Differences of Power Flows in Transmission and Distribution Networks and Implications on Inverter Droop Control

Control of distribution networks and thus knowledge about their characteristics gain in importance. As the R/X ratio is significantly higher in distribution networks than in transmission networks, the coupling between real and reactive power flows cannot be neglected anymore. This is usually discussed based on the analysis of the power flow equations. This paper provides calculations for two exemplary networks: a high-voltage transmission and a low-voltage distribution network. By randomly drawing the loads' power consumption from a predefined set, initial values are generated which are used to compute a large amount of power flows by means of power system state estimation. The comparison of these power flows in both networks reveals that besides the difference in R/X ratio, the differences in the magnitudes of the impedances are also remarkably important for system behavior and control. The system variables, active and reactive powers, magnitudes and phase angles of voltages are coupled differently in distribution networks compared to transmission networks. The implications of these results on primary controllers of inverters are discussed and the impact of the findings is demonstrated by analyzing the performance of different primary controllers in both test networks.