Intro + 1st

Marco Liserre [email protected]

Grid-connected PWM voltage source converters: opportunities and challenges

Control of PWM converters for renewable energy systems

Marco Liserre

[email protected]



Aims, pre-requisites, teaching methodsGrid-connected PWM converters are gaining increasing importance in view of a growing contribution of Distributed Power Generation Systems (DPGS) to the total power flow in the European electric utility. This is also owed to an increasing inflow from Renewable Energy Sources (RES). The course reviews some of the most important aspects related to the advanced control of grid-connected PWM converters with attention paid to DPGS based on RES.Pre-requisites

Basic power converters. Control theory

Lectures, supported by projector and blackboard, personalized feedback and coaching to improve every aspect of the student's work. Slides and exercises will be available at http://www.tf.uni-kiel.de/etech/LEA/?a=links the day before the lecture. Slides will be available in printed version the day of the lecture for students.

Teaching methods




Course contents

Grid-connected PWM voltage source converters: opportunities and challenges Overview of Distributed Power Generation Systems (DPGS) and Renewable Energy Systems (RES) Grid requirements to connect DPGS based on RES

Review of modulation and basic control system, harmonic compensation Grid filter design and stability of the current control loop Grid converter operation (dc and ac control loops) Grid synchronization Grid Converter control and future functions Modulation and control for cascaded multilevel converters Non-linear control

Power Converters for Distributed Power Generation Systems

Control of the grid-connected power converter




Course contents

Anti-islanding techniques for small DPGS Control of Grid Converters Under Grid Faults (Low Voltage Ride Through- LVRT) Micro-grid operation Droop control HVDC, STATCOM, Active filter

Modulation, PI control and P+res control, Harmonic control LCL-filter: stability issues Synchronization of the converter Cascaded control of grid converter Anti-islanding LVRT

Control of DPGS

Exercises (computer simulations)




Course contents

LCL-filter stability problems STATCOM operation of the grid converter to support the grid voltage

Exercises (laboratory)

Expected knowledgeKnowledge of the main issues related to power conditioning in DPGS based on renewable energy systems, function of the grid converter

Examination methodOral based on a presentation of a research described in a scientific paper. A general knowledge of the course contents is expected


Course assistant Dipl. Ing. Jörg Dannehl [[email protected]]



Bibliography1. N. Mohan, T. M. Undeland and W. P. Robbins, “Power Electronics: Converters, Applications, and

Design” Wiley, 2002, ISBN-10: 04712269392. B. Bose, “Modern Power Electronics and A.C. Drives”, Prentice Hall, 2001, ISBN 013016743.3. D.G. Holmes and T. Lipo, Pulse Width Modulation for Power Converters : Principles and Practice,

2003, ISBN 0471208140.4. M. P. Kazmierkowski, R. Krishnan, F. Blaabjerg, “Control in Power Electronics”, Academic Press,

2002, ISBN 0-12-40277205.5. J. Machowski, J. Bialek, J. Bumby, “Power System Dynamics: Stability and Control ” Wiley, 2008,

ISBN-10: 0470725583. 6. T. Ackermann, “Wind Power in Power Systems”. John Wiley & Sons, Ltd., 2005.7. 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, October 2006, vol. 53, no. 5, pp. 1398-1408. 8. R. Teodorescu, F. Blaabjerg, M. Liserre and P. Chiang Loh, “Proportional-Resonant Controllers and

Filters for Grid-Connected Voltage-Source Converters”, IEE proceedings on Electric Power Applications, September 2006, vol. 153, no. 5, pp. 750-762.

9. M. Liserre, R. Teodorescu, F. Blaabjerg, “Stability of Photovoltaic and Wind Turbine Grid-Connected Inverters for a Large Set of Grid Impedance Values”, IEEE Transactions on Power Electronics, January 2006, vol. 21, no.1, pp. 263-272. 10. P. Rodriguez, A. Timbus, R. Teodorescu, M. Liserre and F. Blaabjerg, “Flexible Active Power Control of Distributed Power Generation Systems During Grid Faults”, IEEE Transactions on Industrial Electronics, October 2007, vol. 54, no. 5, pp. 2583-2592.





Marco Liserre

[email protected]



Power Electronics Scenario

Multi-level converters (particularly diode-clamped and cascaded H-bridges) will be the most important

Multi-MW induction and synchronous motor drives now routinely use multi-level PWM converters (instead of traditional cycloconverters)

Converter

GTOs are already obsolete. IGBT and IGCT will compete

High voltage high power silicon carbide power devices will play important roles

Device




Intelligent control and optimal design are indispensable tools Controllers based on PWM will be the dominant technology (average-based

or on-off) The choice in high power system will be between the frequency-domain

approach or time-domain approach (predictive) and efficiency will be the driver

Diagnosis and fault-tolerant control will be a standard feature for high power converters

Optimal design and control

0

50

100

150 G.M.: 2.08 dB Freq: 2.07e+003 HzStable loop

Mag

nitu

de (d

B)

Open-Loop Bode Editor (C)

100

101

102

103

104

-400

-300

-200

-100

0

P.M.: 28.1 degFreq: 597 Hz

Frequency (Hz)

Pha

se (d

eg)

notch filter

*i

i

sTONt

k 1k 2k

ONt

sT

frequency shaping

predictivecontrol




Power electronics is revolutionizing the field of power engineering Voltage-fed multi-terminal HVDC will be very important FACTS and STATCOM will be very important for P and Q control Renewable energy systems (wind and photovoltaic) are becoming very

important

Utility applications

Grid-connected PWM voltage source converters will be the intelligent interface for loads, generation systems, storage systems and flexible

transmission

production

consumption



The Importance



The PWM grid converter, a kind of new synchronous machine ?

P

gI

V

LV

E

Q

The synchronous machine has a central role in the centralized power system

The “synchronous converter” major player in the future power system

Interfacing power production, consumption, storage and transportation within the future power system based on smart grids

Based on semiconductor technology and signal processing



The PWM grid converter frequency behavior

P

gI

V

LV

E

Q

The PWM grid converter is equivalent to multiple synchronous machines

The grid converter can control the active and reactive power flow in a vast frequency range

P

gI

V

LV

E

Q

P

gI

V

LV

E

Q

1 h n

. . . .

volta

ge

harmonic order

1

hn

PWM carrier and sideband harmonics



The Need



A glance to the distributed power generation

Future Power System

Less central power plants and more Distributed Power Generation Systems

Current Power System



A glance to the renewable energy systems

Rotor

Power conversion & power control

Power transmission

Gearbox (obtional) Generator

Power conversion

Power converter(obtional)

Power conversion & power control

Supply grid

Power transmission

Wind power

Mechanical power Electrical power

Electrical control

Power control

Pref Qref

Consumer

Wind systems require optimized grid converter at high power 3.6-6 MW prototypes running 2 MW WT are still the "best seller" on the market!




Doubly-fed is the most adopted soltion in wind systems

PM-synchronousGeneratorMulti-pole

Pitch

GridDC

AC

AC

DC

Pref Qref

Gear

Inductiongenerator

Pitch

DC

AC

AC

DC

Pref Qref

Gear

Doubly-fedinduction generator

Pitch

Grid

DC

AC

AC

DC

Pref Qref

Full power converter can be used either with asynchronous generator or synchronous generator (multipole permanent magnet gearless solution is the most promising)




PV PanelsString

dc-dcboost

LCLLow pass

filterC

VPV

IPV

+

-

L

N

Trafo&

Grid

Anti-IslandingProtections

Grid /PV plant Monitoring

Ig

Vg

dc-acPWM-VSI

VdcPWM PWM

MPPT

Active filtercontrol

Grid support(V,f,Q)

Ancillary functions

PV specific functions

Basic functions (grid conencted converter)

CurrentControl

VdcControl

MicroGridControl

GridSynchronization

Photovoltaic systems require high-efficient and multi-functional grid converters



A glance to the transmission system

Right Of Way (ROW) restrictions

Need of connection: distance between production and consumers, economics of scale, wider choice of generating plants, reduction in reserve capability, etc

Increase of power carrying capability vs transient stability



A glance to the transmission system

separate control loops active and reactive power active power control

one station controls the active power other station controls the DC-link voltage

reactive power control reactive power or AC side voltage

HVDC based on PWM grid converter offers . .



A glance to the power quality

STATCOMPCCV

LV

GV

q

Series and parallel active filters enhance grid power quality compensating voltage sag, harmonic, reactive power, etc .



A glance to the load demand

Active rectifier is adopted as active front-end for medium and high power systems like multi-drive systems and single drives working frequently in regenerative operation like cranes, elevators . .



The opportunities and challenges



The increase in the power leads to the use of more voltage levels:

Single-cell converter

Multi-cell converter

Design and Control challenges and opportunities:

Lower switching frequency

More powerful computational device

Solutions:

Non-linear analysis

Optimization with deterministic and stochastic techniques



Single-cell converter

Wind turbine systems: high power -> 5 MW converter

Photovoltaic systems: many dc-links for a transformerless solution

0V 0V0V

2dcV

2dcV

A B C predictive control to achieve the best control performance with

minimum commutation

advanced grid filter design to deal with a low switching frequency



Multi-cell converter

Many converters forming cells connected in series to share the power

Both for wind and photovoltaic solutions

Passivity-based control to manage the power transfer from each cell independently

Reliability study to optimize each component and the choice of the cell structure



Passivity-based control of a cascade converter



Dynamical test

Measured DC voltages [50 V/div] and grid current [4 A/div] (2330 F)

dc voltage reference step on one bus

dc load steps on the two buses leading to different loads



Modified Phase Shifted Carrier PWM

The original PSC-PWM angles can be obtained as a particular solution Asymmetrical PWM angles can be obtained dividing the obtained results

by 2

originalShifting angles =0º, 120º and 240º

modifiedShifting angles =0º, 36º and 191º

1 1

01201

2122cos1cos214cos)(

m n

N

iic

dcin

N

i

dci mtntmVnmMmJ

mtVMtv

The different dc voltages can be managed using a proper modulation



Grid monitoring: detection and synchronization

Current control: harmonic rejection and stability

Micro-grid management and grid support: power control strategies

Phase Detector

LoopFilter

VoltageControlledOscillator

fvvvdv

converter

BUS 2 BUS 3

BUS m BUS m+13Ø

3Ø

grid interaction

BUS n BUS n+1

2Ø

systembacktobackWT )( parkwindofpartusually

BUS 1

0

50

100

150 G.M.: 2.08 dB Freq: 2.07e+003 HzStable loop

Mag

nitu

de (d

B)

Open-Loop Bode Editor (C)

100

101

102

103

104

-400

-300

-200

-100

0

P.M.: 28.1 degFreq: 597 Hz

Frequency (Hz)

Pha

se (

deg)

notch filter

Main topics



Grid monitoring: detection and synchronization



Detection of grid conditionsislanding detection

Test to verify the detection of the islanding condition in a short time

Test to verify immunity of the method (no false trip) to frequency variation



Synchronization

vk

v v

qv

SOGI-QSG

qv

/ dq

PI

v

SRF-PLL

ff

qv

dv

Synchronization will be crucial for all the grid connected inverters to adapt their behavior in any grid condition

Single PLL based on a second order integrator acting as a sinusoidal follower is the building block of a class of advanced synchronization methods

2 2( ) ( )v k sD s sv s k s



Synchronization Detection of the positive and negative sequences will be important

during grid-faults Three-phase system synchronization needs a vectorial approach and

a dual PLL

v

v

v

v

qv

v

qv

v

v

v

v

12

12

DSOGI

PNSC

v

v

SOGI-QSG()

e

qv’ v’

w’

v

e

qv’ v’

w’

v

SOGI-QSG()

PI[Tdq]

qv

dv

v

SRF-PLL ff

2 2q d qv v v

2

2 22o

o os s

qv



Current control: harmonic rejection and stability



Current Control and LCL-filter

harmonic limit

5th 5-6 %

7th 3-4 %

11th 1.5-3 %

13th 1-2.5 %

1.IEEE Std 1547-2003 "IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems", 2003.

1.IEC Standard 61000-3-6, “Electromagnetic Compatibility, Assessment of Emission Limits for Distorting Loads in MV and HV Power Systems”, 1996.



Using Multiple Synchronous Reference Frames (MSRFs)

Harmonic rejection

d7

q7

d5 q5

i

i

i

i

5je

7je

5

7

Using selective filters based on resonant controllers

7,5,322 2

2)(

h c

cihh

hss

sKsG



Hybrid solution



Repetitive current control

1

0

2 2cosh

N iDFT ai h N

F z h i N zN N

i

i iG

e

pG*i i 'icG

DFTF FIRk hi

RepF z

The resonant controller can track a sinusoidal signal, a repetitive controller can track a periodic signal

The control action should be limited



Rejection of grid voltage background distortion

no harmonic control

harmonic control



1 1 1

22 2 1

33 2 1 2 3 1

2 2 44 2 1 3 2 2 3 1 2 4 1

2 2 3 55 2 1 4 3 1 3 3 1 2 4 1 2 5 1

2

2 3

2 3 3 4

t L i t

t L i t

t L i t i t L i t

t L i t i t L i t L i t i t L i t

t L i t i t L i t i t L i t i t L i t i t L i t

Rejection of harmonics caused by non-linearitiesThe frequency behaviour of the non-linear inductance can be studied splitting the model in a linear part and a non-linear part in accordance with the Volterra theory.

The Volterra-series expansion of the flux is 5

1i

i

t t

v e

L1 ii1

1 1

1

,...,n nn

i ii

L

2 12

1

ii

L

3 1 2

31

,i ii

L

non-linear inductance



flux spectrum of the non-linear inductance

input current at ω1= 50 Hz

input current at ω2= 150 Hz

input current at (ω1 + ω2 )

When two sinusoids of different frequencies are applied simultaneously intermodulation components are generatedThey increase the frequency components in the response of the system and the complexity of the analysis

Volterra-series expansion inductor model

5

5 5 5 11 1 1 1 1 110 5 3 5

16I

i t I sen t sen t sen t sen t



Grid current with non-linear inductor and resonant controller: a) (1) grid current [10A/div]; (2) grid voltage [400V/div]; (A) grid voltage spectrum [10V/div]; (B) grid current spectrum [0.5A/div]; (C) a period of the grid voltage; (D) a period of the grid current; b) a period of the grid current (simulation results) [10A/div].

High current non-linearityresonant controller repetitive controller

THD= 8.1%

Grid current with non-linear inductor and repetitive controller: a) (1) grid current [10A/div]; (2) grid voltage [400V/div]; (A) grid voltage spectrum [10V/div]; (B) grid current spectrum [0.5A/div]; (C) a period of the grid voltage; (D) a period of the grid current; b) a period of the grid current (simulation results) [10A/div].

THD= 4.9%

a

b

a

b



101

102

103

104

105

-70

-60

-50

-40

-30

-20

-10

0

frequency (Hz)

magnitude (Db)

L1

L1+L2

LCL

swres

LC

sw

swg zhihi

22

2

ripple attenuation

Grid converters connected through an LCL-filter

vi



Passive damping

L1L2

Cf

Rd

vvce

iig ic

2

21

2

2

2

2

1

1)()(

resdT

LCd

sLLRLs

zsLR

s

sLsvsi

As the damping resistor increases, both stability is enforced and the losses grow but at the same time the LCL-filter effectiveness is reduced.

Frequency [Hz]

-50

0

50D(z)G(z)

D(z)Gd(z)

Magnitude [dB]

102 103

0 100 200 300 400 500-10

0

10

0 50 100 150 200 250 300 350 400 450 500-15

-10

-5

0

5

10

15



Active damping The aim is to shape the harmonic spectrum around the resonance frequency

GAD

Gf

z-1GAD Gf



Genetic algorithm active damping

-1.5 -1 -0.5 0 0.5 1 1.5-1.5

-1

-0.5

0

0.5

1

1.5

-1.5 -1 -0 .5 0 0.5 1 1.5-1.5

-1

-0.5

0

0.5

1

1.5optimal position of the poles optial position of the poles

final result of GA

4 3 24 3 2 1 0

4 3 24 3 2 1 0

d i

a i z a i z a i z a i z a iD z

b i z b i z b i z b i z b i

GA optimize this controller



Comparison with the non-linear Levenberg-Marquardt optimisation method already used for passive damping design

The non-linear least-square method finds a point characterized by 1.12 while the absolute minimum is 0.92

0.92

1.12

Comparison with non-linear optimisation method



Micro-grid management and grid support: power control

strategies



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)



Droop control for grid forming or supporting 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



Instantaneous Active Reactive Control (IARC)

• Distorted and unbalanced current • Instantaneous power perfectly controlled• Overcurrent trip risk

P&Q Control Strategies Under Grid Fault

*2

*2

;

;

p

q

Pg g

Qb b

i vv

i vv -1

0

1

2

p, q

[kW

, kva

r]pq

*2 2

*2 2

;

;

p

q

Pg g

Qb b

i v vv v

i v vv v

-10

-5

0

5

10i [

A]

Positive- Negative-Sequence Compensation (PNSC)

*2

*2

;

;

p

q

PG G

QB B

i vv

i vv -40 -30 -20 -10 0 10 20 30 40 50 60

-10

-5

0

5

10

t [ms]

i [A

]

Balanced Positive Sequence (BPS)

• Active or reactive power without oscillations

• Bothe active and reactive power with oscillations



Conclusions Key drivers

Technology for different country voltages/frequencies and codes/standards

Avoid bulky transformers, reduce part count, increase efficiency Stability of new power systems based on DPGS

Major challenges are: Synchronization with the grid Stability of current and voltage loops Proper harmonic control Detecting the grid disconnection without communication Managing soft re-connection to the grid Micro-grid control and optimum management of energy storage

Intro + 1st

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