Using Grid-Forming Inverters to Improve Transient ...
Transcript of Using Grid-Forming Inverters to Improve Transient ...
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Using Grid-Forming Inverters to Improve Transient Stability of Autonomous Microgrids and Dynamic
Distribution Systems
Wei Du
April 29, 2019, Seattle
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OUTLINE
Grid-Forming Inverter & Control
Modeling Work at PNNL & Validation against CERTS Microgrid Field Test Results
Transient Stability Study of Large-Scale Distribution Systems with High Penetration of PVs
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Grid-Following vs. Grid-Forming
EXL
ω
E δ
P,Q
I θ+φ V θ
P,Q
grid
I (θ+φ)
+ Current Control (PLL+ Current loop)
+ Control P & Q
- Do not control voltage & frequency
- ‘Negative Load’ to Power System
- Reduce System Inertia
+ Voltage & Frequency Control
+ Can Work in islanded mode
+ Autonomous Load Tracking
- No Direct Control of Current
- Overload Issues
Grid-Following (Current Source) Grid-Forming (Voltage Source)
Most grid-connected sources (PV, fuel
cells) use grid-following control
Grid-forming source is crucial for
islanded microgrid operation
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Overload Mitigation Controller
• Droop control has already been widely used in microgrids
• Grid-Forming inverters have overload issues: currents are not controlled
• Address the two overload issues by actively controlling sources’ frequency
• Part of the sources are overloaded: Overload Transfer
• All the sources are overloaded: Under Frequency Load Shedding
Overload Mitigation Controller
-+
mp
Pset
P ++
ω0
+
kppmax+kipmax/s
+
+
-
-+
+
Pmin
0
0
Pmax
P
ω
kppmax+kipmax/s
Δω +
5
0
20
40
60
80
A1 Power kW
A2 Power kW
-500
0
500
Load Voltage Phase a
Load Voltage Phase b
Load Voltage Phase c
0 0.2 0.4 0.6 0.8 1
-100
0
100
Time Seconds
A1 3 Phase Currents
A2 3 Phase Currents
Overload
Overload Transfer
Without Pmax Controller
• When one source is dispatched near its
maximum generation, a load step can result in
overload
• Collapse the dc bus of inverters, stall the
synchronous generators, etc.
A Two-Source System
Z1
A1
XLA1
Z2
K
A2
XLA2
(EA1,δA1) (EA2,δA2)
Load 1 Load 2
0 10 20 30 40 50 60 70 8059
59.2
59.4
59.6
59.8
60
60.2
60.4
60.6
Power[kW]
Fre
quency[H
z]
P
max=60kW
Pset-A2
=5kW
Pset-A1
=55kW
PA2
=20kW PA1
=70kW
Without Pmax Controller
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0
20
40
60
80
A1 Power kW
A2 Power kW
-500
0
500
Load Voltage Phase a
Load Voltage Phase b
Load Voltage Phase c
0 0.2 0.4 0.6 0.8 1
-100
0
100
Time Seconds
A1 3 Phase Currents
A2 3 Phase Currents
Overload
Overload Transfer
Without Pmax Controller
With Pmax Controller
• Transfer the extra load to non-overloaded
sources by reducing the frequency rapidly
• The change of phase angle redistributes
power flow between sources
0
20
40
60
80
59.6
59.8
60
0 0.2 0.4 0.6 0.8 1
2
3
4
Time Seconds
A1 Power kW
A2 Power kW
A1 Frequency Hz
A2 Frequency Hz
Phase Angle Degree
-500
0
500
-100
0
100
Load Voltage Phase a
Load Voltage Phase b
Load Voltage Phase c
A1 3 Phase Currents
A2 3 Phase Currents
Overload
Change of Phase Angle
A Two-Source System
Z1
A1
XLA1
Z2
K
A2
XLA2
(EA1,δA1) (EA2,δA2)
Load 1 Load 2
0 10 20 30 40 50 60 70 8059
59.2
59.4
59.6
59.8
60
60.2
60.4
60.6
Power[kW]
Fre
quency[H
z]
P
max=60kW
Pset-A2
=5kW
Pset-A1
=55kW
PA2
=20kW PA1
=70kW
With Pmax ControllerWithout Pmax Controller
0 10 20 30 40 50 60 70 8059
59.2
59.4
59.6
59.8
60
60.2
60.4
60.6
Power[kW]
Fre
quency[H
z]
P
max=60kW
PA2
=30kW
Pset-A1
=55kW
Pset-A2
=5kW
PA1
=60kW
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How do thousands of grid-forming/grid-following
inverters impact the transient stability of large-scale
distribution systems?
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• Challenges in simulation: Neither the electromagnetic simulation nor the positive
sequence-based transient stability simulation works for large-scale, three-phase,
unbalanced distribution systems simulation
• Solutions: PNNL developed an open-source software—GridLAB-D, which can
conduct three-phased transient stability simulations for distribution systems.
Dynamic models include:
• Synchronous generators
• Motors
• Grid-Forming/Grid-Following inverters
• …
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XLE∠δa
E ω
XL
E ω
XLE∠δc
E ω
E∠δb Distribution
System
Network
Solution
Grid-Forming Controller
Pi,Qi,Vgi
E,ω
Inverter model and the interface to the
distribution system
Modeling of Grid-Forming Inverters in GridLAB-D
• Grid-Forming Inverter: Three-phase balanced voltage source behind XL &
Controller
• Network Solution: Extension of Current Injection Method, which allows three-
phase power flow calculation
P-f Droop and Overload Mitigation
Q-V Droop
Inverter model and the interface to the
distribution system
Phase-Lock Loop
Current Loop
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Modeling of Grid-Following Inverters in GridLAB-D
• Grid-Following inverters are usually modeled as controllable PQ node in
traditional transient stability analysis software, PLL and current loops are ignored
• In GridLAB-D, they are modeled as controllable voltage sources, and detailed
PLL and current loop are modeled
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-50
0
50
100
150
A1 Power kW
B1 Power kW
ESS Power kW
LB4 Power kW
-200
0
200
A1 Current phase a A
A1 Current phase b A
A1 Current phase c A
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
-200
0
200
Time Seconds
B1 Current phase a A
B1 Current phase b A
B1 Current phase c A
AEP Test Data 3/18/2016 10:53:09.5829
-50
0
50
100
150
A1 Power kW
B1 Power kW
ESS Power kW
LB4 Power kW
-200
0
200
A1 Current phase a A
A1 Current phase b A
A1 Current phase c A
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
-200
0
200
Time Seconds
B1 Current phase a A
B1 Current phase b A
B1 Current phase c A
Simulation
Field Test Results
PSCAD Simulation, Time
Step = 50 μs
Under-Frequency Load Shedding: Generator vs. Inverter
Feeder A
Feeder B
Inverter-based Source Inverter-based Source
Energy Storage Synchronous Generator
Load Bank 3 Load Bank 4
Load Bank 5
Feeder C
Load Bank 6Grid
Static Switch
A1
ESS B1
A2
• In most microgrids, load shedding is achieved through fast
communication. The CERTS microgrid can do under-frequency
load shedding.
• By ignoring electromagnetic transients, GridLAB-D is able to
conduct transient analysis of large-scale, three-phase,
unbalanced distribution systems
CERTS/AEP Microgrid
Time Step = 5 ms
Validation against CERTS/AEP Microgrid Test Results
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An Islanded IEEE 123-Node Test Feeder with High Penetration of PVs
• Synchronous Generators: 3*600 kW
• 6 Distributed PV Inverters: 1400 kW
• Peak Load: 2150 kW + 490 kvar
• PV Penetration: 65% (compared to
peak load)
• Substation voltage is lost due to
extreme weather events, three
microgrids work as an islanded
networked microgrid
• Contingency: Loss of Generator 3
Simulation on a Modified IEEE 123-Node Test Feeder
Gen PV
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PV: Grid-Following controlled at MPP
• PV operates at Maximum Power Point
• Contingency: Loss of Generator 3
• Frequency drops fast due to the low inertia, 12%
Loads are tripped
GridLAB-D Simulation
Simulation on a Modified IEEE 123-Node Test Feeder
Load shedding
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• As the penetration of PVs continues increasing,
dynamic reserve of PVs has the potential to
improve the transient stability
• How do we use this reserve?
33% Reserve
Simulation on a Modified IEEE 123-Node Test Feeder
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Simulation on a Modified IEEE 123-Node Test Feeder
33% Reserve
• PV: Grid-Following with Frequency-Watt
• Not only reduces the system inertia, but also results in
oscillations between generators and inverters
• Loads are still tripped
Load shedding
Grid-Following with Frequency-Watt
0.05s 4% droop 0.25s
Frequency-Watt Control
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Load shedding
Grid-Following with Frequency-Watt
Improve frequency control
Simulation on a Modified IEEE 123-Node Test Feeder
• Grid-Forming PV with dynamic reserve can improve
frequency stability, load shedding is avoided
• Grid-forming inverters response to load changes
instantaneously
Grid-Forming
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Load shedding
Grid-Following with Frequency-Watt
Mitigate voltage unbalance
Grid-Forming
Simulation on a Modified IEEE 123-Node Test Feeder
• Grid-Forming inverters can improve voltage control,
voltage unbalance is mitigated
• All PV inverters become three-phase, balanced voltage
source behind XL, system is stiffer
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Simulation on a Modified IEEE 123-Node Test Feeder
• The voltage profile of the entire distribution feeder is
significantly improved
• The three-phase unbalanced characteristic cannot be
easily reflected by positive-sequence-based
simulation
Grid-FormingGrid-Following
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Conclusion
• A three-phase, electromechanical modelling approach is proposed to model Grid-Forming/Grid-Following inverters for large-scale distribution system study
• The developed Grid-Forming inverter model in GridLAB-D is validated against CERTS Microgrid test results
• PV Grid-Forming inverters with dynamic reserve can improve Frequency & Voltage stability of distribution systems
• More work needs to be done in the Dynamic Distribution Systems research area
Thank you
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-50
0
50
100
150
A1 Power kW
A2 Power kW
ESS Power kW
LB4 Power kW
-200
0
200
A1 Current phase a A
A1 Current phase b A
A1 Current phase c A
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
-200
0
200
Time Seconds
A2 Current phase a A
A2 Current phase b A
A2 Current phase c A
AEP Test Data 5/12/2016 09:18:51.8316
-50
0
50
100
150
A1 Power kW
A2 Power kW
ESS Power kW
LB4 Power kW
-200
0
200
A1 Current phase a A
A1 Current phase b A
A1 Current phase c A
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8
-200
0
200
Time Seconds
A2 Current phase a A
A2 Current phase b A
A2 Current phase c A
Simulation
Under-Frequency Load Shedding: Inverter vs. Inverter
Feeder A
Feeder B
Inverter-based Source Inverter-based Source
Energy Storage Synchronous Generator
Load Bank 3 Load Bank 4
Load Bank 5
Feeder C
Load Bank 6Grid
Static Switch
A1
ESS B1
A2
Validation against CERTS/AEP Microgrid Test Results
CERTS/AEP Microgrid
Time Step = 5 ms
By ignoring electromagnetic transients, GridLAB-D is
able to conduct transient analysis of large-scale,
three-phase, unbalanced distribution systemsPSCAD Simulation, Time
Step = 50 μs
Field Test Results
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Tecogen’s response to fault
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Current Loop PI gains: kp=0.05, ki=5, but with feedforward term F=0.3
Coordinate
Transformation
ugqi
δPLLi
kpPLL
∆ωi
1/sδPLLi Ugi∠δgi
ω0
fPLLi
2π
+
+
kiPLL/s
++
+
-
+-
kpc+
-
+
++
+
igdi_ref
igdi
igqi_ref
igqi
ωL
ωL
ugdi
ugqi
edi
eqi
kic/s+
+
kpc
kic/s+
+
Pref changes from 0.5 pu to 1 pu at 2.1 s
Grid-Following Inverters
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Grid-Forming Inverter & Droop Control
• Grid-Forming inverter: voltage source behind a coupling reactance XL
A well designed XL is important for controller design: P∝δ, Q∝E
• Droop Control: make multiple inverters work together in a microgrid
P vs. f droop ensure sources are synchronized
Q vs. V droop avoids large circulating reactive power between sources
P vs. f droop Q vs. V droop