IEEE PES T&D Conference & Exposition 2022 - 99

interruption of 10 to 15 minutes, batteries can be used to
bridge the gap and keep critical loads functional.
The last type of voltage event is a long-duration RMS
variation, which is classified by events that last longer than
1 minute - sustained interruptions (0pu), undervoltages (0.80.9pu),
and overvoltages (1.1-1.2pu). Similar to short-term
RMS variations, these long-term events require a fast response
initially to avoid interrupting service, but solutions can vary
depending on the type of event. For sustained undervoltages
or overvoltages, a fast-acting reactive power regulation device
can be used to bring the grid voltage within an acceptable
continuous range (0.9-1.1pu), but after that, slower regulation
devices, such as tap-changing regulators or switched capacitor/reactive
banks, can be used to bring the voltage up to or
down to the nominal value. This allows the operator to optimally
size the devices to avoid unnecessary costs.
In contrast, for sustained interruptions, the real power augmented
D-STATCOM is a good solution to avoid interrupting
service for a section of the grid, and the real power source
can be designed for as long as interruptions are expected to
last. At a certain point, this solution crosses over from pure
voltage regulation to a dispatchable battery energy storage
system that can also be used for additional power quality
controls such as frequency regulation and peak-shaving. In
a shunt configuration, this type of solution allows for supporting
a section of the grid, but a double-conversion series
configuration can also be used to protect a specific load.
As the penetration of PV generation continues to increase, utility
planners are now considering these more distributed voltage
regulation solutions as another major asset to solve these issues.
Flicker
Voltage fluctuations are becoming more common on the distribution
grid with the increase in DGPV due to the higher
number of power electronics devices and effects such as
clouding. The effect that the voltage fluctuation has on lamp
illumination is called " flicker. " These fluctuations can be
periodic or random but typically will not exceed the voltage
range of 0.95pu to 1.05pu. They may be perceived as a
change in lamp illumination intensity, causing headaches or
irritation. In an industrial setting, voltage fluctuations may
cause electric components to misoperate or malfunction. IEC
61000-4-15 describes how to monitor flicker and defines the
limits of flicker from a specific piece of equipment. These
flicker meters use grid voltage as an input, then filter out
only the fluctuations defined as perceptible to humans and
output a normalized perceptibility factor. The instantaneous
perceptibility factor (Pinst) is then aggregated over periods of
10 minutes and 2 hours to create a short-term (Pst) and longterm
perceptibility factor (Plt), respectively.
For larger power systems, IEEE 1453 provides recommended
analysis techniques and acceptable flicker levels for
both utility and industrial customers to follow. The recommended
planning emission limits vary based on system voltage,
with the limits for Pst/Plt on MV systems being 0.9/0.7
April 2022 Show Issue
and the limits for Pst/Plt on high voltage or extra-high voltage
systems being 0.8/0.6. The analysis of flicker levels includes
collecting the Pst flicker trend over a week, calculating the Plt
based on the Pst, and finally performing a statistical analysis
on these datasets to show the flicker has remained below
recommended emission limits for at least 95% of the time.
The 99% probability value is also calculated, and this value
may exceed emission limits by a factor determined by the
system operator, but typically no more than 1 to 1.5 times
the planned emission limit. An example of this analysis is
shown later in the Field Validation and Mitigation section.
Harmonics
By definition, harmonics in power systems can appear on voltage
or current signals as a sinusoid having a frequency that
is an integer multiple of the fundamental power frequency
(60Hz in the United States). Harmonic distortion on the grid
is a growing concern for many utilities and their customers
with the increased use of power electronics-based equipment,
such as the pulse width-modulated inverters used to convert
solar cells' DC power to AC power usable on the grid. These
distortions in the voltage and current can create issues for
many pieces of distribution equipment, such as increased
heating and early insulation wear-out in motors, generators,
and transformers. In addition, electronic controls that rely on
zero-crossings or a clean sine-wave input may not operate
properly if exposed to higher harmonic distortion.
Harmonic limits for a given piece of equipment are defined
in IEEE 519. They are defined for both current and voltage
in the form of both individual and total harmonic distortion.
Each distortion can be calculated by taking the ratio of a chosen
harmonic voltage or current magnitude and the value of
the voltage or current at the fundamental frequency. For total
harmonic distortion, the root sum of all harmonics (typically
up to the 50th harmonic) is divided by the voltage or current at
the fundamental frequency. The IEEE 519 limits for voltage at
the distribution level (<69kV) are 3% for individual harmonics
and 5% for total voltage harmonic distortion.
Current harmonic limits are slightly more complex as
they depend on the ratio of the short-circuit capacity of the
feeder to the load, which can be thought of as a measure of
how much a specific load affects the overall grid characteristics.
The larger the ratio, the smaller the load is in comparison
to the whole system. The harmonic limits are relaxed in
these cases since their high harmonic content will not have
much effect on the grid as a whole. The current harmonics
are also combined, similar to the total voltage harmonic
distortion, to a value called the total demand distortion. To
calculate the total demand distortion, the root sum of the
individual current harmonics is divided by the maximum
demand load current (taken over a 15 or 30m interval).
IEEE 519 is a common requirement for any distribution
or transmission power electronics-based device being installed
on the grid. But even if all devices meet individual equipment
limits, the increase in solar inverter installations can tip the
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IEEE PES T&D Conference & Exposition 2022

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