PART I POWER ELECTRONICS FOR THE INTEGRATION OF RENEWABLE ENERGY SOURCES › eno › Papers2009 ›...
Transcript of PART I POWER ELECTRONICS FOR THE INTEGRATION OF RENEWABLE ENERGY SOURCES › eno › Papers2009 ›...
PART IPOWER ELECTRONICS FOR THE INTEGRATION OFRENEWABLE ENERGY SOURCES
Presentation at University of Alcala de Henares, SpainSeptember 14, 2009
Marta MolinasNTNU
Topics in Today’s presentation (Part 1)
Low Voltage Ride Through (LVRT) in wind energy conversion systems Full converter solution
STATCOM solution, Capacitor bank solution
STATCOM-SVC performance comparison
Torque control with STATCOM: Gearbox stress alleviation
Reactive power ancillary service provided by distributed power electronics loads
Application of power electronics in Distributed Energy Systems
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 Systems
Large scale 20-50% Grid capacity.
GRIDPower ElectronicsInterface
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
Specific problem statement
Ride Through capability
Why ride through? A short circuit initialise a voltage drop down stream
Most wind turbines are programmed to disconnect themselves from the grid if voltage drops by 30% for 50msThe transmission system operators in many countries require ride-through capability for the wind farms to be integrated into the power network.
Fault near load center
Const. Torque motor load descel.
Fault clears and motor draws high current attempt to accel.
Weak PS: long voltage recovery; possible voltage collapse.
Loss of generation will increase the risk of voltage collapse and will not support frequency
IPEC 2005
LVRT Profiles in Grid Codes
75% voltage drop for 250 ms
Transient95% -0,5 sec.after fault 95%
Small reduction of Output power (10%)
Nordic Grid code
Generation system choices
Induction generator: squirrel cage rotor + Full converter
Proven technology: robust cheap solution
Full speed range
No brushes on the generator
Complete control of active and reactive power
Full scale power converter
Need for a gear
SCIG
IPEC 2005
High marketshare PM coming
technologyMature tech. Large scale problem in weak grid
Comparisson between cost of geared technology and direct drive
IPEC 2005
Source: Böhmeke et al. [Böh 1997]
Full converter
Standard power devices Decoupling effect between grid and generator
(compensating for unbalance and power quality issues) Complete control of active and reactive power Need for energy storage in the DC link Power losses (switching and conduction losses)
DC link
SCIGis
ulil
idc
us
Gear
IPEC 2005
Case-study (1): Full Converter Solution
G Electric Grid
Grid side converter
Generator side converter
DC-linkCage InductionGenerator
Gear
Wind turbine
GRIDPower Electronics
Interface
Wind Generation System proposed
G Electric Grid
Grid side converter
Generator side converter
DC-linkCage InductionGenerator
Gear
Wind turbine
Wind Park
G
Generator side converter
Cage InductionGenerator
Gear
Wind turbine
G
Generator side converter
Cage InductionGenerator
Gear
Wind turbine
DC-link
DC-link
Electric Grid
Grid side converter
Electric Grid
Grid side converter
Electric Grid
Grid side converter
Electric Grid
Grid side converter
DC-Grid
IPEC 2005
Experimental Investigation
1mH
0.5mH
0.2mHGM
DSP 1 DSP 2
Utility Bus400 V
Host PC
Turbine Emulator
RS-232
Vgrid
Vdc
Vgen
Igen
Igrid
RS-232
CAN
Commercialconverter
Short circuit
55 kW M-Gset-up
IGBT PWM
Inverter
IGBT PWM
Inverter
100 msSet to give Constant nominal torque
Grid side converter
Generator side converter
DC-link
G
Cage InductionGenerator
Gear
Wind turbine
ElectricGrid
Control of generator side converter
Sensorless vector control
IG
Clark
Park
Flux & Speed
Observer
Flux Regulator
Inverse Park
Inverse Clark
D,Q Current
Regulator
DC-Link control
PWM
IGBT converter
Speed Regulator
,,abci
refφ
,rrefω
φ̂
φ̂∠
,iαβ ,vαβ
,,abcv
,dqv
,drefi
,qrefi
diqi
,dcLIMVdcV
dcV
ˆrω
refT
( )s s s sv R i dtψ = − ⋅∫
r s sL iσψ ψ= −
23
me d q
r
LT P iL
ψ=
From DSP1 through CANbus
IPEC 2005
Decoupled control of Te and fluxSampling 200 µsec.
Control of grid side converter
1tanv
vv
β
α
θ − =
32dc dc d dV i v i=Clark Clark
ParkPark
βα ,v βα ,i
di qidv
PI PI
Lω
Lω
PIrefqi ,
refdcV ,
dcV
refdi ,
refqv
refdv
PWM
Inv. Clark
Inv. Park
refv βα ,
refabcconv
v.
+
+
+
-
-
+
-
++
-
PLL
IGBTconverter
bav , bai ,
DC link
fL
0qi =
To DSP2through CANbus
IPEC 2005
Decoupled control of VDC and Q 200 µsec.
Motor-Generator set and short circuit device
Trigger signal for a short circuit duration of 100 ms55 kW, 380 V, 6 poles, 50 Hz
Cage induction machines
Thyristor regulated
IPEC 2005
Set-up of convertersused in Experiments
VDC Setting
Digital Control
DC link voltage regulated to 650 V
Current rating of IGBTs reg. to withstand 110 A
5 kHz switching freq.
CAN bus between both DSPs.
IPEC 2005
Results (1)
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
1.5
Line
vol
tage
(pu
)
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20
0.5
1
1.5
Line
vol
tage
and
DC
-Lin
k (p
u)
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
1.5
Line
-sid
e co
nver
ter
pow
er (p
u)
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
1
1.5
Line
-sid
e co
nver
ter
curr
ents
(pu)
Time (s)
The Id rises up to the limit of 1 pu
Current limit of grid side Conv. set to 1 pu
DClink is very stiff
P is kept relatively constant
Results (2)
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
1.5
Line
vol
tage
(pu
)
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20
0.5
1
1.5
Line
vol
tage
and
DC
-Lin
k (p
u)
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
1.5
Line
-sid
e co
nver
ter
pow
er (p
u)
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
1
1.5
Line
-sid
e co
nver
ter
curr
ents
(pu)
Time (s)
Current limit of grid side Conv. set to 0.8 pu
Excess power from generator-DC link rises above safety limit
Converter protection acts
And trips for overvoltage
Restults (3)
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
1.5
Line
vol
tage
(pu
)
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16 0.18 0.20
0.5
1
1.5
Line
vol
tage
and
DC
-Lin
k (p
u)
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
1.5
Line
-sid
e co
nver
ter
pow
er (p
u)
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
1
1.5
Line
-sid
e co
nver
ter
curr
ents
(pu)
Time (s)
Current limit of grid side Conv. set to 0.8 pu
Excess power from generator-DC link rises-active DClink control activated on gen.side
Generated power is reduced to keep DClink under control
The Id rises up to the limit of 0.8 pu
Analysis of Experimental results
1.Control performance depends on control algorithm
2.Rating of semiconductor devices is relevant for success of ride through
3.The ride through capability depends on the dynamics of the DC link
Conclusion & Discussion
For this specific case with specific disturbance, the system proposed performed well (50% drop, 100ms)
The response of the system is almost entirely dictated by the embedded control algorithm
Over current rating of power electronics switches is relevant to ride through
Ride-through capability strongly depends on DC link dynamics
For voltage sags deeper than 50% IGBT current limit Control on gen. side can be aided by a
fast pitch control
When the voltage sag approaches zero
High overcurrent limit Braking choppers in DC link And/or large DC link capacitors Maximize generator losses during fault (
Q)
Future wind farms will be designed as power plants withCapabilities to respond to power system needs
Considering that WG will trip when 30% voltage sag is detected, the results shown are very promising
IPEC 2005
Case-study (2): STATCOM Solution
Electric Grid
Electric Grid
G
G
G
STATCOMTorque
Time
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
Effect of nr. of turbines on voltage drop
Between the speed range 1.0-1.02 p.u. , the voltage variation at PCC:• 10 turbines: 1%• 200 turbines: 15%
When the voltage drops, motor and other inductive loads draw too much current overheating the equipment and possibly damaging gearboxes.
Voltage and Torque profiles
Effect of nr. of turbines on torque capability
Runaway condition:Te-peak drops below the Taero-peak if number of turbines is above 120. This is the operating point when instability occurs (runaway condition)
Without any control• Runaway condition• Voltage collapse• Overheated windings• Large currents slip
n=120,Te-peak at 3% slip, voltage drop is 14% (Vdrop can be indicator of runaway conditon), then a 10% drop can be a safe limit and with control the drop can be kept withing that range
Series and parallel capacitors bank
FeaturesParallel
• Improves the power factor of each turbine (loss reduc., voltage reg.)
Series• Compensation of line impedance (effective reactance reduction)• Improves power transfer capabilities of transmission lines but there is increase of stator current• level of compensation increases with line current
Parallel-Series• parallel compensates the individual IGreactive power• series compensates the line impedance(XS-XC) stiffer grid
Problems
•as power factor and output power also fluctuates, ideal compensation will require variable reactive comp (SVC).• switching a big block of capacitance in and out can swing the voltage up or down and this variation is felt as an abrupt change in torque on the turbine gearboxes
A variable reactive compensation-type is needed (SVC)
Switched shunt capacitor
Compensation Schemes
1. Mechanically switched shunt capacitors
2. Static var Compensators (TCR, TCSC)
3. STATCOM
4. Power electronics converters (Inverter/Rectifier-DC link-inverter)
Steady State events
Transients events
The power electronics interface depends on the source characteristics
SVC Solution
SVC systems are solutions that are made up of thyristor (power electronics) switched capacitors and reactors. SVC outputs are continuous (infinitely variable) and do not cause sudden voltage changes on the system and are highly effective in regulating voltage. SVCs must operate at all times within its rated output, if it is desired that the device is to react strongly to a voltage event, it must be rated to do so. SVCs typically operate poorly at lower than nominal voltages.
Typical SVC Scheme
• Max. comp. current is proportional to system voltage• losses with capacitive output steeper than with inductive • harmonics filtering required• to regulate the voltage in a narrow range of nominal value (V1-V2) 29
V=Vo+xIs
Ltn ωα = Ctn
ωα 1
=
STATCOM Solution
STATCOM devices are pure power electronic devices made from IGBT, IGCT or GTO based converters to directly generate reactive currents. Compared to SVCs, STATCOMs are faster, smaller, and have better performances at reduced voltages. STATCOMS have the capability to address transient events.
STATCOM
• Max. comp. current is independent of system voltage• Losses increase smoothly with cap. and inductive• No harmonics filtering required • Faster response
Compensation with Statcom
GElectric Grid
InductionGenerator
Gear
Wind turbine
Q
STATCOM
Ground fault
System configuration
G
Gear
Wind turbine
Electric Grid
STATCOM
Electric Grid
C
0.2 pu
1 pu
Simulation of Stationary &Dynamic Operation
15 20 25 30 35 40 450.8
0.9
1
1.1
Simulation time [s]
Vol
tage
[pu]
v__grid v__pcc v__pcc with STATCOM
15 20 25 30 35 40 45-20
0
20
40
Simulation time [s]STA
TCO
M R
eact
ive
pow
er [M
VA
r]
25 30 35 40 45 500.98
1
1.02
1.04
Simulation time [s]
Vol
tage
[pu]
v__grid v__pcc v__pcc with STATCOM
25 30 35 40 45 50-2
0
2
4
6
8
10
Simulation time [s]
Q S
TATC
OM
[MV
Ar]
Simulation of Flicker
30 30.5 31 31.5 320.99
0.992
0.994
0.996
0.998
1
1.002
Simulation time [s]
Vol
tage
[pu]
v__grid v__pcc v__pcc with STATCOM
• turbulence• blade tower passage• uneven wind shear on
blades
Torque pulsations that cause voltage fluctuations
5Hz, 0.15 pu torque ripple imposed
Requirements of higher penetration
Voltage stability
Fault ride through capability
Low-Voltage Ride-Through
0%
15%
90%
100%110%
0150 1000 3000Time in ms
Time Fault Occurred
Normal Operating Voltage Band
Grid VoltageU/Un
Tripping allowed below red line
Transient mode: The LVRT Challenge
Why ride through?
• Loss of generation can provoke VOLTAGE COLLAPSE• Loss of generation will provoke frequency excursions•A short circuit can originate VOLTAGE COLLAPSE
9 10 11 12 13 140
0.5
1
Simulation time [s]
Vol
tage
[pu]
v__grid v__pcc v__pcc with STATCOM
9 10 11 12 13 140
20
40
Simulation time [s]
Q S
TATC
OM
[MV
Ar]
9 10 11 12 13 14
-1
-0.5
0
Simulation time [s]
iq S
TATC
OM
[pu]
Simulation of LVRT
Experimental Model
Set to give Reference torque
GRID
DC motor
Induction Generator
0.23pu
0.07 pu
AC
DC
DC
AC
Wind Turbine + Wind Generator
IndependentAC GRID
Weak Grid
STATCOM
Short Circuit Device
PCC
0.14 pu
Host PC
15 kW
Voltage regulation to wind
0 5 10 15 20 250.86
0.88
0.9
0.92
0.94
0.96
Grid
vol
tage
[pu]
0 5 10 15 20 25-1
-0.5
0
0.5
1
Grid
pow
er [p
u]
0 5 10 15 20 25
-0.55
-0.5
-0.45
-0.4
Sta
tcom
cur
rent
[pu]
Time [s]
STATCOM current
Power to Grid
ControlledQ
Q from Grid (uncontrolled)
Controlled voltage
Uncontrolled voltage
LVRT test without STATCOM
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1
-0.5
0
0.5
1
Grid
vol
tage
[pu]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1
0
1
Grid
pow
er [p
u]
Time [s]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.9
1
1.1
1.2
1.3
Gen
erat
or s
peed
[pu]
Time [s]
Voltage collapse
Generator speedaccelerates
Power is around Zero
LVRT-STATCOM 0.5 pu
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1
-0.5
0
0.5
1
Grid
vol
tage
[pu]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1.5
-1
-0.5
0
0.5
1
1.5
Grid
pow
er [p
u]
Time [s]
25% voltage
Terminal voltage
Power recovers fast
WT Power
LVRT-STATCOM 1pu
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1
-0.5
0
0.5
1
Grid
vol
tage
[pu]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1.5
-1
-0.5
0
0.5
1
1.5
Grid
pow
er [p
u]
Time [s]
25% voltage
Terminal voltage
Faster Power recovery
LVRT- STATCOM 1pu
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
-1
-0.5
0
0.5
1
Grid
vol
tage
[pu]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1.5
-1
-0.5
0
0.5
Sta
tcom
cur
rent
[pu]
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80.9
1
1.1
1.2
1.3
Gen
erat
or s
peed
[pu]
Time [s]
Statcom d-current
Statcom q-current
WT generator speed
Summary
Types of compensation depend on the source characteristics: DFIG, conventional IG, conventional SG, PMSG
Wind farms are required to meet many of the grid interconnected standards of conventional power generation
The fixed-speed wind turbine generator is simple and low maintenance. In a weak grid, needs reactive power compensation with the proper timing and control strategy.
If a disturbance pushes the machine beyond its pull out torque, the machine will become unstable: the generator will speed up, voltage will collapse and protection systems will separate the unit from the system
Consequences of STATCOM for LVRT
0 0.5 1 1.50
0.2
0.4
0.6
0.8
1
Vm
ains
[pu
]
0 0.5 1 1.5
1
1.2
1.4
1.6
1.8
IG s
peed
[pu
]
0 0.5 1 1.5-2
-1
0
1
2
I st
atco
m [
pu]
Time [s]
a)
b)
c)
No control
ISTATCOM = 1.8 pu ISTATCOM = 1 pu
ISTATCOM = 0.5 pu
No-control
ISTATCOM = 0.5 puISTATCOM = 1 puISTATCOM = 1.8 pu
NO STATCOM - UNSTABLE
Voltage
Speed
STATCOM current
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 generator torque
Accelerating torque
0 0.5 1 1.5-2.5
-2
-1.5
-1
-0.5
0
0.5
1
1.5
2
2.5
IG T
orqu
e [p
u]
Time [s]
No control
ISTATCOM = 0.5 pu
ISTATCOM = 1 puISTATCOM = 1.8 pu
NO STATCOM - UNSTABLE
Higher Iq gives:
• Faster recovery
• More stable system
• But higher peak torque
Why torque transient alleviation...Vital for the drive train life time....
Windturbines gearboxes > 20 years Grid disturbances > transient loads
fatigue and failure of drive train Fixed speed wind turbines no torque smoothing capability
Indirect control of torque by STATCOM
System under investigation
G
Cage InductionGenerator
Gear
Wind turbine
Electric Grid
Transformer
STATCOM
Electric Grid
PCC
Three line to ground fault
STATCOM
PWM Clark Clark
bav ,
ParkPark
bai ,
DC linkfL
βα ,v βα ,i
di qiPark-inv.dv
PI
Lω
*qi
*dcV
dcV
*di
*qv*
dv +
+
-
PI
ITC
refV
dV
refT
gn
puVref 1=
Voltage OrientedVector Current Control
Normal STATCOMITC
STATCOM
PWM Clark Clark
bav ,
ParkPark
bai ,
DC linkfL
βα ,v βα ,i
di qiPark-inv.dv
PI
Lω
*qi
*dcV
dcV
*di
*qv*
dv +
+
-
PI
ITC
refV
dV
refT
gn
puVref 1=
Voltage OrientedVector Current Control
Normal STATCOMITC
Torque Control with STATCOM
( , )ref ref genV f T n=
Analytical approach...
0gv ∠
2jx
mjx
1jx1r1i 2i
2rs
mi
gjxgrli
STATCOMi
1v
22,, 2,
iem i i
i
rs
τ = i ( )2,2, ,
1, 2,,
ii m i
ii i
m i
rj x x
sjx
+ +=i i
( )( ),1, 1, 1, , , 1, , ,i refSTATCOMi i i eq r i i eq r ir r j x x= = + + +v v i
Torque control illustration...-1 0 1 2 3 4 5 6 7 8
0
0.2
0.4
0.6
0.8
1
1.2
Time [s]
Vol
tage
[pu]
Normal STATCOM controlSTATCOM used for ITC
Reactive current response...
-1 0 1 2 3 4 5 6 7 8-1.5
-1
-0.5
0
0.5
1
Time [s]
Cur
rent
[pu]
Normal STATCOM controlSTATCOM used for ITC
Torque trayectory...1 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
Speed [pu]
Torq
ue [p
u]
Torque trajectory with normal STATCOM controlTorque trajectory with ITC
Current trayectory...1 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Speed [pu]
Cur
rent
[pu]
Current trajectory with normal STATCOM controlCurrent trajectory with ITC
Inductive region
Capacitive region
Current trayectory...
1 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
Speed [pu]
Cur
rent
[pu]
Current trajectory with normal STATCOM controlCurrent trajectory with ITC
Inductive region
Capacitive region
Torque Control Effect
0 1 2 3 4 5 6 7 8 9 10-0.5
0
0.5
1
1.5
Gen
erta
tor t
orqu
e [p
u]
Normal STATCOMITC
0 1 2 3 4 5 6 7 8 9 10-0.5
0
0.5
1
1.5
Time [s]
Sha
ft to
rque
[pu]
Normal STATCOMITC
STATCOM In a Wind Park
Electric Grid
Electric Grid
PCCThree line to ground faultG
Cage InductionGenerator 1
GearBox
Wind turbine 1
STATCOMWind turbine 2
Transformer
A
G
Cage InductionGenerator 2
GearBox
STATCOM
B
Results: Grid side
-2 -1 0 1 2 3 4 5 6 7 80
0.2
0.4
0.6
0.8
1
Time [s]
Term
inal
vol
tage
[pu]
Generator 1 - Normal STATCOMGenerator 1 - ITCGenerator 2 - Normal STATCOMGenerator 2 - ITC
-2 -1 0 1 2 3 4 5 6 7 80
0.2
0.4
0.6
0.8
1
Time [s]
Vol
tage
at P
CC
[pu]
Normal STATCOMITC
X
X
X
XO
O
*
*
-1 0 1 2 3 4 5 6 7 8
0
1
2
3
4
5
6
7
Time [s]
Grid
Pow
er [M
W]
Normal STATCOMITCX
X
Results
-1 0 1 2 3 4 5 6 7 8-6
-4
-2
0
2
Time [s]
PC
C R
eact
ive
Pow
er [M
VA
r]Turbine 1 - Normal STATCOMTurbine 1 - ITCTurbine 2 - Normal STATCOMTurbine 2 - ITC
-1 0 1 2 3 4 5 6 7 8
-10
-5
0
Time [s]
Grid
Rea
ctiv
e P
ower
[MV
Ar]
Normal STATCOMITCX
X
XXO
O
*
*
-2 -1 0 1 2 3 4 5 6 7 8-1.5
-1
-0.5
0
0.5
1
Time [s]
STA
TCO
M c
urre
nt [p
u] Unit 1 - Normal STATCOMUnit 1 - ITCUnit 2 - Normal STATCOMUnit 2 - ITC
XX O
O
**
1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 20
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Speed [pu]
Torq
ue [p
u]
Constant terminal voltageNo compensation0.5 pu STATCOM1.0 pu STATCOM1.8 pu STATCOM1 pu SVC2 pu SVC
Rating of STATCOM vs. SVC
Analytical investigation
0gv ∠
2jx
mjx
1jx1r1i 2i
2rs
mi
gjxgrli
STATCOMi
1v
C
Rating of STATCOM-SVC
c) STATCOM and SVC current rating as function of speed at clearing for various grid reactances
d) STATCOM and SVC current rating as function of grid reactances for various critical speeds
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 20
1
2
3
4
5
6
7
Speed at clearing [pu]
STA
TCO
M o
r SV
C ra
ting
on th
e st
abili
ty li
mit
[pu]
STATCOM - xg = 0.1 [pu]SVC - xg = 0.1 [pu]STATCOM - xg = 0.15 [pu]SVC - xg = 0.15 [pu]STATCOM - xg = 0.2 [pu]SVC - xg = 0.2 [pu]STATCOM - xg = 0.3 [pu]SVC - xg = 0.3 [pu]
0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50
1
2
3
4
5
6
Grid reactance [pu]S
TATC
OM
or S
VC
ratin
g on
the
stab
ility
lim
it [p
u]
STATCOM - n = 1.25 [pu]SVC - n = 1.25 [pu]STATCOM - n = 1.5 [pu]SVC - n = 1.5 [pu]STATCOM - n = 1.75 [pu]SVC - n = 1.75 [pu]
Rating of STATCOM-SVC
a) STATCOM and SVC current rating as function of speed at clearing for various stator reactances
b) STATCOM and SVC current rating as function of speed at clearing for various rotor reactances
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 20
1
2
3
4
5
6
7
Speed at clearing [pu]
STA
TCO
M o
r SV
C ra
ting
on th
e st
abili
ty li
mit
[pu]
STATCOM - x1 = 0.05 [pu]SVC - x1 = 0.05 [pu]STATCOM - x1 = 0.1 [pu]SVC - x1 = 0.1 [pu]STATCOM - x1 = 0.15 [pu]SVC - x1 = 0.15 [pu]STATCOM - x1 = 0.2 [pu]SVC - x1 = 0.2 [pu]
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 20
1
2
3
4
5
6
7
Speed at clearing [pu]
STA
TCO
M o
r SV
C ra
ting
on th
e st
abili
ty li
mit
[pu]
STATCOM - x2 = 0.05 [pu]SVC - x2 = 0.05 [pu]STATCOM - x2 = 0.1 [pu]SVC - x2 = 0.1 [pu]STATCOM - x2 = 0.15 [pu]SVC - x2 = 0.15 [pu]STATCOM - x2 = 0.2 [pu]SVC - x2 = 0.2 [pu]
Reactive Power Ancillary Service by Distributed Responsive Loads
Power Electronics dominated power systems
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.
R
Vpwm
PI
Vector current control
Iq,ref
Id,refPref
P
PIVref
V
Induction motorLoad
Induction motor drive system with active rectifier CPL
CPL controller
to AC distributed system
Typical examples of CPL load• motor drives• power supplies• interface with diode/thyristor rectifier• large rectifiers for DC loads• aluminum plants, paper mills
Active Rectifier Interfaced Load
=
Grid
Asynchronous Generator
L
RP
L
LP
R
C
FixedCapacitor
=
RP
LP
R
C
Lg
Line to ground fault
PCC
=R
C
RP
LP
CPL1 CPL2 CPL3
L
STATCOM
RP
LP
Distribution System
A
Point of voltage measurement
L 0,02 pu
Lg 0,2 pu
One CPL=25% of generated power
System Investigated
22
2 L
dv P Vi P R
di i P−= − = − = − = −
CPLs operate with Negative incremental Resistance
• Voltage drop at the VSC terminals 0.1 – 0.6• Incremental current rating moderated
q
dI
qItI
t dI I−
d
d
PI
V=
( )2 2
t d qI I I= +
*
*t
t
tI II
I∆ =
−
Total current when Iq is disabled
Incremental Current Rating
In this region it is beneficial to have it in a CPL than in a STATCOM
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.50.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
iq [pu]
i [pu
]
id(iq) for vg=1.0
itot(iq) for vg = 1.0
id(iq) for vg=0.8
itot(iq) for vg = 0.8
id(iq) for vg=0.8 and reduced reactance
itot(iq) for vg = 0.8 and reduced reactance
xg 1.8 puCPL 0.25 pu
Reduced xg 0.4 pu
=
P const=gR gXgV
Total current rating as function of grid parameters
Minimum required Iq for reducing stresses in the grid
Minimum total current does not apper at PF=1
Required Total Current
Total power drawn by CPLs is kept constant = 80% of generated power
Distributed Iq versus STATCOM• distributed reactive current support by CPL less than with STATCOM • > 300 ms fault with 2 CPLs more convenient than STATCOM• 3 CPLs with Iq always more convenient
4.8 5 5.2 5.4 5.6 5.8 6 6.2
-1
-0.5
0
0.5
1
Time [s]
VC
PL [p
.u.]
4.8 4.9 5 5.1 5.2 5.3 5.4 5.5 5.6 5.7 5.8
-1
-0.5
0
0.5
1
Time [s]
CCTs for different loading types and regulation of CPLsType of loading Regulation CCT
Case 1: 80% CPL P constant and Iq = 0 162 ms
Case 2: 20% CPL,60% induction motor
P constant and Iq 187 ms
Case 3: 40% CPL,40% induction motor
P constant and Iq 238 ms
Case 4: 80% CPL P constant and Iq=38% 510 ms
Voltage measured at PCC
CCTCCT
Critical Clearing Time and Iq
A Reactive Power Investigation: the System
Kondoh’s lab 220V bus
DC
AC
NTNU Inverter 1 as CPL
DC
AC
NTNU Inverter 2 as Grid
r x P+jQ
I
Pload/Vload
jQload/Vload
Or M-G set from SIEI Spa
Vs VloadVg
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-6
-5
-4
-3
-2
-1
0
1
2
Voltage at point of load connection [pu]
Rea
ctiv
e po
wer
com
pens
atio
n Q
c [p
u]
Distribution line with z=0.257+j0.4 pu, X/R=1.56Transmission 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 Characteristic
M. Molinas, J. Kondoh, “Reactive Power Ancillary Service with Power Electronic Loads: Analytical and Experimental Investigation," EPE 2009, Barcelona.
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
Sub-sea cable 0.005+j0.041 8.2
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2.5
-2
-1.5
-1
-0.5
0
0.5
Voltage at the point of load connection [pu]
Rea
ctiv
e po
wer
com
pens
atio
n [p
u]
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)
Converter control influence
0 20 40 60 80 100 120 140 160 180 200 220-5000
-4000
-3000
-2000
-1000
0
1000
Voltage at point of compensation [Volts]
Rea
ctiv
e po
wer
com
pens
atio
n [V
AR],
Activ
e po
wer
P[W
], D
C li
nk v
olta
ge V
dc[V
]
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
0 20 40 60 80 100 120 140 160 180 200 220
-3000
-2000
-1000
0
1000
2000
Voltage at point of compensation[Volts]
Rea
ctiv
e po
wer
com
pens
atio
n, A
ctiv
e po
wer
, DC
link
vol
tage
Vdc
[VAR
, 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
Wave Energy Conversion Systems: Control Challenges for Power Electronics
Electric Grid
Interface Technology
Electric Grid
GWECWEC
GWECWEC
GWECWEC
Case 1: Induction generator + STATCOM Case 2: Doubly fed induction generator with rotor converterCase 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.
Challenges
Cost-effective: active control for increased extraction Active control for Grid Code compliance
Power electronics for both:
Active control of WEC and Power quality
Power Electronic Interfaces: lessons from wind
Case1: Induction generator +STATCOM
Case 2: Doubly fed induction generator with rotor converter
Case 3: Induction generator with full converter in series
IG
DC
AC
Grid
DC
ACDC
ACIG
Energy Storage(Batt/Supercap)
Grid
IG Grid
DC
ACDC
AC
IG
Power extraction traces for irregular waves
0 100 200 300 400 500 600 700 800 900 10000
10
20
30
40
50
60
Time (seconds)
Pow
er (k
W)
Passive control
Pinst Pavg
0 100 200 300 400 500 600 700 800 900 10000
50
100
150
200
250
300
350
Time (seconds)P
ower
(kW
)
Latching control
Pinst Pavg
Highly fluctuating power poses difficulties for voltage stability in case of large scale wave power penetration
Pav-latching/Pav-passive =5
Ppeak/Pav =7 Ppeak/Pav =7
Induction Generator+STATCOM
IG
DC
AC
Grid
Induction generator with a shunt connected STATCOM as grid interface
Platform
Buoy
High pressureaccumulator
IG
Hydraulic PTO with the induction generator-STATCOM as the grid interface technology
Power and Voltage Quality
0 100 200 300 400 500 600 700 800
0
5000
10000
15000
P, Q
[W,V
A]
0 100 200 300 400 500 600 700 800
0.8
0.85
0.9
0.95
1
PCC
Volta
ge [p
u]
Time [s]
P-No STATCOM Q-No STATCOM
0 100 200 300 400 500 600 700 800
0
5000
10000
15000
P, Q
[W,V
A]
0 100 200 300 400 500 600 700 8000.97
0.98
0.99
1
PCC
Volta
ge [p
u]
0 100 200 300 400 500 600 700 800
0
0.5
1St
atco
m c
urre
nts
[pu]
Time [s]
P(higher storage) Q(higher storage) P(lower storage) Q(lower storage)
Id(lower storage) Iq(higher storage) Id(higher storage) Iq(lower storage)
Active and reactive powers, and PCC voltage without reactive support by the STATCOM
Active and reactive powers, PCC voltage and STATCOM currents for the Case Study 1 with lower and higher energy buffering capacities
Induction Generator+ series Back-to-Back converteres
DC
ACDC
ACIG
Energy Storage(Batt/Supercap)
Grid
Generator system
Hydrodynamic forces
Power quality easier to handle Complete decoupling between
WEC and grid
Power Extraction with Active Control
110 111 112 113 114 115 116 117 118 119 120-1.5
-1
-0.5
0
0.5
1
1.5
Time (seconds)
Pow
er (k
W)
Wave elevation [m]Buoy position [m]Power extraction with passive control [100 kW]Average power [100 kW]
Power extraction with passive control for the configuration of full converter in series
110 111 112 113 114 115 116 117 118 119 120-5
-4
-3
-2
-1
0
1
2
3
Time (seconds)
Wave elevation [m]Buoy position [m]Power extraction with latching control [100kW]Average power [100kW]
Power extraction with latching control for the configuration of full converter in series
• active control of WEC with Power Electronics • Power quality not a big issue because of grid side converter
Pav-latching/Pav-passive =6
Concluding Remarks
Power Electronic components are going to dominate the future electric power system
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 work
• Influence of modeling approaches for stability investigations of grid dominated by power electronics
• Detailed mathematical model versus software implemented models for investigating small signal stability
• Multi domain design approach for energy conversion systems
High Frequency Transformer
0
0,2
0,4
0,6
0,8
1
1,2
1,4
0 0,2 0,4 0,6 0,8 1 1,2 1,4 1,6 1,8 2
Resistive load
1 non-controlled CPL
2 controlled CPLs with Vdc control
1 controlled CPL
1 STATCOM+1 non-controlled CPL
Effect of Iq on the Nose Curve…
ISIE 2008
xg 1.8 puCPL 0.25 pu
Loading can be increased at the expense of a flat P-V curve
Control structure and tuning have an important influence
Power
Volta
ge
Hybrid Thyristor-Transistor Based HVDC Link for Wind Energy
0
0,2
0,4
0,6
0,8
1
1,2
0 1 2 3 4 5 6
L [km]
I p_q [p.u.]
WITH DROOP
WITHOUT DROOP
Influence of the Droop Control Total reactive current injection with droop control is lower
Simple to implement, no need of communication and good result
It will influence the nose curve by allowing for increased loading
CPL
rea
ctiv
e cu
rren
t
Discussions and future work
ISIE 2008
Results CPLs increases the chances of voltage instability (voltage collapse) 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 Required increase of current rating of converters depend on grid parameters Droop control reduces needed amount of Iq and therefore rating of converter
Future work Role of the control structure on overall stability Thorough analytical investigation of small signal stability 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 Behavior
0 0.1 0.2 0.3 0.4 0.5 0.6-400
-300
-200
-100
0
100
200
300
400
Time [s]
Volta
ge a
t poi
nt o
f con
verte
r con
nect
ion
Vuv,
Con
verte
r DC
link
vol
tage
Vdc
[V]
Voltage at point of converter coonnectionConverter DC link voltage Vdc
0 0.1 0.2 0.3 0.4 0.5 0.6-300
-200
-100
0
100
200
300
400
500
Time [s]
Volta
ge a
t poi
nt o
f con
verte
r con
nect
ion
Vuv,
Con
verte
r DC
link
vol
tage
Vdc
[V]
Voltage at point of converter connection VuvConverter DC link voltage Vdc
Doubly Fed Induction Generator
IG Grid
DC
ACDC
AC
IG
• Non suitable for direct drive (speed variation…)• Together with hydraulics PTO• Limited LVRT (similar to case 1)
Concluding Remarks