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Load Modeling in WECC
DOE-NERC FIDVR Workshop September 2009 Washington DC
Dmitry Kosterev(*) and John Undrill (**) (*) Bonneville Power Administration (**) John Undrill LLC
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Load Modeling Timescale of Interest
Time
cycle minute
hour
day
year
10 years
Our primary interest is the dynamic behavior of loads in cycle to minute time frame, not projections of future demand.
second
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Load Model Development
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History Of Load Modeling in WECC • 1990’s – Constant current real, constant impedance
reactive models connected to a transmission bus • 1990’s – IEEE Task Force recommends dynamic load
modeling, however the recommendation does not get traction in the industry
• 1996 – Model validation study for July 2 and August 10 system outages: – Need for motor load modeling to represent oscillations and
voltage decline • 2000 – 2001 – WECC “Interim” Load Model:
– 20% of load is represented with induction motors – Tuned to match inter-area oscillations for August 10 1996 and
August 4, 2000 oscillation events – Recognized model limitations and the need to continue…
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o
o
o o o o
o
o
o o o o
Simulations – instantaneous voltage recovery
This is what we thought would happen using “interim” load model…
Motivation for Better Load Modeling
30 seconds
-
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
1.05
1.10
0 5 10 15 20 25 30Seconds
Volta
ge (p
u)
… and this is what actually happened
Reality – 30-second voltage recovery, 12 seconds below 80%
Motivation for Better Load Modeling
30 seconds
100%
75%
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History Of FIDVR Load Modeling • Late 1980’s – Southern California Edison observed events
of delayed voltage recovery attributed to stalling of residential air-conditioners
• 1994 – Florida Power published an IEEE paper on the similar experiences of delayed voltage recovery following transmission faults
• 1997 – SCE model validation study of Lugo event: – Need to represent a distribution equivalent – Need to have special models for air-conditioning load
• 2005–2008 – Similar observations are made by Southern Company in their analysis of FIDVR events in Atlanta area
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Looking Forward • Loads will play much more influential role in power system
stability • Resistive-type loads are phasing out (incandescent lights
and resistive heating) – Energy inefficient but “grid-friendly”
• Electronic loads, VFDs, AC compressors, heat pumps, CFLs are moving in – Energy efficient but have undesirable characteristics from
standpoint of grid dynamic stability – Increasing penetration of residential air-conditioners
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WECC Load Modeling Task Force
• WECC LMTF was formed in 2002
• Component-based approach – Understand load behavior over the wide range
of disturbances
• Bottom-up model development
• Top-down model validation
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Questions • How to capture the variety of electrical end-
uses in the model ? • How to represent electrical distance between
the transmission bus and a end-user buses with a distribution equivalent ?
• How to represent seasonal / daily / geographical variations in load composition ?
• How to validate load model performance ?
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Modeling Electrical End-Use
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LBNL Electricity Use In California Study
0
1
2
3
4
5
6
7
8
Res. - AirConditioning
Com'l. - AirConditioning
Com'l. -InteriorLighting
Com'l. - Other Res. -Miscellaneous
Res. -Refrigerator
Com'l. -Ventilation
Res. -Cooking
Res. - Dryer Com'l. -Refrigeration
0
5
10
15
20
25
30
35
Peak Demand
Annual Consumption
Source: CEC Demand Analysis Office
Peak Demand (GW) Consumption (TWh)
Residential AC
Commercial AC
Lighting
Refrigeration Ventilation
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Residential Single-Phase Air-Conditioners
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A/C Compressor Motors are Prone to Stall
7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 120
50
100
150
200
250Voltage (Volts)
7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 120
50
100
Current (Amps)
Time (sec)
7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 120
5
10
15
20Active Power (kW)
7 7.5 8 8.5 9 9.5 10 10.5 11 11.5 120
5
10
15
20
Time (sec)
Reactive Power (kVAR)
Dip to 55% for 3 cycles
Stall Thermal Trip
To find out WHY, SCE, BPA and EPRI tested more than 30 AC units
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(a) A/C Compressor Motors are non-symmetric
R
S
C Run Capacitor
Thermal Relay
Auxiliary Winding Main
Winding
1 phase supply, 2 windings
capacitor-run motor
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(a) A/C Compressor Motors are non-symmetric
Non-symmetry becomes more pronounced as the supply voltage is declining The forward-rotating torque component is counter-acted by backward-rotating component at lower voltages
0 10 20 30 40 50 60 700
50
100
150
200
250Voltage
0 10 20 30 40 50 60 700
20
40
60Current
0 10 20 30 40 50 60 700
1000
2000
3000
4000
5000Compressor Power
TotalPositive Seq.Negative Seq.
TotalMainAuxiliary
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(a) Single-phase motors produce much less electrical torque than a comparable 3-phase motor
at lower voltages and slower speeds
0 50 100 150 200 250 300 350 4000
2
4
6
8
10
12
14
16
18
rotor speed
torq
ue
Critical speed = 30% of rated
Critical voltage near 60%
Torque/speed and load torque curves Plot prepared by Dr. Bernard Lesieutre, LBNL (now with University of Wisconsin)
Electrical torque for 1-phase motor at various voltages Electrical torque for 3-phase motor at nominal voltage Mechanical load torque
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(b) Compressor Motors Inertia is Very Low
310 mm
75 mm
E.g. 3.5-ton compressor motor: Weight: 4.6 kg
H = 0.03 – 0.05 seconds
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(c) Compressor Load Torque
360 720
Torq
ue
Rotor Position
It is very possible that the motor stalls at the next compression cycle
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Types of Compressors • There are two types of compressors – reciprocating and
scroll – Reciprocating compressors represent majority of the installed
capacity – Scroll units will be majority of new units moving forward
• Scroll compressors have more favorable characteristics: – Less prone to stall (lower voltage, longer sag) – Can re-accelerate when the voltage recovers above 90% – Pressure seems to equalize faster – However, can restart and run backwards for hours
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0 10 20 30 40 50 60 70 800
50
100
Cur
rent
(Am
ps)
Current
0 10 20 30 40 50 60 70 8050
100
150
200
Pre
ssur
e (P
si)
Compressor Pressure
Time (sec)
What do the motors do after they stall ?
stall thermal trip
thermal trip
thermal re-close
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0 5 10 15 20 25 30 35 40 45 50 550
20
40
60
80Current
Cur
rent
[pu]
0 5 10 15 20 25 30 35 40 45 50 550
50
100
150
200Compressor Pressure
Pre
ssur
e [p
si]
Time [sec]
What do the motors do after they stall ?
stall thermal trip
successful restart
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Compressor Motor Steady-State Loading
80 85 90 95 100 105 110 1152.6
2.8
3
3.2
3.4
3.6P
ower
(kW
)
Ambient Temperature (F)80 85 90 95 100 105 110 115
0.56
0.58
0.6
0.62
0.64
0.66
Sta
ll V
olta
ge (p
er u
nit)
* Compressor motors have high power factor ~0.97
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How to Model Single-Phase AC Motors ?
• We were unable to fit the observed behavior into a three-phase motor model structure
• Several Modeling Approaches are considered, prototyped and tested: – Phasor Model (MOTORC) – Differential equations, used as a
benchmark
– Performance Model – empiric model based on the test results
– Hybrid model – three-phase model for running motor, short-circuit impedance when stalled (original SCE approach)
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Performance Model Static Empirical Model
0 0.2 0.4 0.6 0.8 10
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5Real Power
Rea
l Pow
er (p
er u
nit)
Voltage (per unit)
RUNSTALL
STALL
0 0.2 0.4 0.6 0.8 10
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5Reactive Power
Rea
ctiv
e P
ower
(per
uni
t)
Voltage (per unit)
RUN
STALL
STALL
RUN STATE - Polynomial STALL STATE - Impedance Switch from RUN to STALL when voltage drops below Stall Voltage
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Single-Phase Motor Model • 1-phase model includes:
– Performance model to represent a single-phase motor model
– Thermal relay model
– Under-voltage relay model
– Contractor model
• Model is validated against actual tests
• Model is tested for numeric stability
• Implemented in PSS®E (ACMTBL) and PSLF (LD1PAC)
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Composite Load Model Structure
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Composite Load Model Structure
Electronic
M
M
M 115-kV 230-kV
Static
M UVLS
UFLS
1-phase AC
3-phase compressor
Fan
Pump
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Load Model Data
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WECC Composite Load Model
Electronic
M
Load Model Composition Data
M
M 115-kV 230-kV
Static
Load Component Model Data
Distribution Equivalent Data
UVLS and UFLS Data
M
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Load Model Data
• Load model data records include: – Distribution equivalent model data
• PG&E developed methodology, proven to work, feel very confident
– Model data for load model components (e.g. motor inertia, driven load, electrical data, etc)
• Getting test data, develop better understanding of end-use characteristics
• Motor protection and control remains least known
– Fractions of total load assigned to each load model component
– UFLS and UVLS data
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Load Composition • The most challenging part of model data
• Sources of data: – LBNL Reports on Electricity Use in California
– California Energy Commission: 2006 Commercial End-Use Survey
– SCADA load profiles
– BPA end-use monitoring data
• PNNL-BPA Load Composition Model
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California Commercial End-Use Survey • 15 climate zones in California • Four seasons • Typical, Hot, Cold, Weekend • 24-hour data
0 5 10 15 20 250
2
4
6
8
10
12
14x 10
5
FCZ10 Season:Summer Day:Typical
AirCompMotorsProcessMiscOfficeEquipIntLightExtLightRefrigCookingWaterHeatVentCoolingHeating
Data is available on CEC web-site
< 1%4%< 1%5%3%
25%
< 1%8%
3%< 1% 10%
41%
< 1%
FCZ10 Season:Summer Day:Typical Hour:
16
AirCompMotorsProcessMiscOfficeEquipIntLightExtLightRefrigCookingWaterHeatVentCoolingHeating
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Load Composition Data • PNNL-BPA Load Composition Model • WECC LMTF provided regional defaults
– winter, summer, and shoulder seasons – 6:00, 9:00, 15:00, 18:00 hours – Multiple climate zones in WECC – Composition for residential, commercial and mixed load
types
• Utilities are encouraged to provide bus-specific load composition information
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Load Model Studies
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Load Model Studies
• Event validation studies, initial focus is on on-peak summer conditions: – Local FIDVR events in Southern California
– Interconnection-wide disturbances
• System impact studies (in progress) – Large generation outages in WECC
– 3-phase faults on major inter-tie
– 3-phase faults in large load centers
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Event Validation Studies
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Load Model Studies – Initial Observations • What is important for FIDVR studies:
– Load Composition Data • Bus-specific information is recommended
– Motor control and protection • Sensitivity studies are required
– With respect to “reasonable” variations of “key parameters”
– Scenario planning • Generators and SVC may trip because of depressed voltages • Plants due to internal protection and control issues • Generator protection operation (OEP during low voltages,
Volts/Hertz during high voltages) • Line protection operation
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FIDVR Scenario Planning
Time
Bus Voltage
Sensitivity to load composition
Sensitivity to motor protection Line protection Generator and SVC tripping
Consequential plant trips Volts Per Hertz
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Load Model Implementation in WECC • Load Model Approval:
– Composite Load Model structure (WE ARE HERE) – Load model data – Validation and system impact studies
• Process for Load Model Building
– Tools for managing load model data
• WECC Voltage Dip Criteria
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Closing Remarks • Composite load model is capable of reproducing in
principle the phenomenon of delayed voltage recovery that is known to occur
• Composite load model is adequate to evaluate FIDVR exposure risks when used appropriately
• Composite load model is not intended to provide “accurate” representation of details of a particular feeder response
• Composite load model is a tool to help make an engineering judgment when appropriate scenario planning is used
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Thank You
Load Modeling in WECC��DOE-NERC FIDVR Workshop�September 2009�Washington DCLoad Modeling �Timescale of InterestLoad Model DevelopmentHistory Of Load Modeling in WECCMotivation for Better Load ModelingMotivation for Better Load ModelingHistory Of FIDVR Load ModelingLooking ForwardWECC Load Modeling Task ForceQuestions Modeling Electrical End-Use Slide Number 12Residential�Single-Phase �Air-ConditionersA/C Compressor Motors �are Prone to Stall(a) A/C Compressor Motors are non-symmetric (a) A/C Compressor Motors are non-symmetric Single-phase motors produce much less �electrical torque than a comparable 3-phase motor �at lower voltages and slower speeds(b) Compressor Motors Inertia is Very Low(c) Compressor Load TorqueTypes of CompressorsWhat do the motors do after they stall ?What do the motors do after they stall ?Compressor Motor Steady-State LoadingHow to Model Single-Phase AC Motors ?Performance Model�Static Empirical ModelSingle-Phase Motor ModelComposite Load Model StructureComposite Load Model StructureLoad Model DataWECC Composite Load ModelLoad Model DataLoad CompositionCalifornia Commercial End-Use SurveyLoad Composition DataLoad Model StudiesLoad Model StudiesEvent Validation StudiesLoad Model Studies – Initial ObservationsFIDVR Scenario PlanningLoad Model Implementation in WECCClosing RemarksThank You
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