Abstract
Problem Statement (Relevance): Modern high-power electric drives used in rolling mills are made on the basis of synchronous and induction motors and frequency converters (FC), which are, in turn, designed according to the symmetrical principal and include active rectifiers (AR) and inverters. A typical feature of ARs is that they generate high frequency voltage and current with harmonic numbers higher than 40, which is due to the applicaton of the pulse width modulation (PWM) necessary to control the power keys. The in-house power supply systems of metallurgical plants with the medium output of 1 to 2 mln tons of steel per year usually rely on 6-35 kV distributed medium voltage power networks. With high cable runs, the total distributed capacitance of the cables may reach several microfarads. The interaction between the network step-down transformer inductance and the power cables capacitance causes a current resonance to occur in the network frequency characteristic. If the cable capacitance is high, the current resonance may occur in the area of high frequency harmonics generated by the AR. This can cause a considerable high-frequency voltage distortion occurring on the shared sections of the switchgear which may lead to frequency converter failures due to faults occuring in the pulse generation blocks of the active rectifier power keys. Thus, finding ways to ensure the electromagnetic compatibility (EMC) of high-power FCs and ARs in the conditions of current resonances present in the frequency characteristic of the power network presents an important problem. Objectives: This article focuses on identifying the causes of high voltage distortion that can occur in the 10 kV in-house power network of a metallurgical plant which houses a rolling mill with high-power electric drives designed on the basis of multi-level frequency converters with active rectifies. Based on the results of an experimental study and mathematical simulation, the authors offered new ways to ensure the electromagnetic compatibility between the FCs and the power network. Methods Applied: The experimental data were processed with the help of the Matlab package comprising the Simulink application. Using the spectral analysis methods and some original data processing algorithms the main coefficients of 10 kV network voltage harmonics were calculated. The ways to improve the frequency characteristic of a 10 kV power network were examined by simulating an in-house power supply system. Originality: As of today, the problem of ensuring the EMC of high-power FCs and ARs is little studied and only limitedly covered in either Russian or foreign literature. Thus the research results presented in this paper can be described as original and theoretically and practically relevant. Findings: This research provides recommendations on the use of a special correction filter ensuring a current resonance shift towards the safety area of the frequency characteristic of a 10 kV power network which does not have high harmonics generated by power converters. Practical Relevance: This research may be applicable when designing power supply systems for industrial application as it can help choose the right configuration of medium voltage power networks. This research can also be of relevance for the existing sites which face power quality issues caused by a significant high-frequency harmonic distortion of voltage resultant from the use of high-power FCs and ARs.
Keywords
Frequency converter, active rectifier, upper harmonics, power quality, electromagnetic compatibility, current resonance, harmonic filter.
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