Resynchronization of islanded unbalanced ADNs: Control, synchrocheck and experimental validation

The problem of securely reconnecting active distribution networks (ADNs) - e.g. microgrids - to their upstream grids at the point of common coupling (PCC) has been extensively discussed by the existing literature. The latter is commonly referred to as resynchronization and has to be done with care in order to avoid large transient current flows resulting from differences of nodal voltage phasors at both sides of the PCC. The active resynchronization process can be split into two tasks: the PCC-control and the synchrocheck. The PCC-control refers to the process used to steer the PCC nodal voltage at the ADN's side (i.e. downstream) towards the PCC nodal voltage at the upstream-grid's side (i.e. upstream). The synchrocheck refers to the algorithm used to check the synchronization (i.e. phasor alignment within tolerances) of the upstream and downstream PCC nodal voltages. Methods for PCC-control and synchrocheck presented in the literature commonly ignore the ADN's operational constraints and rely on the assumption of a balanced system. In this respect, the contribution of this paper is twofold. First, an approximated optimal-power-flow is proposed to control ADNs' resources in order to rapidly steer their PCC downstream nodal voltages close to their non-controllable upstream counterparts. Second, an Interpolated-Discrete-Fourier-Transform (IpDFT)-based synchrocheck that verifies the alignment of all three-phases of both upstream and downstream nodal voltages at the PCC, is proposed. The algorithms associated to both contributions are experimentally validated on the CIGRE-low-voltage-benchmark-microgrid at the Distributed Electrical Systems Laboratory (DESL) at the ecole Polytechnique Federale de Lausanne (EPFL) where the results of the developed synchrocheck are further benchmarked against the Schneider Electric's Micom P143 grid relay.