Variable Frequency Drives
Harmonic drive compliance becomes more critical every day. At SPOC, we focus on the entire system, providing low-harmonic drives to ensure that you continue running reliably, efficiently and in compliance of IEEE519 and EN-6100-3-2.
Using inductive reactance is a cost-effective way for lowering harmonics. By placing a 3% AC input reactor in line with the incoming power to the rectifier or by installing a DC reactor (choke) after the rectifier, the distorted current is lowered to around 35%. Without these devices, the harmonics can be typically up to 80%. Inductive reactance is a simple and inexpensive first step towards reducing harmonics and should be considered a minimum practice for any drive. As a side benefit, the reduced harmonics reduce the incoming amperage to the VFD, reducing heating and extending its expected life.
Passive filters create a low-impedance path to ground for the harmonic frequencies and may be tuned to a frequency between two prevalent harmonics to help attenuate both. In addition, they normally include a large line reactor on the input. These filters work well in many applications to help achieve IEEE519 compliance. Due to the capacitors inside of them, care should be taken in application to use the fusing and contactors to protect the lift and remove them from the line when the VFD is not running. These work on all PWM drives which make up 99% of drives made today. Due to the way 6-step drives are made, you should consult your filter manufacturer to help you size a filter for one of these drives.
Like what noise cancelling headphones do with sound waves, these filters inject equal and opposite harmonics onto the power system to cancel those generated by the other equipment. This method has proven effective in reducing harmonics well below required levels. The drawback is that this method can be costly.
This method employs two separate rectifier bridges, which supply a single DC bus. The two bridges are fed from phase shifted supplies of the output of special transformers. These transformers must be installed to create 6 phases from the 3 utility phases and the phase shifting causes some lower order harmonics to cancel out each other. This method however, is highly sensitive to voltage imbalance as well as background distortion. In addition, if they are lightly loaded, 12 pulse drives still have quite high harmonics. While they are a proven technology, the reality is that 12 pulse drives cannot be counted on to meet IEEE519 in many cases.
This method employs three separate rectifier bridges, which supply a single DC bus. These bridges are fed from a special transformer that supplies three phase shifted sets of three phase power. This method is effective in eliminating a much wider range of harmonics than the 12-pulse. These transformers are often air-cooled with fans that can be problematic for outdoor applications due to dust and dirt contamination. Another drawback of this method is that voltage imbalances greatly exasperate harmonics. 18 pulse drives have similar harmonic mitigation to the high-performance filters and normally can help meet IEEE519 standards.
This method uses a bidirectional power converter for the front end of a common DC bus drive line up. An external LCL filter is used at the input. This method reduces total harmonics to 2-3% and is immune to voltage imbalances. Active front end is by far the most effective means at reducing harmonics. In addition, active front end drives can boost voltage inside the drive, countering poor power that may exist on site. The drawback of this method is that it is expensive.