On composite load modeling for voltage stability and under voltage load shedding

Undervoltage load shedding (UVLS) has been considered by some utilities as a cost-effective corrective tool to overcome voltage instability and abnormal voltage conditions. Load modeling is an important aspect for voltage stability studies and real time system controls pertinent to UVLS. At a given bus, a load would be an aggregate of load elements that would differ in their static and dynamic performance. Also at load centers, aggregate loads would encompass system components of particular characteristics such as load tap changers, SVC's and in-plant generation. Modeling aggregated loads should reflect the nature of load components and fit the applications of the model in studies and real time controls. In a tutorial style, This work discusses the evolution of static and dynamic load models for voltage stability and examines their applicability to UVLS. Load modeling approaches were listed as deterministic, generic and stochastic. Concerns and recommendations were given for adopting one approach or a combination of approaches in aggregate load modeling for UVLS. Load models validation and verification is emphasized especially when using generic or stochastic models.