State of charge based droop control for coordinated power exchange in low voltage DC nanogrids

Decentralized battery and solar photovoltaic (PV) system organized in the form of an autonomous low voltage DC nanogrid is a potentially low cost and scalable solution for electrifying rural areas without access to the national grid. Each DC nanogrid can be installed on a single home and used to supply basic lighting, charge mobile phones and power a television set. To provide enough power to meet productive energy uses such as irrigation, the DC nanogrid can be connected to neighboring DC nanogrids to form a cluster and exchange power. However, to achieve a coordinated power exchange in the cluster, new control strategies are required. In this paper, we propose a decentralized droop control method which uses a state of charge of the battery to coordinate the power exchange. The power exchange is achieved by scheduling a terminal voltage set point at each DC nanogrid based on the state of charge of the battery. The performance of the proposed method at achieving the power exchange is analyzed through simulations in Matlab/Simulink. The method does not require inter-unit communication. Therefore, the method is reliable, robust and scalable. Also, the method maintains low amounts of power flow in distribution lines during power exchange to reduce distribution line power losses

Modelling and stability analysis for a variable speed DC tidal/wind turbines with MPPT in low voltage DC microgrid

DC Microgrid is a new candidate for Multi-Purpose Platforms (MPP) which highly supported by the international community for sustainably exploiting our oceans. Considering DC-systems for the MPP requires all renewable energy sources to be interfaced to a DC-bus. Wind turbines as proven developed energy for offshore use is a crucial energy source for MPP. Normally, a wind turbine in a DC-environment uses a diode rectifier for interfacing to the grid and a DC-DC boost or buck converter for extracting maximum power. Previous studies included wind turbine in a DC-microgrid did not analyze the turbine stability with wind variations. Moreover, tidal turbines, as an immature technology, are rarely considered in such DC-systems. So, this paper provides a modelling and stability analysis for a wind/tidal turbines based Permanent Magnet Synchronous Generator (PMSG). A linearized small-signal model is derived for a wind/tidal turbines based PMSG and boost converter. Two control loops are used for speed and current control. Stability analysis shows that the controller can effectively keep the system stable under wind/tidal speed variations. Simulation analysis is carried out to shows the effectiveness of the controller at various speeds considering tidal and wind turbines