Fluke 435 I, 435 II, 437, 437 II Service Guide

Common power

Unbalanced load:
kVA

TOTAL

= kVA1 + kVA2 + kVA

3

kVA

1

Red

Red

Red

Black

Black

Black

kVA

2

kVA

3

ø1

ø2

ø3

N

quality factors

affecting transformers

Application Note

Commercial buildings commonly
have a 208/120 V transformer in
a delta-wye configuration to
feed receptacles. Single-phase,
non-linear loads connected to
the receptacles produce triplen
harmonics, which add up in the
neutral. When this neutral cur­rent reaches the transformer, it is
reflected into the delta primary
winding where it causes over­heating and transformer failures.

Another transformer problem
results from core loss and copper
loss. Transformers are normally
rated for a 60 Hz phase current
load only. Higher frequency har­monic currents cause increased
core loss due to eddy currents
and hysteresis, resulting in more
heating than would occur at the
same 60 Hz current.

Transformers supplying non­linear loads should be checked
periodically to verify operation

within acceptable limits. Trans­formers are also critical to the
integrity of the grounding system.

Factors

1. Transformer loading (kVA)

Start by measuring kVA and
determine wether the transformer
load is balanced.

Connect voltage probes on

•

Phase 1 and Neutral and
clamp current probe on same
phase. Repeat for Phase 2 and

3.
Use a single phase power

•

quality analyzer to read kVA of
each phase and sum all three
for total transformer kVA.
Or, connect all four current

•

clamps and all five test leads
for the three phase power
quality analyzer to read kVA
for each phase and the total.

Compare actual load kVA to

•

nameplate kVA rating to deter­mine % loading.

When using a single phase
analyzer on a balanced load, a
single measurement is suffic

ient.
Transformers loaded at less than
50 % are generally safe from
overheating. However, as loads
increase, measurements should
be made periodically. At some
point the transformer may require
derating.

Figure 2. Harmonic spectrum.

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 sin

•

-

gle-phase loads, the principal

n is the

er

onc

ill add arith-

ye transformer

y

.

harmonic of c

. The 3rd w

3rd
metically in the neutral and

irculate in the delta primar

c
of a delta-w
The good news is that the
delta-w
rest of the system from the 3rd
(though not the 5th, 7th or

ye tends to isolate the

other non-triplen harmonics).

e 1

Figur

. Measuring transformer load (unbalanc

From the Fluke Digital Library @ www.fluke.com/library

ed) using a single phase power quality analy

The bad new
transformer pa
w

.

zer

ith additional heat

s is that the

ys the pric

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e

Table 1: Measurements at the distribution transformer

Measurement

. kVA Transformer loading. If loading exceeds 50 %, check for harmonics

1

2. Harmonic
spectrum 3rd harmonic (single-phase loads)

3. THD Harmonic loading within limits:

-factor Heating effect on transformer from harmonic loads

4. K

5. Ground currents

In a transformer feeding three-

•

phase loads which include
drives or UPS systems with 6­pulse converters, the 5th and
7th harmonic will tend to pre­dominate. 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 ampli­tude than lower frequencies,
we can suspect a resonant

Look for

and possible need for derating.

Harmonic orders/amplitudes present:

•

5th, 7th (primarily three-phase loads)

e of higher order harmonics

Resonanc

•

Effectiveness of harmonic trap filters

•

Voltage %THD < 5 %
Current %T

•

•

•

HD < 5-20 % (Table 2)

Objectionable ground currents are not quantified but are
prohibited by the N

eutral-ground bond in place

N
ESG (Electrical Safety Ground) connector to ground electrode
(typically building steel) in place

EC

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
extremely valuable to deter­mine 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).

Harmonic Sequences

Name F 2nd 3rd 4th 5th 6th 7th 8th 9th

180 240 300 360 420 480 540

20

Frequenc
Sequence +—0+—0+—0

Rule: If waveforms are symmetrical, even harmonics disappear.

Eff

Sequence Rotation Effects (from skin effect, eddy currents, etc.)

Positive Forward Heating of conductors, circuit breakers, etc.
N
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

2.

3. Sequence:

y

ects of Harmonic Sequences

egative

0 Hz

8

1
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

onverter c

c

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

•

Reverse

.

.

ircuit

e. Add in neutral of 3-phase, 4-w

1

0

6

Heating as above + motor problems

ire system

. Mainly 3rd harmonic.

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 util­ity PQ.

3a. Voltage THD

THD has a long history in the
industry. The underlying concept
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.

2 Fluke Corporation Common power quality factors affecting transformers