The stability of dc microgrids (MGs) depends on the control strategy adopted for each mode of operation. In an islanded operation mode, droop control is the basic method for bus voltage stabilization when there is no communication among the sources. In this paper, it is shown the consequences of droop implementation on the voltage stability of dc power systems, whose loads are active and nonlinear, e.g., constant power loads. The set of parallel sources and their corresponding transmission lines are modeled by an ideal voltage source in series with an equivalent resistance and inductance. This approximate model allows performing a nonlinear stability analysis to predict the system qualitative behavior due to the reduced number of differential equations. Additionally, nonlinear analysis provides analytical stability conditions as a function of the model parameters and it leads to a design guideline to build reliable (MGs) based on safe operating regions.

The non-inverting buck-boost converter has attracted significant attention in the photovoltaic (PV) module integrated application, as it offers high efficiency while maintaining a low cost and a simple topology. When this converter is employed, special care must be taken at the transition between buck and boost operating modes, as a dead-zone in the voltage transfer function can cause abrupt perturbations in the controlled voltage, decreasing the regulation quality and ultimately lowering the power conversion efficiency. In this paper a new dead-zone compensation technique is proposed with the scope of smoothing the transition between operating modes, eliminating the voltage ripple and improving the regulation performance, while maintaining high efficiency. The converter under analysis is studied together with its gate driving circuit, which is based on a bootstrap capacitor power supply for the high side switches. The proposed dead-zone compensation technique is deduced using the principle of maintaining the ideal voltage gain function across the converter operating range. The technique is analyzed, implemented and tested on a purposely built PV module integrated non-inverting buck-boost converter. The experiments reveal a sensible improvement of the voltage regulation during mode transition, confirming the effectiveness of the proposed technique and its fitness for the PV application.

In this paper the nonlinear dynamics of interconnected power converters in an islanded direct current (DC) microgrid is analyzed. By using a simplified scheme based on two cascaded converters we analyze the dynamical behavior that can arise from the interconnection of these devices on a common DC bus. Furthermore, in order to address the bus voltage control problem, we propose a Sliding Mode Controller for a DC-DC bidirectional power converter to control the DC bus voltage under instantaneous constant power loads. This class of loads introduces a destabilizing nonlinear effect on the converter through an inverse voltage term that can lead to significant oscillations in the DC bus voltage. Simulation results are shown to illustrate the nonlinear analysis.

The strive to increase the energy yield of photovoltaic (PV) power systems has made PV module integrated dc-dc converters (dc-MICs) a reality of modern PV plants. These converters regulate their input voltage, and their dynamic behavior is heavily influenced by the non-linear characteristic of the PV module. The regulation of the PV module voltage and average inductor current by means of a linear cascaded controller is a popular control technique, simplifying the converter dynamics and providing inherent current limiting, however it is prone to instability depending on the interaction between the PV source and the interfacing converter, as well as the value of the controller parameters. These factors present a clear challenge for control design; moreover, the converter transient response undesirably depends on the PV module operating point. In order to solve these issues, while maintaining regulation of PV module voltage and average inductor current, this paper proposes to adopt a non-linear controller designed with the feedback linearization control (FLC) technique. The control laws are derived and implemented in a non-inverting buck-boost dc-MIC, as this is a favorite topology for the PV interfacing application. A digitally controlled converter prototype is built and used to obtain experimental results, where the FLC technique is compared with a linear cascaded control technique. The results confirm the superior performance of the presented FLC technique, which is robust and able to regulate the converter input voltage with fast and consistent dynamics, regardless of the PV module or load operating conditions.