Research On Harmonic Oscillation Mechanism And Countermeasures For Sending-end System Of HVDC Power Transmission System

With the rapid growth of the scale and capacity of the Southern Power Grid and the continuous advancement of the strategy of power transmission from west to east,the scale of the power grid is constantly expanding.Large-capacity,long-distance,cross-province multi-circuit AC and DC projects run in parallel.In 2016,Yunnan Power Grid and the main network have been connected asynchronously.It is planned to put into operation the interconnected DC project of Yungui Power Grid in 2020 and the Kunliulong multi-terminal DC project in 2021.Therefore,the characteristics of the grid at the DC transmitting and receiving end are becoming increasingly complex.In order to solve the problem of harmonic instability in the DC transmission system of China Southern Power Grid,it is necessary to analyze the mechanism of the problem in principle and summarize the relevant analysis and evaluation methods.At the same time,effective suppression control measures are proposed to assist grid planning and operation decisions.This article has carried out a detailed study on the harmonic instability mechanism of the DC transmission system.Through a detailed summary of the cases of harmonic instability at home and abroad,this article further analyzes the mechanism of harmonic instability and the calculation method of harmonic impedance.The key factor of the strength of the DC transmission system is included in the analysis.According to the modulation principle of the converter and the switching function,it proposes two types of harmonic instability criteria that consider the effects of different factors.Based on the Nyquist criterion and impedance transfer function,it proposes a class of harmonic instability criterion.At the same time,combining with the mechanism that DC bias magnetic saturation of the converter transformer core leads to harmonic instability,it proposes a class of harmonic instability criterion.By establishing the CIGRE timedomain electromagnetic simulation model on the PSCAD / EMTDC simulation platform,the proposed criterion is simulated and verified.It is concluded that the harmonic instability criterion based on Nyquist criterion and impedance transfer function is more suitable for harmonic judgment.This paper further validates the accuracy and applicability of the conclusions based on the Niu Cong DC approximate engineering model.It elaborates the harm of harmonic oscillation and analyzes the mechanism of the harmonic suppression measures of the DC transmission terminal system.Combined with the simulation results of the CIGRE model,it is verified that adjusting the generator start-up method,DC step-down operation and switching AC filter which can effectively suppress the occurrence of harmonic instability of the sending end system.