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Digital filters to separate the first and second harmonics of signals in microprocessor-bases protection of electrical installations equipped with transformers DOI:10.15199/48.2018.07.11

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In electrical installations equipped with power transformers when they are turned on with no load under voltage, an inrush of the magnetizing current (IMC) occurs in some cases of voltage recovery after disconnection of external short circuits (SC) and in some other cases. Under the influence of mentioned inrush overcurrent protection of electrical installations might falsely trigger. Prevention is possible either by means of an increase of the current of protection operation (that is not always acceptable), or by the implementation of the protection blocking at the IMC occurrence. The classic way of locking protection when IMC takes place is based on tuning-out by harmonics content. The content of low harmonics in the current can be the criterion for protection actuation, and the presence of the current harmonics of a higher order can indicate the appearance of the IMC and their use to block the action of protection. Various options of the lock, based on the above provision, are presented in [1]. Herewith, one of these implementations involves the use of the amplitude of the second harmonic current of negative sequence and of the amplitude of the current of the first harmonic of direct sequence, according to which a lock parameter is determined [2]. In order to obtain the necessary information, from the corresponding currents with the aid of frequency filters the harmonics with frequencies of 100 and 50 Hz are separated, according to which their sine and cosine orthogonal components (OC) are formed. In accordance with the specified OC or with the readouts of instantaneous values of one of them [3] amplitudes of currents of the first and second harmonics for the subsequent determining of the lock parameter are calculated. The article describes the principle of implementation of the respective filters that makes it possible to obtain eventually current amplitudes of the first and the second harmonics currents and examines [...]

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