Refined Calculation of Zero Sequence Current Protection Setpoints for 110–220 kV Overhead Power Lines under the Transformer Inrush Current Conditions

Possible ways of taking into account the transformer inrush current in selecting the zero sequence protection pickup parameters for overhead power lines of 110 kV and higher voltage levels are analyzed. As a part of the analysis, the main advantages and shortcomings of existing methods were identified. It has been determined that the most promising approach involves the application of calculation methods using the new curves to determine the magnetization inrush coefficient for the effective current value developed at the Moscow Power Engineering Institute Department of Relay Protection and Automation of Power Systems. The new approach makes it possible to perform a more accurate calculation of the inrush current, since it takes into account all types of circuit breaker making operations, as well as specific steel grades of transformers. In addition, this approach takes into account the operation peculiarities of microprocessor protections and enables the user to perform a more accurate calculation of the pickup parameters for both electromechanical and microprocessor-based protections. A detailed description of the procedure for calculating the magnetization inrush current according to the new curves is given together with an example of calculating the zero sequence current protection pickup parameters for an overhead line subject to detuning from the inrush current for a real electric network section.