俞敏洪附录Ⅰ：专业相关文献翻译
Faults on power system
Each year new designs of equipment bring about incread reliability of operation. Nevertheless, equipment failures and interference by outside sources occasionally result in faults on electric power system. On the occurrence of power from the generating stations to the loads may be unsatisfactory over a considerable area, and if the faulted equipment is not promptly disconnected from the remainder of the system, damage may result to other pieces of operating equipment.
A fault is the unintentional or intentional connecting together of two or more conductors which ordinarily operate with a difference of potential between them. The connection between the conductions may be by physical metallic contact or it may be through an arc. At the fault, the voltage between the two parts is reduced to zero in the ca of metal-to-metal contacts, or to a very low value in ca the connection is through an arc. Currents of abnormally high magnitude flow the network to the point of fault. The short-circuit currents will usually be much greater than the designed thermal ability of the conductors in the lines or machines feeding the fault. The resultant ri in temperature may cau damage by the annealing of conductors and by the charring of insulation. In the period during which the fault is permitt
ed to exist, the voltage on the system in the near vicinity of the fault will be so low that utilization equipment will be inoperative. It is apparent that the power system designer must anticipate points at which fault may occur, be able to calculate conditions that exist during a fault, and provide equipment properly adjusted to open the switches necessary to disconnect faulted equipment from the remainder of the system. Ordinarily it is desirable that no other switches on the system are opened, as such behavior would result in unnecessary modification of the system circuits.
A distinction must be made between a fault and an overload. An overload implies only that loads greater than the designed value have been impod on system. Under
such a circumstance the voltage at the overload point may be low, but not zero. This undervoltage condition may extend for some distance beyond the overload point into the remainder of the system. The currents in the overloaded equipment are high and may exceed the thermal design limits. Nevertheless, such currents are substantially lower than in the ca of a fault. Service frequently may be maintained, but at below-standard voltage.
Overloads are rather common occurrence in homes. For example, a houwife might plug five waffle irons into the kitchen circuit during a neighborhood party. Such an over-load, if permitted to continue,
would cau heating of the wires from the power center and might eventually start a fire. To prevent such trouble, residential circuits are protected by fu or circuit breakers which open quickly when currents above specified values persist. Distribution transformers are sometimes overloaded as customers install more and more appliances. The continuous monitoring of distribution circuits is necessary to be certain that transformer sizes are incread as load grows.
Faults of many types and caus may appear on electric power systems. many of us in our homes have en frayed lamp cords which permitted the tow conductors of the cord to come in contact with each other. When this occurs, there is a resulting flash, and if breaker or fu equipment functions properly, the circuit is opened.
Overhead lines, for the most part, are constructed of bare conductors. The are sometimes accidentally brought together by action of wing, sleet, trees, cranes, airplanes, or damage to supporting structures. Overvoltages due to lightning or switching may cau flashover of supporting or from conductor to conductor. Contamination on insulators sometimes results in flashover even during normal voltage conditions.
The conductors of underground cables are parated from each other and from ground by solid insul
ation, which may be oil-impregnated paper or a plastic such as polyethylene. The materials undergo some deterioration with age, particularly if overloads on the cables have resulted in their operation at elevated temperature. Any
small void prent in the body of the insulating material will result in ionization of the gas contained therein, the products of which react unfavorably with the insulation. Deterioration of the insulation may result in failure of the material to retain its insulating properties, and short circuits will develop between the cable conductors. The possibility of cable failure is incread if lightning or switching produces transient voltage of abnormally high values between the conductors.
cockroachTransformer failures may be the result of insulation deterioration combined with overvoltages due to lightning or switching transients. Short circuits due to insulation failure between adjacent turns of the same winding may result from suddenly applied overvoltages. Major insulation may fail, permitting arcs to be established between primary and condary windings or between a winding and grounded metal parts such as the core or tank.
mountainousGenerators may fail due to breakdown of the insulation between adjacent turns in the same slot, resulting in a short circuit in a single turn of the generator. Insulation breakdown may also occur betwtrainees
een one of the windings and the grounded steel structure in which the coils are embedded. Breakdown between different windings lying in the same slot results in short-circuiting extensive ctions of machine.weeds
Balanced three-pha faults, like balanced three-pha loads, may be handled on a lineto-neutral basis or on an equivalent single-pha basis. Problems may be solved either in terms of volts, amperes, and ohms. The handling of faults on single-pha lines is of cour identical to the method of handling three-pha faults on an equivalent single-pha basis.
V oltage transformers
V oltage transformers are ud with voltmeters, watt-meters, watt-hour meters, power-factor meters, frequency meters, synchroscopes and synchronizing apparatus, protective and regulating relays, and the no-voltage and over-voltage trip coils of automatic circuit breakers. One transformer can be ud for a number of instruments at the same time if the total current taken by the instruments does not exceed that forwords fail me
which the transformer is designed and compensated.
V oltage transformers are generally designed for a capacity of about 200 volt-amp. There are two caus of errors in voltage transformers, namely, ratio error and pha-angle error. The part of the errors due to the exciting current is constant for any particular voltage. It can be reduced to a minimum by choosing the best quality of iron and working it at a low magnetic density. The part of the errors due to the load current varies directly with the load and can be minimized by making the resistance of the windings very slow.
V oltage transformers are compensated for their iron loss at rated voltage. When ud on some other voltage, either higher or lower, an error is introduced. In general this error will not be more than 0.15 percent of rated voltage. A voltage transformer should never be ud on a circuit who voltage is more than 10 percent above the rated voltage of the transformer.
The condary terminals of a voltage transformer should never be short-circuited, a heavy current will flow which, if continued, will burn out the windings. In order to protect the system against sustained short circuits in the transformer circuit, it is generally recognized as good practice to introduce into the primary circuit a resister and fu, the been connected in ries. The resistors are designed to limit the current to about 20 to 40 amp., while the fus are designed to break such current. In normal operation the current which the resistor carries is only the very small primary curre
nt of the voltage transformer, and the drop in voltage that they cau is inappreciable.
Current transformers
Current transformers are ud with ammeters, watt-meters, power-factor meters, watt-hour meters, compensators, protective and regulating relays, and the trip coil of circuit breakers. One current transformer can be ud to operate not to exceed that for which the transformer is designed and compensated.
糟糠之妻俱乐部韩语The current transformer is connected directly in ries with the line, and usually has a fixed number of instruments in the condary. A ri or fall in the line current requires a corresponding ri or fall in the condary voltage to force the condary current through the impedance of the meter load. the magnetic flux in the iron, which
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supplies the voltage, thus follows the ri and fall of the primary or line current.
The instruments connected in the condary circuit of the transformer are placed in ries, so that the condary current will pass through each instrument. As the instrument are added, higher voltage is required to force the current through the instruments. This requires a high magnetic densit
y in the iron. A higher magnetic density increas both the iron loss and the magnetizing current; hence both the ratio and the pha-angle errors are magnified. For the sake of accuracy, therefore, there is a limit to the number of instruments that should be placed on a single current transformer.
The condary circuit of a current transformer should never be opened while the primary is carrying current. If it is necessary to disconnect instruments, the condary should first be short-circuited. If the condary circuit is opened, a difference of potential is developed between terminals which is dangerous to anyone coming in contact with the meters of leads. The cau of this high voltage is that with open condary circuit all the primary ampere turns are effective in producing flux in the core, whereas normally but a small portion of the total performs this function. The danger is magnified by the fact that the wave form of this condary voltage is peaked, produced in this way may also permanently change the magnetic condition or the core, so that the accuracy of the transformer were be impaired.
Arresters
One of the means of protecting transmission equipment is the surge arrester. two types of surge arresters may be ud for this reason: active gap (SiC) and gapless (ZnO) metal oxide surge arresters.
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Active gap (SiC) arrester
The two principal components of active gap surge arresters (diverters) are the spark gap and the non-linear resister. One of the earlier designs was the lightning arrester with plate gaps, which is still ud today in some medium voltage networks. At still higher voltages, arresters with magnetically blow spark gaps are more commonly ud, in particular in EHV networks (300—750kV). The consist mainly of three parts: spark gaps, discharge resistors and a grading system that monitors the
