Methods

An Analysis of Sub-Hourly Ramping Impacts of Wind Energy and 

Balancing Area Size

Michael Milligan and Brendan Michael Milligan and Brendan Kirby, consultants, NREL

NREL/PO-500-43084 | The information contained in this poster is subject to a government rights | WINDPOWER 2008 | Houston, TX | June 1–4, 2008

Objective

•Explain why ramping can be costly when marginal generators cannot ramp fast enough

•Examine the impact of within-hour (5-minute) ramping impacts of a high wind penetration

•Quantify the benefits of Balancing Area combined operations on mitigating these ramp impacts.

Approach

•Obtain 5-minute load and wind power data from the Minnesota 20% Wind Integration Study

•Utilize the 25% (by energy) wind penetration

•Calculate ramp requirements:

–Separate Balancing Area Operations vs. Combined Balancing Area Operations.

3200

3000

2800

2600

2400

4:00 AM 6:00 AM 8:00 AM 10:00 AM 12:00 PM 2:00 PM 4:00 PM 6:00 PM

dat

Base Load - $10/MWh

Peaking -

$90/MWh

Energy Price $10/MWh Energy Price $10/MWh Energy Price Increases

to $90/MWh because

base unit can't ramp

fast enough

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Why can ramping be costly?

Ramping capability may be limited in some circumstances, even though there is 

sufficient overall capacity. This can cause higher dispatch costs, as illustrated in this 

figure. The base load generation cannot ramp fast enough, so a peaking unit is 

necessary to provide the ramp. This increases costs. Ramping can be part of energy if 

it is abundant or it may need to be a separate service if it is scarce.

We used data from the MN 20% Wind Study, divided by 

Balancing Area: Great River, Minnesota Power, Northern 

States Power, and Minnesota Power.

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2

Load (MW)

0

5-Minute Periods for Approximately 9 Months

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0

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-1000

5-minute changes

6,600 Hours of 5-minute Electric Load

Combined load (no wind) has considerable variability in the 

5-minute time scale. The top panel shows the load itself, 

the bottom panel shows that some 5-minute load ramp 

requirements can be as high as 500 MW to 1000 MW 

within 5 minutes.

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Load (MW)

0

5-Minute Periods for Approximately 9 Months

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5-minute changes

6,600 Hours of 5-minute Electric Load and Wind

Ramp requirements increase with 25% wind energy 

penetration. The upper panel also shows the importance of 

being able to achieve lower minimum loads by the 

conventional generation fleet. 

14x103

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0

MW

0 5-Minute Periods for Approximately 9 Months

 Load Only

 Load and Wind

The need for additional turn-down capability is also shown 

by this load duration curve, based on 5-minute data. Lower 

minimum loads require greater flexibility but additional 

capacity is not needed.

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Ramp (MW/5min)

One 4-hour Period

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Excess Ramping (MW/5min

Operating separate balancing areas causes 

extra ramping compared to combined operations.

Blue: up-ramp

Green: down-ramp

Yellow: combined ramp

Ramping that could be eliminated by combining operations

Some areas are ramping up 1220 MW/5min

while other areas are ramping down more than 700 MW/5min

Combining Balancing Area operations can result in the 

‘cancelling’ out of some ramp requirements when one BA 

needs upward ramps at the same time another BA needs 

lower ramps. This graph shows an illustration from the MN 

data for load and wind together.

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Excess Ramp (MW)

0 5-Minute Periods for Approximately 9 Months

Excess Ramps: Load Only

Even without wind there is excess ramping when Balancing 

Areas operate separately. This graph shows the ramps that 

could be eliminated with combined operations.

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0

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Excess Ramp (MW)

0 5-Minute Periods for Approximately 9 Months

Excess Ramp: Load and Wind

With a large wind penetration, there is an increase in 

excess ramping when Balancing Areas operate separately. 

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0

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200

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Excess Ramp (MW)

0 5-Minute Periods for Approximately 9 Months

Excess Ramp: Load and Wind Compared to Excess Ramp: Load Alone

 Load and Wind

 Load Alone

This graph combines the previous two graphs. The red 

trace shows the excess ramping from load alone, and the 

blue shows excess ramping from load and wind together. It 

is clear that the excess ramping is larger with wind and 

load together, compared to load alone.

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500,000

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Load Combined

Load

Load and Wind Combined

Load and Wind

Total Ramp MW-5min

For load alone, there is a 22.4% reduction in ramping 

requirements when operations are combined. For load and 

wind together, this reduction is 23.8%.This graph shows the 

total ramping, measured in MW-5min (one MW ramp for 5 

minutes)

Combined operations can offer a significant 

improvement on tail events: large but 

infrequent ramps

Conclusions

•Combined operations of separate Balancing 

Areas can reduce 5-minute ramping needs, 

both without wind, and with wind

•Additional turn-down is needed at high 

penetrations, but not extra capacity

•Combined operations can significantly 

reduce large but infrequent ramps. -600

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Excess Ramp Comparison: Separate vs. Combined Operations

0 200 400 600 800 1000

5-Minute Periods

 Load and W ind

 Load Only

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600

Excess Ramp Comparison: Separate vs. Combined Operations

0 2 4 6 8 10

5-Minute Periods

 Load and Wind

 Load Only

This graph shows the difference in excess ramping

requirements between wind and no-wind cases.