Study On Probability-based Seismic Performance Assessment Method For Substation Systems

Power systems play a key role in emergency rescue after an earthquake.Substations are among the most important components of a power system.The seismic performance of substations has to be carefully studied.Most studies,however,have focused on the seismic behavior of major electrical equipment or the reliability of the substation under ordinary operational conditions.As a whole system,its seismic performance has not been thoroughly studied.This study proposes a new probability-based method,namely the state tree method,to evaluate the seismic performance of complex engineering systems with multi-inputs and multi-outputs characteristics.The effectiveness and practicability of the method was validated through a case study of a typical 220 kV substation in China.The method includes four steps:(1)definition of system functionality;(2)system analysis with the proposed state tree method;(3)vulnerability definition of components;(4)calculation of the system's fragility curves using Monte-Carlo simulation.Primary research contents are stated as follows:1.The seismic design codes for structures and electrical equipment in different countries were compared and analyzed,the problems and insufficiencies of the current regulations of Chinese seismic design code was identified.Pseudo-acceleration response spectra of over 10 thousands earthquake ground motion records collected from NGAWest2 and KiK-net strong motion database were calculated.These records were separated into four categories based on their site conditions according to Code for Seismic Design of Buildings(GB 50011-2010).The least square method was used to calibrate multiple parameters of the averaged spectra.The dynamic magnification factors,characteristic periods and attenuation coefficients of horizontal and vertical spectra were respectively discussed.Several suggestions were made to revise the seismic design spectra for structures and electrical equipment respectively.Moreover,programs used for searching spectral-matched records and for generating artificial records were developed.Thirty-six groups of strong ground motion records were selected for calculating the equipment's fragility curves.2.The commonly used methods for system analysis were briefly introduced and their limitations were identified.A new method,namely the state tree method,which integrates the fault tree method and the success paths was proposed.A case study was conducted on a typical 220 kV substation.The system functionality of the substation was defined according to the power accessibility at the end of electrical out-lines.The state tree of the substation was established with clearly defined logical relationships between different components such as transformers,circuit breakers,disconnecting switches,current transformers,potential transformers,lightning arresters,insulators and supports,etc.3.The dynamic characteristics and seismic performance of an 110 kV potential transformer,an 110 kV current transformer and a 10 kV medium voltage drive board were obtained through shaking table tests.Numerical models of the potential transformer and current transformer were established.Their fragility curves were calculated through time history analyses with the preselected ground motion records.Literature research was also conducted on the fragility curves of various 110 kV and 220 kV equipment.Double lognormal distribution expressed by median capacity,randomness and uncertainty was used to define the components' fragility.Based on the results of shaking table tests,numerical simulations and previous researches,the fragility parameters for all components were determined.4.The substation's median seismic capacities and the corresponding logarithmic standard deviations with different level of functionality were calculated through MontelCarlo simulation based on the system's state tree model and the components' fragilities.Parametric analyses were conducted to explore the effects of components' fragilities and the types of fragility curves.Key components(weakest links)were identified according to the level of their impacts to the whole system.The effect of key components with improved seismic capacity were also studied at the substation's level.