Analysis Of Power System Voltage Stability L Index And Its Improved Evaluation Method

The network structure of the current power system becomes more and more complex,which puts forward higher requirements for the system's rapid real-time calculations.Therefore,there is a need for an index that can monitor the voltage stability of the system online.This index must evaluate the system state not only accurately and quantitatively,but also satisfy the requirements of rapidity.The voltage stability evaluation of a complex system can be simplified using the dynamic equivalence principle.The impedance modulus index based on this principle can accurately evaluate the voltage stability of the system.At the system voltage stability boundary,the load impedance modulus margin index of each node strictly follows the theoretical limit values.That is,the index has a distributed voltage stability evaluation feature.The impedance modulus margin of any load node can determine the voltage stability boundary of the system.However,the impedance modulus margin index is based on the power flow equation.The calculation is relatively complex in the large-scale power system,therefore,a fast and accurate real-time evaluation index should be sought.The power system voltage stability L index is widely quoted for its simplicity and speed of calculation,but the speed of calculation is at the expense of accuracy.This paper profoundly analyzes its essence from the mechanism,and uses the system node voltage equation as an entry point to establish its Thevenin analytical equivalent model;it reveals the inherent reasons for the limitation of the index,and proposes an improved evaluation method accordingly.The main work of the thesis is as follows:Firstly,it analyzes the nature of the L index.Looking from the load node to the system,the PV node and the PQ node in the system are equivalent to the voltage source and the current source respectively,and the internal system is simplified according to the Thevenin equivalence principle of the linear circuit.Secondly,the Thevenin analytical equivalent model of L index was established.Based on the voltage equations of the network nodes,the analytical equivalent circuit and the method of solving the Thevenin equivalent parameters are given.The index is defined as the ratio of the system equivalent impedance modulus to the load equivalent impedance modulus.According to the load equivalent transfer theory,the correctness of the equivalent model and its parameters is proved.Finally,an improved evaluation method for this index is proposed.In the critical state of voltage stability,the L index of each load node cannot uniformly approach the theoretical extreme value,so there is a certain degree of conservation.In order to overcome the limitations of its existence,an improved method based on the non-linear fitting of the Thevenin equivalent parameter was proposed.This method uses Polynomial Curve Fitting with the same point as Thevenin equivalent parameter to accurately predict the power limit of the system and avoids the problem that the P-V curve does not converge at the saddle junction bifurcation point.Taking the impedance modulus margin index as the reference object,the evaluation results of the L index and its improved index were comprehensively compared.The IEEE standard system simulation analysis shows that compared with the original index,the improved index not only achieves the accuracy of the impedance modulus margin,but also quickly converges to the current limit.It is suitable for on-line evaluation of distributed voltage stability.