IEEE Power Electronics Magazine - March 2020 - 31

also occur abruptly or have more gradual rates of change,
although testing is done using abrupt dips. For example, the
dip in Figure 2 shows a 40% dip (20% on each phase) lasting
20 ms and resulting in 60% of normal voltage.
A dropout or short interruption is defined as a dip that
lasts up to 1 min and is between 95 and 100% deep. Dropouts are also mostly the result of load switching and fault
clearance as well as switching between mains and alternative supplies, such as emergency backup systems or uninterruptible power supplies (UPSs). As in dip testing, dropouts
are applied at zero crossing, such as in the 60-ms example
Figure 3 shows, but can happen at any phase angle. A dropout that lasts longer than 1 min is not covered by the testing
standard and is commonly referred to as a power failure.
Voltage variations also include the longer-lasting and
slowly varying sags (or brownouts) and swells, which can
occur over periods of hours. Although sags and swells can
briefly go as low as 0 V, they would also be considered power
failures if the duration exceeds a minute (Figure 4). Swells
are different from transient surges or spikes, which typically last for less than 1 ms. Most mains supplies have a tolerance-typically ±6%-and equipment should be designed
and tested with that in mind. IEC 61000-4-11 does not specify
any swells testing, but sags can be a problem for ac motors
and for some dc motors due to possible stalling. Although
most motor controls have undervoltage trips to prevent that,
sags can still cause outages that affect operations.
Variations in supply frequency can arise from the rotational generators. For example, if a generator's speed
changes due to a suddenly connected large load, a variation
can occur. AC generation systems (other than large public
utilities) can experience their own frequency variations.
These systems include the small generators that homes or
businesses use during power outages, hospital emergency
backup systems, and independent power facilities for server
farms, military installations, and the like. Such power

supplies can change frequency significantly when loads are
applied. Small home generators, for example, audibly change
pitch when a significant load is turned on. Inverter-based
sources like UPSs have their frequencies set by their internal
circuitry, so these are mostly unaffected by load changes.
Equipment that relies on the supply frequency for timing can experience errors or fall out of synchronization

60 ms

FIG 3 A 60-ms short dropout. The Preen AFV-P Series is set up
to trigger on the 0° phase angle but can be programmed for
any phase angle as well.

Volts rms
230 V

115 V

FIG 4 A sag test showing a 50% sag with a duration of 1 s.

Mains
Supply

Unregulated dc Input
V

When Mains
Supply Dip
Occurs
(60 ms)

3.3 V

3.3-V
Regulator's
Output
Voltage

When Input Voltage
Drops Below the
Threshold, Circuit
Malfunctions May Occur

Minimum Voltage
Required for the 3.3-V
ICs Function Properly

FIG 5 An example of a 3.3-V circuit shows how dips and dropouts can cause malfunctions in the internal electronics of the DUT.

March 2020

z IEEE POWER ELECTRONICS MAGAZINE

IEEE Power Electronics Magazine - March 2020

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