NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

The Impact of High Wind Power 

Penetration on Hydroelectric 

Unit Operations

Preprint

Bri-Mathias Hodge, Debra Lew, 

and Michael Milligan

To be presented at the 10th International Workshop on Large-Scale 

Integration of Wind Power into Power Systems

Aarhaus, Denmark

October 25-26, 2011

Conference Paper

NREL/CP-5500-52202

October 2011
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1

Abstract — The Western Wind and Solar Integration Study 

(WWSIS) investigated the operational impacts of very high levels 

of variable generation penetration rates (up to 35% by energy) in 

the western United States. This work examines the impact of this 

large amount of wind penetration on hydroelectric unit 

operations. Changes in hydroelectric unit operating unit 

patterns are examined for an aggregation of all hydro generators. 

The cost impacts of maintaining hydro unit flexibility are 

assessed and compared for a number of different modes of 

system operation.

Index Terms— power systems, wind power generation,

hydroelectric power generation, stochastic systems

I. INTRODUCTION

he growing amounts of variable generation (VG) being 

integrated into the electricity system are already starting 

to change system operations. As the penetration rates of these 

technologies increases further, significant changes in system 

operations are expected. The Western Wind and Solar 

Integration Study [1] (WWSIS) investigated the operational 

impacts of very high levels of VG penetration rates (up to 35% 

by energy) in the western interconnection of the United States. 

In the largest scenarios, 30% of energy in the WestConnect 

footprint was supplied by wind power, with an additional 5% 

split between concentrating solar thermal (CSP) and solar 

photovoltaic (PV) generators. While the WestConnect area 

was the focus of the study, the entire WECC area was 

modeled due to the strong interactions between the two areas. 

WWSIS studied system operations in the year 2017, based 

on expected system loads, generation unit retirements and 

expansions and transmission build-out. While such high 

levels of VG penetration are not expected by 2017, the choice 

of this year allows for reasonable assumptions about the rest 

of the system, and most importantly, the transmission grid, to 

be made without requiring long-term forecasting of system 

changes. To capture the operational effects of this influx of 

VG, a transmission-constrained hourly production cost model 

(GE MAPS) was used in modeling the WECC area. Because 

there can be large differences in operations on an inter-annual 

basis, the same year was modeled three times, utilizing timesynchronized load, wind, and solar data from the years 2004, 

2005, and 2006. Because there are not currently as many wind 

and solar sites in WECC as were modeled in the study, wind 

 The authors are with the National Renewable Energy Laboratory, Golden, 

CO 8041 USA (email: bri-mathias.hodge@nrel.gov, debra.lew@nrel.gov, 

michael.milligan@nrel.gov)

and solar power output time-series data had to be simulated 

using a numerical weather prediction (NWP) model [2]. 

Though the goal of the study was to model a system close to 

current system operations, some simplifying assumptions were 

made. The most important of these was using only five 

regional balancing areas in the study, instead of the 37 that 

existed in WECC at the start of the study. Balancing area 

consolidation has long been a trend in the United States 

electricity system, and has also been proposed as a change in 

system operations that would be helpful in integrating 

additional amounts of VG; though this degree of consolidation 

is not anticipated in the near future. The transmission system 

was reduced to 20 transmission zones with only inter-zonal

lines modeled.

In this work, we examine how the high wind penetration 

levels modeled in the WWSIS impact hydroelectric unit 

operations. Hydroelectric power is often seen as the perfect 

complement to wind power because hydro units are quickstarting and, therefore, can balance variations in wind power 

output. However, the reality is not quite as simple due to nonpower constraints on hydro units. Since hydroelectric units 

often have water use constraints, they may be required or 

prevented from operating at certain times to maintain reservoir 

levels, or required to generate in order to avoid spilling.

Belanger and Gagnon examined the effects of using a 

hydroelectric plant as a backup system to balance the 

variability of a single wind plant [3]. While they chose a

system with large amounts of hydro power generation, 

specifically Quebec, Canada, they did not look at the systemwide effects on all hydro generators, and instead chose to 

focus on a single hydro plant. A number of other studies have 

also examined combined wind-hydro systems, usually on 

small island systems where close to one–to-one backup of 

wind power is necessary because of the paucity of other 

generators [4, 5]. However, one must be careful about 

applying the results generated with this approach to larger 

systems because wind power does not require one-to-one 

backup in larger systems. Instead, balancing the reduced 

variability of a number of different wind plants together is a 

more efficient approach [6]. One study that considers a larger 

system is Benitez et al. [7] which looked at using an 

aggregation of hydropower plants to balance wind power in 

two high wind scenarios. Specific concerns addressed were the 

need for new thermal generation, the costs of wind integration,

and the costs of reducing CO2 emissions.

The Impact of High Wind Power Penetration on 

Hydroelectric Unit Operations

Bri-Mathias Hodge, Debra Lew, and Michael Milligan

T
2

II. WWSIS METHODS AND DATA

In this section, we describe some of the important details of 

the methods and data utilized in the study. Section II-A 

contains information on the various scenarios run in the 

WWSIS, in terms of wind penetration rates and wind plant 

locations. Section II-B provides details on how the 

hydroelectric units were modeled within the production cost 

simulation model.

A. Scenarios

The wind resources available in the Western 

Interconnection can vary widely in quality from location to 

location. The choice of where to site the proposed wind plants 

that would combine to meet the study’s renewable penetration 

goals is influenced by many factors. For this reason, three 

different siting scenarios were devised: the In-Area, Mega 

Project and Local Priority scenarios. The In-Area scenario 

forced the areas within WestConnect to fulfill their renewable 

goals with resources located in their area of responsibility, and 

will be the focus of our analysis of the effect of high wind 

power penetration on hydro generator operations.

Though a goal of the WWSIS was to examine the impact of 

VG penetration rates above 30% on electricity system 

operations, the changes that would occur in the system at 

lower penetration rates on the trajectory to 30% are also 

important. For this reason, four penetration rate scenarios 

were included in the study. The lowest penetration rate case 

examined the impact of 10% wind energy and 1% solar energy 

in both the WestConnect area and the rest of WECC. There 

were two 20% cases created. One examined 20% wind energy 

and 3% solar energy penetration in both WestConnect and 

WECC as a whole. The other included only 10% wind energy 

and 1% solar energy in the rest of WECC, while keeping 

WestConnect at 20% and 3% for wind and solar energy 

penetration respectively. The final case examined penetration 

rates of 30% wind energy and 5% solar energy for 

WestConnect, with the balance of WECC only having a 

20%/3% split. In our analysis of the impacts on hydro 

generators, we will continually use the different penetration 

cases to examine the impact of increasing VG penetration.

B. Hydroelectric Modeling

Unlike the wind and solar data used in the WWSIS, the 

hydroelectric modeling does not use time series data specific 

to the climate patterns of 2004-2006. Since hydro units are 

dispatchable, modeling their operation solely on a historical 

dataset would have increased the perceived integration cost by 

neglecting the hydro unit flexibility. Instead, each plant was 

assigned monthly energy and plant capacity limits based on 11 

year averages of these generator specific variables. This 

allowed the production cost model to dispatch the hydro 

resources, subject to their monthly constraints. One limitation 

of this approach is that it always assumes that the full hydro 

generator nameplate capacity is available, regardless of 

current reservoir levels. By not explicitly modeling the 

reservoir levels at all of the hydro units, some of the water 

usage and non-power constraints and requirements found in 

actual unit operation are neglected. Some of these issues 

include: recreational water levels, irrigation, flood control, and 

dissolved gas levels. While they may greatly affect hydro 

generator production levels, the above named issues are also 

very unit- and situation-specific, making a full accounting of 

all possible constraints in all contingencies beyond the scope 

of a WECC-wide study such as WWSIS.

III. RESULTS

Hydroelectric units can behave quite differently than 

thermal units in normal power system operations, and so case 

studies were performed to examine how the hydroelectric 

units might operate in a high VG scenario. Hydroelectric units 

have the advantage of being quick-start, however, they also 

have constraints on their generation based on maintaining 

reservoir levels within appropriate bounds. Additional 

analysis was conducted on the impact of using hydro 

generation as flat capacity instead of flexible generation on 

total system costs. Since the high levels of variable generation 

modeled require greater system flexibility, hydro power 

becomes a very valuable resource due to its flexibility; as 

evidenced by the higher costs associated with flat hydro 

production when compared with flexible production.

A. Aggregate Hydroelectric Generation

Since some of the modeling assumptions in the WWSIS 

study differ from current system operations, even in the no 

wind case, the operation of individual hydroelectric units 

should not be expected to perfectly mimic current operational 

practices. However, this does not mean that changes in 

operational patterns over the entire class of hydro units cannot 

be discerned. For example, as seen in Figure 2, the hourly 

output levels do change slightly as the wind penetration level 

increases. Whereas, the total monthly energy for hydro units 

remains constant and so the daily output may shift slightly 

from the day to the night, or vice versa, depending on the 

corresponding wind output. This difference can be quite 

significant. In Figure 1, on April 14th the difference between 

the no wind hydro output and 30% case is as large as 15 GW 

of power at one point.

While the higher wind penetration rates can cause the 

hydroelectric units to operate quite differently at specific time 

points, overall changes in operational patterns are more 

important. One way to see the large scale differences in 

operation is by examining a generation duration curve for all 

hydro units over the course of a year. Figure 2 shows these 

curves for the no wind, 10%, 20%, and 30% in footprint 

scenarios. As may be observed, the curves are very similar for 

all four cases, showing that while the combined hydro 

production may be quite different at certain moments in time, 

the general pattern of usage is not significantly changed.
3

Figure 1. Total hydro generation in the different In-Area wind 

penetration scenarios for the week of April 10th.

Figure 2. Annual generation duration curve for all hydro units in WECC 

in the different In-Area wind penetration scenarios.

One place where the high wind power penetration rates 

make a significant impact is on the spot prices that occur 

during hydropower operation. As may be seen in Figure 6, 

there are noticeably lower spot prices across the entire year in 

the high wind penetration rates than in the no wind case. At 

the end of the spectrum with the lowest prices, the 30% wind 

cases have spot prices that are approximately $50/MWh lower 

than in the no wind case. These changes are to be expected 

due to the essentially zero marginal operating costs of wind 

plants, however, the overall loss in revenue for hydro 

generators implied by the lower spot prices is very important.

Finally, we examined an operational decision that can have 

a large effect on total system operating costs; whether to 

schedule hydro units based on total system load or net system 

load. We define net load in this case as the load remaining 

after subtracting expected wind generation from the load. 

Scheduling hydro units based only on load limits the 

utilization of their flexibility for balancing wind variability. 

The additional operating costs to the system for the 10%, 20%,

and 30% wind penetration scenarios may be seen in Figure 4. 

The cost of this lost flexibility is relatively modest in the two 

lower wind penetration cases, but increases significantly in the 

30% wind case.

Figure 3. Spot price duration curve of hydro unit operation for the 

different In-Area wind penetration scenarios.

Figure 4. Operating cost increases for dispatching hydro based on 

only load versus net load for the different In-Area wind penetration 

scenarios.

B. Flat Block Hydro

The flexibility of hydroelectric generators to start and ramp 

quickly is expected to be an important attribute with 

increasing VG penetration. To help assess the value of 

hydropower being able to balance system variability, a 

comparison was performed between two modes of hydro 

operation: allowing hydro units to be dispatched and grouping 

all hydropower together as a flat block. In the flat block case,

the hydro units were restricted to producing at a fixed rate 

during all times of the day, with the production level varying 

from month to month, based on the seasonal monthly 

production averages.

The total system operating costs for a variety of wind 

penetration scenarios, both with and without flat hydro blocks, 

are displayed in Figure 5. Immediately noticeable are the 

decreasing total system operating costs with higher 

penetrations of wind energy due to the zero marginal cost of 

wind power. Also immediately apparent are the higher costs 

in every scenario when a flat hydro block is used instead of 

allowing hydro to be dispatched.
4

Figure 5. Total system operating cost changes due to flat block hydro 

operation for the different In-Area wind scenarios.

Figure 6 provides a closer look at the differences in cost 

between the dispatchable and flat hydro cases. An interesting 

result is that the incremental decreases in operating cost 

between the two forms of hydro operation is lower for each of 

the wind penetration cases than for the no wind case. 

However, one must also remember that the total operating 

costs are lower in the high wind penetration scenarios. 

Therefore, the percentage decrease in total operating costs are 

fairly similar for all the cases, with the highest wind 

penetration rates having slightly higher percentage cost 

increases when using the flat block operation.

Figure 6. Incremental operating cost increase due to flat block hydro 

production in the various In-Area penetration rate scenarios.

Another way to assess the impact of the flat block hydro 

operating policy is through the examination of the spot price 

duration curve. Figure 7 shows this curve for the no wind and 

30% penetration scenarios for both hydro operation modes. 

The flat block operation of hydro units has an amplifying 

effect in the no wind case. During times of high spot prices,

the flat block creates even higher prices by not being available 

as a cheap dispatch solution. In the lower price cases, which 

most often occur during times of low system load, the flat 

hydro block further reduces system prices. In the 30% wind 

scenario, the dispatchable and flat block cases are very similar 

for most of the year. It is only during the lowest cost hours 

that the flat block case further reduces the spot price.

Figure 7. Spot price impact of flat block hydro in the no wind and 30% 

wind penetration In-Area scenarios.

IV. CONCLUSION

In this work, we have examined the impact that high wind 

power penetration rates will have on hydroelectric unit 

operations in the western United States. Though hydroelectric 

generators’ flexibility is often seen as the perfect complement 

to variable and uncertain wind power, the physical flexibility 

of the generators is often reduced by non-power constraints. 

Another important result is the establishment of the significant 

total system cost increases that arise from not utilizing this 

flexibility, as was determined by examining flat-block hydro 

operation. In order to more accurately establish the effects of 

high wind penetrations on hydro system operations, the nonpower constraints must be modeled on a unit-by-unit basis. In 

summary, the flexibility of hydro units can be an important 

factor in reducing total system costs, so long as that flexibility 

is available to the system.

V. REFERENCES

[1] GE, "Western Wind and Solar Integration Study," NREL/SR-550-

47434, 2010.

[2] C. Potter, D. Lew, J. McCaa, S. Cheng, S. Eichelberger, and E. 

Grimit, "Creating the Dataset for the Western Wind and Solar 

Integration Study," Wind Engineering, vol. 32, pp. 325 - 338, 

2008.

[3] C. Belanger and L. Gagnon, "Adding wind energy to hydropower," 

Energy Policy, vol. 30, pp. 1279 - 1284, 2002.

[4] J. K. Kaldellis, K. Kavadias, and E. Christinakis, "Evaluation of 

the wind-hydro energy solution for remote islands," Energy 

Conversion and Management, vol. 42, pp. 1105 - 1120, 2001.

[5] C. Bueno and J. A. Carta, "Wind powered pumped hydro storage 

systems, a means of increasing the penetration of renewable energy 

in the Canary Islands," Renewable and Sustainable Energy 

Reviews, vol. 10, pp. 312 - 340, 2006.

[6] M. Milligan, K. Porter, E. DeMeo, P. Denholm, H. Holttinen, B. 

Kirby, N. Miller, A. Mills, M. O'Malley, M. Schuerger, and L. 

Soder. (2009) Wind Power Myths Debunked: Common Questions 

and Misconceptions. IEEE Power & Energy. 89 - 99.

[7] L. Benitez, P. Benitez, and G. C. van Kooten, "The economics of 

wind power with energy storage," Energy Economics, vol. 30, pp. 

1973 - 1989, 2008.