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POWER QUALITY phần 5

Việc sử dụng máy bơm, máy nén ly tâm hoặc loại bánh công tác và làm giảm các vấn đề nhấp nháy đáng kể. Các vấn đề nhấp nháy được không, đối với hầu hết các phần, loại bỏ cho đến khi các trạm lớn tạo đưa vào sử dụng. Ánh sáng nhấp nháy do | FIGURE 4.1 Sinusoidal voltage and current functions of time t . Lagging functions are indicated by negative phase angle and leading functions by positive phase angle. FIGURE 4.2 Nonsinusoidal voltage waveform Fourier series. The Fourier series allows expression of nonsinusoidal periodic waveforms in terms of sinusoidal harmonic frequency components. frequency of f the second harmonic has a frequency of 2 X f the third harmonic has a frequency of 3 X f and the nth harmonic has a frequency of n X f. If the fundamental frequency is 60 Hz as in the U.S. the second harmonic frequency is 120 Hz and the third harmonic frequency is 180 Hz. The significance of harmonic frequencies can be seen in Figure 4.3. The second harmonic undergoes two complete cycles during one cycle of the fundamental frequency and the third harmonic traverses three complete cycles during one cycle of the fundamental frequency. V1 V2 and V3 are the peak values of the harmonic components that comprise the composite waveform which also has a frequency of f. 2002 by CRC Press LLC 1 L I FUNDAMENTAL V1 sin wt 1 CYCLE 1 SECOND HARMONIC 1 V2 sin 2wt 1 cycle A A A THIRD HARMONIC 1 1 V3 sin 3wt V V FIGURE 4.3 Fundamental second and third harmonics. The ability to express a nonsinusoidal waveform as a sum of sinusoidal waves allows us to use the more common mathematical expressions and formulas to solve power system problems. In order to find the effect of a nonsinusoidal voltage or current on a piece of equipment we only need to determine the effect of the individual harmonics and then vectorially sum the results to derive the net effect. Figure 4.4 illustrates how individual harmonics that are sinusoidal can be added to form a nonsinusoidal waveform. The Fourier expression in Eq. 4.3 has been simplified to clarify the concept behind harmonic frequency components in a nonlinear periodic function. For the purist the following more precise expression is offered. For a periodic voltage wave with fundamental .