**0. Preface**
In China's metallurgical industry, intermediate frequency (IF) furnaces are widely used due to their advantages such as high heating speed, high production efficiency, low oxidative decarbonization, material and cost savings, uniform heating, small temperature difference between core and surface, and high temperature control accuracy. However, the IF furnace consists of a series of rectification and inverter devices, making it a typical nonlinear load compared to the power supply. It contains numerous harmonic components, leading to high power consumption and a low power factor. These harmonics and reactive power can deteriorate power quality, imposing stricter requirements on the power grid for sensitive loads. Therefore, it is essential to implement harmonic control measures for IF furnaces.
Currently, common methods to address harmonic issues include controlling the harmonic source and installing filter compensation devices. Among these, installing filter compensation devices is the most commonly used approach. This paper takes a steel plant as a case study, analyzing the impact of the IF furnace load on the power grid using ETAP (Electrical Transient Analysis Program), and proposes a reasonable and efficient harmonic control solution.
**1. Theory and Method of Harmonic Analysis of Intermediate Frequency Furnaces**
The power rectifier section of an IF furnace typically includes six-pulse, twelve-pulse, and twenty-four-pulse rectifier circuits. Each type has different characteristic harmonic contents and frequencies, requiring specific handling. The single-group full-bridge six-pulse rectifier circuit is widely used in industrial and mining enterprises due to its mature technology and low cost [3, 4].
Under ideal conditions, the six-pulse rectifier ignores the commutation process and current ripple, allowing AC-side currents to be approximately represented by square waves. Considering the impedance drop of the circuit, the motor input voltage becomes distorted. Fourier transform is used to decompose harmonic currents:
$$ I_n = \frac{I_{dc}}{n} $$
Where $ I_{dc} $ is the DC current and $ n $ is the harmonic order.
**2. Case Study**
**2.1 Introduction to ETAP Software and Modeling**
ETAP (Electrical Transient Analysis Program) is a graphical interface-based power system simulation and analysis software developed by OTI (Operation Technology Inc.) in the United States. It integrates modules such as power flow analysis, short-circuit calculation, transient stability analysis, harmonic analysis, and reliability analysis. It allows users to quickly build power system models and visualize results directly on the graphical interface [6].
In this study, the power flow and harmonic analysis modules of ETAP were used to simulate an IF furnace with a 1000kVA transformer capacity in a steel plant. A harmonic load model was established, as shown in Figures 1 and 2. Due to the unbalanced load of the IF furnace, the three-phase harmonic current amplitudes at busbar 2 differ. For the worst-case scenario, the phase with the highest subharmonic current amplitude was selected as the input.
**2.2 Background of the Case**
The IF furnace in this steel plant is a major source of harmonics. Its demand for reactive power lowers the power factor of the grid, affecting the normal operation of other electrical equipment. Each IF furnace is supplied through a 10kV power line via a 1000kVA, 10/0.4kV transformer. The IF furnace uses a six-pulse rectifier, with rated power and average power during actual operation. The main harmonic orders are 5th, 7th, and 11th.
**2.3 Harmonic Analysis and Its Solution**
**2.3.1 Harmonic Analysis**
The test measurement point is the secondary side bus of the transformer. Data from several measurements were collected and analyzed, with the data showing the most severe harmonics selected for further study.
A one-line diagram of the steel plant’s power supply system was created (as shown in Figure 1). Parameters were entered for simulation analysis. The harmonic distortion voltage and voltage curves of the transformer’s secondary bus are shown in Figures 3 and 4, while the harmonic current values are listed in Table 1.
From Figures 5 and 6, it can be observed that after installing a passive filter, the harmonic distortion voltage of the transformer’s secondary bus decreases, the voltage curve becomes smoother, and the harmonic current content is significantly reduced. The total voltage distortion rate is 4.9%, meeting national standards, and the current distortion rate is 10.41%. The power factor increases to 0.98, greatly improving the harmonic and reactive power environment. Thus, the optimized passive filter solution proves effective.
**3. Conclusion**
This paper simulates and analyzes the load of an IF furnace in a steel plant using ETAP software and designs a harmonic treatment scheme. The effectiveness of the scheme is verified through simulation. After implementation, the issue of the IF furnace causing problems with other loads and reactive power compensation cabinets is resolved, enhancing the overall performance of the IF furnace and the power grid. This provides valuable guidance for practical projects.
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