Electrical Distribution Systems

• Introduction
• Switching Stations
• Switching Substations
• High-Voltage Conductors and Circuits
• Electric Power Generation
• System Protection
• Instruments
• Additional Resources

Introduction

Electricity is a unique power source in that it must be used the instant it is generated—it cannot be stored or stockpiled in large capacities to meet future requirements. Generating capacities must be able to meet peak load requirements instantaneously. Since the first system to sell lighting to New York City was installed in 1882, this has created two problems: how to deal with the voltage drop during transmission across long distances, and how to meet widely varying voltage requirements. High-voltage alternating current (AC) and the development of transformers in the late 19th century resolved these problems and made possible efficient distribution systems that would meet wide-ranging customer requirements. They also led to the phenomenal growth of the electric industry.

The electric systems in use before the turn of the century were direct current systems. The generation systems were small, the distribution network was limited, and the voltage levels were low. Alternating current distribution systems developed rapidly, when it was recognized that interconnecting generation sites produced economic benefits. The need for higher voltages became apparent as electric networks increased in size and power-carrying capability.

Although simple in concept, current electric distribution equipment and systems are characterized by highly sophisticated technologies that continue to develop rapidly. Because electricity is invisible and its effects are not readily discernible, a mathematical approach is needed to achieve a full understanding of the design and operation of modern distribution systems. This requires highly technical training and is beyond the scope of this manual; the intent here is to describe the physical devices, their purposes, and their relationships to provide a more general understanding of the systems.

Presently, transmission lines in the United States carry voltages of 345,000, 500,000, and 765,000 volts (V). For distribution systems, utilities use 12,000, 12,470, 13,200, 69,000, and 138,000 V. The primary voltages for medium to large customers are 12,000, 12,470, 13,200, 4,160, and 2,400 V. The following three main research projects dealing with higher transmission line voltages were conducted:

1. 1,000-kV line, by Bonneville Power Administration
2. 1,000-kV line, by Electric Power Research Institute (EPRI) and General Electric Company
3. 2,000-kV line, by American Electric Power (AEP) and the Swedish corporation Allamanna Svenska Electriska Atievalaget (ASEA)

The studies revealed that transmitting at higher voltages results in higher power transfers over long distances due to increase in surge impedance loading (SIL), reduction in current and losses.

However the studies also revealed that a higher inventory of spare equipment would be needed to reduce the exposure to extended outages. In addition, industry standardization for UHV (Ultra High Voltage) equipment is not as advanced as for comparable EHV (Extra High Voltage) equipment. There were also insulation challenges for lines passing through high altitude and polluted areas. The studies also determined that Increased impedance of a transformer at each line terminal increases losses and cost. Adding a new voltage overlay also requires a large investment in tools, equipment, and training.

The above transmission voltages thus remain unproven in commercial application. In addition, dramatic reductions in load growth and the development of natural gas-fired generation close to load centers subsequently deferred the need for long distance transmission projects.

The first 735-kV transmission lines were built by Hydro-Québec in 1965. Although efforts continued to establish the technical feasibility of power transmission in the range of 1000-1500 kV, practical implementation of transmission systems at these voltages was not feasible because of a steady decline in load growth following the energy crisis that began in 1973. Transmission lines in the range of 1000-1200 kV were built in Russia (the former USSR) and Japan, but the Russian line, after a few years of operation at the design voltage of 1150 kV, has been operated at the lower level of 500 kV, while the Japanese 1000-kV line has also been operated at 500 kV since it was built. Thus, the highest operating transmission voltage in different countries around the world continues to be in the range of 700-800 kV.

The introduction of high-voltage direct current (HVDC) transmission lines has opened new horizons. The advantage of HVDC over AC for long distances is its lower cost. Presently, the break-even point is 500 miles. For transmission lines longer than 500 miles, HVDC is cheaper than AC, and for lines shorter than 500 miles, AC is cheaper. When using HVDC, doubling the voltage of a cable increases the power-carrying capability of the cable by fourfold. However, as the network voltages increase, so do the costs of design, installation, and maintenance.

The electric distribution systems described here are typical of university-owned facilities where electricity (whether generated on campus, purchased, or both) is received and further distributed to points on campus. Not covered are situations where a municipally or commercially owned electric utility furnishes electricity in utilization voltages to individual buildings.

College and university electric distribution systems generally consist of (1) a switching station for receiving the electricity into the university system, (2) switching substations (which include transformers), (3) high-voltage conductor circuits, (4) electric power generation, and (5) system protection.