CHALLENGES AND CHALLENGES AND OPPORTUNITIES FOR POWER ELECTRONICS IN THE INTEGRATION OF IN THE INTEGRATION OF DISTRIBUTED ENERGY SYSTEMSSeminar at University of PadovaJuly 2, 2009
Marta MolinasNTNU
Topics in Today’s presentationTopics in Today s presentation
Low Voltage Ride Through (LVRT) of wind energy conversion systems
STATCOM based torque control in a wind energy conversion system STATCOM based torque control in a wind energy conversion system
Reactive power ancillary service provided by distributed power electronics Reactive power ancillary service provided by distributed power electronics loads
High frequency direct AC link for reducing the nacelle weight in wind energy conversion systems for offshore solutions
Wave energy conversion systems with all electric power take off systems: control challenges for STATCOM and Back to Back converters
Low Voltage Ride Through (LVRT) in Wind Energy Conversion Systemsin Wind Energy Conversion Systems
GRIDGRIDPower Electronics GRIDGRIDInterface
Large scale Large scale 2020--50% Grid capacity.50% Grid capacity.
M. Molinas et.al. “Robust Wind Turbine System Against Voltage Sag with Induction Generators Interfaced to the Grid by Power Electronic Converters," IEEJ 2006, vol. 127D, no. 7pp. 865-871
LVRT Profiles in Grid CodesLVRT Profiles in Grid Codes
Transient95% -0,5 sec.after fault 95%
Nordic Grid code
75% voltage drop for 250 ms
Small reduction of of Output power (10%)
Case-study (1): Back-to-BackCase study (1): Back to Back
Gear
Wind turbine
G ElectricG id
Grid side converter
Generator side converter
Gea
GRIDGRIDPower ElectronicsG Grid
DC-linkCage InductionGenerator
GRIDGRIDInterface
Experimental InvestigationExperimental Investigation
RS-232
Wind turbine
DSP 1 DSP 2Host PCVgridVgen
IIgrid
RS-232
CANGrid side converter
Generator side converter
Gear
1mH 0.2mHGM
Utility Bus400 V
Turbine Emulator Vdc
Igen
g
Commercialconverter
IGBT PWM
Inverter
IGBT PWM
Inverter
G ElectricGrid
0.5mH
Short 55 kW M-G
set-up
DC-linkCage InductionGenerator
circuit 100 msSet to give Constant nominal torque
Results (1) Current limit of grid side Results (1)
0
0.5
1
1.5lta
ge (p
u)
gConv. set to 1 pu
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-1.5
-1
-0.5
0
Line
vol
u) DClink is very stiff
0.5
1
1.5
olta
ge a
nd D
C-L
ink
(p
y
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20Li
ne v
o
0 5
1
1.5
pow
er (p
u) P is kept relatively constant
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-1.5
-1
-0.5
0
0.5
Line
-sid
e co
nver
ter
Th I i h li i f 1
0.5
1
1.5
onve
rter c
urre
nts
(pu) The Id rises up to the limit of 1 pu
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-0.5
0
Line
-sid
e co
Time (s)
Results (2) Current limit of grid sideResults (2)0 5
1
1.5e
(pu)
Current limit of grid side Conv. set to 0.8 pu
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-1.5
-1
-0.5
0
0.5
Line
vol
tage
0.5
1
1.5
age
and
DC
-Lin
k (p
u) Excess power from generator-DC link rises above safety limit
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20
0.5
Line
vol
ta
1
1.5
wer
(pu) Converter protection acts
0 0 02 0 04 0 06 0 08 0 1 0 12 0 14 0 16 0 18 0 2-1.5
-1
-0.5
0
0.5
1
Line
-sid
e co
nver
ter p
ow
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2L
0.5
1
1.5
erte
r cur
rent
s (p
u) And trips for overvoltage
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-0.5
0
0.5
Line
-sid
e co
nve
Time (s)
Restults (3) Current limit of grid side Restults (3)0.5
1
1.5ge
(pu)
Conv. set to 0.8 pu
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-1.5
-1
-0.5
0
Line
vol
tag
) Excess power from generator-DC link rises-active DClink control activated on gen side
0.5
1
1.5
tage
and
DC
-Lin
k (p
u) Excess power from generator DC link rises active DClink control activated on gen.side
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20Li
ne v
ol
1
1.5
pow
er (p
u)
Generated power is reduced to keep DClink under control
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-1.5
-1
-0.5
0
0.5
Line
-sid
e co
nver
ter p
0.5
1
1.5
nver
ter c
urre
nts
(pu) The Id rises up to the limit of 0.8 pu
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.2-0.5
0
Line
-sid
e co
n
Time (s)
Case-study (2): STATCOMCase study (2): STATCOM
G
Electric Grid
Electric Grid
G
G
STATCOM
Wind or Wave Farms with Asynchronous generators
M. Molinas et.al. “Low Voltage Ride Through of Wind Farms With Cage Generators: STATCOM Versus SVC," IEEE Trans. PE 2008, vol. 23, no. 3, pp. 1104-1117
Experimental ModelExperimental Model
Set to give Reference torqueReference torque
15 kW
H PCHost PC
Voltage regulation to wind Voltage regulation to wind 0.96
u]
Controlled voltage
0 88
0.9
0.92
0.94d
volta
ge [p
u
Uncontrolled voltage
0 5 10 15 20 250.86
0.88
Grid
1]
Power to Grid
-0.5
0
0.5
rid p
ower
[pu]
C ll d0 5 10 15 20 25
-1
Gr
-0.4
[pu] STATCOM
ControlledQ
Q from Grid (uncontrolled)
-0.55
-0.5
-0.45
com
cur
rent
[ current
0 5 10 15 20 25Sta
tc
Time [s]
LVRT test without STATCOMLVRT test without STATCOM1u]
Voltage collapse
-0.5
0
0.5rid
vol
tage
[p
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1Gr
1pu] Power is
around Zero
-1
0
1
Grid
pow
er [p around Zero
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
G
Time [s]
1 3d [p
u]
1
1.1
1.2
1.3
nera
tor s
peed
Generator speedaccelerates
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.9G
en
Time [s]
LVRT-STATCOM 0.5 puLVRT STATCOM 0.5 pu25% voltage
0.5
1e
[pu]
25% voltage
-0.5
0
Grid
vol
tage Terminal voltage
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1
1 5Power recovers
0.5
1
1.5
wer
[pu]
fastWT Power
-1
-0.5
0
Grid
pow
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1.5
Time [s]
LVRT-STATCOM 1puLVRT STATCOM 1pu25% voltage
0.5
1ge
[pu]
-1
-0.5
0
Grid
vol
tag
Terminal voltage
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
1
1.5 Faster Power
0
0.5
1
ower
[pu]
recovery
-1 5
-1
-0.5
Grid
po
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.81.5
Time [s]
LVRT- STATCOM 1puLVRT STATCOM 1pu1u]
-0.5
0
0.5
1rid
vol
tage
[pu
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1Gr
0
0.5
nt [p
u] Statcom d-current
1 5
-1
-0.5
0
tatc
om c
urre
n
Statcom q-0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1.5St
1.3
eed
[pu]
current
WT generator
0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 80.9
1
1.1
1.2
Gen
erat
or s
pe WT generator speed
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8G
Time [s]
Consequences of STATCOM for LVRT
0.4
0.6
0.8
1
Vm
ains
[pu]
ISTATCOM = 1.8 pu ISTATCOM = 1 pu
ISTATCOM = 0.5 pu
Voltage0 0.5 1 1.5
0
0.2
a)
No control
NO STATCOM - UNSTABLE
g
1.4
1.6
1.8
eed
[pu]
No control
NO STATCOM UNSTABLE
Speed
0 0.5 1 1.5
1
1.2IG s
pe
b)
No-control
ISTATCOM = 0.5 puISTATCOM = 1 puISTATCOM = 1.8 pu
p
1
2
u]
b)
STATCOM
-1
0
I sta
tcom
[pu
current
0 0.5 1 1.5-2
Time [s]
c)M. Molinas et.al. “Extending the Life of Gear Box in Wind Generators by Smoothing Transient Torque with STATCOM," under review process in IEEE Trans. IE, 2009
Influence of STATCOM operation on t tgenerator torque
2.5
1.5
2
NO STATCOM - UNSTABLE
0.5
1
No control
Accelerating torque 0
IG T
orqu
e [p
u]
-1
-0.5
ISTATCOM = 0.5 pu
ISTATCOM = 1 puISTATCOM = 1 8 pu
Higher Iq gives:
• Faster recovery
-2
-1.5
STATCOM pISTATCOM 1.8 puFaster recovery
• More stable system
0 0.5 1 1.5-2.5
Time [s]
• But higher peak torque
STATCOM based Torque ControlDC linkDC link
STATCOM based Torque Control
STATCOM
bv bi
fL
STATCOM
bv bi
fL
PWM Clark Clark
bav , bai ,
v i
PWM Clark Clark
bav , bai ,
v i
ParkPark
,v ,i
di iPark-inv. v
Voltage OrientedVector Current Control
ParkPark
,v ,i
di iPark-inv. v
Voltage OrientedVector Current Control( )V f T n di qidv
L
*V
dcV
*i
*qv*
dv +
-refT
di qidv
L
*V
dcV
*i
*qv*
dv +
-refT
( , )ref ref genV f T n
PI
*i
dcV di
+
PI
ITC
refV
gn
puVref 1
PI
*i
dcV di
+
PI
ITC
refV
gn
puVref 1
qi
dV Normal STATCOMITC
qi
dV Normal STATCOMITC
Torque Control Effect Torque Control Effect
STATCOM In a Wind ParkSTATCOM In a Wind ParkWind
GearBox
turbine 1
PCCThree line to ground faultG
A
ElectricElectric
Cage InductionGenerator 1
STATCOMWind turbine 2
Grid Gridturbine 2
TransformerGearBox
GB
Cage InductionGenerator 2
STATCOM
Results: 0 6
0.8
1
age
[pu] X
O*
Grid side0.2
0.4
0.6
Term
inal
vol
ta
Generator 1 - Normal STATCOMGenerator 1 - ITCGenerator 2 - Normal STATCOMGenerator 2 ITC
XO*
-2 -1 0 1 2 3 4 5 6 7 80
Time [s]
Generator 2 - ITC
1] X
*
0.4
0.6
0.8
age
at P
CC
[pu X
-2 -1 0 1 2 3 4 5 6 7 80
0.2
Time [s]V
olta
Normal STATCOMITCX
5
6
7
W]
Normal STATCOMITCX
2
3
4
5
Grid
Pow
er [M
W
X
-1 0 1 2 3 4 5 6 7 8
0
1
Time [s]
G
Reactive Power Ancillary Service by Distributed Responsive Loads Distributed Responsive Loads
GridP
Power Electronics dominated power systems
P
P P P
PP
Q Q Q
DistributedGeneration
P P PP
Q Q Q Three-phaseline
Loads LoadsLoads
M. Molinas et.al. “Investigation on the role of power electronic controlled constant power loads for voltage support in distributed AC systems," IEEE PESC2008, Rhodes 2008.
Active Rectifier Interfaced Load
to AC distributed system
CPL controller
to AC distributed system
Vpwm
PI
Vector current control
Iq,ref
Id,refPref
PIVref
VCPL controllerTypical examples of CPL load
• motor drives• power supplies• interface with diode/thyristor rectifier
P
y• large rectifiers for DC loads• aluminum plants, paper mills
RInduction motor drive system with active rectifier CPL
Induction motorLoad
System InvestigatedPoint of voltage measurement
y g
Asynchronous Generator
Grid
L L
FixedCapacitor
Lg
Line to ground
PCC L A
RP RP
Line to ground fault
RPRP
Distribution System
LP LP LPLP
L 0,02 pu
Lg 0,2 pu
=R
C
=R
C
=R
C
CPL1 CPL2 CPL3STATCOM
One CPL=25% of generated power
CPLs operate with Negative i l R i incremental Resistance
22
2 L
dv P Vi P R
di i P
Incremental Current Ratingq
g
• Voltage drop at the VSC terminals 0.1 – 0.6• Incremental current rating moderate
ddI
qItI
t dI IP t d
d
d
PI
V
2 2
t d qI I I
*
*ttI I
II
In this region it is beneficial to have it in a CPL than in a STATCOM
tI
Total current when Iq is disabled
0.6 id(i ) for v =1.0
Required Total C
0.5
0.55
id(iq) for vg 1.0
itot(iq) for vg = 1.0
id(iq) for vg=0.8
itot(iq) for vg = 0.8
Current
0.4
0.45
q g
id(iq) for vg=0.8 and reduced reactance
itot(iq) for vg = 0.8 and reduced reactance
Total current rating as function of grid parameters
Minimum required Iq for
0.3
0.35
i [pu
]
q qreducing stresses in the grid
Minimum total current does not apper at PF=1
0 2
0.25
xg 1.8 pu
apper at PF 1
0 1
0.15
0.2g p
CPL 0.25 pu
Reduced xg 0.4 pu0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5
0.1
iq [pu]
g p
gR gXgV
P constg g
Distributed Iq versus STATCOMDistributed Iq versus STATCOM• distributed reactive current support by CPL less than with STATCOM • > 300 ms fault with 2 CPLs more convenient than STATCOM• > 300 ms fault with 2 CPLs more convenient than STATCOM• 3 CPLs with Iq always more convenient
Total power drawn by CPLs is kept constant = 80% of generated power
Critical Clearing Time and Iq
1 1 CCTCCT
g q
0.5
1
p.u.
]
0.5
1 CCT
-0.5
0
VC
PL [p
-0.5
0
4.8 5 5.2 5.4 5.6 5.8 6 6.2
-1
Time [s]4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8
-1
Time [s]Voltage measured at PCC
CCTs for different loading types and regulation of CPLsType of loading Regulation CCT
Voltage measured at PCC
Case 1: 80% CPL P constant and Iq = 0 162 ms
Case 2: 20% CPL,60% induction motor
P constant and Iq 187 ms
C 3 40% CPL P t t d I 238Case 3: 40% CPL,40% induction motor
P constant and Iq 238 ms
Case 4: 80% CPL P constant and Iq=38% 510 ms
A Reactive Power Investigation: h Sthe System
Reactive Power Characteristic2 Distribution line with z=0.257+j0.4 pu, X/R=1.56
Transmission line with z=0.01869+j0.17726 pu, X/R=9.48Subsea cable with z=0.005+j0.041 pu, X/R=8.2
Reactive Power Characteristic0.5
0
1[p
u]
j p ,
0 5
0u]
-2
-1
wer
com
pens
atio
n Q
c
-1
-0.5
e po
wer
com
pens
atio
n [p
u
-4
-3
Rea
ctiv
e po
-2
-1.5Rea
ctiv
e
C diti 0 294 0 247 i
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-6
-5
Voltage at point of load connection [pu]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
-2.5
Voltage at the point of load connection [pu]
centralized compensationdistributed compensation
Conditions:r=0.294, x=0.247 inpu (very long distribution lines fromUMIST source)Q=0.2; P=0.2; Vs=1; n=3 (for distributedcompensation)
Voltage at point of load connection [pu]
Type of line Impedance [pu] X/R ratioVery long distribution line 0.257+j0.4 1.56
Transmission line 0.01869+j0.177726 9.48
M. Molinas, J. Kondoh, “Reactive Power Ancillary Service with Power Electronic Loads: Analytical and Experimental Investigation," EPE 2009, Barcelona.
Sub-sea cable 0.005+j0.041 8.2
Converter control influenceConverter control influence
1000
Vdc[
V]
2000
W, V
]
Measured reactive power compensation QcMeasured active power PAnalytically obtained Qc curve with measured values of impedanceMeasured converter DC link voltage for controlling Qc
1000
0
P[W
], D
C li
nk v
olta
ge V
1000
k vo
ltage
Vdc
[VAR
,
-2000
-1000
on [V
AR],
Activ
e po
wer
-1000
0
Activ
e po
wer
, DC
link
4000
-3000
tive
pow
er c
ompe
nsat
io
-2000
ower
com
pens
atio
n, A
0 20 40 60 80 100 120 140 160 180 200 220-5000
-4000
Voltage at point of compensation [Volts]
Rea
ct
Measured Qc curve obtained in the labMeasured active power PAnalytically obtained Qc curve with measured values of impedanceMeasured DC link voltage during control of Qc
-3000Rea
ctiv
e po
0 20 40 60 80 100 120 140 160 180 200 220Voltage at point of compensation[Volts]
High Frequency Direct AC Link for Wi d E C i Off hWind Energy Conversion Offshore
A. B. Mogstad, M. Molinas, “A Power Conversion System for Offshore Wind Parks," IEEE IECON2008, Florida 2008.
Standard Solution: offshore AC grid with centralized converter
Proposed Series ConnectionProposed Series Connection
Direct AC linkDirect AC link
F ll B id Full Bridge
Bi-direction SwitchesDirect AC-AC converter
Comparing the LossesComparing the Losses
Wave Energy Conversion Systems: Control Challenges for Power ElectronicsControl Challenges for Power Electronics
GWECWEC
Electric Grid
Interface Technology
Electric Grid
GWECWEC
GWECWEC
GWECWEC
C 1 I d ti t + STATCOM Case 1: Induction generator + STATCOM Case 2: Doubly fed induction generator with rotor converterCase 3: Induction generator wiht full converter Case 3: Induction generator wiht full converter
Molinas et.al. , “Power electronics as grid interface for actively controlled wave energy converters," IEEE ICCEP, Capri 2007, pp. 188-195.
ChallengesChallenges
C t ff ti ti t l f i d t ti Cost-effective: active control for increased extraction Active control for Grid Code compliance
Power electronics for both: Power electronics for both: Power electronics for both: Power electronics for both:
Active control of WEC and Power quality
Power Electronic Interfaces: lessons from wind
Case1: Induction generator Case1: Induction generator +STATCOM+STATCOM
IG
AC
Grid
DC
Case 2: Doubly fed Case 2: Doubly fed induction generator with induction generator with rotor converterrotor converter
IG Grid
DCAC
IG
rotor converterrotor converter
C 3 I d ti t C 3 I d ti t
ACDC
Case 3: Induction generator Case 3: Induction generator with full converter in serieswith full converter in series
DC
ACDC
ACIG
Energy Storage
Grid
Energy Storage(Batt/Supercap )
Power extraction traces for irregular waves
Latching control
50
60Passive control
Pinst Pavg
300
350Latching control
Pinst Pavg
Ppeak/Pav =7 Ppeak/Pav =7
30
40
wer
(kW
)
150
200
250
ower
(kW
)
10
20
Pow
50
100
150Po
0 100 200 300 400 500 600 700 800 900 10000
Time (seconds)
0 100 200 300 400 500 600 700 800 900 10000
Time (seconds)
Highly fluctuating power poses difficulties for voltage stability in
Pav-latching/Pav-passive =5
Highly fluctuating power poses difficulties for voltage stability in case of large scale wave power penetration
Induction Generator+STATCOMInduction Generator+STATCOM
IG Grid Platform High pressurel t
IGaccumulator
DC
AC
Buoy
Induction generator with a shunt connected STATCOM as grid interface
Hydraulic PTO with the induction generator-STATCOM as the grid interface technology S CO as g d te aceSTATCOM as the grid interface technology
Power and Voltage QualityPower and Voltage Quality
10000
15000
VA
]
P-No STATCOM Q-No STATCOM
5000
10000
15000
Q [W
,VA
]
P(higher storage) Q(higher storage) P(lower storage) Q(lower storage)
0 100 200 300 400 500 600 700 800
0
5000
10000
P, Q
[W,
0 100 200 300 400 500 600 700 800
0
5000
P, Q
1
pu]
0 100 200 300 400 500 600 700 800
0.95
1
0 100 200 300 400 500 600 700 8000.97
0.98
0.99
PC
C V
olta
ge [p
0.85
0.9
PC
C V
olta
ge [p
u]
0.5
1
com
cur
rent
s [p
u]
Id(lower storage) Iq(higher storage) Id(higher storage) Iq(lower storage)
0 100 200 300 400 500 600 700 800
0.8
Time [s]0 100 200 300 400 500 600 700 800
0Sta
tc
Time [s]
Active and reactive powers, and PCC voltage without reactive Active and reactive powers, PCC voltage and STATCOM currents support by the STATCOM
p , CC g S COfor the Case Study 1 with lower and higher energy buffering capacities
Induction Generator+ series Back-to-Back converteres
DC
ACDC
ACIG Grid
ACDC
Energy Storage(Batt /Supercap )
Generatorsystem
(Batt /Supercap )
Hydrodynamicforces
Power quality easier to handle Complete decoupling betweenComplete decoupling between
WEC and grid
Power Extraction with Active ControlPower Extraction with Active Control• active control of WEC with Power Electronics
1 5 3
• Power quality not a big issue because of grid side converter
Pav-latching/Pav-passive =6
1
1.5
Wave elevation [m]Buoy position [m]Power extraction with passive control [100 kW]Average power [100 kW]
1
2
3
Wave elevation [m]Buoy position [m]Power extraction with latching control [100kW]Average power [100kW]
0
0.5
Pow
er (k
W)
2
-1
0
-1
-0.5
-4
-3
-2
110 111 112 113 114 115 116 117 118 119 120-1.5
Time (seconds)
P t ti ith i t l f th
110 111 112 113 114 115 116 117 118 119 120-5
Time (seconds)
Power extraction with latching control forPower extraction with passive control for the configuration of full converter in series
Power extraction with latching control for the configuration of full converter in series
Concluding RemarksConcluding Remarks
Power Electronic components are going to dominate the future electric power s stemelectric power system
Transient and dynamic interactions of these components with the Transient and dynamic interactions of these components with the power system is not yet well understood
But it appears clear that control structure and strategy will have a dominant role in a system with a large share of power electronics
Future workFuture work
• Influence of modeling approaches for stability investigations of grid dominated by power electronics of grid dominated by power electronics
• Detailed mathematical model versus software implemented pmodels for investigating small signal stability
• Multi domain design approach for energy conversion systems
Current trayectory...
Concluding RemarksConcluding Remarks
High Frequency TransformerHigh Frequency Transformer
Effect of Effect of IIqq on the Nose Curve…on the Nose Curve…ec oec o qq o e ose Cu eo e ose Cu e
1,4 Loading can be
i d h
1
1,2increased at the expense of a flat P-V curve
Control structure and t i h i t t e
0,4
0,6
0,8
Resistive load
1 t ll d CPL
ISIE
tuning have an important influence Vo
ltage
0
0,2
0,
0 0 2 0 4 0 6 0 8 1 1 2 1 4 1 6 1 8 2
1 non-controlled CPL
2 controlled CPLs with Vdc control
1 controlled CPL
1 STATCOM+1 non-controlled CPL
E 2008
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2
xg 1.8 puPower
CPL 0.25 pu
Hybrid Thyristor-Transistor Based HVDC Link for Wind EnergyLink for Wind Energy
Influence of the Droop ControlInfluence of the Droop Controlpp Total reactive current injection with droop control is lower
Simple to implement, no need of communication and good result
1, 2
It will influence the nose curve by allowing for increased loading
0, 6
0, 8
1I p _ q [ p . u . ]
WI T H DROOP
ve c
urre
nt
0, 2
0, 4
WI T HOUT DROOP
CPL
rea
ctiv
0
0 1 2 3 4 5 6
L [ k m ]
Discussions and future workDiscussions and future workDiscussions and future workDiscussions and future work
Results CPLs increases the chances of voltage instability (voltage collapse) Voltage source converters as preferable interface for loads (controllability, flexibility, Voltage source converters as preferable interface for loads (controllability, flexibility,
ability of Iq control) Transient stability improved by Iq and distributed Iq lower than STATCOM Steady state stability influenced by control structure and tuning
ISIE
Required increase of current rating of converters depend on grid parameters Droop control reduces needed amount of Iq and therefore rating of converter
E 2008
Future work Role of the control structure on overall stability Thorough analytical investigation of small signal stability
C i i l h f CPL i h i h i Critical share of CPLs in the system with reactive current support Customized design of converters for CPLs Influence of several CPLs control in the system stability
Transient BehaviorTransient Behavior
300
400
volta
ge V
dc [V
]
Voltage at point of converter coonnectionConverter DC link voltage Vdc
400
500
olta
ge V
dc [V
]
Voltage at point of converter connection VuvConverter DC link voltage Vdc
0
100
200
ter c
onne
ctio
n Vu
v, C
onve
rter D
C li
nk v
0
100
200
300
erte
r con
nect
ion
Vuv,
Con
verte
r DC
link
vo
0 0.1 0.2 0.3 0.4 0.5 0.6-400
-300
-200
-100
Volta
ge a
t poi
nt o
f con
vert
0 0.1 0.2 0.3 0.4 0.5 0.6
-300
-200
-100
Time [s]
Volta
ge a
t poi
nt o
f con
ve
0 0.1 0.2 0.3 0.4 0.5 0.6Time [s]
Doubly Fed Induction GeneratorDoubly Fed Induction Generator
IG GridIG
DCAC
ACDC
• Non suitable for direct drive (speed • Non suitable for direct drive (speed variation…)• Together with hydraulics PTO• Limited LVRT (similar to case 1)
Results 0 5
1
[pu]
Unit 1 - Normal STATCOMUnit 1 - ITCXXResults
-0.5
0
0.5
TCO
M c
urre
nt
Unit 2 - Normal STATCOMUnit 2 - ITC
X O
O
**
2
-2 -1 0 1 2 3 4 5 6 7 8-1.5
-1
Time [s]
STA
T O
2
0
2
Pow
er [M
VA
r]
O
*
6
-4
-2
PC
C R
eact
ive
P
Turbine 1 - Normal STATCOMTurbine 1 - ITCTurbine 2 - Normal STATCOMTurbine 2 - ITC
XXO*
*
-1 0 1 2 3 4 5 6 7 8-6
Time [s]
P
0
MV
Ar]
10
-5
activ
e P
ower
[
X
-1 0 1 2 3 4 5 6 7 8
-10
Time [s]
Grid
Rea Normal STATCOM
ITCX