Application Note

10 Fluke Corporation Power Quality Troubleshooting
Unbalanced load:
kVA
TOTAL
= kVA
1
+ kVA
2
+ kVA
3
kVA
1
Red
Red
Red
Black
Black
Black
kVA
2
kVA
3
ø 1
ø 2
ø 3
N
For a balanced load: kVA
TOTAL
= kVA
1
• 3
= (kVA
1
)(1.732)
Red
Black
Transformers are subject to
overheating from harmonic cur-
rents. Transformers supplying
non-linear loads should be
checked periodically to verify
operation within acceptable
limits. Transformers are also
critical to the integrity of the
grounding system.
Measurements
1. Transformer loading (kVA)
If the transformer has a four-
wire wye secondary, which is
the standard configuration for
commercial single-phase loads,
actual kVA can be easily
determined. (See Figure 3.2)
•
Connect voltage probes on
Phase 1 and Neutral and
clamp current probe on same
phase. Repeat for Phase 2
and 3.
•
Read kVA of each phase and
sum all three for total trans-
former kVA.
•
Compare actual load kVA
to nameplate kVA rating to
determine % loading.
If the load is balanced, a single
measurement is sufficient. (see
Figure 3.1) Transformers loaded
at less than 50% are generally
safe from overheating. However,
as loads increase, measure-
ments should be made periodi-
cally. At some point the
transformer may require derat-
ing (see page 15).
2. Harmonic spectrum
The harmonic spectrum of the
secondary (load) current will
give us an idea of the harmonic
orders and amplitudes present:
•
In a transformer feeding
single-phase loads, the prin-
cipal harmonic of concern is
the 3rd. The 3rd will add
arithmetically in the neutral
and circulate in the delta pri-
mary of a delta-wye trans-
former. The good news is
that the delta-wye tends to
isolate the rest of the system
from the 3rd (though not the
5th, 7th or other non-triplen
harmonics). The bad news is
that the transformer pays the
price with additional heat.
•
In a transformer feeding
three-phase loads which in-
clude drives or UPS systems
with 6-pulse converters, the
5th and 7th harmonic will
tend to predominate. Exces-
sive 5th is of particular con-
cern because it is negative
sequence. It will tend to
produce counter-torque and
overheating in polyphase
motors.
Section 3
Transformers
Figure 3.1 Measuring transformer load (balanced).
Figure 3.2 Measuring transformer load (unbalanced).
Figure 3.3 Harmonic spectrum.