grid integration of large offshore wind farms using statcom-controlled hvdc power transmission :

1

Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC

Power Transmission:

Control and Engineering Issues

S. Bozhko , G. M. Asher, J. C. Clare, L. Yao, and M. Bazargan

Reporter: Dr S. Bozhko

May, 8th, 2007

2

Grid Integration of Large Offshore Wind Farms Using STATCOM-Controlled HVDC Power Transmission: Control and Engineering Issues

Introduction

• World electricity demand – to be covered for up to 12% by 2020

• Offshore: wind conditions are better, planning restrictions are reduced

• HVDC vs HVAC

• VSC HVDC vs LCC HVDC

• SG vs DFIG

• DFIG + STATCOM + LCC HVDC: well studied as separate components

• Existing studies consider the overall system concept and possible

control paradigms – no detailed study or rigorous design procedure

3

AC filters

Local loads

STATCOM

CS

TS

TC Rectifier HVDC Inverter HVDCSubmarine

DC cable TI

Collection Bus

Ons

hore

AC

gri

d

TechnologicalPlatform (island)

CoastLine

TWF Submarine AC cable

10…20km 80…150km

Total wind farm power: 1GW (set of DFIG-based WTG 3.3MVA each)

Collection Bus Voltage: 33kV; Offshore Bus Voltage: 132kV;

Onshore Grid: 400kV @ SCR=2,5; HVDC Link: 1GW (2kA@500kV)

The power system studied:


4

Control system should provide:

• optimal tracking of collected wind power and its transfer into the HVDC link

• control of voltage and frequency of the offshore grid

• Detailed mathematical study of the system

• The controlled plant model appropriate for rigorous control design and understanding of the power system interactions

• Engineering studies of the designed control system

Main steps of our investigation include :


5

System model development for control design:

• aggregation of multiple WTG into a single one with similar DFIG control strategy

• aggregated DFIG as a controlled current source

• harmonic filters are represented by their low-frequency capacitive properties

• no power losses in the STATCOM and HVDC rectifier

• HVDC inverter is in voltage-control mode and can be replaced by an equivalent DC voltage source


6

Control Approach

Simplified diagram of the studied system:

VGIC

IG

ISTC

TSCS

ES

L0 R0

E0

V0

+

_

V*S ABC AOR (α)

VS VC

STATCOM HVDC

Cf

AC F


7

Control Approach

Reduced plant of control

GdV*0R

20S

S

GdfGqCq*Sq

Gqf

GdCd*Sd

Gdf

IKIKdt

dEC

VCIIIdt

dVC

IIIdt

dVC

Controlled Plant

x2

PI

PI

PI

I*0

I*Sd

I*Sq

ES DC

VGd

VGq

E*S DC

V*Gd

V*Gq

Proposed control structure


8

∫

CS

VG

2π50

PI

IC

IG

ISLS

RS

ES

L0

R0

E0

I0 V0+

_

2/3

V*Sα

VGβ

ejθ

3/2

3/2

3/2

Cf

V*S ABC

VGα

V*Sβ

ωe*

θ

ICβ

VGd

e-jθ

x2

ISq

AOR (α)

ISd

ICd

ICq

ISβ

ISα

I0

IGd

IGq

ωLS

3/2

ICα

IGβ

IGα

V*Sd

V*Sq

VGq e-jθ

e-jθ

ωLS

e-jθ

PI

PI

controlled plant

VGd*

ωCf

*20S )E(

ωCf

VGq*

(= 0)

3VGd0/CS

PI

PI

PI

controllers

Isd*

I0*

Isq*

Detailed block-diagram of the proposed control structure


9

PSCAD/EMTDC simulations of the proposed control system

Control Approach

• Detailed PSCAD/EMTDC simulation model is used

TG

TC1

TC2 TS

CS

AC Filters

V0

_

+

L0/2 R0/2

HVDC Rectifier

L0/2 R0/2

C0

HVDC Inverter

StatCom

33kV

132kV

Local Offshore Bus TW 1kV

Wind Farm 1000MVA

10kV E0

_

+ TI1

TI2 400kV

Main Onshore Grid Connection

SCR=2.5


10

Simulation results:

Control Approach

0

0.5

1

-0.4

-0.2

0

0.2

0

1

2

100

120

140

49.8

50

50.2

32

34

36

38

0 0.5 1 1.5 2 2.5 3 3.50

20

40

60

0

0.5

1Wind Power Active (1) and Reactive (2) Powers, pu

-1

0

1

STATCOM Active (1) and Reactive (2) Powers, pu

0

2

4

HVDC DC-Link Current, kA

100110120130140150

Offshore Grid Voltage, kV

49.9

50

50.1

50.2Offshore Grid Frequency, Hz

40

50

60

70STATCOM DC-Link Voltage, kV

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.10

50

100Rectifier Firing Angle, deg

Time, s

1

2

1

2

1

2 1

2

Wind Power Active (1) and Reactive (2) Powers, pu

STATCOM Active (1) and Reactive (2) Powers, pu

HVDC DC-Link Current, kA

Offshore Grid Voltage, kV

Offshore Grid Frequency, Hz

STATCOM DC-Link Voltage, kV

Rectifier Firing Angle, deg

Time, s

• Confirm high performance in both normal conditions and during a severe fault

•Raise engineering concerns regarding STATCOM rating (1.3pu in order to handle the fault)

• Also raise concerns regarding STATCOM capacitor overvoltage (1.92pu)

• Some measures must be undertaken to improve the system practicality


11

STATCOM DC-link capacitor sizing

• Energy stored in this capacitor:2

ECdt)PPP(e)t(e

2SdcS

t

tlCG0

0

• Overvoltage factor:

0 Sdcmax SdcV E/Ek


Power to HVDC link

PC0

t t

Generator’s power

PG0

τd

tt

Losses

PL0 tt

• Power balancing equation:

)e1(PtP)e1(PP)1k(2

ECCG

d t

C0C0L

t

G0Gd0G2V

20SdcS

12

STATCOM DC-link capacitor sizing

• Can be used to derive a criterion for the STATCOM capacitor sizing in order to guarantee that the capacitor overvoltage during a fault will not exceed the acceptable level:

CS MIN = F(tf, τd, τG, τC, kV, PG0, PC0, PL0)

Communication delay τd, s

CS m

in,

F

n=1

n=2

n=3

0 0.005 0.01 0.015 0.020

0.02

0.04

0.06

0.08

0.10


13

Power system operation during a fault

PI

132kVLocal Offshore Grid

I0*AC Filters PI

TSCS

STATCOM

10kV

0/2 R0/2 L0/2 R0/2

C0

E0

TI1

TI2 400kV


SCR=2.5

HVDC Inverter

TC1

TC2

V0 L

HVDC Rectifier

ES2 *

TG33kVTW

Wind Farm

PI I*rd

I*rq

Q*wf

P*wf

I*fd

I*fq

Q*f=0

Efdc=E*fdc

Fault Detected

0 pu

I*rq=0

τd

0.25 pu


14

Influence of communication delay τd on STATCOM rating

The dynamics of HVDC rectifier AC currents is twice as faster than the dynamics of HVDC DC-link current loop!


fin0t

fin0ini00 Ie)II()t(I • If assume ideal performance of HVDC current control loop during a fault:

)(t • then the behavior of AOA can be found as:

• and HVDC rectifier AC-side currents then can be derived as follows:

1)(

)γ(

3

2sin

3

22

0020

γ2222γ200

20

0

LRI

eVkke

V

LRIkII

ini

tGdTrt

Gd

iniCq

t

Gd

iniCd e

V

LRIkII γ200

20

0)γ(

3

2cos

3

2

15

STATCOM rating issue (continued)

• If communication delay exceeds some value, the STATCOM apparent power demand during faults can reach the value of wind farm delivered apparent power

d

q

VG

IC0d

IC0q

IC0

IS0

IG0

• STATCOM rating can be reduced substantially only if no communication delay or if it is very small compare to HVDC DC-link current control time constant

STATCOM S, P and Q vs communication delay

2 3 4 5 6 7 8 9 10

x 10-3

0

0.2

0.4

0.6

0.8

1.0

0.6kA

0.2kA

0.2kA

0.6kA

0.2kA

0.6kA Sst

Pst

Qst

2 3 4 5 6 7 8 9 10

x 10-3

0

0.2

0.4

0.6

0.8

1.0

7.95ms

2.65ms

2.65ms

7.95ms

2.65ms

7.95ms

Sst

Pst

Qst


16

Power system operation during a fault

PI

132kVLocal Offshore Grid

I0*AC Filters PI

TSCS

STATCOM

10kV

0/2 R0/2 L0/2 R0/2

C0

E0

TI1

TI2 400kV


SCR=2.5

HVDC Inverter

TC1

TC2

V0 L

HVDC Rectifier

ES2 *

TG33kVTW

Wind Farm

PI I*rd

I*rq

Q*wf

P*wf

I*fd

I*fq

Q*f=0

Efdc=E*fdc

Fault Detected

0.25pu

I*rq=0

τd

3/2ICFq

ICFd


17

Reduction of the STATCOM rating can be achieved by:

• Suppression of STATCOM DC-link voltage control: fault detection scheme can set the HVDC current demand I0

* to some value I0fin in order to absorb the AC filters

reactive power by HVDC link, not by STATCOM;

• Reduction of wind farm output power via fast DFIG current control loops;

• Communication delay τd due to distant location if WTGs: should be lowered

• Reactive power capabilities of DFIGs front-end converters: the reactive current reference as a function of reactive current component at HVDC input;

• Active power support through rotor q-current controls: the q-current reference as a function of active current component at HVDC/filters input;

• Improvement of HVDC DC-link current control: need adaptation to fault conditions;

• Lowering the bandwidth of offshore grid voltage and frequency controls;

• Hard Limits on STATCOM currents.


18

0

0.5

1

Wind farm active (1) and reactive (2) pow ers, pu

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-1

0

1STATCOM active (1) and reactive (2) pow ers, pu

0

1

2

3

HVDC DC-link current, kA

60

80

100

120

140

160Offshore grid voltage, kV

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

49

50

51

Offshore grid frequency, Hz

Time, s

35

40

45STATCOM DC-link voltage, kV

0

50

100

150Rectif ier f iring angle, deg

1.3

1.4

1.5

DFIG shaft speed, pu

1

2

1

2

Time, s

Simulation of fault in the enhanced system

• STATCOM active and reactive power demand is significantly lowered

• STATCOM DC-link overvoltage is reduced from 94% to 25%

0.998 1 1.002 1.004 1.006 1.008 1.01 1.012 Time,s

0

0.2

0.4

0.6

0.8

1

Td=2ms

Td=4ms

Td=6ms

Td=6ms Td=10ms

Fault

STATCOM apparent power during fault development


19

Conclusions

Acknowledgement

THANK YOU!

• A large offshore wind farm with a LCC HVDC connection to the main onshore grid is considered

• The proposed control system is proven to provide high performance control of the offshore grid and wind power transfer to onshore

• Engineering issues related to the STATCOM sizing is considered

• Recommendations for control system enhancement are given

• The proposed system can be a satisfactory solution for integrating large offshore DFIG-based wind farms into existing AC networks

Authors would like to express their appreciation for the partial funding support from the New and Renewable Energy Programme of the DTI, UK under the contract K/KL/00340/00/00.


grid integration of large offshore wind farms using statcom-controlled hvdc power transmission :

Documents

wind power

statcom capacitor overvoltage

apparent power statcom

statcom capacitor sizing

statcom apparent power

hvdc linkcontrol of

kvthe power system

hvdc current demand