Decoupled Control Scheme of Grid-Connected Split-Source Inverters

During the last few years, single-stage power conversion systems has undergone a fast evolution to replace the conventional two-stage architecture, which includes a front-end dc-dc boost converter (BC) and an output voltage source inverter (VSI) [1]. This evolution has grown up to improve the overall system performance in terms of reducing its size, weight, and complexity. Most of these single-stage topologies and their different modulation schemes have been reviewed in [1]. Among these different single-stage options, the split-source inverter (SSI), shown in Figure 1, has been recently proposed in [2] as a single-stage dc-ac power converter topology to overcome some demerits in the other single-stage topologies, like the discontinuity of the input current and the dc-link voltage. According to [1], the SSI has the following merits: continuous dc-link voltage; continuous input current; lower switch voltage stresses with higher voltage gains, i.e. for lower input dc voltages, compared to the other equivalent topologies; lower passive component-count; no need for additional active switches compared to the standard VSI; same standard modulation schemes as the VSI for basic operation; same switching states as the VSI. Meanwhile, it suffers from the following demerits: higher current stresses of the lower switches; higher voltage stresses and higher THD of the output voltage for lower voltage gains, i.e. for higher input dc voltages; high frequency commutations of the input diodes. Several research works have been done on the SSI, as in [3]-[6], where the authors in [3]-[5] are discussing its three-level operation using the diode-clamped and flying capacitors bridges, while its single-phase operation is discussed in [6]. Meanwhile, its control scheme in grid-connected mode of operation has not been investigated yet. The SSI is modulated using the same eight standard states of the VSI, unlike the ZSI that utilizes an additional state, called the shoot-through state, to achieve the boosting capability. Such additional state gives an additional degree of freedom to control its dc side independently from the ac one, in which the two-stage conventional control method is utilized. Hence, it is of paramount importance to investigate the possibility of using the conventional synchronous reference frame control technique, that is commonly used with the two-stage architecture, with the so-called SSI, which is convenient for many applications. Accordingly, this paper models the SSI dc side and proposes a modified modulation scheme combined with the synchronous reference frame control technique, which is shown in Figure 2, to achieve a decoupled control scheme of the SSI in grid-connected mode, i.e. the dc and the ac sides of the SSI can be controlled independently. In this decoupled control scheme, the common mode term of the ac modulating signals is used to regulate the dc side, leading to an additional degree of freedom of having two control parameters like the two-stage architecture.