Background: The third step of distance protection is used as backup protection against phase-to-phase short circuits on 110-220 kV transmission lines. The main problem when using these protections on a line with several branches is to ensure the effectiveness of distant backup protection in case of phase-to-phase short circuits behind branch transformers of substations. The distant backup protection effectiveness problem solution is possible by expanding the information base of protection. Currently, backup protection of lines with branches is being developed with control of currents and their components when using a communication channel, as well as based on algorithmic models of the facility. In this regard, the urgent task is to develop an algorithm for the distance protection, ensuring the required sensibility during short circuits on the lower voltage side of the branch transformers.
Materials and Methods: Analytical methods of determining the impedance measurement and simulation in the Simulink and SimPowerSystems of the Matlab modeling system are used. Evaluation of the effectiveness of distant backup protection can be made on the basis of recognition possibility of short circuit modes behind the branch transformers.
Results: Analytical expressions were obtained to determine the impedance measurement during phase-to-phase short-circuits behind a branch transformer and under load conditions. Criteria have been developed to assess the proposed protection possibility to recognize the mode of phase-to-phase short circuits behind the branch transformer. Studies have shown that the main factor determining the possibility of mode recognition is the ratio between the protected transformer power and the total power of the branches loads. The use of several impedance measuring elements with their own response characteristics for branches with transformers of different capacities provides mode recognition for any really possible corelations of the power of branch substations.
Conclusions: The use of the distance protection implementation option developed by the authors allows us to provide the required sensitivity for short circuits behind branch transformers and to solve the problem of distance backup line protection on a line with several branches. The reliability of the data obtained is confirmed by the correspondence of analytical research and simulation results.