Flexibility service design for mitigating voltage unbalance in the distribution network

It is expected that unbalanced voltages will increase in future distribution systems as the level of single-phase distributed energy resources (DERs) rises significantly, and this paper presents a methodology to address such problems. The approach is based on a three-phase optimal power flow (OPF) method seeking to mitigate the voltage unbalance factor (VUF) by using demand-side flexibility provided by DERs. A novel flexibility service proposed here allows Distribution System Operators (DSOs) to mitigate VUF through local flexibility resources. Considering the differences in preferences among DSOs, we compare and analyze three different flexibility service design options to provide a reference for DSOs to develop their strategies. The proposed convex relaxation model was tested using an IEEE network and the results showed that various flexibility service schemes could lead to differences in the allocation of flexibility resources and hence changes in the system state. Furthermore, it was found that the activation of flexibility resources might further increase the unbalance of the system if the voltage unbalance constraint was ignored.