IEEE Power & Energy Magazine - March/April 2015 - 25

important considerations and are interdependent with system
strength and customer loads. A holistic plan must take into account the unique performance of the entire distribution system
as well as the characteristics of DERs and how they affect the
grid. Such an integrated approach is the foundation of a new
methodology developed by EPRI for incorporation into distribution planning.

The Plan: An Integrated Approach

Voltage
Utilities design the distribution system so as to maintain primary voltages within standard ranges; typically, primary
voltages are kept within ±5% of the nominal rating. DER
installations have the potential to change voltage along a distribution feeder because of the power they inject into the grid.
For variable generation, such as PVs and wind, unacceptable
voltages can occur on either the primary or secondary systems,
causing overvoltages and/or excessive voltage deviations that
can affect regulation equipment and result in flicker. Alternatively, when coordinated with utility regulation, additional
voltage support can be achieved by means of dispatchable
DERs and DERs with reactive power control capability.
Standard load flow analyses coupled with appropriate
approximations for the distribution of DERs can effectively
analyze the potential voltage benefits and impacts. Voltage
magnitude, voltage deviation, and voltage imbalance are
examined at all feeder primary and secondary nodes as well
as at voltage regulation points. Various load levels, which
also dictate baseline response, are considered as well to better capture the range of impacts that can be expected.

march/april 2015

Energy

Energy Losses
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Current

Watts

Voltage

Distribution system assessment for DERs must focus on incorporating DERs while maintaining established standards of
reliability and power quality. In other words, the first step in the
plan is to establish the distribution feeder's ability to host DERs
(determine its hosting capacity). As discussed previously, hosting capacity is the amount of DERs that can be accommodated
under current grid conditions without affecting power quality
or reliability.
Understanding the hosting capacity under the system's
current arrangement is, however, only the first step. To provide the proper analytical perspective, the analysis must be
taken further to:
✔✔ analyze the ability to expand hosting capacity through
a series of mitigation options (if necessary)
Protection
✔✔ investigate the benefits that DERs can provide to the Utilities must retain the ability to detect and isolate faults
particular distribution system.
as well as provide service restoration to all customers in
EPRI has developed and
demonstrated methods for
conducting feeder-specific
Voltage
analyses that take all of these
Unacceptable
factors into account. Based on
Overvoltage
experience from detailed individual feeder analyses, EPRI
Limit
has developed a new methodology that makes possible the
Time
Protection
Capacity
practical characterization of
the impacts of various levels
Relay
and arrangements of DERs on
Load Only
Desensitization
all of a utility's feeders. The
methodology uses commerLoad
Integrated
and PV
cially available analysis tools
Approach
Impedance
Impedance
for most of its steps, in particular, for determining the hosting capacity based on voltage
and protection evaluations.
EPRI's methods for charUnserved Energy
acterizing the benefits and
impacts on distribution of
Energy
DERs are contained in the
Exceeding Normal
following five root-cause catTime
Month
egories, as shown in Figure 4:
✔✔ voltage
Energy
Reliability
✔✔ protection
✔✔ energy
✔✔ capacity
figure 4. The five categories used in EPRI's performance assessment methodology for
✔✔ reliability.
characterizing distribution feeders.
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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2015