Frequency converter harmonics management method - Database & Sql Blog Articles

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(1) Isolation, shielding, and grounding of the inverter: To minimize electromagnetic interference, it's essential to isolate the inverter’s power supply from other equipment. This can be achieved by using an isolating transformer on the input side or placing the inverter inside a grounded metal enclosure. Additionally, the inverter’s output cables should be kept at least 50mm away from control cables. If they must cross, do so at a right angle. When laid parallel, keep the length as short as possible (not exceeding 1 meter), and route the cables through grounded steel conduits for added protection.

(2) Installing AC and DC reactors: When the distribution transformer has a capacity greater than 500 KVA and is more than ten times the inverter’s capacity, an AC reactor is recommended on the input side. If the voltage from the transformer is unbalanced with a ratio above 3%, it may cause high current peaks that could overheat conductors. In such cases, an AC reactor should be used, and in severe conditions, a DC reactor might be necessary to further reduce harmonic distortion.

(3) Using passive filters: Passive filters are installed on the AC side of the inverter and consist of inductors (L), capacitors (C), and resistors (R) to form a harmonic resonance circuit. These filters can prevent higher harmonics from entering the grid when their frequency matches that of the LC circuit. They are cost-effective, simple in design, and reliable, but they can be sensitive to system parameters and may amplify certain subharmonics. They are also bulky and expensive compared to other solutions.

(4) Active filters: Introduced in the early 1970s, active filters detect and compensate for harmonic currents in real-time by injecting opposite-phase currents. Compared to passive filters, they offer better controllability, faster response, and multi-functionality. They also avoid resonance issues with the system impedance and can automatically track and adjust to harmonic changes. However, they require larger capacity and come with a higher cost.

(5) Static reactive power compensation devices: For systems with large fluctuating loads, static reactive power compensation devices can help quickly meet reactive power demands, improve the power factor, and reduce harmonic injection into the grid. The self-saturated reactance type (SR type) is particularly effective due to its reliability, fast response, and ease of maintenance, and it is commonly manufactured in China.

(6) Separation of power lines: Harmonic currents generated by nonlinear loads can distort the voltage waveform. To prevent this, separate the power lines that supply harmonic-generating loads from those that serve sensitive equipment. By feeding linear and nonlinear loads from different circuits at the same point of common coupling (PCC), the distorted voltage caused by nonlinear loads won't affect the linear ones.

(7) Diversification and multiplexing of circuits: Parallel operation of multiple inverter units can cancel out harmonic components through waveform superposition. Multi-pulse rectification (e.g., 12-pulse, 18-pulse, 24-pulse) also reduces harmonics. Series-connected power cells with multi-pulse configurations can further minimize distortion. New modulation techniques, such as voltage vector deformation, are also being explored to enhance performance.

(8) Enhancing inverter control methods: With advancements in power electronics, microelectronics, and computer networks, modern inverters now use digital control systems. These systems, such as those based on MCS51, 80C196MC, SLE4520, or EPLD, offer improved performance. Combining multiple control strategies helps overcome individual limitations, leading to reduced harmonics and increased efficiency.

(9) Green inverters: A green inverter aims to be free of harmonic pollution. Its ideal characteristics include sinusoidal input and output currents, controllable power factor, and the ability to provide any desired output frequency. This ensures clean power delivery and minimizes negative impacts on the electrical grid.