PRINCIPLES OF IMPLEMENTATION OF BLOCKING OF CURRENT PROTECTION OF ELECTRICAL POWER UNITS WITH TRANSFORMERS

Principles of implementation of current protection of elements of electric power systems with power transformers are considered. In some cases, in facilities with power transformers there could be an inrush of a magnetizing current that can cause a false positive current protection. The special measures of blocking of current protection ought to be taken to avoid this. The methods of implementation of blocking digital current protection when magnetizing current inrush takes place are described. The main way of blocking protection from magnetizing current inrush is tuning-out by harmonics content. The presence of lower harmonics content serves as a criterion of permitting operation of the protection and the higher harmonics content, on the contrary, indicates the appearance of magnetizing current inrush and is used to block the protection action. The operation condition of blocking of digital protection from magnetizing current inrush is presented. A realization of digital part of implementation of current protection blocking is described and the functional scheme of the latter is considered. The scheme contains conditioners of orthogonal components of signals of the first and second harmonics as well as the filters of positive and reverse sequences and blocks of determination of amplitudes. The blocks compute current amplitudes of first harmonic of positive sequence and the negative sequence current of the second harmonic with the use of the orthogonal components. By these values the characteristic quantity of blocking is calculated, which is compared with the activation setpoint. The improvement of performance of the principles of blocking current protection of electric transformers with current magnetizing inrush has been suggested by incorporating the digital part the block of the maximum value formation that determines the maximum amplitude of the first harmonic of the total current into the functional block scheme. 

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