harmonic rejection

Marco Liserre [email protected]

Grid converter control


Marco Liserre

[email protected]



A glance at the lecture content

• Introduction • Dc-voltage control• Power control• Islanding, microgrid, droop

control, grid supporting



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, power quality)



+

- dcV

1L 2L gLgii

cV

+

-

+

-V

+

-gV

POWER CONTROL

CURRENT CONTROL

VOLTAGE CONTROL

DC VOLTAGE CONTROL

GRID MANAGEMENT

dcV i cV gV

+

-

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

a controlled current source

a controller voltage source

Introduction

with the LCL-filter both the options

can be integrated within a multiloop

structure



Dc voltage control In the grid connected converter a change of the produced power causes

transient 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 discharge processes



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



Linear control 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



1 1ˆd 3ˆ ˆ ˆˆ ˆ ˆ ˆ

d 2dc dc

dc dc dc dc o o d d d d q q q qV v

V v C V v I i E e I i E e I it

Dc current [A]

ˆ 3ˆ 2 1dc o

od

v RR Csi

1

ˆˆ 1dc o

oo

v RR Csi

ˆ 3ˆ 1dc

d o

ve R Cs

constant power case

plant

generator disturbance

grid disturbance

Linear control: small signal analysis



Voltage-oriented control The Voltage Oriented Control (VOC) of the grid converter is based on

the use of a dq-frame rotating at speed and oriented such as the d-axis is aligned on the grid voltage vector

The reference current d-component i*d is controlled to perform the active power regulation while the reference current q-component i*q is controlled to obtain reactive power regulation

Similar results can be achieved in a stationary -frame

i

i

qi di

i



ipKKs

dcv

dcV

P

Q

+ -

di

qi

dcV controller

PLLgvgv

gv

fgV

je

gdv

gqv

*v

abcje

i

i

di

qi

+-

di

di

gdv

+-

qi Current

controller

Currentcontroller

gqv

je

v

v

ipKKs

ipKKs

L

L

i αβ

abc

αβ

αβ

abc

2 2

1 gd gq

gq gdgd gq

v vv vv v

gdv gqv

X

+ -

• active and reactive power feed-forward control • Vdc control acts on

the power reference

Synchronous frame VOC: PQ open loop control



• active and reactive power PI-based control

• Vdc control acts on id*

Synchronous frame VOC: PQ closed loop control

ipK

Ks

gd d gq q

gq d gd q

P v i v i

Q v i v i

dcv

dcV

P

Q

di

qiP

Q+-

+-

+ -

di

qi

Q controller

P controller

dcV controller

PQ controller

+ -

PLLgvgv

gv

fgV

je

gdv

gqv

*v

abcje

i

i

di

qi

+-

di

di

gdv

+-

qi Current

controller

Currentcontroller

gqv

je

v

v

ipK

Ks

ipK

Ks

ipK

Ks

ipK

Ks

L

L

i αβ

abc

αβ

αβ

abc

gdv gqv



• PLL may be avoided but it is used for making the control freq. adaptive

Stationary frame VOC: PQ open loop control

ip

K sKs

2 2

i

i

ii

i

+-

i

+-

i Current

controller

Currentcontroller

v

v

ip

K sKs

2 2

abc

αβ *v

αβ

abc

dcV

P

Q

+ -

dcV controller

ipK

Ks

2 2

1 g g

g gg g

v vv vv v

gv gv

X

+ -

dcv

PLLgv

gv

gv

f ,

• active and reactive power feed-forward control • Vdc control acts on the power reference



• PLL is still indispensable for reference generation

Stationary frame VOC: PQ closed loop control

ip

K sKs

2 2

θ

sin

cos

dcv

dcV

P

Q

P

Q+-

+-

+ -

i

i

I

Q controller

P controller

dcV controller

PQ calculation

+ -

ii

i

+-

i

+-

i Current

controller

Currentcontroller

v

v

ip

K sKs

2 2

ipKKs

ipK

Ks

ipK

Ks

abc

αβ *v

αβ

abc

g g

g g

P v i v i

Q v i v i

ii

gv gv

PLLgvgv

gv

f ,

• active and reactive power PI-based control

• Vdc control acts on I



Microgrid operation with controlled storage

G +-

+-

+-

G

FLYWHEELVOLTAGE CONTROL

STORAGE CONTROL

VOLTAGE CONTROL

CURRENT CONTROL

WT

MICRO-GRID MANEGEMENT

*CV *

dcV *dc,ESV*

dcI

dcidcV i

cV dc,ESV

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



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 VPZ Z

2

sin sinA A BAV V VQZ Z

cos A Ad A B

A

RP XQV V VV

sin A A

q BA

XP RQV VV

A

A B

XPV V

AA B

A

XQV VV



Droop control The angle δ 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



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 forming



Improving grid power quality: voltage dips

Vload

Ic

IloadIg

E

CURRENT CONTROLLEDgrid-feeding component adjusting reactive power according to grid

voltage variations

Vload

Vc

Iload

Ig

E

VOLTAGE CONTROLLEDgrid-forminng component

controlling its output voltage in order to stabilize load voltage

Voltage dip compensation by means of reactive power injection



Improving grid power quality: voltage dips

Iload

Ic’

E Ig

PIRepetitive

control

Ic’

+

- -

+

Iref

Vc’

grid

Vref

Vpeak

mpptd+ + MPPTalgorithm

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)



Conclusions• AC Power control is performed through voltage and/or current

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 and

harmonics)



Bibliography1. 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. Cárdenas, R. Peña, M. Pérez, J. Clare, G. Asher, and F. Vargas, “Vector Control of Front-End Converters for Variable-Speed Wind–Diesel 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. García de Vicuña, 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.



Bibliography9. 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 ISIE’00, Vol. 2, pp. 453-458, 2000.

13. R. A. Mastromauro, M. Liserre, A. Dell’Aquila, 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. 1707–1719, Nov/Dec. 2005.



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 Systems – in 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

harmonic rejection

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