Research On Relative Issues Of UHVDC Links On AC Power Grid

To satisfy the needs of long-distance transmission of large-scale energy resources in China, UHV grid has begun to construct extensively from technology innovation and engineering demonstrative phase. The UHVDC project with highest voltage and largest capacity is in operation in China. So many large capacity UHVDC projects connected to AC power grid must effect on grid structure, transient characteristics, system operation and controls which are more and more complicated. As a result, risk of security and stability caused by DC faults has gradually increased. Meanwhile, due to DC's fast response and controllability, they can prove helpful in stability control and security redispatching. In this context, this thesis studies relative issues of UHVDC links on AC power grid in three aspects of maximum DC capacity limited by AC system, applicability of guide or code related to power system stability and reserve redispatch after UHVDC block. The main work includes the following three parts:In direct method for transient stability assessment with detailed generator model, changes of generator's equivalent internal impedance in its Thevenin equivalent circuit and its effects on the transient stability analysis are analyzed. Then, a method for calculating equivalent internal impedance is proposed and the transient energy function method has been improved. Simulation results of IEEE three-machine and nine-bus system demonstrate that the proposed method can improve the accuracy of the stability prediction to some degree.Calculation method of transient stability-constrained maximum DC power considering DC blockout as reference event by using the improved TEF is proposed. This approach is applied to Central-North China power grid equivalent system and results are cheeked against the simulation results. Effects on enhancement of transient stability after DC blocking by utilizating overload capacity of non-fault pole and other DC lines are researched and results show that DC power modulation can improve permissible maximum DC power. Based on the simulation results, applicability of the current domestic power system security and stability standards is discussed and some revision recommendations are put forward.Aiming at reserve dispatch after UHVDC block, scenarios of DC power modulation participating dispatching are concluded. Mathematical models of generator redispatch in-corporating control of DC in AC/DC hybrid grid in three situations in terms of enough reserve capacity in the disturbed province, regional reserve capacity sufficient and insufficient to make up for power vacancy are presented. Then, a decision-aid system for optimal reserve dispatch after UHVDC block is proposed. Simulation results of Central China Power grid indicate that the proposed scheduling scheme can reduce the operation risks of security and load loss if necessary.