Input filters or 12 pulse rectifiers?
With current recommendations demanding lower levels of input harmonics (THDi) it is more common for larger systems to require some form of input filtering to achieve this. Twelve pulse rectifiers can improve upon more standard 6 pulse systems and offer the ability to correct the problem across the whole load range. Passive input filters added to 6 pulse designs are usually cost effective and improve the input power factor, but often only effective at higher load levels of the load, typically above 50%. For very large installations it is quite common to have a combination of a 12 pulse rectifier with extra input filtering, to dramatically reduce harmonics as a result of a restricted input supply or standby generator limitations. Recent developments in rectifier design have resulted in IPFC (input power factor corrected) IGBT rectifiers offering very low THDi and a high Input PF as standard.