Interaction of Transmission-Distribution System in the Presence of DER Units–Co-simulation Approach

The effects of distribution-connected solar farms on the transmission and distribution systems are studied in this paper using a practical power system network. Both the transmission and distribution networks are modeled and co-simulated for intermittent effects of solar power and changing load profiles. These effects include flicker in both the transmission and the distribution systems, operation of the substation Load Tap Changer (LTC) and feeder regulators, and distribution feeder voltage profiles. The results of the study show a potential hunting effect causing excessive LTC operation especially during periods of high reverse power flow condition and a significant visible flicker in the distribution system but lower levels of flicker at the transmission level. It is shown that the hunting behavior of the LTC can be mitigated and downstream feeder voltage profiles can be improved via enhanced bandwidth, block operation mode, and adjusting time delay of the substation LTC controller. In addition, the inverter Volt-Var control can be used to effectively diminish excessive LTC operation based on IEEE Std 1547-2018 Volt-Var characteristics with modified settings. Finally, an enhanced load-flow algorithm with modified Jacobian is presented that incorporates the Distributed Energy Resource (DER) Volt-Var characteristics in the conventional Newton-Raphson method.