IEEE Power & Energy Magazine - March/April 2015 - 21

By Jeff Smith, Matthew Rylander,
Lindsey Rogers, and Roger Dugan
with equipment for providing both voltage control and system protection; this equipment is operated using
custom settings to enable the utility to serve all customers in an efficient and reliable manner.
Within each feeder there are tens or hundreds of service transformers that convert power from the medium
voltage down to a more usable, low-voltage service level. These transformers distribute this service through
multiple secondary systems that connect each service transformer with individual residences, commercial
buildings, and industrial complexes.
Therefore, customers located at the very "edge" of the grid-and distribution utilities often
have hundreds of thousands or even millions of customers-are served by a vast and
diverse network of feeders, substations, planning areas, and, ultimately, an entire
distribution service territory (see Figure 1). In evaluating the grid from the transmission system all the way down to each individual customer, the amount
of visibility (measurement data, models, and so on), controllability (voltage and protection), and flexibility (automation) lessens. Therefore, the
vast majority of the change is occurring where utilities have the least
amount of information.
Regardless of whether the DER installations are connected to the
grid at the customer side of the meter or directly to the medium-voltage grid, to properly evaluate the impact of DERs the complexity
of the distribution system and the uniqueness of each location and
individual feeder must be considered.

The Grid Is Changing
Given the breadth of current DER additions, conflicts have
already arisen between distribution systems designed for
one-way power flow and DERs that want to force power
flow in the opposite direction. The structural diversity of the
distribution system poses an even greater challenge, as each
distribution feeder can have a unique response to this new
resource. Screening methods exist to avoid adverse impacts
due to DERs, but this is addressing the abundance of DER
interconnect requests and can result in higher overall costs if the
resource isn't fully integrated and located appropriately.
One example of this shift in planning philosophy is in designing
a voltage regulation scheme for a feeder. Without DERs, a planner
designs for a voltage drop from the substation to the feeder extremities.
If the voltage is projected to drop too low during peak demand periods,
a capacitor bank or voltage regulator is added to boost the voltage. Feeder
voltage control is designed to yield voltages within ANSI C84.1. With DERs,
the planner must also design for the voltage rise resulting from DER power output
and must consider time in the analysis. This requires new or enhanced planning tools.
An example of the time and location evaluation of PVs on a distribution feeder is captured in
Figure 2. The interaction of control devices and variable resources is evident in the embedded video
(to view the video, scan the QR code with a smartphone).
The daylight hours are simulated at a 1-s resolution to capture the solar variability impacts on voltage
magnitude and on voltage regulator and capacitor operations. Three snapshots captured during the day are
shown in the figure to represent the impact of solar generation on voltage profiles. At the beginning of the
day, the solar output is low, as shown by the surface plot of location-based solar resource potential for power
production. The surface represents the solar resource as measured from several ground-based measurement
units, indicated by vertical lines. The actual power production is dependent on the location of the actual
PV systems on the feeder. During the day, the solar resource and total power production fluctuate as clouds
march/april 2015

ieee power & energy magazine

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2015

IEEE Power & Energy Magazine - March/April 2015 - Cover1
IEEE Power & Energy Magazine - March/April 2015 - Cover2
IEEE Power & Energy Magazine - March/April 2015 - 1
IEEE Power & Energy Magazine - March/April 2015 - 2
IEEE Power & Energy Magazine - March/April 2015 - 3
IEEE Power & Energy Magazine - March/April 2015 - 4
IEEE Power & Energy Magazine - March/April 2015 - 5
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IEEE Power & Energy Magazine - March/April 2015 - Cover3
IEEE Power & Energy Magazine - March/April 2015 - Cover4
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