Research On Voltage And Reactive Power Optimization Method For AC/DC System With LCC-HVDC

The construction of UHVDC transmission project is the objective requirement of energy security,high efficiency and clean supply in China,and it is also an important part of energy transition.However,with the continuous UHVDC projects put into operation and a large number of new energy connected to the grid in China,the problem of frequent operation of discrete voltage regulator equipment in converter stations caused by the change of AC power grid is becoming prominent.Meanwhile,in the actual engineering,the reactive power devices of converter station and AC system are controlled independently,and there is the problem of insufficient utilization of the steady state regulation for DC reactive power supply(eg.synchronous condensers in conveter station)which is set for solving the problem of insufficient local dynamic reactive power support capacity of the power grid at the sending and receiving end.Therefore,how to realize the coordinated control between synchronous condenser and DC system in the converter station,and how to coordination of AC system to converter station in the power grid near UHVDC converter station become the key issues to be solved.It is of great significance to give full play to the steady-state reactive power support of DC reactive power supply and to reduce the action frequency of the discrete voltage regulator equipment in the converter station.Thus,this paper deeply studies the voltage and reactive power optimization method for AC/DC system with LCC-HVDC from the two aspects of UHVDC converter station and UHVDC near area grid.In order to realize the coordinated optimal control between synchronous condenser and DC system in a UHVDC converter station,the time-varying characteristics of the transient standby demand of synchronous condenser should be considered.Meanwhile,the current reactive power optimization model of AC/DC system cannot satisfy the minimum filter capacity requirement of converter station and the switching sequence requirement of AC filters and shunt capacitors.To solve this problem,a dynamic reactive power optimization model of UHVDC converter station in coordination with synchronous condenser is proposed in this paper.On the basis of the existing research,the adjustable range constraint with non-fixed boundary of synchronous condenser,the minimum filter capacity constraint of the converter station,and the switching sequence constraint of AC filters and shunt capacitors are added.In addition,considering the characteristics of voltage balance control for the UHVDC converter station with hierarchical connection mode,its dynamic reactive power optimization model is established by increasing the symmetric operation constraints for the high-side and low-side converters.A two-stage relaxation decoupling algorithm is used to solve the model effectively.Numerical examples of ZLT converter station in Northeast China and TZ converter station with hierarchical connection mode in East China are given to verify the effectiveness of the proposed method.The dynamic reactive power optimization method of UHVDC converter station in coordination with synchronous condenser can effectively reduce the operation times of discrete equipment in the converter station,but it is assumed that all equipment can be scheduled uniformly and load power can be accurately predicted.However,in the actual system,synchronous condenser and DC system adopt relatively independent control mode,and there will be errors between the predicted and the actual working conditions.To solve this problem,this paper proposes a parameter setting method of coordinated control between synchronous condenser and DC system based on dynamic reactive power optimization.Firstly,an area-diagram strategy of synchronous condenser is designed.The area-diagram is divided by the converter bus voltage with the two modes of reactive power control mode and voltage control mode for synchronous condenser.The conversion logic and ideal value setting method between different control modes are proposed.Secondly,the dividing-stage strategy of reactive power exchange for AC/DC system is proposed.Based on the principle of dead-zone control for AC filters and shunt capacitors and taking the prediction error into account,the ideal value of reactive power exchange for AC/DC system is proposed according to the dynamic reactive power optimization results of UHVDV converter station in coordination with synchronous condenser.Numerical examples of ZLT converter station in Northeast China is given to verify the effectiveness of the proposed method.In the tertiary voltage control of the power grid near UHVDC,the coordination of AC system to converter station can effectively reduce the risk of extra action of discrete voltage regulator equipment in converter station.Therefore,a tertiary voltage control optimization model for the power grid near UHVDC considering independent converter station control constraints is proposed in this paper.On the basis of the conventional static reactive power optimization model of AC/DC system,all the discrete voltage regulator devices do not participate in the tertiary voltage control optimization,and the dead zone constraint of local reactive power balancing of DC system is added.In addition,in view of the problem that the adjustable range of the reactive power supply is fixed in the existing research,the adjustable range constraint with non-fixed boundary of auxiliary power supply for DC system is considered.The prediction-correction primordial dual interior point method is used to solve the model effectively.Numerical examples of ZLT converter station with its nearby AC power grid in Northeast China is given to verify the effectiveness of the proposed method.Dynamic reactive power optimization of AC/DC system can realize the spatial and temporal coordination of all continuous and discrete voltage regulator devices,which provides an ideal decision-making basis for the evaluation of reactive power resources and AVC strategy of AC/DC interconnected power grid.Therefore,a dynamic reactive power optimization method based on ADMM decoupling algorithm for the power grid near UHVDC is proposed in this paper.In terms of the model,on the basis of existing research,the time-varying characteristics of the converter bus voltage and the DC power supply are considered,and the constraints of the variable operating range of the converter bus voltage and the adjustable range of the steady state reactive power of DC power supply are added.Considering the switching requirements of AC filters and shunt capacitors,the minimum filter capacity constraint of the converter station,and the switching sequence constraint of AC filters and shunt capacitors are added.In terms of algorithm,in view of the low efficiency of the current dynamic reactive power optimization algorithm for AC/DC system with LCC-HVDC,high nonlinearity and large linearization error of characteristic equations of LCC converters,a dynamic reactive power optimization algorithm for the power grid near UHVDC based on Alternating Direction Multiplier Method is proposed,which takes the coupling bus of AC/DC system as the boundary node and iteratively calculates the decomposition and coordination model of AC and DC systems.Specifically,the AC system adopts a high-precision linearization model and is solved by CPLEX.The DC system adopts an accurate nonlinear model,which is solved alternately by interior point method and dynamic programming method,in which the state reduction strategy greatly reduces the search space of discrete variables and avoids the problem of dimension disaster.In view of the problem of difficulties in setting the multi-objective weight values for the model,a multi-objective weight coefficient is proposed.Numerical examples of ZLT converter station with its nearby AC power grid in Northeast China is given to verify the effectiveness of the proposed method.