| In China,the energy resources are not distributed evenly as required by the allocation of electric power loads.With the increasing power demand of load center,the demand for long-distance power transmission is also increasing.Line Commutated Converter Based High Voltage Direct Current(LCC-HVDC)transmission is one of the most widely used longdistance power transmission modes at present.The thyristor converters in LCC-HVDC convert electricity between AC and DC sides by coupling the AC system and DC system which are different in properties.In practical applications,it is found that subsynchronous oscillation(SSO)may occur between LCC-HVDC and its connected generator units,which jeopardizes the safe and stable operation of the power grids,as well as the shafting of generator units.The potential hazards caused by the SSO widely exist in Gaoling Back-to-Back HVDC transmission project,Guizhou-Guangzhou II HVDC transmission project,Hulun Buir-Liaoning HVDC transmission project,and Tianshan – Zhongzhou HVDC transmission project,and even resulted in serious accidents.Along with the expanding application of high-power electronic technologies in power system,the building of power grids across regions and countries,and the increasing grid-connection of new energy,the problems caused by the SSO will become more complicated.Therefore,it is necessary to further study the mechanism and suppression of SSO.Against such a backdrop,this paper mainly builds a pulsating switch function model for the thyristor converters,which are essential to couple the AC and DC sides;conducts further studies on the system models,oscillation mechanisms and influence factors of the AC-DC coupling oscillation;and puts forward a universal SSO suppression solution.The main contents are as follows:This paper builds a pulsating switch function time domain model for the AC-DC coupling power system,to reflect the nolinear coupling functions of the thyristor converters.By transforming the system model from the time domain to the frequency domain through the frequency shifting function,this paper builds a pulsating switch function frequency domain model for the AC-DC power system coupled by thyristor converters.The time domain model and frequency domain model provide a theoretical basis for analyzing the stability of the system.Using the established pulsating switch function model of the AC-DC coupling power system,this paper studies the oscillation mechanism and influential factors of the system,analyzes the coupling paths of the oscillation components of subsynchronous frequency on the DC and AC sides of converter,deduces the stability judgment criteria for the generatorgrid power system based on DC and AC coupled by thyristor converters,and demonstrates the influences of the AC side capacitive impedance,converter control parameters and direct current transmission power on the SSO of the system.On the basis of analyzing the fluctuating current and power during SOO,this paper provides an inverse power injection method against the SSO: to suppress the SSO in the system by controlling the high-power converters to send inverse oscillation signals to the system via the treating process of the physical signals containing the SSO oscillation components.Upon the simulation experiments,this paper verifies the effectiveness of the inverse power injection method.This method,which has been implemented in a coal energy base connected to an AC and DC hybrid network in China,is proved that it can effectively suppress the SSO in the generator-grid power n system based on DC and AC coupled by thyristor converters,and is expected to provide certain inspiration for other similar projects. |
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