IEEE Power & Energy Magazine - November/December 2015 - 39

Conclusions: The West and Beyond
This study found that "the Western Interconnection can be
made to work well in the first minute after a big disturbance
with both high wind and solar and substantial coal displacement, using good, established planning and engineering
practice and commercially available technologies." The
technical review committee of Western grid operators, utilities, and renewable energy and national reliability experts
for the study was careful with the wording of that conclusion
and rightly insisted on the qualifications in that sentence.
There is valuable and broadly applicable insight there. The
quantitative investigation used to illustrate concepts did not
identify any fundamental reasons why the Western Interconnection cannot meet transient stability performance criteria
with high levels of wind and solar generation. However,
good system planning and power system engineering practices must be followed. That's a diplomatic way of saying
"just because the power comes from renewable resources,
you can't forget power system fundamentals." Systems can
handle large amounts of variable renewables, but care is
needed. The consensus of the expert committee is "Don't
panic and don't be complacent".
At a minimum, for substantial additions of wind and
utility-scale solar, some transmission system improvements
will probably be needed to avoid local voltage and thermal
problems. The dynamic behavior of distributed PV generation has the potential to substantially impact the bulk power
system. Distribution is not decoupled from transmission and
will impact bulk power system operation. From a transient
stability perspective, the system appears to tolerate substantial displacement of thermal generation. However, care will be
november/december 2015

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Terminal Voltage (p.u.)

For disturbances that cause large generation/load imbalances, the reduction in system inertia affects the initial trajectory of the system. The design-basis event in the Western
Interconnection is the simultaneous trip of two of the Palo Verde
nuclear units. When the penetration of wind and solar in the
United States portion of the Western Interconnection increases
by 27 GW, from 29% penetration to 56% penetration (between
the light spring base case and the high-mix case), the initial
rate of change of frequency (ROCOF) increases about 18%.
The impact of this increased ROCOF on the system stability is
nearly invisible in terms of frequency response: both the nadir
and the settling frequency are essentially unchanged. It should
be noted that these levels of ROCOF, on the order of 0.1 Hz/s,
are quite small compared to some of the smaller systems around
the world that have ROCOF concerns primarily driven by the
use of ROCOF relays. This reinforces other results that suggest
the loss of system inertia associated with increased wind and
solar generation is of little consequence for up to at least 50%
levels of instantaneous penetration for large U.S. interconnections as long as adequately fast primary frequency responsive
resources are maintained. The results support industry efforts
to monitor and incentivize fast frequency response, rather than
to mandate arbitrary minimum levels of system inertia.

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DG with LVRT

DG Without LVRT

figure 7. The impact of common-mode tripping of DG
without low-voltage ride-through (LVRT).

needed in the event that the system is driven to near zero commitment of synchronous generation (e.g., coal plants). Both
traditional means of improving transient stability, such as adding transmission lines and other reinforcements, and emerging
methods, such as providing ancillary services on renewable
plants (as discussed elsewhere in this issue), conversion of
thermal plants to synchronous condensers, and weak grid controls on wind plants, were shown in the study to be effective.
This investigation is not a substitute for thorough system
planning studies: the West will need to exercise good practice and care as it proceeds to integrate more wind and solar
generation. While every grid has its own unique dynamic
characteristics, there is also a great deal of commonality:
basic physics does not change between grids. These conclusions apply broadly to other grids. Concerns about transient
stability and other dynamic performance impacts of wind
and solar generation are often overstated and, in the possible cases where they occur, can be mitigated by a spectrum
of traditional and nontraditional, but commercially available, technologies.

For Further Reading
NREL. Western Wind and Solar Integration Study Phase 3-
Frequency response and transient stability. (2014). [Online].
Available: http://www.nrel.gov/docs/fy15osti/62906.pdf
NREL. (2013, June). Eastern Frequency Response Study.
[Online]. Available: http://www.nrel.gov/docs/fy13osti/
58077.pdf
N. W. Miler, C. Loutan, K. Clark, and M. Shao "Emergency response," IEEE Power Energy Mag., Nov./Dec. 2013.

Biography
Nicholas W. Miller is with GE Energy Consulting, Schenectady, New York.
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ieee power & energy magazine

http://www.nrel.gov/docs/fy15osti/62906.pdf http://www.nrel.gov/docs/fy13osti/

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2015