Six-Step Modulation

7.2 Six-Step Modulation

Michael Giesselmann

Six-step modulation represents an early technique to control a three-phase inverter. Six-step modulation uses a sequence of six switching patterns for the three phase legs of a full-bridge inverter to generate a full cycle of three-phase voltages. A switch pair connected between the positive DC bus and the negative DC bus represents a phase leg. The output terminal is the midpoint of the two switches. Only one switch of a phase leg may be turned on at any given time to prevent a short circuit between the DC buses. One state of the inverter leg represents the case when the upper switch is turned on whereas the opposite state is represented by the lower switch being turned on. If each phase leg has these two states, the inverter has

2 3 = 8 possible switching states. Six of these states are active states, whereas the two states in which either all of the upper or all of the lower switches are turned on are called zero states, because the line-to-line output voltage is zero in these cases. The six discrete switching patterns for six-step modulation are shown in Fig. 7.1a to f. For clarity, free-wheeling diodes have been omitted. After the switching pattern shown in Fig. 7.1f , the cycle begins anew with the switching pattern shown in Fig. 7.1a . Note that in subsequent patterns, only a single inverter leg changes states. The switching patterns shown in Fig. 7.1a to f represent the following inverter states in the following order:

• Positive peak of Phase A • Negative peak of Phase C • Positive peak of Phase B • Negative peak of Phase A • Positive peak of Phase C • Negative peak of Phase B

The aforementioned inverter states are equally spaced in a circle with 60 ° of phase shift between them. This is illustrated in Fig. 7.2 . The hexagon in Fig. 7.2 represents the trace of a voltage vector around a circle for six-step modulation. This scheme could be extended to space vector modulation, if the voltage vector would not make discrete 60 ° steps, but would alternate at high speed between two adjacent states. The switching control would be such that the average time spend in the previous state is gradually decreasing, whereas the average time spent in the next state is gradually increasing. Also by inserting zero states, the magnitude of the output voltage could be controlled.

Figure 7.3 shows the phase to neutral waveform of one inverter leg for six-step operation if the neutral point is considered the midpoint between the positive and negative bus. The resulting line-to-line output voltage is shown in Fig. 7.4 . This waveform is closer to a sinusoid than the phase to neutral voltage but it still has a considerable amount of harmonics. Figure 7.5 shows the spectrum of the line-to-line voltage for six-step operation normalized to the fundamental frequency. The lowest harmonic component is the 5th harmonic.

The advantages of six-step modulation are the simplicity of the procedure and the ability to use slow- switching, high-power devices like GTOs. However, the harmonic content of the output voltage and the inability to control the magnitude of the output voltage are serious drawbacks. Because of these drawbacks and due to the recent advances in high-power IGBT technology, this modulation scheme is today seldom considered for new designs.