| The vigorous development of renewable energy such as wind power is conducive to the optimization and adjustment of energy structure of our country.Reactive power compensation equipment,as a key device connecting new energy to the public power grid,determines power quality directly.The early design of reactive power compensation devices for power electronics was small and could not satisfy dynamic reactive power compensation requirements when large-scale wind power is connected to the grid,and the problem of the power system fluctuations has become increasingly serious.In order to ensure the stability of key nodes,the stable operation of the power grid,and the power quality standards,reactive power compensation devices are particularly important.The cascaded H-bridge static var generator(SVG)has been widely used in practical fields because of its advantages such as large reactive power compensation capacity and high voltage resistance.The continuous reactive power is input to the power grid which makes the grid operate stably due to the use of SVGTo ensure reasonable reactive power distribution and voltage regulation,a large number of static var generators(SVGs)are connected to wind farms and afflux stations.However,the issue of negative interaction caused by the interaction between SVGs emerges,which threatens the stable operation of the grid-connected system when it is serious.Therefore,it is necessary to study the harmonic interaction mechanism of each reactive power compensation device and its DC voltage instability.The parallel operation of the static reactive power generator(SVG)of the wind farm afflux stations is taken as an object to study the DC voltage instability caused by the interaction between the machines.Aiming at this problem,firstly,the cascaded H-bridge topology of two-parallel SVGs is analyzed.Then5 inter-converter harmonic voltage difference is derived under the condition of parallel SVGs with asynchronous carriers,and the effect of higher harmonic circulation on dynamic stability of the DC voltage of the H-bridge sub-module in SVGs is studied.The intrinsic relationship between non-characteristic harmonics and DC 'voltage imbalance of H-bridge sub-module is found out by deriving and analyzing the difference of active powers flowing through DC capacitor.When the non-characteristic harmonics exceed limit due to harmonic circulation,H-bridge DC voltage imbalance will be induced.A DC voltage balancing strategy is proposed to solve the SVG DC voltage imbalance problem,so that the DC voltage of the H-bridge sub-module keeps stable.Finally,through simulation and experimental analysis,it is found that when carriers of the parallel SVGs are asynchronous,the DC voltage of the parallel SVGs appear to diverge;And,DC voltage balancing control strategy which is proposed is applied to the simulation system of two-machine parallel SVGs,which suppresses the DC voltage unbalance phenomenon.The correctness of theoretical analysis and the proposed control strategy are verified with simulation results. |
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