Digital transformer differential protection setting using its mathematical models

The purpose of protecting the electric power system is one of the most complicated and urgent tasks since the stability of the whole electric power system largely depends on the correct operation of relay protection and automation. The main difficulty lies in determining the settings that ensure the correct operation of relay protection in power systems. The reason for this is the lack of methods and means providing the opportunity for adequate consideration of the variation dynamics of the electric power system mode during the accident. This is confirmed by the application of the most simplified models of electric power system equipment and corresponding mathematical methods for calculating short-circuit currents used in the formation of relay protection settings, which allow a very rough estimate of the effective value of short-circuit currents at the time period t = 0 s for only one phase. To calculate the currents of remaining phases, the method of symmetric components is used. In some cases, significant discrepancy of such settings to the actual conditions of protection operation is proved by the statistics of electric power system accidents, as well as by the statistical data of incorrect relay protection actions. The authors propose a new approach to relay protection setting. Its feature is the use of detailed mathematical models describing the processes simultaneously in the totality of all elements in the relay protection circuit (including primary converters) in combination with a hybrid simulator of power systems of an all-time simulation complex of electric power system electrical equipment real-time, which implements a methodically accurate analog-digital solution of mathematical models of electric power system equipment. Using this approach, the digital differential protection of a transformer is configured using a standard four-zone tripping characteristic. Also, the authors propose a new approach to the formation of a flexible multi-zone tripping characteristic that envelopes the emergency characteristics of I_Д = f(I_Т) of those modes in which the digital differential protection of the transformer should not be triggered. The introduction of this procedure will allow to determine relay protection settings in specific conditions of its operation as well as to avoid excessive roughening of settings, which will ensure reliable all-time operation of protection.

mathematical modeling, hybrid modeling, tuning, relay protection, all-mode modeling complex, electrical equipment of electric power systems