Page 110 - Power Quality in Electrical Systems
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Power Harmonic Filters  93


                        Resultant Fourier spectrum of system without filter
        400 V



        300 V



        200 V




        100 V



          0 V
            0 Hz   100 Hz  200 Hz  300 Hz  400 Hz  500 Hz  600 Hz  700 Hz
               V(Vo)                   Frequency
                                          (c)
        Figure 6.18 (Continued)



          The PSPICE simulation result (Figure 6.18b) shows the result in the
        time domain. In the frequency domain, we see harmonic distortion, as
        expected, at the 5th, 7th, and 11th harmonics, as shown in the Fourier
        spectrum of Figure 6.18c.
          We can reduce the harmonic distortion in the load voltage using the
        circuit of Figure 6.19a. The three series-resonant circuits are tuned
        4 percent below the 5th, 7th, and 11th harmonics. In Figure 6.19b, we
        see an improvement in the waveform distortion. Looking at the spectrum
        (Figure 6.19c), note that the 5th, 7th, and 11th harmonic distortion has
        been largely eliminated and that the resultant THD of the load voltage
        in the filtered case is roughly 1 percent.



        TABLE 6.2 Calculating THD for Example 6.3
                           Magnitude of line
                             impedance at     Voltage drop at
        Harmonic number    harmonic frequency  harmonic frequency
               1               0.08 Ω              8.4
               5               0.41 Ω             20.7
               7               0.58 Ω             14.5
              11               0.91 Ω             13.7
   105   106   107   108   109   110   111   112   113   114   115