Reactive power provision by distribution system operators — Optimizing use of available flexibility

Distributed Energy Resources (DER) connected to High Voltage (HV) level must provide controllable reactive power output as a part of grid connection codes in many countries. This reactive power is usually available to the system operator free of charge. A scheme is proposed in which the Distribution System Operator (DSO) provides the transmission system operator (TSO) with controllable reactive power based on controlling HV DER and tap changers. It is shown using real measurement data how delivery of reactive power could be distinguished from the natural variation of reactive power exchange between DSO and TSO. The suggested provision scheme incorporates the capabilities of DER providing reactive power only during non-zero active power output. It exploits the possibility to forecast the active power output and therefore the related reactive power capability and does not require a closed-loop Optimal Power Flow (OPF) on the side of the TSO.