Liserre Lecture 8

7/14/2019 Liserre Lecture 8

1/23

Grid converter control

Marco Liserre [email protected]


Marco Liserre

[email protected]


2/23



A glance at the lecture content

Introduction

Dc-voltage control Power control

Islanding, microgrid, droop

control, grid supporting


3/23



Introduction

The grid converter can operate as grid-feeding or grid-forming device Main control tasks

manage the dc-link voltage (if there is not a dc/dc converter in charge of it)

inject ac power (active/reactive)

A third option is the operation as grid-supporting device (voltage, frequency, powerquality)

ACPOWER

CONTROL

GRID

SUPPORTING

GRID

FORMING

DC VOLTAGE

CONTROL

GRID

FEEDING

POWER

THE

ORY

stationary

frameab

natural

frame abc

rotating

frame dq

AC CURRENT

CONTROL

AC VOLTAGE

CONTROL


4/23



+

-dcV

1L 2L gL

gii

cV

+

-

+

-V

+

-

gV

POWER CONTROL

CURRENT

CONTROL

VOLTAGE

CONTROL

DC VOLTAGE

CONTROL

GRID

MANAGEMENT

dcVi

cV gV

+

-

Another major difference is that the grid converter could be requested to operate onthe grid side as:

a controlled current source

a controller voltage source

Introduction

with the LCL-filter

both the options

can be integrated

within a multiloopstructure


5/23



Dc voltage control

In the grid connected converter a change of the produced power causestransient conditions hence charge or discharge processes of the dc capacitor

The increase of the produced power results in voltage overshoot while its

decrease results in voltage undershoot

So, from the point of view of the dc voltage control, power changes result in

voltage variations that should be compensated by charge or dischargeprocesses

dcV

1L i

cV

+

-

+

-V

+

-

2L gLgi

cV

+

-

gV

+

-

produced power


6/23



Dc voltage control

The dc voltage control is achieved through the control of the power exchanged by

the converter with the grid or through the control of a dc/dc converter

In the first case the decrease or increase of the dc voltage level is obtained

injecting more or less power to the grid respect to that one produced by the

WTS

In the second case the grid converter does not play a role in the

management of the dc-link

*

(3 )

2

so

o

P n Tv

C v

Voltage error as a result

of power change


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Linear control

dcV

1L i

cV

+

-

+

-

V

CC

i

+

-

2L gL

gi

cV

+

-

gV

+

-

VCi*

synch

*

dcV

The control of the dc voltage through the ac current can result in the identification of

two loops, an outer dc voltage loop and an internal current loop

The internal loop is designed to achieve short settling times

On the other hand, the outer loop main goals are optimum regulation and stability

thus the voltage loop could be designed to be some what slower

Therefore, the internal and the external loops can be considered decoupled


8/23



dcV

TOTL

i+

-

V+

- E

oio1i +

-

1 1d 3

d 2

dc dcdc dc dc dc o o d d d d q q q q

V vV v C V v I i E e I i E e I i

t

Dccurrent

[A]

3

2 1

dc o

od

v R

R Csi

1

1

dc o

oo

v R

R Csi

3

1

dc

d o

v

e R Cs

constant power case

plant

generator disturbance

grid disturbance

Linear control: small signal analysis


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Voltage-oriented control

The Voltage Oriented Control (VOC) of the grid converter is based onthe use of a dq-frame rotating at w speed and oriented such as the d-axis is aligned on the grid voltage vector

The reference current d-

component i*d is controlled toperform the active powerregulation while the referencecurrent q-component i*q iscontrolled to obtain reactive

power regulation Similar results can be

achieved in a stationary ab-frame

ia

ib

qi di

b

d

qi

a


10/23



ip KK

s

dcv

dcV

P

Q

+-

di

qi

dcV controller

PLLgv

gv a

gv b

fgV

je

gdv

gqv

*v

abc

je

ia

ib

di

qi

+-

di

di

gdv

+-

qi Current

controller

Currentcontroller

gqv

je

va

vb

i

p

KK

s

i

p

KK

s

L

L

i

abc

abc

2 2

1 gd gq

gq gdgd gq

v v

v vv v

gdv gqv

X

+-

active and reactive

power feed-forwardcontrol

Vdc control acts on

the power reference

Synchronous frame VOC: PQ open loop control

qi di

d

q

i


11/23



active and reactive

power PI-based control

Vdc control acts on id*

Synchronous frame VOC: PQ closed loop control

qi di

d

q

i

i

pKKs

gd d gq q

gq d gd q

P v i v i

Q v i v i

dcv

dcV

P

Q

di

qi

P

Q

+-

+-

+-

di

qi

Q controller

P controller

dcV controller

PQ controller

+-

PLLgv

gv a

gv b

fg

V

je

gdv

gqv

*v

abc

je

ia

ib

di

qi

+-

di

di

gdv

+-

qi Current

controller

Currentcontroller

gqv

je

va

vb

i

p

KK

s

i

p

KK

s

i

p

KK

s

i

p

KK

s

L

L

i

abc

abc

gdv

gqv


12/23



PLL may be avoided but it is used for

making the control freq. adaptive

Stationary frame VOC: PQ open loop control

ia

ib

b

i

a

i

p

K sK

s

2 2w

ia

ib

i

ia

ib

+-

ia

+-

ib Current

controller

Currentcontroller

va

vb

i

p

K sK

s

2 2w

abc

*v

abc

dcV

P

Q

+-

dcV controller

i

p

KK

s

2 2

1 g g

g gg g

v v

v vv v

a b

b aa b

gv a gv b

X

+-

dcv

PLLgv

gv a

gv b

f ,w

active and reactive

power feed-forwardcontrol

Vdc control acts on the

power reference


13/23



PLL is still indispensable

for reference generation

Stationary frame VOC: PQ closed loop control

ia

ib

b

i

a

i

p

K sK

s

2 2w

sin

cos

dcv

dcV

P

Q

P

Q

+-

+-

+-

ia

ib

I

Q controller

P controller

dcV controller

PQ calculation

+-

i

ia

ib

+-

ia

+-

ib Current

controller

Currentcontroller

va

vb

i

p

K sK

s

2 2w

i

p

KK

s

i

p

KK

s

i

p

KK

s

abc

*v

abc

w

a a b b

b a a b

g g

g g

P v i v i

Q v i v i

iaib

gv a gv b

PLLgv

gv a

gv b

f ,w

active and reactive

power PI-based control

Vdc control acts on I


14/23



Microgrid operation with controlled storage

G+

-

+

-

+

-

G

FLYWHEELVOLTAGE

CONTROLSTORAGECONTROL

VOLTAGE

CONTROL

CURRENT

CONTROL

WT

MICRO-GRID MANEGEMENT

*CV

*

dcV *dc,ESV*dcI

dci dcV i cV dc,ESV

w

ENERGY STORAGE

load

Multiloop control for microgrid operation with WT system

The management of the dc voltage is in charge of the controlled storage

unit ESS

A flyweel is used


15/23



Droop control

Using short-line model and complex phasors, the analysis below is valid for

both single-phase and balanced three-phase systems.

At the section A

For a mainly inductive line

I

VA

Z

Vd

VBI

VB0

VA

Vq

RIXI

BA

cos cosA A BAV V V

PZ Z

2

sin sinA A B

A

V V VQ

Z Z

cosA A

d A BA

RP XQV V V

V

sin A A

q BA

XP RQV V

V

A

A B

XP

V V AA B

A

XQV V

V


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Droop control The angle can be controlled regulating the active power P whereas the inverter

voltage VA is controllable through the reactive power Q. Control of the frequency dynamically controls the power angle and, thus, the real

power flow.

Thus by adjusting P and Q independently, frequency and amplitude of the grid

voltage are determined

However, low voltage distribution lines have a mainly resistive nature.

0 0pf f k P P 0 0qV V k Q Q

Q

V

V0

Q0 P

f

f0

P0


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Droop control for grid supporting operation

The droop control is not only used in island application when it is needed

to a have a wireless load sharing but also in order to support the grid

In this case grid-feeding and grid-forming schemes can be modified

accordingly including droop control

grid feeding grid formingVSI

Modulator

dcv+

-

1L

v

i

CvC fC

*

v

+

-

Current

control

+

-

Voltage

control

*i

Power

calculation

Droop

control

*

cvsynch

P

Q

VSI

Modulator

dcv+

-

1L

v

i

CvC fC

*v

+

-

Current

control

+

-

*i

Inverse

droop

control

synch


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Improving grid power quality: voltage dips

Vload

E

Iload

Ig

Ic

Vload

E

Iload

Ig

Ic

compensation of a voltage dip of 0.3 pu

Vload

E

Iload

Ic

Ig

VLg

VLg VLg

current-controlled converter in normal

conditionsvoltage-controlled converter in normal

conditions

Vload

Ic

IloadIg

E

CURRENT CONTROLLED

grid-feeding component adjusting

reactive power according to grid

voltage variations

Vload

Vc

IloadIg

E

VOLTAGE CONTROLLED

grid-forminng component

controlling its output voltage in

order to stabilize load voltage

Voltage dip compensation by means of reactive power injection


19/23



Improving grid power quality: voltage dips

Iload

Ic

EIg

PIRepetitive

control

Ic

+

- -

+

Iref

Vc

grid

Vref

Vpeak

mpptd+ + MPPT

algorithm

PV

Ipv

Upv

PLL

Vg

Also low power PV systems can be designed to improve the power

quality. They can provide grid voltage support and compensation of

harmonic distortion at the point of common coupling (PCC)


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Conclusions

AC Power control is performed through voltage and/orcurrent control

Grid forming, grid feeding or grid supporting

Grid supporting: harmonics, reactive power, dips

Droop control for amplitude and frequency control

Voltage control can be used for supporting voltage (dips andharmonics)


21/23



Bibliography

1. F. Blaabjerg, R. Teodorescu, M. Liserre, A. V. Timbus, Overview of Control and Grid Synchronization for

Distributed Power Generation Systems IEEE Transactions on Industrial Electronics, Ottobre 2006, Vol. 53,No. 5, pagg. 1398-1408.

2. R. Crdenas, R. Pea, M. Prez, J. Clare, G. Asher, and F. Vargas, Vector Control of Front-End Converters

for Variable-Speed WindDiesel Systems IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL.

53, NO. 4, AUGUST 2006 1127.

3. K. De Brabandere, B. Bolsens, J. Van den Keybus, A. Woyte, J. Driesen and R. Belmans, A voltage and

frequency droop control method for parallel inverters Proc. of Pesc 2004, Aachen 2004

4. J. M. Guerrero, L. Garca de Vicua, J. Matas, M. Castilla, and J. Miret, A Wireless Controller to Enhance

Dynamic Performance of Parallel Inverters in Distributed Generation Systems IEEE TRANSACTIONS ON

POWER ELECTRONICS, VOL. 19, NO. 5, SEPTEMBER 2004, 1205-1213.

5. C. Klumpner, M. Liserre, F. Blaabjerg, Improved control of an active-front-end adjustable speed drive with a

small de-link capacitor under real grid conditions PESC 04, Vol. 2, 20-25 June 2004, pp. 1156 1162.

6. T. Ohnishi, Three phase PWM converter/inverter by means of instantaneous active and reactive power

control, Proc. of IECON 91, Vol. 2, 1991, pp. 819-824.

G id l


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Bibliography

9. L. Malesani, L. Rossetto, P. Tenti and P. Tomasin, AC/DC/AC PWM converter with reduced energy storage in

the dc link, IEEE Transactions on Industry Applications, 1995, 287-292.

10. Poh Chiang Loh and Donald Grahame Holmes, Analysis of Multiloop Control Strategies for LC/CL/LCL-

Filtered Voltage-Source and Current-Source Inverters IEEE TRANSACTIONS ON INDUSTRY

APPLICATIONS, VOL. 41, NO. 2, MARCH/APRIL 2005, 644-654.

11. Jinn-Chang Wu; Hurng-Liahng Jou, "A new UPS scheme provides harmonic suppression and input power

factor correction," IEEE Transactions on Industrial Electronics, , vol.42, no.6, pp.629-635, Dec 1995

12. J.R. Espinoza, G. Joos, M. Perez, T.L.A Moran, Stability issues in three-phase PWM current/voltage source

rectifiers in the regeneration mode, Proc. of ISIE00, Vol. 2, pp. 453-458, 2000.

13. R. A. Mastromauro, M. Liserre, A. DellAquila, T. Kerekes, A Single-Phase Grid Connected Photovoltaic

System with Power Quality Conditioner Functionality, accepted for future publication on IEEE Transactions

on Industrial Electronics.

14. Y. W. Li, D. M. Vilathgamuwa and P. C. Loh, Micro-grid power quality enhancement using a three-phase four-

wire grid-interfacing compensator, IEEE Trans. Ind. Applicat., vol. 41, pp. 17071719, Nov/Dec. 2005.

G id t t l


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Marco Liserre liserre@ieee org

Acknowledgment

Part of the material is or was included in the present and/or past editions

of the

Industrial/Ph.D. Course in Power Electronics for Renewable Energy

Systemsin theory and practice

Speakers: R. Teodorescu, P. Rodriguez, M. Liserre, J. M. Guerrero,

Place: Aalborg University, Denmark

The course is held twice (May and November) every year

Liserre Lecture 8

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