Power Quality Enhancing Systems Industrial Electrical

Industrial Electrical Engineering and AutomationLund University, Sweden

Power Quality Enhancing Systems

© Mats Alaküla L7&L8 – Power Quality Enhancing Systems

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1. ObjectivesDiscuss the problems related to harmonics in the power system and present some methods to reduce the problems.

• Single-phase diode rectifiers and power factor correctors

• Three-phase diode rectifiers

• Controlled three-phase diode rectifiers: Minnesota and Vienna

• The voltage source converter

• Series and shunt active filters, the unified power flow controller

• Detection and identification of harmonic currents

• Effect of blanking time and forward voltage drop

• Multiple rotating coordinate systems

• Simulation and measurement results


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Acronyms

• APF – Active Power Filter• UPFC – Unified Power Flow Controller• SVC – Static Var Converter• HVDC – High Voltage Direct Current• ...

All made to improve ”Power Quality”


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Non-ideal loads

• are loads that:– are non-resistive -> consume reactive power– vary with time or phase -> consume harmonic

current components.– are different in different phases -> consume

negative sequence currents


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nWays to improve the loads

• Self improvement– Solutions that draw as “ideal” current as

possible from the grid• Compensation

– A parallel unit is used to counteract the non ideal currents drawn by the main load


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2. Single-phase diode rectifiers (I)A single-phase diode rectifier draws a line current with a large harmonic content since the diodes must be forward biased to conduct.

Lline

iline

eLN C

dcv

dc

D1

D2

D3

D4 0 0.005 0.01 0.015 0.02

−100

−75

−50

−25

0

25

50

75

100

t [s]

i line [A

]

0 300 600 900 1200 1500 1800 21000

5

10

15

20

f [Hz]

i line

,pk(f

) [A

]

0 0.005 0.01 0.015 0.02−400

−300

−200

−100

0

100

200

300

400

t [s]

v dc [V

]

eLN

−eLN


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2. Single-phase diode rectifiers (II)The current harmonics create voltage harmonics at the point of common coupling (PCC), i.e. at the point where other loads are connected. Note that all single phase diode rectifiersdraws current in the vicinity of the peak voltage.

+

-

V4

100

D11

D12 D14

D13

R16

1

1

1

1

CDC1m

0

Time

0s 2ms 4ms 6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms

V(R16:1)- V(R16:2) V(V4:+)- V(V4:-)

-200V

0V

200V

-325V

325V


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2. Power Factor Corrector (PFC)

22

0.01

0.01

+

-

V4

Driver

0.1

30

D12 D14

D13

L1

1000uH

D1

Z2

IXGH40N60

+

-

360V V3

10000+

- G=1E6U4R10

D11

+-

G=1U5

+-

G=0.1/325U6

0

0

Time

0s 2ms 4ms 6ms 8ms 10ms 12ms 14ms 16ms 18ms 20ms

I(L1)

0A

10A

20A

30A

SEL>>

V(V4:+)- V(V4:-)

0V

-325V

325V

Implemented with tolerance-band (or hysteresis) current controller.


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n3. Three-Phase diode rectifiers

Note that for three-phase diode rectifiers, only two phases carry current simultaneously. This means that each diode has a maximum conduction interval equal to 120 for each half period. Therefore a three-phase PFC is not possible to implement. However there are two types of controllable diode rectifiers derived from three single-phase diode rectifiers (which is also possible to implement):

• The Minnesota rectifier invented by Ned Mohan

• The Vienna rectifier invented by Johann Kolar


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3. The Minnesota Rectifier


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3. The Vienna Rectifier


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Load model

2

00

tj

ttj

t

eE

je

dteEdte ie

t

2t

d

q

e Loadi


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nReactive power

• A phase lag between the voltage and thecurrent:

• In flux coordinates:ie

t

2t

d

q

2ˆ2

3 jei

tjei2

3


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Assymetric load

• The phase currents are not equal inamplitude or phase lag ...

0 1 2 3 4 5 6 7-10

-5

0

5

10


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An assymetric load current vectorin the ( )-frame

2ˆˆ

3

2ˆ2

3,ˆ,ˆ

ˆ2

3

2

ˆ3

3

2

2

ˆ3

3

2,ˆ

2ˆ

2ˆ

2ˆ

3

2

2ˆ

2ˆ1

2ˆ

3

2

3

2

0

3

2

3

4

3

2

3

4

3

43

4

3

4

3

23

2

3

2

cosˆ

ctjctj

ctj

xcx

tjtjtj

tjtjxx

ctjctj

c

btjbtj

batjatj

a

jctjctj

c

jbtjbtj

b

atai

atjatj

a

eeiieiiiii

eieeei

ei

ii

eei

eei

eei

eee

ieee

iee

ii


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An assymetric load current vectorin the (d,q)-frame

!2

2322

!

22

2

23

2

2

2

2

ˆˆ6

1ˆ23

ˆ23

2ˆˆ

32ˆ

23,ˆˆ,ˆˆ

ˆ23,ˆˆ

withbackwardsRotating

tj

movingNot

j

c

j

j

tjtj

tj

c

tjtj

xcx

tjtj

xx

tjdq

cc

cc

eeiiei

ei

eeeiieeiiiii

eeiii

eii


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• Symmetricload

• Assymetric

M-file ”Load current vectors” - demo

-10 0 10-15

-10

-5

0

5

10

15Alfa - Beta

-10 0 10-15

-10

-5

0

5

10

15d - q

-10 0 10-15

-10

-5

0

5

10

15Alfa - Beta

-10 0 10-15

-10

-5

0

5

10

15d - q


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Harmonics

• Non-linear load impedance

0 1 2 3 4 5 6 7-20

-10

0

10

20


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5’th and 7’th harmonic example with Simulink

tjtjj

dq

tjtjtj

eieieii

eieieii

67

65

21

77

551

ˆ2

3ˆ2

3ˆ2

3

ˆ2

3ˆ2

3ˆ2

3


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AFActive currentReactive currentAssymetric currentHarmonic current

- Reactive current- Assymetric current- Harmonic current

Active current

Active filtering


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nVoltage Source Converters

R Lia

+Cdc

+- ea u1au1bu1c


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Types of Active Filters

Grid LoadiLine iLoad

iAF

Cdc

VSC

Lf

Cdc

Grid Load

VSC

iLine iLoadvAF

Shunt (top) and series (bottom) active filters.


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Active Filter Combinations

Load

ShuntPassive FilterShunt AF

Load

ShuntPassive FilterSeries AF

Load

Shunt AFSeries AF

vAF

iAF

iLine iLoad

Shunt and series hybrid active filters (top) andUnified Power Flow Controller, UPFC (bottom).


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Shunt Active Filter

A d d itio n a llo ad

L o ad tob e filte red

1d ci 2d ci

1i

ge

gL

1L

lo a di

1u

cpe

Cd cU

gi


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nAC side Current Control (I)

• Vector Control with Field Orientation

)(ˆ)(ˆ

)(

2

)(ˆ

)(2

)(1

01

*11

*1

11*1 kenini

T

RL

Tkiki

R

T

Lku cp

kn

ns

s

s


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AC side Current Control (II)

• Note the cross-coupling in the line model

• This cross-coupling should be incorporated in the current controller

cpqddqq

cpdqddddqcp

dqdqdqdq

eiLiRidtdLu

eiLiRidtdLu

eiRiLjidtdLu

11111111

11111111

11111111

)(ˆ)(ˆ)(2

)(ˆ)(

2

)(ˆ)(2

)( 1*

11

01

*11

*1

11*1 kekikiLjnini

TRLTkikiR

TLku cp

kn

ns

s

s


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DC link Voltage Control System (I)

sC1

sTs11

dcidcp sT

K.

.1

1*dcu dcu*

Ci

2dci

*1dci 1dci Ci

+

+ +

-

- -

Additionalload

1dci 2dci

1i1L 1uC

dcU


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• Transfer function from reference to actual DC link voltage

• Desired denominator polynomial

DC link Voltage Control System (II)

sC1

sTs11

dcidcp sT

K.

.1

1*dcu dcu*

Ci

2dci

*1dci 1dci Ci

+

+ +

-

- -

CTTKsCTKsTssTs

sUsUsG

sUsCsTsT

KsUsU

sdcidcpsdcps

s

dc

dcUdc

dcsdci

dcpdcdc

,,,23

23

*

,,

*

11

11

111

22 2 nnnn ssssA


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• Use Symmetric Optimum

• Note that for a = 2 the poles are placed as for a Butterworth filter, i.e. maximum flat pass-band. However, the bandwidth is by far to high for practical operation.

Controller Parameters ...

sdcidci

dcp

snn

TaTTCaK

aTaa

2.

.. ,

system)stablefor1Note(,2

1


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Convert DC to AC current references

*2

.

*1

.

*11

.1

1.11.11

112

121)(

Cdcdcqcp

dcdc

qcp

dcqq

dc

qcpdc

qqcpdcdcqqcpqq

dd

qd

iieui

euii

ue

i

ieiuieidtdiLi

dtdiLRiRitp


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DC link voltage controller

Additionalload

1dci 2dci

1i1L 1uC

dcU

*

*

..2

*1

11

Cdci

dcdcdci

dcpdccp

dcq uu

sTKi

eu

i


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Active filter control

C u rren t C o n tro l

Id en tif ic a tio n

G rid L o ad

A F


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nExample of DC voltage control

>> L=0.01;>> R=1;>> Ts=0.0005;>> Tidc=9*Ts;>> Kpdc=3*Cdc/Tidc;>> Cdc=1e-4;

0 0.02 0.04 0.06 0.08 0.1600

700

800

900Udc

0 0.02 0.04 0.06 0.08 0.1-20

0

20iq* & iq


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Example with active filtering

lo a di

lo a dL

lo a dR 0 0.01 0.02 0.03 0.04 0.05-500

0

500ua

0 0.01 0.02 0.03 0.04 0.05-20

0

20ia


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Filter Current References (I)

fdcdc

dcidcpdc

cp

dc

f

floadqerActiveFiltq

loadderActiveFiltd

Tsuu

sTKi

eu

TsTs

ii

ii

1111

1*

..2,

*,

,*

,

• Note that the q-direction current reference based on the load current is high-pass filtered since the active filter should not provide active power to the load

• Note that the DC link current reference is low- pass filtered since it is important that it does not contain any 100 Hz (twice fundamental) component which would result in a negative sequence component consumed by the active filter


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Filter Current References (II)

• To avoid that the DC link current reference contains a negative sequence component Tf 10 ms is a proper choice.

• To assure that the negative sequence current consumed by the load is provided by the active filter Tf 10 ms is a proper choice.

• Use Tf = 10 ms


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nFilter Current References (III)

• Since Tf = 10 ms >> Ts 100 s (typically) the converter dynamics do not have to be included in the selection of the DC link voltage controller parameters, i.e. Gi(s)1.

• However a new low-pass filter shows up in the transfer function, e.g. the one with time constant Tf .

• This means that the transfer function looks exactly the same but Ts is replaced by Tf .

• In this case a = 2 is a proper choice for the DC link voltage controller parameters.

• Note that a = 3 gives real poles since the closed loop damping of all poles is equal to 1.


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Filter Current References (IV)

• The rather long time constant of the low-pass filter (Tf)results in the need of a comparably large DC link capacitor (C) for converters used for active filters.

• In this case the DC side load current (idc2 ) is also filtered with Tf which increases the need of a large DC link capacitor. The DC side load current (idc2 ) should be fed forward without any low-pass filtering for proper operation.


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Filter Currents

>> L=0.01;>> R=1;>> Ts=0.00005;>> Rload=50;>> Lload=0.1;>> Tf=10e-3;>> Tidc=9*Tf;>> Kpdc=3*Cdc/Tidc;

0 0.01 0.02 0.03 0.04 0.05-20

0

20ia,load

0 0.01 0.02 0.03 0.04 0.05-10

0

10ia,ActiveFilter

0 0.01 0.02 0.03 0.04 0.05-20

0

20ia,grid


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Spectra

0 0.02 0.04 0.06 0.08 0.1-15

-10

-5

0

5

10

15Original signal (Blue) and resampled signal (Red)

0 200 400 600 8000

2

4

6

8

10

Frequency spectra with 10 Hz resolution

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1-20

-10

0

10

20Original signal (Blue) and resampled signal (Red)

0 100 200 300 400 500 600 700 800 900

2

4

6

8

10

Frequency spectra with 10 Hz resolution


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nX. Effects of non-idealities (I)

L Re

i

u

T1 D1

T2 D2

D3 T3

D4 T4

Tp1

Tp2

Tp3

Tp4

CDCVDC

+ vA vB

T4 D3 T4 T4 T4 D3 T4 T4 T4

T1 T1 T1 D2 T1 T1 T1 D2 T1A

B

v

+vtri

-vtri

vA, ref

vB, ref

vA

VDC/2

-VDC/2

vB

VDC/2

-VDC/2

i

VDC

u

t

t

t

t

Note that the semiconductor forward voltage drop affects the output voltageLoss of voltage-time area!


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X. Effects of non-idealities (II)

L Re

i

u

T1 D1

T2 D2

D3 T3

D4 T4

Tp1

Tp2

Tp3

Tp4

CDCVDC

+ vA vB

T4 D3 T4 T4

T1 T1 T1 D2A

B

v

+vtri

-vtri

vA, ref

vB, ref

vA

VDC/2

-VDC/2

vB

VDC/2

-VDC/2

i

VDC

u

t

t

t

t

T4

D2

Note that the blanking time affects the output voltageLoss of voltage-time area!


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X. Effects of non-idealities (III)

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02-20

-10

0

10

20

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02-400

-200

0

200

400

0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02-500

0

500

Forward Voltage drop: redBlanking Time: green

11 ,uu

au1

cba iii ,,


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X. Effects of non-idealities (IV)

Forward Voltage drop: redBlanking Time: green

u u


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nX. Effects of non-idealities (V)

• The proportional part of the controller is too “weak” to compensate. The only possible way to make it less weak is to increase the gain which would give an oscillatory behaviour

• The integral part act properly only on dc-components and not on dq-reference frame harmonics. To circumvent the last problem, rotating coordinate systems are included also for the frequencies where the harmonics appear.

The three-phase converter controller can not fully compensate for these non-idealities for two reasons:


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More InformationActive Power Line Conditioners are in most cases notcovered in books on power electronics, however:

On the home page of power electronics course:• The Power Electronics Handbook", edited by Timothy L

Skvarenina, CRC-press, 2002, ISBN 0-8493-7336-0.http://www.engnetbase.com/books/447/7336_pdf_toc.pdf

On Per Karlsson’s homepage:• M. Bojrup, (1999), "Advanced Control of Active Filters in a Battery

Charger Application", Licentiate thesis, Department of IndustrialElectrical Engineering and Automation, Lund Institute of Technology,Lund, Sweden, December 1999, ISBN 91-88934-13-6.http://www.iea.lth.se/~ielper/charger/MB-thesis.pdf


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Y. ConclusionsCommercial Semikron three-phase inverter

Suitable also for active power line conditioning!

73 kVA PWM three phase inverter with a long-life DC-link for motor drives

VDC-link = 600 V ± 10 %Voutput = 3 x 420 V (0,05 Hz ... 60 Hz)Inominal = 100 ARMS, Ioverload = 165 ARMS

fswitching = 1,5 kHz

SKiiP 342GD120 - 314 CTVP 16/280 F

9 900 µF / 800 V at DC-link

Documents

Power Quality Enhancing Systems Industrial Electrical