Application Note

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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 cur-

rent reaches the transformer, it is

reflected into the delta primary

winding where it causes over-

heating 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 har-

monic 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 non-

linear loads should be checked

periodically to verify operation

within acceptable limits. Trans-

formers 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

•

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.

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

Common power

quality factors

affecting transformers

Unbalanced load:

kVA

TOTAL

= kVA

+ kVA

kVA

Red

Black

kVA

ø1

ø2

ø3

•

Compare actual load kVA to

nameplate kVA rating to deter-

mine % 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.

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

harmonic of c

onc

n is the

3rd

. The 3rd w

ill add arith-

metically in the neutral and

irculate in the delta primar

of a delta-w

ye transformer

The good news is that the

delta-w

ye tends to isolate the

rest of the system from the 3rd

(though not the 5th, 7th or

other non-triplen harmonics).

The bad new

s is that the

transformer pa

ys the pric

ith additional heat

Figur

e 1

. Measuring transformer load (unbalanc

ed) using a single phase power quality analy

zer

Figure 2. Harmonic spectrum.

Summary of content (6 pages)

PAGE 1
Common power 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.
PAGE 2
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 spectrum • Harmonic orders/amplitudes present: 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: Voltage %THD < 5 % Current %THD < 5-20 % (Table 2) 4.
PAGE 3
Table 2: IEEE 519 limits for harmonic currents at the point of common coupling (All percentages are % of IL, maximum demand load current) Odd Harmonics <11 11-17 17-23 23-35 >35 TDD 4.0 % 2.0 % 1.5 % 0.6 % 0.3 % 5.0 % 20-50 7.0 % 3.5 % 2.5 % 1.0 % 0.5 % 8.0 % 50-100 10.0 % 4.5 % 4.0 % 1.5 % 0.7 % 12.0 % 100-1000 12.0 % 5.5 % 5.0 % 2.0 % 1.0 % 15.0 % >1000 15.0 % 7.0 % 6.0 % 2.5 % 1.4 % 20.
PAGE 4
standard trade sizes of K-4, K-13, K-20, K-30, etc. K-13 is a common rating for a transformer supplying office loads. The higher ratings tend to be packaged into PDUs (Power Distribution Units) which are specially designed to supply computer and other PQsensitive installations. A final word on measuring THD: the one place not to apply the specs is at the individual harmonic-generating load. This will always be a worst-case distortion and a misleading reading.
PAGE 5
Solutions There are a number of solutions for transformer-related PQ problems: • Install additional distribution transformers (Separately Derived Systems) • Derate transformers • Install K-rated transformers • Used forced air cooling 1. Separately Derived System (SDS) The distribution transformer is the supply for a Separately Derived System (SDS), a term which is defined in the NEC (Article 100).
PAGE 6
order is squared in the equation and that is precisely where the high- frequency heating effects, like eddy current losses, are taken into account. K-rated transformers are designed to minimize and accommodate the heating effects of harmonics. K-rated transformers do not eliminate harmonics (unless additional elements like filters are added).