The Determination Of The Stability Of Power Systems During Transients

During transients,the instability of the power system can attribute to the angle instability as well as the voltage instability.The angle and voltage stability are entangled with each other under such circumstances.It is difficult to determine the instability mode of the system during transients.The angle stability has been well-studied,and many online stability assessment approaches have been put forward to this end.Yet the problem to determine the voltage stability of power systems during transients are seldom discussed,especially when mixing angle instability and the voltage instability interact with each other.In order to determine the stability model of the system easily,this paper proposes two methods to determine voltage instability and angle instability,which can determine the dominant instability mode of the system during transients.Firstly,this paper analyze the interaction of angle and voltage stability,and it shows that voltage stability is determined by the magnitude of the voltage source,the load,and also the impedance of the transmission line.And then,it is verified that voltage stability is influenced by the load characteristic.Then,it can be seen that the change of the angle difference of the generators results in the voltage stability.Generator angle decline rapidly and the supply balance is destructed are two main factors influence the voltage stability.Besides,the location of the source also influence the voltage stability.Next,a direct method approach to determine the types of instability of power systems during transients is proposed.Based on differential algebraic equations of the systems,the equilibriums and singular points of the system are defined.The singular surface consisting of singular points then divides the manifolds of the system trajectory into multiple algebraic manifolds,with each algebraic manifold containing one or more equilibriums.Three types of instability of the system are identified according to the distribution of equilibriums,the angle instability,voltage instability and angle-induced-voltage instability.The proposed method can easily determine the dominant instability mode of the system during transients and the case studies also demonstrate its effectiveness.But this method is not suitable for large systems,and it can’t tell the exact time of the instability occurs.In the last part,an approach to determine the stability of power system during transients is proposed,which can solve the above problems.An online method to identify the voltage stability of the power systems first proposed.The proposed method is based on the nodal equivalent method,and also utilities the measured data to identify the parameters that are applied to determine the voltage stability of the system.We use the EEAC method to determine the angle stability problems.It is easy to determine the instability model of the proposed approach.Simulation studies demonstrate the effectiveness.Finally,a study on generator shedding and load shedding are verified to the power system.The proposed method can apply to ordinary systems and can determine the main instability model easily.besides it is significant to choice the control measure when damage occurs.