REU on Wind Energy Science, Engineering, & Policy Summer 2011 Iowa State University Electric Power Industry Overview, Power System Operation, and Handling Wind Power Variability in the Grid James D. McCalley Harpole Professor of Electrical & Computer Engineering

Outline 1.The electric power industry 2.Control centers 3.Basic problems, potential solutions 4.Wind power equation 5.Variability 6.System Control 7.Comments on potential solutions 2

Organizations comprising the Electric Power Industry Investor-owned utilities: 210 (MEC, Alliant, Xcel, Exelon, …) Federally-owned: 10 (TVA, BPA, WAPA, SEPA, APA, SWPA…) Public-owned: 2009 (Ames, Cedar Falls, Muscatine, …) Consumer-owned: 883 (Dairyland, CIPCO, Corn Belt, …) Non-utility power producers: 1934 (Alcoa, DuPont,…) Power marketers: 400 (e.g., Cinergy, Mirant, Illinova, Shell Energy, PECO- Power Team, Williams Energy,…) Coordination organizations: 10 (ISO-NE, NYISO, PJM, MISO, SPP, ERCOT, CAISO, AESO, NBSO) Oversight organizations: Regulatory: 52 state, 1 Fed (FERC) Reliability: 1 National (NERC), 8 regional entities Environment: 52 state (DNR), 1 Fed (EPA) Manufacturers: GE, ABB, Toshiba, Schweitzer, Westinghouse,… Consultants: Black&Veatch, Burns&McDonnell, HD Electric,… Vendors: Siemens, Areva, OSI,… Govt agencies: DOE, National Labs,… Professional organizations: IEEE PES … Advocacy organizations: AEWA, IWEA, Wind on Wires… Trade Associations: EEI, EPSA, NAESCO, NRECA, APPA, PMA,… Law-making bodies: 52 state legislatures, US Congress 3

4 Big changes between 1992 and 2002….

/2000 G G G G G G G G Transmission Operator Independent System Operator Transmission Operator Transmission Operator Today GGG G G G GG Transmission and System Operator Vertically Integrated Utility Independent System Operator 5

What are ISOs? The regional system operator: monitors and controls grid in real-time The regional market operator: monitors and controls the electricity markets The regional planner: coordinates the 5 and 10 year planning efforts Also the Regional Transmission Organization (RTO) They do not own any electric power equipment! None of them existed before 1996! California ISO (CAISO) Midwest ISO (MISO) Southwest Power Pool (SPP) Electric Reliability Council of Texas (ERCOT) New York ISO (NYISO) ISO-New England (ISO-NE) Pennsylvania- Jersey-Maryland (PJM) 6

What are the North American Interconnections? “Synchronized” 7

What is NERC? NERC: The North American Reliability Corporation, certified by federal government (FERC) as the “electric reliability organization” for the United States. Overriding responsibility is to maintain North American bulk transmission/generation reliability. Specific functions include maintaining standards, monitoring compliance and enforcing penalties, performing reliability assessments, performing event analysis, facilitating real-time situational awareness, ensuring infrastructure security, trains/certifies system operators. There are eight NERC regional councils (see below map) who share NERC’s mission for their respective geographies within North America through formally delegated enforcement authority Western Electricity Coordinating Council (WECC) Midwest Reliability Organization (MRO) Southwest Power Pool (SPP) Texas Reliability Entity (TRE) Reliability First Corporation (RFC) Southeast Electric Reliability Council (SERC) Florida Reliability Coordinating Council (FRCC) Northeast Power Coordinating Council (NPCC) 8

What is a Balancing Authority (BA)? From NERC: A BA is the responsible entity that integrates resource plans ahead of time, maintains load-interchange-generation balance within a Balancing Authority Area, and supports Interconnection frequency in real time. This means it is the organization responsible for performing the load/generation balancing function. All ISOs are BAs but many BAs are not ISOs. Main functions of BA: unit commitment, dispatch, Automatic Gen Control (AGC). Unit commitment: Determine in the next time interval (week, 2 day, 24 hrs, 4 hrs) which gen units should be connected (synchronized). Dispatch: Determine in the next time interval (1 hr, 15 mins, 5 mins), what should be the MW output for each committed gen unit. AGC: Maintain frequency at 60Hz in the interconnection, ensure load changes in the BA are met by gen changes in the BA, maintain tie line flows at scheduled levels. 9

Energy Control Centers Energy Control Center (ECC): SCADA, EMS, operational personnel “Heart” (eyes & hands, brains) of the power system Supervisory control & data acquisition (SCADA): Supervisory control: remote control of field devices, including gen Data acquisition: monitoring of field conditions SCADA components: »Master Station: System “Nerve Center” located in ECC »Remote terminal units: Gathers data at substations; sends to Master Station »Communications: Links Master Station with Field Devices, telemetry is done by either leased wire, PLC, microwave, or fiber optics. Energy management system (EMS) Topology processor & network configurator State estimator and power flow model development Automatic generation control (AGC), Optimal power flow (OPF) Security assessment and alarm processing 10

Substation Remote terminal unit SCADA Master Station Communication link Energy control center with EMS EMS alarm display EMS 1-line diagram 11 ECCs: EMS & SCADA

12 ECCs: EMS & SCADA

More energy control centers 13

More energy control centers 14

15 Electricity “two settlement” markets Day-Ahead Market (every day) Real-Time Market (every 5 minutes) Energy & reserve offers from gens Energy bids from loads Internet system Which gens get committed, at roughly what levels for next 24 hours, and settlement Internet system Energy offers from gens Energy bids from loads Generation levels for next 5 minutes and settlement for deviations from day-ahead market

Basic problems with wind & power balance 1.Wind is a variable resource when maximizing energy production a.Definition: NETLOAD.MW=LOAD.MW-WIND.MW b.Fact: Wind increases NETLOAD.MW variability in grid c.Fact: Grid requires GEN.MW=NETLOAD.MW always d.Fact: “Expensive” gens move (ramp) quickly, “cheap” gens don’t, some gens do not ramp at all. e.Problem: Increasing wind increases need for more and “faster” resources to meet variability, increasing cost of wind. 2.Wind is an uncertain resource a.Fact: Market makes day-ahead decisions for “unit commitment” (UC) based on NETLOAD.MW forecast. b.Fact: Large forecast error requires available units compensate. c.Problem: Too many (under-forecast) or too few (over-forecast) units may be available, increasing the cost of wind. 16

Solutions to variability & uncertainty 1.We have always dealt with variability and uncertainty in the load, so no changes are needed. 2.Increase MW control capability during periods of expected high variability via control of the wind power. 3.Increase MW control capability during periods of expected high variability via more conventional generation. 4.Increase MW control capability during periods of expected high variability using demand control. 5.Increase MW control capability during periods of expected high variability using storage. 17 Groups of 2-3, 5 minutes Identify your preferred approach to the variability problem Consider the below solutions, one, or combination, or other Identify reasons (e.g., economics, effectiveness, sustainability) and have one person report to class at end of 10 minutes 17

Power production Wind power equation v1v1 vtvt v2v2 v xx Swept area A t of turbine blades: The disks have larger cross sectional area from left to right because v 1 > v t > v 2 and the mass flow rate must be the same everywhere within the streamtube: ρ=air density (kg/m 3 ) Therefore, A 1 < A t < A 2 Mass flow rate is the mass of substance which passes through a given surface per unit time. 18

Power production Wind power equation 3. Mass flow rate at swept area: 1. Wind velocity: 2. Air mass flowing: 4a. Kinetic energy change: 5a. Power extracted: 6a. Substitute (3) into (5a): 4b. Force on turbine blades: 5b. Power extracted: 6b. Substitute (3) into (5b): 7. Equate 8. Substitute (7) into (6b): 9. Factor out v 1 3 : 19

Power production Wind power equation 10. Define wind stream speed ratio, a: 11. Substitute a into power expression of (9): 12. Differentiate and find a which maximizes function: This ratio is fixed for a given turbine & control condition. 13. Find the maximum power by substituting a=1/3 into (11): 20

Power production Wind power equation 14. Define C p, the power (or performance) coefficient, which gives the ratio of the power extracted by the converter, P, to the power of the air stream, P in. power extracted by the converter power of the air stream 15. The maximum value of C p occurs when its numerator is maximum, i.e., when a=1/3: The Betz Limit! 21

Power production Cp vs. λ and θ Tip-speed ratio: u: tangential velocity of blade tip ω: rotational velocity of blade R: rotor radius v 1 : wind speed Pitch: θ GE SLE 1.5 MW 22

Power production Wind Power Equation So power extracted depends on 1.Design factors: Swept area, A t 2.Environmental factors: Air density, ρ (~1.225kg/m 3 at sea level) Wind speed v 3 3. Control factors affecting performance coefficient C P : Tip speed ratio through the rotor speed ω Pitch θ 23

Power production Cp vs. λ and θ Tip-speed ratio: u: tangential velocity of blade tip ω: rotational velocity of blade R: rotor radius v 1 : wind speed GE SLE 1.5 MW Important concept #1: The control strategy of all US turbines today is to operate turbine at point of maximum energy extraction, as indicated by the locus of points on the black solid line in the figure. Important concept #2: This strategy maximizes the energy produced by a given wind turbine. Any other strategy “spills” wind !!! Important concept #3: Cut-in speed>0 because blades need minimum torque to rotate. Generator should not exceed rated power Cut-out speed protects turbine in high winds 24

Power production Usable speed range Cut-in speed (6.7 mph)Cut-out speed (55 mph) 25

Wind Power Temporal & Spatial Variability 26 JULY2006 JANUARY2006 Notice the temporal variability: lots of cycling between blue and red; January has a lot more high-wind power (red) than July; Notice the spatial variability “waves” of wind power move through the entire Eastern Interconnection; red occurs more in the Midwest than in the East Blue~VERY LOW POWER; Red~VERY HIGH POWER 26

MW-Hz Time Frames = + Load FollowingRegulation Source: Steve Enyeart, “Large Wind Integration Challenges for Operations / System Reliability,” presentation by Bonneville Power Administration, Feb 12, 2008, available at 27

MW and Frequency 28

How Does Power System Handle Variability 29 Turbine-Gen 1 Turbine-Gen 2 Turbine-Gen … Turbine-Gen N ∆f ∆P tie ACE= ∆P tie +B∆f Primary control provides regulation Secondary control provides load following 29

Characterizing Netload Variability ∆T HISTOGRAM Measure each ∆T variation for 1 yr (∆T=1min, 5min, 1 hr) Identify “variability bins” in MW Count # of intervals in each variability bin Plot # against variability bin Compute standard deviation σ. Regulation Load following Ref: Growing Wind; Final Report of the NYISO 2010 Wind Generation Study, Sep _Final_Report_of_the_NYISO_2010_Wind_Generation_Study.pd f 30 Loads: 2011: MW 2013: MW 2018: MW

Solutions to variability & uncertainty 1.Do nothing: fossil-plants provide reg & LF (and die  ). 2.Increase control of the wind generation a.Provide wind with primary control Reg down (4%/sec), but spills wind following the control Reg up, but spills wind continuously b.Limit wind generation ramp rates Limit of increasing ramp is easy to do Limit of decreasing ramp is harder, but good forecasting can warn of impending decrease and plant can begin decreasing in advance 3.Increase non-wind MW ramping capability during periods of expected high variability using one or more of the below: a.Conventional generation b.Load control c.Storage d.Expand control areas 31 %/min$/mbtu$/kw LCOE,$/mwhr Coal Nuclear NGCC CT Diesel

Why Does Variability Matter?  NERC penalties for poor-performance  Consequences of increased frequency variblty:  Some loads may lose performance (induction motors)  Relays can operate to trip loads (UFLS), and gen (V/Hz)  Lifetime reduction of turbine blades  Frequency dip may increase for given loss of generation  Areas without wind may regulate for windy areas  Consequences of increased ACE variability (more frequent MW corrections):  Increased inadvertent flows  Increase control action of generators  Regulation moves “down the stack,” cycling! 32

How to decide? First, primary frequency control for over-frequency conditions, which requires generation reduction, can be effectively handled by pitching the blades and thus reducing the power output of the machine. Although this action “spills” wind, it is effective in providing the necessary frequency control. Second, primary frequency control for under-frequency conditions requires some “headroom” so that the wind turbine can increase its power output. This means that it must be operating below its maximum power production capability on a continuous basis. This also implies a “spilling” of wind. Question: Should we “spill” wind in order to provide frequency control, in contrast to using all wind energy and relying on some other means to provide the frequency control? Answer: Need to compare system economics between increased production costs from spilled wind, and increased investment, maint, & production costs from using storage & conventional gen. 33