ISA Northern California
Section, South Bay
October 14, 2003
Craig Chidester
909 288 7990
AFDs and Their Effect on
Power Quality
What Kind of Power Quality Effects?
• Power factor?
– PF = kW / kVA
– High motor content means lagging PF
– 100HP motor, 460V, 93% eff, 119A
•
•
•
•
(100HP x 0.746kW/HP) / 0.93 = 80.2kW
119A x 460V x 1.73 / 1000 = 94.8kVA
PF = 80.2kW / 94.8kVA = 84.6% @ FL
But … at actual load, more like 70% or less
– PF is improved with AFDs to 90 – 95%
– AFDs seen as resistive load
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
What Kind of Power Quality Effects?
• 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
What kind of Power Quality Effects?
• Harmonic Distortion
– AFDs, DC Drives, UPSs, DC power supplies (computers,
duplicators, fax’s) will cause current (and voltage)
harmonics
• Single phase – 3rd, 6th, etc (triplens) can cause transformer
neutral conductor overheating
• Three phase – 5th, 7th, 11th, 13th, etc can cause equipment
malfunctions
• Big questions – “How much?” and “How much is too much?”
What are Harmonics?
Definition:
Harmonics are integral multiples of some fundamental
frequency that, when added together, result in a
distorted waveform.
What are Harmonics?
+
sin(5x)
f(x) = sin(x)
f(x) =
5
The resulting wave shows a strong departure from the smooth
waves comprising it:
=
f(x) = sin(x) + sin(5x)
5
What are Harmonics?
In fact, any function may be constructed from a
sine wave and some number of its harmonics:
Where do they come from?
The power company typically supplies a reasonably
smooth sinusoidal waveform:
Where do they come from?
...but nonlinear devices will draw distorted waveforms,
which are comprised of harmonics of the source:
Common sources of Harmonics
Lighting ballasts
UPS systems
AC and DC drives
M
AC drives and Harmonics
Converter
DC bus
&
smoothing
Determine the line-side
harmonics
Inverter
Determines load-side
harmonics
AC drives and Harmonics
EFFECTS OF LOAD-SIDE
HARMONICS:
Have implications for the motor
insulation and windings.
Inverter
Essentially have zero effect on
other equipment on the power
system.
Determines load-side
harmonics
AC drives and Harmonics
LINE-SIDE HARMONICS CAN HAVE
FAR-REACHING EFFECTS ON THE
POWER SYSTEM:
Distribution transformers
Converter
DC bus
&
smoothing
Standby generators
Communications equipment
Switchgear and relays
Determine the line-side
harmonics
Computers, computer systems
Diagnostic equipment
AC drives and Harmonics
Typical 6-step converter waveform:
Harmonic Content
I5 = 22.5%
I7 = 9.38%
I11 = 6.10%
I13 = 4.06%
I17 = 2.26%
I19 = 1.77%
I23 = 1.12%
I25 = 0.86%
Harmonics and transformers
Transformer overheating and potential
insulation failure result from several
conditions caused by harmonics:
Increased skin and proximity effects
Harmonics circulating in the primary
winding
AFC
AFC
Increased hysteresis losses
Increased eddy current losses
DC in the primary windings
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.
AFC
AFC
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.
Calculation of the harmonic resonance frequency for the
system can give an indication of potential resonance
problems.
Harmonics and power-correction capacitors
EXAMPLE:
Assume a 1500 kVA supply xfmr,
with a 5.75% impedance.
1500 kVA
5.75%
Also assume 600 kVA of power
correction capacitors on the system
600 kVAC
The harmonic resonance frequency is defined by:
hr =
kVAsc
kVAC
1500 / 0.0575 = 6.6
=
600
Recommended limits - IEEE 519
The Institute of Electrical and Electronics Engineers (IEEE)
has set recommended limits on both current and voltage
distortion in IEEE 519-1992.
Voltage distortion limits (@ low-voltage bus):
Application class
THD (voltage)
Special system
3%
General system
5%
Dedicated system
10 %
Recommended limits - IEEE 519
MAXIMUM HARMONIC CURRENT DISTORTION
in percent of IL
Isc/IL
<20
20-50
50-100
100-1000
>1000
<11
4.0
7.0
10.0
12.0
15.0
Isc:
IL:
Individual harmonic number (odd harmonics)
11<h<17
17<h<23
23<h<35
2.0
1.5
0.6
3.5
2.5
1.0
4.5
4.0
1.5
5.5
5.0
2.0
7.0
6.0
2.5
Maximum short-circuit current at the Point of Common
Coupling (PCC).
Maximum demand load current (fundamental) at
the PCC.
TDD
5.0
8.0
12.0
15.0
20.0
Attenuation of Harmonics
Inductive Reactance
Method:
Add a line reactor or isolation transformer
to attenuate harmonics.
Benefits:
Low cost.
Technically simple.
Concerns:
Tends to offer reductions in only higher
order harmonics. Has little effect on the 5th
and 7th harmonics.
Because of the associated voltage drop,
there are limits to the amount of reactance
that may be added.
Attenuation of Harmonics
Passive Filters
Method:
Provide a low-impedance path to ground
for the harmonic frequencies.
Benefits:
May be tuned to a
frequency between two prevalent harmonics
so as to help attenuate both.
Concerns:
Tuning the filters may be a labor-intensive
process.
Filters are difficult to size, because they offer
a path for harmonics from any source.
Quite sensitive to any future system changes.
Attenuation of Harmonics
Active Filters
Method:
Inject equal and opposite harmonics onto the
power system to cancel those generated by
other equipment.
Benefits:
Have proven very effective in reducing
harmonics well below required levels.
Concerns:
The high performance inverter required for the
harmonic injection is costly.
Power transistors are exposed to conditions
of the line, so reliability may be a problem.
Attenuation of Harmonics
12-pulse Rectifiers
Method:
Two separate rectifier bridges supply a single
DC bus. The two bridges are fed from phaseshifted supplies.
Benefits:
Very effective in the elimination of 5th and 7th
harmonics.
Stops harmonics at the source.
Insensitive to future system changes.
Concerns:
May not meet the IEEE standards in every
case.
Does little to attenuate the 11th and 13th
harmonics.
Attenuation of Harmonics
18-pulse Rectifier
Method:
An integral phase-shift transformer and rectifier
Input which draws an almost purely sinusoidal
waveform from the source.
Benefits:
Meets the IEEE standards in every case!
Attenuates all harmonics up to the 35th.
Stops harmonics at the source.
Insensitive to future system changes.
Concerns:
Can be expensive at smaller HP’s
Comparison of waveforms
6-pulse converter
note the level of distortion
and steep current rise.
12-pulse converter
the waveform appears more
sinusoidal, but still not very
smooth.
18-pulse converter
virtually indistinguishable
from the source current
waveform.