Reliability Evaluation of UHVDC Transmission Systems with a Hierarchical Connection Mode

Compared with the typical Ultra HVDC(UHVDC)system,the inverters of the UHVDC system with a hierarchical connection mode(UHVDC-HCM),connect two receiving-end systems with different operational conditions.This requires the corresponding conversion units(CUs)at the different terminals to be differentiated in reliability modeling,and the spares should also be separately set.A reliability model of the UHVDCHCM system is proposed in this paper.The operating modes are classified by the capacities of the total system and the transmission powers to the two receiving-ends.Considering the independent spares of the components of the CUs at different terminals,the state space is derived.Two sets of indices are newly proposed to more accurately evaluate the system reliability.Based on the matrix description of the frequency&duration(F&D)method,the sensitivities of the reliability indices to the reliability parameters of the components are quantified.Numerical results validate the feasibility of the proposed model.The vulnerabilities are recognized by the sensitivity analysis,and the impact of different spare schemes on the reliability indices and sensitivities are compared.The proposed model and indices provide a reference for the practical UHVDC-HCM projects.

A tour-based analysis of travel mode choice accounting for regional transit service

The aim of this work is to explore the impact of regional transit service on tour-based commuter travel behavior by using the Bayesian hierarchical multinomial logit model, accounting for the spatial heterogeneity of the people living in the same area.With two indicators, accessibility and connectivity measured at the zone level, the regional transit service is captured and then related to the travel mode choice behavior. The sample data are selected from Washington-Baltimore Household Travel Survey in 2007,including all the trips from home to workplace in morning hours in Baltimore city. Traditional multinomial logit model using Bayesian approach is also estimated. A comparison of the two different models shows that ignoring the spatial context can lead to a misspecification of the effects of the regional transit service on travel behavior. The results reveal that improving transit service at regional level can be effective in reducing auto use for commuters after controlling for socio-demographics and travel-related factors.This work provides insights for interpreting tour-based commuter travel behavior by using recently developed methodological approaches. The results of this work will be helpful for engineers, urban planners, and transit operators to decide the needs to improve regional transit service and spatial location efficiently.

Ontology-Based Semantic Multi-agent Framework for Micro-grids with Cyber Physical Concept

The distributed hierarchical control based on multi-agent system(MAS) is the main control method of micro-grids.By allowing more flexible interactions between computing components and their physical environments,cyber physical system(CPS) presents a new approach for the distributed hierarchical engineering system,with micro-grids included.The object of this paper is to integrate the CPS concept with MAS technology and propose a new control framework for micro-grids.With the analysis of the operating mode and control method of micro-grids,the cyber physical control concepts of ontologybased semantic agent are discussed.Then an MAS-based architecture of cyber physical micro-grid system and an intelligent electronic device(IED) function structure are proposed.Finally,in order to operate and test the cyber physical micro-grid concept,an integrated simulation model is presented.

Impact of Synchronous Condenser on the Dynamic Behavior of LCC-based UHVDC System Hierarchically Connected to AC System

Hierarchical connection(HC)is a very attractive mode for±800 kV line commutated converter based ultra high voltage direct current(LCC-UHVDC)system connected to different AC voltage levels because of its ability to reduce the scale factor of a converter transformer.Faults in the HC-UHVDC system can cause commutation failure(CF).In this paper,impact of synchronous condenser(SC)to mitigate CF in HC-UHVDC system is analyzed.A±800KV HC-UHVDC system along with synchronous condenser is built in PSCAD/EMTDC.Transient performance analysis of HC-UHVDC for single and three phase to ground faults is investigated.Commutation failure immunity index(CFII),commutation failure probability index(CFPI),fault recovery time(FRT),and transient overvoltage(TOV)are used as measures to evaluate the effects of SC at HC-UHVDC system design.The simulation results show that SC can make the HCUHVDC system less susceptible to CF,effectively improve fault recovery performances of the overall system,and reduce transient overvoltage when single or multiple converters are blocked.The results of this research can provide technical assistance in real world HC-UHVDC projects.