j. mccalley
DESCRIPTION
J. McCalley. Wind Power Variability in the Grid: Regulation & Load Following. Outline. AGC AGC and wind Control performance standards (CPS) Effect of AGC on CPS. 2. BA 1. BA 2. P 12. X. Two Area System. Stiffness coefficient:. - PowerPoint PPT PresentationTRANSCRIPT
Two Area System
3
BA 1 BA 2P12
X
XT e0
And if the two areas are operating with only primary control, then their “slow” dynamics are represented by the following block diagram.
Stiffness coefficient:
Two area system with primary control dynamics only
4
Σ ΔPm2(s) + 1
M2s+D2
ΔPtie(s)
Δω2(s)
1
1+sTT,2
ΔPV2(s) 1 1+sTG,2
T2(s) G2(s)
+
1 R2
Σ ΔPref,2(s)
+
-
Σ ΔPm1(s) + 1
M1s+D1 ΔPtie(s)
Δω1(s)
1
1+sTT,1
ΔPV1(s) 1 1+sTG,1
T1(s) G1(s)
-
1 R1
Σ +
-
ΔPref,1(s)
T s Σ
ΔPNL1(s)
ΔPNL2(s)
-
-
+
-
See http://home.eng.iastate.edu/~jdm/ee553/AGC1.pdf
State equations for this system
5
)(1
)(1
)()(1
)(
)(1
)(1
)(
)(1
)(1
)(1
)(
111
11
11
11
11
11
1
11
111
11
1
tPM
tPM
tM
DtP
Mt
tPT
tPT
tP
tPT
tRT
tPT
tP
NLtiem
mT
VT
m
refGG
VG
V
)(1
)(1
)()(1
)(
)(1
)(1
)(
)(1
)(1
)(1
)(
222
22
22
22
22
22
2
22
222
22
2
tPM
tPM
tM
DtP
Mt
tPT
tPT
tP
tPT
tRT
tPT
tP
NLtiem
mT
VT
m
refGG
VG
V
)()()( 21 tTtTtPtie
Form of equations is the same, except for sign of ΔPtie term in 3rd equation of each set.
Will this work?
6
Steady-state values
The system of the previous two slides consists of just primary control, and as we have seen will distribute the generation imbalance to all units, leaving a non-zero steady-state frequency error. Thus, ΔPm2∞≠0; Δω∞≠0, and ΔPtie∞≠0. We need an additional control loop.
Each BA compensates for its own load change.
For a load change in area 1, we desire:∆ Pm1∞=∆PNL1
∆Pm2∞=0∆ω∞=0∆Ptie∞=0
Introduce Area Control Error
7See http://home.eng.iastate.edu/~jdm/ee553/AGC1.pdf
ACE1=-B1∆ω-∆Ptie, ACE2=-B2∆ω+∆Ptie
The additional loop is an integral control loop, which provides the ability to zero the steady-state error of the system output (frequency) in response to a unit step disturbance.
PNL1
PNL2
State equations for this system
8
)()()( 21 tTtTtPtie
tieref
Ltiem
mT
VT
m
refGG
VG
V
PKtKBtP
tPM
tPM
tM
DtP
Mt
tPT
tPT
tP
tPT
tRT
tPT
tP
)()(
)(1
)(1
)()(1
)(
)(1
)(1
)(
)(1
)(1
)(1
)(
222
222
22
22
22
22
22
2
22
222
22
2
tieref
Ltiem
mT
VT
m
refGG
VG
V
PKtKBtP
tPM
tPM
tM
DtP
Mt
tPT
tPT
tP
tPT
tRT
tPT
tP
)()(
)(1
)(1
)()(1
)(
)(1
)(1
)(
)(1
)(1
)(1
)(
111
111
11
11
11
11
11
1
11
111
11
1
AGC and participation factors
9See http://home.eng.iastate.edu/~jdm/ee553/AGC1.pdf
ACE, being a measure of how much the total system generation needs to change, is allocated to the various units that comprise the balancing area via participation factors.The participation factors are obtained by linearizing the economic (market) dispatch about the last base point solution (see Wood & Wollenberg, section 3.8).
Base point calculation is performed by the real-time market every 5 mins.
Summary of power balance control levelsNo. Control Name Time frame Control objectives Function
1 Inertial response 0-5 secsPower balance and
transient frequency dip minimization
Transient frequency control
2 Primary control, governor 1-20 secs
Power balance and transient frequency
recovery
Transient frequency control
3 Secondary control, AGC
4 secs to 3 mins
Power balance and steady-state frequency Regulation
4 Real-time market Every 5 mins Power balance and economic-dispatch
Load following and reserve provision
5 Day-ahead market Every day Power balance and
economic-unit commitmentUnit commitment and
reserve provision
10
We are addressing the system’s ability to control steady-state frequency. Why consider the real-time market?
The real-time market has a secondary influence on the system’s ability to control steady-state frequency because it computes base points based on a net load forecast. The accuracy of this forecast determines how much the units will be moved by AGC and as a result, how much frequency variability is present.
So let’s take a look at how the real time market uses a net load forecast.
Base point calculation via real-time market
11
Source: Y. Makarov, C. Loutan, J. Ma, and P. de Mello, “Operational impacts of wind generation on California power systems,” IEEE Trans on Power Systems, Vol. 24, No. 2, May 2009.
Focus on interval 2, { t+5, t+10}.
For interval 2, a short-term net load forecast is made 7.5 min before interval 2 begins, at t-2.5, and generation set points are computed accordingly.
At t+2.5, which is 2.5 minutes before interval 2 begins, the units start to move.
The units are ramped at a rate which provides that they reach the desired base point at t+7.5 min, which is 2.5 min after the interval begins.
ADS: automatic dispatch systemDOT: dispatch operating target
Key point: The base point is computed from a net load forecast. There is error in this forecast, which typically increases as wind penetration increases. This error contributes to frequency deviation.
Wind farm participation in AGC
12
Most windfarms do not participate in AGC today.
However, windfarms do affect the net load seen by AGC, as indicated here.
PNL1
PNL2
13
Control performance standardsControl Performance Standards CPS1 and CPS2 evolved from earlier metrics and were enacted by NERC in 1997 to evaluate a balancing area’s frequency control performance in normal interconnected power system operations.
The motivation underlying CPS is to ensure a targeted long term frequency control performance of the entire interconnection.
CPS measures each balancing area’s frequency control performance in achieving control objectives.
N.Jaleeli and ,L.VanSlyck, “Control performance standards and procedures for interconnected operation,” Electric Power Research Institute, Dublin, Ohio, Tech.Rep. TR-107813, Apr.1997.N.Jaleeli and L.S.Vanslyk, “NERC’s new control performance standards. IEEE Trans. Power Syst.,” vol.14, pp.1092-1099, Aug.1999.
14
Control performance standards
NERC Standard BAL-001-0.1a — “Real power balancing control performance,” http://www.nerc.com/files/BAL-001-0_1a.pdf.
CPS1 CPS2
15
CPS1:a measure of a balancing area’s long term (12 mo) frequency performance. • Control objective - bound excursions of 1-minute average frequency error over 12 months in the interconnection. • Measures control performance by comparing how well a balancing area’s ACE performs in conjunction with the frequency error of the interconnection.
FBPPACE
tieP
stieatie
||)( ,,
min1min1
min1 ||10F
B
ACECP
Ref: M. Terbrueggen, “Control Performance Standards” 2002
Average ACE, ΔF over 1 min to compute:
10B to give units of Hz.
ΔF is an interconnection measure. ΔPtie is a balancing area measure. When ΔF<0, the interconnection needs generation, so desire BA to make ΔPtie large ACE>0 (helping). If ACE<0, it means BA is undergenerating “hurting.”
So we want to see CP negative, large in mag.
16
CPS1:a measure of a balancing area’s long term (12 mo) frequency performance. • Control objective - bound excursions of 1-minute average frequency error over 12 months in the interconnection. • Measures control performance by comparing how well a balancing area’s ACE performs in conjunction with the frequency error of the interconnection.
ε1 =target bound for 12 month of 1min avg freq error. e.g., 0.018Hz in EI, 0.228Hz in WECC, 0.020 Hz for ERCOT. Must be squared to normalize Hz2 in numerator.
FBPPACE stieatie ||)( ,,
%100100)2(1 CFCPS
21
12min1
)(
)(
MonthCP
CF
min1min1
min1 ||10F
B
ACECP
Ref: M. Terbrueggen, “Control Performance Standards” 2002
Average ACE, ΔF over 1 min to compute:
Average CP 1min over 12 mo to compute:
17
CPS1:a measure of a balancing area’s long term (12 mo) frequency performance. • Control objective - bound excursions of 1-minute average frequency error over 12 months in the interconnection. • Measures control performance by comparing how well a balancing area’s ACE performs in conjunction with the frequency error of the interconnection.
Problem: balancing area can grossly over- or under-generate (as long as it is opposite frequency error) and get very good CPS1, yet impact its neighbors with excessive flows (large ACEPtie,a>>Ptie,s).
FBPPACE stieatie ||)( ,,
%100100)2(1 CFCPS
21
12min1
)(
)(
MonthCP
CF
min1min1
min1 ||10F
B
ACECP
Ref: M. Terbrueggen, “Control Performance Standards” 2002
Average ACE, ΔF over 1 min to compute:
Average CP 1min over 12 mo to compute:
CPS2: measure of a balancing area’s ACE over all 10-minute periods in a month. • Control objective – limit ACE variations & bound unscheduled power flows between balancing areas. • Developed to address “problem” of previous slide.
10 101.65 10 10i sL B B
• BS=sum of B values for all control areas.
• ε10 =target bound for 12 mo RMS of10-min avg freq error: e.g., 0.0057Hz in EI, 0.0073 for the WI and ERCOT.• In 2003, the 10Bs were ~ -5692 mw/0.1hz for EI, -1825 mw/0.1hz for WEEC, -920 mw/0.1Hz for ERCOT.
Requirement: |ACE10min |<
CPS2=100%-(Percent of 10 min periods in violation)>90%
• L10 is max value within which ACE10min must be controlled
18
Simulation System•Two Area System (Area A and Area B)
Wind power is assumed in area A •Each area consists of 10 conventional units, with inertia and with speed governing• Based points are computed from net load forecast made 7.5 min ahead, with an assumed error of 1% for load and 4.5% for wind.•Wind penetration levels- 6%, 10%, 13%, 17%, 21%, 25% (Pw/Pnw) are considered (by capacity).• Wind is assumed to displace conventional units• Actual sec-by-sec p.u. value of load and of wind power data from one wind farm is used.
A BWind units
Con units Con
units
19C. Wang and J. McCalley, “Impact of Wind Power on Control Performance Standards,” under review, IEEE Trans on Pwr Sys.
2 Area Simulation System
FBPPACE stieatie ||)( ,,
)(1 tP regNL
)(2 tP regNL
C. Wang and J. McCalley, “Impact of Wind Power on Control Performance Standards,” under review, IEEE Trans on Pwr Sys.
)()(
)()()()(
)()()()(
)]([)]([)(
11
,11,11
,1,111
,111
tPtP
tPtPtPtP
tPtPtPtP
tPtPtP
regw
regL
fcstwwfcstLL
fcstwfcstLwL
RTEDGNLregNL
)(
)()(
)()(
)]([)]([)(
2
,22
,22
,222
tP
tPtP
tPtP
tPtPtP
regL
fcstLL
fcstLL
RTEDGNLregNL
Area 1 input Area 2 input
Inputs for 2 Area Simulation System
The sec-by-sec generation levels in each case (PG1,RTED and PG2,RTED) are determined by linearly interpolating between their respective 5 minute load and wind forecasts.
C. Wang and J. McCalley, “Impact of Wind Power on Control Performance Standards,” under review, IEEE Trans on Pwr Sys.
Study results
0% 6.67% 10.14% 13.71% 17.37% 21.12% 24.94%0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Nor
mal
ized
CP
S1
Sco
re
Wind Energy Penetration Level in Area 1
Case A
Case B
0% 6.67% 10.14% 13.71% 17.37% 21.12% 24.94%0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1N
orm
aliz
ed C
PS
2 S
core
in A
rea
1
Wind Energy Penetration Level in Area 1
Case A
Case B
Normalized CPS1
Normalized CPS2
Case A: Area 1, Area 2 have same size.Case B: Area 1 unchanged. Area 2 load and gen scaled up by 10.
22
Conclusions:1.CPS1 and CPS2 deteriorates with increasing wind penetration.2.The effect is larger for “smaller” interconnections.
C. Wang and J. McCalley, “Impact of Wind Power on Control Performance Standards,” under review, IEEE Trans on Pwr Sys.
Study resultsMeasures to improve CPS1, CPS2:•M1: Increase primary frequency control capability in Area 1 •M2: Increase the forecast accuracy of wind power•M3: Control wind power output to be no more than a band around forecasted value•M4: Combining control areas.
CPS1 AND CPS2 SCORE WITH DIFFERENT MEASURES AT 25% WIND POWER ENERGY PENETRATION LEVEL IN AREA1, CASE A
Measures CPS1 Improvement over Original
CPS1
CPS2 Improvement over Original
CPS2 M1 52.93% 31.74% 59.71% 3.75%
M2 * 65.58% 63.21% 71.22% 23.75% M2 ** 92.90% 131.21% 100% 75.00%
M3 60.76% 51.22% 66.19% 15.00% M4 73.84% 83.78% - -
In M2*, NRMSE of wind power forecast is assumed to be 3%; In M2**, NRMSE of wind power forecast is assumed to be 0%.
CPS1 AND CPS2 SCORE WITH DIFFERENT MEASURES AT 25% WIND POWER ENERGY PENETRATION LEVEL IN AREA 1, CASE B
Measures CPS1 Improvement over Original
CPS1
CPS2 Improvement over Original
CPS2 M1 91.50% 4.61% 96.52% 2.21% M2 * 93.78% 7.21% 98.61% 4.41% M2 ** 96.56% 10.40% 100% 5.88% M3 91.16% 4.22% 97.92% 3.68% M4 99.07% 13.25% - -
In M2*, NRMSE of wind power forecast is assumed to be 3%; In M2**, NRMSE of wind power forecast is assumed to be 0%.
23C. Wang and J. McCalley, “Impact of Wind Power on Control Performance Standards,” under review, IEEE Trans on Pwr Sys.
Solutions to variability & uncertainty1. Do nothing: fossil-plants provide reg & LF (and die ).2. Improve forecasts (M2)3. Increase control of the wind generation
a. Control wind to band around forecasted value (M3)b. Provide wind with primary control
• Reg down (4%/sec), but spills wind following the control • Reg up, but spills wind continuously
c. 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 advance4. Increase non-wind MW ramping capability during periods of
expected high variability using one or more of the below (M1):a. Conventional generation b. Load controlc. Storaged. Expand control areas
5. Combine control areas (M4)
24
%/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 13.8124