Degree Type


Date of Award


Degree Name

Doctor of Philosophy


Electrical and Computer Engineering

First Advisor

Venkataramana Ajjarapu


A novel Continuation-based Quasi-Steady-State (CQSS) analysis is developed and integrated with trajectory sensitivity which in turn can be used to address various aspects of control strategies to mitigate long-term voltage collapse.;In this research, two scenarios are defined according to the severity of the contingency: Scenario One. The post-contingency long-term load characteristic intersects the system's PV curve; Scenario Two . The post-contingency long-term load characteristic doesn't intersect the system's PV curve.;First, the CQSS simulation, which is based on two different parameterizations, is utilized to trace the system trajectory after the contingency. One is for Scenario One where load change and OLTC action are considered. The other is for Scenario Two where load restoration and OLTC action are taken into account simultaneously. Secondly, the identification of the saddle node bifurcation point (SNB) and singularity-induced bifurcation (SIB) point can be accomplished by either continuation parameter or trajectory sensitivity. A new approach is developed in the CQSS simulation to approximate the differential representation of the thermostatic load restoration. It also avoids the numerical problem around the singularity point.;The salient features of this research are listed below: (1) A new CQSS simulation is developed. (a) It is numerically well-conditioned. (b) It can readily identify the SIB point and the SNB point. (c) The time information of the controls can be obtained automatically. (d) Combined effects of the OLTCs and the load change on voltage stability are taken into account. (2) A computationally-fast approximation of the generic load restoration is developed. (a) Parameterization of the load exponent provides a new way to approximate the load restoration in the long-term time scale. (b) The change of load types and compositions with the time can be considered. (3) Trajectory sensitivity is derived and calculated in two ways. (a) It is applied to identify the long-term SNB point. (b) It is related to margin sensitivity by using continuation method. (c) It is used to formulate the control problem to maintain a sufficient stability margin. (4) A systematic and comprehensive control strategy to mitigate longterm voltage instability is developed and implemented.;This proposed methodology is tested on two systems.



Digital Repository @ Iowa State University,

Copyright Owner

Qin Wang



Proquest ID


File Format


File Size

168 pages