Study On Fast Voltage Sag Assessment Of Distribution Network And Active Reconfiguration

With the wide application of precision devices and semiconductors in daily life and production,the demand of power consumers for power quality is increasing.Power quality problems such as voltage sag caused by short circuit fault cannot be ignored.How to accurately calculate the voltage sag under the short circuit fault of distribution network is the basis of quantifying the impact of voltage sag on power users,and further quantifying the impact of random fault on distribution network is the key link of voltage sag prevention and control research.For this reason,this paper carried out the following research.First of all,aiming at the problem that the calculation of short-circuit faults in distribution network only considers the distribution network,but neglects the influence of transformer and the coupling relationship of transmission and distribution network,a method based on compensation is proposed in this paper.Based on the topology and operation data of distribution network,the distribution network normal model is established.And according to the type and location of distribution network short circuit fault,the equivalence of distribution network under short circuit fault is established.The three-sequence model of transformer is used to establish the equivalence of distribution network at the bus bar of high voltage side of transformer.Combining with the variation of equivalence of distribution network before and after the fault occurs,the variation on the state of the transmission network is calculated by using the compensation method.After the variation is superposed with the network state before the fault,the state of transmission and distribution network is determined successively.The feasibility and accuracy of the proposed algorithm are verified by simulating in certain urban power systems.Then,in the study of voltage sag risk assessment under random short-circuit fault,Monte Carlo simulation of network fault is usually needed,but the amount of calculation is huge.In order to improve the efficiency of single short-circuit fault calculation,a practical calculation method of distribution network voltage sag is proposed;in order to improve the speed of distribution network voltage sag assessment,a fast algorithm based on fitting is proposed in this paper.The concepts of voltage sag sensitivity threshold are introduced by considering sensitivity difference of different power equipment to voltage sag while making assessment and the assessment is realized by integral and weighted summation finally.Experiments were carried out on the IEEE33 three-phase distribution network system,which verify the accuracy of the practical calculation method of distribution network voltage sag.The fast algorithm and Monte Carlo method are used to evaluate the voltage sag of random short-circuit fault respectively.The difference between the two evaluation results is very small,and the former has obvious advantages in evaluation speed,and the evaluation result is guiding significance for the implementation of voltage sag control measures.Finally,an active reconfiguration method that takes into account both reducing the risk of the voltage sag and the purpose of controlling line losses is proposed.First establish an active reconfiguration model considering both voltage sag assessment and line loss,and then propose an improved simulated annealing particle swarm algorithm.The improved algorithm improves the diversity of particles by introducing mutation and exchange mechanism,and proposes the improved Metropolis criterion considering the stagnation degree of local optimal value as the basis of judging the inferior solution is absorbed or not.Finally,IEEE33 three-phase distribution system is taken as the simulation object,and the goal of reconfiguration is to reduce network loss,improve the reliability of distribution network under random fault and the combination of the two,respectively,to verify the effectiveness and superiority of the improved algorithm,and to prove that active reconfiguration strategy can not only reduce the line loss,but also effectively reduce the harm of random fault to distribution network system and power consumers,so that it has practical value of engineering.