Activities on power electronics in power systems at Chalmers

Chalmers University of Technology

Activities on power electronics in power

systems at Chalmers

Massimo Bongiorno

Department of Energy and Environment




Two main campuses

17 Departments and 8 Areas of

Advance

Motto: Avancez


Division of Electric Power Engineering at Chalmers

45 (25 Ph.D Students)

Generation

Transmission

Consumption

Work with energy flows, where at least

on part is electricity


Our interaction with society


Division of Electric Power Engineering at Chalmers

One vision: contribute to decreased life cycle cost in sustainable energy

production, distribution and usage

Power electronics in

power systems

Wind power

Electrical machines and drives

E-mobility

Four main research areas


The real breakthrough in the electric power systems

AC transmission was first demonstrated at an

exhibition in Frankfurt am Main 1891

170 kW transferred 175 km from

Lauffen hydropower station to the

exhibition area at 13000-14700 V


The real breakthrough in the electric power systems

The AC transmission system is based on several key inventions that

have been implemented during a few decades only

Transformer

(Blathy, Déri, Zipernowsky, 1885)

Synchronous

machine

(Tesla, 1884)

Asynchronous

3-phase motor

(Ferraris, Dobrowolsky, 1891)

Three-phase systems

(Jonas Wenström, 1889)

Practical handling

of high voltage

(Brown, 1891)


How will the future power system look like?

Today’s electric power system Tomorrow’s electric power system

Bulk dc

import/export

Connecting ac system

Traditional

power plant

Industrial plant

ac

dc

Distribution feeder

How should the electric power system

change in order to become fully

sustainalbe, efficient and at the same

time be available and reliable?


Vision of the group

Tomorrow’s electric power systems must drastically change in order to become

sustainable and efficient, meeting at the same time the changing societal

challenges.

Power electronic technologies in electrical energy generation, transportation and

consumption will play a key role in enhancing the system feasibility, safety,

availability, reliability and efficiency.

Chalmers will be in the lead of this process by proposing, investigating and

evaluating new solutions for reinforcing or supplant today’s technologies and

infrastructures.


The way forward

Bulk dc

import/export

Connecting ac system

Traditional

power plant

Industrial plant

ac

dc

Distribution feeder

1879 1891 Today Tomorrow?

How has the power systems evolved over the years?


The way forward

Beijing3000MW

Kangbao1500MW Fengning

1500MW

Yudaokou

Zhangbei1500MW

Tangshan

Current projects

in China

Proposed dc-transmission between

Trollhättan and Hamburg/Berlin (1909)

Is this too futuristic?


Power electronics will play a key

role in the future power system


Example of future power grid


Two main research tracks

Research on converter technology

Research on converter application and system analysis

Aim is to conduct research on power converters, with main focus on converters for

grid applications. Main topics of research are:

• Multilevel converter topologies for grid applications

• Converter control and modulation

• Emerging technologies (such as SiC) and their impact on the future converter

topologies

Aim is to conduct research on the application of power converters in power

systems and analysis of grids with high penetration of power electronics. Main

research topics are:

• Control (outer control loops) of grid-connected power converters

• Investigation of new functionalities for power converters

• Stability studies

• Hybrid ac/dc grids


Our small team

Ehsan Behrouzian

PhD student

Panagiotis Asimakopoulos

PhD student

Selam Chernet

PhD student

Nicolas Espinoza

PhD studentJoachim Härsjö

PhD student

Gustavo Pinares

PhD studentGeorgios Stamatiou

PhD student

Mebtu Beza

PostDoc

Massimo Bongiorno

Btr. Professor


Research in VSC technology


Specification priorities in VSC technology

Size/weight

Every system that we deal with has to consider five basic design parameters

Cost

EfficiencyPerformance

Reliability

OBS! ”Performance” includes

controllability, dynamic response, EMC

and harmonic output


CHB Converters for STATCOM Applications

phase cphase a

To VSC’s ac terminals

phase b

Star-connected Cascaded H-Bridge (CHB) Converter

Electronic

ac supplyFilter

Control computer

H-bridges

Y/D selector



phase cphase a

To VSC’s ac terminals

phase b

Star-connected CHB converter

Advantages :

Modular design

Reduced switching losses

Allows transformerless operation

Reduced harmonic pollution

Disadvantages:

Increased number of components

Problematic capacitor balancing

Reduced controllability under

unbalanced conditions


Modular Multilevel Converters for grid applications

Two configurations available in the market

phase cphase a phase b phase cphase a phase b

Star-connected MMC Delta-connected MMC



The problem of capacitor balancing under unbalanced conditions

Star-connected MMC

va vb vc

ia ib ic

v0Uneven power distribution among the three phases!



The problem of capacitor balancing under unbalanced conditions

Star-connected MMC Delta-connected MMC

va vb vc

ia ib ic

v0

va vb vc

ia ib ic

i0

Zero-sequence injection is needed to guarantee capacitor balancing


CHB Converters for STATCOM Applicationsva vb vc

ia ib ic

v0

va vb vc

ia ib ic

i0

Currents in phase

Currents in opposite phase Voltages in opposite phase

Voltages in phase


CHB Converters for STATCOM Applicationsva vb vc

ia ib ic

i0

Voltages in opposite phase

Voltages in phase

What does this means?

Measured single-phase fault

Measured double-phase fault



What does this means?

va vb vc

ia ib ic

v0

Currents in phase

Currents in opposite phase

We need more converter topologies/configurations!

VDE, “E VDE-AR-N 4120:2012-11 Technische Bedingungen für den

Anschluss und Betrieb von Kundenanlagen an das Hochspannungsnetz.”

.


Research in VSC integration and system analysis


Example: power oscillation damping using STATCOM

Mebtu Beza – Power System Stability Enhancement Using Shunt-connected Power Electronic Devices

with Active Power Injection Capability

generatorStiff AC

Bus 1Pg

Bus 2

E-STATCOM

sourceSynchronous

Uniform damping can be achieved

by proper control of active and

reactive power


Example: power oscillation damping using STATCOM

Mebtu Beza – Power System Stability Enhancement Using Shunt-connected Power Electronic Devices

with Active Power Injection Capability

generatorStiff AC

Bus 1Pg

Bus 2

E-STATCOM

sourceSynchronous

Classical (blue) vs proposed (red) control strategy

time [s]

tran

smit

ted

pow

er[p

u]

Ener

gy [

kJ]


Risk for controller interaction in the ac grid

High penetration of controllable objects can lead to unwanted phenomena in

the grid

Example: subsynchronous resonances in wind farms. First world incidend: Zorillo Gulf

Wind,Texas 2009

345 kV series

compensated lines

Edinburg

Rio Hondo

circuit breaker

Nelson

Ajo

Lon Hill

33% series compensation 17% series

compensation

93 MW

96 MW


Investigated system and modeling approach

Gear

Box DFIG

Transformer

Power Electronic Converter

udc

iR if

vf

is

Cdc

vs

+

_

Pout, Qout

PR Pf

vR Eg

infinite bus

XT RL XL Xc

WT

(s)GZ (s)LY

WT WT

WT WT WT

WT WT WT

ig iL

vg

gvD

sZG

sYL

*iD

iD


Analysis approach

Nyquist Stability Criterion

u y sG

sH

𝑇𝑐𝑙 (𝑠) =𝐺 𝑠

1 + 𝐺 𝑠 𝐻 𝑠

jω

gvD

sZG

sYL

*iD

iD

Analysis

Closed-loop approach (eigen-value

analysis)

Open-loop approach (Nyquist)


Risk for controller interaction in the ac grid

Real (top) and imaginary part of open-loop transfer

function for 55% series-compensated line as a function

of closed-loop current controller bandwidth.

Transmitted active power. Top: acc=1 pu;

bottom: acc reduced to 0.5 pu

infinite bus

XT RL XL Xc

WT

(s)GZ (s)LY

WT WT

WT WT WT

WT WT WT

ig iL

vg


Converter interactions in VSC-based HVDC systems

dc-transmission

link

AC

fil

ters

VSC1AC

Grid

#1

AC

Grid

#2

VSC2Pin Pout

dc,1+

-

+

-

AC

fil

ters

dc,2Phase

reactor

Phase

reactor

Power flow direction

Direct-voltage

controlled converter

Active-power

controlled converter

Prated=1000MW

udc, rated=±320kV

dc-link length=300km

-1000

-500

0

P o

ut [M

W]

0

500

1000

P in [

MW

]

5 5.5 6 6.5 7 7.5 8 8.5 9 9.5

600

650

time [s]

d

c1 [

kV

]



Interactions might occur between the sending station (direct-voltage

controlled) and the remaining dc grid

Direct-voltage

control

Power control

VSC2VSC1

CL12

+e1 +e2

ac grid

ac bus 1 ac bus 2

VSC phase

reactorVSC phase

reactor

CL12

Pg1 Pg2

ac grid

-e1 -e2



New approaches for stability analysis

Direct-voltage

control

Power control

VSC2VSC1

CL12

+e1 +e2

ac grid

ac bus 1 ac bus 2

VSC phase

reactorVSC phase

reactor

CL12

Pg1 Pg2

ac grid

-e1 -e2

10-1

100

101

100

|G(j

)| (

pu

)

Frequency (pu)

dc-grid resonances

10-1

100

101

100

|F(j

)| (

pu

)

Frequency (pu)

appearance of resonance for largeincrease in power

10-1

100

101

0

5

10

15

Re[1

/F(j

)] +

Re[G

(j

)] (

pu

)

Frequency (pu)

: Stable system (Net-

damping above zero)

: Unstable system (Net-

damping below zero at

resonant frequencies)



New approaches for stability analysis

Direct-voltage

control

Power control

VSC2VSC1

CL12

+e1 +e2

ac grid

ac bus 1 ac bus 2

VSC phase

reactorVSC phase

reactor

CL12

Pg1 Pg2

ac grid

-e1 -e2

-1.5 -1 -0.5 0

-2

0

2

Real part (pu)

Imag

inary

part

(p

u)

0 0.1 0.2 0.3 0.4 0.5 0.60

0.1

0.2

0.3

0.4

Net-

dam

pin

g a

t

N [

pu

]

Damping factor of concerned poles

concerned poles

-1.5 -1 -0.5 0

-1

-0.5

0

0.5

1

Real part (pu)

Imag

inary

part

(p

u)

0 0.1 0.2 0.3 0.40

0.1

0.2

0.3

0.4

Net-

dam

pin

g a

t

N [

pu

]

Damping factor of concerned poles

concerned poles

Correlation between dominant

poles and net-damping

Top: increase of power flow;

bottom: increase of dc-cable length


Ongoing investigation: DC-side input admittance of MMC converter

in HVDC (direct-voltage controlled)


Investigation on damping properties of MMC

converter and comparison with two-level

converter

MC 1

MC 2

MC N

MC 1

MC 2

MC N

MC 1

MC 2

MC N

MC 1

MC 2

MC N

s

g

2

d

2

d

u

+

-

l

+

-

MC 1

MC 2

MC N

MC 1

MC 2

MC N

ui

li

L

R

R

L

Zs(s)

dci

PCC

s

f

sis

0P

MMCY

Include impact of

• Number of submodules

• AC Current Control

• Circulating Current Control

• Direct-Voltage Control

• Time-varying direct voltage

• AC-grid strength

• PLL dynamics


Risk for controller interaction in the dc grid

System modeling

2 4 6 8 10-4

-3

-2

-1

0

1

2

Frequency [pu]

Real part

of

F(j

) [p

u]

2 4 6 8 10-3.5

-3

-2.5

-2

-1.5

-1

-0.5

0

0.5

Frequency [pu]

imagin

ary

part

of

F(j

) [p

u]

VSC Based AC/

DC Terminal-1

VSC Based AC/

DC Terminal-2

VSC Based AC/

DC Terminal-3

VSC Based AC/

DC Terminal-4

3 phase grid

3 phase grid 3 phase grid

3 phase grid

External DSP

controlled

External DSP

controlled

Controller

inside RTDS

Controller

inside RTDS

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10.5

1

1.5

dc v

olta

ge [

pu]

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-2

0

2

Time [s]

Id c

urre

nt [

pu]

1 2 3 4 5 6 7 8 9 100

0.2

0.4

0.6

0.8

1

1.2

1.4

Frequency [pu]

real part

of

dc g

rid im

pendace [

pu]

Frequency response of one of the converters on the MTDC system

Resonance peak at the dc node where the converter is connected

Voltage and current at VSC1 for power increase. Instability

develops at VSC1


Electromagnetic Transient study of wind Farms

connected by HVDC

Offshore-WindParks in the Nordic See


The BorWin Offshore-WindFarm


However…


Study approach

Frequency dependent impedance models

should be developed for each component

(Wind turbine converters and HVDC

converters)

Useful method if detail model of

components is not easy to get. Moreover,

stability analysis will be easier as the

various subsystems can be studied

separately.

Control and system settings have

significant impact on the results.

DC cable

AC

fil

ters

Offshore

VSC

AC

fil

ters

Phase

reactor

Phase

reactor

Onshore

VSC

ACC Bus

AC cableWind

turbine 1

AC cableWind

turbine N

.

.

.

.

.

.

.

.

.

Wind FarmHVDC System

AC grid

System Nyquist plot in case of short (blue)

and long (ref) long AC cable


Risk for controller interaction in the dc grid

If an existing HVDC network shows an acceptable performance, will the

addition of a new element (converter, cable) influence the performance of it?


Which is the most suitable dc-grid topology?

AC Gridlevel

DC Gridlevel

AC Gridlevel

DC Gridlevel

AC Gridlevel

DC Gridlevel

AC Gridlevel

DC Gridlevel

Independent HVDC links Radial connection

Ring connection Meshed connection

The meshed connection seems to be the preferred solution both from

industries and from the research community



AC Gridlevel

DC Gridlevel

Meshed connection

Represents the dc replica of the

existing ac-transmission system

Interconnection of transmission lines in the ac grid gives:

increased reliability in power supply

effective use of power generation installations

Reduced power peaking

Allows utilization of uncontrolled energy sources (e.g. renewables)



What do we need for a meshed dc grid?

Good models and understanding of system dynamics

“FACTS kind” controllers (such as power-flow controllers)

Dc/dc transformers

Dc breakers

Are we sure that this is the way to go?

AC Gridlevel

DC Gridlevel

Meshed connection

Represents the dc replica of the

existing ac-transmission system


To sum up…

Tomorrow's electric power systems must drastically change in

order to become sustainable and efficient

Power electronics will play a key role in the future power system

The technology is changing…

We must have a system approach to understand the benefits and

possible pitfalls of high-penetration of power electronics in the

future power system


Thank you very much for your attention!

Activities on power electronics in power systems at Chalmers

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