Research On Control And Protection Strategies Of Hybrid Multi-infeed HVDC Transmission System Under Commutation Failure

At present,traditional Line Commutated Converter High Voltage Direct Current(LCC-HVDC) has been extensively put into commercial operation in the world,due to its technical and economic advantages. Because of the unbalanced distribution of regional economic development and energy,more than one DC transmission lines have been pointed to the same receiving ac systems at some parts of the load center. As the development of high power full-controllable power electronic devices and the the large-scale utilization of renewable energy sources like wind and solar,Voltage Source Converter Based High Voltage Direct Current(VSC-HVDC) projects have been favoured. Following the two-level,three-level voltage source converter topology,the modular multi-level converter has appeared. As a new generation of DC transmission technology,VSC-HVDC system is being developed and applied worldwide. Thus,the hybrid multi-infeed DC power(MIDC) system pattern will appear in future power grid constructed by VSC-HVDC and LCC-HVDC lines,which brings new opportunities and challenges to safe and steady operation of future power grid. In traditional MIDC transmission system,the voltage interaction between two converter ac buses is affected by the relative strength of the ac system and commutation failure of the inverter station may cause an accident of one DC subsystem " spread " to other DC subsystems. A new hybrid MIDC system was investigated synthetically to utilize the advantages of LCC-HVDC and VSC-HVDC transmission system playing their respective advantages.1) The transmission principle and control strategy of LCC-HVDC and VSC-HVDC transmission systemAiming at the technical characteristics of the LCC-HVDC and VSC-HVDC transmission system,the principle of AC-DC transmission system,the basics structure and the control strategies of LCC-HVDC and VSC-HVDC system are investigated separately. VSC-HVDC “imbedding” LCC-HVDC transmission system can improve the characteristics of ac busbar voltage,reduce LCC-HVDC commutation failure and enhance the stability of the entire transmission system.2)System strength calculation and performance of simulation analysis in a hybrid multi-infeed HVDC SystemCommutation failure have certain correlation with the strength of receiving system. A hybrid MIDC transmission system taking into account the AC system load characteristics is established. To compared with hybrid MIDC,a single LCC-HVDC transmission system considering load characteristics of receiving ac system is also established. From the effective short circuit ratio( ESCR),commutation failure of immune index(CFII) and recovery features et al multiple indexes,the steady state and transient performance of LCC-HVDC transmission system of each type are theoretically deduced and compared with simulation analysis. The results shows that the VSC-HVDC system fed into can help enhance the ESCR of receiving ac system,increase CFII,reduce the chances of commutation failure,and recover failure more smoothly and rapidly.3) performance of impedance directional protection based on variation of power-frequency components in hybrid multi-infeed HVDC systemBase on the analysis of model of the traditional DC equivalent impedance based on variation of power-frequency components under the receiving AC power failure in the traditional AC/DC power system,the hybrid MIDC equivalent impedance based on variation of power-frequency components is evaluated qualitatively and quantitatively under the ac power line fault of LCC-HVDC fed into in the MIDC system. Meanwhile,reveals the mechanism of the influence of VSC-HVDC fed into. The simulations of a single LCC-HVDC fed into system and the hybrid MIDC system which are both not considered load characteristics by PSCAD/EMTDC comprehensively and systematically testify the accuracy and validity of the theory analysis.4) novel comprehensive Impedance directional protection principle based on power-frequency variation components of segregated phase in hybrid multi-infeed HVDC interconnected systemBase on the analysis of DC system equivalent impedance based on variation of power-frequency components under the receiving ac power failure and inducing DC system transient process, the directional pilot protection based on variation of power-frequency components in receiving ac power transmission lines under hybrid MIDC fed into is theoretically analyzed and improved.Then,the protection criterion of comprehensive impedance directional protection principle based on power-frequency variation components of segregated phase is put forward,and finally systematically verify the theoretical analysis is correct by PSCAD/EMTDC simulation.