Research On Overvoltage And Insulation Coordination Of ±1100kV UHVDC System

Our country has proposed some new energy development strategies such as "Electricity Sent from West to East" and "Electricity Sent from South to North" in this new century, but the traditional AC transmission system suffers great loss in power when huge amount of electric power is transferred in a long distance, and the stability of AC system may face big challenges too. The UHVDC system gives out a solution to those problems and its rapid development allows our country to set up the new energy plan. The operational statistics of the existed ± 800kV UHVDC projects shows that overvoltage due to faults or operations is the key cause that determines the devices’insulation level which are at the DC side of converter station. So it has great value to do some studies on the characteristics of the overvoltage at UHVDC converter station’s DC side for both scientific research and practical engineering uses. The main work done in the thesis is to carry out some studies on the overvoltage at the DC side of UHVDC converter station and the system’s coordination insulation.In this thesis the reasons that lead to the overvoltage at the DC side are analyzed, then the way how the overvoltage due to faults or operations is transferred to the DC side is studied. After the parameters of every device is confirmed, the simulation model is set up in PSCAD for simulating the real project. The characteristic of the overvoltage at important points in the converter station, such as the valve、neutral bus and DC line under many typical operations and fault conditions is analyzed. Some key devices’ insulation level is given out after referring to some guidelines.This thesis points out the advantage of the arrester configuration in which arrester A2 is used to protect the valve side of converter transformer at the higher end and building smoothing reactor in DC line and neutral bus respectively for+1100kV UHVDC system. Combining the simulation results and guidelines for overvoltage, this thesis gives out the insulation level for the devices both on the AC side and DC side of ± 1100kV UHVDC system. For example, the bus line and devices on the AC 750 kV side take 2100kV(1950kV for transformer)/1550kV as their LIWL/SIWL; the LIWL/SIWL for the devices at the transformer’s valve side is 2350kV/2100kV It also discusses the possibility to lower LIWL/SIWL for the transformer by taking Uref of the MOA at the rectifier station down to 576kV.The conclusions of this thesis have great value to study and design insulation coordination of ± 1100kV UHVDC transmission system.