Research On Static Security Mechanism And Analysis Method Of Electromagnetic Loop Network

The electromagnetic loop network(EMLN)is a power network connected by different voltage lines through transformers.In the weak period of power grid,China adopts the strategy of careful use of EMLN for avoiding its security problem.With the growing strength of the power grid,the EMLN security problem gradually disappears.Nevertheless,EMLN is still not encouraged in China.However,the parallel power transmission on high and low voltage lines,as well as the loop power distribution characteristic of EMLN,plays an active role in the power network security and reliability.In the modern power grid environment where the power grid is strong enough,re-examining and correctly determining the security property of EMLN is not only of theoretical significance to promote the power network security analysis,but also of practical significance to further enhance the security and reliability of the modern power grid.This paper establishes a general model of EMLN,analyses and examines online the EMLN problem,and demonstrates the time-series evolution relation between EMLN and the power network security.First,a general model of EMLN is established and accordingly the EMLN security problem is analyzed.Based on the established model,1)the influence mechanism of the network framework on the security of EMLN is analyzed,and 2)the robustness of the grid from the perspective of EMLN is described.The model shows that the security of EMLN is closely related to the robustness of the power grid.If the parallel flow phenomenon or the flow transfer phenomenon in EMLN is slight,and the adequacy of low voltage lines is sufficient,then the power network presents high robustness,therefore the EMLN possesses no security problem.Conversely,the EMLN possesses security problems.Second,the master-slave splitting algorithm of the post-contingency steady state power flow is proposed to examine more accurately the static security of EMLN.Postcontingency steady state is the steady state in which the power balance is restored after contingency.The frequency regulation action after contingency should be considered while calculating the post-contingency steady state power flow.The data with different voltage classes in EMLN may be located in different energy management systems(EMS),then the post-contingency steady state power flow better adopts the splitting algorithm.The master-slave splitting algorithm splits the power network into master network and slave networks according to the voltage class,calculates the system frequency in the master network,responds to the frequency in the slave networks,and achieves an accurate global solution by means of alternating iterations between the master network and the slave networks with few data interchanges.The improved master-slave splitting algorithm based on the current decomposition formula of the slave network branches in EMLN model can effectively improve the calculation speed.The test results illustrate that the proposed algorithm is more accurate than the post-contingency steady state power flow that is calculated only in the master network environment,and the accuracy is more noticeable in the case of EMLN.Third,the post-contingency steady state power transfer method is proposed to analyze online the static security of weak branches in EMLN.Post-contingency steady state power transfer method is the algorithm that ranks all the anticipated contingencies according to the degree of their influences on the inspected branch.The standard queue to verify the algorithm should be obtained from the post-contingency steady state power flow solution to accommodate the significant influence of the system frequency regulation on ranking results.The core formula of the proposed method is based on the first real power iteration of the fast decomposition method of the post-contingency steady state power flow.The formula shows that the post-contingency steady state power increment of the weak branch in EMLN depends on four factors: power network framework,initial operating condition,contingency power and frequency regulation pattern.The postcontingency steady state power transfer method can concentrate on calculating only the post-contingency power increment of the inspected branch on the basis of reserving a small number of constant coefficients.It is proved that this method can significantly improve the accuracy and speed of the post-contingency steady state security assessment of the weak branch in EMLN in a general sense.Finally,the security of EMLN in the time-series invariant power network,as well as the time-series variant power network,is analysed.In the time-series invariant power network,a sufficient condition,as well as a necessary condition,for the security operation of EMLN is derived.Such conditions,together with the proposed two security analysis methods,constitute the method of on-line monitoring the security of EMLN.Accordingly,a variety of distribution EMLN structures with high power supply reliability are proposed.In the time-series variant power network,the security of EMLN is time-series variant.On one hand,the security and the resource utilization efficiency of EMLN is increasing during the evolution process of power grid.On the other hand,the security of EMLN is decreasing during the degeneration process of power grid,while the influence of EMLN on cascading failure process may be promoted,restrained or ineffective.For the strong EMLN which is easy to form normally weak EMLN in the process of cascading failure,maintaining the EMLN operation mode can improve the security of power grid.The cascading failure analysis results of an actual provincial power grid case support the conclusions above.In summary,the security of EMLN is closely related to the robustness and the timeseries variance process of power network.The EMLN in the power network with high robustness possesses no security problem,and can even improve the power supply reliability of the power network.