Karady, G.G. Energy Distribution The Electrical Engineering Handbook Ed. Richard C. Dorf Boca raton crc Press llc. 2000
Karady, G.G. “Energy Distribution” The Electrical Engineering Handbook Ed. Richard C. Dorf Boca Raton: CRC Press LLC, 2000
65 Energy Distribution 65.1 Introduction 65.2 Primary Distribution System 65.3 Secondary Distribution System 65.4 Radial Distribution System 65.5 Secondary Networks 65.6 Load characteristics George G. Karad 65.7 Voltage Regulation Arizona State University 65.8 Capacitors 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 lectric power system. The power plants convert the energy stored in the fuel( coal, oil, gas, nuclear)or hydro nto 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-volta 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 ibstations 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 c 2000 by CRC Press LLC
© 2000 by CRC Press LLC 65 Energy Distribution 65.1 Introduction 65.2 Primary Distribution System 65.3 Secondary Distribution System 65.4 Radial Distribution System 65.5 Secondary Networks 65.6 Load Characteristics 65.7 Voltage Regulation 65.8 Capacitors 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 highvoltage 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
Power plant High voltage network Transmission ■ CB Open Step-down astore Primary Distribution System 日来 dary Dis Secondary FIGURE 65.1 Electric energy system TABLE 65.1 Typical Primary Feeder Voltages(line-to-line) wire delta 4-wire y 4-wire y wire delta/4-wire y 4-wire Y 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
© 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
Circuit Breaker istnibution Reclosing Circuit I Feeder 1 Feeder 4 Feeder i 3 Phase, 4 Wire Main Feeder A Phase Lateral Feeder 济动 B- Phase Lateral Feeder Open Tie-switch 气十 Open Tie-switch to adjacent feeder FIGURE 65.2 Radial primary distribution system. The circulating y using the open-loop connection. This is a popular, frequently used circuit. Figure 65. loop primary feeder. The distribution substation has four outgoing main feeders. each rent 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 c 2000 by CRC Press LLC
© 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
der. 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 ne lighting loads are supplied by the 120V 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-Vfour 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 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 l-P 0/240 Three-wire Residential 208/120 Four-wire Commercial/ residential 3-Phase 480/277 Four-wire High-rise buildings 3-Phase 380/220 Four-wire General system, Europ 3-Phase 120/240 Four 3-Phase 240 Three-wire Commercial/industrial 3-Phase 480 Three-wire Industrial 3-Phase 240/480 Four-wire Industrial c 2000 by CRC Press LLC
© 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 transformers 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 fourwire network system is used. This network assures higher reliability but has significantly higher costs. Underground 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