parallel operation of distributed generators by virtual synchronous
TRANSCRIPT
Parallel Operation of Distributed Generators by Virtual Synchronous Generator Control in Microgrids
Jia Liu* and Toshifumi Ise Osaka University, Japan
October 21, 2016
Niagara 2016 Symposium on Microgrids October 20-21, 2016 Niagara, Canada
Contents
• Introduction
• Parallel Inverters
• Synchronous Generator + Inverter
• Conclusion
2
Contents
• Introduction
• Parallel Inverters
• Synchronous Generator + Inverter
• Conclusion
3
4
Synchronous Generator
Swing Equation
Kinetic Energy of Rotating Mass
Frequency fluctuation under active power transition
limited by the inertia
DC/AC Inverter
Inertial feature of Synchronous Generators
No intrinsic relation
Undesirable frequency dynamics
5
Virtual Synchronous Generator (VSG) Control
Conventional Droop Control Load Sharing
Swing Equation Emulation
Smooth Transition between Islanding
and Grid-connection
Inertia Support
A New Concept of Inverter Control in AC Microgrid
Concept of VSG Control
6
To apply VSG control to DC-AC inverters in microgrids Conventional droop control
Objectives
• Two topics to be discussed • Parallel Inverters • Synchronous Generator + Inverter
What is the benefit?
7
With Frequency
Restoration
Without Frequency
Restoration
Less maximum frequency deviation
Frequency during Loading transition
Slower frequency variation rate
Less maximum frequency deviation Frequency during Three-phase ground fault cleared in 0.1 s
Contents
• Introduction
• Parallel Inverters
• Synchronous Generator + Inverter
• Conclusion
8
Microgrid: Parallel Inverters 9
Sbase1=10kVA
Lf1:1mH
0.0942pu R*line1:0.035pu
L*line1:0.00375puDG1
BK3
Sbase2=5kVADG2
100m 22mm2
(0.14+j0.015)Ω
100m 22mm2
(0.14+j0.015)Ω
Grid
0.0471pu
Cf1:10μF66.31pu
R*line2:0.0175pu
L*line2:0.00188pu
Cf2:10μF33.16pu
BK1
BK2
Load1
Load2
.
.
BUS
MGCC
Vout1 Iout1
Vout2 Iout2
Vbus*_0 iP *
_0 iQ,
Lf2:1mH
2 parallel DGs equipped with VSG control
Fluctuated loading
Microgrid Central Controller (MGCC) Frequency and voltage restoration
SwingEquation Function
PowerMeter
PWM
1/s
Pin
Frequency Detector
Pout
EnergyStorage
DistributedGenerator
PI ++
+
Governor Model
Q DroopVbusQref
E0
ωm
Lf
-
Iout(αβ)Vout(αβ)
abc/αβ
Stator ImpedanceAdjuster
Vbus Estimator
Vout(abc) Iout(abc)
ωg
Vpwm
θpwm
θm
E
Cf BUS
Zline
^Q0
P0
QoutLPF.
. Zls
VSG Control Scheme (Basic Part)
10
Swing Equation
ω–P Droop Controller
V–Q Droop Controller
kp: ω–P droop coef. P0:Set value of active power
kq:V–Q droop coef. Q0:Set value of reactive power
J:Virtual inertia D:Damping factor ωm:Virtual rotor frequency
ωm
P0
Pin+
+pk−
+
-
ω0 (Constant) .
.
Vbus
Q0
Qref+
+qk−
+
-
E0
.
. (Constant)
^
VSG Control Scheme (Enhanced Part)
11
SwingEquation Function
PowerMeter
PWM
1/s
Pin
Frequency Detector
Pout
EnergyStorage
DistributedGenerator
PI ++
+
Governor Model
Q DroopVbusQref
E0
ωm
Lf
-
Iout(αβ)Vout(αβ)
abc/αβ
Stator ImpedanceAdjuster
Vbus Estimator
Vout(abc) Iout(abc)
ωg
Vpwm
θpwm
θm
E
Cf BUS
Zline
^Q0
P0
QoutLPF.
. Zls
Eθm ρθ/αβ
iout_α
-+
iout_β
αβ/ρθeαeβ
-+
vpwm_α
vpwm_β
Lls
Vpwm
θpwm
ωm×
×
vZls_α
vZls_β
-+
++
Rls
Rls
+
+
Iout +-
Ithresh
0kZ
Lls0
ΔRls = Rls
ΔZls
2)/(1
1
XR+1/ω0
++
LlsΔLls
2)/(1
1
RX+
.
.
Stator Impedance Adjuster
Virtual Stator
Impedance Calculator
iout_α
-+
iout_β
αβ/ρθ-+
Llineωm
×
×
vline_α
vline_β
-+
++Rline
Rline
vout_α
vout_β
vbus_α
vbus_βVbus
^
^^
Stator Impedance Adjuster
• Constant part - Transient power sharing - Increased damping
• Transient part - Overcurrent limiting
Bus voltage estimator
Estimate bus voltage for proper reactive power sharing
Control Parameters 12
Parameter Value Parameter Value 10 kVA
5 kVA
200 V 0.0125 pu
376.99 rad/s
8 s 1 pu
17 pu 0.05 s
1 pu 6.39 mH
0 pu 13.81 mH
20 pu 1.79 pu
5 pu 5
Simulation Results Loading Transition
13
Droop Control VSG Control
Reduced frequency deviation
Loading Transition Islanding Loading
Transition Islanding
Simulation Results Three-Phase Ground Fault
14
Droop Control VSG Control
Reduced frequency deviation
3P Ground Fault
3P Ground Fault
Fault Current During Three-Phase Ground Fault
15
Without Transient Stator Impedance With Transient Stator Impedance
Fault Current is limited by Transient Stator Impedance
Eθm ρθ/αβ
iout_α
-+
iout_β
αβ/ρθeαeβ
-+
vpwm_α
vpwm_β
Lls
Vpwm
θpwm
ωm×
×
vZls_α
vZls_β
-+
++
Rls
Rls
+
+
Iout +-
Ithresh
0kZ
Lls0
ΔRls = Rls
ΔZls
2)/(1
1
XR+1/ω0
++
LlsΔLls
2)/(1
1
RX+
.
.
Stator Impedance Adjuster
Virtual Stator
Impedance Calculator
16
Oscillation damped
Without Constant Stator Reactance With Constant Stator Reactance
Effect of Constant Stator Reactance
Simulation
Experiment
Oscillation eliminated
Loading Transition Islanding P0_2
Change Loading
Transition Islanding P0_2 Change
Loading Transition
Loading Transition
P0_2 Change
P0_2 Change
Oscillation damped Oscillation eliminated
State-Space Model of Islanded Microgrid
17
Loading Transition
Change of Active Power Set Value
Frequency Active Power
Disturbance:
Output:
• The damping ratio of oscillation in output parameters is determined by the eigenvalues of state matrix A
18
• Total output reactance X increases • ⇒Damping ratio ζ increases
Oscillation can be damped by increasing
output reactance
Relation between Output Reactance and Damping Ratio
Transient Load Sharing 19
• When the disturbance is a loading transition – If the total output reactance of each VSG is of the same
per unit value, poles are cancelled by zeros and oscillation is eliminated
Design of Constant Stator Reactance
20
Eθm ρθ/αβ
iout_α
-+
iout_β
αβ/ρθeαeβ
-+
vpwm_α
vpwm_β
Lls
Vpwm
θpwm
ωm×
×
vZls_α
vZls_β
-+
++
Rls
Rls
+
+
Iout +-
Ithresh
0kZ
Lls0
ΔRls = Rls
ΔZls
2)/(1
1
XR+1/ω0
++
LlsΔLls
2)/(1
1
RX+
.
.
Stator Impedance Adjuster
Virtual Stator
Impedance Calculator
21
Without Constant Stator Reactance With Constant Stator Reactance
Effect of Constant Stator Reactance
Simulation
Experiment
Loading Transition Islanding P0_2
Change Loading
Transition Islanding P0_2 Change
Loading Transition
Loading Transition
P0_2 Change
P0_2 Change
Reactive power sharing improved
Reactive power sharing improved
22
ω–P Droop V–Q Droop
If the input of droop controller is equal, reactive power should be shared properly
Cause of Poor Reactive Power Sharing
Bus Voltage Estimator for Proper Reactive Power Sharing
23
SwingEquation Function
PowerMeter
PWM
1/s
Pin
Frequency Detector
Pout
EnergyStorage
DistributedGenerator
PI ++
+
Governor Model
Q DroopVbusQref
E0
ωm
Lf
-
Iout(αβ)Vout(αβ)
abc/αβ
Stator ImpedanceAdjuster
Vbus Estimator
Vout(abc) Iout(abc)
ωg
Vpwm
θpwm
θm
E
Cf BUS
Zline
^Q0
P0
QoutLPF.
. Zls
iout_α
-+
iout_β
αβ/ρθ-+
Llineωm
×
×
vline_α
vline_β
-+
++Rline
Rline
vout_α
vout_β
vbus_α
vbus_βVbus
^
^^
Bus voltage estimator
Estimate bus voltage for proper reactive power sharing
V–Q Droop Controller kq:V–Q droop coef. Q0:Set value of reactive power
Vbus
Q0
Qref+
+qk−
+
-
E0
.
. (Constant)
^
Contents
• Introduction
• Parallel Inverters
• Synchronous Generator + Inverter
• Conclusion
24
Microgrid: SG + Inverter 25
• Stand-alone microgrid in remote area • SG (10 kVA): Round-rotor moved by gas engine • DG (10 kVA): Photovoltaic panels • Load: Three-phase loads and single-phase Loads
Issues of Small Rating SG 26
Parameter Value Parameter Value
200 V 1 pu
10 kVA 0 pu
376.99 rad/s 20 pu
0.16 s 5 pu
20 pu 1 s
0.025 s AVR LPF cut-off frequency 20 Hz
Impedance Model
0.219 pu 0.027 pu 0.01 pu
6.55 s 0.039 s 0.85 s 0.071 s
SG Controller SG Parameters
Poor inertia
Slow governor response
Simulation of Single SG Operation
27
Initial loading : 3P 1 kW, 0.5 kvar Connected loading : 1P 4.8 kW, 2.1 kvar
Rotor frequency decreased rapidly due to poor inertia
Slow governor response cannot restore this frequency drop immediately
Ripples due to 3P unbalance introduced by 1P loading
Control Requirements of DG 28
• 1. Reduce SG rotor frequency deviation – If SG rotor frequency drop below 90%, SG may be unstable – Virtual Synchronous Generator (VSG) Control
• Generate virtual inertia to provide transient frequency support • Share active and reactive power with the SG
• 2. Eliminate Negative-Sequence Current from SG – Prevent SG from overheating and torsional stresses – Active power filter (APF)
• 3. Both 1+2 should be realized in ONE inverter
Proposed Modified VSG Control 29
SwingEquation Function
PWM
1/s
Pin_dg
Pout_dg
EnergyStorage
DistributedGenerator
PI ++
+
Qout_dg
Governor Model
Vout_dg
Zls
Qref_dg
E0
ωm_dg
Lf
-
Vcomp(αβ)
abc/αβ
Iout_dg(αβ)
Stator ImpedanceAdjuster
Vout_dg(abc)
Iout_dg(abc)ωg_dg
Vpwm
θpwmθm_dg
Edg
Cf BUS
Zline
Vout_sg(abc)
Iout_sg(abc)
−)(_ dqsgoutI
SG Neg.-Seq. Compensation
DDSRF
Q Droop
Iout_dg
Q0
P0
LPF
θg_sg
.
.
DDSRF 30
SwingEquation Function
PWM
1/s
Pin_dg
Pout_dg
EnergyStorage
DistributedGenerator
PI ++
+
Qout_dg
Governor Model
Vout_dg
Zls
Qref_dg
E0
ωm_dg
Lf
-
Vcomp(αβ)
abc/αβ
Iout_dg(αβ)
Stator ImpedanceAdjuster
Vout_dg(abc)
Iout_dg(abc)ωg_dg
Vpwm
θpwmθm_dg
Edg
Cf BUS
Zline
Vout_sg(abc)
Iout_sg(abc)
−)(_ dqsgoutI
SG Neg.-Seq. Compensation
DDSRF
Q Droop
Iout_dg
Q0
P0
LPF
θg_sg
.
.
ωg
T +Vαβ
-
+
T +2
θg
θg
LPF−
dqfV
−dqV
T –2 LPF+
dqfV
+dqV
T – +
T +Iαβ-
+
T +2
θg
LPF−dqfI
−dqI
T –2 LPF+dqfI
+dqI
T – +
PLL+qv
.
.
• Applied to both DG and SG voltage and current to extract positive and negative sequence components
• Output power, voltage and current of DG are calculated from only positive sequence components
• Prevent ripples due to negative sequence from entering the controller
LPF: 1st order low pass filter (cut-off frequency 40 Hz)
Double Decoupled Synchronous Reference Frame (DDSRF) Decomposition
31
SG Neg.-Seq. Compensation
SwingEquation Function
PWM
1/s
Pin_dg
Pout_dg
EnergyStorage
DistributedGenerator
PI ++
+
Qout_dg
Governor Model
Vout_dg
Zls
Qref_dg
E0
ωm_dg
Lf
-
Vcomp(αβ)
abc/αβ
Iout_dg(αβ)
Stator ImpedanceAdjuster
Vout_dg(abc)
Iout_dg(abc)ωg_dg
Vpwm
θpwmθm_dg
Edg
Cf BUS
Zline
Vout_sg(abc)
Iout_sg(abc)
−)(_ dqsgoutI
SG Neg.-Seq. Compensation
DDSRF
Q Droop
Iout_dg
Q0
P0
LPF
θg_sg
.
.
SG Neg.-Seq. Compensation
Stator Impedance Adjuster
Control SG Neg.-Seq. to be 0
Parameter Design of Inverter 32
Parameter Value Parameter Value 200 V 1 pu
10 kVA 0 pu
376.99 rad/s 20 pu
0.16 s 5 pu
8.7 pu 0 s
1.122 mH 0.69 pu
1.836 mH 5
PI Controller for Reactive Power
0.05 pu
PI Controller for SG Neg.-Seq. Compensation
0.1 pu 0.01 s
20 Hz
All parameters in red should be set equal to SG in per unit value to ensure proper transient and steady power sharing
Design of Parameters in blue will be discussed in detail
33
Set equal to SG in order to share transient active power?
Low frequency oscillation indicated by oscillatory conjugated eigenvalues
Constant Virtual Stator Inductance
34
Output reactance of DG and SG should be set equal to share transient power, but how about the value?
Connection of DG
Simulation of 1P Loading Transition
35
Initial loading : 3P 1 kW, 0.5 kvar Connected loading : 1P 4.8 kW, 2.1 kvar
Improved SG rotor frequency deviation
(Frequency Support of VSG)
SG only
SG + VSG
Initial loading : 3P 2 kW, 1 kvar Connected loading : 1P 9.6 kW, 4.2 kvar
(2x of SG only case)
Simulation of 1P Loading Transition
36
Ripples due to unbalance have been eliminated
w/o DDSRF and SG Neg. Seq.
with DDSRF and SG Neg. Seq.
Phase Current 37
w/o DDSRF and SG Neg. Seq.
Balanced SG current
with DDSRF and SG Neg. Seq.
Contents
• Introduction
• Parallel Inverters
• Synchronous Generator + Inverter
• Conclusion
38
Conclusion 39
• VSG control can provide inertia support for microgrids, leading to less fluctuant frequency
• Parallel inverters and SG + inverter operations were established, and several related issues were solved
Conclusion
• Operation of Multiple SGs + Multiple inverters
Future Plan
Thank you for your kind attention!
• J. Liu, Y. Miura, H. Bevrani, and T. Ise, “Enhanced virtual synchronous generator control for parallel inverters in microgrids,” IEEE Transactions on Smart Grid., doi: 10.1109/TSG.2016.2521405.
• J. Liu, Y. Miura, T. Ise, J. Yoshizawa, and K. Watanabe, “Parallel operation of a synchronous generator and a virtual synchronous generator under unbalanced loading condition in microgrids,” 8th International Power Electronics and Motion Control Conference (IPEMC-ECCE Asia), Hefei, China, 2016, pp. 3741-3748.
• J. Liu, Y. Miura, and T. Ise, “Power quality improvement of microgrids by virtual synchronous generator control,” 10th Electric Power Quality and Supply Reliability Conference (PQ2016) , Tallinn, Estonia, 2016.
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