Voltage Characteristics Analysis Arnd Control Strategies Design For Receiving End Power Networks With Multiple HDVC Infeeds

With the development of high-voltage and large-capacity power electronics technology,the HVDC technology has been widely used in long-distance power transmission,asynchronous power network interconnection and renewable power connection.With HVDC inverter stations densely concentrated,the multi-infeed HVDC configuration will be formed for a power network at the load center.The operation of a HVDC transmission system with the half-controlled thyristor converters as the commutators requires strong support from the receiving end AC power network.If the receiving end power network is weak,a series of security and stability problems may occur.In a receiving end AC power network with multiple HVDC infeeds,a fault in the power network may cause simultaneous commutation failures of multiple HVDC inverter stations due to the interactions between various HVDC infeeds.The commutation failures may even trigger HVDC blocking for a single infeed or multiple infeeds,which will cause the loss of large-capacity generation and instability for the receiving end power network with huge outage costs produced.It is of great theoretical and practical value to study the characteristics and control strategies for multi-infeed HVDC systems for maintaining the stable operation of power networks.This thesis focuses on the voltage characteristics and control strategies of receiving end power networks with multiple HVDC infeeds.The main work and research results of the thesis are as follows.(1)A static voltage stability assessment method is proposed for receiving end power networks with multiple HVDC infeeds.The main factors influencing the static voltage stability of the receiving end power network are analyzed,which are the strength of the receiving end AC power system and the electrical couplings between HVDC inverter stations.Indices are extracted to quantify these factors with the information compression technique applied.By using the multivariate regression analysis method,a regression model is established with static voltage stability index as the dependent variable while the indices for the strength of the receiving end AC power system and the electrical couplings between HVDC inverter stations as the independent variables.Simulation results of a test power system verify the effectiveness of the proposed assessment method.(2)A cooperative secondary voltage control strategy for multi-infeed power systems is proposed.A secondary voltage control model is built,which is applicable to the hybrid multi-infeed HVDC power systems consisting of line-commutated converter-based HVDC(LCC-HVDC)infeeds and voltage source converter-based HVDC(VSC-HVDC)infeeds.By using a consensus algorithm,a cooperative secondary voltage control strategy for multi-infeed power systems is proposed,for which the consensus term and correction term are derived via the Lagrangian multiplier approach.Based on the H2 performance standard,a communication network optimization model for the cooperative secondary voltage control strategy is established and solved by the mixed integer programming method.Simulation results of a test power system verify the effectiveness of the proposed control strategy.(3)A commutation failure immunity analysis and enhancement method is proposed for multi-infeed power systems.An electromagnetic transient simulation-based approach for computing the commutation failure immunity index is presented.Accounting for the reactive power and voltage distribution characteristics of the receiving end power network,the explicit expression for the voltage correlation factor is derived which can adapt to the changing power system conditions.By using the voltage correlation factor and the critical voltage correlation factor,an identification method is proposed for localizing zones in the receiving end power network where a fault will trigger simultaneous commutation failures of multiple HVDC infeeds.Various types of dynamic reactive power compensation devices are compared in their performance of enhancing commutation failure immunity by using electromagnetic transient simulation.The placement plan for dynamic reactive power compensation devices is introduced,which aims to enhance commutation failure immunity and eliminate zones triggering simultaneous commutation failure for multi-infeed power systems.Simulation results of a test power system verify the effectiveness of the proposed analysis method and placement plan.