IEEE Power & Energy Magazine - March/April 2017 - 62

In future grids, the number of inverter-based devices
may be more than 50% of the rated power at any time,
thereby forming an inverter-dominated grid.
contributor, and with the reduction in costs for these technologies during the last five years, large-scale deployments
are happening around the world.
Countries such as Denmark, Ireland, and Germany have
aggressively installed VRE systems and are operating with
annual VRE penetrations of more than 20% at the national
level. The annual penetration level is used to describe how
much VRE is produced on an annual basis, but instantaneous
penetrations vary widely during the year to reach those levels
(see "VRE Penetration Levels"). The instantaneous penetration may be an even more important factor when considering
the actual stability of a power system at high VRE levels. For
example, Ireland currently limits its instantaneous penetration to 55%. Currently, VRE provides approximately 13% of
the annual energy in Europe and 5% in the United States, but
VRE technologies have seen rapid deployment during the
last five years. So what would a power system dominated by
these technologies look like?

Dealing with Variability and Uncertainty
Several studies have examined increasingly higher levels of
renewable penetrations in the United States. Figure 1(a) and
(b) shows potential future deployments from the National
Renewable Energy Laboratory's "Eastern Renewable Grid
Integration Study," which examined up to 30% of the VRE
deployed in the eastern United States; generators are color
coded: blue represents wind locations, and yellow represents
solar locations. The future scenario in Figure 1(b) shows the
highly distributed nature of potential future deployments
of VRE technologies across the Eastern Interconnection.
Reaching 100% renewable energy would have even more
VRE systems distributed and deployed across the grid.
Wind and solar power are different from most thermal
generators because they have variable and uncertain power
output determined by local weather conditions. Conventional generators, such as coal and gas plants, are considered

VRE Penetration Levels
*	 	Annual	 VRE	 penetration	 level	 =	 Fraction	 of	 annual	energy	(kWh)	met	by	VRE	
*	 	Instantaneous	 VRE	 penetration	 level	 =	 Fraction	
of	instantaneous	power	(kW)	met	by	VRE	at	any	
point	in	time

62

ieee power & energy magazine

dispatchable because they can more easily change their
power output (both up and down) to meet changes in load.
As the penetration of VRE increases within a system, many
factors require greater grid flexibility to accommodate the
changes in generation. PV power, in particular, has a natural
challenge associated with its diurnal cycle because it does
not produce any power during the night. This makes the
power output between individual PV generators very well
correlated, with large amounts of energy in relatively small
windows of time. This can lead to larger net load ramps than
might otherwise be seen in the evenings.
Wind energy also has a diurnal cycle, albeit one that is
less pronounced than that for PV power. In many locations
within the United States, there tends to be more wind energy
produced during nighttime hours than during daytime hours.
Wind power can also produce ramps in power output when
there are large changes in weather conditions across large
geographic areas. Generally, these ramps tend to occur during multiple hours when there is a sufficient amount of geographic diversity in the wind power resource. In addition,
because of the time coincidence of VRE, there can be times
when there is too much supply, and the curtailment of VRE
makes sense for economic or reliability reasons. An example of this is shown in Figure 2, which depicts the generation dispatch stack from the "Renewable Electricity Futures
Study" during a week of low load and 80% renewable energy
penetration. During times when both wind and solar power
have high output values, curtailing some of these generators is the most economic option; however, these curtailment
decisions rely on a complex set of variables, including the
flexibility of the remaining generation fleet.
Although there are a number of additional challenges
associated with large penetrations of renewable energy,
there are also many solutions that can provide the flexibility needed to handle these challenges. These include the
smoothing of overall VRE power output through sufficient
amounts of geographic diversity when siting the VRE generators. Another solution is an expansion of the transmission system to be able to move large amounts of power more
efficiently from regions where VRE generators are currently
producing to the areas where load is currently needed. The
increased effective geographic diversity of VRE generators and utilization of transmission resources can both be
accomplished through greater coordination among balancing authority areas and faster interchange intervals.
A technology that allows for the temporal shifting of
VRE is energy storage. Energy storage has value in the power
march/april 2017



Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - March/April 2017

IEEE Power & Energy Magazine - March/April 2017 - Cover1
IEEE Power & Energy Magazine - March/April 2017 - Cover2
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IEEE Power & Energy Magazine - March/April 2017 - Cover3
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