Harmonic Filter
In Electrical Systems, Harmonics is a voltage otherwise current occurred at different basic frequencies. In laymen’s term, we call it NOISE within the power system.
Generally, we categorize Harmonics into two types – voltage harmonics and current harmonics. Harmonics usually comes from non-linear loads!
e.g VFD, SCR.
Current Harmonics might be induced through VFD (non-linear load), and the effect of the current Harmonics using source impedance will generate voltage Harmonic.
We need Harmonic Filters to minimize, if not to eliminate, electrical systems Harmonics.
CASE STUDY:
A recent Power Quality Analysis was carried out on an offshore rig that consist of VFD and SCR drilling equipment. The rig is powered by six 600VAC diesel engines & generators.
The SCR equipment will distort supply voltage, causing commutation notches in the voltage sine wave when the DC Drive loading is high.
High levels of voltage notching will cause functional issues and potentially cause problems to associated loads, including AC VFD’s.
Max THDV measured was 17%!
These distortion levels of the voltage sinewave are due to the presence of harmonic currents which are circulating in the system.
These harmonic currents have the potential to overload and overheat the busbar conductors in the main switchboard, the SCR room and the VFD House, to cause nuisance tripping of associated circuit breakers and to damage sensitive equipment such as printed circuit boards throughout the rig.
There are significant levels of voltage notching occurring in this installation. This voltage notching is being major cause of the failures of the AC VFD’s at the rig.
One mitigation technique for voltage notching in this case is the installation of in-line reactors/inductors in series to the SCR input power, or installation of Harmonic Filter either tied to the main Bus (ACTIVE) or in series to the selected system (PASSIVE).
Below is a sample for installation in series.
