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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. - PowerPoint PPT Presentation
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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
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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
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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
xAtmQ
3. Mass flow rate at swept area:
22
212
1 vvmKE
1. Wind velocity:txv
xAm 2. Air mass flowing:
4a. Kinetic energy change:
5a. Power extracted: 2
221
22
21 2
121 vvQvv
tm
tKEP t
6a. Substitute (3) into (5a):)()2/1( 2
221 vvvAP tt
4b. Force on turbine blades: 21 vvQv
tm
tvmmaF 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 vvvvAvvvvAP t
t
9. Factor out v13: )1)()(1(
4 1
22
1
231
vv
vvvAP t
19
Power productionWind power equation
10. Define wind stream speed ratio, a: 1
2vva
)1)(1(4
231 aavAP 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
aaaaaaaaa
aaavAaP t
This ratio is fixed for a given turbine & control condition.
13. Find the maximum power by substituting a=1/3 into (11):
278
34
98
4)
34)(
911(
4
31
31
31 vAvAvAP 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 aavAP t
31
211
211
21 2
121
210
21 vAvvAvQv
tm
tKEP ttin
power extracted by the converter
power of the air stream
)1)(1(21
21
)1)(1(4 2
31
231
aavA
aavA
PPC
t
t
inp
15. The maximum value of Cp occurs when its numerator is maximum, i.e., when a=1/3:
5926.02716)
34)(
98(
21
in
p PPC
The Betz Limit!
312
1 vACPCP tPinp
21
Power productionCp vs. λ and θ
Tip-speed ratio:11 vR
vu
u: tangential velocity of blade tipω: rotational velocity of blade R: rotor radiusv1: wind speed
Pitch: θ
GE SLE 1.5 MW 22
Power productionWind Power Equation
31),(
21 vACPCP 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 vR
vu
u: tangential velocity of blade tipω: 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
REG
ULAT
ION
IN M
EGAW
ATTS
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.
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