Optimal Power Flow in Stand-alone DC Microgrids

Direct-current microgrids (DC-MGs) can operate in either grid-connected or stand-alone mode. In particular, stand-alone DC-MG has many distinct applications. However, the optimal power flow problem of a stand-alone DC-MG is inherently non-convex. In this paper, the optimal power flow (OPF) problem of DC-MG is investigated considering convex relaxation based on second-order cone programming (SOCP). Mild assumptions are proposed to guarantee the exactness of relaxation, which only require uniform nodal voltage upper bounds and positive network loss. Furthermore, it is revealed that the exactness of SOCP relaxation of DC-MGs does not rely on either topology or operating mode of DC-MGs, and an optimal solution must be unique if it exists. If line constraints are considered, the exactness of SOCP relaxation may not hold. In this regard, two heuristic methods are proposed to give approximate solutions. Simulations are conducted to confirm the theoretic results

Multiferroic heterostructures and tunneling junctions

AbstractMultiferroic heterostructures showing both electric and magnetic orders have attracted much attention because of their promising applications in the next generation of memories, sensors, and microwave devices and so on. The complex electronic and magnetic orders at the interface in multiferroic heterostructures will cause abundant physical phenomena due to the interplay among spin, charge, orbit, and lattice degrees of freedom, and various prototype devices have been achieved. In this review, we summarize some recent progresses mainly in the strain- and charge-mediated effects on the magnetic and electronic transport properties manipulated by electric/magnetic fields in multiferroic heterostructures. The recent advances in multiferroic tunnel junctions with ferroelectric barriers by using the spin polarized nature of magnetic materials are particularly presented, which exhibit magnetoelectric coupling effects at the interface and multi-stable resistance states in a single memory unit cell. Finally, the new inspiration for the design of spintronic devices having more energy efficiency and higher density is discussed