01-06-2012, 12:52 PM
AFDs and Their Effect on Power Quality
AFDs and Their Effect on Power Quality .ppt (Size: 2.18 MB / Downloads: 41)
What Kind of Power Quality Effects?
Incoming Sine Wave Notching?
Arises from SCR front ends on AFD’s
Forced commutation causes line notches
But … modern AFDs use diode front ends
Self commutating … no notching
Voltage sag?
Standard motor starters allow 650% inrush
“Weak” power systems are affected
500HP motor on 1000kVA, 5.75%Z Xfmr
650% X (500 / 1000) X 0.0575 = 19% sag
AFD limits inrush to 110% (or 150%)
110% X (500 / 1000) X 0.0575 = 3% sag
Harmonics and transformers
Many transformers are rated by
“K factor” which simply describes
their ability to withstand harmonics.
Transformers may also be derated
to compensate for the additional
heating caused by harmonics.
Improved transformer designs have
also been developed, with oversized
neutral busses, special cores, and
specially designed coils.
Harmonics and power-correction capacitors
Power correction capacitors can cause
series and parallel resonance effects on
a power system.
If a harmonic is generated which excites
a system resonance, amplification of that
harmonic may occur.