Japan‐Norway Energy Science Week 2015
Tokyo, 27.05.2015
Functionalities of Power Electronic
Converters in the context
of SmartGrids
Jon Are Suul
pos.doc. researcher
Department of Electric Power Engineering
Norwegian University of Science and Technology
Outline
• Introduction to converters in power systems
• Added functionalities in traditional power systems
–
–
–
–
–
Contributions to primary frequency control
Control of voltage or reactive power
Damping of oscillations
Active filtering
Inertia emulation
• Operation in MicroGrids and isolated systems
– Example: Virtual Synchronous Machine
– Control for islanding with local loads
– Resynchronization to large-scale power system
• Summary
2
Traditional grids vs. "SmartGrids"
Traditional Utility grid
"SmartGrid" Scenario
F. Rahimi, A. Ipakchi, “Demand Response as a Market Resource Under the Smart Grid Paradigm,” in IEEE Transactions on Smart Grid, Vol. 1, No. 1, June 2010, pp. 82‐88
3
Local actuation - Power Electronics
•
•
Power Electronic converters has become the main enabling
technology for grid integration of renewable energy sources and
controllable loads
Typical examples:
–
–
–
–
–
•
Variable speed wind turbines
Photovoltaic power generation
Regenerative loads
Electric vehicle battery chargers
Energy Storage Systems
The local actuation and control in a Smart Energy System will be
mainly performed by power electronic converters
Grid equivalent source
Vg
Rg
Lg
LCL Filter
PCC
R2
L2
Vf
R1
Converter
L1
Ic
Vc


4
Production/Load/Storage
CDC
VDC
Two main types of converter control systems
•
Grid integrated operation
•
– Depends on Phase Locked
Loop for synchronization to the
grid voltage
– Usually operated with inner loop
current control
– Stand-alone operation requires
change of control system
Stand-alone operation
– Must be able to operate in
voltage control mode
– Need control loops for sharing
of active and reactive power
– Synchronization based on
power-frequency control
characteristics
Z load
I c ,ab
L2
Vs ,ab
vs
Grid
Grid information
Synchronization
L1
VDC
q
p
I c ,ab
Active
Power
Control
Current
Reference
Reactive Q*
Power
Control
ic
Synchronizing
signals
P*
Calculation
I
*
Current
Control
&
Modulation
I c ,ab
VDC
L1 C1
p&q
g16


q0
p0
CDC
Reactive
Power
Control
Active
Power
Control
v̂*
Voltage

Control
ic*
Current
Control
&
Modulation
I c ,abc
g PWM

CDC
VDC
vDC
Outline
• Introduction to converters in power systems
• Added functionalities in traditional power systems
–
–
–
–
–
Contributions to primary frequency control
Control of voltage or reactive power
Damping of oscillations
Active filtering
Inertia emulation
• Operation in MicroGrids and isolated systems
– Example: Virtual Synchronous Machine
– Control for islanding with local loads
– Resynchronization to large-scale power system
• Summary
6
Power-frequency control
•
Relevant for contribution to power system primary frequency
regulation
– All converters with controllable generation or load can apply
frequency-dependent power control
– Traditional power-frequency droop is the simplest option
• Can be applied to most control schemes
– Implies loss of generation or load
for units without energy storage
•
Can be used in addition to load
shifting or other techniques for
improving the match between
generation and load profiles
p*
*
 grid


k
r*
 p
Frequency Droop
7
Reactive power or voltage control
•
Inverters and regenerative rectifiers can control their reactive
power flow
– Free capacity can be used to regulate grid voltage in inductive
transmission systems
•
•
Droop control can be applied to share
load between parallel or nearby units
Dual control option
– Voltage control with reactive power
droop
– Reactive power control with voltage
droop
•
8
Droop-based Reactive Power Controller
v̂*
q*


q
f
kq


qm
s f
Dedicated compensation units are needed in case of generation
units and loads without reactive power control capability
vˆ r*
Consequences of reactive current injection
• Dependent on grid
impedance
• Required incremental
rating can be small
It 
I
2
d
Id
It
 Iq
2

i [pu]
q
Characteristics of system with 1.0 pu generation, x eq = 0.25, req = 0.05
d
Iq
2
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
1.1
1
0.9
0.8
0.7
0.6
id(iq) for vg = 1.0
itot(iq) for vg = 1.0
id(iq) for vg = 0.8
itot(iq) for vg = 0.8
-0.1
0
0.1
0.2
0.3
0.4
0.5 0.6
iq [pu]
0.7
0.8
0.9
1
1.1
1.2
It  I d
M. Molinas, D. Moltoni, G. Fascendini, J. A. Suul, R. Ferranda, T. Undeland: “Investigation on the role of power electronic controlled constant power loads for voltage support in AC systems,” in Proceedings of the IEEE 39th Power Electronics Specialists Conference, PESC’08, Rhodes, Greece, 15‐19 June, 2008, pp. 3597‐3602
J. A. Suul, M. Molinas; "Properties of Reactive Current Injection by AC Power Electronic Systems for Loss Minimization," in Proceedings of the 15th International Power Electronics and Motion Control Conference, EPE‐PEMC 2012 ECCE Europe, Novi Sad, Serbia, 3‐6 September 2012, 8 pp.
9
Active Damping of oscillations
• Basic concept
– Inject voltage or current component in counter-phase with the
measured oscillations
• Effectively cancels poorly damped oscillations
– Oscillatory components usually isolated by high-pass filter
• Can be applied for wide range of oscillation phenomena
– LC oscillations in filters or transmission cables
– Low frequency oscillations in mechanical components
• Can require corresponding energy storage capability
Active Damping of LCL filter oscillations
vo , dq
 AD
s   AD
dq


k AD
*
vdq
, AD
Example: Active Damping of LC oscillations
•
Hysteresis controlled converter
– Wide spectrum harmonics generated
by hysteresis control initiates
oscillations in LCL filter
– Active damping is a simple way of
alleviating this problem
•
Experimental results without and with
active damping
•
Fixed frequency PWM can also
trigger LC oscillations
– Can be damped using the same
approach
J. A. Suul, K. Ljøkelsøy, T. Midtsund, T. Undeland, "Synchronour Reference Frame Hysteresis Current Control for Grid Converter Applications, in Proceedings of the International Conference on Power Electronics and Motion Control, EPE‐PEMC 2010, Ohrid, Republic of Macedonia, 6‐8 September 2010, 9 pp
Active Power Filtering
• Conventional approach:
– Dedicated converter
compensating for distortions
from non-linear loads
• High control bandwidth is
required
• High switching frequency
• Possibility:
– Active filtering as auxiliary
service from converter for
generation, load or energy
storage
H. Akagi, E. Watanabe, M. Aredes, "Instantaneous Power Theory and Applications to Power Conditioning," Wiley/IEEE Press, 2007
Inertia emulation
•
Conventional approach
– Based on frequency derivative
pIE  J IE
•
13
d g
dt
Operation depends on the
presence of conventional
generation with physical inertia
and power-frequency droop
control
G. Delille, B. François, and G. Malarange, "Dynamic Frequency Control Support by Energy Storage to Reduce the Impact of Wind and Solar Generation on Isolated Power System’s Inertia" in IEEE Transactions on Sustainable Energy, Vol. 3, No. 4, October 2012, pp. 931‐939
Outline
• Introduction to converters in power systems
• Added functionalities in traditional power systems
–
–
–
–
–
Contributions to primary frequency control
Control of voltage or reactive power
Damping of oscillations
Active filtering
Inertia emulation
• Operation in MicroGrids and isolated systems
– Example: Virtual Synchronous Machine
– Control for islanding with local loads
– Resynchronization to large-scale power system
• Summary
14
MicroGrids and isolated systems
•
Requirements for power converters in isolated systems
– Distributed control of:
• Frequency and active power sharing
• Voltage and reactive power sharing
– Possibility to operate without physical inertia
– Possibility to operate in mixed systems with generation from both
synchronous machines and converter-interfaced sources
•
Additional requirements for MicroGrids
– Converter control systems must also be adapted to grid-connected
operation
– Seamless transition between grid-connected and islanded operation
– Smooth reconnection to the main power system
15
Virtual Synchronous Machine (VSM)
•
•
Power Electronic converter controlled
to emulate the behavioral
characteristics of synchronous
machines
Concept first introduced about one
decade ago
– Several possible implementations
•
•
Parameters are not limited by
physical design constraints
Emulated characteristics:
– Inertia and damping
– Power-balance-based grid
synchronization mechanism
– Active and reactive power control
mechanisms
• Can operate in both grid
connected and islanded modes
16
S. D'Arco, J. A. Suul, "Virtual Synchronous Machines –
Classification of Implementations and Analysis of Equivalence to
Droop Controllers for Microgrids," in Proceedings of IEEE PES
PowerTech 2013, Grenoble, France, 16-20 June 2013, 7 pp.
Overview of implementation
•
•
•
•
•
•
Synchronous Reference Frame cascaded voltage and current controllers
Virtual Impedance
Active damping of LC filter
Local frequency tracking by Phase Locked Loop only needed for damping emulation
Power Controller including Virtual Swing equation ensuring inertial characteristics
Reactive Power Controller
V
g
PLL
Zl
Phase
Locked
Loop
Zg
Measurement
io ,dq
q
q*
v̂*
Reactive vˆ r*
Power
Control
PLL
p
p
*
*
VSM
Virtual 
VSM
Inertia
and
VSM
Power
Control
vo , dq
io
vo
Processing
icv , dq
v*AD ,dq
Active
Damping
*
*
mabc
Virtual vo,dq Voltage icv ,dq Current mdq dq
PWM

Control
Control
Impedance
Lf
icv
vo , dq
Cf
I cv
g PWM
CDC
vDC
S. D'Arco, J. A. Suul, O. B. Fosso, "Small‐signal modelling and parametric sensitivity of a Virtual Synchronous Machine in islanded operation," in International Journal of Electric Power and Energy Systems, Vol. 72, November 2015, pp. 3‐15
17
Response to sudden islanding
•
•
VSM operated with power reference in grid connected mode
Sudden islanding to a local load
•
•
Transition is immediate and smooth while maintaining voltage on the local load
Local frequency settles to a value depending on the steady-state frequency droop
Power [pu]
VSM speed
0.75
1.025
0.7
1.02
0.65
1.015
VSM [pu]
p [pu]
0.6
0.55
0.5
1.01
1.005
0.45
1
0.4
0.35
0
0.5
1
Time [s]
1.5
0.995
0
0.5
1
1.5
2
Time [s]
2.5
3
S. D'Arco, J. A. Suul, O. B. Fosso, "Small‐signal modelling and parametric sensitivity of a Virtual Synchronous Machine in islanded operation," in International Journal of Electric Power and Energy Systems, Vol. 72, November 2015, pp. 3‐15
18
3.5
Resynchronization to main grid
•
•
Amplitude, frequency and phase of the voltage in the isolated system
must be aligned with the grid voltage before reconnection
A centralized controller can act on the references to the local
controllers for eliminate the voltage across the breaker
Zg
VB , g VB ,l
Local Bus
L2
VSM 1
Vo ,1
Vg
I c ,1 Vc ,1


Cf
Synchronizing
Controller
VSM 2
L2
Zl
L1
Vo ,2
L1
Cf
I c ,2 Vc ,2


Source/Storage
VDC
CDC
Source/Storage
VDC
CDC
S. D'Arco, J. A. Suul, " A Synchronization Controller for Grid Reconnection of Islanded Virtual Synchronous Machines," in Proceedings of the 6th
International Symposium on Power Electronics for Distributed Generation, PEDG 2015, Aachen, Germany, 22‐25 June 2015, 8 pp.
19
Incremental VSM frequency reference
•
Acting on frequency reference to be compatible with different VSM
implementations as well as synchronous machine controllers
PLL
Phase
Locked
Loop
Lg
Measurement
io ,dq
q
q*
v̂
*
Reactive vˆ r*
Power
Control
PLL
p
p
*
*

* 
VSM
20
*
VSM
Virtual 
VSM
Inertia
and
VSM
Power
Control
vo , dq
io
vo
Processing
icv , dq
v*AD ,dq
Active
Damping
*
*
mabc
Virtual vo,dq Voltage icv ,dq Current mdq dq
PWM

Control
Control
Impedance
Lf
icv
vo , dq
Cf
I cv
g PWM
CDC
vDC
Smooth reconnection
 [pu]
0.02
0
-0.02
-0.04
-0.06
•
•
•
When phase difference is eliminated
the breaker can be closed
VSM is smoothly transferred into gridconnected operation without any
change of internal control system
VSM can start tracking the power
reference
10
15
20
Time [s]
25
30
35
40
-pi
0
5
10
15
20
Time [s]
25
30
35
40
0
5
10
15
20
Time [s]
25
30
35
40
0
5
10
15
20
Time [s]
25
30
35
40
 [rad]
pi
pi/2
0
-pi/2
Active power - po [pu]
•
In islanded operation the phase
of the voltage across the breaker
is drifting with the frequency
difference
Synchronization controller must
eliminate the frequency and
phase difference across the
breaker
5
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Current - |io| [pu]
•
0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
S. D'Arco, J. A. Suul, " A Synchronization Controller for Grid Reconnection of Islanded Virtual Synchronous Machines," in Proceedings of the 6th
International Symposium on Power Electronics for Distributed Generation, PEDG 2015, Aachen, Germany, 22‐25 June 2015, 8 pp.
Summary
•
Power electronic converters will be among the main actuators of the
emerging SmartGrid scenario
• High controllability of grid converter enables provision of additional
functions
• Active and reactive power control for contribution to the power system
operation
• Damping of oscillations
• Harmonic filtering
• Inertia emulation for increasing equivalent rotating mass of the power system
•
Additional control system requirements apply to converters in isolated
power systems and MicroGrids
• Virtual Synchronous Machines presented as an example of control
systems that can ensure:
• Operation in both grid-connected and islanded modes with smooth transitions
and without need for on-line control system reconfiguration
• Operation in isolated systems with both power converters and machines
• Active and reactive power control with inertial characteristics in both gridconnected and islanded operation
22
Thank you for your attention!
Questions?
References
• S. D'Arco, J. A. Suul, O. B. Fosso, "Small-signal modelling and parametric sensitivity of a Virtual Synchronous Machine in islanded
operation," in International Journal of Electric Power and Energy Systems, Vol. 72, November 2015, pp. 3-15
• S. D'Arco, J. A. Suul, O. B. Fosso, "A Virtual Synchronous Machine Implementation for Distributed Control of Power Converters in
SmartGrids," in Electric Power System Research, Vol. 122, May 2015, pp. 180-197
• S. D'Arco, J. A. Suul, O. B. Fosso, "Small-Signal Modeling and Parametric Sensitivity of a Virtual Synchronous Machine," in Proceedings
of the 18th Power Systems Computation Conference, PSCC 2014, Wrocław, Poland, 18-22 August 2014, 9 pp.
• S. D'Arco, J. A. Suul, "Equivalence of Virtual Synchronous Machines and Frequency-Droops for Converter-Based MicroGrids," in IEEE
Transactions on Smart Grid, Vol. 5, No. 1, January 2014, pp. 394-395
• S. D'Arco, J. A. Suul, " A Synchronization Controller for Grid Reconnection of Islanded Virtual Synchronous Machines," in Proceedings of
the 6th International Symposium on Power Electronics for Distributed Generation, PEDG 2015, Aachen, Germany, 22-25 June 2015, 8 pp.
• J. A. Suul, S. D'Arco, G, Guidi, "Virtual Synchronous Machine-based Control of a Single-phase Bi-directional Battery Charger for Providing
Vehicle-to-Grid Services," in Proceedings of the 9th International Conference on Power Electronics, ICPE – ECCE Asia, Seoul, Korea, 1-5
June 2015, 8 pp.
• J. A. Suul, S. D'Arco, G. Guidi, "A Single-Phase Virtual Synchronous Machine for Providing Vehicle-to-Grid Serviced from Electric Vehicle
Battery Chargers," in Proceedings of the International Electric Vehicle Technology Conference & Automotive Power Electronics, EVTeC &
APE Japan, Yokohama, Japan, 22-24 May 2014, 7 pp.
• S. D'Arco; J. A. Suul; O. B. Fosso, "Control System Tuning and Stability Analysis of Virtual Synchronous Machines," in Proceedings of the
2013 IEEE Energy Conversion Congress and Exposition, ECCE 2013, Denver, Colorado, USA, 15-19 September 2013, pp. 2664-2671
• S. D'Arco, J. A. Suul, "Virtual Synchronous Machines – Classification of Implementations and Analysis of Equivalence to Droop
Controllers for Microgrids," in Proceedings of IEEE PES PowerTech 2013, Grenoble, France, 16-20 June 2013, 7 pp.
• M. Molinas, D. Moltoni, G. Fascendini, J. A. Suul, R. Ferranda, T. Undeland: “Investigation on the role of power electronic controlled
constant power loads for voltage support in AC systems,” in Proceedings of the IEEE 39th Power Electronics Specialists Conference,
PESC’08, Rhodes, Greece, 15-19 June, 2008, pp. 3597-3602
• J. A. Suul, M. Molinas; "Properties of Reactive Current Injection by AC Power Electronic Systems for Loss Minimization," in Proceedings
of the 15th International Power Electronics and Motion Control Conference, EPE-PEMC 2012 ECCE Europe, Novi Sad, Serbia, 3-6
September 2012, 8 pp.
• J. A. Suul, K. Ljøkelsøy, T. Midtsund, T. Undeland, "Synchronous Reference Frame Hysteresis Current Control for Grid Converter
Applications, in Proceedings of the International Conference on Power Electronics and Motion Control, EPE-PEMC 2010, Ohrid, Republic
of Macedonia, 6-8 September 2010, 9 pp.
24