Research On Virtual Branch Method In Static Voltage Stability

As the sale and complexity of electric power system and long-distance transmission lines increasing continually, as well as the innovation of electric power market advancing, distance safety domain boundaries in present power system are getting close, so severe demands are required on system safety analysis and the control after fault-clearing. Voltage stability which covers a wide range with great danger is an important course on power safety analysis study. Branch outages and branch power flow off-limits are important factors of voltage instability. The outage of fault branches can cause the system to lose static load flow solutions, which increases the difficulty of analysis and control. Up to now, there are no systematic approaches to solve the problem of branch power flow off-limit. This paper mainly studies the assessment of the severity of the instability fault branches, and the problems of recovering the feasible solution of power flows after the outage of fault branches and solving the issues of branch power flow off-limits.Firstly, the problem of reactive power constrains of generator at slack bus is studied, and a load margin algorithm of continuation power flow of reactive power constrain at slack bus is proposed. Based on the analysis of the character of AC power grid load flow equations and the special grid parameter structure in power systems, an improved power flow algorithm regarded reactive power limits at slack bus is puted forward. When the slack bus reaches its reactive power maximum value, a bus type change method, i.e., the Vθbus is turned to Qθbus, is proposed. By using the variation dynamic admittance algorithm of the load power, the ill-conditional number problem of the node admittance matrix is solved. The improved power flow algorithm is applied to load margin calculation of the continuation power flow, and the results of simulation display that the reactive power constrain of the slack bus decrease the load margin, and speed up the coming of voltage collapse. So, the problem of reactive power constrains of generator at slack bus should be considered when calculating the load margin and critical voltage by using the method of continuation power flow.Secondly, in order to evaluate the severity of instability fault branches and solve the problem that the system will lose power flow feasible solutions after the faults, a virtual branch control method of the instability fault branch is puted forward, basing on Substitution Theorem and the continuation power flow method of reactive power constrains at slack bus. This method regards the removed-to-be fault branch as a virtual branch, and the removed branch issue into the increasing problem of the injected power in fault branches, and using the continuation power flow to calculate the maximum value of the virtual parameter in the voltage safety domain. If the maximum virtual parameter is lower than the upper limits, then this branch is an instability fault branch, and the virtual parameter can act as an important indictor of evaluating the severity of fault branches. Control measures on instability fault branches are classified as fault-clearing process control and post-fault optimal control, and this paper primarily studies the first part. This method selects the optimal control direction by using the participation factor of the minimum eigenvalue of Jacobian matrix, and the control is gradually carried out according to the increasing of the virtual power source parameter, thus the minimum load-shedding and the time optimal control become available.Furthermore, in order to remove the branch power flow off-limit, a virtual branch control method of the branch power flow off-limit is puted forward. Based on Thevenin's Theorem, a shunting virtual branch model in power flow off-limit branches is adopted. Admittance parameters of the virtual branch can be obtained by object shunting coefficient, Thevenin's equivalent resistance and the admittance of the primary branch of power flow off-limit. After using the quasi-margin model, the process of virtual branch removal is transferred into the increasing process of the parameter of virtual power sources. By using the sensitivity of the primary branch's apparent power to the load and generator power, the minimum charge scheme of control can be obtained to eliminate the problem of branch power flow off-limit. This method can utilize the minimum control charge to reduce the currents of the primary power flow off-limit branches to the object boundary.Finally, according to the state that the"concavity and convexity"effect of the security domain can be obtained by the minimum load margin of the voltage security domain, this paper has proposed a new radius algorithm in the minimum load margin of the voltage security domain which is based on the method of the continuation power flow. A voltage absolute security domain suprasphere is defined, and then select the fastest direction of the instability voltage by the participation factor of Jacobian matrix minimum eigenvalue, and finally based on the method of continuation power flow, trace the static operating point gradually to the critical point of system collapse in the direction where the system deteriorates fastest, thus the minimum load margin radius of voltage security domain can be obtained. Under the conditions of N-1, this method could provide guidance in obtaining the maximum absolute control of the security domain without the"concavity and convexity"effect.An IEEE bus system is used for simulating and analyzing the approach proposed in this paper. The testified result fully indicates the correctness and effectiveness of the presented algorithm and also demonstrates that the approach has certain theoretical value and practical engineering value.