Download Control of DPGS Under Grid Faults

Control of DPGS Under Grid Faults
Control of DPGS Under Grid Faults
Marco Liserre
[email protected]
Marco Liserre
[email protected]
Control of DPGS Under Grid Faults
Outline
• Introduction
• Crowbar protection for DFIG
• Effect of voltage sag over the IG connected to grid
• Effect of voltage sag over the wind farm
• Solutions for dynamic reactive injections
• LVRT with variable-speed wind turbines
• Control strategies under unbalanced voltage
• Conclusions
Marco Liserre
[email protected]
Control of DPGS Under Grid Faults
Introduction
 Control under grid faults has become very actual after new grid codes (Germany,
Spain) has requested not only ride-through but also reactive power injection under
fault
 Low voltage ride-through LVRT can be achieved by quickly limiting the active power
using a crowbar and inject reactive power to support grid voltage restoring
 Reactive power injection during asymmetrical fault requires a positive sequence angle
extraction
 Advanced control specially designed for unbalance voltage requires positive/negative
sequence extraction and current controllers for both sequences
 P+Resonant current controllers can handle both sequences
 Using the instantaneous power theory different control strategies can be designed in
order to minimize the dc voltage oscillations or output P and Q oscillations during
unbalanced voltage
 New grid codes are expected to require even more controllability under grid fault in
order to support the grid recovering (for ex. Non-oscillating P or Q)
Marco Liserre
[email protected]
Control of DPGS Under Grid Faults
Grid Faults and their consequences
 In general when a fault occurs in the electric grid and the fault is detected, the
minimum possible portion of the grid containing the fault should be isolated and
the neighboring parts should try to go back to normal operation. If a component
fails then a larger area can be affected.
 Then the fault is removed and the damage is repaired, finally the normal service is
restored.
 Faults can lead to both angle and voltage instabilities
 The angle instability is cause by active power imbalance and by the excitation of
mechanical generator dynamics. As a consequence the generators lose
synchronism and they are disconnected.
 Voltage instability arises due lack of reactive power and as a consequence leads
to overload of power lines and as a consequence disconnection of loads.
Marco Liserre
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Control of DPGS Under Grid Faults
Low Voltage Ride Through
 Two requirements can be defined for WTS during grid fault:
to remain connected even during severe under-voltage
to control reactive power to support the grid voltage
 These two requirements are associated with:
Measure and maintain full control on the reactive and active power injected hence
the capability of limiting it
Limit the power quality decrement in terms of power fluctuations and harmonic
distortion that may have severe effects on the grid.
 LVRT is strictly related to the capability of
the WTS to preserve the converter safety
and avoid overcurrent tripping. The
semiconductor over-load capability can be
increased only reducing the switching
frequency
Marco Liserre
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Control of DPGS Under Grid Faults
Crowbar protection
Resistive crowbar
Rotor short-circuited
 When the resistive crowbar is
implemented, the stator and
rotor transient current decay
rapidly to value with amplitude
lower than 1 p.u.
 Amplitude of transient
electromagnetic torque is
reduced when the resistive
crowbar is activated.
 On the other hand the
electromagnetic torque
oscillates longer with the
resistive crowbar.
Marco Liserre
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Control of DPGS Under Grid Faults
Effect of voltage dips over the IG connected to grid
During the fault period:
 Voltage at the wind turbine terminal drops as well as the air-gap flux in the generator.
 Reduction in the electromagnetic torque which causes the rotor to accelerate.
After the clearance of the fault:
 Reactive power is supplied by the power system to recover the air-gap flux.
 This causes a high inrush current to be drawn by the wind turbine from the power system, which
in turn causes a voltage drop at the wind turbine terminal.
Effect of a sustained voltage drop after fault clearance:
 If the electromagnetic torque is not strong enough in comparison with the aerodynamic torque,
the rotor speed will continue to increase and the induction generator could draw high inrush current
from the external power system until appropriate protection devices trip it.
 In this condition, voltage at the wind turbine terminal dips and the output power of wind turbine
drops.
The system loses stability and the wind turbine has to be disconnected.
Marco Liserre
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Control of DPGS Under Grid Faults
Effect of voltage dips over the IG connected to grid
Electromagnetic torque
Active power delivered
by the SCIG-WT
Marco Liserre
Rotor speed
Reactive power delivered
by the SCIG-WT
Positive-sequence
voltage at the stator
Negative sequence
voltage at the stator
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Control of DPGS Under Grid Faults
Effect of voltage dips over the wind farm
without STATCOM
Source: American Superconductor
Marco Liserre
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Control of DPGS Under Grid Faults
Effect of voltage dips over the wind farm
With D-VAR STATCOM
Source: American Superconductor
Marco Liserre
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Control of DPGS Under Grid Faults
Dynamic reactive power injection
LVRT retrofit solution for wind farms – D-VAR,Dynamic Capacitors
Source:
American
Superconductor
Marco Liserre
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Control of DPGS Under Grid Faults
LVRT with STATCOM

STATCOM is a potential technology to retrofit LVRT to existing wind farms based
on fixed-speed generators

As LVRT is compulsory in some countries and will be spread worldwide a big
market is open for STATCOM industry

Along with providing LVRT, the STATCOM is providing also voltage regulation
especially important for weak grids

Reactive power grid support is an ancillary service that will be paid by TSO in the
future making it more attractive
Marco Liserre
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Control of DPGS Under Grid Faults
LVRT with DFIG. Stability issue
DFIG stability and reactive power
 Conventional DFIG rotor current control based on synchronous d-q reference
frames is prone to oscillations when reactive power is fed to grid from the rotor side
under fault conditions
 The generator will be unstable if:
 v

RS
ird  2 

irq 
 Lmo LS o 
 This is only twice the rotor current that is required for operation at unity power
factor
 In voltage dips the stability limit is further lowered
 However, generators that used Direct Torque Control (DTC) instead of vector
current control turned out to be reasonable stable allowing the rated reactive current
production down to very low grid voltages
Marco Liserre
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Control of DPGS Under Grid Faults
LVRT with DFIG example
Control of reactive power injection on the synchronous d-q frame
Stable, but dampnig is low
Slow decay of the dc
component
Slow
Source: ABB
Marco Liserre
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Control of DPGS Under Grid Faults
LVRT with DFIG example
Control of reactive power injection using DTC active crow bar
Very stable dc voltage
Fast decay of the dc
component
Fast
100ms
Source: ABB
Marco Liserre
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Control of DPGS Under Grid Faults
LVRT with DFIG example
Control of reactive power injection using DTC active crow bar
Boost, voltage level
without reactive
injection
Fast reactive power support
Marco Liserre
Source: ABB
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Control of DPGS Under Grid Faults
LVRT with Variable Speed WT

Two technologies:



Full-scale converter with SG, PMSG or IG
Double-fed IG
Both are capable of injecting reactive power to the extent of the kVA rating
Both are capable of quickly reduce the generated power (vector control or DTC of
generator, chopper)


Both can reduce the aerodynamic torque after fault by pitching control (few seconds)

Full-scale converter can control faster the grid currents being directly connected
Marco Liserre
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
∙ Power oscillations occurs due to negative sequence
∙ These translates into dc voltage oscillations  trip
∙ The aim of the control is to reduce these oscillations and
inject positive sequence reactive power
∙ The following additional control issues are necessary to be
addressed to the conventional control structure:
▫
Positive/negative sequence calculation
◦
◦
▫
Current control that can handle both sequences
◦
◦
▫
Marco Liserre
Dual synchronous (PI-dq) reference frame
Single stationary reference frame (P+Resonant)
Current reference calculator to address a specific control strategy like:
◦
◦
◦
▫
in synchronous dq frame (LP.BP, Notch filtering)
In stationary frame with (DSC, SOGI-FLL)
minimization of dc voltage oscillations
maximizing the transferred power
minimization of oscillations in the instantaneous P&Q
Power reference calculator (P* and Q*)
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Current Reference Calculation – instantaneous power theory
Grid voltage under generic conditions:
V  V  Vh

V 


n 1

va  va   vh 
v    v   v 
 b   b   h 
 vc   vc  vh 

 sin( n t    n ) 
 sin( n t    n )  
 n

n
n
n
2 
2 
V sin( n t    3 )  V sin( n t    3 ) 

sin( n t    n  23 )
sin( n t    n  23 ) 






vh 
V
0n
sin( n t   0 n )
n 1
Generic current injected by the three-phase grid inverter:
i 
  a 
I  ib 
 
 ic  
Marco Liserre


n 1
  sin( n t    n ) 
 sin( n t    n )  
 n

n
n
n
2 
2 
I
sin(

t



)

I
sin(

t



)
 
n
n
3 
3 

 sin( t    n  2 )
sin( n t    n  23 ) 
n
3



 
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Current Reference Calculation – instantaneous power theory
Instantaneous active power delivered to the grid:
p3 
3 
  V  n I  n cos(  n    n )  V  n I  n cos(  n    n ) 
2 n1
p3 
 
3 

    V  m I  n cos((m  n )t    m    n ) 
2  m1  n1

m

n

  m n

   V I cos((m  n )t    m    n ) 
m 1  n 1


m n


    V  m I  n cos((m  n )t    m    n ) 
m 1  n 1



m n
m
n  
    V I cos((m  n )t     )  .
m 1  n 1


Marco Liserre
[email protected]
Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Current Reference Calculation – instantaneous power theory
Instantaneous reactive power delivered to the grid:
q3 
3 
   V  n I  n sin(  n    n )  V  n I  n sin(  n    n ) 
2 n1
q3 
 
3 

     V  m I  n sin((m  n )t    m    n ) 
2  m1  n1

m

n

  m n

   V I sin((m  n )t    m    n ) 
m 1  n 1


m n


    V  m I  n sin((m  n )t    m    n ) 
m 1  n 1



m n
m
n  
    V I sin((m  n )t     )  .
m 1  n 1


Marco Liserre
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Current Reference Calculation – instantaneous power theory
∙ Experimental evaluation
Source: [4,5]
Simulated
fault
Pgreen
DC power
source
vdc
PWM
inverter
v pwm
y
v, i
LC
filter
Grid
simulator
Transformer
Local
load
s
SVM
modulator
v*
D
Sensors
i
v
Current
controller
i*
Current
reference
.- Grid Code
Requirements
.- Fault strategy
.- MPPT
.- Protection
Marco Liserre


P* Q* v ,v
Sequence
detector
Power
reference
POWER PROCESSOR CONTROL
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Current Reference Calculation – P control laws
Instantaneous Active Reactive Control (IARC)
∙Basis:
p  vi
∙Reference constrains:
P  v  i*p
∙Control law:
;
g
P
v
2
5
i [A]
i gv
*
p
10
0
-5
• Distorted and unbalanced current
• Instantaneous power perfectly
controlled
• Overcurrent trip risk
Marco Liserre
-10
2
p, q [kW, kvar]
∙Comments:
q  v  i*p  0
1
p
q
0
-1
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Current Reference Calculation – P&Q control laws
Instantaneous Active Reactive Control (IARC)
∙Basis:
400
∙Reference constrains:
0
-200
-400
Q  v  i
10
*
q
5
i [A]
P  vi
*
p
200
v [V]
q  v  i  v  i
p  vi
v  is a 90-degrees leaded version
of the voltage vector v
∙Control law:
0
-5
i*q  b v 
Marco Liserre
;
g
P
2
v
;
b
Q
v
-10
2
2
p, q [kW, kvar]
i*p  g v
1
0
-1
p
q
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Current Reference Calculation – P control laws
Positive- Negative-Sequence Compensation (PNSC)
∙Basis:
p   v  v   i  i





400
200

*
p

*
p

v i  v i  P
v [V]
∙Reference constrains:
*
p

*
p
v i  v i  0
0
-200
-400
∙Control law:
;
5
P
g 
v
 2
v
i [A]
i*p  g   v   v  
10
0
 2
-5
-10
∙Comments:
 v   i*p  v   i*p  v   i*p  v   i*p
2
p, q [kW, kvar]
q  vi
*
p
1
p
q
0
-1
0
q
• Sinusoidal unbalanced currents
• Oscillations in the instantaneous reactive power
• Maximun value of current is a known function of power and sag deep
Marco Liserre
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Current Reference Calculation – P&Q control laws
Positive- Negative-Sequence Compensation (PNSC)
400
p   v  v   i  i 
v [V]
200
q   v   v     i   i  
-200
-400
10

*
p

*
p
v i  v i  P
v i  v i  0
v  i*q  v  i*q  Q
v  i*q  v  i*q  0
*
p

*
p
i*q  b   v   v  
Marco Liserre
0
-5
-10
∙Control law:
i*p  g   v   v  
5
i [A]
∙Reference constrains:

0
2
;
P
g 
v
;
 2
v
 2
v
 2
v
1
0
-1
Q
b 
p, q [kW, kvar]
∙Basis:
p
q
 2
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Current Reference Calculation – P control laws
Balanced Positive Sequence (BPS)
p   v  v   i
400
200
v [V]
∙Basis:
∙Reference constrains:

v i
*
p
v
P

v

i
*
p
0

v i
*
p
p
0
-200
-400
∙Control law:
10

;

G 
P
v
0
 2
∙Comments:
q  v  i*p  v   i*p  v   i*p
-5
-10
2
p, q [kW, kvar]

i G v
*
p
i [A]
5
1
p
q
0
0
q
-1
• Sinusoidal balanced current
• Oscillations in both instantaneous powers
• Maximun value of current is easily calculated
Marco Liserre
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Current Reference Calculation – P&Q control laws
Balanced Positive Sequence (BPS)
400
p   v  v   i
v [V]
200
q   v   v    i 
-200
-400
-40
10
∙Reference constrains:
v   i*p  P


v i
*
q
Q
v
v
 v    i*p  0



v


i
*
q
0
0
v   i*p  p


*
q
v i  q
i B v
Marco Liserre

;

G 
P
v

0
10
20
30
40
50
60
20
30
40
50
60
20
30
40
50
60
t [ms]
-10
-40
-30
-20
-10
0
10
t [ms]
i G v
*
q
-10
-5


;

B 
 2
2
1
0
-1
-40
Q
v
p, q [kW, kvar]

-20
0
∙Control law:
*
p
-30
5
i [A]
∙Basis:
p
q
-30
-20
-10
0
10
t [ms]
2
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
e
ow
uip
Eq line
Current Reference Calculation – P control laws
∙ IARC allows full control of the power
delivered to the grid at the expense of
 v
v
the current quality.
This strategy
should be used when the control of
the dc-bus voltage is the main issue.
v

v
∙ PNSC cancels instantaneous active
power oscillation keeping unbalanced
i p ( IARC )
sinusoidal currents. However no full
v
i p ( BPS )
 power can be delivered since
 q( BPS ) 
i p ( PNSC )
oscillations exist in the reactive power

 q( PNSC ) 
and the maximum value of the

v   v  injected current should be limited.


∙ BPS achieves sinusoidal balanced
currents and gives rise to oscillations
T
in the instantaneous powers. This
1




x y 
x  y dt  0
strategy should be used when current
T 0
quality is the main issue.
r

Marco Liserre
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Power Reference Calculation
Active Power Delivery - APD
PBS
∙Goal: deliver max. available P
and no Q without exceeding
nominal current
∙Control law:

P  3 V I N
*
Q 0
*
IARC
∙Comments:
∙V+-RMS value of pos seq. voltage
∙IN – RMS value of nominal current
PNSC
• PBS – sinusoidal, balanced, limited.
Oscillations in P and Q
• IARC – distorted hard to predict
peak. No P and Q oscillations
•PNSC – unbalanced,sinusoidal.
Oscillations in Q only
Source: [6]
Marco Liserre
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Power Reference Calculation
Grid Voltage Supporting - GVS
PBS
∙Goal: deliver max. available Q
and no P without exceeding
nominal current
∙Control law:
P 0
*

Q  3 V I N
*
IARC
∙Comments:
∙V+-RMS value of pos seq. voltage
∙IN – RMS value of nominal current
∙GVS: ON when V+ <0.9VN, OFF V+
≈ VN
• PBS – sinusoidal, balanced,
limited.
Oscillations in P and Q
• IARC – distorted hard to predict
peak. No P and Q oscillations
•PNSC – unbalanced,sinusoidal.
Oscillations in P only
Marco Liserre
PNSC
Source: [6]
[email protected]
Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Power Reference Calculation
Active Reactive Power Delivering- ARPD
PBS
∙Goal: deliver P and Q according
to E.ON guidlines. Deliver P &
supporting grid with Q
∙Control law:
P 
*
 3 V

I N   Q
2

* 2
,

V 
Q  6  V I N 1 

VN 

*

IARC
∙Comments:
∙V+-RMS value of pos seq. voltage
∙IN, VN – RMS value of nominal
current(voltage
∙ARPD: ON when V+ <0.9VN, OFF V+ ≈ VN
• PBS – sinusoidal, balanced, limited.
Oscillations in P and Q
• IARC – distorted hard to predict
peak. No P and Q oscillations
•PNSC – unbalanced,sinusoidal.
Oscillations in P and Q
Marco Liserre
PNSC
Source: [6]
[email protected]
Control of DPGS Under Grid Faults
Experimental results
Analyzing the reference current expressions
it is possible to determine the maximum
power to be delivered to the grid during
unbalanced voltage sages without reaching
the overcurrent limit in any of the phases at
any time
PNSC
Marco Liserre
IARC
BPS
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Control of DPGS Under Grid Faults
P&Q Control Strategies Under Grid Fault
Current Reference Calculation – P&Q control laws
 Sinusoidal balanced currents:
 High quality in the injected current
 Maximum current at the ac-side can be readily predicted
 Oscillations in injected active power  moderated dc-voltage oscillations
 Sinusoidal unbalanced currents:
 Acceptable quality in the injected current, no high order harmonics.
 Maximum current at the ac-side can be predicted
 Depending on the selected strategy, active power oscillations can be cancelled when no reactive power is injected
 no dc-voltage oscillations
 Nonsinusoidal currents:
 Poor current quality in the injected current
 Maximum value of the injected currents can not be easily predicted
 No oscillations in the injected active power independently of the reactive power  no dc-voltage oscillations
Possible
fault-tolerant
scenario
Marco Liserre
Unbalance
small (<2-5%)
medium (5-15%)
high (>15 %)
Output power
capability
Full
Full
Partial
Input currents
Should be
sinusoidal &
balanced
May be sinusoidal
but unbalanced
May be not
sinusoidal &
unbalanced
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Control of DPGS Under Grid Faults
Conclusions








Marco Liserre
LVRTcan be implemented with resistive crow-bar and reactive power
injection to existing wind farms.
Variable speed wind turbines can do LVRTas they are capable of
quickly reduce the torque of the generator and inject reactive power.
The capability of the system to allow ride-through grid disturbances is
strictly related to the inverter rating (maximum voltage and current at
the ac-side) as well as its dc-link energy storage (maximum/minimum
voltage at the dc-side).
A robust control strategy for the grid inverter should ride-through
transient faults avoiding under-/over-voltage trip in the dc-bus and
output over-currents due to distortion and imbalance
Reactive power injection is helping the grid voltage to recover
Several strategies for reference current generations. Overcurrent and
under/over voltage should be avoided to ride-through transient faults
Different power delivery strategies under fault are also presented
New grid codes with even more sestrictions during grid faults are
expected in the future an more advanced control strategies have to
be developed.
[email protected]
Control of DPGS Under Grid Faults
Bibliography
1.
S. Seman, J. Niiranen, S. Kanerva, and A. Arkkio, ”Analysis of a 1.7 MVA Doubly Fed Wind-Power Induction Generator
during Power Systems Disturbances”,
2.
Molinas, Marta; Suul, Jon Are; Undeland, Tore , “Improved grid interface of induction generators for renewable energy by
use of STATCOM” , International Conference on Clean Electrical Power, 2007. ICCEP '07., Vol., Iss., 21-23 May 2007
Pages:215-222
3.
G. Saccomando, J. Svensson, “Transient Operation of Grid-connected Voltage Source Converter Under Unbalanced
Voltage Conditions,” 2001 IEEE Industry Applications Conference, 36th IAS Annual Meeting, September 30 - October 5,
2001, Chicago, USA, 2001.
4.
Rodriguez, P.; Timbus, A. V.; Teodorescu, R.; Liserre, M.; Blaagjerg, F, “Independent PQ Control for Distributed Power
Generation Systems under Grid Faults”, IEEE Industrial Electronics, IECON 2006 - 32nd Annual Conference on, Vol.,
Iss., Nov. 2006 Pages:5185-5190
5.
P. Rodriguez, A.V. Timbus, R. Teodorescu, M. Liserre, F. Blaabjerg, “Flexible Active Power Control of Distributed Power
Generation Systems During Grid Faults”. Industrial Electronics, IEEE Transactions on Volume 54, Issue 5, Oct. 2007
Page(s):2583 – 2592
6.
P. Rodriguez, Luna, A., Teodoresc R., Blaabjerg F., - “Fault Ride-through Capability Implementation in Wind Turbine
Converters Using a Decoupled Double Synchronous Reference Frame PLL” – Proceedings of EPE 2007, 2-5 sep. 2007
(on CD)
7.
A. Luna, P. Rodriguez, R. Teodorescu and F. Blaabjerg, “Low voltage ride through strategies for SCIG wind turbines in
distributed power generation systems,” in Proc. IEEE Power Electron. Conf. PESC 2008. Jun. 2008
8.
F. Blaabjerg, R. Teodorescu, M. Liserre, and A. V. Timbus, “Overview of Control and Grid Synchronization for Distributed
Power Generation Systems”, IEEE Trans. on Ind. Electronics, Vol. 53, Oct. 2006 Page(s):1398 – 1409
9.
P. Rodríguez, A. Timbus, R. Teodorescu, M. Liserre and F. Blaabjerg, “Reactive Power Control for Improving Wind
Turbine Systems Behaviour under Grid Faults,” IEEE Trans. on Power Electronics, Aug. 2008 (in press)
Marco Liserre
[email protected]
Control of DPGS Under Grid Faults
Acknowledgment
Part of the material is or was included in the present and/or past editions of the
“Industrial/Ph.D. Course in Power Electronics for Renewable Energy Systems – in
theory and practice”
Speakers: R. Teodorescu, P. Rodriguez, M. Liserre, J. M. Guerrero,
Place: Aalborg University, Denmark
The course is held twice (May and November) every year
Marco Liserre
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