Karady, G.G. “Energy Distribution” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000 65 Energy Distribution 65.1Introduction 65.2Primary Distribution System 65.3Secondary Distribution System 65.4Radial Distribution System 65.5Secondary Networks 65.6Load Characteristics 65.7Voltage Regulation 65.8Capacitors and Voltage Regulators 65.1 Introduction Distribution is the last section of the electrical power system. Figure 65.1 shows the major components of the electric power system. The power plants convert the energy stored in the fuel (coal, oil, gas, nuclear) or hydro into electric energy. The energy is supplied through step-up transformers to the electric network. To reduce energy transportation losses, step-up transformers increase the voltage and reduce the current. The high-voltage network, consisting of transmission lines, connects the power plants and high-voltage substations in parallel. The typical voltage of the high-voltage transmission network is between 240 and 765 kV. The high-voltage substations are located near the load centers, for example, outside a large town. This network permits load sharing among power plants and assures a high level of reliability. The failure of a line or power plant will not interrupt the energy supply. The subtransmission system connects the high-voltage substations to the distribution substations. These stations are directly in the load centers. For example, in urban areas, the distance between the distribution stations is around 5 to 10 miles. The typical voltage of the subtransmission system is between 138 and 69 kV. In high load density areas, the subtransmission system uses a network configuration that is similar to the high- voltage network. In medium and low load density areas, the loop or radial connection is used. Figure 65.1 shows a typical radial connection. The distribution system has two parts, primary and secondary. The primary distribution system consists of overhead lines or underground cables, which are called feeders. The feeders run along the streets and supply the distribution transformers that step the voltage down to the secondary level (120–480 V). The secondary distribution system contains overhead lines or underground cables supplying the consumers directly (houses, light industry, shops, etc.) by single- or three-phase power. Separate, dedicated primary feeders supply industrial customers requiring several megawatts of power. The subtransmission system directly supplies large factories consuming over 50 MW. 65.2 Primary Distribution System The most frequently used voltages and wiring in the primary distribution system are listed in Table 65.1. Primary distribution, in low load density areas, is a radial system. This is economical but yields low reliability. In large cities, where the load density is very high, a primary cable network is used. The distribution substations George G. Karady Arizona State University ? 2000 by CRC Press LLC are interconnected by the feeders (lines or cables). Circuit breakers (CBs) are installed at both ends of the feeder for short-circuit protection. The loads are connected directly to the feeders through fuses. The connection is similar to the one-line diagram of the high-voltage network shown in Fig. 65.1. The high cost of the network limits its application. A more economical and fairly reliable arrangement is the loop connection, when the main feeder is supplied from two independent distribution substations. These stations share the load. The problem with this connection is the circulating current that occurs when the two supply station voltages are different. The loop arrangement significantly improves system reliability. FIGURE 65.1 Electric energy system. TABLE 65.1 Typical Primary Feeder Voltages (line-to-line) Class, kV Voltage, kV Wiring 2.5 2.4 3-wire delta 5 4.16 4-wire Y 8.66 7.2 4-wire Y 15 12.47 3-wire delta/4-wire Y 25 22.9 4-wire Y 35 34.5 4-wire Y ? 2000 by CRC Press LLC The circulating current can be avoided by using the open-loop connection. This is a popular, frequently used circuit. Figure 65.2 shows a typical open-loop primary feeder. The distribution substation has four outgoing main feeders. Each feeder supplies a different load area and is protected by a reclosing CB. The three-phase four-wire main feeders supply single-phase lateral feeders. A recloser and a sectionalizing switch divide the main feeder into two parts. The normally open tie-switch connects the feeder to the adjacent distribution substation. The fault between the CB and recloser opens the reclosing CB. The CB recloses after a few cycles. If the fault is not cleared, the opening and reclosing process is repeated two times. If the fault has not been cleared before the third reclosing, the CB remains open. Then the sectionalizing switch opens and the tie-switch closes. This energizes the feeder between the recloser and the tie-switch from the neighboring feeder. Similarly, the fault between the recloser and tie-switch activates the recloser. The recloser opens and recloses three times. If the fault is not cleared, the recloser remains open and separates the faulty part of the FIGURE 65.2 Radial primary distribution system. ? 2000 by CRC Press LLC feeder. This method is particularly effective in overhead lines where temporary faults are often caused by lightning, wind, and metal balloons. A three-phase switched capacitor bank is rated two-thirds of the total average reactive load and installed two-thirds of the distance out on the feeder from the source. The capacitor bank improves the power factor and reduces voltage drop at heavy loads. However, at light loads, the capacitor is switched off to avoid overvoltages. Some utilities use voltage regulators at the primary feeders. The voltage regulator is an autotransformer. The secondary coil of the transformer has 32 taps, and a switch connects the selected tap to the line to regulate the voltage. The problem with the tap changer is that the lifetime of the switch is limited. This permits only a few operations per day. The lateral single-phase feeders are supplied from different phases to assure equal phase loading. Fuse cutouts protect the lateral feeders. These fuses are coordinated with the fuses protecting the distribution transformers. The fault in the distribution transformer melts the transformer fuse first. The lateral feeder fault operates the cutout fuse before the recloser or CB opens permanently. A three-phase line supplies the larger loads. These loads are protected by CBs or high-power fuses. Most primary feeders in rural areas are overhead lines using pole-mounted distribution transformers. The capacitor banks and the reclosing and sectionalizing switches are also pole-mounted. Overhead lines reduce the installation costs but reduce aesthetics. In urban areas, an underground cable system is used. The switchgear and transformers are placed in underground vaults or ground-level cabinets. The underground system is not affected by weather and is highly reliable. Unfortunately, the initial cost of an underground cable is significantly higher than an overhead line with the same capacity. The high cost limits the underground system to high-density urban areas and housing developments. Flooding can be a problem. 65.3 Secondary Distribution System The secondary distribution system provides electric energy to the customers through the distribution trans- formers and secondary cables. Table 65.2 shows the typical voltages and wiring arrangements. In residential areas, the most commonly used is the single-phase three-wire 120/240-V radial system, where the lighting loads are supplied by the 120 V and the larger household appliances (air conditioner, range, oven, and heating) are connected to the 240-V lines. Depending on the location, either underground cables or overhead lines are used for this system. In urban areas, with high-density mixed commercial and residential loads, the three-phase 208/120-V four- wire network system is used. This network assures higher reliability but has significantly higher costs. Under- ground cables are used by most secondary networks. High-rise buildings are supplied by a three-phase four-wire 480/277-V spot network. The fluorescent lighting is connected to a 277-V and the motor loads are supplied by a 480-V source. A separate local 120-V system supplies the outlets in the various rooms. This 120-V radial system is supplied by small transformers from the 480-V network. TABLE 65.2 Secondary Voltages and Connections Class Voltage Connection Application 1-phase 120/240 Three-wire Residential 3-phase 208/120 Four-wire Commercial/residential 3-phase 480/277 Four-wire High-rise buildings 3-phase 380/220 Four-wire General system, Europe 3-phase 120/240 Four-wire Commercial 3-phase 240 Three-wire Commercial/industrial 3-phase 480 Three-wire Industrial 3-phase 240/480 Four-wire Industrial ? 2000 by CRC Press LLC 65.4 Radial Distribution System A typical overhead single-phase three-wire 120/240-V secondary system is shown in Fig. 65.3. The three distribution transformers are mounted on separate primary feeder poles and supplied from different phases. Each transformer supplies 6 to 12 houses. The transformers are protected by fuses. The secondary feeders and the service drops are not protected individually. The secondary feeder uses insulated No. 1/0 or 4/0 aluminum conductors. The average secondary length is from 200 to 600 ft. The typical load is from 15 to 30 W/ft. The underground distribution system is used in modern suburban areas. The transformers are pad-mounted or placed in an underground vault. A typical 50-kVA transformer serves 5 to 6 houses, with each house supplied by an individual cable. The connection of a typical house is shown in Fig. 65.4. The incoming secondary service drop supplies the kW and kWh meter. The modern, mostly electronic meters measure 15-min kW demand and the kWh energy consumption. It records the maximum power demand and energy consumption. The electrical utility maintains the distribution system up to the secondary terminals of the meter. The homeowner is responsible for the service panel and house wiring. The typical service panel is equipped with a main switch and circuit breaker. The main switch permits the deenergization of the house and protects against short circuits. The smaller loads are supplied by 120 V and the larger loads by 240 V. Each outgoing line is protected by a circuit breaker. The neutral has to be grounded at the service panel, just past the meter. The water pipe was used for grounding in older houses. In new houses a metal rod, driven in the earth, provides proper grounding. In addition, a separate bare wire is connected to the ground. The ground wire connects the metal parts of the appliances and service panel box together to protect against ground-fault-produced electric shocks. FIGURE 65.3 Typical 120/240-V radial secondary system. ? 2000 by CRC Press LLC 65.5 Secondary Networks The secondary network is used in urban areas with high load density. Figure 65.5 shows a segment of a typical secondary network. The secondary feeders form a mesh or grid that is supplied by transformers at the node points. The multiple supply assures higher reliability and better load sharing. The loads are connected directly to the low-voltage grid, without any protection equipment. The network is protected by fuses and network protector circuit breakers installed at the secondary transformers. A short circuit blows the fuses and limits the current. The network protectors automatically open on reverse current and reclose when the voltage on the primary feeder is restored after a fault. 65.6 Load Characteristics The distribution system load varies during the day. The maximum load occurs in the early evening or late afternoon, and the minimum load occurs at night. The design of the distribution system requires both values, because the voltage drop is at the maximum during the peak load, and overvoltage may occur during the minimum load. The power companies continuously study the statistical variation of the load and can predict the expected loads on the primary feeders with high accuracy. The feeder design considers the expected peak load or maximum demand and the future load growth. FIGURE 65.4 Residential electrical connection. FIGURE 65.5 Typical segment of a secondary distribution network. ? 2000 by CRC Press LLC The economic conductor cross-section calculation requires the determination of average losses. The average loss is calculated by the loss factor (LSF), which is determined by statistical analyses of load variation. The average load is determined by the load factor (LF), which is the ratio of average load to peak load. The load factor for an area is determined by statistical analyses of the load variation in past years. The approximate relation between the loss factor and load factor is This equation is useful because the load factor is measured con- tinuously by utilities, and more accurate values are available for the load factor than for the loss factor. Typical values are given in Table 65.3. The connected load or demand can be estimated accurately in residential and industrial areas. The connected load or demand is the sum of continuous ratings of apparatus connected to the system. However, not all equipment is used simultaneously. The actual load in a system is significantly lower than the connected load. The demand factor is used to estimate the actual or maximum demand. The demand factor (DF) is defined by The demand factor depends on the number of customers and the type of load. Typical demand factor values are given in Table 65.4. 65.7 Voltage Regulation The voltage supplied to each customer should be within the ±5% limit, which, at 120 V, corresponds to 114 and 126-V. Figure 65.6 shows a typical voltage profile for a feeder at light and heavy load conditions. The figure shows that at heavy load, the voltage at the end of the line will be less than the allowable minimum voltage. However, at the light load condition the voltage supplied to each customer will be within the allowable limit. Calculation of the voltage profile, voltage drop, and feeder loss is one of the major tasks in distribution system design. The concept of voltage drop and loss calculation is demonstrated using the feeder shown in Fig. 65.6. To calculate the voltage drop, the feeder is divided into sections. The sections are determined by the loads. Assuming a single-phase system, the load current is calculated by Eq. (65.1): (65.1) where P is the power of the load, V is the rated voltage, and j is the power factor. The section current is the sum of the load currents. Equation (65.2) gives the section current between load i and i – 1: TABLE 65.3Typical Annual Load Factor Values Type of Load Load Factor Residential 0.48 Commercial 0.66 Industrial 0.72 TABLE 65.4Typical Demand Factors for Multifamily Dwellings Number of Dwellings Demand Factor, % 3 to 5 45 18 to 20 38 39 to 42 28 62 & over 23 LSF average loss loss at peak load = LSF 0LF+0.7LF 2 =.3 DF maximum demand total connected demand = ** **I P V II i i i ii i i ==+ cos , (cos sin ) j jj ? 2000 by CRC Press LLC (65.2) The electrical parameters of the overhead feeders are the resistance and reactance, which are given in W/mi. The underground feeders have significant capacitance in addition to the reactance and resistance. The capac- itance is given in mF/mi. The actual values for overhead lines can be calculated using the conductor diameter and phase-to-phase and phase-to-ground distances [Fink and Beaty, 1978]. The residential underground system generally uses single-conductor cables with polyethylene insulation. The older systems use rubber insulation with neoprene jacket. Circuit parameters should be obtained from manufacturers. The distribution feeders are short transmission lines. Even the primary feeders are only a few miles long. This permits the calculation of the section resistance and reactance by multiplying the W/mi values by the length of the section. The length of the section in a single-phase two-wire system is two times the actual length. In a balanced three-phase system, it is the simple length. In a single-phase three-wire system the voltage drop on the neutral conductor must be calculated. Further information may be obtained from Pansini [1991]. Equation (65.3) gives the voltage drop, with a good approximation, for section i, (i – 1). The total voltage drop is the sum of the sections voltage drops. (65.3) Equation (65.4) gives the losses on the line: (65.4) The presented calculation method describes the basic concept of feeder design; more details can be found in the literature. FIGURE 65.6 Feeder voltage profile. II ii i i (, –) – 1 1 1 = ? eIR X ii ii ii ii ii ii,( ) ,( ) ,( ) ,( ) ,( ) ,( ) ( cos sin ) ------ =+ 111111 ** jj Loss ii i i IR= -- - ? () ,( ) ,( )1 2 1 1 1 ? 2000 by CRC Press LLC 65.8 Capacitors and Voltage Regulators The voltage drop can be reduced by the application of a shunt capacitor. As shown in Fig. 65.7, a properly selected and located shunt capacitor assures that the voltage supplied to each of the customers will be within the allowable limit at the heavy load condition. However, at light load, the same capacitor will increase the voltage above the allowable limit. Most capacitors in the distribution system use switches. The capacitor is switched off during the night when the load is light and switched on when the load is heavy. The most frequent use of capacitors is on the primary feeders. In an overhead system, three-phase capacitor banks with vacuum switches are installed on the poles. Residential underground systems require less shunt capacitance for voltage control due to the reduced reactance. Even so, shunt capacitors are used for power factor correction and loss reduction. The optimum number, size, and location of capacitor banks on a feeder is determined by detailed computer analyses. The concept of optimization includes the minimization of the operation, installation, and investment costs. The most important factor that affects the selection is the distribution and power factor of loads. In residential areas, the load is uniformly distributed. In this case the optimum location of the capacitor bank is around two-thirds of the length of the feeder. The effect of capacitor bank can be studied by adding the capacitor current to the load current. The capacitor current flows between the supply and the capacitor as shown in Fig. 65.7. Its value can be calculated from Eq. (65.5) for a single-phase system: (65.5) where C is the capacitance, f is the frequency (60 Hz), and V is the voltage to ground. The capacitive current is added to the inductive load current, reducing the total current, the voltage drop, and losses. The voltage drop and loss can be calculated from Eqs. (65.2) to (65.5). The voltage regulator is a tap-changing transformer, which is located, in most cases, at the supply end of the feeder. The tap changer increases the supply voltage, which in turn increases the voltage above the allowable minimum at the last load. The tap changer transformer has two windings. The excitation winding is connected in parallel. The regulating winding is connected in series with the feeder. The latter has taps and a tap changer FIGURE 65.7 Capacitor effect on voltage profile. I jCV f c ==wwp,2 ? 2000 by CRC Press LLC switch. The switch changes the tap position according to the required voltage. The tap changing requires the short interruption of load current. The frequent current interruptions reduce the lifetime of the tap changer switch. This problem limits the number of tap changer operations to between one to three per day. Defining Terms Capacitor bank: Consists of capacitors connected in parallel. Each capacitor is placed in a metal can and equipped with bushings. Feeder: Overhead lines or cables which are used to distribute the load to the customers. They interconnect the distribution substations with the loads. Recloser: A circuit breaker which is designed to interrupt short-circuit current and reclose the circuit after interruption. Substation: A junction point in the electric network. The incoming and outgoing lines are connected to a busbar through circuit breakers. Tap changer: A transformer. One of the windings is equipped with taps. The usual number of taps is 32. Each tap provides a 1% voltage regulation. A special circuit breaker is used to change the tap position. Related Topics 1.2 Capacitors and Inductors ? 3.1 Voltage and Current Laws ? 3.2 Node and Mesh Analysis ? 3.4 Power and Energy ? 67.4 Load Management References D.F.S. Brass et al., in Electric Power Distribution, 415 V–33 kV, E.O. Taylor and G.A. Boal (eds.), London: Edward Arnold, 1966, p. 272. D.G. Fink and H.W. Beaty, Standard Handbook for Electrical Engineers, 11th ed., New York: McGraw-Hill, 1978, sec. 18. T. G?nen, Electric Power Distribution System Engineering, New York: Wiley, 1986. T. G?nen, Electric Power Transmission System Engineering, New York: Wiley, 1988, p. 723. A.J. Pansini, Power Transmission and Distribution, Liburn, Ga.: The Fairmont Press, 1991. E.P. Parker, McGraw-Hill Encyclopedia of Energy, New York: McGraw-Hill, 1981, p. 838. Various, Electrical Transmission and Distribution Reference Book, W. Central Station Engineers, East Pittsburgh: Westinghouse Electric Corporation, 1950, p. 824. Various, Distribution Systems. Electric Utility Engineering Reference Books, J. Billard (ed.), East Pittsburgh: Westinghouse Electric Corporation, 1965, p. 567. Various, EHV Transmission Line Reference Book, G.E.C. Project EHV (ed.), New York: Edison Electric Institute, 1968, p. 309. B.M. Weedy, Underground Transmission of Electric Power, New York: Wiley, 1980, p. 294. W.L. Weeks, Transmission and Distribution of Electrical Energy, New York: Harper & Row, 1981, p. 302. Further Information Other recommended publications include J. M. Dukert, A Short Energy History of the United States, Edison Electric Institute, 1980. Also, the IEEE Transactions on Power Delivery publishes distribution papers sponsored by the Transmission and Distribution Committee. These papers deal with the latest development in the distribution area. Every-day problems are presented in two magazines: Transmission & Distribution and Electrical World. ? 2000 by CRC Press LLC