Reliability And Contingency Reserve Research Of Power Grids Considering UHVDC Infeed

China's power grid blueprint is to form a 1000kV UHVAC(ultra-high voltage alternating current)backbone with ± 800kV UHVDC(ultra-high voltage direct current)transmission support.Compared with UHVAC transmission,UHVDC transmission system possesses flexible and fast control mode,which is advantageous to undertake point-to-point,high-capacity and long-distance power delivery from energy center to load center.UHVDC transmission system is helpful to relieve the unbalanced distribution of energy base and load center in China and effective in alleviating the pressure of atmospheric pollution prevention and control in central and eastern regions.What UHVDC transmission brings to the receiving end grid,is not only sufficient power support,but also problems of reliability and contingency reserve.Huge power shortage caused by bipolar block of the UHVDC transmission system will impose great impact on the safe and reliable power supply of receiving end grid.At present,the contingency reserve of power system is usually in accordance with the capacity of the maximum generator unit,or bipolar capacity in DC transmission infeed background.In most traditional methods,DC transmission infeed is usually equivalent to a two-state generator,and then the AC/DC hybrid grid is regarded as a pure AC network.Power flow and derating operation of UHVDC transmission system are not considered.As a result,contingency reserve is often too large and uneconomical.Research on reliability and contingency reserve of power grids with UHVDC infeed,in order to enhance the reliability and economy level of receiving end grids,is of great significance.This paper discusses related issues of reliability and contingency reserve of power grids considering UHVDC infeed in depth.First,a reliability assessment method of UHVDC transmission system considering the characteristic of bipole symmetry is proposed.Based on the analysis on the Markov process of the unipolar system state space,a combination operator is used to evaluate the reliability indexes of UHVDC transmission system by taking standby effect of AC filters and recovery effect of available converter unit into consideration.Sensitivity analysis method is also introduced to locate the weak element of the UHVDC transmission system.Secondly,on the basis of reliability assessment of UHVDC transmission system,a reliability evaluation method of receiving end grid considering UHVDC infeed is proposed.In order to truly reflect the actual situation of UHVDC transmission system,innovative per unit values are introduced so that the hybrid AC/DC power flow can directly consider the derating operation status of the UHVDC transmission system.The proposed method covers all N-2 fault conditions,including UHVDC transmission system,generators and lines of receiving end grid,to evaluate reliability indexes like LOLP and EDNS,which substantially reflect the reliability level of receiving end grid.Finally,a fault-oriented method of contingency reserve arrangement is proposed by using after-the-fact analysis to determine the distribution of reserve between the online generators.A comprehensive index of contingency reserve evaluation considering reliability and economy is proposed by applying reliability assessment of receiving end grid considering UHVDC infeed,which provides reference basis for actual contingency reserve allocation.The calculation result shows that,compared with traditional method,the proposed reliability assessment method of UHVDC transmission system is more concise,but of the same accuracy.The evaluation results of modified IEEE24 reliability test system demonstrates that the proposed method can be used to quantify the reliability level of complex power grid with different contingency reserve capacity in the background of UHVDC transmission system infeed,and provide reference for grid dispatching and operation.