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890 POWER CONDITIONING FOR PHOTOVOLTAIC POWER SYSTEMS<br />

S 1<br />

D<br />

V PV<br />

C 1 L<br />

C 2<br />

V load<br />

i L<br />

Figure 19.33<br />

Step-down/step-up converter during “off” state<br />

19.2.4.3.4 Combination of a step-down converter with an inverter<br />

The combination of a step-down converter with a voltage inversion is shown in<br />

Figure 19.34. It should be noted that in this configuration the voltage of the DC input<br />

sources must always be equal to or bigger than the AC peak voltage.<br />

L1<br />

S0<br />

S1<br />

C1<br />

D<br />

C1<br />

AC output<br />

DC sources<br />

S2<br />

Figure 19.34 The combination of a step-down converter with inverters allows to produce<br />

sine-shaped AC output voltage. Wave shapes correspond to the voltage at the terminals over them<br />

19.2.4.3.5 Voltage shaping by digital synthesis<br />

In a further concept, the step-down converter can be replaced by digital synthesis as<br />

shown in Figure 19.34. In this concept, the desired voltage is obtained by binary addition<br />

of individual voltage sources [5–7]. Depending on the sum of these sources, the sine-shape<br />

can be approximated.<br />

When using 5 sources, 32 voltage steps according to 2 5 adjustable levels can be<br />

obtained as shown in Figure 19.35. The resulting total harmonic distortion (THD) 1 can<br />

be kept well below 5%.<br />

1 Definition of THD: sum of the amplitudes of all harmonic frequencies compared to the amplitude of the<br />

fundamental signal (the 50- or 60-Hz frequency).

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