| With the construction of extra-high voltage power grid and the increase of trans-regional and trans-national transmission,the scale of power system interconnection is constantly expanding,the form of power grid gets more complex,and the vulnerability of power system itself is also gradually highlighted.Since 2003,a dozens of large-scale blackout have taken place around the world,the economic losses and social influence for this reason was extremely serious.Therefore,it is of great significance to study the distribution and change characteristics of transient energy in the network under different operation states and instability conditions,identify vulnerable links in the network,and take effective monitoring and control measures to prevent and suppress the further expansion of disasters.Focusing on the structural vulnerability of large power grid,the intrinsic relationship between transient potential energy distribution and structural vulnerability of power grid are studied theoretically this paper,and the vulnerable links of power network can be identified according to the flow or distribution of transient potential energy.In order to enhance the network's ability absorbing transient potential energy,it is proposed to improve the inherent structure of the network,and control strategies of vulnerable links are studied.Finally,the application of transient potential energy distribution in out-of-step splitting is discussed.The main research contents are as follows:(1)The voltage variation track,the phase angle difference distribution variation and transient potential energy distribution law of different position between the equivalent two-machine system are are studied from a geometric point of view,and the internal relationship among them is revealed.The mathematical model of the phase angle difference distribution along the line is established,and its changes under different operating conditions are studied.Based on branch transient potential energy function,the relationship between potential energy distribution and phase angle difference of unit reactance in different position are studied,and the potential energy distribution in different operation states and different positions are compared and analyzed.On this basis,the distribution law of voltage,phase angle difference and transient potential energy in different positions are analyzed for equivalent three machine system,and the relationship between transient potential energy distribution and power network structure is further revealed.At some point,the distribution law of transient potential energy and phase angle difference is consistent.The distribution law of phase angle difference and transient potential energy can provide important indication for the emergency operation of power system,and it can provide accurate criterion for out-of-step splitting.(2)For multi-machine power system,based on graph theory,the vulnerable links of power network are identified in two steps.The first step is to set up network vulnerability indicators from the perspective of pure network structure and parameters.The coupling matrix is formed by using the coupling idea,and the corresponding elements in the coupling matrix are weighted to form the cut-set weight coefficients between generator group.According to the order of the cut-set weight coefficients from small to large,the inherent vulnerable links in network can be identified from macro perspective.In the second step,combining transient potential energy distribution law,the vulnerable links in the network are identified by means of WAMS system.The critical cut-set presented by system instability depend on the topology structure of the network,and also depend on the flow and distribution of transient potential energy.The influence of fault location and power flow size on the vulnerable link are analyzed,and the operation status of power system can be displayed better on the condition of taking into account the power flow size.The simulation results has verified the effectiveness of the proposed method.(3)Aiming at the problem that the ability of the network absorbing transient potential energy is limited due to the uneven power network connection,network homogenization control strategy is proposed.In order to relieve local accumulation of transient potential energy in vulnerable links of the network,firstly,the inherent vulnerable links of the network are fundamentally improved by using cut-set weight coefficients.Secondly,the measures which include increasing voltage in vulnerable link,reducing reactance in vulnerable link and reducing transmission power in vulnerable link on line are taken.All these methods can effectively alleviate the accumulation degree of transient potential energy in vulnerable links,thus enhance the power network's ability to absorb transient potential energy.In order to avoid cascading failures occurrence caused by vulnerable degree increasing of network and the transfer of vulnerable links,it is proposed that the existing relay protection trip should take additional control measures in combination with the operating state of the system.(4)In view of the fact that the existing out-of-step splitting criterion is not accurate,make use of the characteristics that the phase angle difference of the branch in which oscillation center is located exceeds 180°,while the phase angle difference of all other branches varies within 90°,the out-of-step criterion using frequency difference or branch phase angle difference exceeding 180° are proposed.Real-time dates are obtained by means of WAMS system,but in order to reduce data transmission,only vulnerable branches information whose phase angle difference are more than 90°are uploaded as a starting signal of out-of-step splitting.Considering comprehensively to calm the oscillation and maintain the power balance in each subsystem as much as possible after splitting,the splitting section should be selected at the out-of-step section,but the load and generator power corresponding to the out-of-step section should be cut off at the same time. |
PDF Full Text Request