Fundamental Performances Reasearch On The Novel Hybrid Dual-Infeed HVDC System

At present, Line-Commutated-Converter High Voltage Direct Current (LCC-HVDC) has been extensively put into operation in the world, due to its technical and economic advantages. As the development of high power full-controllable power electronic devices and the exploitation of large-scale renewable energy sources, Voltage Source Converter based High Voltage Direct Current (VSC-HVDC) projects have been expanding rapidly. Thus, the hybrid dual-infeed or multi-infeed HVDC system will appear in future power grid, which brings new opportunities and challenges to security and stabilization control of future power grid. Therefore, the operation mechanism of hybrid dual-infeed or multi-infeed HVDC system needs to be investigated deeply. In this dissertation, the hybrid dual-infeed HVDC system, combined with LCC-HVDC and VSC-HVDC, is researched fundamentally and comprehensively.1) Research on the control strategies and operation performances of LCC-HVDC and VSC-HVDCAiming at studying the operation mechanism of hybrid dual-infeed HVDC system, the control strategies and operation performances of LCC-HVDC and VSC-HVDC system are investigated separately. According to LCC-HVDC, various aspects, such as the Short Circuit Ratio (SCR), Maximum Available Power (MAP), Transient Overvoltage,(TOV), Commutation Failure Immunity Index (CFII)and fault recovery performance are considered to comprehensively analyze the steady state and transient performances of LCC-HVDC. The results show that the AC system strength is the key factor to impact the performance of LCC-HVDC system.A new control strategy for VSC-HVDC is proposed using coordinate conversion and variables substitution, and it can control the active and reactive power independently. The corresponding power controllers were designed. Based on the proposed strategy, the transmission limits of the active and reactive power were deduced. According to the power circle characteristics, it was theoretically proved that the control strategy has the ability to control the active and reactive power independently. Hooke and Jeeves algorithm was adopted to optimize the PI parameters of the presented controllers. Simulation results obtained using PSCAD/EMTDC show that, the operation characteristics of VSC-HVDC are quite improved with the optimized PI parameters. Moreover, the proposed controllers can independently control the active and reactive power, and has fast response speed, desirable stability and strong robustness.2) Research on the operation mechanism of hybrid dual-infeed HVDC system supplying power to active networkFirstly, the model of hybrid dual-infeed HVDC system for supplying power to active AC network is developed, coordinated control strategy between LCC-HVDC and VSC-HVDC is presented, and its steady state performance is simulated and analyzed. Then, a conventional dual-infeed HVDC system, composed of two LCC-HVDC links, is established. The various aspects, such as SCR, MAP, TOV, CFII and fault recovery performance, are considered to compare the steady state and dynamic performances between hybrid dual-infeed HVDC system and conventional dual-infeed HVDC system. The results show that VSC-HVDC has the ability to improve AC voltage regulation, increase the MAP value of LCC-HVDC, reduce the TOV of LCC-HVDC, make the LCC-HVDC less susceptible to commutation failure, accelerate the LCC-HVDC fault recovery speed, and finally improve the stabilization of LCC-HVDC system.3) Impact of VSC-HVDC on LCC-HVDC system performances in hybrid dual-infeed HVDC systemBased on the Maximum Power Curve (MPC) and MAP concept, a new index called the "Apparent Increase in Short Circuit Ratio (AISCR)" is proposed to quantify the performance improvement of the LCC-HVDC by VSC-HVDC in hybrid dual-infeed HVDC system. Then, from the "equivalent operation performance" point of view, a single LCC-HVDC system with increased SCR is developed. The various aspects, such as SCR, MAP, TOV, CFII and fault recovery performance, are considered to compare the steady state and dynamic performances between hybrid dual-infeed HVDC system and the developed single LCC-HVDC system. In order to further testify the effectiveness of AISCR index, the TOV and CFII feathers are deeply investigated. The results show that the proposed AISCR index can describes the steady-state behaviors improvements of LCC-HVDC by VSC-HVDC infeed; however, it is not accurate for transient behaviors.4) Research on the operation mechanism of hybrid dual-infeed HVDC system during the initial stage of power gird restoration after receiving end blackoutBased on the technical features of LCC-HVDC and VSC-HVDC, a new concept, supplying power to an entirely passive AC network through a hybrid dual-infeed HVDC system, is proposed during the initial stage of power gird restoration after receiving end blackout, and the corresponding model is developed in PSCAD/EMTDC software. The principle of LCC-HVDC start-up using VSC-HVDC is presented, and the control strategies of hybrid dual-infeed HVDC system to supply passive network is also provided. Then, the interaction mechanism between LCC-HVDC and VSC-HVDC is further analyzed. Aiming at studying the steady state and transient performances of hybrid dual-infeed HVDC system with passive receiving AC network, the process of HVDC start-up using VSC-HVDC and dynamic performance under common AC busbar faults are simulated in PSCAD/EMTDC. Simulation results show that during the initial stage of power gird restoration after receiving end blackout, LCC-HVDC can start successfully using VSC-HVDC, and the concept, supplying power to passive network using proposed hybrid dual-infeed HVDC system, is feasible. In additon, the whole system has favorable power recovery performances under common AC busbar faults conditions.