How to Deal with Harmonics Generated by Air Compressor Frequency Converters
During operation of an air compressor frequency converter, high-power diodes are used for rectification of the input power supply (or transistors/inverter modules for inversion). High-order harmonics are generated in the input and output circuits during the inversion process. These harmonics interfere with the power supply system, loads and other adjacent electrical equipment.
Harmonics will inevitably be produced when the frequency converter operates. They are acceptable as long as they do not exceed the scope specified in national standard GB12668.3-2003. The standard stipulates that the total harmonic content on the input side of the frequency converter shall be less than 10%.
Harmonic Mitigation Measures for Frequency Converters
Technical methods such as shielding, isolation and grounding can be adopted to tackle harmonic problems and suppress radiation interference and power supply system interference.
Install Appropriate Reactors
Suitable reactors are connected in series on the input and output sides of the frequency converter to absorb harmonics and increase the impedance of the power supply or load, so as to suppress harmonics and reduce electromagnetic radiation during transmission. By restraining harmonic current, the power factor can be improved from the original 0.5–0.6 to 0.75–0.85.
Power Isolation or Installation of Isolation Transformers
Install isolation transformers to block harmonic current.
Separate the power supply of the frequency conversion system from that of other equipment, or install isolation transformers on the input side between the frequency converter and other electrical loads.
Prevent Radiated Interference
Cables between the motor and frequency converter shall be routed through steel conduits or armored cables. These cables shall be laid separately from other weak-current signal cables in different cable trenches to avoid radiated interference.
Proper Grounding of Frequency Converters
Proper grounding enables the system to effectively suppress external interference and reduce interference emitted by the equipment itself. The frequency converter shall adopt a dedicated, short and thick grounding wire. Ground wires of adjacent electrical equipment must be separated from the wiring of the frequency converter and kept short. This can effectively suppress radiated interference of current harmonics to nearby equipment.
Shorten Wiring Length
Minimize cable length. Power cables and signal cables shall be laid separately and avoid crossing. If crossing is unavoidable, they must intersect vertically; parallel laying is prohibited. The shielding layer of signal cables shall not be connected to the ground of the motor or frequency converter, but to the common terminal of the control circuit.
Install EMI Power Filters
The adoption of EMI power filters represents an important trend for harmonic suppression of frequency converters. Connected in series or parallel to the main circuit, the filter detects harmonic current of the frequency converter from the compensated object in real time. The compensation device generates a compensating current with equal magnitude and opposite direction to the harmonic current, so that the grid current only contains fundamental current.
EMI power filters can track and compensate harmonics of frequency converters with varying frequency and amplitude. Their performance is not affected by the system, and there is no risk of harmonic amplification, which makes them widely applied.
Harmonics will inevitably be produced when the frequency converter operates. They are acceptable as long as they do not exceed the scope specified in national standard GB12668.3-2003. The standard stipulates that the total harmonic content on the input side of the frequency converter shall be less than 10%.
Harmonic Mitigation Measures for Frequency Converters
Technical methods such as shielding, isolation and grounding can be adopted to tackle harmonic problems and suppress radiation interference and power supply system interference.
Install Appropriate Reactors
Suitable reactors are connected in series on the input and output sides of the frequency converter to absorb harmonics and increase the impedance of the power supply or load, so as to suppress harmonics and reduce electromagnetic radiation during transmission. By restraining harmonic current, the power factor can be improved from the original 0.5–0.6 to 0.75–0.85.
Power Isolation or Installation of Isolation Transformers
Install isolation transformers to block harmonic current.
Separate the power supply of the frequency conversion system from that of other equipment, or install isolation transformers on the input side between the frequency converter and other electrical loads.
Prevent Radiated Interference
Cables between the motor and frequency converter shall be routed through steel conduits or armored cables. These cables shall be laid separately from other weak-current signal cables in different cable trenches to avoid radiated interference.
Proper Grounding of Frequency Converters
Proper grounding enables the system to effectively suppress external interference and reduce interference emitted by the equipment itself. The frequency converter shall adopt a dedicated, short and thick grounding wire. Ground wires of adjacent electrical equipment must be separated from the wiring of the frequency converter and kept short. This can effectively suppress radiated interference of current harmonics to nearby equipment.
Shorten Wiring Length
Minimize cable length. Power cables and signal cables shall be laid separately and avoid crossing. If crossing is unavoidable, they must intersect vertically; parallel laying is prohibited. The shielding layer of signal cables shall not be connected to the ground of the motor or frequency converter, but to the common terminal of the control circuit.
Install EMI Power Filters
The adoption of EMI power filters represents an important trend for harmonic suppression of frequency converters. Connected in series or parallel to the main circuit, the filter detects harmonic current of the frequency converter from the compensated object in real time. The compensation device generates a compensating current with equal magnitude and opposite direction to the harmonic current, so that the grid current only contains fundamental current.
EMI power filters can track and compensate harmonics of frequency converters with varying frequency and amplitude. Their performance is not affected by the system, and there is no risk of harmonic amplification, which makes them widely applied.
