Analysis Of Transient Voltage Stability Of AC/DC System With Wind Power Based On Controlling Unstable Equilibrium Point Method

Transient voltage stability analysis has always been one of the most important and difficult problems in the field of power system stability research.With the development of renewable energy technology and High Voltage Direct Current(HVDC)transmission technology,the proportion of wind power generators and HVDC lines in the grid will continue to increase,and the dynamic characteristics of the power system have undergone profound changes.At present,there are not enough researches on transient voltage stability based on the idea of energy function method,and the related theoretical basis and calculation methods still need to be further improved.There is of great significance to study the transient voltage stability of AC/DC systems containing wind power generators based on the energy function method.This paper analyzes the transient voltage stability of the power system based on the controlling unstable equilibrium point(CUEP)method.Based on stability region theory and energy function theory,the mechanism of transient voltage instability is discussed and analyzed.It is pointed out that under the idea of energy function method,the transient voltage instability is due to the fault trajectory passing through the stable manifold of voltage-type unstable equilibrium point.Based on the reactive power conservation law,the numerical energy function of the constant speed wind turbine,doubly-fed induction generator and HVDC link is derived.The energy function of the constant speed wind turbine is derived from two strategies.Through theoretical analysis,the energy function of the constant speed wind turbine obtained with equivalent to the PQ dynamic load is less conservative.For those devices whose mathematical models are more complex and it is difficult to obtain their analytical energy functions,such as doubly-fed induction generator and HVDC,the idea of PQ dynamic load equivalence is recommended to derive the energy function.Then,the research of the reliability of the method to search for the voltage-type CUEP method is carrying on.Since the methods of obtaining CUEP based on Newton-Raphson method requires high initial value,if the initial value is unreasonably selected,the iteration may be time-consuming or divergent.Combining the heuristic method and Newton homotopy method,the voltage-type CUEP of the power system can be obtained.Through theoretical analysis and demonstration of examples,it is shown that the Newton homotopy method can reduce the influence of the fractal boundary of the convergence regions in the Newton-Raphson method,which improves the reliability of numerical calculations converging to CUEP.It also further shows that the transient voltage instability is dominated by the unstable manifold of the voltage-type unstable equilibrium point.The energy function method was used in the transient voltage stability analysis of AC system and AC/ DC system with wind power generators,and they all obtained conservative results,reflecting the conservativeness of the energy function method,and verifying the effectiveness of the energy function and the voltage-type unstable equilibrium point.Finally,from the perspective of energy function method,the influence of HVDC on system stability is analyzed,and the results consistent with time domain simulation are obtained,which provides a new way for analyzing the influence of equipment on system stability.Simulation results show that it is feasible to apply the energy function method to the analysis of transient voltage stability of AC/DC systems with wind power.