Applications based on renewable energy sources such
as photovoltaic (PV), wind, and fuel cells are fast in creasing due
to their environmental friendly nature as well as rapid development of power electronic technologies [1]-[15].Usually, a single renewable energy source varies its output power from time to time. For example, PV panels can supply maximum power during sunny days and almost no power on rainy and cloudy days; on the other hand, winds are often strong during bad weather days. Therefore, hybrid
power generation system incorporating multiple different renewable
energy sources is more reliable because of its ability of producing continuous power to the grid [9].Fig. 1 shows two alternative structures of multi-in put renewable energy power systems. Usually, a single-source renewable energy power system incorporates a step-up DC-DC converter and a DC-AC inverter in cascade. The step-up converter boosts and conditions the wide-range input DC
voltage to a desired level, and thus the inverter can operate at
minimum device stress and optimum efficiency. ln Fig. 1(a), separated converter- inverter systems are used to interface different power sources and generate power to the grid individually. Therefore, in each sub-system, there are two power stages incorporated, which inevitably increases the system cost and lowers the overall reliability. The alternative approach in Fig. 1(b) collects the outputs of the first-stage DC-DC converters to a common DC bus, and uses only one inverter to connect with grid [9]. Even though the structure in Fig. 1(b) reduces the required number of power stages, an independent DC-DC converter is still indispensable for each source. Three phase Z-source inverter (Fig. 2) was firstly proposed in 2002 [16] and investigated by many researchers a fterwards.It incorporates an X-shape impedance network coupling the DC source and inverter main devices, and introduces the extra shoot-through (or open-circuit) zero state such that the inverter output can be both stepped up and stepped down. This promising feature enables Z-source inverter to adapt a wide
input range based on the low-cost single stage topo logy.Moreover, the intrinsic immunity to EMI noises beca use of the coexistence of shoot-through and open circuit greatly enhances reliability. us recall the original three-phase voltage-fed Z-source inverter.The extra shoot-through zero state is utilized to boost the input DC voltage. Each time when performing the shoot-through operation, all the three phase legs are shorted simultaneously.However, intuitively, each phase must have a shoot-through control freedom independently. Shooting through three phase legs at the same time indeed results in a waste of two control freedoms. If all legs in the three-phase Z-source inverter can be decoupled, each one would have its own shoot-through control freedom such that an independent DC source can be adapted.