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