Overview of calculation verification methods of current transformers which work in relay protection circuits

Abstract

Systematized existing methods of calculation verification of current transformers that work in the circuits of modern relay protection devices. The requirements of the Ukrainian rules for the arrangement of electrical installations differ from the requirements of the standard of the International Electrotechnical Commission IES 61869-2. The difference concerns the absence of contact vibration and not exceeding the maximum allowable current error of 50% in the case of an internal short circuit is a separate case of the requirements for electromechanical protections and these requirements cannot be used for microprocessor protection devices. For any relay protection device, there is no concept of the permissible value of "voltages at the current inputs". The voltage at the terminals of the secondary winding and the voltage at the "input of the relay protection device" are two completely different voltages. There are no methods for calculating errors in the case of switching on protection based on the sum of the currents. Companies producing microprocessor relay protection devices specify in their instructions individual requirements for current transformers for each individual device. Compliance with these requirements guarantees the correct operation of the relay protection even for large values of the time constant of the primary network. Leading manufacturers of microprocessor relay protection devices develop and recommend software tools for checking the correct selection of current transformers. But they are not freely available. To increase the accuracy of the calculation of the limit multiplicity, it is advisable to use complex supports of the secondary winding and load of current transformers. Depending on the initial data and conditions, the permissible load resistance can be calculated using four methods. The question of whether the allowable load of a current transformer can exceed its nominal load requires further research. The scientific novelty is consists in the systematization of existing methods of calculation verification of current transformers that work in the circles of modern relay protection devices. The practical significance of the work is that checking current transformers that are connected to microprocessor relay protection devices do not need to check the maximum permissible current error of 50%; to check the correct operation of microprocessor relay protection devices, it is advisable to use the instructions for the requirements for current transformers of device manufacturers; for a more accurate calculation of the limit multiplicity, it is advisable to use complex supports of the secondary winding and load of current transformers.