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Contract No. DE-AC36-08GO28308

Wind and Solar Curtailment

Preprint

Debra Lew,

1 Lori Bird,1 Michael Milligan,1 Bethany Speer,1

 

Xi Wang,

1 Enrico Maria Carlini,2 Ana Estanqueiro,3 Damian Flynn,4

Emilio Gomez-Lazaro,

5 Nickie Menemenlis,6 Antje Orths,7

 

Ivan Pineda,

8 J. Charles Smith,9 Lennart Soder,10 Poul Sorensen,11

Argyrios Altiparmakis,

11 and Yasuda Yoh12

1

National Renewable Energy Laboratory

2

Terna Rete Italia

3

LNEG

4

University College Dublin

5

University of Castilla-La Mancha

6

Hydro Quebec

7

Energinet.dk

8

European Wind Energy Association

9

UVIG

10KTH

11Technical University of Denmark

12Kansai University

To be presented at the International Workshop on Large-Scale 

Integration of Wind Power Into Power Systems as Well as on 

Transmission Networks for Offshore Wind Power Plants

London, England

October 22 – 24, 2013

Conference Paper

NREL/CP-5500-60245

September 2013
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Wind and Solar Curtailment 

Debra Lew, Lori Bird, Michael Milligan, 

Bethany Speer, Xi Wang

National Renewable Energy Laboratory, Colorado, USA

Enrico Maria Carlini 

Terna Rete Italia, Rome, Italy

Ana Estanqueiro

LNEG, Lisbon, Portugal

Damian Flynn

University College Dublin, Dublin, Ireland

Emilio Gomez-Lazaro

University of Castilla-La Mancha, Spain

Hannele Holttinen

VTT, Finland

Nickie Menemenlis

Hydro Quebec, Quebec, Canada

Antje Orths

Energinet.dk, Fredericia, Denmark

J. Charles Smith

UVIG, North Carolina, USA

Lennart Soder

KTH, Stockholm, Sweden

Poul Sorensen, Argyrios Altiparmakis

Technical University of Denmark, Denmark

Yasuda Yoh

Kansai University, Osaka, Japan

Abstract—High penetrations of wind and solar generation on power 

systems are resulting in increasing curtailment. Wind and solar 

integration studies predict increased curtailment as penetration 

levels grow. This paper examines experiences with curtailment on 

bulk power systems internationally. It discusses how much 

curtailment is occurring, how it is occurring, why it is occurring, 

and what is being done to reduce curtailment. This summary is 

produced as part of the International Energy Agency Wind Task 25 

on Design and Operation of Power Systems with Large Amounts of 

Wind Power.

Keywords-wind; solar; curtailment; transmission congestion

I. INTRODUCTION

In many regions, wind and solar generation are preferred 

instead of conventional generation because of their emissions 

benefits; policy, legislation, and/or incentives may be established 

to encourage the use of wind and solar instead of conventional 

generation. Additionally, wind and solar have zero or very low 

marginal costs, which means that as long as a system operates 

within transmission and operating constraints, wind and solar 

tend to displace conventional generation. However, increasing

wind and solar penetration levels may drive a system to

encounter transmission or operational constraints, forcing the 

system operator to accept less wind or solar than is available. 

High levels of wind and solar power can be challenging to 

integrate into power systems because of their variability and 

limits in predictability. When high levels are planned, 

infrastructural, operational, or institutional changes to the grid

may be necessary. During this transition phase, curtailment may

be higher than after the changes are made. We use the term 

curtailment broadly to refer to the use of less wind or solar power

than is potentially available at that time. 

There are many reasons for curtailment, and system operators 

may distinguish between these reasons for compensation and 

accounting purposes. The main reasons for wind and solar 

curtailment are listed below.

Transmission congestion, or local network constraints, is a 

common reason for system operators to utilize higher marginalpriced resources instead of less expensive resources. Related to 

congestion is insufficient transmission availability. Because of 

the mismatch in construction times, wind power plants may be 

built somewhat in advance of the necessary transmission to 

transport those energy resources to load centers. These new wind 

power plants may be curtailed until transmission infrastructure is 

commissioned.

Minimum operating levels on thermal generators are another 

driver for curtailment. Wind, in particular, is often stronger at 

night, when loads are low and thermal units are pushed down 

against their minimum operating constraints. A related issue is 

the requirement for downward reserves. If wind and solar are

unable to provide downward reserves, then sufficient downward 

capability may need to be held on thermal units, raising their 

operating levels. 

Hydro plants may also have minimum operating levels

because of environmental, recreational, or irrigation constraints. 

For example, to comply with limits on dissolved gases to protect 

fish, operators may be required to run water through their 

turbines rather than spill water over a dam.

Curtailment can also occur in the distribution system to avoid 

high penetrations or back-feeding, in which a feeder produces 

more energy than it consumes, of distributed generation on 

feeders, which can lead to voltage control issues as a result of

variability of the wind or solar resource. Back-feeding can be 

problematic if protection devices and other infrastructure were

not designed or are not yet adapted for this type of operation. 

Finally, limits may be placed on nonsynchronous generation 

levels to maintain frequency requirements and stability issues, 
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especially on small, isolated grids. Modern wind and solar power 

plants interconnect to the grid through power electronics. 

Because they displace conventional synchronous generation, 

which provides inertia and may provide governor response, 

system frequency response might suffer if a contingency event

occurs when there is a high penetration of nonsynchronous 

generation. 

II. EXPERIENCES WITH CURTAILMENT

A. Canada

Hydro Quebec (HQ) currently has 1,500 MW of installed 

wind power capacity (5% energy penetration) and expects to have 

3,000 MW of installed capacity (10% energy penetration) by 

2016. TransÉnergie, the transmission system operator (TSO), 

assumes responsibility for system security. The distributor 

purchases power from wind developers, estimates the cost of lost 

energy using wind resource data, and compensates the wind plant

operator. 

To date, curtailments have been caused by technical faults, 

and neither voluntary nor mandatory losses have occurred. 

However, additional installed wind capacity is planned for the 

Gaspésie Peninsula and will likely result in curtailment caused by

congestion. It may also result in increased voluntary curtailment 

to ensure system stability. 

Given the current projection for 2016 wind penetration levels, 

HQ does not anticipate significant curtailments in the near future. 

For similar reasons, HQ has not yet considered valuing curtailed 

generation as a source of system operating reserve or strategized 

how to reduce future losses. 

B. Denmark

At the end of 2012, 4,443 MW of wind power was installed in 

Denmark (3,280 MW onshore, 1,163 MW offshore), mostly in 

the Western Danish system. These installations produced 

10.3 TWh electricity (6.8 TWh onshore, 3.5 TWh offshore), 

which is a 30.1 % share of Danish electricity consumption (peak 

load in 2012 was 6,106 MW). According to Danish Energy 

Policy, wind capacity is expected to increase by 44% by the year

2022, leading to 20.5 TWh annual production. Policy targets 

include a 30% renewables share of Danish energy consumption in 

2020 (heat and electricity), corresponding to 52% of 2020 

electricity consumption sourced from wind. 

Wind integration on this scale is supported by strong 

interconnections to neighboring systems and functioning 

international electricity markets, including negative price signals 

to incentivize wind to dispatch down. All wind power is traded in 

the markets (day-ahead and intraday power markets), either by 

production balancing actors or by the Danish TSO. Wind owners 

are compensated at slightly more than the market price. 

Compensation for large offshore wind plants is negotiated during 

the tendering procedure. 

In 2012, there were 405 hours during which wind production 

exceeded consumption in the Western Danish system and none in 

the Eastern Danish system. During these hours, the high-voltage 

direct current (HVDC) connections between the Danish systems 

and the hydro-based systems in Norway and Sweden are valuable 

because wind power can be sent to where it can be used. 

An unusual situation occurred on December 25, 2012: spot 

prices in Denmark and Germany were negative because of

surplus energy in both systems. (See Fig. 1.) However, German 

prices were below Danish ones, causing an import to Denmark 

while Danish producers’ bids had been curtailed to balance 

demand and supply at the minimum price floor of -200 €/MWh. 

(See Fig. 1.) During these hours, consumption was very low, 

market actors with heating boilers were not paying enough 

attention to the market and hence did not activate their boilers, 

and interconnection capacity to Sweden was reduced to limited

export possibilities. Most importantly, wind in both Denmark and 

Germany was high, and total Danish central thermal production 

was at a low level of 630 MWh/h to 900 MWh/h during the 

critical hours, but electricity was imported from Germany despite 

negative prices. Among the bids, which were curtailed pro rata, 

may have been those from wind.

Figure 1. Spot prices in Denmark and Germany on December 25, 2012.

Figure 2. Exchange between Denmark and Germany on December 25, 2012. 

To avoid this situation in the future, the following

improvements have been recommended. Market actors should 

increase their focus on risk for extreme prices and be prepared to 

act accordingly. Offshore wind power plants should not be 

compensated by fixed feed-in tariffs during periods of negative 

prices. Import capacity to Germany should either be reduced 

during these situations, or the negative prices should be 

harmonized at both sides of the border. The process toward 

harmonization of prices will probably include a reduction of the 

current minimum price floor in DK from -200 €/MWh 

to -500 €/MWh, because this will increase the probability of the 

market being balanced by price rather than by curtailment in 

surplus supply situations. The spot market should be reopened for 
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a second auction in case prices appear to be out of a predefined 

interval. (This is treated differently at both power exchanges.) 

C. Ireland

In 2012, Ireland, with an installed wind capacity of 2.1 GW,

dispatched down 110 GWh (2.1% of available wind energy). 

Approximately 80% of this was caused by systemwide reasons, 

whereas the remainder was a result of local network constraints. 

(See Fig. 3.) 

Typically, curtailment occurs during periods of low demand, 

most often from 23:00 at night to 09:00 in the morning, when the 

minimum generation levels of conventional plants are imposed. 

Five types of security limits have been defined that could 

necessitate curtailment, including system stability (synchronous 

inertia, dynamic and transient stability); operating reserve; 

voltage control; morning load rise; and, exceedance of the system 

nonsynchronous penetration (SNSP) limit. The SNSP limit, 

defined as the ratio of nonsynchronous generation (wind and 

HVDC imports) to demand plus HVDC exports, follows from the 

Facilitation of Renewables studies [1], which proposed an upper 

limit to this ratio (currently 50%) for system stability reasons.

Figure 3. Dispatch-down of wind in Ireland in 2012 [2]. 

In contrast, local network constraints may occur throughout 

the day and are typically imposed because of network limitations, 

with the northwest and southwest part of the transmission system 

being most affected. Other parts of the network have also 

experienced constraints primarily because of maintenance 

outages. 

A number of grid rollout programs are in place to minimize 

constraint levels. Similarly, the Delivering A Secure Sustainable 

Electricity System Program [2] proposes various plant 

performance monitoring and new ancillary services, including a 

synchronous inertial response, fast frequency response, and fast 

post-fault active power recovery, which will enable the SNSP 

limit to be raised from 50% to 75%, which is estimated to reduce 

the level of curtailment from more than 15% to approximately 

6%. As part of grid code requirements, wind turbines have the 

capability to provide reserve when dispatched down; however, 

this capability is not currently being utilized. 

D. Italy

In Italy at the end of 2012, the installed capacity of wind and 

photovoltaic (PV) was 8.1 GW and 16.6 GW, respectively, with 

total energy penetration at 9.8% in 2012. (In southern Italy, the 

energy penetration was 31%.) Most wind power plants (more 

than 95%) are connected to the transmission grid at the 150-kV 

voltage level and located in the south of Italy. Most PV plants are 

connected to the distribution grid at the medium- and low-voltage 

level (more than 95% of the PV capacity, corresponding to 

approximately half a million plants). 

Wind curtailment occurs sometimes during low-load days 

when there is consistent solar and wind production. Curtailment 

is undertaken to relieve congestion and maintain adequate reserve 

margins when no other actions on conventional generators are 

possible (conventional power plants at minimum generation 

level). In fact, in these situations, the instantaneous penetration of 

PV and wind can exceed safe system operation levels in terms of 

reserve margins, voltage profiles, and dynamic stability issues.

In the past four to five years, significant grid investments 

have been made that have increased the transmission capacity 

within and between market zones, reducing wind curtailment, as 

shown in Table I. 

TABLE I. CURTAILMENT IN ITALY

Year 2009 2010 2011 2012

Total wind 

generation (GWh) 6,543 9,126 9,856 13,407

Curtailment (GWh) 700 527 264 166

Curtailment/Total 

wind generation 10.70% 5.77% 2.67% 1.24%

The cost of curtailment is borne by the system, and the TSO 

has an incentive by the Italian regulator to reduce the amount of 

curtailments that are determined ex post by a third-party 

company. 

Investments in battery storage (energy and power intensive)

are in progress in southern Italy to increase the flexibility of the 

system to reduce levels of wind and solar curtailment, local 

congestion, and spinning reserves. 

E. Japan

In Japan at the end of 2011, the installed capacity of wind and 

PV was 2.4 GW and 4.9 GW, respectively, with total energy 

penetration at 0.9%. Even after the Feed-in Tariff (FIT) Act (The 

Act on Special Measures concerning the Procurement of 

Renewable Electric Energy by Operators of Electric Utilities, Act 

No. 108 of 2011) was enforced in July 2012, utilities can curtail 

wind and solar for a maximum of 30 days (8% annual) without 

compensation. These curtailment requirements are stipulated in 

the Ministerial Ordinance for Enforcement of the FIT Act (No. 46 

of 2012). Some utilities have required wind developers/owners to

install secondary batteries onsite to mitigate wind variability in 

addition to the 30-day uncompensated curtailment.

Priority dispatch is not given for renewables in Japan. 

Description on the priority dispatch is not in either the FIT Act or 

the Ministerial Ordinance. According to the latest grid rule stated 

by the Electric Power Council of Japan, curtailment of wind and 

solar should be undertaken before other measures such as 

interchanges between utilities, curtailment of other power 

producers and suppliers (independent generators with mainly 

conventional combustion plants), and/or curtailment of baseload 

generators. Information is not available on the actual amount of 
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curtailment that has occurred because utilities have not published 

this data. 

F. Portugal

At the end of 2012, Portugal had 4,517 MW of wind power 

capacity installed (20% energy penetration). Legislation restricts 

curtailment of renewable energy generation except in the case of 

technical problems. Despite several instances of excess wind (and 

other nondispatchable plants) in recent years, the Portuguese TSO 

has yet to invoke technical problems to curtail wind. For the 

purpose of the current paper, excess generation in the Portuguese 

control area that is transmitted to Spain at zero value is called

curtailment.

Excess renewable energy generation occurs in times of low 

loads and high wind. Generation from nondispatchable combined 

heat and power (CHP) industrial plants tend to further increase 

the supply of energy with high generation during nighttime 

periods of low demand. 

Portugal typically manages high-penetration events by 

controlling production from run-of-river hydro plants, exporting 

excess power to the Spanish energy market, and halting the 

import of electricity from France through Spain. Portugal has two 

dispatch centers that enable the monitoring, control, and 

curtailment of a large fraction of wind generation during highsupply/low-demand periods [3,4]. 

The FIT legislation requires wind producers to be 

compensated only for losses that exceed 50 hours at full capacity, 

the value of which is calculated at the tariff payment level. 

Because of the economic burden introduced by these situations

and significant installed wind power capacity, new connections of 

wind power plants to the transmission system have been halted 

until 2020, the only exception being one 75-MW offshore 

installation. 

The Portuguese regulator is currently analyzing the halting of 

electricity imports during excess electricity events and has made

an initial recommendation that this procedure be substituted by 

countertrading. 

G. Spain

As of 2012, Spain had 22.6 GW of wind installed (18.1% 

energy penetration). Red Eléctrica de España (REE) is the 

Spanish system operator that manages the distribution and 

transmission system. In Spain, wind curtailment typically occurs 

when demand is low and wind production is high. Through most 

of 2009, curtailments were primarily because of inadequate 

transmission and distribution system capacity. However, since the 

end of 2009, REE has also curtailed wind to give priority to 

scheduled energy and to improve system stability. The need for 

curtailment is somewhat reduced during high wind speeds 

because many wind power plants are stopped to protect the 

turbines from damage or are operating at lower production with 

speed regulation controls in place. 

Curtailment peaked in 2010 at 202.2 GWh of wind. Energy 

curtailed in January – March 2013 was 132.5 GWh. The Spanish 

Wind Energy Association estimated economic losses as a result 

of curtailments from January 2013 to April 2013 to total 

approximately €70 million. If the 2020 Spanish wind targets are 

met, curtailment is expected to reach 2.3 TWh, or 3.1% of wind 

energy generation, in 2020. 

Future policies to mitigate renewable energy curtailment 

include providing wind power plant managers with production set

points, which will be developed by the Control Centre of 

Renewable Energy. Installing new pumped hydro storage systems 

and establishing international transmission interconnections are 

additional future strategies Spain is exploring to balance 

significant fluctuations in wind power production. The country 

may also look to demand-side management, such as smart grid 

technologies, to improve the supply-demand balance. Valuing 

curtailed generation as a source of operating reserves has not yet 

been considered. 

H. Sweden

In Sweden, the only curtailments that are currently performed 

are those caused by grid limitations. If grid reinforcements are 

needed when a wind power plant is installed, then the wind 

power plant owner has to pay for these. The Swedish TSO may 

curtail wind power plants, e.g., in a situation in which a wind 

power plant is interconnected before needed grid reinforcements 

have been completed and a line outage occurs. A signal is sent 

directly to the wind power plant controller to decrease the output.

In the Jönköping Energi distribution system operator (DSO), 

there is a wind plant with an installed capacity of 29.3 MW. But 

because of possible voltage problems, the wind plant owner has 

agreed to reduce the infeed to 26 MW. This limitation is 

implemented within the plant. The alternative was for the wind 

plant owner to pay to strengthen the grid, but analysis of how 

often the wind plant would produce more than 26 MW showed 

that this was not economically viable. This formally means that 

the plant is sometimes curtailed up to 3.3 MW. The agreement 

also stipulates that the DSO can decrease the output even farther 

below 26 MW if there are voltage problems in the grid. However, 

this has not been needed since commissioning in 2010. Again, the 

wind plant owner had the option to strengthen the grid at a cost 

that was higher than the value of expected wind curtailment [5]. 

I. United States

The United States has many balancing areas, each of which 

may have its own curtailment practices. Some key balancing 

areas are discussed here. The areas with the greatest amount of 

curtailment of wind and solar power to date include the 

Bonneville Power Administration (BPA) balancing area, the 

Southwest Power Pool (SPP), and Hawaii, which has struggled 

primarily with excess wind at low-load periods and minimum 

generation requirements. Both the Midcontinent Independent 

System Operator (MISO) and the Electric Reliability Council of 

Texas (ERCOT) have recently implemented market-based 

solutions and have seen reductions in curtailment levels. 

BPA has approximately 4,500 MW of wind in its balancing 

area and a peak demand of 11,500 MW, although some wind 

resources have shifted out of the balancing area recently. There 

are two types of curtailments: curtailments caused by exhaustion 

of balancing reserves and those that result from seasonal hydro 

oversupply. Curtailments caused by the former averaged 7,000 

MW in 2012, but have dropped to 1,700 MW so far in 2013. 

Curtailments caused by the latter reached approximately 5% to 

6% of total available wind generation in spring 2011, but these 
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levels dropped to approximately half that amount in spring 2012 

[6]. Curtailments have been attributed to a lack of system 

flexibility and balancing challenges, which is, in part, caused by

environmental constraints on hydro units. BPA has been 

modifying its curtailment protocols and implementing or 

exploring measures to help reduce the amount of curtailment, 

including faster scheduling, better use of forecasts, and improved

methods of committing and de-committing reserves. 

In the SPP, lack of transmission access has been the primary 

cause of curtailment. Wind has grown rapidly—essentially 

doubling in 2012—and has come online ahead of planned 

transmission. There are approximately 7,000 MW of wind in the 

system, which has a peak demand of 54,000 MW [7]. According 

to wind generators, at times approximately 40% to 50% of 

potential output is being curtailed. The use of manual processes 

to implement curtailments has exacerbated the impacts. 

Transmission lines scheduled for completion in 2014 or 2015 

should help alleviate curtailment in the future. In the meantime, 

SPP is holding stakeholder processes to improve and increase 

transparency of its curtailment practices, and moving toward an 

automated, market-based approach. 

MISO, which has more than 12,000 MW of wind capacity

and a peak demand of 98,000 MW, implemented the 

Dispatchable Intermittent Resource (DIR) protocol in mid-2011, 

which includes wind generators in 5-minute dispatch. As a result, 

manual curtailments have decreased. Of wind generators not 

participating in the DIR program, curtailment levels range from 

1% to 3% of total wind generation [8]. Similar programs are in 

place at PJM and the New York Independent System Operator 

(NYISO). 

ERCOT shifted from a zonal congestion market with 15-

minute dispatch to a nodal market with 5-minute dispatch in 

December 2010. In addition, new transmission was added, which 

helped reduce curtailment levels from 17% in 2009, to 8% to 9% 

in 2010 to 2011, and 4% in 2012 [7].

III. HOW WIND IS CURTAILED

There are various approaches to how wind and solar are 

curtailed. There is emerging interest in performing curtailment as 

part of the market function. The advantage to this approach is that 

economic signals regarding the cost-effectiveness of alternative 

curtailments (dispatch down) are transparent. If all market 

participants, including wind power plants, participate, then the 

solution will be economically efficient. In the United States, 

MISO, ERCOT, and NYISO are examples that use the market 

mechanism. Conversely, when no market mechanism exists or is 

available for curtailment, a system operator must typically make 

a decision in real time concerning which plant(s) to curtail. The 

absence of price signals in this case will likely result in an 

economically inefficient outcome. SPP and BPA are example 

U.S. regions that use manual curtailment.

For example, in 2010, wind was being increasingly curtailed 

in MISO via manual phone calls from operators. The DIR

program allows wind to participate in the real-time market like 

other generation. MISO obtains a wind forecast for each plant for 

each 5-minute interval that sets the maximum for that plant in 

that time interval. Wind is part of the co-optimized 5-minute 

market dispatch. This allows wind to set price and to choose not 

to generate if prices are severely negative. 

Grid-congestion events result in local curtailments, and 

balancing-related events can, in principle, impact either all wind 

and PV plants or part of them, depending also on possibilities to 

control the output of the plants. First experience on using 

curtailed plants for up or down regulation is emerging. 

An example of a balancing-related event is shown for the 

vertically-integrated utility of Xcel/Public Service of Colorado 

(PSCO), which needs to balance its own load with its own 

generators (mostly thermal) and long-term contracts with wind 

power plant owners. At night, when winds are high and loads are 

low, PSCO runs its thermal units down to minimum generation 

levels and still needs to curtail wind at times to balance the load. 

Fig. 4 depicts an example of this. At 02:45, the system operator 

initiated a manual block curtailment of a wind power plant to 300 

MW because of a high positive area control error (ACE). ACE 

decreased but began to run too low in the negative direction, so at 

4 a.m., the operator put the wind power plant on automatic 

generation control (AGC), by which ACE signals are sent to the 

wind power plant every 4 seconds to 6 seconds to keep ACE 

within a prescribed band. This resulted in much better control of 

ACE, as shown in Fig. 4. In fact, PSCO has gone a step further, 

and when wind is curtailed, they have used wind to provide down 

regulating reserves. 

Figure 4. ACE during wind on block curtailment and wind on AGC [9]. 

In Ireland, given the distinction made between constraints 

(resolved by reducing the output of particular units) and 

curtailment (resolved by reducing the output of any or all units), 

constraints are normally applied first, followed by curtailment, if 

both are required simultaneously. Curtailment and constraint 

instructions are shared out based on the active power control set

point of each unit. When a dispatch-down instruction is required, 

noncontrollable wind plants will be prioritized first (by opening 

relevant circuit breakers), then controllable wind plants, followed 

by wind plants that are still in a commissioning phase.

IV. MITIGATION OPTIONS

Reducing curtailment typically involves finding additional 

sources of flexibility in the system. These can be physical 

additions (e.g., storage), grid capacity, institutional changes (e.g., 

access to a new market) or operational changes. 
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Portugal has a high wind penetration (20% wind energy in 

2012), with limited interconnections to other regions (1,600 MW 

to 2,250 MWto Spain) [10]. Other constraints include run-ofriver hydro and significant CHP plants. To reduce wind 

curtailment, the Portuguese operator uses pumped hydro and the 

intertie to Spain.

Figure 5. Portuguese load and generation profiles for May 15, 2011 [3]. 

For example, on May 15, 2011, at 06:45, wind penetration 

reached 81% of load. See Fig. 5. Most of the hydro generation 

was shut down, with a few plants staying online to provide 

balancing services. The pumped hydro storage facilities were 

pumping water up at full output. One thermal plant remained in 

operation, and its output was reduced by 25% near 04:00 and 

closely coordinated with the hydro units. Imports of electricity 

from France through Spain were replaced by exports.

In the United States, PSCO has examined in detail the tradeoffs between cycling their coal units and curtailing their wind in 

high (2-GW to 3-GW) wind scenarios [11]. They conducted a 

detailed analysis of their cycling costs for their coal units to 

understand the costs of starts, shallow ramps (to economic 

minimum generation levels), and deep ramps (to emergency 

minimum generation levels). They determined that shutting down 

a coal unit during low-load periods at night was uneconomic. 

They then analyzed the trade-off in cycling cost for deep ramps 

versus shallow ramps with wind curtailment. Costs were found to 

be essentially the same with either mode of operation. 

In Ireland, when priority dispatch generation must be 

dispatched down, a specific ranking order has been defined with 

the principle of preserving least-cost dispatch: the TSO may 

perform countertrades on HVDC interconnectors after gate 

closure, followed by set point reductions for peat plant, highefficiency CHP, biomass and hydro generation, and finally wind 

generation. the output of the peat and CHP units is reduced to 

their minimum stable generation levels, rather than decommitted, as such units represent the major source of negative 

reserves for the system.

The European Union’s Twenties project has studied market 

scenarios for 2020 and 2030 in Northern Europe including the 

countries with significant plans for offshore wind power 

development: United Kingdom, France, Germany, Belgium, 

Netherlands, Denmark, Norway, Sweden, Finland, and Poland. 

The studies are based on detailed scenarios of offshore wind 

power development. Large-scale offshore wind development is 

expected to increase wind curtailment, mainly because of

increased wind generation, but also because of additional 

variability from geographically concentrated offshore wind 

plants. Market simulations show that wind curtailment is 

expected to increase from 0.4 TWh in 2020 to 9.3 TWh in 2030 

[12].

The Twenties project studied different mitigation options. 

One is introduction of virtual power plants—aggregations of 

distributed generation and demand response that replace 

conventional generators—in Denmark in 2030 that will reduce 

wind power curtailment by 0.18 TWh [12]. This is marginal 

compared to the 9.3-TWh curtailment in Northern Europe, but the 

consumption in Denmark is only 2.2% of the consumption in this 

region, so the mitigating impact of virtual power plants can be 

significant. Anotheris a recommended scenario for feasible 

expansion of hydro power and transmission capacity that will 

reduce wind power curtailment by 1.5 TWh [13]. This result 

confirms that the flexibility of hydro power has a significant 

impact on wind power curtailment. Finally, Twenties also 

demonstrated the use of dynamic line rating (DLR) technology 

for increasing transmission capacity without building additional 

lines. A reported 10% to 15% increase in transmission capacity 

using DLR would mean a significant reduction of wind 

curtailment [14].

V. IMPLICATIONS OF CURTAILMENT

Curtailment is a growing concern in the wind and solar 

industry. It has implications for wind and solar developers and 

owners as well as utilities that purchase wind and solar power. In 

markets or contracts in which curtailment is not compensated, 

developers need to assess the risk and quantify the amount of 

future curtailment, which depends on many factors (future 

markets, future penetration levels of wind and solar, future 

flexibility in the balance of generation, etc.). Even when 

curtailment is compensated, developers need to assess the risk 

that these rules may change. Utilities with renewable energy 

targets also need to assess their risk and quantity of future

curtailment, which is likely to increase with increased 

penetrations of wind and solar power. Utilities that are signing 

long-term contracts for wind and solar may want to consider who 

bears the curtailment risk during the lifetime of the contract. No 

easy answers have been identified for how the curtailment risk 

should be borne. Legal battles have already been fought about 

how wind and solar are curtailed and how much has been 

curtailed. This issue is ripe for future research.

ACKNOWLEDGMENT

D.L. thanks the U.S. Department of Energy, which supported 

this work under Contract No. DE-AC36-08-GO28308 with the 

National Renewable Energy Laboratory. 

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