Phase Unbalance Impacts on Black-Start Service Restoration of Distribution Networks

The emergence of the microgrid (MG) concept, along with the increasing integration of distributed generators (DGs), is enabling the rapid, emergency restoration of customers before the main grid gets restored. However, the unbalanced operation of the network can pose limitations in the restoration procedure, especially since the MGs operate in an islanded mode. In this regard, this paper studies the impact of network imbalance on the black-start service restoration of MGs. A two-level simulation-assisted framework was used to implement the restoration model. The first level is a mathematical optimization method that solves the restoration problem over a horizon. Then, a transient model of the DGs is simulated in the second level to manage frequency stability during the restoration procedure. To study the impact of the system imbalance on the operation of the DGs, the restoration model was applied to a modified version of the IEEE 123 node test feeder. Based on the results, ignoring the unbalanced operation of the DGs can result in operating the units in a severely unbalanced mode and can damage the generators.