【正文】 Electric power system1 Technical Characteristics of Electric Power The electric power has unique technical characteristics which give the power industry certain unique characteristics.1 Intangibility. The customer cannot directly detect a kilowatthour with any of his physical senses.2 Quality. The quality of service can be measured by service continuity or reliability, uniformity of voltage at the proper level, proper and uniform frequency of the alternating voltage.3 Product storage. Unlike most businesses, the electric power utility must create its product simultaneously with its use because there is no storage of electricity.4 Responsibility for power service. Because the utility delivers its product to the customer’s premises it must assume responsibility for the safe and reliable delivery of its product.5 Public Safety. The utility must provide reasonably adequate protection for the public and its own skilled workers.2 Power System PlanningIn anticipation of continued growth in the loads served by the electric utilities, power systems must be continually expanded in capability. Longrange planning is essential to assure that necessary additions are technically adequate, reasonable in cost and fit into a growth pattern. The difficulties encountered by the longrange planner include: uncertainty of load growth with respect to both geography and time, the probability of new invention or technological development. Good system planning strives for optimum design on a systemwide basis, not necessarily for minimum cost in one part of the system without regard to the effect on the other parts. In recent years, there has been an emphasis on economy in planning and operation. Now there is increased emphasis on reliability and environmental factors.Before planning decisions are made, many factors must be carefully considered:(1)Equipment decision have longterm effects requiring a forecast and study period of 1525 years.(2)There are many alternate means of generating electric power nuclear, baseload fossil, midrange bustion turbines or hydro, and in largemedium or smallsize plants, and different forms of energy storage.(3)There are several alternate means of transmitting electric power, for example, by alternating or direct current, overhead or underground cable, and all over a wide range of voltages.(4)The planning decisions are affected by load management techniques and the load patterns.(5)Uncertainty exists concerning the factors, such as future fuel cost, interest rates on money and capital availability, equipment forcedoutage rates, new technologies and environmental restrictions.3 Electrical Distribution Primary Distribution SystemsThe wiring between the generating station and the final distribution point is called the primary distribution systems. There are several methods used for transmitting the power between these two points. The two most mon methods are the radial system and the loop system.（1）The Radial SystemsThe term radial es from the word radiate, which means to send out or emit from one central point. A radiate system is an electrical transmission system which begins at a central station and supplies power to various substations.In its simplest from, a radial system consists of a generating station which produces the electrical energy. This energy is transmitted from the generator(s) to the central station, which is generally part of, or adjacent to, the generating station. At the central station the voltage is stepped up to a higher value for longdistance transmission.From the central station, several lines carry the power to various substations. At the substations the voltage is usually lowered to a value more suitable for distribution in populated areas. From the substations, lines carry the power to distribution transformers. These transformers lower the voltages to the value required by the consumer.（2） The loop systemThe loop system starts from the central station or a substation and makes a plete loop through the area to be served, and back to the starting point. This results in the area being supplied from both ends, allowing sections to be isolated in case of a breakdown. An expanded version of the loop system consists of several central stations joined together to from a very large loop.（3）Consumer Distribution SystemsThe type of distribution system that the consumer uses to transmit power within the premises depends upon the requirements of the particular installation. Residential occupancies generally use the simplest type. Commercial and industrial systems vary widely with load requirements. Singlephase SystemsMost singlephase systems are supplied from a threephase primary. The primary of a singlephase transformer is connected to one phase of the threephase system. The secondary contains two coils connected in series with a midpoint tap to provide a singlephase, threewire system. This arrangement is generally used to supply power to residential occupancies and some mercial establishments. For residential occupancies, the service conductors are installed either overhead or underground. Singlefamily and small multifamily dwellings have the kilowatthour metes installed on the outside of the building. From the kilowatthour meter, the conductors are connected to the main disconnect.Three separate disconnecting means are used with one mon ground.From the main disconnect, the conductors suppl