Power Flow Modelling And Reliability Evaluation To UHVDC Line And Its Hierarchical Connection Mode

The ultra-high voltage direct current(UHVDC)transmission lines are capable of bulk power transmission over long distance,with the merits of flexible controllability.Compared with the traditional HVDC line,the UHVDC line has more complex structure and more versatile operation modes.Under the hierarchical connection mode,the inverters connect two AC grids,and the outage may cause the great influence on the power grids at both sending-end and receiving-end.In this paper,the power flow modelling,reliability modelling,and sensitivity analysis to the AC/UHVDC systems are studied.The main work is as follow:1)The power flow models of the AC/DC system with UHVDC line and its hierarchical connection mode are newly proposed.Considering the balanced and unbalanced operation modes of the UHVDC line and its hierarchical connection mode,the independent DC variables are determined.The equivalent powers of the converters are derived,and the elements of the extended Jacobian matrix are defined,thus to derive the power flow constraint equation.According to the power transfer schemes,the changes of the DC currents of the positive and negative poles,and the voltages and powers of both converters are derived,thus to compare the impacts of them on the grid power flow.2)Based on the matrix description of the frequency & duration(F&D)method,the reliability model of the UHVDC line with hierarchical connection mode is proposed.The components in the conversion units(CUs)of the rectifier,inverter at high-terminal,and inverter of low-terminal are differentiated,and the corresponding spares are considered independently.According to the influence of the components on the total capacity,the AC filter(ACF)of the inverters belong to the subsystem of the CU,while the ACF of the rectifier is part of the subsystem of the pole equipment.Thus the process of the reliability equivalence is determined,yielding the state space with 8 states of the UHVDC line with hierarchical connection mode.Considering the structure and operating characteristic,two sets of the reliability indices are newly proposed.3)A sensitivity-based optimal reliability design model is newly proposed.The corresponding sensitivity provides references to identify the non-coherency equipments and to adjust the reliability parameters,while avoiding the repeated calculation of the system reliability.The optimal load shedding model of the hybrid system with UHVDC line is proposed mode,considering its different operation modes.The incremental transition rate of the UHVDC is derived,thus to quantify the system reliability indices.Based on the probabilities of the system state,UHVDC,and its components,a direct solution of the corresponding sensitivity is derived,while avoiding the repeated calculation of the derivatives,and the error of the sensitivity is reduced.By combining the sensitivity,the calculation of the system reliability is converted to the linear problem with higher efficiency.In addition,the non-coherency equipments and components are identified by the corresponding sensitivities,thus the unnecessary investment and operation costs are reduced.4)With the sequential Monte Carlo simulation(MCS),the system state probability is not the explicit function of the reliability parameters of all equipments.A new sensitivity model for the sequential MCS is proposed in this paper,and the sensitivity model is applied to the hybrid system with the UHVDC line.The sequence of the UHVDC line is obtained with considering the possible reached state,mean duration,and probability for each state transition.Based on the system state,the states and durations of the related equipments from the beginning of the simulated years to the generated time of the system state are considered and differentiated,to describe the derivative of the system state duration to the reliability parameters.By combining the sensitivity model of the UHVDC line itself,the equivalent transition rates are applied to connect the system state duration and the reliability parameters of the UHVDC components,yielding the corresponding sensitivities.