Grid Converters for Photovoltaic and Wind Power Systems

by R. Teodorescu, M. Liserre and P. RodriguezISBN: 978‐0‐470‐05751‐3Copyright Wiley 2011

Chapter 9p

Grid Converter Control for WTS

A glance at the lecture content

• Introduction

• Model of the converter

• Ac‐voltage and Dc‐voltage control

• VOC and DPC

• Stand‐alone, micro‐grid, droop control andgrid supporting

2

Introduction

• The grid converter can operate as grid‐feeding or grid‐forming device

• Main control tasks Manage the dc‐link voltage

Inject ac power (active/reactive)

• A third option is the operation as grid‐supporting device (voltage,frequency power quality)frequency, power quality)

WER

O

L POW

ER

THEO

RY

AC

PO

WC

ON

TRO T

3

Introduction

• The grid converter of a distributed generation system operates as acontrolled power source

• The power control can be performed using: Voltage Oriented Control in analogy with the Field Oriented Control

Di t P C t l i l ith th Di t T C t l Direct Power Control in analogy with the Direct Torque Control

• The power control can be synchronized on the grid voltage or on thevirtual flux that is the integral of the grid voltage (from another pointg g g ( pof view it is a filtered grid voltage)

• The power control can be performed in view of controlling voltage andf PCC d/ i i li d id h hfrequency at PCC and/or improving power quality and ride‐throughcapability

4

Introduction

• Another major difference is that the grid converter could be requested to• Another major difference is that the grid converter could be requested tooperate on the grid side as: A controlled current source

A controller voltage source

i i1L 2L gL

With the LCL‐filter both the options

i gi1 2 g

dcv gvCfvv e

both the options can be integrated within a multiloop 

structure

idcv gvCfv

structure

5

Model of the L‐filter converter

dci

dcvA

B

i R Le

n

C

• Converter switching function

AC l i

22( ) ( ) ( ) ( )3 a b cp t p t p t p t

• AC voltage equation

d ( ) 1 ( ) ( ) ( ) ( )i t Ri t e t p t v t 6

( ) ( ) ( ) ( )d dcRi t e t p t v tt L

Model of the L‐filter converter

Use of a synchronous frame

1 1 2 1 223 0 3 2 3 2

a

b

pp

pp

v

v

cp

2 2cos cos cos ap

vqv

dv

3 32 2sin sin sin3 3

ad

bq

c

pp

pp

p

‐frame dq‐frame

1

1

dcdi t

Ri t e t p t v tdt L

di t

1

1

dq d d d dc

di ti t Ri t e t p t v t

dt Ldi t

1

dcdi t

Ri t e t p t v tdt L

1q

d q q q dcdi t

i t Ri t e t p t v tdt L

7

Model of the LCL‐filter converterdci

2 gL L1L 2 gR R1R

ai

bi

agi

bgi

ae

bendcv

A

B1L

1L

1R

1R

ci cgi ce

nB

C2 gL L 2 gR R

2 gL L 2 gR RfC av

fC bvfC cv

1 1R

fCfff

1

1 1

1

1 1

1 0 0 0

10 0 0

1 10 0 0

f f

d d

q q

f fC d C d

RL L

RL Li i

i iC Cv vd

2

0 0 00 00 0 00 0

dc

g

dc

d d

vL L

ve pL L

2

2 2

2

1 10 0 0

10 0 0

10 0 0

f f

f f

C d C d

C q C qf f

gd gdg

gq gq

g g

g

dv vdt

C Ci i

R Ri iL L L L

R R

2

1

1

0 01 0 00

0 01 0 00

0 0

d dg

q q

e pL Le p

L

L

8

2

2 2

10 0 0 g

g gL L L L

AC voltage controlAC voltage control:• Stand‐alone mode or micro‐gridSta d a o e ode o c o g d

• Supporting local loads, the local electrical power system or even the power grid

A multiloop control should be adopted: the AC capacitor voltage isp p p gcontrolled through the AC converter current. In fact, the current‐controlledconverter is operated as a current source used to charge/discharge thecapacitorcapacitor.

+ +

i1L

-

+

-+- v Cfvdcv

CC VC +-+-i Cfv *

Cfv

Poh Chiang Loh and Donald Grahame Holmes, “Analysis of Multiloop Control Strategies for LC/CL/LCL‐Filtered Voltage‐Source and Current‐SourceInverters” IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 41, NO. 2, MARCH/APRIL 2005, 644‐654 9

DC voltage control

• In the grid connected converter a change of the produced power causestransient conditions hence charge or discharge processes of the dcg g pcapacitor

• The increase of the produced power results in voltage overshoot whileit d lt i lt d h tits decrease results in voltage undershoot

• So, from the point of view of the dc voltage control, power changesresult in voltage variations that should be compensated by charge orresult in voltage variations that should be compensated by charge ordischarge processes

1Lii

v fC CvdcvC

10

DC voltage control

Voltage error as a result of (3 )P n T Voltage error as a result ofpower change *

(3 )2

sdc

dc

P n Tv

C v

• The dc voltage control is achieved through the control of the powerexchanged by the converter with the grid or through the control of aexchanged by the converter with the grid or through the control of adc/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 bythe WTS

In the second case the grid converter does not play a role in theg p ymanagement of the dc‐link

L. Malesani, L. Rossetto, P. Tenti and P. Tomasin, “AC/DC/AC PWM converter with reduced energy storage in the dc link”, IEEE Transactions onIndustry Applications, 1995, 287‐292 11

DC voltage control

• The control of the dc voltage through the ac current can result in theidentification 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 andOn the other hand, the outer loop main goals are optimum regulation andstability thus the voltage loop could be designed to be some what slower

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

1L i 2L gi gL

dcv vCfv gv e

dcv

ii

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 12

Linear control: small signal analysis

oi dciiTOTL

dcv v e

ˆd 3ˆ ˆ ˆd dV v

Constant power casePlantˆ 3dc ov R

d 3ˆ ˆ ˆˆ ˆ ˆ ˆd 2dc dc

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

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

A]

Constant power casePlant

Generator disturbance

ˆ 2 1dc o

od R Csi

ˆdc ov R

Dc curren

t [AGenerator disturbance ˆ 1 oo R Csi

ˆ 3dcvGrid disturbance ˆ 1

dc

d oe R Cs

13

Linear control: PI tuning procedure

3 1

2 1 3P I

ovk T s

H sT T C

( 1)

oRR Cs

oi 2 1 3ov

I sT s T s Cs

13c

saT

11k

dcv 1 3 oR

( 1)oR Cs

dcv

1 cossin

s

I

a

Ta

2 3Ps

CkaT

1pI

ksT

1 3 ssT 2 ( 1)oR Cs

3de

150

3I

s

aT

3( 1)RCs

de

-50

0

50

100

150

Mag

nitu

de (D

b)

0.8

1

1.2

1.4

0.8

1

1.2

1.4

10-1 100 101 102 103 104-50

160

-140

-120

e (D

egre

e)

0

0.2

0.4

0.6

0

0.2

0.4

0.6

10-1 100 101 102 103 104-180

-160

Frequency (Hz)

Pha

s

100 Hz0 50 100 150 200 250

00 50 100 150 200 250

0

Without pre‐filter With pre‐filter

14

Voltage‐oriented control

Th V lt O i t d C t l (VOC) f th id t i b d th• The Voltage Oriented Control (VOC) of the grid converter is based on theuse of a dq‐frame rotating at ω speed and oriented such as the d‐axis isaligned on the grid voltage vector

• The reference current d‐componentThe reference current d componenti*d is controlled to perform theactive power regulation while thef t t i*

i i

reference current q‐component i*qis controlled to obtain reactivepower regulation

qi di

• Similar results can be achieved in astationary ab‐frame

i

15

Synchronous frame VOC: PQ open loop control

Ikk

di

di

gdvCurrentcontroller

i

Pk s

*vje

i

i

di

qije

v

v

L

L

i

abc

abc

qi diqi

gvgv

v

f gV

qi

gqv

IPkks

Currentcontroller

abc

v

gv

je

gdv

gqv

abc

• Active and reactivepower feed‐forwardcontrol

dcv

dV controller

IPkks

gdv gqv

dcV

• Vdc control acts on thepower reference

di

qi

dcV controller

2 2

1 gd gq

gq gdgd gq

v vv vv v

gd gq

P

Q

16

Synchronous frame VOC: PQ closed loop control

di Current

controller gdv

jei

i

i

di

+-di

Li

je je

abcv

vv

IPkks

i

ei qiL

e

v

+-

Currentcontroller

qi

gqvv gv

f gV

IPkks

abc

qi diqi

PLLgq

abc

gvgv

je

gdv

gqvv

• Active and reactivepower PI‐based control

P controller

Vdc controller

+-

dcv

dcV

IPkks

P

v v• Vdc control acts on i*d

PQ controller

+-

+-di

P

+-Q qi

gd d gq q

gq d gd q

P v i v i

Q v i v i

gdv gqvdiqi

Q

IPkks

IPkks

17

Q controllerPQ controllerQ

Stationary frame VOC: PQ open loop control

i Current

controller

i i

i2 2I

Pk sk

s

ii

i

v

v

*v

+-

abc

ii

+-

Currentcontroller

abc

2 2I

Pk sk

s

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

PLLgvgv

gv

,f

• Active and reactive powerfeed‐forward control

IPkks

X

dcv

+-dcV

feed forward control

• Vdc control acts on thepower reference

i

i

dcV controller

2 2

1 g g

g gg g

v vv vv v

gv gv

+-P

Q

18

Stationary frame VOC: PQ closed loop control

i Currentcontroller

i i

i 2 2I

Pk sk

s

ii

i

i

v

v

*v

controller

abc

i

i Current

controller

2 2I

Pk sk

s

abc

• PLL is still indispensable for referencegeneration

dcv

gvgv

gv

,f

• Active and reactive powerPI based control P controller

dcV controllerI

Pkks

v v

P

dcV

PI‐based control

• Vdc control acts on Ii

i

I

PQ calculation

g g

g g

P v i v i

Q v i v i

ii

gv gv

PQ

IPkks

IPkks

19

Q controllerPQ calculation

Q

Virtual flux

Current controller

+

di

++

gdv

Ikk

• The idea is to model thegrid as a big electricalmachine and estimate

ixi

yi

di

qije

-

Li

Li

++

je

v

v

v

abc

abc

Pk s

the equivalent air‐gapflux for control purposes

• The estimation obtained

Current controller

atan

+-

qi

+++

gv

gv

gqv

abc

gv

IPkks

The estimation obtainedintegrating the measuredgrid voltage can be usedfor synchronization Vdc controller

je

gdv

gqv

dcv

abc

for synchronizationpurposes and/or toestimate the powerinjected into the grid for 1

a a b b c cP v i v i v i

P controller

dcv

di

P

, ,a b ci

vP +-Q

+-

+-

IPkks

IPkks

injected into the grid forcontrolling power fluxes

13 ab c bc a ca bQ v i v i v i

Q controllerPQ calculation

, ,a b cv

Q

Q+- qi

IPkks

Malinowski, M.; Kazmierkowski, M.P.; Hansen, S.; Blaabjerg, F.; Marques, G.D., “Virtual‐flux‐based direct power control of three‐phase PWMrectifiers”,, IEEE Transactions on Industry Applications, pp. 1019‐1027, Volume: 37, Issue: 4, Jul/Aug 2001 20

Direct power control

• The direct power control developed in analogy to the well known direct torquecontrol used for drives

• In DPC there are no internal current loops and no PWM modulator block becausethe converter switching states are appropriately selected

By a switching table

dcv

dcv

dci dcP

P

IP

kks

y a s tc g tab ebased on theinstantaneous errorsbetween the

P

g g

g g

P v i v i

Q v i v i

gv gv

i i

P

QS

between thecommanded andestimated values ofactive and reactive

Q

gvgv

gv

fgVabc

active and reactivepower

gabc

ii

i

abc

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.T. Noguchi, H. Tomiki, S. Kondo, I. Takahashi, “Direct power control of PWM converter without power‐source voltage sensors”, IEEETrans. on Ind. App., vol. 34, May/June 1998, pp. 473‐479 21

Direct power control with modulator

• Introduced to solve the problem related to the request of high samplingfrequency of the previous schemefrequency of the previous scheme

• If the grid is stiff the active and reactive power loops behave like classicald and q current loopsq p

dcv

dcv

dci dcPIP

kks

• In case the grid voltageis weak substantialdifferences may arise

P

g g

g g

P v i v i

Q v i v i

gv gv

i i

P

Q

dv

qv je

IP

kks

v

v

v

abc

IP

kks

differences may ariseQ

gvgv

gv

f

abc

g

gVabc

ii

i

b

22

abc

Direct power control with modulator and virtual flux

gv

gv

gv

ii

i

abc

dcv

dcv

dci dcP

abc

Ikk

P i i

P

P dv

v

Pk s

IP

kks

P i i

Q i i

i i Q

qv je

Q

v

v

abc

s

IP

kks

23

Q

Islanding operation: Gaia project• The converter voltage is controlled directly

• There is not a current control but a current limiter add a voltage contribution inorder to limit the current if it is too highorder to limit the current if it is too high

• The dc voltage controller gives a contribution only if the dc voltage is below thenatural dc‐link voltage in order to avoid the PWM saturation, if the dc voltage isabove the dc natural voltage the chopper dissipates the power in excess

dc-link voltage controllerdcv

-dcv

Ikk

Gaia project: 11 kW Current Limiter

output voltage controller

+++

+dc

dV

+-

dV

dV

+++

dv

je

V i di

v

v

v

IP

kks

IP

kks

IG for household and remote locations

+2 2d qi i Current

limiter

limdV

i

ije

qirmsi

limi

0qv abc

abc

gvgv

gv je

gdv

gqv

abc

R. Teodorescu, F. Blaabjerg, “Flexible Control of Small Wind Turbines with Grid Failure Detection Operating in Stand‐Alone and Grid‐Connected Mode”, IEEE Trans. on Power Electronics, Vol. 19, No. 5, 2004, pp. 1323‐1332 24

Microgrid operation with controlled storage

• Multiloop control for microgrid operation with WT system

• The management of the dc voltage is in charge of the controlled storageunit ESS

A fl l i d• A flyweel is used

+-

+-

+-

ENERGY STORAGE

load

Generator

i i

Generator

FLYWHEELVOLTAGE CONTROL

STORAGE CONTROL

VOLTAGE CONTROL

CURRENT CONTROL

WT

dci

i

dcvi Cv

Cv

,dc ESv

MICRO-GRID MANEGEMENT

dci Cv

dcv ,dc ESv

R. Cárdenas, R. Peña, M. Pérez, J. Clare, G. Asher, and F. Vargas, “Vector Control of Front‐End Converters for Variable‐Speed Wind–DieselSystems” IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 53, NO. 4, AUGUST 2006 1127 25

Droop control

• Using short‐line model and complex phasors, the analysis below is valid forboth single‐phase and balanced three‐phase systemsboth single phase and balanced three phase systems

• At the section A cos cosA A B

AV V VPZ Z

2

sin sinA A BAV V VQZ Z

At the section A

0

cos A Ad A B

RP XQV V VV

sin A A

q BXP RQV VV

• For a mainly inductive line

AVq

AV

AXP AA B

XQV V • For a mainly inductive lineA BV V

A BA

V VV

26

Droop control

• The angle δ can be controlled regulating the active power P whereas the invertervoltage VA is controllable through the reactive power Q

• Control of the frequency dynamically controls the power angle and, thus, the realpower flow

• Thus by adjusting P and Q independently, frequency and amplitude of the gridvoltage are determined

V V k Q Q f f k P PV

0 0qV V k Q Q f

0 0pf f k P P

V0 f0

QQ0 PP0

• However, low voltage distribution lines have a mainly resistive nature27

Droop control Implementation• The droop control is not only used in island application when it is needed to a

have a wireless load sharing but also in grid connection mode

• In this case grid feeding and grid forming schemes can be modified accordingly• In this case grid‐feeding and grid‐forming schemes can be modified accordinglyincluding droop control

Grid feeding Grid forming

1Li

dcvC v fC Cv

1Li

dcvC v fC Cv

v

i

v

i

cv

P

Q

sync

h

K. De Brabandere, B. Bolsens, J. Van den Keybus, A. Woyte, J. Driesen and R. Belmans, “A voltage and frequency droop control method for parallelinverters” Proc. of Pesc 2004, Aachen 2004

J. M. Guerrero, L. García de Vicuña, J. Matas, M. Castilla, and J. Miret, “ A Wireless Controller to Enhance Dynamic Performance of Parallel Inverters inDistributed Generation Systems” IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 19, NO. 5, SEPTEMBER 2004, 1205‐1213

28

Droop control Implementation: impedance emulation

• The multiloop approach can be modified into finite‐output impedanceemulationemulation

• The aim is to control the virtual impedance on the grid side of theconverter in order to optimize the power sharing of different units inparallel or to provide hot‐swap capability

1Li 2L gLi

+

-

1Li

dcvC-

+v fC

+

-Cv

2L ggi

+

-e

v

29

Conclusions

• Three operation modes: grid forming, grid feeding or grid supporting

A lt t l i d t f hi h WTS• Ac voltage control is mandatory for higher power WTS

• Dc voltage control can be implemented using a cascade structure

P t l th h V lt i t d t l di t• Power control through Voltage oriented control or direct powercontrol (with or without virtual flux)

• Droop control for voltage amplitude and frequency controlDroop control for voltage amplitude and frequency control

30