毕业设计英文文献翻译(电力方向附带中文)

大学毕业设计 英文文献翻译 ,关于电力系统方向,电力谐波!绝对原创!

Harmonics

Service reliability and quality of power have become growing concerns for many facility managers, especially with the increasing sensitivity of electronic equipment and automated controls. There are several types of voltage fluctuations that can cause problems, including surges and spikes, sags, harmonic distortion, and momentary disruptions. Harmonics can cause sensitive equipment to malfunction and other problems, including overheating of transformers and wiring, nuisance breaker trips, and reduced power factor.

What Are Harmonics?

Harmonics are voltage and current frequencies riding on top of the normal sinusoidal voltage and current waveforms. Usually these harmonic frequencies are in multiples of the fundamental frequency, which is 60 hertz (Hz) in the US and Canada. The most common source of harmonic distortion is electronic equipment using switch-mode power supplies, such as computers, adjustable-speed drives, and high-efficiency electronic light ballasts.

Harmonics are created by these ―switching loads‖ (also called “nonlinear loads,‖ because current does not vary smoothly with voltage as it does with simple resistive and reactive loads): Each time the current is switched on and off, a current pulse is created. The resulting pulsed waveform is made up of a spectrum of harmonic frequencies, including the 60 Hz fundamental and multiples of it. This voltage distortion typically results from distortion in the current reacting with system impedance. (Impedance is a measure of the total opposition—resistance, capacitance, and inductance—to the flow of an alternating current.) The higher-frequency waveforms, collectively referred to as total harmonic distortion (THD), perform no useful work and can be a

significant nuisance.

Harmonic waveforms are characterized by their amplitude and harmonic number. In the U.S. and Canada, the third harmonic is 180 Hz—or 3 x 60 Hz—and the fifth harmonic is 300 Hz (5 x 60 Hz). The third harmonic (and multiples of it) is the largest problem in circuits with single-phase loads such as computers and fax machines. Figure 1 shows how the 60-Hz alternating current (AC) voltage waveform changes when harmonics are added.

大学毕业设计 英文文献翻译 ,关于电力系统方向,电力谐波!绝对原创!

The Problem with Harmonics

Any distribution circuit serving modern electronic devices will contain some degree of harmonic frequencies. The harmonics do not always cause problems, but the greater the power drawn by these modern devices or other nonlinear loads, the greater the level of voltage distortion. Potential problems (or symptoms of problems) attributed to harmonics include:

■ Malfunction of sensitive equipment

■ Random tripping of circuit breakers

■ Flickering lights

■ Very high neutral currents

■ Overheated phase conductors, panels, and transformers

■ Premature failure of transformers and uninterruptible power supplies (UPSs)

■ Reduced power factor

■ Reduced system capacity (because harmonics create additional heat, transformers and other

distribution equipment cannot carry full rated load)

Identifying the Problem

Without obvious symptoms such as nuisance breaker trips or overheated transformers, how do you determine whether harmonic current or voltages are a cause for concern? Here are several suggestions for simple, inexpensive measurements that a facility manager or staff electrician could take, starting at the outlet and moving upstream:

■ Measure the peak and root mean square (RMS) voltage at a sample of receptacles. The ―crest factor‖ is the ratio of peak to RMS voltage. For a perfectly sinusoidal voltage, the crest factor will be 1.4. Low crest factor is a clear indicator of the presence of harmonics. Note that these measurements must be performed with a ―true RMS‖ meter—one that doesn‘t assume a perfectly sinusoidal waveform.

■ Inspect distribution panels. Remove panel covers and visually inspect components for signs of overheating, including discolored or receded insulation or discoloration of terminal screws. If you see any of these symptoms, check that connections are tight (since loose connections could also cause overheating), and compare currents in all conductors to their ratings.

■ Measure phase and neutral currents at the transformer secondary with clamp-on current probes. If no harmonics are being generated, the neutral current of a three-phase distribution system carries only the imbalance of the phase currents. In a well-balanced three-phase distribution system, phase currents will be very similar, and current in the neutral conductor should be much lower than phase current and far below its rated current capacity. If phase currents are similar and neutral current exceeds their imbalance by a wide margin, harmonics are present. If neutral current is above 70 percent of the conductor‘s rated capacity, you need to mitigate the problem.

■Compare transformer temperature and loading with nameplate temperature rise and capacity ratings. Even lightly loaded transformers can overheat if harmonic current is high. A transformer that is near or over its rated temperature rise but is loaded well below its rated capacity is a clear sign that harmonics are at work. (Many transformers have built-in temperature gauges. If yours does not, infrared thermography can be used to detect overheating.)

大学毕业设计 英文文献翻译 ,关于电力系统方向,电力谐波!绝对原创!

In addition to these simple measurements, many power-monitoring devices are now commercially available from a variety of manufacturers to measure and record harmonic levels. These instruments provide detailed information on THD, as well as on the intensity of individual harmonic frequencies. After taking the appropriate measureme …… 此处隐藏：6320字，全部文档内容请下载后查看。喜欢就下载吧 ……