Effect Of Subsynchronous Oscillation On Distance Protection And New Approach To Damp Subsynchronous Oscillation

In China, centers of power consumption mostly stand in the east, far away from t centers of hydropower resources or fossil fuels in the southwest and northwest. More and more FACTs based on power electronics are used to transmit electric power over a long distance, including series compensation and HVDC. Sub synchronous Oscillation (SSO) problem caused by these shall not be neglected, which will threaten the safety of power grids. Studying the influence of SSO on the power grids and looking for economic and effective approaches to damp SSO are becoming more and more important.This Thesis mainly focuses on the effect of the subsynchronous oscillation on distance protection and a new approach to damp SSO based on the widely-used voltage-source converters. Main content of this thesis includes:With a simulation model based on the actual parameters of the system, by describing the measuring impedance trajectory diagram, the effect of subsynchronous oscillation on distance protection is studied. By observing impedance trajectory whether to enter the action zone and analysis of the enter duration, the distance protection movement situation is determined. The results that the subsynchronous resonance has an influence on Zone-Ⅰ but not on Zone-Ⅱ or Zone-Ⅲ are showed. When there is an outer fault in the series compensated line, the Zone-Ⅰ maybe has misoperation, and when the line has internal fault, the protection will not refuse action. In the transmission line without series compensation, if this fault line, from protection installation point to the point of failure, has a series compensation capacitor, Zone-Ⅰ maybe has misoperation. Influence factors of the subsynchronous resonance on Zone-Ⅰ are series compensation degree, fault point, fault type and installation locations of series compensation capacitor, and so on. By the spectrum analysis of voltage and current measured on protection installation point, it is verified that voltage and current contain the synchronization harmonic component, resulting in the deviation of measuring impedance.The effect of voltage-source converters on the system electric damping is analyzed with the help of output impedance. Then the effect of converters on the system stability of subsynchronous oscillations can be analyzed. A new approach based on voltage-source converters to damp subsynchronous oscillations is proposed, in which a voltage controller is used to reshaping the output impedance to introduce positive electric damping. Time domain simulation results show the effectiveness of the proposed new damping approach.