Advanced TSO-DSO Interface for Provision of Ancillary Services by DER in Distribution Networks

In Danish electricity networks, the large fossil-fuel based power plants are being phased out and replaced by distributed energy resources (DER) such as wind and solar based power generators. Substantial amount of variable power generation from renewable sources are being integrated to the medium and low-voltage distribution networks. In this context, ancillary services of DERs located at distribution level will be required to support the transmission system operator
(TSO) to perform frequency and voltage control in the transmission networks. To facilitate this, an improved interface between the TSO and the distribution system operator (DSO) is necessary. This paper proposes an advanced TSO-DSO interface based on state of the art review for congestion free utilization of ancillary services by the TSO from DERs at medium-voltage distribution networks. The proposed interface includes timely exchange of information about the DERs from DSOs to the TSO. The DER information is used by the TSO to purchase ancillary services through the market for load balancing. The DSO receives the DER dispatch information (active power profile) from the TSO and runs an optimal power flow (OPF) routine to check whether the activation of ancillary services may cause network congestion. Based on the feasibility, the DSO either sends the reactive power setpoints to the DERs or request the TSO to re-evaluate the DER dispatch accordingly. Hence, the congestion, which may occur at distribution networks due to direct interactions between the TSO and DERs can be avoided. This paper employs an OPF method based on semidefinite programming to calculate reactive power dispatch of the DERs. Simulation studies are done on a simplified Danish medium-voltage network of Støvring area using Matlab to illustrate the efficacy of the proposed OPF method and its effect on proposed TSO-DSO interface. Comparison of the study cases performed without and with the proposed TSO-DSO interface shows that effective utilization of DERs for ancillary service is possible using the proposed interface.