Question

what is the z-source inverter?

Answer 1

z source inverters are DC to AC converting circuits..Rectifiers convert AC to DC, inverters do the opposite.
Normally, three phase inverters have 8 vector states (6 active states and 2 zero states). But ZSI along with these 8 normal vectors has an additional state known as the shoot through state, during which the switches of one leg are short circuited. In this state, energy is stored in the impedance network and when the inverter is in its active state, the stored energy is transferred to the load, thus providing boost operation. Whereas, this shoot through state is prohibited in VSI.

To achieve the buck-boost facility in ZSI, required Pulse-width modulation is as shown in figure. The normal Sinusoidal PWM (SPWM) is generated by comparing carrier triangular wave with reference sine wave. For shoot through pulses, the carrier wave is compared with two complementary DC reference levels. These pulses are added in the SPWM, highlighted in figure. ZSI has two control freedoms: modulation index of the reference wave which is the ratio of amplitude of reference wave to amplitude of carrier wave and shoot through duty ratio which can be controlled by DC level.