Research Of Series Compensation Impact On The Transmission Line Protection And Its Coutermeasures

With the increase of the power transported by transmission lines year by year, the technique of series capacitor compensation has been widely applied in high-voltage or ultrahigh pressure long-distance transmission system. The safety and stability of electric power system is facing up to more rigorous tests. Because this technique can improve the ability of power delivery of transmission lines without building new lines. At the same time, this technique can also increase the utilization of electric equipments and enhance the safety and stability of electric power system. However,the series capacitor will break the homogeneity of the transmission lines, which makes traditional protections face new challenge. For example, with the scale-up of power network, the problems of current reverse and voltage reverse are increasing more serious due to the appearing of this technique.Firstly, this thesis investigated the impact of series capacitor on the current differential protection and distance protection of transmission lines. On one hand, effects of this technique on different kinds of current differential protections were studied, such as current phase-mutation protection, split-phase full current protection, zero-sequence current differential protection, and so on. Also, this thesis discussed the reasons of the reduction of the differential protection sensitivity, even to fail to act. On the other hand, effects of the “voltage reverse” on the distance protection were discussed. Also, this thesis investigated the performance of distance protection with the technique, pointing out the shortcomings of related coping strategy.Secondly, combined with practical engineering, this thesis built transmission network simulation model with series compensation technique. For the current differential protection, effects of fault types, fault position, load current, and transition resistance on the performance characteristics were investigated by the simulation method. For the distance protection, comparing the traditional distance protection and the new distance protection, this thesis showed the advantages of the new one, and also explained the unsolved problem. Related simulation results verified the theoretical results.Lastly, to solve the problems mentioned above, this thesis put forward corresponding improved schemes. For the current differential protection, this thesis proposed that the protection sensitivity can be improved by properly reducing ratio braking coefficient and using three-broken line characteristic curve. For the distance protection, this thesis presented a new distance protection method. The relative position between the fault position and the series capacitor can be judged effectively based on the convergence time of the measurement results, and the protection distance can be extended to the whole line. Based on PSCAD/EMTDC simulation method, this thesis validated the effectiveness of the related improved measurement.