Research On Model And Algorithm Of Voltage Stability Constrained Optimal Power Flow

With the foundation of large interconnected network and the development of electricity market, it is more and more important for power systems to operate economically and safely. As a powerful tool to solve these problems, optimal power flow(OPF) has been widely used and studied. Voltage instability is an outstanding problem in large interconnected network, and voltage stability constrained OPF(VSCOPF) becomes the hot topic in recent years. This study, based on the model and algorithm of VSCOPF, mainly include following achievements:Several common voltage stability margin indices(VSMI) are analyzed, and the physical meanings and computation methods for these indices are also introduced. Based on simulation results of IEEE 30-bus test systems, these indices are compared and their advantages and disadvantages of reflecting system voltage stability are pointed out, which play the important role in choosing and applying VSMI reasonably.In this study, the effect of operation constraints of the stator and rotor's current on the upper and lower limit of the generator's reactive power output is fully considered, and a set of piecewise functions are proposed to describe reactive power capability of the generation. On the basis of that, considering power flow constraints and voltage security constraints for normal state and critical state respectively, a new voltage stability constrained optimal reactive power flow(VSCORPF) model is proposed with minimum total active power losses as the objective and reactive powers of generators, on-load tap changer transformer(OLTC) ratios, injected reactive powers of capacitors as control variables. This model is also extended to voltage stability constrained optimal active power flow (VSCOAPF) model, optimal reactive power flow model and integrated optimal power flow(VSCIOPF) model. Different from VSCORPF model, in VSCOAPF model, control variables are active and reactive powers of generators, while in VSCIOPF model, control variables are reactive powers of generators, OLTC ratios, injected reactive powers of capacitors. New models are of more practical value since more accurate operation constraints of the generator are considered.In VSCORPF model, with minimum total active power losses as the objective, power flow equality constraints, inequality constraints of control variables and state variables and voltage stability constraints are considered. In VSCOAPF and VSCIOPF model, with minimum cost of production of generators the objective, constraints consist of power flow constraints, generators output capability constraints, inequality constraints of control variables and state variables and stability constraints.The above OPFs are solved by predictor-corrector primal-dual interior point method. Simulation results of IEEE 30-bus test systems show that conventional models may cause generators to exceed operation limits because in these models voltage stability constraints are determined by fixed upper and lower limits of generators. The effects of considering voltage regulation control variables as independent variables or common variables in VSCOAPF model and VSCIOPF model on voltage stability margin are analyzed as well.