【導(dǎo)讀】電力具有獨特的技術(shù)特點，這使得電力工業(yè)具有獨特的行業(yè)特點。用戶不能用人體感官直接察覺千瓦時的用電量。供電質(zhì)量可由供電連續(xù)性或供電可靠性、在標(biāo)準(zhǔn)電壓等級下。的電壓均等性、交流電壓頻率的正確不變性來度量。求生產(chǎn)出電力來，因為電能無法貯存。電由電力部門輸送到用戶，因此必須對安全、可靠供。電力部門須對公眾及其技術(shù)人員提供穩(wěn)妥的保護。遠期規(guī)劃主要是保證這種擴建在技術(shù)上是適宜的，在造價上是合理的，與增長模式是相符的。負荷增長的不確定性、新發(fā)明新技術(shù)發(fā)展的可能性。其它部分的影響。近年來，已經(jīng)強調(diào)了規(guī)劃和運行的經(jīng)濟性。電或水電，以及大型、中型、小型電廠和各種形式的蓄能。這將造成所有的電站供應(yīng)同樣的電能,最后。因為隔離絕緣而崩潰。接到三向系統(tǒng)中。這種接地系統(tǒng)運行制度意味著。相變壓器，三繞組變壓器或三相變壓器系統(tǒng)供應(yīng)的。設(shè)計和安裝人員操作這些系統(tǒng)一定要遵從NEC的要求和當(dāng)?shù)氐男?。放在地面上的目的是?

　　

【正文】 a general term covering switching and interrupting devices, also associated devices with control, metering, protective and regulatory equipment. Switchgear mainly includes circuit breaker, disconnecting switch, loadbreak switch and fuse. The disconnect switch is the simplest switch on the basis of function, operating only in the absence of appreciable current. This switch cannot open normal load current and its function is to disconnect or connect transformers, circuit breakers, other pieces of equipment and short length of high voltage conductors only after current through them has been interrupted by opening a circuit breaker or loadbreak switch. A loadbreak switch will switch normal load currents but will not interrupt short circuit currents. However, circuit breakers will perform the switching functions of the above two classes, but will, if applied within rating, interrupt all short circuit currents that may occur on the system. Fuses consist essentially of a fusible element and an arcextinguishing means. Circuit breakers and disconnect switches should not be blown open or otherwise damaged by short circuit currents within their short time ratings. The circuit breakers and disconnecting switches should be designed or protected to withstand normal operating voltages across the device in the open position. 6 Means of Reactive Power Compensation The capacitance of a line has two related voltage effects. One is the rise in voltage along the line resulting from the capacitive current of the line flowing through the line inductance. The second effect is the rise in voltage 20 resulting from the capacitive current of the line flowing through the source impedance. These effects are corrected by the generator voltage regulators. If the line delivers too much charging current, the generator field excitation will bee very low which reduces the stability limit and is unacceptable. These voltages can be reduced by the application of shunt reactors. The degree of pensation provided by a reactor is usually quantified by the percentage of the line capacitance that is pensated. The percent shunt pensation of EHV lines in service ranges from 0% to 90% with the reactors located in the substations at one or both ends of the line. The basic purpose of a shunt capacitor bank is to increase the local circuit voltage or improve the load power factor carried by the circuit. Many large capacitor banks are switched on and off as the system need for reactive kilovolt amperes changes. System requirements govern whether a certain bank should or should not be switched. If the voltage at the capacitor would be too high during knight load, some or all of the capacitors are switched off. Very large banks are usually switched in steps. This procedure has the disadvantage of requiring more switches and thus increasing the total equipment cost per kilovar. It, however, provide a means of keeping the voltage change per step within permissible limits. A synchronous condenser is nothing more than a synchronous machine running at synchronous speed witch no mechanical load. The condenser has a control circuit that controls the field excitation to provide voltage control. When the system voltage starts to fall below the desired values, the control circuit will automatically increase the field excitation which causes the synchronous condenser to supply vats to the system. This will increase the system voltage at the point. 7 Overvoltage and Insulation Coordination An area of critical importance in the design of power system is the 21 consideration of the insulation requirements for lines, cables and stations. When lightning strikes a phase conductor of transmission line, the current of the lightning stroke will encounter the surge impedance of the conductor so that overvoltage will be built up and propagate to the substation along the transmission line in wave form. This type of overvoltage is called lightning ining wave. It will danger electric equipment in substations. Insulation coordination is the process of determining the proper impulse insulation level and switching insulation level required in various electrical equipments together with the proper surge arrester. This process is determined from the known surge characteristics of equipment and the characteristics of surge arresters. 8 Grounding Grounding in power system is for the purpose of operation consideration, lightning proof and safety of personnel and equipment. Grounding means connecting to a low resistance earth electrode or an excellent earthing system. The earthing installations must have a currentcarrying capacity sufficient to deal with the maximum fault currents, and a grounding resistance low enough to prevent a dangerous voltage appearing between any points which a man could reach simultaneously. The earthing arrangements should also be such as to ensure that, under fault conditions, the lowest practical voltage appears between earthed points of the equipment and the main body of the earth, so that insulation breakdown or burning does not occur on equipment which is earthed. During a fault, the flow of current to earth will result in voltage gradients on the surface within and around a substation. Unless proper precautions are taken, the voltage differences along the ground may be great enough to endanger a person walking there. In addition, such voltage differences can sometimes exist. Between“ grounded” structures or equipment frames and the nearby earth. As a 22 result of these concerns, it is mon practice for substations to have an electrical ground system consisting of a gird of horizontal and buried conductor.