The neutral point converter proposed by Nabae, Takahashi, and Akagi in 1981 was
essentially a three-level diode-clamped inverter [5]. In the 1990s several researchers published
articles that have reported experimental resultsfor four-, five-, and six-level diode-clamped
converters for such uses as static var compensation,variablespeed motor drives, and highvoltage systeminterconnections [18-31]. A three-phase six-level diode-clamped inverteris
shown in Figure 31.5. Each of the three phases of the inverter shares a common dc bus, which
has been subdivided byfive capacitors into six levels. The voltage across each capacitor is Vdc,
and the voltage stress across each switching device is limited to Vdcthrough the clamping diodes.
Table 31.1 lists the output voltage levels possible for one phase of the inverter with the negative
dc rail voltage V0as a reference. State condition 1means the switch is on, and 0means the
switch is off. Each phasehas five complementary switch pairs such that turning on one of the
switches of the pair requires that the other complementaryswitch be turnedoff.The
complementary switch pairs for phase leg aare (Sa1, Sa’1), (Sa2, Sa’2),(Sa3, Sa’3),(Sa4, Sa’4), and
(Sa5, Sa’5). Table 31.1 also shows thatin a diode-clamped inverter, theswitches that are on for a
particular phase leg are always adjacent and in series.For a six-level inverter,a set of five
switches is on at any given time.
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