REU on Wind Energy Science, Engineering, & PolicySummer 2011

Iowa State University

Electric Power Industry Overview, Power System Operation, and

Handling Wind Power Variability in the Grid

James D. McCalleyHarpole Professor of

Electrical & Computer Engineering

Outline1. The electric power industry2. Control centers3. Basic problems, potential solutions4. Wind power equation5. Variability6. System Control7. Comments on potential solutions

2

Organizations comprising the Electric Power IndustryOrganizations 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

Apr 1990: UK Pool

opens

Jan. 1991: Norway launches Nordpool

Jan. 1996: Sweden in Nordpool

Oct 1996: New

Zealand NZEM

Jan 1998: PJM ISO created

Mar 1998: Cal ISO opens

Jan. 1998: Finland in Nordpool

Dec 1998: Australia

NEM opens

Nov 1999: NY ISO launches

May 1999: ISO-NE opens

Jan. 2000: Denmark in

Nordpool

Mar 2001: NETA

replaces UK Pool

July 2001: ERCOT becomes

one control

area May 2002:

Ontario IMO

launches

North America

1990 1992 2000 1998 1996 1994

Jan. 2001: Alberta Pool opens

Overseas

2002 2004 2006

Dec 2001 MISO becomes first RTO

Feb 1996 MISO formed.

April 2005 MISO Markets Launch

1996: ERCOT becomes ISO.

Jan 2002 ERCOT opens retail zonal mrket

2008 Feb 2007 SPP Markets Launch

Dec 2008 ERCOT Nodal Market

Launched

Big changes between 1992 and 2002….Big changes between 1992 and 2002….

1900-1996/2000

G G

G

G

G

G

G

G

TransmissionOperator

IndependentSystem

Operator

TransmissionOperator

TransmissionOperator

Today

G G G

G

G

G

G G

Transmission and System Operator

Vertically Integrated Utility

IndependentSystem

Operator

5

What are ISOs?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?What are the North American Interconnections?

“Synchronized”

7

What is NERC?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)?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 CentersEnergy 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

SubstationRemote terminal unit

SCADA Master Station

Com

mun

icati

on li

nk

Energy control center with EMS

EMS alarm displayEMS 1-line diagram 11

ECCs: EMS & SCADA

Today’s real-time market functions

SCADA

Overloads & Voltage Problems

Potential Overloads &

Voltage Problems

Breaker/Switch Status Indications

System Model Description

Telemetry & Communications equipment

State Estimator

Network Topology program

AGC

Economic Dispatch

OPF

Contingency Selection

Contingency Analysis Security Constrained OPF

Display Alarms

Updated System Electrical Model

Analog Measurements

Display to Operator

Power flows, Voltages etc.,

Display to Operator

Bad Measurement Alarms

Generator Outputs Generation Raise/Lower Signals

State Estimator Output

Substation RTUs and

power plants

EMS

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 balance1. Wind is a variable resource when maximizing

energy productiona. Definition: NETLOAD.MW=LOAD.MW-WIND.MWb. Fact: Wind increases NETLOAD.MW variability in gridc. Fact: Grid requires GEN.MW=NETLOAD.MW alwaysd. 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 resourcea. 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 productionWind power equation

v1 vt v2

v

x

Swept area At of turbine blades:

The disks have larger cross sectional area from left to right because• v1 > vt > v2 and• the mass flow rate must be the same everywhere within the streamtube:

ρ=air density (kg/m3)

Therefore, A 1 < At < A 2

2211

21

vAvAvA

QQQ

tt

t

Mass flow rate is the mass of substance which passes through a given surface per unit time.

18

Power productionWind power equation

ttt

t vAt

xA

t

mQ

3. Mass flow rate at swept area:

22

212

1vvmKE

1. Wind velocity:t

xv

xAm 2. Air mass flowing:

4a. Kinetic energy change:

5a. Power extracted: 2

221

22

21 2

1

2

1vvQvv

t

m

t

KEP t

6a. Substitute (3) into (5a):)()2/1( 2

221 vvvAP tt

4b. Force on turbine blades:

21 vvQvt

m

t

vmmaF t

5b. Power extracted:

21 vvvQFvP ttt

6b. Substitute (3) into (5b):)( 21

2 vvvAP tt

ttttt vvvvvvvvvvvvvvvvv ))(2/1()())(()2/1()()()2/1( 12212

21212122

221

7. Equate

8. Substitute (7) into (6b): ))((4

)()))(2/1(( 2122

2121

221 vvvv

AvvvvAP t

t

9. Factor out v13: )1)()(1(

4 1

22

1

231

v

v

v

vvAP t

19

Power productionWind power equation

10. Define wind stream speed ratio, a: 1

2

v

va

)1)(1(4

231 aa

vAP t

11. Substitute a into power expression of (9):

12. Differentiate and find a which maximizes function:

1,3/10)1)(13(

0123122

0)1()1(24

222

231

aaaa

aaaaa

aaavA

a

P t

This ratio is fixed for a given turbine & control condition.

13. Find the maximum power by substituting a=1/3 into (11):

27

8

3

4

9

8

4)

3

4)(

9

11(

4

31

31

31 vAvAvA

P ttt

20

Power productionWind power equation

14. Define Cp, the power (or performance) coefficient, which gives the ratio of the power extracted by the converter, P, to the power of the air stream, Pin.

)1)(1(4

231 aa

vAP t

31

211

211

21 2

1

2

1

2

10

2

1vAvvAvQv

t

m

t

KEP ttin

power extracted by the converter

power of the air stream

)1)(1(2

1

21

)1)(1(4 2

31

231

aavA

aavA

P

PC

t

t

inp

15. The maximum value of Cp occurs when its numerator is maximum, i.e., when a=1/3:

5926.027

16)

3

4)(

9

8(

2

1

inp P

PC

The Betz Limit!

312

1vACPCP tPinp

21

Power productionCp vs. λ and θ

Tip-speed ratio:11 v

R

v

u u: tangential velocity of blade tip

ω: rotational velocity of blade

R: rotor radiusv1: wind speed

Pitch: θ

GE SLE 1.5 MW 22

Power productionWind Power Equation

31),(

2

1vACPCP tPinp

So power extracted depends on 1.Design factors:

• Swept area, At 2.Environmental factors:

• Air density, ρ (~1.225kg/m3 at sea level)• Wind speed v3

3. Control factors affecting performance coefficient CP: • Tip speed ratio through the rotor speed ω• Pitch θ 23

Power productionCp vs. λ and θ

Tip-speed ratio:11 v

R

v

u u: tangential velocity of blade tip

ω: rotational velocity of blade

R: rotor radiusv1: 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 productionUsable speed range

Cut-in speed (6.7 mph) Cut-out speed (55 mph)

25

Wind Power Temporal & Spatial Variability

26

JULY2006JANUARY2006

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

-100

-80

-60

-40

-20

0

20

40

60

80

100

07:00 07:20 07:40 08:00 08:20 08:40 09:00 09:20 09:40 10:00

RE

GU

LA

TIO

N I

N M

EG

AW

AT

TS

Regulation

=

+

Load Following Regulation

Source: Steve Enyeart, “Large Wind Integration Challenges for Operations / System Reliability,” presentation by Bonneville Power Administration, Feb 12, 2008, available athttp://cialab.ee.washington.edu/nwess/2008/presentations/stephen.ppt.

27

27

MW and Frequency

2828

How Does Power System Handle Variability

29

Turbine-Gen 1Turbine-Gen 2Turbine-Gen …Turbine-Gen N

∆f∆Ptie

ACE=∆Ptie +B∆f

Primary control provides regulation

Secondary control provides load following

29

Characterizing Netload Variability∆T HISTOGRAMMeasure each ∆T variation for 1 yr (∆T=1min, 5min, 1 hr)Identify “variability bins” in MWCount # of intervals in each variability binPlot # against variability binCompute standard deviation σ.

Regulation

Load following

Ref: Growing Wind; Final Report of the NYISO 2010 Wind Generation Study, Sep 2010.www.nyiso.com/public/webdocs/newsroom/press_releases/2010/GROWING_WIND_-

_Final_Report_of_the_NYISO_2010_Wind_Generation_Study.pdf

30

Loads:2011: 12600 MW2013: 12900 MW2018: 13700 MW

Solutions to variability & uncertainty1. 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 controlc. Storaged. Expand control areas

31

%/min $/mbtu $/kw LCOE,$/mwhr

Coal 1-5 2.27 2450 64

Nuclear 1-5 0.70 3820 73

NGCC 5-10 5.05 984 80

CT 20 5.05 685 95

Diesel 40 13.8131

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

33