Research On Low Frequency Oscillation Of Gaoling HVDC System With Suizhong Plants Adjusted To The North China Side

With the developing of high voltage direct current transmission (HVDC) projects in China, the power system changes to a large-scale hybrid AC and DC grid. The grid becomes much more complex and the low frequency oscillation problems are aggravated. At present, the east China power system is connected to north China power system by Gaoling HVDC. The AC grid structure of this system is weak but the DC part is strong. As the Suizhong plants adjusted to the north China grid, the active power feed will increase to 4900MW which makes the static state and dynamic stability margin decreasing. The hybrid DC and AC feeding large active power into the grid brings high pressure to the operation. It is necessary to analyze the influence to stability caused by typical operation modes after the adjusting project getting into operation.The eigenvalue analysis and Prony analysis are the two typical analysis method in low frequency oscillation dynamic stability studies. They both have strong points and weakness in AC-DC system. This paper focuses on low frequency oscillation problems of the north China AC-DC system. It combines the eigenvalue analysis and Prony analysis method to analyze the mechanism then gives measures to suppress low frequency oscillation. The detailed processes are as follows:Modeling and simulating the north China AC-DC power system in detail on PSCAD platform. Based on the simulation of low frequency oscillation, recognizing the mode of low frequency oscillation by improved Prony method with energy theory and verifying the results by eigenvalue analysis method. The energy-sorted Prony method is able to overcome the drawbacks of traditional method on mode recognition and offers a technical support to find the dominant mode of low frequency oscillation caused by connection changing project.This paper focuses on the possible low frequency oscillation typical operation mode in practical engineering; attempts to explain the cause of low frequency oscillation; analyzes the dominant oscillation mode and the failure commutation occurring concurrently. Referred to the conclusion of analysis and the dominant oscillation mode recognition, the paper establishes HVDC control and improved PSS coordination control strategy. The effectiveness of the proposed suppression measures are all validated by simulation.The results of this paper is useful to analyze the low frequency oscillation problems in hybrid DC and AC power systems. The analysis conclusions and suppression strategy of low frequency oscillation problems in north China side power system of Gaoling HVDC after Suizhong plants adjusted to the north China grid provides guiding suggestions to the practical engineering operation.