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22 ieee power & energy magazine november/december 2017
A
An indisputAble fAct cAnnot be 
rebutted. it is supported by theory and experi­
ence. over the past 25 years, wind and solar 
generation has undergone dramatic growth, 
resulting in a variety of experiences that model 
the integration of wind and solar into the plan­
ning and operation of modern electric power 
systems. in this article, we bring together ex ­
amples from europe, north America, and Aus­
tralia to identify five indisputable facts about 
planning and operating modern power systems. 
taken together, we hope these experiences 
can help build consensus among the engineer­
ing and public policy communities about the 
current state of wind and solar integration and 
also facilitate conversations about evolving 
future challenges.
Fact One: The Grid Can Handle 
More Renewable Generation 
Than Previously Believed
Modern power systems are more flexible than 
was previously thought possible. the first mod­
ern wind and solar power plants were built in 
the 1980s. their capacity factors were low, and 
their output was unpredictable. Most serious 
engineers considered these systems incapable 
of serving meaningful amounts of load with­
out massive batteries. only dreamers imagined 
100% instantaneous wind penetration levels 
such as those achieved today in denmark, south 
Australia, and portugal. even if renewables 
could work in remote locations, most 
operators and engineers had reserva­
tions about whether large regional 
systems could operate reliably with 
wind and solar at scale.
initial analyses focusing on the 
feasibility of integrating wind and 
solar generation technologies were 
not optimistic. one of the earliest as ­
sessments of wind power integration 
occurred in denmark in the early 1990s. At that 
time, the system operator of West denmark 
concluded that the power system could tolerate 
fewer than 300 MW of installed wind capac­
ity. today, that same system has experienced a 
Digital Object Identifier 10.1109/MPE.2017.2729079
Date of publication: 18 October 2017
It’s Indisputable
Five Facts 
About Planning 
and Operating 
Modern Power 
Systems
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november/december 2017 ieee power & energy magazine 23
tenfold increase in wind capacity, and denmark is planning to 
obtain 50% of its electricity from wind by 2020. 
california independent system operator’s (cAiso’s) initial 
2007 20% renewable portfolio standard integration study found 
that it was technically feasible but suggested significant increases 
in frequency regulation and load­following requirements and 
potential overgeneration with approximately 7–8 GW of wind 
and under 1 GW of solar. today, cAiso successfully integrates 
almost 5 GW of wind and more than 10 GW of new solar in 
addition to the more than 6 GW of behind­the­meter 
photovoltaic (pV) affecting the load shape. in 
2016, according to the california energy 
commission, the full california power 
system had approximately 24 GW of 
in­state renewable generation, represent­
ing all types and serving approximately 
27% of annual electricity consumption. 
table 1 shows recent records for wind 
and solar integration in north America, 
europe, and Australia.
the ability to integrate thousands of 
megawatts of generation from wind and solar 
has not been accomplished by accident. Along the way, 
system planners and grid operators have called on tools 
and techniques used to manage peak days, low demand, 
and daily variations in demand to unlock the flexibility of 
modern power systems. for example, previous experi­
ence using weather data to forecast load has been part of 
utilities’ unit commitment process for decades. this made it 
relatively easy for system operators to implement wind and 
solar forecasts into operations. by leveraging old tools in 
new ways, many systems have been able to accommodate 
penetration levels of more than 20% annually. 
in denmark, the key to successfully integrating wind and 
solar has been the use of interconnectors to neighboring coun­
tries. trading electricity with neighbors enables countries to 
more efficiently balance supply and demand and, in this case, 
to reach local penetration levels of more than 20% annually. 
in portugal and spain, the use of online information of all 
renewable power plants has complemented the somewhat lim­
ited use of interconnectors. When large interconnectors with 
neighboring systems are not available, new techniques and 
methods are being developed to reach instantaneous penetra­
tion levels of 75% asynchronous generation. the small island 
system of ireland regularly receives 40–60% of its generation 
from wind and is solving stability issues to allow 75% instant 
penetration levels of asynchronous generation. in south Aus­
tralia, the key has been subhourly, real­time electricity mar­
kets to dispatch the system and match supply and demand 
within 5­min time frames; the same is done in most parts 
of the united states.
fast­responding natural gas and hydropower 
plants’ demand response, renewables forecasting, 
and pumped storage have been used to efficiently 
and reliably add wind and solar to electric power 
systems at a scale that caught many in the industry 
By Aaron Bloom, Udi Helman, Hannele Holttinen, 
Kate Summers, Jordan Bakke, Gregory Brinkman, 
and Anthony Lopez
table 1. The wind and solar penetration level records for various regions.
Region Country
Instantaneous Penetration 
of Asynchronous Generation 
as a Percentage of Load
Annual Penetration of 
Asynchronous Generation 
as a Percentage of Load Peak Load (MW)
CAISO United States 49% (2017) 27% (2016) 46,232 (2016)
Denmark Denmark 140% (2015) 42% (2015) 6,000 (2013)
EirGrid Ireland 60% (2017) 22% (2016) 4,700 (2016)
Electric Reliability 
Council of Texas
United States 50% (2017) 15% (2016) 71,000 (2016)
MISO United States 22% (2016) 8% (2016) 120,700 (2016) 
Portugal Portugal 105% (2016) 23% (2015) 8,300 (2015)
South Australia Grid Australia 119% (2016) 35% (2016) 2,895 (2016)
Southwest Power Pool United States 52% (2017) 14% (2015) 50,083 (2016)
24 ieee power & energy magazine november/december 2017
by surprise. However, recent research in india suggests that 
even systems relying on coal­fired generation can extract sig­
nificant flexibility from the existing fleet and transmission sys­
tem to enable the integration of gigawatts of wind and solar 
generation. Variable renewables such as wind and solar are no 
longer being built only because of mandates; they are being 
built because they make economic sense and because opera­
tors agree that modest penetration levels of wind and solar, 
such as 20–30%, can be reliable, profitable, and affordable.
However, much progress to date has been a result of the 
inherent flexibility that was built into the grid decades ago. 
leaders in renewables integration are efficiently making use 
of the inherent flexibility in their systems and exploring new 
technologies and tools to reach ever increasing levels. At 
the point where renewables’ penetration levels fully utilize 
the grid’s inherent flexibility, subsequent variable renewable 
generation will face rapidly rising integration challenges due 
to excessive curtailment. using the existing bulk transmis­
sion network and building new capacity to enable sharing 
among regions can provide one solution to rising curtailment. 
Another approach is using energy storage that enables gen­
eration to be shifted to hours when it is needed most.
south Australia is one region that is pushing the limits of 
what was previously thought possible. this small system—
2,895­MW peak load—is interconnected to Australia’s much 
larger national electricity Market (neM) through a series of 
small interties. the system has experienced instantaneous pen­
etration levels of 100%, and it reached an annual penetration 
level of 35% in 2015–2016. in south Australia, the challenges 
of accommodatingvery high penetration levels of wind are 
spurring new emphasis on maintaining sufficient system inertia. 
the precision with which thermal generation follows dispatch 
signals in Australia has changed considerably in recent years, 
with numerous cases of generators blocking their governors 
to prevent an automatic generation control (AGc) signal from 
being followed. this has resulted in a documented decrease in 
frequency control. figure 1 shows the impact of synchronous 
generation (scheduled) on system frequency compared to asyn­
chronous generation (semischeduled and nonscheduled).
the documented change in thermal power plant perfor­
mance in neM is largely driven by market design features 
that value following 5­min dispatch more than responding 
to frequency deviations. the declining frequency response 
of neM is driven by the frequency control market and 
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NEM Frequency Stabilization FrequencyFrequency Standards
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figure 1. System frequency and generator schedules in NEM.
november/december 2017 ieee power & energy magazine 25
compounded by the rise of wind and solar generation. com­
petitive pressures from wind and solar generation are push­
ing many thermal plants to retire, thereby further reducing 
the frequency response of the system. these challenges 
are most evident in the contributing activities preceding a 
blackout that occurred in south Australia during an extreme 
weather event in september 2016; they also highlight the 
need for reform on frequency control arrangements. 
initial responses to the south Australia blackout indicate 
that a variety of capabilities are available for natural gas gen­
eration and renewable generators that could have been used to 
arrest or minimize the cascading failure experienced during 
the storm, which included tornados and caused fallen trans­
mission towers. the key issue facing Australia’s regulators 
now is figuring out how to value resources that provide ser­
vices contributing to grid reliability when penetration levels 
are more than 75% of demand.
Fact Two: Geographic and 
Resource Diversity Provide 
Additional Reliability to the System
At high penetration, renewable resources need to contribute signifi­
cantly to the reliability of power systems. if power system planners 
and regulators want to mitigate all risk of loss of load, the solution 
is relatively straightforward: build 
lots of redundant generation and 
transmission. the problem with this 
approach is that it is very expensive; 
in addition, because electricity is a 
fundamental input for the modern 
economy, excessive electricity costs 
that result from efforts to mitigate 
all risk are financially challenging. 
one generally successful approach 
for reducing the costs of manag­
ing power system risks is to share 
capacity and reserves with neigh­
boring regions. this approach dates 
to when the first interconnections 
were created in the northeastern 
united states and to the creation of 
the first power pools in the united 
states in the 1940s.
Another risk faced by the in ­
dustry relates to the overdepen­
dence on a single fuel type. A dra­
matic example of this challenge 
arose during the energy crisis of the 1970s in the united states. 
At that time, the concern was that international disputes regard­
ing oil and gas could negatively impact the electricity, indus­
trial, and transportation sectors. As a result, the u.s. congress 
passed the power plant and industrial fuel use Act of 1978 to 
reduce power plant reliance on international fuel sources. then, 
as now, resource diversity was considered a key strategy to miti­
gate risk and manage costs. these strategies continue to show 
value for modern power systems.
under current system conditions, wind and solar generation 
increases resource diversity. they reduce reliance on fossil­fuel 
resources and can provide alternative sources of generation 
when hydropower systems are faced with extreme conditions; 
however, they are subject to their own diversity challenges. 
compared to traditional resources, for which weather chal­
lenges (e.g., hurricanes, flooding, and extreme cold) are infre­
quent, wind and solar resources that are concentrated in one 
area experience a variety of seasonal, diurnal, and weather­
related challenges.
spatial variability of solar and wind resources differs by 
region and can impact the geographic distribution required 
to achieve geographic diversity within a system. spatial vari­
ability is influenced by local and regional climates as well 
as local terrain complexity. figure  2 illustrates this point 
We bring together ex amples from Europe, North America, 
and Australia to identify five indisputable facts about 
planning and operating modern power systems.
Jaisalmer
Chennai
DNI Correlation Coefficient
200 km
0.68–0.75
0.75–0.8
0.8–0.85
0.85–0.9
0.9–0.95
0.95–1
figure 2. The spatial variability of DNI in India. 
26 ieee power & energy magazine november/december 2017
by comparing the spatial variability of direct normal irradi­
ance (dni) for two regions in india. in both circled images, 
the center­point daytime hourly dni was correlated to each 
neighboring pixel’s daytime hourly dni out to 200 km (over 
many years). the top left of the figure shows Jaisalmer, a city 
in the arid desert region of Rajasthan, india. At the bottom 
right is chennai, a city off the coast of the bay of bengal in 
tamil nadu, india. As the illustration shows, the dni in Jais­
almer is highly correlated and homogeneous out to 200 km; 
in chennai, on the other hand, the correlation coefficient 
drops rapidly when moving inland, away from the bay—and 
it declines more slowly when moving north and south along 
the coastlines.
Geographic diversity in variable renewables decreases 
this risk. spreading resources among large regions mitigates 
the impact of weather events. for wind and solar resources, 
spreading the capacity among multiple areas decreases the 
impact of any one meteorological event, such as a weather 
front or a cloud. for wind, the benefits of transmission are 
large; for solar, temporal diversity can be increased by spread­
ing resources among lines of longitude. As the earth rotates, 
the irradiance for a given location changes—for example, 
solar noon in new York city happens before it occurs in buf­
falo, new York. if solar resources are spread among time 
zones, the peak in solar generation broadens, reducing the 
impacts of sunrise and sunset in addition to those caused by 
weather events.
experience with wind and solar confirms these theoreti­
cal conclusions. in the united states, most regional inde­
pendent system operators (isos) are experiencing rapidly 
increasing wind and solar penetration. each has a unique 
geography and resource mix. the Midcontinent system 
operator (Miso) manages one of the world’s largest 
energy and operating reserves markets. it ensures reliable 
delivery of electricity at the lowest cost across high­volt­
age power lines in the midcontinental region of the united 
states and canada. 
in 2016, Miso’s annual wind energy penetration level 
was 8%, and it experienced a record instantaneous penetra­
tion level of more than 22% of load served (12.5 GW) on 
13 november 2016 at 4 a.m. Miso’s wind peak output was 
13.6 GW, which occurred on 7 december 2016 at 11 p.m. 
Miso’s large geographic scope enables it to balance large 
amountsof wind generation with a diverse resource port­
folio, even though most of the wind is located in Miso’s 
northern region. this region includes most of the load for 
Minnesota, iowa, north dakota, south dakota, Wisconsin, 
Michigan, and Manitoba. the Miso north region experi­
enced an annual wind energy penetration level of 26.8% in 
2016 with a maximum penetration of 80% of load served. As 
wind penetration increases, Miso has also been pioneering 
more sophisticated methods of counting wind’s contribution 
to resource adequacy. these simulations include a look­
ahead to higher penetrations over time.
Fact Three: Wind and Solar Forecasting 
Provide Significant Value
Wind and solar generation can be predicted with some accu­
racy. diurnal patterns such as sunrise and sunset can be cal­
culated precisely at every point on the globe, and large­scale 
weather systems can be monitored as they cross oceans 
and continents. these observable facts enable meteorolo­
gists to work with power system planners and operators to 
 anticipate, prepare for, and mitigate a variety of weather­
driven events that impact the generation of wind and solar 
power plants.
Weather forecasting is not new. the predecessor to the u.s. 
national oceanic and Atmospheric Administration, the u.s. 
Weather bureau, was created in 1870 by president ulysses s. 
Grant to assist the military in anticipating storms that could 
impact military activities and commercial operations on the 
nation’s waterways. the benefits of weather forecasting were 
later realized by broader economic needs for agriculture and 
transportation. no farmer ignores the weekly weather fore­
cast, and many people look at the radar when their flight is 
delayed. in the electricity industry as in the agriculture and 
airline industries, weather forecasts—specifically, composite 
wind, solar, and load forecasts—are critical to minimize risk 
and maximize efficiency.
in modern power systems, weather forecasts become 
increasingly important. Grid operators have been using 
weather forecasts to anticipate challenging load conditions 
for decades. early research in power systems operations with 
wind energy showed that power forecasts can provide signifi­
cant value in their simplest form. for example, if high winds 
are anticipated for a period of time, thermal generation can 
reduce output, thereby reducing fuel costs to the system. More 
sophisticated forecasts that leverage machine learning are 
becoming increasingly popular as one method to reduce 
system costs. 
the unit commitment decision that power system planners 
face is complex. failure to commit sufficient resources to 
meet expected conditions can result in operator actions that 
may be expensive—e.g., starting a combustion turbine in real 
Spatial variability of solar and wind resources differs 
by region and can impact the geographic distribution 
required to achieve geographic diversity within a system.
november/december 2017 ieee power & energy magazine 27
time. this places a significant value on improving wind and 
solar power forecasting in the day­ahead unit commitment 
process—although the industry is also pursuing advances in 
short­term intraday and intrahour forecasting.
solar power forecasting follows one of two approaches: 
1) pV performance models aim to use irradiance forecasts 
to determine the performance of solar generation facili­
ties, and 2) statistical models use artificial neural networks, 
regressions, and other statistical methods to predict future 
behavior based on historical data. Hybrid models are being 
developed that incorporate irradiance forecasts into neural 
networks. in all cases, forecasts can be created for individual 
power plants or for an ensemble of plants based on the pur­
pose of the forecast. 
A forecast tuned for an individual plant might be designed 
to maximize market value, whereas a regional forecast used 
by a system operator might be designed to minimize risk. Many 
early forecasting programs focused on creating determinis­
tic predictions for power output, providing a single value 
for a moment in time; however, more recent advances use 
probabilistic methods to incorporate information such as 
the upper and lower bounds of possible forecasts and/or con­
fidence intervals around a particular forecast value. increas­
ingly, forecasting is being used not only to schedule thermal 
and hydropower generation but also to optimize energy stor­
age assets.
Miso has been using weather­based load forecasting since 
the launch of its energy market in 2005. in 2009, with wind 
penetration increasing, Miso began implementing weather­
based wind forecasts into all aspects of markets and opera­
tions, which is one of many factors that enabled penetration 
levels to exceed 25% of annual energy in 2016 for Miso’s 
northern region. building on the success of wind forecasting, 
Miso started forecasting solar in 2016. 
Miso’s wind and solar forecasts are a blend of five 
forecasting methods designed to accurately predict gen­
eration throughout various time scales and levels of 
fidelity. Maximum economic potential is obtained by look­
ing at the feasible weather­based output of wind and solar 
power plants along with transmission limitations to gain 
a realistic understanding of future wind and solar output 
throughout Miso. in 2010, Miso designed and imple­
mented a market mechanism to take advantage of advances 
in wind technology that make the concept of nondispatch­
ability less applicable. the introduction of dis patchable 
intermittent resources (diRs) allows such resources to 
fully participate in the energy markets and has resulted 
in more economic and reliable grid operations. cur­
rently, 85% of the wind resources in Miso are regis­
tered as diRs.
Fact Four: While Our Electric Power 
Markets Were Not Originally 
Designed for Variable Renewables, 
They Can Be Adapted
in the 1990s, as capacity margins and natural gas prices 
increased, electricity regulators, economists, and engineers 
began to implement new ways to reduce electric power costs. 
large portions of electricity systems in the united states and 
europe, along with those in several other countries, were sub­
jected to regulatory reforms that created competitive whole­
sale markets. transmission owners gave operational control 
of their assets to isos, which established centralized mar­
kets for energy, ancillary services, and capacity that aimed to 
minimize the cost of wholesale power in serving load while 
meeting the complicated operating and reliability require­
ments of power systems within different time frames (from 
seconds to years). in some regions, retail competition was 
also introduced and sustained.
the iso markets—which vary dramatically in geographic 
size (among many different countries), from one state or 
province to very large multistate regions and entire nations—
have generally been effective at reflecting changes in the 
fixed and variable costs of power supply and the resulting 
market prices while minimizing market power. As a result, 
market prices fluctuate, sometimes significantly, but usually 
within regulated limits. Although the transparency of mar­
ket rules and price results have been an ongoing issue, there 
is no question that market participants, including renewable 
energy suppliers, have much more information about how 
they are valued within a market than before these institu­
tional changes were implemented. in principle, the markets 
are technology agnostic, but they need to adapt to public 
policies as well.
these iso markets have been very effective at integrat­
ing a variety of resources and have also been flexible enough 
to handle dramatic changes in the composition of the gen­
eration fleet as well as extreme events such as frigid winter 
conditions, hurricanes, and drought. they have an excellent 
reliability record. However, the addition of wind and solar 
to the system, combined with the shale gas revolution in the 
unitedstates and overcapacity in some regions of the united 
states and europe, has—with a few exceptions—suppressed 
electricity prices for several years.
Increasingly, forecasting is being used not only to schedule 
thermal and hydropower generation but also to 
optimize energy storage assets.
28 ieee power & energy magazine november/december 2017
in some markets, the impact of renewables is very clear. 
for example, as shown in figure 3, by May 2017 the more 
than 9 GW of grid­connected solar production in cAiso—
combined with more than 6 GW of solar behind­the­meter 
and high hydropower conditions—had pushed average day­
ahead energy prices during solar production hours down to 
nearly zero (the prices were actually negative during many 
hours, but on average they remained above zero). Although 
low electricity prices may be good for wholesale buyers, 
such prices make many existing generation plants unprofit­
able in the short run, resulting in their planned or threat­
ened retirement. 
Herein lies a key challenge for renewable expansion in 
competitive markets. Weather­driven energy sources exhibit 
variability among multiple time scales. their output can 
change based on diurnal patterns and seasonal trends and as 
a result of larger global weather events, such as the el niño 
southern oscillation. the challenges become how to 1) design 
market rules and operating schemes that take advantage of 
wind and solar when they are available, while ensuring suffi­
cient operational flexibility from all resources, and 2) ensure 
that adequate capacity is available during times when the sun 
does not shine and the wind does not blow.
the potential solutions to these challenges are likely to 
lead to new types of market arrangements (as already evi­
dent in several regions), but starting with adaptations of 
existing wholesale market designs. transparent prices for 
the range of power services are still the best guide to what 
is happening on the grid. the degree of regulatory and mar­
ket modifications will be influenced by the timing of the 
changes in power system operations and markets—i.e., these 
systems were originally intended to respond to incremental 
changes in the overall stock of infrastructure, but renewable 
expansion is happening extremely quickly in some regions 
and so requires a rapid market design response.
A key question in adapting the wholesale markets is 
whether the full set of market products is sufficient to facili­
tate the relatively efficient entry and exit of resources while 
also ensuring reliability (recognizing the fact that, depend­
ing on the region, renewable resources are entering markets 
with financial incentives or procurement requirements dif­
ferent from those of conventional resources). the metric is 
revenue sufficiency: if one source of market value declines 
but the resource is needed for operations and/or reliability, 
then another revenue source must be available. Hence, as 
wholesale energy loses value for the reasons described pre­
viously, markets for capacity, ancillary services, and other 
reliability services can help pick up the slack.
of these, the most important will be capacity pay­
ments, which (after energy) constitute the largest source of 
resource revenue in most u.s. iso markets (texas being 
the exception). capacity markets have proved difficult to 
design: challenges include 1) how to pay for the needed 
capacity but still value some surplus and 2) how to limit 
capacity payments to only the amount needed, in addition 
to other energy market revenues, while keeping the gen­
erators available and operable. However, capacity markets 
have attracted new types of resources—notably, demand 
response—and their role as a backstop for keeping exist­
ing conventional generation revenue sufficient is likely to 
become more important.
several factors now considered in only a limited fashion 
by most wholesale market designs will have to be improved 
to make this transition work. first, renewable resources them­
selves will have to be accurately rated for their contribution 
to resource adequacy. iso market operators have only a few 
years of experience with the simulations needed to calculate 
wind and solar effective load­carrying capability; most still 
rely on simpler approximations. if a wide range of possible 
capacity contributions arises as a function of weather, then, as 
renewable capacity expands and more is counted toward capac­
ity requirements, longer­term capacity mechanisms might be 
required to provide payments to conventional resources (which 
are needed despite declining energy market value). Most iso 
capacity markets are one year or three years ahead. Are longer­
term arrangements needed, based on renewable energy pen­
etration forecasts? in addition, in markets such as california, 
capacity requirements and value are also being affected by 
behind­the­meter solar, over which there is currently little con­
trol or production forecasting.
A key question in adapting the wholesale markets is whether 
the full set of market products is sufficient to facilitate the relatively 
efficient entry and exit of resources while also ensuring reliability.
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figure 3. CAISO’s Southern California Edison load 
 aggregation point prices (hourly averages), January–April 2017.
november/december 2017 ieee power & energy magazine 29
second, the right types of generation must be provided 
with capacity payments. Generators that are flexible will be 
increasingly valuable in modern power systems—especially, 
generators that can handle part­load and part­time opera­
tion. A shift is occurring in markets that have high shares of 
renewables, where less generation is operated as base load 
and more is needed to cycle at faster ramp rates. At the same 
time, with the right incentives, this might not require a major 
increase in new, expensive peaking plants; some currently 
base­loaded or start­limited generators could be upgraded 
to provide more flexibility. cAiso has introduced a forward 
“flexible capacity” product to create a ramping requirement 
for some quantity of resource adequacy capacity; other isos 
have preferred to leave the capacity product denominated 
only in megawatts and rely on the energy and ancillary ser­
vice markets to reward flexibility. time will tell which of 
these approaches is better when faced with rapid changes in 
the resource mix.
note that not all markets with high renewables penetra­
tion levels have capacity markets. for example, in the united 
states, texas runs an energy­only wholesale market that 
relies on volatile energy prices—from negative prices to 
us$9,000/MWh during times of scarcity—thus providing 
income to the generators that are used during only a small 
part of the year. High price volatility also incentivizes the 
need for demand response and storage; however, it remains 
to be determined how far energy­only markets can go to sup­
port investments, as renewables potentially suppress prices 
during both current peak and off­peak periods.
third, related to our observations about operational flex­
ibility, there is some expectation that the demand and prices 
for ancillary services will grow, as forecast uncertainty 
increases because of more generation from wind and solar. 
Revenues from these services could partially offset the loss 
of energy revenues and limit the need to rely on capacity 
payments. new ancillary products and market mechanisms 
are needed, and several of these are currently being devel­
oped to help integrate renewables. for example, in several 
u.s. regions, ramping reserves have recently been imple­
mented, reflecting the impact of wind and solar on real­time 
operations; however, as of now, these product markets are 
small, and the amount of extra income they provide to gen­
erators will be affected by their design.
Moreresearch and analysis are required to understand 
potential solutions and help the market provide the appropri­
ate incentives to keep the grid reliable. the problem is one of 
economics, so that generation not being used can be retired 
as soon as possible—but not so early that doing so leaves the 
power system vulnerable to unforeseen constraints on renew­
able energy production and potential reliability problems.
Fact Five: Modern Power Electronics 
Are Creating New Sources of Essential 
Reliability Services
Modern power systems have more options for providing essen­
tial reliability services than ever before. traditionally, fos­
sil­fueled and hydropower resources were considered the 
sole sources of services necessary to balance the system and 
maintain transmission network security; however, advanced 
inverter technologies coupled with modern communications 
networks are adding new sources of grid f lexibility and 
responsiveness to the market. these capabilities enable wind, 
demand, solar, and storage to offer a full suite of essential reli­
ability services. planners and regulators face the challenge of 
determining how to enable these resources to come to market 
and contribute to grid reliability, resiliency, and security.
one requirement of power systems is to maintain electri­
cal frequency within a safe range. frequency response is an 
essential component of grid reliability. it measures an inter­
connection’s ability to stabilize the frequency immediately 
following the sudden loss of generation or load. A variety of 
terms are used to describe the services procured to improve 
a system’s frequency response. in the united states, the 
most common is primary frequency response, although pri-
mary control and frequency responsive reserve are also used 
in referring to the automatic, local response to frequency 
excursions through turbine speed governors as well as fre­
quency responsive demand that adjusts to counter­frequency 
deviations and so stabilizes the system. Another aspect of 
the power system that contributes to system­wide stability is 
system inertia, the aggregate inertia of load and synchronous 
generation that injects or extracts stored kinetic energy from 
the rotating mass of a machine. the combined impact of sys­
tem inertia and primary frequency response is essential to bal­
ancing system frequency.
Recent studies in the united states and Australia have 
shown that the frequency response of various interconnected 
systems is declining. this decline results from a variety of 
physical reasons, including generators that operate in modes 
that do not offer primary frequency response, excessive 
governor dead bands, and blocked governors. other reasons 
might be institutional or caused by inappropriate incentives 
in electricity markets. finally, the addition of asynchronous 
generation such as wind and solar can reduce the amount 
Futures with 50–80% penetration levels of renewables are being 
studied in detail by system operators and researchers, and, in many 
areas, legislated requirements already exist for these levels.
30 ieee power & energy magazine november/december 2017
of system inertia because these resources do not generate 
electricity in the same manner as synchronous machines.
However, recent analysis also indicates that appropriately 
equipped wind and solar resources can contribute to the 
stability of interconnected power systems. simulations con­
ducted with General electric’s positive sequence load flow 
dynamic simulation software have shown that wind and solar 
resources with commercially available active power controls 
could be used to contribute to frequency response. A pio­
neering applied demonstration project performed jointly by 
cAiso, first solar, and the national Renewable energy lab­
oratory evaluated a 300­MW utility­scale solar pV plant’s 
ability to contribute to grid reliability. that work shows that 
properly equipped utility­scale solar generation can provide 
active and reactive power controls (including participation 
in AGc), primary frequency control, ramp­rate control, and 
voltage regulation. 
figure 4 shows the response of a first solar facility to 
AGc signals provided by cAiso in August 2016. compari­
sons between the 300­MW utility­scale solar pV plant and 
conventional technologies showed that the accuracy with 
which solar pV follows AGc was significantly faster than 
that of gas turbine technologies. the regulation response rate 
by the pV plant observed in the 2016 tests outperformed con­
ventional technologies such as combined­cycle, gas turbine, 
and hydropower generation by 24–30 points.
Conclusions
As a mainstream source of electricity production, wind 
and solar generation is growing considerably around the 
world and can be used to help reliably and economically 
meet the electricity needs of modern economies. Modest 
penetration levels of wind and solar (20–30% of annual 
demand) appear to be well within the capabilities of cur­
rent technologies and have already been demonstrated in 
various diverse power systems (e.g., california, ireland, and 
denmark). How far these resources can go toward meeting 
the world’s seemingly insatiable need for energy remains 
to be seen. one thing, however, is clear: policy makers and 
engineers are setting ever higher goals for wind, solar, and 
modern power systems. futures with 50–80% penetration 
levels of renewables are being studied in detail by system 
operators and researchers, and, in many areas, legislated 
requirements already exist for these levels. even futures 
with nearly 100% of all electricity generated by renewable 
resources dominated by wind and solar pV are garnering 
attention worldwide and will likely be subject to detailed 
operational study soon.
For Further Reading
d. palchak, J. cochran, A. ehlen, b. Mcbennett, M. Mil­
ligan, i. chernyakhovskiy, R. deshmukh, n. Abhyankar, 
s. K. soonee, s.R. narasimhan, and M. Joshi. (2017). 
Greening the grid: pathways to integrate 160 gigawatts of 
wind and solar energy into india’s electric grid. nRel/
tp­6A20­68530. Golden, co: national Renewable energy 
laboratory. [online]. Available: http://www.nrel.gov/docs/
fy17osti/68530.pdf
J. Antonanzas, n. osorio, R. escobar, R. urraca, f. J. Mar­
tinez­de­pison, and f. Antonanzas­torres. Review of photo­
voltaic power forecasting. Solar Energy, vol. 136, pp. 78–111, 
2016. [online]. Available: http://dx.doi.org/10.1016/j.solener 
.2016.06.069
c. loutan, p. Klauer, s. chowdhury, s. Hall, M. Morja­
ria, V. chadliev, n. Milam, c. Milan, and V. Gevorgian, 2017. 
“demonstration of essential reliability services by a 300­MW 
solar photovoltaic power plant,” nRel/tp­5d00­67799. 
Golden, co: national Renewable energy laboratory. 
[online]. Available: http://www.nrel.gov/docs/fy17osti/ 
67799.pdf 
epRi. (2016). Wholesale electricity market design initia­
tives in the united states: survey and research needs. epRi, 
palo Alto, cA. [online]. Available: https://www.epri.com 
/#/pages/product/000000003002009273/ 
[online]. Available: http://www.aemo.com.au/­/media/ 
files/electricity/neM/planning_and_forecasting/sA_Ad 
visory/2016/2016_sAeR.pdf 
Biographies
Aaron Bloom is with the national Renewable energy labora­
tory, Golden, colorado.
Udi Helman is with Helman Analytics, san francisco, 
california.
Hannele Holttinen is with the Vtt technical Research 
centre of finland, espoo.
Kate Summers is with pacific Hydro, Melbourne, Australia.
Jordan Bakke is with the Midcontinent independent system 
operator, eagan, Minnesota.
Gregory Brinkman is with the national Renewable energy 
laboratory, Golden, colorado.
Anthony Lopez is with the national Renewable energy lab­
oratory, Golden, colorado.
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figure 4. The results of a midday AGC test using a First 
Solar300-MW PV plant in the CAISO service area.