IEEE Power & Energy Magazine - November/December 2015 - 33

options for where power is generated relative to where it is consumed,
their economic impact can be huge.

NERC Interconnections

Qu├ębec
Interconnection

Enter Wind and Solar
As wind and photovoltaic (PV)
solar power have grown as a portion of the overall energy mix,
these zero marginal cost resources
tend to displace generation that
incurs fuel costs (i.e., thermal
plants) or opportunity costs (i.e.,
hydro plants with finite pondage)
WECC
when there is available wind and
sunlight. However, unlike in the
past, when one type of synchronous generation would tend to
Western
displace another during economic
Eastern
Interconnection
commitment and dispatch, wind
Interconnection
and solar PVs do not have synERCOT
chronous generators. The dynamic
Interconnection
behavior of these inverter-based
resources can be quite different. figure 1. Major North American interconnections and NERC regional reliability
The fact that inverter-based gen- entities.
eration does not contribute inertia to the grid, unless explicitly controlled to do so, is often including those directed by the National Renewable Energy
mentioned as a critical difference. (See "Transient Stability Laboratory (NREL) and sponsored by U.S. Department of
in a Nutshell.") This has given rise to concerns that tran- Energy (DOE), have examined the West under possible nearsient stability-and, therefore, system reliability-will be future conditions for which a substantial amount of wind and
adversely affected. The question, in the vernacular, is "Will solar are built. These NREL Western Wind and Solar Intethe system stay stable with all those inverters instead of syn- gration studies, initially appearing at the beginning of 2008,
have been mentioned often in IEEE Power & Energy Magachronous machines?"
zine special issues on wind and solar integration.
One recently completed project sponsored by the DOE's
Think Power, Not Energy
Energy
Efficiency and Renewable Energy Wind and Water
In the ongoing evolution of both energy policy and power
systems operation, there are objectives and constraints-just Power Technologies Office and the SunShot Initiative builds
as there must be for all problems. In broad terms, policy on the preceding studies. It looked at the dynamic behavior
objectives such as "reduce carbon footprint," "increase fuel of the system at specific instants of time under energy-based
independence," or "grow renewables" are all energy based- scenarios with even higher levels of annual wind and solar
typically met or missed in terms of annual metrics such as energy penetration than are currently under consideration
MMBTU/year, MWh/year, or, often most important for through standard planning processes in the West.
wind and solar, percent of annual energy derived from wind
and solar, aka energy penetration.
A Credible Build-Out Scenario
By comparison, power systems operational constraints, In the evaluation of transient stability, initial conditions
such as "maintain transient stability criteria," "meet fre- and modeling details and assumptions are critical. This
quency response obligation," and, fundamentally, "keep the is the analytical constraint that accompanies the broader
lights on" are all power based. That is to say, grid operators power versus energy tension. Industry practice for evaluatand planners need to keep the grid safe and secure at every ing transient stability relies on detailed simulations of a few
instant of time, not just on average over the year.
snapshots in time. In simple terms, planners select specific
The importance of this distinction is illustrated in the operating conditions, like the peak-load or a light-load hour,
study results presented in the balance of this article. The apply the load, synchronize and dispatch a subset of the
Western Interconnection is one of the more transient sta- available generating resources, solve a load flow, and then
bility-constrained of the five major North American grids. perturb the system. The behavior following the disturbances
Economics and policy are driving the development of wind dictates whether the performance is acceptable. And, then,
and PV solar across the Interconnection. Several studies, the industry practice extrapolates the findings for those
november/december 2015

ieee power & energy magazine

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

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