Fundamental factors influencing bus coherency in distribution networks with distributed energy resources

With the participation of distributed energy resources (DERs) in inertia emulation, voltage control and primary frequency response, distinct angle and voltage dynamics are introduced in a conventional distribution network. These additional low-frequency dynamics emanating from a pre-dominantly resistive distribution network influence the mode properties of system oscillations uniquely. For better control and partitioning of active distribution network during emergency conditions coherent participation of DERs in local and inter-area modes need to be evaluated. In this work, fundamental factors affecting the bus coherency of distribution system are analyzed. It introduces a concept of coherency duality between pre-dominantly resistive and inductive distribution networks along with unique effect of source and control interactions by DERs. The work discovers pre-dominance of voltage based coherency criteria for active power flow control/islanding in resistive sub-networks and angle based criteria for inductive sub-networks, due to the reverse relation between state and power variables. Dynamic coherency changes inflicted by DER variabilities like different control and source interactions, increased penetration level, change in the spatial distribution of DERs, and dynamic loading are presented in the work. The proposition is tested for modified IEEE-39 bus systems with 33 bus 132/33/11/0.4 kV and 2182 bus 20/0.4 kV distribution networks in DigSILENT/PowerFactory.