Study On Stability Feature Extraction Of Hybrid HVDC Receiving System

The world’s largest AC/DC hybrid power grid with the highest voltage level has been built in China,which is beneficial to China’s resource optimization.HVDC transmission technology based on line-commutated converter(LCC)has been widely used in practical engineering due to its large capacity and low cost.However,LCC itself also has problems such as commutation failure,needs of AC power grid support and so on.The flexible HVDC technology based on voltage source converter(VSC)has advantages of autonomous commutation and capability of passive inversion.Therefore,hybrid HVDC transmission systems that can combine the advantages of the two has emerged.However,hybrid HVDC transmission systems also bring new challenges to the security and stability analysis of power system.There are many kinds of components and controllers in the hybrid HVDC transmission systems,and the coupling relationships between the stations and the AC/DC system are complex.How to find the key factors related to the stability of the system is an urgent problem to be solved.At the same time,with the participation of a large number of power electronic devices,the voltage stability of the receiving-end AC system is also facing new challenges.How to realize the rapid identification of voltage instability scenarios is an urgent problem to be solved.In this paper,aiming at solving the oscillation problem of hybrid HVDC system and the voltage stability problem of receiving-end power grid,the stability feature extraction methods of the two are studied.The interactive coupling relationship related to the control system in hybrid HVDC transmission system is studied,and the DC side control parameters which have great influence on the small signal stability of the system are extracted.It is proposed that near strong modal resonance(NESMOR)can be used as the precursor of oscillation of hybrid HVDC transmission system.From the perspective of the receiving-end AC system,the extraction method of the receiving-end AC system’s voltage stability characteristics is studied,and a voltage transient instability criterion is proposed based on the stability characteristics.The main research contents of this paper are as follows:(1)In order to meet the needs of stability analysis,the modeling methods of LCC and VSC stations are discussed to highlight the leading role of the studied control system in system stability.Based on the small signal analysis theory,the influence of control parameters on the small signal stability of the system under specific topology is studied,and the control parameter set strongly related to the small signal stability of the system is screened.(2)The near strong modal resonance that may exist in the hybrid HVDC transmission system is studied.The similarities between NESMOR and strong modal resonance are discussed,and the necessary conditions for the emergence of NESMOR are summarized.Then,based on eigenvector analysis,it is found that NESMOR can characterize the interaction relationship in the system.A method is proposed to quantify the influence of parameter changes on the system eigenvalue by the changing speed of the real part of the eigenvalue getting close to the imaginary axis in NESMOR.Since there is a mode that advances rapidly in the direction of damping reduction in the mode participating in resonance after the occurrence of NESMOR,NESMOR is used as the precursor of system oscillation.(3)The extraction method of voltage transient stability characteristics of receiving-end power grid is studied.It is found that in the process of voltage collapse,the load has a positive feedback process in which the slip ratio increases while the active power decreases.Based on this process,the voltage transient stability characteristics composed of slip ratio and active power are proposed,and the quantity which is difficult to measure is described by the quantities which are easy to measure.The voltage transient stability characteristics based on active power,reactive power and its derivative are obtained.The stability characteristic quantities are used to construct the voltage transient instability criterion of the receiving-end grid,and the possible misjudgment scenarios and their causes are analyzed.Additional criteria are added to improve the original criteria.Through the analysis of practical examples,the validity of the criterion and the correctness of the stability characteristics are proved.