【正文】 ipment, and transformers. In a large substation, circuit breakers are used to interrupt any short circuits or overload currents that may occur on the network. Smaller distribution stations may use recloser circuit breakers or fuses for protection of distribution circuits. Substations themselves do not usually have generators, although a power plant may have a substation nearby. Other devices such as capacitors and voltage regulators may also be located at a substation.Substations may be on the surface in fenced enclosures, underground, or located in specialpurpose buildings. Highrise buildings may have several indoor substations. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions.Where a substation has a metallic fence, it must be properly grounded (UK: earthed) to protect people from high voltages that may occur during a fault in the network. Earth faults at a substation can cause a ground potential rise. Currents flowing in the Earth39。s surface during a fault can cause metal objects to have a significantly different voltage than the ground under a person39。s feet。 this touch potential presents a hazard of electrocution.Transmission substationA transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains highvoltage switches that allow lines to be connected or isolated for fault clearance or maintenance. A transmission station may have transformers to convert between two transmission voltages, voltage control/power factor correction devices such as capacitors, reactors or static VAr pensators and equipment such as phase shifting transformers to control power flow between two adjacent power systems.Transmission substations can range from simple to plex. A small switching station may be little more than a bus plus some circuit breakers. The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, many circuit breakers and a large amount of protection and control equipment (voltage and current transformers, relays and SCADA systems). Modern substations may be implemented using international standards such as IEC Standard 61850.Distribution substationA distribution substation transfers power from the transmission system to the distribution system of an area. It is uneconomical to directly connect electricity consumers to the main transmission network, unless they use large amounts of power, so the distribution station reduces voltage to a value suitable for local distribution.The input for a distribution substation is typically at least two transmission or subtransmission lines. Input voltage may be, for example, 115kV, or whatever is mon in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between kV and 33kV depending on the size of the area served and the practices of the local utility.The feeders run along streets overhead (or underground, in some cases) and power the distribution transformers at or near the customer premises.In addition to transforming voltage, distribution substations also isolate faults in either the transmission or distribution systems. Distribution substations are typically the points of voltage regulation, although on long distribution circuits (of several miles/kilometers), voltage regulation equipment may also be installed along the line.The downtown areas of large cities feature plicated distribution substations, with highvoltage switching, and switching and backup systems on the lowvoltage side. More typical distribution substations have a switch, one transformer, and minimal facilities on the lowvoltage side.DesignThe main issues facing a power engineer are reliability and cost. A good design attempts to strike a balance between these two, to achieve sufficient reliability without excessive cost. The design should also allow expansion of the station, when required.Selection of the location of a substation must consider many factors. Sufficient land area is required for installation of equipment with necessary clearances for electrical safety, and for access to maintain large apparatus such as transformers. Where land is costly, such as in urban areas, gas insulated switchgear may save money overall. The site must have room for expansion due to load growth or planned transmission additions. Environmental effects of the substation must be considered, such as drainage, noise and road traffic effects. A grounding (earthing) system must be designed, and ground potential rise must be calculated to protect passersby during a shortcircuit in the transmission system. The substation site must be reasonably central to the distribution area to be served.The first step in planning a substation layout is the preparation of a oneline diagram which shows in simplified form the switching and protection arrangement required, as well as the ining supply lines and outgoing feeders or transmission lines. It is a usual practice by many electrical utilities to prepare oneline diagrams with principal elements (lines, switches, circuit breakers, transformers) arranged on the page similarly to the way the apparatus would be laid out in the actual station.In a mon design, ining lines have a disconnect switch and a circuit breaker. In some cases, the lines will not have both, with either a switch or a circuit breaker being all that is considered necessary. A disconnect switch is used to provide isolation, since it cannot interrupt load current. A circuit breaker is used as a protection device to interrupt fault currents automatically, and may be used to switch loads on and off, or to cut off a line when power is flowing in the 39。wrong39。 direction. When a