Two-Level Versus Three-Level PWM Inverters
Two-Level versus Three-Level PWM Inverters:
- The three-phase bridge inverter consisting of six switches can output pole voltages of only two levels 0.5Edc and -0.5Edc.
- In contrast to a two-level inverter, a three-level inverter is capable of producing three different pole-voltage levels, namely, 0.5Edc, zero and -0.5Edc.
- The circuit details of three-level inverter will not be discussed in this course but it can easily be shown that the three-level inverter will have better harmonic spectrum in comparison to the two-level inverter
- Any reduction in the fundamental output voltage magnitude of a two level inverter results in increased rms magnitude of unwanted ripple in the output waveform.
- Since the pole voltage can now have zero level too, the rms magnitude of the pole voltage can be brought below 0.5Edc. For lower magnitude of fundamental pole-voltage, suitable intervals of zero voltage level may be introduced such that with lowering of fundamental voltage the rms of the overall pole voltage also reduces.
- Thus the rms of the ripple voltage, in case of three-level inverter, can be made lower than that of the two-level inverter. The three-level versus two-level comparison can be applicable to a single-phase PWM inverter too.
- For the circuit of A 1-phase full-bridge VSI, for this circuit if all the time one of the two diagonal pair of switches, (Sw1 and Sw4) or (Sw2 and Sw3), conduct the load voltage will have two levels; E or –E. By suitably switching between one diagonal pair to another diagonal pair one can obtain a PWM waveform similar to the pole voltage waveform of a three-phase PWM inverter (only change is in the voltage magnitude).
- Now if the allowed switching combination includes conduction of Sw1 along with Sw3 (or Sw2 along with Sw4) the load voltage may have three-levels, i.e., E, zero and –E. As with a three-phase inverter, the single phase PWM inverter too will have lower voltage distortion in case of three-level load voltage (than the corresponding distortion in two level output).