K. Matsui, H. Kojima, I. Yamamoto, M. Hasegawa, H. Mori, and F. Ueda (Japan)
SVC, Active filter, Passive filter, NPC-inverter, Symmetrical modulation, Asymmetrical modulation
NPC-inverters have begun to be used in applications such as large ac motor drives, SVC's (Static Var Compensators) and the like. In this paper, various harmonic characteristics of SVC's are discussed, comparison being made between symmetrical and asymmetrical modulation strategies. It appears that the asymmetrical modulation strategy is the better approach in higher power factor applications such as ac motor drives. However, in a case requiring a passive filter, for example, with an extremely large scale SVC, a symmetrical modulation strategy is superior. Therefore, in this research, a symmetrical modulation is preferred for a NPC-PWM-inverter,. Since the harmonics appear in a limited narrow frequency range, the output harmonics can be easily eliminated using a simple passive filter. Furthermore, harmonics such as the fifth can be suppressed using the proposed SVC scheme, in which the feedback signal for the fifth harmonic is injected into the SVC. Using this feedback signal, the harmonics in the power system can be significantly reduced. This result is confirmed by simulation. Thus, in addition to the proper operation of the SVC, it is possible to perform an active filtering function.
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