An Overview of Coordinated Control via TSO-DSO Interface for Congestion Management in Distribution Networks

Integration of renewable energy sources (RES) such as wind power plants and solar PV to electric power systems is a global phenomenon to reduce emissions of green house gases and combat climate change. Majority of the RES are connected to medium and low-voltage distribution grids. Power transmission grids, owned and operated by transmission grid operators (TSOs), are generally well interconnected, monitored on a continuous basis and have standardized control policies. On the other hand, distribution grids operated by distribution system operators (DSOs) have limited observability, communication infrastructure with slow bandwidth, less automated functions and lack of coordination with TSOs. Power production from RES, for example, wind power plants is intermittent depending upon the wind speed. In future distribution grids, at higher penetration levels of wind power plants, grid congestion in the form of node over-voltages, over-currents in the lines may be experienced. To alleviate grid congestion, DSOs can curtail the power output of the wind generators. However, curtailment of wind power will cause revenue loss to wind power plant owners, and it can hamper the green energy transition. At present, the control and coordination among TSOs and DSOs are limited and not automated. Assuming an improved interface between TSOs and DSOs, during grid congestion, controllable resources in the transmission grid can be utilized for mitigation. In this paper, a detailed review of the current interface between TSOs and DSOs, in the context of few European countries including Denmark, will be conducted. A practical example of grid congestion in a representative distribution grid due to varying power production from wind power plants will be analyzed by simulation studies. The required functionalities on the TSO-DSO interface to address the grid congestion problem will be studied. The possibilities of utilizing the transmission grid assets such as on-load tap changing transformers, STATCOMs etc., to aid the DSO in managing the grid congestion will be explored. A simple case study will be performed illustrating the effectiveness of improved TSO-DSO interface for congestion management in a representative distribution grid. Recommendations for improving the TSO-DSO interface to tackle the future operational challenges of power system with large share of renewable generation will be provided.