IEEE Power & Energy Magazine - March/April 2017 - 93

in my view (continued from p. 96)

march/april 2017

Actual Frequency Response to a Generating Unit Trip-O'ahu
Unit Trip
p

60.2
60.0
59.7
59.5

Legacy DER Trip (59.3)

59.2
(Hz)

this type of disturbance. Much of the
system inertia is created by the rotating
mass or energy in the turbine-generators.
Today, conventional thermal generation is turned down or cycled off to make
room for additional renewables-which
reduces the amount of inertia on the
system. During these underfrequency
events, which can occur multiple times
a year, the load shed scheme activates
to rebalance the load and generation. It
is also notable that multiple underfrequency load shed blocks are activated
to arrest the steep frequency decay.
Because rooftop PVs masks or offsets
the load the utility serves on these load
shed blocks, to prevent a collapse of
the power system, more load blocks are
disconnected than previously. The underfrequency condition worsens when
PV inverters trip offline at 59.3 Hz, the
previous IEEE 1547 trip point for distributed generation.
We are pursuing innovative solutions to address this vulnerability in
three ways:
✔✔ conducting research to develop
advanced frequency stabilizing
functions for distributed resources
✔ ✔ evaluating fast frequency response capabilities of customersited resources
✔✔ improving ride-through settings
of existing and future inverters.
These DER-based solutions, developed in
collaboration with inverter manufacturers, distributed energy developers, U.S.
Department of Energy national laboratories, and others, will help to preserve system stability while enabling customers to
interconnect more PV systems.
First, together with the National Renewable Energy Laboratory and Sandia National Laboratory, we pursued
the Department of Energy's Grid Modernization Lab Call to partner with
inverter manufacturers in developing
bulk system frequency-stabilizing capabilities. At the end of the two-year
research effort, we expect that behindthe-meter PVs and battery energy storage systems can autonomously respond

59.0

Sh
h Block 1
Underfrequency (58.9) Load Shed

58.7

Sh
h Block 2
Underfrequency (58.7) Load Shed

58.5

Sh
h Block 3
Underfrequency (58.4) Load Shed

58.2

Underfrequency (58.1) Load Shed Block 4

58.0
57.7

Underfrequency (57.8) Load Shed Block 5
-5

10
Time (s)

figure 2. The frequency response of the Hawaiian Electric power system during
the loss of the grid's largest generator in April 2013.

bidirectionally-frequency response
up and down-most quickly to system
disturbances.
Second, to prevent multiple blocks
of load shedding during underfrequency disturbances, we plan to leverage
demand-response resources to supplement utility-scale frequency response
resources. In collaboration with distributed energy providers, we seek to demonstrate that customer-owned batteries,
PVs, and controllable loads respond
within 30 cycles during a loss of generator (underfrequency) event to reduce the
rate of change of the system frequency.
Third, we worked closely with Enphase Energy to expand ride-through
capabilities by retroactively and remotely
reprogramming settings for more than
28,000 rooftop PV systems, or 154 MW.
These new ride-through settings make
it mandatory for PV systems to continue to produce power through frequency disturbances to lessen the rapid
frequency decay during loss of generator events.
This collaboration produced innovation in two ways. 1) It demonstrated
the capability to remotely reprogram PV
inverters, a key component in the devel-

opment of DERs that is adaptable and
flexible to dynamically changing grids
of the future. 2) It led the industry in expanding ride-through settings beyond
the IEEE standard for interconnection
of distributed energy resources. This
highlighted the need to update these
standards to recognize specific island
grid requirements.
Intermittent resources can make
balancing the power system difficult for
grid operators, particularly those with
steam turbine-generators, which we
use at Hawaiian Electric. The unpredictability of wind patterns and cloud
movements can disrupt reliable operations on a minute-to-minute and even a
second-to-second basis.
Consider, for example, Figure 3,
which shows how the combination of intermittent resources and customer load
changes cause the grid frequency to
fluctuate. Adding firm, flexible sources
of generation to the power system can
solve the problem. However, the modern utility should demonstrate shared
benefits that can increase the value of
significant capital investments.
Through a partnership with the U.S.
Army, Hawaiian Electric will build a
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