Study On Active Power Balance Scheme Of Receiving-end Grid Considering Randomness And DC Blocking Fault

The "west power-to-east" strategy promotes rapid development of High Voltage Direct Current transmission technology in China.Meanwhile,a huge active power shortage will be generated after the DC blocking fault in the receiving-end grid with multiple DC connections,which will cause frequency drop,substantial power transfer,AC transmission lines overloaded,even results in cascading failures or large-scale power outage.In order to avoid the serious impact,it is of great practical significance to deeply study the balance scheme of active power shortage in receiving-end grid after the fault.However,existing research on this issue has not solved the following problems.Firstly,how to take into account the influence of new energy output and load randomness.Secondly,how to accurately calculate the active power shortage after the fault.Moreover,how to optimize and coordinate resources when the system safety is guaranteed after the fault to make the corrective scheme more economical and practical.Therefore,in view of the unresolved problems above,specific contributions of this paper are threefold:1)A load shedding(LS)scheme considering the static characteristics of stochastic load and the randomness of new energy output in receiving-end grid after DC blocking fault is proposed in this paper.In order to accurately calculate the active power shortage,inertial link of traditional generator and stochastic load model with static characteristics consideration are taken into account in the scheme.Finally,the solution is based on probabilistic power flow(PPF).Meanwhile,this paper deeply studies various factors which affect the amount of active power shortage and their mechanism with new energy output and load randomness consideration.In addition,multiple LS coefficients are used in this paper to allocate the active power shortage.A safety index to evaluate system operation under stochastic scenarios is proposed to compare the performance of different LS coefficients.Based on the proposed safety index,this paper further determines the optimal LS scheme and unifies LS coefficient in stochastic scenarios,making the LS scheme more practical.Based on related experiments of modified IEEE-39 bus system,the effectiveness of proposed LS scheme in this paper is demonstrated.2)Given the proposed LS scheme in 1)can neither optimize the cost of balancing active power shortage,nor restrict the off-limit probability of voltage amplitude and branch active power in stochastic scenarios,this paper establishes stochastic optimization model for balancing active power shortage after DC blocking fault.Emergency DC power support of unfaulted DC lines is introduced to participate in balancing active power shortage,to make full use of the available resources of power system.To reasonably evaluate the impact of the method participated in balancing active power shortage on the system operation,the overload risk of DC lines and the contribution of LS to the operational safety are taken into account in comprehensive cost,which makes the objective function composed of the comprehensive cost more reasonable.Corresponding chance constraints are established on the voltage amplitude,branch active power flow and DC power flow,in order to control the off-limit probability in stochastic scenarios.Based on the proposed model,the balance scheme of active power shortage can change the off-limit probability of chance constraints according to the safety requirements of system operation,and make the scheme more flexible and practical in the case of coordinating economy and safety.This paper verified the effectiveness and superiority of the proposed model based on the modified IEEE-39 bus system.3)Given the stochastic optimization model proposed in 2)can neither calculate the system frequency,nor take into account the effect of voltage and frequency on the actual load,this paper proposed stochastic optimization model considering frequency for balancing active power shortage after DC blocking fault.To reflect the actual system operation after the fault,the proposed model considers the stochastic load model affected by the static frequency/voltage characteristics and the generator model with the static frequency characteristics consideration.Not only the conventional constraints corresponding to the voltage amplitude,branch active power and DC power flow,but also the frequency,need to be converted into chance constraints in order to control the offlimit probability of these variables in stochastic scenarios.The problem transformed into an iterative solution process of optimal power flow(OPF)considering frequency and PPF considering frequency by introducing uncertainty boundary.Since the proposed model introduces more complex variable function constraints,it is quite difficult to solve the corresponding OPF considering frequency problem.The Path Following method is adopted in this paper to solve the OPF problem considering frequency,and the solution process is described in detail.Based on the modify IEEE-39 bus system,the effectiveness and superiority of the proposed model is demonstrated in this paper.