Ideally, all loads and sources have a pure sinusoidal current waveform. However, the true waveform of most equipment is very different. Nowadays almost every load is a non-linear load. Typical examples are the power supply of a computer or the diode input bridge of a 6 pulse drive.

These loads cause a distortion of the main voltage. This distortion, evaluated by the Total Harmonic distortion THD, typically evaluates frequencies from the 2nd up to the 40th or 50th harmonic. Looking at a 50Hz network this is the area between 100Hz and 2.5kHz.

The impact of harmonic distortion is diverse, but the most typical effects are:

• Significantly reduced lifetime of electronic components
• Transformer overload
• Capacitor bank (PFC) overload
• Increased energy costs
• Decreased equipment lifetime
• Prone to failure equipment
• Vibration damage on rotating equipment

The working principle of the RHF-Active is completely different from any other harmonic solution. Instead of working as a line filter with tuned passive filter circuits, the active solution is connected in parallel and injects harmonics. These injected harmonics are of inversed polarity and therefore eliminate the harmonics taken from the supply.

The RHF-Active may be used for pf correction, imbalance compensation, harmonic mitigation or altogether. It reduces the harmonic current distortion of non-linear loads and sources to any required THDi. This is necessary to reach various standards and recommendations, such as IEEE 519-2014.

1. Compensation of unknown harmonics: A passive filter is always designed for a specific load with a specific harmonic spectrum. Our RHF-5P and RHF-8P, for example, are designed as filters for a drive. Active filters, on the other hand, can be used for many different loads. The composition of the harmonics can vary or be dynamic. Our active harmonic filters are therefore ideal for eliminating the sum of different consumers with an unknown harmonic spectrum.

2. Decentralized compensation of harmonics: An active harmonic filter offers an enormous advantage for the compensation of a group of drives or as a retrofit installation. The filter can be installed at any point within the low voltage network and therefore the retrofit is significantly easier than adding a passive filter. Additionally, the space requirement is usually significantly lower.

3. Avoidance of reactive power: Passive harmonic filters generally cause reactive power in the load range of 0- 30%. This is a disadvantage for systems that work permanently in this partial load range. The active harmonic filter does not cause any unwanted reactive power. And can therefore achieve a pf of almost 1, regardless of the load.

Active Harmonic Filters are parallel filter circuits injecting harmonics into the supply. Based on the parallel topology, the required filter current is not equal to the load current.

The first digit in the type code of the RHF-Active identifies the corresponding nominal compensation current. The RHF-Active 55-400-50-20-C is therefore designed for 55A compensation current. A de-rating of the VFD due to the voltage drop is not necessary using a REVCON RHF-5P or RHF-8P.

The required compensation current can be calculated in many different ways, or can be taken from a power quality analysis.

Following this equation based on existing THDi and input current, an estimated required harmonic filter current IHFC can be achieved.

When improving the line current with an active filter, the mains voltage distortion will improve, which leads to a higher distortion of the load current. Therefore it is recommended to add a safety factor (1.062). This factor is based on experience and may differ significantly depending on the main condition. The factor is based on RHF-Active filter with SiC technology. Using the 3-Level IGBT a factor of 1,071 should be used.