On-Load Tap-Changer and Flexibility for Local Ancillary Services

The flexibility of users’ demand profile can contribute to the management of distribution systems; the local flexibility useful to Distribution System Operators (DSOs) should be exploited through market mechanisms. User-side flexibility is useful both for local congestion relief and voltage support services. The concept of the flexibility aggregation perimeter is useful for both DSOs and market operators; it is based on the load area approach, a cluster of nodes with a similar impact on the operation of the grid. The voltage profile in a distribution grid strongly depends on the tap setting of the On-Load Tap-Changer (OLTC) of the supplying transformer. Flexibility can be determined in conjunction with the OLTC tap settings to achieve optimal use. In the paper, the problem of optimal simultaneous procurement of flexibility and OLTC tap settings is presented, with a simplified representation of OLTC itself. The optimization problem is solved for two case studies, each with different grid partitions and flexibility offers; the results show that using the flexibility for voltage support services may benefit from the simultaneous determination of OLTC tap settings.