PSERC SeminarFebruary 9, 2010
Iowa State University
Frequency control (MW-Hz) with wind
James D. McCalley
Harpole Professor of Electrical & Computer Engineering
1
Outline1. MW-Hz time frames
2. Transient frequency control
3. Variability
4. Regulation
5. Load following
6. Storage
7. Conclusions
2
MW-Hz Time Frames
0+<t<2s; Inertial
t=0+; Proximity
2s<t<10s; Speed-governors 10s<t<3m; AGC
1m<t; AGC, ED
3
MW-Hz Time Frames
Source: H. Holttinen, “The impact of large‐scale power production on the Nordic electricity system,” VTT Publications 554, PhD Dissert, Helsinki U. of Technology, 2004.
Transient frequency response
Inertia & governor
RegulationGovernor & AGC
Load followingAGC and ED
SchedulingED & UC
4
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
REGU
LATI
ON IN
MEG
AWAT
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.
5
Transient frequency control
What can happen if frequency dips too low?• For f<59.75 Hz, underfrequency relays can trip load.• For f<59 Hz, loss of life on turbine blades• Violation of NERC criteria with penalties
• N-1: Frequency not below 59.6 Hz for >6 cycles at load buses• N-2: Frequency not below 59.0 Hz for >6 cycles at load buses
6
Transient frequency control
fn
i
i
L m
H
fP
dt
fd
1
Re
2
60
t1
mf1
mf2
mf3
Time (sec)
Frequency(Hz)
60-mf1t1
60-mf2t1
60-mf3t1
60
t1
mf1
mf2
mf3
Time (sec)
Frequency(Hz)
60-mf1t1
60-mf2t1
60-mf3t1
The greater the rate of change of frequency (ROCOF) following loss of a generator ∆PL, the lower will be the frequency dip. ROCOF increases as total system inertia ΣHi decreases.Therefore, frequency dip increases as ΣHi decreases.
7
Transient frequency control
49.35
Nadir
2.75 sec
sec/227.0475*2
)50(32.4
21
Re Hz
H
fP
dt
fdm
n
i
i
Lf
Example: Ireland: ∆PL =432 MW=4.32 pu. ΣHi =475 sec
1. Governors2. Load frequency sensitivity
50-0.227*2.75=49.38Hz
8
Transient frequency controlExample: Estrn Interconnection: ∆PL =2900 MW=29 pu. ΣHi =32286 sec
Nadir59.9828 Hz
59.9725z
sec/0269.032286*2
)60(29
21
Re
Hz
H
fP
dt
fdm
n
i
i
Lf
60-0.0269*1.5=59.9597Hz
9
Transient frequency control
So what is the issue with wind….?1. Increasing wind penetrations tend to displace
(decommit) conventional generation.2. DFIGs, without specialized control, do not contribute
inertia. This “lightens” the system(decreases denominator) fn
i
i
L m
H
fP
dt
fd
1
Re
2
Let’s see an example….
10
Transient frequency control
• Green: Base Case
• Dark Blue: 2% Wind Penetration
• Light Blue: 4% Wind Penetration
• Red: 8% Wind Penetration
Estrn Interconnection: Frequency dip after 2.9GW Gen drop for Unit De-Commitment scenario at different wind penetration levels (0.6, 2, 4, 8%)
11
Transient frequency controlWhy do DFIGs not contribute inertia?
They do not decelerate in response to a frequency drop.
FUELSteam Boiler
Generator
CONTROL SYSTEM
Steam valve controlFuel supply control
MVAR-voltage control
Wind speed
Gear Box
Generator
CONTROL SYSTEM
MVAR-voltage control
Real power output control
STEAM-TURBINE
WIND-TURBINE
The ability to control mech torque applied to the generator using pitch control & electromagnetic torque using rotor current control (to optimize Cp and to avoid gusting) enables avoidance of mismatch between mechanical torque and electromagnetic torque and, therefore, also avoidance of rotor deceleration under network frequency decline.
12
Transient frequency control
Ireland sees significant ∆f for loss of largest unit. Estrninterconnection (EI) sees small ∆f for loss of largest unit.
Ireland total system inertia is 475 sec. Estrn interconnection total system inertia is 32286 sec.(ERCOT and WECC are between these extremes.)The “heavier” the system, the less frequency moves.
Hard to cause trans freq dip problem in the EI with N-1 outage, but frequency stability is still of concern because:1. Islanding conditions can be susceptible2. Control performance standards may be impacted.
13
Transient frequency control – CPS1, CPS2
2min min 1
min10ute ute
ute
ACECF f
B
min min1 2 100%ute uteCPS CF
10 10
2 1010
1.65 10 10
1i sL B B
CF ACEL
2
2
Number of intervals that CF 1Total number of intervals100(1 )%
R
CPS R
1 10min 11
*10
i
i
ACEC AVG F
B
2 10 min ( )ute iC AVG ACE
2
1min{ } periodRMS dF AVG F
14
Transient frequency controlWhat is the fix for this? Consider DFIG control system
Source: J. Ekanayake, L. Holdsworth, and N. Jenkins, “Control of DFIG Wind Turbines,” Proc. Instl Electr. Eng., Power Eng., vol. 17, no. 1, pp. 28-32, Feb 2003.
15
Transient frequency controlAdd “inertial emulation,” a signal dω/dt scaled by 2H
-2H
dω / dt
16
Transient frequency controlSeveral European grid operators have imposed requirements on wind plants in regards to frequency contributions, including Nordic countries [1,2]. North American interconnections have so far not imposed requirements on wind farms in regards to frequency contributions, with the exception of Hydro-Quebec. (We better fix this before installing 600 GW of wind!!!!)
Hydro Quebec requires that wind farms be able to contribute to reduce large (>0.5 Hz), short-term (< 10 sec) frequency deviation [3]. The Hydro-Quebec requirement states [4], “The frequency control system must reduce large, short-term frequency deviations at least as much as does the inertial response of a conventional generator whose inertia (H) equals 3.5 sec.”[1] “Wind Turbines Connected to Grids with Voltages above 100 kV – Technical Regulation for the Properties and the Regulation of Wind Turbines, Elkraft System and Eltra Regulation, Draft version TF 3.2.5, Dec., 2004. [2] “Nordic Grid Code 2007 (Nordic Collection of Rules), Nordel. Tech. Rep., Jan 2004, updated 2007. [3] N. Ullah, T. Thiringer, and D. Karlsson, “Temporary Primary Frequency Control Support by Variable Speed Wind Turbines – Potential and Applications,” IEEE Transactions on Power Systems, Vol. 23, No. 2, May 2008. [4] “Technical Requirements for the Connection of Generation Facilities to the Hydro-Quebec Transmission System: Supplementary Requirements for Wind Generation,” Hydro Quebec, Tech. Rp., May 2003, revised 2005.
17
Temporal Variability
Source: Task 25 of the International Energy Agency (IEA), “Design and operation of power systems with large amounts of wind power: State-of-the-art report,” available at www.vtt.fi/inf/pdf/workingpapers/2007/W82.pdf.
18
Spatial Variability (geo-diversity)
Source: Task 25 of the International Energy Agency (IEA), “Design and operation of power systems with large amounts of wind power: State-of-the-art report,” available at www.vtt.fi/inf/pdf/workingpapers/2007/W82.pdf.
19
Spatial Variability (geo-diversity)
Source: Task 25 of the International Energy Agency (IEA), “Design and operation of power systems with large amounts of wind power: State-of-the-art report,” available at www.vtt.fi/inf/pdf/workingpapers/2007/W82.pdf.
20
Variability of net load
1 hour 10 minσ max σ max
Load (MW) 123 400 22 135Net load (MW) 130 499 23.6 158
Source:V. Vittal, J. McCalley, V. Ajjarapu, “Impact of Increased DFIGWind Penetration On Power Systems And Markets,” Final report, Power System Engineering Research Center, 2009.
21
Evaluating regulation share of a gen or load
T
TLwX2
222
ORNL Method – allocation of regulation [1]: The basic concept of this method stems from the following: • If a wind farm’s natural diurnal cycle is positively-correlated
with the 24 hour load cycle, then ; the wind will ramp with the load, and there will be less need for load following.• If a wind farm’s natural diurnal cycle is negatively-correlated
with the 24 hour load cycle, then ; the wind will ramp against the load, and there will be less need for load following.[1] B. Kirby, M. Milligan, Y. Makarov, D. Hawkins, K. Jackson, H. Shiu “California Renewables Portfolio Standard Renewable Generation Integration Cost Analysis, Phase I: One Year Analysis Of Existing Resources, Results And Recommendations, Final Report,” Dec. 10, 2003, available at http://www.consultkirby.com/files/RPS_Int_Cost_PhaseI_Final.pdf.
wind.less load totalofVar :
load. totalofVar :
plant windofVar :
2
2
2:
T
L
w
22LT
22LT
Contribution of wind variability to net load variability
22
Solutions to increased variability
1. Increase control of the wind generation via pitch controla. Provide regulation and/or load following capabilityb. 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
2. Increase non-wind MW ramping capability during periods of expected high variability using one or more of the below:a. Conventional generation b. Storage (e.g., pumped storage, CAES, batteries…)c. Load control
23
Ensure availability of high-ramp rate units
Source: www.xcelenergy.com/COMPANY/ABOUT_ENERGY_AND_RATES/RESOURCE%20AND%20RENEWABLE%20ENERGY%20PLANS/Pages/2007_Minnesota_Resource_Plan.aspx
Steam turbine plants 1- 5 %/minNuclear plants 1- 5 %/minGT Combined Cycle 5 -10 %/min Combustion turbines 20 %/min Diesel engines 40 %/min
“Coal units typically have ramp rates that are in the range of 1% to 1.5% of their nameplate rating per minute between minimum load and maximum load set points. Coal unit minimum load set-points range from 20% to 50% of nameplate, depending on the design of the air quality control system being used. For example, a 500 MW coal plant may have a minimum load of 100 MW and would be able to ramp up at the rate of 5 MW per minute. In addition, it can take a day or more to bring a coal plant up to full load from a cold start condition. Natural gas-fired combustion turbines, on the other hand, can normally be at full load from a cold start in 10 to 30 minutes (which results in an effective ramp rate of 3.3% to 10% of their nameplate rating per minute).”
24
Regulation via rotor speed & pitch control
FUELSteam Boiler
Generator
CONTROL SYSTEM
Steam valve controlFuel supply control
MVAR-voltage control
Wind speed
Gear Box
Generator
CONTROL SYSTEM
MVAR-voltage control
Real power output control
STEAM-TURBINE
WIND-TURBINE
Whereas speed control may be well suited for continuous, fine, frequency regulation, blade pitch control can provide fast acting, coarse control both for frequency regulation as well as emergency spinning reserve.
Pitch control
Rotor speed control
Sources: Rogério G. de Almeida and J. A. Peças Lopes, “Participation of Doubly Fed Induction Wind Generators in System Frequency Regulation,” IEEE Trans On Pwr Sys, Vol. 22, No. 3, Aug. 2007. B. Fox, D. Flynn, L. Bryans, N. Jenkins, D. Milborrow, M. O’Malley, R. Watson, and O. Anaya-Lara, “Wind Power Integration: Connection and system operational aspects,” Institution of engineering and technology, 2007.
25
Regulation via rotor speed & pitch control
[1] “Wind Generation Interconnection Requirements,” Technical Workshop, November 7, 2007, available at www.bctc.com/NR/rdonlyres/13465E96-E02C-47C2-B634-F3BCC715D602/0/November7WindInterconnectionWorkshop.pdf. [2] [North American Electric Reliability Corporation, “Special Report: Accommodating High Levels of Variable Generation,” April 2009, available at http://www.nerc.com/files/IVGTF_Report_041609.pdf.
Review of the websites from TSOs (in Europe), reliability councils (i.e., NERC and regional organizations) and ISOs (in North America) suggest that there are no hard requirements regarding use of primary frequency control in wind turbines.There are soft requirements [1]:•BCTC will specify “on a site by site basis,” •Hydro Quebec requires that wind turbines be “designed so that they can be equipped with a frequency control system (>10MW)”•Manitoba Hydro “reserves the right for future wind generators”NERC [2], said, “Interconnection procedures and standards should be enhanced to address voltage and frequency ride-through, reactive and real power control, frequency and inertial response and must be applied in a consistent manner to all generation technologies.”
26
Regulation via rotor speed & pitch control
[15] Draft White Paper, “Wind Generation White Paper: Governor Response Requirement,” Feb, 2009, available at www.ercot.com/content/meetings/ros/keydocs/2009/0331/WIND_GENERATION_GOVERNOR_RESPONSE_REQUIREMENT_draft.doc..
ERCOT says [1], “…as wind generation becomes a bigger percentage of the on line generation, wind generation will have to contribute to automatic frequency control. Wind generator control systems can provide an automatic response to frequency that is similar to governor response on steam turbine generators. The following draft protocol/operating guide concept is proposed for all new wind generators: All WGRs with signed interconnect agreements dated after March 1, 2009 shall have an automatic response to frequency deviations. …”
27
But are we sure….?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 conventional generation to provide the frequency control? Answer: Need to compare system economics between increased production costs from spilled wind, and increased production and investment costs from using storage and conventional generation.
28
Advanced Dispatch SCADA
Wind ProductionForecasting
Wind Production ForecastMeasured Hybrid Wind System Production Data
Storage Status
System-level Dispatch
Plant-level Control
Production Smoothing Control Command
Hybrid Wind Farm Control
Component-level Control
Energy Storage
Wind Plant
Power Storage
- Pumped Hydro + Hydraulic Turbine - CAES + Gas Turbine- Biomass + Gas Turbine- Hydrogen + Fuel Cell
- Super Capacitor + PE- SMES + PE- Flywheel + PE- Battery + PE PE: Power Electronics
Production Firming Control Command
Hybrid Wind Systems
"Energy" Storage "Power" Storage Storage
Technology
Conversion
Technology
Storage
Technology
Conversion
Technology
Pumped
Hydro
Hydraulic
Turbine (HT)Ultracapacitor
Power
Electronics
(PE)
Compressed
Air Energy
Storage
(CAES)
Gas Turbine
(GT)
Superconducti
ng Magnetic
Energy Storage
(SMES)
Power
Electronics
(PE)
BiomassGas Turbine
(GT)Flywheel
Power
Electronics
(PE)
Hydrogen Full Cell (FC) Battery
Power
Electronics
(PE)
Hybrid Wind Systems – Architecture
Storage and Conversion Technologies
A suite of models facilitate a plug-and-
play modular approach to configure
hybrid wind systems.
i
iwiw
i
igig PCPCpf ,,,,mini
i
i
i RCSP
Stochastic SCOPF
Plant-Level Inter-Device Control
Predictive Control
Grid,Wind turbine, Storage devices, per table right
Component Modeling and Control
Design and Control Hybrid Wind System to Firm and Smooth Wind Variability
29
Hybrid Wind Systems –Save Money, Enhance Frequency Regulation
HOLDEN REDBRIDG CHENAUX CHFALLSMARTDALE
HUNTVILL
NANTCOKE
WALDEN COBDEN MTOWN
GOLDEN BVILLE STRATFRDJVILLE
WVILLE
STINSONPICTON
CEYLON RICHVIEWLAKEVIEW
MITCHELL
PARKHILL
BRIGHTON
HANOVER KINCARD
HEARN
DOUGLAS
Number of buses 60Number of generators 25Number of branches 96Peak Load 6,110MWTotal Generation Capacity 10,995MW
Wind Power Capacity 545MWCAES
Power CapacityCompressor 30MWGas Turbine 75MW
CAES Energy Capacity 17,000MWhNaS Battery Power Capacity 5.5MWNaS Battery Energy Capacity 1.25MWh
0 200 400 600 800 1000 1200 1400 1600 1800-50
0
50
100
150
200
250
300
350
400
Time (s)
Po
we
r C
om
ma
nd
(M
W)
Wind Power
CAES Power NaS Battery Power ×10
0 200 400 600 800 1000 1200 1400 1600 180059.96
59.97
59.98
59.99
60
60.01
60.02
60.03
60.04
Time (S)
Syste
m F
req
ue
ncy (
Hz)
Wind plant
Hybrid Wind Systems
0 200 400 600 800 1000 1200 1400 1600 18002
4
6
8
10
12
Time (s)
Win
d S
pe
ed
(s)
Cost ($M) Saving ($M)Investment Cost Operation Cost
155.15 221.83 481.40
Life time: 20 years 0 200 400 600 800 1000 1200 1400 1600 1800-100
-80
-60
-40
-20
0
20
40
60
80
100
Mis
matc
h (M
W)
With StorageNo Storage
30
Conclusion: Select solution portfolioWind energy attrbute
Grid prblemcaused by wind attrbute
SolutionsDFIG Control Inc.
reservesStorage Load Cntrl Stoch-
asticUnit Cmmtprgrm
Dec fore-cast error
Wind plant remote trip (SPS)
HVDC control
Geo-diversity of wind
Inrtialemu-lation
Freq reg via pitch+ cnvrtr
Fast rmping
Spnng/10 min
1 hour Fast Slow Fast Slow
Estimated relative costs/MW of solution technology (to be refined)5 5 6 10 10 9 9 9 9 4 4 6 10 10
Decreased inertia
Transient frequency dips, CPS2 perfrmance
√ √ √ √Increased 1 min MW variability
CPS2 perfrmance √ √ √ √ √ √
Increased 10 min MW variability
CPS1, CPS2 perfrmance √ √ √ √ √ √ √ √
Increased 1 hr MW variability
Balancing market perfrmance √ √ √ √ √ √ √
Increased day-ahead MW variability
Day-ahead market perfrmance √ √ √ √ √ √ √
Increased transmission loading
Increased need for transmssion
√ √ √Low, variable capacity factor
More planning uncertainty √ √ √ √
31
Solar, 1.0
Nuclear, 15
Hydro, 2.95
Wind, 8.1
Geothermal 3.04
Natural Gas 23.84
Old Coal10.42
Biomass 3.88
Petroleum15.13
26.33
8.58
25.7
8.5
Unused Energy
(Losses)43.0
Electric Generation
49.72
12.68
Used Energy42.15
Residential11.48
Commercial8.58
Industrial23.94
Trans-portation
15.5
15
6.82
20.54
6.95
Reducing 2008 CO2 by 35% sees wind at 34%
INCREASE Non-CO2
12Q to 30Q
32
IGCC, 3
REDUCE PETROLEUM 37Q15Q LightDuty: 8.56QFreight: 3.75QAviation: 3.19Q
32