ECE 636– B2

Dynamics and Control of Voltage Source Converters (VSCs)

Winter 2013

Instructor: Dr. Yasser Mohamed Electrical and Computer Engineering Dept. University of Alberta Office W2-014 – ECERF email: yasser2@ualberta.ca

1

Lecture 1 – Introduction

Outline

• What is a Voltage Source Converter?

• The importance of Voltage Source Converters in modern energy systems

• Course Organization

2

What is a Voltage Source Converter? A Voltage Source Converter can be seen as a

Flexible Power Electronic-Based Voltage Source

- Similar to a synchronous machine, which is is a key component in power systems, VSCs are key components in future distribution and power system. A VSC can be seen as flexible synchronous machine (motor or generator). 3

gI

V

LV

E

Two-way power flow

fRgLi fL

fC

gRgidcV

DC

Sy

stem

PWM

v+

-

e+

-

Grid

AC system

AC-side filter (LC)

VSC

Control

signal

))()(cos()( ttttVv mc

Control and signal

processing

MeasurementsMeasurements

Semiconductor

switching cells

vo

lta

ge

harmonic order

1

h n

Pulse width modulation

(PWM)carrier and sideband

harmonics

What is a Voltage Source Converter?

The frequency behavior of VSCs

- - The VSC can operate at wide range of frequencies, multiple

frequencies can be intentionally included in the control signal.

1 h n

. . . .

Unwanted harmonics!

Fundamental

component Injected harmonics

(for specific control

functions)

4

gI

V

LV

E

gI

V

LV

E

gI

V

LV

E

The importance of VSCs Flexible AC Transmission Systems (FACTs)

High voltage DC (HVDC) transmission system based on VSC ABB – HVDC Light up to 1,200 MW at ± 500kV dc (modular VSCs)

VSC HVDC system is utilized in - Connecting wind farms to power grids - Underground power links - Connecting asynchronous grids - City center in-feed - Increasing power carrying capability versus transient stability - “Firewall” connection in large AC systems

Rectifier

station

Inverter

station

5

The importance of VSCs

High voltage DC (HVDC) transmission system based on VSC

330 MW HVDC light converter

station.

Source: ABB 6

The importance of VSCs

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 . .

7

The importance of VSCs

Multi-terminal VSC-based Network 8

Average

Model of

Ideal

Three

Phase VSC

Vnull

PLL

abc to dq ρ ρ

Vsq

Vsd

R+ron ia

ib

ic

L

L

L

R+ron

R+ron

+Vsa-

iqid

ia ib icmcmbma

Vta

Vtb

Vtc

iLoss C

iDCiext

-

V

DC

+

PextPDC

Pt Ps and Qs

+Vsb-

+Vsc-

VSC-m

Average

Model of

Ideal

Three

Phase VSC

Vnull_2

PLL

abc to dq

ρNVsq

Vsd

R+ron ia

ib

ic

L

L

L

R+ron

R+ron

+Vsa-

iqid

ia ib icmcmbma

Vta

Vtb

Vtc

iLoss

iDC

-

V

DC

+

Ps and Qs

+Vsb-

+Vsc-

VSC-PQ

K (s)+-

Id _ ref V DC2

_ ref

++

G-1

i (s)

P f (s ) G N -i (s)e(t)

(.)2

V DC _ ref

(.)2

V DC _ ref

Id _ ref

+-PI

Controller

I d

V sd

Iq _ ref PI

Controller

I q

V sq

+-

-2 / Vdc

2 / Vdc

m d

m q

K (s)+-

V DC2

_ ref

++

G-1

i (s)

P f (s ) G N -i (s)e(t)

(.)2

V DC _ ref

(.)2

V DC _ ref

Ps

Id _ ref

+-PI

Controller

I d

V sd

Iq _ ref PI

Controller

I q

V sq

+-

-2 / Vdc

2 / Vdc

m d

m q

dq to abc

m qm d

m cm bm a

Sequence Analyzer

and

Phase - Locked - Loop ( PLL )

V sa V sb V sc V ta V tb V tc

V sd+ V sd

- V sq+ V sq

- V td+ V td

- V tq+ V tq

-

Dual - Sequence Current -

Command Generator

Id- Iq

+ Iq-

Qs _ ref

Proposed DC -Voltage Control System

Zone I: VSC-m: DC-Voltage Power Port connected to Grid1

L1

CLVL

S1

S2

iL1iESS

Average

Model of

Ideal

Three

Phase VSC

abc to dq ρ

ron

ron

ron

iqid

ia ib icmcmbma

CW

PMSG

CH

Zone II: PQ VSC

connected to grid 2

Zone IV: DC Micro-grid with

Energy Storage System (ESS)

Id_ref

+-Current

Controller

Id ωLs

Σ

Vsd

Iq_ref Current

Controller

Iq ωLs

Σ

Vsq

+-

-2/Vdc

2/Vdc

md

mq

Wind Turbine Current Controller:

VL

VL_Ref+-

PI

ControllerPWM

S2

iL1

iL_Ref+-

PI

ControllerPWM

S1

S1=0

S2=0

Bidirectional DC-DC Converter:

Voltage Controller; buck mode

Current Controller; boost mode

Ccable

Grid 1

Grid 2

PDCPt

DC Energy

Pool

Main

Breaker

VCap_Charger

Interlocked

breaker #1

ρωL

ωL

Zone III: Full-Scale Wind Turbine

with AC/DC VSC Positive-sequence

current controller

Dual Sequence Current

Controller

Ps_ref

and

PLL

Vsa Vsb Vsc Vta Vtb Vtc

Vsd+ Vsd

- Vsq+ Vsq

- Vtd+ Vtd

- Vtq+ Vtq

-

Dual-Sequence Reference-

Current Generator

Id+ Id

- Iq+ Iq

-

Qs_ref

Sequence Analyzer

Rcable

Lcable

Rcable

Lcable

Rcable

Lcable

Rcable

Lcable

RDC

Mode 2:

Buck Mode

Mode 1:

Boost Mode

+

VL

-

Σ

Σ

VSC-W

Energy

storage

DC-micro-

grid load

ρN

The importance of VSCs VSC STATic Compensator (STATCOM) for reactive

power compensation or voltage regulation

Flexible wide range capacitive/inductive reactive power control AC voltage control at the point of common coupling

to support the transmission system

9

STATCOMPCCVLV

GV

q

Holly STATCOM – Austin –Texas – Source: ABB.

The importance of VSCs VSC STATic Compensator (STATCOM) for reactive

power compensation or voltage regulation

10

Field measurement of STATCOM response during a transmission line fault (Austin,Texas, 2004): a) Three VSC phase currents b) DC voltage c) VSC reactive current reference calculated by the control system d) 138 kV system line-to-ground voltages.

The importance of VSCs

Distributed Power Grids

11

Future Power System

Less central power plants and more Distributed Power Generation Systems

VSCs are the main interfacing medium for renewable/clean generation units and modern loads

Current Power System

The importance of VSCs

Interfacing Renewable/Clear Energy Resources

12

Rotor

Power conversion &

power controlPower 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 New turbines go up 7.5 MW with full scale power converter (VSC)

The importance of VSCs

Interfacing Renewable/Clear Energy Resources

13

Doubly-fed is the most adopted soltion in wind systems

PM-synchronous

Generator

Multi-pole

Pitch

GridDC

AC

AC

DC

Pref Qref

Gear

Induction

generator

Pitch

DC

AC

AC

DC

Pref Qref

Gear

Doubly-fed

induction 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)

The importance of VSCs

Interfacing Renewable/Clear Energy Resources

14

PV Panels

String

dc-dc

boost

LCL

Low pass

filter

C

VPV

IPV

+

-

L

N

Trafo

&

Grid

Anti-Islanding

Protections

Grid /PV plant

Monitoring

Ig

Vg

dc-ac

PWM-VSI

VdcPWM PWM

MPPT

Active filter

controlGrid support

(V,f,Q)

Ancillary functions

PV specific functions

Basic functions (grid conencted converter)

Current

Control

Vdc

Control

MicroGrid

Control

Grid

Synchronization

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

The importance of VSCs

15

Power Quality Conditioners A

CT

IVE

FIL

TE

R

load

AC

TIV

E

FIL

TE

R

load

LVGV

Series and parallel (or hybrid series/parallel) active power conditioners enhance grid power quality compensating voltage sag, harmonic, reactive power, etc .

The importance of VSCs

16

Converter Interfaced Loads

PCCVLVGV

ACTIVE

RECTIFIER

p

load

load Active rectifier is adopted as

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

Motor drive load

The importance of VSCs

17

Converter-Based Machine Drives Loads

Induction motor drive

The importance of VSCs

18

Converter-Based Machine Drives Loads

Permanent-magnet motor drive

The importance of VSCs

19

Distributed Generation Micro-grids AC Micro-grid

Local-generation to meet the load demand Higher efficiency and power quality High reliability by providing backup power during utility supply outage

The importance of VSCs

20

Distributed Generation Micro-grids

Hybrid AC/DC Micro-grid

Vin Iin

Cdc

Lf Rf

Cf

vo io

Utility Grid

L g Rg

i

Lf R f

Cf

vo iov i

Feeder Feeder

AC-

Loads

AC-Bus

AC-BusVSC-1

VSC-2

DC Bus

Microturbine

Fuel Cell

Storage

dc-load Direct

dc-load

DC Micro-grid

Utility Grid

Interface

AC micro-grid

Interface

vg

Islanded Micro-grid

Utility Grid interfacingDG Park

Idc

DC Bus

v

Common

DC-Link

PMSM

M

Vdc

SW

Yload1

Yload2

Yload3

Ysource Idc1

Idc2

Idc3

The importance of VSCs

21

Hybrid Electric and All-Electric Vehicle

High efficiency energy conversion by VSCs

Two-way power flow (motoring, generation)

Fast low-level control for robust and fast high-level energy management and control

Course Scope and Content Voltage source converters (VSCs) are gaining increased acceptance and importance in energy systems. This course presents key Modeling, Analysis and Control aspects of VSCs. Key topics include: 1) DC/AC Half-Bridge Converter 2) Control of DC/AC Half-Bridge Converter 3) Space Phasors and Two-Dimensional Frames 4) Two/Three-Level 3-phase VSC 5) Control of Grid-Connected VSC in Stationary Reference Frame 6) Control of Grid-Connected VSC in Rotating Reference Frame 7) Control of VSC in Autonomous Mode (isolated micro-grid mode) 8) Variable-Frequency Control of VSC 9) Applications: 1) Static Compensator (STATCOM) 2) ) HVDC 3) Distributed Generation Micro-grids 4) Active Power Filters 5)Wind Energy Conversion Systems

Course Organization

22

Course Objectives

Course Organization

• Review fundamentals (circuit construction, principle of operation and modeling) of half-bridge, 2-level VSCs and multi-level VSCs

• Understand different operation modes of VSCs (current-controlled, voltage-controlled, grid-connected, isolated micro-grid, variable frequency, etc.)

• Understand and implement switched and average models of VSCs

• Understand and design simple and effective control algorithms for VSCs

• Understand VSC converter operation and control in key applications

23

Course Prerequisites

Course Organization

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• Basic understanding of power electronic converters, power systems and control theory.

• Familiarity with a programming language and/or a simulation package such as MATLAB/Simulink or EMTDC/PSCAD.

A. Yazdani and R. Iravani, Voltage-sourced converters in power systems, Modeling, Control, and Applications, John Willy, 2010, ISBN 978-0-470-52156-4.

ECE636 Lectures Notes (slides). Will be available on eClass.

Main Teaching Materials

Other References

Course Organization

25

M. P. Kazmierkowski, R. Krishnan, F. Blaabjerg, “Control in Power Electronics”,

Academic Press, 2002, ISBN 0-12-40277205. B. Bose, “Modern Power Electronics and A.C. Drives”, Prentice Hall, 2001, ISBN

013016743 Background references: N. Mohan, T. M. Undeland and W. P. Robbins, “Power Electronics: Converters,

Applications, and Design” Wiley, 2002, ISBN-10: 0471226939 B. Kuo, Digital Control Systems., Oxford University Press, New York, NY, USA,

1992. Chi-Tsong Chen, Linear Systems Theory and Design, - Oxford University Press,

1999 – ISBN 0195117778

Reading Assignment (25%)

Simulation Project (25%)

Final Exam (50%)

Grading Scheme

Course Organization

26

Reading Assignment (25%)

A well-written report detailing a comprehensive critical survey on an application related to VSCs. Report should be based on a fair number of IEEE journal papers (at least 10~15 papers) related to the reading topic.

Simulation Project (25%)

Reproduction of results reported in a journal paper by simulation. One of the papers used in the reading assignment can be used for a simulation project. Softcopy of developed models should be submitted with your simulation project report.

Final Exam (50%)

Based on a good understanding of course topics; this is a grad-course!

Grading Scheme Description

Course Organization

27

One page Abstract on proposed reading topic and project for Instructor’s approval: Jan. 31, 2013

Reading Assignment Report and Simulation Project Report: April 18, 2013

20-minute presentation summarizing Reading Assignment and Simulation Project findings: (Last 3 lectures)

Final Exam: Date: TBD

Important Dates

Course Organization

28

Grid interfacing system for photovoltaic arrays

Doubly-fed induction generator-based wind turbine system.

Full-scale permanent-magnet synchronous generator-based wind turbine system

Grid interfacing system for fuel cells

Grid interfacing system for micro-turbines

VSC-based high voltage DC transmission (HVDC) system

Multi-level VSCs in renewable energy applications

Multi-converter distributed generation AC micro-grid

Multi-converter DC micro-grid

Parallel operation of VSCs

Fault-ride-through control of doubly-fed-based wind turbines

Reading Assignment/Simulation Project Topics

Course Organization

29

VSC-based unified power quality conditioner

VSC-based dynamic voltage restorer

Coupled inductor VSCs

Modeling and real-time simulation of VSCs

VSC for energy storage applications

VSC-based STATCOM (or distribution STATCOM – DSTATCOM)

Mechanical-sensorless VSC-fed induction motor drive

Mechanical-sensorless VSC-fed permanent-magnet synchronous motor drive

Flexible VSC-based distributed generation

VSC versus Z-source converter

VSC versus current source converter

Reading Assignment/Simulation Project Topics

Course Organization

30

Notes: Reading Assignment/Simulation Project

Cannot be something that you have already done.

Must have a strong VSC modeling and control components.

New ideas and justified improvement suggestions have higher weights.