  The large-scale integration of distributed renewable energy sources into the
electricity grid requires the investigation of new methods to ensure stability.
For example, Active Distribution Networks (ADNs) can be used at (sub-)
transmission levels for emergency operation, provided robust and efficient
control is available. This paper investigates the use of Feasible Operating
Regions (FORs) and Flexibility Regions (FRs) for Cross-Voltage-Level Power Flow
Control (CPFC). The enhancement of network stability due to the provision of
ancillary services is illustrated, as is the need for strengthened cooperation
between Transmission (TSOs) and Distribution System Operators (DSOs). Optimal
power flow methods are considered, focusing on computational advances through
PieceWise Linearization (PWL) and convex relaxation techniques aiming to speed
up runtime while keeping high accuracy. To illustrate the algorithms' benefits
and drawbacks, they are analyzed using exemplary medium voltage grids.
