Voltage Adaptive Dynamic Control And Steady-state Optimization Control Strategy For AC/DC Distribution Network

It is the trend of the future to develop AC-DC hybrid distribution network(AC-DC HDN).However,the problems of AC-DC voltage deviations and fluctuations are becoming more and more sophisticated.On the one hand,the large voltage deviation of each node in the system and uneveness of the overall voltage distribution deteriorate the problem.On the other hand,local AC-DC nodes’ susceptibility to power fluctuations leads to the weak stability of AC grid voltage and the further fall of DC voltage inertia.In this context,the Soft Normally Open Points(SNOP)is introduced in this paper,the voltage adaptive dynamic control and steady state optimization control strategy is proposed,which is able to improve the overall voltage distribution and the dynamic stability of the local AC-DC nodes’ voltage.In this paper,research is carried out from two levels: voltage adaptive dynamic control and voltage steady-state optimization,taking the three-port SNOP-based AC-DC HDN as the research subject.The main research results are as follows:1.Aiming at the problem of poor voltage stability of local AC nodes,firstly,the effect mechanism of AC grid voltage disturbance on the dynamic response of DC voltage is analyzed on basis of the dq model of SNOP converter unit.The influence of AC grid disturbance can be effectively suppressed by designing the voltage feedforward control method.Secondly,considering the weak capacity of AC power grid regulation,an adaptive power compensation control method is introduced which takes SNOP to adaptively compensate power shortage according to the voltage variation at the operating point,thereby improving voltage stability.2.In view of the problem of low inertia of DC voltage,this paper devises an improved adaptive virtual inertial control method compared with the existing DC virtual inertial control strategy,considering both the DC voltage deviation and voltage change rate.With this method inertia of the DC system which is increasingly disturbed can be improved to the next level.Secondly,the influence of control parameters on the system stability are also discussed through small signals of the AC-DC HDN,which clarifing the value range of the control parameters.3.In order to improve the overall voltage distribution of AC-DC HDN,firstly,voltage optimization model is established by build of the objective function considering the voltage deviation and voltage distribution evenness,and of the constraint condition of AC-DC power flow,SNOP operation control and voltage adaptive control,etc.Secondly,in light of the operating state of SNOP,it is divided into three operating scenarios where the solving algorithms is designed respectively.4.In view of the problem of large voltage deviation and uneven voltage distribution of the overall nodes,the reference command value of the SNOP port converter is obtained by solving the voltage optimization model under the scenario of the SNOP rated operation.For the issue that original control parameters of the SNOP port converter are difficult to adapt to the new reference command value,the optimization values of adaptive power compensation control parameters and adaptive virtual inertial control parameters are obtained through joint optimization of adaptive control parameters under the scenario of SNOP changed operating state,thus improving the voltage distribution,meanwhile,strengthening the flexibility of dynamic control.Finally,a SNOP-based AC-DC HDN simulation model is built in Matlab/Simulink.Then,this paper carries out the comparison and analysis of control effects under different operating states.The voltage optimization model is solved in Optimization Toolbox through a modified IEEE 33-node system simulation example,and the effectiveness of proposed steady-state voltage optimization method is verified under three type scenarios.The validity of the control strategy proposed in this paper is verified.