Assessing wind power impacts on the flexibility of a power system

Mark O’Malley, Michael Milligan, Hannele Holttinen

Reserve requirements

Flexibility assessment

Production cost simulations: Unit commitment and

Economic Dispatch

2Flowchart of a Wind Integration study

Inputs

Important input data: Wind power (hourly or faster) Load (synchronized with wind) Generation data including

Ramp rate Minimum run levels Min up-time, down-time Heat rate Other relevant constraints

Dispatch time step Commitment time step; whether rolling, lock-down time,

etc. Inputs from other boxes in the flow chart

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Additional Wind Induced Reserve

Contingency reserve has basic (N-1) rules – wind rarely (if ever) influences

Operating reserves for normal operation (load variability and forecast errors) do not have set rules (e.g. U.S. Vs. European reserve categories !) this is the part that wind will influence

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Reserve requirements

-10 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150

Minutes

ContingencyOccurs

Spinning & Non-Spinning Reserve

FrequencyResponse

Reserves "Should" be Restored

Reserves "Must" be Restored

Market Response

Supplemental Operating Reserve

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Estimating Reserve Requirements

Many methods used to estimate increase due to wind power Drawback of deterministic methods adding up extreme cases

from load / contingency event / wind is that will over-estimate Simple statistical method combining wind and load variability

and forecast errors exceedence better than n times σ as wind not normally distributed.

Risk/reliability based more advanced methods also recommended.

Dynamic reserve is recommended.

Forecast errors day-ahead will bring high reserve requirements – needed for Unit Commitment but operationally we can update

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250

300

350

400

450

500

550

1 6 11 16 21 26 31 36

Forecast horizon [h]

Re

pla

cem

ent

rese

rve

s [M

W]

80th 85th 90th 95th 97th 99th

300

500

700

900

1100

1300

1500

1 6 11 16 21 26 31 36

Forecast horizon [h]

Re

pla

cem

ent

rese

rve

s [M

W]

Portfolio 1 Portfolio 2 Portfolio 3 Portfolio 4 Portfolio 5 Portfolio 6

Increasing demand for Reserve

Recommendations for Reserves

Method: not just adding up extreme events but

combining with risk/reliability levels. dynamic not static Timing is important - horizon etc.

Beware Definitions are different

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Flowchart of a Wind Integration study

Reserve requirements

Flexibility assessment

Production cost simulations: Unit commitment and

Economic Dispatch

9

WHAT IS FLEXIBILITY?

“FLEXIBILITY IS THE ABILITY OF A SYSTEM TO DEPLOY ITS RESOURCES TO RESPOND TO CHANGES IN NET LOAD, WHERE NET LOAD IS THE REMAINING SYSTEM DEMAND NOT SERVED BY VARIABLE GENERATION”

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Flexibility in a power system

Installed

Committed

Dispatched

Internal resources

External resources

Operational (short-term) Constrained by previous unit

commitment What can be dispatched or

accessed via markets? Operational (Mid-Term)

Constrained by existing (planned and built) sources of flexibility

Unit commitment problem to ensure sufficient committed generation for flexibility requirements

Planning (long-run) Necessary but not sufficient

for real-time flexibility provision

Needs an assessment of long-term flexibility requirements

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12Production Cost Simulation

Meibom, P., Barth, R., Hasche, B., Brand, H., Weber, C. and O´Malley, M.J., “Stochastic optimisation model to study the operational impacts of high wind penetrations in Ireland”, IEEE Transactions on Power Systems, Vol. 26, pp. 1367 - 1379, 2011.

Source: http://www.nerc.com/docs/pc/ivgtf/IVGTF_Task_1_4_Final.pdf

Win

d P

ower

Out

put

(MW

)

+3,039 MW in 1 hour

Ramp ERCOT – 18/19th April 2009 13

SinkFacilitatorsSources

Ph

ysic

al

Insti

tuti

on

al

Econ

om

ic

•Load•Outages•Solar•Wind etc..

•Conventional Generation•Electricity Storage •Interconnection•Demand Side•Variable Generation

• Transmission Networks

• Fuel Storage

• Forecasting• Gate Closure• Grid Codes

• Market Resolution• Balancing Area Size• Unit Commitment

• Ancillary Services Reparation

• Cycling Costs

Marke

tsO

pera

tions

Pla

nn

ing

Sources of Flexibility Need to modelled 14

UNIVERSITY COLLEGE DUBLIN

ELECTRICITY RESEARCH CENTRE

15

Transmission playing its part

Note the sag on the line

16Available Flexibility

http://www.aeso.ca/gridoperations/14246.html

The purpose of this graph is to demonstrate the variable flexibility that exists in the market place to accommodate wind.

Quantifying Flexibility

© OECD/IEA 2011

Step 1: Identify flexible resources

Step 2: are they available?

Step 3: what are the needs?

Step 4: Compare need & resource

Optimise resource /deploy

additional

The FAST Method

19Flexibility Metrics

Lannoye, Flynn & O’Malley, Evaluating Power System Flexibility, IEEE Trans. Power Systems, in press, 2012.

Effects of Cycling 20

Production Cost Simulation

Meibom, P., Barth, R., Hasche, B., Brand, H., Weber, C. and O´Malley, M.J., “Stochastic optimisation model to study the operational impacts of high wind penetrations in Ireland”, IEEE Transactions on Power Systems, Vol. 26, pp. 1367 - 1379, 2011.

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Comparison methods e.g. varying output or block output ?.

Recommendations

Flexibility assessment Dispatch and Unit Commitment models Iterate between them or develp a new method of

assessment

Key Messages Representativeness of input data for wind power variability and

uncertainty capturing system characteristics and response through

operational simulations and modelling; examining actual costs independent of tariff design structure and

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Assessing wind power impacts on the flexibility of a power system

Mark O’Malley, Michael Milligan, Hannele Holttinen