assessing wind power impacts on the flexibility of a power system
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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
3
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
5
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”
10
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.
21
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
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