Application Note

Power Quality Troubleshooting Fluke Corporation 11
•
Harmonic amplitudes
normally decrease as the
frequency goes up. If one
frequency is significantly
higher in amplitude 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 overcurrent/
overvoltage conditions.
•
Before-and-after harmonic
spectrum measurement is ex-
tremely valuable to deter-
mine if harmonic mitigation
techniques, like trap filters,
which are tuned to specific
frequencies, are sized prop-
erly and are working as
expected.
•
Different harmonic frequen-
cies affect equipment in dif-
ferent 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% (Table 3.2)
IEEE 519 sets limits for harmon-
ics 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 (al-
though 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 eas-
ily 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 fa-
cility, such as on the feeder sys-
tem, 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 af-
fecting utility PQ.
3a. Voltage THD
THD has a long history in the
industry. The underlying con–
cept is that harmonic currents
generated 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
create harmonic currents in that
linear load. By setting maximum
limits for voltage distortion, we
set limits for the system-wide
impact of harmonics.
Table 3.1 Measurements at the distribution transformer.
Measurement Look for Instrument
1. kVA Transformer loading. If loading 43, 41B
exceeds 50%, check for harmonics
and possible need for derating.
2. Harmonic spectrum
•
Harmonic orders/amplitudes present: Same
3rd harmonic (single-phase loads)
5th, 7th (primarily three-phase loads)
•
Resonance of higher order harmonics
•
Effectiveness of harmonic trap filters
3. THD Harmonic loading within limits: Same
Voltage %THD <5%
Current %THD <5-20% (Table 3.2)
4. K-factor Heating effect on transformer from Same
harmonic loads
5. Ground currents
•
Objectionable ground currents are Same
not quantified but are prohibited by True-rms Clamp
the NEC.
•
Neutral-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
Frequency 60 120 180 240 300 360 420 480 540
Sequence +—0+—0+—0
Rule: If waveforms are symmetrical, even harmonics disappear.
Effects of Harmonic Sequences
Sequence Rotation Effects (from skin effect, eddy currents, etc.)
Positive Forward Heating of conductors, circuit breakers, etc.
Negative 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 180 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 converter circuit.
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 sequence. Add in neutral of 3-phase, 4-wire system.
Mainly 3rd harmonic.