Power Systems

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Kinds of Systems: Nomenclature

Some understanding should be reached regarding the designation of different kinds of systems and apparatus. A general nomenclature has been widely used, and, for our purposes, systems might be defined as follows, bearing in mind that there may be some differences of opinion and some overlapping of one group into another.

A transmission or power system will be considered a system operating at voltages above 15 kV, and the apparatus used for the transmission of large blocks of power will be considered power equipment. It is true that lower-voltage systems may be power systems using power equipment, such as a generator circuit, but the definition will, in general, serve useful purposes as far as lightning protection is concerned. Protection for transmission systems may be segregated primarily into protection for the apparatus connected to the system and protection for the lines themselves in order to prevent insulator flashover.

A distribution system will be considered one operating at voltages above 750 volts, but not higher than 15 kV. By some power distributors, 23 kV may be considered a distribution voltage, and in some cases, a lower voltage may actually appear to be transmission, as, for example, a long rural system operating at 12 or 13.8 kV. A distribution service is a system that brings electricity to the transformers that step the voltage down to utilization voltages. These systems usually fall in the above-mentioned voltage classes; therefore, the convention of 750 to 15,000 volts will be used.

A secondary system is considered one that distributes power to the machines or devices that utilize it, such as motors, lights, etc. These circuits are largely 240/120, 440, 550, or 660 volts. Therefore, in this text, secondary circuits will be considered circuits delivering appreciable power at voltages less than 750. There are, especially in industrial plants, circuits that feed machines at higher voltages, such as 2,300-volt motors in steel mills and coal mines, and there are 400-volt circuits that might be considered distribution circuits or even transmission, such as long 440-volt circuits paralleling railroads to supply small amounts of power to the transformers feeding railway signal installations. However, to draw a distinguishing line, secondary circuits will be considered those operating at less than 750 volts.

Signal or control circuits are considered those of low power, usually operating at very low voltages, from a few volts to perhaps 120 and sometimes 240 volts.

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The 'Preface' states:

The structure of the electric power system is very large and complex. Nevertheless, its main components (or subsystems) can be identified as the generation system, transmission system, and distribution system. These three systems are the basis of the electric power industry. [...] This text is unique in that it is written specifically for in-depth study of modern power transmission engineering. This book has evolved from the content of courses given by the author at the California State University at Sacramento, the University of Missouri at Columbia, the University of Oklahoma, and the Florida International University. [...]

(Additional information from this book is located on the Poles and Towers page, the Interference Between Power and Telecom Lines page, and the Insulator Usage page.)

The following information is excerpted from this book.

Transmission System Planning

An electrical power system can be considered to consist of a generation system, a transmission system, a subtransmission system, and a distribution system. In general, the generation and transmission systems are referred to as bulk power supply, and the subtransmission and distribution systems are considered to be the final means to transfer the electric power to the ultimate customer.

Bulk power transmission is made of a high-voltage network, generally 138 - 765 kV alternating current, designed to interconnect power plants and electrical utility systems and to transmit power from the plants to major load centers. Table 1.1 gives the standard transmission voltages as dictated by ANSI Standard C-84 of the American National Standards Institute. The subtransmission refers to a lower voltage network, normally 34.5 - 115 kV, interconnecting bulk power and distribution substations.

Table 1.1 - Standard System Voltages

Rating (kV)

Rating (kV)





















The voltages that are in the range of 345 - 765 kV are classified as extra-high voltages (EHVs). [...] The high-voltage transmission systems up to 230 kV can be built in relatively simple and well-standardized designs. [...] The voltages above 765 kV are considered as the ultrahigh voltages (UHVs). Currently, the UHV systems, at 1000-, 1100-, 1500-, and 2250-kV voltage levels, are in the R&D stages. [...]

Steady-State Performance of Transmission Lines


One-Line Diagrams

[...] Usually, the neutrals of the transformers used in transmission lines are solidly grounded. [...] Transmission lines with overhead ground wires have a ground connection at each supporting structure to which the ground wire is connected. In some circumstances, a 'counterpoise' - that is, a bare conductor - is buried under a transmission line to decrease the ground resistance. The best known example is the one that has been installed for the transmission line crossing the Mohave Desert. The counterpoise is buried alongside the line and conencted directly to the towers and the overhead ground wires. [...]

Environmental Effects of Overhead Lines

Recently, the importance of minimizing the environmental effects of overhead transmission lines has increased substantially due to increasing use of greater extra-high- and ultrahigh-voltage levels. Therefore, the magnitude and effect of radio noise, television interference, audible noise, electric field, and magnetic fields must not only be predicted and analyzed in the line design stage, but also measured directly. [...]

Most environmental measurements are highly affected by prevailing weather conditions and transmission line geometry. The weather conditions include temperature, humidity, barometric pressure, precipitation levels, and wind velocity. [...]

Direct-Current Power Transmission


Comparison of Power Transmission Capacity of High-Voltage DC and AC

Assume that there are two comparable transmission lines; one is the AC and the other the DC line. [...] Both lines have the same transmission capability and can transmit the same amount of power. However, the DC line has two conductors rather than three, and thus requires only two-thirds as many insulators. Therefore, the required towers and rights-of-way are narrower in the DC line than the AC line. Even though the power loss per conductor is the same for both lines, the total power loss of the DC line is only two thirds that of the AC line. Thus, studies indicate that a DC line generally costs about 33 percent less than an AC line of the same capacity. [...] In general, the cost advantage of the DC line increases at higher voltages. The power losses due to the corona phenomena are smaller for DC than for AC lines. [...]

The major advantages of the DC transmission can be summarized as:

  1. If the high cost of converter stations is excluded, the DC overhead lines and cables are less expensive than AC overhead lines and cables. The break-even distance is about 500 mi for the overhead lines, somewhere between 15 and 30 mi for submarine cables, and [between] 30 and 60 mi for underground cables. Therefore, in the event that the transmission distance is less than the break-even distance, the AC transmission is less expensive than DC; otherwise, the DC transmission is less expensive. The exact break-even distance depends on local conditions, line performance requirements, and connecting AC system characteristics.

  2. A DC link is asynchronous; that is, it has no stability problem in itself. Therefore, the two AC systems connected at each end of the DC link do not have to be operating in synchronism with respect to each other, or even necessarily at the same frequency.

  3. The corona loss and radio interference conditions are better in the DC than the AC lines.

  4. The power factor of the DC line is always unity, and therefore no reactive compensation is needed.

  5. Since the synchronous operation is not demanded, the line length is not restricted by stability.

  6. The interconnection of two separate systems via a DC link does not increase the short-circuit capacity, and thus the circuit-breaker ratings, of either system.

  7. The DC line loss is smaller than for the comparable AC line.

The major disadvantages of the DC transmission can be summarized as follows:

  1. The converters generate harmonic voltages and currents on both AC and DC sides, and therefore filters are required.

  2. The converters consume reactive power.

  3. The DC converter stations are expensive.

  4. The DC circuit breakers have disadvantages with respect to the AC circuit breakers because the DC current does not decrease to zero twice a cycle, contrary to the AC current.


Multibridge (B-Bridge) Converter Stations

Figure 5.10 shows a typical converter station layout. For such a station, the general arrangement of a converter station with 12-pulse converters is shown in Figure 5.11.

Figure 5.10. Typical converter station layout [2]. Figure 5.11. General arrangement of converter station with 12-pulse converters: (1) AC busbar; (2) converter transformer; (3) valve-side bushing of converter transformer; (4) surge arresters; (5) quadruple valves; (6) valve-cooling fans; (7) air core reactor; (8) wall bushing; (9) outgoing DC buswork; (10) smoothing reactor; (11) outgoing electrode line connection [2].

[Ref. 2 = Fink, D. G., and Beaty, H. W. "Standard Handbook for Electrical Engineers", 11th ed. McGraw-Hill, New York, 1978.]

[...] The converter bank is made of two or more three-phase bridges, and each bridge contains up to six mercury arc valves or thyristors. [...] The number of bridges required is dictated by the direct voltage level selected for economical transmission. [...]

This information comes from

The 'Preface' states:

[...] The continued popularity of the book in its original form over a period of twenty-one years has been very gratifying, and it is hoped that, in its new form, it will be equally acceptable.

(Additional information from this book is located on the Overvoltages and Flashovers page, the Interference Between Power and Telecom Lines page, the Poles and Towers page, and the Insulator Usage page.)

The following information is excerpted from this book.

Supply Systems

Feeders and Distributors

The conductor system, by means of which electrical energy is conveyed from a power station to a consumer, can, in general, be divided up into two distinct parts, viz. the transmission and distribution systems. These again can be subdivided into primary and secondary transmission, and primary and secondary distribution, and finally, there is the system of supply to the individual consumer. [...] In general, in an A.C. system there will be a change of voltage at each point where the subdivision takes place, this change being affected at a substation, and it therefore follows that there may be several working voltages in the same system. For obvious reasons, it has been necessary to standardize voltages, the values in use in this country [Great Britain] being as follows:

  1. Generating voltages: 6600, 11000, and up to 33000.
  2. High-voltage transmission: 275000, 132000, 66000, down to 11000.
  3. High-voltage distribution: 11000 and 6600.
  4. Low-voltage distribution:
    • A.C.: 400 between phases, 230 to neutral.
    • D.C.: 3-wire, 2x240 and 2x250.

For A.C. working, the standard frequency which has been adopted in this [UK] and many other countries is 50 cycles per second. In America, a frequency of 60 is adopted for lighting and some of the power load, but 25 cycles is also in use for power. For single-phase traction, much lower frequencies are necessary, e.g. 15 or 16 2/3 cycles.

The distribution system, i.e. not including the transmission lines, can be subdivided into feeders, distributors, and the service mains. The feeders are the conductors which connect the substations, or in some cases the generating stations, to the areas served by these stations. The distributors are characterised by the numerous tappings which are taken from them for the supply to consumers, and the service mains are the connecting links between distributors and consumers' terminals. The essential difference between feeders and distributors is that, whereas the current loading of a feeder is the same along the whole of its length, a distributor has a distributed loading, with consequent variations of current along its length. [...] In some cases, smaller feeders are tee'd off from a main feeder, but this does not make any exception to the general rule that tappings are not taken from a feeder to a consumer's premises.

Influence of Working Voltage on Size of Feeders and Distributors

Because of the inevitable drop in voltage along all conductors, it is obvious that it is impossible to keep the potential difference absolutely constant at the terminals of any consumer. On the other hand, large fluctuations in voltage may be very undesirable from the point of view of the consumer, as for example in the case of a lighting load in which the luminous output of a lamp is proportional to the voltage. [...] The only means of reducing this drop in voltage is to reduce the conductor resistance, and therefore to increase the cross-section and cost of the conductor. [...] The inevitable compromise is fixed by a Board of Trade regulation, which states that the voltage at the consumer's terminals must not vary by more than plus or minus 6 per cent. of the 'declared pressure'. In the case of an A.C. supply, there is an additional regulation fixing the variation in frequency at not more than 2 1/2 per cent. above or below the 'declared frequency'. [...]

Effect of System Voltage on Transmission Efficiency

[...] The supply voltage affects the transmission system as follows:

  1. The line loss is inversely proportional to the supply voltage: it is also inversely proportional to the power factor.
  2. The efficiency of transmission increases as the voltage increases, and also as the power increases.
  3. For a given current density, the resistance drop per line is a constant, the percentage resistance drop therefore decreasing as the voltage is increased.
  4. The volume of copper required in the transmission line is inversely proportional to the voltage, and also inversely proportional to the power factor.

From the above argument, it is clear that for long-distance transmission, very high voltages are essential, and with A.C. systems a high power factor is also desirable. Increase in voltage introduces difficulties associated with the insulation of the conductors, whether bare or covered; and also, in the case of bare conductors, difficulties associated with their mechanical support and the necessity for very large clearances. For these reasons, voltage is largely fixed by the length of the line. Since there is no hard-and-fast rule, the old rule of 1,000 volts per route mile is still a useful, though very rough, guide. [...]

Comparison of Conductor Costs - Cable Systems

[...] With insulated conductors, the D.C. system is not so greatly advantageous as with bare conductors, and since transmission is generally carried out by bare conductors, and low-voltage distribution by cables, it follows that, although D.C. has a very decided advantage for transmission, this advantage is not so marked with distribution. In fact, when other factors are taken into account, the most important robably being the ease with which the voltage in an A.C. system can be changed by means of static transformers, the A.C. system is, on the whole, the most suited to distribution. [...]



Best Position for Substation

In the early days of electricity supply, there was very little long-distance transmission, and each undertaking was more in the nature of a comprehensive distributing system. Consequently, it was fairly common practice to locate the power station near the centre of gravity of the load, in order to economise in the cost of cables. This procedure led to the choice of many unfortunate sites for power stations, but within recent years the majority of such stations have been converted to substations, for which such a basis of calculation is justifiable. [...]



Working Voltage

The working voltage depends, of course, on the distance of transmission. We have already stated the rough rule of 1,000 volts per mile, but it is to be noted that in practice the voltage per mile varies from about 600 to 1,500. The following table forms an approximate guide in the case of overhead transmissions:


Kilovolts per mile

Up to 10 miles

1.5 to 1.2

10 to 15 miles

1.2 to 1.0

50 to 75 miles

1.0 to 0.9

75 to 100 miles

0.9 to 0.8

100 to 150 miles

0.8 to 0.7

The volts per mile increase with the amount of power to be transmitted, in order that the line loss and line drop may be kept within reasonable limits. The above values can be taken as applying to a base load of 1,000 kVA., and 1 per cent. can be added to the kilovolts per mile for each additional 1,000 kVA. [...]

Cables v. Overhead Lines

The primary consideration in the comparison of competitive systems is that of cost, and overhead lines score heavily over insulated cables in this respect. [...] With both systems, the cost per kW per mile decreases enormously as the voltage and amount of power are increased, but in all cases the figures are in favour of overhead transmission, which system shows up to particular advantage (viz. a ratio of about 1:2) for low voltages and powers. It is largely for these reasons that the overhead system is used not ony for high-voltage transmission of bulk supply, but also low-voltage distribution to small towns and in rural districts. In large cities, overhead lines are not suitable, and buried cables are the rule, in spite of their much greater cost. [...]

Line Constants


For overhead transmission by bare conductors, the only metals used for short or moderate spans are hard-drawn copper and aluminium. For long spans, copper-clad steel or steel-cored aluminium are used, the justification forthese constructions being that they enable the use of long spans without increase in tower height, because of their high breaking loads.

So far as the combined properties of conductivity and weight are concerned, aluminium has an advantage over copper, but the chief objection to aluminium is its low tensile strength and high coefficient of expansion. [...] The great advantage of hard-drawn copper is its high mechanical strength: it should have a minimum ultimate tensile strength of 55,000 lb. per sq. in., which is more than twice the value for aluminium. [...]



Substation Layout

The choice between outdoor and indoor substations depends on their relative costs and the atmospheric conditions. An outdoor substation saves building costs but requires a larger site and more maintenance; a major factor in the design is the provision of adequate clearance so that maintenance work can be carried out without making more of the substation dead than is absolutely necessary. At and above 132 kV., outdoor construction is almost universal. Case must also be taken to ensure that all metal work (structures, fencing, circuit-breaker tanks, etc.) is effectively bonded to the earthing system, the latter comprising a network of wires earthed by buried rods or plates at a number of points. Indoor stations are usual in urban areas where sites are expensive and difficult to obtain, and where voltages are not usually above 66 kV. or occasionally 132 kV. A major factor in their design is the minimization of the fire risk; sectionalising by fire-resisting walls is usually necessary and fire-extinguishing apparatus must be installed. Carbon-dioxide is a very effective fire-extinguishing medium, and causes no damage to sound equipment; the gas is stored in cylinders and released automatically by fusible plugs. Where extremely rapid extinction is essential -- e.g. to prevent damage to adjacent premises -- chemical foam is effective, although the plant may suffer further damage from its use.


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The following information is excerpted from this book.


The Transmission of Electrical Energy


Types of Power Lines

[...] We distinguish four types of power lines, according to their voltage class:

  1. Low-voltage (LV) lines are installed inside buildings, factories, and houses, to supply power to motors, electric stoves, lights, and so on. The service entrance panel constitutes the source, and the lines are made of insulated cable or bus-bars, operating at voltages below 600 V. In some metropolitan areas, the distribution system consists of a grid of underground cables operating at 600 V or less. Such a network provides dependable service, because even the outage of one or several cables will not interrupt customer service. Today, however, we prefer to install medium-voltage radial distribution systems in the larger cities. In radial systems, the transmission lines spread out like fingers from one or more substations to feed power to various load centers.
  2. Medium-voltage (MV) lines tie the load centers to the main substation of the utility company. The voltage is usually between 2.4 kV and 69 kV.
  3. High-voltage (HV) lines connect the main substations ot the generating stations. The lines are composed of aerial wire or underground cable operating at voltages below 230 kV. In this category, we also find lines that transmit energy between two large systems, to increase the stability of the network.
  4. Extra-high voltage (EHV) lines are used when generating stations are very far from the load centers. We put them in a separate class because of their special properties. Such lines operate at voltages up to 800 kV and may be as long as 1000 km. High-tension direct lines are also included in this group.

Standard Voltages

To reduce the cost of distribution apparatus and to facilitate its protection, standards-setting organizations have established a number of standard voltages for transmission lines. These standards, given in Table 29A, reflect the various voltages presently used in North America. Voltages that bear the symbol * are preferred voltages.

Table 29A: Voltage Classes as applied to Industrial and Commercial Power
Voltage Class
Nominal System Voltage




Low Voltage (LV)


120/240 * (127/254 max)

120 / 208 *


480 * (508 max)

277 / 480 *


600 (635 max)

347 / 600

Medium Voltage (MV)


2,400 (2,540 max)



4,160 * (4,400 max)



4,800 (5,080 max)



6,900 (7,260 max)



13,800 * (14,520 max)

7,200 / 12,470 *


23,000 (24,340 max)

7,620 / 13,200 *


34,500 (36,500 max)

7,970 / 13,800


46,000 (48,300 max)

14,400 / 24,940 *


69,000 * (72,500 max)

19,920 / 34,500 *

High Voltage (HV)


115,000 * (121,000 max)



138,000 * (145,000 max)



161,000 (169,000 max)



230,000 * (242,000 max)


Extra High Voltage (EHV)


345,000 * (362,000 max)



500,000 * (550,000 max)



735,000-765,000 *
(800,000 max)


Note: voltage class designations were approved for use by the IEEE Standards Board (September 4, 1975)


Components of a Transmission Line

[...] For voltages above 70 kV, we always use suspension-type insulators, strung together by their ball and socket metallic parts. The number of insulators depends upon the voltage: for 110 kV, we generally use from 4 to 7; for 230 kV, from 13 to 16. [On a] 735 kV line, it is composed of 4 strings of 35 insulators each, to provide both mechanical and electrical strength.

The supporting structure must keep the conductors at a safe height from the ground and at an adequate distance from each other. For voltages below 70 kV, we can use single wooden poles equipped with cross-arms, but for higher voltages a wooden H-frame must be used. The wood is treated with creosote or special metallic salts to prevent it from rotting. For very high-voltage lines, we always use steel towers made of galvanized angle-iron, bolted together.


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The 'Foreword' states:

This publication is prepared by the Industrial Plants Power Systems Subcommittee of the IEEE Industrial and Commercial Power Systems Committee, which is a technical committee of the IEEE Industry Applications Society. It has been approved by the IEEE Standards Board [on December 12, 1975] as an IEEE standards document to provide current information and recommended practices for the design, construction, operation, and maintenance of electric power systems in industrial plants. It was thirty-one years ago that "Electric Power Distribution for Industrial Plants" was first published by the AIEE. It was given the nickname of the "Red Book" because of the color of its cover... and it became the first of the present IEEE Color Book series. The second edition of the Red Book was produced in 1956, and identified as AIEE No. 952. This was followed by the third edition in 1964; it was also identified as IEEE No. 141. The fourth edition was produced in 1969; it was approved as a Recommended Practice of the Institute and identified as IEEE Std 141-1969. This, the fifth edition, is now identified as IEEE Std 141-1976. It was initiated in 1970 with the participation of more than fifty electrical engineers from industrial plants, consulting firms, and equipment manufacturers. [...]

(Additional information from this book is located on the History of Electrical Systems and Cables page.)

The following information is excerpted from this book.

Power Switching, Transformation, and Motor-Control Apparatus

[...] Electric equipment must be installed to be safe and accessible to persons frequently in the area. Sufficient access and working space should be provided and maintained about all electric apparatus to permit ready and safe operation and maintenance of such equipment. [...]

Table 55 - Minimum Clear Working Space in Front of Electric Equipment

Voltage to Ground

Working Space (feet) for Conditions *




0 - 150

2 1/2

2 1/2


151 - 600

2 1/2

3 1/2


601 - 2,500




2,501 - 9,000




9,001 - 25,000




25,0001 - 75,000




Based on the NEC 1975 [National Electric Code ANSI C1-1975,
Sections 110-16 and 110-34]

* Conditions:

  1. Exposed live parts on one side and no live or grounded parts on the other side of the working space, or exposed live parts on both sides effectively guarded by suitable wood or other insulating materials. Insulated wire or insulated bus bars operating at not over 300 V shall not be considered live parts.

  2. Exposed live parts on one side and grounded parts on the other side. Concrete, brick, or tile walls will be considered as grounded surfaces.

  3. Exposed live parts on both sides of the work space [not guarded as provided in Condition 1] with the operator between.

Exception: Working space is not required in back of equipment such as deadfront switchboards or control assemblies where there are no renewable or adjustable parts such as fuses or switches, and when all connections are accessible from other locations than the back.



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