Voltage Stability Analysis And Load-shedding Strategy Research Based On The Impedance Modulus Margin

In recent years,the interconnection of regional power network,on the one hand,greatly improves the ability of the resource optimization allocation,but it also has brought more complex problems about operation and control of the system.Voltage collapse accidents happened at home and abroad in the last century had a tremendous effect on the economic and people's life of all countries,so it is extremely important to find the suitable indexes for evaluating the voltage stability and to determine the effective measures to prevent the voltage instability.The impedance modulus margin based on the comprehensive dynamic equivalent circuit of the power system is an effective index among the indexes for evaluating the system voltage stability to monitor the maximum transmission capacity of the power system.According to the size of the index,the strength of the voltage stability of the system load nodes can be ranked and monitoring the load node of the largest impedance modulus margin is the most valuable.In order to study the internal relationship between this index and the load power,the relationship between the load power and the impedance margin index is obtained by increasing the load in this paper,and the mechanism of the impedance margin index in improving the voltage stability during the implementation of the load shedding is revealed.As an important means of preventing voltage instability,in previous studies,load shedding is usually implemented in the weak nodes of system,but after the resectable loads of the weak note itself are all removed,voltage instability may not still be unable to prevent,so other suitable nodes to continue shedding load have to be found.From this the two-stage identification method of the optimal load-shedding point set with the control order is put forward and the determined shedding-load position by the method considering the coupling relationship between the nodes and the load power factor of each node will be more in line with the actual situation.According to the sensitivity index of node voltage to power parameter variable in the first stage,the nodes who are more sensitive to power changes in the system can be determined and at the same time they are also the weak nodes of system,but the index can not consider the influence of other load to the low voltage node.In order to further reduce the load-shedding point set and determine the optimal load-shedding sequence,according to the sensitivity of the impedance modulus margin index to the power in the second stage,the optimal load-shedding point set with the control order is determined.As for the calculation of load shedding,according to the pressure drop of every load on the system equivalent impedance,the changed pressure drop on the system equivalent impedance when reaching the target impedance modulus margin index is calculated so that calculats the needed load-shedding amount in the load-shedding node and the correction factor is introduced to correct the load-shedding amount of each round.According to the optimal load-shedding point set,implementing load shedding will make the total amount minimum to reach the target impedance modulus margin value and the implementation process of load shedding is reviewed simultaneously.Finally,the correctness and effectiveness of impedance modulus margin index and the proposed shedding-load algorithm in this paper are verified through the IEEE14-bus and the IEEE30-bus systems.