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US NREL's study: Grid expansion needed to reduce wind curtailment [free access]

February 17, 2017

Wind power is one of the fastest growing sources of new electricity generation in the United States (US) and is already providing substantial economic, energy security and health benefits. The effective policies and commitment of the former US President Barack Obama have spurred this recent historical growth in wind power in the country. According to the Energy Information Administration (EIA), in 2015, wind generation comprised 4.7 per cent of the total electricity production capacity of nearly 74 GW.  This represents a doubling of the share of wind generation since 2010, when it was 2.3 per cent.


Given the recent increase in wind energy penetration across the country, curtailment of variable generation is becoming more widespread. While curtailment of generation is normal in a complex power system, owners of wind power plants, which have no fuel costs, are concerned about its impact on project economics.


Operator-induced generation curtailment typically occurs because of congestion in the transmission network or due to lack of transmission access. The large body of recent renewable grid integration research suggests that transmission expansion is one option to increase system flexibility and thereby mitigate curtailment and other integration challenges.


With this background, in January 2017, the US Department of Energy (DoE) released a report which confirms that adding even limited electricity transmission capacity can significantly reduce the costs of expanding wind energy to supply 35 per cent of US electricity by 2050. The report, titled “Reducing Wind Curtailment through Transmission Expansion in a Wind Vision Future” and authored by the National Renewable Energy Laboratory (NREL), affirms the findings of the DoE’s 2015 Wind Vision, which showed that a future in which wind provides 20 per cent of US electricity in 2030 and 35 per cent in 2050 is achievable and would provide significant economic, energy security and health benefits to the nation.


The report attempts to better assess the operational feasibility of a high wind future as portrayed in DoE’s 2015 Wind Vision study, in the western US. The analysis is based on a commercially available production cost model, PLEXOS, to model the hour-to-hour, chronological unit commitment and economic dispatch of generators in the Western Interconnection.


Global Transmission Research presents the key findings of the latest DoE reports.


Core wind vision scenarios

According to the study, variable energy curtailment is caused by various reasons such as insufficient transmission capacity also referred to as grid congestion, or the inability to reduce output from other generating sources. Higher penetration and integration of renewable energy along with minimum curtailment require infrastructure changes, such as increased transmission capacity or more flexible, fast-start generation sources such as gas-fired combustion turbines.



At a high level of wind energy penetration, the study indicates that the generation mix is largely similar between the two scenarios. (See Figure 1) Particularly, the small differences are largely caused by differences in capacity as the fleets in the two analyses were not identical. However, one major difference between the two scenarios is the level of curtailment. As depicted in the figure given below, curtailment of renewable resources (in red) is much higher in the Reference simulation as compared to the 2050 Wind Vision Study Scenario.


Thus the study emphasises that without new transmission, the wind capacity deployed in the Wind Vision scenarios faces significant curtailment. Although the Wind Vision Study Scenario indicates an achievement of 37 per cent wind penetration in the western United States (with around 12 per cent solar penetration) by 2050, the Reference system only achieves 28 per cent wind penetration due to the curtailment. The simulation indicates insufficient transmission capacity, a generator fleet with insufficient flexibility, or both.


The study also assesses the relative impacts of transmission or generator inflexibility on the curtailment. The findings suggest that retiring baseload coal in an unchanged transmission network only reduces curtailment marginally for a high wind penetration scenario. In fact, the study weighs the curtailment at 15.5 per cent to 14.9 per cent. This is because generation from gas combined cycle and gas combined thermal largely makes up for the deficit in coal generation.


The study further discovers that high wind penetration with no transmission constraints, referred to as copper sheet scenario, has only 0.5 per cent curtailment. Specifically, the annual curtailment is reduced from 15.5 per cent under the reference to 0.5 per cent in the copper sheet scenario (see table 1). Another important finding shows that the elimination of transmission constraints results in a relatively flexible output by coal generators. Conclusively, the study claims that transmission congestion is the primary cause of curtailment observed in the reference scenario. Conversely, coal generator inflexibility does not appear to have a major impact on curtailment.


Transmission buildout scenario

Under this scenario, the study attempts to determine the amount and value of transmission that may be required to effectively transmit the targeted level of wind energy throughout the western US.





Furthermore, the new transmission buildouts also alleviate imports and exports in each state. The rise in imports depicts better use of wind energy in every state, not just the states in which each line begins and terminates. This is especially obvious in the difference between the Reference and Transmission 1 buildout scenarios. Although the second and third transmission builds show less dramatic changes, the Reference case is the most transmission-constrained system (as mentioned above). Thus, on the basis of the above discussion, it can be claimed that the addition of the first 10 GW of transmission capacity into a more congested system has a greater impact on wind curtailment than the subsequent additions.


In practice, additional transmission capacity can reduce total generation costs, primarily by transmitting lower cost energy to displace higher cost energy elsewhere. The other important finding of the study suggests that the proposed transmission of wind energy can help avoid generation costs and emissions, but with diminishing returns.


Suboptimal transmission flows

In the real world, non-physical constraints prevent full optimisation of power flow, which in turn have an adverse effect on generation costs and renewable energy integration. The study therefore also attempts to incorporate the impact of suboptimal transmission scheduling on wind curtailment and generation costs. It emphasises that the low levels of error can be absorbed with a relatively small impact, but in the high error case, annual curtailment nearly doubles, and generation cost increases by USD5 billion annually. Overall, this sensitivity provides a method to begin the quantification of the benefits of a centrally operated transmission and generation system, also considered to be a key factor in limiting renewable integration problems.


Summing up

The best wind resources in the western US are often not near the most populated regions. Therefore, taking full advantage of wind energy in the western US requires transporting the wind energy hundreds of miles from where it is generated to where it is needed. This study highlights the inefficacy of existing transmission facilities, which result in large wind energy curtailment.


To assess the value of transmission in mitigating wind curtailment and generation costs, the study models a suite of three transmission expansion scenarios. On the basis of multiple scenario analysis, it discovers that with no transmission constraints the wind energy curtailment can be reduced to 0.5 per cent.  Moreover, reducing wind curtailment can substantially lower annual productions costs and reduce carbon dioxide emissions.


In specific, greater transmission expansion was found to bring further benefits in terms of curtailment and cost reduction. However, the marginal benefits of additional transmission projects were found to decline beyond the initial buildout. Overall, transmission expansion is likely to play a vital role in allowing for efficient usage of renewable resources with 37 per cent wind penetration (and 12 per cent solar penetration). Going forward, further research is needed to understand how transmission infrastructure is utilised and the degree of sub-optimality existent in real transmission networks.


Figure 1: Total generation breakdown of the Reference study run (left) compared to the 2050 Wind Vision Study Scenario from ReEDS (right)



 Source: NREL

Table 1: Summary statistics for the study


Annual curtailment (%)





Copper Sheet


Source: NREL