Background. At present, in 110–220 kV transmission lines the problem of interphase short-circuits in the long-distance backup protection zone, in most cases, is solved by a backup level of current or distance protection. The existing protection solutions for lines with branching are based on the use of special high-voltage equipment, various current components and their ratios, a virtual resistance relay; on the control of emergency and orthogonal current components with adaptive braking and signal correction as well as with correction of the response characteristics according to the data on the voltage change and the state of the switching devices of the main substation. A number of developments are aimed at increasing the sensitivity of back-up protection of branch substations by changing the connection circuit, improving the response characteristics and refining the method for selecting the settings. However, these protection solutions in many cases do not provide the required sensitivity during short circuits on the low voltage side of a low-power branch transformer. All this means that an urgent problem to be solved is developing an algorithm for the operation of backup protection, providing the required sensitivity to short circuits on the low voltage side of the branch transformer.
Materials and methods. Analytical methods and simulation in the Simulink and SimPowerSystems packages of the Matlab modeling system were used to determine the impedance measurement of the distance protection measuring element. A mathematical model of the lumped-parameter line was used. When deriving analytical expressions, the impedances of the positive and negative sequences were assumed to be equal.
Results. Analytical expressions were obtained for determining the impedance measurement of the proposed and existing distance protection during phase-to-phase short-circuits appearing after a branch transformer and under load conditions. The studies have shown that the proposed distance protection has the required sensitivity to short circuits on the low voltage side of the branch transformer and the necessary detuning from load modes. The characteristics and methods for selecting response parameters of the additional stage of distance protection were determined using information on the currents of the line supply ends.
Conclusions. The use of an additional distance protection stage developed by the authors based on the sum of currents of the line supply ends can ensure the required sensitivity to phase-to-phase short circuits on the low voltage side of the branch transformer with a star-delta connection circuit and solve the problem of distant backup protection of low power branch substations. The reliability of the data obtained is confirmed by the correspondence of the results of the analytical research and simulation modeling.