Study On Characteristics Of Protections For Power Grid With Multi-infeed HVDC

In recent years, China has accelerated the construction of direct current transmission projects. HVDC system has many advantages, but also inevitably brings many problems to the network. Among them, the impact of the HVDC infeed on the the operation characteristics of the AC protections is one of the issues that should be focused. In the AC-DC interconnected system, there is a close electrical coupling relationship between AC system and HVDC system. The fault transient process of two systems'interaction will change the operation environment of the AC system protections inevitably, so that it will influence the actions of those protections. In the worst situation, it will lead to the malfunctions or rejecting operations of the AC protections. At present, relevant research results are still rare, but the lack of systematic and theoretical analysis and research. Therefore, this paper deep studies the operation characteristics of the receiving AC network's protections under the situation of multi-infeed HVDC system. The main contents are decribed below:1. Based on the in-depth study of HVDC commutation failure characteristics, transient process of commutation failures caused by the different receiving AC network faults is analyzed systematically.2. The impedance model of HVDC equivalent power-frequency component variation is eatablished to analyze the AC-DC interconnected system faults. On this basis, considering the principle of power-frequency componet variation direction relay, the impacts of commutation failure on power-frequency component variation elements of the fault line are analyzed. And the factors leading to the rejecting operation of power-frequency component variation directional pilot protection are also analyzed quantitatively in different situations. The issue of the transient power direction sudden change of the non-fault line direction element caused by the commutation failure transient process is analyzed. And then, based on the different action logic of direction pilot protection, the malfunction operation of directional pilot proctection of the non-fault line caused by the commutation failure transient process is analyzed. Then the action characteristics of the directional pilot protection under the situation of multi-infeed HVDC system are analyzed. The validity of theoretical analysis is proved based on the PSCAD/EMTDC simulation at last.3. Based on the theoretical analysis of fault component criterion and steady state component criterion of transmission line current differential protection, considering the transient characteristics of HVDC equivalent fault current component under the different AC fault situations, the influence mechanisms of the infeed HVDC on the fault component criterion and steady state component criterion of transmission line current differential protection are analyzed comprehensively. The HVDC impacts on the fault component criterion and steady state component criterion of transmission line current differential protection are theoretically analyzed and compared in different operation situations.Then the action characteristics of transmission line current differential protection under the situation of multi-infeed HVDC system are analyzed. At last, the validity of theoretical analysis is proved based on the PSCAD/EMTDC simulation.4. Considering the characteristics of distance protection, the impacts of infeed HVDC system on the transmission line distance protection are analyzed from the point of measured impedance variation angle of distance protection caused by the infeed HVDC system. Based on the faults of receiving network and commutation failures caused by the faults of receiving network, the impacts of HVDC system on the transmission line distance protection are theoretically analyzed and compared under the different operation situations of receiving network. Then the action characteristics of distance protection are analyzed under the situation of multi-infeed HVDC system. At last, the validity of theoretical analysis is proved based on the PSCAD/EMTDC simulation.This dissertation is supported by Key Project of the National Eleventh-Five Year Research Program of China (2006BAA02A30) ,National Natural Science Foundation of China(50977032) and Guangdong-Hongkong Technology Cooperation Funding (2009498B3201) .