FIGURE 1.11: Radial Feeder.
T1 T2 T3 T4 T5
Fault
Power On Power Off
Single-Phase, Pad-Mounted Transformers
in this section, provides information on the economics of radial versus open-loop systems.
Open-Loop Feeder
As mentioned earlier, the open-loop feeder has two sources and, therefore, provides better system availability. Large subdivisions or com- mercial shopping areas are ideal applications of open-loop systems. Figure 1.12 shows an open- loop feeder in a shopping center. Utility crews can isolate any section of faulted cable and restore power to all transformers. This feature makes the open-loop feeder a preferred design
for UD systems serving multiple or critical consumers. An open-loop feeder also requires that the designer consider the ampacity of the primary cables and devices while serving all possible loop segments, which may dictate the use of a larger cable size than otherwise needed.
Multiple-Loop Feeder
In heavy load areas, multiple-loop feeders are necessary to improve sectionalizing and to allow the coordination of overcurrent protective devices. A typical multiple-loop system is shown in Fig- ure 1.13. This type of system usually has a sub-
FIGURE 1.12: Open-Loop Feeder in Shopping Center.
Three-Phase Feeder
Normally Open Point
Three-Phase, Pad-Mounted Transformers
Riser Pole Riser Pole
FIGURE 1.13: Multiple-Loop System.
Riser Pole Riser Pole
Sectionalizing Switch Sectionalizing Switch N.O. N.O. N.O. N.O. Legend
N.O. Normally Open Point Single-Phase, Pad-Mounted Transformer
N.O. Three-Phase, Pad-Mounted Transformer
feeder that serves as an open-loop system be- tween two sources. The sectionalizing switches on the sub-feeder have fused taps that serve other open-loop feeders. This arrangement provides excellent system availability. It also speeds up fault location because the large load area has been sectionalized into small load groups. A multiple-loop feeder also requires that the designer consider the ampacity of the feeder cables and devices while serving all possible loop segments, which may dictate the use of a larger cable size than otherwise needed.
TRANSFORMER AND SECONDARY SYSTEMS
Pad-mounted transformers and underground secondary-voltage cable constitute the final seg- ment of a UD system. To properly design this part of the system, the engineer must first select the appropriate equipment rating and cable ampacity.Section 4provides information for making these selections.
Second, the engineer must consider reliability. Most secondary cable faults are the result of me- chanical damage to the cable. Utilities can mini- mize mechanical damage by following the prop- er installation techniques described inSection 9 and by specifying cable with an abrasion-resistant
Ground Electrode Cable Riser Pole Lighting Package Underground Secondary-Voltage Cable
FIGURE 1.14: Area Lighting System.
or self-healing insulating jacket (seeSection 2). Cable dig-ins by other utilities or consumers also damage cable. To minimize dig-ins by con- sumers, cable should be installed two to three feet off the property line. Doing so helps pre- vent cable damage if the consumer installs a fence on the property line. Another method for minimizing dig-in damage is to use conduit. The conduit offers some mechanical protection, par- ticularly from hand digging. As noted, the coop- erative may particularly want to use conduit in areas congested with other utilities.
A third design concern with secondary systems is voltage drop and voltage flicker. The engineer must design a system that provides the consumer with acceptable voltage levels throughout the day and during motor starting.Appendix Blists the acceptable voltage levels and gives methods for calculating voltage drop and flicker.
STREET AND AREA LIGHTING
Public safety and consumer convenience require street and area lighting in the area served by a large percentage of underground projects. Most cooperatives furnish this service, so the engineer must make accommodations in underground sys- tems to include it. The engineer needs to devel- op a plan at the start of the project for eventual (if not actual) street and area lighting. Conduits and pedestals can then be installed at strategic locations that will minimize future trenching in lawns or around consumer facilities.
This type of UD system is shown in Figure 1.14. It uses a combination of overhead components (poles and a lighting package) and underground components (underground secondary-voltage cable, surge arresters, and grounding electrodes).
Street and area lights are generally self-con- tained units with an integral photoelectric cell for control. These standard light packages usually operate from 120-Volts single phase or 120/240- Volts single phase. The cooperative may want to consider using the same lighting package that it uses in overhead areas. Doing so will avoid unnecessary duplication of stock and minimize confusion during installation and maintenance.
If the lighting package requires a 120-Volt, two-wire power supply, service may be pro- vided through a two-wire duplex underground
cable. If the cooperative has a large amount of underground street lighting, purchasing a twisted duplex cable with a ruggedized insulation system will be most economical. This cable will essentially comply with the secondary cable spec- ification presented inAppen- dix C. When this duplex is used, the conductor may be either copper or aluminum.
When aluminum is used, the size should not be smaller than No. 6 American Wire Gauge (AWG). Satisfactory performance may be achieved with copper conductors as small as No. 10 AWG. In areas where deep frost lines are routine, larger aluminum conductors, possibly No. 2 AWG, might be considered as a minimum gauge.
In cases of infrequent use or where ruggedi- zed duplex cable is not readily available, Type UF (underground feeder) commercial cable may be substituted. This cable should be purchased only with copper conductors No. 10 AWG or larger. The Type UF cable must be rated as sunlight-resistant. Otherwise, the cable may deteriorate where it is exposed to sunlight between the pole riser conduit and the bottom of the lighting support bracket.
Lighting packages may be installed on wood poles at a height appropriate for the size of the lamp and the area to be lighted. On wood poles, polyvinyl chloride (PVC) conduit should be used to protect the cable riser. Schedule 40 PVC is recommended as a minimum. U-guards are not recommended because irregularities in wood poles may allow the smaller cable used for lighting service to protrude or be pinched between the U-guard and the pole surface. Each wood pole installation must be equipped with a pole-grounding conductor (No. 6 AWG copper) that is attached to a driven ground rod. This is particularly important because street and area lights are often among the highest objects in a subdivision served by an underground system. In cases of lightning strikes, the lightning must have a relatively low impedance path into the earth. If pole grounding conductors are not in- stalled, a much larger portion of the lightning
current will travel along the lighting conductors and be propagated into the secondary of the transformer and into all connected services. In areas with intense lightning activity, the cooperative should consider installing secondary lightning arresters on each transformer that serves a lighting installation.
Where aesthetics are of prime importance, cooperatives may choose to install metal lighting poles. In such cases, the height of the fixture mounting should not be compromised; it should be installed in accordance with standard practices for the partic- ular type of light and the size of the area to be lighted. With metal poles, the pole interior may generally be used as a raceway to conceal the conductor along its entire length. In these cases, sunlight resistance will not be required on Type UF cables if the cables are shielded from sun- light along their entire length. Metal poles will still require adequate grounding to avoid prob- lems with lightning surges. Metallic poles should also be directly connected to this same ground- ing system, which is also positively connected to the neutral of the secondary supply conductors. If the poles are direct buried, they generally have an insulating coating for corrosion protec- tion. If direct-buried poles are installed or if the poles are installed on poured concrete founda- tions, a ground rod is also recommended. If poles are installed on a metal screw anchor base, the ground rod may be eliminated.
The main limitation on the layout of street lighting conductors is voltage drop. As most contemporary lighting systems are either mer- cury vapor, metal halide, or high-pressure sodium systems, the most critical case is during starting of the most distant light. This is the time of highest current draw and lowest power factor. The magnitude and power factor of the starting current depend on the type of ballast, as does the acceptable voltage range for satisfactory op- eration. Table 1.1 gives examples of typical light characteristics. It is obvious that the regulator ballasts offer a substantial advantage in allowing long runs of small secondary voltage conductors