PV Module-Level CHB Inverter with Integrated Battery Energy Storage System

Abstract

In this paper, a photovoltaic (PV) module-level Cascaded H­Bridge (CHB) inverter with an integrated Battery Energy Storage System (BESS) is proposed. The advantages and drawbacks of the CHB circuit architecture in distributed PV generation systems are highlighted. The main benefits are related to the higher granularity of the PV power control, which mitigates mismatch effects, thus increasing the power harvesting. Nevertheless, heavy unbalanced configurations due to the intermittent nature of PV sources need to be properly addressed. In order to smooth the PV fluctuations, a Battery Energy Storage System is used to provide both an energy buffer and coordination of power supply and demand to obtain a flat profile of the output power. In particular, by exploiting the inherent modularity of the conversion circuit, a distributed storage system is also implemented by splitting the battery into smaller units each of which represents the backup module of a single power cell of the PV CHB. The proposed design and control strategy allows overcoming the operating limits of PV CHB inverter. Simulation results, carried out on a single­phase nineteen­level PV CHB inverter, evidence the effectiveness of the proposed design and control approach to minimize the adverse impact of deep mismatch conditions, thus enabling continuous power output by compensating PV power fluctuations.

Power Scheduling Method for Grid Integration of a PV-BESS CHB Inverter With SOC Balancing Capability

“…The evaluation of output inverter reference voltage is carried out by the time domain resonant controller [17] [28], [29]. This latter processes the actual feedback grid current error, ε ref…”

“…The practical implementation can find a suitable solution in multilevel inverters (MLI), in which the energy storage elements can be integrated in a distributed way, with evident advantages in terms of power control and management [15], [16]. When using the MLI topology, the power fluctuation problem can be addressed in two different ways: in some solutions there is full integration of the batteries and the PV panel, i.e., one group for each submodule (i.e., inverter stage) of the MLI [17], [18], [19]; other solutions provide for partial integration, with one or more batteries connected to the dc-link instead of to the PV panel [20], [21], [22] (so-called hybrid system). However, in all cases mentioned, the integration of each battery always requires an interface dc/dc converter capable of managing the charging/discharging phases during operation.…”

“…A unidirectional dc/dc converter is used only for the connection of the photovoltaic generator (PVG) to the dc-link. The proposed circuit configuration in PV-BESS CHB inverter represents a novelty with respect to the solutions of [17], [18], [19], [20], [21], [22].…”

Inverter Operation Mode of a PhotoVoltaic Cascaded H-Bridge Battery Charger

“…Therefore, the CHB is connected to the main grid, and it operates as an active rectifier to provide the difference in power needed for battery charging. One of the advantages of this configuration relies on the possibility of splitting the storage system into smaller units each of which is connected to the separate dc-links of the CHB circuit, leading to a distributed storage system [2][3][4][5][6][7][8].…”

“…In [6] the authors proposed a PV module-level single-phase CHB inverter with an integrated BESS to smooth the PV fluctuations and to provide both an energy buffer and coordination of power supply and demand to obtain a flat profile of the output power, also in case of PV generators mismatch conditions, by ensuring balanced dc-link voltage at the price of SOCs unbalancing. The authors of [7] are the same as [4], so the same issue of unbalanced power distribution among the HBs of hybrid BESS in a single-phase CHB/MMC inverter is considered by taking into account the effect of the capacitor voltage ripple of each sub-module in the multilevel configuration.…”