| The voltage sag events caused by short circuits can seriously threaten the reliability of power supply along with the power qualities of customers.Usually,the short circuits in the grids occur randomly.Under such a circumstance,it is significant to study the accurate assessment approach of voltage sag according to associated factors and the appropriate governance schemes to assess the mitigation benefit.In this paper,the evaluation and mitigation methods of voltage sag are studied.The major contributions of this paper can be summarized as follows:(1)The Monte Carlo simulation technique is implemented for the evaluation of voltage sag due to the occurrence of faults,including the severity and financial losses evaluation of a node.(1)In the aspect of severity evaluation,an average influence degree at a node is designed.The designed index takes multiple characteristic parameters into account,considers the event classification and improves the defect of data skewness.Then the system-side index set is constructed with the average influence degree as well as the voltage sag frequency index,further forms a comprehensive evaluation index set with the failure rate index.The nodes which suffer from greater impacts of voltage sag events can be found out by means of the proposed comprehensive evaluation method.(2)In the aspect of financial evaluation,the direct financial losses of sensitive load nodes are estimated by the frequency,severity and sensitivity characteristics of nodes,while the potential financial losses of nodes are calculated by the node voltage offset values.By quantifying the financial losses,the severity of sags can be comprehensively and intuitively reflected.(II)Considering the preventive effect of prevention-type mitigation equipment and the remedial effect of compensation-type mitigation equipment,this paper proposes a governance scheme to reduce the voltage sag induced financial losses from the perspective of programming.A variety of prevention-type mitigation equipment and compensation-type mitigation equipment are selected to participate in the governance.Then the Pareto optimization is carried out to determine the allocation plan of the governance equipment.In order to further discuss about the investment plan,this paper studies the pricing of voltage sag value-added service and puts forward the pricing satisfaction functions of the power supply companies and customers under the value-added service.The price of voltage sag value-added service is determined on the basis of mutual satisfation so as to improve the business environment.(III)Considering the topological characteristics of distribution network,the optimization strategy of distribution network operation mode is proposed to reduce the voltage sag induced financial losses from the perspective of operation.Initially,a single objective optimization model is established to minimize the voltage sag induced financial losses,which takes the topological characteristics of distribution network into account.The case studies of IEEE 16 node benchmark and 18 nodes distribution system show that the optimization of operation modes can do reduce the financial losses.Secondly,according to the influence mechanism of voltage sag,the structural indicators for voltage sag are established.The feasible grid structures of IEEE 16 node benchmark are selected to analyze the relationship between voltage sag levels and the grid structures by means of the structural evaluation method proven to be reasonable.The results show that the voltage sag levels are indeed affected by the structural attributes of power grid.Finally,the structural evaluation method is briefly compared with the financial evalution method.The optimization method of operation mode based on the combination of structural indicators and financial losses can provide ideas for low-cost voltage sag governance.All in all,the purpose of this paper is to provide some ideas for comprehensive evaluation and mitigation schems of voltage sags. |
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