How to achieve “harmonic mitigation” ?
Several different solutions are proposed for Harmonic mitigation. The right choice is always dependent on a variety of factors, such as the activity sector, the applicable standards, the power level …. Several solutions are relative to Variable Speed Drives, as this type of electrical equipment represents a large part of the installed power in industrial installations and the most significant power harmonic current generators.
- AC-Line or DC-link chokes for Drives
They are commonly used up to about 500kW unit power or 1,000kW total drives power. Depending on the transformer size and cabling, the resulting THDu will be ~ 5%, which is usually well accepted in industrial networks.
- C-less technology Drives
This technology applies to Variable Speed Drives and offers a reduced current distortion compared to traditional technology. Combined with an advanced control algorithm, this solution is suitable for applications with low over-torque requirements like centrifugal pumps, fans and HVAC machines.
- Multi-pulse arrangement
This solution includes a dedicated transformer directly supplied from the MV network. Standard is the use of a 3-winding transformer providing a 12-pulse supply for one or multiple rectifiers or drives. This limits the power harmonic emission considerably and usually no further mitigation is necessary. Besides, multi-pulse solutions are the most efficient in terms of power losses. This is usually used for drives above 400 kW, but could also be reasonable for smaller power ratings.
- Active Front End (AFE)
An Active Front End is a sophisticated electronic circuit connected on the supply side of a Variable Speed Drive. This is the best performing solution concerning harmonic mitigation, limiting the THDi below 5%. All the applicable standard requirements can be met. No detailed system evaluation is necessary, making this solution the easiest to implement.
- Passive filter
A passive filter consists of reactors and capacitors set up in a resonant circuit configuration, tuned to the frequency of the power harmonic order to be eliminated. A system may be composed of a number of filters to eliminate several harmonic orders.
- Active filter
An active filter is an electronic equipment which injects, in opposite phase, the same harmonic current as drawn by the load, such that the line current remains sinusoidal.
- Hybrid filter
A hybrid filter is a combination of a passive filter and an active filter in a single unit.