Application Note

2 Fluke Corporation Common power quality factors affecting transformers
•
In a transformer feeding three-
phase loads which include
drives or UPS systems with 6-
pulse converters, the 5th and
7th harmonic will tend to pre-
dominate. Excessive 5th is of
particular concern because it is
negative sequence. It will tend
to produce counter-torque and
overheating in polyphase
motors.
•
Harmonic amplitudes normally
decrease as the frequency
goes up. If one frequency is
significantly higher in ampli-
tude than lower frequencies,
we can suspect a resonant
condition at that frequency. If
such a condition is detected,
be sure to take readings at
capacitor banks to see if the
caps are experiencing overcur-
rent/overvoltage conditions.
•
Before-and-after harmonic
spectrum measurement is
extremely valuable to deter-
mine if harmonic mitigation
techniques, like trap filters,
which are tuned to specific
frequencies, are sized properly
and are working as expected.
•
Different harmonic frequencies
affect equipment in different
ways (see below).
3. Total Harmonic Distortion
Check for THD of both voltage
and current:
•
For voltage, THD should not
exceed 5 %
•
For current, THD should not
exceed 5-20 % (see Odd
Harmonics table)
IEEE 519 sets limits for har-
monics at the PCC (Point of
Common Coupling) between the
utility and customer (EN50160 is
the European standard). IEEE 519
is based on THD measurements
taken at the PCC. Technically, the
PCC is the primary of the utility
supply transformer (although
there are cases where the PCC is
at the secondary if the secondary
feeds a number of customers). In
practice, these measurements are
often made at the secondary of
the customer’s main transformer,
since that is the point most easily
accessible to all parties (and also
since that is generally a Low
Voltage measurement).
Some PQ practitioners have
broadened the concept of PCC to
include points inside the facility,
such as on the feeder system,
where harmonic currents being
generated from one set of loads
could affect another set of loads
by causing significant voltage
distortion. The emphasis is on
improving in-plant PQ, rather
than on simply not affecting util-
ity PQ.
3a. Voltage THD
THD has a long history in the
industry. The underlying concept
is that harmonic currents gener-
ated by loads will cause voltage
distortion (E=IZ) as they travel
through the system impedance.
This voltage distortion then
becomes the carrier of harmonics
system-wide: if, for example, the
distorted voltage serves a linear
load like a motor, it will then cre-
ate harmonic currents in that
linear load. By setting maximum
limits for voltage distortion, we
set limits for the system-wide
impact of harmonics.
Table 1: Measurements at the distribution transformer
Measurement
Look for
1
. kVA Transformer loading. If loading exceeds 50 %, check for harmonics
and possible need for derating.
2. Harmonic
•
Harmonic orders/amplitudes present:
spectrum 3rd harmonic (single-phase loads)
5th, 7th (primarily three-phase loads)
•
Resonanc
e of higher order harmonics
•
Effectiveness of harmonic trap filters
3. THD Harmonic loading within limits:
Voltage %THD < 5 %
Current %T
HD < 5-20 % (Table 2)
4. K
-factor Heating effect on transformer from harmonic loads
5. Ground currents
•
Objectionable ground currents are not quantified but are
prohibited by the N
EC
•
N
eutral-ground bond in place
•
ESG (Electrical Safety Ground) connector to ground electrode
(typically building steel) in place
Harmonic Sequences
Name F 2nd 3rd 4th 5th 6th 7th 8th 9th
Frequenc
y
6
0
1
20
180 240 300 360 420 480 540
Sequence +—0+—0+—0
Rule: If waveforms are symmetrical, even harmonics disappear.
Eff
ects of Harmonic Sequences
Sequence Rotation Effects (from skin effect, eddy currents, etc.)
Positive Forward Heating of conductors, circuit breakers, etc.
N
egative
Reverse
Heating as above + motor problems
Zero None Heating, + add in neutral of 3-phase, 4-wire system
Harmonics are classified as follows:
1. Order or number: Multiple of fundamental, hence, 3rd is three times the fundamental, or
1
8
0 Hz
.
2.
Odd or even order: Odd harmonics are generated during normal operation of nonlinear
loads. Even harmonics only appear when there is dc in the system. In power circuits, this
only tends to occur when a solid state component(s), such as a diode or SCR, fails in a
c
onverter c
ircuit
.
3. Sequence:
•
Positive sequence. Main effect is overheating.
•
Negative sequence. Create counter-torque in motors, i.e., will tend to make motors go
backwards, thus causing motor overheating. Mainly 5th harmonic.
•
Zero sequenc
e. Add in neutral of 3-phase, 4-w
ire system
. Mainly 3rd harmonic.