| 1000kV UHV AC transmission lines are of advantages when transporting electric power over a long distance, this facilities renewable energy resource development from remote sites, and gives incentives to sustainable development and optimal allocation of power resources. However, UHV AC transmission lines are coupled with huge distributed capacitance currents, which may have a serious impact on the phase segregated current differential protection scheme, leading to poor sensitivity and security, may not meet the requirements of UHV AC relay protection. Therefore, an in-depth and detailed study on this issue, and put forward a solution is of practical significant importance.This paper firstly analyzes the physical characteristics and mathematical model of a UHV AC transmission line, its distributed capacitance current and related calculation method, and the associated protection schemes and philosophies. A transmission line PSCAD/EMTDC is constructed to gain good understanding of line charging current at various voltage level.Secondly, theoretical analysis of the UHV transmission lines distributed capacitance current on the full current differential protection scheme, the superimposed current differential protection scheme, and zero-sequence current differential protection are carried out. Three EMTDC relaying models are developed, to study the relay performance under various fault conditions with different locations, fault resistances, source phase angles, or fault switching-on points.Lastly, the paper summarizes cons and pros of three compensation methods of line charging currents:1) high voltage shunt reactor compensation,2) the vector compensation method,3) and the time domain compensation method, and proposes the self-adaptive time domain compensation method to reduce the error. Extensive EMTDC simulation studies, have verified the proposed adaptive compensation method is of better performance, and is able to improve the sensitivity of current differential protection schemes. |
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