Real-Time Voltage Stability Monitoring in Smart Distribution Grids

As electric distribution grids continue in hosting high penetration levels of renewable Distributed Generation (DG), several challenges will be brought to system operation and stability. When distribution networks face either an outage (or unavailability) of DG unit or an increasing in load demand, the loss of voltage stability may result. Moreover, some types of DGs, especially fixed speed induction generators, can cause voltage instability since they always consume reactive power. This paper uses a nodal model for voltage stability monitoring, which is applicable for online voltage control in distribution networks. The load impedance can be easily calculated and the entire distribution system can be simplified as an equivalent impedance based on SCADA and PMUs data. Since the proposed method is based on nodal method, the equivalent impedance will reflect the nonlinear dynamic nature of the load of multi bus power system. An index based on impedance matching theorem will be derived to calculate the voltage stability margin and determine the weak buses, and enable a warning in case of voltage instability detection. The proposed voltage stability monitoring will be performed on a 77-bus, 11 kV radial distribution system under different operational conditions.