[原创]电气英语原文2 Five-wire Distribution System Demonstration Project A four-wire multl-grounded distribution system is the predominant system used in North America.One of the features of a four-wire design is that unbalanced load current can return in the earth. The presence of ground current results in the drawbacks that high-impedance faults are different to detect, stray voltages can be caused, and magnetic fields are higher. Other types of distrbution disigns have been proporsed to address these concerns including there-wie designs. The five-wire design is a new approach that may reduce stray voltage and magnetic fields and also make high-impedance faults more easily derectable. The didth wire is an isolated neutral that carries all of the unbalaunced return current. Under normal conditions, the five-wire design operates very similar to the four-wire design with one major exception: all the return current is confined to the neutral that is isolated from ground. The multi-grounded ground wire continues to perform the safety functions associated with a multi-grounded system. The purpose of this study was to simulate, construct, and monitor a five-wire distribution system over a two-year period. The first year of the two-year minitoring period invoved extensive monitoring of an existing four-wire system. After completion of the first year of monitoring, the four-wire system was converted to a five-wire system, and another year of monitoring was performed. A 12.47 KV three phase wye distribution system circuit in South Cooperstown, New York was chosen as the five-wire demonstration site. The circuit consists of approximately ten to thirteen kiometers of overhead line. Most of the line is of armless construction, with some cross-arm construction scattered throughout the system. Approximately four kilometers of South Cooperstown circuit wasconverted to a five-wire configuration. The two-bushing transformer has an arrester on both bushings, and one bushing is attached to the fifth wire. The main conclusion of the five-wire demon-stration project is that the five-wire system improved performance for high impedance faults, stray voltage, and magnetic fields relative to a four-wire system: High-impedance faults can be detected more easily on five-wire system. High-impedance faults as low as 5A may be detected on a five-wire system. Stray voltages are much less on a pure five-wire system than on a four-wire system. If only a portion of a circuit is converted to a five-wire system, end effects will impact the system’s ability to conntrol stray voltage. Some of the unbalanced return in the four-wire system may return via the ground wire in the five-wire system causing stray voltage on the five-wire portion. Magnetic fields are lower on the five-wire system. This is especially ture when the overhead line is near conducting objects in the ground like buried phone wires or water pipes. Balancing transformers are an effective way of conversion from a five-wire system to a four-wire system. A balancing transformer offers lower impedance that helps with over current coordination. A balancing transformer will also provide some protection for an open neutral condition. The fifth wire (the neutral) should be treated as an energized conductor and installed as such. Surge arresters of the same rating as the phase arrester should be mounted on the neutral bushings of all transformers. Crew practices are important when considering a five-wire system. The location of the neutral and its insulation require proper attention. Initial installations should have the fifth wire labeled to warn crews that it is not to be treated as a grounded conductor. ----------------------------------------------