Why can isolation transformers suppress 3rd and multiples of 3rd harmonics?
It is a transformer using a D,yn11 connection configuration. Because the third and multiples of the third harmonic have their own unique characteristics—a frequency of 150Hz and a phase difference between each phase phasor that is three times the fundamental frequency—and because 3 × 120° = 360°, the phase phasors of each phase form a zero-sequence configuration. In a D,yn11 connection distribution transformer, the primary winding is delta-connected, allowing for the circulation of zero-sequence current and third and multiples of the third harmonic currents. The zero-sequence magnetic flux on the auxiliary winding side induces a zero-sequence circulating current in the primary winding. This suppresses the third and multiples of the third harmonic.
Isolation transformers can isolate harmonic pollution from frequency converters.
Inverter harmonics are high-order harmonics generated in the input and output circuits during the inverter's operation, which requires high-power diode rectification (or transistor/inverter module) of the input power supply. Inverter harmonics can interfere with the power supply system, load, and other nearby electrical equipment.
Variable frequency drive harmonic mitigation measures
To address inverter harmonic issues and suppress radiated interference and power supply system interference, technical measures such as shielding, isolation, and grounding can be employed.
1. Install appropriate reactors
Connecting appropriate reactors in series between the input and output sides of the frequency converter absorbs harmonics and increases the impedance of the power supply or load, thereby suppressing harmonics and reducing electromagnetic radiation during transmission. By suppressing harmonic currents, the power factor is increased from the original (0.5-0.6) to (0.75-0.85).
2. Power supply isolation or installation of an isolation transformer
The power supply of the frequency converter system should be independent of the power supply of other equipment, or an isolation transformer should be installed on the input side of the frequency converter and other electrical equipment to cut off harmonic currents;
3. Avoid interference with radiation
The cable between the motor and the frequency converter should be laid in steel pipe or armored cable, and should be laid in a separate cable trench from other low-voltage signals to avoid radiation interference.
4. Correct grounding of the frequency converter
Proper grounding can effectively suppress external interference and reduce the interference of the equipment itself to the outside world. The frequency converter uses a dedicated grounding wire, and the grounding wire should be a thick and short wire. The grounding wires of other nearby electrical equipment must be separated from the frequency converter wiring and use short wires. This can effectively suppress the radiation interference of current harmonics to nearby equipment.
5. Shorten the line length
Shorten the line length, lay power lines and signal lines separately to avoid crossing. If crossing is unavoidable, they must cross perpendicularly and never be laid parallel. The shielding layer of the signal line should not be connected to the ground of the motor or frequency converter, but should be connected to the common terminal of the control line.
6. Install EMI power filter
A key trend in suppressing inverter harmonics is the use of EMI power filters, which are connected in series or parallel to the main circuit. These filters detect the inverter harmonic current in real time from the object being compensated, and the compensation device generates a compensation current that is equal in magnitude and opposite in direction to the inverter harmonic current, thus ensuring that the grid current contains only the fundamental current.
