Top FAQ’s (Frequently Asked Questions) About Utility Integration of Renewables…and Answers

1. Top FAQ’s (Frequently Asked Questions) About Utility Integration of Renewables…and Answers Michael Milligan & Debbie LewNational Wind Technology CenterNational Renewable Energy LaboratoryGolden, Colorado USA

2. The Questions

3. 1. Overview of the Grid • Interconnections, markets, BAs

4. There are 3 interconnections in the U.S. 1 2 3

5. There are many control areas (balancing areas) in the East and in the West

6. General Operating Requirements • The interconnection must be balanced ∑ loads = ∑ generation * • Implication for individual balancing areas (BAs) ∑ loads = ∑ generation + Imports – Exports * DC ties can span two interconnections, but these are limited

7. Markets cover part of the U.S. NPPD is Joining SPP

8. 2. How does the utility schedule resources? • Non-market areas (regulated by state PUCs) • Balancing Authority is responsible for scheduling generation to meet expected loads • Individual utilities will often provide schedules to BA, based on economics • Market areas • Similar, except the schedules are induced by the energy market, not single entity

9. Economic DispatchLoad Duration Curve

10. Economic Dispatch Peaking gens provide capacity, not much energy Intermediate gens change output in response to load Base load generation does not change its output

11. A Chronological View Hours to Day ahead: use load forecast to commit units 10 minutes to few hours: manually adjust generator set points Seconds to minutes: AGC automatically adjusts generator output

12. 3. What are reserves? There are different kinds of reserves • Contingency reserves • Extra spinning capacity to guard against sudden loss of generator or transmission • Contingency event is big and fast (wind does not behave this way) • Contingency reserves are often shared in reserve pools or groups • Largest hazard • Expensive • Regulating Reserves • Operating Reserves

13. 3. What are reserves? • Contingency reserves • Regulating reserves • Capacity that is reserved for small up-down changes in load • Other operating reserves

14. 3. What are reserves? • Contingency reserves • Regulating Reserves • Operating Reserves • Generation that can be used to meet changing or unexpected load conditions • No universally-accepted, rigorous definition • May be spinning or non-spinning (10-minute availability) • CA study of 33% renewables found increased operating reserves of 3-7%

15. 4) Does a wind plant start or stop very suddenly? • Large wind farms have many individual wind turbines • The turbines are spread over many miles and do not experience the same wind at the same time • TX event Feb 24, 2007: drop of 1,500 MW over 2 hours is similar to behavior of load

18. Typical monthly wind-speed state transition matrix

19. 5. Does Geographic diversity help?

20. 6. Which generation is displaced when the wind plant generates power?

21. Wind displaces the most expensive (marginal) generation, absent dispatch or other constraints • Reduced CT and gas-oil generation in this scenario (your mileage will vary)

22. Do other generators reduce output by 1kWh when wind/solar produces 1kWh? • Yes – if not in this balancing area, then another • (unless generation is charging a storage system) • System must be balanced ∑ loads + reserves = ∑ generation • Unit or combination of units run at reduced output • May be a heat-rate penalty (generators are less efficient at partial output) • Some units could perhaps be de-committed • CA study • Mostly combined cycle units were displaced (40% CA and 60% WECC) • 7500 MW wind and 1900 MW solar reduced NOx ~1500 tons and SOx ~725 tons

23. 7. What happens in the control center when the wind stops generating power? • Operating reserves (other generation) is deployed • Committed, spinning resource in short time frame, in merit order subject to ramp constraints • Units may be started over longer time frame if economic • Import/export from neighboring Balancing Area

24. 8. Does wind need backup or storage? • Increased operating reserves may be necessary, but not dedicated backup • Although new storage has value, it may not be cost effective • There is typically already storage on the system • Natural gas in the pipeline or storage facility • Controllable hydro • A recent study by Xcel/PSCO found • Using existing pumped storage to offset variability provided $1.30/MWh reduction in wind integration cost • Enlarging existing gas storage facility was economic at large wind penetration

25. 9. Does wind have capacity value? How is it measured and what does it mean? • System Resource Adequacy: is there enough installed generation to meet load? • Reliability methods applied to power systems • Loss of load probability (or related metric) • Loss of load expectation (LOLE) and reliability target such as 1 day/10 years LOLE

26. Effective Load Carrying Capability (ELCC) Each generator added to the system helps increase the load that can be supplied at all reliability levels Gi Gi+1Gi+2 Added Generators

27. What is ELCC? • What ELCC is Not • a minimum generation value • a schedule or forecast for wind • ELCC is • Measure of wind (or other resource’s) contribution to overall system adequacy • Decomposition of the generator’s contribution to adequacy

28. 10. What is wind’s impact on power systems operation, and is there a cost? • Wind integration studies can help answer this question • Two primary impacts: variability and uncertainty • Data requirements • High fidelity wind power data set • Detailed system specifications (generator characteristics, loads) • System simulation and statistical analysis

29. Each relevant time frame for system operations is analyzed to determine wind’s impact • Evaluate operating impacts and associated costs • Regulation • Load Following • Unit Commitment • Evaluate reliability impacts • Effective Load Carrying Capability/Loss of Load Probability

30. 10. What is wind’s impact on power systems operation, and is there a cost? • Minnesota 25% wind energy penetration (by energy) causes an increase in variability that must be met by power system operators and the non-wind generation fleet

31. Impact on Uncertainty • Increase in regulating and operating reserves • Operating reserve will depend on wind generation in real-time • Change in unit commitment (wind forecasts can help – more later)

32. Comparison of Cost-BasedU.S. Operational Impact Studies * 3-year average; total is non-market cost ** highest integration cost of 3 years; 30.7% capacity penetration corresponding to 25% energy penetration; 24.7% capacity penetration at 20% energy penetration *** found $4.37/MWh reduction in UC cost when wind forecasting is used in UC decision

33. 11. How much does wind forecasting help? • Wind forecasts are derived from weather prediction models • Wind forecast accuracy is improving • Several wind forecasting firms in U.S. Courtesy: WindLogics, Inc. St. Paul, MN

34. Wind forecasting benefits for NYISO • 10% penetration, 2005 operating costs with wind forecasts

35. Forecasting benefits for CAISO

36. 12. Is there a limit to how much wind can be accommodated on the grid? • Current studies in the U.S. have analyzed up to 25% of all electric energy from wind • Based on work done so far, the question is not whether wind can be accommodated at high penetrations, the question is how and at what cost of integration

37. Recent International Energy Agency Report: Design and operation of power systems with large amounts of wind power http://www.uwig.org/IEA_Annex25-State_of_the_Art_Report.pdf

38. 12. Is there a limit to how much wind can be accommodated on the grid? Denmark has access to large export markets Lennart Söder,KTH, Sweden, presented at UWIG, Oct 23-25, 2006

39. 13. How do wind, concentrating solar power, and PV plants impact the grid? • Investigation in process in Western Wind & Solar Integration Study • 30% Wind, 3.5% CSP, 1.5% PV by energy in study footprint • Our prediction: • CSP (with thermal storage) will help the grid • Wind will have load following impacts • Distributed PV will have little impact, but centralized PV could have impacts on regulation and possibly load following

40. Need for Subhourly PV Data and Analysis

41. Wind power output

42. PV and CSP output on sunny dayConfidential Data

43. Wind output (113 MW Storm Lake, Iowa)

44. PV and CSP on a windy cloudy dayConfidential Data

45. Wind output (113 MW Storm Lake, Iowa)

46. PV and CSP on a cloudy day with some windConfidential Data

47. Wind output (113 MW Storm Lake, Iowa)

48. 14. How can more wind be accommodated on the grid? • Utility balancing areas can combine or cooperate – large electricity markets (example: Denmark/Europe)

49. 14. How can more wind be accommodated on the grid? • Utility balancing areas can combine or cooperate – large electricity markets • Example: Ramping, or changing output of generators that can be eliminated with larger balancing areas

50. 14. How can more wind be accommodated on the grid? • Power system operations practices and wind farm control/curtailment • Integration of wind forecasting and real time measurements into control room operations (WindLogics/EnerNex/UWIG/ Xcel study underway in Minnesota) • Hydro dispatch, pumped hydro • Longer term: other storage and markets (plug-hybrid electric vehicles, hydrogen)