Contents I. Introduction
Large-scale photovoltaic (PV) power systems interacting with a medium-or high-voltage (MV/HV) distribution grid are obtaining more and more attentions. For such MV/HV application, modular cascaded multilevel converter has been one of the most attractive topologies. Modularity and scalability allow MPPT to extract maximum power from distributed PV panels, which is crucial to achieving high energy efficiency, low cost and high power density, etc.. When it comes to each PV/converter module, although traditional single-stage structure [1], [2] is an option in lower-voltage applications owing to its simple structure and fewer devices integration, its extending to high-voltage application is precluded by many drawbacks like over-design etc.. Despite two-stage structure [3], [4] with extra dc-dc converter for voltage boosting was then proposed, additional stage will increase the cost and impoverish power density. In the last decade, Z-source network and quasi-Z-source network(ZSN/qZSN)[5], [6] receive more and more attentions due to their promising characteristics. In ZSN/qZSN based inverter, the buck-boost function for PV system is fulfilled. What's more, the reliability and output waveform quality are highly improved because of no dead-zone consideration which is one of tough barriers in traditional two-level VSIs. Nowdays, the main contributions of researches on qZSN are focused on modeling, design, operation and control under normally condition etc.. Moreover, as to the reliable and economic operation of the system, a proper modulation method is critical. Comparing with traditional two-stage VSIs, there exist more modulation strategies for cascaded multilevel converter. Complementary to control algorithms, the status of modulation strategies are raised day by day. Not only proper strategies can limit the switching loss in reasonable range, but also customized strategies according to existing control objectives can solve some special issue in multilevel systems like fluctuation of dc voltages, circulating current, [switching harmonics etc.. Comparing with traditional cascaded PV system in which the PD-PWM(phase disposition pulse width modulation) and PS-PWM(phase-shifted pulse width modulation) are two basic strategies, ZSN/qZSN based one allows the existing of shoot-through state, which determines that its modulation will be different with traditional ones. Current using strategies for ZSN/qZSN based cascaded system are mostly based on PS-PWM[7], [8]: the duty cycle of shoot-through state is decided by the requirements of dc voltage boosting and ac modulation index. The differences of these strategies are mainly reflected in the placing of shoot-through states. Taking sawtooth waves as carriers makes the original dis-continuous shoot-through states in a switching period continuous [7]. This strategy can reduce the switching loss but not that obvious. Phase-shifted pulse-width-amplitude(PS-PWAM) modulation method[8] centralizes the shoot-through states to one third of fundamental cycle. Although in this strategy the switching times can be reduced greatly, it can be only used in three-phase qZSN based inverters for 3rd order harmonics consideration. What's more, in these strategies, the positions of shoot-through states are relative fixed. Being susceptible to diversities in operating environment, manufacture etc., maximum power points (MPP) of PV modules won't be uniform consequently. Individual MPPT control for each module will deteriorate the symmetry of active-power distribution, which can bring in over-modulation in the phase suffering module-mismatch and power imbalance in three-phase level, both of which will degrade quality of electric energy and introduce negative effects to other electrical equipment interacted with the same power grid.
