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Background. Powerful electric drives of modern rolling mills are based on AC medium voltage motors (asynchronous and synchronous) and frequency converters with active rectifiers. Such electric drives are the source of higher harmonics, which makes the problem of their electromagnetic compatibility with the internal distribution network an important issue. In particular, this issue is relevant to enterprises with systems of internal power supply using extended 6–35 kV cable lines, in which the total value of distributed capacity can reach several microfarads. As a result of the interaction of the transformer inductor of the main stepdown substation and the total capacity of the cable lines, the current resonance extremum in the network frequency response can be located at the frequencies of frequency converter harmonics. This causes strong voltage distortions in the common sections of the switchgear, which can lead to electrical equipment failures. The existing ways of minimizing the negative impact of resonant phenomena on the electricity quality in the internal power networks are reduced to the installation of special filters and require certain economic costs, which makes it an urgent task to investigate alternative ways of eliminating strong distortions caused by frequency converters. Materials and methods. An improved PWM algorithm has been developed in the MATLAB software with a Simulink application based on the created imitation model taking into account the actual frequency response of the internal power network AO «Severstal – Grading Plant Balakovo» and the parameters of the frequency converter with an active rectifier. Results. We have developed an advanced pulse width modulation algorithm that adapts to resonant phenomena in internal power networks due to the changes in the angles of rectifier valve activation depending on the frequency extremum position. We have also developed a simulation model taking into account frequency characteristics and frequency converter parameters. With this model, we were able to obtain the network voltage curves by applying the original and the improved PWM algorithms, and to compare the results. It has turned out that adaptive PWM reduces the coefficient of total harmonic distortions of the network voltage curve by 2,5 times. Conclusions. The reliability of the obtained result is confirmed by the ability of the applied mathematical model to adequately describe the research subject, as well as by the real-world nature of the initial data used for modeling. The proposed PWM algorithm makes it possible to improve the electromagnetic compatibility of the frequency converter with the network without using special filter circuits. The results obtained can be used in the design of frequency converters with active medium voltage rectifiers.

Development of an advanced PWM algorithm for active rectifier with adaptation to current resonances in internal power supply system