POWER April 2021 - 14

O&M
Using Distribution-Class
STATCOMs to Resolve
Distributed Generation
Plant Voltage Issues
When distributed generation causes
net reverse power flow on a distribution
feeder, several voltage-related issues can
occur. According to studies published by
the National Renewable Energy Laboratory
(NREL) and Sandia National Laboratories,
utility voltage violations constitute
the most commonly occurring constraint
that limits solar generation capacity on a
distribution feeder.
Some common causes of voltagerelated
impacts include:
■ Excessive voltages (above 105%
nominal). Significant reverse power
flow on a feeder, typically greater than
1 MW, can cause excessive voltage
rise and result in voltages greater than
105% of nominal voltage.
■ Over-operation of load tap changers
(more than 30 daily taps). Solar and
wind are variable generation sources.
For solar generation, partly cloudy
days, especially windy ones, can result
in up to 100 daily cloud pass events.
When combined with the smaller variations
of existing feeder loading, this
can result in substantially overtaxed
tap changers.
■ Poor power quality and rapid voltage
changes (low voltages below 95%
and sudden voltage variations greater
than 3%). Variability due to solar and
wind can cause a sudden loss of generation.
This is known to result in voltages
below normal operating limits
and poor power factors, as well as
aggravating voltage variations, such as
customers experiencing persistent appliance
noise.
Figure 1 shows actual field voltage
recordings from a feeder experiencing
problematic reverse power flow due to
variable solar generation. Conventional
line regulators and switched capacitor
banks are far too slow to compensate
for these negative impacts, leaving utilities
and project developers with limited
choices for resolving these issues. Traditional
utility methods of reconductoring a
feeder or constructing a dedicated feeder
for a generating plant can incur costs of
up to $800,000 per MW of plant rating.
14
1. This graph shows voltage on a distribution
feeder with variable solar generation causing
reverse power flow. Courtesy: AMSC
Such traditional wire-based approaches
can easily render an otherwise attractive
investment economically infeasible.
Land of Solar and Wind
Opportunity
Many attractive sites for distributed solar
and wind plants are located in semi-rural
or rural areas where land is readily available
and well-suited for solar plant development.
Unfortunately, the best sites for
land are often the worst when it comes
to available grid infrastructure. Specifically,
with respect to voltage, downline
feeder locations corresponding to more
rural areas, such as locations more than
5 miles from the nearest utility substation,
are the ones most sensitive to the
effects of reverse power flow and variable
generation explained above.
Agricultural communities in particular
can benefit from leasing land for solar
and wind plants to create an attractive
and non-seasonal income supplement
to farming. However, agricultural communities
are commonly served by longer
distribution lines that confound the connection
of generating plants. This lack of
a strong grid infrastructure can be a serious
barrier to making such a local economy-boosting
option feasible.
Recent progress in power electronics
is enabling the industry to cost-effectively
address these distributed generation
challenges. An example of one such
solution is a power electronics device
known as a STATCOM (STATic synchronous
COMpensator).
STATCOMs are shunt-connected grid
inverters that are optimally designed
for controlling reactive power like that
created by solar or wind. For more than
30 years, STATCOMs have been used
www.powermag.com
2. This graph shows the performance of a
distribution-class STATCOM regulating voltage
on a circuit with variable solar generation.
Compared to the performance obtained with
traditional utility regulation equipment shown
in Figure 1, the D-VAR VVO utilized in this
case significantly improves system operation.
Courtesy: AMSC
extensively in transmission systems
to address a variety of applications, including
utility voltage control and grid
code compliance for large renewable
generator plants.
AMSC's D-VAR VVO is a distributionclass
STATCOM that meets both the
high-performance and low-cost requirements
of distribution applications. Figure
2 shows a more consistent daily voltage
profile with a single distribution-class
STATCOM installed on the feeder.
AMSC's distribution-class D-VAR VVO
solution was designed from the ground
up for installation and operation within
distribution circuits, and features the following
design characteristics:
■ No routine maintenance.
■ Excellent safety, without batteries.
■ Extremely low operating losses (less
than 1% at full output).
■ Excellent reliability due to no moving
parts and a completely sealed design.
■ Compact feeder-ready design enables
installation within existing utility rightof-way
boundaries.
■ Compliance with stringent distribution
equipment standards, including
dielectric integrity, short-circuit withstand
rating, and enclosure integrity.
Figure 3 shows a typical D-VAR VVO
installation on an existing right-of-way.
It's All About Voltage
Since the inception of modern power delivery
systems, distribution utilities have
taken responsibility for controlling and
POWER | April 2021
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POWER April 2021

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