IEEE Power & Energy Magazine - November/December 2017 - 51

Some power systems now have enough distri-
buted energy resources (DERs) that the bulk power
system is impacted. This article examines
these impacts, the role of interconnection
standards, and mitigation options. We
focus on distributed PVs (DPVs), the
most common DER, but much of this
applies also to other DERs, such as
distributed storage, distributed wind,
and electric vehicles.

Bulk System Impacts
in a Nutshell
There are two types of bulk system
impacts resulting from high penetra-

tions of DPVs. The first relates to high penetrations of PVs,
whether distributed or utility scale, including impacts on sys-
tem balancing, i.e., managing low net load (oversupply con-
ditions) during the middle of the day, with steeper net load
ramps to peak demand. Inverter-based generation, such as
PVs, can affect system reliability by displacing conventional
units that may be providing inertia or primary frequency
response (PFR). While PVs can contribute to essential reli-
ability services such as PFR, it is not typically required or
incentivized to do so, and so it does not.
The second main category of impacts resulting from
DPVs relates to DERs and includes how the DPVs are inter-
connected and whether it has the ability to appropriately
detect and ride through voltage or frequency events and sup-
port the grid by restoring power output shortly thereafter.
DPVs may undermine the effectiveness of underfrequency
load shedding (UFLS) because blocks of load may also con-
tain generation. Finally, DPVs can create challenges for load
forecasting because load forecasting models now must take
into account potential sunny versus cloudy days, as well
as the extent to which installations of DPVs will increase
over time.
When new technologies such as DPVs outpace the stud-
ies that analyze their impact on the grid, unintentional con-
sequences can be costly and difficult to fix. Very fast DPV
growth in Germany, combined with a legacy interconnec-
tion standard that required DPVs to trip offline at an over-
frequency of 50.2 Hz (where nominal frequency is 50 Hz),
led to potentially severe reliability impacts. The European
system was designed to handle the loss of 3,000 MW, but up
to 13,000  MW of DPVs potentially became the new single
largest contingency. A loss of load could result in frequency
exceeding 50.2 Hz, at which point common-mode tripping
of the DPV capacity could occur, potentially leading to very
severe consequences. This issue led to retrofitting the set-
tings of hundreds of thousands of DPV inverters at an esti-
mated cost of hundreds of millions of dollars. This under-
scores the need for good planning, engineering practices,
and system studies.

How DPVs Impact Bulk Power
System Performance in Hawaii
Hawaii is a leader in the deployment of distributed renew-
ables. The Hawaiian Electric Companies (HECO) operate
the utilities on the islands of Hawaii, Oahu, Maui, Molokai,
and Lanai. Today, 15% of customers across these islands have
DPV systems. On Oahu (the most densely populated island),
about one-third of single-family homes have DPVs. These
high penetration levels have led to technical challenges on
the distribution system (e.g., half the circuits are now back-
feeding at the substation) and the transmission system; we
will elaborate on the latter here.
A key system-level issue regarding DPVs is the degra-
dation of bulk power system reliability. Figure 1 shows the
actual frequency response on Oahu resulting from the 2013
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