliver ac power where it is needed by customers. The primary elements of an ac power system can be divided into the following general areas of technology:
• Power generators
• Power transformers
• Capacitors
• Transmission circuits
• Control and switching systems, including voltage regulators, protection devices, and fault isolation devices
The path that electrical power takes to end users begins at a power plant, where electricity is generated by one of several means and is then stepped-up to a high voltage (500 kV is common) for transmission on high-tension lines. Step-down transformers reduce the voltage to levels appropriate for local distribution and eventual use by customers. Figure 1.7 shows how these elements interconnect to provide ac power to consumers.
Transmission Circuits
The heart of any utility power-distribution system is the cable used to tie distant parts of the network together. Conductors are rated by the American Wire Gauge (AWG) scale. The smallest is no. 36, and the largest is no. 0000. There are 40 sizes in between. Sizes larger than no. 0000 AWG are specified in thousand circular mil units, referred to as “MCM” units (M is the Roman numeral expression for 1000). The cross- sectional area of a conductor doubles with each increase of three AWG sizes. The diameter doubles with every six AWG sizes.
Most conductors used for power transmission are made of copper or aluminum. Copper is the most common. Stranded conductors are used where flexibility is required. Stranded cables usually are more durable than solid conductor cables of the same AWG size. For long distances, utilities typically use unin- sulated aluminum conductors or aluminum conductor steel-reinforced cables. For shorter distances, insulated copper wire normally is used.
Ampacity is the measure of the ability of a conductor to carry electric current. Although all metals will conduct current to some extent, certain metals are more efficient than others. The three most com- mon high-conductivity conductors are
• Silver, with a resistivity of 9.8 Ω/circular mil-ft
• Copper, with a resistivity of 10.4 Ω/circular mil-ft
• Aluminum, with a resistivity of 17.0 Ω/circular mil-ft
The ampacity of a conductor is determined by the type of material used, the cross-sectional area, and the heat-dissipation effects of the operating environment. Conductors operating in free air will dissipate heat more readily than conductors placed in a larger cable or in a raceway with other conductors will. Table 1.1 lists the principal parameters of common wire sizes.
Types of Conductors
Wire and cable designed for use in a power-distribution system can be roughly divided into two catego- ries:
• Overhead conductors
• Underground cables
Each will be examined in the following sections.
Overhead Conductors
Many different types of conductors are used on overhead distribution lines [3]. They vary in both size and number, depending on the voltage level and the type of circuit. Copper, aluminum, and steel are the most commonly used materials for overhead lines. Copper is used in three forms: hard drawn, medium-hard drawn, and soft drawn or annealed. Hard-drawn copper has the greatest strength and is used for circuits of relatively long spans (200 ft or more). However, its inflexibility makes it harder to
work with. The soft-drawn variety is the weakest of the copper conductors. Its use is limited to short spans. The medium-hard-drawn copper conductor has found widespread use in medium-range distribution circuits.
Steel wire is only about one tenth as good a conductor as copper and, hence, is rarely used alone. However, it offers an economic advantage over the other types of conductors. Also, because steel wire is much stronger than copper, it permits longer spans and requires fewer supports.
Aluminum is only 60 to 80% as good a conductor as copper and only half as strong as copper. How- ever, its property of lighter weight, as compared to copper and steel, and its relative advantage in trans- mitting ac power because of reduced skin effect makes it suitable for overhead lines. Usually, the aluminum wires are stranded on a core of steel wire to form what is termed an aluminum conductor steel- reinforced (ACSR) conductor. The more strands in the ACSR conductor, the greater flexibility it will have. Hence, the larger conductors used today are all stranded and twisted in layers concentrically around a central steel wire.
Table 1.2 lists the characteristics of various conductors that are typically used on overhead distribution lines.
Underground Cables
Underground construction of distribution lines is designed mostly for urban areas and is dictated by economics, congestion, and density of population [3]. Although overhead lines have been ordinarily considered to be less expensive and easier to maintain, developments in underground cable and construction technology have narrowed the cost gap to the point where such systems are competitive in many urban and suburban residential installations.
The conductors used underground are different from overhead lines in that they are insulated for their entire length, and several of them may be combined under one protective sheath. The whole assem- bly is called an electric cable. These cables are either buried directly in the ground, or they may be installed in ducts buried in the ground. The conductors in cables are usually made of copper or aluminum and are usually stranded. They are made of soft-drawn copper because they do not have to support any appreciable weight. Cables can be either single conductor or multiple conductors enclosed in a single sheath for economy.
Skin Effect
The effective resistance offered by a conductor to high frequencies is considerably greater than the ohmic resistance measured with direct currents (dc). This is because of an action known as the skin effect, which causes the currents to be concentrated in certain parts of the conductor and leaves the remainder of the cross section to contribute little toward carrying the applied current.
When a conductor carries an alternating current, a magnetic field is produced that surrounds the wire. This field continually is expanding and contracting as the ac current wave increases from zero to its maximum positive value and back to zero, then through its negative half-cycle. The changing magnetic lines of force cutting the conductor induce a voltage in the conductor in a direction that tends to retard the normal flow of current in the wire. This effect is more pronounced at the center of the conductor. Thus, current within the conductor tends to flow more easily toward the surface of the wire. The higher the frequency, the greater the tendency for current to flow at the surface. The depth of current flow is a function of frequency and is determined from
It can be calculated that at a frequency of 100 kHz, current flow penetrates a conductor by 8 mils. At 1 MHz, the skin effect causes current to travel in only the top 2.6 mils in copper, and even less in almost all other conduc- tors. Therefore, the series impedance of conductors at high frequencies is significantly higher than at low fre- quencies. Figure 1.8 shows the distribution of current in a radial conductor.
When a circuit is operating at high frequencies, the skin effect causes the current to be redistributed over the conductor cross section in such a way as to make most of the current flow where it is encircled by the smallest number of flux lines. This general principle controls the distribution of current regardless of the shape of the conductor involved. With a flat-strip conductor, the current flows primarily along the edges, where it is surrounded by the smallest amount of flux.
It is evident from Equation 1.9 that the skin effect is minimal at power-line frequencies for copper conductors. For steel conductors at high current, however, skin effect considerations are often important.
Dielectrics and Insulators
Dielectrics are materials that are used primarily to isolate components electrically from each other or ground or to act as capacitive elements in devices, circuits, and systems [6]. The insulating properties of dielectrics are directly attributable to their large energy gap between the highest filled valence band and the conduction band. The number of electrons in the conduction band is extremely low because the energy gap of a dielectric (5 to 7 eV) is sufficiently large to maintain most of the electrons trapped in the lower band. As a consequence, a dielectric subjected to an electric field will allow only an extremely small conduction or loss current. This current will be caused by the following:
• The finite number of free electrons available
• Other free charge carriers (ions) typically associated with contamination by electrolytic impurities
• Dipole orientation losses arising with polar molecules under ac conditions
Often, the two latter effects will tend to obscure the minuscule contribution of the relatively few free electrons available. Unlike solids and liquids, vacuum and gases (in their nonionized state) approach the conditions of a perfect insulator — i.e., they exhibit virtually no detectable loss or leakage current.
Two fundamental parameters that characterize a dielectric material are its conductivity σ and the value of the real permittivity or dielectric constant ε’. By definition, σ is equal to the ratio of the leakage current density Jl to the applied electric field E
Because Jl is in A cm–2 and E is in V cm–1, the corresponding units of σ are in S cm–1 or Ω –1 cm–1. Under ac conditions, dielectric losses arise mainly from the movement of free charge carriers (electrons and ions), space charge polarization, and dipole orientation. Ionic, space charge, and dipole losses are temperature and frequency dependent, a dependency that is reflected in the measured values of σ and ε’. This necessitates the introduction of a complex permittivity e defined by ε = ε’ – jε”, where ε” is the imaginary value of the permittivity.
As the voltage is increased across a dielectric material, a point is ultimately reached beyond which the insulation will no longer be capable of sustaining any further rise in voltage and breakdown will ensue, causing a short circuit to develop between the electrodes. If the dielectric consists of a gas or liquid medium, the breakdown will be self-healing in the sense that the gas or liquid will support anew a reap- plication of voltage. In a solid dielectric, however, the initial breakdown will result in a formation of a permanent conductive channel, which cannot support a reapplication of full voltage.
The breakdown strength of a dielectric under dc and impulse conditions tends to exceed that at ac fields, thereby suggesting the ac breakdown process is partially of a thermal nature. An additional factor, which may lower the ac breakdown strength, is that associated with the occurrence of partial discharges either in void inclusions or at the electrode edges. This leads to breakdown values much lower than the intrinsic value. In practice, breakdown values are generally of an extrinsic nature, and the intrinsic values are useful conceptually insofar as they provide an idea of an upper value that can be attained only under ideal conditions.
All insulating materials will undergo varying degrees of aging or deterioration under normal operating conditions. The rate of aging will be contingent upon the magnitude of the electrical, thermal, and mechanical stresses to which the material is subjected. It will also be influenced by the composition and molecular structure of the material itself, as well as the chemical, physical, and radiation environment under which the material must operate. The useful life of an insulating system will, thus, be determined by a given set and subset of aging variables. For example, the subset of variables in the voltage stress
variable are the average and maximum values of the applied voltage, its frequency, and the recurrence rate of superposed impulse or transient voltage surges. For the thermal stress, the upper and lower ambient temperatures, the temperature gradient in the insulation, and the maximum permissible operating temperature constitute the subvariable set. In addition, the character of the mechanical stress will differ, depending upon whether torsion, compression, or tension and bending are involved.
Furthermore, the aging rate will be differently affected if all stresses (electrical, thermal, and mechanical) act simultaneously, separately, or in some predetermined sequence. The influence exerted on the aging rate by the environment will depend on whether the insulation system will be subjected to corrosive chemicals, petroleum fluids, water or high humidity, air or oxygen, ultraviolet radiation from the sun, and nuclear radiation. Organic insulations, in particular, may experience chemical degradation in the presence of oxygen. For example, polyethylene under temperature cycle will undergo both physical and chemical changes. These effects will be particularly acute at high operating temperatures (90 to 130°C). At these temperatures, partial or complete melting of the polymer will occur, and the increased diffusion rate will permit the oxygen to migrate to a greater depth into the polymer. Ultimately, the antioxidant will be consumed, resulting in an embrittlement of the polymer and, in extreme cases, in the formation of macroscopic cracks. Subjection of the polymer to many repeated overload cycles will be accompanied by repeated melting and recrystallization of the polymer — a process that will inevitably cause the formation of cavities, which, when subjected to sufficiently high voltages, will undergo discharge, leading eventually to electrical breakdown.
The 60 Hz breakdown strength of a 1 cm gap of air at 25°C at atmospheric pressure is 31.7 kV cm–1. Although this is a relatively low value, air is a most useful insulating medium for large electrode separations, as is the case for overhead transmission lines. The only dielectric losses in the overhead lines are those resulting from corona discharges at the line conductor surfaces and leakage losses over the insulator surfaces. In addition, the highly reduced capacitance between the conductors of the lines ensures a small capacitance per unit length, thus rendering overhead lines an efficient means for transmitting large amounts of power over long distances.
Insulating Liquids
Insulating liquids are rarely used by themselves. Rather, they are intended for use mainly as impregnants with cellulose or synthetic papers [6]. The 60 Hz breakdown strength of practical insulating liquids exceeds that of gases; for a 1-cm gap separation, it is of the order of about 100 kV cm–1. However, because the breakdown strength increases with decreasing gap length and the oils are normally evaluated using a gap separation of 0.254 cm, the breakdown strengths normally cited range from approximately 138 to 240 kV cm–1 (Table 1.3). The breakdown values are more influenced by the moisture and particle con- tents of the fluids than by their molecular structure.
Mineral oils have been extensively used in high-voltage electrical apparatus. They constitute a cate- gory of hydrocarbon liquids that are obtained by refining crude petroleum. Their composition consists of paraffinic, naphthenic, and aromatic constituents and is dependent upon the source of the crude as well as the refining procedure followed. The inclusion of the aromatic constituents is desirable because of their gas absorption and oxidation characteristics. Mineral oils used for cable and transformer applica- tions have low polar molecule contents and are characterized by dielectric constants extending from about 2.10 to 2.25, with dissipation factors generally between 2 × 10 –5and 6 × 10 –5 at room temperature, depending upon their viscosity and molecular weight. Their dissipation factors increase appreciably at higher temperatures when the viscosities are reduced. Oils may deteriorate in service because of oxida- tion and moisture absorption.
Alkyl benzenes are used as impregnants in high-voltage cables, often as substitutes for the low-viscosity mineral oils in self-contained, oil-filled cables. The electrical properties of alkyl benzenes are com- parable to those of mineral oils, and they exhibit good gas inhibition characteristics. Because of their detergent character, alkyl benzenes tend to be more susceptible to contamination than mineral oils.
Since the discontinued use of the nonflammable polychlorinated biphenyls (PCBs), a number of unsaturated synthetic liquids have been developed for use in high-voltage capacitors, where, because of
high stresses, evolved gases can readily undergo partial discharge. Most of these new synthetic capacitor fluids are, thus, gas-absorbing, low-molecular-weight derivatives of benzene, with permittivities ranging from 2.66 to 5.25 at room temperature (compared to 3.5 for PCBs). None of these fluids have the non- flammable characteristics of the PCBs; however, they do have high boiling points.
Silicone liquids consist of polymeric chains of silicon atoms alternating with oxygen atoms and with methyl side groups. For electrical applications, polydimethylsiloxane (PDMS) fluids are used, primarily in transformers as substitutes for the PCBs because of their inherently high flash and flammability points, and reduced environmental concerns.
Insulating Solids
Solid insulating materials can be classified into two main categories: organic and inorganic [13]. There are a large number of solid inorganic insulants available, including the following:
• Alumina, produced by heating aluminum hydroxide or oxyhydroxide; it is widely used as a filler for ceramic insulators. Further heating yields the corundum structure, which in its sapphire form is used for dielectric substrates in microcircuit applications.
• Porcelain, a multiphase ceramic material that is obtained by heating aluminum silicates until a mullite phase is formed. Because mullite is porous, its surface must be glazed with a high-melt- ing-point glass to render it smooth and impervious to contaminants for use in overhead line insulators.
• Electrical-grade glasses, which tend to be relatively lossy at high temperatures. At low temperatures, however, they are suitable for use in overhead line insulators and in transformer, capacitor, and circuit breaker bushings. At high temperatures, their main application lies with incandescent and fluorescent lamps as well as electronic tube envelopes.
• Mica, a layer-type dielectric (mica films are obtained by splitting mica blocks). The extended two-dimensionally layered strata of mica prevents the formation of conductive pathways across the substance, resulting in a high dielectric strength. It has excellent thermal stability and, because of its inorganic nature, is highly resistant to partial discharges. It is used in sheet, plate, and tape forms in rotating machines and transformer coils.
Solid organic dielectrics consist of large polymer molecules, which generally have molecular weights in excess of 600. Primarily (with the notable exception of paper, which consists of cellulose that is com- prised of a series of glucose units), organic dielectric materials are synthetically derived. Some of the more common insulating materials of this type include:
• Polyethylene (PE), perhaps one of the most common solid dielectrics. PE is extensively used as a solid dielectric extruded insulator in power and communication cables. Linear PE is classified as a low- (0.910 to 0.925), medium- (0.926 to 0.940), or high- (0.941 to 0.965) density polymer. Most of the PE used on extruded cables is of the cross-linked polyethylene type.
• Ethylene-propylene rubber (EPR), an amorphous elastomer that is synthesized from ethylene and propylene. It is used as an extrudent on cables where its composition has a filler content that
usually exceeds 50% (comprising primarily clay, with smaller amounts of added silicate and carbon black). Dielectric losses are appreciably enhanced by the fillers, and, consequently, EPR is not suitable for extra-high-voltage applications. Its use is primarily confined to intermediate voltages (< 69 kV) and to applications where high cable flexibility (due to its inherent rubber properties) may be required.
• Polypropylene, which has a structure related to that of ethylene with one added methyl group. It is a thermoplastic material having properties similar to high-density PE, although because of its lower density, polypropylene has also a lower dielectric constant. Polypropylene has many electrical applications, both in bulk form as molded and extruded insulations, as well as in film form in taped capacitor, transformer, and cable insulations.
• Epoxy resins, which are characterized by low shrinkage and high mechanical strength. They can also be reinforced with glass fibers and mixed with mica flakes. Epoxy resins have many applica- tions, including insulation of bars in the stators of rotating machines, solid-type transformers, and spacers for compressed-gas-insulated busbars and cables.
Impregnated-paper insulation is one of the earliest insulating systems employed in electrical power apparatus and cables. Although many current designs use solid- or compressed-gas insulating systems, the impregnated-paper approach still constitutes one of the most reliable insulating techniques available. Proper impregnation of the paper results in a cavity-free insulating system, thereby eliminating the occurrence of partial discharges that inevitably lead to deterioration and breakdown of the insulating sys- tem. The liquid impregnants employed are either mineral oils or synthetic fluids.
Low-density cellulose papers have slightly lower dielectric losses, but the dielectric breakdown strength is also reduced. The converse is true for impregnated systems utilizing high-density papers. If the paper is heated beyond 200°C, the chemical structure of the paper breaks down, even in the absence of external oxygen, because the latter is readily available from within the cellulose molecule. To prevent this process from occurring, cellulose papers are ordinarily not used at temperatures above 100°C.
Control and Switching Systems
Specialized hardware is necessary to interconnect the elements of a power-distribution system. Utility control and switching systems operate under demanding conditions, including high voltage and current levels, exposure to lightning discharges, and 24-hour-a-day use. For reliable performance, large margins of safety must be built into each element of the system. The primary control and switching elements are high-voltage switches and protection devices.
High-voltage switches are used to manage the distribution network. Most disconnect switches function to isolate failures or otherwise reconfigure the network. Air-type switches are typically larger versions of the common knife switch device. To prevent arcing, air switches are changed only when power is removed from the circuit. These types of switches can be motor driven or manually operated.
Oil-filled circuit breakers are used at substations to interrupt current when the line is hot. The contacts usually are immersed in oil to minimize arcing. Oil-filled circuit breakers are available for operation at 500 kV and higher. Magnetic air breakers are used primarily for low-voltage indoor applications.
Protection devices include fuses and lightning arresters. Depending upon the operating voltage, various types of fuses can be used. Arc suppression is an essential consideration in the design and operation of a high-voltage fuse. A method must be provided to extinguish the arc that develops when the fuse ele- ment begins to open. Lightning arresters are placed at numerous points in a power-distribution system. Connected between power-carrying conductors and ground, they are designed to operate rapidly and repeatedly if necessary. Arresters prevent flashover faults between power lines and surge-induced trans- former and capacitor failures. The devices are designed to extinguish rapidly, after the lightning discharge has been dissipated, to prevent power follow-on damage to system components.
A fault in an electrical power system is the unintentional and undesirable creation of a conducting path (a short circuit) or a blockage of current (an open circuit) [7]. The short-circuit fault is typically the most common and is usually implied when most people use the term “fault.” The causes of faults include
lightning, wind damage, trees falling across lines, vehicles colliding with towers or poles, birds shorting out lines, aircraft colliding with lines, vandalism, small animals entering switchgear, and line breaks resulting from excessive ice loading. Power system faults can be categorized as one of four types:
• Single line-to-ground
• Line-to-line
• Double line-to-ground
• Balanced three-phase
The first three types constitute severe unbalanced operating conditions.
It is important to determine the values of system voltages and currents during fault conditions so that protective devices can be set to detect and minimize their harmful effects. The time constants of the associated transients are such that sinusoidal steady-state methods can typically be used.
High-voltage insulators permit all of the foregoing hardware to be reliably interconnected. Most insulators are made of porcelain. The mechanical and electrical demands placed on high-voltage insula- tors are stringent. When exposed to rain or snow, the devices must hold off high voltages. They also must support the weight of heavy conductors and other components.
Fault Protection Devices
Fuses are designed to melt and disconnect the circuit within which they are placed should the current in the circuit increase above a specified thermal rating [3]. Fuses designed to be used in circuits operating above 600 V are classified as fuse cutouts. Oil-filled cutouts are mainly used in underground installations and contain the fusible elements in an oil-filled tank. Expulsion-type cutouts are the most common devices used on overhead primary feeders. In this class of device, the melting of the fusible element causes heating of a fiber fuse tube, which, in turn, produces deionizing gases to extinguish the arc. Expul- sion-type cutouts are classified as:
• Open-fuse cutouts
• Enclosed-fuse cutouts
• Open-link-fuse cutouts
The automatic recloser is an overcurrent device that automatically trips and recloses a preset number of times to clear or isolate faults. The concept of reclosing is derived from the fact that most utility system faults are temporary in nature and can be cleared by de-energizing the circuit for a short period of time. Reclosers can be set for a number of operation sequences, depending on the action desired. These typi- cally include instantaneous trip and reclose operation followed by a sequence of time-delayed trip opera- tions prior to lockout of the recloser. The minimum pick-up for most reclosers is typically set to trip instantaneously at two times the nominal current rating.
An automatic line recloser is constructed of an interrupting chamber and the related contacts that operate in oil, a control mechanism to trigger tripping and reclosing, an operator integrator, and a lock- out mechanism. An operating rod is actuated by a solenoid plunger that opens and closes the contacts in oil. Both single-phase and three-phase units are available.
The line sectionalizer is yet another overcurrent device. It is installed in conjunction with backup cir- cuit breakers or reclosers. The line sectionalizer maintains coordination with the backup interrupting device and is designed to open after a preset number of tripping operations of the backup element. Line sectionalizers are installed on poles or crossarms in overhead distribution systems. The standard contin- uous current rating for sectionalizers ranges from 10 to 600 A. Sectionalizers also are available for both single-phase and three-phase systems.
The function of a circuit breaker is to protect a circuit from the harmful effects of a fault, in addition to energizing and de-energizing the same circuit during normal operation. Breakers are generally installed on both the incoming subtransmission lines and the outgoing primary feeders of a utility sub- station. These devices are designed to operate as quickly as possible (less than 10 cycles of the power fre- quency) to limit the impact of a fault on the distribution and control system. At the same time, the arc
that forms between the opening contacts must be quenched rapidly. Several schemes are available to extinguish the arc, the most common being immersion of the contacts in oil. Some circuit breakers have no oil, but quench the arc by a blast of compressed air. These are referred to as air circuit breakers. Yet another type encloses the contacts in a vacuum or a gas, such as sulfur hexafluoride (SF6).
Air circuit breakers are typically used when fault currents are relatively small. These devices are characteristically simple, are low cost, and require little maintenance. The fault current flows through coils, creating a magnetic field that tends to force the arc into ceramic chutes that stretch the arc, often with the aid of compressed air. When the arc is extinguished through vacuum, the breaker is referred to as a vac- uum circuit breaker. Because a vacuum cannot sustain an arc, it can be an effective medium for this appli- cation. However, owing to imperfections present in a practical vacuum device, a small arc of short duration can be produced. The construction of vacuum circuit breakers is simple, but the maintenance is usually more complex than with other devices.
Lightning Arrester
A lightning arrester is a device that protects electrical apparatus from voltage surges caused by lightning [3]. It provides a path over which the surge can pass to ground before it has the opportunity to pass through and damage equipment. A standard lightning arrester consists of an air gap in series with a resistive element. The resistive element is usually made of a material that allows a low-resistance path to the voltage surge, but presents a high-resistance path to the flow of line energy during normal operation. This material is known as the valve element. Silicon carbide is a common valve element material. The voltage surge causes a spark that jumps across the air gap and passes through the resistive element to ground.