Voltage Constraint Limitations of the Feasible Operation Region for Ancillary Service Provision between Different Voltage Levels

Abstract:
The transition of electric power systems towards power systems exclusively relying on renewable energy sources leads to an increasing number of decentral energy resources (DERs), which are primarily located in the distribution grids of lower voltage levels. As a result, the flexibility potential of DERs located in lower system levels to participate in the provision of ancillary services in upper system levels is increasing. To effectively utilize this potential for ancillary services, enhanced coordination between distribution system operators and transmission system operators is crucial. A promising approach involves aggregating the flexibility potential of a lower system level at the vertical grid interface, as the Feasible Operation Region (FOR). The literature presents a variety of aggregation methods, which differ in terms of speed and accuracy. This paper assesses the accuracy of voltage band constraint approximations derived from an analytical method based on linear DistFlow equations on the FOR. These approximations are compared to results obtained via a nonlinear optimization-based aggregation method for the calculation of FORs for different voltage levels: low, medium, and high voltage. The analysis highlights the influence of the R/X-ratio on the slope of the FOR limits given by voltage constraints in radial networks.