Assessing wind power impacts on the flexibility of a power system

1
Assessing wind power impacts on the flexibility
of a power system

• Mark OMalley, Michael Milligan, Hannele Holttinen

2

Flowchart of a Wind Integration study

• Reserve requirements
• Flexibility assessment
• Production cost simulations
• Unit commitment and Economic Dispatch

3
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

4
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

5
Reserve requirements
6
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 s 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

7
Increasing demand for Reserve
8
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

9
Flowchart of a Wind Integration study

• Reserve requirements
• Flexibility assessment
• Production cost simulations
• Unit commitment and Economic Dispatch

10
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
11
Flexibility in a power system
Installed

• 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

Committed
Dispatched
Internal resources
External resources
12
Production Cost Simulation
Meibom, P., Barth, R., Hasche, B., Brand, H.,
Weber, C. and OMalley, 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.
13
Ramp ERCOT 18/19th April 2009
Wind Power Output (MW)
3,039 MW in 1 hour
Source http//www.nerc.com/docs/pc/ivgtf/IVGTF_T
ask_1_4_Final.pdf
14
Sources of Flexibility Need to modelled
Facilitators
Sources
Sink
Physical
Planning

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

• Load
• Outages
• Solar
• Wind etc..

• Transmission Networks
• Fuel Storage

Institutional
Operations

• Forecasting
• Gate Closure
• Grid Codes
• Market Resolution
• Balancing Area Size
• Unit Commitment

Economic
Markets

• Ancillary Services Reparation
• Cycling Costs

15
Transmission playing its part Note the sag on
the line
16
Available Flexibility
The purpose of this graph is to demonstrate the
variable flexibility that exists in the market
place to accommodate wind.
http//www.aeso.ca/gridoperations/14246.html
17
Quantifying Flexibility
18
The FAST Method
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
19
Flexibility Metrics
Lannoye, Flynn OMalley, Evaluating Power
System Flexibility, IEEE Trans. Power Systems,
in press, 2012.
20
Effects of Cycling
21
Production Cost Simulation
Comparison methods e.g. varying output or block
output ?.
Meibom, P., Barth, R., Hasche, B., Brand, H.,
Weber, C. and OMalley, 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.
22
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

23
Assessing wind power impacts on the flexibility
of a power system

• Mark OMalley, Michael Milligan, Hannele Holttinen