Voltage and Frequency Stability of Weak Power Distribution Networks with Droop-Controlled Rotational and Electronic Distributed Generators

Distributed generations (DG’s), based on both synchronous generators (SG’s) and fast inverters, are incorporated to improve power quality and reliability of power grids. These DG’s, however, may cause voltage and frequency stability issues due to weakness in power distribution networks. Weakness not only means significant ratios between resistance and reactance components in impedances (lossy), but also large power flow compared with rated power level (stressful). Weak networks complicate voltage stability and frequency synchronization analysis, because of coupled network dynamics. Additionally, droop controlled rotational and electronic DG’s have different dynamics, making a comprehensive stability analysis even more difficult. This paper derives sufficient conditions for voltage stability and frequency synchronization of a weak power distribution network coupled with droop-controlled DG’s, which are based on both fast inverters and SG’s. These conditions are inequality constraints on network parameters, load levels and generation control commands. Simulation tests show that asymptotic voltage stability and frequency synchronization are ensured in a weak network, in both a modified 37-node test feeder and a rural electrification network model. Moreover, these stability conditions also provide guidance in robustly integrating DG’s into existing power distribution networks.