Research On Influence And Control Of The DC-link Ripples Of The Cascaded H-Bridge STATCOM With Low Capacitance

With the rapidly increased new energy as well as the nonlinear,impact and unbalance loads connected to the grid,the stability of grid voltage has become a prominent problem.Immediate reactive power compensation at the loads can effectively reduce the power transimission loss,cut down the cost of the power equipment and maintain the stability of the grid voltage.As the most advanced equipment for reactive power compensation,the static synchronous compesnsator(STATCOM)has been well studied and widely applied for the advantages of fast dynamic response,high compensation ability and high quality of output currents.Cascaded H-bridge STATCOM is often adopted in high-voltage and high-power application due to its simple structure and modularity.Apparently,the reduction of H-bridge capacitance value is of great significance for decreasing the cost,volume and weight of the whole system.However,low capacitance value will definitely result in high dc-link ripples,which will influence the system compensation capacity,the quality of the output currents and the control of the dc side voltages.In this paper,the Cascaded H-Bridge STATCOM with low capacitance has been chosen as the research object,the issues mentioned above will be analyzed and discussed in detail.Differenet from the conventional STATCOM,the dc-link ripples of the Cascaded H-Bridge STATCOM with low capacitance can not be ignored and will have an effect on the output voltages of the converter,thus the range of the output currents is changed.In this paper,the relationship between the dc-link voltages and the reactive power currents is established in detailed by taking the ripple voltages across the dc-link into consideration.Thus,the compensation capacity of the system,which is related to the capacitance value,is derived under the limits of modulation,maximum and minimum dc side voltages.A new design method of the capacitance value based on the system compensation capacity is proposed as well.Low dc capacitance will definitely result in high dc-link ripples among three clusters,and thus cause severe third harmonic currents under asymmetric grid.To mitigate the third harmonic currents,this paper proposes a selective components compensation approach for the Cascaded H-Bridge STATCOM with low capacitance.To study the effect of the dc-link ripples on the harmonic currents,sequence component decomposition are employed to analyze the relationship between the dc-link ripples and output voltages.The analysis reveals that the output voltages contain positive,negative and zero sequence in fundamental and third order under asymmetric grid,while only the positive and negative sequence components in third order result in third harmonic currents.Therefore,the third harmonic currents suppression method is proposed by only compensating the positive and negative components in third order.The dc-link voltage of the Cascaded H-Bridge STATCOM with low capacitance will becomes more sensitive to the changes in instantaneous active and reactive powers of the system and is susceptible to external factors such as the drop of the grid voltages,resulting in the overvoltage protection.In order to increase the reliability of the system,based on the traditional three layer control strategy of dc-link voltages,the dc-link peak voltages are controlled by employing the Seconde Order Generalized Intergrator(SOGI)for the raal-time extraction of the dc-link peak voltages in this paper.Based on the Matlab/Simulink simulation and experimental platform,the range of the compessation and the third harmonic currents suppression method proposed in this paper as well as the dc-link peak voltages control strategy are validated,simulation and experimental results show that the dirivations are precise and the proposed methods are effective.