IEEE Power Electronics Magazine - March 2020 - 29

IEC 61000-4-28 addresses the immunity of devices to
variations in the frequency of their ac power sources. Like
IEC 61000-4-14, it is considered a basic test standard. The
tests specified by IEC 61000-4-28 help meet generic standards called for under the EMC directive. They also provide, if followed completely, a high level of confidence
and reliability that can help assure conformance with the
EMC directive's protection requirements. However, even
if a device meets the product or generic standards listed
by the EMC directive, it could still be unreliable or fail in
real life because it is not sufficiently immune to the actual
electromagnetic environments in which it is normally used.
Variations in the frequency of ac power sources are related
Test Standards Address ac Fault Protection
to the rotational speed of the generators, which increase
The test standards that directly address these issues are IEC
and decrease in response to the multiple loads on the sys61000-4-11, IEC 61000-4-14, IEC 61000-4-28, and the Aircraft
tem. Because the frequency of ac mains supplies is usually
Electric Power Characteristics standard, MIL-STD-704F (the
50-60 Hz, the effects they will have on equipment are fairly
latest version).
simple to calculate. However, the problems that ac power freIEC 61000-4-11 is the standard for testing for immunity
quency variations can cause should not be underestimated [3].
(or susceptibility) to voltage variations. Variations includFor example, control systems that use the ac frequency
ing dips, dropouts, sags, and interruptions are the most
as a timing reference can enter error states or fall out of
common forms of disturbance in the mains supply and are
synch with other elements in a sysoften caused by load switching, use
tem that is controlled by a more accuincreases/decreases, and other tranrate dc timing source. AC motors
sient interruptions. Such faults are
IEC 61000-4-11's comincrease and decrease rotational
considered the most common causes
speed according to frequency, which
of poor mains supply quality.
prehensive and varied
can affect their ability to handle
IEC 61000-4-11's comprehensive
set of tests enables
loads. Frequency variations can also
and varied set of tests enables detecadversely affect other electrometion of problems in great detail and
detection of problems
chanical elements, such as transformfocuses on the particular type of
in great detail and
ers or relays, depending on their indierror. Although this set of tests and
vidual characteristics.
focuses on the particulevels goes beyond what is generically
The MIL-STD-704 standard is now
required by most common immunity
lar type of error.
up to revision F. The new revision,
standards, they set a level of complihowever, does not negate the earlier
ance that can go a long way toward
versions. Aircraft systems have long
assuring reliability and customer satlives, and they are not renovated every time a revision to the
isfaction. As compliance with these standards has become
standard is adopted. Therefore, it is very important to know
more widely required, a need for integrated test equipment
what revision of the power platform an aircraft is using
has been created, for example, the Preen AFV-P Series
before testing for compliance to the standard. Although it
from AC Power Corp., to ease the burden on developers
is an American military standard, MIL-STD-704 has been
and manufacturers.
adopted by a number of countries around the world, includIEC 61000-4-14 is directed at detecting the effects of
ing China [4].
disturbances in the mains supply voltage and is often
referred to as a basic test standard [1]. The voltage fluctuThe standard originally described a voltage waveform
ations addressed by IEC 61000-4-14 are commonly caused
of 150/120 V at 400 Hz. Six additional specifications were
by randomly varying heavy loads like arc furnaces, rolladded later, including a three-phase version, two 115-V
ing mills, or large motors with varying loads-things that
variable-frequency versions, a 115-V/60-Hz version, and two
can cause voltage drops until the supply can compensate.
dc versions-270 VDC and 28 VDC. The variations on the
A common symptom is transient flicker in a lighting sysMIL-STD-704 standard describe the power characteristics
tem, so IEC 61000-4-14 offers a basic immunity test for
of various classes of aircraft systems. They do not, however,
voltage fluctuations. Although IEC 61000-4-14 is a test
describe tests for those systems. Instead, a series of handmethod, the EMC standard the testing must meet includes
books describe the tests. For example, the handbook MILthe generic requirements plus those for the product stanHDBK-704F-2 defines tests for single-phase 115-V 400-Hz
dard. The product-family standard determines the voltsystems. Handbooks are available for each class of aircraft
age fluctuation levels and duration for which the product
power system described by the standard and should be used
is tested for conformity [2].
for the proper version and revision of that standard (Figure 1).
types of outputs, such as three-phase 400 Hz, 28 VDC,
and several others.
All of these sources are prone to errors, such as dips,
sags, brownouts, voltage and/or frequency variations, and
interruptions that can affect the operation of electronic
devices. The European Union Electromagnetic Compatibility (EMC) directive addresses these issues along with other
compatibility matters. The result has been that the International Electrotechnical Commission (IEC) has come up
with a set of standards by which devices can be tested and
certified to comply with requirements for protection.

March 2020

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IEEE Power Electronics Magazine - March 2020

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