View
16
Download
0
Category
Preview:
Citation preview
1
Using Grid-Forming Inverters to Improve Transient Stability of Dynamic Distribution Systems
Wei DuSenior Research EngineerElectricity & Infrastructure GroupPacific Northwest National LaboratoryMay 30, 2019, Salt Lake City
This work is funded by the Microgrid R&D program, which is funded by the U.S. Department of Energy’s (DOE) Office of Electricity. The Microgrid R&D Program is managed by Mr. Dan Ton.
2
OUTLINE
Grid-Forming Inverter & Grid-Following Inverter
Inverter Modeling Work at PNNL & Model Validation against CERTS/AEP Microgrid Field Test Results
Transient Stability Study of Large-Scale Distribution Systems with High Penetration of PV Grid-Forming Inverters
3
Grid-Following vs. Grid-Forming
E XL
ω
E∠δ
P,Q
I θ+φ V∠θ
P,Qgrid
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+ 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
Inverter Control
4
• 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• …
How do thousands of grid-forming/grid-following inverters impact the transient stability of large-scale
distribution systems?
5
XLE∠δa Eω
XL
E ω
XLE∠δc E ω
E∠δb Distribution
System NetworkSolution
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
6
-50
0
50
100
150
A1 Power kWB1 Power kWESS Power kWLB4 Power kW
-200
0
200
A1 Current phase a AA1 Current phase b AA1 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 AB1 Current phase b AB1 Current phase c A
AEP Test Data 3/18/2016 10:53:09.5829
-50
0
50
100
150
A1 Power kWB1 Power kWESS Power kWLB4 Power kW
-200
0
200
A1 Current phase a AA1 Current phase b AA1 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 AB1 Current phase b AB1 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
3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8
Time (sec)
-50
0
50
100
150
P [k
W]
PA1
PB1
3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8
Time (sec)
-50
0
50
Q [k
var]
QA1
QB1
3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8
Time (sec)
0.9
0.95
1
1.05
1.1
Vol
tage
[pu]
Va
Vb
Vc
3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8
Time (sec)
58.5
59
59.5
60
60.5Fr
eque
ncy
[Hz]
fA1
fB1
fLB4
7
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 islandednetworked microgrid
• Contingency: Loss of Generator 3
Simulation on a Modified IEEE 123-Node Test Feeder
Gen PV
8
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
0 200 400 600 800 1000 1200
Voltage[V]
0
280
560
840
1120
1400
1680
Pow
er[k
W]
dP/dV>0
stableunstable
dP/dV<0
Maximum Power Point
T=25 ° C
G=600W/m 2
Simulation on a Modified IEEE 123-Node Test Feeder
4 6 8 10 12 14
57
58
59
60
61
Freq
uenc
y [H
z]
4 6 8 10 12 14
Time [s]
0.9
0.95
1
1.05
1.1
Vol
tage
[V]
Va-97
Vb-97
Vc-97
4 6 8 10 12 140
500
1000
1500
Act
ive
Pow
er [k
W]
Diesel:1+2
Diesel:3
Total PV
Load shedding
9
• 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
0 200 400 600 800 1000 1200
Voltage[V]
0
280
560
840
1120
1400
1680
Pow
er[k
W]
dP/dV>0
stableunstable
dP/dV<0
Maximum Power Point
T=25 ° C
G=600W/m 2
10
Simulation on a Modified IEEE 123-Node Test Feeder
33% Reserve
0 200 400 600 800 1000 1200
Voltage[V]
0
280
560
840
1120
1400
1680
Pow
er[k
W]
dP/dV>0
stableunstable
dP/dV<0
Maximum Power Point
T=25 ° C
G=600W/m 2
• 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
4 6 8 10 12 1458
59
60
61
Freq
uenc
y [H
z]
4 6 8 10 12 14
Time [s]
0.9
0.95
1
1.05
1.1
Vol
tage
[V]
Va-97
Vb-97
Vc-97
4 6 8 10 12 140
500
1000
1500
Act
ive
Pow
er [k
W]
Diesel:1+2
Diesel:3
Total PVLoad shedding
Grid-Following with Frequency-Watt
0.05s 4% droop 0.25s
Frequency-Watt Control
11
4 6 8 10 12 1458
59
60
61
Freq
uenc
y [H
z]
4 6 8 10 12 14
Time [s]
0.9
0.95
1
1.05
1.1
Vol
tage
[V]
Va-97
Vb-97
Vc-97
4 6 8 10 12 140
500
1000
1500
Act
ive
Pow
er [k
W]
Diesel:1+2
Diesel:3
Total PVLoad shedding
Grid-Following with Frequency-Watt
4 6 8 10 12 1458
59
60
61
Freq
uenc
y [H
z]
4 6 8 10 12 14
Time [s]
0.9
0.95
1
1.05
1.1
Vol
tage
[V]
Va-97
Vb-97
Vc-97
4 6 8 10 12 140
500
1000
1500
Act
ive
Pow
er [k
W]
Diesel:1+2
Diesel:3
Total PV
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
instantaneously0 200 400 600 800 1000 1200
Voltage[V]
0
280
560
840
1120
1400
1680
Pow
er[k
W]
dP/dV>0
stableunstable
dP/dV<0
Maximum Power Point
T=25 ° C
G=600W/m 2
Grid-Forming
12
4 6 8 10 12 1458
59
60
61
Freq
uenc
y [H
z]
4 6 8 10 12 14
Time [s]
0.9
0.95
1
1.05
1.1
Vol
tage
[V]
Va-97
Vb-97
Vc-97
4 6 8 10 12 140
500
1000
1500
Act
ive
Pow
er [k
W]
Diesel:1+2
Diesel:3
Total PVLoad shedding
Grid-Following with Frequency-Watt
4 6 8 10 12 1458
59
60
61
Freq
uenc
y [H
z]
4 6 8 10 12 14
Time [s]
0.9
0.95
1
1.05
1.1
Vol
tage
[V]
Va-97
Vb-97
Vc-97
4 6 8 10 12 140
500
1000
1500
Act
ive
Pow
er [k
W]
Diesel:1+2
Diesel:3
Total PV
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
13
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
Thank you
14
15
Backup Slides
16
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
4 4.5 5 5.5 6 6.5 7 7.5 8
Time (s)
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
Vol
tage
[pu]
4 4.5 5 5.5 6 6.5 7 7.5 8
Time (s)
0.9
0.92
0.94
0.96
0.98
1
1.02
1.04
1.06
1.08
1.1
Vol
tage
[pu]
17
Grid-Following with Frequency-Watt Grid-Forming
Currents Currents
• Grid-Forming inverters provide larger currents than grid-following inverters, because they need to provide reactive power to support the voltage
4 5 6 7 8 9 10 11 12 13 14 15
Time (sec)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Tota
l Cur
rent
[pu]
Ia
Ib
Ic
4 5 6 7 8 9 10 11 12 13 14 15
Time (sec)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Tota
l Cur
rent
[pu]
Ia
Ib
Ic
18
-50
0
50
100
150
A1 Power kWA2 Power kWESS Power kWLB4 Power kW
-200
0
200
A1 Current phase a AA1 Current phase b AA1 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 AA2 Current phase b AA2 Current phase c A
AEP Test Data 5/12/2016 09:18:51.8316
-50
0
50
100
150
A1 Power kWA2 Power kWESS Power kWLB4 Power kW
-200
0
200
A1 Current phase a AA1 Current phase b AA1 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 AA2 Current phase b AA2 Current phase c A
Simulation
3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8
Time (sec)
-50
0
50
100
150
P [k
W]
PA1
PA2
3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8
Time (sec)
-50
0
50
Q [k
var]
QA1
QA2
3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8
Time (sec)
0.9
0.95
1
1.05
1.1
Vol
tage
[pu]
Va
Vb
Vc
3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6 4.8
Time (sec)
59
59.5
60
Freq
uenc
y [H
z]
fA1
fA2
fLB4
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 systems
PSCAD Simulation, Time Step = 50 μs
Field Test Results
19
Tecogen’s response to fault
20
2 2.05 2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.51.01
1.015
1.02
1.025
volta
ge [p
u]
ugd-pu-A
2 2.05 2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.50.4
0.6
0.8
1
curr
ent [
pu]
igd-pu-A
igd-ref-A
2 2.05 2.1 2.15 2.2 2.25 2.3 2.35 2.4 2.45 2.5
Time (sec)
-0.4
-0.2
0
curr
ent [
pu]
igq-pu-A
igq-ref-A
Current Loop PI gains: kp=0.05, ki=5, but with feedforward term F=0.3
CoordinateTransformation
ugqi
δPLLi
kpPLL
∆ωi1/s
δPLLi 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
21
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
Inverter model and the interface to the distribution system
Phase-Lock Loop
Current Loop
22
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
23
Synchronous Generator• Why synchronous generators need inertia?
• A synchronous generator behaves as a voltage source behind Xd’’ in transient • Pe responses to load changes instantaneously• Pm from prime mover changes slowly• The unbalance between Pe and Pm results in the change of speed
Engine
Governor
ValveGenerator
Pm Pe
Load PL
Fuel
Speed ω
E∠δ Xd
’’
2 m edH P Pdtω= −
Pe responses fastPm responses slow
Recommended