Reliability of protection schemes used for multi-terminal transmission lines

Transelectrica's substations (Romanian Transmission and System Operator) are facing with various retrofit projects throughout the entire power system. Consequently, the initial network topology is changing during the retrofit process and different configurations of the network are adopted to maintain the stability and security of the power system. Multi-terminal transmission lines are often used in this situation. In this article is presented a protection scheme of a multi-terminal transmission line from Romanian power system. The security and selectivity criteria are assured with a grading plan of the protection functions. For a proper coordination of the protected zones, short circuit currents computation is realized in a modeling platform in which the entire 400/220/110 kV transmission and distributions network of the Romanian power system is designed. The protection system of the studied network area must detect and remove the unwanted condition within the fault clearing time. Taking into account what is mentioned above, a hybrid teleprotection scheme realized through a digital communication is proposed in order to improve functionality of multi-terminal transmission lines.