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